Nicole A. Kotulak

Transparent conducting oxide-based, passivated contacts for high efficiency crystalline Si solar cells

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 (S_eff,max) of <;30 cm/s. The samples with ultrathin Al₂O₃ layer <;3 nm, however, shows degradation in passivation quality, reaching the S_eff,max<;500 cm/s. When Al-doped zinc oxide (ZnO:Al) as TCO contact is directly deposited onto a ~10.6 nm thick Al₂O₃ coated p-Si via RF magnetron sputtering, the final passivation quality (p-Si/Al₂O₃/ZnO:Al) is characterized by the saturation current density at contact (J_0,contact) of 92.1 fA/cm² with the implied open-circuit voltage (iVoc) of 653 mV, showing the passivation quality is not severely degraded after sputtering without thermal treatment. Further process optimization of PEALD is in progress to produce an improved quality of surface passivation with the S_eff,max<;10 cm/s for ultrathin passivation layers less than 2 nm, enabling a passivated tunneling contact.

Imaging Atomic-Scale Clustering in III–V Semiconductor Alloys

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.

Enhanced surface passivation of epitaxially grown emitters for high-efficiency ultrathin crystalline Si solar cells

In this work, we demonstrated an enhanced surface passivation of epitaxially grown boron-doped emitters by replacing thermal SiO₂ 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) Al₂O₃/PECVD SiN_x stack. For the Al₂O₃/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.

Rapid thermal annealing of InAlAsSb lattice-matched to InP for top cell applications

The effect of rapid thermal annealing on the optical properties of In_xAl_1-xAs _ySb _1-y was analyzed and compared to that for In_0.52 Al_0.48As. 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 In_0.52 Al_0.48As, the emission energy and the full-width-half-maximum remain constant during the annealing study. The elimination of sub-bandgap states in In_0.52 Al_0.48As is critical for achieving a realistic path towards high efficiency multijunction cells lattice-matched to InP.

Analysis of gaas photovoltaic device losses at high MOCVD growth rates

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.

Hole-selective molybdenum oxide as a full-area rear contact to crystalline p-type Si solar cells

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 <100 cm/s at p-Si/MoO x .

Three-dimensional composition reconstruction of InAlAsSb lattice-matched to InP for top cell implementation

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.

High-temperature (450°C) operation of InGaP solar cell under N 2 ambient using refractory metal contacts

We have developed an InGaP solar cell structure capable of operating at 450°C under N2 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.

Split InAlAs top cell enabled four-junction solar cell lattice matched to InP

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.

Native oxide and optical constant determination of InP lattice-matched quaternary materials using variable angle spectroscopic ellipsometry

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.