Non-conjugated polymers as thickness-insensitive electron transport materials in high-performance inverted organic solar cells

Journal Pre-proof

Non-conjugated polymers as thickness-insensitive electron transport materials in high-performance inverted organic solar cells Zhiquan Zhang , Zheling Zhang , Yufu Yu , Bin Zhao , Sheng Li , Jian Zhang , Songting Tan PII: DOI: Reference:

S2095-4956(19)30932-5 https://doi.org/10.1016/j.jechem.2019.12.011 JECHEM 1037

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

9 November 2019 18 December 2019 24 December 2019

Please cite this article as: Zhiquan Zhang , Zheling Zhang , Yufu Yu , Bin Zhao , Sheng Li , Jian Zhang , Songting Tan , Non-conjugated polymers as thickness-insensitive electron transport materials in high-performance inverted organic solar cells, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.12.011

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Non-conjugated polymers as thickness-insensitive electron transport materials in high-performance inverted organic solar cells Zhiquan Zhanga,b,1, Zheling Zhangb,1, Yufu Yua, Bin Zhaoa,*, Sheng Lic,*, Jian Zhangb,*, Songting Tana,* a

Key Laboratory for Green Organic Synthesis and Application of Hunan Province，

Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China b

College of Material Science & Engineering, Guangxi Key Laboratory of Information

Materials, Guilin University of Electrical Technology, Guilin 541004, Guangxi, China c

Hunan Research Institute of Light Industry, Changsha 410015, Hunan, China

*

Corresponding authors.

E-mail addresses: [email protected] (B. Zhao), [email protected] (J. Zhang), [email protected] (S. Li), [email protected] (S. Tan). 1

These authors contributed equally to this work.

Abstract Two

non-conjugated

quaternization

of

polymers

PEIE-DBO

polyethyleneimine

and

ethoxylate

PEIE-DCO, by

prepared

1,8-dibromooctane

by and

1,8-dichlorooctane respectively, are developed as electron transport layer (ETL) in high-performance inverted organic solar cells (OSCs), and the effects of halide ions on polymeric photoelectric performance are fully investigated. PEIE-DBO possesses 1

higher electron mobility (3.68×10−4 cm2 V−1 s−1), higher conductivity and more efficient exciton dissociation and electron extraction, attributed to its lower work function (3.94 eV) than that of PEIE-DCO, which results in better photovoltaic performance in OSCs. The inverted OSCs with PTB7-Th: PC71BM as photoactive layer and PEIE-DBO as ETL exhibit higher PCE of 10.52%, 9.45% and 9.09% at the thickness of 9, 35 and 50 nm, respectively. To our knowledge, PEIE-DBO possesses the best thickness-insensitive performance in polymeric ETLs of inverted fullerene-based OSCs. Furthermore, PEIE-DBO was used to fabricate the inverted non-fullerene OSCs (PM6:Y6) and obtained a high PCE of 15.74%, which indicates that PEIE-DBO is effective both in fullerene-based OSCs and fullerene-free OSCs. Keywords: Organic solar cells; Electron transport materials; Thickness-insensitive; Non-conjugated polymer

2

1. Introduction Organic solar cells (OSCs) are considered as a promising solar energy conversion technology because of their excellent merits in cost-efficient roll-to-roll (R2R) processing and outstanding mechanical flexibility [1,2]. To date, various strategies have been carried out to promote power conversion efficiencies (PCEs) to over 16% [3–9], such as exploiting novel active materials [10,11] and interfacial materials [12], optimizing device architecture [13] and device processing [14]. Among them, high-performance electron transport layers (ETLs) have been introduced between ITO (indium tin oxide) electrode and photoactive layer to achieve efficient inverted OSCs, which results in superior long-term stability [15] and improved device performance [16]. So far, a great variety of efficient ETLs have been exploited for inverted OSCs, which include metal oxide [17–25] and organic materials [26–31]. To adapt the requirements for large-area R2R processing, challenges including thickness independence and low-temperature processing, have to be overcome. Therefore, it is important to exploit low-cost, low-temperature solution-processed thickness-insensitive ETLs. Compared with metal oxide-based ETLs, organic ETLs possess distinct merits including chemical diversity, low-temperature processing and good compatibility with flexible substrates. However, the thickness of most organic ETLs is usually less than 10 nm due to their low conductivity and electron mobility. Small molecular conjugated electrolytes [32–35] and conjugated polyelectrolytes (CPEs) [36–39] have been exploited for thickness-insensitive ETLs in traditional OSCs, but these ETLs are seldom used for thick ETLs in inverted OSCs because of

3

their strong absorption in visible waveband. So far, only a few n-doping fullerene derivatives [40–42] and polyfluorene derivatives [43] have been developed as thickness-insensitive ETLs in inverted OSCs because doping via anion-induced electron transfer [40] is an available strategy to improve electron mobility and electrical conductivity of these organic ETLs. Typically, the PTB7-Th: PC71BM OSC showed a PCE of 9.08% when the thickness of the polymeric ETL (20%PN4N@x-N2200) was 20 nm [43]. However, complex synthesis routes and tedious purification process of these ETLs result in an increased cost and complex manufacturing procedures, which restrict their future applications. Polyethyleneimine ethoxylate (PEIE) [26,27] and polyethyl-eneimine (PEI) [44] possess advantages of non-conjugated polymers, such as low cost and low-temperature processing, but they are only used as ultrathin ETLs (< 10 nm) due to their intrinsically insulativity. Hence, it is a formidable challenge but significant progress to achieve a thickness-insensitive ETL by using a non-conjugated polymer. In our recent study, a non-conjugated polymer ETL, named as PEIE-DIO, was exploited as thickness-insensitive ETL. It exhibited PCE values of 9.19% and 8.88% under the thickness of 34 and 50 nm in inverted OSCs [45]. In this contribution, two non-conjugated polymers PEIE-DBO and PEIE-DCO (Fig. 1) have been developed by quaternization of PEIE with 1,8-dibromooctane (DBO) or 1,8-dichlorooctane (DCO) in order to exploit higher-performance ETL and study the effects of halogen anions on electron extraction and electron transport properties. PEIE-DBO based on quaternary ammonium bromide possesses lower

4

work function (WF), higher conductivity and electron mobility than the reported PEIE-DIO [45] as well as PEIE-DCO, so the inverted OSCs based on PEIE-DBO with PTB7-Th:PC71BM as the active layer show higher photovoltaic performance with PCE values of 10.52%, 9.45% and 9.09% under the thickness of 9, 35 and 50 nm respectively. Therefore, PEIE-DBO is an outstanding thickness-insensitive ETL, which may be potentially applied for R2R processing. 2. Experimental 2.1. Materials PEIE was purchased from Sigma Aldrich. PTB7-Th (1-Material Inc.), PM6, PC71BM (Solarmer Materials Inc.), Y6 (Shenzhen Derthon Optoelectronic Materials Science & Technology Co.), 1,8-diiodooctane (Sigma Aldrich), 1,8-dibromooctane (Alfa Aesar) and 1,8-dichlorooctane (Tokyo Chemical Industry) were purchased and used directly as received. The PEIE-DXO precursor solution, containing PEIE (Mw=70000 kDa, 35 wt%–40 wt% in H2O) with different weight concentrations (0.2 wt% for 9 nm, 1.2 wt% for 50 nm) and corresponding DXO (PEIE: DBO=1:3 (w/w), DBO: DCO=1:1 (mol/mol)) in 2-methoxyethanol, was prepared by stirring at 70 °C for 8 h before spin-coating. The thickness of the ETLs was obtained by measuring the difference between the thickness of ETL/Ag bilayer and the thickness of the corresponding Ag layer. The PEIE-DBO precursor solution for non-fullerene OSCs containing PEIE (Mw =110000 kDa, 35 wt%–40 wt% in H2O) with 0.05 wt% and DBO with 0.15 wt% in 2-methoxyethanol. 2.2. Device fabrication

5

2.2.1. Fabrication of OSCs The inverted OSC device configuration was ITO/ETL/PTB7-Th: PC71BM (100 nm)/MoO3/Al. The ITO glass substrates were cleaned by sequential ultrasonication in acetone, detergent, deionized water and isopropyl alcohol and then dried with a flowing nitrogen stream. ETLs were prepared by spin coating at a speed of 5000 rpm for 60 s and subsequently annealing at 100 °C for 10 min in ambient air. The substrates were transferred into nitrogen-filled glove box, then active layers (about 100 nm) were deposited by spin-coating from a blended solution of PTB7-Th (10 mg) and PC71BM (15 mg) in chlorobenzene (1 mL) with 3 vol% of DIO under 1500 rpm for 60 s. The nonfullerene active layer was prepared with a blended solution of PM6 (6 mg) and Y6 (7.2 mg) in chloroform (1 mL) under 1800 rpm for 40 s and subsequently annealing at 110 °C for 10 min. Finally, MoO3 (6 nm) and Al (100 nm) layers were thermally evaporated through a shadow mask onto the active layer under 2.0 × 10−4 Pa. 2.2.2. Fabrication of electron-only devices The electron-only devices were fabricated with the configurations of ITO/Al/ETL (8–9 nm)/Al, and the mobilities were determined by fitting the dark current to the single-carrier SCLC (space charge-limited currents) model at low voltage, which is described as J=(9/8)ε0εrμ((V2)/(L3)), where J is the current, μ is the electron mobility, ε0 is the permittivity of free space, εr is the relative permittivity of the material, V is the effective voltage, and L is the

6

thickness of the ETL layers. The effective voltage equals the applied voltage (Vappl) because the built-in voltage (Vbi) is zero in these devices. 2.2.3. Fabrication of devices for conductivity characterization The devices were fabricated with the configuration of ITO/ETLs (100 nm)/Ag (100 nm). The conductivities of different layers were recorded under dark and AM 1.5G irradiation (100 mW cm−2). The conductivity was defined as σ = h/(RA), where R is electrical resistance (V/I), A is the active area (6 mm2), and h is film thickness. 2.3. Instruments and measurements The current density voltage (J-V) characteristics for the devices were measured in a glove box using a Keithley 2400 source meter and an AM 1.5 Global solar simulator (Kaohsiung, Enlitech). The irradiation intensity of the light source was calibrated by a standard silicon solar cell with a KG5 filter, modulated a value of 100 mW cm−2. EQE values were tested with a commercial EQE measurement system (Kaohsiung, Enlitech, QE-R) during illumination with chopped monochromatic light from a xenon lamp. Absorption and transmittance spectra were measured using a UV-Vis spectrophotometer (Perkin Elmer Lambda 365). The film thickness was measured using a surface profiler (XP-100). The film morphologies were investigated using via atomic force microscopy (Bruker, Innova, Germany) in tapping mode. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Thermo Fisher Scientific PHI Quantera II system with a monochromatic Al Kα source (1,486.6

7

eV). Ultraviolet photoelectron spectrometer (UPS) measurements of the onset of photoemission for determining the WF were performed using standard procedures with a −10 V bias applied to the sample. Impedance measurements were obtained on the CHI660E electrochemical workstation (CH instruments, Inc.) under a bias of 0.6 V with the amplitude of 50 mV and a dark condition (1M to 100 Hz). The contact angle measurement was carried out by JC2000D1 Contact Angle Meter (Powereach Inc.), and the surface energies were directly obtained from the instrument (Ziman one-liquid method). 3. Results and discussion The schematic diagram of PEIE-DBO and PEIE-DCO are exhibited in Fig. 1. XPS was used to characterize the chemical compositions of two polymers. As shown in Fig. 2(a, b), the N 1s peaks of two polymers are mainly attributed to secondary amine nitrogen atoms (400.9 eV), tertiary amine nitrogen atoms (398.9 eV) and quaternary ammonium nitrogen atoms (402.0 eV) [46]. According to the integral area ratio between peaks at 402.0 eV and the N 1s peaks, the N+/N ratios of PEIE-DBO and PEIE-DCO are close to 23% and 18%, respectively. According to Fig. 2(c, d), the Br−/Br [47] and Cl−/Cl [48] ratios of PEIE-DBO and PEIE-DCO are calculated to be 65%, and 60% respectively. It is reasonable that bromoalkane possesses higher reactive activity than chloroalkane during quaterisation reaction.

8

Fig. 1. The schematic diagram of PEIE-DBO and PEIE-DCO.

Fig. 2. High-resolution XPS spectra of (a) N 1s for PEIE-DCO, (b) N 1s for PEIE-DBO (c) Cl 2p for PEIE-DCO and (d) Br 3d for PEIE-DBO. The WFs of ITO and ETLs-coated ITO were characterized by ultraviolet photoelectron spectroscopy (UPS). Compared with pristine ITO (WF=4.83 eV), PEIE-DBO- and PEIE-DCO- covered ITO show reduced WFs to 3.94 and 4.22 eV respectively (Fig. 3a). The lower WFs of ETL/ITO originate from the intrinsic molecular dipole, associated with ethylamine, hydroxyethyl groups and halogenated quaternary ammonium salt, and the interfacial dipole attributed to halogenated quaternary ammonium salt in the ETL/ITO interface [49]. Moreover, different halogen anion shows an influence on the WFs. Theoretically, chloride quaternary ammonium salt would possesses a slightly larger molecular dipole than bromide quaternary ammonium salt for its larger electronegativity, but the content of bromide quaternary ammonium salt in PEIE-DBO (23%) is significantly higher the content of 9

chloride quaternary ammonium salt (18%) in PEIE-DCO, which leads to lower WF of PEIE-DBO modified ITO because of more quaternary ammonium salt [50]. Moreover, the larger size and deformability of bromide ion would results in larger interfacial dipole between PEIE-DBO and ITO, which also leads to lower WF of PEIE-DBO modified ITO. Therefore, PEIE-DBO/ITO shows obviously smaller WF than PEIE-DCO/ITO, which would lead to better ohmic contact and electron extraction than the later [49,50]. High optical transparency is a key for high performance ETLs in order to maximize light-harvesting efficiency of active layers in inverted OSCs. The transmittance spectra of the ETLs-covered ITO glass and bare ITO glass are presented in Fig. 3(b). All the ETL-covered ITO glass obviously possesses high light transmittance (>80%) in the waveband between 350 and 800 nm. The ETLs-covered ITO glass shows a slightly lower light transmittance than bare ITO glass, but the difference is less than 2%, which indicates these polymers are suitable for ETLs in inverted OSCs. Simultaneously, the ETLs-covered ITO glass exhibits similar light transmittance, so the ETLs would lead to insignificant effects on light harvest of active layers.

10

Fig. 3. (a) UPS spectra and (b) transmittance spectra of PEIE-DBO-, PEIE-DCO -modified ITO and bare ITO glass. The hydrophilicity property of PEIE-DBO and PEIE-DCO was characterized by the contact angle measurements. PEIE-DBO and PEIE-DCO covered ITO exhibit the contact angles of 48.8° and 37.4° (Fig. S1) respectively, so their corresponding surface energies are 48.3 and 51.9 mN m−1, respectively. This result indicates that the hydrophilicity of the ETLs increases as the degree of crosslinking (reaction variable of quaterisation) decreases, and the same phenomenon was observed and reported in our early study [45]. The film morphology of PEIE-DBO and PEIE-DCO is characterized by AFM (atomic force microscopy) and shown in Fig. S2(a–h). Two kinds of ETL films possess smooth and homogenous surface morphology with similar RMS (root mean square roughness) values of 0.93 nm for PEIE-DBO and 1.03 nm for PEIE-DCO. According to Fig. S2(c, d), the RMS of the PTB7-Th: PC71BM blend films on PEIE-DBO and PEIE-DCO are calculated to be 1.15 nm and 1.47 nm respectively. Moreover, the blend film on PEIE-DBO shows smaller domain size (Fig. S2g) than

11

the blend film on PEIE-DCO (Fig. S2h). As we all know, smaller domain size would provide more effective interfacial area for exciton dissociation [51]. The inverted OSCs (ITO/ ETL/ PTB7-Th: PC71BM/ MoO3/ Al) with PEIE-DBO and PEIE-DCO as the ETLs were fabricated. The J-V curves of the OSCs with thin ETLs (8–9 nm) are shown in Fig. 4(a), and the detailed photovoltaic parameters are summarized in Table 1. The OSCs based on PEIE-DBO (9 nm) shows a slightly higher Voc value of 0.80 V than that (0.79 V) of the OSC based on PEIE-DCO (8 nm). It is reasonable that the lower WF (3.94 eV) of PEIE-DBO/ITO leads to better ohmic contact between ITO and the photoactive layer. These OSCs exhibit obviously different Jsc values, which are 18.17±0.13 mA cm−2 for PEIE-DBO (9 nm) and 17.14±0.15 mA cm−2 for PEIE-DCO (9 nm), respectively. The OSCs based on PEIE-DBO possesses larger Jsc than that of the OSCs based on PEIE-DCO because of lower WF and probably higher electron transport efficiency of PEIE-DBO. According to the external quantum efficiency (EQE) spectra (Fig. 4(b)), the integrated Jsc’ values are calculated to be 18.07, and 17.09 mA cm−2 for the OSCs with PEIE-DBO and PEIE-DCO as the ETLs, respectively, which are similar to the measured Jsc values within 3% experimental errors. The OSCs based on PEIE-DBO show FF values of 0.71±0.01, and the OSCs based on PEIE-DCO show lower FF values of 0.69±0.01, which implies that PEIE-DCO could possess lower electron mobility and stronger charge recombination. As a result, the OSCs based on PEIE-DBO and PEIE-DCO exhibit the maximum PCE values of 10.52% and 9.56%, respectively.

12

Table 1. Device parameters of the OSCs based on the ETLs with various thicknesses under the illumination of AM 1.5G, 100 mW cm−2. ETLs 9 nm PEIE-DBO 16 nm PEIE-DBO 35 nm PEIE-DBO 50 nm PEIE-DBO 8 nm PEIE-DCO 15 nm PEIE-DCO 33 nm PEIE-DCO 49 nm PEIE-DCO

Jsc (mA cm−2) 18.17±0.13 17.94±0.15 17.37±0.18 16.73±0.16 17.14±0.15 16.67±0.17 15.79±0.19 14.75±0.22

Voc (V) 0.80±0.00 0.79±0.00 0.78±0.00 0.77±0.01 0.79±0.00 0.78±0.00 0.78±0.00 0.77±0.00

FF 0.71±0.01 0.69±0.01 0.68±0.01 0.68±0.01 0.69±0.01 0.68±0.01 0.63±0.02 0.58±0.03

PCE (%) 10.32±0.20 9.78±0.23 9.21±0.24 8.76±0.33 9.34±0.22 8.84±0.22 7.76±0.34 6.68±0.36

Best PCE (%) 10.52 10.01 9.45 9.09 9.56 9.06 8.10 7.04

Fig. 4. (a) J−V curves and (b) EQE curves of the inverted OSCs with thin PEIE-DBO and PEIE-DCO as ETLs. Along with the increasement of ETLs thickness, all photovoltaic parameters of the OSCs begin to go down (Table 1 and Fig. S3) because thick ETLs result in large series resistance (Rs) and inferior electron transport. It is noticed that the photovoltaic parameters of the OSCs based on PEIE-DBO slowly decrease, but those of the OSCs based on PEIE-DCO rapidly decrease. When the thickness is close to 50 nm, the maximum PCEs of the OSCs based on PEIE-DBO and PEIE-DCO are 9.09% and 7.04% respectively (Table 1 and Fig. 5), which are attributed to depressed Jsc values and FF values. The Jsc values of the OSCs based on PEIE-DBO and PEIE-DCO are reduced to 16.73±0.16 and 14.75±0.22 mA cm−2 (Fig. 5). The depressed Jsc values

13

should be attributed to decreased electron transport efficiency under thick ETL condition and increased charge recombination since the ETLs possess similar light transmittance. The Voc values of the OSCs gradually decrease to 0.77±0.01 V which could be due to smaller shunt resistance (Rsh) and more charge recombination. The FF values of the OSCs based on PEIE-DBO and PEIE-DCO are reduced to 0.68±0.01 and 0.58±0.01 because thicker ETLs result in larger Rs values and smaller Rsh. It is noticed that the PCE value still retains 86.4% when the thickness of PEIE-DBO increases from 9 to 50 nm. However, the reported PCE value only retains 5% when the thickness of P5 CPE increases from 8 to 50 nm [50].

Fig. 5. PCE and Jsc as a function of various ETLs thickness. To investigate the effects of different halide ions on their electron transport performance, the conductivities and electron mobilities have been characterized. According to Fig. 6(a), the conductivities of PEIE-DBO and PEIE-DCO are obtained to be 3.3×10−5 and 1.7×10−5 S cm−1. Usually, a bromide polyelectrolyte possesses higher ionization degree than the analogous chloride polyelectrolyte under the same condition, which would result in higher ionic conductivity, so PEIE-DBO exhibits higher conductivity than PEIE-DCO. Moreover, there is no difference in the I−V

14

curves when the devices are characterized under darkness or illumination at AM 1.5G 100 mW cm−2, which indicates that no photo-induced doping occurs in the ETLs [21]. According to the J1/2−V curves of the electron-only devices (Fig. 6b), the electron mobilities of the devices based on PEIE-DBO (50 nm) and PEIE-DCO (49 nm) are 3.68×10−4 and 1.91×10−4 cm2 V−1 s−1, respectively. Compared with PEIE-DCO, PEIE-DBO based on bromide ions possesses higher electron mobility. These results indicate that the kinds of halide ions affect the conductivities and electron mobilities of the polymers. Huang et al. reported that various anions affect electron mobilities of ETLs because various anions lead to different doping degree [33,52]. However, this mechanism cannot be applied to explain this phenomenon since these polymers do not comprise any π–π conjugated backbone and doping is inexplicable. It is inferred that various halide ions could result in different dipole moment and varying degrees of molecular reorientation [50], which affect the conductivities and electron mobilities of the polymers.

Fig. 6. (a) I−V curves of the devices (ITO/polymers (100 nm)/Ag) and (b) J1/2−V curves of the electron-only devices (ITO/Al/ETLs/Al). Moreover, charge-transfer resistances (RCTs) of the OSCs have been 15

characterized by electrochemical impedance spectroscopy. As shown in Fig. 7(a), the OSCs with PEIE-DBO and PEIE-DCO as the ETLs exhibit the RCT values of 22.1 and 39.8 Ω cm2, respectively. The OSC based on PEIE-DBO shows smaller RCT than that based on PEIE-DCO, which means PEIE-DBO possesses lower leakage current and better electron transport capacity. Time-resolved photoluminescence (PL) spectra of the photoactive layer on PEIE-DBO and PEIE-DCO are shown Fig. 7(b), and the exciton lifetimes (τexciton) of the photoactive layers on PEIE-DBO and PEIE-DCO are calculated to be 1.51 and 2.46 ns, respectively. The photoactive layer on PEIE-DBO shows obviously faster exciton decay rate than that on PEIE-DCO, which indicates PEIE-DBO film leads to more efficient exciton disassociation and electron extraction. As a result, the OSC based on PEIE-DBO shows higher Jsc and FF values because PEIE-DBO possesses higher electron mobility, less charge recombination, more efficient exciton disassociation and electron extraction.

Fig. 7. (a) Electrical impedance spectra of the PSC devices and (b) time-resolved PL spectra of the photoactive layers on the ETLs.

16

Table 2. Device parameters of the nonfullerene OSCs under the illumination of AM 1.5G, 100 mW cm−2. Active layers PTB7-Th: IEICO-4F PTB7-Th: IEICO-4F PM6: IT-4F PM6: IT-4F PM6: Y6 PM6: Y6

ETLs ZnO PEIE-DBO ZnO PEIE-DBO ZnO PEIE-DBO

Jsc (mA cm−2) 24.92 26.15 22.37 22.92 26.09 27.26

Voc (V) 0.70 0.72 0.82 0.82 0.83 0.84

FF 0.67 0.64 0.71 0.71 0.71 0.69

Best PCE (%) 11.69 12.05 13.05 13.34 15.37 15.74

The inverted nonfullerene OSCs (ITO/ETL/Polymer: Acceptor/MoO3/Al) with ZnO and PEIE-DBO as the ETLs were fabricated, and the J−V curves of the optimized OSCs are shown in Fig. 8. It is seen that the OSCs based on PEIE-DBO exhibit slightly larger Voc values than those of the ZnO-based OSCs, which indicates that PEIE-DBO leads to better ohmic contact and smaller potential barrier in the interface between ITO and the photoactive layers. Simultaneously, the OSCs based on PEIE-DBO also show obviously higher Jsc values than those of the ZnO-based OSCs, which imply that PEIE-DBO leads to more efficient electron extraction. However, the formers exhibit lower FF values than those of the latters, which implies that PEIE-DBO still possess a larger series resistance than ZnO for its low conductivity. As a result, the OSCs based on PEIE-DBO with PTB7-Th:IEICO-4F, PM6:IT-4F and PM6:Y6 as the active layers show PCE values of 12.05%, 13.14% and 15.74% respectively, which are slightly higher than those of the ZnO-based OSCs.

17

Fig. 8. J−V curves of the nonfullerene OSCs based on ZnO and PEIE-DBO as ETLs. 4. Conclusions In conclusion, two non-conjugated polymers PEIE-DBO and PEIE-DCO have been developed for thickness-insensitive ETLs in inverted OSCs, and the effects of halogen anions on electron extraction and electron transport properties have been fully investigated. The results indicate that PEIE-DBO based on bromide ions possesses lower WF, higher conductivity, higher electron mobility, more efficient exciton disassociation and electron extraction than the congener polymers based on iodide ions and chloride ion, so the corresponding OSCs show better photovoltaic performance. As a result, the inverted OSCs (PTB7-Th:PC71BM) based on PEIE-DBO showed a high PCE of 10.52% at the thickness 9 nm, which still retained high PCE values of 9.45% and 9.09% at the thickness of 35 and 50 nm respectively. Moreover, the inverted PM6: Y6 OSCs based on PEIE-DBO shows a PCE of 15.74%, which is slightly higher than that of the ZnO-based OSC (15.37%). All these results indicate that PEIE-DBO is a high-performance ETL. This work not only brings the highest performance thickness-insensitive polymeric ETL but also provides an efficient strategy to design low-cost and thickness-insensitive polymeric ETLs for R2R

18

industry in future.

Acknowledgments The authors express gratitude for the support from the National Natural Science Foundation of China (51873177, 51573153, 61564003 and 21875204), the group of Advanced Photoelectricity and Supermolecule Function Materials of Ministry of Education (IRT-17R90) and the Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization. J. Zhang acknowledges the support from Guangxi Bagui Scholar Program and Guangxi Natural Science Foundation (2015GXNSFGA139002). We thank Zhongyun Ma and Yong Pei (Xiangtan University) for theoretical calculation.

Conflict of interest The authors declare that they have no conflict of interest.

19

References [1] J. Kalowekamo, E. Baker, Solar Energy 83 (2009) 1224–1231. [2] M. Kaltenbrunner, M.S. White, E.D. Głowacki, T. Sekitani, T. Someya, N.S. Sariciftci, S. Bauer, Nat. Commun. 3 (2012) 770. [3] Q.P. Fan, W.Y. Su, Y. Wang, B. Guo, Y.F. Jiang, X. Guo, F. Liu, T.P. Russell, M.J. Zhang, Y.F. Li, Sci. China Chem., 61 (2018) 531-537. https://doi.org/10.1007/s11426-017-9199-1 [4] J.Q. Zhang, L.Y. Zhu, Z.X. Wei, Small Methods 1 (2017) 1700258. [5] J.W. Zhang, R.M. Xue, G.Y. Xu, W.J. Chen, G.-Q. Bian, C.A. Wei, Y.W. Li, Y.F. Li, Adv. Funct. Mater. 28 (2018) 1705847. [6] J. Yuan, Y.Q. Zhang, L.Y. Zhou, G.C. Zhang, H.-L. Yip, T.-K. Lau, X.H. Lu, C. Zhu, H.J. Peng, P.A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y.F. Li, Y.P. Zou, Joule 3 (2019) 1140-1151. [7] B.B. Fan, D.F. Zhang, M.J. Li, W.K. Zhong, Z.M.Y. Zeng, L. Ying, F. Huang, Y. Cao, Sci. China Chem. 62 (2019) 746-752. [8] J.L. Song, C. Li, L. Zhu, J. Guo, J.Q. Xu, X.N. Zhang, K.K. Weng, K.N. Zhang, J. Min, X.T. Hao, Y. Zhang, F. Liu, Y.M. Sun, Adv. Mater. 31 (2019) 1905645. [9] X.P. Xu, K. Feng, Z.Z. Bi, W. Ma, G.J. Zhang, Q. Peng, Adv. Mater. 31 (2019) 1901872. [10] Y. Cui, H. Yao, J. Zhang, T. Zhang, Y. Wang, L. Hong, K. Xian, B. Xu, S. Zhang, J. Peng, Z. Wei, F. Gao, J. Hou, Nat. Commun. 10 (2019) 2515. [11] C. Li, J.L. Song, Y.H. Cai, G.C. Han, W.Y. Zheng, Y.P. Yi, H.S. Ryu, H.Y. Woo, Y.M. Sun, J. Energy Chem. 40 (2020) 144-150. [12] X.H. Ou Yang, R.X. Peng, L. Ai, X.Y. Zhang, Z.Y. Ge, Nat. Photonics 9 (2015) 520-524. [13] L.T. Dou, J.B. You, J. Yang, T. Moriarty, K. Emery, G. Li, C.-C. Chen, Y. Yang, Nat. Photonics 6 (2012) 180-185. [14] J. Huang, J.H. Carpenter, C.-Z. Li, J.-S. Yu, H. Ade, A.K.-Y. Jen, Adv. Mater. 28 (2016) 967-974. [15] G. Li, C.-W. Chu, V. Shrotriya, J. Huang, Y. Yang, Appl. Phys. Lett. 88 (2006) 253503. [16] L.-M. Chen, Z.R. Hong, G. Li, Y. Yang, Adv. Mater. 21 (2009) 1434-1449. [17] C.S. Kim, S.S. Lee, E.D. Gomez, J.B. Kim, Y.-L. Loo, Appl. Phys. Lett. 94 (2009) 113302. [18] H. Schmidt, K. Zilberberg, S. Schmale, H. Flügge, T. Riedl, W. Kowalsky, Appl. Phys. Lett. 96 (2010) 243305. [19] B.X. Bulliard, S.-G. Ihn, S. Yun, Y. Kim, D. Choi, J.-Y. Choi, M. Kim, M. Sim, J.-H. Park, W. Choi, K. Cho, Adv. Funct. Mater. 20 (2010) 4381-4387. [20] Y.M. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Adv. Mater. 23 (2011) 1679-1683. [21] L. Nian, W.Q. Zhang, N. Zhu, L.L. Liu, Z.Q. Xie, H.B. Wu, F. Würthner, Y.G. Ma, J. Am. Chem. Soc. 137 (2015) 6995-6998. [22] Z.L. Jiang, D. Yang, N. Wang, F.J. Zhang, B. Zhao, S.T. Tan, J. Zhang, Sci. China Chem. 56 (2013) 1573-1577. [23] M.B. Upama, N.K. Elumalai, M.A. Mahmud, C. Xu, D. Wang, M. Wright, A. Uddin, Sol. Energy Mater. Sol. Cells 187 (2018) 273-282. [24] W. Yu, L. Huang, D. Yang, P. Fu, L. Zhou, J. Zhang, C. Li, J. Mater. Chem. A 3 (2015) 10660-10665. [25] H. Liu, Z.-X. Liu, S. Wang, J. Huang, H. Ju, Q. Chen, J. Yu, H. Chen, C.-Z. Li, Adv. Energy Mater. 9 (2019) 1900887. [26] Y.H. Zhou, C. Fuentes-Hernandez, J. Shim, J. Meyer, A.J. Giordano, H. Li, P. Winget, T.

20

Papadopoulos, H. Cheun, J. Kim, M. Fenoll, A. Dindar, W. Haske, E. Najafabadi, T.M. Khan, H. Sojoudi, S. Barlow, S. Graham, J.-L. Brédas, S.R. Marder, A. Kahn, B. Kippelen, Science 336 (2012) 327-332. [27] S.X. Xiong, L. Hu, L. Hu, L.L. Sun, F. Qin, X.J. Liu, M. Fahlman, Y.H. Zhou, Adv. Mater. 31 (2019) 1806616. [28] Z.A. Tan, W.Q. Zhang, Z.G. Zhang, D.P. Qian, Y. Huang, J.H. Hou, Y.F. Li, Adv. Mater. 24 (2012) 1476-1481. [29] S.J. Liu, K. Zhang, J.M. Lu, J. Zhang, H.-L. Yip, F. Huang, Y. Cao, J. Am. Chem. Soc. 135 (2013) 15326-15329. [30] J.S. Yu, Y.Y. Xi, C.C. Chueh, J.Q. Xu, H.L. Zhong, F. Lin, S.B. Jo, L.D. Pozzo, W.H. Tang, A.K.Y. Jen, Nano Energy 39 (2017) 454-460. [31] Z.J. Li, X.P. Xu, G.J. Zhang, M. Deng, Y. Li, Q. Peng, Solar RRL 2 (2018) 1800182. [32] Z.-G. Zhang, B.Y. Qi, Z.W. Jin, D. Chi, Z. Qi, Y.F. Li, J.Z. Wang, Energ. Environ. Sci, 7 (2014) 1966-1973. [33] Z. Wang, N. Zheng, W. Zhang, H. Yan, Z. Xie, Y. Ma, F. Huang, Y. Cao, Adv. Energy Mater. 7 (2017) 1700232. [34] S.C. Wang, Z.J. Li, X.P. Xu, M.L. Zhang, G.J. Zhang, Y. Li, Q. Peng, J. Mater. Chem. A 6 (2018) 22503-22507. [35] J.J. Liu, N.N. Zheng, Z.C. Hu, Z.F. Wang, X.Y. Yang, F. Huang, Y. Cao, Sci. China Chem. 60 (2017) 1136-1144. [36] Z.H. Wu, C. Sun, S. Dong, X.-F. Jiang, S. Wu, H.B. Wu, H.-L. Yip, F. Huang, Y. Cao, J. Am. Chem. Soc. 138 (2016) 2004-2013. [37] R.G. Xu, K. Zhang, X. Liu, Y.C. Jin, X.-F. Jiang, Q.-H. Xu, F. Huang, Y. Cao, ACS Appl. Mater. Inter. 10 (2018) 1939-1947. [38] Z.M. Chen, Z.C. Hu, Z.H. Wu, X. Liu, Y.C. Jin, M.J. Xiao, F. Huang, Y. Cao, J. Mater. Chem. A 5 (2017) 19447-19455. [39] J.C. Jia, B.B. Fan, M.J. Xiao, T. Jia, Y.C. Jin, Y. Li, F. Huang, Y. Cao, Macromolecules 51 (2018) 2195-2202. [40] C.-Z. Li, C.-Y. Chang, Y. Zang, H.-X. Ju, C.-C. Chueh, P.-W. Liang, N. Cho, D.S. Ginger, A.K.-Y. Jen, Adv. Mater. 26 (2014) 6262-6267. [41] J.W. Zhang, R.M. Xue, G.Y. Xu, W.J. Chen, G.Q. Bian, C.A. Wei, Y.W. Li, Y.F. Li, Adv. Funct. Mater. 28 (2018) 1705847. [42] Y. Liu, Z. Page, S. Ferdous, F. Liu, P. Kim, T. Emrick, T. Russell, Adv. Energy Mater. 5 (2015) 1500405. [43] S. Dong, Z.C. Hu, K. Zhang, Q.W. Yin, X.F. Jiang, F. Huang, Y. Cao, Adv. Mater. 29 (2017) 1701507. [44] S. Woo, W.H. Kim, H. Kim, Y. Yi, H.-K. Lyu, Y. Kim, Adv. Energy Mater. 4 (2014) 1301692. [45] Z.Q. Zhang, Z.L. Zhang, B. Zhao, Y.H. Huang, J. Xiong, P. Cai, X.G. Xue, J. Zhang, S.T. Tan, J. Mater. Chem. A 6 (2018) 12969-12973. [46] W. Lee, J.W. Jung, J. Mater. Chem. A 4 (2016) 16612-16618. [47] N.D. Hutson, B.C. Attwood, K.G. Scheckel, Environ. Sci. Technol. 41 (2007) 1747-1752. [48] A.F. Perez-Cadenas, F.J. Maldonado-Hodar, C. Moreno-Castilla, Carbon 41 (2003) 473-478. [49] C.-C. Chueh, C.-Z. Li, A.K.-Y. Jen, Energ. Environ. Sci. 8 (2015) 1160-1189. [50] B.H. Lee, I.H. Jung, H.Y. Woo, H.-K. Shim, G. Kim, K. Lee, Adv. Funct. Mater. 24 (2014)

21

1100-1108. [51] D.L. Liu, Y. Zhang, G. Li, J. Energy Chem. 35 (2019) 104-123. [52] Z.F. Wang, N. Zheng, W.Q. Zhang, H. Yan, Z.Q. Xie, Y.G. Ma, F. Huang, Y. Cao, Adv. Energy Mater. 7 (2017) 1700232.

Graphical abstract The inverted PTB7-Th:PC71BM OSC achieves a PCE 9.09% at 50 nm, which indicates that PEIE-DBO is the best thickness-insensitive polymeric ETL in inverted OSCs. The inverted PM6:Y6 OSC achieves a PCE 15.74%.

22