Formation of new bulk-heterojunction structure in organic thin film solar cells

Organic solar cells were fabricated by stacking aromatic amine and C60 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 about 140 °C, and the maximum energy conversion efficiency reached 1.1% under AM 1.5, 100 mW/cm2 simulated solar light. The effect of the heat treatment was attributed to the infiltration of the amorphous aromatic amine compound into grain boundaries of the microcrystalline C60 layer. 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 C60 and aromatic amine phases wedge each other in a direction normal to two electrodes.