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Explaining the Fill‐Factor and Photocurrent Losses of Nonfullerene Acceptor‐Based Solar Cells by Probing the Long‐Range Charge Carrier Diffusion and Drift Lengths
Author(s) -
Tokmoldin Nurlan,
Vollbrecht Joachim,
Hosseini Seyed Mehrdad,
Sun Bowen,
PerdigónToro Lorena,
Woo Han Young,
Zou Yingping,
Neher Dieter,
Shoaee Safa
Publication year - 2021
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.202100804
Subject(s) - organic solar cell , materials science , photocurrent , diffusion , photovoltaic system , acceptor , charge carrier , short circuit , optoelectronics , open circuit voltage , range (aeronautics) , voltage , physics , electrical engineering , condensed matter physics , polymer , thermodynamics , quantum mechanics , composite material , engineering
Organic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open‐circuit voltage loss and fill‐factor, with state‐of‐the‐art OSCs rarely displaying fill‐factor of 80% and above. The fill‐factor of transport‐limited solar cells, including organic photovoltaic devices, is affected by material and device‐specific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long‐range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady‐state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices—including the high efficiency nonfullerene acceptor blends—and show that the diffusion length of the free carriers provides a good correlation with the fill‐factor. It is, finally, concluded that most state‐of‐the‐art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor.