Search

We recently demonstrated an ultra-thin solar cell with increased radiation tolerance as compared to a traditionally thick absorber counterpart. However, as the active region of the device was reduced so was the absorption with respect to state of the art devices. Therefore, we discuss a light trapping design, which implements nanosphere natural photolithography to fabricate a micropillared diffraction grating, combined with a transparent spacer layer and a distributed Bragg reflector. We discuss the fabrication process and illustrate the effectiveness of integrating light trapping structures for enhanced optical confinement within an ultra-thin solar cell design.

In this paper, we present findings on micro-concentrator photovoltaic cells composed of lattice matched subcells grown on GaAs and InP substrates, which are stacked into single, four-terminal devices using micro-transfer printing. The design, modeling, growth, fabrication and assembly of the devices will be described, and potential interconnection schemes to achieve efficient, two-terminal strings of cells with flexible current and voltage outputs and resilience to defects is discussed.

Quaternary alloys are essential for the development of high-performance optoelectronic devices. However, immiscibility of the constituent elements can make these materials vulnerable to phase segregation, which degrades the optical and electrical properties of the solid. High-efficiency III-V photovoltaic cells are particularly sensitive to this degradation. InAlAsSb lattice matched to InP is a promising candidate material for high-bandgap subcells of a multijunction photovoltaic device. However, previous studies of this material have identified characteristic signatures of compositional variation, including anomalous low-energy photoluminescence. In this work, atomic-scale clustering is observed in InAlAsSb via quantitative scanning transmission electron microscopy. Image quantification of atomic column intensity ratios enables the comparison with simulated images, confirming the presence of nonrandom compositional variation in this multispecies alloy.

We have developed multijunction solar cells on both GaAs and InP platforms to split the solar spectrum efficiently among subcells that maintain lattice-match to their respective source substrates. The GaAs-based dual-junction solar cell has achieved 31% AM1.5D concentrator efficiency. The InP-based triple junction solar cell has achieved 8.7% AM1.5D efficiency under concentration. Using micro-transfer printing, the wide-bandgap GaAs-based subcells will be stacked on top of the narrow-gap InP-based subcells and characterized to determine 4-terminal AM1.5D efficiency of the five-junction device. The microcells, less than 200 μm on a side, are assembled on transparent glass substrates, permitting diffuse light transmission to be collected by a silicon backplane. The approach facilitates efficient collection of both direct and diffuse sunlight for potential applications in a wide variety of climates. Major milestones in III-V device development are detailed.

The Sb concentration profile in an nBn photodetector containing an InAs/InAsSb type-II superlattice is collected and analyzed using atom probe tomography. A 3D reconstruction comprises the full composition of 31 periods. The Sb concentration profile is evaluated for the entire 31 period stack, as well as each individual period using segregation models from Muraki and Wood. Trends in the asymmetric Sb profile show a consistent non-negligible Sb concentration in the InAs layers and a lower Sb concentration in the InAsSb with respect to the target concentration.

In this report, we use atom probe tomography (APT) to produce 3D compositional maps of one-step growth of perovskite films (CH 3 NH 3 PbI 3 , MAPbI 3 . Initial studies indicate that further grain size enhancement and improved crystallinity can be achieved with the addition of Cd to these films. For MAPbI 3 films, perovskite specimens for APT were prepared and analyzed, which highlighted several challenges in the development of an APT procedure that reduces the effects of sample preparation and data collection on the fundamental morphology of the perovskite being analyzed. This optimization of the APT process, specifically for perovskite films, must be achieved before meaningful comparisons can be drawn between baseline films and those with Cd additives. We present some of the observed changes in morphology of the perovskite due to the APT process, which will enable future process optimization.

The surface passivation of p-benzoquinone (BQ) and hydroquinone (HQ) when dissolved in methanol (ME) has been examined through effective lifetime testing of crystalline silicon (c-Si) wafers treated with the aforementioned solutions. Changes in the availability of both photons and protons in the solutions were demonstrated to affect the level of passivation achieved. The requirement of both excess protons and ambient light exposure to maintain high effective lifetimes supports the presence of a free radical species that drives the surface passivation. Surface analysis suggests a 1:1 ratio of HQ-like bonds to methoxy bonds on the c-Si surface after treatment with a BQ/ME solution.

We examine the potential to enhance cell performance using wide bandgap metal oxide films as full-area rear contacts to p-type crystalline Si (c-Si) solar cells. We aim to introduce a band offset through wide bandgap nickel oxide rather than introducing a band bending through transition metal oxides (e.g. MoO x , V 2 Ox, WO x ). Our numerical simulation shows it is possible to achieve one-sun efficiency of 21.6% with the open-circuit voltage (V oc ) of 652 mV, the short-circuit current density (J sc ) of 39.9 mA/cm2 and the fill factor (FF) of 82.5% when the back surface recombination velocities is 100 cm/s at p-Si/NiO x .

Devices were demonstrated making use of benzoquinone passivation of silicon surfaces in a PEDOT:PSS-Silicon hybrid device structure. It is shown that the insertion of the benzoquinone layer does not provide an impediment to current flow across the PEDOT:PSS-Silicon interface and device efficiencies of 9.6% can be obtained. Devices using this passivation exhibited exceptionally high external quantum efficiencies in the low wavelength region, indicative of a high-quality front surface. Multiple rear contact schemes were evaluated, with Al giving the best results. Front-side texturing is shown to boost the short-circuit current of the devices by 20%, yielding significant gains to device fill factors.

To evaluate the effects of high temperature terrestrial ambient conditions on InGaP films for use in PV devices, InGaP with a bandgap of 1.87 eV was grown on GaAs by MOVPE. The films were annealed in ambient conditions at 400°C and 450°C for a cumulative duration of 16 hours, respectively. At intervals of 1, 6, and 16 hours of annealing, VASE measurements were taken to determine the composition and thickness of the developing oxide layer. The oxide was found to be like GaP-oxide in nature, with parallel growth rates over 16 hours of annealing time. This trend was independent of temperature. Final oxide thickness was dependent on annealing temperature, with InGaP annealed at 450° C having a thicker oxide. Surface roughness also increased after annealing, as shown by AFM.

An InAlAsSb random alloy grown by molecular beam epitaxy (MBE) to be lattice-matched to InP with an intended bandgap of 1.57 eV, has been analyzed using atom probe tomography (APT). The InAlAsSb material has demonstrated optoelectronic properties that deviate from theoretically predicted values, potentially due to the presence of compositional variation and defects. The mass spectrum displays sharp peaks with minimal molecular evaporation. 3D reconstruction of the film shows no variant regions on the visual inspection length scale. 2D concentration profiles for each element on a 20×20×10 nm3 region of interest (ROI) show compositional variations spanning 2–4 nm, with In and Al segregating opposite each other (i.e. regions of high/low In concentration are low/high in Al content). Frequency distribution analysis indicates that the In distribution for the ROI is statistically nonrandom. Thus, there exists compositional variations in the InAlAsSb film at nanometer length scales, with the behavior of In likely being a main factor in the production of this variation.

We have developed an InGaP solar cell structure capable of operating at 450°C under N 2 ambient. This structure has been annealed for over 70h without degradation in room temperature performance. This type of structure has applications in hybrid solar energy plants which combine photovoltaic and thermal collection systems to maximize overall conversion efficiency. We anticipate that this device will be able to achieve up to 17% efficiency at 400°C and 500x concentration based on simulations with incorporated optical and electrical high-temperature semiconductor parameters.

In this work, we demonstrated an enhanced surface passivation of epitaxially grown boron-doped emitters by replacing thermal SiO 2 as a passivation layer of p+-emitter employed in a 16.8% efficient 18-μm Si solar cell on stainless steel with plasma assisted atomic layer deposition (ALD) Al 2 O 3 /PECVD SiN x stack. For the Al 2 O 3 /SiN x stacks on epitaxial p+-emitter after post-deposition anneal, the emitter saturation current density (J 0e ) values were decreased to 19.5 fA/cm2 with the corresponding iV oc of 688 mV By using advanced surface passivation scheme, further improvement in the V oc of a present 16.8% efficient ultrathin Si solar cell on steel can be expected.

We have demonstrated for the first time a proof of concept four junction solar cell grown on an InP substrate. The split top cell design uses two 1.45 eV InAlAs subcells in series to take advantage of the large portion of the solar spectrum with energies higher than this bandgap. The prototype cell has an open circuit voltage of 2.44 V and short circuit current density of 1.8 mA/cm2. Further optimization may allow this cell to have performance comparable with high quality triple-junction GaAs cells. This architecture may be useful in other systems where high quality wide bandgaps are not available or in materials systems where carrier collection is strongly limited by the diffusion length within the material.

Gallium phosphide (GaP), with its wide band gap of 2.26 eV, is a good candidate for the top junction solar cell in a multijunction solar cell system. Here, we design, fabricate, characterize, and analyze GaP solar cells. Liquid phase epitaxy is used to grow the semiconductor layers. Four generations of GaP solar cells are developed and fabricated with each solar cell structure being designed and improved based on the first principles analyses of the predecessor solar cells. Quantum efficiency and current-voltage measurements are used to analyze the solar cell performance and to develop predictive models. We create a GaP solar cell with an efficiency of 2.42% under AM 1.5G one sun illumination.

Quaternary materials lattice-matched to InP have been examined using variable angle spectroscopic ellipsometry (VASE). The optical constants of InGaAsP and InAlGaAs, lattice-matched to InP with bandgaps of 1.2eV, have been determined. The native oxide layer of the materials was simulated using a weighted Bruggeman effective medium approximation (EMA) approach comprising the known optical constants of the native oxides of the end-point materials. The extracted optical constants from VASE measurements are compared with interpolated values from literature data, in order to assess the effectiveness of such an approach for quaternary alloys. Additionally, the absorption coefficients of InGaAsP and InAlGaAs are compared, the results of which can be used to evaluate the effectiveness of both material systems in high efficiency multijunction (MJ) photovoltaic (PV) devices.

The effect of rapid thermal annealing on the optical properties of InxAl1−xAsySb1−y was analyzed and compared to that for In0.52Al0.48As. Initial ellipsometry and photoluminescence experiments performed before the annealing indicate the presence of a low energy Urbach tail in the absorption spectrum. Rapid thermal annealing produces a blue-shift in the PL emission when annealed at 650°C for 60s and a decrease in the full-width-half-maximum, which originates from a reduction of the emission from the longer wavelength states. For the In0.52Al0.48As, the emission energy and the full-width-half-maximum remain constant during the annealing study. The elimination of sub-bandgap states in InxAl1−xAsySb1−y is critical for achieving a realistic path towards high efficiency multijunction cells lattice-matched to InP.

In order to be considered a viable option as a window layer, a number of requirements must be met. These include high transparency, small valence band offset, high conduction band offset, low surface recombination velocity, ability to be passivated in air, and availability of a selective etchant with the contact layer. In this paper, we demonstrate that strained In0.3Al0.7As meets all of these requirements for widebandgap, lattice-matched InAlAs grown on InP substrates. We expect the results to be applicable to the InAlAsSb system which could enable triple junction photovoltaic devices with record efficiencies.

Gallium arsenide material has been deposited via metal organic chemical vapor deposition (MOCVD) at growth rates varying between 14 µm/hr and 56 µm/hr. Photovoltaic device results indicate a 6–7% relative decrease in efficiency between 14 and 56 µm/hr GaAs solar cells, due to a reduction in short-circuit current and open-circuit voltage. By simulating the experimental characterization data, it is established that performance losses are associated with rear surface recombination velocity and Shockley-Read-Hall lifetime. The relative impact of these loss mechanisms will be quantified and conclude with discussions on their mitigation.

In this work, we investigate a transparent conducting oxide (TCO)-based, passivated contact for the potential use as a passivated tunnel contact to p-type Si. As a surface passivation layer, the Al2O3 films with varying the thickness are deposited using plasma-enhanced atomic layer deposition (PEALD) at 200 °C, followed by post-deposition annealing. For a ∼15 nm thick Al2O3 layer, a high level of surface passivation is achieved, characterized by the effective surface recombination velocity (Seff,max) of

We examine thermally evaporated MoO x films as a full-area rear contact to crystalline p-type Si solar cells for efficient hole-selective contacts. Prior to front- and rear-metallization, the implied open-circuit voltage (iV oc) is evaluated to be 646 mV with implied fill factor (iFF) of 82.5% for the tunnel SiO x /MoO x rear contacted cell structure with the passivated emitter on the textured surface, showing it is possible to achieve an implied 1-sun efficiency of 20.8%. Numerical simulation reveals that the electron affinity (χ) of the MoO x material strongly influences the performance of the MoO x contacted p-Si cell. Simulated band diagrams show that the values in χ of the MoO x layer must be sufficiently high in order to lower junction recombination, indicating that the highest efficiency of 21.1% is achievable for a high χ of 5.6 eV of MoO x films and back surface recombination velocity of

Quinhydrone has two constituent parts p-benzoquinone and hydroquinone and these components have been dissolved in methanol to passivate hydrogen-terminated silicon surfaces. It has been confirmed that p-benzoquinone is the active component; however, hydroquinone does improve in performance when in contact with the substrates for a longer duration. This effect is independent of substrate doping. The effect of light was also examined. Substrates passivated in full ambient light conditions displayed the highest effective lifetimes. The passivation effect from solutions exposed to light with measurements taken in the dark were compared with that of substrates passivated using a process completely performed in the dark. The passivation effect from these procedures is substrate doping dependent, but not as effective as full light passivation. It is confirmed that the presence of light is facilitating the bonding reaction. The labile proton on methanol is also found to play an important role in passivation. Additionally, protons that help with the effectiveness of passivation process predominantly come from methanol, but not water.

The wide band gap of GaP (2.26eV) makes it a very good candidate for the top junction solar cell in a multi-junction solar cell system. A wide band gap solar cell can increase the efficiency of the system by absorbing and converting the high energy photons more efficiently. Quantum efficiency (QE) is a powerful tool in analyzing the solar cell's performance by identifying the recombination from different regions of a solar cell. In this work, a QE model has been developed in curve fitting the measured QE curve to further analyze the solar cell and improve the solar cell design. With the continuous improved designs based on the QE analysis results, the best reported efficiency of a GaP solar cell has been achieved. This solar cell has an open circuit voltage (Voc) of 1.55V, short circuit current density (Jsc) of 1.97mA/cm2, fill factor (FF) of 79.4% and efficiency of 2.42% compared to the previous best reported efficiency of 1.17%.