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

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-empirica...

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Bibliographic Details
Published inSustainable energy & fuels Vol. 6; no. 3; pp. 756 - 765
Main Authors Li, Shaoxian, Hamada, Fumiya, Nishikubo, Ryosuke, Saeki, Akinori
Format Journal Article
LanguageEnglish
Published London Royal Society of Chemistry 01.02.2022
Subjects
Absorptivity
Capacitance
Carrier transport
Charge transport
Current carriers
Diffusion length
Empirical analysis
Fullerenes
Heterojunctions
Hole mobility
Irradiation
Mobility
Photoabsorption
Photovoltaic cells
Polymers
Solar cells
Space charge
Thickness
Time measurement
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Summary: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.
ISSN:2398-4902
2398-4902
DOI:10.1039/D1SE01228B