Organic solar cells were fabricated by stacking aromatic amine and C<sub>60</sub> 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/cm<sup>2</sup> 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 C<sub>60</sub> 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
C<sub>60</sub> and aromatic amine phases wedge each other in a direction normal to two electrodes.
A buffer layer is often placed between an ITO (indium-tin- oxide) electrode and a hole transport layer (HTL) of organic EL devices made of low-molecular-weight materials. Cu- phthalocyanine is the representative material of the buffer layer. Form the analysis of the current-voltage properties of the devices, we found that buffer layer hinders the current when it is very thin, 60 nm or less. On the other hand, when it is thick, it enhances the current. In order to clarify these effects of the buffer layer, we studied the hole injection process from the ITO electrode into the HTL in devices with and without the buffer layer. The results indicate that the holes are accumulated at the buffer layer/HTL interface because of the energy gap. These holes are injected into the HTL across the barrier. However, some of the holes are back transferred to the ITO. This rate is dependent on the thickness of the buffer layer and the voltage applied to the devices. The competition between forward and backward movements of the accumulated holes determines the current- voltage characteristics of the organic EL devices.