| Summary: | The open-circuit voltage (<em>V</em><sub>OC</sub>) of an organic solar cell is limited by the donor-acceptor material system. The effective gap <em>E</em><sub>g</sub><sup>eff</sup> between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for <em>V</em><sub>OC</sub>, 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 <em>V</em><sub>OC</sub> is identical to <em>E</em><sub>g</sub><sup>eff</sup>. Furthermore, the influence of the hole transport material in a p-doped hole transport layer and the donor-acceptor mixing ratio on this limit <em>V</em><sub>0</sub> is found to be negligible. Varying the active material system, the quantitative relation between <em>V</em><sub>0</sub> and <em>E</em><sub>g</sub><sup>eff</sup> is found to be identity. A comparison of <em>V</em><sub>0</sub> 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. © 2013 WILEY-VCH Verlag GmbH and Co.
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