Quantifying Charge Recombination in Solar Cells Based on Donor - Acceptor P3HT Analogues

The creation of semi-random donor acceptor analogues of poly(3-hexylthiophene) (P3HT) yields polymers that exhibit pan-chromatic absorption spectra extending into the near-infrared. Despite this extended absorption however, different semi-random polymers exhibit markedly different photovoltaic performance when blended as a bulk-heterojunction with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM). To understand the physical origin of these differences, we performed transient absorption (TA) measurements and device characterization of blends of two representative semi-random polymers, poly(3-hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), with PCBM. Although both polymers absorb strongly throughout the visible and near-infrared, devices based on P3HTT-DPP-10%:PCBM exhibit a power conversion efficiency of similar to 6%, while films consisting of P3HTT-TP-10%:PCBM blends display values under 1%. TA experiments reveal that polarons generated upon excitation of a P3HTT-TP-10%:PCBM blend undergo a high degree of geminate recombination (survival percentage, Phi(s) similar to 45%) independent of excitation wavelength, explaining its lower efficiency. In contrast, P3HTT-DPP-10%:PCBM blends show excitation wavelength-dependent polaron recombination dynamics. While excitation of the polymer in the visible region leads to less geminate recombination (Phi(s) similar to 65%) compared to P3HTT-TP-10%:PCBM, this loss process is similar to 1.5 times more deleterious following near-infrared (NIR) excitation. Despite this observation, a significant fraction (Phi(s) similar to 45%) of the charges formed following NIR excitation escape recombination, partly explaining the high performance of P3HTT-DPP-10%:PCBM devices.