Improvement of power conversion efficiency by a stepwise band-gap structure for silicon quantum dot solar cells.

As a promising next-generation solar cell, the power conversion efficiency of a silicon quantum dot (Si-QD) solar cell is still low. In this work, the band-gap structure of a Si-QD layer was modified to improve the power conversion efficiency of a Si-QD solar cell. A stepwise band-gap Si-QD (SB Si-QD) layer with a high bandgap top layer (about 2.22 eV) and a low band-gap bottom layer (about 1.98 eV) was grown on a Si (100) substrate. The open circuit voltage and short circuit current were improved by band-gap engineering of the Si-QD absorption layer. As a result, the power conversion efficiency of the SB Si-QD solar cell increased from 16.50% to 17.50%, compared to that of a Si-QD solar cell with a uniform band gap. This results will provide a guide to design advanced Si-QD solar cells by considering the band-gap structure in the Si-QD absorption layer.

High efficiency Si quantum dot heterojunction solar cells using a single SiOX:B layer.

Si quantum dots (QDs) have been fabricated from SiO2/SiOx multilayer structures to create a homogeneous size. However, this structure achieved much lower efficiencies than would be expected in the Si QD photovoltaic field. This is because Si QD generation and photoexcited carrier transport is restricted by the adjacent SiO2 layer. In this study, we applied a single SiOx:B layer fabrication method to the Si QD heterojunction solar cells. The number of generated Si QDs and the photo-excited carrier lifetime was maximized when the oxygen partial pressure and boron doping concentration parameters were 2.7 × 10-5 Torr and 2.27 × 1021 atoms cm-3, respectively. As a result, in excess of 17% power conversion efficiency for the Si QD heterojunction solar cell was achieved using the single layer method.

Ultraviolet responses of a heterojunction Si quantum dot solar cell.

We investigated the ultraviolet (UV) responses of a heterojunction Si quantum dot (QD) solar cell consisting of p-type Si-QDs fabricated on a n-type crystalline Si (p-Si-QD/n-c-Si HJSC). The UV responses were compared with a conventional n-type crystalline Si solar cell (n-c-Si SC). The external and internal quantum efficiency results of the p-Si-QD/n-c-Si HJSC exhibited a clear enhancement in the UV responses (300-400 nm), which was not observed in the n-c-Si SC. Based on the results of the cell reflectance and bias-dependent responses, we expect that almost all UV responses occur in the p-Si-QD layer, and the generated carriers can be transported via the Si-QD layer due to the formation of a sufficient electric filed. As a result, a high power conversion efficiency of 14.5% was achieved from the p-Si-QD/n-c-Si HJSC. By reducing the thickness of the n-Si substrate from 650 µm to 300 µm, more enhanced power conversion efficiency of 14.8% was obtained which is the highest value among the reported Si-QD based solar cells to date.