Light management design in ultra-thin chalcopyrite photovoltaic devices by employing optical modelling

Abstract

In ultra-thin chalcopyrite solar cells and photovoltaic modules, efficient light management is required to increase the photocurrent and to gain in conversion efficiency. In this work we employ optical modelling to investigate different optical approaches and quantify their potential improvements in the short-circuit current density of Cu(In, Ga)Se2 (CIGS) devices. For structures with an ultra-thin (500 nm) CIGS absorber, we study the improvements related to the introduction of (i) highly reflective metal back reflectors, (ii) internal nano-textures applied to the substrate and (iii) external micro-textures by using a light management foil. In the analysis we use CIGS devices in a PV module configuration, thus, solar cell structure including encapsulation and front glass. A thin Al2O3 layer was considered in the structure at the rear side of CIGS for passivation and diffusion barrier for metal reflectors. We show that not any individual aforementioned approach is sufficient to compensate for the short circuit drop related to ultra-thin absorber, but a combination of a highly reflective back contact and textures (internal or external) is needed to obtain and also exceed the short-circuit current density of a thick (1800 nm) CIGS absorber.

Efficient broadband light absorption in thin-film a-Si solar cell based on double sided hybrid bi-metallic nanogratings

“…A key technique for obtaining good light trapping and enhancing the efficiency of solar cells is to engineer the light behavior by using photonic crystals, 4 diffraction gratings, 5 anti-reection coatings, 6 surface texturing, 7 and metallic nanoparticles. 3,8 Plasmonic nanogratings among other structures show a substantial progress as they can be used either as an innovative back reector patterned on a metal mirror [9][10][11][12] and/or on transparent and conductive oxides (TCO), [13][14][15][16] to improve both the optical path length and optical absorption over a broad spectrum. The effective coupling between metallic nanograting modes and the incident light essentially presents an efficient light trapping developing from surface plasmon resonances and their resultant near-eld light concentration.…”

Modelling Supported Design of Light Management Structures in Ultra-Thin Cigs Photovoltaic Devices

“…FEM enables modelling of the exact morphology of (periodic) nanotextured interfaces in three dimensional structures. Such rigorous analysis in combination with realistic complex refractive indices of materials [12] enables to include various effects in simulation, such as light scattering and anti-reflection due to nanotexturing, as well as plasmonic absorption that can occur at textured metallic surfaces. However, it only considers coherent propagation of light in thin layers.…”

“…As lateral dimensions of the structures are expected to be much larger than vertical ones, edge effects in simulations are neglected (but are possible to be included). All models were calibrated according to the literature [12] and measurement results -see Section 5.…”

“…To improve optical performance of thin CIGS, in addition to alternative highly reflective BR, we also introduce internal nano-textures and external microtextures (LM foil) in simulations. The position of the internal and external textures can be seen in Figure 1(a), while its shape and size can be seen in Figure 1 In our previous work, a detailed optical analysis of different internal textures introduced at the rear side of the device was performed [12], thus here we only use an optimal texture in our analysis. We selected a periodic two-dimensional, sine-like nano-texture with P = 800 nm and h = 300 nm (see inset of Figure 1(b)), that can be fabricated on silicon or glass master by e-beam or etching techniques [19].…”

“…By optimization of the LM foil texture (e.g. aspect ratio) or by using different textures, even higher J sc could be achieved [12].…”