S. Edmiston

Improved modeling of grain boundary recombination in bulk and p‐n junction regions of polycrystalline silicon solar cells

This article provides a theoretical investigation of recombination at grain boundaries in both bulk and p‐n junction regions of silicon solar cells. Previous models of grain boundaries and grain boundary properties are reviewed. A two dimensional numerical model of grain boundary recombination is presented. This numerical model is compared to existing analytical models of grain boundary recombination within both bulk and p‐n junction regions of silicon solar cells. This analysis shows that, under some conditions, existing models poorly predict the recombination current at grain boundaries. Within bulk regions of a device, the effective surface recombination velocity at grain boundaries is overestimated in cases where the region around the grain boundary is not fully depleted of majority carriers. For vertical grain boundaries (columnar grains), existing models are shown to underestimate the recombination current within p‐n junction depletion regions. This current has an ideality factor of about 1.8. An im...

Limits to the efficiency of silicon multilayer thin film solar cells

Abstract 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, at present, 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.

Simplified buried contact solar cell process

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.

Innovative structures for thin film crystalline silicon solar cells to give high efficiencies from low quality silicon

The multilayer solar cell has been specifically designed with the aim to obtain high solar cell efficiency using low quality, thin film, polycrystalline silicon material. The structure consists of multiple p- and n-type silicon layers. This paper examines the tolerance of the cell design to a range of metallic impurities and grain boundaries using computer simulation. The modelled results indicate that the device can tolerate impurity concentrations up to 250 times greater than a conventional, thick solar cell. Further, the results indicate that the structure has excellent tolerance to grain boundaries present in bulk regions of the device. The simulations indicate that grain boundaries present in depletion regions will limit efficiencies considerably if the effective recombination velocity of the grain boundary approaches 10/sup 7/ cm/s. This extreme case should be largely avoided utilizing grain boundary passivation techniques during device fabrication.

Anomalously high collection probability in thin film polycrystalline silicon solar cells from numerical modeling

The impact of grain size on the average collection probability of fine grained (<10 μm) polycrystalline silicon has been investigated using a numerical device simulator. This model predicts that the collection probability can improve dramatically once the depletion regions around adjacent grain boundaries overlap. Thus, contrary to most analytical models, the collection probability is not monotonic with grain size. The collection probability has a local maximum at a grain size determined by the grain boundary trap state density. The magnitude of the improvement is dependent upon the grain boundary recombination velocity.

Grain boundary modeling and characterization of thin-film silicon solar cells

An analytical model is developed to describe recombination currents arising from recombination at grain boundaries (GBs) in the depletion region of a p-n junction solar cell. Grain boundaries are modeled as having a single energy level in the energy gap, and partial occupancy of these states gives rise to a charge on the GB. The analytical model is compared to a complete numerical simulation package (DESSIS) and found to be in good agreement. Additionally, cross sectional EBIC images of a multilayer device containing vertical GBs are presented. Results derived from an analytical model are compared qualitatively to the experimental data and found to give good agreement.

Capitalizing on two dimensional minority carrier injection in silicon solar cell design

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