Takeyuki Sekimoto

Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis

We report on a highly improved CO2 to HCOOH conversion system using a tandem photo-electrode (TPE) of InGaN and two Si p-n junctions. To improve its efficiency, narrow-band-gap InGaN was applied as the photo-absorption layer. In the TPE structure, the current matching between GaN-based photo-absorption layer and two Si p-n junctions is crucial for the improvement of the efficiency. The energy conversion efficiency for HCOOH production reached 0.97%, which is greater than average of global biological photosynthetic one.

Designing band offset of a-SiO:H solar cells for very high open-circuit voltage (1.06 V) by adjusting band gap of p–i–n junction

We have matched the world’s highest open-circuit voltage (Voc: 1.06 V) achieved to date for a layered structure comprised of a glass/tin oxide (SnO2)/hydrogenated amorphous silicon oxide (a-SiO:H) (p–i–n)/back electrode. For the purposes of this study, we adjusted the band gaps of each layer (p–i–n) to improve overall film quality. Fine-tuning of band profiles with reference to activation energy and optical band gap allowed us to offset the conduction band and the valence band of each layer (p–i–n) and thus improve the built-in potential rather than the electron conductivity, Fourier transform infrared spectroscopy, transmittance or reflectance ratio, resulting in a high Voc. To fully exploit the characteristics of wide-band-gap materials and prevent problems with absorbance, we employed commercially available SnO2 in the front transparent conductive oxide instead of zinc oxide. Using our deposition and evaluation technologies to build a wide-band-gap single solar cell, we succeeded in matching the world’s highest Voc of 1.06 V (Eff: 5.38%, Jsc: 8.15 mA/cm2, FF: 0.624).

Impact of microcrystalline-silicon surface-morphology on film quality and solar cell performance

Atomic force microscopy (AFM) and Raman and infrared spectroscopy were used to measure and compare hydrogenated microcrystalline silicon (µc-Si:H) thin films. The height-height correlation function was calculated from the AFM profiles, and roughness parameters of surface roughness w, longitudinal correlation length ξ and roughness component α were estimated for µc-Si:H thin films with various film thicknesses. Results for film thickness dependence showed w and α to be more sensitive to changes in film quality than Xc. A rapid increase in w and/or α indicated the formation of defective regions with low Xc in µc-Si:H thin film, due to the shadowing effect. The Raman crystallinity Xc dependences of roughness parameters and the stretching modes (SM) of Si–H bonds were investigated for µc-Si:H thin films with almost the same film thickness. The properties of µc-Si:H thin film (high SM fraction, Xc, α) were compared with cell parameters of a hydrogenated amorphous silicon (a-Si:H)/µc-Si:H tandem cell. Consequently, α was a better index of a-Si:H/µc-Si:H tandem solar cell performance than Xc or high SM fractions. We consider w and α to be promising new indexes of film and solar cell quality in addition to the conventional Xc and SM.

Identification of defective regions in thin-film Si solar cells for new-generation energy devices

We developed a high-conversion-efficiency, a-Si/μc-Si tandem solar module using μc-Si thin film on a Gen. 5 class glass substrate. The stabilized module efficiency was 10.7% (initial module efficiency: 12.0%). We prepared high-performance a-Si/μc-Si tandem solar cells based on our original technology for μc-Si thin films with localized plasma confinement chemical vapor deposition, optical confinement techniques, and laser patterning. To obtain higher conversion efficiency, optical confinement techniques are crucial, especially transparent conductive oxide (TCO) controlling technology. However, high-performance TCO includes steep valleys in the texture structure. Therefore, many defective regions are generated in the deposition of μc-Si thin film that degrade the solar cell performance. In this study we structurally identified these defective regions and propose a new formation model of the defective regions.

Use of microcrystallinity depth profiling in an actual tandem silicon solar cell by polishing to achieve high conversion efficiency

In order to perform an accurate evaluation of the crystallinity (Xc) of hydrogenated microcrystalline silicon (µc-Si:H), polishing was performed from the electrode layer side after measuring the current–voltage characteristics of thin-film silicon tandem solar cells. The angle of polishing was about 1.4°, and the µc-Si:H was exposed in the horizontal direction. The polishing could facilitate the measurement of Xc in the vertical direction of µc-Si:H by Raman analysis, and it succeeded in revealing a variation of Xc in the depth direction of the solar cell, which even further clarified its characteristics of the solar cells. Additionally, an Xc-adjustment film was used to adjust the Xc profile of i-µc-Si:H. It could control Xc in the depth direction of µc-Si:H, and a high Xc was obtained, which did not decrease during the process of forming a thick film. As a result, the conversion efficiency was improved by 1.8% (0.21 points) compared with that under normal conditions. In this paper, we propose an index of the Xc profile of µc-Si:H for obtaining high conversion efficiency.

Wireless InGaN–Si/Pt device for photo-electrochemical water splitting

We demonstrate a wireless device comprising a gallium nitride (GaN)–silicon-based photo-electrode, and a platinum cathode. Compared with conventional two-electrode photo-electrochemical systems, this wireless monolithic device showed potential for a wider range of applications, and reduced the resistance losses resulting from the wiring and aqueous solution. The efficiency was improved when the electrolyte was changed from KHCO3 to NaOH because water oxidation capability of the surface of the GaN was enhanced. A wider solar spectrum wavelength range was exploited by adopting InGaN as a photo-absorption layer; the improved efficiency for hydrogen generation was 0.90%.

Electrochemical application of Ga2O3 and related materials: CO2-to-HCOOH conversion

We report on the complex catalytic behavior of Ga2O3 for the electrochemical reduction of CO2 to formic acid (HCOOH). Although the experiments were reproducible, the behavior observed during the reaction was complex. A characteristic feature of the reaction was that Faradaic efficiency was strongly dependent on the electric charge during electrolysis. This result implied that the produced HCOOH affected the CO2 reduction reaction on the surface of the electrode, which was confirmed by experiments with initially added acid. The Faradaic efficiency for HCOOH production (η_HCOOH) increased with electric charge, and was further increased by the presence of initially added acid. We also show electrochemical CO2 reduction over other Ga compounds such as GaN and GaP, for which it can be assumed that p electrons and the Ga–Ga distance on the surface of the catalyst have important roles in selective HCOOH production as in the case of Ga2O3.

Panasonic's R&D on Photovoltaic Technologies

The history and recent activity of R&D on photovoltaic technologies in Panasonic, such as thin film silicon and HIT solar cells were reviewed.