On the collection of photocurrent in solar cells with a contact grid

The exact solution for the dependence of specific power of photoconversion in the mode of maximum collected power on distance l between lines of the contact grid has been obtained. It is shown that in an optimized case, when the change of the potential of heavily doped front layer under contacts and between contacts is less than kT/q, the characteristic length L can be introduced with a meaning of the distance at which the photocurrent reduces by a factor of e due to recombination. Variation of the filling factor of SC IVC due to the presence of contact grid is then analytically expressed via this length. It is found that in unoptimized case, when the distance between contact strips l is much longer than L, the photocurrent collection is determined by lesser, as compared to L, distance, at which the front layer potential changes from the value of Vm under contacts to the open- circuit voltage between the contacts. In this case the change of IVC filling factor due to the presence of contact grid is expressed again analytically via this new characteristic length. In the intermediate case, when l ≈ L, the solution of the problem can be found by numerical methods only

Conversion efficiency in silicon solar cells with spatially non-uniform doping

The conversion efficiency of diffusion-type silicon solar cells, η, is studied theoretically in assumption of different doping levels existing under collection grid contacts and within the inter-contact spacing. It is shown that at high under-contact doping levels and at relatively low inter-contact doping ones the conversion efficiency increases as compared to uniform doping case. The dependence of η on Shockley-Reed-Hall carrier lifetimes both in the base and in the top-surface n⁺-layer as well as on the depth of p-n-junction and the shape of electron concentration profile, N(x), in the n⁺-region is analysed

Effect of emitter proprties on the conversion efficiency of silicon solar cells

The effect of donor concentration distribution N(x) in the n⁺-emitter on the conversion efficiency h of silicon n⁺-p-p⁺ solar cells is studied theoretically. Shockley-Reed-Hall recombination in the emitter is taken into consideration together with band-to-band Auger recombination. The calculation is performed in the approximation when in the region with changing concentration a small part of generated electron-hole pairs recombines. It is shown that, in general, the correlation between h and p-n - junction depth is absent

Efficiency limit for diffusion silicon solar cells at concentrated illumination

A general approach has been developed to calculation of photoconversion efficiency of thinbase silicon solar cells with double-sided metallization for concentrated solar illumination. The full absorption of photoactive radiation has been theoretically simulated, the light absorption by free charge carriers in heavily doped regions in AM0 conditions was taken into account. It was found that the efficiency of photoconversion η at K ≈ 100 can be as high as 27%

Exciton effects in band-edge electroluminescence of silicon barrier structures

A theoretical analysis of the band-edge electroluminescence efficiency in silicon diodes and p-i-n-structures has been made. We have shown that maximal possible efficiency can achieve 10 % both at room and liquid nitrogen temperatures. Maximal values of the efficiency are restricted by the interband Auger recombination process. It is found that electroluminescence efficiency decreases rapidly with the decrease of characteristic Shockley- Reed-Hall nonradiative lifetime for minority carriers. It is shown that even at room temperatures the main contribution into the edge electroluminescence in silicon barrier structures is given by excitonic effects. Dark I-V characteristics of directly biased silicon diodes measured both at room and nitrogen temperatures are used to explain anomalous temperature dependencies of silicon diode electroluminescence

The study of solar cells with back side contacts at low illumination

Theoretical analysis and experimental research of Si solar cells (SC) with interdigitated back side contacts (BSC) photovoltaic parameters and photoconversion efficiency at low light level have been done in presence of floating p⁺-n junctions and isotype n⁺-n junctions on frontal (illuminated) surface. It has been found that in case of floating junction the magnitudes of short-circuit current, open-circuit voltage and efficiency, as well as of internal quantum efficiency of photocurrent can decrease significantly due to recombination in the space charge region (SCR) rather than to surface recombination. In case of isotype junction, this decrease is absent. These results allow to conclude that the floating p⁺-n junctions at the front surface of the silicon BSC SC would be appropriate for use only in case of an illumination intensity ≥ 1000 W/m²

Polarization unstabilities in a quasi-isotropic He-Ne laser in axial magnetic field

The exact solution for the dependence of specific power of photoconversion in the mode of maximum collected power on distance l between lines of the contact grid has been obtained. It is shown that in an optimized case, when the change of the potential of heavily doped front layer under contacts and between contacts is less than kT/q, the characteristic length L can be introduced with a meaning of the distance at which the photocurrent reduces by a factor of e due to recombination. Variation of the filling factor of SC IVC due to the presence of contact grid is then analytically expressed via this length. It is found that in unoptimized case, when the distance between contact strips l is much longer than L, the photocurrent collection is determined by lesser, as compared to L, distance, at which the front layer potential changes from the value of Vm under contacts to the open- circuit voltage between the contacts. In this case the change of IVC filling factor due to the presence of contact grid is expressed again analytically via this new characteristic length

Exciton-enhanced recombination in silicon at high concentrations of charge carriers

The interrelation between processes of electron-hole recombination and annihilation of excitons in silicon is examined. It is shown, that recombination processes can be essentially influenced by the exciton annihilation at high concentrations of non-equilibrium or equilibrium charge carriers. In n-type material a correlation between Shockley-Reed-Hall lifetime values and a square-law recombination coefficient is found. This correlation is explained in terms of assumption that both Shockley-Reed-Hall lifetime, and non-radiative exciton annihilation time constant responsible for a square-law recombination, are determined by the same deep level. It is stated, that the mentioned regularities should essentially affect the bulk lifetime values in n-type silicon at doping concentration exceeding 10¹⁶ cm⁻³

Effect of excitons on photoconversion efficiency in the p⁺-n-n⁺- and n⁺-p-p⁺-structures based on single-crystalline silicon

We have performed theoretical simulation of the photoconversion efficiency in silicon solar cells for AM0 conditions with regard to excitonic effects. Along with known effects, we have taken into account both radiative and nonradiative exciton annihilation. They manifest themselves as square-law recombination of electron-hole pairs. It was shown that the effect of nonradiative exciton annihilation on the photoconversion efficiency is particularly profound in p⁺-n-n⁺-structures. In solar cells based on them the total action of all the excitonic effects leads to an about 10% decrease in the limiting value of photoconversion efficiency. At the same time for n⁺-p-p⁺-structures the reduction of this value due to the excitonic effects is about 5%. As a result, their limiting value of photoconversion efficiency is higher than that in p⁺-n-n⁺-structures