Correlative Influence of Charge Carrier Recombination and Extraction Processes on the Fill Factor in Bulk Heterojunction Organic solar Cells
DOUGLAS YEBOAH a, Jai Singh a
a Charles Darwin University
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Benidorm, Spain, 2018 May 28th - 31st
Organizers: Emilio Palomares and Rene Janssen
Oral, DOUGLAS YEBOAH, presentation 004
DOI: https://doi.org/10.29363/nanoge.hopv.2018.004
Publication date: 21st February 2018

A new figure of merit (FOM) as a ratio of the non-geminate recombination rate to the extraction rate of free charge carriers in bulk heterojunction (BHJ) organic solar cells (OSCs) is derived as a function of the effective carrier mobility, effective carrier concentration, active layer thickness, bimolecular recombination coefficient and internal voltage. We further derive a relation between the FOM and fill factor (FF) and find that the FF increases with a decrease in FOM, which means an increase in multiple material parameters such as effective mobility and dielectric constant, and a decrease in light intensity, active layer thickness and dimensionless reduction prefactor. This shows a strong correlation between extraction and recombination, which controls the performance of BHJ OSC and particular FF which decreases rapidly from ~0.85 to ~0.30 in MDMO-PPV:PC61BM and ~0.76 to ~0.20 in PCDTBT:PC60BM if the FOM increases from ~0.05 to ~10 in both blends. The results indicate that the influence of charge carrier extraction dominates at high FF and high power conversion efficiency (PCE) whereas the influence of charge carrier recombination dominates at low FF and low PCE, hence further improvement in the synthesis of new materials for the fabrication of BHJ OSCs is required to reduce carrier recombination as well as increase carrier extraction to the respective electrodes.

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