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Solar Cells
Phosphonium Halides as Both Processing Additives and Interfacial Modifiers for High Performance PlanarHeterojunction Perovskite Solar Cells Chen Sun, Qifan Xue, Zhicheng Hu, Ziming Chen, Fei Huang,* Hin-Lap Yip,* and Yong Cao Organic–inorganic hybrid perovskite solar cells based on organolead halide (e.g., CH3NH3PbX3, X = Cl, Br, or I) light absorbers are considered as a new generation photovoltaic technology and have attracted tremendous attention over the past few years.[1–8] The perovskite semiconductors can be easily processed from solution and therefore can be potentially produced at very low cost based on continuous coating methods. Recent studies revealed that organolead halide perovskites possess several important properties including high absorption coefficients for efficient light harvesting,[9] high dielectric constants for efficient exciton dissociation,[10] long carrier diffusion lengths,[11,12] insensitive to electronic trap states,[13] and facile tuning of bandgaps by varying compositions.[14] These combinations of advanced physical properties made it an ideal material for photovoltaic applications, and as a result, the power conversion efficiencies (PCE) of perovskite solar cells improved drastically from ≈4% to over 18% in just a few years.[3,7,15,16] Recent theoretical calculations suggested that the PCE of optimized perovskite solar cells may reach 25%–30%[17] and can be even higher when advanced architecture such as tandem structure is introduced, making it a strong competitor to the current silicon-based photovoltaic technology. Another advantage of perovskite semiconductors is that they can be adopted in different types of solar cell architectures. The early studies of perovskite hybrid solar cells were based on mesoporous TiO2 scaffold, in which perovskite and TiO2 were considered as a sensitizer and an electron transporter, respectively.[18,19] In a later study, Snaith et al. found that perovskite solar cells showed even better performance when replacing the mesoporous TiO2 scaffold with an insulating Al2O3 scaffold,[20] suggesting the perovskite semiconductors can also efficiently transport charges.[21] This finding C. Sun, Q. Xue, Z. Hu, Z. Chen, Prof. F. Huang, Prof. H.-L. Yip, 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/smll.201403344 small 2015, DOI: 10.1002/smll.201403344
led to the further development of p-i-n planar-heterojunction solar cells. The p-i-n structure is particularly attractive due to the simplicity of the device architecture and solar cells with similar device geometry that were widely adopted in other well-developed inorganic thin film (e.g., CdTe and CIGS) photovoltaic technology.[22,23] In general, two types of p-i-n structure for perovskite solar cells were demonstrated. In the first case, metal oxide such as TiO2 or ZnO is applied as the electron-selective layer on the transparent conductive oxide (FTO or ITO) electrode and spiro-OMeTAD or other organic transporting materials is used as the hole selective layer adjacent to the reflective metal electrode.[3,24–28] The other device structure composes of a PEDOT:PSS conducting polymer as a hole transporting layer coated on the transparent electrode and a fullerene electron-selective layer adjacent to the metal electrode.[16,29–31] The latter structure is advantageous as both the interlayers can be processed at low temperatures on flexible substrates and they have proven processability using largearea coating technique as demonstrated in roll-to-roll manufactured organic photovoltaic devices.[32] The performance of perovskite solar cells is critically dependent on both the microstructures of the perovskite films and also the interfacial properties of the devices. There were several processing techniques developed to control the morphology and crystallization of perovskite thin films including physical vapor deposition,[27,33] sequential solution deposition method,[6,34–36] vapor-assisted solution process,[28] and one-step solution deposition process.[37–39] The latter one is a relatively simple method and therefore, several groups had been focused on improving the one-step solution deposition process for preparing high quality perovskite films by tuning the precursor compositions, the annealing process, the processing solvents, and also the introduction of processing additives to alter the film formation kinetics. Snaith et al.[11,20] reported a mixed halide precursor solution containing both Cl− and I− that could effectively tune the nucleation and growth kinetics of the perovskite films. As a result, the CH3NH3PbI3–xClx films show better coverage, crystallinity, and also photophysical properties when compared with those derived from tri-iodide perovskites. Another approach to improve the perovskite films is to control the solvent–solute interactions during the film formation process through solvent engineering. Seok et al.[7,38] demonstrated that the use
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of mixed solvent of γ-butyrolactone (GBL) and dimethylsulphoxide (DMSO) to prepare perovskite films can lead to extremely uniform and dense layers through the formation of a CH3NH3I–PbI2–DMSO intermediate phase, which alters the film formation kinetic and enables the fabrication of very high performance solar cells. Alternatively, introduction of processing additives is another effect approach to modulate the growth of perovskite films. Jen and co-workers[40] reported that alkyl halide additives could provide specific interactions with the perovskite, such as ligation and chelation, and therefore could alter the kinetics of the crystallization for the perovskite films. Recently, the same group also suggested that the halide atoms of the alkyl halide additives might be ionized during the thermal annealing process, providing free halide ions which could participate in the formation of perovskite crystals.[41] In addition to the control of crystallization process and morphology of perovskite films, proper interface engineering to optimize to charge-collection property of the perovskite solar cells is also required to improve the device efficiency. For the case of perovskite/fullerene-based planar-heterojunction solar cells, the major efforts had been focused on the optimization of the fullerene and metal cathode interface. Thermally evaporated C60/BCP double layer or thin LiF layer were deposited on top of the phenyl-C61-butyric acid methyl ester (PCBM) or ICBA layer in order to improve the electron-selective property of the cathode.[16,37,42] Compared with vacuum-deposited interfacial materials, solution-processed interfacial materials are more favorable choices as they can be printed and can be potentially applied to produce allsolution processed perovskite solar cells at low cost. Several solution-processed interfacial materials including fullerene surfactant and amino-functionalized polymer electrolytes, which had been successfully applied in polymer solar cells, were therefore employed to modify the cathode interface in perovskite solar cells.[40,43,44] It was found that all those interfacial materials could efficiently enhance the performance of perovskite solar cells by improving the electron-selective property and reducing the contact resistance at the cathode. These findings proved that interface engineering is a critical factor for improving perovskite solar cell performance and the development of new interfacial materials, particularly
for those that are available in low cost and widely accessible, remains an important topic for perovskite solar cell research. In this study, a series of organic halide salts with different organic cations and halide anions, as shown in Figure 1, were applied as processing additives and their effect on the microstructures of the CH3NH3PbI3–xClx perovskite films as well as the corresponding perovskite/fullerene planar-heterojunction solar cell performances were investigated. We found that both the morphology and crystallinity of the perovskite films were critically dependent on the types and concentrations of the organic halide salts. Among different additives, tetraphenylphosphonium iodide (TPPI) and chloride (TPPCl) were found to be the most efficient ones in improving the crystallinity and coverage of the perovskite films as the I− and Cl− anions may chelate to the Pb2+ cations and alter the kinetic of film formation. As a result, the PCE of the solar cells improved from less than 10% for the reference cell to ≈11.5% for the additive-enhanced cells. To further optimize the solar cell performance, we also explored using TPPI as an interfacial n-dopant to modify the cathode interface between PCBM and Al, which resulted in reduced contact resistance at the cathode interface. As a result, the PCE of planar-heterojunction perovskite solar cells employed these dual-functional phosphonium halides further enhanced to 13%, providing a simple and efficient strategy to improve the performance of perovskite solar cells. The solar cell structure and the chemical structures of the processing additives are shown in Figure 1. We adopted a typical p-i-n planar heterojunction with the configuration of ITO/PEDOT:PSS (≈40 nm)/CH3NH3PbI3– xClx(≈300–400 nm)/PCBM (≈80 nm)/Al in our study. The detailed information for the preparation of the perovskite precursor solutions, deposition of perovskite thin films, and fabrication of the solar cells is described in the Experimental Section. Several commercially available organic halide salts were chosen for this study including tetrabutyl ammonium and phosphonium iodide salts (TBAI and TBPI) and tetraphenyl phosphonium salts with Cl− (TPPCl), Br− (TPPBr), and I− (TPPI) as the anions. It is well-known that alkyl or aromatic ammonium cations can form layered organic–inorganic hybrid structure with metal halides,[45–47] which may totally alter the physical properties of the CH3NH3PbI3–xClx perovskites. Therefore, the choice of phosphonium salt against ammonium salt is aimed to minimize the possible chemical interactions with the perovskite crystal. The bulky and more rigid aromatic groups in the tetraphenylphosphonium salts may also alter the growth kinetic of the perovskite films while the free halide ions from the salts may participate in the perovskite crystal formation.[41] The current–density (J–V) curves of the best-performing perovskite solar cells processed from precursor solutions doped with 1 wt% of the organic halide salts are shown in Figure 2a and the corresponding Figure 1. Device configuration of the planar-heterojunction solar cell and the chemical photovoltaic performance data are summarized in Table 1. The reference cell structures of different organic halide salts used in this study.
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perovskite film. The crystal size is ≈1 µm while some pinholes with size of ≈1 µm were also observed. It is worth noting that the formation of pinholes is detrimental to the performance of perovskite solar cells due to the formation of undesired PEDOT:PSS/PCBM interfaces that cause charge recombinations.[37] In the cases of TPPI- and TPPCl-doped films, an even higher surface coverage with reduced pinhole size was achieved but in the meanFigure 2. a) Current density–voltage characteristics and b) EQE spectra of pristine and while, the size of the perovskite crystal additive-doped perovakite solar cells. was also slightly decreased to less than 1 µm. Interestingly, despite a reduction of without doping showed a PCE of 9.7%, which is comparable crystal size in those doped films, the degree of crystallizato other reported performance based on similar device struc- tion measured by the XRD was indeed much higher when ture and processing conditions.[31,40,41] The device perfor- compared with that of the pristine perovskite film. Figure 2b mance deceased significantly in both TBAI- and TBPI-doped shows the XRD patterns of all the perovskite films, the strong perovskite solar cells with PCE dropped down to 1.4% and diffraction peaks at 14.1°, 28.3°, 43.0°, and 58.6° are corre2%, respectively. All the three photovoltaic parameters sponding to the (110), (220), (330), and (440) crystal planes including open-circuit voltage (Voc), short-circuit current den- of the CH3NH3PbI3–xClx tetragonal structure. The intensity sity (Jsc), and fill factor (FF) were simultaneously decreased of the XRD peaks for the TPPI- and TPPCl-doped perovsand the S-shaped J–V curves indicated that the devices show kite films was about two times larger than that of the prispoor diode behavior. In contrast, solar cells doped with both tine film. As the thicknesses of all the films are similar, the TPPI and TPPCl showed positive effect on the device per- increase in XRD peak intensity could therefore be attributed formance with PCE of 11.7% and 11.5%, respectively. The to the higher phase purity and preferential orientation of the major factor contributed to the enhanced PCEs was the FF, perovskite crystals, which are important factors for improving which improved from 0.57 of the reference devices to 0.66– the photophysical properties[20] and performance of perovs0.68 for the doped devices. The Jsc also slightly increased kite solar cells.[36,40,48,49] The morphology of the TPPBr-doped film is quite difin both cases, while the Voc largely remained the same at 0.90 V. The external quantum efficiency (EQE) spectra of ferent from those of the TPPI- and TPPCl-doped films. those doped cells and reference cells are shown in Figure 2b The film showed a lower coverage and the perovskite crysand the shape and magnitude of the three spectra are very tals appeared to be larger in size but with smoother edges. similar with average EQE over 70% covering the visible to The XRD result revealed that the film had a relatively poor near infrared region. In the case of TPPBr-doped solar cells, crystallinity even though the crystal size is larger. These an adverse effect on the device performance was observed results suggest that the anions of the additives could play a with both Jsc and FF dropped compared with the reference critical role in affecting the growth kinetics of the perovscell, leading to a decrease in PCE to 8.4%. kite films. The free Br− ions that did not originally exist in To investigate the film structure-device property relation- the CH3NH3I and PbCl2 precursor solution may participate ship of the additive-doped perovskite solar cells, a combina- in the formation of even more complex perovskite compotion of experiments including scanning electron microscopy sitions involving all three halide ions and therefore may (SEM), X-ray diffraction (XRD), absorption spectrum, alter the crystal growth behavior. The free Cl− and I− ions and photoluminescence (PL) were used. The SEM images from the additives might slightly vary the initial concentrain Figure 3a reveal that a wide range of morphologies was tion and ratio of halide ions in the precursor solutions, which formed in the additive-doped films. A polycrystalline film also changed the crystal growth and film formation properwith high surface coverage was observed for the pristine ties. The effect of additive concentration was also studied in the case of TPPI-doped films and the morphology and device Table 1. Photovoltaic performance data of planar-heterojunction solar performance of the resulted films are provided in the Supcells based on the perovskite films doped with different additives at a porting Information (Figure S1–S3, Supporting Information). concentration of 1 wt%. The results suggested that there is an optimal doping concenPerovskite Layer Voc Jsc FF PCE tration at 1 wt%. For the case of TBAI- and TBPI-doped per[V] [mA cm–2] [%] ovskite films, the morphologies are very different from the 0.90 18.8 0.57 9.6 CH3NH3PbI3–xClx others. It appeared that the crystals were more favorable to 0.90 19.2 0.68 11.7 CH3NH3PbI3–xClx + TPPI grow in the lateral direction with large crystal sizes of over 10 µm. The XRD study suggested that these films had much 0.90 19.3 0.66 11.5 CH3NH3PbI3–xClx + TPPCl lower crystallinity but the tetragonal perovskite structure was CH3NH3PbI3–xClx + TPPBr 0.89 17.8 0.53 8.4 preserved as the peak positions are the same as the others. 0.51 15.8 0.25 2.0 CH3NH3PbI3–xClx + TBPI We speculate that both the alkyl-containing ammonium 0.65 10.1 0.21 1.4 CH3NH3PbI3–xClx + TBAI and phosphonium cations may interact with the perovskite small 2015, DOI: 10.1002/smll.201403344
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Figure 3. a) SEM images and b) XRD spectra of perovskite films with different dopants on glass/PEDOT:PSS substrates. c) UV–vis and PL spectra of the perovskite films deposited on ITO/PEDOT:PSS and glass substrates, respectively. The excitation wavelength for the PL study was 580 nm.
crystallographic plane terminated with the corner-sharing metal-halide octahedrals and formed a layered organic–inorganic hybrid structures. The layered structure showed good lateral charge transport property[50] and light emitting property[51,52] but may not be suitable for photovoltaic applications as the vertical charge transport property is hindered by the insulating organic layer; therefore, poor solar cell performance was found in our study. To study the effect of additives on the optical properties of the perovskite films, absorption and photoluminescence experiments were performed and the data are shown in Figure 3c. Typical absorption and emission spectra for the CH3NH3PbI3–xClx perovskite were shown in the pristine film with absorption edge at ≈770 nm, while the emission peak was at 780 nm. Enhanced absorption between 350 and 600 nm region were observed in the TPPI- and TPPCldoped films, while for all the other doped films, the absorption were decreased. The change of absorption property can be attributed to the difference in crystallinity of the films as the absorbance increases with the degree of crystallinity as suggested by previous reports.[24,40,41,53] In contrary, from the photoluminescence study, we found that the emission intensity decreased with the degree of crystallinity. This result is also in agreement with the photovoltaic performance of the resultant devices as the larger carrier diffusion lengths found in highly crystalline perovskite films reduces the possibility of radiative recombination but improves the charge transport and charge collection properties in the perovskite solar cells. Most additives employed in previous reports to improve the performance of perovskite solar cells are in liquid form
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and they were removed from the films either during the spin-coating process or post-thermal annealing process. In this study, all the additives are solid with high melting points (150–350 °C), so it is very likely the additives will remain in the perovskite films and it is important to study how the residual additives distributed in the final film structures and also their effect on the device performance. From the optical images of the additive-doped films (see Figure S4, Supporting Information), we observed that crystals of the organic halide salts were formed on the surface of the perovskite films and the shape, size, and number of crystals were depended on the type of additives. Taking TPPI as an example, elongated crystals with size of ≈10 µm were formed. We also found that the crystals remained in the device even after the deposition of the PCBM layer as revealed by the SEM image in Figure 4. Cross-section SEM images also confirmed that the TPPI crystals were embedded in the perovskite and PCBM films (see Figure S5, Supporting Information). Those additive crystals are unlikely to cause a negative effect to the solar cell as
Figure 4. SEM images of a) pristine and b) TPPI-doped perovskite films covered by a PCBM layer on glass/PEDOT:PSS substrates (the scale bar is 20 µm).
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Figure 5. Current density–voltage characteristics of pristine perovskite and TPPI-doped perovskite solar cells with TPPI interlayer.
the overall performance of the devices was improved as discussed in the previous section. Indeed, we speculate that the close contact of the additive crystal with the PCBM layer and the metal cathode may introduce a positive effect to improve charge-collection property of the cathode. This hypothesis triggered us to also explore the use of the TPPI additive as an interfacial modifier to improve the interface between the PCBM and Al cathode. TPPI was chosen because it is known that TPPI can cocrystallize with C60 to form a TPP+C60– charge transfer complex with high electrical conductivities.[54] So, it is also likely that TPPI could interact with PCBM and create an n-doped electron transport layer with improved electrical property. However, it is difficult to directly dope the TPPI into the PCBM solution as it has very poor solubility in nonpolar organic solvents such as chlorobenzene, which was used as processing solvent for the PCBM layer. Therefore, instead of modifying the bulk property of the PCBM layer, we took an alternative route and modified the interface between the PCBM and Al cathode by spin coating an ultrathin layer (5 nm) of TPPI on the PCBM surface from isopropanol solution, which is an orthogonal solvent to the bottom layers. The J–V curves of the interface-modified solar cells comprising a pristine perovskite layer and a TPPI-doped perovskite layer are shown in Figure 5a and their photovoltaic parameters are summarized in Table 2. Compared with the reference cell, the interface-modified device with a pristine perovskite layer showed an enhancement of PCE from 9.7% to 11.1%, which contributed from both the improvement of Jsc and FF. When the TPPI was used as both the processing additive and interface modifier, the PCE of the solar cells further
improved to 13% with a high FF of 0.73. As the FF is governed by both the shunt resistance (Rp) and series resistance (Rs) of the solar cells, those parameters were therefore calculated from the J–V curves for further analysis. Compared with the reference cell, the solar cells with the TPPI-doped perovskite layer and TPPI interlayer showed an obvious increase in Rp from 424.2 to 873.6 Ω cm2 and also a decrease in Rs from 9.4 to 6.0 Ω cm2. Since the loss of charge carriers through leakage paths in the films and the recombination of carriers during their transit through the cell will both affect the Rp, we therefore attributed the increase in Rp to the improvement of coverage and crystallinity of the TPPI-doped perovskite film. The reduction of Rs was attributed to the improved electrical property of the cathode interface. It is also possible that the interfacial modification can reduce the recombination of charges at the cathode interface by removing the trap states at the PCBM/metal interface through interfacial n-doping, which was reported in the case of polymer solar cells employing halide ions-based interfacial materials.[55,56] To further study the effect of interfacial modification on the charge transporting properties of PCBM and the cathode interface, electron-only devices based on the architectures of ITO/Al/PC61BM/Al with and without TPPI interlayer were prepared and the electron mobilities of the devices were extracted by fitting the data using the space-chargelimited current (SCLC) model.[57] The TPPI-modified electron-only device showed a much higher current density at the region of low applied field (Figure 6a), which suggested a smaller ohmic loss probably resulted from the reduced contact resistance at the PCBM/TPPI/Al interface. The electron mobilities of the PCBM layer were calculated from the slopes of the J1/2–V curves and determined to be 1.9 × 10−3 and 8.5 × 10−4 cm2 V−1 s−1 for the devices with and without the TPPI interlayer, respectively (Figure 6b). We suggest that an electron transfer process may occur between the I− anion and PCBM and result in an n-doped PCBM interface with reduced contact resistance as similar doping mechanism was also observed in several PCBM/organic halide systems.[56] The improved mobility of the PCBM layer could be attributed to the filling of surface trap states of the PCBM and this finding is in good agreement with the result from a recent report, which suggested that n-type doping of fullerenes can improve electron mobility up to one order of magnitude owning to the dopant-assisted filling of shallow electron traps.[58] These results suggested that interface engineering is a simple but effective approach to optimize the charge collection efficiency as well as the performance of perovskite solar cells. In summary, we have introduced organic halide salts as both a processing additive to modulate the morphology and crystallinity of perovskite light-absorbing layer and also
Table 2. The photovoltaic performance data of perovskite planar-heterojunction solar cells with TPPI interlayer. Perovskite Layer
Interface Layer
Voc [V]
Jsc [mA cm–2]
FF
CH3NH3PbI3–xClx
–
0.90
18.8
CH3NH3PbI3–xClx + TPPI
–
0.90
19.2
CH3NH3PbI3–xClx
TPPI
0.90
CH3NH3PbI3–xClx + TPPI
TPPI
0.90
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PCE [%]
Rs [Ω cm2]
Rp [Ω cm2]
0.57
9.6
9.4
424.2
0.68
11.7
6.2
808.8
19.9
0.62
11.1
6.7
816.1
19.7
0.73
13.0
6.0
873.6
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Figure 6. J–V curves and J1/2–V characteristics of the electron-only devices with or without TPPI interlayer.
an interfacial modifier to improve the electrical contact of the PCBM/Al electron-collecting electrode in perovskite/ fullerene planar-heterojunction solar cells. Among different organic halide salts that have been tested in this study, phosphonium halides with bulky aromatic cations, such as TPPI or TPPCl, were found to be the most efficient ones in improving the morphology of the perovskite films. The free I– and Cl– anions provided by those additives could alter the growth kinetic of the perovskite films and led to improved film coverage, crystallinity, and solar cell performance. Moreover, TPPI was also explored as an efficient interfacial n-dopant to improve the electrical property of the PCBM/metal contact by reducing the contact resistance at that cathode interface. As a result, planar-heterojunction perovskite solar cells employed these dual-functional phosphonium halides showed enhanced performance with PCE up to 13%. These findings provide valuable design guidelines for new device process and efficient interfacial materials for high performance perovskite solar cells.
Experimental Section Materials and Sample Preparation: Methylammonium iodide (CH3NH3I) was synthesized by reacting 0.12 mol methylamine (33 wt% in absolute ethanol, Aldrich) and 13.2 mL of 0.1 mol hydroiodic acid (57 wt% in water, Alfa Aesar) at 0 °C for 2 h with constant stirring under nitrogen atmosphere. Then the CH3NH3I was crystallized by rotary evaporation to remove the solvent and the white powder was washed by diethyl ether for more than three times. Finally, the CH3NH3I powder was collected and dried at 50 °C in a vacuum overnight. To prepare the perovskite precursor solution, CH3NH3I and PbCl2 (Sigma-Aldrich) were mixed in anhydrous N,N-dimethylformamide (DMF) (Sigma-Aldrich) with a molar ratio of 3:1. The perovskite/dopants precursor solution was prepared via adding 1 wt% of dopants into perovskite precursor solution. The solutions were stirred at 60 °C overnight and then filtered with 0.45 µm PTFE filters before used for device fabrication. The concentration of the perovskite precursor solution was 35 wt%. Poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), (CleviosPEDOT:PSS, Clevios P VP AI 4083), and [6,6]-phenylC61-butyric acid methyl ester (PCBM) were purchased from Heraeus Precious Metals GmbH & Co.,Germany and Nano-C Inc., US, respectively. Other reagents we used were purchased from commercial sources (Aldrich, Acros, and Alfa Aesar) and used as received. Fabrication of Thin-Film Perovskite Solar Cells: The indium tin oxide (ITO, 15 ohm sq–1) glass substrates were cleaned sequentially under sonication for 20 min with acetone, detergent,
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deionized water, and isopropyl alcohol and then dried under 80 °C in baking oven overnight, followed by an plasma treatment for 4 min. PEDOT:PSS (filtered through a 0.45 µm PES filter) was spun on ITO substrate and dried at 140 °C for 20 min in air. The CH3NH3PbI3–xClx or CH3NH3PbI3–xClx /dopants precursor solution was spun onto PEDOT:PSS layer to form a thin-film perovskite layer with thickness of 300–400 nm and then further annealed at 100 °C for 2 h. The PCBM (30 mg mL–1) was spun cast onto the perovskite layer at 1200 rpm for 30 s and tetraphenylphosphonium iodide (TPPI) dissolved in isopropyl alcohol at a concentration of 0.5 mg mL–1 was spun onto PCBM layer at 2000 rpm for 30 s for some devices. An aluminum electrode (100 nm) was deposited by thermal evaporation finally through a shadow mask under a base pressure of 1 × 10−6 mbar. The active area was defined at 0.16 cm2. Device Characterization: The device photocurrent was measured under an AM 1.5G solar simulator (Japan, SAN-EI, XES-40S1). The current density–voltage (J–V) characteristics for the devices were recorded with a Keithley 2400 source meter. The illumination intensity of the light source was calibrated before the testing using a standard silicon solar cell with a KG5 filter, calibrated using a National Renewable Energy Laboratory (NREL)-calibrated silicon photodiode, giving a value of 100 mW cm−2 in the test. All the J–V curves reported in this manuscript are measured under reversed voltage bias. The hysteresis property was tested by sweeping the J–V measurement from both forward and reversed direction (Figure S6, Supporting Information). The average PCEs of the different devices and the statistic distributions of the cell performance are shown in Figure S7 (Supporting Information). UV–vis absorption, photoluminescence (PL) spectra, scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and optical polarizing microscope (OPM) were recorded on a HP 8453 spectrophotometer, an Instaspec IV CCD spectrophotometer (Oriel Co.), Zeiss EVO 18 SEM, PANalytical X’pert PRO X-ray diffractometer, and Nikon eclipse E600 POL, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the Ministry of Science and Technology (Grant No. 2014CB643500), the Natural Science Foundation of China (Grant Nos. 51323003 and 21125419), and the Guangdong Natural Science Foundation (Grant No. S2012030006232). The authors acknowledge Dr. Jiang Liu and Jiahui Lin from Chengdu Green Energy and Green Manufacturing Technology R&D Centre for the cross-sectional SEM study.
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www.MaterialsViews.com [4] O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, M. K. Nazeeruddin, H. J. Bolink, Nat. Photonics 2014, 8, 128. [5] Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan, J. Huang, Energy Environ. Sci. 2014, 7, 2359. [6] Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao, J. Huang, Energy Environ. Sci. 2014, 7, 2619. [7] N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. I. Seok, Nat. Mater. 2014, 13, 897. [8] H. S. Jung, N.-G. Park, Small 2014, 11, 10. [9] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 2013, 52, 9019. [10] C. Homes, T. Vogt, S. Shapiro, S. Wakimoto, A. Ramirez, Science 2001, 293, 673. [11] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 341. [12] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344. [13] N. K. Noel, A. Abate, S. D. Stranks, E. S. Parrott, V. M. Burlakov, A. Goriely, H. J. Snaith, ACS Nano 2014, 8, 9815. [14] G. E. Eperon, S. D. Stranks, C. Menelaou, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 2014, 7, 982. [15] H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. [16] J. Y. Jeng, Y. F. Chiang, M. H. Lee, S. R. Peng, T. F. Guo, P. Chen, T. C. Wen, Adv. Mater. 2013, 25, 3727. [17] W. J. Yin, T. Shi, Y. Yan, Adv. Mater. 2014, 26, 4653. [18] I. Chung, B. Lee, J. He, R. P. Chang, M. G. Kanatzidis, Nature 2012, 485, 486. [19] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, Sci. Rep. 2012, 2, 1. [20] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643. [21] J. M. Ball, M. M. Lee, A. Hey, H. J. Snaith, Energy Environ. Sci. 2013, 6, 1739. [22] R. Noufi, K. Zweibel, in 4th World Conf. Photovoltaic Energy Conversion (WCPEC-4)IEEE, Waikoloa, HI 2006, p. 317. [23] M. J. Currie, J. K. Mapel, T. D. Heidel, S. Goffri, M. A. Baldo, Science 2008, 321, 226. [24] B. Conings, L. Baeten, C. De Dobbelaere, J. D’Haen, J. Manca, H. G. Boyen, Adv. Mater. 2014, 26, 2041. [25] H. Li, K. Fu, A. Hagfeldt, M. Grätzel, S. G. Mhaisalkar, A. C. Grimsdale, Angew. Chem. Int. Ed. 2014, 53, 4085. [26] N. J. Jeon, H. G. Lee, Y. C. Kim, J. Seo, J. H. Noh, J. Lee, S. I. Seok, J. Am. Chem. Soc. 2014, 136, 7837. [27] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. [28] Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu, G. Li, Y. Yang, J. Am. Chem. Soc. 2013, 136, 622. [29] P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, H. J. Snaith, Nat. Commun. 2013, 4, 2761. [30] S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T. C. Sum, Y. M. Lam, Energy Environ. Sci. 2014, 7, 399. [31] J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou, Y. Yang, ACS Nano 2014, 8, 1674.
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[32] C. Roldan-Carmona, O. Malinkiewicz, A. Soriano, G. Minguez Espallargas, A. Garcia, P. Reinecke, T. Kroyer, M. I. Dar, M. K. Nazeeruddin, H. J. Bolink, Energy Environ. Sci. 2014, 7, 994. [33] C.-W. Chen, H.-W. Kang, S.-Y. Hsiao, P.-F. Yang, K.-M. Chiang, H.-W. Lin, Adv. Mater. 2014, 26, 6647. [34] Y. Wu, A. Islam, X. Yang, C. Qin, J. Liu, K. Zhang, W. Peng, L. Han, Energy Environ. Sci. 2014, 7, 2934. [35] L. Zheng, Y. Ma, S. Chu, S. Wang, B. Qu, L. Xiao, Z. Chen, Q. Gong, Z. Wu, X. Hou, Nanoscale 2014, 6, 8171. [36] Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan, J. Huang, Adv. Mater. 2014, 26, 6503. [37] Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan, J. Huang, Energy Environ. Sci. 2014, 7, 2359. [38] S. Ryu, J. H. Noh, N. J. Jeon, Y. Chan Kim, W. S. Yang, J. Seo, S. I. Seok, Energy Environ. Sci. 2014, 7, 2614. [39] A. Abrusci, S. D. Stranks, P. Docampo, H.-L. Yip, A. K. Y. Jen, H. J. Snaith, Nano Lett. 2013, 13, 3124. [40] P. W. Liang, C. Y. Liao, C. C. Chueh, F. Zuo, S. T. Williams, X. K. Xin, J. Lin, A. K. Y. Jen, Adv. Mater. 2014, 26, 3748. [41] C.-C. Chueh, C.-Y. Liao, F. Zuo, S. T. Williams, P.-W. Liang, A. K. Y. Jen, J. Mater. Chem. A 2015, DOI: 10.1039/C4TA05012F. [42] J. Seo, S. Park, Y. Chan Kim, N. J. Jeon, J. H. Noh, S. C. Yoon, S. I. Seok, Energy Environ. Sci. 2014, 7, 2642. [43] H. Zhang, H. Azimi, Y. Hou, T. Ameri, T. Przybilla, E. Spiecker, M. Kraft, U. Scherf, C. J. Brabec, Chem. Mater. 2014, 26, 5190. [44] Q. Xue, Z. Hu, J. Liu, J. Lin, C. Sun, Z. Chen, C. Duan, J. Wang, C. Liao, W. M. Lau, F. Huang, H.-L. Yip, Y. Cao, J. Mater. Chem. A 2014, 2, 19598. [45] D. B. Mitzi, Chem. Mater. 1996, 8, 791. [46] Z. Cheng, J. Lin, CrystEngComm 2010, 12, 2646. [47] D. B. Mitzi, C. A. Feild, W. T. A. Harrison, A. M. Guloy, Nature 1994, 369, 467. [48] J. H. Kim, S. T. Williams, N. Cho, C.-C. Chueh, A. K. Y. Jen, Adv. Energy Mater. 2014, DOI: 10.1002/aenm.201401229. [49] A. Dualeh, N. Tétreault, T. Moehl, P. Gao, M. K. Nazeeruddin, M. Grätzel, Adv. Funct. Mater. 2014, 24, 3250. [50] C. R. Kagan, D. B. Mitzi, C. D. Dimitrakopoulos, Science 1999, 286, 945. [51] K. Chondroudis, D. B. Mitzi, Chem. Mater. 1999, 11, 3028. [52] E. R. Dohner, A. Jaffe, L. R. Bradshaw, H. I. Karunadasa, J. Am. Chem. Soc. 2014, 136, 13154. [53] C. Zuo, L. Ding, Nanoscale 2014, 6, 9935. [54] A. Penicaud, A. Perez-Benitez, R. Gleason, V. E. Munoz, P. R. Escudero, J. Am. Chem. Soc. 1993, 115, 10392. [55] H.-L. Yip, A. K. Y. Jen, Energy Environ. Sci. 2012, 5, 5994. [56] C.-Z. Li, C.-C. Chueh, F. Ding, H.-L. Yip, P.-W. Liang, X. Li, A. K. Y. Jen, Adv. Mater. 2013, 25, 4425. [57] V. Mihailetchi, J. Wildeman, P. Blom, Phys. Rev. Lett. 2005, 94, 126602. [58] S. Rossbauer, C. Müller, T. D. Anthopoulos, Adv. Funct. Mater. 2014, 24, 7116.
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Received: November 11, 2014 Revised: December 4, 2014 Published online:
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Solar Cells
Phosphonium Halides as Both Processing Additives and Interfacial Modifiers for High Performance PlanarHeterojunction Perovskite Solar Cells Chen Sun, Qifan Xue, Zhicheng Hu, Ziming Chen, Fei Huang,* Hin-Lap Yip,* and Yong Cao Organic–inorganic hybrid perovskite solar cells based on organolead halide (e.g., CH3NH3PbX3, X = Cl, Br, or I) light absorbers are considered as a new generation photovoltaic technology and have attracted tremendous attention over the past few years.[1–8] The perovskite semiconductors can be easily processed from solution and therefore can be potentially produced at very low cost based on continuous coating methods. Recent studies revealed that organolead halide perovskites possess several important properties including high absorption coefficients for efficient light harvesting,[9] high dielectric constants for efficient exciton dissociation,[10] long carrier diffusion lengths,[11,12] insensitive to electronic trap states,[13] and facile tuning of bandgaps by varying compositions.[14] These combinations of advanced physical properties made it an ideal material for photovoltaic applications, and as a result, the power conversion efficiencies (PCE) of perovskite solar cells improved drastically from ≈4% to over 18% in just a few years.[3,7,15,16] Recent theoretical calculations suggested that the PCE of optimized perovskite solar cells may reach 25%–30%[17] and can be even higher when advanced architecture such as tandem structure is introduced, making it a strong competitor to the current silicon-based photovoltaic technology. Another advantage of perovskite semiconductors is that they can be adopted in different types of solar cell architectures. The early studies of perovskite hybrid solar cells were based on mesoporous TiO2 scaffold, in which perovskite and TiO2 were considered as a sensitizer and an electron transporter, respectively.[18,19] In a later study, Snaith et al. found that perovskite solar cells showed even better performance when replacing the mesoporous TiO2 scaffold with an insulating Al2O3 scaffold,[20] suggesting the perovskite semiconductors can also efficiently transport charges.[21] This finding C. Sun, Q. Xue, Z. Hu, Z. Chen, Prof. F. Huang, Prof. H.-L. Yip, 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/smll.201403344 small 2015, DOI: 10.1002/smll.201403344
led to the further development of p-i-n planar-heterojunction solar cells. The p-i-n structure is particularly attractive due to the simplicity of the device architecture and solar cells with similar device geometry that were widely adopted in other well-developed inorganic thin film (e.g., CdTe and CIGS) photovoltaic technology.[22,23] In general, two types of p-i-n structure for perovskite solar cells were demonstrated. In the first case, metal oxide such as TiO2 or ZnO is applied as the electron-selective layer on the transparent conductive oxide (FTO or ITO) electrode and spiro-OMeTAD or other organic transporting materials is used as the hole selective layer adjacent to the reflective metal electrode.[3,24–28] The other device structure composes of a PEDOT:PSS conducting polymer as a hole transporting layer coated on the transparent electrode and a fullerene electron-selective layer adjacent to the metal electrode.[16,29–31] The latter structure is advantageous as both the interlayers can be processed at low temperatures on flexible substrates and they have proven processability using largearea coating technique as demonstrated in roll-to-roll manufactured organic photovoltaic devices.[32] The performance of perovskite solar cells is critically dependent on both the microstructures of the perovskite films and also the interfacial properties of the devices. There were several processing techniques developed to control the morphology and crystallization of perovskite thin films including physical vapor deposition,[27,33] sequential solution deposition method,[6,34–36] vapor-assisted solution process,[28] and one-step solution deposition process.[37–39] The latter one is a relatively simple method and therefore, several groups had been focused on improving the one-step solution deposition process for preparing high quality perovskite films by tuning the precursor compositions, the annealing process, the processing solvents, and also the introduction of processing additives to alter the film formation kinetics. Snaith et al.[11,20] reported a mixed halide precursor solution containing both Cl− and I− that could effectively tune the nucleation and growth kinetics of the perovskite films. As a result, the CH3NH3PbI3–xClx films show better coverage, crystallinity, and also photophysical properties when compared with those derived from tri-iodide perovskites. Another approach to improve the perovskite films is to control the solvent–solute interactions during the film formation process through solvent engineering. Seok et al.[7,38] demonstrated that the use
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of mixed solvent of γ-butyrolactone (GBL) and dimethylsulphoxide (DMSO) to prepare perovskite films can lead to extremely uniform and dense layers through the formation of a CH3NH3I–PbI2–DMSO intermediate phase, which alters the film formation kinetic and enables the fabrication of very high performance solar cells. Alternatively, introduction of processing additives is another effect approach to modulate the growth of perovskite films. Jen and co-workers[40] reported that alkyl halide additives could provide specific interactions with the perovskite, such as ligation and chelation, and therefore could alter the kinetics of the crystallization for the perovskite films. Recently, the same group also suggested that the halide atoms of the alkyl halide additives might be ionized during the thermal annealing process, providing free halide ions which could participate in the formation of perovskite crystals.[41] In addition to the control of crystallization process and morphology of perovskite films, proper interface engineering to optimize to charge-collection property of the perovskite solar cells is also required to improve the device efficiency. For the case of perovskite/fullerene-based planar-heterojunction solar cells, the major efforts had been focused on the optimization of the fullerene and metal cathode interface. Thermally evaporated C60/BCP double layer or thin LiF layer were deposited on top of the phenyl-C61-butyric acid methyl ester (PCBM) or ICBA layer in order to improve the electron-selective property of the cathode.[16,37,42] Compared with vacuum-deposited interfacial materials, solution-processed interfacial materials are more favorable choices as they can be printed and can be potentially applied to produce allsolution processed perovskite solar cells at low cost. Several solution-processed interfacial materials including fullerene surfactant and amino-functionalized polymer electrolytes, which had been successfully applied in polymer solar cells, were therefore employed to modify the cathode interface in perovskite solar cells.[40,43,44] It was found that all those interfacial materials could efficiently enhance the performance of perovskite solar cells by improving the electron-selective property and reducing the contact resistance at the cathode. These findings proved that interface engineering is a critical factor for improving perovskite solar cell performance and the development of new interfacial materials, particularly
for those that are available in low cost and widely accessible, remains an important topic for perovskite solar cell research. In this study, a series of organic halide salts with different organic cations and halide anions, as shown in Figure 1, were applied as processing additives and their effect on the microstructures of the CH3NH3PbI3–xClx perovskite films as well as the corresponding perovskite/fullerene planar-heterojunction solar cell performances were investigated. We found that both the morphology and crystallinity of the perovskite films were critically dependent on the types and concentrations of the organic halide salts. Among different additives, tetraphenylphosphonium iodide (TPPI) and chloride (TPPCl) were found to be the most efficient ones in improving the crystallinity and coverage of the perovskite films as the I− and Cl− anions may chelate to the Pb2+ cations and alter the kinetic of film formation. As a result, the PCE of the solar cells improved from less than 10% for the reference cell to ≈11.5% for the additive-enhanced cells. To further optimize the solar cell performance, we also explored using TPPI as an interfacial n-dopant to modify the cathode interface between PCBM and Al, which resulted in reduced contact resistance at the cathode interface. As a result, the PCE of planar-heterojunction perovskite solar cells employed these dual-functional phosphonium halides further enhanced to 13%, providing a simple and efficient strategy to improve the performance of perovskite solar cells. The solar cell structure and the chemical structures of the processing additives are shown in Figure 1. We adopted a typical p-i-n planar heterojunction with the configuration of ITO/PEDOT:PSS (≈40 nm)/CH3NH3PbI3– xClx(≈300–400 nm)/PCBM (≈80 nm)/Al in our study. The detailed information for the preparation of the perovskite precursor solutions, deposition of perovskite thin films, and fabrication of the solar cells is described in the Experimental Section. Several commercially available organic halide salts were chosen for this study including tetrabutyl ammonium and phosphonium iodide salts (TBAI and TBPI) and tetraphenyl phosphonium salts with Cl− (TPPCl), Br− (TPPBr), and I− (TPPI) as the anions. It is well-known that alkyl or aromatic ammonium cations can form layered organic–inorganic hybrid structure with metal halides,[45–47] which may totally alter the physical properties of the CH3NH3PbI3–xClx perovskites. Therefore, the choice of phosphonium salt against ammonium salt is aimed to minimize the possible chemical interactions with the perovskite crystal. The bulky and more rigid aromatic groups in the tetraphenylphosphonium salts may also alter the growth kinetic of the perovskite films while the free halide ions from the salts may participate in the perovskite crystal formation.[41] The current–density (J–V) curves of the best-performing perovskite solar cells processed from precursor solutions doped with 1 wt% of the organic halide salts are shown in Figure 2a and the corresponding Figure 1. Device configuration of the planar-heterojunction solar cell and the chemical photovoltaic performance data are summarized in Table 1. The reference cell structures of different organic halide salts used in this study.
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perovskite film. The crystal size is ≈1 µm while some pinholes with size of ≈1 µm were also observed. It is worth noting that the formation of pinholes is detrimental to the performance of perovskite solar cells due to the formation of undesired PEDOT:PSS/PCBM interfaces that cause charge recombinations.[37] In the cases of TPPI- and TPPCl-doped films, an even higher surface coverage with reduced pinhole size was achieved but in the meanFigure 2. a) Current density–voltage characteristics and b) EQE spectra of pristine and while, the size of the perovskite crystal additive-doped perovakite solar cells. was also slightly decreased to less than 1 µm. Interestingly, despite a reduction of without doping showed a PCE of 9.7%, which is comparable crystal size in those doped films, the degree of crystallizato other reported performance based on similar device struc- tion measured by the XRD was indeed much higher when ture and processing conditions.[31,40,41] The device perfor- compared with that of the pristine perovskite film. Figure 2b mance deceased significantly in both TBAI- and TBPI-doped shows the XRD patterns of all the perovskite films, the strong perovskite solar cells with PCE dropped down to 1.4% and diffraction peaks at 14.1°, 28.3°, 43.0°, and 58.6° are corre2%, respectively. All the three photovoltaic parameters sponding to the (110), (220), (330), and (440) crystal planes including open-circuit voltage (Voc), short-circuit current den- of the CH3NH3PbI3–xClx tetragonal structure. The intensity sity (Jsc), and fill factor (FF) were simultaneously decreased of the XRD peaks for the TPPI- and TPPCl-doped perovsand the S-shaped J–V curves indicated that the devices show kite films was about two times larger than that of the prispoor diode behavior. In contrast, solar cells doped with both tine film. As the thicknesses of all the films are similar, the TPPI and TPPCl showed positive effect on the device per- increase in XRD peak intensity could therefore be attributed formance with PCE of 11.7% and 11.5%, respectively. The to the higher phase purity and preferential orientation of the major factor contributed to the enhanced PCEs was the FF, perovskite crystals, which are important factors for improving which improved from 0.57 of the reference devices to 0.66– the photophysical properties[20] and performance of perovs0.68 for the doped devices. The Jsc also slightly increased kite solar cells.[36,40,48,49] The morphology of the TPPBr-doped film is quite difin both cases, while the Voc largely remained the same at 0.90 V. The external quantum efficiency (EQE) spectra of ferent from those of the TPPI- and TPPCl-doped films. those doped cells and reference cells are shown in Figure 2b The film showed a lower coverage and the perovskite crysand the shape and magnitude of the three spectra are very tals appeared to be larger in size but with smoother edges. similar with average EQE over 70% covering the visible to The XRD result revealed that the film had a relatively poor near infrared region. In the case of TPPBr-doped solar cells, crystallinity even though the crystal size is larger. These an adverse effect on the device performance was observed results suggest that the anions of the additives could play a with both Jsc and FF dropped compared with the reference critical role in affecting the growth kinetics of the perovscell, leading to a decrease in PCE to 8.4%. kite films. The free Br− ions that did not originally exist in To investigate the film structure-device property relation- the CH3NH3I and PbCl2 precursor solution may participate ship of the additive-doped perovskite solar cells, a combina- in the formation of even more complex perovskite compotion of experiments including scanning electron microscopy sitions involving all three halide ions and therefore may (SEM), X-ray diffraction (XRD), absorption spectrum, alter the crystal growth behavior. The free Cl− and I− ions and photoluminescence (PL) were used. The SEM images from the additives might slightly vary the initial concentrain Figure 3a reveal that a wide range of morphologies was tion and ratio of halide ions in the precursor solutions, which formed in the additive-doped films. A polycrystalline film also changed the crystal growth and film formation properwith high surface coverage was observed for the pristine ties. The effect of additive concentration was also studied in the case of TPPI-doped films and the morphology and device Table 1. Photovoltaic performance data of planar-heterojunction solar performance of the resulted films are provided in the Supcells based on the perovskite films doped with different additives at a porting Information (Figure S1–S3, Supporting Information). concentration of 1 wt%. The results suggested that there is an optimal doping concenPerovskite Layer Voc Jsc FF PCE tration at 1 wt%. For the case of TBAI- and TBPI-doped per[V] [mA cm–2] [%] ovskite films, the morphologies are very different from the 0.90 18.8 0.57 9.6 CH3NH3PbI3–xClx others. It appeared that the crystals were more favorable to 0.90 19.2 0.68 11.7 CH3NH3PbI3–xClx + TPPI grow in the lateral direction with large crystal sizes of over 10 µm. The XRD study suggested that these films had much 0.90 19.3 0.66 11.5 CH3NH3PbI3–xClx + TPPCl lower crystallinity but the tetragonal perovskite structure was CH3NH3PbI3–xClx + TPPBr 0.89 17.8 0.53 8.4 preserved as the peak positions are the same as the others. 0.51 15.8 0.25 2.0 CH3NH3PbI3–xClx + TBPI We speculate that both the alkyl-containing ammonium 0.65 10.1 0.21 1.4 CH3NH3PbI3–xClx + TBAI and phosphonium cations may interact with the perovskite small 2015, DOI: 10.1002/smll.201403344
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Figure 3. a) SEM images and b) XRD spectra of perovskite films with different dopants on glass/PEDOT:PSS substrates. c) UV–vis and PL spectra of the perovskite films deposited on ITO/PEDOT:PSS and glass substrates, respectively. The excitation wavelength for the PL study was 580 nm.
crystallographic plane terminated with the corner-sharing metal-halide octahedrals and formed a layered organic–inorganic hybrid structures. The layered structure showed good lateral charge transport property[50] and light emitting property[51,52] but may not be suitable for photovoltaic applications as the vertical charge transport property is hindered by the insulating organic layer; therefore, poor solar cell performance was found in our study. To study the effect of additives on the optical properties of the perovskite films, absorption and photoluminescence experiments were performed and the data are shown in Figure 3c. Typical absorption and emission spectra for the CH3NH3PbI3–xClx perovskite were shown in the pristine film with absorption edge at ≈770 nm, while the emission peak was at 780 nm. Enhanced absorption between 350 and 600 nm region were observed in the TPPI- and TPPCldoped films, while for all the other doped films, the absorption were decreased. The change of absorption property can be attributed to the difference in crystallinity of the films as the absorbance increases with the degree of crystallinity as suggested by previous reports.[24,40,41,53] In contrary, from the photoluminescence study, we found that the emission intensity decreased with the degree of crystallinity. This result is also in agreement with the photovoltaic performance of the resultant devices as the larger carrier diffusion lengths found in highly crystalline perovskite films reduces the possibility of radiative recombination but improves the charge transport and charge collection properties in the perovskite solar cells. Most additives employed in previous reports to improve the performance of perovskite solar cells are in liquid form
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and they were removed from the films either during the spin-coating process or post-thermal annealing process. In this study, all the additives are solid with high melting points (150–350 °C), so it is very likely the additives will remain in the perovskite films and it is important to study how the residual additives distributed in the final film structures and also their effect on the device performance. From the optical images of the additive-doped films (see Figure S4, Supporting Information), we observed that crystals of the organic halide salts were formed on the surface of the perovskite films and the shape, size, and number of crystals were depended on the type of additives. Taking TPPI as an example, elongated crystals with size of ≈10 µm were formed. We also found that the crystals remained in the device even after the deposition of the PCBM layer as revealed by the SEM image in Figure 4. Cross-section SEM images also confirmed that the TPPI crystals were embedded in the perovskite and PCBM films (see Figure S5, Supporting Information). Those additive crystals are unlikely to cause a negative effect to the solar cell as
Figure 4. SEM images of a) pristine and b) TPPI-doped perovskite films covered by a PCBM layer on glass/PEDOT:PSS substrates (the scale bar is 20 µm).
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Figure 5. Current density–voltage characteristics of pristine perovskite and TPPI-doped perovskite solar cells with TPPI interlayer.
the overall performance of the devices was improved as discussed in the previous section. Indeed, we speculate that the close contact of the additive crystal with the PCBM layer and the metal cathode may introduce a positive effect to improve charge-collection property of the cathode. This hypothesis triggered us to also explore the use of the TPPI additive as an interfacial modifier to improve the interface between the PCBM and Al cathode. TPPI was chosen because it is known that TPPI can cocrystallize with C60 to form a TPP+C60– charge transfer complex with high electrical conductivities.[54] So, it is also likely that TPPI could interact with PCBM and create an n-doped electron transport layer with improved electrical property. However, it is difficult to directly dope the TPPI into the PCBM solution as it has very poor solubility in nonpolar organic solvents such as chlorobenzene, which was used as processing solvent for the PCBM layer. Therefore, instead of modifying the bulk property of the PCBM layer, we took an alternative route and modified the interface between the PCBM and Al cathode by spin coating an ultrathin layer (5 nm) of TPPI on the PCBM surface from isopropanol solution, which is an orthogonal solvent to the bottom layers. The J–V curves of the interface-modified solar cells comprising a pristine perovskite layer and a TPPI-doped perovskite layer are shown in Figure 5a and their photovoltaic parameters are summarized in Table 2. Compared with the reference cell, the interface-modified device with a pristine perovskite layer showed an enhancement of PCE from 9.7% to 11.1%, which contributed from both the improvement of Jsc and FF. When the TPPI was used as both the processing additive and interface modifier, the PCE of the solar cells further
improved to 13% with a high FF of 0.73. As the FF is governed by both the shunt resistance (Rp) and series resistance (Rs) of the solar cells, those parameters were therefore calculated from the J–V curves for further analysis. Compared with the reference cell, the solar cells with the TPPI-doped perovskite layer and TPPI interlayer showed an obvious increase in Rp from 424.2 to 873.6 Ω cm2 and also a decrease in Rs from 9.4 to 6.0 Ω cm2. Since the loss of charge carriers through leakage paths in the films and the recombination of carriers during their transit through the cell will both affect the Rp, we therefore attributed the increase in Rp to the improvement of coverage and crystallinity of the TPPI-doped perovskite film. The reduction of Rs was attributed to the improved electrical property of the cathode interface. It is also possible that the interfacial modification can reduce the recombination of charges at the cathode interface by removing the trap states at the PCBM/metal interface through interfacial n-doping, which was reported in the case of polymer solar cells employing halide ions-based interfacial materials.[55,56] To further study the effect of interfacial modification on the charge transporting properties of PCBM and the cathode interface, electron-only devices based on the architectures of ITO/Al/PC61BM/Al with and without TPPI interlayer were prepared and the electron mobilities of the devices were extracted by fitting the data using the space-chargelimited current (SCLC) model.[57] The TPPI-modified electron-only device showed a much higher current density at the region of low applied field (Figure 6a), which suggested a smaller ohmic loss probably resulted from the reduced contact resistance at the PCBM/TPPI/Al interface. The electron mobilities of the PCBM layer were calculated from the slopes of the J1/2–V curves and determined to be 1.9 × 10−3 and 8.5 × 10−4 cm2 V−1 s−1 for the devices with and without the TPPI interlayer, respectively (Figure 6b). We suggest that an electron transfer process may occur between the I− anion and PCBM and result in an n-doped PCBM interface with reduced contact resistance as similar doping mechanism was also observed in several PCBM/organic halide systems.[56] The improved mobility of the PCBM layer could be attributed to the filling of surface trap states of the PCBM and this finding is in good agreement with the result from a recent report, which suggested that n-type doping of fullerenes can improve electron mobility up to one order of magnitude owning to the dopant-assisted filling of shallow electron traps.[58] These results suggested that interface engineering is a simple but effective approach to optimize the charge collection efficiency as well as the performance of perovskite solar cells. In summary, we have introduced organic halide salts as both a processing additive to modulate the morphology and crystallinity of perovskite light-absorbing layer and also
Table 2. The photovoltaic performance data of perovskite planar-heterojunction solar cells with TPPI interlayer. Perovskite Layer
Interface Layer
Voc [V]
Jsc [mA cm–2]
FF
CH3NH3PbI3–xClx
–
0.90
18.8
CH3NH3PbI3–xClx + TPPI
–
0.90
19.2
CH3NH3PbI3–xClx
TPPI
0.90
CH3NH3PbI3–xClx + TPPI
TPPI
0.90
small 2015, DOI: 10.1002/smll.201403344
PCE [%]
Rs [Ω cm2]
Rp [Ω cm2]
0.57
9.6
9.4
424.2
0.68
11.7
6.2
808.8
19.9
0.62
11.1
6.7
816.1
19.7
0.73
13.0
6.0
873.6
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Figure 6. J–V curves and J1/2–V characteristics of the electron-only devices with or without TPPI interlayer.
an interfacial modifier to improve the electrical contact of the PCBM/Al electron-collecting electrode in perovskite/ fullerene planar-heterojunction solar cells. Among different organic halide salts that have been tested in this study, phosphonium halides with bulky aromatic cations, such as TPPI or TPPCl, were found to be the most efficient ones in improving the morphology of the perovskite films. The free I– and Cl– anions provided by those additives could alter the growth kinetic of the perovskite films and led to improved film coverage, crystallinity, and solar cell performance. Moreover, TPPI was also explored as an efficient interfacial n-dopant to improve the electrical property of the PCBM/metal contact by reducing the contact resistance at that cathode interface. As a result, planar-heterojunction perovskite solar cells employed these dual-functional phosphonium halides showed enhanced performance with PCE up to 13%. These findings provide valuable design guidelines for new device process and efficient interfacial materials for high performance perovskite solar cells.
Experimental Section Materials and Sample Preparation: Methylammonium iodide (CH3NH3I) was synthesized by reacting 0.12 mol methylamine (33 wt% in absolute ethanol, Aldrich) and 13.2 mL of 0.1 mol hydroiodic acid (57 wt% in water, Alfa Aesar) at 0 °C for 2 h with constant stirring under nitrogen atmosphere. Then the CH3NH3I was crystallized by rotary evaporation to remove the solvent and the white powder was washed by diethyl ether for more than three times. Finally, the CH3NH3I powder was collected and dried at 50 °C in a vacuum overnight. To prepare the perovskite precursor solution, CH3NH3I and PbCl2 (Sigma-Aldrich) were mixed in anhydrous N,N-dimethylformamide (DMF) (Sigma-Aldrich) with a molar ratio of 3:1. The perovskite/dopants precursor solution was prepared via adding 1 wt% of dopants into perovskite precursor solution. The solutions were stirred at 60 °C overnight and then filtered with 0.45 µm PTFE filters before used for device fabrication. The concentration of the perovskite precursor solution was 35 wt%. Poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), (CleviosPEDOT:PSS, Clevios P VP AI 4083), and [6,6]-phenylC61-butyric acid methyl ester (PCBM) were purchased from Heraeus Precious Metals GmbH & Co.,Germany and Nano-C Inc., US, respectively. Other reagents we used were purchased from commercial sources (Aldrich, Acros, and Alfa Aesar) and used as received. Fabrication of Thin-Film Perovskite Solar Cells: The indium tin oxide (ITO, 15 ohm sq–1) glass substrates were cleaned sequentially under sonication for 20 min with acetone, detergent,
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deionized water, and isopropyl alcohol and then dried under 80 °C in baking oven overnight, followed by an plasma treatment for 4 min. PEDOT:PSS (filtered through a 0.45 µm PES filter) was spun on ITO substrate and dried at 140 °C for 20 min in air. The CH3NH3PbI3–xClx or CH3NH3PbI3–xClx /dopants precursor solution was spun onto PEDOT:PSS layer to form a thin-film perovskite layer with thickness of 300–400 nm and then further annealed at 100 °C for 2 h. The PCBM (30 mg mL–1) was spun cast onto the perovskite layer at 1200 rpm for 30 s and tetraphenylphosphonium iodide (TPPI) dissolved in isopropyl alcohol at a concentration of 0.5 mg mL–1 was spun onto PCBM layer at 2000 rpm for 30 s for some devices. An aluminum electrode (100 nm) was deposited by thermal evaporation finally through a shadow mask under a base pressure of 1 × 10−6 mbar. The active area was defined at 0.16 cm2. Device Characterization: The device photocurrent was measured under an AM 1.5G solar simulator (Japan, SAN-EI, XES-40S1). The current density–voltage (J–V) characteristics for the devices were recorded with a Keithley 2400 source meter. The illumination intensity of the light source was calibrated before the testing using a standard silicon solar cell with a KG5 filter, calibrated using a National Renewable Energy Laboratory (NREL)-calibrated silicon photodiode, giving a value of 100 mW cm−2 in the test. All the J–V curves reported in this manuscript are measured under reversed voltage bias. The hysteresis property was tested by sweeping the J–V measurement from both forward and reversed direction (Figure S6, Supporting Information). The average PCEs of the different devices and the statistic distributions of the cell performance are shown in Figure S7 (Supporting Information). UV–vis absorption, photoluminescence (PL) spectra, scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and optical polarizing microscope (OPM) were recorded on a HP 8453 spectrophotometer, an Instaspec IV CCD spectrophotometer (Oriel Co.), Zeiss EVO 18 SEM, PANalytical X’pert PRO X-ray diffractometer, and Nikon eclipse E600 POL, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the Ministry of Science and Technology (Grant No. 2014CB643500), the Natural Science Foundation of China (Grant Nos. 51323003 and 21125419), and the Guangdong Natural Science Foundation (Grant No. S2012030006232). The authors acknowledge Dr. Jiang Liu and Jiahui Lin from Chengdu Green Energy and Green Manufacturing Technology R&D Centre for the cross-sectional SEM study.
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Received: November 11, 2014 Revised: December 4, 2014 Published online:
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