The Influence of Contacts in Bulk Heterojunction Solar Cells: A Drift-diffusion Approach
Ronald Österbacka a, Mathias Nyman a, Oskar J. Sandberg a
a Abo Akademi University, Porthaninkatu 3, Turku, 20500, Finland
International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Ecublens, Switzerland, 2014 May 11th - 14th
Organizers: Michael Graetzel and Mohammad Nazeeruddin
Oral, Oskar J. Sandberg, presentation 117
Publication date: 1st March 2014

One major loss mechanism in bulk heterojunction (BHJ) solar cells is reduced Fill Factors due to imperfect contacts, the perhaps most prominent feature being the appearance of an S-kink in the JV curves under illumination. An imperfect contact could for example be caused by unintentional oxidation at the electrode giving rise to reduced surface recombination, increased injection barrier, or a doping profile (of minority carrier) in the vicinity of the contact. In BHJ solar cells the driving force for charge extraction is ideally provided by the built-in voltage. However, imperfect contacts are known to limit both the open-circuit voltage and the built-in voltage. It is thus crucial to obtain a comprehensive understanding of how imperfect contacts alter the driving force for charge extraction, giving in some cases rise to S-shaped JV curves.

 

In this work the influence of imperfect contacts on the driving force for charge extraction in BHJ solar cells has been numerically and analytically investigated in terms of a drift-diffusion model. The effect of increased injection barriers, reduced surface recombination for majority carriers, and interfacial minority carrier doping at the electrodes has been clarified and compared to the case with ideal contacts.

 

An analytical approximation for the open-circuit voltage, explaining the injection barrier dependence and the role of the built-in voltage, could be obtained. Furthermore, two distinctly different underlying mechanisms leading to S-shaped features were found, both leading to an effective shift of the built-in voltage. In case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S-kink is due to an induced diffusion potential. In case of minority carrier doping in the vicinity of the contact, the S-kink results from band-bending caused by the fixed (or trapped) space charge. We furthermore found analytical expressions describing the effective reduction of the built-in voltage, providing means of how to quantify and distinguish between the different mechanisms.



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