Rear Optical Reflection and Passivation Using a Nanopatterned Metal/Dielectric Structure in Thin-Film Solar Cells

Abstract

Currently, one of the main limitations in ultrathin Cu(In,Ga)Se2 (CIGS) solar cells are the optical losses, since the absorber layer is thinner than the light optical path. Hence, light management, including rear optical reflection and light trapping is needed. In this work we focus on increasing the rear optical reflection. For this, a novel structure based on having a metal interlayer in between the Mo rear contact and the rear passivation layer is presented. In total, eight different metallic interlayers are compared. For the whole series, the passivation layer is aluminum oxide (Al2O3). The interlayers are used to enhance the reflectivity of the rear contact and thereby increasing the amount of light reflected back into the absorber. In order to understand the effects of the interlayer in the solar cell performance both from optical and/or electrical point of view, optical simulations were performed together with fabrication and electrical measurements. Optical simulations results are compared with current density-voltage (J-V) behavior and external quantum efficiency (EQE) measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising rear reflective layer since it provides solar cells with light to power conversion efficiency values of 9.9 %, which is 2.2 % (abs) higher than the passivated ultrathin sample and 3.7 % (abs) higher than the unpassivated ultrathin reference sample.

High‐Performance and Industrially Viable Nanostructured SiOx Layers for Interface Passivation in Thin Film Solar Cells

“…[ 18 ] One approach to mitigate the recombination in the rear CIGS surface is focused on the use of an insulator passivation layer, in between the rear contact and the CIGS layer. [ 18,19,20-28 ] To establish electrical contact between CIGS and the rear electrode, nanocontacts are opened in the insulator layer, with e‐beam lithography being commonly used. [ 4,18,19,29 ] However, photolithography has the advantage of being cheaper and to have higher machine throughput compared with e‐beam lithography.…”

Encapsulation of Nanostructures in a Dielectric Matrix Providing Optical Enhancement in Ultrathin Solar Cells

“…Such diffusion constitutes an issue for the CIGS solar cell performance, because it is known that the metal diffusion into the CIGS during its growth may degrade the solar cell electrical performance. [ 35 ]…”

“…From the J – V curves analysis, it is clear that both devices show evidences of shunting, a typical situation of ultrathin devices, where the reduced thickness facilitates pin holes. [ 35 ] To understand the impact of the Au aggregates in the CIGS solar cell optoelectronic properties, the obtained figures of merit values for this device were compared with those of the Ref device. Lower open‐circuit voltage ( V oc ) and fill factor ( FF ) values were obtained for the NPep device with respect to the Ref device.…”

“…[ 33 ] Absorption enhancement in CIGS ultrathin layers has been explored by the implementation of different light management strategies. [ 4,8,15,26,34,35 ] In this work, we present, for the first time, an innovative substrate architecture to enhance the optical path length in the CIGS layer: we used Au NPs (diameter of (24.6 ± 3.8) nm) aggregates synthesized by a wet chemical process, to create a randomly texturized rear interface. However, it is well known that metal NPs are not thermally compatible with the high‐temperature deposition process of the CIGS layer.…”

Interface engineering of ultrathin Cu(In,Ga)Se2 solar cells on reflective back contacts

“… the formation of a detrimental Ga oxide layer at the CIGS/TCO back interface, 26–30 which is promoted when an external supply of Na is used 31,32 ; the diffusion of metallic elements from the back contact to the absorber 21,24 …”