An Effective Area Approach to Model Degradation in Organic Solar Cells

Beat Ruhstaller^a^,^b, Simon Züfle^a^,^b, Stephane Altazin^b, Martin Neukom^b, Ton Offermans^c

^aICP, ZHAW, Wildbachstr. 21, Winterthur, 8401, Switzerland

^bCSEM Muttenz, Tramstr. 99, Muttenz, 4132, Switzerland

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

Poster, Martin Neukom, 047
Publication date: 5th February 2015

We find that in standard P3HT:PCBM organic solar cells comprising a PEDOT:PSS anode exposed to humid air, the ingress of water through the PEDOT:PSS layer is the main reason for device failure, leading to an oxidation of the aluminum cathode. The developing oxide layer gives rise to an injection/extraction barrier and leads to a rapid decrease of the device current [1]. This lateral degradation is mainly governed by the employed hole transport material, with its rate being determined by the material’s water diffusion constant. The decay of the short-circuit photocurrent can be analysed to largely follow a square-root time-dependence. In order to study the lateral degradation we exploit various steady-state and transient measurement techniques in combination with numerical simulations based on a previously established charge drift-diffusion model [2]. We attribute the lateral loss in contact area to the local formation of an insulating oxide layer at the electrode interface arising from the water ingress. The resulting energy barrier leads to the substantial reduction of steady-state conduction current in the aged cell, while transient charge displacement current still flows. With an effective area approach we model any steady or transient state of the device during degradation as a combination of a “good” and a “bad” device in very good agreement with experimental data. We thus have identified the underlying and dominating degradation mechanism.
Figure 1. Transient photocurrent during degradation
1: Voroshazi, E.; Verreet, B.; Buri, A.; Müller, R.; Di Nuzzo, D.; Heremans, P.. Influence of cathode oxidation via the hole extraction layer in polymer:fullerene solar cells. Organic Electronics 2011, 12,736-744. 2: Neukom, M.; Züfle, S.; Ruhstaller, B.. Reliable extraction of organic solar cell parameters by combining steady-state and transient techniques. Organic Electronics 2012, 13, 2910-2916.

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