Quantifying the optimal thickness in polymer:fullerene solar cells from the analysis of charge transport dynamics and photoabsorption. Issue 3 (4th January 2022)

• Record Type: Journal Article
• Authors: Li, Shaoxian; Hamada, Fumiya; Nishikubo, Ryosuke; Saeki, Akinori
• Is Part Of: Sustainable energy & fuels. Volume 6:Issue 3(2022)
• Page Start: 756
• Page End: 765
• Publication Date: 2022-01-04
• Subjects: Renewable energy sources -- Periodicals; Fuel cells -- Periodicals; Electric batteries -- Periodicals; Electrochemistry -- Periodicals; 660.297
• DOI: 10.1039/d1se01228b
• Languages: English
• ISSNs: 2398-4902

Abstract

Abstract : We provide a semi-empirical equation that quantifies the optimal layer thickness in bulk heterojunction organic solar cells, which is based on time-of-flight and time-resolved microwave conductivity measurements and photoabsorption of a film. Abstract : The optimal photoactive layer thickness ( L opt ) of bulk heterojunction (BHJ) organic solar cells (OSCs) is typically below 200 nm, which is unlikely suitable for large scale production. Toward increasing the L opt, a deeper understanding of L opt is indispensable. Here, we propose a semi-empirical model for the quantitative determination of L opt in OSCs, which considers the limited charge carrier transport length, space charge accumulation, and photoabsorption. Three widely studied BHJs of polymer:fullerene OSCs (P3HT, PffBT4T, and PCPDTBT each blended with PCBM ) were used for the validation of this model. Simultaneous measurements of time-of-flight (TOF) and time-resolved microwave conductivity (TRMC) revealed the electron/hole mobility relaxation and effective carrier diffusion length. The space charge effect on L opt was examined by considering the electron/hole mobility balance. In addition, the photoabsorption effect was incorporated by calculating the effective absorption coefficient under the standard solar irradiation. Based on this model, L opt was calculated to be ∼100 nm for PCPDTBT /PC71 BM, ∼150 nm for P3HT /PCBM, and ∼300 nm for PffBT4T /PCBM, showing good consistence with the experimental values. The effectiveness of our model was further supported by solar cell capacitance simulator (SCAPS) calculations. Thus, our work provides a feasible method for quantifying L opt in OSCs and an insight into the charge relaxation dynamics.