Composition Dependent Open-Circuit Voltage and Efficiency in Ternary Blend Bulk Heterojunction Solar Cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Nikolaos Felekidis, presentation 119
Publication date: 5th February 2015

The tunability of the open-circuit voltage (Voc) in ternary blend bulk heterojunction (BHJ) solar cells has been demonstrated for a large number of material systems. Rather than being pinned by the lowest effective bandgap, Voc is -surprisingly- found to be a smooth function of the ratio of the two donor polymers or the two acceptor fullerenes. For practical purposes this is important as it allows to enhance the overlap with the solar spectrum, and thereby the short circuit current, without disproportionally suffering from Voc losses. So far a plausible and general mechanism for the composition dependence of Voc has been absent, hampering a quantitative assessment of the possibility to enhance efficiency in ternary systems. Combining experiments and modeling we arrive at a simple physical understanding of the composition dependence of Voc in ternary BHJ. This insight is subsequently used to assess the potential efficiency increase in these systems. For the experimental results we constructed TQm6:TQm12:PC₇₁BM and TQp6:TQm6:PC₇₁BM devices of constant overall D:A ratio of 1:1. For each ternary device the Donor1:Donor2 ratio was increased in steps of 0.1 and the samples were constructed following the ITO/PEDOT:PSS/Active layer/LiF-AL sequence. The resulting Voc values are shown in the figure along with the modeling results. Our modeling is based on state filling in the total density of states that is broadened by Gaussian disorder. Using a simple approximation for the charge carrier concentration it predicts the electron and hole Fermi levels and hence Voc which is their difference. The predicted values of the open-circuit voltage follow closely both our own experimental data (figure) as well as those reported by others ^[1-3]. A characteristic feature we find is that the bowing of the Voc vs. composition curve goes up with increasing difference in Voc between the two binary compounds. Efficiency modeling shows this to be unfortunate as it implies that ternary compounds that benefit more from enhanced absorption also suffer most from voltage losses. In general we consistently find that systematic but modest efficiency improvements can be reached in ternary systems as compared to binary ones.
Plot of Voc of ternary organic BHJ vs. donor ratio; binary compounds sit at 0 and 1. Symbols and lines are experimental data and model results for the indicated two-donor-single acceptor systems.
[1] Lu, L; Xu, T.; Chen, W.; Landry, E.; Yu, L. Ternary blend polymer solar cells with enhanced power conversion efficiency. Nature Photonics 2014, 8, 716-722 [2] Khlyabich, P. P.; Burkhart, B.; Thompson, B. Compositional Dependence of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells Based on Two Donor Polymers. J. Am. Chem. Soc. 2012, 134, 9074−9077 [3] Khlyabich, P. P.; Burkhart, B.; Thompson, B. Efficient Ternary Blend Bulk Heterojunction Solar Cells with Tunable Open-Circuit Voltage. J. Am. Chem. Soc. 2011, 133, 14534–14537.

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