Amorphous Silicon-germanium Alloys Research Articles

Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films

Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si1-xGex:H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors.

Mean Free Path of Photoelectronic Excitations in Hydrogenated Amorphous Silicon and Silicon Germanium Alloy Semiconductors

Hydrogenated amorphous silicon germanium alloy (a‐Si1−x Ge x :H) films are prepared by plasma enhanced chemical vapor deposition (PECVD) and characterized by in situ real time spectroscopic ellipsometry (RTSE). From complex dielectric function spectra extracted, the broadening width energy (Γ) of the primary absorption feature centered near 3.7 eV is quantified using the Cody–Lorentz oscillator model. Mean free path length of photoelectronic excitations is calculated from Γ based on reasonable estimates for speed of electron–hole photoexcitations. A model is applied with excited state lifetime assumed to be limited by scattering from network disorder and provides a relative measure of short‐range order. The simple model applied is an extension of that widely used to characterize broadening of optical transitions in polycrystalline semiconductors. Decreases in mean free path of up to ∼30% occur with germanium. Relative increases in mean free path with increased hydrogen during PECVD a‐Si:H, a‐Si0.73Ge0.27:H, and a‐Si0.60Ge0.40:H occur from ∼5% to ∼8% and with hydrogen plasma treatment of a‐Si:H by ∼6%. Mean free path changes are tracked with thickness to provide short range order evolution and the effect of the underlying material. Mean free path of photoexcitations in electronic quality a‐Si1−x Ge x :H is estimated at ∼3.5 Å, on the same order as interatomic spacing.

Mode localization and suppressed heat transport in amorphous alloys

Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-Si$_{1-x}$Ge$_x$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasi-harmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-Si$_{1-x}$Ge$_x$ as a function of $x$ exhibits a smoother U-shape than in crystalline mass-disordered alloys. The main contribution to the initial drop of thermal conductivity at low Ge concentration stems from the localization of otherwise extended modes that make up the lowest 8\% of the population by frequency. Contributions from intermediate frequency modes are decreased more gradually with increasing Ge to reach a broad minimum thermal conductivity between concentrations of Ge from $x=0.25$ to $0.75$.

Performance evaluation of composition graded layer of aSi1-xGex: H in n+aSi:H/i-aSi:H/p+aSi1-xGex:H graded band gap single junction solar cells

The work investigates the role of composition grading of amorphous silicon germanium alloy on single junction graded band gap amorphous silicon solar cells, for the first time. The fabrication and optical characterization of composition graded aSi1-xGex: H alloy by varying the gas flow rate of germane (GeH4) are presented. We propose the composition graded p+aSi1−xGex: H layer to be considered as one of the active layers in amorphous silicon based solar cells. The principle of operation of the solar cell is illustrated with the energy band diagram of proposed n+aSi:H/i-aSi:H/p+aSi1-xGex:H solar cell structure. Further, the behavior of the structure is evaluated through simulation in terms of proposed layer thickness variation and temperature variation. It is observed that the proposed structure is capable of delivering an open circuit voltage (Voc), short circuit current density (Jsc), Fill Factor (FF) and conversion efficiency (η) of 1.1 V, 15.56 mA/cm2, 0.885 and 15.19% respectively, which is at par with any other single junction amorphous silicon solar cells. The cell performance parameters were compared with related state of the art as well as our previous works. The performance parameters of proposed structure show that it is better to use a single composition graded layer instead of using a number of non-graded composition layers in photovoltaic structures, which reduces complexity and cost of fabrication.

Improved power conversion efficiency in radial junction thin film solar cells based on amorphous silicon germanium alloys

Advanced material combining with novel 3D radial junction design is an attractive strategy to fabricate low-cost high-efficiency thin film solar cells. For improving the power conversion efficiency of traditional a-Si:H thin film solar cells, a conceptually new method incorporating Ge doping technology with 3D radial junction architecture is proposed here. By doping Ge with the optimal amount, the eternal quantum efficiency response of a-Si:Ge alloy thin film solar cell is greatly improved in the near-infrared wavelength. This enhanced spectrum response contributes to the improved power conversion efficiency according to the experimental results and theoretical simulations. On the other hand, by introducing the 3D radial junction architecture, both the excellent light trapping effect and the fast carrier separation contribute to the significantly improved short current density. With the optimal Ge doping amount, the radial junction a-Si:Ge alloy thin film solar cell achieves an open circuit voltage of 0.58 V, short current density of 20.14 mA/cm2, and fill factor of 53.7%, which contributes to the high power conversion efficiency up to 6.26%. From beginning to end, the fabrication temperature is lower than 400 °C and the whole fabrication process shows remarkable compatibility with current photovoltaic industry application. All above results suggest a potential route to the next generation of stable and highly efficient a-Si:H thin film solar cells.

Performance control for amorphous silicon germanium alloys by in situ optical emission spectroscopy

The relationship between processing conditions and specific chemical reactions led to film growth in plasma is identified by optical emission spectroscopy to simplify the optimization process of film properties. SiH⁎ transient behavior after plasma ignition in parallel plate silane/germane/hydrogen plasma is investigated with the variation of germane, hydrogen flow rate and power. The effect on interface property between p layer and intrinsic layer in amorphous silicon germanium solar cell is obtained. Hα⁎, Hβ⁎, SiH⁎ and GeH⁎ emission intensity is recorded when germane and silane/germane flow rate changes. Ge content is analyzed by optical band gap and Raman spectra of amorphous silicon germanium films. The results are expected to serve as a guide for improving the performance of solar cells.

Improvement in carrier collection at the i/n interface of graded narrow-gap hydrogenated amorphous silicon germanium solar cells

We examined different buffer layers at the i/n interface of narrow-gap amorphous silicon germanium alloy (a-SiGe:H)-based thin-film solar cells. These buffers included a conventional hydrogenated amorphous silicon (a-Si:H), an inversely graded hydrogenated amorphous silicon germanium, and a crystalline seed buffer (CSB). The solar cell with the CSB shows the highest performance, of 10%. The better carrier extraction at the rear side of the device is attributed to the role of the CSB layer. The effect of CSB thickness from 50 nm to 100 nm on cell performance was examined. Cell efficiency increased with the buffer thickness up to 80 nm and decreased with buffer thickness of 100 nm. This decrease can be attributed to increased defect densities of the buffer due to less efficient passivation of amorphous phase at the crystalline column boundaries.

Hydrogenated Amorphous Silicon Germanium Active Layer for Top Cell of a Multi Junction Cell Structure.

Intrinsic hydrogenated amorphous silicon-germanium (a-SiGe:H) alloy is generally used in the bottom cell because of its low band gap. The a-SiGe:H has a higher photo conductivity in comparison to the a-Si:H; thus, it is expected that the a-SiGe:H can show better short circuit current density than that of the a-Si:H based solar cell. Therefore, we optimized a-SiGe:H active layer that can be a suitable choice for the front cell of a multi junction.solar cell. Furthermore, we carried out a comparative study of the solar cells that have a-SiGe:H and a-Si:H as respective active layers. The a-SiGe:H based solar cells show higher short circuit current density, while the a-Si:H based cells show higheropen circuit voltage. The current-voltage characteristics of these cells are as follows: (a) V(oc) = 770 mV, J(sc) = 15.0 mA/cm2, FF = 64.5%, and η = 7.47% for a-SiGe:H based cell; and (b) V(oc) = 826 mV, J(sc) = 13.63 mA/cm2, FF = 72.0%, and η = 8.1% for a-Si:H based cell.

Ag nanoparticle enhanced light trapping in hydrogenated amorphous silicon germanium solar cells on flexible stainless steel substrate

Silver nanoparticle (Ag NP) enhanced back reflector (BR) has been used for effective light trapping in thin-film silicon solar cells. In this letter, we demonstrate experimentally that the photocurrent density of hydrogenated amorphous silicon–germanium alloy (a-SiGe:H) based solar cells are enhanced by as much as 3.0mA/cm2 or 19.5% with medium sized (320nm) Ag NPs as compared to the specular BR. In addition, it is demonstrated that the optimized Ag NP BR is also effective in multi-junction solar cells. It is believed that it is the appropriated surface morphology of the Ag NPs that leads to the improved light-trapping. It is found that the average haze of the Ag NP enhanced BR can be improved to as much as >70% throughout the wavelength range 600–900nm. The atomic-force-microscopy measurement shows that the optimized root mean square roughness of Ag NP BR is about 35.2nm.

Anomalous small-angle X-ray scattering of nanoporous two-phase atomistic models for amorphous silicon–germanium alloys

The present work deals with a detailed analysis of the anomalous small-angle X-ray scattering in amorphous silicon–germanium alloy using the simulation technique. We envisage the nanoporous two-phase alloy model consisting in a mixture of Ge-rich and Ge-poor domains and voids at the nanoscale. By substituting Ge atoms for Si atoms in nanoporous amorphous silicon network, compositionally heterogeneous alloys are generated with various composition-contrasts between the two phases. After relaxing the as-generated structure, we compute its radial distribution function, and then we deduce by the Fourier transform technique its anomalous X-ray scattering pattern. Using a smoothing procedure, the computed X-ray scattering patterns are corrected for the termination errors due to the finite size of the model, allowing so a rigorous quantitative analysis of the anomalous small-angle scattering. Our simulation shows that, as expected, the anomalous small-angle X-ray scattering technique is a tool of choice for characterizing compositional heterogeneities coexisting with structural inhomogeneities in an amorphous alloy. Furthermore, the sizes of the compositional nanoheterogeneities, as measured by anomalous small-angle X-ray scattering technique, are X-ray energy independent. A quantitative analysis of the separated reduced anomalous small-angle X-ray scattering, as defined in this work, provided a good estimate of their size.

Electrical properties of hydrogenated microcrystalline silicon carbon alloys: effect of deposition parameters and light soaking

New materials with electrical properties similar to those of hydrogenated amorphous silicon (a-Si:H) or hydrogenated amorphous silicon germanium alloys, but that are stable under light soaking, could be a breakthrough for multi-junction thin-film silicon solar cell technology. With this aim, hydrogenated microcrystalline silicon-carbon alloy (c-:H) thin films seem like promising candidates as the variation of the carbon in the alloy composition can be used to tune the effective bandgap, keeping a high crystalline fraction. In this study, a highly hydrogen-diluted silane and methane gas mixture was used to deposit c-:H thin films by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The effect of the independent variation of three deposition parameters (variation of methane flow rate, addition of a small amount of silicon tetrafluoride and variation of the RF-power density) on the electronic transport and defect properties was investigated. Selected samples were studied in their ‘as deposited’ state, after light soaking until saturation, and after a high temperature annealing. Particular care was paid to comparing the behaviour of this new material system to that of a reference a-Si:H sample, deposited by standard RF-PECVD.

Small-angle X-ray scattering of two-phase atomistic models for amorphous silicon–germanium alloys

The present work deals with a detailed analysis of the small-angle X-ray scattering in amorphous silicon–germanium alloy using the simulation technique. We envisage the two-phase alloy model consisting in a mixture of Ge-rich and Ge-poor domains at the nanoscale. By substituting Ge atoms for Si atoms in a continuous random network, compositionally nanoheterogeneous alloys are generated with various composition-contrasts between the two phases. After relaxing the as-generated structure, we compute its radial distribution function, and then we deduce by the Fourier transform technique its X-ray scattering pattern. Using a smoothing procedure, the computed small-angle X-ray scattering patterns are corrected for the termination errors due to the finite size of the model, allowing so, for the first time at our best knowledge, a rigorous quantitative analysis of this scattering. Our simulation shows that the relative alloy composition of the two phases is a determinant parameter for the occurrence of small-angle X-ray scattering. A composition-contrast threshold, independent of the overall composition of the alloy as well as the relative volume fraction of the phases, has been demonstrated. Thus, two-phase silicon–germanium alloy with composition-contrast smaller than 0.3 behaves as compositionally homogeneous alloy in the context of small-angle X-ray scattering experiment. Our simulation also shows that the experimentally derived characteristics of the compositional heterogeneity in the alloy, such as radius of gyration and correlation volume, are composition-contrast dependent; they are generally smaller or equal to the actual values.

Radial Junction Architecture: A New Approach to Stable and Highly Efficient Silicon Thin Film Solar Cells

ABSTRACTIncorporation of properly designed nanostructures in solar cells improves light trapping and consequently their power conversion efficiencies. Due to its unique structure, a silicon nanowire (SiNW) matrix provides excellent light trapping and thus offers a promising approach for cost-effective, stable and efficient silicon thin film photovoltaics. Moreover, by decoupling the light absorption and carrier collection directions, radial junction solar cells built around the SiNWs allow the use of very thin active layers. As a matter of fact, radial PIN junctions with 9.2% power conversion efficiency have already been demonstrated on glass substrates with only 100 nm thick intrinsic hydrogenated amorphous silicon layers. The most straightforward way to further improve the short circuit current density is to use an active layer with a lower band gap. In this work, the performances of devices with two different low band gap materials, e.g., hydrogenated microcrystalline silicon (μc-Si:H) and hydrogenated amorphous silicon germanium alloy (a-SiGe:H) are presented. To the best of our knowledge, this is the first demonstration of a-SiGe:H radial junction solar cell.

Properties of a‐SiGe:H thin films: correlation between photosensitivity and density of states

AbstractThin films of hydrogenated amorphous silicon germanium (a‐Si1‐xGex:H) alloy are prepared using DC magnetron sputtering method, at a substrate temperature of 150 °C. Silicon and germanium are co‐pulverized. All our films are deposited in the same preparation conditions except for those having a germanium content from 27% to 40%. Structural, optical and electrical properties are systematically studied from IR absorption, optical transmission and electrical measurements. Hydrogen content is determined by integrating the wagging mode infrared absorption. The increase of the Ge content from 27% to 40% is responsible for a decrease in optical bandgap of 1.59 to 1.43 eV and a reduction of the total content of hydrogen bonds in both Si‐H and Ge‐H configurations. A good correlation is observed between the reduction of the optical gap and the increase of one order of magnitude of the dark conductivity.The white light sensitivity shows a significant degradation due to germanium incorporation in the silicon matrix. Therefore, this degradation increases with germanium content. The density of states (DOS) located at the Fermi level g(EF) is estimated from the analysis of capacitance‐frequency measurements on Au/a‐Si1‐xGex:H/c‐Si(N+) Schottky structure, in zero bias condition. In our layers, the turn on condition which permits to measure the complete response of the deep states, for 1 kHz frequency, occurs at a relatively high temperature (over 75 °C). The measured density of states increases from 3x1016 cm‐3 eV‐1 to 3x1018 cm‐3 eV‐1 when the germanium content increases. This can highlight the narrow correlation between sensitivity degradation and density of states augmentation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Applications of real-time and mapping spectroscopic ellipsometry for process development and optimization in hydrogenated silicon thin-film photovoltaics technology

Four applications of real-time spectroscopic ellipsometry (RTSE) and ex-situ mapping spectroscopic ellipsometry (SE) in thin-film hydrogenated silicon (Si:H) photovoltaics (PV) technology are reviewed with the common theme being the development and application of SE-derived growth evolution diagrams. The goals of these applications are to understand and consequently further advance this technology. In the first application, fabrication of engineered thin films consisting of periodic arrays of silicon (Si) nanocrystallites in an amorphous Si:H (a-Si:H) host matrix has been guided by a growth evolution diagram developed by RTSE for radio-frequency plasma-enhanced chemical vapor deposition (PECVD) using SiH4+H2 mixtures. Such precisely controlled microstructures are of interest as possible intrinsic-layer components of p–i–n and n–i–p thin-film PV devices, and RTSE is shown to be a key technique for guidance in fabrication and for structure verification. In the second application of growth evolution diagrams, very-high-frequency PECVD intrinsic a-Si:H, hydrogenated amorphous silicon–germanium alloys (a-Si1−xGex:H), and hydrogenated nanocrystalline silicon (nc-Si:H) have been investigated for use as the top, middle, and bottom-cell i-layer components, respectively, of triple-junction n–i–p solar cells. The growth evolution diagram for the bottom-cell i-layer, starting from an underlying mixed-phase amorphous+nanocrystalline silicon [(a+nc)-Si:H] n-layer, reveals a bifurcation at a critical H2-dilution flow ratio R (R=[H2]/[Si2H6], in this application) between mixed-to-amorphous phase evolution [(a+nc)→a] at low R and mixed-to-nanocrystalline phase evolution [(a+nc)→nc] at high R. The highest performance single-step nc-Si:H solar cell is found at minimal R while remaining on the nanocrystalline side of the identified bifurcation where suitable grain boundary passivation can be assured. Because of the importance of the roll-to-roll flexible substrate configuration in such multijunction Si:H-based PV technology, RTSE has been demonstrated in a third application for monitoring PECVD of a-Si:H n–i–p solar cell structures on back-reflector-coated flexible roll-to-roll polymer substrates. RTSE has been used for probing along the center line of the moving substrate during deposition, and ex-situ mapping SE has been used over the full substrate area after deposition. Detailed studies of the top-most p-layer of the n–i–p solar cell have been performed, with the goal being to develop spatially-dependent (in contrast to R-dependent) growth evolution diagrams in order to evaluate uniformity across the width of the substrate and thus to enable optimization of the resulting a-Si:H PV modules. In this study, efficiency optimization occurs at the p-layer transition region in which a-Si:H nucleates from the i-layer surface, but evolves to predominantly nc-Si:H for improved contact to the top-most In2O3:Sn layer. In the fourth and final application reviewed here, the mapping-SE-deduced properties of the Si:H i and p-layers have been spatially correlated with device performance parameters from an array of n–i–p a-Si:H-based dot cells over a 13×13cm2 substrate area. Analysis of the SE data acquired over the full area provides property maps of i-layer thickness and band gap, p-layer thickness and band gap, and p-layer surface roughness thickness for the n–i–p structure. The mapped values adjacent to the PV devices have been correlated with the device performance parameters. When sufficient non-uniformity exists, these correlations enable optimization based on specific ranges of values that characterize the fundamental properties of the material and the film structure.

Pulsed–light Crystallization of Thin Film Silicon, Germanium, and Silicon Germanium Alloy

ABSTRACTWe report the recent progress of crystallization of amorphous silicon (a-Si), amorphous germanium (a-Ge) and amorphous silicon germanium alloy (a-SiGe) using a pulsed-Xenon-lamp system with multiple lamps. The precursor materials were deposited using a sputtering machine on display glass substrates maintained on a rotary holder. The RF powers on the silicon and germanium targets were varied to control the Ge/Si ratio in the materials. The film thickness was in the range of 50-100 nm, targeting the application in thin film transistors (TFT). The samples were pre-heated to 350-450°C in a conveyer chamber with nitrogen flow before the crystallization. The materials were characterized using AFM, Raman and Spectroscopic Ellipsometry. We demonstrated that we can uniformly crystallize a-Si, a-SiGe, and a-Ge with a single-pulse or multiple-pulse process on 10×5 cm2 glass substrates. We found that the required crystallization power for a-Ge is much lower than for a-Si. The power needed to crystallize a-SiGe is between the power required for a-Ge and a-Si crystallizations, and it increased with increasing Si fraction. No Raman signal was measurable in the as-deposited films. Strong Raman peaks at 520 cm-1 and 290 cm-1 were observed in the pulsed-lamp crystallized poly-Si and poly-Ge films, respectively. Distinct Ge-Ge, Si-Ge, and Si-Si vibration modes were observed at ~285 cm-1, ~390 cm-1, and ~470 cm-1, respectively, in the poly-SiGe films formed after the pulsed-light treatments. Their intensity ratios and the peak positions depended on the Ge/Si ratio and the light intensity used for the crystallization. AFM images showed the formation of large grains with increased surface roughness.

Hydrogenated amorphous silicon germanium alloy with enhanced photosensitivity prepared by plasma enhanced chemical vapor deposition at high temperature

Hydrogenated amorphous silicon germanium (a-SiGe:H) alloys were prepared by plasma enhanced chemical vapor deposition (PECVD) at different deposition temperatures. The optical, optoelectronic properties and the microstructure of the prepared a-SiGe:H alloys were investigated systematically by transmission, photo/dark conductivity, Raman, and Fourier transform infrared (FTIR) spectroscopy measurements. The Ge atom content was further determined by energy dispersive spectroscopy (EDS) and the surface roughness was checked by atomic force microscopy (AFM). It was found that the narrow bandgap (Eg) a-SiGe:H alloy with the enhanced photosensitivity (Ratio of photo conductivity to dark conductivity) and the rapid growth rate could be prepared when the deposition temperature was relatively high. The underlying mechanism was analyzed carefully. The results indicated that high temperature deposition might be an effective way to prepare a-SiGe:H alloys for solar cell application.

Amorphous and nanocrystalline silicon thin film photovoltaic technology on flexible substrates

This paper reviews our thin film silicon-based photovoltaic (PV) technology, including material and device studies as well as roll-to-roll manufacturing on a flexible substrate. Our current thin film silicon PV products are made with hydrogenated amorphous silicon (a-Si:H) and amorphous silicon germanium (a-SiGe:H) alloys. The advantages of a-Si:H-based technology are low cost, capability of large scale manufacturing, abundance of raw materials, and no environmental concerns. One disadvantage of a-Si:H PV technology is lower energy conversion efficiency than solar panels made of crystalline and polycrystalline silicon and compound crystal thin film semiconductors. Significant efforts have been made to improve efficiency. First, a-Si:H and a-SiGe:H material quality has been improved by optimizing deposition conditions, especially using high hydrogen dilution to deposit the amorphous materials close to the amorphous/nanocrystalline transition. Second, cell efficiency has been improved by engineering the device structure, such as bandgap profiling. In order to use the solar spectrum effectively, multijunction structures that incorporate a-SiGe:H in the middle and bottom cells have been used. The authors achieved record high solar cell efficiency with an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure. Using the same structure, we fabricated solar laminates on flexible stainless steel coils with roll-to-roll production systems. Our current product has a stable aperture area efficiency of 8.2%. In recent years, hydrogenated nanocrystalline silicon (nc-Si:H) has emerged as a potential replacement for the a-SiGe:H bottom cell in multijunction structures. The authors have conducted a great deal of research and development of a-Si:H- and nc-Si:H-based multijunction PV technology. The authors have significantly improved the efficiency of a-Si:H and nc-Si:H multijunction solar cells and modules by optimizing the nc-Si:H material quality and device structure. The authors achieved an initial active-area (∼0.25 cm2) solar cell efficiency of 16.3% using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure and an initial aperture-area module (∼400 cm2) efficiency of 12.0% and a stable aperture-area module (∼800 cm2) efficiency of 11.3% using an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. The authors expect to launch a new a-Si:H/nc-Si:H/nc-Si:H triple-junction product in near future with much higher efficiency than the current product.

High-Efficiency, Multijunction nc-Si:H-Based Solar Cells at High Deposition Rate

Hydrogenated nanocrystalline silicon (nc-Si:H) is a promising candidate to replace the hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin-film silicon solar cells due to its superior long-wavelength response and stability against light-induced degradation. Due to its indirect bandgap, the absorbing nc-Si:H layer needs to be much thicker than its amorphous counterpart. For nc-Si:H-based solar cells to be commercially viable, the challenge is to deposit the nc-Si:H layer at a high rate with good quality. In this paper, we report on the development of our proprietary high-frequency glow discharge deposition technology to fabricate high-efficiency, large-area, a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells at a high deposition rate >;1 nm/s. The National Renewable Energy Laboratory (NREL) has confirmed stable efficiency of 12.41% on a 1.05-cm2 solar cell. We have attained initial efficiency of 12.33% on an encapsulated cell of aperture area ~400 cm2 ; the corresponding stable efficiency is projected to be 11.7-11.9%.

12.0% Efficiency on Large-Area, Encapsulated, Multijunction nc-Si:H-Based Solar Cells

Because of its superior long-wavelength response and stability against light-induced degradation, hydrogenated nanocrystalline silicon (nc-Si:H) has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin-film silicon solar cells. In this paper, we report on the development of our proprietary high-frequency (HF) glow discharge deposition technology for nc-Si:H solar cells that has resulted in high-quality nc-Si:H materials with good spatial uniformity. We studied the HF-deposited nc-Si:H material using various analytical techniques. We fabricated a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells that are deposited on textured Ag/ZnO back reflectors. Large-area cells were fabricated and encapsulated using our proprietary lightweight flexible encapsulants. National Renewable Energy Laboratory (NREL) has confirmed (1) initial aperture-area efficiency of 11.8% on an 807.8-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> encapsulated cell and (2) stable aperture-area efficiency of 11.2% on a 400-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> encapsulated cell.