Kazuya Takahira

Impedance analysis of PbS colloidal quantum dot solar cells with different ZnO nanowire lengths

The photoconversion efficiency of colloidal quantum dot (QD) solar cells has been markedly improved by optimizing the surface passivation and device structure, and details of device physics are now under investigation. In this study, we investigated the resistance and capacitance components at the ZnO/PbS-QD interface and inside a PbS-QD layer by measuring the impedance spectrum while the interface area was controlled by changing the ZnO nanowire length. By evaluating the dependence of optical intensity and DC bias voltage on the ZnO nanowire length, only the capacitance was observed to be influenced by the interface area, and this indicates that photoinduced carriers are generated at the surface of PbS-QD. In addition, since the capacitance is proportional to the surface area of the QD, the interface area can be evaluated from the capacitance. Finally, photovoltaic performance was observed to increase with increasing ZnO nanowire length owing to the large interface area, and this result is in good agreement with the capacitance measurement.

Organic photovoltaic cell fabricated by electrospray deposition using non-halogenated solvent

An electrospray deposition (ESD) is a solution process, which has several advantages, such as the utilization of low concentration solution, the large device area using a continuous roll-to-roll process, and the simple setup without vacuum condition. By applying a high voltage to a solution, mists are generated from a tip of glass capillary, leading to the formation of organic thin film. Halogenated solvents are often used, but the amount of solvent should be reduced from the environmental aspect. Thus, by using the ESD and the non-halogenated solvent, the manufacturing process is friendly to environment. In this research, by measuring current density-voltage (J-V) characteristics and atomic force microscope (AFM) images, we evaluated OPV characteristics and root mean square (RMS) roughness of devices fabricated using the non-halogenated solvent. By adding 1,8-diiodooctane (DIO), the photo conversion efficiency (PCE) was drastically improved, and the PCE of 5.45% was achieved.