Connecting charge transfer kinetics to device parameters of a narrow-bandgap polymer-based solar cell

Abstract

To achieve intrinsically light-weight flexible photovoltaic devices, a bulk-heterojunction-type active layer with a narrow-bandgap polymer is still considered as one of the most important candidates. Therefore, detailed information about the charge transfer efficiency from a photo-excited species on an electron-donating polymer to an electron acceptor is an important factor, given that it is among the most fundamental quantitative measures to understand the solar power conversion efficiency, in particular at the initial stage followed by primary exciton formation. To obtain accurate information in this regard, wide-range acceptor concentration-dependent transient absorption spectroscopy with femtosecond laser pulse excitation was performed using a representative narrow-bandgap polymer, commonly known as PTB7. The investigated acceptor concentration range covered was from 0.01 wt% up to 10 wt%, in addition to a 0 wt% pristine polymer sample and a sample with a conventional acceptor concentration of 60 wt%, which is important for high efficiency. From the kinetic data, an almost two orders of magnitude faster acceptor-induced charge transfer rate constant in addition to the native primary exciton lifetime of about 100 picoseconds could be extracted. These data were used to verify the suggested kinetic model and compare with device properties that show no meaningful loss during the extraction of photo-generated charge carriers.

Comparing Donor- and Acceptor-Originated Exciton Dynamics in Non-Fullerene Acceptor Blend Polymeric Systems

“…This means that the system suffers an inadvertent loss of approximately 7% owing to, e.g., relatively fast unimolecular recombination. A similar instinctive loss in a narrowband-gap polymer and fullerene blend system was previously reported [38]. Contrarily, when the PE(P) is dominant, the situation is different owing to the insensitive behavior of P upon the incorporation of I and/or I solid .…”

Spatial Balance of Photogenerated Charge Carriers in Active Layers of Polymer Solar Cells

“…Through this conversion, exciton diffusion-controlled and/or monomolecular-recombination-induced losses can occur. However, those losses are not explicitly included in the TMM-aided J SC calculation because many research groups support that the CC photogeneration is very effective [26], and monomolecular recombination is also by far depressed in such a well-optimized BHJ blend system [27]. These are also the reasons why spatial balancing must be a thickness-dependent loss process, which will be addressed later in this study.…”