The effect of composition on energetics and kinetics at open-circuit in OPV

Scot Wheeler^a, James Durrant^a

^aImperial College London, United Kingdom, South Kensington, Londres, Reino Unido, United Kingdom

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Scot Wheeler, presentation 275
Publication date: 5th February 2015

Organic bulk-heterojunction photovoltaics remain a viable technology for flexible and stable photovoltaic systems, manufactured on a large area, with an efficiency that makes them competitive with current and emerging alternatives. Morphology plays a key role in device performance, influencing charge transport, charge recombination as well as energetics. Therefore, understanding what constitutes a ‘good’ morphology is of importance for further development of these systems. One way to control the morphology is by varying the ratio of donor and acceptor in the blend. We have reported before on the pBTTT:PC60BM system, how increases in PCBM loading leads to PCBM aggregation and larger fullerene domains An energetic change in PCBM LUMO was observed which favoured charge generation, but reduced the VOC . However, such a change could also lead to a reduction in the recombination dynamics, leading to an increase in VOC. The Charge extraction (CE) and transient photovoltage (TPV) techniques are commonly used to explore the in-situ density of states and recombination dynamics, especially at VOC. Through the use of these techniques, we aim to explore the competition between energetics and kinetics which arise from changes in blend composition, which lead to changes in the VOC. A variety of systems have been investigated, from crystalline polymers such as DPPTT-T to amorphous polymers such as PBDTTT-CT, at a range of compositions. The effect of some common additives such as DIO and CN has also been investigated. In general, through the increased amount of PCBM or addition of additive, we see increased aggregation of the PCBM and appearance of the exciton photoluminescence. With this we expect a reduction in the quasi-Fermi level splitting due to a lowering of the PCBM LUMO, which opposes a contribution from longer carrier lifetimes.

Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney, M.; Stingelin, N.; Durrant, J. R., Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells. Chemical Science 2012, 3 (2), 485-492. Hawks, S. A.; Deledalle, F.; Yao, J.; Rebois, D. G.; Li, G.; Nelson, J.; Yang, Y.; Kirchartz, T.; Durrant, J. R., Relating Recombination, Density of States, and Device Performance in an Efficient Polymer:Fullerene Organic Solar Cell Blend. Advanced Energy Materials 2013.

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