Surface passivation and open-circuit voltage in ultra-thin silicon solar cells.

摘要

Silicon solar cells are the most widely used and optimized solar cells. Because of this, of any solar cell technology they have achieved efficiencies which are closest to their theoretical performance limits of any material. The record one-sun silicon solar cell has an efficiency of 24.7%, compared to a theoretical efficiency of approximately 31% (depending on the solar spectrum). In these devices, the short circuit currents are exceptionally close to their theoretical values. However, the open circuit voltages even in record solar cells are substantially below their theoretical limits.;The highest voltage silicon solar cells attain voltages of approximately 739 mV by Sanyo. Other groups have shown the capability of achieving ∼ 720 mV by several device structures using different methods of surface passivation. However, while these experimental results suggest a fundamental limiting mechanisms near 739 mV, the theoretical limit from detailed balance calculations is between 830 mV and 860 mV (depending on concentration and spectrum). The more than 100 mV discrepancy still remains a challange.;In order to achieve silicon solar cells which approach the detailed balance voltage limits, the work will examine and reduce the fundamental mechanisms controlling the open circuit voltage. The three fundamental factors limiting the open circuit voltage of a solar cells are: (1) fundamental recombination parameters, which for silicon is dominated by Auger recombination; (2) the volume of material in which recombination takes place (which for a given concentration ratio is determined by the thickness of the material), and (3) the surface passivation. In order to overcome existing open circuit voltage limitations, each of these loss mechanisms require new approaches.;If all of photo-generated carriers in the ultra-thin device can be extracted out of the device, in the form of current and voltage, then the efficiencies are bound to reach the detail balance limit. This can be possible only with a passivation scheme that allows very low surface recombination velocity and a light trapping scheme that allows the optical path length of 50, i.e. the light bounces back and forth in the device multiple times hence increasing the chances of absorption. Since the device thickness is reduced to a considerable amount, a low surface recombination velocity and an optical path length ∼ 50 becomes extremely hard to achieve in ultra-thin wafers with today's technology.;The goal of the thesis is to improve the understanding of the open circuit voltage in the silicon solar cells by means of theoretical demonstration of analytical modeling, and by practical means of reduced thickness and surface passivation schemes relevant for ultra-thin silicon solar cells.