InGaP Solar Cells Research Articles

Improvement of InGaP solar cells grown with TBP in planetary MOVPE reactor

In this study, the impact of various growth parameters, such as wafer temperature, device structures, and substrate misorientation, on the quality of InGaP epitaxial layers and solar cells grown in a planetary MOVPE reactor using TBP as a P source were investigated and optimized. Results showed that the carrier lifetime in unintentionally n-doped InGaP significantly decreased as the growth temperature increased from 560 to 590 °C. At temperatures above 600 °C, the growth of InGaP was unsuccessful due to extremely rough surface morphology. The study revealed that InGaP rear homojunction and rear heterojunction solar cells using n-type InGaP base layers displayed improved photo-carrier collection and performance compared to traditional front junction solar cells with p-type InGaP base layers. Notably, InGaP grown on GaAs (001) with a 5° miscut towards (111)A was found to have better quality, while InGaP grown on GaAs (001) with a 6° tilt towards (111)B had worse results in terms of carrier lifetime, PL intensity, and solar cell performance.

Al0.35Ga0.65As/InGaP heterojunction solar cell based on temperature-graded growth

The p-Al0.35Ga0.65As/n-InGaP heterojunction solar cells are promising competitors compared with conventional InGaP or AlGaAs solar cells as the heterogeneous combination can overcome the demerits of each material. However, InGaP has an optimized growth temperature much lower than that of AlGaAs in metal-organic vapor phase epitaxy (MOVPE) growth. Therefore, a challenge arises from oxygen contamination at the hetero-interface during temperature adjustment for obtaining MOVPE-grown high-quality p-AlGaAs/n-InGaP heterojunction. Here we report that a temperature-graded layer (TGL) of AlGaAs can solve the issue of growth temperature mismatch, and significantly improve the voltage performance of AlGaAs/InGaP solar cells. Absolute electroluminescence measurement and atomic force microscopy confirmed a smooth interface with less non-radiative recombination after TGL was applied. The temperature-graded growth is verified able to improve the AlGaAs/InGaP interface quality and provides a scalable method for AlGaAs-based heterogeneous growth.

MOCVD growth and characterization of high efficiency (Al)InGaP solar cells for luminescent concentrators

The employment of Luminescent Solar Concentrators (LSCs) is a well known strategy for the integration of photovoltaic technologies in buildings. One of the main challenges in this area lies in the developement of effective solar cells, whose band gap and structures are adjusted to match the emisson peak of the related LSCs. The cells are placed at the edges of the concentrator plate and should be designed to benefit of quasi-monochromatic irradiation. In this work we investigate the growth and characterization of high-efficiency (Al)InGaP-based solar cells designed to be coupled with LSCs: their performance is evaluated upon variation of their structure and composition and compared with Si cell efficiency. In particular, by EQE theoretical simulations, we evidence how InGaP structures optmized for LSCs differ from those to be used under direct solar spectrum. Results obtained not only unveil the characteristics for an improved performance of the cells, but also represent an advance in the elucidation of the behaviour of InGaP-based devices in LSCs for future optimization.

Differences in radiation damage to carrier lifetimes in the neutral and depletion regions of InGaP and GaAs solar cells

We investigated the radiation damage to carrier lifetimes in the neutral and depletion regions of the InGaP and GaAs subcells of an InGaP/GaAs/Ge triple-junction solar cell. It is difficult to clarify the degradation characteristics of the carrier lifetime in each region using the conventional analysis method (e.g., the dark current–voltage characteristic analysis), so we propose a novel method using the internal luminescence efficiency. The radiation damage coefficients for the carrier lifetimes in the neutral region of InGaP and GaAs subcells were two or three orders of magnitude larger than those in the depletion region. This result suggests that the effective radiation-induced defects in the regions, which significantly impact a solar cell's electrical characteristics, differ. This paper discusses the effective radiation-induced defects that contribute to the output degradation in each region. We compare the radiation damage coefficients for the carrier lifetimes obtained in our analysis, the product of the capture cross section, and the defect introduction coefficient of each defect reported in previous studies.

Modeling diffusion length damage coefficient in GaAs and InGaP solar cells under electron irradiation

A method is proposed for calculating the diffusion length damage coefficient for minority carriers (KL) in GaAs and InGaP solar cells under electron irradiation using the Shockley–Read–Hall (SRH) model for defect-assisted recombination. In the SRH model, the damage coefficient KL is proportional to the product kσc, where k is the defect introduction rate under particle radiation and σc is the minority carrier capture cross section of the said defects. The introduction rate k is evaluated using the atomic theory for displacement under electron radiation, and the calculation for σc is adapted from Henry and Lang’s high-temperature multiphonon emission formulation. A linear scaling relationship is observed between k,KL and nonionizing energy loss—validated by bibliographic data in the radiation energy range E≈0.7–12 MeV. Our model reproduces the increasing trend in KL with doping, as observed in the literature, while also capturing the anisotropy between the p-type and n-type materials, with the p-type exhibiting greater radiation resistance than its n-type counterpart. The calculated KL is fed into the physical model for solar cell operation to obtain the post-irradiated Isc,Voc,Pmax at a given fluence Φ. The degradation of the electrical quantities is consistent with the measurements recorded in the literature. The findings show that InGaP is more radiation resistant than GaAs. It is demonstrated that calculating k not only aids in determining the degradation of solar cell parameters from first principles, but also in obtaining the empirical function for degradation: a−blog(1+ckΦ), used in fitting the experimental measurements. The limitations and potential scope of improvements in the model are also discussed.

Material and device characterization of InGaP solar cells grown on GaAs misoriented substrates by metal-organic chemical vapor deposition

We grew and characterized Zn- and Si-doped In0.48Ga0.52P grown on (001) GaAs substrates misoriented by θS = 0°, 2°, 6°, and 10° toward (111)A using metal-organic chemical vapor deposition. The dopant incorporation efficiency is improved with higher θS because of a larger surface free energy. The lattice constant of InGaP is increased with higher doping concentration, whereas it is decreased with higher θS as a result of reduced CuPt-type ordering in InGaP. This reduced ordering with θS from 0° to 10° is supported by a bandgap increase of ~60 meV. The open-circuit voltage of InGaP solar cell with θS = 10° is more than 100 meV higher than that with θS = 2° due to the increased bandgap and the decrease of the ordering-related defects. Besides, a larger CuPt-type ordering results in a greater reduction of the thermal activation energy of InGaP solar cell.

Numerical simulation and performance analysis of InGaP, GaAs, Ge single junction and InGaP/GaAs/Ge triple junction solar cells

Recent trends in the field of solar cells show that Multi-junction III-V semiconductor solar cells are an attractive option because of their high efficiency and cost effectiveness. As the first step to develop an InGaP/GaAs/Ge triple junction solar cell, we first simulated three individual InGaP, GaAs and Ge single junction solar cells using Silvaco ATLAS Software and obtained the efficiency of 13.42%, 12.99%, 3.37% for each cell respectively. Further in order to increase the efficiency of solar cell, we numerically simulated the InGaP/GaAs/Ge triple Junction solar cell by stacking InGaP, GaAs and Ge single junction solar cells and achieved high efficiency of 29.78%. The current–voltage curve, power-voltage curves and solar cell performance parameters and spectral response of each single junction solar cell as well as performance parameters and spectral response of InGaP/GaAs/Ge triple Junction solar cell have been extracted and analysed in detail. The aim of the study is to contribute to the investigation for experimental research work on the improvement of efficiency of InGaP/GaAs/Ge triple junction solar cell, through the numerical simulation, Since systematic cost of experimental investigation of the solar cell is very high and also it requires large time therefore technology computer aided design (TCAD) based simulation and performance optimization of the solar cells is important.

High-Speed MOVPE Growth of InGaP Solar Cells

Impacts of reactor pressure and growth temperature on the qualities and properties of InGaP epitaxial layers grown in a narrow-channel horizontal metalorganic vapor-phase epitaxy (MOVPE) reactor were investigated with a growth rate of 10 μm/h. The roughness of InGaP was improved remarkably when the pressure was increased from 6 to 10 and 15 kPa. The reason is most likely related to the absolute pressure of P and the migrations of In and Ga adatoms on the wafer surface. Owing to the gas-phase reactions and desorption of P from InGaP, the surface of InGaP became rough, and the pit dislocation was increased with an increase in growth temperature. Although a low temperature was beneficial in order to obtain a smooth InGaP surface, the carrier lifetime in InGaP was shortened as a drawback. Consequently, InGaP n-on-p solar cells grown at a low temperature suffered from the degradations of both short-circuit current density and open-circuit voltage. Background impurities and defect densities in low temperature grown InGaP were attributed to these deteriorations. At the optimized growth temperature of 650 °C and the reactor pressure of 15 kPa, the growth rate of InGaP was accelerated to 20 and 30 μm/h. InGaP solar cells were successfully fabricated with the rate of 30 μm/h that opened up the way for the fabrication of III–V multijunction solar cells in the high-speed MOVPE reactor.

Application of InGaP space solar cells for a radiation dosimetry at high dose rates environment of Fukushima Daiichi nuclear power plant

ABSTRACT Decommissioning the Fukushima Daiichi nuclear power plant (1F) after the accident caused by a tsunami in 2011 requires characterization of the fuel debris by dose distribution measurement. This paper describes the experimental and theoretical behavior of a radiation detector applied with InGaP solar cells is investigated and allow the localization and characterization of the fuel debris. In the irradiation test, it was observed that the radiation-induced current output of the InGaP solar cells increases linearly with increasing dose rates of 60Co γ-rays. For measurements at low dose rates, it becomes clear that the minimum detectable dose rate and resolution can be determined by analyzing the noise characterization. The maximum detectable level of radiation dosimetry for the InGaP solar cell was found to be higher than the highest γ-ray dose rate observable at the reactor core for 1F plants. Additionally, as an analysis of the radiation-induced current, it is attempted to express a relational expression between the absorbed dose rate and the creation of radiation-induced current pairs in the solar cells. The experimental and simulation results suggest that solar cells can be powerful tools for radiation dosimetry in high dose rate environments near the debris of the 1F plant.

Effects of front InGaP layer thickness on solar cell characteristics in InP/InGaP quantum dot solar cells

To investigate the impact of the front InGaP layer thickness on solar cell characteristics in InGaP-based InP quantum dot (QD) solar cells, we prepared InP/InGaP QD solar cells with a front i-InGaP layer and compared their performance with those of a device without i-InGaP layer and InGaP solar cell. Even though the QD solar cells show open-circuit voltage (Voc) reduction compared to the InGaP solar cell, the device with thin i-InGaP layer shows slightly higher Voc compared to the device with no i-InGaP layer. This result is explained by the fact that the saturation current in dark current characteristics decreases in the device with thin i-InGaP layer. This likely originates from the suppressed thermal carrier escape from QDs. Our findings reveal that the i-InGaP layer in the QDs significantly affects the photovoltaic performance. For further improvement of photovoltaic performance of such devices, optimization of the i-InGaP layer thickness is necessary.

Recovery from radiation-induced degradation in InGaP solar cells by light soaking

In this paper, we report that InGaP solar cells irradiated with electrons exhibit a significant recovery of open-circuit voltage after AM0 light soaking. The soaking-light intensity dependence of defect reduction and the change in the carrier concentration profile upon light soaking suggests that the recombination of photogenerated carriers contributes to the recovery. Because 0.2 MeV electrons are only responsible for phosphorus recoil in InGaP, the absence of correlation between the activation energy for voltage recovery and the electron irradiation energies of 0.2, 0.4, and 1 MeV implies that the annihilation of radiation-induced phosphorus-related defects contributes to the recovery.

Optimization and Comparison between the Efficiency of InGaP and GaAs Single Solar Cells with and without a Window Layer

Optimization and Comparison between the Efficiency of InGaP and GaAs Single Solar Cells with and without a Window Layer

Enhancement of open-circuit voltage in InGaP solar cells grown by solid source molecular beam epitaxy

The growth of ternary InGaP alloys is often susceptible to atomic ordering, which can lead to an anomalous bandgap reduction as well as the formation of antiphase boundaries (APBs). In this study, we report on the effect of growth rate and growth temperature on the performance of lattice-matched In0.52Ga0.48P solar cells grown on GaAs(001) substrates miscut 2° toward (111)B by solid-source molecular beam epitaxy. The bandgap of the InGaP film was widened from 1.855 eV for a film grown at 480 °C and 1.0 µm/h to 1.880 eV for a film grown at 510 °C and 1.5 µm/h owing to the suppression of atomic ordering in the alloy. In terms of solar cell performance, the open-circuit voltage was improved from 1.243 to 1.311 V by increasing the growth temperature and the growth rate due to not only the widening Eg but also the improved electrical characteristics (due to the fewer APBs present). In addition, the decreased number of APBs improved the fill factor and short-circuit current density. Consequently, the highest η of 14.43% was obtained for an InGaP single-junction solar cell grown at 510 °C and 1.5 µm/h and that of 25.57% for a dual-junction InGaP/GaAs solar cell.

Efficiency improvement of single-junction InGaP solar cells by advanced photovoltaic device modeling

A model to optimize single-junction InGaP based solar cells for soundly high photovoltaic characteristics is proposed. The simulated photovoltaic (PV) characteristics include current density (J-V) plots, open circuit potential (VOC), short circuit photocurrent density (JSC), fill factor (FF), conversion efficiency (η) spectral response and photo generation rates, taking into account different structural parameters. The results are superior to earlier experimental multi-junction InGaP based solar cells, in terms of η and FF values. They are also soundly comparable to earlier simulated PV characteristics. Starting with a single-junction InGaP solar cell, the results are potentially useful in future enhancements of multi-junction (III–V semiconducting materials, with different band gap values, such as InGaP/GaAs/Ge) solar cell models, which involve InGaP as front stack. The design concept of InGaP single-junction solar cells is described here, together with key technologies to achieve high efficiency of 18.55% at AM1.5 Sun, using the numerical modeling TCAD tool Silvaco ATLAS.

Investigation of the open-circuit voltage in wide-bandgap InGaP-host InP quantum dot intermediate-band solar cells

To analyze the open-circuit voltage (Voc) in intermediate-band solar cells, we investigated the current–voltage characteristics in wide-bandgap InGaP-based InP quantum dot (QD) solar cells. From the temperature dependence of the current–voltage curves, we show that the Voc in InP QD solar cells increases with decreasing temperature. We use a simple diode model to extract Voc at the zero-temperature limit, V0, and the temperature coefficient C of the solar cells. Our results show that, while the C of InP QD solar cells is slightly larger than that of the reference InGaP solar cells, V0 significantly decreases and coincides with the bandgap energy of the InP QDs rather than that of the InGaP host. This V0 indicates that the Voc reduction in the InP QD solar cells is primarily caused by the breaking of the Fermi energy separation between the QDs and the host semiconductor in intermediate-band solar cells, rather than by enhanced carrier recombination.

InGaP solar cell on Ge-on-Si virtual substrate for novel solar power conversion

InGaP single-junction solar cells are grown on lattice-matched Ge-on-Si virtual substrates using metal-organic chemical vapor deposition. Optoelectronic simulation results indicate that the optimal collection length for InGaP single-junction solar cells with a carrier lifetime range of 2–5 ns is wider than approximately 1 μm. Electron beam-induced current measurements reveal that the threading dislocation density (TDD) of InGaP solar cells fabricated on Ge and Ge-on-Si substrates is in the range of 104–3 × 107 cm−2. We demonstrate that the open circuit voltage (Voc) of InGaP solar cells is not significantly influenced by TDDs less than 2 × 106 cm−2. Fabricated InGaP solar cells grown on a Ge-on-Si virtual substrate and a Ge substrate exhibit Voc in the range of 0.96 to 1.43 V under an equivalent illumination in the range of ∼0.5 Sun. The estimated efficiency of the InGaP solar cell fabricated on the Ge-on-Si virtual substrate (Ge substrate) at room temperature for the limited incident spectrum spanning the photon energy range of 1.9–2.4 eV varies from 16.6% to 34.3%.

Radiation degradation prediction for InGaP solar cells by using appropriate estimation method for displacement threshold energy

InGaP solar cells are not predicted to be susceptible to displacement damage by irradiation with electrons at energies lower than 100 keV from non-ionizing energy loss (NIEL) calculations. However, it is recently observed that InGaP solar cells are shown to degrade by irradiation with 60 keV electrons. This degradation is considered to be caused by radiation defects but is not clear. In this study, the kind of the defects generated by electrons at energies lower than 100 keV is found by deep-level transient spectroscopy (DLTS). The result of DLTS indicates that the prediction of primary knock-on atoms by using the radiation damage model is different from the experiment. In order to suggest the generation mechanism of radiation defects, we propose a new displacement threshold energy (Ed) by using a new technique in which NIEL and the introduction rate of radiation defects are combined. The degradation prediction by using estimated Ed is found to agree well with the degradation of electric power of InGaP solar cells irradiated by low-energy electrons. From the theory of radiation defects, we propose a new obtaining process of suitable degradation prediction by the displacement damage dose method.

Performance degradation of InGaP solar cells due to 70 keV electron irradiation

The performance of InGaP solar cells irradiated with electron beams at energies lower than the damage threshold is degraded. To investigate the irradiation effect, GaAs and InGaP solar cells are irradiated with 70 keV electron beams. Measurements of the electroluminescence and external quantum efficiency, in addition to the light current voltage measurement, are conducted. The results show that the performance of the InGaP solar cell, mainly open-circuit voltage, is degraded and affected by nonradiative recombination centers, while the performance of the GaAs solar cell is hardly degraded. The comparison of the experimental results with the prediction from the displacement damage dose model suggests that the non-ionizing energy loss of electrons is the main factor for evaluating the degradation of the InGaP cell caused by electron beams in a relatively low energy range.

Effects of substrate miscut on the properties of InGaP solar cells grown on GaAs(001) by solid-source molecular beam epitaxy

The growth of ternary InGaP alloys is often susceptible to atomic ordering, which leads to an anomalous bandgap reduction as well as the formation of antiphase boundaries (APBs). The effect of substrate miscut on the performance of lattice-matched In0.52Ga0.48P solar cells grown on GaAs(001) substrates by solid-source molecular beam epitaxy (SS-MBE) is investigated. A B-type miscut enhanced single-variant atomic ordering even with SS-MBE, resulting in a bandgap (Eg) reduction from 1.87 eV for an alloy grown on an exact substrate to 1.85 eV for that grown on the substrate miscut 6° toward (111)B. Conversely, an A-type miscut suppressed the formation of atomic ordering, resulting in the Eg widening of the alloy grown on the substrate miscut 6° toward (111)A to 1.89 eV. With regard to solar cell performance, InGaP solar cells grown on A-type miscut substrates enhanced the open-circuit voltage (VOC) and WOC (= Eg/q − VOC) because of the low degree of atomic ordering. Large improvements in WOC and efficiency to 0.58 V and 10.93%, respectively, were obtained for the cell grown on the substrate miscut 2° toward (111)B. A reduction in the number of APBs due to single-variant atomic ordering was related to this latter result.

Influence of Growth Temperature on the Characteristics of Single-Junction p–i–n InGaP Solar Cells.

Single-junction p–i–n InGaP solar cells are grown at various temperatures from 620 to 700 °C by low pressure metalorganic chemical vapor deposition on GaAs (001) substrates. The short circuit current density of the p–i–n InGaP solar cells increases by up to 38.8% when the growth temperature is reduced from 700 to 620 °C, while the open circuit voltage and fill factor show relatively small changes. The external quantum efficiency, especially, in the wavelength regime below 500 nm, is improved for the p–i–n InGaP solar cells grown at lower temperatures. The improvement might be attributed to the reduced absorption loss of the photons in the n-InGaP emitter region. The highest conversion efficiency of 11.01% is attributed from the p–i–n InGaP solar cell grown at 640 °C. Electron mobility and concentration of undoped InGaP layers are investigated as a function of the growth temperature and correlated with the p–i–n InGaP solar cell performance.