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Chunhui Duan, Alice Furlan, Jacobus J. van Franeker, Robin E. M. Willems, Martijn M. Wienk, and René A. J. Janssen*
Solution-processed polymer solar cells (PSCs) attract growing attention in the prospect of producing high-efficient, large-area, and flexible photovoltaic modules via cheap roll-to-roll processing.[1] The core component, the photoactive layer of PSCs, is usually a blend of a conjugated polymer acting as electron donor and a fullerene derivative as electron acceptor. In the past years, significant progress has been achieved in this field with power conversion efficiencies (PCEs) in excess of 9%.[2] Further improvements are required to commercialize this technology. Efforts are thus devoted to extend the spectral coverage of PSCs to the near-infrared (NIR) region. Narrow-bandgap (NBG) semiconducting polymers, which can absorb NIR light, often suffer from either modest external quantum efficiency (EQE) for photon-to-electron conversion or substantially reduced open-circuit voltage (Voc) and hence provide unsatisfactory overall performance levels.[3] The champion single-junction PSCs achieved to date have been constructed by applying medium-bandgap polymers, which convert light up to ≈800 nm but are not able to exploit the NIR light of solar radiation.[2b–d] A feasible and successful strategy to enhance the performance of PSCs and extend their spectral coverage is to use multijunction device architectures.[2e,f ] In multijunction PSCs, photons with different energy are spatially separated and absorbed by different subcells that have complementary absorption ranges to reduce thermalization and transmission losses that are inevitable in single-junction devices. Notably, several record PCEs for PSCs have been reported for multijunction devices.[2f–h] The highest PCEs of 11.5% and 11.8% have recently been achieved in triple-junction PSCs.[2g,h] Generally, an efficient multijunction PSC features multiple absorbers with complementary optical bandgaps (Eg) to minimize spectral overlap between subcells and to guarantee
Dr. C. Duan, A. Furlan, J. J. van Franeker, R. E. M. Willems, Dr. M. M. Wienk, Prof. R. A. J. Janssen Molecular Materials and Nanosystems Institute for Complex Molecular Systems Eindhoven University of Technology P. O. Box 513, 5600 MB, Eindhoven, The Netherlands E-mail: [email protected] Dr. M. M. Wienk, Prof. R. A. J. Janssen Dutch Institute for Fundamental Energy Research De Zaale 20, 5612 AJ, Eindhoven, The Netherlands
DOI: 10.1002/adma.201501626
Adv. Mater. 2015, 27, 4461–4468
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Wide-Bandgap Benzodithiophene–Benzothiadiazole Copolymers for Highly Efficient Multijunction Polymer Solar Cells
a high current output for each subcell. Benefiting from the widespread efforts in developing high performance semiconducting polymers for efficient single-junction PSCs in the past decade, the methodologies toward state-of-the-art medium-bandgap polymers[2c,4] and NBG polymers[2e,f,5] have successfully been established and outstanding materials have emerged.[6] In sharp contrast, the viable choice of wide-bandgap (WBG) semiconducting polymers is very limited.[7] In fact, poly(3-hexylthiophene) (P3HT) and poly[[9-(1-octylnonyl)-9Hcarbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7diyl-2,5-thiophenediyl] (PCDTBT) remain the most common choices despite their unsatisfactory performance.[2e–g,8] For example, although impressive PCEs of 9.6% and 11.6% for triple-junction PSCs have been realized, the WBG cells give PCEs of only 4.73% and 5.79% when used as a single-junction cell at the layer thickness used in the triple-junction PSCs.[2e,g] It is therefore fair to say that the real potential of multijunction PSCs is presently mostly limited by the lack of efficient WBG materials. From a device point of view, there are several prerequisites for an ideal WBG material for multijunction PSCs application. First, because the Voc of any PCS is limited by the lowest Eg of the two components (either the electron donor or the electron acceptor),[9] an Eg of ≈1.75 eV for the WBG electron donor is optimal because it coincides with the Eg of the most widely used electron acceptors [60]PCBM and [70]PCBM ([6,6]-phenyl-Cn-butyl acid methyl ester, with n = 61 and 71, respectively).[10] A higher Eg would lead to a lower photocurrent, but not increase the maximum attainable Voc. A lower Eg would cause substantial spectral overlap with NBG subcell and reduce the Voc. Second, a high EQE of >0.65 is needed to ensure current matching between each subcell and maximize current output of the multijunction PSCs. Third, the WBG polymer should possess suitable energy level to afford high Voc (>0.85 V at least in a single-junction solar cell) especially by taking into account that NBG absorbers usually offer low Voc. Fourth, the WBG subcell should be able to tolerate a broad range of active layer thicknesses with high PCE to allow freedom in optimizing the optical electric field distribution within multijunction device to balance the performance of the subcells. For semiconducting polymers, increasing the thickness of the active layer has proved highly challenging, because it often leads to a sharp decrease in fill factor (FF) and hence PCE due to exacerbated bimolecular charge recombination and buildup of space charge at large active layer thickness.[11] WBG polymers possessing all of these four merits are presently unknown.
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Figure 1. BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu. a) Chemical structures. b) Absorption spectra in o-DCB solution and in films. c) Energy levels.
Among the various donor units used in photovoltaic π-conjugated polymers, benzo[1,2-b:4,5-b′]dithiophene (BDT) is one of the most promising units because of its excellent planarity and adjustable Eg and energy levels in resulting polymers by copolymerizing with suitable acceptor units.[12] Several BDT-based polymers with exceptional performance in singlejunction PSCs have been developed.[4,13] Moreover, introducing fluorine in semiconducting polymers is effective in shifting the frontier molecular orbital energy levels down via the electronwithdrawing nature of the fluorine atom and improving the coplanarity of the polymer via weak intramolecular non-covalent interactions (C–H…F, S…F).[14] The decreased highest occupied molecular orbital (HOMO) level offers higher Voc, while improved coplanarity leads to higher hole mobility for polymers and thereby more efficient charge extraction in PSCs. Based on these two characteristics, we designed and synthesized a series of novel WBG copolymers, BDT-BT-2T, BDT-FBT-2T, BDT-BT2Fu, and BDT-FBT-2Fu (Figure 1a) based on thienyl-substituted BDT and benzo[2,1,3]thiadiazole (BT). Different flanking units (thiophene and furan) between BDT and BT, and substituents (H and F) on BT were introduced to systematically investigate structure–property relationships and establish empirical rules for developing WBG polymers for multijunction PSCs application. From this comparison, BDT-FBT-2T emerges as a very promising candidate, possessing all four aforementioned merits. Single-junction PSCs based on BDT-FBT-2T exhibit PCEs over 6.5% in a wide active layer thickness range from 90 to 250 nm, with a maximum PCE of 7.7% achieved at both ≈100 and ≈250 nm thick active layer. So far, only few semiconducting polymers show PCEs over 7% in PSCs with active layer thickness over 200 nm.[2c,7a,13g,14d,15] Incorporating BDT-FBT-2T as WBG absorber and our previously reported diketopyrrolopyrrole-based copolymer PMDPP3T as NBG absorber,[2e] an efficient double-junction PSC with a high PCE of 8.9% is achieved.
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The synthesis route (Figure S1, Supporting Information) and detailed procedure for the preparation of the monomers and polymers are described in the Supporting Information. 4,8-di(2,3-Didecylthiophen-5-yl)-benzo[1,2-b:4,5-b′]dithiophene was chosen as the electron-rich building block because its BDT core provides small steric hindrance and good coplanarity, while the pendant didecylthienyl substituents afford the required solubility for the resulting polymers even at high molecular weight. No additional alkyl chains were introduced onto the flanking units to avoid any undesired torsion along conjugated main chain. The target polymers were synthesized by Stille cross-coupling reaction between the bis(trimethyltin) BDT monomer and relevant dibrominated BT monomers. The molecular weight and polydispersity index (PDI) of the polymers was determined by gel permeation chromatography (GPC) using ortho-dichlorobenzene (o-DCB) as eluent at 140 °C. The resulting number-average molecular weights (Mn) are 47.6, 65.2, 25.5, and 34.5 kg mol−1 for BDT-BT-2T, BDT-FBT-2T, BDTBT-2Fu, and BDT-FBT-2Fu, respectively (Table 1). The furanflanked polymers are more soluble than their thiophene-flanked counterparts (BDT-BT-2Fu > BDT-BT-2T; BDT-FBT-2Fu > BDT-FBT-2T), while the fluorinated polymers are less soluble than their non-fluorinated counterparts (BDT-FBT-2T < BDTBT-2T; BDT-FBT-2Fu < BDT-BT-2Fu). In particular, BDT-FBT-2T displayed limited solubility in almost all solvents at room temperature, but it is readily soluble in warm (80−90 °C) chlorinated aromatic solvents, thus offering good solution-processing condition for a high quality film. UV–vis absorption spectra of the polymers in dilute solutions and thin films spin-coated from o-DCB are shown in Figure 1b and relevant data are listed in Table 1. Compared to dilute solutions, all polymers show a bathochromic shift absorption in solid-state films as a result of aggregation. The absorption spectra of thiophene-flanked polymers are red-shifted
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Mna) [kg mol−1]
PDIa)
BDT-BT-2T
47.6
BDT-FBT-2T
65.2
BDT-BT-2Fu BDT-FBT-2Fu
Polymer
Solution
Film
Egopt [eV]
HOMO [eV]
LUMO [eV]
EgCV [eV]
ΔELUMOb) [eV]
λmax [nm]
λonset [nm]
λmax [nm]
λonset [nm]
2.7
609
718
642
740
1.68
−5.18
−3.55
1.63
0.61
2.1
631
710
650
720
1.72
−5.33
−3.63
1.70
0.53
25.5
2.2
589
685
593
705
1.76
−5.36
−3.59
1.77
0.57
34.5
2.3
581
680
637
696
1.78
−5.28
−3.60
1.68
0.56
Determined with GPC at 140 °C using o-DCB as the eluent; b)ΔELUMO = ELUMO (polymer) − ELUMO ([70]PCBM).
a)
compared to those of furan-flanked polymers. In the normalized absorption spectra, the thiophene-flanked polymers show a less intense second absorption peak (located at ≈440 nm) than the furan-flanked polymers, indicating stronger electronic coupling and intramolecular charge transfer between BDT units and BT units along the conjugated main chain. Compared to their non-fluorinated counterparts, the fluorinated polymers possess a less intense second absorption peak and exhibit a more distinct vibronic fine structure. These observations suggest enhanced coplanarity along the conjugated main chain and stronger interchain aggregation upon fluorination of the polymers. Except for BDT-BT-2T, all polymers show an absorption onset around 710 nm, corresponding to an Eg of ≈1.75 eV, which is an ideal value for WBG absorbers in multijunction PSCs. Especially, the absorption onset of BDT-FBT-2T is identical to that of [70]PCBM. Notably, the absorption onset of BDT-BT-2T is red-shifted compared to that of BDT-FBT-2T, possibly caused by homo-coupling defects of BT-2T units in BDTBT-2T as similar phenomena are observed in numerous DPP polymers.[16] The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were determined by cyclic voltammetry (CV) experiments and are referenced to a work function of ferrocene of −5.23 eV, used as the internal standard (Figure 1c, Table 1, and Table S1 and Figure S2, Supporting Information). The polymers show similar LUMO levels but different HOMO levels. As expected, BDT-FBT-2T and BDTBT-2Fu exhibit deeper-lying HOMO levels than BDT-BT-2T due to the electron-withdrawing effect of fluorine and the less electron-rich nature of furan compared to thiophene, respectively. Surprisingly, the furan-flanked polymer gives rise to a higher HOMO level upon fluorination. The reason for this behavior is unclear. The electrochemical bandgaps of the polymers, which are determined from the difference between the onsets of the oxidation and reduction waves in CV measurements, match well with their optical bandgaps, except for that of BDT-FBT2Fu. The offset, ΔELUMO, between the LUMO levels of the polymers and [70]PCBM are 0.53−0.61 eV and significantly higher than the empirical threshold value of 0.30 eV for efficient exciton dissociation in PSCs.[17] The performance of these polymers in single-junction PSCs was evaluated in a conventional device architecture of ITO/PEDOT:PSS (35 nm)/polymer:fullerene/LiF (1 nm)/Al (100 nm) under AM1.5G illumination (100 mW cm−2). EQE calibrated current densities were used to accurately calculate the PCEs of the solar cells. For each polymer, the blend layer was systematically optimized in terms of type of fullerene,
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Table 1. Molecular weights, optical properties, and energy levels of BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu.
polymer/fullerene ratio, amount, and type of solvent additive used for spin coating, and active layer thickness to maximize the PCE (see Table S2 in the Supporting Information for results on BDT-FBT-2T:[70]PCBM as an example). Typical current-density−voltage (J−V) curves for each polymer are shown in Figure 2a, the corresponding EQE curves are presented in Figure 2b and device parameters are summarized in Table 2 (statistics on device parameters are given in Table S3 in the Supporting Information). BDT-BT-2T-based solar cells display
Figure 2. a) Current-density−voltage characteristics of the PSCs based on BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu under AM1.5G illumination (100 mW cm−2). b) EQE curves of the corresponding solar cells.
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www.MaterialsViews.com Table 2. Performance parameters of single-junction PSCs based on BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu and field-effect transistor mobility of the polymers. Polymer
Acceptor
Ratio
Solvent
Jsc [mA cm−2]
Voc [V]
FF [−]
PCE [%]
EQEmax [−]
µh [cm2 V−1 s−1]
BDT-BT-2T
[70]PCBM
1:2
o-DCB
11.2
0.75
0.64
5.4
0.62
4.8 × 10−2
BDT-FBT-2T
[70]PCBM
1:1.5
CB/CN
13.3
0.85
0.68
7.7
0.72
9.3 × 10−2
BDT-BT-2Fu
[70]PCBM
1:2
o-DCB
7.9
0.86
0.55
3.7
0.50
3.6 × 10−3
BDT-FBT-2Fu
[60]PCBM
1:1.5
o-DCB
8.7
0.86
0.59
4.4
0.51
1.5 × 10−2
the lowest Voc at 0.748 V. Upon fluorination, the Voc of the BDTFBT-2T solar cell increases by 0.105 V. Replacing the flanking thiophenes by furans also results in an increased Voc (from 0.748 V for BDT-BT-2T to 0.858 V for BDT-BT-2Fu). No substantial enhancement in Voc is observed going from BDT-BT2Fu to BDT-FBT-2Fu. These observations on Voc roughly agree with the HOMO level alignment of the polymers. The thiophene-flanked polymers BDT-BT-2T and BDT-FBT-2T showed much higher short-circuit current densities (Jsc) and FFs than the corresponding furan-flanked polymers. A remarkable high Jsc of 13.3 mA cm−2, which is confirmed by a high EQE, is achieved in BDT-FBT-2T-based solar cells. With an FF of 0.68, the BDT-FBT-2T-based solar cell shows a maximum PCE of 7.7%, whereas the BDT-BT-2T, BDT-BT-2Fu, and BDT-FBT-2Tbased devices show PCEs of 5.4%, 3.7%, and 4.4%, respectively. From the EQE spectra, it is clear that BDT-FBT-2T shows a significantly higher EQE than the other polymers. The maximum EQE values are 0.62, 0.72, 0.50, and 0.51 for BDT-BT-2T, BDTFBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu, respectively. The higher Jsc, FF, and EQE of BDT-FBT-2T-based solar cell is probably due to a higher hole mobility of BDT-FBT-2T (9.3 × 10−2 cm2 V−1 s−1) as compared to that of BDT-BT-2T (4.8 × 10−2 cm2 V−1 s−1), BDT-BT-2Fu (3.6 × 10−3 cm2 V−1 s−1), and BDT-FBT-2T (1.5 × 10−2 cm2 V−1 s−1) as measured in field-effect transistors (Table 2 and Figure S3, Supporting Information). The morphology of the blend films, which plays a critical role on the efficient exciton dissociation at the polymer/fullerene interfaces and charge transport in the polymer domains, was probed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). In the TEM images finely dispersed fibrillar structures and bicontinuous networks can be seen in both BDT-BT-2T:[70]PCBM and BDT-FBT-2T:[70]PCBM blend films, whereas for BDT-BT-2Fu:[70]PCBM and BDT-FBT-2Fu:[70] PCBM the films show serious phase separation with large
domain size but no fibrillar network (Figure S4, Supporting Information). The fibrillar structure of BDT-FBT-2T:[70]PCBM film that gives a higher EQE seems somewhat more finely dispersed and more compact as compared to that of the BDT-BT-2T:[70]PCBM film. In the AFM height images the BDTBT-2T:[70]PCBM and BDT-FBT-2T:[70]PCBM blend films show a root mean square roughness (Rq) of 1.17 and 1.06 nm, while BDT-BT-2Fu:[70]PCBM and BDT-FBT-2Fu:[70]PCBM films show Rq equal to 1.75 and 1.33 nm, respectively, supporting the smoother surfaces and the more favorable morphology of the former two (Figure S5, Supporting Information). The high hole mobility of BDT-FBT-2T and the favorable morphology of the BDT-FBT-2T:[70]PCBM film indicate that BDT-FBT-2T-based single-junction PSCs tolerate large thickness variations of the active layer. High efficiency single-junction PSCs with small sensitivity to variations in active layer thickness are not only very much in demand to allow optimization of the optical electrical field distribution in multijunction devices, but are also highly desired for large-scale fabricating of uniform and pinhole-free PSCs via high speed printing technologies. However, thicker active layers have proved challenging for PSCs, mainly because the increased distance over which photogenerated charges need to travel toward the electrodes enhances the bimolecular charge recombination while the buildup of space charge caused by less mobile charges further lead to decreased FF and PCE.[11] BDT-FBT-2T:[70]PCBM-based single-junction PSCs were thus tested at different layer thicknesses in a conventional device configuration. The J−V characteristics and device performance parameters of these devices are presented in Figure S6 and Table S4 (Supporting Information), respectively. Figure 3a shows the PCE, Jsc, Voc, and FF values of PSCs versus active layer thickness. In the 90−250 nm thickness range, the Jsc of the PSCs initially increases, then slightly decreases, and then slowly increases again with
Figure 3. a) Solar cell performance parameters (PCE, Jsc, Voc, and FF) versus active layer thickness of BDT-FBT-2T:[70]PCBM solar cells; b) EQE spectra of corresponding solar cells; c) spectrally averaged IQE of BDT-FBT-2T:[70]PCBM solar cells as a function of layer thickness.
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as compared to the 65.2 kg mol−1 polymer. Our preliminary experience suggests that the different aggregation behavior of BDT-FBT-BT with different molecular weights influences the morphology of the resulting BDT-FBT-2T:[70]PCBM films. To obtain homogeneous active layer films for extra-high molecular weight BDT-FBT-BT, a high content of high boiling point solvent 1-chloronaphthlene (30% in volume ratio) which can suppress the aggregation of BDT-FBT-2T was added into the host solvent. The slow evaporation of a high fraction of 1-chloronaphthlene changes the drying kinetics of the thin film and affects the selforganization and morphology of the BDT-FBT-2T:[70]PCBM blend. Interestingly, both low and extra-high molecular weight samples of BDT-FBT-2T also show a small sensitivity on the variation of active layer thickness (Figure S10 and Table S6, Supporting Information), suggesting the inherent superiority of BDT-FBT-2T in thick film PSCs. The effect of the molecular weight on the photovoltaic performance of BDT-FBT-BT suggests that the lower efficiencies of the solar cells obtained for BDT-BT-2T, BDT-BT-2Fu, and BDTFBT-2Fu (Table 2) are, at least in part, due to their lower molecular weights. However, in addition to molecular weight, also the chemical structure is important. For comparable molecular weights BDT-BT-2T affords a significantly lower PCE than BDT-FBT-2T. In our view, the molecular structure, the molecular weight, the nature of the solvent combination, and drying conditions determine the morphology of the photoactive layer, which then largely governs the solar cell efficiency. To test the potential of BDT-FBT-2T in multijunction PSCs, double-junction tandem cells in a two-terminal series connection were constructed. As near infrared absorber PMDPP3T, which is one of the most efficient NBG polymers, and has previously been utilized in constructing efficient multijunction PSCs was selected for the back subcell.[2e,h,20] The device structure is ITO/PEDOT:PSS (35 nm)/BDT-FBT2T:[70]PCBM/ZnO (30 nm)/pH neutral PEDOT:PSS (15 nm)/ PMDPP3T:[60]PCBM/LiF (1 nm)/Al (100 nm). The chemical structure of PMDPP3T and device structure of the tandem PSCs are shown in Figure S11 (Supporting Information). It is worth pointing out that [60]PCBM rather than [70]PCBM was used as electron acceptor in combination with PMDPP3T to minimize unfavorable absorption of high-energy photons by the NBG back subcell. Before device making of tandem cells, optical and electrical modeling was conducted to determine the optimal thickness of the front and back subcells. The details on simulation are shown in the Supporting Information. From the simulation results, PCEs >10% are anticipated for a broad range of tandem cells, with a front subcell thickness between 120 and 170 nm and a back subcell thickness between 120 and 160 nm (Figure S12, Supporting Information). Tandem PSCs were fabricated within these optimized subcell thickness ranges. Typical J−V characteristics of the tandem PSCs and corresponding single-junction reference subcells are shown in Figure 4a, and the performance parameters are summarized in Table 3. With respect to corresponding WBG (BDTFBT-2T:[70]PCBM, 153 nm) and NBG (PMDPP3T:[60]PCBM, 121 nm) reference single-junction solar cells that show PCE = 6.6% and 5.8%, respectively, the optimized tandem cell with the same layers and thickness provides PCE = 8.9% along with Jsc = 10.1 mA cm−2, Voc = 1.420 V, and FF = 0.62, representing an
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increasing active layer thickness. This trend correlates with the number of photons absorbed in active layer, which is governed by interference between incident and reflected light which causes an undulation in the absorbance of the active layer with increasing thickness.[18] The Voc shows little changes, but the FF decreases slightly at large thickness. Correspondingly, the PCEs of the single-junction PSCs show similar trend to Jsc with maximum PCEs of 7.7% at active layer thickness of 89 and 246 nm, respectively. Notably, all devices give PCEs >6.5% and FF >0.60 at the active layer thickness range of 89−246 nm. The high FF suggests efficient and balanced transport of charge carriers and also evidences limited bimolecular recombination even in very thick BDT-FBT-2T:[70]PCBM films. The high Jsc is also verified by the high EQE (Figure 3b). The maximum EQE reaches 0.72 at 550 nm for the 100 nm thick device, and 0.76 at 530 nm for the 246 nm thick device (Table S4, Supporting Information). In sharp contrast, the single-junction PSCs based on BDT-BT-2T, BDT-BT-2Fu and BDT-FBT-2Fu display serious decrease in Jsc and FF and thereby PCE with increasing active layer thickness. Correspondingly, the EQEs of each of these PSCs decrease significantly with the increase of active layer thickness (Figure S7, Supporting Information). To better understand the origin of the exceptional performance of BDT-FBT-2T-based single-junction PSCs over such a wide photoactive layer thickness range, we determined the internal quantum efficiency (IQE) for these devices. To this end, we first measured the average spectral extinction coefficient and refractive index for three BDT-FBT-2T:[70]PCBM films (Figure S8, Supporting Information). Using optical modeling that involved the entire stack of layers (glass/ITO/PEDOT:PSS/ BDT-FBT-2T:[70]PCBM/LiF/Al), the IQE of the PSCs were calculated by dividing the EQE by the calculated fraction of photons absorbed by the active layer (for details see the Supporting Information). By calculating the total number of photons absorbed by the active layer from the 1.5 air mass (AM1.5G) solar spectrum and comparing this to the AM1.5G integrated EQE, we obtain the spectrally averaged IQE of BDT-FBT-2T:[70]PCBM-based single-junction PSCs as a function of the active layer thickness (Figure 3c). We find that the BDT-FBT-2T:[70]PCBM solar cells have remarkably high IQE values. The spectrally averaged IQE of almost of all PSCs (except for the 89 nm thick one) approaches, or even exceeds, 90%, suggesting that nearly every absorbed photon generates a pair of free charges. It is well known that the molecular weight of the polymer plays a critical role in the performance of PSCs.[13h,19] BDT-FBT-2T with both relative low molecular weight (Mn = 45.5 kg mol−1, PDI = 2.4) and extra-high molecular weight (Mn = 108.8 kg mol−1, PDI = 4.2) were thus synthesized to examine this dependence (for synthetic details see the Supporting Information). The photovoltaic characterization reveals that both lower and higher molecular weight lead to somewhat reduced performances (Figure S9a and Table S5, Supporting Information). The optimal single-junction PSCs show maximum PCE of 6.9% and 6.7% for the 45.5 and 108.8 kg mol−1 polymer, respectively. The inferior PCEs are primarily because of the reduced Jsc, which is reflected in lower EQE. From Figure S9b (Supporting Information), it is clear that the solar cells made from 45.5 and 108.8 kg mol−1 polymer show significant EQE losses in the wavelength range of 460−570 nm
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Figure 4. a) Current-density−voltage characteristics of tandem PSC under illumination of AM1.5G (100 mW cm−2) and b) EQE spectra of the corresponding subcells.
increase of about 36% in PCE compared to the single-junction cells. EQE measurements of the individual subcells under relevant illumination conditions and correct electrical bias[21] (see Supporting Information) reveal that the front and back subcell have high quantum efficiencies with maxima of 0.61 and 0.56, respectively (Figure 4b). Integrating the EQE with the AM1.5G solar spectrum shows highly balanced current generation in each subcells with current density of 10.2 mA cm−2 for the front subcell and 10.0 mA cm−2 for the back subcell. The EQE-integrated current densities of the subcells are consistent with the measured Jsc of the tandem solar cell under AM 1.5G Table 3. Performance parameters of tandem PSC and reference subcells. Device
Jsca) [mA cm−2]
Voca) [V]
FFa)
PCEa) [%]
Ref. front cell
11.6 (11.5)
0.84 (0.84)
0.67 (0.68)
6.6 (6.6)
Ref. back cell
14.6 (14.8)
0.61 (0.61)
0.65 (0.63)
5.8 (5.7)
Tandem measured
10.1 (9.9)
1.42 (1.42)
0.62 (0.62)
8.9 (8.7)
Tandem predicted
10.5
1.45
0.67
10.2
a)Average
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values from six devices are given in parentheses.
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illumination, corroborating the accuracy and consistency of our measurement. Despite the good performance, there are non-negligible differences between measured and predicted device performance. All measured performance parameters Jsc, Voc, FF, and consequently PCE, are lower than the predicted values (Table 3). In particular, the measured FF of the tandem solar cell is not only lower than predicted value but also lower than those of both single-junction reference cells. This differs from the observation that the FFs of tandem PSCs normally lie between the lower subcell FF and the higher subcell FF when balanced current are generated in each subcell.[2e,f,22] To reveal the cause for these differences, we constructed the J−V curves of the tandem subcells by interpolating the J−V curves of two reference single-junction cells with the current densities acquired from EQE convoluting of the tandem subcells as the Jscs of the constructed subcells. Assuming a loss-free intermediate layer, the J−V curve of the tandem cell can thereby be reconstructed by adding the two curves using Kirchhoff’s law.[23] This reconstruction yields a J−V curve (magenta line in Figure 4a) that is almost identical to the predicted J−V curve of the tandem cell. The comparison between the reconstructed J−V curve and the measured J−V curve shows that the PCE loss of the tandem cell mainly comes from the losses in FF and Voc, suggesting a suboptimal contact at active layer/intermediate layer interface. The reduced light intensity illuminated on the back cell compared with one sun illumination after photon absorbing by the front cell will also lead to a slight Voc drop of the back cell.[2f,24] Anyway, it is notable that the optimal active layer thickness of the WBG subcell is around 155 nm for balancing and maximizing the current generation in each subcell of the tandem PSC (Table S7, Supporting Information). At such a relative high thickness, however, common photovoltaic polymers normally affords limited FF and PCE, which suggests the importance of developing WBG polymers with small sensitivity to thickness variation. A recent paper published by Hou and co-workers corroborates the effectiveness of varying the front cell layer thickness to achieve high performance tandem PSCs.[25] Notably, the current output and overall efficiency of their tandem cells increased significantly with increasing front cell thickness, but the FF dropped seriously. We thus envision that higher efficiencies may be expected in multijunction PSCs that can fully exploit the advantages of BDT-FBT-2T through optimal intermediate layer design, and relevant work is underway in our laboratory. In summary, a series of new WBG semiconducting polymers (BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT2Fu) based on BDT and BT was designed and synthesized for application in multijunction PSCs. Systematic characterization suggest that the new polymer BDT-FBT-2T possesses ideal bandgap, relative deep HOMO energy level, and high hole mobility. Single-junction PSCs based on BDT-FBT-2T achieved PCEs over 6.5% for a broad range of active layer thickness with a maximum PCE of 7.7% at active layer thickness of 89 and 246 nm, respectively. Double-junction tandem PSCs based on BDT-FBT-2T with high efficiency of 8.9% were achieved at the active layer thickness of 155 nm for WBG subcell. Toward highly efficient multijunction PSCs, the significance of developing WBG materials with an ideal bandgap around ≈1.75 eV
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[5]
[6]
[7]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
[8]
Acknowledgements The authors thank Weiwei Li for providing PMDPP3T, Ralf Bovee for GPC analysis, Wijnand Dijkstra, and Marco van der Sluis for AFM measurements. The work was performed in the framework of the Mujulima project that received funding from the European Commission’s Seventh Framework Programme (Grant Agreement No. 604148). The research leading to these results has also received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 339031. The research forms part of the Solliance OPV program and has received funding from the Ministry of Education, Culture and Science (Gravity Program 024.001.035). Received: April 6, 2015 Revised: May 21, 2015 Published online: July 2, 2015 [1] a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789; b) L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street, Y. Yang, Adv. Mater. 2013, 25, 6642. [2] a) Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu, Y. Cao, Nat. Photonics 2012, 6, 591; b) J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li, J.-X. Tang, Adv. Mater. 2015, 27, 1035; c) Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Commun. 2014, 5, 5293; d) S.-H. Liao, H.-J. Jhuo, P.-N. Yeh, Y.-S. Cheng, Y.-L. Li, Y.-H. Lee, S. Sharma, S.-A. Chen, Sci. Rep. 2014, 4, 6813; e) W. Li, A. Furlan, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen, J. Am. Chem. Soc. 2013, 135, 5529; f) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446; g) C.-C. Chen, W.-H. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, Adv. Mater. 2014, 26, 5670; h) A. R. b. M. Yusoff, D. Kim, H. P. Kim, F. K. Shneider, W. J. da Silva, J. Jang, Energy Environ. Sci. 2015, 8, 303; i) S. Zhang, L. Ye, W. Zhao, B. Yang, Q. Wang, J. Hou, Sci. China: Chem. 2015, 58, 248. [3] a) Y. J. Xia, L. Wang, X. Y. Deng, D. Y. Li, X. H. Zhu, Y. Cao, Appl. Phys. Lett. 2006, 89, 081106; b) F. L. Zhang, J. Bijleveld, E. Perzon, K. Tvingstedt, S. Barrau, O. Inganäs, M. R. Andersson, J. Mater. Chem. 2008, 18, 5468; c) A. P. Zoombelt, M. Fonrodona, M. M. Wienk, A. B. Sieval, J. C. Hummelen, R. A. J. Janssen, Org. Lett. 2009, 11, 903; d) Y. Dong, W. Cai, M. Wang, Q. Li, L. Ying, F. Huang, Y. Cao, Org. Electron. 2013, 14, 2459; e) Y. S. Park, Q. Wu, C.-Y. Nam, R. B. Grubbs, Angew. Chem. Int. Ed. 2014, 53, 10691. [4] a) Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135; b) L. Ye, S. Zhang, W. Zhao, H. Yao, J. Hou,
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and small sensitivity to active layer thickness variation is clearly demonstrated. The advantageous inherent properties and relative large photon energy loss (Eg − eVoc ≈ 0.85 eV) of BDT-FBT2T in single-junction PSCs indicate further room for developing more efficient WBG polymers based on this system via optimal molecule design, advanced synthesis technology, and optimized purification procedure. Moreover, multijunction PSCs with record efficiency are thus conceivable through further interface engineering of intermediate layer.
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Chunhui Duan, Alice Furlan, Jacobus J. van Franeker, Robin E. M. Willems, Martijn M. Wienk, and René A. J. Janssen*
Solution-processed polymer solar cells (PSCs) attract growing attention in the prospect of producing high-efficient, large-area, and flexible photovoltaic modules via cheap roll-to-roll processing.[1] The core component, the photoactive layer of PSCs, is usually a blend of a conjugated polymer acting as electron donor and a fullerene derivative as electron acceptor. In the past years, significant progress has been achieved in this field with power conversion efficiencies (PCEs) in excess of 9%.[2] Further improvements are required to commercialize this technology. Efforts are thus devoted to extend the spectral coverage of PSCs to the near-infrared (NIR) region. Narrow-bandgap (NBG) semiconducting polymers, which can absorb NIR light, often suffer from either modest external quantum efficiency (EQE) for photon-to-electron conversion or substantially reduced open-circuit voltage (Voc) and hence provide unsatisfactory overall performance levels.[3] The champion single-junction PSCs achieved to date have been constructed by applying medium-bandgap polymers, which convert light up to ≈800 nm but are not able to exploit the NIR light of solar radiation.[2b–d] A feasible and successful strategy to enhance the performance of PSCs and extend their spectral coverage is to use multijunction device architectures.[2e,f ] In multijunction PSCs, photons with different energy are spatially separated and absorbed by different subcells that have complementary absorption ranges to reduce thermalization and transmission losses that are inevitable in single-junction devices. Notably, several record PCEs for PSCs have been reported for multijunction devices.[2f–h] The highest PCEs of 11.5% and 11.8% have recently been achieved in triple-junction PSCs.[2g,h] Generally, an efficient multijunction PSC features multiple absorbers with complementary optical bandgaps (Eg) to minimize spectral overlap between subcells and to guarantee
Dr. C. Duan, A. Furlan, J. J. van Franeker, R. E. M. Willems, Dr. M. M. Wienk, Prof. R. A. J. Janssen Molecular Materials and Nanosystems Institute for Complex Molecular Systems Eindhoven University of Technology P. O. Box 513, 5600 MB, Eindhoven, The Netherlands E-mail: [email protected] Dr. M. M. Wienk, Prof. R. A. J. Janssen Dutch Institute for Fundamental Energy Research De Zaale 20, 5612 AJ, Eindhoven, The Netherlands
DOI: 10.1002/adma.201501626
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Wide-Bandgap Benzodithiophene–Benzothiadiazole Copolymers for Highly Efficient Multijunction Polymer Solar Cells
a high current output for each subcell. Benefiting from the widespread efforts in developing high performance semiconducting polymers for efficient single-junction PSCs in the past decade, the methodologies toward state-of-the-art medium-bandgap polymers[2c,4] and NBG polymers[2e,f,5] have successfully been established and outstanding materials have emerged.[6] In sharp contrast, the viable choice of wide-bandgap (WBG) semiconducting polymers is very limited.[7] In fact, poly(3-hexylthiophene) (P3HT) and poly[[9-(1-octylnonyl)-9Hcarbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7diyl-2,5-thiophenediyl] (PCDTBT) remain the most common choices despite their unsatisfactory performance.[2e–g,8] For example, although impressive PCEs of 9.6% and 11.6% for triple-junction PSCs have been realized, the WBG cells give PCEs of only 4.73% and 5.79% when used as a single-junction cell at the layer thickness used in the triple-junction PSCs.[2e,g] It is therefore fair to say that the real potential of multijunction PSCs is presently mostly limited by the lack of efficient WBG materials. From a device point of view, there are several prerequisites for an ideal WBG material for multijunction PSCs application. First, because the Voc of any PCS is limited by the lowest Eg of the two components (either the electron donor or the electron acceptor),[9] an Eg of ≈1.75 eV for the WBG electron donor is optimal because it coincides with the Eg of the most widely used electron acceptors [60]PCBM and [70]PCBM ([6,6]-phenyl-Cn-butyl acid methyl ester, with n = 61 and 71, respectively).[10] A higher Eg would lead to a lower photocurrent, but not increase the maximum attainable Voc. A lower Eg would cause substantial spectral overlap with NBG subcell and reduce the Voc. Second, a high EQE of >0.65 is needed to ensure current matching between each subcell and maximize current output of the multijunction PSCs. Third, the WBG polymer should possess suitable energy level to afford high Voc (>0.85 V at least in a single-junction solar cell) especially by taking into account that NBG absorbers usually offer low Voc. Fourth, the WBG subcell should be able to tolerate a broad range of active layer thicknesses with high PCE to allow freedom in optimizing the optical electric field distribution within multijunction device to balance the performance of the subcells. For semiconducting polymers, increasing the thickness of the active layer has proved highly challenging, because it often leads to a sharp decrease in fill factor (FF) and hence PCE due to exacerbated bimolecular charge recombination and buildup of space charge at large active layer thickness.[11] WBG polymers possessing all of these four merits are presently unknown.
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Figure 1. BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu. a) Chemical structures. b) Absorption spectra in o-DCB solution and in films. c) Energy levels.
Among the various donor units used in photovoltaic π-conjugated polymers, benzo[1,2-b:4,5-b′]dithiophene (BDT) is one of the most promising units because of its excellent planarity and adjustable Eg and energy levels in resulting polymers by copolymerizing with suitable acceptor units.[12] Several BDT-based polymers with exceptional performance in singlejunction PSCs have been developed.[4,13] Moreover, introducing fluorine in semiconducting polymers is effective in shifting the frontier molecular orbital energy levels down via the electronwithdrawing nature of the fluorine atom and improving the coplanarity of the polymer via weak intramolecular non-covalent interactions (C–H…F, S…F).[14] The decreased highest occupied molecular orbital (HOMO) level offers higher Voc, while improved coplanarity leads to higher hole mobility for polymers and thereby more efficient charge extraction in PSCs. Based on these two characteristics, we designed and synthesized a series of novel WBG copolymers, BDT-BT-2T, BDT-FBT-2T, BDT-BT2Fu, and BDT-FBT-2Fu (Figure 1a) based on thienyl-substituted BDT and benzo[2,1,3]thiadiazole (BT). Different flanking units (thiophene and furan) between BDT and BT, and substituents (H and F) on BT were introduced to systematically investigate structure–property relationships and establish empirical rules for developing WBG polymers for multijunction PSCs application. From this comparison, BDT-FBT-2T emerges as a very promising candidate, possessing all four aforementioned merits. Single-junction PSCs based on BDT-FBT-2T exhibit PCEs over 6.5% in a wide active layer thickness range from 90 to 250 nm, with a maximum PCE of 7.7% achieved at both ≈100 and ≈250 nm thick active layer. So far, only few semiconducting polymers show PCEs over 7% in PSCs with active layer thickness over 200 nm.[2c,7a,13g,14d,15] Incorporating BDT-FBT-2T as WBG absorber and our previously reported diketopyrrolopyrrole-based copolymer PMDPP3T as NBG absorber,[2e] an efficient double-junction PSC with a high PCE of 8.9% is achieved.
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The synthesis route (Figure S1, Supporting Information) and detailed procedure for the preparation of the monomers and polymers are described in the Supporting Information. 4,8-di(2,3-Didecylthiophen-5-yl)-benzo[1,2-b:4,5-b′]dithiophene was chosen as the electron-rich building block because its BDT core provides small steric hindrance and good coplanarity, while the pendant didecylthienyl substituents afford the required solubility for the resulting polymers even at high molecular weight. No additional alkyl chains were introduced onto the flanking units to avoid any undesired torsion along conjugated main chain. The target polymers were synthesized by Stille cross-coupling reaction between the bis(trimethyltin) BDT monomer and relevant dibrominated BT monomers. The molecular weight and polydispersity index (PDI) of the polymers was determined by gel permeation chromatography (GPC) using ortho-dichlorobenzene (o-DCB) as eluent at 140 °C. The resulting number-average molecular weights (Mn) are 47.6, 65.2, 25.5, and 34.5 kg mol−1 for BDT-BT-2T, BDT-FBT-2T, BDTBT-2Fu, and BDT-FBT-2Fu, respectively (Table 1). The furanflanked polymers are more soluble than their thiophene-flanked counterparts (BDT-BT-2Fu > BDT-BT-2T; BDT-FBT-2Fu > BDT-FBT-2T), while the fluorinated polymers are less soluble than their non-fluorinated counterparts (BDT-FBT-2T < BDTBT-2T; BDT-FBT-2Fu < BDT-BT-2Fu). In particular, BDT-FBT-2T displayed limited solubility in almost all solvents at room temperature, but it is readily soluble in warm (80−90 °C) chlorinated aromatic solvents, thus offering good solution-processing condition for a high quality film. UV–vis absorption spectra of the polymers in dilute solutions and thin films spin-coated from o-DCB are shown in Figure 1b and relevant data are listed in Table 1. Compared to dilute solutions, all polymers show a bathochromic shift absorption in solid-state films as a result of aggregation. The absorption spectra of thiophene-flanked polymers are red-shifted
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Mna) [kg mol−1]
PDIa)
BDT-BT-2T
47.6
BDT-FBT-2T
65.2
BDT-BT-2Fu BDT-FBT-2Fu
Polymer
Solution
Film
Egopt [eV]
HOMO [eV]
LUMO [eV]
EgCV [eV]
ΔELUMOb) [eV]
λmax [nm]
λonset [nm]
λmax [nm]
λonset [nm]
2.7
609
718
642
740
1.68
−5.18
−3.55
1.63
0.61
2.1
631
710
650
720
1.72
−5.33
−3.63
1.70
0.53
25.5
2.2
589
685
593
705
1.76
−5.36
−3.59
1.77
0.57
34.5
2.3
581
680
637
696
1.78
−5.28
−3.60
1.68
0.56
Determined with GPC at 140 °C using o-DCB as the eluent; b)ΔELUMO = ELUMO (polymer) − ELUMO ([70]PCBM).
a)
compared to those of furan-flanked polymers. In the normalized absorption spectra, the thiophene-flanked polymers show a less intense second absorption peak (located at ≈440 nm) than the furan-flanked polymers, indicating stronger electronic coupling and intramolecular charge transfer between BDT units and BT units along the conjugated main chain. Compared to their non-fluorinated counterparts, the fluorinated polymers possess a less intense second absorption peak and exhibit a more distinct vibronic fine structure. These observations suggest enhanced coplanarity along the conjugated main chain and stronger interchain aggregation upon fluorination of the polymers. Except for BDT-BT-2T, all polymers show an absorption onset around 710 nm, corresponding to an Eg of ≈1.75 eV, which is an ideal value for WBG absorbers in multijunction PSCs. Especially, the absorption onset of BDT-FBT-2T is identical to that of [70]PCBM. Notably, the absorption onset of BDT-BT-2T is red-shifted compared to that of BDT-FBT-2T, possibly caused by homo-coupling defects of BT-2T units in BDTBT-2T as similar phenomena are observed in numerous DPP polymers.[16] The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were determined by cyclic voltammetry (CV) experiments and are referenced to a work function of ferrocene of −5.23 eV, used as the internal standard (Figure 1c, Table 1, and Table S1 and Figure S2, Supporting Information). The polymers show similar LUMO levels but different HOMO levels. As expected, BDT-FBT-2T and BDTBT-2Fu exhibit deeper-lying HOMO levels than BDT-BT-2T due to the electron-withdrawing effect of fluorine and the less electron-rich nature of furan compared to thiophene, respectively. Surprisingly, the furan-flanked polymer gives rise to a higher HOMO level upon fluorination. The reason for this behavior is unclear. The electrochemical bandgaps of the polymers, which are determined from the difference between the onsets of the oxidation and reduction waves in CV measurements, match well with their optical bandgaps, except for that of BDT-FBT2Fu. The offset, ΔELUMO, between the LUMO levels of the polymers and [70]PCBM are 0.53−0.61 eV and significantly higher than the empirical threshold value of 0.30 eV for efficient exciton dissociation in PSCs.[17] The performance of these polymers in single-junction PSCs was evaluated in a conventional device architecture of ITO/PEDOT:PSS (35 nm)/polymer:fullerene/LiF (1 nm)/Al (100 nm) under AM1.5G illumination (100 mW cm−2). EQE calibrated current densities were used to accurately calculate the PCEs of the solar cells. For each polymer, the blend layer was systematically optimized in terms of type of fullerene,
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Table 1. Molecular weights, optical properties, and energy levels of BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu.
polymer/fullerene ratio, amount, and type of solvent additive used for spin coating, and active layer thickness to maximize the PCE (see Table S2 in the Supporting Information for results on BDT-FBT-2T:[70]PCBM as an example). Typical current-density−voltage (J−V) curves for each polymer are shown in Figure 2a, the corresponding EQE curves are presented in Figure 2b and device parameters are summarized in Table 2 (statistics on device parameters are given in Table S3 in the Supporting Information). BDT-BT-2T-based solar cells display
Figure 2. a) Current-density−voltage characteristics of the PSCs based on BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu under AM1.5G illumination (100 mW cm−2). b) EQE curves of the corresponding solar cells.
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www.MaterialsViews.com Table 2. Performance parameters of single-junction PSCs based on BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu and field-effect transistor mobility of the polymers. Polymer
Acceptor
Ratio
Solvent
Jsc [mA cm−2]
Voc [V]
FF [−]
PCE [%]
EQEmax [−]
µh [cm2 V−1 s−1]
BDT-BT-2T
[70]PCBM
1:2
o-DCB
11.2
0.75
0.64
5.4
0.62
4.8 × 10−2
BDT-FBT-2T
[70]PCBM
1:1.5
CB/CN
13.3
0.85
0.68
7.7
0.72
9.3 × 10−2
BDT-BT-2Fu
[70]PCBM
1:2
o-DCB
7.9
0.86
0.55
3.7
0.50
3.6 × 10−3
BDT-FBT-2Fu
[60]PCBM
1:1.5
o-DCB
8.7
0.86
0.59
4.4
0.51
1.5 × 10−2
the lowest Voc at 0.748 V. Upon fluorination, the Voc of the BDTFBT-2T solar cell increases by 0.105 V. Replacing the flanking thiophenes by furans also results in an increased Voc (from 0.748 V for BDT-BT-2T to 0.858 V for BDT-BT-2Fu). No substantial enhancement in Voc is observed going from BDT-BT2Fu to BDT-FBT-2Fu. These observations on Voc roughly agree with the HOMO level alignment of the polymers. The thiophene-flanked polymers BDT-BT-2T and BDT-FBT-2T showed much higher short-circuit current densities (Jsc) and FFs than the corresponding furan-flanked polymers. A remarkable high Jsc of 13.3 mA cm−2, which is confirmed by a high EQE, is achieved in BDT-FBT-2T-based solar cells. With an FF of 0.68, the BDT-FBT-2T-based solar cell shows a maximum PCE of 7.7%, whereas the BDT-BT-2T, BDT-BT-2Fu, and BDT-FBT-2Tbased devices show PCEs of 5.4%, 3.7%, and 4.4%, respectively. From the EQE spectra, it is clear that BDT-FBT-2T shows a significantly higher EQE than the other polymers. The maximum EQE values are 0.62, 0.72, 0.50, and 0.51 for BDT-BT-2T, BDTFBT-2T, BDT-BT-2Fu, and BDT-FBT-2Fu, respectively. The higher Jsc, FF, and EQE of BDT-FBT-2T-based solar cell is probably due to a higher hole mobility of BDT-FBT-2T (9.3 × 10−2 cm2 V−1 s−1) as compared to that of BDT-BT-2T (4.8 × 10−2 cm2 V−1 s−1), BDT-BT-2Fu (3.6 × 10−3 cm2 V−1 s−1), and BDT-FBT-2T (1.5 × 10−2 cm2 V−1 s−1) as measured in field-effect transistors (Table 2 and Figure S3, Supporting Information). The morphology of the blend films, which plays a critical role on the efficient exciton dissociation at the polymer/fullerene interfaces and charge transport in the polymer domains, was probed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). In the TEM images finely dispersed fibrillar structures and bicontinuous networks can be seen in both BDT-BT-2T:[70]PCBM and BDT-FBT-2T:[70]PCBM blend films, whereas for BDT-BT-2Fu:[70]PCBM and BDT-FBT-2Fu:[70] PCBM the films show serious phase separation with large
domain size but no fibrillar network (Figure S4, Supporting Information). The fibrillar structure of BDT-FBT-2T:[70]PCBM film that gives a higher EQE seems somewhat more finely dispersed and more compact as compared to that of the BDT-BT-2T:[70]PCBM film. In the AFM height images the BDTBT-2T:[70]PCBM and BDT-FBT-2T:[70]PCBM blend films show a root mean square roughness (Rq) of 1.17 and 1.06 nm, while BDT-BT-2Fu:[70]PCBM and BDT-FBT-2Fu:[70]PCBM films show Rq equal to 1.75 and 1.33 nm, respectively, supporting the smoother surfaces and the more favorable morphology of the former two (Figure S5, Supporting Information). The high hole mobility of BDT-FBT-2T and the favorable morphology of the BDT-FBT-2T:[70]PCBM film indicate that BDT-FBT-2T-based single-junction PSCs tolerate large thickness variations of the active layer. High efficiency single-junction PSCs with small sensitivity to variations in active layer thickness are not only very much in demand to allow optimization of the optical electrical field distribution in multijunction devices, but are also highly desired for large-scale fabricating of uniform and pinhole-free PSCs via high speed printing technologies. However, thicker active layers have proved challenging for PSCs, mainly because the increased distance over which photogenerated charges need to travel toward the electrodes enhances the bimolecular charge recombination while the buildup of space charge caused by less mobile charges further lead to decreased FF and PCE.[11] BDT-FBT-2T:[70]PCBM-based single-junction PSCs were thus tested at different layer thicknesses in a conventional device configuration. The J−V characteristics and device performance parameters of these devices are presented in Figure S6 and Table S4 (Supporting Information), respectively. Figure 3a shows the PCE, Jsc, Voc, and FF values of PSCs versus active layer thickness. In the 90−250 nm thickness range, the Jsc of the PSCs initially increases, then slightly decreases, and then slowly increases again with
Figure 3. a) Solar cell performance parameters (PCE, Jsc, Voc, and FF) versus active layer thickness of BDT-FBT-2T:[70]PCBM solar cells; b) EQE spectra of corresponding solar cells; c) spectrally averaged IQE of BDT-FBT-2T:[70]PCBM solar cells as a function of layer thickness.
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as compared to the 65.2 kg mol−1 polymer. Our preliminary experience suggests that the different aggregation behavior of BDT-FBT-BT with different molecular weights influences the morphology of the resulting BDT-FBT-2T:[70]PCBM films. To obtain homogeneous active layer films for extra-high molecular weight BDT-FBT-BT, a high content of high boiling point solvent 1-chloronaphthlene (30% in volume ratio) which can suppress the aggregation of BDT-FBT-2T was added into the host solvent. The slow evaporation of a high fraction of 1-chloronaphthlene changes the drying kinetics of the thin film and affects the selforganization and morphology of the BDT-FBT-2T:[70]PCBM blend. Interestingly, both low and extra-high molecular weight samples of BDT-FBT-2T also show a small sensitivity on the variation of active layer thickness (Figure S10 and Table S6, Supporting Information), suggesting the inherent superiority of BDT-FBT-2T in thick film PSCs. The effect of the molecular weight on the photovoltaic performance of BDT-FBT-BT suggests that the lower efficiencies of the solar cells obtained for BDT-BT-2T, BDT-BT-2Fu, and BDTFBT-2Fu (Table 2) are, at least in part, due to their lower molecular weights. However, in addition to molecular weight, also the chemical structure is important. For comparable molecular weights BDT-BT-2T affords a significantly lower PCE than BDT-FBT-2T. In our view, the molecular structure, the molecular weight, the nature of the solvent combination, and drying conditions determine the morphology of the photoactive layer, which then largely governs the solar cell efficiency. To test the potential of BDT-FBT-2T in multijunction PSCs, double-junction tandem cells in a two-terminal series connection were constructed. As near infrared absorber PMDPP3T, which is one of the most efficient NBG polymers, and has previously been utilized in constructing efficient multijunction PSCs was selected for the back subcell.[2e,h,20] The device structure is ITO/PEDOT:PSS (35 nm)/BDT-FBT2T:[70]PCBM/ZnO (30 nm)/pH neutral PEDOT:PSS (15 nm)/ PMDPP3T:[60]PCBM/LiF (1 nm)/Al (100 nm). The chemical structure of PMDPP3T and device structure of the tandem PSCs are shown in Figure S11 (Supporting Information). It is worth pointing out that [60]PCBM rather than [70]PCBM was used as electron acceptor in combination with PMDPP3T to minimize unfavorable absorption of high-energy photons by the NBG back subcell. Before device making of tandem cells, optical and electrical modeling was conducted to determine the optimal thickness of the front and back subcells. The details on simulation are shown in the Supporting Information. From the simulation results, PCEs >10% are anticipated for a broad range of tandem cells, with a front subcell thickness between 120 and 170 nm and a back subcell thickness between 120 and 160 nm (Figure S12, Supporting Information). Tandem PSCs were fabricated within these optimized subcell thickness ranges. Typical J−V characteristics of the tandem PSCs and corresponding single-junction reference subcells are shown in Figure 4a, and the performance parameters are summarized in Table 3. With respect to corresponding WBG (BDTFBT-2T:[70]PCBM, 153 nm) and NBG (PMDPP3T:[60]PCBM, 121 nm) reference single-junction solar cells that show PCE = 6.6% and 5.8%, respectively, the optimized tandem cell with the same layers and thickness provides PCE = 8.9% along with Jsc = 10.1 mA cm−2, Voc = 1.420 V, and FF = 0.62, representing an
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increasing active layer thickness. This trend correlates with the number of photons absorbed in active layer, which is governed by interference between incident and reflected light which causes an undulation in the absorbance of the active layer with increasing thickness.[18] The Voc shows little changes, but the FF decreases slightly at large thickness. Correspondingly, the PCEs of the single-junction PSCs show similar trend to Jsc with maximum PCEs of 7.7% at active layer thickness of 89 and 246 nm, respectively. Notably, all devices give PCEs >6.5% and FF >0.60 at the active layer thickness range of 89−246 nm. The high FF suggests efficient and balanced transport of charge carriers and also evidences limited bimolecular recombination even in very thick BDT-FBT-2T:[70]PCBM films. The high Jsc is also verified by the high EQE (Figure 3b). The maximum EQE reaches 0.72 at 550 nm for the 100 nm thick device, and 0.76 at 530 nm for the 246 nm thick device (Table S4, Supporting Information). In sharp contrast, the single-junction PSCs based on BDT-BT-2T, BDT-BT-2Fu and BDT-FBT-2Fu display serious decrease in Jsc and FF and thereby PCE with increasing active layer thickness. Correspondingly, the EQEs of each of these PSCs decrease significantly with the increase of active layer thickness (Figure S7, Supporting Information). To better understand the origin of the exceptional performance of BDT-FBT-2T-based single-junction PSCs over such a wide photoactive layer thickness range, we determined the internal quantum efficiency (IQE) for these devices. To this end, we first measured the average spectral extinction coefficient and refractive index for three BDT-FBT-2T:[70]PCBM films (Figure S8, Supporting Information). Using optical modeling that involved the entire stack of layers (glass/ITO/PEDOT:PSS/ BDT-FBT-2T:[70]PCBM/LiF/Al), the IQE of the PSCs were calculated by dividing the EQE by the calculated fraction of photons absorbed by the active layer (for details see the Supporting Information). By calculating the total number of photons absorbed by the active layer from the 1.5 air mass (AM1.5G) solar spectrum and comparing this to the AM1.5G integrated EQE, we obtain the spectrally averaged IQE of BDT-FBT-2T:[70]PCBM-based single-junction PSCs as a function of the active layer thickness (Figure 3c). We find that the BDT-FBT-2T:[70]PCBM solar cells have remarkably high IQE values. The spectrally averaged IQE of almost of all PSCs (except for the 89 nm thick one) approaches, or even exceeds, 90%, suggesting that nearly every absorbed photon generates a pair of free charges. It is well known that the molecular weight of the polymer plays a critical role in the performance of PSCs.[13h,19] BDT-FBT-2T with both relative low molecular weight (Mn = 45.5 kg mol−1, PDI = 2.4) and extra-high molecular weight (Mn = 108.8 kg mol−1, PDI = 4.2) were thus synthesized to examine this dependence (for synthetic details see the Supporting Information). The photovoltaic characterization reveals that both lower and higher molecular weight lead to somewhat reduced performances (Figure S9a and Table S5, Supporting Information). The optimal single-junction PSCs show maximum PCE of 6.9% and 6.7% for the 45.5 and 108.8 kg mol−1 polymer, respectively. The inferior PCEs are primarily because of the reduced Jsc, which is reflected in lower EQE. From Figure S9b (Supporting Information), it is clear that the solar cells made from 45.5 and 108.8 kg mol−1 polymer show significant EQE losses in the wavelength range of 460−570 nm
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Figure 4. a) Current-density−voltage characteristics of tandem PSC under illumination of AM1.5G (100 mW cm−2) and b) EQE spectra of the corresponding subcells.
increase of about 36% in PCE compared to the single-junction cells. EQE measurements of the individual subcells under relevant illumination conditions and correct electrical bias[21] (see Supporting Information) reveal that the front and back subcell have high quantum efficiencies with maxima of 0.61 and 0.56, respectively (Figure 4b). Integrating the EQE with the AM1.5G solar spectrum shows highly balanced current generation in each subcells with current density of 10.2 mA cm−2 for the front subcell and 10.0 mA cm−2 for the back subcell. The EQE-integrated current densities of the subcells are consistent with the measured Jsc of the tandem solar cell under AM 1.5G Table 3. Performance parameters of tandem PSC and reference subcells. Device
Jsca) [mA cm−2]
Voca) [V]
FFa)
PCEa) [%]
Ref. front cell
11.6 (11.5)
0.84 (0.84)
0.67 (0.68)
6.6 (6.6)
Ref. back cell
14.6 (14.8)
0.61 (0.61)
0.65 (0.63)
5.8 (5.7)
Tandem measured
10.1 (9.9)
1.42 (1.42)
0.62 (0.62)
8.9 (8.7)
Tandem predicted
10.5
1.45
0.67
10.2
a)Average
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illumination, corroborating the accuracy and consistency of our measurement. Despite the good performance, there are non-negligible differences between measured and predicted device performance. All measured performance parameters Jsc, Voc, FF, and consequently PCE, are lower than the predicted values (Table 3). In particular, the measured FF of the tandem solar cell is not only lower than predicted value but also lower than those of both single-junction reference cells. This differs from the observation that the FFs of tandem PSCs normally lie between the lower subcell FF and the higher subcell FF when balanced current are generated in each subcell.[2e,f,22] To reveal the cause for these differences, we constructed the J−V curves of the tandem subcells by interpolating the J−V curves of two reference single-junction cells with the current densities acquired from EQE convoluting of the tandem subcells as the Jscs of the constructed subcells. Assuming a loss-free intermediate layer, the J−V curve of the tandem cell can thereby be reconstructed by adding the two curves using Kirchhoff’s law.[23] This reconstruction yields a J−V curve (magenta line in Figure 4a) that is almost identical to the predicted J−V curve of the tandem cell. The comparison between the reconstructed J−V curve and the measured J−V curve shows that the PCE loss of the tandem cell mainly comes from the losses in FF and Voc, suggesting a suboptimal contact at active layer/intermediate layer interface. The reduced light intensity illuminated on the back cell compared with one sun illumination after photon absorbing by the front cell will also lead to a slight Voc drop of the back cell.[2f,24] Anyway, it is notable that the optimal active layer thickness of the WBG subcell is around 155 nm for balancing and maximizing the current generation in each subcell of the tandem PSC (Table S7, Supporting Information). At such a relative high thickness, however, common photovoltaic polymers normally affords limited FF and PCE, which suggests the importance of developing WBG polymers with small sensitivity to thickness variation. A recent paper published by Hou and co-workers corroborates the effectiveness of varying the front cell layer thickness to achieve high performance tandem PSCs.[25] Notably, the current output and overall efficiency of their tandem cells increased significantly with increasing front cell thickness, but the FF dropped seriously. We thus envision that higher efficiencies may be expected in multijunction PSCs that can fully exploit the advantages of BDT-FBT-2T through optimal intermediate layer design, and relevant work is underway in our laboratory. In summary, a series of new WBG semiconducting polymers (BDT-BT-2T, BDT-FBT-2T, BDT-BT-2Fu, and BDT-FBT2Fu) based on BDT and BT was designed and synthesized for application in multijunction PSCs. Systematic characterization suggest that the new polymer BDT-FBT-2T possesses ideal bandgap, relative deep HOMO energy level, and high hole mobility. Single-junction PSCs based on BDT-FBT-2T achieved PCEs over 6.5% for a broad range of active layer thickness with a maximum PCE of 7.7% at active layer thickness of 89 and 246 nm, respectively. Double-junction tandem PSCs based on BDT-FBT-2T with high efficiency of 8.9% were achieved at the active layer thickness of 155 nm for WBG subcell. Toward highly efficient multijunction PSCs, the significance of developing WBG materials with an ideal bandgap around ≈1.75 eV
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements The authors thank Weiwei Li for providing PMDPP3T, Ralf Bovee for GPC analysis, Wijnand Dijkstra, and Marco van der Sluis for AFM measurements. The work was performed in the framework of the Mujulima project that received funding from the European Commission’s Seventh Framework Programme (Grant Agreement No. 604148). The research leading to these results has also received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 339031. The research forms part of the Solliance OPV program and has received funding from the Ministry of Education, Culture and Science (Gravity Program 024.001.035). Received: April 6, 2015 Revised: May 21, 2015 Published online: July 2, 2015 [1] a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789; b) L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street, Y. Yang, Adv. Mater. 2013, 25, 6642. [2] a) Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu, Y. Cao, Nat. Photonics 2012, 6, 591; b) J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li, J.-X. Tang, Adv. Mater. 2015, 27, 1035; c) Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Commun. 2014, 5, 5293; d) S.-H. Liao, H.-J. Jhuo, P.-N. Yeh, Y.-S. Cheng, Y.-L. Li, Y.-H. Lee, S. Sharma, S.-A. Chen, Sci. Rep. 2014, 4, 6813; e) W. Li, A. Furlan, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen, J. Am. Chem. Soc. 2013, 135, 5529; f) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446; g) C.-C. Chen, W.-H. Chang, K. Yoshimura, K. Ohya, J. You, J. Gao, Z. Hong, Y. Yang, Adv. Mater. 2014, 26, 5670; h) A. R. b. M. Yusoff, D. Kim, H. P. Kim, F. K. Shneider, W. J. da Silva, J. Jang, Energy Environ. Sci. 2015, 8, 303; i) S. Zhang, L. Ye, W. Zhao, B. Yang, Q. Wang, J. Hou, Sci. China: Chem. 2015, 58, 248. [3] a) Y. J. Xia, L. Wang, X. Y. Deng, D. Y. Li, X. H. Zhu, Y. Cao, Appl. Phys. Lett. 2006, 89, 081106; b) F. L. Zhang, J. Bijleveld, E. Perzon, K. Tvingstedt, S. Barrau, O. Inganäs, M. R. Andersson, J. Mater. Chem. 2008, 18, 5468; c) A. P. Zoombelt, M. Fonrodona, M. M. Wienk, A. B. Sieval, J. C. Hummelen, R. A. J. Janssen, Org. Lett. 2009, 11, 903; d) Y. Dong, W. Cai, M. Wang, Q. Li, L. Ying, F. Huang, Y. Cao, Org. Electron. 2013, 14, 2459; e) Y. S. Park, Q. Wu, C.-Y. Nam, R. B. Grubbs, Angew. Chem. Int. Ed. 2014, 53, 10691. [4] a) Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135; b) L. Ye, S. Zhang, W. Zhao, H. Yao, J. Hou,
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and small sensitivity to active layer thickness variation is clearly demonstrated. The advantageous inherent properties and relative large photon energy loss (Eg − eVoc ≈ 0.85 eV) of BDT-FBT2T in single-junction PSCs indicate further room for developing more efficient WBG polymers based on this system via optimal molecule design, advanced synthesis technology, and optimized purification procedure. Moreover, multijunction PSCs with record efficiency are thus conceivable through further interface engineering of intermediate layer.
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