AlGaAs/GaAs/InGaAs composite MQW structures for photovoltaic applications

AlGaAs/GaAs/InGaAs composite MQW structures were theoretically studied and simulated. The computer simulation indicated that an appropriate composite MQW, both with symmetrical and non-symmetrical structures, could keep |ψ|² of quantized carriers at proper locations in electric-field-tilted quantum wells, so the efficient transition by photon absorption would be possible and applicable for photovoltaic cells which have the composite MQW as the active region. The AlGaAs/GaAs MQW and GaAs/InGaAs (4x) SQW structures were separately prepared by MBE and were evaluated for their spectral responses. The AlGaAs/GaAs MQW has a high response at a short wavelengths (peak at 685 nm) due to the quantized states in GaAs wells, while the GaAs/InGaAs SQW has a broader spectral response covering longer wavelengths (600–850 nm) because of the strong absorption in the GaAs barrier and substrate. However, (4x) photoluminescence peaks at 900–1100 nm that werefound from GaAs/InGaAs strained quantum wells at room temperature are promising evidence for the longer wavelength spectral response. The AlGaAs/GaAs/InGaAs composite SQW and MQW samples were experimentally prepared by MBE techniques and tested for their optical properties. The broader photoluminescence peak was observed and reflected the nature of the composite structure. The study on the photospectral response of composite MQW structures has been conducted which provides the basic information for high performance solar cell design.     

Numerical prediction of InGaAs/GaAs MQW solar cell characteristics under concentrated sunlight

The characteristics of InGaAs/GaAs multi-quantum well (MQW) solar cells under concentrated sunlight were analyzed by self-consistent numerical calculation and were compared with GaAs p-i-n cells that have the same structure except for the QWs. Prior to the calculations, the absorption coefficients of the InGaAs MQW cell were determined by fitting the calculated quantum efficiencies to experimental ones. In the simulation, mainly the influences of concentrated sunlight and temperature were investigated. Results of the simulation showed that the rate of the conversion efficiency and open-circuit voltage (V_oc) of MQW cells increased faster than that of p-i-n cells as the light intensity increased, while the rate of efficiency of the MQW cell decreased a little slower than that of the p-i-n cell with the temperature increase. MQW cells compensate for the disadvantage of lower efficiency and V_oc than p-i-n cells in a concentrated sunlight system.     

Experimental analysis of the efficiency of heterostructure GaAsAlGaAs solar cells

Some years ago Multiple Quantum Wells (MQW) solar cells were introduced as an alternative to obtain high efficiencies. Based on the simple Shockley diode model, the short-circuit current could be increased without loss in the open-circuit voltage. Applying the Detailed Balance Theory, including radiative recombination in the i-region, leads to less optimistic predictions of the limiting efficiency of MQW cells. We present an experimental study in order to compare the efficiency of MQW solar cells with heterostructure cells with graded Al compositions and single bandgap solar cells. Compared to the homojunction AlGaAs cell, an increase of the short-circuit current is observed by the incorporation of GaAs in the i-region. However, the open-circuit voltage is reduced by the implementation of GaAs, due to an increase of the non-radiative recombination current. To estimate the maximum possible open-circuit voltage, the radiative recombination current is determined by measuring the light emission as a function of the applied voltage. From this experiment we conclude that the maximum possible open-circuit voltage of all the heterostructure cells is considerably lower than the homogeneous AlGaAs cell and close to the value of the GaAs cell, showing the relation between the open-circuit voltage and the smallest bandgap in the cell. The measured curves can be well predicted by calculations based on the Detailed Balance Theory. We find no principal advantage of MQW cells over cells with graded composition or single bandgap cells.     

Experimental analysis of the efficiency of heterostructure GaAs---AlGaAs solar cells

Some years ago Multiple Quantum Wells (MQW) solar cells were introduced as an alternative to obtain high efficiencies. Based on the simple Shockley diode model, the short-circuit current could be increased without loss in the open-circuit voltage. Applying the Detailed Balance Theory, including radiative recombination in the i-region, leads to less optimistic predictions of the limiting efficiency of MQW cells. We present an experimental study in order to compare the efficiency of MQW solar cells with heterostructure cells with graded Al compositions and single bandgap solar cells. Compared to the homojunction AlGaAs cell, an increase of the short-circuit current is observed by the incorporation of GaAs in the i-region. However, the open-circuit voltage is reduced by the implementation of GaAs, due to an increase of the non-radiative recombination current. To estimate the maximum possible open-circuit voltage, the radiative recombination current is determined by measuring the light emission as a function of the applied voltage. From this experiment we conclude that the maximum possible open-circuit voltage of all the heterostructure cells is considerably lower than the homogeneous AlGaAs cell and close to the value of the GaAs cell, showing the relation between the open-circuit voltage and the smallest bandgap in the cell. The measured curves can be well predicted by calculations based on the Detailed Balance Theory. We find no principal advantage of MQW cells over cells with graded composition or single bandgap cells.     

Temperature dependence of photocurrent components on enhanced performance GaAs/AlGaAs multiple quantum well solar cells

The performance of Al_0.36Ga_0.64As p/i/n solar cells with multiple quantum wells (MQW) of GaAs/Al_0.36Ga_0.64As in the i-region has been investigated at various temperatures, ranging from −10°C to 100°C, and compared with that of conventional solar cells composed of either the quantum well material (GaAs) or the barrier material (Al_0.36Ga_0.64As) alone. The dark currents of the MQW cells were found to lie between those of the conventional cells. The increase of dark current with temperature was accompanied by a slight decrease of the diode ideality factor. A linear dependence of open-circuit voltage (V_oc) on temperature was observed for all cells when illuminated with a 100W halogen lamp. V_oc for the MQW cells was found to be independent of the number of wells, lying between the V_oc's for the two conventional cells. The MQW cells exhibited performance improvement with temperature when compared to the conventional cells and there was a significant enhancement in the short-circuit current with temperature of those MQW cells that exhibited poorer performance at lower temperatures. Theoretical calculations have quantified the contribution of the tunneling current component to the total observed photocurrent at the various temperatures examined. It was found that tunneling currents are present at all temperatures and can be the dominant component in MQW cells of thinner wells at low temperatures. These results suggest that GaAs/Al_0.36Ga_0.64As MQW structures, of good-quality material, when processed as conventional solar cells with antireflective coatings should deliver more output power under intense illumination than conventional solar cells composed of the quantum well material alone.     

The effects of temperature and light concentration on the GaInP/GaAs multijunction solar cell’s performance

Monolithic Ga_0.49In_0.51P/GaAs cascade solar cells with a p⁺/n⁺ GaAs tunnel junction were grown by MOCVD technique. The variation of the photovoltage, photocurrent, fill factor, efficiency, I–V characteristics and spectral response under different temperatures (25–75 °C), and light intensity values (1–40 sun), were investigated experimentally.The open-circuit voltage of the multijunction cell decreases with the temperature increase at a rate of 5.5 mV/°C. The efficiency of the cascade structure under investigation was increased with an increase in the light concentration up to a point where the series resistance and the tunnel junction effects become significant.     

Advances in Bragg stack quantum well solar cells

Strain-balanced quantum well solar cells (SB-QWSC) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a p–i–n device. The addition of a distributed Bragg reflector (DBR) can substantially increase the photocurrent with little or no detriment to the dark-current. Experimental results are presented that show improvements of DBR cell efficiencies over SB-QWSC's without DBR's. In addition, at high dark-current levels appropriate to high concentration, we observe that the dark-currents of the SB-QWSC's exhibit ideal diode behaviour. We present evidence that the ideality n=1 dark-current is reduced in the DBR cells and discuss the possible efficiency improvements if the dark-current is radiatively dominant.     

Lowering of thickness of boron-doped microcrystalline hydrogenated silicon film by seeding technique

By using a seeding technique it has been possible to reduce the thickness of p-μc-Si:H film to 230 Å, with an improved electrical conductivity (0.93 S cm⁻¹) and lower optical absorption compared to those of conventional p-μc-Si:H layers without a seed layer, for use at the tunnel junction and as the top layer of a double junction n–i–p structured a-Si solar cell. Undoped-μc-Si:H has been used as the seed layer. The layers were prepared by the radio frequency plasma-enhanced chemical vapour deposition (RF-PECVD) method (13.56 MHz) at 40 mW/cm² rf power density and low substrate temperature (200 °C). The ultrathin seed layer (30 Å) enhances the growth of microcrystallinity of the p-type μc-Si:H film as confirmed by the results of transmission electron microscopy (TEM) analysis and Raman spectroscopy.     

Potentially modulated multi-quantum wells for high-efficiency solar cell applications

The expected high performance of multi-quantum well (MQW) solar cells depends not only on the material quality ensuring minimum non-radiative recombination losses at the heterointerfaces, but also on the escape rate of photogenerated carriers out of QWs into the “base” region having minimum radiative losses within QWs. For this, we propose and investigate potentially modulated MQW solar cell structures, for which the MQW structure is modified from a series of conventional square-shaped QWs to “step-like” potentially modulated QWs. We have studied “two-step” and “three-step” MQWs introduced into the intrinsic region of GaAs p–i–n solar cells. Compared with a square-shaped MQW cell, a three-step modulated MQW cell having the same average potential well depth showed photoluminescence characteristics with a smaller thermal activation energy and higher average carrier temperature. The dark current characteristics also indicated that photogenerated carriers escape out of QWs more efficiently in this type of structure.     

Photovoltaic cell characteristics for high-intensity laser light

The series resistance value of a photovoltaic (PV) cell required for high-intensity light and the effects of both the α parameter (the ratio of the open-circuit voltage to the bandgap) and temperature on conversion efficiency are investigated by a calculation method derived from the fundamental characteristics of PV cell. The PV cell characteristics for high-intensity laser light, including Si, GaAs, InGaAs PV cells and InGaAs uni-traveling-carrier photodiode (UTC-PD), are experimentally investigated. The small series resistance as large as 20–30 μΩ cm² and the suppression of recombination are important for obtaining higher conversion efficiency, especially for high-intensity laser light.     

III–V compound multi-junction solar cells: present and future

As a result of top cell material quality improvement, development of optically and electrically low-loss double-hetero structure tunnel junction, photon and carrier confinements, and lattice-matching between active cell layers and substrate, the last 15 years have seen large improvements in III–V compound multi-junction (MJ) solar cells. In this paper, present status of R&D program for super-high-efficiency MJ cells in the New Sunshine Project in Japan is presented. InGaP/InGaAs/Ge monolithic cascade 3-junction cells with newly recorded efficiency of 31.7% at AM1.5 (1-sun) were achieved on Ge substrates, in addition to InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with world-record efficiency of 33.3%. Future prospects for realizing super-high-efficiency and low-cost MJ solar cells are also discussed.     

Electrosynthesis and characterization of GaAs in acid solutions by potentiostatic method

Gallium arsenide (GaAs) is one of the important materials used for the fabrication of light emitting diodes, solar cells, microwave devices, etc. In the present work, electrodeposition of GaAs was successfully carried out potentiostatically from an aqueous solution mixture of gallium chloride (GaCl₃) and arsenic oxide (As₂O₃). The optimum deposition potential, pH and bath temperature to synthesize GaAs thin films are found to be −0.8 V versus SCE, 2.0±0.1 and 60 ^°C, respectively. The effects of solution pH, bath temperature and deposition potential on the gallium content of GaAs films are studied. Photoelectrochemical (PEC) solar cells using n-GaAs photo-anode in a polysulphide electrolyte is constructed and I–V, C–V studies are carried out. Various semiconductor parameters such as, flat-band potential, band bending, donor density, depletion layer width are evaluated and the results are discussed.     

Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells

Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells with a world-record efficiency of 26.9% (AM0, 28°C) has been evaluated by 1 MeV electron irradiation. Degradation in tandem cell performance has been confirmed to be mainly attributed to large degradation in the GaAs bottom cell. Similar radiation resistance with GaAs-on-Ge cells has been observed for the InGaP/GaAs tandem cell. Moreover, recovery of the tandem cell performance has been found due to minority-carrier injection under light illumination or forward bias, which causes defect annealing in InGaP top cells. The optimal design of the InGaP base layer thickness for current matching at end of life (EOL) (after irradiation with 10¹⁵ electrons cm⁻²) has been examined.     

GaInP single junction and GaInP/GaAs two junction thin-film solar cell structures by epitaxial lift-off

The epitaxial lift-off (ELO) technique was used in forming a thin-film GaInP/GaAs two-junction monolithic tandem solar cell structure. First, the GaInP single junction solar cell to be used in the tandem cell structure as a top cell was thinned by the ELO process. Although the ELO process and the transfer to the quartz substrate caused a strain in the thin-film cell after separation from the GaAs substrate, the photoluminescence peak intensity was not decreased. This shows that defects, such as those causing carrier loss, were not introduced on the thin-film cell during the thinning process. The key issue for thin-film cell fabrication is to avoid damaging the AlInP window layer during the selective etching (HF etchant), by which the thin-film cell is released from the GaAs substrate. A GaInP/GaAs monolithic tandem structure was also thinned by the same process with a GaInP single junction cell. Characteristics of the single-junction GaInP cell and individual cells in the GaInP/GaAs tandem structure were examined. It was found that the spectral response remains almost the same as that for cells with a GaAs substrate, thus confirming the feasibility of using the ELO process to fabricate thin-film GaInP/GaAs cells.     

用于高效级联太阳电池的ZnSe材料研究

研究了用于高效Znse／GaAs/Ge(硒化锌绅化镓／锗)级联太阳电池顶电池的ZnSe材料。用MBE技术制备了ZnSe p-n结样品，测量了其外量子效率；提出了改进ZnSe顶电池性能的方法；分析了ZnSe／GaAs／Ge结构比GaInP／GaAs／Ge结构的优越之处。     

Performance study of solar cell arrays based on a Trough Concentrating Photovoltaic/Thermal system

The performances of solar cell arrays based on a Trough Concentrating Photovoltaic/Thermal (TCPV/T) system have been studied via both experiment and theoretical calculation. The I–V characteristics of the solar cell arrays and the output performances of the TCPV/T system demonstrated that among the investigated four types of solar cell arrays, the triple junction GaAs cells possessed good performance characteristics and the polysilicon cells exhibited poor performance characteristics under concentrating conditions. The optimum concentration ratios for the single crystalline silicon cell, the Super cells and the GaAs cells were also studied by experiments. The optimum concentration ratios for the single crystalline silicon cells and Super cells were 4.23 and 8.46 respectively, and the triple junction GaAs cells could work well at higher concentration ratio. Besides, some theoretical calculations and experiments were performed to explore the influences of the series resistances and the working temperature. When the series resistances R_s changed from 0 Ω to 1 Ω, the maximum power P_m of the single crystalline silicon, the polycrystalline silicon, the Super cell and the GaAs cell arrays decreased by 67.78%, 74.93%, 77.30% and 58.07% respectively. When the cell temperature increased by 1 K, the short circuit current of the four types of solar cell arrays decreased by 0.11818 A, 0.05364 A, 0.01387 A and 0.00215 A respectively. The research results demonstrated that the output performance of the solar cell arrays with lower series resistance was better and the working temperature had a negative impact on the current under concentration. In addition, solar irradiation intensity had certain effects on the solar cell’s performance. For the crystalline silicon solar cell arrays, when the solar direct radiation exceeded a certain value, the I–V curves almost became a straight line and the output performances decreased due to the high series resistance leading to the high power loss. For the triple junction GaAs solar cell array, its performance was always excellent.     

Study on GaAs/GaAlAs MQW structure for photovoltaic applications

GaAs/GaAlAs MQW structures with varying well widths having narrowest wells on the top layers and gradually wider wells for the inner layers were studied and confirmed by Auger spectroscopy and photoluminescence measurements. Multiplication of quantum wells gives stronger photoluminescence due to higher density of quantized states in the structure. This MQW structure was experimented in the photoconductivity and photocurrent measurements at room temperature. It is found that at low multiplication of quantum wells in MQW structure, the photoconductivity effect was mainly controlled by bulk material as shown in the spectral response. Photocurrent at low bias voltage shows relatively better spectral sensitivity at shorter wavelength. The photoconductivity and photocurrent measurements indicate that appropriate MQW structures acting as broader absorbers due to graded characters of quantized energy states are needed for photovoltaic application. Integration of MQW structure to solar cell structure was also investigated.     

Analysis of thin film surface barrier solar cells with a microrelief interface

The influence of a microrelief interface between a thin conductive film (emitter) and a semiconductor substrate (absorber) on the optical and recombination losses in surface barrier solar cells is analyzed.Equations for the calculation of monochromatic light transmission through a thin absorbing film with one or two rough surfaces, on an absorbing substrate, are presented. For a weakly texturized interface and a normal direction of the incident light, the influence of the microroughness is taken into account by the formulae obtained. A model of patches with microsurfaces parallel to the structure surface and ones inclined to it at a certain angle (the most probable statistically) was used to describe the experimental results. Au/GaAs surface barrier structures with interface microrelief of a quasigrating or a dendritic type, obtained by chemical anisotropic etching of GaAs, were investigated. The geometric and statistical parameters of the microrelief were determined by atomic force microscopy techniques. The recombination parameters of the textured interfaces were determined from the experimental spectra of the external quantum efficiency, taking into consideration the calculated spectra of light transmittance. A comparison of the internal quantum efficiency spectra with the calculated ones allows a determination of the electronic parameters of the interface. Both the photoconversion parameters under AM0 simulated illumination and the tolerance for ⁶⁰Co γ-irradiation, obtained for structures with various interface microrelief morphologies, allowed identification of microrelief of the quasigrating type as more suitable for solar cell applications.     

Charge transport study and spectral response of GaSb/GaAs heterojunctions prepared by MOVPE

The purpose of our work was the evaluation of GaSb/GaAs heterostructures grown on GaAs substrates for thermophotovoltaics (TPV). Heterojunctions p-GaSb/n-GaAs with p-layer prepared by metal organic vapour phase epitaxy (MOVPE) method at growth temperatures ranging from 500°C to 560°C were investigated. We have studied the charge transport in these structures and its influence on photovoltage spectral response of the cells. Measurement of I–V characteristics in the temperature range from 200 to 350 K show that the charge transport can be described by a combination of emission and diffusion processes. There is a spike and hence a discontinuity in the band diagram of the junction. The discontinuity increases with increasing GaSb growth temperature. Photovoltage spectral response shows higher signal from GaAs than that from GaSb. The experimental curves were compared with theoretically calculated ones accounting for the reduction of electron current crossing the barrier. The discontinuity is very probably connected with the lattice mismatch between both materials rather than with the affinity difference. Our results show that p-GaSb/n-GaAs heterojunctions prepared by this MOVPE method are not suitable enough for use in TPV.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.