David C. Bobela

Anneal treatment to reduce the creation rate of light-induced metastable defects in device-quality hydrogenated amorphous silicon

We observe a dramatic reduction in the Staebler–Wronski metastable defect creation efficiency in device-quality films of hydrogenated amorphous silicon after they undergo a 20 min anneal treatment at 350 to 400 °C. After several hours of rapid degradation with a high intensity pulsed laser, there are about half as many total dangling bond defects in the annealed samples as in unannealed control samples. This reduction is observed in both 1.02-μm- and 0.22-μm-thick films, indicating it is not a surface-related artifact. The improved stability is likely caused by H motion, which restructures the Si–Si network and H-related nanovoids.

The relation between the band gap and the anisotropic nature of hydrogenated amorphous silicon

The bandgap of hydrogenated amorphous silicon (a-Si:H) is studied using a unique set of a-Si:H films deposited by means of three different processing techniques. Using this large collection of a-Si:H films with a wide variety of nanostructures, it is demonstrated that the bandgap has a clear scaling with the density of both hydrogenated divacancies (DVs) and nanosized voids (NVs). The presence of DVs in a dense a-Si:H network results in an anisotropy in the silicon bond-length distribution of the disordered silicon matrix. This anisotropy induces zones of volumetric compressed disordered silicon (larger fraction of shorter than longer bonds in reference to the crystalline lattice) with typical sizes of ~0.8 up to ~2 nm. The extent of the volumetric compression in these anisotropic disordered silicon zones determines the bandgap of the a-Si:H network. As a consequence, the bandgap is determined by the density of DVs and NVs in the a-Si:H network.The network and nature of hydrogenated amorphous silicon (a-Si:H) are conventionally interpreted in terms of a continuous random network (CRN) of Si-Si bonds, weak Si-Si, Si-H bond and dangling bonds. A CRN requires that the smallest anisotropic features like dangling bonds and bonded hydrogen are randomly distributed and reside as isolated configurations in the network. However, in recent years more and more theoretical and experimental evidence have been found that both the isolated dangling bond and the isolated hydrogen are not present in the a-Si:H network. To the contrary, all studies come to the conclusion that the real nature of the a-Si:H is to contain more local structural order than expected from a CRN. These insights offer new opportunities to revisit the origin of several properties of a-Si:H, which are up to now explained within the framework of the CRN model. In this contribution we will discuss that many diagnostics like nuclear magnetic resonance, positron annihilation, small angle x-ray spectroscopy, density analysis and infrared spectroscopy on a-Si:H consistently demonstrate that a-Si:H exhibits an anisotropic network. In dense disordered networks the hydrogen predominantly resides in hydrogenated divacancies, whereas for less dense networks the hydrogen predominantly resides in poly-vacancies up to nanosized voids. We will discuss that hydrogenated divacancies in a disordered network contribute to the amorphous nature of a-Si:H and its electronic structure like the band gap, gap tails and the defect gap states.

Device Physics of Heteroepitaxial Film c-Si Heterojunction Solar Cells

We characterize heterojunction solar cells made from single-crystal silicon films grown heteroepitaxially using hot-wire chemical vapor deposition (HWCVD). Heteroepitaxy-induced dislocations limit the cell performance, providing a unique platform to study the device physics of thin crystal Si heterojunction solar cells. Hydrogen passivation of these dislocations enables an open-circuit voltage VOC close to 580 mV. However, dislocations are partially active, even after passivation. Using standard characterization methods, we compare the performance of heteroepitaxial absorbers with homoepitaxial absorbers that are free of dislocations. Heteroepitaxial cells have a smaller diffusion length and a larger ideality factor, indicating stronger recombination, which leads to inefficient current collection and a lower VOC than homoepitaxial cells. Modeling indicates that the recombination in the inversion layer of heterojunction cells made from defective absorbers is comparable with the overall recombination in the bulk. Temperature-dependent VOC measurements point to significant recombination at the interface that is attributable to the presence of dislocations.

A SAXS Study of Hydrogenated Nanocrystalline Silicon Thin Films

We have used small-angle x-ray scattering (SAXS) in conjunction with X-ray diffraction (XRD) to study the nanostructure of hydrogenated nanocrystalline silicon (nc-Si:H). The crystallite size in the growth direction, as deduced from XRD data, is 24 nm with a preferred [220] orientation in the growth direction of the film. Fitting the SAXS intensity shows that the scattering derives from electron density fluctuations of both voids in the amorphous phase and H-rich clusters in the film, probably at the crystallite interfaces. The SAXS results indicate ellipsoidal shaped crystallites about 6 nm in size perpendicular to the growth direction. We annealed the samples, stepwise, and then measured the SAXS and ESR. At temperatures below 350◦C, we observe an overall increase in the size of the scattering centers on annealing but only a small change in the spin density, which suggests that bond reconstruction on the crystallite surfaces takes place with high efficacy.

Epitaxial crystal silicon absorber layers and solar cells grown at 1.8 microns per minute

We have grown device-quality epitaxial silicon thin films at growth rates up to 1.85 μm/min, using hot-wire chemical vapor deposition from silane, at substrate temperatures below 750°C. At these rates, which are more than 30 times faster than those used by the amorphous and nanocrystalline Si industry, capital costs for large-scale solar cell production would be dramatically reduced, even for cell absorber layers up to 10 μm thick. We achieved high growth rates by optimizing the three key parameters: silane flow, depletion, and filament geometry, based on our model developed earlier. Hydrogen coverage of the filament surface likely limits silane decomposition and growth rate at high system pressures. No considerable deterioration in PV device performance is observed when grown at high rate, provided that the epitaxial growth is initiated at low rate. A simple mesa device structure (wafer/epi Si/a-Si(i)/a-Si:H(p)/ITO) with a 2.3 μm thick epitaxial silicon absorber layer was grown at 0.7 μm/min. The finished device had an open-circuit voltage of 0.424 V without hydrogenation treatment.

Low-Cost CdTe/Silicon Tandem Solar Cells

Achieving higher photovoltaic efficiency in single-junction devices is becoming increasingly difficult, but tandem modules offer the possibility of significant efficiency improvements. Device modeling shows that four-terminal CdTe/Si tandem solar modules offer the prospect of 25%–30% module efficiency, and technoeconomic analysis predicts that these efficiency gains can be realized at costs per Watt that are competitive with CdTe and Si single junction alternatives. The cost per Watt of the modeled tandems is lower than crystalline silicon, but slightly higher than CdTe alone. However, these higher power modules reduce area-related balance of system costs, providing increased value especially in area-constrained applications. This avenue for high-efficiency photovoltaics enables improved performance on a near-term timeframe, as well as a path to further reduced levelized cost of electricity as module and cell processes continue to advance.

High efficiency large area multi-junction solar cells incorporating a-SiGe∶H and nc-Si∶H using MVHF technology

We fabricated five different types of a-SiGe∶H and nc-Si∶H based multi-junction solar cell structures using modified Very High Frequency (MVHF) technology. After optimization, all five structures reached similar initial cell performance, i.e. ∼12% small active-area (0.25 cm2) efficiency and 10.6–10.8% large aperture-area (≥ 400 cm2) efficiency after encapsulation. However, they showed quite different light soaking stability behavior, which can be attributed to the degradation of component cells. We conducted a comparative study between the MVHF deposited solar cells with those deposited by RF. Materials studies were also conducted to understand the mechanism responsible for better stability for the MVHF deposited a-SiGe∶H solar cells. The best stable efficiency achieved for the large-area encapsulated cells is approaching 10% for both a-SiGe∶H and nc-Si∶H based multi-junction cells.

Simulated Potential for Enhanced Performance of Mechanically Stacked Hybrid III-V/Si Tandem Photovoltaic Modules Using DC-DC Converters

Abstract. This work examines a tandem module design with GaInP2 mechanically stacked on top of crystalline Si, using a detailed photovoltaic (PV) system model to simulate four-terminal (4T) unconstrained and two-terminal voltage-matched (2T VM) parallel architectures. Module-level power electronics is proposed for the 2T VM module design to enhance its performance over the breadth of temperatures experienced by a typical PV installation. Annual, hourly simulations of various scenarios indicate that this design can reduce annual energy losses to ∼0.5% relative to the 4T module configuration. Consideration is given to both performance and practical design for building or ground mount installations, emphasizing compatibility with existing standard Si modules.

Optical absorption in co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films

The conductivity of amorphous/nanocrystalline hydrogenated silicon thin films (a/nc-Si:H) deposited in a dual chamber co-deposition system exhibits a non-monotonic dependence on the nanocrystal concentration. Optical absorption measurements derived from the constant photocurrent method (CPM) and preliminary electron spin resonance (ESR) data for similarly prepared materials are reported. The optical absorption spectra, in particular the subgap absorption, are found to be independent of nanocrystalline density for relatively small crystal fractions (

Hydrogenation of dislocation-limited heteroepitaxial silicon solar cells

Post-deposition hydrogenation by remote plasma significantly improves performance of heteroepitaxial silicon (Si) solar cells. Heteroepitaxial deposition of thin crystal Si on sapphire for photovoltaics (PV) is an excellent model system for developing the PV technology platform of film c-Si on inexpensive Al2O3-coated (100) biaxially-textured metal foils. Without hydrogenation PV conversion efficiencies are less than 1% in our model system, due to carrier recombination at electrically-active dislocations and other growth defects. Hydrogenation dramatically improves performance, with low-temperature hydrogenation at 350°C being more effective than hydrogenation at 610°C. Spectral quantum efficiency, secondary ion mass spectrometry (SIMS), and vibrational Si-Hx Raman spectroscopy measurements elucidate the effects of hydrogenation on the materials and devices. Quantum efficiency increases at wavelengths >;400 nm, indicating hydrogenation is mostly affecting the bulk of the cells. SIMS detects nearly 100 times more hydrogen atoms in our cells than available dangling bonds along all dislocations. Yet, Raman spectroscopy indicates that only low temperature hydrogenation creates Si-Hx bonds; trapped hydrogen does not stably passivate dangling-bond recombination sites at high temperatures.