Industrially Viable Rear Surface Passivation Approach for Cu( In,Ga)Se2 Solar Cells

Abstract

Reducing the thickness of Cu(In,Ga)Se2 (CIGS) absorber layers has potential to decrease its cost significantly, but has drawbacks like incomplete absorption and increased back-contact recombination, both resulting in power conversion efficiency losses. One solution is to implement a rear surface passivation layer, which has potential to reduce rear surface recombination velocity and increase rear internal reflection [1]. Alumina (Al2O3) is such an ideal passivation layer, but unfortunately also acts as an electron and diffusion barrier layer, and thus prevents current flow and sodium (Na) diffusion. As is discussed below, our novel approach to generate point contact openings in this passivation layer overcomes both problems at once. The proposed method is to use sodium fluoride (NaF) on top of the Al2O3 passivation layer, which will generate contact openings during selenization. This applied approach is industrially viable, as compared to proven methods – e.g. using nanoparticles, e-beam or nano-imprint lithography – which are too expensive, time-consuming or not applicable for larger areas. D. Ledinek et al. previously proposed using a very thin layer of Al2O3 rear surface passivation in combination with NaF deposition to enhance the electrical characteristics of CIGS solar cells. In their study this surface passivation is claimed to allow tunneling [2]. In our study, we prove that point contacts have been generated in this Al2O3 surface passivation layer. Atomic layer deposition (ALD) was used to deposit very thin Al2O3 layers on molybdenum (Mo) rear contact, and NaF was deposited on these layers by spin coating. Thereafter CIGS layers (±500 nm thick) were grown by single stage co-evaporation at 550°C, followed by standard solar cell process described in [3]. To detect the contact openings in the passivation layer, scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy was applied. Glass/Mo/Al2O3/NaF characterization samples were used for this analysis, where the co-evaporation process was mimicked by using a selenization step. After the selenization, point openings in the thin Al2O3 passivation layers were determined by SEM, and supported by EDX measurement. To investigate the impact of Al2O3 thickness on passivation of the CIGS rear surface, time resolved photo-luminescence (TR-PL) measurements on finished absorber layers (capped with CdS) were used. According to these TR-PL results, a 6 nm thick passivation layer gave the highest free charge carrier lifetime and PL response in all sets of samples. This was confirmed in full solar cell devices, where a significant increase in power conversion efficiency, and gain in open-circuit voltage, current density and fill-factor values was measured for the 6 nm Al2O3 rear passivated CIGS cells, as compared to unpassivated reference cells. Hence, it can be concluded that by using a simple, cost-effective and fast way, i.e. ALD for Al2O3 and spin coating for NaF, we succeeded to make point contact openings to passivate the back surface of ultra-thin CIGS solar cells. The ongoing work focuses on different aspects: (i) At present the point openings are still submicron size, now we try to make them nano-size. (ii) We also concentrate on controlling the density of the contact openings, e.g. by changing the temperature of ALD process or the molarity of NaF solution. (iii) This approach also enables us to investigate different metal oxides as passivation layers, such as HfO2 and TiO2, in combination with the same contact opening approach. (iv) And finally, we are investigating this rear surface passivation approach for other absorber layer deposition/growth techniques, e.g. a 2-step sputtering + selenization method.