Thin-film silicon solar cells applying optically decoupled back reflectors

We compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5-μm-thick low-pressure chemical vapor deposition (LPCVD)-ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVDZnO layer (with thickness below 200 nm). For this latter design, the optical loss in the nano-rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar-plasma treatment. Because of its superior lateral conductivity, the thin-ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD-ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin-ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active-layer thickness of only 1.1 μm. We thereby highlight the necessity to optimize simultaneously the front and back electrodes.

Section: Introductionmentioning

confidence: 97%

Silver versus white sheet as a back reflector for microcrystalline silicon solar cells deposited on LPCVD‐ZnO electrodes of various textures

Khazaka

Moulin

Boccard

et al. 2014

“…Enhanced light trapping is thus the mechanism behind the superior performance achieved with the thin‐ZnO/Ag BR. Based on previous work, the reason can be found in the more favorable angular intensity distribution for the light that is scattered back into the absorber layer achieved with an adjacent BR design .…”

Section: Resultsmentioning

confidence: 99%

Metal versus dielectric back reflector for thin‐film Si solar cells with impact of front electrode surface texture

Mercaldo

Usatii

Esposito

et al. 2016

Thin‐film Si solar cells employ a back reflector (BR) for a more efficient use of the long wavelength light. Here, we have carried out a cross evaluation of metal (Ag‐based) and dielectric (white paint‐based) BR designs. Conclusive results have been reached regarding the most suitable BR type depending on the front electrode morphology, both with crater‐like and pyramidal texture. The ZnO/Ag BR is found to be optically more efficient because of improved light trapping, although the gain tends to vanish for rougher front electrodes. Thanks to non‐conventional Raman intensity measurements, this dependence on the front texture has been linked to the different weight of front and back interfaces in the light trapping process for the different morphologies. With rougher substrates, because the minor optical gain is accompanied by sputter‐induced electronic deterioration of the solar cell during the ZnO buffer layer deposition, the white paint‐based BR design is preferred. Copyright © 2016 John Wiley & Sons, Ltd.

“…In practice, it is difficult to find such materials, because transparent rear dielectrics of very low refractive index are often not good conductors. In the realistic case where ZnO is applied as a rear dielectric, we have found in another study that a BR adjacent to the µc‐Si:H layers provides significantly better optical performance than a BR of higher surface roughness separated from the µc‐Si:H by several micrometers . Even by replacing the Ag BR by a white paint (concept widely adopted in the industry), a better light trapping has been obtained with an 80 nm‐ZnO/Ag BR design.…”

Section: Resultsmentioning

confidence: 99%

Investigation of the impact of the rear‐dielectric/silver back reflector design on the optical performance of thin‐film silicon solar cells by means of detached reflectors

Moulin

Paetzold

Bittkau

et al. 2013

Thin-film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear-dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (mc-Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non-conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the mc-Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell.