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“Enhanced sub-bandgap efficiency of a solid-state organic intermediate band solar cell using triplet-triplet annihilation,” Y.L. Lin, M. Koch, A.N. Brigeman, D.M.E. Freeman, L. Zhao, H. Bronstein, N.C. Giebink, G.D. Scholes, B.P. Rand, Energy Environ. Sci., 10, 1465 (2017).