The future of sustainable photovoltaics (PV) depends on the possibility to lower the cost of electricity below the cost based on burning fossil fuels without the need of subsidies. Thin Film PV based on II â?" VI semiconductors has become presently the major challenger of crystalline Si and poly-Si based PV technologies. Nevertheless the present industrial efficiencies are far from the theoretical one, even though many years have passed since the introduction of CdTe in the use of thin film PV. Thermal evaporation, in one form or another has been the industrial fabrication tool up to now. The major obstacle for the development of thin film photovoltaics is the lack of a sustainable high quality wide area thin film deposition technology. Siena Solar Nanotech (2SN) has implemented the technology, developed by Organic Spintronics, of Pulsed Plasma Deposition (PPD), based on ablation by means of nanosecond pulsed electron beam, to the deposition of II â?" IV photovoltaics. 2SN has successfully developed a linear c.w. deposition system that allows the deposition of a very high quality CdTe with a rate approaching 1 micron per minute. The efficiency of the PPD deposition, associated with the low thermal budget, the low film roughness and the safety of the closed vacuum process, allows to drastically reduce the production costs proving to achieve grid parity. Moreover, the high quality of the absorber makes it eventually possible to follow, step by step the learning curve up to the theoretical limits of approximately 30%. Furthermore, The PPD technology that is compatible both with substrate and super-strate configurations is eventually applicable to flexible thin film fabrication. A wide area (300 mm ) R2R compatible c.w. deposition system is presently under construction at Siena Solar Nanotech.

Spectroscopic ellipsometry has been applied in both ex situ and in situ imaging/mapping studies of the large area spatial uniformity of partial and complete multilayer stacks in hydrogenated silicon (Si:H), cadmium telluride (CdTe), and Cu(In,Ga)Se₂ (CIGS) thin film photovoltaics (PV) technologies. A critical component of these studies is the development of an optical property database for each PV technology in which the index of refraction and extinction coefficient (n, k) spectra of the layers (or layer components) are expressed as analytical functions of photon energy. The amplitudes, energies, and widths of the interband electronic oscillators that define the analytical functions for semiconductors, transparent conductors, and metals can be described in turn as functions of film density, alloy composition, stress, temperature, and excited carrier mean free path. The amplitude and width of the intraband electronic component for transparent conductors and metals can be expressed in terms of the optical resistivity and the free carrier scattering time. Thus, through least squares regression, one can determine not only layer thicknesses and phase volume fractions [e.g., Cu(In_1-xGa_x)Se₂+Cu_2-xSe or a-Si:H+nc-Si:H] directly from the ellipsometric spectra, but also the physical properties of the phases through the parameterized dielectric function. Surface and interface roughness on a wide range of scales can be modeled using effective medium theory and coherent/incoherent superposition, depending on whether the in-plane scale is much smaller than the optical wavelengths or is smaller/larger than the in-plane optical coherence length, respectively. Examples are provided from the three PV technologies in which thickness, surface roughness, phase, and property maps have been deduced for both flexible roll-to-roll substrate and rigid superstrate configurations. Once such information for the completed PV stack is extracted, predictions of the quantum efficiency and AM1.5 short circuit current of local dot cells can be generated on the basis of the assumption that all electron-hole pairs created in the active region of the device are collected. Differences between predictions and measurements provide the spectrally-resolved electronic losses. Existing ex situ instruments for such studies use linear detector arrays for spectroscopic mapping and involve translation of ellipsometer heads over the surface, measuring point-by-point -- a relatively time-consuming procedure. Recent, novel instrumentation employs a 2d imaging detector, exploiting one array index for spectroscopy and the second array index for imaging across the PV panel. This latter instrumention enables imaging width-wise and mapping length-wise for uniformity evaluation at the high speeds required for in situ and on-line analysis.

Compound semiconductor Cu₂ZnSnSe₄ (CZTSe) has been considered as a promising candidate for the absorber layer in the next generation thin film solar cells instead of Cu(In,Ga)Se₂ and CdTe because of its high absorption coefficient (>10⁴ cm^-1) and abundance of elements. Here we report, for the first time, the growth of Cu₂Zn(Sn_1-xGe_x)Se₄ (x=0.1, 0.3, 0.5 and 1) thin films, and their electrical and optical properties for possible photovoltaic applications. In this work, the films were grown by RF sputtering at room temperature and annealed in a tube furnace in N₂+Se atmosphere. Two inch targets were prepared by cold pressing of mixed powders of individual elements. The optical band gap was controllable with Ge content from ~1.1 eV to ~1.5 eV. Temperature dependence of photoluminescence spectra showed the decrease of PL peak as increasing temperature. The conversion efficiency of ~4% was observed at the samples with x value of 0.5. Effects of Ge content on structural, optical and electrical properties of the CZTSe thin films are the main focus of this presentation, with possibility of utilizing the scalable sputtering technique.

A novel non-vacuum technique based on electrophoretic deposition of colloidal nanoparticles has been developed for the fabrication of CuInSe₂ (CIS) solar cells. Composite Cu-In nanoparticles were first produced by pulsed laser ablation of a bulk alloy target in liquid solvents. A stable colloid was formed without adding stabilizing chemical agents. Transmission electron microscopy (TEM) study of the nanoparticles revealed a composite structure within single nanoparticle and an overall composition inherited from the target, indicating composition control of the method. Another uniqueness of the method was that the surface charge density of the nanoparticles can be modified, enabling control of the electrokinetic migration of the particles in solvents under an external electric field. Electrophoretic deposition (EPD) was successfully employed to deposit smooth and compact Cu-In precursor thin films. Secondary ion mass spectrometry (SIMS) profiling showed that the Cu-In precursor films, although produced under non-vacuum conditions in organic solvents, contained very low levels of oxygen (below 0.2 at.%) and carbon (near the detection limit). The composition depth profile was uniform throughout the film. CIS solar absorber layers were formed after annealing the Cu-In precursor films in selenium vapor under atmospheric pressure. X-ray diffraction (XRD) and cross-sectional scanning electron microscopy (SEM) imaging showed highly crystalline CIS absorber layers with large grains on the order of several microns. Undesired secondary phases such Cu_2-xSe and In₂O₃ were undetectable. Solar cell devices were finished with the standard coating of the CdS window layer and the transparent conductive oxide layers. A robust average cell efficiency of 5-6% has been achieved for CIS solar cells fabricated directly on flexible Mo sheet metal substrates. Overall, with a high coating speed on the order of 1 μm/min for metal precursor films, involving no hazardous chemicals, and only producing minimal waste of common organic solvents (which is recyclable), the technique is suitable for conformal coating on large scale flexible substrates and/or surfaces with complex shapes. Our results open up a new route for non-vacuum fabrication of CIS and CIGS solar cells, which can be extended to the fabrication of other chalcogenide absorber layers such as Cu₂ZnSnS₄.

Thermal co-evaporation of individual elements in a three-stage process has proven to produce high quality Cu(In_xGa_1-x)Se₂ (CIGS) materials for photovoltaic (PV) devices. Such multi-stage processing provides a high level of flexibility, but also generates greater challenges in run-to-run reproducibility of the optimized process. Real time spectroscopic ellipsometry (RTSE) has been used successfully for monitoring and control of multi-stage PV deposition processes -- including three-stage CIGS film and device fabrication. Information extracted from RTSE includes the evolution of the bulk layer and one or more surface roughness layer thicknesses, as well as the layer dielectric functions. The dielectric functions of the component layers can be further analyzed to extract the alloy and phase compositions as well as the defect density or grain size, all of which can assist in understanding the fabrication process, in optimizing solar cells, and ultimately in monitoring and controlling the optimized process for improved reproducibility. In this study, the focus is on analysis of (Ï^, Î") spectra acquired by RTSE in order to characterize the structural evolution of the (In_xGa_1-x)₂Se₃ growth in the first stage, the transition from Cu-poor to Cu-rich CIGS which defines the end of the second stage, and the transition from Cu-rich to Cu-poor CIGS which defines the end of the third stage. The most commonly used monitoring method is based on changes in emissivity of the film, but the method is largely dependent on the apparatus design, the substrate, and the CIGS bulk layer thickness. An optical monitoring tool is expected to be more direct since, in the transition from Cu-poor to Cu-rich material, a semi-liquid Cu_2-xSe phase is believed to form on top of the bulk layer. In the Cu-rich to Cu-poor transition, this Cu_2-xSe phase fully reacts with In, Ga, and Se to form CIGS. To investigate the RTSE signature of this behavior, a CIGS deposition was performed on a smooth Si wafer substrate to a thickness of 0.7 μm (or ~1/3 of the thickness used in devices), an approach that minimizes surface roughness and enhances optical sensitivity to surface Cu_2-xSe. Although changes in emissivity cannot be detected in this situation, the RTSE spectra in (Ï^, Î") show clear trends that can be quantified through an in-depth analysis in terms of the near-surface Cu_2-xSe volume fraction. Studies using a standard Mo substrate and 2 μm thick CIGS have also revealed features in the (Ï^, Î") spectra characteristic of the anticipated changes in the near surface phase composition. Although careful analysis of RTSE is expected to provide quantitative information on the surface properties and their evolution in this case, control of the deposition has been successful simply by monitoring the real time changes in the (Ï^, Î") spectra.

Copper indium gallium diselenide (CIGS) thin film solar cells already exceed 20 % overall power conversion efficiencies. These high efficiencies are achieved using an n-type cadmium sulfide (CdS) buffer layer deposited on the CIGS absorber. CdS buffer layers are also used in the emerging copper zinc tin sulfide/selenide (CZTSSe) based solar cells. In wide band gap copper indium aluminum gallium selenide (CIAGS) for multi-junction CIGS based devices it is desired to widen the band gap of the buffer layer by alloying with Zn to form Zn(1-x)Cd(x)S films. Specifically, depending on the band gap of the CIGS absorber layer, it is desired to manipulate the band gap of buffer layer to reduce â?ocliffâ? and â?ospikeâ? type discontinuities. While Zn(1-x)Cd(x)S films have been deposited by chemical bath deposition, the structure and composition of these layers have not been studied in detail. Herein, we demonstrate the Zn to Cd ratio control in Zn(1-x)Cd(x)S films by chemical bath deposition from an aqueous solution. The reactant concentrations, hence the deposition rates, are optimized by adjusting the chelating of cadmium sulfate and zinc sulfate (CdSO4 and ZnSO4) with ethylenediaminetetraacetic acid (EDTA) in the solution. The complexation of Zn and Cd ions with EDTA leads to the slow release of ions into solution due to the complexation and equilibrium constant and enables better control of the film structure and stoichiometry especially for ZnS films. Addition of EDTA also improves the film crystallinity. ZnS films are cubic but with increasing Cd in the growth solution and in the film, the film structure shifts from cubic ZnS to hexagonal CdS on molybdenum-coated Si (100) substrates. Using Auger depth profiling, we show that a CdS rich layer forms at the film/substrate interface due to the faster reaction of Cd than Zn. The formation of CdS rich layer at film/substrate interface will lead to a graded band gap buffer layer film with the band gap increasing away from the CIGS surface as the increasing Zn contribution leads to a wider band gap away from the p-n junction. Thus, larger shunt resistances can be achieved without any loss of light transmission, and hence higher efficiencies in chalcopyrite based solar cells.

Chalcopyrite CuInSe2 has been widely studied due to their low-cost terrestrial photovoltaic applications. A low cost non-vacuum technique for CuInSe2 deposition is inherently applicable for large area solar cells and mass-production. In this work, CuInSe2 polycrystalline thin films were prepared from either non toxic solution-processed Cu-In multilayers or the binary combination of In2Se3 and Cu2Se layers. Each Cu and In precursor or the binary solutions were deposited on glass substrate either sequentially or simultaneously by the sol-gel spin-coating method. As-deposited films were then selenized with Se powder in a tubular furnace from 200 to 550 oC. A number of process parameters, i.e., stacking sequence, thicknesses of individual layer, and annealing temperatures, varied in order to determine the optimal growth condition of each approach. The formation of CuInSe2 was more effective with a larger grain size of ~1 μm at the lower temperature of ~400 oC in a shorter selenization duration in the case of the binary In2Se3-Cu2Se layers compared to the individual Cu-In approach. The binary solution-driven CuInSe2 films demonstrated a preferred orientation along theã?^112ã??direction with a conversion efficiency of 3.0 %. Minimizing the residue of carbon is discussed in conjunction with the optimized processing parameters. Conclusively, the binary solution approach may be more suitable for the large-scale device fabrication.

Cu(In,Ga)Se2 (CIGS)-based thin film solar cell has been commercialized recently due to its high conversion efficiency. A thinner CIGS absorber layer is desirable because of the high costs of indium and gallium materials. However, thinner absorber leads to a decreased short circuit current resulted from incomplete light absorption. It is known that the reflectivity of molybdenum back contact is quite low, which is responsible for the decreased absorption in CIGS solar cells. We are proposing an alternative sputtered-metal back contact silver (Ag) as a substitution of the traditionally used molybdenum because of its much higher reflectivity. A TCO barrier layer is also sputter-deposited between silver contact and CIGS absorber to protect silver from reacting with CIGS layer. The TCO sputtering conditions such as pressure, substrate temperature are strictly followed to ensure an efficient diffusion barrier. The cell performance, particularly photocurrent, has been improved for thinner cell by using TCO/Ag back contact.

An efficient and stable ohmic back contact for thin film n-CdS/p-CdTe solar cells has been developed, which utilizes a vapor-deposited MoOx thin film as the buffer layer between the p-CdTe and the back electrode. The low-resistance behavior of the back contact is ascribed to the extraordinarily high work function of MoOx, which reportedly is as high as 6.8 eV, and thus adequately matches that of p-CdTe. With MoOx as the buffer, a variety of common metals, even those with a low work function such as Mg and Al, have been found to be useful as the electrode in forming the back contact. One significant advantage of the MoOx buffer is to retard the in-diffusion of metal electrode to CdTe as a diffusion barrier and therefore stabilize the cell efficiencies during thermal stress and light soaking tests. Other advantages of the MoOx buffer include dry application by vacuum deposition, and thus it is particularly suitable for the fabrication of ultra-thin CdTe solar cells without introducing additional shorting defects. For making the MoOx back contact, surface of the CdTe film should be rinsed after VCC treatment to remove Cd3Cl2O2 residues. Thermal evaporation method should be adopted to deposit MoOx buffer with high work function. Besides MoOx, other transition metal oxides including thermally evaporated V2O5 and WO3 have been successfully employed for making the ohmic back contact to p-CdTe. To summarize, CdTe cells with high efficiency and good stability can be achieved with MoOx or other transition metal oxides as the back contact buffer.

In the past two decades, the growing global demand for solar energy has spurred scientific interest in alternative technologies to conventional silicon. In particular, CuIn(1-x)Ga(x)Se2 (CIGS) has emerged as a competitor. Current state-of-the-art CIGS devices are produced using a three-stage thermal coevaporation process that has resulted in laboratory efficiencies of up to 20%, but this process is difficult to implement at a commercial scale. Our work has focused on developing a scalable deposition technique using RF magnetron sputtering of quaternary CIGS. Notably, the resulting films do not require post-selenization, reducing processing time and cost. We have fabricated devices above 10% efficiency using this approach, showing its promise as a production method for high-performance CIGS photovoltaics. However, the morphology of the sputtered CIGS layer is markedly different from conventional evaporated films; grain sizes vary through the thickness of the film, with numerous small grains dominating at the Mo/CIGS interface that then either terminate or grow in an inverted-pyramid fashion to form large, columnar grains at the CIGS/CdS interface. To better understand the origin of this morphology, we have studied the growth behavior of the CIGS layer using a combination of atomic force microscopy and electron microscopy to observe initial nucleation and grain growth behavior of quaternary-sputtered CIGS. We also discuss the effects of interfacial layers at the Mo/CIGS interface and sputtering conditions on nucleation behavior and the resulting microstructure. Finally, the growth behavior and morphology are compared with that of conventional thermally evaporated CIGS.

Non-vacuum methods for the deposition of Cu(In,Ga)Se₂ (CIGS) absorber layers have received growing attention due to inherent savings in equipment cost and material consumption, eased scale-up, and high throughput fabrication which may translate in reduced energy pay-back time of modules and cost per watt output power. Most successful approaches for CIGS absorber growth are based on nanoparticle dispersions or true solutions, however, often require highly toxic and/or combustible materials for reduction or conversion of the precursor, or to solubilize chalcogen containing species, respectively. In this study, we present a non-vacuum deposition method that is based on binder-free solutions of metal salts in alcohol solvents and conversion with selenium vapors that resulted in up to 7.7% solar cell efficiency. Despite the utilization of low boiling point solvents, a residual carbon-rich layer between the absorber layer and back-contact was obtained. To illuminate the process chemistry and the origin of the carbon-rich layer a systematic investigation by means of XRF, XRD, SEM, EDX, WDX, TGA, DTA, mass spectrometry, and EXAFS measurements has been conducted. Experiments confirm the in-situ formation of an intermediate carboxylic chelate complex in the precursor layer that decomposes at elevated temperatures to facilitate the selenization of metal cations. The organic coordination enables handling of the precursor inks at ambient conditions without hydrolysis, segregation and evaporation of metals but can lead to compositional gradients and unreacted carbon-rich residuals. We propose a complete reaction mechanism towards CIGS and discuss the conductive properties of the obtained carbon-rich layer.

For the achievement of substantial cost reduction of crystalline silicon based photovoltaics a dramatic reduction in the use of silicon, specifically the reduction of wafer thickness, is required. For achieving the maximum conversion efficiency it is only necessary to have wafer thickness of the order of 40 to 50 microns. The current ~ 180 micron thickness of wafers results in wasteful consumption of expensive silicon but such thicknesses are needed in order to enable the handling, processing and packaging of the wafers and solar cells. In this paper we discuss a new technology for the manufacture, handling, processing and packaging of wafers and solar cells of ~ 50 microns thickness utilizing a direct vapor to wafer process involving epitaxial deposition of silicon. A technology is being developed for the high rate production of single crystal epitaxial films and the fabrication of solar cells and modules from the thin silicon wafers. The current status of this technology including the achieved conversion efficiency in ~ 50 micron thick solar cells will be discussed. This technology combines the cost reduction advantages of thin film solar along with the high efficiency, reliability and non toxicity of silicon.

We are developing heteroepitaxial crystalline Si thin films deposited on a cube-textured copper substrate to enable high efficiency, flexible solar cells at low cost by roll-to-roll manufacturing; however, direct Si-on-Cu epitaxy is not possible. To circumvent this problem we developed a multi-layer architecture consisting of a binary oxide as the seed layer (oxygen diffusion barrier), perovskite oxide as the metal diffusion barrier layer and γ-Al₂O₃ as the cap layer. This specific buffer architecture was selected to satisfy two key requirements for heteroepitaxial Si growth on Cu: 1) deposition of a highly crystalline γ-Al₂O₃ layer without compromising the integrity of the metal/buffer interface by forming Cu-oxide, and 2) provide a barrier for Cu upward diffusion during the high-temperature Si deposition. The buffer layers are dense and x-ray diffraction confirms that they replicate the biaxial texture of the foil substrate, thus enabling Si epitaxy while avoiding formation of copper silicide. As an additional advantage the buffer layers are transparent, allowing excellent red reflectivity from the Cu foil substrate. The high red reflectivity of the Cu foil is critical to obtain the light trapping required of film c-Si photovoltaics 3-20 microns thick. Using this architecture, we fabricated the first heteroepitaxial film c-Si solar cells on Cu foils. We grew a heavily doped n⁺ silicon back contact and then a 1 micron n^- absorber layer at growth rates of about 130 μm/min, using hot-wire chemical vapor deposition from silane gas mixed with phosphine. The absorber was then hydrogen passivated and completed with an amorphous silicon heterojunction emitter. We then deposited an ITO top contact and made simple mesa structures with chemical etching techniques, allowing us to contact the n⁺ layer. Our non-optimized Si solar cells, still made without intentional light-trapping features, have an open-circuit voltage up to V_OC=387 mV indicating considerable promise for this approach. We will present our latest device results, buffers and Si microstructure, and discuss the effect of epitaxial strain on the cubic texture of the buffer layers and silicon thin films.

Thin film techniques have been touted as a less expensive manufacturing approach for photovoltaics (PV) because of the use of much fewer materials and roll-to-roll (R2R) continuous processing, but none of them have resulted in PV efficiency levels that are needed for broad market penetration. The University of Houston recently established a program to develop technology that combines the low-cost advantages of thin film PV with the high efficiencies only achieved with single crystalline photovoltaics. The innovation lies in the creation of an architecture that yields single-crystalline-like thin films even on polycrystalline or amorphous substrates. This technology has been very successfully demonstrated and being commercialized in the superconductor field and inserted in the U.S. electric power grid. The enabler that we have employed to achieve such an architecture is a biaxially-textured template made by Ion Beam-Assisted Deposition (IBAD). In this work, strongly (400) textured Ge films with an in-plane texture spread of just 1° FWHM were deposited epitaxially on IBAD templates on metal substrates using intermediate oxide buffer layers. Germanium as well as all intermediate layers was deposited by R2R continuous processing using magnetron sputtering. Optical measurements of the germanium films reveal properties that are comparable to that single crystal Ge. Epitaxial (100) GaAs has also been successfully grown by molecular beam epitaxy (MBE) on the Ge films. While excellent epitaxial growth has been achieved in GaAs on flexible metal substrates, the defect density of the films shows a high value of 5 * 10^8 per cm^2. It has been found that the defect characteristics can be substantially modified by changing intermediate buffer layers in the multilayer architecture. The strongest preferred (400) orientation of Ge as well as the highest hall mobility in Ge have been achieved with SrTiO3 intermediate layer. Hall mobility values of the germanium film are found to increase with film thickness and reach as high as 670 cm2/Vs. Cross sectional transmission electron microscopy (TEM) images of thick germanium films on IBAD templates on flexible metal substrates show a high density of defects near the germanium-oxide interface and a significant decrease in defect density in the mid and top part of the film. In addition to using our single-crystalline-like templates for III-V PV, we are also developing them for epitaxial silicon. Single-crystalline-like silicon films have been grown atop IBAD templates on flexible substrates, directly on oxides or on germanium by R2R continuous processing. Progress in use of our R2R processed single-crystalline-like templates on low-cost, flexible substrates for III-V and silicon photovoltaics will be discussed in this presentation.

Photovoltaics are a promising solution to many problems currently associated with fossil fuel energy sources. Sunlight energy is extraordinarily abundant and available globally. In order to offer a scalable alternative to fossil fuels, however, photovoltaic devices must have a few properties. They must be robust to environmental degradation or internal reactions, allowing a 20 year or longer lifetime. They must be made from earth-abundant materials to allow manufacturing to scale quickly and without constraining supply. And they must be possible to manufacture at low cost with relatively minimal energy input. Photovoltaic cells made entirely from silicon and carbon offer these properties. In previous work, we have successfully fabricated Schottky barrier devices using monolayer graphene in contact with both crystalline and amorphous silicon. Unfortunately, these devices have poor transport characteristics, limiting output current. In this work, we investigate post-fabrication treatments that can dramatically improve power conversion efficiency in these devices. These treatments include both thermal treatment to improve the interface transport between the silicon and carbon and light management strategies to allow a reduction in amorphous silicon layer thickness and improve charge extraction in these cells. Research supported in part by Department of Energy, National Science Foundation, and Office of Naval Research.

The vast majority of photovoltaic solar cells that have been produced to date have been based on silicon wafers, with this dominance likely to continue well into the future. The surge in manufacturing volume over the last decade has resulted in greatly decreased costs. Multiple companies are now breaking through the US$1/Watt module manufacturing cost benchmark that was once regarded as the lowest possible with this technology. Despite these huge cost reductions, there is obvious scope for much more of the same, particularly as the market for the polysilicon source material becomes more competitive, the new â?oquasi-monoâ? directional solidification processes are brought on-line, wafer slicing switches to much quicker diamond impregnated approaches and cell conversion efficiencies increase towards the 25% level. This makes module costs of $0.50/Watt and the US Governmentâ?Ts â?oSunShotâ? target of $1/Watt installed system cost by 2020 very achievable with silicon photovoltaics. Evolutionary paths to lower cost beyond this point are also explored.

We have grown good device-quality epitaxial crystal silicon (c-Si) films at a rate of ~1 micron/min at display glass-compatible temperatures to fabricate PV solar cells with open circuit voltages above 560 mV using c-Si wafer substrate as seed. Using hot-wire CVD epitaxy with optimized growth start conditions, filament design, and bulk growth conditions, we have also achieved epitaxial growth rates above 1.8 micron/min, about 30 times higher than used by amorphous and nanocrystalline Si industry. These advances enable high quality, inexpensive â?owafer replacementâ? c-Si PV devices on inexpensive, seeded substrates. At 1.8 µm/min, adequate absorber layers can be grown in only a few minutes at display glass-compatible temperatures of about 700 ^oC, and excess thicknesses can be added to allow etched pyramids or other light-trapping structures into the front surface. This ensures effective utilization of both short and long wavelength light, despite the indirect bandgap of silicon. Our high deposition rate was obtained by designing the hot-filament geometry to concentrate the radical flux, and by optimizing the hot-wire to substrate distance, system pressure, and silane flow, based on our earlier growth model [1]. In-situ film quality measurements by spectroscopic ellipsometry were complemented by residual gas analysis in the growth chamber. We found that the best growth conditions are near a borderline depletion regime that retains both high gas flow and high pressure with good silane gas utilization. However, increasing total pressure above 80 mTorr leads to reduction of growth rate at the same partial pressure of silane. From the gas flow, pressure, and distance dependences of the growth rate, we conclude that the most likely rate-limiting factor is hydrogen atoms temporarily bonding to the W filament and their subsequent reactions with Si atoms on the filament surface. The epitaxial film quality is most sensitive to the initial moments of growth, with dislocations forming at the growth interface. We therefore initiate epitaxy at low rate ~ 200 nm/min and then increase the rate to above 1μm/min for the bulk of growth. We fabricate test solar cells on heavily doped electronically â?odeadâ? silicon wafers by epitaxially growing the absorber layer, hydrogenating and finishing with an a-Si:H heterojunction, and an ITO top contact. The finished device had an open-circuit voltage above 560 mV with corresponding dislocation densities ~1x10⁵ cm^-2 (determined by EBIC), comparable to similar devices grown entirely at low 200 nm/min rate. We discuss further progress in improving high-rate devices by interface-conditioning treatments and initial growth conditions. This work was supported by the U.S. DOE Solar Energy Technology Program under Contract No. DE-AC36-08GO28308. 1. I. T. Martin, C. W. Teplin, J. R. Doyle, H. M. Branz, and P. Stradins. J. Appl. Phys. 107, 054906 (2010).

We fabricated waferless heterojunction (SHJ) solar cells using epitaxial film crystal silicon grown on display glass coated with a (100) silicon seed and achieved an open circuit voltage (Voc) of 539 mV and efficiency >4%. The 1.5-μm thick n-type absorber layers were grown using hot-wire CVD at glass compatible temperatures (<750°C) on 0.5 μm layer-transfer c-Si seed bonded to Corning EAGLE XG® display glass (SiOG). Similar 2-μm thick absorber layers grown on highly doped (â?~deadâ?T) wafers resulted in Voc of 633 mV and >7% efficiency, without any light trapping. We find that our low-temperature epitaxy is extremely sensitive to impurities in the source gas at the start of deposition that nucleate recombination-active dislocations. Gains in Voc and efficiency have been facilitated by improved growth achieved through the reduction of these impurities just before and during deposition. We have also verified the absence of elemental diffusion of glass constituents into the absorber layer. A significant improvement in device performance is achieved via passivation of defects through hydrogenation using a remote plasma source. The quality and thickness of the absorber layers and the device series resistance are being optimized. Performance of the solar cells should improve further when light trapping is applied. Measurements including quantum efficiency (QE) and transient lifetime have confirmed diffusion lengths ~3 times greater than the base layer thickness on wafers and ~2 times greater on Corning SiOG. Electron-beam-induced current (EBIC) images confirm the implied low defect density in the epi-films. The high diffusion lengths enable efficient collection of carriers in cells with absorbers up to 3 μm thick and the low defect density enables a clean SHJ interface and low shunt resistance. The good material quality is also reflected in the near ideal QE in the red and the absence of reverse-bias dependence of the QE. Our high-quality high-deposition rate (1.8 μm/min) epitaxial films are promising for ultra-thin absorber silicon solar cells on low cost substrates seeded with highly crystalline silicon or foreign seed layers, with efficiencies comparable to multi-crystal silicon solar cells. The demonstration of good epitaxial quality on layer-transfer seeds, 400 °C lower than thermal CVD, brings us a step closer to achieving 15% efficient modules at less than 50 cents per Watt. Research at NREL was mainly supported by the U.S. DOE Solar Energy Technology Program under Contract No. DE-AC36-08GO28308.

Incorporating a semiconductor with a smaller bandgap such as Si_1-xGe_x to absorb more of the solar spectrum is attractive for increasing the output current and efficiency of solar cells. The challenge in using Si_1-xGex is the reduction in Voc due to the smaller bandgap. Heterojunction emitter based solar cells (HIT) use a large band gap (1.7 eV) amorphous Si (a-Si) emitter to increase the open circuit voltage (V_oc) in crystalline Si (c-Si) cells. This HIT cell design can be extended to epitaxial Si_1-xGe_x based cells to potentially increase the Voc and solar output current. In this work, thin film a-Si(n+)/c-Si_0.6Ge_0.4(p)/c-Si(p+) heterojunction solar cells (HIT cells) are studied by experiment and TCAD. LPCVD was used to grow 4, 2, and 1 μm-thick epitaxial cap layers of p-type Si_0.6Ge_0.4 on top of 5 μm Si_1-xGe_x graded buffer layers. Comparing the 4, 2, 1 μm Si_0.6Ge_0.4 solar cells, Jsc drops from 18.1 mA/cm² to 17.2 mA/cm² to 17 mA/cm² respectively with no change in Voc (0.41V). The rather small drop in Jsc and the constant Voc indicates that the effective lifetime does not change significantly with Si_0.6Ge_0.4 cap thickness. The effective lifetime is a combination of the higher lifetime Si1-xGex cap layer and the lower lifetime Si_xGe_1-x graded layer. The Si_xGe_1-x graded layer has a large dislocation density due to the lattice mismatch between Si and Si_0.6Ge_0.4. These results bode well for thin film solar cells with even larger Ge percentages as the absorption depth will decreases allowing for more electron-hole pairs to be collected. This will reduce the material quality requirement needed to overcome a Si based solar cell. To this point, an a-Si(n+)/Si(p)/c-Si(p+) based solar cell with 2-μm-thick Si(p) was fabricated for comparison. The results show a 3 mA/cm² increase in the Jsc for the 2-μm-thick-cap Si_0.6Ge_0.4 cell compared to the 2 μm Si based cell due the smaller bandgap which allows for more of the optical spectrum to be absorbed. The Voc dropped from 0.6 V for the Si cell to 0.41 V for the Si_0.6Ge_0.4 due to the reduction in bandgap. Physics based TCAD simulations combined with the experimental results are used to extract the effective lifetime and surface recombination velocity. These results indicate that with proper design, Si_xGe_1-x graded buffer layers can be used as substrate test vehicles to study the material quality requirements for heterojunction solar cell applications.