A. S. Abramov

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.

Study of the light-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters

The photo-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters with an initial efficiency of 10.4% under light flux densities of 1 and 10 kW m−2 (AM1.5G) is studied. It is shown that the stabilized state is reached after 500 h of exposure to the standard light-flux density and after 300 min at a flux 10 times higher in density. In both cases, the efficiency decreases by 1.2–1.4 abs. %. The experimentally measured spectral and current-voltage characteristics of the photovoltaic converters are used to determine the nonequilibrium carrier lifetimes and to calculate variation dependences of the dangling-bond concentration in i-α-Si:H and i-μc-Si:H layers. The dependences are approximated in terms of the floating-bond model. The calculated dangling-bond concentrations after various exposure times are used to simulate the dependences of the photovoltaic-converter parameters on light exposure. The results obtained show good coincidence between the simulated degradation rates of the current and efficiency of a tandem photovoltaic cell and the experimental data.

Method for optimizing the parameters of heterojunction photovoltaic cells based on crystalline silicon

An approach is proposed to calculate the optimal parameters of silicon-based heterojunction solar cells whose key feature is a low rate of recombination processes in comparison with direct-gap semiconductors. It is shown that at relatively low majority-carrier concentrations (Nd ∼ 1015 cm–3), the excess carrier concentration can be comparable to or higher than Nd. In this case, the efficiency η is independent of Nd. At higher Nd, the dependence η(Nd) is defined by two opposite trends. One of them promotes an increase in η with Nd, and the other associated with Auger recombination leads to a decrease in η. The optimum value Nd ≈ 2 × 1016 cm–3 at which η of such a cell is maximum is determined. It is shown that maximum η is 1.5–2% higher than η at 1015 cm–3.

High-efficiency plasma treatment for surface modification of LPCVD ZnO

Plasma treatment of LPCVD Boron-doped ZnO aimed at surface modification of the films has been performed. We have shown that five minutes treatment with RF magnetron Ar plasma can be sufficient to transform surface morphology from as-deposited V-type to U-type, which better suits the growth and enhances the properties of post-deposited microcrystalline silicon as a material for PV modules. Effect of plasma treatment on optical and electrical properties and surface morphology has been studied. Comparative analysis of the acquired results has shown that short time treatment can provide required changes in surface morphology without significant deterioration of structure and electrical and optical properties of treated films, while long time treatment results in reduction of electronic properties most probably caused by excess defect formation at the surface of ZnO films. These results show that, despite promising outlooks, RF magnetron plasma treatment of ZnO for the production of PV modules requires careful optimization.

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.

Study of the properties of solar cells based on a-Si:H p-i-n structures by admittance spectroscopy

Properties of solar cells based on a-Si:H p-i-n structures are studied by admittance spectroscopy. The responses of the density of states in the (i)a-Si:H layers and a-SiC:H layers in the p-type region of the structure are distinguished in the admittance spectra. The density of states in the middle of the mobility gap for (i)a-Si:H is estimated to be 5 × 1016 cm−3 eV−1. It is shown that this value increases during the course of photoinduced degradation to ∼1017 cm−3 eV−1. For the wide-gap a-SiC:H layers, the observed response of the density of states in the valence-band tails made it possible to estimate the lower bound for the density of states at the Fermi level (1018 cm−3 eV−1) and to find the Fermi level position to be 0.4 eV above the valenceband edge. The suggested procedure can be used to optimize the design of solar cells in order to improve their efficiency.

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.

Investigation of the Characteristics of Heterojunction Solar Cells Based on Thin Single-Crystal Silicon Wafers

The operating characteristics of heterojunction solar cells based on single-crystal silicon wafers with a reduced thickness are investigated experimentally. It is found that a decrease in the wafer thickness by 40% as compared to the standard values leads to degradation of the photoelectric-conversion efficiency to 5%. The obtained results could be used for estimating the commercial feasibility of the production of solar cells with a reduced wafer thickness.

Electroluminescent study of the efficiency of silicon heterostructural solar cells

A strong (by more than an order of magnitude) change in the electroluminescence intensity is observed for the first time in high-quality heterojunction solar cells that are based on a single-crystal silicon and have an efficiency of 18 to 20.5%. This effect occurs due to the sharp change in the concentration of the recombination centers on the surface of single-crystal silicon wafers in the course of their pyramidal texturing and also due to the rise in the series resistance. The effect can be used for a quantitative highly sensitive characterization of the texturing, which is a fundamentally important stage in fabricating highly efficient silicon solar cells.

Study of the photoinduced degradation of tandem photovoltaic converters based on a-Si:H/μc-Si:H

The photoinduced degradation of photovoltaic converters based on an a-Si:H/µc-Si:H tandem structure under a standard illuminance of 1000 W/m2 is studied. The spectral and current–voltage characteristics of specially fabricated samples with various degrees of crystallinity of the intrinsic layer in the lower (microcrystalline) cascade are measured in the course of the tests.