M. A. Mintairov

Experimental characterization and self-consistent modeling of luminescence coupling effect in III-V multijunction solar cells

In this paper, we focused on developing an accurate model to describe the luminescent coupling (L-C) effect in multijunction solar cells (MJSC) under light concentration. We present here a transcend current-voltage (I-V) formula combined with a self-consistent simulation algorithm to derive the coupling yield γ dependence on light intensity by including the electrical parameters such as shunt resistance (Rsh) and series resistance (Rs), which were ignored in previous simulation models. The effects of both Rsh and Rs on γ were revealed, and the dependence of γ on the external voltage bias Vbias was investigated. Meanwhile, we have performed experiments to determine coupling yield γ by measuring the I-V curves of individual subcell of InGaP/GaAs/Ge triple junction solar cell under varied light intensity. We found that the measured results are only in good agreement with the simulated data obtained from the model where the resistance parameters were included. Based on these results, we calculated the convers...

Method for direct measurements of luminescent coupling efficiency in concentrator MJ SCs

A theoretical model describing the coupling effect for p-n junctions being in a direct optoelectronic contact in the multi-junction solar cell structures is elaborated. The experimental method for determination of the transfer function (coupling yield) is proposed. The method is realized at investigation of triple-junction GaInP/GaAs/Ge solar cells.

The Segmental Approximation in Multijunction Solar Cells

The segmental approach has been considered to analyze dark and light I-V curves. The photovoltaic (PV) dependence of the open-circuit voltage (Voc), the maximum power point voltage (Vm), the efficiency (η) on the photogenerated current (Jg), or on the sunlight concentration ratio (X), are analyzed, as well as other photovoltaic characteristics of multijunction solar cells. The characteristics being analyzed are split into monoexponential (linear in the semilogarithmic scale) portions, each of which is characterized by a definite value of the ideality factor A and preexponential current J0. The monoexponentiality ensures advantages, since at many steps of the analysis, one can use the analytical dependences instead of numerical methods. In this work, an experimental procedure for obtaining the necessary parameters has been proposed, and an analysis of GaInP/GaInAs/Ge triple-junction solar cell characteristics has been carried out. It has been shown that up to the sunlight concentration ratios, at which the efficiency maximum is achieved, the results of calculation of dark and light I-V curves by the segmental method fit well with the experimental data. An important consequence of this work is the feasibility of acquiring the resistanceless dark and light I-V curves, which can be used for obtaining the I-V curves characterizing the losses in the transport part of a solar cell.

Photoelectric determination of the series resistance of multijunction solar cells

A method for determining the series resistance Rs of multijunction solar cells is suggested and sub-stantiated. The method uses the presence of a maximum in the dependence of the efficiency on the sunlight concentration ratio η(X) or in that of the operating voltage on the photogenerated current, Vm(Jg). The study employs the concept that, in a limited but practically important range of photogenerated currents (up to the maximum η), the series resistance can be represented by a fixed quantity that is linear and independent of Jg. It is analytically substantiated that this resistance can be found from the formula RS = (E/Jg)η = max, where E = AkT/q and A and Jg are local values of the ideality factor and photogenerated current at the maximum η (or Vm). It is shown that the value of Rs, determined by this method, is independent of the spectral composition of the incident light, which was experimentally confirmed in a study of the photovoltaic characteristics of triple-junction InGaP/GaAs/Ge solar cells. The method is suitable for both multi- and single-junction photoelectric converters.

Counter-photo-electromotive force at heterointerfaces in MJ SC: Study by spectral method

The influence of p-n junction heating on dependence of the open circuit voltage on the photogenerated current density (Voc-Jg) has been observed and procedure for Voc correction has been suggested. Using this procedure the corrected Voc-Jg dependence has been obtained for triple-junction GaInP/GaAs/Ge solar cells. The shape of Voc-Jg dependence has been discussed, and it was suggested that it indicates the appearance of a counter EMF of about 10 mV at Jg = 100 A/cm2. It is assumed that this counter EMF is generated on some isotype heterointerface. The spectral method for determining the position of such heterointerface has been considered.

Estimation of the potential efficiency of a multijunction solar cell at a limit balance of photogenerated currents

A method is proposed for estimating the potential efficiency which can be achieved in an initially unbalanced multijunction solar cell by the mutual convergence of photogenerated currents: to extract this current from a relatively narrow band-gap cell and to add it to a relatively wide-gap cell. It is already known that the properties facilitating relative convergence are inherent to such objects as bound excitons, quantum dots, donor-acceptor pairs, and others located in relatively wide-gap cells. In fact, the proposed method is reduced to the problem of obtaining such a required light current-voltage (I–V) characteristic which corresponds to the equality of all photogenerated short-circuit currents. Two methods for obtaining the required light I–V characteristic are used. The first one is selection of the spectral composition of the radiation incident on the multijunction solar cell from an illuminator. The second method is a double shift of the dark I–V characteristic: a current shift Jg (common set photogenerated current) and a voltage shift (−JgRs), where Rs is the series resistance. For the light and dark I–V characteristics, a general analytical expression is derived, which considers the effect of so-called luminescence coupling in multijunction solar cells. The experimental I–V characteristics are compared with the calculated ones for a three-junction InGaP/GaAs/Ge solar cell with Rs = 0.019 Ω cm2 and a maximum factual efficiency of 36.9%. Its maximum potential efficiency is estimated as 41.2%.

Heterointerfaces in MJ SC: IV curves and their peculiarities

The heterointerface IV curves at different intensity illumination levels have been extracted from full IV curves of triple-junction GaInP/GaAs/Ge solar cell. For this purpose a special approach for multi-junction solar cells has been proposed. The solar cell equivalent circuit consisting of two parts – generation and connecting ones were considered. The generation part includes all photovoltaic p-n junctions, while connecting one incudes a resistance of the layers, spreading resistance, tunnel diodes and heterointerfaces. The equivalent circuit was used for analysis of «kink» free IV curves of Ge and GaAs solar cells. In this case the IV curves of connecting part are completely described by the spreading resistance. The analysis of GaInP/GaAs/Ge allows to obtain heterointerface IV curves and it has been shown that their shape depends on illumination level. The empirical formula was used for an approximation of heterointerface IV curves to show the nature of current flow mechanism which was established as tunneling one.

Current flow mechanism in GaAs solar cells with GaInAs quantum dots

Resistance-less dark IV-curves of GaAs solar cells with and without quantum-dimensional objects allied to both quantum dot and quantum well have been obtained and analyzed. Anomalous large increase of the pre-exponential factors was observed in tested (with quantum objects) p-n junctions. According to proposed interpretation embedding of quantum objects in p-n junction created new current flow mechanisms. As a result current flows throught recombination transitions inside quantum objects in space charge region of p-n junction.

Overcoming the luminescent coupling effect in experimental search for the actual quantum efficiency values in multi-junction solar cells

A new experimental procedure to overcome luminescent coupling effect in a multi-junction solar cell is proposed. The procedure was used to obtain the true (hereinafter the actual) external quantum efficiency EQE(λ) values of Ge subcell, and consists of the following steps: 1) photon-coupled characteristic determination; 2) initial EQE(λ) measurement conditions choice and fixation; 3) EQE(λ) measurement with the conventional method; 4) actual EQE(λ) data search with use of photon-coupled characteristic.

Current mismatch violation in concentrator multijunction solar cells

The work presents an experimental techniques aimed at investigating the subcells photocurrent nonlinear behavior in MJ SC with irradiance increase and the revealing of a possible shift in the current matching between the subcells of the MJ SC at sunlight concentration ratio rise and determining the causes of possible current mismatch.