Hisashi Uzu

High efficiency solar cells combining a perovskite and a silicon heterojunction solar cells via an optical splitting system

We have applied an optical splitting system in order to achieve very high conversion efficiency for a full spectrum multi-junction solar cell. This system consists of multiple solar cells with different band gap optically coupled via an “optical splitter.” An optical splitter is a multi-layered beam splitter with very high reflection in the shorter-wave-length range and very high transmission in the longer-wave-length range. By splitting the incident solar spectrum and distributing it to each solar cell, the solar energy can be managed more efficiently. We have fabricated optical splitters and used them with a wide-gap amorphous silicon (a-Si) solar cell or a CH3NH3PbI3 perovskite solar cell as top cells, combined with mono-crystalline silicon heterojunction (HJ) solar cells as bottom cells. We have achieved with a 550 nm cutoff splitter an active area conversion efficiency of over 25% using a-Si and HJ solar cells and 28% using perovskite and HJ solar cells.

Minimizing optical losses in monolithic perovskite/c-Si tandem solar cells with a flat top cell.

In a monolithic perovskite/c-Si tandem device, the perovskite top cell has to be deposited onto a flat c-Si bottom cell without anti-reflective front side texture, to avoid fabrication issues. We use optical simulations to analyze the reflection losses that this induces. We then systematically minimize these losses by introducing surface textures in combination with a so-called burial layer to keep the perovskite top cell flat. Optical simulations show that, even with a flat top cell, the monolithic perovskite/c-Si tandem device can reach a matched photocurrent density as high as 19.57 mA/cm2.

High Efficiency Silver-Free Heterojunction Silicon Solar Cell

In this work, we present the results of the replacement of silver screen printing on heterojunction crystalline silicon (c-Si) solar cells with a copper metallization scheme that has the potential to reduce the manufacturing cost while improving their performance. We report for the first time silver-free heterojunction c-Si solar cells on 6-in. wafers. The conversion efficiency reached is a record 22.1% for c-Si technology for this wafer size (Voc = 729 mV, Jsc = 38.3 mA/cm2, FF= 79.1%). The total power generated is more than 5 W for 1-sun illumination, which is a world record. Heat-damp reliability tests show comparable performance for mini-modules fabricated with copper metalized as for conventional silver screen printed heterojunction c-Si solar cells.

High-efficiency heterojunction crystalline Si solar cell and optical splitting structure fabricated by applying thin-film Si technology

Thin-film Si technology for solar cells has been developed for over 40 years. Improvements in the conversion efficiency and industrialization of thin-film Si solar cells have been realized through continuous research and development of the thin-film Si technology. The thin-film Si technology covers a wide range of fields such as fundamental understanding of the nature of thin-film Si, cell/module production, simulation, and reliability technologies. These technologies are also significant for solar cells other than the thin-film Si solar cells. Utilizing the highly developed thin-film Si solar cell technology, we have achieved ~24% efficiency heterojunction crystalline Si solar cells using 6-in. wafers and >26% efficiency solar cells with an optical splitting structure. These results indicate that further improvement of thin-film Si technology and its synergy with crystalline Si solar cell technology will enable further improvement of solar cells with efficiencies above 26%.

High-current perovskite solar cells fabricated with optically enhanced transparent conductive oxides

We focused on fluorine tin oxide (FTO)-coated glass substrates for perovskite solar cells (PVSCs) and studied the effects of the optical properties and surface morphology on the short-circuit current density (J sc). The PVSC on our FTO substrate demonstrated a gain in J sc by 1.4–1.6 mA/cm2, compared with the PVSCs on commercial FTO substrates. This is attributed not only to the low absorption of the FTO substrate but also to the suppression of reflection loss, caused by the light trapping effect on the textured surface. Finally, the power conversion efficiency of our PVSC reached >21% with less hysteresis.

6 inch High efficiency back contact crystalline Si solar cell applying heterojunction and thinfilm technology

We have developed a 6 inch heterojunction interdigitated back contact (HJBC) solar crystalline Si cell by applying our heterojunction technology and thinfilm technology. To form rear junction and front side passivation, we applied our heterojunction technology that allows efficiency of 24.5% with top/rear contact heterojunction solar cell. To secure this high quality passivation during the HJBC solar cell fabrication process, we have developed deposition technology of insulator layer originally used in thinfilm silicon solar cells. Furthermore, new structure has been designed to drastically suppress the rear electrode resistance. By adapting all these technologies, our HJBC solar cell has reached conversion efficiency of 24.91% independently confirmed at AIST (VOC=737 mV, ISC=10.07 A, FF=80.2 %, PMAX=5.95 W) under 1 sun illumination by total area measurement (239.0 cm2).

Optical Simulation of Multi-junction Solar Cells

We introduce our optical model for solar cell design. It is especially suitable for optimization of light-trapping schemes in multi-junction devices. We illustrate this for triple junction thin-film silicon and for perovskite/c-Si tandem solar cells.