InGaAs Solar Cells Research Articles

Defect analysis of 1-MeV electron irradiated flexible InGaAs solar cells by deep-level transient spectroscopy and photoluminescence

1-MeV electron irradiated InGaAs flexible solar cells were investigated based on solar cell I–V characteristics, external quantum efficiency, photoluminescence, and deep-level transient spectra (DLTS) mesurements. The electrical parameters of the solar cells, including Rs and Rsh, were extracted from the I–V curves. With an increase in irradiation fluence, the electrical and optical characteristics of the solar cells continuously degraded. The Voc, Isc, and Pmax values decreased to 83.16%, 81.96%, and 68.88% of their initial values, respectively, when the irradiation fluence was increased to 2 × 1015 e/cm2. Photoluminescence (PL) emission peak intensity decreased to 0.54% of its original value, and the minority carrier lifetimes derived from fitting PL emission spectra were reduced to 1.25%. DLTS technique was used to analyse the irradiation-induced defect properties. Similar fitting values for the minority carrier lifetimes were obtained from the PL and DLTS measurements.

Identifying and Investigating Spatial Features in InGaAs Solar Cells by Hyperspectral Luminescence Imaging.

Hyperspectral luminescence imaging adds high-resolution spectral data to electroluminescence and photoluminescence images of photovoltaic materials and devices. This enables absolute calibration across a range of spectra, and subsequently enhances the information that can be gained from such measurements. We present a temperature-dependent luminescence hyperspectral imaging study of dilute InGaAs solar cells. We are able to identify the cause of dark spots on the device as local areas with increased defect-related recombination and identify a likely candidate for the type of defect. Hyperspectral images also reveal a device-wide pattern in low-energy-tail luminescence and In alloy fraction, which corresponds with increased nonradiative recombination. This pattern would not be identifiable with conventional imaging methods. Detailed information on such features is useful as, paired with knowledge of fabrication processes and device design features, it can help identify ways to reduce associated non-radiative recombination and improve device performance.

Epitaxy and characterization of InP/InGaAs tandem solar cells grown by MOVPE on InP and Si substrates

The integration of III-V multi-junction solar cells on Si substrates is currently one of the most promising possibilities to combine high photovoltaic performance with a reduction of the manufacturing costs. In this work, we propose a prospective study for the realization of an InP/InGaAs tandem solar cell lattice-matched to InP on a commercially available Si template by direct MOVPE growth. The InP top cell and the InGaAs bottom cell were firstly separately grown and optimized using InP substrates, which exhibited conversion efficiencies of 13.5% and 11.4%, respectively. The two devices were then combined in a tandem device by introducing an intermediate InP/AlInAs lattice-matched tunnel junction, showing an efficiency of 18.4%. As an intermediate step towards the realization of the tandem device on Si, the InP and InGaAs single junction solar cells were grown on top of a commercial InP/GaP/Si template. This transitional stage enabled to isolate and evaluate the effects of the growth of III-V on Si on the photovoltaic performance through the comparison with the aforementioned devices on InP. Each cell was electrically characterized by external quantum efficiency and dark and illuminated current-voltage under solar simulator. The material quality was also analyzed by means of X-ray diffraction, Atomic-Force Microscopy, Transmission Electron and Scanning Electron Microscopy. The III-V on Si devices showed efficiencies of 3.6% and 2.0% for the InP and InGaAs solar cells, respectively.

Local Voltage Mapping of Solar Cells in the Presence of Localized Radiative Defects.

Hyperspectral electroluminescence and photoluminescence imaging of photovoltaic materials and devices produces three-dimensional spatially and spectrally resolved luminescence data, which can be calibrated to an absolute scale, enabling the extraction of high resolution maps of quantities such as the local voltage (quasi-Fermi-level splitting). This extraction requires supplemental measurements of external quantum efficiency (EQE), but these do not have the same spatial resolution. Previously, assumptions have been made to overcome this limitation. In this work, we evaluate these assumptions for InGaAs solar cells with significant spatial variation in the luminescence spectrum shape due to small regions with elevated concentrations of radiative defects. Although appropriate for small variations in spectral shape, we find that with more significant variation, these assumptions can result in non-physical EQEs and too-low voltages. Combining multiple methods can help alleviate this, or a minimum voltage map can be extracted, which will be similar to the actual voltage when EQE is high.

Optimal spectra management for self-power producing greenhouses for hot arid climates

The air conditioning in hot and arid climates remains one of the grand challenges for food production in greenhouses. Therefore, significant efforts are given to determine the suitable techniques for effective cooling of greenhouses. Usually, greenhouses are used in colder climates to gather sunlight for heating purposes. However, hot climates with elevated solar irradiation characteristics suffer from high temperatures within the greenhouse. In this study, we propose a unique method solving the extensive cooling requirement of greenhouses in hot arid climates by employing an optimal spectra management strategy. The novel sun-tracking roof design of the greenhouse incorporates hot dielectric mirrors and solar panels, which help to reduce the cooling load and provide electricity to the vapor compression cooling unit. The spectrum above 750 nm is reflected to vertically aligned InGaAs solar cells for additional power generation, whereas the c-Si solar cells are able to provide effective shadowing at noontime without significant comprimise on photosynthetically active radiation (PAR) while producing power. The results showed an average reduction of 28.9% and 25.4% in the cooling load of the proposed greenhouse compared to the conventional greenhouse during the summer and winter seasons for sun tracking hours (10:00 a.m. to 1:00 p.m.), respectively, at a set greenhouse temperature of 28 °C. The proposed greenhouse produced an average of 22.18 kWh/day of electrical energy throughout the year. The novel roof significantly lowers the cooling load and partially meets the energy demand of greenhouses without compromising on photosynthetically active radiations to enter the greenhouse.

Pd-mediated mechanical stack of III–V solar cells fabricated via hydride vapor phase epitaxy

In this study, we present the first demonstration of mechanical stacking of solar cells grown via hydride vapor phase epitaxy (HVPE) to fabricate tandem cells. Smart stack technology, in which Pd nanoparticles were used as the bonding mediator, was used for bonding the cells. To prepare the smart stack, the top cells of the tandem structure were peeled from the substrate using the epitaxial lift-off technique. For this purpose, the cell structure was grown on an AlAs sacrificial layer. GaAs solar cells with good performance were fabricated using HVPE, in which the surface roughened by AlAs was restored to flatness by growing a thick GaAs film. The GaAs//InGaAs tandem cells achieved by bonding the GaAs solar cells grown via HVPE with the InGaAs solar cells grown by metal–organic vapor phase epitaxy exhibited a conversion efficiency of 22.6%.

Inhomogeneous in-plane distribution of preferential glide planes of β dislocations in a metamorphic InGaAs solar cell

By using synchrotron X-ray diffraction, we investigated the in-plane distribution of preferential glide planes in a metamorphic InGaAs solar cell with a relatively low open-circuit voltage (V oc) compared to other cells fabricated on the same wafer. The reciprocal-space maps revealed that the low-V oc cell contains several domains with different preferential glide planes of β dislocations. Since fluctuations of the average β-glide plane have not been observed for high-V oc cells, the observed inhomogeneous distribution should be related to the V oc degradation. Understanding preferential glide-plane changes within a cell can help to improve the uniformity of cell properties over the whole wafer.

Failure analysis of thin‐film four‐junction inverted metamorphic solar cells

AbstractInverted metamorphic solar cells play an important role in the field of photovoltaics, because it can directly grow stacked tandem junctions with different bandgaps according to the spectrum. We have found that the four‐junction AlGaInP/AlGaAs/InGaAs/InGaAs solar cells with the bandgap of 1.96/1.55/1.17/0.83 eV on the basis of the inverted metamorphic three‐junction AlGaInP/AlGaAs/InGaAs materials will cause a serious decrease in short‐circuit current density but with a normal open‐circuit voltage. The sharp decrease in short‐circuit current density is not attributed to the mismatched buffers dislocations penetrating into the active region of the InGaAs subcells but resulted from the minority carrier recombination due to defects in the AlGaInP subcell, which is observed directly from transmission electron microscopy, external quantum efficiency, electroluminescence, and secondary ion mass spectrometry measurements. The process of growing AlGaInP materials by metal–organic chemical vapor deposition easily introduces Al‐O deep‐level defects, resulting in the poor collection of minority carriers in AlGaInP materials. After improving the growth conditions of AlGaInP materials, a four‐junction solar cell with a photoelectric conversion efficiency of 34.9% and an open‐circuit voltage of 3.53 V was obtained.

Fabrication and simulation of GaInAs Solar cells using compositionally step-graded AlGaInAs buffers on GaAs substrate

In the direction of the industrial application of invert metamorphic GaInAs/AlGaInAs/GaAs/GaInP quadruple junction solar cells, another 0.67 eV band gap lattice-mismatched GaInAs junction was added into the three-junction 1.8/1.4/1.0 eV solar cells. In this paper, we performed an investigation of a 0.67 eV-GaInAs solar cell on the compositionally step-graded InP/AlGaInxAs/GaAs virtual substrate in order to identify the metamorphic buffers qualities after growing the solar cell. The transmission electron microscopy, X-ray diffraction and atomic force microscope were carried out to analyze material structure properties. The high surface roughness is observed and a region where a significant number of external dislocations are formed in the InGaAs tunnel junction of solar cells is identified. The I–V and EQE characterizations of metamorphic 0.67 eV InGaAs solar cell were measured under 1 sun AM1.5D conditions. To research the impact of threading dislocation densities on the InGaAs solar cell, the simulation results of I–V and EQE have also been discussed with Crosslight APSYS’s some applicable features.

Cathodoluminescence Study of Dislocations in Step-Graded InGaP Buffer Layers of Metamorphic Single-Junction InGaAs Solar Cells

Epitaxial growth of lattice-mismatched materials is useful for solar cells, but lattice dislocations must be controlled for best device performance. It has been shown that metamorphic growth enables fabrication of InGaAs p–n junctions with good performances on GaAs substrates due to the insertion of buffer layers. Here, we investigate misfit and threading dislocations inside the step-graded InGaP buffer layers of a single-junction InGaAs solar cell by cathodoluminescence microscopy. Prior to measurement, the device edges were polished at various angles (less than 10° with respect to the substrate surface). By using this technique, cross sections of very thin layers can be directly imaged with a resolution that allows us to observe misfit and threading dislocations. In the present device, the densities of the two types of dark lines depend on the position in the buffer structure. In particular, near the InGaAs base layer, the density of the dark lines extending in the [110] direction is higher than that of the dark lines extending in the [1-10] direction. We believe that this difference in the dark line density is related to the surface morphology.

Growth of InGaAs Solar Cells on InP(001) Miscut Substrates Using Solid‐Source Molecular Beam Epitaxy

Herein, the effects of both the growth temperature and the substrate miscut on the properties of lattice‐matched InGaAs solar cells grown on InP substrates via solid‐source molecular beam epitaxy are investigated. The growth temperature is varied from 420 to 490 °C. InP(001) miscut by 2° toward (111)A and (111)B denoted by 2°A and 2°B, respectively, and exactly‐cut substrates are used. Material quality is evaluated by photoluminescence (PL) and atomic force microscopy (AFM) measurements. At room temperature, the PL emissions become more intense at higher growth temperatures and with miscut substrates, indicating less nonradiative recombination. AFM results show a streaky surface with step structures along the direction for cell grown at 490 °C on 2°A, suggesting that the substrate promotes step‐flow growth. Consequently, the highest conversion efficiency (12.3%) for the cell grown at 490 °C on 2°A is obtained. Especially, the open‐circuit voltage (VOC) increases from the baseline of 0.350 V (for the cell grown at 420 °C on an exact substrate) to 0.374 V. The Eg−VOC deficit, where , of 369 mV is obtained, which is a standard benchmark for high‐material‐quality solar cells.

Epitaxial lift-off of InGaAs solar cells from InP substrate using a strained AlAs/InAlAs superlattice as a novel sacrificial layer

In this work we present a new epitaxial lift-off (ELO) approach based on the use of a strained AlAs/InAlAs superlattice (SL) as sacrificial layer for InP related materials. Such an ELO process enables the fabrication and transfer of a thin active III-V heterostructure via its separation from its III-V parent substrate using selective chemical etching. The strategy is of particular interest for large area devices such as solar cells. The process studied here also allows the substrate reuse for a low-cost approach based on III-V-based device fabrication. In order to realize the ELO process on InP substrates, the main difficulty is the lack of lattice-matched materials offering the high chemical etching selectivity needed over both the substrate and the lattice-matched alloys of the active heterostructure. The present study therefore contributes effective strategy for overcoming the latter constraints. The AlAs/InAlAs SL was thus explored as a potential candidate as sacrificial layer for the InP lattice matched materials. The growth conditions of such SLs were investigated to produce low defect SLs compatible with the properties of an optimal sacrificial layer. The under-etching behavior of such SLs in a hydrofluoric acid-based solution was also studied in detail. The results show that advantageous under-etching rates, high enough for a full wafer detachment, combined with a low defect density, can be obtained with novel sacrificial layers based on such thin AlAs/InAlAs SL. Finally, the fabrication of solar cells via an active heterostructure grown over an optimized SL on a monolithic substrate and via a thin reported active heterostructure was performed. The solar cells perform well and demonstrate the suitability of such SLs as a sacrificial layer for InP related materials.

Performance comparison of III\u2013V//Si and III\u2013V//InGaAs multi-junction solar cells fabricated by the combination of mechanical stacking and wire bonding

The integration of III–V and Si multi-junction solar cells as photovoltaic devices has been studied in order to achieve high photovoltaic conversion efficiency. However, large differences in the coefficients of thermal expansion and the lattice parameters of GaAs, Si, and InGaAs have made it difficult to obtain high-efficiency solar cells grown as epilayers on Si and InP substrates. In this paper, two types of devices, including GaInP/GaAs stacked on Si (GaInP/GaAs//Si) and GaInP/GaAs stacked on InGaAs (GaInP/GaAs//InGaAs), are fabricated via mechanical stacking and wire bonding technologies. Mechanically stacked GaInP/GaAs//Si and GaInP/GaAs//InGaAs triple-junction solar cells are prepared via glue bonding. Current-voltage measurements of the two samples are made at room temperature. The short-circuit current densities of the GaInP/GaAs//Si and GaInP/GaAs//InGaAs solar cells are 13.37 and 13.66 mA/cm2, while the open-circuit voltages of these two samples are measured to be 2.71 and 2.52 V, respectively. After bonding the GaInP/GaAs dual-junction with the Si and InGaAs solar cells, the conversion efficiency is relatively improved by 32.6% and 30.9%, respectively, compared to the efficiency of the GaInP/GaAs dual-junction solar cell alone. This study demonstrates the high potential of combining mechanical stacked with wire bonding and ITO films to achieve high conversion efficiency in solar cells with three or more junctions.

Research on influence of parasitic resistance of InGaAs solar cells under continuous wave laser irradiation

InGaAs solar cells were irradiated by 1060-1080nm continuous wave (CW) laser, and studied the laser-electrical conversion and damage experiment with the power density as 97mW/cm2 and 507W/cm2 respectively. The result indicated that there is no obvious damage phenomenon but air layer appeared in the damaged region, and there is no direct relationship between the area and the extent of damage. Moreover, the p-n junction in the damage zone was destroyed, lost the ability of photoelectric conversion. The region acts as a resistance between the two electrodes, resulting in an increase in the leakage current of the solar cells and a decrease in the parallel resistance, which is the main reason leading to the decline of open circuit voltage, short circuit current and conversion efficiency. This paper would provide a reference for wireless energy transmission and the subsequent laser damage of solar cells.

Comparison of Ge, InGaAs p-n junction solar cell

In this paper, the effect of material parameters on the efficiency of Ge and InGaAs p-n junction solar cells which are most commonly used as the sub-cell of multi-junction solar cells are investigated and the results due to these two cells are compared. The efficiency of Ge (EG=0.67 eV) solar cell which is easy to manufacture and inexpensive in cost, is compared with the efficiency of InGaAs (EG=0.74 eV) solar cell which is coming with drawback of high production difficulties and cost. The theoretical efficiency limit of Ge and InGaAs solar cells with optimum thickness were determined by using detailed balance model under one sun AM1.5 illumination. Since the band gap values of two cells are close to each other, approximate detailed balance efficiency limits of 16% for InGaAs and 14% for Ge are obtained. When drift-diffusion model is used and the thicknesses and doping concentrations are optimized, the maximum efficiency values are calculated as 13% for InGaAs and 9% for Ge solar cell. For each solar cell external quantum efficiency curves due to wavelength are also sketched and compared.

Design of InP-based metamorphic high-efficiency five-junction solar cells for concentrated photovoltaics

We propose an InP-based upright five-junction (5J) solar cell structure for high conversion efficiency under concentration. In the structure, three bottom subcells are composed of lattice-matched (LM) InGaAsP materials, while two top subcells employ metamorphic InGaP materials. The two InGaP subcells are designed to have the same Ga composition of 30%. The first InGaP subcell is thinned so as to transmit half of the photon flux to the second InGaP subcell, thus forming an upright 5J InGaP(1.64 eV)/InGaP(1.64 eV)/InGaAsP(1.3 eV)/InGaAsP(1.02 eV)/InGaAs(0.74 eV) solar cell structure on the InP substrate. The subcell bandgap energies are chosen in such a way that a current matching condition can be achieved. Because no Al- or N-contained materials are used in the absorbers and only one metamorphic growth is required (with a lattice mismatch of 2.1%), the novel InP-based solar cell architecture is considered practically achievable with current growth technology. By comparing it with a InGaP/GaAs/Ge reference cell and adding additional nonideal factors in the modeling, an efficiency as high as 46.2% is estimated under concentration at ∼1500 suns.

Performance Characterization of Thin-Film InGaAs Solar Cells with Double-Hetero-Structure and InP Window-Layers of Various Thicknesses

Performance Characterization of Thin-Film InGaAs Solar Cells with Double-Hetero-Structure and InP Window-Layers of Various Thicknesses

III–V compound semiconductor multi-junction solar cells fabricated by room-temperature wafer-bonding technique

We have developed III–V compound semiconductor multi-junction solar cells by a room-temperature wafer-bonding technique to avoid the formation of dislocations and voids due to lattice mismatch and thermal damage during a conventional high-temperature wafer-bonding process. First, we separately grew an (Al)GaAs top cell on a GaAs substrate and an InGaAs bottom cell on an InP substrate by metal solid source molecular beam epitaxy. Thereafter, we successfully bonded these sub-cells by the room-temperature wafer-bonding technique and fabricated (Al)GaAs ∥ InGaAs wafer-bonded solar cells. To the best of our knowledge, the obtained GaAs ∥ InGaAs and AlGaAs ∥ InGaAs wafer-bonded solar cells exhibited the lowest electrical and optical losses ever reported. The AlGaAs ∥ InGaAs solar cells reached the maximum efficiency of 27.7% at 120 suns. These results suggest that the room-temperature wafer-bonding technique has high potential for achieving higher conversion efficiencies.

Fabrication of sub-wavelength antireflective structure to enhance the efficiency of InGaAs solar cells

Large differences in the refractive index between semiconductors (Si, GaAs, etc.) and air produces considerable Fresnel loss, which can seriously hinder the absorption of sunlight by photovoltaic solar cells. This study presents a cost-effective roller nanoimprinting technique for the fabrication of sub-wavelength structures (SWSs) as an alternative to conventional anti-reflective coatings used to reduce reflectance in triple-junction InGaP/InGaAs/Ge solar cells. The proposed nanoimprinting technology uses a soft PDMS mold duplicated from a hard silicon template, which is fabricated using PS sphere lithography and dry etching processes. To evaluate the anti-reflective performance of SWSs, we employed rigorous coupled wave analysis (RCWA) to simulate the propagation of electromagnetic plane waves in a GaAs substrate. Simulation results demonstrated a considerable reduction in reflectance resulting from a gradual change in the refractive index provided by SWSs. Photoelectric conversion efficiency was also increased.

Broadband and omnidirectional anti-reflection layer for III/V multi-junction solar cells

We report a novel graded refractive index antireflection coating for III/V quadruple solar cells based on bottom-up grown tapered GaP nanowires. We have calculated the photocurrent density of an InGaP–GaAs–InGaAsP–InGaAs solar cell with a MgF2/ZnS double layer antireflection coating and with a graded refractive index coating. The photocurrent density can be increased by 5.9% when the solar cell is coated with a graded refractive index layer with a thickness of 1μm. We propose to realize such a graded refractive index layer by growing tapered GaP nanowires on III/V solar cells. For a first demonstration of the feasibility of the growth of tapered nanowires on III/V solar cells, we have grown tapered GaP nanowires on AlInP/GaAs substrates. We show experimentally that the reflection from the nanowire coated substrate is reduced and that the transmission into the substrate is increased for a broad spectral and angular range.