Publications associated with Advanced Functional Materials and Devices Group

Open-circuit voltage and effective gap of organic solar cells

Advanced Functional Materials 23 (2013) 5814-5821

J Widmer, M Tietze, K Leo, M Riede

The open-circuit voltage (V OC ) of an organic solar cell is limited by the donor-acceptor material system. The effective gap E g eff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for V OC , which is only reached for T → 0 K. This relation is confirmed for a number of small-molecule bulk heterojunction p-i-n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of V OC is identical to E g eff . Furthermore, the influence of the hole transport material in a p-doped hole transport layer and the donor-acceptor mixing ratio on this limit V 0 is found to be negligible. Varying the active material system, the quantitative relation between V 0 and E g eff is found to be identity. A comparison of V 0 in a series of nine different donor-acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material. The effective gap of a photovoltaic donor-acceptor system equals the open-circuit voltage extrapolated to temperature zero. The extrapolation is independent of the illumination intensity, and material variations in the doped transport layers do not affect the measurement result. This is shown for bulk-heterojunction devices with different mixing ratios and with small-molecular materials from various classes. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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