V. M. Emelyanov

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.

Photovoltaic laser-power converter based on AlGaAs/GaAs heterostructures

Photovoltaic laser-power converters for a wavelength of λ = 809 nm are developed and fabricated on the basis of single-junction AlGaAs/GaAs structures grown by metal-organic vapor-phase epitaxy. The parameters of the photovoltaic structure constituted by an optical “window” and a cladding layer are optimized by mathematical simulation. Photovoltaic converters with areas of S = 10.2 and 12.2 mm2 and 4 cm2 are fabricated and studied. For photocells with S = 10.2 mm2, the monochromatic efficiency (η) was 60% at a current density of 5.9 A/cm2. A photovoltaic module with a working voltage of 4 V (η = 56.3% at 0.34 A/cm2) is assembled.

Study of minority carrier diffusion lengths in photoactive layers of multijunction solar cells

A technique for determining a minority carrier’s diffusion length in photoactive III–V layers of solar cells by approximating their spectral characteristics is presented. Single-junction GaAs, Ge and multi-junction GaAs/Ge, GaInP/GaAs, and GaInP/GaInAs/Ge solar cells fabricated by hydride metal-organic vapor-phase epitaxy (H-MOVPE) have been studied. The dependences of the minority carrier diffusion length on the doping level of p-Ge and n-GaAs are determined. It is shown that the parameters of solid-state diffusion of phosphorus atoms to the p-Ge substrate from the n-GaInP nucleation layer are independent of the thickness of the latter within 35–300 nm. It is found that the diffusion length of subcells of multijunction structures in Ga(In)As layers is smaller in comparison with that of single-junction structures.

Simulation of the ohmic loss in photovoltaic laser-power converters for wavelengths of 809 and 1064 nm

The method of mathematical simulation is used to examine the influence exerted by the characteristics of the epitaxial structure and contact grid of photovoltaic laser-power converters on their ohmic loss. The maximum attainable photoconverter efficiency at a Gaussian distribution of the laser-beam intensity on the surface of a photovoltaic converter and at dark-current densities of p–n junctions typical of structures grown by the metal-organic vapor-phase epitaxy (MOVPE) technique are determined. An approach to finding the optimal parameters of GaAs and In0.24Ga0.76As/GaAs photovoltaic converters in relation to the optical power being converted is suggested, and the structural parameters for incident-power values of 5, 20, and 50 W at wavelengths of 809 and 1064 nm are determined. It is found that, at laser-light intensities of up to 5 W, >60% efficiency can be achieved in laser-light conversion at a wavelength of 809 nm and >55% efficiency, at a wavelength of 1064 nm.

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.

Heterostructures of metamorphic GaInAs photovoltaic converters fabricated by MOCVD on GaAs substrates

Heterostructures of metamorphic GaInAs photovoltaic converters (PVCs) are on GaAs substrates by the metal-organic chemical vapor deposition (MOCVD) method. It is shown that using a multilayer metamorphic buffer with a step of 2.5% in indium content and layer thicknesses of 120 nm provides the high quality of bulk layers subsequently grown on the buffer up to an indium content of 24%. PVCs with a long-wavelength photosensitivity edge up to 1300 nm and a quantum efficiency of ~80% in the spectral range 1050–1100 nm are fabricated. Analysis of the open-circuit voltage of the PVCs and diffusion lengths of minority carriers in the layers demonstrates that the density of misfit dislocations penetrating into the bulk layers increases at an indium content exceeding 10%.

Multijunction GaInP/GaInAs/Ge solar cells with Bragg reflectors

Effect of subcell parameters on the efficiency of GaInP/Ga(In)As/Ge tandem solar cells irradiated with 1-MeV electrons at fluences of up to 3 × 1015 cm−2 has been theoretically studied. The optimal thicknesses of GaInP and GaInAs subcells, which provide the best photocurrent matching at various irradiation doses in solar cells with and without built-in Bragg reflectors, were determined. The dependences of the photoconverter efficiency on the fluence of 1-MeV electrons and on the time of residence in the geostationary orbit were calculated for structures optimized to the beginning and end of their service lives. It is shown that the optimization of the subcell heterostructures for a rated irradiation dose and the introduction of Bragg reflectors into the structure provide a 5% overall increase in efficiency for solar cells operating in the orbit compared with unoptimized cells having no Bragg reflector.

Simulation of the characteristics of InGaAs/InP-based photovoltaic laser-power converters

A method for mathematical simulation is used to analyze the efficiencies attainable in photovoltaic laser-power conversion at wavelengths of 1.3 and 1.55 μm in In0.53Ga0.47As/InP heterostructures with light input on the side of the n-InP substrate. The influence exerted on the efficiency by the parameters of the In0.53Ga0.47As/InP heterostructure and by the design of the photovoltaic laser-power converter is examined. The simulated characteristics of In0.53Ga0.47As/InP photovoltaic converters are compared with those of GaAs-based photovoltaic converters for a wavelength of 809 nm. It is shown that efficiencies of 40% at a wavelength of 1.3 μm and nearly 50% at 1.55 μm can be attained at a laser power of about 2–6 W, but the efficiency noticeably decreases at higher laser-light intensities. It is found that the main factor that hinders the achievement of a high efficiency in the conversion of high-intensity laser light is loss in the n-InP substrate. The optimal doping level of n-InP substrates to be used in the photovoltaic converters intended for varied laser-light intensities is estimated.

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.

Multijunction solar cell with intermediate IR reflector

In the present work, for a system with the sunlight spectrum splitting, a multilayer mirror built-in into the GaInP/Ga(In)As/Ge TJ SC structure is proposed as a selective optical mirror. The function of such a mirror is to extract the IR part of the radiation in the wavelength range of 900-1100 nm from the SC structure for its following utilization by a SC based on Si or InGaAs. Being located between the middle GaAs and the bottom Ge subcells, the mirror based on doubled Bragg reflector (2BR) decreases controllably the portion of IR radiation incident on the Ge subcell and creating conditions for matching the Ge subcell photocurrent with the photocurrents of two other subcells in a TJ SC. The merit of the proposed approach is the entire absence of additional radiation losses in the ranges of the top GaInP and middle GaAs subcells sensitivity (usually resulting from diffraction effects in dichroic mirrors), which conserves their photocurrent at the maximum achievable level without violation of the current matching conditions. Thus, the efficiency of a TJ SC with 2BRs remains practically unchangeable compared with that of a conventional cell structure. The efficiency of 36.6% is demonstrated for a pair of cells: GaInP/Ga(In)As/2BR/Ge TJ (32.6%) and Si SJ (4%) the latter being operated under the radiation reflected by a 2BR.In the present work, for a system with the sunlight spectrum splitting, a multilayer mirror built-in into the GaInP/Ga(In)As/Ge TJ SC structure is proposed as a selective optical mirror. The function of such a mirror is to extract the IR part of the radiation in the wavelength range of 900-1100 nm from the SC structure for its following utilization by a SC based on Si or InGaAs. Being located between the middle GaAs and the bottom Ge subcells, the mirror based on doubled Bragg reflector (2BR) decreases controllably the portion of IR radiation incident on the Ge subcell and creating conditions for matching the Ge subcell photocurrent with the photocurrents of two other subcells in a TJ SC. The merit of the proposed approach is the entire absence of additional radiation losses in the ranges of the top GaInP and middle GaAs subcells sensitivity (usually resulting from diffraction effects in dichroic mirrors), which conserves their photocurrent at the maximum achievable level without violation of the current ...