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Recombination Losses Above and Below the Transport Percolation Threshold in Bulk Heterojunction Organic Solar Cells
Author(s) -
Yazmaciyan Aren,
Stolterfoht Martin,
Burn Paul L.,
Lin Qianqian,
Meredith Paul,
Armin Ardalan
Publication year - 2018
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201703339
Subject(s) - materials science , fullerene , percolation (cognitive psychology) , organic solar cell , heterojunction , percolation threshold , polymer solar cell , electric field , chemical physics , acceptor , charge carrier , recombination , percolation theory , electron mobility , condensed matter physics , optoelectronics , energy conversion efficiency , conductivity , electrical resistivity and conductivity , polymer , physics , chemistry , composite material , biochemistry , quantum mechanics , neuroscience , gene , biology
Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second‐order recombination losses dominate the shape of the current density–voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric‐field dependence of first‐order losses, which includes electric‐field‐dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first‐ and second‐order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.