Design of novel thiazolothiazole-containing conjugated polymers for organic solar cells and modules

Solar Energy 198 (2020) 605–611

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Design of novel thiazolothiazole-containing conjugated polymers for organic solar cells and modules

T

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Alexander V. Akkuratova, , Sergey L. Nikitenkoa, Andrey S. Kozlova, Petr M. Kuznetsova, Ilya V. Martynova, Nikita V. Tukacheva, Andriy Zhugayevychb, Iris Visoly-Fisherc,d, Eugene A. Katzc,d, Pavel A. Troshina,b a

Institute for Problems of Chemical Physics of the Russian Academy of Sciences (IPCP RAS), Academician Semenov avenue 1, Chernogolovka, Moscow Region 142432, Russian Federation b Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld.1, Moscow 121205, Russian Federation c Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 8499000, Israel d Ilse Katz Institute for Nanoscale Science & Technology, Ben-Gurion University of the Negev, Be’er Sheva 8410501, Israel

A R T I C LE I N FO

A B S T R A C T

Keywords: Thiazolothiazole Conjugated polymers Organic solar cells Photovoltaic modules Slot-die coating

One of the major challenges in the field of organic photovoltaics is associated with high-throughput manufacturing of efficient and stable organic solar cells. Practical realization of technologies for production of largearea organic solar cells requires the development of novel materials with a defined combination of properties ensuring sufficient reliability and scalability of the process in addition to good efficiency and operation stability of the devices. In this work, we designed two novel polymers comprising thiazolothiazole units and investigated their performance as absorber materials for organic solar cells and modules. Optimized small-area solar cells based on P1/[70]PCBM ([6,6]-phenyl-C71-butyric acid methyl ester) blends exhibited promising power conversion efficiency (PCE) of 7.5%, while larger area modules fabricated using slot die coating showed encouraging PCE of 4.2%. Additionally, the fabricated devices showed promising outdoor stability maintaining 60–70% of the initial efficiency after 20 sun days being exposed to natural sunlight at the Negev desert. The obtained results feature the designed polymer P1 as a promising absorber material for a large-scale production of organic solar cells under ambient conditions.

1. Introduction Organic solar cells (OSCs) are recognized as a promising renewable energy technology (Dong et al., 2019; Wang et al., 2018; Fan et al., 2019). Advanced properties of OSCs such as light weight, flexibility, low cost, and solution processability have attracted a considerable attention from academia and industry. Particularly, OSCs can be used as promising energy sources for low-power indoor electronic devices (You et al., 2019). State-of-the-art OSCs demonstrate commercially competitive power conversion efficiencies (PCEs) of > 17% (Meng et al., 2018). However, this superior performance was achieved for small-area devices (0.05–0.1 cm2) while using laboratory spin-coating technique. Unfortunately, the performance of organic solar cells drops dramatically when going to larger-area devices fabricated using roll-to-roll compatible coating and printing techniques such as slot-die coating, doctor blading, inkjet printing, screen printing, etc. (Gu et al., 2017;

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Krebs, 2009). Severe efficiency roll-offs are mainly attributed to the unsuitable film formation properties, poor charge mobilities in thicker films required for scalable production, optical losses, and high series resistance of electrodes (Zhang et al., 2018; Mao et al., 2017; Lucera et al., 2017). The majority of reported efficiencies of OSCs or modules with an active area of 10–20 cm2 range from 5% to 7.5%. Average PCEs of 3–4% were reached for solar cells with an active area of 20–50 cm2 (Huang et al., 2019). Few papers reported large-area modules (> 100 cm2) demonstrating reasonable efficiencies of 4.5–5.6% (Mao et al., 2017; Lucera et al., 2017; Berny et al., 2016). In this work, we report the synthesis of two novel conjugated polymers P1 and P2 comprising thiazolo[5,4-d]thiazole (Tz), benzo [1,2,5-c]thiadiazole (B), and thiophene (T) units and the fabrication of large-area photovoltaic cells and modules based on these polymers using slot-die coating as a roll-to-roll compatible and industry-relevant film deposition technique (Fig. 1).

Corresponding author. E-mail address: [email protected] (A.V. Akkuratov).

https://doi.org/10.1016/j.solener.2020.01.087 Received 29 October 2019; Received in revised form 5 January 2020; Accepted 31 January 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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Fig. 1. Conjugated polymers P1 and P2.

4 and 5 were obtained via Stille cross-coupling reaction using 4,7-dibromo-2,1,3-benzothiadiazole or 4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole and trimethyl(3-(2-hexyldecyl)thiophene-2-yl)stannane. Then compounds 4 and 5 were treated with 2,5-bis(trimethylstannyl) thiophene with further bromination of the products with N-bromosuccinimide in 1,2-dichlorobenzene to afford the target monomers. Conjugated polymers P1 and P2 were synthesized using monomers M1, M3 and M2, M4 via palladium-catalyzed Stille polycondensation reaction. Crude polymers were purified successively by Soxhlet extraction with acetone, hexane, dichloromethane, and finally with chlorobenzene for 4–5 h. Chlorobenzene fractions were then concentrated and polymers were precipitated with ethanol. Gel permeation chromatography was used to determine relative molecular weight characteristics of polymers (Akkuratov et al., 2017). Obtained weightaverage molecular weights (Mw) and polydispersity indexes of polymers are summarized in Table 1. As can be seen, Mw and Mw/Mn values for both polymers are very similar. Thermal properties of polymers were analyzed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Decomposition temperatures (Td) with 5% weight loss were above 420 °C (Fig. S1, Supporting Information) for both polymers, which indicates their sufficiently good thermal stability.

Structural rigidity and planarity of Tz block and its tendency to induce material crystallization suggest that it can be used in design of highly ordered polymers demonstrating outstanding charge transport characteristics and hence good photovoltaic performance. Furthermore, OSCs based on Tz-containing polymers show excellent stability that is an important prerequisite for their practical application (Guo et al., 2018; Peng et al., 2018; Liu et al., 2018; Saito et al., 2015; Osaka et al., 2010).

2. Results and discussion The synthesis of the polymers P1 and P2 is presented in Scheme 1. First, 3-alkylthiophenes 1a and 1b were brominated with N-bromosuccinimide in acetic acid. Lithiation of the resultant compounds 2a,b followed by quenching of the metallated intermediates with N,N-dimethylformamide yielded corresponding aldehydes 3a,b. 2-Formyl-3alkylthiophenes 3a,b were heated with dithiooxamide to afford thiazolothiazoles, which were converted into key stannanes M1 and M2 by lithiation and further treatment of dilithiated precursors with trimethylchlorostannane. Monomers M3 and M4 were synthesized in three steps. Compounds

Scheme 1. Synthesis of conjugated polymers P1 and P2. Conditions: i – dithioxamide, DMF, reflux; ii – BuLi, THF, −78 °C, Me3SnCl; iii –toluene, Pd(PPh3)4, reflux; iv –NBS, 1,2-DCB; v - Pd2(dba)3, (2-MePh)3P, toluene, reflux. 606

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Table 1 Physical, electrochemical and optical properties of polymers P1 and P2.

P1 P2 a b c

Mw, kDa

Mw/Mn

Td, °C

Tm, °C

Tc, °C

film sol λmax / λmax , nm

λedge, nm

96 90

2.0 1.8

428 445

266 251

242 224

560/646 549/608

781 746

a

Egopt , eV

ox Eonset ,V

HOMO, eV

1.59 1.66

0.52 0.58

−5.58 −5.64

b

LUMO, eV

c

−3.99 −3.98

λedge in thin film. HOMO energies were estimated from onsets of the oxidation potentials using Fermi energy of −5.1 eV for the Fc+/Fc redox couple. LUMO energies were calculated as Egopt + EHOMO.

charge carrier mobilities and, consequently, enhanced short circuit current densities (JSC) and fill factors (FF) in OSCs. The optical band gaps of 1.59 and 1.66 eV were estimated for P1 and P2, respectively, from the low energy band onsets in thin film spectra. As revealed by our DFT calculations, electronic properties of P1 and P2 in the ideal planar geometry are very similar (Tables S9–S10, Supporting Information). It should be noted that the experimentally observed red shift by 0.07 eV of the absorption band for P1 versus that of P2 is in a good agreement with the theoretically predicted value of 0.08 eV. Frontier orbitals computed using DFT approach have a typical form for conjugated polymers: HOMO is delocalized along the conjugated backbone while LUMO is localized mainly on A-D-A fragment (Fig. 3). Computed natural transition orbitals (NTOs, Fig. S10, Supporting Information) are similar and thus we believe that complex charge transfer and exciton localization phenomena could be described, at least qualitatively, using a simplified concept of HOMO-LUMO transition. The energy of the highest occupied molecular orbitals (HOMO) of polymers P1 and P2 were estimated from the cyclic voltammetry data (CVA, Fig. 2b). The CVA curves were recorded vs. the potential of standard Ag/AgNO3 electrode, which was calibrated by the ferroceneferrocenium (Fc+/Fc) redox couple. Onset oxidation potentials (Eox onset) of P1 and P2 were determined to be 0.52 and 0.58 V, respectively. Under the same conditions, E1/2 of the Fc+/Fc redox couple was 0.04 V versus Ag/AgNO3 reference electrode. Assuming that the redox potential of Fc+/Fc is −5.1 eV in the Fermi energy scale (Cardona et al., 2011), the HOMO energy levels of P1 and P2 were estimated to be −5.58 eV and −5.64 eV, respectively. The LUMO (lowest unoccupied molecular orbital) energy levels were close to −4.0 eV as calculated from the HOMO energy and Egopt. It should be noted that the theoretically estimated ionization potential (IP) and electron affinity (EA) values for P1 (P2) polymers were 5.6 eV (5.73 eV) and 2.56 eV (2.62 eV) (see Table S11, Supporting Information), respectively, which qualitatively corresponds to the trend found in experiment. Both experimental and computational data reveal that HOMO level of fluorine-loaded polymer P2 is down-shifted in comparison to that of polymer P1, which is beneficial for achieving higher open-circuit voltages in OSCs (Brabec et al., 2001). LUMO levels of both polymers are

Fig. 2. Absorption spectra (a) and cyclic voltammograms (b) of conjugated polymers P1 and P2.

Thermal transition parameters are summarized in Table 1. The melting temperatures (Tm) of 242 °C and 224 °C, and crystallization points (Tc) of 266 °C and 251 °C were estimated based on DSC data for P1 and P2, respectively. The obtained results suggest crystallinity of the designed polymers. Optical properties of the polymers were investigated using UVvisible absorption spectroscopy in 1,2-dichlorobenzene solution and in thin films. Absorption spectra are shown in Fig. 2a, corresponding data are presented in Table 1. Peak maxima and long-wavelength onsets of absorption bands of polymer P1 are red-shifted compared to those of fluorinated analogue P2 in both solution and thin films (Table 1). The high electronegativity of fluorine substituents weakens the π-conjugation in the polymer backbone resulting in a wider bandgap of fluorineloaded polymers compared to non-fluorinated counterparts (Cui et al., 2015; Nielsen et al., 2015; Leclerc et al., 2016). Similar behavior of fluorine-containing conjugated polymers was described previously by other groups (Meng et al., 2018; Kim et al., 2015; Akkuratov et al., 2017; Li et al., 2017). The observed bathochromic shifts of absorption bands in the spectra of the films as compared to solutions suggest strong interchain interactions of polymer molecules in the solid-state (e.g. π-π stacking)(Wang et al., 2017). Such supramolecular ordering usually leads to increased

Fig. 3. Frontier orbitals of conjugated polymers P1 and P2 according to DFT calculations. 607

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Fig. 5. J-V characteristics (a) and EQE (b) spectra for optimized OSCs based on P1 and P2 with [70]PCBM.

characteristics of the devices were measured under the simulated AM1.5G (100 mW/cm2) illumination provided by a KHS Steuernagel solar simulator (Lichttechnik GmbH, Morfelden-Walldorf, Germany) inside the glovebox at room temperature. J-V curves were recorded in an argon atmosphere using Advantest 6240A source-measurement units. In order to reach an improved solar cells performance, we optimized polymer/PCBM weight ratios, film thickness (measured by AFM) and thermal annealing (TA) regimes of blend films. Different processing additives (1,8-diiodooctane (DIO), diphenyl ether (DPE), 1-chloronaphtalene (CN)) and solvent vapor annealing (SVA) were used to control phase separation between donor and acceptor components in photoactive layer (Chen et al., 2019; Park and Kim, 2019; Babics et al., 2018; Zheng et al., 2014). Finally, n-type fullerene derivative BLF (Li et al., 2012) was introduced as electron selective buffer layer at the interface between the active layer and top metal electrode (Fig. 4c). Using electron transport interlayers in OSCs is recognized as an effective strategy to improve device performance (Steim et al., 2010; Xiao et al., 2015). The performance of solar cells was strongly influenced by the absorber film processing conditions (Table 2, Fig. 5, and Tables S1–S7, Supporting Information). The best performing P1-based devices were fabricated with 1:1.75 w/w donor/acceptor ratio and BLF as charge transport layer. OSCs incorporating polymer P2 displayed the best performance while using D/A ratio of 1:2, TA at 95 °C for 10 min. and CHCl3-SVA treatment of blend films. Maximal power conversion efficiency of 7.5% was achieved for devices based on P1/[70]PCBM blends. The obtained short-circuit current density of 19.3 mA/cm2 is in a good agreement with JSC value estimated by integration of EQE spectrum over true AM1.5G solar

Fig. 4. Schematic layout of the solar cell architecture (a), band alignment of the functional components (b) and chemical structure of the interfacial buffer layer BLF (c).

about 0.3 eV higher than that of [70]PCBM (Sun et al., 2011). Thus, the donor/acceptor interfacial energy offset is large enough to overcome coulombic binding of the light-induced polaron pairs and generate free charge carriers (Fig. 4b) (Hains et al., 2010). Photovoltaic properties of polymers P1 and P2 were evaluated in bulk heterojunction solar cells with the conventional architecture: ITO/ PEDOT:PSS (60 nm)/active layer (110–195 nm)/BLF/Mg(30 nm)/Al (60 nm) and an active area of 0.3 cm2 (Fig. 5a). Fullerene derivative [70]PCBM was used as acceptor component in the photoactive blends with polymers P1 and P2 (Wienk et al., 2003). The current–voltage 608

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Table 2 Optimized processing conditions and characteristics of small-area OSCs and modules based on polymers P1, P2 and [70]PCBM.

P1a P2a a b

Conditions

VOC, mV

JSC, mA/cm2

FF, %

η,

1:1.75, 155 nm, not ann. BLF 1:2, 1 vol% DIO not ann., 150 nm, SVA with CHCl3 for 30 sec.

670 786

19.3 13.6

58 58

7.5 6.2

b

%

μh, cm2V-1s−1

μe, cm2V-1s−1

μh/μe

2.8 × 10-4 2.2 × 10-5

3.1 × 10-4 2.4 × 10-5

0.90 0.92

spin-coated cells with active area of 0.30 cm2. the average of 10 devices.

JPH = 0 mA/cm2, and Vappl is the applied bias voltage. We obtained the sat Jph at Veff > 2 V and the resulting P(E,T) values were revealed to be 79% and 72% for P1 and P2 blends with [70]PCBM, respectively. These results also support more efficient charge collection in case of P1-based solar cells, which is leading to higher JSC. Charge carriers generation and their transport to electrodes is known to be strongly affected by the nanomorphology of the fullerene/ polymer blend films. Atomic-force microscopy (AFM) was used to probe the surface topography of the composites. The films were very smooth and showed root-mean-square (RMS) roughness of 1.4 nm and 2.0 nm for P1/[70]PCBM, and P2/[70]PCBM, respectively. Small nanosized grains revealed on the film surface suggested appropriate miscibility of donor polymers with [70]PCBM thus avoiding large-scale phase segregation (Fig. S4, Supporting Information). Therefore, both systems seem to have optimal nanoscale morphology, which should not limit their photovoltaic performance. We explored photochemical and thermal stability of the designed conjugated polymers as well as the operational stability of photovoltaic cells (Figs. S5–S7, Supporting Information). The thin films of polymers P1, P2, and two reference materials (P3HT and PCDTBT) were exposed to UV or white light illumination (incident power ~10 mW/cm2 and ~300 mW/cm2 for UV and white light sources, respectively) under well-controlled inert atmosphere conditions (O2 and H2O levels below 0.1 ppm, ~45 °C, Fig. S5, Supporting Information) shows that all polymers reveal no any changes in their optical characteristics after 185 h of light exposure, which suggests their superior photostability. Moreover, the absorption spectra of polymer films were not affected by continuous thermal annealing at 80 °C as shown in Fig. S6 (Supporting Information), which indicates good thermal stability. Outdoor tests were performed in Sede Boker (the Negev desert, Israel) for solar cells with inverted configuration ITO/ZnO/active layer/ MoO3/Ag encapsulated with light/UV curable adhesive DELO575. The procedure for performing the outdoor tests was reported previously (Akkuratov et al., 2019). Shortly, the behavior of OSCs under outdoor conditions was assessed using periodic indoor measurements of their JV characteristics under the simulated AM1.5G (100 mW/cm2) illumination provided by an Oriel LCS-100 solar simulator (Newport). The solar cells based on P1 demonstrated severe burn-in effect (Mateker and McGehee, 2017; Rafique et al., 2018) during the first day of solar light exposure with VOC and FF decrease accounting for ~40% loss of the initial PCE. P2-based OSCs showed superior stability for the same period while maintaining > 80% of original efficiency. Both types of cells showed stabilization of their performance 3 days after start of the experiment. As a result, P1-based devices maintained about 60% of their initial efficiency after 20 days of operation, while solar cells based on fluorine-containing polymer P2 showed retention of 70% of initial PCE. These results are encouraging since we did not perform special optimization of interfacial charge-transport layers, electrodes and encapsulation for reaching the best stability. Following the investigation of spin-coated small-area solar cells, we scaled up the device area and fabricated modules with conventional structure using roll-to-roll compatible slot die coating method. The active layers were deposited under ambient conditions without postdeposition of interfacial buffer layers. The schematic layout of the modules incorporating three serially connected cells are shown in

irradiance spectrum (Fig. 5b). It is worth mentioning that EQE maximum of P1/[70]PCBM-based solar cells is quite high and exceed 80% in the wide range of wavelengths. Under the optimized conditions, solar cells based on polymer P2 exhibited efficiency of 6.2% with VOC of 786 mV, JSC of 13.6 mA/cm2, and FF of 58%. Notably, P2-based solar cells demonstrated much higher (by 116 mV) VOC, which is consistent with the lower HOMO level energy of P2 as compared to that of P1. Clearly, the observed lower efficiency of the solar cells based on P2/[70]PCBM blends is due to severely reduced JSC pointing to significant recombination losses. Some insight was gained while studying hole and electron mobilities in two polymerfullerene blends using the space-charge limited current (SCLC) method (Podzorov et al., 2003). The hole-only and electron-only devices were assembled with the following structures: ITO/PEDOT:PSS(60 nm)/ blend/F4TCNQ(1 nm)/MoO3(22 nm)/Ag(120 nm) and (ITO/Yb (15 nm)/blend/Sm(100 nm)), respectively (Fig. S2, Supporting Information). The hole and electron mobilities were fairly balanced in both active blends (Table 2). However, the absolute mobility values for P1/[70]PCBM blends were about one order of magnitude higher as compared to the P2/[70]PCBM blends, which explains different photovoltaic performance of these two systems. Indeed, lower mobility in case of the P2/[70]PCBM blend leads to significant recombination losses. In order to assess the charge recombination processes we studied the dependence of JSC and VOC versus light intensity (Fig. S2, Supporting Information). The impact of bimolecular recombination was investigated by measuring the dependence of JSC against Plight. The exα ponential factor α in the power law JSC ∝ Plight is used to reveal the degree of bimolecular recombination in the blend films. The closer the estimated α to 1, the weaker is the bimolecular recombination occuring in the device (Mateker et al., 2017). As can be seen, OSCs based on both P1 and P2 polymers demonstrate suppressed bimolecular recombination with α equal to 0.94 and 0.96, respectively. The open circuit voltage is in a direct proportion to the illuminated light intensity (Plight) in the semilogarithmic plot with a slope of kT/q (where k is the Boltzmann constant, T is temperature, and q is the elementary charge) (Wang et al., 2019). The slopes larger than kT/q suggest trap-assisted or monomolecular recombination (Huang et al., 2019). Solar cells based on the P1/[70]PCBM blends exhibited the slope of 1.04 kT/q, which suggests that trap-assisted recombination does not affect significantly photovoltaic performance of this system. On the contrary, larger slope of 1.32 kT/q revealed for P2/[70]PCBM-based devices suggest higher concentration of trap states leading to massive monomolecular recombination of charge carriers. The obtained results indicate that the P1/[70]PCBM blend has superior and balanced charge carrier mobilities and demonstrates reduced recombination losses thus enabling high short-circuit densities. We explored also the photocurrent density (Jph) as a function of the effective voltage (Veff) in order to reveal charge dissociation and charge collection probability P(E,T) (Fig. S3, Supporting Information),(Lu sat et al., 2014). P(E,T) is defined as Jph/ Jph . Jph represents JL-JD, where JL is the current density recorded under illumination, while JD is the sat current density in the dark. The Jph corresponds to the current density measured at high Veff values when all photogenerated excitons are dissociated into free charge carriers, which are collected at the device electrodes. The Veff = V0 - Vappl, where V0 is the voltage at which 609

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and encouraging efficiency of 4.2% was obtained. These results feature the designed low-bandgap P1 and similar polymers as promising absorber materials for production of large-area efficient and stable organic photovoltaic devices under ambient conditions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the Russian Foundation for Basic Research (project No. 18-33-20025). General support was also provided by the Ministry of Science and Higher Education of the Russian Federation within the project No. 0089-2019-0010. I. M. acknowledges the support from the Israeli Council for Higher Education in the framework of the Research Academic Internship Program for PhD students. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.01.087. References Dong, S., Zhang, K., Xie, B., Xiao, J., Yip, H.-L., Yan, H., Huang, F., Cao, Y., 2019. Highperformance large-area organic solar cells enabled by sequential bilayer processing via nonhalogenated solvents. Adv. Energ. Mater. 9, 1802832. https://doi.org/10. 1002/aenm.201802832. Wang, G., Adil, M.A., Zhang, J., Wei, Z., 2018. Large-area organic solar cells: material requirements, modular designs, and printing methods. Adv. Mater. 1805089. https:// doi.org/10.1002/adma.201805089. Fan, B., Zhang, D., Li, M., Zhong, W., Zeng, Z., Ying, L., Huang, F., Cao, Y., 2019. Achieving over 16% efficiency for single-junction organic solar cells. Sci. China Chem. 62, 746–752. https://doi.org/10.1007/s11426-019-9457-5. You, Y., Song, C.E., Hoang, Q.V., Kang, Y., Goo, J.S., Ko, D., Lee, J., Shin, W.S., Shim, J.W., 2019. Highly efficient indoor organic photovoltaics with spectrally matched fluorinated phenylene-alkoxybenzothiadiazole-based wide bandgap polymers. Adv. Func. Mater. 1901171. https://doi.org/10.1002/adfm.201901171. Meng, L., Zhang, Y., Wan, X., Li, C., Zhang, X., Wang, Y., Ke, X., Xiao, Z., Ding, L., Xia, R., Yip, H.-L., Cao, Y., Chen, Y., 2018. Organic and solution-processed tandem solar cells with 17.3% efficiency. Science 361, 1094–1098. https://doi.org/10.1126/science. aat2612. Gu, X., Zhou, Y., Gu, K., Kurosawa, T., Guo, Y., Li, Y., Lin, H., Schroeder, B.C., Yan, Hongping, Molina-Lopez, F., Tassone, C.J., Wang, C., Mannsfeld, S.C.B., Yan, He, Zhao, D., Toney, M.F., Bao, Z., 2017. Roll-to-roll printed large-area all-polymer solar cells with 5% efficiency based on a low crystallinity conjugated polymer blend. Adv. Energ. Mater. 7, 1602742. https://doi.org/10.1002/aenm.201602742. Krebs, F.C., 2009. Fabrication and processing of polymer solar cells: A review of printing and coating techniques. Sol. Energ. Mater. Sol. Cells 93, 394–412. https://doi.org/10. 1016/j.solmat.2008.10.004. Zhang, K., Chen, Z., Armin, A., Dong, S., Xia, R., Yip, H.-L., Shoaee, S., Huang, F., Cao, Y., 2018. Efficient large area organic solar cells processed by blade-coating with singlecomponent green solvent. Solar RRL 2, 1700169. https://doi.org/10.1002/solr. 201700169. Mao, L., Tong, J., Xiong, S., Jiang, F., Qin, F., Meng, W., Luo, B., Liu, Y., Li, Z., Jiang, Y., Fuentes-Hernandez, C., Kippelen, B., Zhou, Y., 2017. Flexible large-area organic tandem solar cells with high defect tolerance and device yield. J. Mater. Chem. A 5, 3186–3192. https://doi.org/10.1039/C6TA10106B. Lucera, L., Machui, F., Schmidt, H.D., Ahmad, T., Kubis, P., Strohm, S., Hepp, J., Vetter, A., Egelhaaf, H.-J., Brabec, C.J., 2017. Printed semi-transparent large area organic photovoltaic modules with power conversion efficiencies of close to 5 %. Org. Electron. 45, 209–214. https://doi.org/10.1016/j.orgel.2017.03.013. Huang, K.-M., Wong, Y.Q., Lin, M.-C., Chen, C.-H., Liao, C.-H., Chen, J.-Y., Huang, Y.-H., Chang, Y.-F., Tsai, P.-T., Chen, S.-H., Liao, C.-T., Lee, Y.-C., Hong, L., Chang, C.-Y., Meng, H.-F., Ge, Z., Zan, H.-W., Horng, S.-F., Chao, Y.-C., Wong, H.Y., 2019a. Highly efficient and stable organic solar cell modules processed by blade coating with 5.6% module efficiency and active area of 216 cm 2. Prog. Photovolt. Res. Appl. 27, 264–274. https://doi.org/10.1002/pip.3078. Berny, S., Blouin, N., Distler, A., Egelhaaf, H.-J., Krompiec, M., Lohr, A., Lozman, O.R., Morse, G.E., Nanson, L., Pron, A., Sauermann, T., Seidler, N., Tierney, S., Tiwana, P., Wagner, M., Wilson, H., 2016. Solar trees: first large-scale demonstration of fully solution coated, semitransparent flexible organic photovoltaic modules. Adv. Sci. 3, 1500342. https://doi.org/10.1002/advs.201500342.

Fig. 6. Module architecture comprising three serially connected cells (active area of a single cell is 4.7 cm2) (a); J-V characteristics of the modules based on the blends of polymers P1 and P2 with [70]PCBM (b). Insert shows photograph of the module.

Fig. 6. Active area of each single cell was of 4.7 cm2, while the total active area of the module was of 14.1 cm2. Photovoltaic parameters of modules and J-V curves are shown in Fig. 6b. The modules based on polymer P1 showed PCEs of up to 4.2%, which is more than twice higher as compared to the performance of P1-based module. The results achieved for P1-based modules are encouraging particularly considering that they were processed under ambient conditions and there is a big room for further optimization and improvements.

3. Conclusion Two novel conjugated polymers comprising thiazolothiazole units were synthesized and investigated as electron-donor materials for bulk heterojunction organic solar cells. Small-area devices based on P1/[70] PCBM blends displayed promising PCEs of 7.5% originating from superior charge-transport characteristics of the absorber films and suppressed charge carrier recombination. Organic solar cells based on the designed materials exhibited decent operational stability while maintaining 60–70% of initial efficiency after 20 days of outdoor exposure to natural illumination conditions in the Negev desert. The larger-area photovoltaic modules with the active area of 14.1 cm2 were successfully fabricated under ambient conditions using slot-die coating technique 610

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