Open circuit voltage and efficiency in ternary organic photovoltaic blends

Please do not adjust margins Please do not adjust margins Organic bulk heterojunction solar cells based on ternary blends of two donor absorbers and one acceptor are investigated by experiments and modeling. The commonly observed continuous tunability of the open circuit voltage í µí±½ í µí± ¶í µí±ª with donor1:donor2 ratio can quantitatively be explained as quasi-Fermi level splitting due to photocreated charges filling a joint density of states that is broadened by Gaussian disorder. On this basis, a predictive model for the power conversion efficiency that accounts for the composition-dependent absorption and the shape of the current-voltage characteristic is developed. When all other parameters, most notably the fill factor, are constant, we find that for state-of-the-art absorbers, having a broad and strong absorption spectrum, ternary blends offer no advantage over binary ones. For absorbers with a more narrow absorption spectrum ternary blends of donors with complementary absorption spectra, offer modest improvements over binary ones. In contrast, when, upon blending, transport and/or recombination kinetics are improved, leading to an increased fill factor, ternaries may offer significant advantages over binaries. Solar cells based on blends of organic semiconductors almost routinely reach power conversion efficiencies (PCE) in the 10% range. One of the strategies that has been proposed to push this limit further upward is the use of so-called ternary blends in which two instead of one absorbing donor material is blended with the electron acceptor. Systematic improvement of PCE by ternary blending has been reported. Despite the experimental success, the current understanding of both the PCE enhancement and the underlying puzzling tunability of the open circuit voltage Voc is limited and essentially qualitative. In this contribution we develop and validate a quantitative and predictive framework for both the Voc and PCE of ternary organic solar cells, which allows us to propose design rules. In contrast to current wisdom, we demonstrate that PCE enhancement in ternary blends hardly results from optimizing the overlap with the solar spectrum but predominantly from synergetic improvement of the charge transport and recombination kinetics. The model can be easily extended to include more detail and will allow other researchers to upfront evaluate the photovoltaic potential of 'any' ternary material combination. † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here. See Please do not adjust margins Please do not adjust margins …