Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)
Publication date: 11th May 2021
One of the most important photocurrent loss mechanisms limiting the power-conversion efficiency in all solar cells is trap-assisted recombination caused by localized sub-gap states. The presence and relevance of this first-order recombination in organic photovoltaic devices is still a subject of current debate, hindering the field as it seeks to push the boundaries of efficiency towards inorganic and perovskite semiconductor counterparts. In this work we conduct wide dynamic range, sensitive intensity-dependent photocurrent measurements combined with one-dimensional drift-diffusion simulations. Our key finding is that first-order, trap-assisted recombination appears to be universally present in a large variety of fullerene and non-fullerene acceptor systems - including state-of-the-art PM6:BTP-eC9 achieving above 15 % power conversion efficiency. The trap states are found to be situated ~ 0.35-0.6 eV below the transport level edges of acceptor: donor blends with trap densities lying between 1016-1017 cm-3. We show that the trap-assisted recombination via these deep sub-gap states induces not only losses in the photocurrent but also limits the open-circuit voltage leading to ideality factors between 1 and 2. Hence, our findings deliver new insight into the role and nature of trap states in organic light-harvesting devices, and shed new light on the complexity and variability of ideality factors in solar cells.
This work was supported by the Sêr Cymru Program through the European Regional Development Fund, Welsh European Funding Office, and Swansea University strategic initiative in Sustainable Advanced Materials. A.A. is a Sêr Cymru II Rising Star Fellow and P.M. a Sêr Cymru II National Research Chair. N.Z. was supported by a studentship through the Sêr Cymru II Program.
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