Linda Koschier

ABSTRACT Increased interest in renewable energy sources and concern for the environment are giving impetus to the search for viable energy alternatives. One step to increasing the economic viability of PV technology is to improve the energy conversion efficiency. Most commercially produced solar cells suffer from poor rear surface passivation, including the buried contact solar cell (BCSC), with a poor back surface field (BSF) produced by an aluminium alloying process. In this work, the low temperature process of metal mediated epitaxial growth (MMEG) is used to produce an improved back surface field (BSF) and surface passivation for the BCSC in conjunction with reduced area contacts. The material produced by MMEG gives a p+ epitaxial silicon layer doped with Al at approximately 2×1018 atoms-cm-3. Experimental devices achieve an improvement of 30-40 mV in open circuit voltage over the standard BCSC indicating a significant improvement in rear surface passivation

ABSTRACT Increased interest in renewable energy sources and concern for the environment are giving impetus to the search for viable energy alternatives. One step to increasing the economic viability of PV technology is to improve the energy conversion efficiency. Most commercially produced solar cells suffer from poor rear surface passivation, including the buried contact solar cell (BCSC), with a poor back surface field (BSF) produced by an aluminium alloying process. In this work, the low temperature process of metal mediated epitaxial growth (MMEG) is used to produce an improved back surface field (BSF) and surface passivation for the BCSC in conjunction with reduced area contacts. The material produced by MMEG gives a p+ epitaxial silicon layer doped with Al at approximately 2×1018 atoms-cm-3. Experimental devices achieve an improvement of 30-40 mV in open circuit voltage over the standard BCSC indicating a significant improvement in rear surface passivation

Alloys that have a lower bandgap than silicon can extend the infrared response of a silicon cell and hence increase the current generation. One group of materials that are compatible with silicon is Si1-xGex alloys as silicon is completely miscible with germanium. One problem associated with this method is that, because the introduced material has a lower bandgap, it will therefore also cause the device to suffer a loss in voltage. Most research to date has focused on single-junction bulk devices and shows that the loss in voltage overrides the increase in current except for very low germanium content alloys. This work looks at incorporating these Si1-xGex alloys into a thin-film multilayer structure where the flexibility offered through controlling the number and location of junctions facilitates the achievement of high collection probabilities even in thin regions of high germanium concentration where the diffusion lengths are extremely short. PC1D (a one-dimensional circuit simulation package) has been used to simulate the effect of incorporating a layer of Si1-xGex alloy into the multilayer structure. Results show that considerable efficiency enhancement is achieved with this structure, especially for high germanium concentration alloys. The whole range of germanium concentrations is explored

The buried contact solar cell (BCSC) was originally developed as a high performance technology capable of taking full advantage of the high voltages and efficiencies able to be achieved with floatzone substrates. In this process, three lengthy high temperature processes are necessary. In recent months, a simplified process has been developed that eliminates these three lengthy high temperature processes associated with groove diffusion, back surface field formation and the growth of a thick thermal oxide on the surface. Although considerable performance loss is sustained when applied to floatzone substrates, the simplified process appears capable of achieving similar efficiencies to the conventional BCSC process when applied to solar grade Czochralski and multicrystalline substrates. Just as importantly, the simplified BCSC process has been developed for implementation onto existing screen printing production lines using virtually all the same equipment except for the addition of the grooving process which is applied to the virgin wafer prior to any other processing. This process appears to be particularly well suited to existing manufacturers of screen printed solar cells, where the higher performance and lower cost of the BCSC can be achieved without the need for the decommissioning of existing equipment or large scale investment in new infrastructure and equipment

Injection effects can be effectively utilized in multijunction solar cells to provide new device design rules and higher efficiency cells. A multijunction cell with multiple layers can take advantage of injection effects to de-couple the thickness of each individual layer from the overall series resistance. This allows improved collection efficiency in the presence of high surface recombination, reduced series resistance, and reduced metal shadowing losses

Thin film crystalline silicon solar cells can only achieve high efficiencies if light trapping can be used to give a long optical path length, while simultaneously achieving near unity collection probabilities for all generated carriers. This necessitates a supporting substrate of a foreign material, with refractive index compatible with light trapping schemes for the silicon. The resulting inability to nucleate growth of crystalline silicon films of good crystallographic quality on such foreign substrates, prevents the achievement of high efficiency devices using conventional single junction solar cell structures. The parallel multijunction solar cell provides a new approach for achieving high efficiencies from very poor quality material, with near unity collection probabilities for all generated carriers achieved through appropriate junction spacing. Heavy doping is used to minimise the dark saturation current contribution from the layers, therefore allowing respectable voltages. The design strategy, corresponding advantages, theoretical predictions and experimental results are presented