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Environmentally Friendly Solvent-Processed Organic Solar Cells that are Highly Efficient and Adaptable for the Blade-Coating Method Wenchao Zhao, Shaoqing Zhang,* Yun Zhang, Sunsun Li, Xiaoyu Liu, Chang He, Zhong Zheng, and Jianhui Hou* improvements in the power conversion efficiency (PCE) of OSCs.[5–11] Recently, the leading role of fullerene derivatives as electron acceptors in BHJ OSCs was replaced by non-fullerene (NF)-based organic semiconductors.[12–30] Currently, the record PCE of the NF acceptor-based OSCs (NF-OSCs) has exceeded that of their fullerene-based counterparts (i.e., 13.1% vs 11.7%), implying that NF-OSCs have strong potential for further practical applications.[9,27] Although there is still a long road to transfer OSC technology from the laboratory to the factory, studies have increasingly attempted to solve the potential problems in the solution-coating process of NF-OSCs. In fact, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical productions.[9,27,31–35] Chlorinated and aromatic solvents, which are detrimental to the environment and human health, are usually used for making the solutions.[9,36] Although few studies have demonstrated that low toxic solvents are applicable for highly efficient OSCs (the detailed information of the halogen-free solvents and the photovoltaic parameters of the corresponding devices are listed in Table S2 in the Supporting Information), all of the devices in these works have been fabricated using the spin-coating method.[9,37–45] As is known, wet films will be dried with the help of rapid airflow due to the centrifugal effect during the spin-coating process, while wet films will be dried differently when using other coating methods, such as blade-coating, slotdie coating, and gravure, which are suitable for making large area devices.[33,46,47] A change in the drying process will lead to different phase separation morphologies in the active layers and result in a large variation in photovoltaic performance. In other words, the optimized processing solvents developed based on the spin-coating method may not be adaptable for a practical production. Therefore, it will be of great importance to take the solution-coating method into consideration while seeking environmentally friendly processing solvents for making highly efficient NF-OSCs. In this work, we focus on demonstrating the potential in making a highly efficient NF-OSC by blade-coating with an environmentally friendly processing solvent. Initially, we take

The power conversion efficiencies (PCEs) of state-of-the-art organic solar cells (OSCs) have increased to over 13%. However, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical production. Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this study, a highly efficient PBTA-TF:IT-M-based device processed with environmentally friendly solvents, tetrahydrofuran/ isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, is fabricated; a high PCE of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. More importantly, a blade-coated non-fullerene OSC processed with THF/IPA is demonstrated for the first time to obtain a promising PCE of 11.7%; even for the THF/IPA-processed large-area device (1.0 cm2) made by blade-coating, a PCE of 10.6% can still be maintained. These results are critical for the large-scale production of highly efficient OSCs in future studies.

Bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted extensive research interest because of their unique advantages in making large-area and flexible solar panels through low-cost solution-coating techniques.[1–4] Over the last several decades, numerous research studies in materials chemistry and device engineering have yielded dramatic W. C. Zhao, Y. Zhang, S. S. Li, C. He, Z. Zheng, Prof. J. H. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences CAS Research/Education Center for Excellence in Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected] W. C. Zhao, Y. Zhang, S. S. Li, C. He, Z. Zheng, Prof. J. H. Hou University of Chinese Academy of Sciences Beijing 100049, P. R. China S. Q. Zhang, X. Y. Liu School of Chemistry and Biology Engineering University of Science and Technology Beijing Beijing 100083, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201704837.

DOI: 10.1002/adma.201704837

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a polymer donor based on benzodithiophene and benzotriazole (PBTA-TF) to blend with three NF acceptors (ITCC, IT-M, and IT-4F).[14,23,27,48] We use chloroform as the main processing solvent to spin-coat the active layers and find that the blend of PBTA-TF:IT-M can give the best photovoltaic performance due to the well-matched energy levels and complementary absorption between the donor and the acceptor. Next, based on the blend of PBTA-TF:IT-M, we select two representative non chlorinated solvents, o-xylene and tetrahydrofuran (THF), as the main processing solvents to make the devices, and we observe that the devices processed with these two solvents and fabricated using the spin-coating method can provide similarly high PCEs that are between 12% and 13%. Interestingly, when blade-coating is used to form the active layer, the PCEs of the devices processed with o-xylene as the main processing solvent significantly drop to below 8.2%, whereas that of the THF-processed device could remain at 11.7%. In addition, when we fabricate the THF-processed large-area device (1.0 cm2) using the blade-coating method, a PCE of 10.6% can still be maintained. In this study, we select PBTA-TF as the polymer donor due to two reasons: (1) it has excellent solubility in various solvents,[48] and (2) it possesses complementary absorption with our recently reported small molecular acceptors, ITCC, IT-M and IT-4F.[14,23,27] The chemical structures, absorption spectra and molecular energy levels of the polymer donor and NF acceptors are shown in Figure 1a–c. The absorption edge of PBTA-TF is

approximately at 650 nm, corresponding to an optical bandgap (E gopt) of 1.90 eV, which is complementary with the three acceptors. The lowest unoccupied molecular orbital (LUMO) of PBTA-TF is considerably higher than that of the three acceptors, which can meet the driving force requirement for exciton dissociation.[49] However, for the highest occupied molecular orbital (HOMO) levels, the smallest offset observed for PBTA-TF:ITCC is 0.11 eV, which is favorable for realizing low energy loss (Eloss) and has been observed in the highly efficient NF-OSCs.[13,41] According to the absorption spectra and energy levels, the blend of PBTA-TF:IT-4F has more potential in realizing high short-circuit current density (Jsc) due to its broad absorption, while the blend of PBTA-TF:ITCC will provide higher open-circuit voltage (Voc) because the offset between the HOMO of the donor and the LUMO of the acceptor is larger in PBTA-TF:ITCC than in the other two blends.[50] To screen out a donor–acceptor combination with the best photovoltaic performance for further investigation on coating methods, we first take chloroform and a trace amount of diiodooctane (DIO, 0.5% in volume ratio)—which are commonly used as a main solvent and solvent additive, respectively—to make the solutions of the photoactive materials and fabricate the active layers by spin-coating. The NF-OSCs with a configuration of indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly(styrene-sulfonate)/polymer donor: NF acceptor/PFN-Br/Al were fabricated (see Figure 2),

Figure 1. a,b) Chemical structures (a) and energy levels (b) of PBTA-TF, ITCC, IT-M, and IT-4F. c) Normalized optical absorption spectra of PBTA-TF, ITCC, IT-M, and IT-4F.

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Figure 2. a) Device architecture of NF-OSCs. b,c) Schematics of spin-coating (b) and blade-coating (c) processes. d,e) J–V (d) and EQE (e) curves of the NF-OSCs based on the PBTA-TF blends with ITCC, IT-M, and IT-4F processed with CF/DIO solvents.

where PFN-Br was used as the cathode interlayer,[51] and later tested under simulated AM1.5G illumination with an intensity of 100 mW cm−2. The weight ratio between the donor and the acceptor in the three types of blends was kept at 1:1, and the optimal thickness of the blend film was ≈100 nm. The current density versus voltage (J–V) curves of the NF-OSCs are shown in Figure 2d. As expected, in these three devices, the PBTATF:IT-4F-based device yields the highest Jsc (20.24 mA cm−2) because of the low E gopt of IT-4F, and the PBTA-TF:ITCC-based device shows the highest Voc (1.00 V), caused by the high-lying LUMO level of ITCC (Table 1). For the PBTA-TF:IT-M-based device, a high Voc of 0.97 V and a good Jsc of 17.76 mA cm−2 are simultaneously obtained along with a high fill factor (FF) of 0.71; as a result, this device demonstrated a PCE of 12.2%, which is the best one across these three devices. Figure 2e depicts the external quantum efficiency (EQE) spectra of the devices. It is clear that the three devices have different EQEs in the long wavelength region, which are ascribed to the varied E gopt of the acceptors. The current density values integrated from the EQE curves are 15.32, 17.24, and 19.53 mA cm−2, which coincide with those of the devices based on PBTATF:ITCC, PBTA-TF:IT-M, and PBTA-TF:IT-4F, respectively. Overall, the initial photovoltaic characterizations of the devices clearly indicate that the blend of PBTA-TF:IT-M is the best candidate for further investigation due to its high photovoltaic performance. Next, taking PBTA-TF:IT-M as the photoactive material, two nonchlorinated solvents, o-xylene and THF, are used as the processing solvents to make the NF-OSCs (Figure 2b), and the corresponding devices display PCEs of 11.3% and 11.6% (Figure S1 and Table S3, Supporting Information), respectively. As is known, solvent additives have been broadly used to optimize the photovoltaic performance of solution-processable OSCs. In this study, in order to avoid using the chlorinated/

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halogenated or highly toxic chemicals in the film formation process, 1-phenylnaphthalene (PN) and isopropyl alcohol (IPA) are employed as solvent additives to the photoactive material solutions of o-xylene and THF, respectively, as was observed in similar studies. As shown in Figure 3a and Table 2, the photo voltaic performance of the devices processed with the neat solvents can be slightly improved after adding the additives. Impressively, the PBTA-TF:IT-M-based device processed with XY/PN (95:5, v/v) exhibits a very high PCE of 13.1% along with a Voc of 0.97 V, a Jsc of 18.74 mA cm−2, and an FF of 0.72. To the best of our knowledge, a PCE of 13.1% is the highest value obtained for OSCs fabricated with environmentally friendly solvents. PCEs over 12% can also be achieved for the devices processed from THF/IPA. Similarly, the EQEs of the corresponding devices were measured and shown in Figure 3b, and the integrated current densities coincide with the results from the J–V measurements. Based on these results, we can see that the PBTA-TF:IT-M-based devices can be fabricated by spincoating with the different environmentally friendly solvents and yield similar PCEs. However, the solvents that are suitable for making the highly efficient PBTA-TF:IT-M-based device through blade-coating have not been thoroughly elucidated to date. In this study, the same blade and blading speed were used to coat the active layers (Figure 2c). The blade-coating process was employed in ambient atmosphere and temperature, and the active layer thickness was controlled by tuning the concentrations of the solutions. The same device architecture as that used in the spin-coated devices was adopted. The J–V curves of the blade-coated devices are shown in Figure 3c. Very interestingly, the o-xylene/PN-processed device exhibits dramatically different device performance when the coating method was changed from spin-coating to blade-coating. The o-xylene/ PN-processed device has a low PCE of 8.2% (see Table 2); however, the THF/IPA-processed device exhibits an average PCE of

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Table 1. Photovoltaic parameters of NF-OSCs based on the PBTA-TF blends with ITCC, IT-M and IT-4F. Voc [V]

Jsc (Jcalca)) [mA cm−2]

FF

PCEmax [%]

PCEavgb) [%]

ITCC

1.00

15.54 (15.32)

0.67

10.4

10.1 ± 0.3

IT-M

0.97

17.76 (17.24)

0.71

12.2

11.9 ± 0.3

IT-4F

0.73

20.24 (19.53)

0.72

10.6

10.1 ± 0.3

Solvents

Acceptors

CF/DIO

a)

The Jsc calculated from EQE spectrum; b)Average PCE values are obtained from over 10 devices fabricated in parallel.

11.4% with a maximum PCE of 11.7%, which is comparable to the performance of the spin-coated device processed with THF/IPA. As shown in Figure S2a (Supporting Information), it is clear that high EQE values of THF/IPA-processed devices can be achieved, and the EQE values of o-xylene/PN-processed devices are relatively low over the whole wavelength region. The exciton generation and dissociation behaviors and recombination kinetics were determined to characterize the differences in performance between the o-xylene/PN-processed and THF/IPAprocessed blade-coated devices.[52,53] Figure S2b (Supporting Information) shows that the exciton dissociation probability value in the THF/IPA-processed device (86%) is higher than that of the o-xylene/PN-processed device (73%). According to Figure S2c (Supporting Information), the logarithmic plots of Voc versus light intensity show slopes of 1.16 kT/e and 1.64 kT/e for the THF/IPA-processed and o-xylene/PN-processed devices, respectively. The o-xylene/PN-processed device exhibits strong geminate and nongeminate recombinations, which are responsible for the low PCE of the corresponding device. We fabricated

the inverted blade-coated devices using THF/IPA solvent system, and the best PCE of 11.3% was achieved (see Table 2 and Figure S3, Supporting Information). In addition, as illustrated in Figure 3d, we further fabricated a THF/IPA-processed large-area device (1.0 cm2) using the blade-coating method. This large-area device exhibits a high PCE of 10.6% with a Voc of 0.95 V, a Jsc of 17.13 mA cm−2, and an FF of 0.65, indicating the THF/IPA solvent system has potential in the application of large-area printing techniques. As demonstrated above, although these two solvents can be used with the spin-coating method for making highly efficient devices, only THF/IPA is adaptable to the blade-coating method. To determine the reason for the significant difference, atomic force microscopy (AFM) was employed to investigate phase separation morphologies in the blended films processed with different solvents using the two coating methods. As shown in Figure 4, when the spin-coating method was employed to fabricate the devices, the blended films processed with o-xylene/ PN and THF/IPA solvents show very similar phase separation

Figure 3. a,b) J–V (a) and EQE (b) curves of the spin-coated device processed with XY/PN and THF/IPA. c) J–V curves of the blade-coated device processed with XY/PN and THF/IPA. d) J–V curve of the blade-coated device processed with THF/IPA, with an inset of a photograph of the 1.0 cm2 blade-coated device processed with THF/IPA.

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Table 2. Summary of photovoltaic parameters of NF-OSCs based on PBTA-TF:IT-M processed with XY/PN and THF/IPA. Solvents

Voc [V]

Jsc [mA cm−2]

FF

PCEmax [%]

PCEavga) [%]

XY/PN

0.96 ± 0.01

18.71 ± 0.51

0.70 ± 0.02

13.1

12.7 ± 0.2

Blade-coating

XY/PN

0.93 ± 0.01

14.28 ± 1.11

0.59 ± 0.03

8.2

7.7 ± 0.4

Spin-coating

THF/IPA

0.96 ± 0.01

18.37 ± 0.60

0.67 ± 0.02

12.0

11.7 ± 0.2

THF/IPA

0.95 ± 0.01

18.14 ± 0.66

0.66 ± 0.02

11.7

11.4 ± 0.2

THF/IPA

0.94 ± 0.01

18.10 ± 0.53

0.64 ± 0.02

11.3

10.9 ± 0.4

Coating method Spin-coating

Blade-coating b)

Blade-coating a)All

parameters are calculated from the statistical values of ten devices; b)Using inverted device structure: ITO/ZnO/PBTA-TF:IT-M/MoO3/Al.

morphologies: both films have smooth surface topographies (Figure 4a,e) and nanoscale phase separation in the phase images (Figure 4b,f). However, when blade-coating is used as the film formation method, the phase separation morphology of the o-xylene/PN-processed film changes greatly, as shown in Figure 4c,d: the surface roughness increases to 20 nm, and aggregates of over several hundred nanometers are formed on the film. Obviously, such a severe phase separation leads to strong geminate and nongeminate recombinations and hence is detrimental to device performance.[54,55] By contrast, the phase separation morphologies of the blended films processed with THF/IPA are almost not affected by the different coating method; therefore, the corresponding devices show similar photovoltaic performance. The significant influence in the morphologies by changing the coating method can be interpreted by the differences in the drying processes of the coating methods and the volatilities of the solvents. With the help of the rapid airflow caused by the centrifugal effect during the spin-coating process, the wet film will be dried solely due to the volatility of the solvent. When using a low boiling point (bp) solvent mixture, such as THF/ IPA (65/83 °C), which also has a high saturated vapor pressure, the wet film will be dried very quickly, regardless of which coating method is employed. However, if the devices are fabricated using a solvent with a low saturated vapor pressure or,

particularly, a binary solvent mixture of a medium bp solvent and a very high bp solvent, such as o-xylene/PN (144/325 °C), the drying process during blade-coating will be greatly prolonged, allowing aggregations to grow in the active layer; under such circumstances, spin-coating is more favorable than blade-coating for the formation of nanoscale phase separation. To support the above assumption, we blade-coated the PBTA-TF:IT-M device using neat o-xylene as the processing solvent and obtained a PCE of 10.5% (Figure S4, Supporting Information). In summary, we identify a new material combination with high photovoltaic performance by employing a wide-bandgap polymer as a donor and screening three newly reported NF acceptors. We also identify two environmentally friendly solvent mixtures, o-xylene/PN and THF/IPA, for making the PBTA-TF:IT-M-based devices. By adopting the spin-coating method, outstanding PCEs of 12.0% and 13.1% were obtained from the THF/IPA and o-xylene/PN-processed devices, respectively. When the blade-coating method was used, the THF/ IPA-processed device maintained a high PCE of 11.7%, while that of the o-xylene/PN-processed device dropped to 8.2%. The photovoltaic results and morphological analysis reveal three important points: first, using environmentally friendly solvents, highly efficient NF-OSCs can be fabricated using either the spin-coating or blade-coating method; second, the high PCEs

Figure 4. a–h) AFM topography (a,c,e,g) and phase images (b,d,f,h) of the spin-coated and blade-coated films cast from o-xylene/PN and THF/IPA.

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obtained from the spin-coated OSCs using low bp solvents can be readily reproduced with the blade-coating method. Third, the evaporation rates of processing solvents during the active layer formation are very important for the morphology of active layer. Overall, this work provides useful information regarding transferring OSC technology from the laboratory to practical applications.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors acknowledge the financial support from NSFC (91333204, 21325419, 51673201, 91633301, and 21734008), the Chinese Academy of Sciences (XDB12030200 and KJZD-EW-J01), the National Basic Research Program 973 (2014CB643501), and the CAS-Croucher Funding Scheme for Joint Laboratories (CAS14601).

Conflict of Interest The authors declare no conflict of interest.

Keywords blade-coating, morphology, organic solar cells, power conversion efficiency, processing solvents Received: August 24, 2017 Revised: September 22, 2017 Published online:

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