Abstract:
Organic solar cell (OSC) is one of the emerging solar cells for the next generation. OSCs have
attracted much interest since it has unique functions such as flexibility, lightweight, availability
of low-cost fabrication processes, and semi-transparency of the photoactive layer, etc. Many
kinds of studies have been conducted to fulfill the requirements for practical use. Especially
the understanding of photovoltaic mechanisms in the OSC is critically important to enhance
its efficiency. In this presentation, we will discuss the working mechanism of OSC thorough
our resent study of semi-transparent OSCs. As a semi-transparent OSC, we have fabricated
OSCs with two transparent electrodes, which are a carbon nanotube (CNT) sheet as the top
transparent electrode and an indium tin oxide (ITO) as the bottom one. The illumination direction dependence on the photovoltaic properties of semi-transparent OSC was studied. It was
found that the power conversion efficiency (PCE) under the CNT side illumination showed a
lower value than that under the ITO side illumination. To understand the origin of the asymmetricity of photovoltaics, we fabricated semi-transparent OSCs with different thickness of the
active layer and explored the illumination direction dependence of the photovoltaic properties.
It was found that the discrepancy of the PCE between the ITO side and the CNT side was increased with the increase of thickness. We consider three possible origins of asymmetry: the
difference in the transparency of electrodes, the polymer segregation, and the difference in the
extraction efficiency of carriers. The CNT electrode had lower transparency than the ITO electrode. In addition, the polymer segregation at the CNT electrode side reduced the light intensity.
These effects lower a PCE of constant value under CNT side illumination but do not explain
the thickness dependence of asymmetry. The main factor for the asymmetry is the extraction
efficiency. From the comparison of experimental photocurrent and simulated carrier densities,
we estimated that the hole extraction efficiency is higher than the electron one. The present
results infer that the detailed analysis of photovoltaic properties of semi-transparent solar cells
provides fruitful insight into the photovoltaic mechanism of OSCs.
