Monocrystalline silicon : future cell concepts

Abstract

An overview of currently used cell processes for monocrystalline silicon in industry is given. Since the screen printed solar cell process has the biggest market share, advanced screen printing processes are presented. The front-side with selective emitter structures are investigated by measuring the emitter saturation current (j0e) on symmetrical test samples with QSSPC. The reference sample with an industrial homogeneous 50 Ω/□ emitter and fired PECVD SiN has a j0e of 220 fA/cm2. On selective emitter structures with two diffusion steps, using first a 100 Ω/□ diffusion then a SiN layer as mask and finally a 10 Ω/□ diffusion, j0e is 140 fA/cm2 using PECVD SiN and 120 fA/cm2 respectively with LPCVD SiN. By changing the sequence of light and heavy diffusion and applying PECVD SiN for surface passivation, j0e was measured to 90 fA/cm2. Solar cells were made with a two step selective emitter and a simplified process with a single diffusion step using laser structured SiN as diffusion suppressing layer and no texture was applied to these cells. The best reference cell with homogeneous 50 Ω/□ emitter has an efficiency of 16.2% and 625 mV VOC. The best selective emitter solar cells with both used processes have a 10 mV increase of VOC leading to 635 mV and an efficiency of 17.0% of a cell using the simplified selective emitter process. The bulk lifetime of Cz-Si was monitored during a selective emitter process with a screen printed aluminium BSF on the rear. The bulk lifetime of the as grown wafer was 32 μs and was subsequently improved by phosphorous gettering to 67 μs. Bulk lifetime was further raised to 120 μs through aluminum gettering of the screen printed BSF. This result has to be taken into account when applying alternative rear sides with dielectric passivation where beneficial aluminium gettering cannot be used. Therefore material that does not strongly depend on aluminum gettering should be used. A dielectric rear side passivation can be integrated at several stages in the production process. Each sequence entails different challenges especially in maintaining the rear side passivation quality at the end of the process.