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Recent Advances in Polymer Solar Cells: Realization of High Device Performance by Incorporating Water/AlcoholSoluble Conjugated Polymers as Electrode Buffer Layer Zhicai He, Hongbin Wu,* and Yong Cao* interface to produce separated charge carriers (the process is often called “photoinduced electron transfer” between donor and acceptor) before recombination. In such a case, formation of an interpenetrating phase-separated D/A network with a large interfacial area and proper percolation for charge transport is believed to be a crucial factor for the charge separation.[2] Due to the ultrafast charge transfer (within 100 fs)[16,17] and the associated high quantum efficiency for charge separation process, quantum efficiency for the absorbed photon-to-electron process approaching unity have been demonstrated,[2,17] rendering blends of conjugated polymers with fullerene derivatives as one of the most promising materials for photovoltaic applications. After exciton dissociation, electron and hole will drift toward electrodes when the blend of the donor–acceptor is sandwiched between two metallic electrodes which can drive the photogenerated charge carriers through their asymmetrical work function.[18] Recent developments in the processing and patterning for active optical and electronic devices[19] via ink-jet printing, rollto-roll manufacturing, micro-contact printing, solution spray deposition and other soft lithography techniques have further verified the potential of polymer solar cells for low-cost, largearea optoelectronic devices. Due to these unique advantages, PSCs stands out as one non-CO2 emission energy source that can be exploited globally, without obvious detriment to the environment, and with sufficient theoretical capacity to meet the increasing electricity needs worldwide. Suppose that the level of solar energy utilization by solar cells will reach 1 TW someday in this century, and the average power conversion efficiency of the devices used is assumed to be 10%, then a total solar cells area of 100 km × 100 km is required. Despite the best power conversion efficiency (PCE) of the first generation solar cells based on crystalline silicon (Si) and gallium-arsenide (GaAs) have surpassed 25% and 29%,[20] respectively, which are approaching the Shockley-Queisser limit of 30%,[21] these devices are too expensive and their fabrication are very energy-intensive to make a significant impact on the energy production nowadays. Furthermore, it is not straightforward to reduce the manufacturing and material costs, and the associated installation costs of the first generation solar cells to a level suitable for widespread utilization in the near future. Therefore, novel manufacturing approaches like polymer solar
This Progress Report highlights recent advances in polymer solar cells with special attention focused on the recent rapid-growing progress in methods that use a thin layer of alcohol/water-soluble conjugated polymers as key component to obtain optimized device performance, but also discusses novel materials and device architectures made by major prestigious institutions in this field. We anticipate that due to drastic improvements in efficiency and easy utilization, this method opens up new opportunities for PSCs from various material systems to improve towards 10% efficiency, and many novel device structures will emerge as suitable architectures for developing the ideal rollto-roll type processing of polymer-based solar cells.
1. Introduction Considering that the sun will continue to provide huge amounts of energy (approximately 1.2 × 105 terawatts (1012 W, TW), which is about 10000 times of the present energy consumed globally) to the earth in the next 5 × 109 years, solar energy represents a renewable, reliable and substantial source of energy that human being can depend on. One of the most effective approaches to utilize solar energy is via photovoltaic technology that can directly convert absorbed sunlight into electricity.[1] As an emerging photovoltaic technology, polymer solar cells (PSCs), which are based on the combination of an electron donor (usually a p-type semiconducting conjugated polymer) and an electron acceptor (typically a n-type fullerene derivative) with a bulk heterojunction structure[2] and solution-processed process, have attracted enormous attention and continue to gain importance in view of their immense potential as promising, lightweight and low-cost energy source.[3–11] Upon absorption of solar photons in semiconducting polymer[5,10,11] or small molecules,[12–15]excitons (bound electron-hole pairs) are generated and may dissociate at the donor–acceptor (D/A)
Dr. Z. C. He, Prof. H. B. Wu, Prof. Y. Cao Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou, 510640, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201303391
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Zhicai He did his PhD research at South China University of Technology (SCUT) under Prof. Hongbin Wu, with a focus on the development of highly efficient polymer solar cells. He received a PhD in materials physics from the SCUT in June 2013. He recently joined the South China University of Technology to develop polymer solar cells.
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cells which are easier to implement with cheaper production cost toward large area (i.e., more than 100 km2) are highly desired. While meeting a portion of future global energy needs with solar energy via PSCs is technically possible, further improving devices performances especially their efficiency of PSCs towards 10–15%, which is the threshold for commercial applications by general consensus,[7] are in urgent need. To impove these device performances, many strategies have been intensively investigated. As a result of continuous efforts, the PCE of PSCs has increased dramatically from about 2.5%[22] in 2001 to exceeding 8% since 2011[23–29] or 10% more recently,[30–32] obtained through the development of new electron donor polyvmers,[3–8,26,33–56] new donor materials from small molecules[12–14] and new electron acceptor,[57–60] the invention of novel device structures,[23–25,61–69] and control of the nanoscale morphology of the photoactive layer[70–73] and the optimization of processing techniques via proper additives.[74] In view of device aspects, the PCE of PSCs can be enhanced through the optimization of the following device parameters, open circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF), which determine the power conversion efficiency of polymer solar cells 0P, via 0P = VOC ·JP SC ·F F , where IN PI N is the power intensity of incident light. Increasing the JSC can be achieved by increasing the thickness of the active layer thickness (in case where Beer-Lambert law still validates) and by maintaining a broad overlap with the solar spectrum by using low-band gap polymers.[75] In fact, developing new low-bandgap donor materials have been demonstrated as one of the most common and successful strategies to enhance the device performance in the past a few years,[3–8,10] although too thick active layer may tend to result in more geminate or nongeminate charge recombination which have been shown to be a significant loss mechanism for reduced photocurrent.[18,76,77] While low-bandgap polymers can shift the absorption spectrum of the active layer to longer wavelengths and usually can absorb more photons from the solar spectrum, however, a gain in efficiency can be expected only if a decrease of the opencircuit voltage is avoided.[18] As compared to other tow device parameters, FF is a more sensitive one and its reduction can be attributed to an increasing recombination losses associated with a combined effects in the photoactive layer, such as low charge carrier mobility, unbalanced charge transport, long exciton lifetime, low dielectric constant, the presence of traps states and a small separation distance between electrons and holes upon photoexcitation.[78] On the other hand, bad electrical properties of the solar cells such as a high series resistance and a low shunt resistance, which are associated with the contact at photoactive layer/electrodes (non-ohmic type, inferior diode quality) and low bulk conductivity of the device, are responsible for the reduced of FF and lead to deteriorated device performance. In addition to attempt to maximize JSC and FF of PSCs, enhancement of the VOC has also lead to an improvement in device performance, although the origin of VOC is still a controversial issue so far.[18,79] It had been suggested that VOC directly correlate with the difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, being nearly
Hongbin Wu is a professor in South China University of Technology, Guangzhou, the People’s Republic of China. He received a PhD in materials physics from the South China University of Technology under the supervision of Prof. Yong Cao in 2006 and joined the University as a research fellow at the same year and later was promoted as professor in 2010. His current research interests are polymer optoelectronic devices including light-emitting devices and solar cells. Yong Cao, physical chemist, academician of the Chinese Academy of Sciences, fellow of the World Academy of Sciences for the Advancement of Science in Developing Countries, professor of South China University of Technology, Guangzhou, the People's Republic of China since 1999. He obtained his B.S. from the department of chemistry, Leningrad (now San Petersburg) University and a Ph.D. degree from Tokyo University, Japan, respectively. He was visiting senior researcher at University of California, Santa Barbara in 1988–1990 and senior scientist at UNIAX Corporation in 1990–1998. His current research interests include polymer optoelectronic materials and corresponding devices.
independent on the work function of the electrode.[80] Consequently, it seems that one of the most promising strategies to enhance the VOC is to shift the LUMO of the donor closer to the vacuum level,[37,42,79,80] or to deepen the HOMO of the acceptor away from the vacuum level,[10,57–60] or the combination of both.[81,82] On the other hand, many previous reports
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clearly demonstrated that an increase in VOC offers a significant opportunity for substantial improvement.[37,83,84] Besides the aforementioned strategies that focus on the bulk properties of photoactive layer in polymer solar cells, engineering and modification the interface between the photoactive layer/metal electrodes also bear a promising potential in the enhancement of device performance of PSCs.[85–87] The performance of polymer solar cells, which are in stacked thin films architecture in nature, critically rely on the properties of interfaces between distinct materials to collect and extract photo-generated current.[88–90] While many fundamental questions concerning the generation of photo current and bulk properties of semiconducting polymers remain,[91,92] engineering and optimization of the interface between photoactive layer with electrode are equally challenging and important, and can be exploited to further enhance device performance. Therefore, the complicated and ill-defined nature of the interface in these devices can appear to be difficult hurdles to overcome, or instead present real opportunities to obtain maximum device performance for given material systems, especially nowadays new donor/acceptor materials catalogs continue to expand rapidly. Through interfacial engineering, energy alignment between at the photoactive layer/metal electrode can be optimized independently, thus can form ohmic contacts for facilitating charge transport and collection. From the lessons learned from polymer light-emitting devices (PLEDs) research,[93–96] incorporation of thin layer of alcohol/water-soluble conjugated polymers as photoactive layer/electrode interlayer into PSCs, which is fully compatibility with flexible substrates and roll-to-roll manufacturing, have been identified as a very effective approach to improve the device performance of PSCs.[23,97,98] Especially we demonstrated high efficiency of 8.37% in 2011 by simultaneously improving the open-circuit, short-circuit current density, and fill factor in a highly efficient PSCs through incorporating alcohol/watersoluble conjugated polymers as the cathode interlayer.[23] The improvements are mainly attributed to the energy barrier modulation of charge collection contacts by the interlayer molecules that have a specific functional group. More recently, we further proposed a new type of inverted device structure for highly efficient PSCs, in which an alcohol/water-soluble conjugated polymer was used as a novel ITO surface modifier.[26] As results of ohmic contact formed at the photoactive layer/ electrode interface for photogenerated charge-carrier collection and optimum photons harvest in the inverted device, a PCE of 9.214% was achieved. Due to drastic improvement in efficiency and easy utilization, we anticipate that this method opens new opportunities for PSCs from various material systems to improve towards 10% efficiency and many novel device structures will emerge as suitable architecture for developing the ideal roll-to-roll type processing of polymer-based solar cells. This Progress Reports focuses on the recent advances in polymer solar cells performance (efficiency, stability), achieved by using thin layer of alcohol/water-soluble conjugated polymers as critical electrode buffer layer, as well as the newly emerging device architectures. The understanding under the interface science, the origin of the enhanced device performance upon the incorporation of the electrode buffer layer and the device physics associated with the new device architecture
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are also addressed. The progress of using other catalogues of electrode buffer materials, such as metal oxides, small molecules for polymer solar cells can be referred in some of recent reviews.[85,86,90,99,100]
2. Efforts of Engineering the Active Layer/ Electrode Interface in Polymer Light-Emitting Devices Using Alcohol/Water-Soluble Conjugated Polymers as Electron Injection Layer In the past few years, water/alcohol soluble conjugated polymers (WSCPs) have attracted particular attention due to their successful applications in organic electronic devices, such as polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs).[101–103] Generally WSCPs are a kind of polymers that contain π-conjugated main chains which render them delocalized electronic structure and polar pendant groups (such as ammonium groups, sulfonate groups, phosphonate groups etc.) on their side chains which can increases their solubility in water and polar organic solvents such as methanol. Due to their unique processibility from water or other polar solvents in which most of conjugated polymers are insoluble, WSCPs can be used as environment-friendly active layer, charge injection/transport layer. This feature is particularly useful for the fabrication of multilayer organic electronic devices by solution processing, in which interfacial mixing active layer with subsequent electron injection layer can be avoided. In 2004, our group reported the synthesis of aminofunctionalized polyfluorene namely poly[(9,9-bis(3′-(N,Ndimethylamino) propyl)-2,7-fluorene) -alt-2,7-(9,9-dioctylfluorene)] (PFN) and poly[(9,9-bis(3’-(N,N-dimethylamino) propyl)2,7-fluorene)-alt-1,4-phenylene] (PFPN). The corresponding conjugated polyelectrolytes PFN-Br and PFPN-Br were prepared by a quaternization treatment of the precursor polymers with bromoethane.[94] The resulted charged polymers (PFN-Br, PFPN-Br) exhibited good solubility in polar solvents such as water, methanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), while the neutral polymers also showed good solubility in these solvents in the presence of a small quantity of acetic acid, due to the weak interaction between the nitrogen atoms in the side chain and the acetic acid.[104] Initial study indicated that when these amino-/ammonium-functionalized polyfluorene WSCPs as emissive layer, the obtained PLEDs using high work function metal Al as cathode exhibited even higher efficiency than those with low-work function metals Ba cathode.[94] For example, PFN showed a maximal external quantum efficiency (EQE) of 0.38% in PLEDs with a configuration of ITO/PVK/PFN/Al, while an EQE of only 0.12% was observed in the device with Ba cathode. These results were unusual since typically low work function metals are more prone to form Ohmic contact for electron injection into organic semiconductors due to its higher lying Fermi level. Later, we further demonstrated a novel device structure of ITO/anode buffer layer/emission layer/thin layer of WSCP/Al for highly efficient PLEDs, and it clearly confirmed that significant improvement of electron injection and device performance can be realized via this simple method.[93] In addition to Al, these amino-/ammonium-functionalized polyfluorene WSCPs
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Figure 1. Photovoltaic characteristics of devices ITO/PEDOT/PVK/PFO/ PFN /Au with varied PFN layer thickness (0–30 nm). Reproduced with permission.[95] Copyright 2005, Elsevier.
also worked well with varieties of other high work-function metals (φ > 4.1 eV), such as In (φ = 4.12 eV), Ag (φ = 4.26 eV), Sn (φ = 4.42 eV), Cu (φ = 4.65 eV), and Au (φ = 5.20 eV).[95] Unlike the performance of conventional electron injection materials, such as lithium fluoride (LiF),[105] cesium fluoride (CsF)[106] and organic surfactants[107] in PLEDs, which is critically dependent on the cathode metals, the use of WSCPs as electron injection layer shows very weak dependence on metals. This fact implies that the role of WSCPs at the active layer/ metal interface differs with that of the LiF/Al cathode. Furthermore, the improved device efficiency upon the incorporation of WSCPs showed direct and strong correlations with their thickness, which is in good agreement with their built-in potential (Vbi) across the device as measured by photovoltaic response. For this reason, the Vbi of the devices can be continuously tuned in a wide range of 1.0 V with different PFN thicknesses (Figure 1). In combination with the J-L-V characteristics of the PLEDs and photovoltaic measurement, we concluded that the increase in an electron injection and the device efficiency is due to the reduction of barrier height in the cathode interface, as the results of change in the energy level alignment between the EL polymer/metal electrode.[93,95] We also proposed that interfacial dipole formation be the most likely origin of the reduction of barrier height between the high work function metals and EL polymers,[88,93,95] which introduce an abrupt, rigid shift of vacuum level at the interface and alter the work function or surface potential of the metals. In contrast with metal-inorganic semiconductor contact, in which band alignment is governed by well-established metalinduced gap states (MIGS),[108] a comprehensive understanding of the energy level alignment in metal-organic interface or metal-organic contact barrier height remains a challenge, thus there are no simple physical pictures that can be provided to
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Figure 2. Possible factors forming and affecting the interfacial dipole layer summarized by Prof. Kazuhiko Seki et al.[88] a1) and a2): Charge transfer across the interface; b) Concentration of electrons in the adsorbate leading to positive charging of the vacuum side; c) Rearrangement of electron cloud at the metal surface, with the reduction of tailing into vacuum; d) Strong chemical interaction between the surface and the adsorbate leading to the rearrangement of the electronic cloud and also the molecular and surface geometries (both directions of dipoles possible); e) Existence of interface state serving as a buffer of charge carriers, and f) Orientation of polar molecules or functional groups. Reproduced with permission.[88] Copyright 1998, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
describe the metal-organic interface properties. In the 1990s it was proposed that the traditional assumption of a common vacuum level alignment (Schottky-Mott rule) turned out to be invalid if one or combinations of the following exists:[88,89] 1) formation of interfacial dipole; 2) reactive metal-organic chemical reaction induced gap states (including chemical doping) or chemisorption or bond formation; 3) metal-organic chemical reaction induced defects and Fermi level pinning occurs; 4) charge neutrality level; 5) band bending. Among all of the above-mentioned approaches/mechanisms that can lead to band offset at the metal-organic interface, interface dipole is of special research interest because it can provide flexible implement approach due to unique characteristic of organic materials.[109] Research carried out all over the world in the last two decades had significantly increased our knowledge about the origin of interface dipole in across metalorganic interface. Various possible origins of interfacial dipole have been proposed and widely discussed,[88] including charge transfer across the interface (Figure 2 a1-a2), molecular polarization induced by mirror image force effect (Figure 2b), “push back” effect on the electron cloud tail out of the metal surface by organic molecule and consequent redistribution of electron cloud (Figure 2c), chemisorption or interfacial chemical reaction and consequent formation of chemical bonds (Figure 2d), and metal-induced interface state (Figure 2e) and surface realignment of the permanent dipole of polar molecular (Figure 2f).
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Therefore, one of the most feasible methods towards interfacial dipole formation is to intentionally attach self-assembled monolayer or polar molecules with favorable permanent dipole to the electrode surface during the process of device fabrication. The surface potential shift (shift in a vacuum level) due to adsorbed dipoles can be written by the Helmholtz equation in a simple electrostatic approach[110]: N = Ng :r gmol , where N is 0 the areal density of absorbed molecules, : mol the net vertical component of the interfacial dipole moment of an individual molecule, g r the relative dielectric constant of the molecules and g 0 the permittivity of vacuum. Campbell et al. demonstrated the control over the work function of Ag substrate (thus the Schottky energy barriers with organic semiconductors) in a wide range of 1 eV via the insertion of a set of self-assembling monolayer polar alkane thiole derivatives with various magnitude and direction of dipole moment between the metal and the organic material.[109] They also verified that the builtin potential of a metal/polymer/metal structure as determined by electroabsorption measurement is in a nearly same amount with the change in work function.
3. Polymer Solar Cells with Alcohol/Water-Soluble Conjugated Polymers as Cathode Buffer Layer Inspired by the success of applying alcohol/water-soluble conjugated polymers as electron injection layer in PLEDs, our group is among the first ones who validated this method in polymer solar cells in 2009.[111] Luo et al. reported that the VOC of PSCs based on specific system can be enhanced by up to 0.35 V by using a thin layer of poly [(9,9-bis{3′-[(N,N-dimethyl)-N–ethylammonium]propyl}-2, 7-fluorine)-2,7(9,9-dioctylfluorene)-co-(4,7,-dithien2-yl)-2,1,3-benzotiadiazole] dibromide (PFNBr-DBT15) or poly[(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorene)alt-1,4-phenylene] dibromide (PFPNBr), as interlayer between active layer and metal electrode. For PSCs based on poly[2,7-(9,9-dioctylfluorene)-co- (4,7-dithien-2-yl)-2,1,3-benzothiadiazole] (PF-DBT35) and PCBM blend, it is worth noting that all of the devices with interlayer were characterized with enhanced open-circuit as high as 0.95 ± 0.05 V, while in contrast only a moderate VOC of 0.65 ± 0.05 V was observed in the routine cathode devices (Figure 3a). The observed enhancement of VOC was found to be in good agreement with increased built-in potential across the devices, as revealed by the J-V characteristics of the devices in the dark (Figure 3b). Similarly, Zhao et al.[112] and Na et al.[113] reported the applications of other fluorene-based alcohol polymers, such as conjugated poly (9,9-bis(60-diethoxylphosphorylhexyl)fluorene) (PF-EP) and a polyfluorene derivative which has hybrid quaternary ammonium end-capped alkyl/alkoxyl side chains named poly[(9,9-bis((6′-(N,N,N-trimethylammonium)hexyl)2,7-fluorene)-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-9-fluorene)) dibromide (WPF-oxy-F), respectively, as cathode interlayer for polymer solar cells in 2009. Zhao et al. found that upon the incorporation of a thin layer PF-EP of 5 nm, the shunt resistance polymer solar cells based on P3HT: PCBM was effectively increased from 30.7 kΩ cm−2 to 1.14 MΩ cm−2, while the VOC was improved to 0.64 V (0.45 V for the control device) and the
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Figure 3. a) J–V characteristics of devices with a structure of ITO/ PEDOT:PSS/PF-DBT35:PCBM (1:2, w/w) (70–80 nm)/various cathode under the illumination of an AM 1.5 G solar simulator (800 W m−2) b) A replot of Fig. 1a in double-log (open) together with the J-V characteristics of the devices in the dark (filled), Ba/Al cathode (circles), Al cathode (squares), PFNBr-DBT15/Al (diamonds) and PFPNBr/Al (triangles). Reproduced with permission.[111] Copyright 2009, American Institute of Physics.
fill factor was improved from 51% to 59%, resulting in a PCE of 3.38% (1.98% for the control device). The overall performance improvements were mainly attributed to more effective preventing penetration of the hot metal atoms, which may lead to significant electrical leakage and quenching cites for photo-generated excitons. In parallel, Na et al. demonstrated that through the use of a thin layer of WPF-oxy-F as interlayer between the photoactive layer and metal electrode, the PCE of polymer solar cells based on poly(3-hexylthiophene) (P3HT): PCBM increased from 2.95% to 3.77%, mainly due to the improved VOC and FF resulting from the reduction of the work function of Al cathode.
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It is noteworthy that the VOC of the devices showed weak dependence on the thickness of the WPF-oxy-F interlayer in range of 3–6 nm, and the obtained VOC of 0.68 V was obviously higher than that of the control device (∼0.57 V). Furthermore, Oh et al. had applied this strategy to other air-stable, high work function metals such as Ag, Au and Cu, resulting in encouraging device performances with PCE above 3% and nearly cathode-independent open-circuit voltage.[114] The improved performance (VOC and PCE) was attributed mainly due to the reduced Schottky-barrier height resulted from the decrease of work function of these noble metals, as indicated by the Kelvin probe under N2 atmosphere. Afterwards, we and the Bazan group[115] investigated the origin of VOC enhancement in PSCs based on different donor materials via interfacial modification using four polyfluorenebased, ionic type WSCPs (PF-X, X for different cationic compensating counterions: Br, trifluoromethanesulfonate, tetrakis(1-imidazolyl)borate and tetrakis[3,5-bis(trifluoromethyl) phenyl]borate), along with PFN.[94] It was found that obvious VOC enhancement can be observed in devices that contain a polyfluorene copolymer as the donor material, regardless of the choice of the choice of compensating ions, while no significant enhancement in VOC was found in devices from P3HT and poly(2-methoxyl-5-((2′-ethylhexyl)oxyl)-1,4-phenylenevinylene) (MEH-PPV), probably due to the highly hydrophobic nature of the active layer.[116] On the other side, it was found that there is a close correlation between the dark J–V characteristics with the VOC enhancement. For instance, the dark currents of those devices with the interlayers and enhanced VOC were found to be 1–2 orders of magnitude lower than that of the control devices, which corresponds to an increase of 0.05–0.10 V in VOC, as revealed by the Shockley equation. Therefore, these observations from the electrical characteristics aspects can lead to deeper understanding on the VOC enhancement in PSCs. Despite the effects of the WSCPs was found to critically depend on chemical structure of the donor materials,[115] we reported that PFN/Al cathode can work very well with a newly synthesized low band gap donor poly[(4,5-ethylene-2,7-carbazole) -5,8-bis(2′-thienyl)-2,3-bis (4-octyloxyphenyl) quinoxaline] (PECz-DTQx) later.[97] With a blend of PECz-DTQx and [6,6]phenyl C71 butyric acid methyl ester (PC71BM) as the photoactive layer, the device with Al cathode displayed an initial PCE of 3.99%, while a high PCE of 6.07% was realized for device with PFN was introduced as the cathode interlayer, probably due to strong N–N interactions between the N-heterocycle-containing polymer donor and the PFN layer. For the past several years, it witnessed the progress of the synthesis of novel low bandgap donor materials and their successful applications in high performance PSCs.[10] However, the trade-off between light harvesting (JSC) and open-circuit voltage (VOC) in low bandgap donor material systems seems to present as a difficult hurdle towards realization of high efficiency ∼10%. Aiming at addressing this problem, more recently we applied the method of incorporating PFN as cathode interlayer in polymer solar cells using promising low bandgap donor materials such as poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5- (4′,7′-di2-thienyl-2′,1′,3′- benzothiadiazole) (PCDTBT)[36] and poly[{4,8bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}]
Figure 4. The effect of PFN interlayer on PCDTBT:PC71BM solar cell performance. a). Current density versus voltage (J–V) characteristics of devices with (solid red circle) and without the PFN interlayer (open black circle) measured under 1000 W m−2 AM 1.5 G illumination. b). J–V characteristics of devices with (solid red circle) and without the interlayer (open black circle) in the dark. Reproduced with permission.[23] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
(PTB7)[45] as electron acceptor, and PC71BM as electron donor, and simultaneously achieved improvements in all of the device performance parameters, including VOC, JSC, and FF (see Figure 4a).[23] For instance, compared with the control device based on Al cathode, a simultaneous enhancement in VOC (0.90 V vs. 0.70 V), JSC (12.1 mA cm−2 vs. 11.4 mA cm−2) and fill factor (62% vs. 49%) was achieved in a device with PFN incorporated as cathode interlayer between PCDTBT:PC71BM and Al, leading to a significant improvement in the PCE by ∼65% (6.73% vs. 4.02%). Figure 4b showed the typical current density
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Figure 5. Electric dipole moment of the interlayer probed by Scanning Kelvin Probe Microscopy (SKPM). a-b) Topographic image (a) and surface potential image (b) of device active layer area partially covered with the interlayer. Scale bars in both images represent 10 mm. c) Schematic illustration of the experimental setup. d) Cross-sectional line profile of the topographic and surface potential images in a and b. e-f) the energy band structure of the devices without (e) and with (f) the interlayer under short-circuit condition (with zero external applied bias) constructed from the observed SKPM results. Reproduced with permission.[23] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
versus voltage (J–V) characteristics of the devices without and with the interlayer, in which a turn-on voltage of about 0.8–1.0 V was clearly seen in the latter device while it is only 0.5–0.6 V for the control devices, implying an enhanced built-in potential across the device. What's more, the PFN interlayer was found to be effective when in combination with low work function metal such as Ca. For example, when a PFN/Ca/Al cathode is used for PCDTBT: PC71BM device, a PCE of 6.79% was achieved, also obviously higher than that based on Ca/Al cathode (4.11%). The abovementioned observation agrees with the literatures that report high work function metals with interlayer may work as low work function electrodes thus can lead to reduced electron injection barrier and increased built-in potential as a result of the formation of interfacial dipole.[93,95] Given these facts, we proposed that interfacial dipole due to the strong interaction between the interlayer and the photoactive layer and a consequent vacuum level shift at the interface were responsible for the origin of the enhancement in VOC. To identify this speculation, the interface dipole was directly probed by scanning Kelvin probe microscopy (SKPM), with the experimental setup depicted in Figure 5c. The surface is quite smooth in both the active layer and the interlayer areas (Figure 5a) and as expected, the surface potential is uniform within both areas. But it is worth mentioning that the surface potential of the interlayer area is about 300 mV more positive than that of the active layer without interlayer (Figure 5b and d), which corresponds to an interfacial dipole moment with the positive charge end pointing toward the Al electrode and the negative charge end pointing toward the photoactive layer. Furthermore, the direction of electric field of the interfacial dipole is aligned with the builtin field inside the device; hence, the actual built-in potential across the device is reinforced as a result of the superposition (Figure 5f). Furthermore, the electrical field associated with the interfacial dipole layer is of 6 × 105 V cm−1, which is about one
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order of magnitude higher than that of average built-in field in devices without the PFN interlayer under short-circuit condition (Figure 5e). Besides the straightforward effect on the enhancement in VOC, we found that the improvements in Jsc and FF upon the incorporation of the PFN interlayer involved multiple origins including enhanced and more balanced charge carrier mobilities, elimination of serve space charge buildup, reduced bimolecular recombination and improved charge collection efficiency under working conditions.[23] In pursuit of highly efficient polymer solar cells, we also examined the effects of introduction of PFN interlayer in devices based on a promising low bandgap donor, PTB7 (see also Table 1). The PCE increased from 5.00% to 8.04%, and 7.13% to 8.22%, when Al and Ca were employed as metal electrode, respectively, both accompanied by simultaneous enhancements in VOC, Jsc and FF (Figure 6a). To the best of our knowledge, this is the first scientific report on PSCs with efficiency over 8%, representing a key advance towards a new regime for PSCs efficiency. Therefore, the results clearly showed that the interlayers provides a new dimension in device design, which enables independent control of VOC, JSC, which can be exploited avoid the trade-off between VOC and JSC, and resulted in simultaneous enhancement of VOC, JSC and FF. Importantly, the device performance of the PTB7 devices was independently verified by the National Center of Supervision & Inspection on Solar Photovoltaic Products Quality of China (CPVT), which is a third-party national PV products test center of China and one of International Laboratory Accreditation Cooperation (ILAC) affiliated laboratories. Moreover, the measurements from this independent source yield results very similar to what we obtained in our laboratory, resulting in a PCE of 8.37%, with a VOC of 0.7563 V, a Jsc of 15.75 mA cm−2 and a FF of 70.15%.
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Interlayer Catalog polyfluorene
Interlayer
Interlayer thickness (nm)
Device structure
VOC (V)
JSC (mA cm−2)
FF (%)
PCE (%)
Ref
PFN
/
ITO/PEDOT/PTB7:PC71BM/Ca/Al
0.70
15.5
66.0
7.13
[23]
5
ITO/PEDOT/PTB7:PC71BM /PFN/Ca/Al
0.76
16.1
69.0
8.22
ITO/PEDOT/PTB7:PC71BM /PFN/Ca/Al
0.7563
15.75
70.15
8.370a)
/
ITO/PEDOT/PCDTBT:PC71BM/Al
0.70
11.3
49.0
3.95
5
ITO/PEDOT/PCDTBT:PC71BM /PFN/Al
0.90
12.1
62.0
6.73
/
ITO/PEDOT/PECz-DTQx :PC71BM/Al
0.78
9.10
56.3
3.99
/
ITO/PEDOT/PECz-DTQx :PC71BM/Ca/Al
0.80
9.84
57.4
4.52
5
ITO/PEDOT/PECz-DTQx :PC71BM/PFN/Al
0.81
11.4
65.8
6.07
5
ITO/PEDOT/PCDTBT:PC71BM/PFN/Al
0.88
6.12
60.1
6.33c)
PEDOT/GMS/PCDTBT:PC71BM/PFN/Al
0.84
6.09
59.0
5.90c)
ITO/PEDOT/PTB7:PC71BM/PFN/Al
0.72
8.24
67.4
7.77c)
PEDOT/GMS/PCDTBT:PC71BM/PFN/Al
0.70
8.22
64.8
7.06c)
/
ITO/PEDOT/PCz-DTBTA:PC61BM/Al
0.80
3.91
48.4
1.51
5
ITO/PEDOT/PCz-DTBTA:PC61BM/PFN/Al
0.90
4.68
65.3
2.75
/
ITO/PEDOT/PFO-DBT35:PC61BM/Al
0.65
3.9
38.0
1.0
5
ITO/PEDOT/PFO-DBT35:PC61BM /PFN-Br/Al
1.00
4.7
45.0
2.1
/
ITO/PEDOT/PFO-DBT35:PC61BM/Al
0.92
3.41
47.5
1.46
5
ITO/PEDOT/PFO-DBT35:PC61BM/PF-BIm4/Al
1.07
3.41
48.8
1.75
PFNBr
PF-(halogen ionic)
PF-EP
WPF-oxy
PFCn6:K+
PNOs
PFNSO
polythiophene
PT3MAHT
PTMAHT
P3HTN
polycarbazole
non-conjugated polymers
Small molecule
PC-NOH
PEO
Rhodamine 101
/
ITO/PEDOT/P3HT:PC61BM/Al
0.45
8.57
51.0
1.98
5
ITO/PEDOT/P3HT:PC61BM /PF-EP/Al
0.64
9.01
59.0
3.38
/
ITO/PEDOT/P3HT:PC61BM/Al
0.57
5.40
35.4
1.09
3∼6
ITO/PEDOT/P3HT:PC61BM/WPF-oxy-F/Al
0.68
6.09
45.6
1.89
/
ITO/PEDOT/P3HT:PC61BM/Al
0.53
9.04
48.0
2.30
3
ITO/PEDOT/P3HT:PC61BM/WPF-6-oxy-F/Al
0.64
10.08
60.0
3.89
/
ITO/PEDOT/P3HT:ICBA/Ca/Al
0.85
10.43
65.2
5.78
5
ITO/PEDOT/P3HT:ICBA/PFCn6:K+/Ca/Al
0.89
11.65
72.6
7.50
/
ITO/PEDOT/PCDTBT:PC71BM/Al
0.65
11.4
53.8
4.0
5∼10
ITO/PEDOT/PCDTBT:PC71BM/ PF6NO25Py/Al
0.91
11.6
66.2
6.9
/
ITO/PEDOT/PTB7:PC71BM/Al
0.55
16.33
56.3
5.03
5∼10
ITO/PEDOT/PTB7:PC71BM /PFNS/Al
0.73
16.38
73.1
8.74
/
ITO/PEDOT/PCDTBT:PC71BM/Al
0.82
9.7
61.0
5.00
PROGRESS REPORT
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Recent Advances in Polymer Solar Cells: Realization of High Device Performance by Incorporating Water/AlcoholSoluble Conjugated Polymers as Electrode Buffer Layer Zhicai He, Hongbin Wu,* and Yong Cao* interface to produce separated charge carriers (the process is often called “photoinduced electron transfer” between donor and acceptor) before recombination. In such a case, formation of an interpenetrating phase-separated D/A network with a large interfacial area and proper percolation for charge transport is believed to be a crucial factor for the charge separation.[2] Due to the ultrafast charge transfer (within 100 fs)[16,17] and the associated high quantum efficiency for charge separation process, quantum efficiency for the absorbed photon-to-electron process approaching unity have been demonstrated,[2,17] rendering blends of conjugated polymers with fullerene derivatives as one of the most promising materials for photovoltaic applications. After exciton dissociation, electron and hole will drift toward electrodes when the blend of the donor–acceptor is sandwiched between two metallic electrodes which can drive the photogenerated charge carriers through their asymmetrical work function.[18] Recent developments in the processing and patterning for active optical and electronic devices[19] via ink-jet printing, rollto-roll manufacturing, micro-contact printing, solution spray deposition and other soft lithography techniques have further verified the potential of polymer solar cells for low-cost, largearea optoelectronic devices. Due to these unique advantages, PSCs stands out as one non-CO2 emission energy source that can be exploited globally, without obvious detriment to the environment, and with sufficient theoretical capacity to meet the increasing electricity needs worldwide. Suppose that the level of solar energy utilization by solar cells will reach 1 TW someday in this century, and the average power conversion efficiency of the devices used is assumed to be 10%, then a total solar cells area of 100 km × 100 km is required. Despite the best power conversion efficiency (PCE) of the first generation solar cells based on crystalline silicon (Si) and gallium-arsenide (GaAs) have surpassed 25% and 29%,[20] respectively, which are approaching the Shockley-Queisser limit of 30%,[21] these devices are too expensive and their fabrication are very energy-intensive to make a significant impact on the energy production nowadays. Furthermore, it is not straightforward to reduce the manufacturing and material costs, and the associated installation costs of the first generation solar cells to a level suitable for widespread utilization in the near future. Therefore, novel manufacturing approaches like polymer solar
This Progress Report highlights recent advances in polymer solar cells with special attention focused on the recent rapid-growing progress in methods that use a thin layer of alcohol/water-soluble conjugated polymers as key component to obtain optimized device performance, but also discusses novel materials and device architectures made by major prestigious institutions in this field. We anticipate that due to drastic improvements in efficiency and easy utilization, this method opens up new opportunities for PSCs from various material systems to improve towards 10% efficiency, and many novel device structures will emerge as suitable architectures for developing the ideal rollto-roll type processing of polymer-based solar cells.
1. Introduction Considering that the sun will continue to provide huge amounts of energy (approximately 1.2 × 105 terawatts (1012 W, TW), which is about 10000 times of the present energy consumed globally) to the earth in the next 5 × 109 years, solar energy represents a renewable, reliable and substantial source of energy that human being can depend on. One of the most effective approaches to utilize solar energy is via photovoltaic technology that can directly convert absorbed sunlight into electricity.[1] As an emerging photovoltaic technology, polymer solar cells (PSCs), which are based on the combination of an electron donor (usually a p-type semiconducting conjugated polymer) and an electron acceptor (typically a n-type fullerene derivative) with a bulk heterojunction structure[2] and solution-processed process, have attracted enormous attention and continue to gain importance in view of their immense potential as promising, lightweight and low-cost energy source.[3–11] Upon absorption of solar photons in semiconducting polymer[5,10,11] or small molecules,[12–15]excitons (bound electron-hole pairs) are generated and may dissociate at the donor–acceptor (D/A)
Dr. Z. C. He, Prof. H. B. Wu, Prof. Y. Cao Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou, 510640, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201303391
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Zhicai He did his PhD research at South China University of Technology (SCUT) under Prof. Hongbin Wu, with a focus on the development of highly efficient polymer solar cells. He received a PhD in materials physics from the SCUT in June 2013. He recently joined the South China University of Technology to develop polymer solar cells.
PROGRESS REPORT
cells which are easier to implement with cheaper production cost toward large area (i.e., more than 100 km2) are highly desired. While meeting a portion of future global energy needs with solar energy via PSCs is technically possible, further improving devices performances especially their efficiency of PSCs towards 10–15%, which is the threshold for commercial applications by general consensus,[7] are in urgent need. To impove these device performances, many strategies have been intensively investigated. As a result of continuous efforts, the PCE of PSCs has increased dramatically from about 2.5%[22] in 2001 to exceeding 8% since 2011[23–29] or 10% more recently,[30–32] obtained through the development of new electron donor polyvmers,[3–8,26,33–56] new donor materials from small molecules[12–14] and new electron acceptor,[57–60] the invention of novel device structures,[23–25,61–69] and control of the nanoscale morphology of the photoactive layer[70–73] and the optimization of processing techniques via proper additives.[74] In view of device aspects, the PCE of PSCs can be enhanced through the optimization of the following device parameters, open circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF), which determine the power conversion efficiency of polymer solar cells 0P, via 0P = VOC ·JP SC ·F F , where IN PI N is the power intensity of incident light. Increasing the JSC can be achieved by increasing the thickness of the active layer thickness (in case where Beer-Lambert law still validates) and by maintaining a broad overlap with the solar spectrum by using low-band gap polymers.[75] In fact, developing new low-bandgap donor materials have been demonstrated as one of the most common and successful strategies to enhance the device performance in the past a few years,[3–8,10] although too thick active layer may tend to result in more geminate or nongeminate charge recombination which have been shown to be a significant loss mechanism for reduced photocurrent.[18,76,77] While low-bandgap polymers can shift the absorption spectrum of the active layer to longer wavelengths and usually can absorb more photons from the solar spectrum, however, a gain in efficiency can be expected only if a decrease of the opencircuit voltage is avoided.[18] As compared to other tow device parameters, FF is a more sensitive one and its reduction can be attributed to an increasing recombination losses associated with a combined effects in the photoactive layer, such as low charge carrier mobility, unbalanced charge transport, long exciton lifetime, low dielectric constant, the presence of traps states and a small separation distance between electrons and holes upon photoexcitation.[78] On the other hand, bad electrical properties of the solar cells such as a high series resistance and a low shunt resistance, which are associated with the contact at photoactive layer/electrodes (non-ohmic type, inferior diode quality) and low bulk conductivity of the device, are responsible for the reduced of FF and lead to deteriorated device performance. In addition to attempt to maximize JSC and FF of PSCs, enhancement of the VOC has also lead to an improvement in device performance, although the origin of VOC is still a controversial issue so far.[18,79] It had been suggested that VOC directly correlate with the difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, being nearly
Hongbin Wu is a professor in South China University of Technology, Guangzhou, the People’s Republic of China. He received a PhD in materials physics from the South China University of Technology under the supervision of Prof. Yong Cao in 2006 and joined the University as a research fellow at the same year and later was promoted as professor in 2010. His current research interests are polymer optoelectronic devices including light-emitting devices and solar cells. Yong Cao, physical chemist, academician of the Chinese Academy of Sciences, fellow of the World Academy of Sciences for the Advancement of Science in Developing Countries, professor of South China University of Technology, Guangzhou, the People's Republic of China since 1999. He obtained his B.S. from the department of chemistry, Leningrad (now San Petersburg) University and a Ph.D. degree from Tokyo University, Japan, respectively. He was visiting senior researcher at University of California, Santa Barbara in 1988–1990 and senior scientist at UNIAX Corporation in 1990–1998. His current research interests include polymer optoelectronic materials and corresponding devices.
independent on the work function of the electrode.[80] Consequently, it seems that one of the most promising strategies to enhance the VOC is to shift the LUMO of the donor closer to the vacuum level,[37,42,79,80] or to deepen the HOMO of the acceptor away from the vacuum level,[10,57–60] or the combination of both.[81,82] On the other hand, many previous reports
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clearly demonstrated that an increase in VOC offers a significant opportunity for substantial improvement.[37,83,84] Besides the aforementioned strategies that focus on the bulk properties of photoactive layer in polymer solar cells, engineering and modification the interface between the photoactive layer/metal electrodes also bear a promising potential in the enhancement of device performance of PSCs.[85–87] The performance of polymer solar cells, which are in stacked thin films architecture in nature, critically rely on the properties of interfaces between distinct materials to collect and extract photo-generated current.[88–90] While many fundamental questions concerning the generation of photo current and bulk properties of semiconducting polymers remain,[91,92] engineering and optimization of the interface between photoactive layer with electrode are equally challenging and important, and can be exploited to further enhance device performance. Therefore, the complicated and ill-defined nature of the interface in these devices can appear to be difficult hurdles to overcome, or instead present real opportunities to obtain maximum device performance for given material systems, especially nowadays new donor/acceptor materials catalogs continue to expand rapidly. Through interfacial engineering, energy alignment between at the photoactive layer/metal electrode can be optimized independently, thus can form ohmic contacts for facilitating charge transport and collection. From the lessons learned from polymer light-emitting devices (PLEDs) research,[93–96] incorporation of thin layer of alcohol/water-soluble conjugated polymers as photoactive layer/electrode interlayer into PSCs, which is fully compatibility with flexible substrates and roll-to-roll manufacturing, have been identified as a very effective approach to improve the device performance of PSCs.[23,97,98] Especially we demonstrated high efficiency of 8.37% in 2011 by simultaneously improving the open-circuit, short-circuit current density, and fill factor in a highly efficient PSCs through incorporating alcohol/watersoluble conjugated polymers as the cathode interlayer.[23] The improvements are mainly attributed to the energy barrier modulation of charge collection contacts by the interlayer molecules that have a specific functional group. More recently, we further proposed a new type of inverted device structure for highly efficient PSCs, in which an alcohol/water-soluble conjugated polymer was used as a novel ITO surface modifier.[26] As results of ohmic contact formed at the photoactive layer/ electrode interface for photogenerated charge-carrier collection and optimum photons harvest in the inverted device, a PCE of 9.214% was achieved. Due to drastic improvement in efficiency and easy utilization, we anticipate that this method opens new opportunities for PSCs from various material systems to improve towards 10% efficiency and many novel device structures will emerge as suitable architecture for developing the ideal roll-to-roll type processing of polymer-based solar cells. This Progress Reports focuses on the recent advances in polymer solar cells performance (efficiency, stability), achieved by using thin layer of alcohol/water-soluble conjugated polymers as critical electrode buffer layer, as well as the newly emerging device architectures. The understanding under the interface science, the origin of the enhanced device performance upon the incorporation of the electrode buffer layer and the device physics associated with the new device architecture
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are also addressed. The progress of using other catalogues of electrode buffer materials, such as metal oxides, small molecules for polymer solar cells can be referred in some of recent reviews.[85,86,90,99,100]
2. Efforts of Engineering the Active Layer/ Electrode Interface in Polymer Light-Emitting Devices Using Alcohol/Water-Soluble Conjugated Polymers as Electron Injection Layer In the past few years, water/alcohol soluble conjugated polymers (WSCPs) have attracted particular attention due to their successful applications in organic electronic devices, such as polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs).[101–103] Generally WSCPs are a kind of polymers that contain π-conjugated main chains which render them delocalized electronic structure and polar pendant groups (such as ammonium groups, sulfonate groups, phosphonate groups etc.) on their side chains which can increases their solubility in water and polar organic solvents such as methanol. Due to their unique processibility from water or other polar solvents in which most of conjugated polymers are insoluble, WSCPs can be used as environment-friendly active layer, charge injection/transport layer. This feature is particularly useful for the fabrication of multilayer organic electronic devices by solution processing, in which interfacial mixing active layer with subsequent electron injection layer can be avoided. In 2004, our group reported the synthesis of aminofunctionalized polyfluorene namely poly[(9,9-bis(3′-(N,Ndimethylamino) propyl)-2,7-fluorene) -alt-2,7-(9,9-dioctylfluorene)] (PFN) and poly[(9,9-bis(3’-(N,N-dimethylamino) propyl)2,7-fluorene)-alt-1,4-phenylene] (PFPN). The corresponding conjugated polyelectrolytes PFN-Br and PFPN-Br were prepared by a quaternization treatment of the precursor polymers with bromoethane.[94] The resulted charged polymers (PFN-Br, PFPN-Br) exhibited good solubility in polar solvents such as water, methanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), while the neutral polymers also showed good solubility in these solvents in the presence of a small quantity of acetic acid, due to the weak interaction between the nitrogen atoms in the side chain and the acetic acid.[104] Initial study indicated that when these amino-/ammonium-functionalized polyfluorene WSCPs as emissive layer, the obtained PLEDs using high work function metal Al as cathode exhibited even higher efficiency than those with low-work function metals Ba cathode.[94] For example, PFN showed a maximal external quantum efficiency (EQE) of 0.38% in PLEDs with a configuration of ITO/PVK/PFN/Al, while an EQE of only 0.12% was observed in the device with Ba cathode. These results were unusual since typically low work function metals are more prone to form Ohmic contact for electron injection into organic semiconductors due to its higher lying Fermi level. Later, we further demonstrated a novel device structure of ITO/anode buffer layer/emission layer/thin layer of WSCP/Al for highly efficient PLEDs, and it clearly confirmed that significant improvement of electron injection and device performance can be realized via this simple method.[93] In addition to Al, these amino-/ammonium-functionalized polyfluorene WSCPs
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Figure 1. Photovoltaic characteristics of devices ITO/PEDOT/PVK/PFO/ PFN /Au with varied PFN layer thickness (0–30 nm). Reproduced with permission.[95] Copyright 2005, Elsevier.
also worked well with varieties of other high work-function metals (φ > 4.1 eV), such as In (φ = 4.12 eV), Ag (φ = 4.26 eV), Sn (φ = 4.42 eV), Cu (φ = 4.65 eV), and Au (φ = 5.20 eV).[95] Unlike the performance of conventional electron injection materials, such as lithium fluoride (LiF),[105] cesium fluoride (CsF)[106] and organic surfactants[107] in PLEDs, which is critically dependent on the cathode metals, the use of WSCPs as electron injection layer shows very weak dependence on metals. This fact implies that the role of WSCPs at the active layer/ metal interface differs with that of the LiF/Al cathode. Furthermore, the improved device efficiency upon the incorporation of WSCPs showed direct and strong correlations with their thickness, which is in good agreement with their built-in potential (Vbi) across the device as measured by photovoltaic response. For this reason, the Vbi of the devices can be continuously tuned in a wide range of 1.0 V with different PFN thicknesses (Figure 1). In combination with the J-L-V characteristics of the PLEDs and photovoltaic measurement, we concluded that the increase in an electron injection and the device efficiency is due to the reduction of barrier height in the cathode interface, as the results of change in the energy level alignment between the EL polymer/metal electrode.[93,95] We also proposed that interfacial dipole formation be the most likely origin of the reduction of barrier height between the high work function metals and EL polymers,[88,93,95] which introduce an abrupt, rigid shift of vacuum level at the interface and alter the work function or surface potential of the metals. In contrast with metal-inorganic semiconductor contact, in which band alignment is governed by well-established metalinduced gap states (MIGS),[108] a comprehensive understanding of the energy level alignment in metal-organic interface or metal-organic contact barrier height remains a challenge, thus there are no simple physical pictures that can be provided to
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Figure 2. Possible factors forming and affecting the interfacial dipole layer summarized by Prof. Kazuhiko Seki et al.[88] a1) and a2): Charge transfer across the interface; b) Concentration of electrons in the adsorbate leading to positive charging of the vacuum side; c) Rearrangement of electron cloud at the metal surface, with the reduction of tailing into vacuum; d) Strong chemical interaction between the surface and the adsorbate leading to the rearrangement of the electronic cloud and also the molecular and surface geometries (both directions of dipoles possible); e) Existence of interface state serving as a buffer of charge carriers, and f) Orientation of polar molecules or functional groups. Reproduced with permission.[88] Copyright 1998, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
describe the metal-organic interface properties. In the 1990s it was proposed that the traditional assumption of a common vacuum level alignment (Schottky-Mott rule) turned out to be invalid if one or combinations of the following exists:[88,89] 1) formation of interfacial dipole; 2) reactive metal-organic chemical reaction induced gap states (including chemical doping) or chemisorption or bond formation; 3) metal-organic chemical reaction induced defects and Fermi level pinning occurs; 4) charge neutrality level; 5) band bending. Among all of the above-mentioned approaches/mechanisms that can lead to band offset at the metal-organic interface, interface dipole is of special research interest because it can provide flexible implement approach due to unique characteristic of organic materials.[109] Research carried out all over the world in the last two decades had significantly increased our knowledge about the origin of interface dipole in across metalorganic interface. Various possible origins of interfacial dipole have been proposed and widely discussed,[88] including charge transfer across the interface (Figure 2 a1-a2), molecular polarization induced by mirror image force effect (Figure 2b), “push back” effect on the electron cloud tail out of the metal surface by organic molecule and consequent redistribution of electron cloud (Figure 2c), chemisorption or interfacial chemical reaction and consequent formation of chemical bonds (Figure 2d), and metal-induced interface state (Figure 2e) and surface realignment of the permanent dipole of polar molecular (Figure 2f).
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Therefore, one of the most feasible methods towards interfacial dipole formation is to intentionally attach self-assembled monolayer or polar molecules with favorable permanent dipole to the electrode surface during the process of device fabrication. The surface potential shift (shift in a vacuum level) due to adsorbed dipoles can be written by the Helmholtz equation in a simple electrostatic approach[110]: N = Ng :r gmol , where N is 0 the areal density of absorbed molecules, : mol the net vertical component of the interfacial dipole moment of an individual molecule, g r the relative dielectric constant of the molecules and g 0 the permittivity of vacuum. Campbell et al. demonstrated the control over the work function of Ag substrate (thus the Schottky energy barriers with organic semiconductors) in a wide range of 1 eV via the insertion of a set of self-assembling monolayer polar alkane thiole derivatives with various magnitude and direction of dipole moment between the metal and the organic material.[109] They also verified that the builtin potential of a metal/polymer/metal structure as determined by electroabsorption measurement is in a nearly same amount with the change in work function.
3. Polymer Solar Cells with Alcohol/Water-Soluble Conjugated Polymers as Cathode Buffer Layer Inspired by the success of applying alcohol/water-soluble conjugated polymers as electron injection layer in PLEDs, our group is among the first ones who validated this method in polymer solar cells in 2009.[111] Luo et al. reported that the VOC of PSCs based on specific system can be enhanced by up to 0.35 V by using a thin layer of poly [(9,9-bis{3′-[(N,N-dimethyl)-N–ethylammonium]propyl}-2, 7-fluorine)-2,7(9,9-dioctylfluorene)-co-(4,7,-dithien2-yl)-2,1,3-benzotiadiazole] dibromide (PFNBr-DBT15) or poly[(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorene)alt-1,4-phenylene] dibromide (PFPNBr), as interlayer between active layer and metal electrode. For PSCs based on poly[2,7-(9,9-dioctylfluorene)-co- (4,7-dithien-2-yl)-2,1,3-benzothiadiazole] (PF-DBT35) and PCBM blend, it is worth noting that all of the devices with interlayer were characterized with enhanced open-circuit as high as 0.95 ± 0.05 V, while in contrast only a moderate VOC of 0.65 ± 0.05 V was observed in the routine cathode devices (Figure 3a). The observed enhancement of VOC was found to be in good agreement with increased built-in potential across the devices, as revealed by the J-V characteristics of the devices in the dark (Figure 3b). Similarly, Zhao et al.[112] and Na et al.[113] reported the applications of other fluorene-based alcohol polymers, such as conjugated poly (9,9-bis(60-diethoxylphosphorylhexyl)fluorene) (PF-EP) and a polyfluorene derivative which has hybrid quaternary ammonium end-capped alkyl/alkoxyl side chains named poly[(9,9-bis((6′-(N,N,N-trimethylammonium)hexyl)2,7-fluorene)-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-9-fluorene)) dibromide (WPF-oxy-F), respectively, as cathode interlayer for polymer solar cells in 2009. Zhao et al. found that upon the incorporation of a thin layer PF-EP of 5 nm, the shunt resistance polymer solar cells based on P3HT: PCBM was effectively increased from 30.7 kΩ cm−2 to 1.14 MΩ cm−2, while the VOC was improved to 0.64 V (0.45 V for the control device) and the
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Figure 3. a) J–V characteristics of devices with a structure of ITO/ PEDOT:PSS/PF-DBT35:PCBM (1:2, w/w) (70–80 nm)/various cathode under the illumination of an AM 1.5 G solar simulator (800 W m−2) b) A replot of Fig. 1a in double-log (open) together with the J-V characteristics of the devices in the dark (filled), Ba/Al cathode (circles), Al cathode (squares), PFNBr-DBT15/Al (diamonds) and PFPNBr/Al (triangles). Reproduced with permission.[111] Copyright 2009, American Institute of Physics.
fill factor was improved from 51% to 59%, resulting in a PCE of 3.38% (1.98% for the control device). The overall performance improvements were mainly attributed to more effective preventing penetration of the hot metal atoms, which may lead to significant electrical leakage and quenching cites for photo-generated excitons. In parallel, Na et al. demonstrated that through the use of a thin layer of WPF-oxy-F as interlayer between the photoactive layer and metal electrode, the PCE of polymer solar cells based on poly(3-hexylthiophene) (P3HT): PCBM increased from 2.95% to 3.77%, mainly due to the improved VOC and FF resulting from the reduction of the work function of Al cathode.
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It is noteworthy that the VOC of the devices showed weak dependence on the thickness of the WPF-oxy-F interlayer in range of 3–6 nm, and the obtained VOC of 0.68 V was obviously higher than that of the control device (∼0.57 V). Furthermore, Oh et al. had applied this strategy to other air-stable, high work function metals such as Ag, Au and Cu, resulting in encouraging device performances with PCE above 3% and nearly cathode-independent open-circuit voltage.[114] The improved performance (VOC and PCE) was attributed mainly due to the reduced Schottky-barrier height resulted from the decrease of work function of these noble metals, as indicated by the Kelvin probe under N2 atmosphere. Afterwards, we and the Bazan group[115] investigated the origin of VOC enhancement in PSCs based on different donor materials via interfacial modification using four polyfluorenebased, ionic type WSCPs (PF-X, X for different cationic compensating counterions: Br, trifluoromethanesulfonate, tetrakis(1-imidazolyl)borate and tetrakis[3,5-bis(trifluoromethyl) phenyl]borate), along with PFN.[94] It was found that obvious VOC enhancement can be observed in devices that contain a polyfluorene copolymer as the donor material, regardless of the choice of the choice of compensating ions, while no significant enhancement in VOC was found in devices from P3HT and poly(2-methoxyl-5-((2′-ethylhexyl)oxyl)-1,4-phenylenevinylene) (MEH-PPV), probably due to the highly hydrophobic nature of the active layer.[116] On the other side, it was found that there is a close correlation between the dark J–V characteristics with the VOC enhancement. For instance, the dark currents of those devices with the interlayers and enhanced VOC were found to be 1–2 orders of magnitude lower than that of the control devices, which corresponds to an increase of 0.05–0.10 V in VOC, as revealed by the Shockley equation. Therefore, these observations from the electrical characteristics aspects can lead to deeper understanding on the VOC enhancement in PSCs. Despite the effects of the WSCPs was found to critically depend on chemical structure of the donor materials,[115] we reported that PFN/Al cathode can work very well with a newly synthesized low band gap donor poly[(4,5-ethylene-2,7-carbazole) -5,8-bis(2′-thienyl)-2,3-bis (4-octyloxyphenyl) quinoxaline] (PECz-DTQx) later.[97] With a blend of PECz-DTQx and [6,6]phenyl C71 butyric acid methyl ester (PC71BM) as the photoactive layer, the device with Al cathode displayed an initial PCE of 3.99%, while a high PCE of 6.07% was realized for device with PFN was introduced as the cathode interlayer, probably due to strong N–N interactions between the N-heterocycle-containing polymer donor and the PFN layer. For the past several years, it witnessed the progress of the synthesis of novel low bandgap donor materials and their successful applications in high performance PSCs.[10] However, the trade-off between light harvesting (JSC) and open-circuit voltage (VOC) in low bandgap donor material systems seems to present as a difficult hurdle towards realization of high efficiency ∼10%. Aiming at addressing this problem, more recently we applied the method of incorporating PFN as cathode interlayer in polymer solar cells using promising low bandgap donor materials such as poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5- (4′,7′-di2-thienyl-2′,1′,3′- benzothiadiazole) (PCDTBT)[36] and poly[{4,8bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}]
Figure 4. The effect of PFN interlayer on PCDTBT:PC71BM solar cell performance. a). Current density versus voltage (J–V) characteristics of devices with (solid red circle) and without the PFN interlayer (open black circle) measured under 1000 W m−2 AM 1.5 G illumination. b). J–V characteristics of devices with (solid red circle) and without the interlayer (open black circle) in the dark. Reproduced with permission.[23] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
(PTB7)[45] as electron acceptor, and PC71BM as electron donor, and simultaneously achieved improvements in all of the device performance parameters, including VOC, JSC, and FF (see Figure 4a).[23] For instance, compared with the control device based on Al cathode, a simultaneous enhancement in VOC (0.90 V vs. 0.70 V), JSC (12.1 mA cm−2 vs. 11.4 mA cm−2) and fill factor (62% vs. 49%) was achieved in a device with PFN incorporated as cathode interlayer between PCDTBT:PC71BM and Al, leading to a significant improvement in the PCE by ∼65% (6.73% vs. 4.02%). Figure 4b showed the typical current density
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Figure 5. Electric dipole moment of the interlayer probed by Scanning Kelvin Probe Microscopy (SKPM). a-b) Topographic image (a) and surface potential image (b) of device active layer area partially covered with the interlayer. Scale bars in both images represent 10 mm. c) Schematic illustration of the experimental setup. d) Cross-sectional line profile of the topographic and surface potential images in a and b. e-f) the energy band structure of the devices without (e) and with (f) the interlayer under short-circuit condition (with zero external applied bias) constructed from the observed SKPM results. Reproduced with permission.[23] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
versus voltage (J–V) characteristics of the devices without and with the interlayer, in which a turn-on voltage of about 0.8–1.0 V was clearly seen in the latter device while it is only 0.5–0.6 V for the control devices, implying an enhanced built-in potential across the device. What's more, the PFN interlayer was found to be effective when in combination with low work function metal such as Ca. For example, when a PFN/Ca/Al cathode is used for PCDTBT: PC71BM device, a PCE of 6.79% was achieved, also obviously higher than that based on Ca/Al cathode (4.11%). The abovementioned observation agrees with the literatures that report high work function metals with interlayer may work as low work function electrodes thus can lead to reduced electron injection barrier and increased built-in potential as a result of the formation of interfacial dipole.[93,95] Given these facts, we proposed that interfacial dipole due to the strong interaction between the interlayer and the photoactive layer and a consequent vacuum level shift at the interface were responsible for the origin of the enhancement in VOC. To identify this speculation, the interface dipole was directly probed by scanning Kelvin probe microscopy (SKPM), with the experimental setup depicted in Figure 5c. The surface is quite smooth in both the active layer and the interlayer areas (Figure 5a) and as expected, the surface potential is uniform within both areas. But it is worth mentioning that the surface potential of the interlayer area is about 300 mV more positive than that of the active layer without interlayer (Figure 5b and d), which corresponds to an interfacial dipole moment with the positive charge end pointing toward the Al electrode and the negative charge end pointing toward the photoactive layer. Furthermore, the direction of electric field of the interfacial dipole is aligned with the builtin field inside the device; hence, the actual built-in potential across the device is reinforced as a result of the superposition (Figure 5f). Furthermore, the electrical field associated with the interfacial dipole layer is of 6 × 105 V cm−1, which is about one
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order of magnitude higher than that of average built-in field in devices without the PFN interlayer under short-circuit condition (Figure 5e). Besides the straightforward effect on the enhancement in VOC, we found that the improvements in Jsc and FF upon the incorporation of the PFN interlayer involved multiple origins including enhanced and more balanced charge carrier mobilities, elimination of serve space charge buildup, reduced bimolecular recombination and improved charge collection efficiency under working conditions.[23] In pursuit of highly efficient polymer solar cells, we also examined the effects of introduction of PFN interlayer in devices based on a promising low bandgap donor, PTB7 (see also Table 1). The PCE increased from 5.00% to 8.04%, and 7.13% to 8.22%, when Al and Ca were employed as metal electrode, respectively, both accompanied by simultaneous enhancements in VOC, Jsc and FF (Figure 6a). To the best of our knowledge, this is the first scientific report on PSCs with efficiency over 8%, representing a key advance towards a new regime for PSCs efficiency. Therefore, the results clearly showed that the interlayers provides a new dimension in device design, which enables independent control of VOC, JSC, which can be exploited avoid the trade-off between VOC and JSC, and resulted in simultaneous enhancement of VOC, JSC and FF. Importantly, the device performance of the PTB7 devices was independently verified by the National Center of Supervision & Inspection on Solar Photovoltaic Products Quality of China (CPVT), which is a third-party national PV products test center of China and one of International Laboratory Accreditation Cooperation (ILAC) affiliated laboratories. Moreover, the measurements from this independent source yield results very similar to what we obtained in our laboratory, resulting in a PCE of 8.37%, with a VOC of 0.7563 V, a Jsc of 15.75 mA cm−2 and a FF of 70.15%.
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Interlayer Catalog polyfluorene
Interlayer
Interlayer thickness (nm)
Device structure
VOC (V)
JSC (mA cm−2)
FF (%)
PCE (%)
Ref
PFN
/
ITO/PEDOT/PTB7:PC71BM/Ca/Al
0.70
15.5
66.0
7.13
[23]
5
ITO/PEDOT/PTB7:PC71BM /PFN/Ca/Al
0.76
16.1
69.0
8.22
ITO/PEDOT/PTB7:PC71BM /PFN/Ca/Al
0.7563
15.75
70.15
8.370a)
/
ITO/PEDOT/PCDTBT:PC71BM/Al
0.70
11.3
49.0
3.95
5
ITO/PEDOT/PCDTBT:PC71BM /PFN/Al
0.90
12.1
62.0
6.73
/
ITO/PEDOT/PECz-DTQx :PC71BM/Al
0.78
9.10
56.3
3.99
/
ITO/PEDOT/PECz-DTQx :PC71BM/Ca/Al
0.80
9.84
57.4
4.52
5
ITO/PEDOT/PECz-DTQx :PC71BM/PFN/Al
0.81
11.4
65.8
6.07
5
ITO/PEDOT/PCDTBT:PC71BM/PFN/Al
0.88
6.12
60.1
6.33c)
PEDOT/GMS/PCDTBT:PC71BM/PFN/Al
0.84
6.09
59.0
5.90c)
ITO/PEDOT/PTB7:PC71BM/PFN/Al
0.72
8.24
67.4
7.77c)
PEDOT/GMS/PCDTBT:PC71BM/PFN/Al
0.70
8.22
64.8
7.06c)
/
ITO/PEDOT/PCz-DTBTA:PC61BM/Al
0.80
3.91
48.4
1.51
5
ITO/PEDOT/PCz-DTBTA:PC61BM/PFN/Al
0.90
4.68
65.3
2.75
/
ITO/PEDOT/PFO-DBT35:PC61BM/Al
0.65
3.9
38.0
1.0
5
ITO/PEDOT/PFO-DBT35:PC61BM /PFN-Br/Al
1.00
4.7
45.0
2.1
/
ITO/PEDOT/PFO-DBT35:PC61BM/Al
0.92
3.41
47.5
1.46
5
ITO/PEDOT/PFO-DBT35:PC61BM/PF-BIm4/Al
1.07
3.41
48.8
1.75
PFNBr
PF-(halogen ionic)
PF-EP
WPF-oxy
PFCn6:K+
PNOs
PFNSO
polythiophene
PT3MAHT
PTMAHT
P3HTN
polycarbazole
non-conjugated polymers
Small molecule
PC-NOH
PEO
Rhodamine 101
/
ITO/PEDOT/P3HT:PC61BM/Al
0.45
8.57
51.0
1.98
5
ITO/PEDOT/P3HT:PC61BM /PF-EP/Al
0.64
9.01
59.0
3.38
/
ITO/PEDOT/P3HT:PC61BM/Al
0.57
5.40
35.4
1.09
3∼6
ITO/PEDOT/P3HT:PC61BM/WPF-oxy-F/Al
0.68
6.09
45.6
1.89
/
ITO/PEDOT/P3HT:PC61BM/Al
0.53
9.04
48.0
2.30
3
ITO/PEDOT/P3HT:PC61BM/WPF-6-oxy-F/Al
0.64
10.08
60.0
3.89
/
ITO/PEDOT/P3HT:ICBA/Ca/Al
0.85
10.43
65.2
5.78
5
ITO/PEDOT/P3HT:ICBA/PFCn6:K+/Ca/Al
0.89
11.65
72.6
7.50
/
ITO/PEDOT/PCDTBT:PC71BM/Al
0.65
11.4
53.8
4.0
5∼10
ITO/PEDOT/PCDTBT:PC71BM/ PF6NO25Py/Al
0.91
11.6
66.2
6.9
/
ITO/PEDOT/PTB7:PC71BM/Al
0.55
16.33
56.3
5.03
5∼10
ITO/PEDOT/PTB7:PC71BM /PFNS/Al
0.73
16.38
73.1
8.74
/
ITO/PEDOT/PCDTBT:PC71BM/Al
0.82
9.7
61.0
5.00
