금속 산화막 패시베이션으로 개선된 나노구조 박형 결정질 실리콘 태양전지

Abstract

Thin crystalline silicon (c-Si) solar cells are beneficial for reducing the materials cost, but require much effective light management because of insufficient light absorption. Si nanostructures increase the optical path of sunlight in a c-Si solar cell by scattering and trapping light. However, when compared to a planar Si(100) surface, the surface exposure of higher-index crystal planes, e.g., (110) and (111), increases the density of surface states on nanostructured (NS) Si. Surface defects act as recombination centers within the bandgap of silicon, thereby deteriorating carrier collection efficiency by decreasing the minority carrier lifetime. Consequently, more effective techniques are required to passivate surface defects in c-Si solar cells employing anti-reflective surface nanostructures. Dielectric materials such as SiO2, SiNx, and Al2O3 have been utilized for efficient passivation of silicon surfaces. In particular, atomic-layer-deposited (ALD) Al2O3 provides a high density of negative fixed charges, resulting in superior field-effect passivation, in a planar geometry, for greatly reducing the surface-induced recombination of charge carriers. In Si solar cells seriously nanostructured for improving anti-reflectance, however, the passivation ability of Al2O3 has been reported to greatly degrade despite fully conformal coating by ALD. It was demonstrated the degradation mechanism of Al2O3 passivation on NS Si with the exposure of high index surface. The change in surface potential built by Al2O3 layer was calculated numerically with the separate consideration of interface state density and fixed charge density at the interface of Al2O3/Si. It was revealed that the positive charges induced to the interface states rather than positive fixed charges in SiO2 interfacial layer would deteriorate seriously the field-effect passivation by Al2O3 in severe NS Si solar cells. Metal-assisted chemical etching is useful and cost-efficient for nanostructuring the surface of c-Si solar cells. It was found that the nanoscale epitaxy of silicon occurs, upon subsequent annealing, at the Al2O3/Si interface amorphized by metal-assisted etching. Since this epitaxial growth penetrates into the pre-formed Al2O3 film, the bonding nature at the newly formed interfaces (by the regrown epitaxy) is deteriorated, resulting in a poor performance of Al2O3 passivation. Compared to the conventional hydrogen (H–) passivation, hydroxyl functionalization by oxygen plasma treatment was more effective as the wafer became thinner. For ultrathin (50 μm) wafers, ~30 % depression in surface recombination velocity led to the improvement of 15.6 % in the short circuit current. The effectiveness of hydroxyl passivation validated by ultrathin wafers would be beneficial for further reducing the wafer cost of nanostructured silicon solar cells. Auger and surface recombinations are major drawbacks that deteriorate a photon-to-electron conversion efficiency in NS Si solar cells. As an alternative to conventional frontside nanostructuring, it was reported how backside nanostructuring is beneficial for carrier collection during photovoltaic operation that utilizes a 50-μm-thin wafer. Ultrathin (4.3 nm) zinc oxide was also effective for providing passivated tunneling contacts at the nanostructured backsides, which led to the enhancement of 24 % in power conversion efficiency. Furthermore, MgF2/ZnS double-layered antireflective coating could optimize the visible (VIS) absorption at the planar frontside. These new surface design could provide all-black-surfaces (ABS) for both sides of 20 µm-thin, silicon absorber, which then resulted in average light absorption of ~90 % at wavelengths from 430 to 830 nm. As mentioned above, the light management and carrier recombination should be considered for low-cost, high-efficiency thin crystalline Si solar cells.