Accurate measurement of charge generation at interfaces of organic solar cell devices printed from non-halogenated solvents

Awwad Nasser Alotaibi

doi:10.7273/000004560

Organic solar cells (OSCs) are promising as printable and massively scalable solar energy sources. The highest performing device architecture, the bulk heterojunction (BHJ) is constructed by printing a single active layer consisting of a mixture of organic electron donating and accepting molecules, and its efficiencies are approaching 20%. The charge generation and recombination processes in BHJ active layers govern device performance in OSCs but separating these two processes is 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, a high 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 OSC 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. On the other hand, solvent processing conditions of BHJ active layers have shown to govern the final OSC nanostructure. In order to manipulate the morphology for a better understanding of its effect on charge generation, we have changed the processing solvent during the fabrication of the investigated systems in this work by using two different solvents, which plays an important role in defining the overall morphology. Where we used halogenated and non-halogenated solvents to track the effect of the morphology on the charge generation and recombination, the non-halogenated solvent gave us a new record with a power conversion efficiency of 7.7%. We correlate the device physics using OP-TDCF with nanomorphology via x-ray techniques, expanding our understanding of how the two aspects are interconnected. 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.

Accurate measurement of charge generation at interfaces of organic solar cell devices printed from non-halogenated solvents

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