Merocyanine/C₆₀ Planar Heterojunction Solar Cells: Effect of Dye Orientation on Exciton Dissociation and Solar Cell Performance

Abstract

In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C 60 organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post-annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric-fielddependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as-deposited cells. Fourier-transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules' orientations on the charge-transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials.

Field-dependent exciton dissociation in organic heterojunction solar cells

“…III A. 28 Here, we demonstrate that also the proposed exciton dissociation tunnel model can account for the observed relation between dye orientation and charge-generation efficiency.…”

“…28 The annealing was found to change the donor molecules' orientation, which decreased the CT-state binding energy and made the annealing effect compatible with the CT-state dissociation model presented in Sec. III A.…”

“…This D/A combination yielded an effective band gap E eff = E LUMO,acceptor − E HOMO,donor (where LUMO stands for lowest unoccupied molecular orbital and HOMO stands for highest occupied molecular orbital) of 1.7 eV and thus allowed open-circuit voltages V oc of more than 1 V. Figure 1 shows an overview of the layer sequence, the molecular structures, and the energy levels of the materials used in the cell stack. 28 Bulk heterojunction device results for similar chromophores have been published, 33,34 and it has been shown that these materials can be processed wetchemically from solution as well as by vacuum thermal evaporation. 35 In this work thermal evaporation was used because it allowed for a simpler fabrication of well-defined multilayer structures like PHJ cells.…”

“…Prior to the layer deposition, the substrates were ex situ treated with UV ozone for 15 min. The following layer sequence was subsequently deposited: MoO 3 (5 nm)/ID583 (7,14,21,28 nm)/C 60 (35 nm)/Bphen (5 nm)/Ag (100 nm) [see Fig. 1(c)].…”

“…This setup allowed temperatures of the glass substrate between 9 • C and 48 • C. More details on the device fabrication and the merocyanine synthesis can be found in Ref. 28. Table I summarizes the corresponding cell operation parameters.…”