Band gap engineering literature review/Spectral response and energy output of concentrator multijunction solar cells

Spectral response and energy output of concentrator multijunction solar cells^[1][edit | edit source]

Abstract: The spectral response of concentrator multijunction solar cells has been measured over a temperature range of 25–75°C. These data are combined with reference spectra representing the AM1·5 standard as well as annual spectral irradiance at representative geographical locations. The results suggest that higher performance in the field may be obtained if multijunction cells are designed for an effective air mass higher than AM1·5.

Radiative coupling effects in GaInP/GaAs/Ge multijunction solar cells^[2][edit | edit source]

Abstract: Direct measurements of radiative coupling effects in GaInP/GaAs/Ge multijunction solar cells are presented. Radiative coupling between the GaInP and GaAs cells is observed by using isotype cells as well as specially fabricated 3-terminal device structures. Spectral response measurements of the GaAs cell in both isotype and 3-terminal approaches are shown to exhibit enhanced quantum efficiency in the short wavelength region under favorable radiative coupling conditions. Additionally, electroluminescence of the GaInP cell is shown to enhance the current output from the GaAs cell using a 3-terminal device structure. One consequence of this effect is the possible influence on the measured J-ratio of a multijunction cell. Consideration of radiative coupling may become increasingly important as multijunction III-V based solar cells - including 4- and 5- junction cells - continue to develop and improve in performance.

High-efficiency space and terrestrial multijunction solar cells through bandgap control in cell structures^[3][edit | edit source]

Abstract: Using the energy bandgap of semiconductors as a design parameter is critically important for achieving the highest efficiency multijunction solar cells. The bandgaps of lattice-matched semiconductors that are most convenient to use are rarely those which would result in the highest theoretical efficiency. For both the space and terrestrial solar spectra, the efficiency of 3-junction GaInP/GaAs/Ge solar cells can be increased by a lower bandgap middle cell, as for GaInAs middle cells, as well as by using higher bandgap top cell materials. Wide-bandgap and indirect-gap materials used in parasitically absorbing layers such as tunnel junctions help to increase transmission of light to the active cell layers beneath. Control of bandgap in such cell structures has been instrumental in achieving solar cell efficiencies of 29.7% under the AMO space spectrum (0.1353 W/cm2, 28°C) and 34% under the concentrated terrestrial spectrum (AM1.5G, 150-400 suns, 25°C), the highest yet achieved for solar cells built on a single substrate.

Advances in High-Efficiency III-V Multijunction Solar Cells^[4][edit | edit source]

Abstract: The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic electricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide-band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga0.44In0.56P/Ga0.92In0.08As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm2, 25∘C), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I2R resistive power loss, is a particularly significant advantage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns.

40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells^[5][edit | edit source]

Abstract: An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP/GaInAs/Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm2, AM1.5D, low aerosol optical depth, 25 °C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

Regional climate consequences of large-scale cool roof and photovoltaic array deployment^[6][edit | edit source]

Abstract: Modifications to the surface albedo through the deployment of cool roofs and pavements (reflective materials) and photovoltaic arrays (low reflection) have the potential to change radiative forcing, surface temperatures, and regional weather patterns. In this work we investigate the regional climate and radiative effects of modifying surface albedo to mimic massive deployment of cool surfaces (roofs and pavements) and, separately, photovoltaic arrays across the United States. We use a fully coupled regional climate model, the Weather Research and Forecasting (WRF) model, to investigate feedbacks between surface albedo changes, surface temperature, precipitation and average cloud cover. With the adoption of cool roofs and pavements, domain-wide annual average outgoing radiation increased by 0.16 ± 0.03 W m − 2 (mean ± 95% C.I.) and afternoon summertime temperature in urban locations was reduced by 0.11–0.53 °C, although some urban areas showed no statistically significant temperature changes. In response to increased urban albedo, some rural locations showed summer afternoon temperature increases of up to + 0.27 °C and these regions were correlated with less cloud cover and lower precipitation. The emissions offset obtained by this increase in outgoing radiation is calculated to be 3.3 ± 0.5 Gt CO2 (mean ± 95% C.I.). The hypothetical solar arrays were designed to be able to produce one terawatt of peak energy and were located in the Mojave Desert of California. To simulate the arrays, the desert surface albedo was darkened, causing local afternoon temperature increases of up to + 0.4 °C. Due to the solar arrays, local and regional wind patterns within a 300 km radius were affected. Statistically significant but lower magnitude changes to temperature and radiation could be seen across the domain due to the introduction of the solar arrays. The addition of photovoltaic arrays caused no significant change to summertime outgoing radiation when averaged over the full domain, as interannual variation across the continent obscured more consistent local forcing.

Internal voltages in GaInP/GaInAs/Ge multijunction solar cells determined by electroluminescence measurements^[7][edit | edit source]

Abstract: We analyze electroluminescence spectra of a GaInP/GaInAs/Ge triple-junction solar cell at different injection currents. Using the reciprocity theorem between electroluminescent emission and external quantum efficiency of solar cells allows us to derive the current/voltage curves and the diode quality factors of all individual subcells.

Spectral response measurements of monolithic GaInP/Ga(In)As/Ge triple-junction solar cells: Measurement artifacts and their explanation^[8][edit | edit source]

Abstract: Procedures for measuring the spectral response of multi-junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non-ideal properties, such as a low shunt resistance or a low reverse breakdown voltage. This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple-junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi-junction cells is developed, specially designed to minimize such measurement artifacts.

Procedures for evaluating multijunction concentrators^[9][edit | edit source]

Abstract: Procedures at NREL and Fraunhofer ISE for evaluating multijunction cells are detailed with a triple-junction GaInP/GaAs/Ge concentrator cell designed and grown at Spectrolab and processed at NREL, and a tandem Fraunhofer ISE Ga0.35In0.85P/Ga0.83 In0.17As cell as examples. The one-sun efficiency and I sc for the triple-junction device measured at Fraunhofer ISE and NREL agreed within 0.2%, well below the ±6% uncertainty estimated by NREL. The procedures for determining the one-sun characteristics involve determining the quantum efficiency and using it for spectral correction during the I-V characterization. The characteristics under concentration are evaluated with a flash simulator

Spectral mismatch correction and spectrometric characterization of monolithic III–V multi-junction solar cells^[10][edit | edit source]

Abstract: III–V monolithic multi-junction (MJ) solar cells reach efficiencies exceeding 30% (AM 1.5 global) and have applications in space and in terrestrial concentrator systems. The subcells of monolithic MJ cells are not accessible separately, which presents a challenge to measurement systems and procedures. A mathematical approach is presented which enables a fast way of spectral mismatch correction for MJ cells, thereby significantly reducing the time required for calibration. Moreover, a systematic investigation of the I–V parameters of a MJ solar cell with variation of the incident spectrum is possible, herein called 'spectrometric characterization'. This analysis method visualizes the effects of current limitation and shifting of the operating voltage, and yields precise information about the current-matching of the subcells. MJ cells can hereby be compared without the need to match the current of the structures to a reference spectrum in advance. Further applications of the spectrometric characterization are suggested, such as for the determination of the radiation response of the subcells of MJ space solar cells or for the prediction of the annual power output of terrestrial MJ concentrator cells.

New methods for measuring performance of monolithic multi-junction solar cells^[11][edit | edit source]

Abstract: The commercialization of multi-junction solar cells for both space and terrestrial applications has increased the need to accurately determine cell performance using typical solar simulators and test equipment. This paper describes specific test methods recently applied in characterizing the performance of both tandem and triple-junction solar cells. Methods applied included: current-voltage measurements in forward and reverse bias using a xenon-arc solar simulator; absolute spectral response measurements of separate junctions using both light and voltage bias; a device simulation model; and a spectral mismatch calculation procedure tailored to multi-junction cells. Procedures are illustrated using measurements for GaInP-GaAs tandem cells, GaInP-GaAs-Ge triple-junction cells, and Ge cells supplied by different manufacturers.

Procedures for evaluating multijunction concentrators^[12][edit | edit source]

Abstract: Procedures at NREL and Fraunhofer ISE for evaluating multijunction cells are detailed with a triple-junction GaInP/GaAs/Ge concentrator cell designed and grown at Spectrolab and processed at NREL, and a tandem Fraunhofer ISE Ga0.35In0.85P/Ga0.83 In0.17As cell as examples. The one-sun efficiency and I sc for the triple-junction device measured at Fraunhofer ISE and NREL agreed within 0.2%, well below the ±6% uncertainty estimated by NREL. The procedures for determining the one-sun characteristics involve determining the quantum efficiency and using it for spectral correction during the I-V characterization. The characteristics under concentration are evaluated with a flash simulator.

High-efficiency GaInP/GaAs/InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction^[13][edit | edit source]

Abstract: The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1 sun global spectrum, space spectrum, and concentrated direct spectrum at 81 suns, respectively. The device consists of 1.8 eV Ga0.5In0.5P, 1.4 eV GaAs, and 1.0 eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural "handle." The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95 V at 1 sun.

The path to 1 GW of concentrator photovoltaics using multijunction solar cells^[14][edit | edit source]

Abstract: This paper presents an overview of the status of the high-concentration photovoltaic (HCPV) module technology and discusses the steps required to take it from to the production of gigawatts in the near future. The paper discusses the impact of the recent advances in multijunction cell technology on the economics of concentrator system.

Raising the Efficiency Ceiling With Multijunction III-V Concentrator Photovoltaics^[15][edit | edit source]

Abstract: In this paper, we look at the question "how high can solar cell efficiency go?" from both theoretical and experimental perspectives. First-principle efficiency limits are analyzed for some of the main candidates for high-efficiency multijunction terrestrial concentrator cells. Many of these cell designs use lattice-mismatched, or metamorphic semiconductor materials in order to tune subcell band gaps to the solar spectrum. Minority-carrier recombination at dislocations is characterized in GaInAs inverted metamorphic solar cells, with band gap ranging from 1.4 to 0.84 eV, by light I-V, electron-beam-induced current (EBIC), and cathodoluminescence (CL). Metamorphic solar cells with a 3-junction GaInP/ GaInAs/ Ge structure were the first cells to reach over 40% efficiency, with an independently confirmed efficiency of 40.7% (AM1.5D, low-AOD, 240 suns, 25°C). The high efficiency of present III-V multijunction cells now in high-volume production, and still higher efficiencies of nextgeneration cells, is strongly leveraging for low-cost terrestrial concentrator PV systems.

First demonstration of multi-junction receivers in a grid-connected concentrator module^[16][edit | edit source]

Abstract: This paper discusses the approach taken by Concentrating Technologies (CT), Spectrolab, and Arizona Public Service (APS) to demonstrate a High Concentration Photovoltaic (HCPV) module using Spectrolab's GaInP/GaInAs/Ge triple-junction solar cells. This module is currently connected to an inverter, feeding electricity into the grid at the Solar Test & Research (STAR) facility of APS in Tempe, AZ. Although the module output is small, under 1 kW AC, it is the world's first demonstration of a grid-connected utility module using the same triple-junction solar cell technology that have been used to power satellites and other spacecrafts. This paper also discusses t