CRIS -Creative Research Institution- Hokkaido University

【Division of Frontier Research】Development of novel solar cells that effectively utilize low-energy photons

Research Center for Highly-Efficient Photoelectric Conversion System Using Optical Nano-Antenna
Development of highly efficient solar cells utilizing near-infrared light
Prof. Dr. Hiroaki MISAWA

Research Institute for Electronic Science and Creative Research Institution 
Hokkaido University

Solar cells utilizing the invisible light in the infrared region

Keywords: Infrared light, Solar cells, Photoelectric conversion, Gold nanorods, Plasmon, Titanium dioxide

The spectral distribution in the infrared wavelength region longer than 800 nm accounts for ca. 40% of the entire solar energy observed on earth as shown in Fig. 1. However, the photoelectric conversion characteristics of an amorphous silicon solar cell commonly used today are known to drastically decrease in wavelength ranges longer than 700 nm. Even in a single crystal silicon solar cell, which can minimize the recombination of excited electrons and electron holes, the photoelectric conversion characteristics also decrease in wavelengths longer than 1000 nm because of the light absorption characteristics of the cell. For photoelectric conversion with wavelengths up to 800 nm, a sensitized solar cell using particulate titanium dioxide (TiO2) electrode covered with sensitizers, such as metal complex and gold nanoparticles, has been suggested in addition to the above described semiconductor solid-state solar cell.

   

  To produce a solar cell with high photoelectric conversion efficiency, therefore, it is necessary to develop a system that responds to the wide spectrum of solar light ranging from visible to near-infrared wavelengths. Recently, we have succeeded in plasmonic photoelectric conversion from visible to near-infrared wavelength by using electrodes in which gold nanorods (Au-NRs) are elaborately arrayed on the surface of TiO2 single crystal electrodes via a top-down nano-structuring process.

  Au-NRs (110 nm × 240 nm × 40 nm) were fabricated within the area of 2.5 x 2.5 mm2 on a TiO2 single crystal (rutile, 0.05 wt% Nb-doped). Fig. 2(a) shows SEM image of Au-NRs. TiO2 substrate modified with Au-NRs was used as a working electrode. Photoelectrochemical measurements were performed using the three-electrode system. The incident photon-to-photocurrent efficiency (IPCE) values of the photocurrent was obtained by dividing the number of generated electrons by the number of the irradiated photons.

  According to the result of measurement on the action spectrum of the photocurrent using linear polarization (Fig. 2(b)), the incident photon-to-photocurrent efficiency (IPCE) values of the photocurrent were 6.2% and 8.4%, corresponding to the localized surface plasmon bands in the transverse mode at 650 nm and the longitudinal mode at 1050 nm, respectively. The photocurrent induced by monochromatic light irradiation at wavelengths of 450 nm, 650 nm and 1000 nm grows in proportion to the first order of the light intensity. Note that no photocurrent was observed at the TiO2 single crystal without Au-NRs under irradiation of light with a wavelength of 450 nm or longer. It is expected, therefore, that a highly efficient photoelectric conversion system can be realized by designing Au-NRs with larger optical field enhancement effects and effective generation of localized surface plasmons.

  Additionally, unlike the conventional solar cell, larger photocurrents can be generated at high temperatures (10-60ºC). Interestingly, it is possible for this system to be used as a solar cell that can be embedded in a living body because it is also able to use light with wavelengths from 800 to 1200 nm, which is the window of the absorbing spectrum of biological systems. Furthermore, there is a possibility that water molecules can serve as an electron source in this system, because stable photocurrents have been obtained for 200 hours, although only the electrolyte solution (KClO4 aq.) was used in this photoelectrochemical measurement, which did not include certain electron donors. Namely, it can be deduced that four-electron oxidation of water molecules will be expected. The oxidation will be important because there is a possibility that the system can become an artificial photosynthesis system using the irradiation from near-infrared light. In this project, we aim at optimization of the design of Au-NRs and development of solar cells capable of photoelectric conversion with higher efficiency.