A research team led by Peng Wang, Niansheng Xu, and Yaohang Cai from Zhejiang University published a research paper titled "Triphenylamine-ethylenedioxythiophene copolymers for perovskite solar cells: impact of substituent type and alternation" in the journal Energy & Environmental Science. Lifei He is the first author, and Peng Wang, Niansheng Xu, and Yaohang Cai are co-corresponding authors.
Key Highlights: Three triphenylamine-ethylenedioxythiophene alternating copolymers were synthesized using a direct arylation polycondensation method. Variations and combinations of substituents resulted in differences in molecular weight, glass transition temperature, highest occupied molecular orbital energy level, and film morphology. Compared to a reference polymer containing only one substituent, the synergistic use of different substituents resulted in polymer semiconductor composite films with smoother morphology and higher conductivity. Using this unique p-type polymer semiconductor, perovskite solar cells with an average power conversion efficiency of 25.4% were fabricated. These cells also exhibited excellent stability under thermal storage conditions of 85°C and operating conditions of 45°C.
Lead halide perovskites are attractive light-absorbing materials for solar cells due to their tunable optical band gaps, extended excited-state lifetimes, and availability of inexpensive raw materials. When fabricating high-efficiency n-i-p perovskite solar cells, donor-acceptor polymer semiconductors typically have narrow optical band gaps, which can lead to depletive absorption when used in PSCs. Therefore, developing cost-effective, efficient p-type polymer semiconductors with excellent figures of merit (e.g., energy levels, hole transport, and mechanical properties) is crucial for improving the performance of formal perovskite solar cells. To date, a wide range of polymer semiconductors have been evaluated for use in PSCs; however, materials that offer both high efficiency and stability at low cost remain scarce.
To this end, a team led by Peng Wang, Niansheng Xu, and Yaohang Cai from Zhejiang University synthesized three alternating triphenylamine-ethylenedioxythiophene copolymers via direct arylation polycondensation. The first polymer possesses three methyl groups on the non-backbone phenyl ring, the second has a single 2-octyldodecyloxy group, and the third contains a combination of half a trimethyl group and half a 2-octyldodecyloxy group. Variations and combinations of different substituents lead to differences in molecular weight, glass transition temperature, highest occupied molecular orbital energy level, and film morphology. Compared to a reference polymer with only one substituent, the synergistic use of different substituents results in polymer semiconductor composite films with smoother morphology and higher conductivity. Using the alternating polymer p-TPA3Me-E-TPAOOD-E, perovskite solar cells achieved an average power conversion efficiency of 25.4%. This surpasses cells fabricated under the same conditions using spiro-OMeTAD (24.1%), PTAA (20.5%), p-TPA3Me-E (23.2%), and p-TPAOOD-E (23.3%). Importantly, cells based on p-TPA3Me-E-TPAOODE exhibited excellent stability under thermal storage at 85°C and operating conditions at 45°C.
This study demonstrates that variations in substituents lead to differences in the HOMO energy levels of the polymer semiconductor, which subsequently influences the degree of air oxidative doping and the hole extraction rate from the perovskite exciton state. These copolymers exhibit hole density-dependent mobility and conductivity. This study provides new insights into design strategies for high-performance polymer semiconductors. Future research should focus on exploring alternative p-type dopants for these organic semiconductors to further enhance the performance of PSCs.
Source：DOI: 10.1039/D4EE03316G
https://pubs.rsc.org/en/content/articlelanding/2024/ee/d4ee03316g