Modulated surface texturing on temporary aluminium substrate for flexible thin-film solar cells

Abstract

Thin-film photovoltaic technologies are gaining momentum over the currently dominated crystalline silicon technologies. In addition to the competitive prices, flexible thin-film technology especially has the added advantage such as in building integrated photovoltaics (BIPV) due to its flexibility and light weight. HyET Solar B.V. is a company based in the Netherlands which state-of-the-art Roll to Roll (R2R) technology to produce such flexible solar cells. A temporary aluminum foil is used as substrate on to which FTO/p-i-n solar cell stack is deposited. The temporary foil is etched away, and the layers are encapsulated in low cost polymer foils. This thesis is part of the on-going FlamingoPV (Flexible Lightweight Advanced Materials In Next Generation of PV) project in collaboration between HyET Solar and TU Delft, to develop single, tandem and triple junction cells with 12, 13 and 14% efficiencies and a lifetime longer than 35 years. This thesis is inspired by the work of Tan et. al [8] where record efficiency cells were achieved on solar cells deposited on ‘Modulated Surface Textured’ (MST) glass substrates. In MST, increased light scattering is obtained by superposition of various scattering mechanisms which is achieved by introducing different textures at different interfaces and stacking them together. The requirements of the MST are two-fold: to provide efficient light trapping and to aid in the growth of high-quality layers. The aim is to introduce MST in the R2R process by developing micro sized crater-shaped features on the temporary Al foil on to which naturally nano sized V-shaped textures of FTO is deposited. The crater-shaped features are developed on the Al foil (~110 um) using wet chemical etching techniques. Various acid and alkaline based etchants are experimented. The etching parameters- concentration and temperature of the etchant and the etching time are varied to achieve the optimum recipe. These parameters should be varied such that the features have a correlation length (an estimate of how wide the feature is) 3-4 um and an aspect ratio (ratio of RMS roughness to correlation length) of 12-14%. These features are characterized using SEM and AFM to measure the aforesaid parameters and reflectance and angular intensity distribution measurements to measure its effectiveness of scattering. Alkaline based etchants (KOH and NaOH) resulted in crater-shaped features unlike acid etchants which resulted in pyramidal features. It was observed that there is an initial ‘induction period’ before which the etching started. For lower temperature, time and concentration, the induction period is longer. To ensure mechanical stability during deposition stages, the foil needs to be thicker than 70 um on lab-scale. The above-mentioned etching parameters were varied, and the best recipe was found to be 1.78M KOH at 70°C for an etching time of 2 minutes, 1.78M KOH at 60°C for 3 minutes and 1.42M NaOH at 70°C for 2.5 minutes. These samples displayed a correlation length 4-4.6 um and aspect ratios from 12-14% which is close to the targeted values and are higher than the existing texturing at HyET (‘factory baseline’) which had correlation length and aspect ratios of 500 nm and 5.7% respectively. These samples also showed higher scattering compared to the factory baseline. The nc-Si:H and a-Si:H/nc-Si:H layers deposited on to these samples resulted in dense high-quality layers. The TCO/p-i-n layers deposited also adapted the texturing pattern of the Al foil, unlike the factory baseline where the Al surface morphology was not adapted by the other layers as the features were significantly smaller. Further, to enhance the uniformity of texturing, various ‘chelating agents’ and ‘surfactants’ such as gluconic acids and glycols as well as varying the speed of etching to ensure homogenous contact of etchant with the foil was carried out. Both these techniques resulted in in an increased etch rate as well as an increase in the density of the craters on the foil. Considering the limitations of the R2R process, the best lab recipes were adapted (50°C, 1.42M NaOH, 1.8 minutes) to implement on the R2R etching machine. The resulting Al foil had higher surface morphology parameters and scattering compared to the factory baseline. The cells deposited on this texturing adapted the morphology of the Al foil. Optical simulations were done using GenPro4 where the AFM data of the textures were given as the input. The best lab samples as well as the R2R testing recipe showed larger absorption in the higher wavelengths in both nc-Si:H single junction and a-Si:H/nc-Si:H tandem cells, compared to the standard factory baseline texturing.