V. P. Kostylyov

Temperature dependence of photoconversion efficiency in silicon heterojunction solar cells: Theory vs experiment

Silicon heterojunction solar cells (HJSC) with the efficiency of about 20% are manufactured. Their short-circuit current, open-circuit voltage, photoconversion efficiency, and fill factor of the current–voltage curve are measured in a broad temperature range from 80 to 420 K. It is established that the open-circuit voltage, the fill factor, and the photoconversion efficiency are non-monotonic functions of temperature, having a maximum in the vicinity of 200 K. A new approach to modeling of HJSCs is proposed, which allows one to obtain quantitative agreement with the experimental results at temperatures above 200 K, as well as to describe the results published in the literature on the solar cells under AM1.5 conditions. The temperature coefficient of photoconversion efficiency in HJSCs is discussed, and its low value is shown to be related to the low surface and volume recombination rates. Finally, a theoretical expression for the SCs temperature under natural working conditions is derived.

The radiative recombination coefficient and the internal quantum yield of electroluminescence in silicon

The results of the analysis of variations in the radiative recombination coefficient with varying doping level and concentration of excess electron-hole pairs are reported. It is shown that, along with the effect of narrowing of the band gap calculated in the many-electron approximation, the effect of screening of the Coulomb interaction responsible for the decrease in the excition binding energy should be taken into account. Both effects produce similar trends and decrease the radiative recombination coefficient with increasing levels of doping or injection. The contributions of excitonic radiative recombination and band-to-band radiative recombination to the total radiative recombination coefficient are separated from each other. It is shown that, in the region of room temperature, both contributions are comparable, while at liquid-nitrogen temperature, the excitonic component dominates over the band-to-band component. The results obtained by refined calculations of the limiting value of the internal quantum yield of electroluminescence for the silicon diodes and p-i-n structures are presented. It is shown that the internal quantum yield of electroluminescence can be as high as 14%. However, this values sharply decreases with increasing surface recombination rate and decreasing lifetime of excess charge carriers in the bulk.

The mechanism of contact-resistance formation on lapped n-Si surfaces

Anomalous temperature dependences of the specific contact resistance ρc(T) of Pd2Si-Ti-Au ohmic contacts to lapped n-Si with dopant concentrations of 5 × 1016, 3 × 1017, and 8 × 1017 cm−3 have been obtained. The anomalous dependences of ρc(T) have been accounted for under the assumption that the current flows along nanodimensional metallic shunts, which are combined with dislocations with a diffusionrelated limit in the supply of charge carriers taken into account. The densities of conducting and scattering dislocations in the surface region of the semiconductor are determined.

Analysis of the possibility of high-efficiency photovoltaic conversion in tandem heterojunction thin-layer solar cells

The possibility of creating tandem heterojunction-with-intrinsic-thin-layer (HIT) solar cells possessing photovoltaic conversion efficiency greater than that of the best existing single-junction HIT structures is analyzed. It is established that, because of small carrier lifetimes and high degrees of compensation, the use of amorphous silicon in tandem HIT cells cannot provide for record high conversion efficiency. Key parameters of the material for a widegap p-n junction on the frontal side of tandem solar cells are determined that will allow a photovoltaic conversion efficiency above 25% under AM1.5 conditions to be reached.

Simulation of the natural characteristics of vertical a-Si:H/μc-Si:H tandem solar cells. 1. General relations

An approach to calculating the characteristics of vertical (series) a-Si:H/μc-Si:H tandem solar cells (SCs) at arbitrary angles of incidence of sunlight is developed. The multiple reflection and refraction of electromagnetic waves at the internal interfaces of a tandem SC, in particular, at the a-Si:H/μc-Si:H interface is taken into account. In the calculation of the ideality factor and saturation current density of the diode component of the I–V characteristic of a tandem SC, as well as in the calculation of all its photovoltaic characteristics on the basis of these parameters at arbitrary sunlight incidence angles, general relations are used that take into account the recombination of excess carriers in both the quasi-neutral regions and in the space charge regions of the investigated structure. Expressions are obtained for determining all the main parameters of the component parts of a tandem SC, which are necessary for calculating the photovoltaic characteristics of the entire tandem SC. The results of calculation for standard AM1.5G (1000 W/m2) illumination conditions are reported.

Comparative Analysis of Photoconversion Efficiency in the Si Solar Cells under Concentrated Illumination for the Standard and Rear Geometries of Arrangement of Contacts

In quite realistic approximations, the theory of photoconversion in Si solar cells with the concentrated illumination is developed for the rear and standard arrangement of current-collecting contacts. The difference between the effect of heating on the efficiency of solar cells with the rear and standard geometries is taken into account. A comparative analysis of the results obtained for two mentioned geometries is carried out. It is shown that the Si solar cells with the rear metallization, in principle, can possess higher photoconversion efficiency. The agreement of the developed theory with the experiment is obtained.

On the ultimate quantum efficiency of band-edge electroluminescence in silicon barrier structures

The ultimate quantum efficiency of electroluminescence in silicon diodes and p-i-n structures at room temperature is calculated. It is shown that the internal quantum yield of electroluminescence is about 10% and is implemented at optimal doping levels for the n-and p-type regions of silicon diodes, ∼1015 and 5×1016 cm−3, respectively. With a decrease in the Shockley-Read-Hall lifetimes of electrons and holes, the internal quantum yield of electroluminescence in silicon barrier structures drops. The physical processes related to the effect of excitons in silicon has much in common with those in electroluminescence, photoluminescence, and photoconversion. It is shown that only electroluminescent p-i-n structures are promising for use in silicon integrated circuits.

Specific features of current flow in α-Si : H/Si heterojunction solar cells

Specific features of the formation of dark I–V characteristics of α-Si: H/Si heterojunction solar cells are investigated taking account the ratio between silicon doping level Nd and excess concentration Δn of electron–hole pairs. It is demonstrated that, at Δn ≥ Nd, the I–V characteristic is fundamentally different from the characteristic of a classical Shockley diode due the effect of the backside surface (additional drop of the applied voltage). The results of analysis are used to describe the experimental I–V characteristics reported in studies on α-Si: H/Si heterojunction solar cells. Numerical values of the ideality factors of the dark I–V characteristics are obtained by comparing the experimental and calculated curves.

The temperature dependence of the characteristics of crystalline-silicon-based heterojunction solar cells

Temperature dependences of the photovoltaic characteristics of (p)a-Si/(i)a-Si:H/(n)c-Si singlecrystalline- silicon based heterojunction-with-intrinsic-thin-layer (HIT) solar cells have been measured in a temperature range of 80–420 K. The open-circuit voltage (VOC), fill factor (FF) of the current–voltage (I–U) characteristic, and maximum output power (Pmax) reach limiting values in the interval of 200–250 K on the background of monotonic growth in the short-circuit current (ISC) in a temperature range of 80–400 K. At temperatures below this interval, the VOC, FF, and Pmax values exhibit a decrease. It is theoretically justified that a decrease in the photovoltaic energy conversion characteristics of solar cells observed on heating from 250 to 400 K is related to exponential growth in the intrinsic conductivity. At temperatures below 200 K, the I–U curve shape exhibits a change that is accompanied by a drop in VOC. Possible factors that account for the decrease in VOC, FF, and Pmax are considered.

Peculiarities of photoconversion efficiency modeling in perovskite solar cells

A theoretical approach to photoconversion efficiency modeling in perovskite p–i–n structures is developed. The results of the theoretical modeling are compared with the experiment. Analysis of the experimental data indicated that the surfaces of the perovskite solar cells (SCs) are spontaneously textured, which leads to more effective light absorption and increased photoconversion efficiency. It is established that photoconversion efficiency as a function of i-layer thickness has a maximum in the region 0.3–0.9 μm.