Xunming Deng

Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics

We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (e1,e2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (e1,e2) can be simulated assuming a single ...

Design considerations for a hybrid amorphous silicon/photoelectrochemical multijunction cell for hydrogen production

Abstract Triple-junction amorphous silicon (a-Si) solar cells demonstrating photovoltaic (PV) efficiencies up to 12.7% and open-circuit voltages up to 2.3 V have recently been deposited onto stainless-steel foil substrates by the University of Toledo for photoelectrochemical (PEC) tests conducted by the University of Hawaii. The fundamental design strategy for producing such high efficiency in multijunction amorphous silicon devices involves careful current matching in each of the junctions by adjustment of the absorption spectra through bandgap tailoring. Integrated electrical/optical models are frequently used to aid in the optimization procedure, as well documented in the PV literature. Typically, the top nip junction in an a-Si triple-junction cell is designed to absorb most strongly in the 350– 500 nm range. In principle, this top cell could be replaced by a PEC junction with strong absorption in a similar range to form a water-splitting photoelectrode for hydrogen production. This photoelectrode could be fabricated on SS with the back surface catalyzed for the hydrogen evolution reaction, and the front surface deposited with an a-Si:nipnip/ITO/SC structure. The top layer semiconductor (SC), which forms the PEC junction with an electrolyte, must have appropriate conduction band alignment for the oxygen evolution reaction, and the junction must be strongly absorbing in the 350– 500 nm region for current matching. Possible candidate SC materials include dye-sensitized titanium dioxide (TiO2), tungsten trioxide (WO3), and iron oxide (Fe2O3). This paper discusses the specific design considerations for high solar-to-hydrogen conversion efficiency in a hybrid solid-state/PEC photoelectrode, and describes the use of integrated electrical/electrochemical/optical models developed at the University of Hawaii for the analysis of such hybrid structures. Important issues include the bias-voltage and current-matching requirements in the solid-state and electrochemical junctions, as well as fundamental quantum efficiency considerations.

Amorphous silicon and silicon germanium materials for high-efficiency triple-junction solar cells

In this paper, we report our recent progress in the amorphous silicon (a-Si)-based photovoltaic research program at The University of Toledo (UT). We have achieved the fabrication of (1) wide bandgap a-Si solar cells with an open-circuit voltage of 0.981 and a fill factor of 0.728 using high hydrogen dilution for the i-layer deposition, (2) mid bandgap a-SiGe solar cells having an open-circuit voltage of 0.815 and a fill factor of 0.65, (3) narrow bandgap a-SiGe solar cells with 9.17% initial efficiency, and (4) triple-junction, spectrum-splitting a-Si/a-SiGe/a-SiGe solar cells with 10.6% initial efficiency.

Amorphous Silicon Alloy Photovoltaic Technology - from R&D to Production

The key requirements for photovoltaic modules to be accepted for large-scale terrestrial applications are (i) low material cost, (ii) high efficiency with good stability, (iii) low manufacturing cost with good yield and (iv) environmental safety. Thin films of amorphous silicon alloy are inexpensive; the products are also environmentally benign. The challenge has been to improve the stable efficiency of these modules and transfer the R&D results into production. Using a Multijunction, Multi-bandgap approach to capture the solar spectrum more efficiently, we have developed one-square-foot modules with initial efficiency of 11.8%. After 1000 h of one-sun light soaking, a stable efficiency of 10.2% was obtained. Both the efficiency values were confirmed by National Renewable Energy Laboratory. The technology has been transferred to production using an automated roll-to-roll process in which different layers of the cell structure are deposited in a continuous manner onto stainless steel rolls, 14” wide and half a mile long. The rolls are next processed into modules of different sizes. This inexpensive manufacturing process produces high efficiency modules with subcell yields greater than 99%. The key features of the technology transfer and future scope for improvement are discussed.

Study of a-SiGe:H films and n-i-p devices used in high efficiency triple junction solar cells

We report our systematic studies on a-SiGe:H thin films and n-i-p solar cells for GeH 4 /Si 2 H 6 ratio varying from 1.43 to 0. This results in a variation of band gap from 1.37 to 1.84 eV. The FTIR studies show that the total hydrogen content in these films decreases as Ge content increases. For Ge rich films, the hydrogen also goes in to Ge-H mode. I V measurements on n-i-p solar cells with i-layer having different Ge content show that as Ge content increase, Short circuit current (J sc ) increases, whereas open circuit voltage (V oc ). fill factor (FF) and conversion efficiency (η) decrease. For Ge rich films, J sc does not significantly increase after GeH 4 /Si 2 H 6 ratio increases beyond 0.72; however V oc , FF and η decrease drastically. The quantum efficiency (QE) measurements in the subgap absorption range show that band gap and Urbach slope of the i-layer can very well be estimated in the devices.

Electroabsorption measurements and built-in potentials in amorphous silicon {ital p}{endash}{ital i}{endash}{ital n} solar cells

We present a technique for using modulated electroabsorption measurements to determine the built‐in potential in semiconductor heterojunction devices. The technique exploits a simple relationship between the second‐harmonic electroabsorption signal and the capacitance of such devices. We apply this technique to hydrogenated amorphous silicon (a‐Si:H)‐based solar cells incorporating microcrystalline Si p+ layers. For one set of cells with a conventional plasma‐deposited intrinsic (i) layer we obtain a built‐in potential of 0.98±0.04 V; for cells with an i layer deposited using strong hydrogen dilution we obtain 1.25±0.04 V. We speculate that interface dipoles between the p+ and i layers significantly influence the built‐in potential.

Impact of hydrogen dilution on microstructure and optoelectronic properties of silicon films deposited using trisilane

We explored the deposition of hydrogenated amorphous silicon (a-Si: H) using trisilane (Si3H8) as a gas precursor in a radiofrequency plasma enhanced chemical vapour deposition process and studied the suitability of this material for photovoltaic applications. The impact of hydrogen dilution on the deposition rate and microstructure of the films is systematically examined. Materials deposited using trisilane are compared with that using disilane (Si2H6). It is found that when using Si3H8 as the gas precursor the deposition rate increases by a factor of similar to 1.5 for the same hydrogen dilution (R = [H-2]/[Si3H8] or [H-2]/[Si2H6])- Moreover, the structural transition from amorphous to nanocrystalline occurs at a higher hydrogen dilution level for Si3H8 and the transition is more gradual as compared with Si2H6 deposited films. Single-junction n-i-p a-Si: H solar cells were prepared with intrinsic layers deposited using Si3H8 or Si2H6. The dependence of open circuit voltage (V-oc) on hydrogen dilution was investigated. V-oc greater than 1 V can be obtained when the i-layers are deposited at a hydrogen dilution of 180 and 100 using Si3H8 and Si2H6, respectively.

Simulation of a-Si/a-SiGe thin film tandem junction solar cells

Amorphous silicon (a-Si) based thin film tandem junction solar cells are simulated based on a uniform field collection model. From the photovoltaic parameters of a single junction a-Si top cell and a few amorphous silicon–germanium (a-SiGe) bottom cells, the optimized a-Si/a-SiGe tandem cell can be predicted. The simulation results are in good agreement with the experiment. The highest efficiency a-Si/a-SiGe tandem cells are obtained with a combination of a-SiGe characteristics and a relatively large mismatch in the short circuit current between the top and bottom cells. A key reason for this behaviour is that the tandem cell may exhibit a larger fill factor than either one of the component cells under a certain current mismatch.

Optimization of a-SiGe based triple, tandem and single-junction solar cells

Recent research activities at the University of Toledo (UT) in the fabrication of high-efficiency triple, tandem and single-junction solar cells, all employing high-quality a-SiGe cells, are reviewed in this paper. Incorporating various improvements in device fabrication, the UT group fabricated 1) triple-junction a-Si/a-SiGe/a-SiGe solar cells with 12.5% initial efficiency and 10.7% stable efficiency, tandem-junction a-Si/a-SiGe solar cells with 12.9% initial efficiency, and single-junction a-SiGe solar cells with 12.5-13% initial efficiency and 10.5% stable efficiency. This review also highlights recent UT work on the nanocrystalline silicon p-layer and light-assisted electrochemical shunt passivation process.

AMPS modeling of nanocrystalline Si p-layer in a-Si nip solar cells

. This paper reports numerical simulations for the impact of a wide bandgap p-type hydrogenated nanocrystalline silicon (nc-Si:H) on the performances of a-Si based component solar cells, using Analysis of Microelectronic and Photonic Structures (AMPS) computer model developed at Penn State University. The effects of band offset and potential barrier formed at the interfaces of player with i-layer and ITO front contact were also investigated. The simulated results show that 1) with increasing bandgap of p-nc-Si:H (E/sub gp/), the V/sub oc/ increases beyond 1 V, then decreases, due to the band offset at the p/i interface, which also leads to an anomalous illuminated I-V characteristics with a bending close to the open circuit point; and 2) the front contact barrier plays a similar role to hinder the hole collection and may cause the illuminated I-V curve to bend seriously.