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

Photoconversion efficiency of quantum-well solar cells for the optimum doping level of a base

Analytical expressions for the maximum obtainable photoconversion efficiency of quantum-well solar cells (QWSCs) under AM0 conditions are given. The modeling of the photoconversion efficiency of QWSCs under AM1.5 conditions using the SimWindows program is fulfilled. It is shown that the photoconversion efficiency of QWSCs with the A₃B₅ p-i-n structure is rather low because of a low photovoltage value. To improve this situation, the base region should be doped heavily enough. Light concentration makes it possible to realize high photoconversion efficiencies for A₃B₅ quantum-well p-i-n structures with a low background level of the base region doping. Their values are comparable to the photoconversion efficiencies for solar cells (SCs) with rather high base region doping levels

Photoeffect Peculiarities in Macroporous Silicon Structures

The effects of increase in photoconductivity in the macroporous silicon structures have been examined as a function of the distance between cylinder macropores. The ratio of macroporous silicon photoconductivity to bulk silicon one has been found to achieve a maximum at the distance between macropores equal to the double thickness of the Shottky layer what corresponds to the experimental results. The relaxation time of photoconductivity for macroporous silicon structures was found to be defined by the light modulation of the barrier on macropore surfaces whereas its relaxation to occur according to the logarithmic law. If T>180 K, the temperature dependence of the relaxation time of photoconductivity is defined by a thermo-emission mechanism of the current transport in the space charge region and below 100 K the relaxation time is controlled by the processes of tunnel current flow.Досліджені ефекти підвищення фотопровідності в структурах макропористого кремнію в залежності від відстані між циліндричними макропорами. Встановлено, що відношення фотопровідності макропористого кремнію до фотопровідності монокристалічного кремнію досягає максимуму при відстані між порами, яка дорівнює двом товщинам шару Шотткі, що відповідає результатам експерименту. Час релаксації фотопровідності структур макропористого кремнію визначається модуляцією світлом бар'єру на поверхні макропор, а її релаксація відбувається за логарифмічним законом. При T>180.К температурна залежність часу релаксації фотопровідності визначається термоемісійним механізмом проходження струму в області просторового заряду, а при T180.К температурная зависимость времени релаксации фотопроводимости определяется термоэмиссионным механизмом прохождения тока в области пространственного заряда, а при Т<100К - туннельными процессами токопереноса

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

A fresh approach to interpretation of visible photoluminescence spectra in silicon nanostructures

To understand both multimodal character of stationary photoluminescence (PL) spectra and observed peculiarities in time-resolved PL in low-dimensional Si structures, it is proposed to take into account an additional effect, which has to emerge in such structures due to indirect-bandgap nature of silicon material. The effect implies that the exciton radiative lifetime becomes a nonmonotonous (oscillating) function of the nanocrystal (NC) size. As a result, in the calculated PL spectra the energy distance between PL peaks or PL minima practically determined by the mean NC size, while dispersion in NC sizes plays a minor role. The qualitative agreement between calculated PL spectra and PL spectra observed experimentally in porous silicon and nanocrystalline silicon (nc-Si) films counts in favor of the used model of radiative exciton recombination

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%

Lateral multijunction photovoltaic cells

In this work, features of solar cells of lateral type were analyzed. The authors offered a design of a monolithic compact solar module with cells electrically connected in series and with a dispersion element (holographic grating). Simulation of a multistage converter was carried out. It has been shown that with increasing the number of cells n the maximum limited efficiency n increases. Its maximum values up to n≈ 53.6% are reached for n = 15 (in the case of perfectly matched semiconductors and conditions AM0). With further increase of n , the efficiency decreases. For a set of concrete semiconductors n ≈ 45% the maximum efficiency for AM0 conditions may be achieved, when the number of cells equals 4. It has been shown that the calculation results agree with experimental data. The possibility of technical implementation of solar cells with using inkjet printers was investigated, too. Briefly discussed have been the properties of these printers, as well as metal, semiconductor and dielectric inks

The influence of the exciton non-radiative recombination in silicon on the photoconversion efficiency. 1. Long Shockley–Read–Hall lifetimes

By comparison of the experimental dependence of bulk lifetime in silicon on the doping and excitation levels with theoretical calculations, it has been shown that a new recombination channel becomes operative when Shockley–Read–Hall lifetime is below 20 ms and the density of doping impurities or the excess electron-hole pair density is of the order of 10¹⁶ cm⁻³. This recombination mechanism is related to the non-radiative exciton Auger recombination assisted by the deep impurities in the bulk. The influence of non-radiative exciton recombination on the photoconversion efficiency in solar cells has been analyzed. It has been shown that the shorter the Shockley–Read–Hall lifetime, τSHR, the stronger its effect. In particular, for τSHR = 100 μs, this recombination channel leads to the reduction of the photoconversion efficiency by 5.5%

Modeling of photo-conversion efficiency for hydrogenated amorphous Si p-i-n structures

An analytical formalism to optimize the photoconversion efficiency η of hydrogenated amorphous silicon-based (a-Si:H) solar cells has been developed. This model allows firstly the optimization of a p⁺ -i-n sandwich in terms of carrier mobilities, thickness of the layers, doping levels, and others. Second, the geometry of grid fingers that conduct the photocurrent to the bus bars and ITO/SiO₂ layers has been optimized, and the effect of non-zero incidence angles of Sun’s light has been included as well. The optimization method has been applied to typical a-Si:H solar cells. The codes allow the optimization of amorphous Si based solar cells in a wide range of parameters and are available on the e-mail request