Formation of a bulk-heterojunction structure in organic solar cells by annealing stacked amorphous and microcrystalline layers

Organic solar cells were fabricated by stacking aromatic amine and C-60 layers. The energy conversion efficiency of these solar cells was low because of poor photoabsorption by these layers and short diffusion length of excitons. However, the photocurrent density was increased by about 3 times by the application of heat treatment to the stacked organic layers at 140 degrees C, and the simulated solar light. The internal quantum maximum energy conversion efficiency reached 1.1 % under AM 1.5, 1.00 mW cm(-2) efficiency of the photocurrent after the annealing reached about 45 %. When the aromatic amine layer was about 100 nm thick, the organic layers after the annealing showed a wrinkled structure under an optical microscope. The annealing temperature needed for the formation of this structure was in good agreement with the temperature needed for the increase in the photocurrent. The morphological change caused by the annealing was attributed to infiltration of the amorphous aromatic amine compound into grain boundaries of the microcrystalline C-60 layer, resulting in expansion of the C-60 layer and the wrinkled structure of the organic layers. From observation by electron microscopy, the mixed form of these two compounds near the interface was found to be suited to solar cells because the C-60 and aromatic amine phases wedge each other in a direction normal to two electrodes. However, the annealing slightly lowered photovoltage of the solar cell. This effect was attributed to a partial contact of the C-60 layer with a counter electrode through the aromatic amine layer.