Miniature silicon solar cells for a tandem cell stack

Abstract

The Defence Advanced Research Project Agency (DARPA) of the United States of America sponsors a project aimed at developing a 50% efficient mechanically stacked solar cell based utilising six sub-cells of differing materials. This thesis examines the development of miniature silicon solar (MSS) cells for the tandem stack. The role of the silicon cell in the tandem stack is to absorb photon energy in the range of 1.42 -1.1 eV, and convert up to 7% of the light incident on the tandem stack into electricity. Other cells in the stack contribute the balance of the electricity. Key design parameters for the silicon cells are that it should have dimensions of 2.5 x 8 mm and it needs to transfer light with energy of less than 1.1ev to the underlying solar cells. In designing MSS cells, considerations such as optical losses, optimum diffusions and substrate doping, substrate thickness, free carrier absorption (FCA), internal quantum efficiency (IQE), recombination and resistive losses were taken into account. The approach of increasing the substrate thickness was used in the absence of texturing and back surface reflectors which interfere with transmission of sub-bandgap light to underlying cells. Reducing the doping density in the base and emitter minimises FCA losses. IQE of the MSS cells operating in the infrared spectrum is less affected by relatively heavy emitter doping because of lower absorption coefficients compared with those for the shorter wavelength light present in normal sunlight. Simulation showed that silicon solar cells with an emitter on both front and rear surface have a superior IQE response than for the case of an emitter on the sunward surface only. The use of nitride as an anti-reflection layer -incorporating a thin oxide passivating the silicon -returns the lowest reflection loss as compared to oxide and titanium oxide layers, for silicon solar cells operating in the infrared spectrum, and surrounded by a pottant material with a refractive index of ~1.4. Recombination in the MSS cells occurs at surface, bulk, contacts, junction and edges. Surface recombination is minimised by high-quality thermal oxide passivation. High bulk lifetime was maintained by the use of high-quality float-zone silicon material, and chlorine-assisted oxidation and cleaning. Recombination in the contacts was suppressed by incorporation of heavy diffusion. Edge recombination was suppressed by dicing the cell nearly completely from the host wafer prior to the final passivation step, and by keeping the emitter diffusion at least 1 mm away from the residual cell edge where the dicing occurs during cell detachment. To achieve an adequately high efficiency of silicon solar cells operating in the infrared spectrum various cell design options were considered: single-junction (SJ), horizontally stacked (HS) and vertically stacked (VS). Factors such as suppressing recombination, increasing current, current matching, series or parallel connection, reflection between cell interfaces, resistive losses and fabrication complexity were compared and contrasted for these cell designs. Modelling showed that VS and HS cells could achieve efficiency higher than SJ cells but at the expense of increased fabrication complexity. A single-junction MSS cell design was adopted since it is easier to implement, has adequate performance potential, and does not require current matching. In fabricating the MSS cells, a wide variety of tools were used. These included infrared and green laser machining, Reactive Ion Etching (RIE), Chemical Vapour Deposition, room temperature passivation, Light-induced and Electrolyte Plating, Palladium Silicide for metallisation and a measurement jig that enables testing the cells under the infrared spectrum. Comprehensive characterisation and development of these tools for their efficacy in fabricating MSS cells was undertaken. Analysis of low shunt resistance and carrier lifetime degradation by RIE on the MSS cells was carried out. RIE induces two main types of degradation on samples: permanent and reversible. Reversible degradation is recoverable by annealing the samples in nitrogen ambient, while permanent degradation is avoidable by wet chemical etch and by limiting the area of sample's surface exposed to RIE. Of all shunts, shunting caused by boron diffusion-induced pinholes was found to be most significant in the fabrication of MSS cells. Silicon solar cells fabricated following characterisation of inferior performances caused by shunting and RIE-induced degradation demonstrated a sharply improved performance but were still short of the expected efficiency. Attention then turned to hydrogenation of LPCVD nitride coated samples which are subjected to prolonged high temperature anneals during the fabrication. Hydrogenation of prolonged high- temperature annealed LPCVD nitride coated samples became ineffective due to the densification of LPCVD nitride. Further characterisation revealed that the carrier degradation arising from laser-scribing which affects a relatively large proportion of the volume of the cell, significantly reduced the high efficiency potential of the miniature silicon solar cells for a tandem stack. Show more