A new method for the direct calculation of the two‐dimensional collection probability in pn junction solar cells is presented. Based on its reciprocity properties, the inhomogeneous continuity equation for excess carriers is transformed to a homogeneous partial differential equation (PDE) for the probability of carriers being collected in the external circuit. The new PDE is easier to solve and directly gives the short‐circuit current. The method is applied to study the impact of grain‐boundary recombination on the performance of polycrystalline silicon solar cells. A critical grain width of four times the carrier diffusion length in the base is found to be the limiting boundary between polycrystalline behavior and monocrystalline behavior of the cell. The sensitivity of short‐circuit AM1.5 collection efficiency to the grain width Wg, the grain‐boundary recombination velocity Sg, minority‐carrier diffusion lengths, and surface recombination velocities, is quantified for a variety of cell types and recombination parameters. The sensitivity analysis indicates that AM1.5 collection efficiency is most sensitive to grain width in narrow grain cells, and to base diffusion length in wide grain cells.

Direct calculation of two‐dimensional collection probability in pn junction solar cells, and study of grain‐boundary recombination in polycrystalline silicon cells

A new relation for the grain boundary potential barrier height under optical illumination is presented. Calculations have been performed of the grain boundary potential barrier heights and effective recombination velocities of minority carriers at the edge of space-charge regions as a function of illumination level. These computations show that the sensitivity of grain boundary potential barrier height to illumination level is determined by the bulk diffusion length and the ratio of hole capture cross-section to electron capture cross-section of the grain boundary recombination centres. The dependence of the barrier height at grain boundary on the grain size has also been investigated theoretically. The available experimental data are found to be in good agreement with the theoretical predictions.

Carrier recombination at grain boundaries in polycrystalline silicon under optical illumination

The effect of grain boundaries upon the efficiency of polycrystalline GaAs thin‐film solar cells is analyzed. Solar‐cell properties are calculated on a simple model where grain boundaries act as recombination centers to reduce the minority‐carrier diffusion length in the solar cell’s active layer and increase the space‐charge layer recombination current. An effective diffusion length is expressed in terms of grain size, allowing the calculation of short‐circuit current density and open‐circuit voltage. Excellent agreement is obtained between theory and experiment. The fabrication of thin‐film GaAs solar cells with an efficiency greater than 18% appears to be possible if the grain size in the thin‐film GaAs layer with thickness of 3 μm is larger than 1000 μm.

Efficiency considerations for polycrystalline GaAs thin-film solar cells

Key assumptions are made, with justification, to simplify the three-dimensional, nonlinear boundary-value problem that defines minority-carrier transport, including recombination, in polysilicon devices. These assumptions enable the separation of the grain-boundary recombination analysis, which is based on quasi-equilibrium in the space-charge region, from the intragrain transport analysis, which is done by partitioning the grain into subregions in which the minority-carrier flow is predominantly one-dimensional. The analyses are coupled through the effective minority-carrier recombination velocity at the grain boundary, which generally is dependent on the minority-carrier density in the quasi-neutral grain. Limitations of the model implied by the quasi-equilibrium assumption are effectively removed by recognizing that when conditions obtain that negate quasi-equilibrium, the effective recombination velocity is fixed at the minority-carrier kinetic-limit velocity. The model development is facilitated by computer-aided numerical analysis of the grain-boundary recombination and is supported by qualitative discussion of the underlying physics.

Analysis of minority-carrier transport in polysilicon devices

Based on a literature review, we explore the material and performance requirements for high-efficiency, potentially low-cost, GaAs solar cells. The goal is a GaAs solar cell, on a low-cost substrate, having an efficiency greater than 20%. An important issue limiting efficiency for polycrystalline GaAs cells is recombination through deep levels associated with grain boundaries, specifically at the part of a grain boundary that intersects the p-n junction. The effect of this junction recombination on cell efficiency is shown as a function of grain size. We explore the potential impact of grain size, grain-boundary passivation, intragrain defects, impurities, and crystal orientation. The impact of intragrain defects on minority-carrier lifetime and cell efficiency is also discussed. We conclude that two critical parameters for achieving high efficiency are the mitigation of intragrain-defect density and perimeter junction recombination. To achieve over 20% efficiency, dislocation densities need to be reduced...

Requirements for a 20%-efficient polycrystalline GaAs solar cell

Numerical calculations have been made of the effect of grain size on the short-circuit current and the AM1 efficiency of polycrystalline thin-film GaAs and InP (2 µm thick) and silicon (25 µm thick) p-n junction solar cells. Junction solar cells are seen to be more efficient than Schottky-barrier cells, due to the higher dark current associated with Schottky diodes. GaAs shows the highest efficiency and both GaAs and InP attain 90 percent of their maximum efficiencies at a grain size of 10 µm, while silicon requires grain sizes of 200 µm to attain 90 percent of maximum efficiency. However, the deleterious effect of poor lifetimes and mobilities is less for silicon polycrystalline cells than for the direct-bandgap devices.

H.J. Hovel

Papers

Efficiency calculations for thin-film polycrystalline semiconductor p-n junction solar cells