J. Cent. South Univ. (2021) 28: 1919-1931

DOI: https://doi.org/10.1007/s11771-021-4741-7

A new low-bandgap polymer acceptor based on benzotriazole for efficient all-polymer solar cells

LI Zhe(李哲), CHEN Hong-gang(陈泓钢), YUAN Jun(袁俊), ZOU Jie(邹洁),LI Jing(李静), GUAN Hui-lan(管慧兰), ZOU Ying-ping(邹应萍)

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: The rational design of polymer acceptors with strong and broad absorption is critical to improve photovoltaic performance. In this work, a new polymer acceptor PY9-T based on heptacyclic benzotriazole (Y9-C16) as a building block and thiophene unit as the linking unit was synthesized, which exhibited a low bandgap (1.37 eV) and a high extinction coefficient of the neat film (1.44×10⁵ cm^-1). When PY9-T was blended with the wide bandgap polymer donor PBDB-T, the all-polymer solar cells (APSCs) showed a high power conversion efficiency (PCE) of 10.45% with both high open circuit voltage of 0.881 V and short-circuit current density of 19.82 mA/cm². In addition, APSCs based on PY9-T show good thermal stability, as evidenced by slight changes morphologies when annealed at 100 ℃. These results suggest that Y9-C16 provides a new building block to develop efficient and stable polymer acceptors.

Key words: all-polymer solar cells; polymer acceptor; low-bandgap; benzotriazole; power conversion efficiency

Cite this article as: LI Zhe, CHEN Hong-gang, YUAN Jun, ZOU Jie, LI Jing, GUAN Hui-lan, ZOU Ying-ping. A new low-bandgap polymer acceptor based on benzotriazole for efficient all-polymer solar cells [J]. Journal of Central South University, 2021, 28(7): 1919-1931. DOI: https://doi.org/10.1007/s11771-021-4741-7.

1 Introduction

Bulk-heterojunction polymer solar cells (BHJ-PSCs) have been regarded as one of the prospective photovoltaic technologies due to the merits of simple device structure, light weight, flexibility, and large-area printed electronics [1-4]. Among the PSCs, all-polymer solar cells based on a blend of polymer donor and polymer acceptor offer unique advantages such as robustness of film morphology, stability of the mechanical stresses, and compatibility of large scale manufacturing [5-8]. Recently, the rapid development of small molecule acceptors (SMAs) has achieved a power conversion efficiency (PCE) of 17.5% (certified) for single-junction devices while the development of polymer acceptors still lags behind. Therefore, developing high-performance polymer acceptor materials is crucial to boost the photovoltaic performance of APSCs.

Many polymer acceptor materials are applied to all-polymer solar cells (APSCs). For example, poly{[N,N`-bis(2-octyldodecyl)-naphthalene-1, 4, 5, 8-bis(dicarboximide)-2,6-diyl]-alt-5,5`-(2,2`-

bithiophene)} (N2200) has demonstrated efficient photovoltaic performance. However, its low absorption coefficient limits the photocurrent and PCE of APSCs [9-11]. Hence, it is necessary to develop new polymer acceptors with strong absorption in visible and near-infrared (vis-NIR) regions and high absorption coefficient. ZHANG et al [5] reported a new design strategy to increase the absorption coefficient of polymer acceptor by using fused-ring SMAs (IDIC-C16) as a building block and thiophene as the linking unit. The corresponding polymer acceptor PZ1 showed a high PCE of 9.19%. Based on this strategy, different types of polymer acceptors are constructed [6, 12-15]. For example, YAO et al [13] reported a new polymer acceptor PFBDT-IDTIC with a fluorinated benzodithiophene (BDT) unit achieving the PCE of 10.3%. Hence, the rational design of materials based on fused ring SMA is an efficient strategy to improve photovoltaic performance.

Recently, “Y-series” non-fullerene acceptors were reported, such as Y6, with high efficiency [16-22]. These acceptor materials have many advantages such as strong absorption in the vis-NIR region, low voltage loss, and high electron mobility. Therefore, reasonable selection of “Y-series” SMAs as building blocks is significant to improve performance. For example, JIA et al [23] chose the derivative of Y6 as building block to prepare the polymer acceptor PJ1 obtaining a PCE of 14.4%. In this work, we designed and synthesized a novel polymer acceptor PY9-T based on a thiophene unit and a SMA of Y9-C16 with a similar molecular backbone to the acceptor Y9 [24]. The PY9-T not only has advantages of Y9 acceptor (e.g., strong absorption in the 500-900 nm region), but merits of

conjugated polymers (e.g., large π-electron delocalization) [25]. The PY9-T exhibits a low-bandgap and a relatively high lowest unoccupied molecular orbital (LUMO) level. Furthermore, blended with wide-bandgap polymer donor PBDB-T (Figure 1(a)), the PY9-T-based device achieved a high PCE of 10.45%. The results indicate that PY9-T is a potential polymer acceptor for efficient APSCs.

2 Results and discussion

2.1 Synthesis and characterization

The synthesis of PY9-T began with compound 1 (Figure S1). Compound 2 was synthesized through the Cadogan reaction of compound 1 and then nucleophilic substitution reaction to introduce the large 2-hexyldecadecyl branched chains. The aldehyde group was introduced into the fused-ring thiophene unit by Vilsmeier-Haack reaction to give compound 3. Y9-C16-Br was obtained by Knoevenagel condensation reaction of compound 3 and IC-Br. The polymer PY9-T (Figure 1(b)) was synthesized by a Stille coupling reaction between Y9-C16-Br and 2,5-bis(trimethylstannyl)thiophene, exhibiting good solubility in organic solvents (e.g., chloroform and chlorobenzene). These synthetic details of Y9-C16-Br monomer, polymerization, and their nuclear magnetic resonance (NMR) spectra are displayed in experimental section in Figures S1-S6. Herein, the PY9-T shows the average molecular weight (M_w) of 1.24 kDa and a polydispersity index (PDI) of 1.9, measured by gel permeation chromatography (GPC) with1,2-dichlorobenzene as the eluent under at 140 °C (Figure S7). Thermal gravimetric analyzer (TGA) and differential scanning calorimetry (DSC) were used to measure the thermal stability of PY9-T (Figure S8). TGA exhibits a decomposition temperature (5% weight loss) of 344 °C under N₂ atmosphere and DSC shows negligible melting phase transition in the temperature range of 20-300 °C, indicating good thermal stability.

Figure 1 (a) Chemical structure of PBDB-T; (b) Chemical structure of PY9-T; (c) Normalized UV-vis absorption spectra of PBDB-T and PY9-T in neat films and PY9-T in chloroform solution; (d) Absorption coefficient of PY9-T and PBDB-T in films; (e) Energy level diagram of PBDB-T and PY9-T; (f) Cyclic voltammograms of PY9-T

2.2 Photophysical and electrochemical properties

The ultraviolet-visible absorption of PBDB-T and PY9-T is shown in Figure 1(c). The film absorption of PY9-T can be observed in the range of 500-900 nm with a maximum peak at 821 nm and two weak shoulder peaks at 654 and 744 nm. Meanwhile, the film absorption of PBDB-T shows complementary absorption in the range of 350-700 nm with a maximum peak at 624 nm. Compared with the diluted solution, the film of PY9-T shows a 23 nm redshift of the maximum absorption peak, which indicates more strong aggregations in the film. The optical bandgap (E_g^opt=1240/λ_onset) of PY9-T deduced from the absorption onset wavelengths (λ_onset) is 1.37 eV and the absorption coefficient of PY9-T is 1.44×10⁵ cm^-1 (Figure 1(d)), which is about five times higher than N2200 (0.3×10⁵ cm^-1) [5]. The low bandgap and complementary absorption are beneficial to light-harvesting to ensure the higher short-circuit current density (J_sc) for PBDT-T: PY9-T based devices. The electrochemical energy levels of PY9-T were measured by cyclic voltammetry　 (CV) method. The highest occupied molecular orbital (HOMO) and LUMO energy levels were calculated from the onset oxidation potential (E_ox) and the onset reduction potential (E_red) via the equation of E_HOMO/LUMO=-(E_ox/red+4.8-E_Fc/Fc⁺) (eV), where E_Fc/Fc⁺ was carried out to be 0.44 eV, and Fc is ferrocene. The HOMO and LUMO levels of PY9-T are -5.56 and -3.72 eV, respectively (Figures 1(e) and (f)). Generally, the higher LUMO energy level of PY9-T is conducive to the larger open circuit voltage (V_oc) [26, 27]. Corresponding optical

properties and energy levels of PY9-T are listed in Table S1.

2.3 Photovoltaic properties

To further investigate the photovoltaic performance of the PY9-T, the conventional device structure with ITO(indium tin oxide)/PEDOT:

PSS (poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate))/PBDB-T:PY9-T /PDINN (2,9-bis(3-((3-(dimethylamino)propyl)amino)propyl) anthrax [2,1,9-def:6,5,10-def]diisoquinoline-1,3,8,10(2H,9H)-tetraone)/Ag was fabricated. The

current density-voltage (J-V) curves were measured and the photovoltaic data are shown in Figure 2(a) and Table 1, respectively. Different weight ratios of PBDB-T: PY9-T were tried and the optimal weight ratio was 1:1. In addition, thermal annealing (TA) at different temperatures was carried out, which increases fill factor (FF) with a slight effect on J_sc and V_oc. Furthermore, ratios of 1-chloronaphthalene (CN) additive were performed to optimize the device performance. As a result, the optimal PCE (10.45%) was obtained under the conditions of PBDB-T: PY9-T mass ratio of 1:1, TA at 100 °C for 10 min and 5% CN additive (Table S2-5). The statistical distribution of the PCE of 15 devices (Figure 2(c)) in optimal conditions is further presented to demonstrate the good reproducibility of PY9-T-based all-PSCs. Here, the energy loss (E_loss) was estimated by using the difference between the E_g^opt of the acceptor and the energy of the extracted charges (qV_oc) of the device (E_loss=E_g^opt-qV_oc). And the optimized APSC exhibited the low E_loss of 0.49 eV [25, 28].

The external quantum efficiency (EQE) spectra of the PBDB-T: PY9-T based-APSCs is shown in Figure 2(b). Compared with the as-cast device, the EQE values of the optimized devices were higher in the range of 430-870 nm, which indicated more efficient photoelectron conversion and matched well with the enhanced J_sc of the optimized devices. The J_sc values of as-cast and optimized devices integrated from the EQE curves were 18.38 and 19.54 mA/cm²,　which agreed well with those from the J-V measurements within 5% errors.

Table 1 Photovoltaic parameters data based on PBDB-T:PY9-T blend films under different conditions

Figure 2 J-V curves (a), EQE spectra (b), bar graphs of PCE counts for 15 individual devices (c), J_sc dependence upon different P_light (d), V_oc dependence upon different P_light (e) and J_ph-V_eff curves of corresponding APSCs based on PBDB-T: PY9-T (f)

2.4 Charge transport properties

The charge carrier mobilities of the PBDB-T: PY9-T film with optimized and as-cast devices were evaluated by fabricating the hole-only device ITO/PEDOT:PSS/PBDB-T:PY9-T/MoO₃/Ag and electron-only device ITO/ZnO/PBDB-T:PY9-T/ PDINN/Ag by using the space charge limited current (SCLC) method (Figure S9). For the as-cast device, electron mobilities (μ_e) and hole mobilities (μ_h) are calculated as 0.883×10^-4 and 0.816×10^-4 cm²/(V·s), respectively, with μ_e/μ_h of 1.08. By comparison, higher and more balanced values with μ_e and μ_h of 1.62×10^-4 and 1.64×10^-4 cm²/(V·s) (μ_e/μ_h of 0.99) are achieved after optimization, which can lead to notably increased FF and J_sc for the optimized devices.

2.5 Exciton dissociation and charge recombination

The exciton dissociation and recombination behaviors have significant influence on J_sc and FF. Generally, J_sc and P_light follow a power law with the relationship J_sc∝P_light^α (P_light is the light intensity and α is a power-law component) [29]. Experimentally, α approaching 1 indicates that the bimolecular recombination is negligible. The α values for the optimized and as-cast devices are 1.006 and 0.999, respectively, implying efficient exciton dissociation and weaker bimolecular recombination in APSCs based on PY9-T (Figure 2(d)). Besides, the V_oc versus P_light (Figure 2(e)) is determined as the formula of V_oc∝(nkT/q)ln(P_light), where n is a constant, K is the Boltzmann constant, T is the temperature and q is the elementary charge. After optimization, the value of slope decreased from 1.48 to 1.21 kT/q, which suggested a lower trap state [30]. The photocurrent (J_ph) and the effective voltage (V_eff) of the active layer were also carried out to study exciton dissociation probabilities (P_diss); J_ph is evaluated by the formula J_ph=J_L-J_D, where J_L and J_D are the current densities under illumination and darkness conditions, respectively; V_eff expresses as V_eff=V₀-V_bias, where V₀ is the voltage when J_ph=0, and V_bias is the applied bias [31]. When V_eff is higher than 2 V, J_ph will reach the saturation current density (J_sat) (Figure 2(f)). Besides, the P_diss follows the formula P_diss=J_ph/J_sat. The P_diss of the as-cast and optimized devices are 91.7% and 93.2% respectively, indicating a high exciton generation and dissociation efficiency.

2.6 Thermal stability and morphology characterization

The thermal stability of APSCs based on PBDT-T: PY9-T was investigated by increasing the TA time at 100 °C, as shown in Figure 10. The device maintains an 86% PCE of the initial efficiency with V_oc of 0.868 V, J_sc of 19.18 mA/cm², and FF of　54.20% after being annealed for 180 min (PCE=9.02%). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) further confirmed these results. As shown in Figures 3(a)-(c), after annealing films for various times, the devices obtain interpenetrating network structure with almost identical root-mean-square surface roughness (R_q) of around 1.40 nm. In addition, fine nanoscale phase separation is also retained in TEM when the active layer is annealed for 180 min (Figures 3(d)-(f)). The results demonstrate that APSC based on PY9-T has potential as a candidate for morphological stable PSCs.

3 Conclusions

A new polymer acceptor PY9-T with benzotriazole fused-ring acceptor as the building block is designed and synthesized. It exhibits strong and broad absorption in 500-900 nm with a high extinction coefficient (1.44×10⁵ cm^-1). When PY9-T was matched with the polymer donor PBDB-T, the devices based on PBDB-T: PY9-T achieved the PCE of 10.45%. PY9-T-based APSCs show good thermal stability owing to no apparent change in morphology. These findings demonstrate that PY9-T is a promising polymer acceptor candidate for high-performance and thermal stable APSCs.

Figure 3 AFM (a-c) and TEM (d-f) images for PBDB-T: PY9-T films with various TA times:

Supporting information

1 Basic characterization

¹H NMR and ¹³C NMR spectra were recorded using a Bruker AV-500 spectrometer in deuterated chloroform solution at 298 K, unless specified otherwise. Chemical shifts were reported as δ values (10^-6) with tetramethylsilane (TMS) as the internal reference. UV-Vis absorption spectra were recorded on the SHIMADZU UV-2600 spectrophotometer. For the solid state measurements, PBDB-T and PY9-T solutions in chloroform were spin-coated on quartz plates. The cyclic voltammetry was recorded with a computer controlled CHI 660E electrochemical workstation using PBDB-T or BZIC films on platinum electrode (1.0 cm²) as the working electrode, a platinum wire as the counter electrode and Ag/AgCl (0.1 mol/L) as the reference electrode in an anhydrous and argon-saturated solution of 0.1 mol/L of tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) in anhydrous acetonitrile at a scanning rate of 50 mV/s. Electrochemical onsets were determined at the position where the current starts to differ from the baseline. The morphologies of the PBDB-T/PY9-T blend films were investigated by atomic force microscopy (AFM, Agilent Technologies,5500 AFM/SPM System, USA).

2 Fabrication and characterization of all-polymer solar cells

The devices were fabricated with an indium tin oxide (ITO) glass as positive electrode and a Ag as negative electrode. Patterned ITO glass with a sheet resistance of 15 Ω was purchased from CSG HOLDING Co. Ltd. (China). The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone and isopropanol, and then treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 20 min. Then PEDOT: PSS (poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate)) (Baytron PVP Al 4083, Germany) was filtered through a 0.45 μm poly(tetrafluoroethylene) (PTFE) filter and spin coated at 6000 r/min for 30 s on the ITO substrate. Subsequently, PEDOT: PSS film was baked at 150 °C for 15 min in the air, and the thickness of the PEDOT:PSS layer is about 30 nm. The polymer PBDB-T and PY9-T (13 mg/mL) were dissolved in CHCl₃ and different volume ratios of 1-chloronaphthalene (CN) (Sigma Aldrich) overnight and spin-cast at 2800 r/min for 30 s onto the PEDOT: PSS layer. After spin-coating, the active layers were annealed at different temperature for various time. The thickness of the active layers is ca. 100 nm. Then methanol solution of PDINN at a concentration of 1.0 mg/mL was deposited atop the active layer at 3000 r/min for 30 s to afford a PDINN cathode buffer layer with thickness of ca. 10 nm. Finally, top Ag electrode was deposited in vacuum onto the cathode buffer layer at a pressure of ca. 1.0×10^-5 Pa. The active area of the devices was 6 mm². Device characterization was carried out under AM 1.5G irradiation with the intensity of 100 mW/cm² (Oriel 67005, 500 W), calibrating by a standard silicon cell. J-V curves were recorded with a Keithley 236 digital source meter. An xenon lamp with AM 1.5 filter was used as the white light source and the optical power was 100 mW/cm². The EQE measurements of organic solar cells were performed by solar cell spectral response measurement system QE-R3-011 (Enli Technology Co., Ltd., Taiwan, China). A calibrated silicon detector was used to determine the absolute photosensitivity at different wavelengths. In addition to EQE, all of these fabrications and characterizations were conducted in a glove box.

3 Materials and synthesis

Tetrakis(triphenylphosphine)palladium

(Pd(PPh₃)₄), bis(triphenyl-phosphine)palladium(II) dichloride(PdCl₂(PPh₃)₂) and n-butyllithium (n-BuLi) were obtained from J&K Energy Chemical Company, China, and they were all used as received. PBDB-T was purchased from Solarmer Energy Inc. IC-Br was obtained from SunaTech Inc. All other reagents and solvents were purchased commercially as analytical pure and used without further purification. 2-(2-ethylhexyl)-5, 6-dinitro-4,7-bis(6-undecylthieno[3,2-b]thiophen-2-yl)-2H-benzo[d] [1, 2, 3] triazole (1) was synthesized according to previous literatures [24].

6-(2-ethylhexyl)-12,13-bis(2-hexyldecyl)-3,

9-diundecyl-12,13-dihydro-6H-thieno[2'',3'':4',5']

thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno

[3,2-b][1,2,3]triazolo[4,5-e]indole (2). Compound 1 (2.00 g, 2.21 mmol) and triphenylphosphorus (5.787 g, 22.07 mmol) were dissolved in the anhydrous N-Methyl pyrrolidone (NMP, 50 mL) into a 250 mL three-necked round bottom flask under argon. The reation was heated at 180 °C overnight. After cooling to room temperature, the brown solution was added 7-(bromomethyl)pentadecane (4.10 g, 13.24 mmol), potassium hydroxide (1.24 g, 22.07 mmol), potassium iodide (0.15 g, 0.88 mmol) at room temperature and the mixture with atmosphere was refluxed at 90 °C for 15 h. The residue was extracted with dichloromethane (DCM) and H₂O three times. The organic layers were combined and dried over MgSO₄, and purified with column chromatography on silica gel using DCM/petroleum ether (PE) (1/8, v/v) as the eluent to give a brownish red liquid 2 (1.06 g, 37.09 % yield). ¹H NMR (400 MHz, CDCl₃) δ_H 6.87 (s, 2H), 4.64 (d, J=7.2 Hz, 2H), 4.49 (d, J=7.6 Hz, 4H), 2.72 (t, J=7.6 Hz, 4H), 2.35-2.22 (m, 1H), 1.97-1.88 (m, 2H), 1.80-1.73 (m, 4H), 1.38-1.16 (m, 44H), 0.94-0.87 (m, 24H), 0.80-0.72 (m, 32H), 0.60 (t, J=6.8 Hz, 12H).

Synthesis of 6-(2-ethylhexyl)-12,13-bis(2-

hexyldecyl)-3,9-diundecyl--12,13-dihydro-6H-

thieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]

thieno[2',3':4,5]thieno[3,2-b][1,2,3]triazolo[4,5-e]

indole-2,10-dicarbaldehyde (3). To a solution of compound 2 (0.72 g, 0.56 mmol) with 1,2-dichloroethane (20 mL) into a 100 mL three-necked round bottom flask under argon and anhydrous N,N-dimethyl-formamide (DMF)(1.7 mL) was added at once. Then, POCl₃ (0.9 mL) was added dropwise slowly at 0 °C for 2 h; the mixture was heated to 90 °C and stirred overnight.

Figure S1 Synthetic routes of PY9-T

The reaction mixture was poured into water (200 mL), and then extracted with DCM-H₂O three times. The combined organic layer was evaporated under reduced pressure and the crude product was purified with column chromatography on silica gel using DCM/PE(1/1, v/v) as the eluent to give a orange red liquid 3 (0.40 g, 53.11 % yield). ¹H NMR (400 MHz, CDCl₃) δ_H 10.05 (s, 2H), 4.65 (d, J=7.2 Hz, 2H), 4.53 (d, J= 7.6 Hz, 4H), 3.11 (t, J=7.2 Hz, 4H), 2.30-2.27 (m, 1H), 1.88-1.81 (m, 6H), 1.40-1.19 (m, 44H), 0.99-0.89 (m, 24H), 0.82-0.73 (m, 32H), 0.60 (t, J=7.2 Hz, 12H).

Synthesis of Y9-C16-Br. Compound 3 (0.39 g, 0.29 mmol), compound IC-Br (0.24 g, 0.88 mmol), pyridine (1 mL) and chloroform (30 mL) were dissolved in a round bottom flask under argon. The mixture was stirred at 65 °C overnight. After cooling to room temperature, the mixture was poured into methanol and filtered. The residue was purified with column chromatography on silica gel using DCM/PE (1/1, v/v) as the eluent to give a blue black solid (0.37 g, 68.04% yield). ¹H NMR (400 MHz, CDCl₃) δ_H 9.15 (s, 2H), 8.82 (d, J=0.8 Hz, 1.27H), 8.53 (d, J=9.2 Hz, 0.70H), 8.01 (d, J=1.6 Hz, 0.63H), 7.88-7.82 (m, 1.70H), 7.80-8.76 (m, 1.71H), 4.74-4.73 (m, 6H), 3.21 (t, J=8.0 Hz, 4H), 2.40-2.34 (m, 1H), 2.08-2.05 (m, 2H), 1.91-1.84 (m, 4H), 1.56- 1.28 (m, 44H), 1.14-0.85 (m, 56H), 0.79 (t, J=7.2 Hz, 12H). ¹³C NMR (CDCl₃, 400 MHz): d_C 187.30, 186.88, 160.07, 159.48, 155.95, 153.69, 149.19, 144.99, 141.43, 138.55, 138.36, 137.96, 137.86, 137.42, 137.01, 136.65, 135.93, 135.49, 133.74, 133.49, 129.97, 129.91, 129.24, 128.09, 126.58, 124.41, 119.70, 119.61, 115.32, 114.76, 112.01, 99.28, 68.25, 67.75, 59.72, 55.44, 40.38, 39.13, 31.92, 31.84, 31.62, 31.57, 31.28, 30.59, 30.49, 29.90, 29.84, 29.76, 29.67, 29.62, 29.53, 29.50, 29.41, 29.35, 29.21, 28.41, 25.51, 23.99, 22.95, 22.69, 22.62, 22.50, 14.12, 14.06, 14.03, 10.51.

4 Mobility measurements

Hole and electron mobilities were measured using the space charge limited current (SCLC) method, with the hole-only device of ITO/PEDOT:PSS/PBDB-T:PY9-T/MoO₃/Ag and electron-only device for hole mobility measurement and electron-only device of ITO/ZnO/PBDB-T:PY9-T/PDINN/Ag for electron mobility measurement. The SCLC mobilities were calculated by MOTT-Gurney equation [32, 33]:

                             (1)

where J is the current density; ε_r is the relative dieletiric constant of active layer material usually 2-4 for organic semiconductor, herein we use a relative dielectric constant of 4; ε₀ is the permittivity of empty space; μ is the mobility of hole or electron and L is the thickness of the active layer; V is the internal voltage in the device, and V=V_app-V_bi, where V_app is the voltage applied to the device, and V_bi is the built-in voltage resulting from the relative work function difference between the two electrodes (in the hole-only and the electron-only devices, and the V_bi values are 0.2 and 0 V, respectively).

5 Additional figures and tables

Table S1 Optical properties and energy levels of PY9-T and PBDB-T

Table S2 Photovoltaic data of all-PSCs based on PBDB-T:PY9-T blend films at different D/A weight ratios with 0.5% CN additive treatment and TA at 100 °C for 10 min, under irridiation of AM 1.5G, 100 mW/cm²

Table S3 Photovoltaic performance parameters of all-PSCs based on PBDB-T:PY9-T (1/1) at different concentration of additive (vol%) with TA at 100 °C for 10 min, under irradiation of AM 1.5G, 100 mW/cm²

Table S4 Photovoltaic performance parameters of all-PSCs based on PBDB-T:PY9-T (1/1) at different temperatures of TA for 10 min with 0.5% CN additive treatment, under irridiation of AM 1.5G, 100 mW/cm²

Table S5 Photovoltaic performance parameters of all-PSCs based on PBDB-T:PY9-T (1/1) at different times of TA 100 °C with 0.5% CN additive treatment, under irradiation of AM 1.5G, 100 mW/cm²

Table S6 Abbreviations of structures

Figure S2 ¹H-NMR spectrum of compound 2 in CDCl₃

Figure S3 ¹H-NMR spectrum of compound 3 in CDCl₃

Figure S4 ¹H-NMR spectrum of compound Y9-C16-Br in CDCl₃

Figure S5 ¹H-NMR spectrum of compound PY9-T in CDCl₃

Figure S6 ¹³C-NMR spectrum of compound Y9-C16-Br in CDCl₃

Figure S7 GPC measurement of PY9-T

Figure S8 TGA curve (a) and DSC thermogram (b) of PY9-T

Figure S9 J^1/2-V characteristics of (a) electron only devices and (b) hole only devices. Solid lines are fitting lines of the data

Figure S10 J-V curves of corresponding APSCs with 100 °C annealing for different time

Contributors

LI Zhe, YUAN Jun and ZOU Ying-ping conceived and directed the experiments. LI Zhe, ZOU Jie, GUAN Hui-lan and LI Jing performed the material synthesis. CHEN Hong-gang performed fabrication of devices. The first draft was written by LI Zhe, YUAN Jun and ZOU Ying-ping revised and edited the manuscript. ZOU Ying-ping supervised this work.

Conflict of interest

LI Zhe, CHEN Hong-gang, YUAN Jun, ZOU Jie, LI Jing, GUAN Hui-lan, and ZOU Ying-ping declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

苯并三氮唑基窄带隙聚合物受体用于高效全聚合物太阳电池

摘要：合理设计具有强和宽吸收的聚合物受体有助于提高光伏性能。本文以七环苯并三氮唑(Y9-C16)为构筑单元，噻吩为连接单元合成一个新型聚合物受体材料PY9-T。该受体材料的薄膜具有较低的带隙(1.37 eV)和较高的消光系数(1.44×10⁵ cm^-1)。当PY9-T与宽带隙聚合物给体PBDB-T共混时，制备全聚合物太阳电池(APSC)实现10.45%的高能量转换效率，器件具有0.881 V的高开路电压和19.82 mA/cm²的高短路电流密度。此外，器件在100°C退火后，共混膜形貌仅发生微小的变化，表明APSC具有好的热稳定性，Y9-C16为开发高效稳定的聚合物受体提供了新的构筑单元。

关键词：全聚合物太阳电池；聚合物受体；窄带隙；苯并三氮唑；能量转换效率

Foundation item: Project(21875286) supported by the National Natural Science Foundation of China

Received date: 2021-01-09; Accepted date: 2021-04-28

Corresponding author: ZOU Ying-ping, PhD, Professor; Tel: +86-18711140102; E-mail: yingpingzou@csu.edu.cn; ORCID: https://orcid. org/0000-0003-1901-7243