Quantifying the optimal thickness in polymer:fullerene solar cells from the analysis of charge transport dynamics and photoabsorption

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/PC₇₁BM, ∼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.