Heavily Doped Carrier-selective Regions for Silicon Solar Cells

Abstract

While heavily doped regions are an integral part of conventional silicon solar cells for carrier separation and selective transport, they also introduce deleterious effects that constrain their performance. This thesis addresses the fundamental limitations imposed by heavy doping in silicon solar cells and explores the potential of utilising doping in a manner that circumvents them. In particular, selectively doped regions and novel carrier-selective passivating contacts are developed. In heavily doped silicon, energy band gap narrowing (BGN) occurs. Subsequently, the pn product in equilibrium increases, which tends to limit the performance of silicon solar cells. In principle, heavily doped silicon should be analyzed using Fermi-Dirac statistics coupled with a BGN model. Nevertheless, applying Schenk’s theoretical BGN model to experimental samples underestimates their corresponding recombination current parameters. Based on a large number of samples and using well-proven models for Auger recombination and carrier mobility, updated empirical expressions for the BGN are derived here for both n-type and p-type silicon. The study confirms that the BGN values in p-type silicon are slightly larger than in n-type silicon. Both updated BGN models contribute to a more complete understanding of the losses and ramifications caused by heavily doped regions in silicon solar cells. A possible approach to reduce the impact of heavily doped regions is to reduce the area they occupy, restricting it to underneath the metal contacts. In this thesis a process to implement such selectively doped (SD) silicon solar cells is developed. The process, based on a controlled etch-back of the diffused region offers the advantage of being self-aligned, which avoids the critical mask alignment step. The TMAH based etch-back solution used here provides a well-controlled etching rate, uniformly etched surface and selectivity to metallic layers (Al or Ag). The poof-of-concept SD silicon solar cells have reached a conversion efficiency of 17.5%. Inspired by silicon heterojunction solar cells and polysilicon emitter BJT, carrier-selective passivating contacts based on heavily doped silicon films are developed. Two approaches of depositing intrinsic polysilicon or amorphous silicon are demonstrated. In both cases, a thermal diffusion process is used to dope and recrystallise the films. The diffusion processes, the intrinsic silicon film thickness, and the interfacial layer conditions are optimized in terms of the trade-off between the recombination current parameter and the contact resistivity. The interfacial layer is found to be critical for blocking the penetration of dopants, for stopping the epitaxial regrowth between the top silicon film and the underlying crystalline silicon, as well as for enhancing the response to a subsequent hydrogenation treatment. Examination of these carrier-selective passivating contact structures before and after the thermal diffusion processes by using XRD, micro-PL and XPS indicates that allowing a moderate level of phosphorus into the silicon oxide and into the silicon substrate is necessary to achieve a low recombination and a low contact resistivity. The electron-selective passivating contacts have been successfully implemented at the rear of n-type silicon solar cells, showing open circuit voltage of > 672 mV and efficiency values up to 20.8%.