Optoelectronic Enhancement of Ultrathin CuIn_1–xGa_xSe₂ Solar Cells by Nanophotonic Contacts

Abstract

Keywords: ultrathin Cu(In,Ga)Se 2 solar cells, dielectric nanopatterns, light trapping, absorption enhancement, back barrier reduction CuIn 1-x Ga x Se 2 (CIGSe) solar cells have achieved record efficiency values as high as 22.6% for small areas, with module efficiency values near 16.5%. However, for economic viability these values must be achieved with reduced material consumption (especially indium), which requires reducing the CIGSe absorber thickness from 2000-3000 nm to below 500 nm. Softimprinted SiO x nanoparticles (NPs) beneath a conformal CIGSe layer enable this thickness reduction. Optically, they enhance the absorption of light through Fabry-Pérot and waveguided resonances within the CIGSe layer, preventing current loss. For CIGSe solar cells on ITO with an absorber thickness of only 390 nm and a nanophotonic contact the current density (J sc) increases from 25.7 to 32.1 mA cm-2. At the same time, the nanopatterned contact reduces the back barrier, leading to an increased open-circuit voltage (518 to 558 mV) and fill factor (50.7% to 55.2%). Combined, these effects increase the efficiency value from 6.8% to 10.0% for initial demonstration. With the addition of an antireflection coating, the champion NP-enhanced cell achieves a J sc of 34.0 mA cm-2 , corresponding to 93% of the J sc achieved by the thick world-record cell. This result shows that optoelectronic nanopatterning provides a path to high efficiency cells with reduced materials consumption. TOC Graphic 1. Introduction CuIn 1-x Ga x Se 2 (CIGSe) solar cells have achieved record light-to-power conversion efficiency values as high as 22.6% for small areas, [1] with 16.5% reached for module production. [2] Compared with other competing PV technologies, CIGSe solar cells offer a remarkably short energy payback time, minimal consumption of high purity materials, high performance over a wide range of illumination intensities, and a reduced sensitivity to shading compared to Si. [3] These features make CIGSe solar cells unique in the competition to surpass crystalline Si, which currently dominates photovoltaic installations with >90% market share. Despite rapid advances in CIGSe technology, leading to performance gains of ~ 0.2% absolute/yr in recent years, [4] additional reductions in manufacturing cost are essential for driving large-scale deployment. Thinning the CIGSe absorber significantly below the typical 2-3 µm can enable a significant reduction in material consumption and reduce the equipment time required to deposit the absorber, thereby simultaneously addressing concerns over indium scarcity [5,6] and lowering manufacturing cost in mass production.

Optical Lithography Patterning of SiO2 Layers for Interface Passivation of Thin Film Solar Cells

“…Hence, the introduction of the SiO 2 layer changes the rear reflection but compared with the reference some light still exits the solar cell. Light trapping is then needed to take advantage of the rear increased optical reflection, in good agreement with other studies . From these simple measurements, it is hard to find a quantification on the increase of the J sc between the increase in the reflection and the passivation effect.…”

“…In this work, we expand the study of using photolithography to create a patterned insulator layer with etched features with 700 and 1400 nm which allow to increase the total passivation area while keeping the distance between the contacts the same order of magnitude as the carrier's diffusion length. Furthermore, according to recent studies indicating that SiO 2 is a promising passivation material, we choose to work with that dielectric material …”

Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer

“…[15][16][17][18] Such passivation layer can lead to three advantageous effects: (i) chemical passivation; (ii) field effect passivation; (iii) increased reflection at the rear contact; all while maintaining good electrical contact and devices with high values of fill factor. While several studies have already shown proof-of-concepts of the point contact structure [17,18,[20][21][22][23][24] and of the passivation properties of Al 2 O 3 , [25][26][27][28][29][30] more detailed studies that analyze the different physical effects involved are needed. Semiconductor surfaces, especially semiconductor-metal interfaces, can host several interface defects, leading to high surface recombination velocities, which in turn lower the electrical performance of solar cells.…”

Novel back contact reflector for high efficiency and double‐graded Cu(In,Ga)Se2 thin‐film solar cells

“…To do so, Mo was replaced by alternative metals such as W, Ta, or Nb, or dielectrics such as ZrN, Al 2 O 3 , or MgF 2 layers were introduced at the CIGS/Mo interface, and in all cases, improved rear contact reflectance was reported to some degree. In addition, texturing of the rear contact to enhance light trapping was proposed . Thicker absorbers >2 μm were thereby usually only discussed as a limiting case or references.…”