Jacob Becker

Ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer

This paper reports the proposal, design, and demonstration of ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer to optimize light management and minimize non-radiative recombination. According to our recently developed semi-analytical model, this design offers one of the highest potential achievable efficiencies for GaAs solar cells possessing typical non-radiative recombination rates found among commercially available III-V arsenide and phosphide materials. The structure of the demonstrated solar cells consists of an In0.49Ga0.51P/GaAs/In0.49Ga0.51P double-heterostructure PN junction with an ultra-thin 300 nm thick GaAs absorber, combined with a 5 μm thick Al0.52In0.48P layer with a textured as-grown surface coated with Au used as a reflective back scattering layer. The final devices were fabricated using a substrate-removal and flip-chip bonding process. Solar cells with a top metal contact coverage of 9.7%, and a MgF2/ZnS anti-reflective coating demonstrated...

Ultra-thin GaAs single-junction solar cells integrated with an AlInP layer for reflective back scattering

This paper proposes and demonstrates the use of a textured and lattice-matched semiconductor layer coated with a Au reflector for reflective back scattering to enhance the efficiency of single-junction solar cells with ultra-thin absorbers. The device structure studied in this work consists of an In_0.49Ga_0.51P/GaAs/In_0.49Ga_0.51P double-heterostructure single-junction solar cell with a GaAs absorber of either 300 nm or 1000 nm thick, as well as a textured Al_0.52In_0.48P layer coated with a highly reflective Au film. The devices, areas ranging from 0.3×0.3 mm² to 1×1 mm², are flip-chip bonded onto Si carrier substrates, covered by a contact metal grid (with a 9.7% or 10.7% shadow area for the 300 nm and 1000 nm devices, respectively) and a MgF₂/ZnS anti-reflective coating. Both types of device designs demonstrate an open-circuit voltage of 1.00 V, short-circuit current densities of 24.5 and 26.1 mA/cm², and power conversion efficiencies of 19.1% and 20.6%, respectively; these measurements are carried out under 1 sun solar radiation (AM 1.5G, 0.1 W/cm²). It is reasonable to expect that the short-circuit current densities and conversion efficiencies of these devices could reach 26.6 mA/cm² and 28.6 mA/cm², and 20.7% and 22.6%, respectively, when a typical contact grid layout with 2% surface coverage is used. These short-circuit current densities are only 3.6 and 1.6 mA/cm² lower than the maximum theoretical value 30.2 mA/cm², respectively.

Monocrystalline CdTe/MgCdTe double-heterostructure solar cells with a ZnTe hole-contact and passivation layer

Monocrystalline p-ZnTe/i-MgCdTe/n-CdTe/n-MgCdTe double-heterostructure (DH) solar cells are grown through a combination of MBE and MOCVD deposition techniques using several different dopants. The adverse effects (high interface recombination velocity) of the lattice mismatched ZnTe/CdTe hetero-interface is suppressed by the use of a DH with an intrinsic MgCdTe top barrier layer that functions as a passivation layer. The steady-state photoluminescence intensity is used to compare the potential device performance with previous ZnTe/CdTe and a-Si/i-MgCdTe/CdTe device structures while quantum efficiency measurements demonstrate the benefit of using ZnTe over previously demonstrated contact layers.

Monocrystalline ZnTe/CdTe/MgCdTe double heterostructure solar cells grown on InSb substrates

Monocrystalline p-ZnTe/p-CdTe/n-CdTe/n-MgCdTe double-heterostructure (DH) solar cells are designed and demonstrated with a maximum efficiency of 10.9 %, an open-circuit voltage (VOC) of 759 mV, a short-circuit current density (JSC) of 21.2 mA/cm2 and a fill factor (FF) of 67.4 %. The low efficiency is mainly due to the combination of the low VOC and FF, which are attributed to high interface recombination at the ZnTe/CdTe, and p-CdTe/n-CdTe interfaces. Activation energies (EA) of 1.54 eV and 1.25 eV are obtained from temperature dependent light-IV measurements, indicating that the dominant recombination mechanism changes from interface recombination for non-annealed devices to bulk recombination for devices annealed at 450 °C.

Characterization and quantitative analysis of ultra-thin GaAs single-junction solar cells with reflective back scattering

This paper studies the impacts of the non-ideal reflective back scattering, non-radiative recombination, and series resistance on the device performance of ultra-thin GaAs single-junction solar cells. The reflectivity of the textured AlInP/Au interface is calculated by averaging the angular reflectivity against the Lambertian distribution, the value of which is 95% at the GaAs absorption edge. The impact of non-ideal scattering on short-circuit current density (Jsc) and external quantum efficiency (EQE) is investigated using Phongs distribution and a Phong exponent m of ~12 is determined by fitting both simulated Jsc and EQE to their experimental values. The measured open-circuit voltage (Voc) is lower than the best achievable value, and the difference is attributed to the non-radiative recombination in the device. Fitting of the measured Voc gives a lifetime of ~130 ns. The impact of series resistance on fill factor is also studied using the single diode equivalent circuit model and the specific series resistivity of the device is determined to be ~1.2 Ω·cm2.

SiO 2 /ZnSe anti-reflection coating for solar cells

This paper reports a proposal and demonstration of a novel anti-reflection coating (ARC) using a dielectric material, such as SiO2, in conjunction with lattice-matched and conductive crystalline ZnSe for GaAs based solar cells. The application of such an ARC to GaAs single-junction solar cell is used for the feasibility study. The transfer matrix method is applied to calculate the reflectance as well as determine the optimal SiO2 and ZnSe layer thicknesses. The simulation results indicate that a minimum reflection loss of 1.5% is achievable when the SiO2 and ZnSe layer thicknesses are 91 nm and 49 nm, respectively. Test structures consisting of ZnSe and SiO2 layers were grown using molecular beam epitaxy and magnetron RF sputtering, respectively. The reflectance measurements of both samples showed 4.0% total reflection loss over the absorbed solar spectrum. This newly proposed ARC can also be used for multi-junction solar cells based on GaAs as well as other single- or multi-junction solar cells based on different substrates. The single-crystal ZnSe layer with proper doping can also be used for current spreading to further improve the overall solar cell efficiencies.

Monocrystalline CdTe/MgCdTe Double-Heterostructure Solar Cells With ZnTe Hole Contacts

Monocrystalline p-ZnTe/i-MgCdTe/n-CdTe/n-MgCdTe double-heterostructure solar cells are grown through a combination of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) deposition techniques using two different dopants within the ZnTe contact layer. The recombination at the ZnTe/CdTe heterointerface is believed to be suppressed by the use of a double heterostructure with an intrinsic MgCdTe passivation layer. A comparison of the steady-state photoluminescence intensity of these cells with record-Voc monocrystalline CdTe solar cells indicates the performance potential of devices with ZnTe contacts, while increases in the internal quantum efficiency demonstrate the benefit of using ZnTe over these previously demonstrated contacts. Solar cells utilizing a copper-doped ZnTe hole contact show promise in terms of built-in voltage but do not realize that potential in terms of Voc with a power conversion efficiency of 9.4% and a Voc of 819 mV. Solar cells utilizing an arsenic-doped ZnTe hole contact exhibit the highest power conversion efficiency, reaching 14.08% with an open-circuit voltage of 867 mV.

CdTe nBn photodetectors with ZnTe barrier layer grown on InSb substrates

We have demonstrated an 820 nm cutoff CdTe nBn photodetector with ZnTe barrier layer grown on an InSb substrate. At room temperature, under a bias of −0.1 V, the photodetector shows Johnson and shot noise limited specific detectivity (D*) of 3 × 1013 cm Hz1/2/W at a wavelength of 800 nm and 2 × 1012 cm Hz1/2/W at 200 nm. The D* is optimized by using a top contact design of ITO/undoped-CdTe. This device not only possesses nBn advantageous characteristics, such as generation-recombination dark current suppression and voltage-bias-addressed two-color photodetection, but also offers features including responsivity enhancements by deep-depletion and by using a heterostructure ZnTe barrier layer. In addition, this device provides a platform to study nBn device physics at room temperature, which will help us to understand more sophisticated properties of infrared nBn photodetectors that may possess a large band-to-band tunneling current at a high voltage bias, because this current is greatly suppressed in the la...

Non-Lambertian Reflective Back Scattering and Its Impact on Device Performance of Ultrathin GaAs Single-Junction Solar Cells

This paper studies non-Lambertian scattering and its impacts on the optical properties and device performance of the ultrathin GaAs single-junction solar cell with a reflective back scattering layer. The Phong distribution is used to quantify the scattering effectiveness of the textured back surface, as well as its impacts on device absorptance, emittance, photon extraction and recycling factor, short-circuit current density (Jsc), external quantum efficiency (EQE), and power conversion efficiency. Both a general GaAs cell design and the ultrathin cell design are carefully investigated. A Phong exponent m of ~12 is determined by fitting both simulated Jsc and EQE to their experimental values, with a more accurate averaged reflectivity of the textured Al0.52In0.48P/Au interface taken into account. Additionally, the measured open-circuit voltage (Voc) is lower than the best achievable value due to the nonradiative recombination in the device, and a limited lifetime of ~130 ns is determined by fitting the simulated and measured Voc; a specific series resistivity of 1.2 Ω·cm2 is determined to account for the 77.8% fill factor.

Monocrystalline 1.7-eV-Bandgap MgCdTe Solar Cell With 11.2% Efficiency

This work demonstrates a monocrystalline 1.7 eV Mg0.13Cd0.87Te solar cell with an open-circuit voltage of 1.176 V and an active-area efficiency of 11.2%. The absorber layer is clad in wider bandgap passivation layers that effectively confine electrons and holes via the resulting band offsets. The potential barriers cladding the absorber are generated using higher magnesium compositions than the absorber and provide excellent carrier confinement. This ultimately leads to long minority carrier lifetimes (>500 ns) and high photoluminescence quantum efficiencies yielding an implied open-circuit voltage of 1.3 V. However, the same barriers at the heterointerfaces reduce fill factor by impeding transport; this is apparent as series resistance losses that can be overcome with operation at higher temperatures. The photocurrent loss mechanisms are simulated and analyzed, laying out the pathway for further improvements in current generation and, thus, efficiency.