Bifacial Cu(In,Ga)Se2 solar cells with submicron absorber thickness: back-contact passivation and light management

Abstract

For bifacial Cu(In,Ga)Se 2 solar cells with submicron absorber thickness, an Al 2 O 3 -layer deposited by a simple, self-organized spray-pyrolysis process onto the transparent SnO 2 :F back-contact is used to increase open circuit voltage and thereby increase power conversion efficiency, especially for rear-illumination, indicating reduced charge carrier back-contact recombination. On non-passivated SnO 2 :F, a thin (10nm) Mo-layer improved the electrical back-contact properties, while on Al 2 O 3 -passivated SnO 2 :F, the solar cell performance was higher without Mo-modification. However, even with Momodification, the solar cell performance increased for Al 2 O 3 -passivated compared to non-passivated back-contacts demonstrating the benefit of the Al 2 O 3 -layer for bifacial solar cells with submicron Cu(In,Ga)Se 2 absorber layers.

Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact

“…In the past, this has been achieved by a selective removal of Al 2 O 3 covered CdS particles, a dispersion of Mo nanoparticles, and by electron beam‐ or photo‐ lithography. Ohm et al applied a 2‐step process, based on spray pyrolysis and selective etching, to deposit a porous Al 2 O 3 layer on FTO and could prove a beneficial passivation effect for the TBC/CIGS interface . The deposition of thick Al 2 O 3 (with openings) probably reduces the impact of the IOH thickness, because a significantly reduced Na supply from the glass is expected.…”

“…A supposedly detrimental Ga 2 O 3 formation at the absorber/TCO interface (for AZO and ITO) or a reduced conductivity of FTO by a fluorine loss during high temperature absorber deposition was suggested as possible origin. On the other hand, Mollica et al have demonstrated ohmic behavior of FTO and AZO layers on CIGS (deposited at T CIGS > 500°C) and used these materials as appropriate TBCs in ultra‐thin CIGS devices . Thus, the applicability of the TCO films as a TBC seems to depend on details in the absorber processing like the incorporation of alkaline atoms.…”

Dielectric-Based Rear Surface Passivation Approaches for Cu(In,Ga)Se2 Solar Cells—A Review

“…In order to investigate the effects and influence of back surface passivation in CIGS solar cells, there are studies that used bi-facial thin CIGS solar cells [28][29][30]. For bi-facial solar cells, all characterizations are made from both the front and back side of the solar cell.…”

Progress and prospects for ultrathin solar cells

“…Still, the light-trapping efficiency of these devices is limited by the low reflectivity of the Mo back contact. The choice of alternatice materials (ZrN 65 ,TCO 66,67,68 ) is constrained by the high temperature of the CIGS deposition process. For this reason, highly reflective metals like Au 69 and Ag 70 have only been introduced in a superstrate configuration.…”

“…CIGS is usually grown by co-evaporation, sputtering or electrodeposition on Mo, a refractory material that forms an ohmic contact with the CIGS thanks to the creation of a thin MoSe 2 interface layer. Alternative back contact materials with higher optical reflectance and lower surface recombination are currently under investigation in several groups 66,67,70 . Silver mirrors encapsulated with transparent conducting oxides appear as a promising candidate 68 .…”