Understanding the open-circuit voltage in organic solar cells: The influence of contacts, recombination and doping

Anton Sundqvist^a, Mathias Nyman^a, Oskar J. Sandberg^a, Ronald Österbacka^a

^aAbo Akademi University, Porthaninkatu 3, Turku, 20500, Finland

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, Oskar J. Sandberg, presentation 226
Publication date: 5th February 2015

In order to achieve commercialization of organic solar cells an improved understanding of the underlying physics leading to reduced device performance is needed. For example, to optimize the open-circuit voltage in bulk heterojunction (BHJ) solar cells it is crucial to understand the influence of the contacts.In the ideal case, the photo-generation is balanced by recombination at open-circuit conditions and the open-circuit voltage is determined by the photo-induced quasi-Fermi level difference between electrons and holes. However, in BHJ solar cells, especially in the case of un-optimized electrodes, the open-circuit voltage has been found to saturate at low temperatures or high light intensities[1-2] and/or become limited by the electrode work functions,[2-3] leading to an injection barrier-dependence. The injection-barrier dependence is typically attributed to an increased surface recombination of minority carriers, i.e. carriers extracted at the wrong electrodes. However, the exact injection barrier-dependence of the open-circuit voltage is still under debate. Furthermore, analytical expressions explaining how the open-circuit voltage is influences by a reduced surface recombination for minority carriers (i.e. increased degree of charge selectivity) at the contacts are still lacking. In this work the influence of contacts on the open-circuit voltage in organic solar cells have been investigated. Guided by a numerical device model, based on a Poisson-drift-diffusion approach, the effect of injection barriers, contact recombination of minority carriers, bulk recombination, and doping have been clarified. We derive analytical expressions explaining the injection barrier dependence of the open-circuit voltage for the various cases and depending on the contact selectivity, bulk recombination, and doping level different working regimes are identified. The underlying physical mechanisms leading to reduced open-circuit voltages are clarified and the analytical predictions are verified by the numerical drift-diffusion simulations.

[1] Kirchartz, T.; Deledalle, F.; Shakya Tuladhar, P.; Durrant, J. R.; Nelson, J; J. Phys. Chem. Lett. 2013, 4, 2371−2376. [2] Rauh, D.; Wagenpfahl, A.; Deibel, C.; Dyakonov, V.; Appl. Phys. Lett. 2011, 98,133301. [3] Mihailetchi, V. D.; Blom, P.W.M.; Hummelen, J. C.; Rispens, M. T.; J. Appl. Phys. 2003, 94, 6849.

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