Nonradiative Recombination via Charge‐Transfer‐Exciton to Polaron Energy Transfer Limits Photocurrent in Organic Solar Cells

• Published in: Advanced energy materials 2022-05, Vol.12 (19), p.n/a
• Main Authors: Khan, Saeed‐Uz‐Zaman, Gui, Manting, Liu, Xiao, Giebink, Noel C., Kahn, Antoine, Rand, Barry P.
• Format: Article
• Language: English
• Subjects: Charge transfer; charge‐transfer‐states; Energy; energy loss; Energy transfer; Excitation spectra; Excitons; Förster energy transfer; non‐radiative recombination; Organic materials; organic solar cells; Photoelectric effect; Photoelectric emission; Photovoltaic cells; Polarons; Quantum efficiency; Solar cells
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.202200551

A recombination and exciton loss mechanism is reported in organic solar cells involving energy transfer between charge transfer (CT) excitons and polarons, impacting photocurrent generation, particularly in the near‐infrared where polaronic transitions typically reside. This process sets a low‐energy cut‐off in the external quantum efficiency spectrum of an excitonic donor/acceptor interface, determined by the low‐energy polaron absorption peak and the CT state reorganization energy. Furthermore, this process explains the deviation from unity and bias dependence of the CT state's internal quantum efficiency at low photon energies. This process is demonstrated in a variety of systems and it is hypothesized that CT state to polaron energy transfer recombination may be responsible for a share of nonradiative recombination in all organic photovoltaics and can explain numerous experimentally observed device trends regarding photocurrent generation and energy losses. Overall, this work enhances the understanding of photophysical processes in organic materials and allows the design of systems that can avoid this recombination pathway. The authors describe an intrinsic loss pathway at organic donor/acceptor interfaces. It is found that a charge‐transfer‐(CT)‐state‐mediated recombination process involving resonant excitation energy transfer from the lowest energy CT state to near‐infrared polaron states contributes to non‐radiative energy loss. This recombination process sets a donor‐material‐dependent fundamental cut‐off to the excitation energy that can produce photocurrent.