Dresden 2011 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 15: Joint Session: Organic Semiconductors I: Solar Cells A

HL 15.9: Talk

Monday, March 14, 2011, 16:30–16:45, ZEU 222

Role of drift and diffusion in organic solar cells measured by transient photocurrents — •Wolfgang Tress, Karl Leo, and Moritz Riede — Institut für Angewandte Photophysik, TU Dresden, Dresden, Germany

The role of the electric field and the contribution of diffusion to photocurrents are controversially discussed in the case of organic solar cells. We investigate flat heterojunction organic solar cells with systematically varied barriers at the contacts that are created by a HOMO offset between donor and hole transport layer. These barriers lead to S-kinks in the IV curve and to strong imbalanced and even reversed forces on charge carriers resulting from the field on the one hand and the concentration gradient on the other hand. By applying a rectangular illumination signal with varied light intensities, we observe in the photocurrent transients a pile-up of charge carriers at an extraction barrier. If an injection barrier is present, there exists a region in the IV curve where charges are extracted against the electric field by a strong diffusion gradient. These findings are based on the analysis of transient current data in the µs regime using electrical simulations based on a drift-diffusion model. The simulations show that observed overshoots in the photocurrent transients result from these imbalance effects and give new insights into the working principle of organic solar cells.