Accurate Measurements of Charge Generation in Bulk Heterojunction Solar Cells with Overpulse TDCF Charge Extraction

Organic solar cells (OSCs) are promising as printable and massively scalable solar energy sources, and their efficiencies are approaching 20%. The charge generation and recombination processes in bulk heterojunction active layers govern device performance in OSCs but separating these two processes are difficult due to the morphological complexity. Time delay collection field (TDCF) is an advanced charge extraction technique that quantitatively separates charge generation and recombination processes at operation-relevant fields, but device capacitances limit the effectiveness of this technique. We introduce an altered experiment, ‘Overpulse TDCF’ that overcomes this issue to successfully extract all generated charges prior to recombination in a BHJ. In this technique, high a voltage pulse accelerates electrode charging by nearly an order of magnitude, thus extracting charges more swiftly than they can recombine. This enables accurate quantification of charge generation efficiency in systems that exhibit even the most diffusive transport as well as studying charge recombination at early times. We demonstrate this capability on PCDTBT:PCBM devices where recombination occurs even at 100s of ns. Using Overpulse TDCF enables the field-independent charge generation properties to be revealed in this system and accurate field dependence of CT-state separation in other systems. Moving forward, this technique will enable understanding the detailed processes of charge generation and recombination to work toward further progress in materials design and device fabrication.