Dopant‐Free Crossconjugated Hole‐Transporting Polymers for Highly Efficient Perovskite Solar Cells

Abstract Currently, there are only very few dopant‐free polymer hole‐transporting materials (HTMs) that can enable perovskite solar cells (PVSCs) to demonstrate a high power conversion efficiency (PCE) of greater than 20%. To address this need, a simple and efficient way is developed to synthesize novel crossconjugated polymers as high performance dopant‐free HTMs to endow PVSCs with a high PCE of 21.3%, which is among the highest values reported for single‐junction inverted PVSCs. More importantly, rational understanding of the reasons why two isomeric polymer HTMs (PPE1 and PPE2) with almost identical photophysical properties, hole‐transporting ability, and surface wettability deliver so distinctly different device performance under similar device fabrication conditions is manifested. PPE2 is found to improve the quality of perovskite films cast on top with larger grain sizes and more oriented crystallization. These results help unveil the new HTM design rules to influence the perovskite growth/crystallization for improving the performance of inverted PVSCs.

Differential scanning calorimetry (DSC) was recorded on a PerkinElmer 13 Diamond DSC in strument under nitrogen at a temperature scan of 10 °C/min. The contact angle was measured by KRÜSS DSA 25 contact angle goniometer. UV−vis−IR absorption spectra were collected using a Perkin Elmer UV-VIS-NIR spectrophotometer Lambda 750S. The ultraviolet photoelectron spectroscopy of HTLs were carried out in a VG ESCALAB 220i-XL surface analysis system equipped with a He-discharge lamp (hv = 21.22 eV). The atomic force microscopy (AFM) of HTLs was measured by using SHIMADZU SPM-9700SPM-9700 under the tapping mode. X-ray diffraction (XRD) characterization of perovskite layers was performed in a D2 Phaser instrument with a Cu Kα (λ=0.154 nm) radiation. The surface morphology of perovskite layers was acquired by scanning electron microscopy (SEM, Philips XL30 FEG). The steady-state photoluminescence (PL) and time-resolved PL spectra of the bi-layered perovskite/dopant-free HTM films were determined with a FLS980 spectrofluorometer from Edinburgh. All theoretical optimizations were done at B3LYP/def2-SVP level with Grimme's D3BJempirical dispersion correction, using Gaussian09 program.

Device fabrication and characterization.
ITO glass substrates were cleaned by sonication with detergent, deionized water, acetone and ethanol for 15 min, sequentially, which are then dried in the dry oven. The cleaned glass substrates were transferred into glove box for following film preparation. Solutions of PPE1 and PPE2 HTMs in chlorobenzene (CB, 2 mg/mL) were spin-coated on the ITO at 6000 rpm for 30 s as the HTLs. The fabricated HTLs were then thermally annealed on the hotplate at 150 ℃ for 10 min. Perovskite precursor solution was prepared by dissolving 171.9 mg FAI, 507.1 mg PbI2, 22.4 mg MABr, 73.4 mg PbBr2 in a 1 mL DMF/DMSO mixed solution (V/V, 5/1), in which 89 μL CsI solution (1.5 M in DMSO) and 1% in volume of NH4BF4 solution (1M in the mixed DMF/DMSO solution, (V/V, 5/1) were added. The perovskite solution was spin-coated on the HTL at 1000 rpm for 5 s and 6000 rpm for 30 s, respectively. Note that 110 μL of anti-solvent (CB) was quickly dropped at last 5 s of the film fabrication, followed by a thermal annealing at 100 ℃ for 30 min. Afterwards, PCBM solution (20 mg/mL in CB) was spin-coated on the perovskite layer at 1500 rpm for 40 s. Finally, 100 nm Ag electrode was evaporated under high vacuum. The device area is 10 mm 2 .
For passivated devices, the processing of PPE2 HTM and perovskite layer is similar to that described in above. The doped PTAA solution was prepared by adding 1 wt% F4-TCNQ (1mg/mL in CB) into PTAA solution (5 mg/mL in toluene), and then heated at 70℃ overnight.
Doped PTAA solution was spin-coated on the ITO substrates at 4000 rpm for 30 s, and then the samples were heated at 150 °C for 10 min. After depositing the perovskite layer onto the polymer HTMs (Dopant-free PPE2 and doped PTAA), 100 µl of PEAI (2 mg/mL in IPA) was spin-coated onto the as-prepared perovskite films at the speed of 5000 rpm for 30s. The film then was heated at 100 °C for 10 min. Noted that the PEAI solution needed to be drop on the perovskite film quickly, and standing for 1-2 s before spin-coating. Then 35 µl PC61BM (20 mg/mL in CB) was spin coated on the top of perovskite film at 1500 rpm for 30 s. Finally, 6 nm BCP and 100 nm Ag electrode were evaporated under high vacuum. The device area is 10 mm 2 .
The J-V characteristic curves were chacracterized under the AM 1.5G sunlight simulated by a simulator of Enlitech, SS-F5, Taiwan with a Keithley 2400 source meter. The steadystate output measurements were carried out without pre-illumination and UV filter. EQE spectra were collected with an EQE measurement system from EnLi Technology (Taiwan).
Compound 1 was synthesized as reported. 1
The organic layer was collected, washed with water and dried with anhydrous Na2SO4. After concentration using a rotary evaporator, the crude product was purified by column chromatography on the silica gel using petroleum ether/DCM (V/V:10/1) as the eluent to obtain 2 as a red solid (0.90 g, 70.9%). 1

Synthesis of PPE1.
A mixture of compound 2 (0.40 g, 0.55 mmol), 1,4-diethynylbenzene (0.07 g, 0.55 mmol), CuI (0.03 g, 0.17 mmol) and Pd(PPh3)4 (26 mg, 0.04 mmol) in iPr2NH (20 mL) and dry THF (40 mL) was heated to 70 ℃ under nitrogen for 72 h. Then the mixture was cooled to room temperature, extracted with DCM and washed with water. After concentrated, the solution was dropped into methanol, and the precipitation was filtrated and washed with methanol. Further purification of the crude products was conducted by exhaustive Soxhlet extraction with methanol (50 mL), acetone (50 mL), and hexane (50 mL) for 24 h successively. The product was collected after being dried under vacuum at 50 ℃ for 24 h and afforded as a yellow solid (0.12 g, 31.6%). 1