Reduced Bimolecular Recombination in Polymer Solar Cells

Abstract

Polymer solar cells with a thick active layer can generate large photocurrent because they absorb many more photons. In most cases, however, the active layer is as thin as ~100 nm under the optimized condition. In this work, we have studied bimolecular charge recombination in highly efficient polymer solar cells with a thick active layer by transient photovoltage and photocurrent measurements. As a result, we found that the bimolecular recombination rate is smaller by three orders of magnitude than that predicted from the diffusion-limited Langevin recombination. We therefore conclude that such reduced bimolecular recombination enables efficient charge collection even in the solar cell with a thick active layer.

Impact of Active Layer Morphology on Bimolecular Recombination Dynamics in Organic Solar Cells

“…Note that in the context of diffusion-reaction theory, our results can be interpreted as using bulk diffusion constant to extract BMR rates due to the spatial diffusion and interfacial reactions. The derived rates (see Table ) are of the same order of magnitude as the experimentally measured rates in P3HT/PC 61 BM and DT-PDPP2T-TT/PC 71 BM (0.7 × 10 6 and 10.2 × 10 6 s –1 , respectively) . However, they are two orders of magnitude smaller than those obtained in the framework of diffusion-limited Langevin theory (0.8 × 10 8 and 2.4 × 10 9 s –1 , respectively).…”

“…The linearity of this decrease in the log− log scale implies a power-law relationship between recombination rate and CT lifetime, which confirms the deviation from the second-order kinetics description and from a constant recombination coefficient (such as that used in the Langevin model), which is consistent with experimental observations. 42,61,94,95 predicts. This can be explained by the fact that in bilayers the reformation of CT states after charge separation is less likely than in BHJs with small domain sizes; in the latter case, it is likely that separated charge carriers coalesce and reform a CT state.…”

Nongeminate charge recombination in organic photovoltaics

“…Fukuhara et al have studied the NGR dynamics in OPVs based on blends of DT-PDPP2T-TT/PC71BM. 115 DT-PDPP2T-TT is also a crystalline polymer and shows a considerably high JSC of 20.07 mA cm −2 and a high FF of 0.70 with an optimum thickness of 340 nm when blended with PC71BM. 136 The reduction factor was estimated through TPV/TPC measurements to be on the order of 10 −3 , indicating that NGR is substantially suppressed in these systems.…”

Ternary Blend Polymer Solar Cells Based on Wide-bandgap Polymer PDCBT and Low-bandgap Polymer PTB7-Th

“…One passible mechanism is the hole transfer from PTB7-Th domains to crystalline PDCBT domains, which would lead to a spatial charge separation at the interface of PTB7-Th/PCBM because of cascaded energy structures, as reported previously. 18,19 The other possible mechanism is ascribed to the improvement in the hole mobility of ternary blends due to reduced trap sites compared to that of the binary blends as reported previously. 20,21 In such case, there would be a balanced charge transport between the hole and electron, giving rise to suppress the non-geminate recombination, and consequently the increase in FF.…”

“…can be efficiently collected even with a thick photoactive layer owing to suppressed non-geminate recombination compared to diffusion-limited Langevin recombination. [15][16][17][18] In other words, the reduced non-geminate recombination is one of the key factors in the polymer solar cells with high FF. There are two possible mechanisms in our ternary blend system taking into account such idea.…”