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Since the photoconversion efficiency $\eta$ of the silicon-based solar cells (SCs) under laboratory conditions is approaching the theoretical fundamental limit, further improvement of their performance requires theoretical modeling and/or numerical simulation to optimize the SCs parameters and design. The existing numerical approaches to modeling and optimization of the key parameters of high-efficiency solar cells based on monocrystalline silicon (c-Si), the dominant material in photovoltaics, are described. It is shown that, in addition to the four usually considered recombination processes, namely, Shockley-Read-Hall, surface, radiative, and band-to-band Auger recombination mechanisms, the non-radiative exciton Auger recombination and recombination in the space charge region (SCR) have to be included. To develop the analytical SC characterization formalism, we proposed a simple expression to model the wavelength-dependent external quantum efficiency (EQE) of the photocurrent near the absorption edge. Based on this parameterization, the theory developed allows for calculating and optimizing the base thickness-dependent short-circuit current, the open-circuit voltage, and the SC photoconversion efficiency. We proved that the approach to optimize the solar cell parameters, especially its thickness and the base doping level, is accurate and demonstrated for the two Si solar cells reported in the literature, one with an efficiency of 26.7 % and the other with the record efficiency of 26.81 %. It is shown that the formalism developed allows further optimization of the solar cell thickness and doping level, thus increasing the SC efficiency to an even higher value.

The recombination rate in the space charge region (SCR) of a silicon-based barrier structure with long Shockley-Reed-Hall lifetime is calculated theoretically taking into account the concentration gradient of excess electron-hole pairs in the base region. The effects of the SCR lifetime and the applied voltage on the structure's ideality factor are analyzed. The ideality factor is significantly reduced by the concentration gradient of electron-hole pairs. This mechanism provides an increase of the effective lifetime compared to the case when it is insignificant, which is realized at sufficiently low pair concentrations. The theoretical results are shown to be in agreement with experimental data.

Presented in this work are the results of our study of the photoelectric properties of perovskite $CH_3NH_3PbI_{2.98}Cl_{0.02}$ films deposited on a glass substrate using the spin-coating method. The unit cell parameters of the perovskite are determined using x-ray diffractometry. It is shown that the film morphology represents a net of non-oriented needle-like structures with significant roughness and porosity. In order to investigate the properties of the films obtained, non-contact methods were used, such as transmission and reflection measurements and the measurements of the spectral characteristics of the small-signal surface photovoltage. The method of spectral characteristics of the low-signal surface photovoltage and the transmission method reveal information about the external quantum yield in the films studied and about the diffusion length of minority carriers in the perovskite films. As a result of this analysis, it has been established that the films obtained are naturally textured, and their bandgap is 1.59 eV. It is shown that in order to correctly determine absorption coefficient and the bandgap values, Urbach effect should be accounted for. Minority carriers' diffusion length is longer than the film thickness, which is equal to 400 nm. The films obtained are promising materials for solar cells., Comment: 17 pages, 7 figures

The transformation of the long-wavelength edge of the external quantum exit (EQE) formation mechanisms in textured silicon solar cells (SCs) is revealed, depending on their thickness. Expressions for experimental EQE dependences on the long-wavelength absorption edge are obtained for a wide range of base thicknesses (100-450 {\mu}m). The expressions allow optimal SC base thickness values calculation from the condition of maximal photoconversion efficiency taking into account surface recombination velocity. In particular, it was found that optimal 100-{\mu}m base thickness corresponds to the surface recombination velocity of about 3 cm/s., Comment: In Russian, 10 pages, 3 figures

Surface recombination affects both light-to-electricity and electricity-to-light conversion in solar cells (SCs). Therefore, quantitative analysis and reduction of surface recombination is an important direction in SC research. In this work, electroluminescence (EL) intensity and photoconversion efficiency of a set of 93 large-area (239\,cm$^2$) a-Si:H/c-Si heterojunction SCs (HJSCs) are measured under AM1.5 conditions at 298 K. The HJSC samples differed only in surface recombination velocity, $S$, but otherwise were identical. Variation in $S$ was due to the variation of the chemical conditions under which the samples were treated. It is established that EL quantum efficiency, is affected by $S$ much more strongly than photoconversion efficiency, $\eta$: namely, the reduction of the latter from 20.5\% to 18\% due to an increase of $S$ is accompanied by a decrease of the former by more than an order of magnitude. In HJSCs with well passivated surfaces, i.e. low $S$, EL efficiency reached 2.1\%, which is notably higher than the known values in silicon homojunction diodes. For temperature-dependent measurements of EL and dark I-V curves, one of the samples was cut into small-area (1 cm$^2$) pieces. It was found that EL intensity as a function of temperature develops a maximum at $T$ = 223 K. At low temperatures, the current at weak bias is shown to be due to tunneling mechanism. A theoretical model is developed that explains all these findings quantitatively.

A theoretical approach to photoconversion efficiency modeling in perovskite p-i-n structures is developed. The results of this modeling compare favorably with the experiment and indicate that the surfaces of the perovskite solar cells (SCs) are naturally textured. It is shown that photoconversion efficiency in the limiting case of negligible Shockley-Read-Hall and surface recombination and in the absence of optical losses reaches the value of 29%. In the realistic case, the current-voltage curve ideality factor equals 2. This value is not due to recombination in the space-charge region; rather, it can be explained by taking into account the effect of the rear surface and high excitation level., Comment: 3 pages, 3 figures

Nonradiative exciton lifetime in silicon is determined by comparison of the experimental and theoretical curves of bulk minority charge carriers lifetime on doping and excitation levels. This value is used to analyze the influence of excitonic effects on internal luminescence quantum yield at room temperature, taking into account both nonradiative and radiative exciton lifetimes. A range of Shockley-Hall-Reed lifetimes is found, where excitonic effects lead to an increase of internal luminescence quantum yield.

We obtain theoretical expressions for the photocurrent in the Heterojunction solar cells with Intrinsic Thin layer (HIT cells). Our calculations take into account tunneling of electrons and holes through wide-bandgap layers of $\alpha$-Si:H or $\alpha$-SiC:H. We introduce the criteria, under which tunneling does not lead to the deterioration of solar cell characteristics, in particular, to the reduction of the short-circuit current and open-circuit voltage. We propose an algorithm to compute the photoconversion efficiency of HIT elements, taking into account the peculiarities of the open-circuit voltage generation, in particular, its rather high values. We test our theoretical predictions against the experimental results. For this, we fabricate HIT elements with the efficiency of about $20\,\%$. We measured the temperature dependence of the short-circuit current, open-circuit voltage, photoconversion power, and fill factor of the current-voltage curve of these elements in a wide temperature range from 80 to 420\,K. In the low-temperature range, the open-circuit voltage and the photoconversion power decrease on cooling. At $T \ge 200$\,K, the theoretical expressions and the experimental curves agree rather well. The behavior of the fill factor and output power at low temperatures is explained by the increase of the series resistance on cooling. We discuss the reasons behind the reduction of the power temperature coefficient in HIT elements. We show that they are related to the low value of the combined surface and volume recombination rate. Finally, we derive a theoretical expression for the HIT element's operation temperature under natural working conditions.

The theoretical analysis of photovoltaic conversion efficiency of highly effective silicon solar cells (SC) is performed for n-type and p-type bases. The case is considered when the Shockley-Read-Hall recombination in the silicon bulk is determined by the deep level of Fe. It is shown that due to the asymmetry of the recombination parameters of this level the photovoltaic conversion efficiency is increasing in the SC with the n-type base and decreasing in the SC with the p-type base with the increase in doping. Two approximations for the band-to-band Auger recombination lifetime dependence on the base doping level are considered when performing the analysis. The experimental results are presented for the key characteristics of the solar cells based on $\alpha-Si:H-n-Si$ heterojunctions with intrinsic thin layer (HIT). A comparison between the experimental and calculated values of the HIT cells characteristics is made. The surface recombination velocity and series resistance are determined from it with a complete coincidence of the experimental and calculated SC parameters' values., Comment: 13 pages, 6 figures

An analysis is made of the conditions for ohmic contacts realization in the case of Schottky contacts. Based on the classical notions about the mechanisms of current flow, we consider the generalized model of Schottky contact that takes into account the thermionic current of majority charge carriers and recombination current of minority charge carriers in Schottky contacts with a dielectric gap. An analysis of the results given by that model made it possible to obtain ohmicity criteria for Schottky contacts and compare the conditions for low injection level and ohmicity of Schottky contacts in the case of silicon-based contacts. It is shown that conditions for Schottky contact ohmicity do not coincide with those for p-n junctions., Comment: 20 pages, in Russian, 4 figures

The selfconsistent model for the temperature dependence of photoconversion efficiency $\eta$ for highly efficient silicon solar cells (SCs) is developed. It is demonstrated that effect of the efficiency decrease due to increasing temperature is less pronounced in the SCs with lower surface recombination velocity, thus offering a possibility to improve the cells' performance. The photoconversion efficiency of the high efficiency silicon solar cells is modeled for the realistic ambient conditions. The SC operating temperature is determined by self-consistently solving the photocurrent, photovoltage, and energy balance equations, considering both radiative and convective cooling mechanisms. The SC temperature is shown to be substantially higher than the ambient temperature even at very high convection coefficients, such as, e.g., 300 $W / (m^2 \cdot K)$, used in our examples. The photoconversion efficiency for this case is substantially below the efficiency of thermally stabilized SC, for which the operating temperature is close to the external temperature. The open-circuit voltage and photoconversion efficiency of the high-quality silicon solar cells under concentrated illumination are also investigated including the tradeoff between SCs heating and cooling processes., Comment: 20 pages, in Russian, 7 figures

Conversion of energy of beta-particles into electric energy in a p-n junction based on direct-bandgap semiconductors, such as GaAs, considering realistic semiconductor system parameters is analyzed. An expression for the collection coefficient, $Q$, of the electron-hole pairs generated by beta-electrons is derived taking into account the existence of the dead layer. We show that the collection coefficient of beta-electrons emitted by a \Tr-source to a GaAs p-n junction is close to 1 in a broad range of electron lifetimes in the junction, ranging from $10^{-9}$ to $10^{-7}$ s. For the combination \Pm/GaAs, $Q$ is relatively large ($\ge 0.4$) only for quite long lifetimes (about $10^{-7}$ s) and large thicknesses (about $100\,\mu$m) of GaAs p-n junctions. For realistic lifetimes of minority carriers and their diffusion coefficients, the open-circuit voltage realized due to the irradiation of a GaAs p-n junction by beta-particles is obtained. The attainable beta-conversion efficiency $\eta$ in the case of a \Tr/GaAs combination is found to exceed that of the \Pm/GaAs combination.

The conversion of energy of electrons produced by a radioactive source into electricity in a Si and SiC $\textit{p-n}$ junctions is modeled. The features of the generation function describing the electron-hole pair production by an electron flow and the emergence of a "dead layer" are discussed. The collection efficiency, $Q$, describing the rate of electron-hole pair production by incident beta particles, is calculated taking into account the presence of the "dead layer". It is shown that in the case of high-grade Si $\textit{p-n}$ junctions, the collection efficiency, $Q$, of electron-hole pairs created by a high-energy electrons flux, e.g. Pm-147 beta flux, is close or equal to 1 in a wide range of electron energies. For SiC junctions, $Q$ is large enough (about 1) only for electrons with relatively low energies of about 5 keV, as produced, e.g., by a tritium source, and decreases rapidly with further increase of electron energy. The conditions, under which the influence of the "dead layer" on the collection efficiency is negligible, are determined. The open-circuit voltage is obtained for realistic values of the minority carriers' diffusion coefficient and lifetime in Si and SiC $\textit{p-n}$ junctions irradiated by a high-energy electrons flux. Our calculations allow us to estimate the attainable efficiency of betavoltaic elements., Comment: 17 pages, 8 figures

To accurately calculate efficiencies $\eta$ of experimentally produced multijunction solar cells (MJSCs) and optimize their parameters, we offer semi-analytical photoconversion formalism that incorporates radiative recombination, Shockley-Read-Hall (SRH) recombination, surface recombination at the front and back surfaces of the cells, recombination in the space charge region (SCR) and the recombination at the heterojunction boundaries. Selfconsistent balance between the MJSC temperature and efficiency was imposed by jointly solving the equations for the photocurrent, photovoltage, and heat balance. Finally, we incorporate into the formalism the effect of additional photocurrent decrease with subcell number increase. It is shown that for an experimentally observed Shockley-Read-Hall lifetimes, the effect of re-absorption and re-emission of photons on MJSC efficiency can be neglected for non-concentrated radiation conditions. A significant efficiency $\eta$ increase can be achieved by improving the heat dissipation using radiators and bringing the MJSC emissivity to unity, that is closer to black body radiation rather than grey body radiation. Our calculated efficiencies compare well with other numerical results available and are consistent with the experimentally achieved efficiencies. The formalism can be used to optimize parameters of MJSCs for maximum photoconversion efficiency., Comment: 40th IEEE Photovoltaic Specialists Conference, June 8-13, 2014, Denver, Colorado, III-V Epitaxy and Solar Cells, F30 168

In this paper, we investigate renormalization of charge carrier effective masses and bandgap narrowing in n-GaAs and wurtzite-type n-GaN over a wide range of temperatures and dopant concentrations. The calculations are based on the Green's function formalism. Contrary to the previous works, we consider the regions below as well as above the Mott transition. Special attention is paid to formation of donor subband and condition for the Mott transition. We also take into account the effects caused by optical phonons. The latter strongly depend on the doping level because of dynamic screening. It is shown that three specific doping levels may be set off in n-GaN. They correspond to 1) Mott transition, 2) resonance amplification of optical phonon-plasmon, and 3) full dynamic screening of optical phonons, respectively. Contrary to the case of n-GaN, the effect of full dynamic screening cannot be implemented in n-GaAs because of stronger nonparabolicity of conduction band., Comment: This paper has been withdrawn by the author due to reconsidering of the approach

We analysed self-consistently photoconversion efficiency of direct-gap A3B5 semicon-ductors based solar cells and optimised their main physical characteristics. Using gallium ar-senide (GaAs) as the example and new efficient optimization formalism, we demonstrated that commonly accepted light re-emission and re-absorption in solar cells (SC) in technologically produced GaAs (in particular, with solid- or liquid-phase epitaxy) are not the main factors re-sponsible for high photoconversion efficiency. As we proved instead, the doping level of the base material and its doping type as well as Shockley-Read-Hall (SRH) and surface recombination velocities are much more important factors responsible for the photoconversion. We found that the maximum photoconversion efficiency (about 27% for AM1.5 conditions) in GaAs with typical parameters of recombination centers can be reached for p-type base doped at $2 \cdot 10^{17}$ cm$^{-3}$. The open circuit voltage $V_{OC}$ formation features are analyzed. The optimization provides a significant increase in $V_{OC}$ and the limiting photoconversion efficiency close to 30%. The approach of this research allows to predict the expected solar cells (for both direct-gap and indirect-band semiconductor) characteristics if material parameters are known. Obtained formalism allows to analyze and to optimize mass production both tandem solar cell (TSC) and one-junction SC parameters., Comment: 32 pages, 16 figures, 3 tables

Contact resistivity rc of InP and GaAs based ohmic contacts was measured in the 4.2/300 K temperature range. Nonmonotonic dependences rc(T), with a minimum at temperature 50 K (150 K) for InP (GaAs) based contacts were obtained. The results can be explained within the framework of the mechanism of current flow through metal shunts (associated with dislocations) penetrating into the semiconductor bulk, with allowance being made for electron freeze-out at helium temperatures. Contact ohmicity in the 4.2/30K temperature range is due to accumulation band bending near shunt ends at the metal/semiconductor interface.

We develop a new approach to calculate the obtainable limit of photoconversion efficiency of tandem solar cells (SCs) and applied it to SCs with both vertical and lateral designs at AM0 and AM1.5 conditions. To get the maximum efficiency, only radiative recombination has been considered using typical radiative recombination parameters of the direct band gap III-V semiconductors, and explicit energy dependence of light absorption. When simulating the efficiency, we selfconsistently took into account the fact that the amount of the heat dissipated by SC decreases as the number of current-matched sub-cells increases. As the operating SCs temperature decreases both the open-circuit voltage and the photoconversion efficiency increase. It is shown that the above effect is especially strong for SCs operating under AM0 conditions. As the number of subcells is increased, narrowing the spectral range for each subcell, the photocurrent is additionally reduced due to the energy dependent light absorption, the factor generally ignored in the standard approaches. Application of our formalism results in a maximum in the theoretical dependence of the efficiency on the number of subcells, which was indeed observed experimentally. Besides agreement with experiment, our theoretical results are also close to other efficiencies calculated using detailed balance based approaches., Comment: 21 pages, 11 figures

The radiative recombination, Shokley-Read recombination, frontal-surface and rear-surface recombination and the recombination at the heterojunction boundaries and the recombination in the space charge region are considered in the calculation of the multijunction solar cell (MSC) efficiency. The calculation is performed by a self-consistent solution of the equations for the photocurrent and photovoltage, as well as the heat balance equation. A cooling of MSC with the increase of the number of cells n and the improvement in the heat dissipation is regarded. It was found that, as the number of cells n is increased, narrowing of spectral range for each cell causes additional reduction of current. A substantial increase in the MSC efficiency can be achieved by improving the heat extraction using radiators and increasing emissivity. A comparison is made between the calculated and experimental efficiency values. A rather good agreement was found. A comparison between this calculation and other formalisms is given., Comment: 21 pages, 7 figures. The paper (in Russian) is submitted for publication in "Fizika i Tekhnika Poluprovodnikov" (Semiconductors)

The results of our researches of the influence of exponentially distributed surface centers on the effective surface recombination rate and the parameters of silicon solar cells (SCs) are reported. In our calculations, we assumed the acceptor and donor surface states to lie in the upper and lower, respectively, parts of the bandgap. The model also supposed a discrete surface level to exist in the middle of the energy gap. In the case where the integrated concentration of continuously distributed centers is comparable with that of deep surface levels, those centers can affect the SC parameters only due to the recombination. If the concentration of continuously distributed centers is comparable or higher than the concentration characterizing a charge built-in into the insulator, those centers directly affect the surface band bending and the photo-induced electromotive force. With the help of a computer simulation, the conditions for the rate of surface recombination through continuously distributed surface centers to exceed that through the deep discrete level are determined. A decrease of the open-circuit voltage in inverted silicon SCs associated with the recombination through continuously distributed centers is calculated. The obtained theoretical results are compared with the experimental data.

We investigated temperature dependence of contact resistance of an Au-Ti-Pd2Si ohmic contact to heavily doped n+-Si. The contact resistance increases with temperature owing to conduction through the metal shunts. In this case, the limiting process is diffusion input of electrons to the metal shunts. The proposed mechanism of contact resistance formation seems to realize also in the case of wide-gap semiconductors with high concentration of surface states and dislocation density in the contact.

The thickness dependences of the photocurrent quantum yield and photoenergy parameters of silicon backside contact solar cells (BC SC) are investigated theoretically and experimentally. The surface recombination rate on the irradiated surface was minimized by means of creating the layers of microporous silicon. A method of finding the surface recombination rate and the diffusion length of minority carriers from the thickness dependences of the photocurrent quantum yield under conditions of the strong absorption is proposed. The performed studies allowed us to establish that the thinning of the BC SC samples in the case of minimizing the surface recombination rate gives a possibility to achieve rather high efficiencies of photoconversion. It is also shown that the agreement between the experimental and theoretical spectral dependences of the photocurrent quantum yield can be reached only with regard for the coefficient of light reflection from the backside surface., Comment: 8 pages, 8 figures; Journal's website: http://www.ujp.bitp.kiev.ua