Transport of Hot Carriers in Polymer-based Organic Solar Cells

Almantas Pivrikas^a

^aThe University of Queensland, Bld 68, Level 9, Cooper Rd., Brisbane, 4072, Australia

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, Almantas Pivrikas, presentation 170
Publication date: 1st March 2014

The photovoltaic performance of organic solar cells is primarily governed by the excitonic nature of organic materials. Mechanisms of exciton dissociation into mobile charge carriers and charge transfer states at the interface between the donor and acceptor remain of intense scientific research. Hot excitons have been proposed to benefit the dissociation efficiency. And it was shown that hot CT excitons could enhance the photocurrent due to the re-excitation in the CT exciton manifold. However, most of these studies were focused on spectroscopic evidence while the critical question regarding the impact of hot excitons to the performance of actual devices remains.

First, we have studied the impact of excess photon energy on excitonic dissociation efficiency in operational and efficient solar cells. We show that hot excitons are not beneficial for operational solar cells, because Internal Quantum Efficiency (IQE) is independent of photon energy [1]. IQE is observed to be essentially flat with no appreciable benefit from higher-energy photons. We demonstrate that methodology of IQE measurement is crucially important for unambiguous result interpretation and clarify the impact of optical interference effects and light absorption in the non-active layers in thin operational devices. An attention is directed to the importance of treating an OSC as a complex optical cavity.

Second, we show the impact of excess photon energy on charge transport in several efficient polymer-based photovoltaic devices. Charge extraction is observed to be independent on photon energy suggesting that excitons lose their excess energy at times scales which are much shorter than the time scales of charge extraction [3]. Moreover, we show that the charge carrier thermalization effects can be neglected in operational solar cells, while charge carrier density loss during transport due to capture by trap states is crucial for efficient device operation. The impact of these results in regards to the present knowledge of charge transport is discussed.

Finally, a number of novel techniques developed for the purpose of unambiguous electron and hole mobility measurements in operational devices is presented. A critical conclusion arising from our results demonstrates, in contrasts the commonly accepted knowledge, that efficient solar cells do not need balanced charge carrier mobilities [3]. This message is further elaborated and clarified that balanced mobility become important only in materials with strong non-Langevin recombination. The impact of Langevin and non-Langevin recombination on the photocurrent and fill factors is demonstrated in actual solar cells at near to operational conditions.

[1] A. Armin, et al. A. Pivrikas, Nature Materials 12, 593 (2013). [2] B. Philippa, et al. A. Pivrikas, Science, submitted (2014). [3] A. Armin, et al. A. Pivrikas, Advanced Energy Materials, Published online (2014).

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