This PDF file contains the front matter associated with SPIE Proceedings Volume 7409, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

The material-quality limiting factors of evaporated solid-phase crystallized (SPC) poly-Si thin films fabricated on planar glass for photovoltaic applications are investigated by a study combining scanning electron microscopy and transmission electron microscopy. The grains in the investigated thin films are found to be randomly oriented, with an average grain size of ~2.1 μm. In general, the grains are found to have a high defect density, although some grains are more defective than others. We also observe a high level of impurity incorporation, in particular, oxygen, into the film. The optical activity of the Si films is dominated by deep band tail states. We conclude that the high intragrain defect densities and the high impurity levels are two major limiting factors for obtaining high-quality evaporated SPC poly-Si thin films for photovoltaics.

Thin film solar cell technology is highly promising to enable clean and low cost generation of solar electricity for various applications. The high efficiency, flexibility and lightweight advantages of thin film solar cells, together with stable performance and potentially low production costs, further enhance their attractiveness for both terrestrial and space applications. A distinct manufacturing advantage of thin film solar cells is the use of fast vacuum deposition methods, providing the high throughput essential to reduce manufacturing costs. However, an essential pre-requisite is the development of deposition techniques which combine capability to deposit the solar cell thin film multilayer preferably within a single vacuum cycle, removing the requirement for certain process steps to be carried out using non-vacuum wet chemistry. Moreover, process development is also needed to provide low temperature processing and low stress multilayer thin film structures which enable photovoltaic devices to be deposited on to low cost flexible polymer or metal substrates. In this paper a new sputtering tool strategy is introduced, utilising high plasma densities (~10mA.cm^-2) and low ion energies, thereby lowering process temperature and film stress for deposition onto both flexible and solid substrates. The technique uses magnetrons of opposing magnetic polarity to create a "closed field" in which the plasma density is enhanced without the need for high applied voltages. A prototype batch system has been designed which employs a rotating vertical drum as the substrate carrier and a symmetrical array of four linear magnetrons. The magnetrons are fitted with target materials for each of the thin films required in the PV stack including the CdTe absorber layer, CdS buffer layer and the back TCO contact. Details of the system design will be provided together with optical, electrical and metrology data already obtained from ITO thin films. The "closed field" sputtering technology allows scale up not only for larger batch system designs but it is also configurable for "in-line" or "roll to roll" formats for large scale production.

Uniform coating of large areas is a technically challenging aspect of physical vapor deposition. This investigation shows that good film uniformity across large areas can be repetitively achieved by a DC magnetron sputtering process by use of multiple sources. A unique feature of this technique is the ability to predict and control the film distribution using the deposition rate, adding flexibility to the deposition system. A model for predicting the material distribution from multiple sources is presented. It will also be demonstrated that this process yields efficient use of the vapor generated from the sources, which results in higher deposition rates and less system maintenance.

We report experimental studies on laser scribing of thin film solar cells using various types of short pulsed lasers (nanosecond, picosecond, and femtosecond temporal pulse widths), aiming to determine the optimum laser parameters for the scribing of multilayer structures of amorphous silicon (a-Si) and copper indium diselenide (CIS) based solar cells. Detailed laser scribing parameters such as repetition rate of the laser pulses, scanning speed of the sample and laser beam, individual pulse energy, laser wavelength, and direction of laser illumination (either from film side or from substrate side) are examined. Characteristics of each scribing conditions are evaluated in terms of morphology by atomic force microscopy (AFM) and scanning electron microscopy (SEM), chemical species analysis by Energy Dispersive X-ray Spectroscopy (EDS), and electrical conductance of interconnects by conductive AFM (c-AFM) and contact resistance measurement to determine the optimal laser scribing conditions. Further issues on defects in the films such as re-deposited debris, elevated molten rim and delamination, thermal damage to surrounding and/or underlying layers and inter-diffusion of materials at the interface are discussed on the basis of thermal/mass diffusion, thermal stress, and ablation-induced plasma formation, in order to demonstrate an efficient laser scribing of P1/P2/P3 of thin film solar cells.

The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock demand scenario have led in the past years to the fast growth of the thin film silicon industry. The high potential for cost reduction and the suitability for building integration have initiated both industrial and research laboratories dynamisms for amorphous silicon and micro-crystalline silicon based photovoltaic technologies. The recent progress towards higher efficiencies thin film silicon solar cells obtained at the IMT-EPFL in Neuchatel in small-area laboratory and semi-large-area industrial Plasma Enhanced Chemical Vapor Deposition (PE-CVD) systems are reviewed. Advanced light trapping schemes are fundamental to reach high conversion efficiency and the potential of advanced Transparent Conductive Oxides (TCO) is presented, together with issues associated to the impact of the substrate morphology onto the growth of the silicon films. The recent improvements realized in amorphous-microcrystalline tandem solar cells on glass substrate are then presented, and the latest results on 1 cm² cells are reported with up to 13.3 % initial efficiency for small-area reactors and up to 12.3 % initial for large-area industrial reactors. Finally, the different strategies to reach an improved light confinement in a thin film solar cell deposited on a flexible substrate are discussed, with the incorporation of asymmetric intermediate reflectors. Results of micromorph solar cells in the n-i-p configuration with up to 9.8 % stabilized efficiency are reported.

United Solar Ovonic is world's largest manufacturer of thin film solar laminates that convert sunlight to electricity. In 1997, we attained initial 14.6% and stable 13.0% cell efficiencies using an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure, which established the foundation of large volume roll-to-roll production. Since then, the power rating of our standard product has been steadily improving from 128 W to 136 W and 144 W. For future generation high efficiency products, we have investigated the possibility of using nc-Si:H to replace the narrow bandgap a-SiGe:H intrinsic layer in the middle and bottom cells of the triple-junction structure. We have investigated various deposition techniques for improving the nc-Si:H material property, solar cell efficiency, and deposition rate. The triple-junction solar cell efficiency incorporating nc-Si:H in the middle and bottom cells has exceeded the record efficiency previously achieved using the conventional a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure. However, in order to insert the nc- Si:H technology into large volume production, several important issues need to be addressed and resolved. We are currently focusing on these key technical challenges, including high efficiency, high rate nc-Si:H deposition, and large-area nc-Si:H uniformity. In this paper, we review recent advances in a-Si:H and nc-Si:H based multi-junction solar cells and modules.

In this work we have experimentally demonstrated the efficiency enhancement of single junction amorphous silicon solar cells fabricated on plastic and glass substrates by using Distributed Bragg Reflectors (DBR). Our results show that the short-circuit current density and as a result the conversion efficiency is enhanced by 10% for the cells fabricated on textured TCO coated glasses.

Silane and hydrogen discharges are widely used for the deposition of silicon thin film solar cells in large area plasma-enhanced chemical vapor deposition reactors. In the case of microcrystalline silicon thin film solar cells, it is of crucial importance to increase the deposition rate in order to reduce the manufacturing costs. This can be performed by using high silane concentration, and usually high RF power and high pressure, all favorable to powder formation in the discharge that generally reduces the deposition rate as well as the deposited material quality. This work presents a study of powder formation using time-resolved optical emission spectroscopy. It is shown that this technique is suitable to detect different regimes in powder formation ranging from powder free discharge to discharge producing large dust particles. Intermediate powder formation regimes include the formation of small silicon clusters at plasma ignition as well as cycle of powder growth and ejection out of the discharge, and both are observable by this low-cost and experimentally simple technique.

Based on Crosslight APSYS, thin film amorphous Si (a-Si:H)/microcrystalline (μc-Si) dual-junction (DJ) and a- Si:H/amorphous SiGe:H (a-SiGe:H)/μc-Si triple-junction (TJ) solar cells are modeled. Basic physical quantities like band diagrams, optical absorption and generation are obtained. Quantum efficiency and I-V curves for individual junctions are presented for current matching analyses. The whole DJ and TJ cell I-V curves are also presented and the results are discussed with respect to the top surface ZnO:Al TCO layer affinity. The interface texture effect is modeled with FDTD (finite difference time domain) module and results for top junction are presented. The modeling results give possible clues to achieve high efficiency for DJ and TJ thin film solar cells.

This paper reports the development of a VHF PECVD process at 40.68 MHz for deposition of device-grade nc-Si:H. It further reports the evaluation of textured ZnO:Al films produced by hollow cathode sputtering as regards their suitability to serve as a TCO substrate for a-Si:H / nc-Si:H tandem device fabrication. The tandem devices were produced using an established VHF PECVD process at 100 MHz. Both VHF processes are capable of producing similar nc-Si:H material based on their analysis using micro-Raman spectroscopy. For the tandem junction devices, a peak in device efficiency was obtained at a Raman crystalline fraction of 50-52 % and a microstructure parameter of 0.60-0.68. A best tandem cell efficiency of 9.9% was achieved on HC ZnO compared to 11.3% on a reference Type-U SnO₂ substrate.

Transport properties are very important for solar cells. The efficiency of solar cells is determined by the competition of carrier collection and recombination. The most important parameter is the carrier mobility-lifetime product. However, methods commonly used for measuring transport parameters require specially designed samples. The results are often not easily correlated to solar cell performance. In this paper, we present our studies of extraction of material properties from conventional current-voltage characteristics and quantum efficiency curves. First, we carried out analyses of shunt resistance as a function of the light intensity. For solar cells with no clear parasitic shunt resistance, the shunt resistance is inversely proportional to the short-circuit current, and its proportionality coefficient is related to the effective carrier mobility-lifetime product. For an a-Si:H solar cell made under an optimized condition with high hydrogen dilution, the effective mobility-lifetime product was estimated to be 1.2x10^-8 cm²/V. For a-SiGe:H solar cells, the effective mobility-lifetime product depends on Ge content. For optimized a-SiGe:H bottom cells used in high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures, their values are ~5.0x10^-9 cm²/V. For high efficiency nc-Si:H solar cells, the effective mobility-lifetime product is ~5.0x10^-7 cm²/V. Second, we measured the quantum efficiency as a function of electrical bias and developed an analytical model to deduce the effective mobility-lifetime product. The results obtained from the second method are consistent with the values from the first method. We will present detailed analyses and interpretations of the transport parameters and their correlation to solar cell performance.

Amorphous and microcrystalline silicon have been proven to be very interesting for low cost thin film photovoltaic devices. Usually these two materials are deposited using the same large area plasma-enhanced chemical vapor deposition reactors from silane and hydrogen gases. The transition from amorphous deposition regime to microcrystalline deposition regime is generally done by reducing the silane concentration in the input gas flow and the optimum deposition parameters to achieve high performance device stands just at the transition between the two microstructures. In the present work, a study of the transition width from amorphous to microcrystalline silicon is presented as a function of the input silane concentration. It is shown that the higher the input silane concentration, the wider is the microstructure transition. As a consequence, the process is less sensitive to fluctuations of the silane concentration when silane concentrations higher than 10 % are used and better uniformity and reproducibility can be then achieved.

The progress of 3D photonic intermediate reflectors for micromorph silicon tandem cells towards a first prototype cell is presented. Intermediate reflectors enhance the absorption of spectrally-selected light in the top cell and decrease the current mismatch between both junctions. A numerical method to predict filter properties for optimal current matching is presented. Our device is an inverted opal structure made of ZnO and fabricated using self-organized nanoparticles and atomic layer deposition for conformal coating. In particular, the influence of ZnO-doping and replicated cracks during drying of the opal is discussed with respect to conductivity and optical properties. A first prototype is compared to a state-of-the-art reference cell.

The monolithically integrated series connection of single solar cell stripes into complete photovoltaic (PV) modules is one of the key advantages of thin film PV technologies. Instead of the well established laser scribing for series connection, this contribution focuses on a novel in situ series connection technology, without breaking the vacuum during module manufacturing, and without the need of costly laser-scribing equipment. Metallic wires or other filaments aligned along the slightly bent substrate, sequentially pattern the solar cell layers for implementing the monolithic series connection, simultaneously with the consecutive evaporation, plasma deposition, and sputtering of the semiconductor and contact layers. In addition to a proof of concept by flexible PV modules, this paper for the first time investigates wire-shading on rigid glass substrates and by multiple adjacent filaments. The results of these studies demonstrate that the in situ series connection is a promising candidate for competing with laser scribing, not only in roll-to-roll production of flexible PV modules, but also in batch or inline processing of standard large-area glass plates. Applying the novel in situ series connection to a laboratory-scale solar cell process, yields 40 cm² sized PV modules, consisting of ten single junction amorphous silicon n-i-p cells on a flexible polymer foil. The modules' total area efficiency of 3 % is close to the non-optimized efficiency of reference cells of 3.3 %. Wire-shading with wire diameters down to 50 μm proves successful, and thereby projects total interconnection losses F < 5 %, whereas the first experimental modules exhibit F = 15 %.

Thin film CdTe/CdS solar modules are currently the most promising photovoltaic technology for cost efficient solar electricity production. Solar modules on glass substrate with more than 10% efficiency are already available on the market. Substituting a flexible substrate material for the rigid and heavy glass will further reduce production costs due to manufacturing advantages and will allow novel applications due to product advantages. In this paper we are presenting the development of CdTe/CdS thin film solar cells with ZnO based TCO on commercially available polyimide. The polyimide is coated with Al doped ZnO as front electrical contact followed by a highly transparent and resistive ZnO buffer layer. Highest efficiency of 12.4% was achieved on 7.5 μm thin polyimide with 823 mV open circuit voltage, 19.6 mA/cm² short circuit current density and 76.5% fill factor. Using 12.5 μm thick polyimide as substrate reduces the current density of the device by approximately 5%. No cracks in the layers or adhesion problems of the solar cell structure on the polyimide are observed even when rolled to small radius of curvatures (few mm).

Relatively high proven efficiencies of CIGS devices are often cited regarding its choice as a semiconductor for photovoltaic manufacturing. Module efficiency is an important parameter, as a number of factors in the cost per watt are driven downward by increasing efficiency. Some of these factors include materials costs, throughput for a given capital investment, and installation costs. Thus, realizing high-efficiency (e.g. 15%) large-area CIGS modules is key in both reducing cost per watt and differentiating the technology from other thin films. This paper discusses the material properties required of each layer of the CIGS device such that large-area CIGS modules can achieve efficiencies 15%, which is substantially higher than the current industrial state-of-the-art. The sensitivity of module performance to the important material parameters is quantified based on both experimental data and modeling. Necessary performance differences between small-area devices and large-area modules imposed by geometry are also quantified. Potential technical breakthroughs that may relax the requirements for each layer are discussed.

We demonstrate photovoltaic integrated circuits (PVIC) with high-quality large-grain Copper Indium Gallium Selenide (CIGS) obtained with the unique combination of low-cost ink-based or Physical Vapor Deposition (PVD) based nanoengineered precursor thin films and a reactive transfer printing method. Reactive transfer is a two-stage process relying on chemical reaction between two separate precursor films to form CIGS, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage, these precursors are brought into intimate contact and rapidly reacted under pressure in the presence of an electrostatic field while heat is applied. The use of two independent thin films provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the synthesis of CIGS. High quality CIGS with large grains on the order of several microns, and of preferred crystallographic orientation, are formed in just several minutes based on compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 14% and module efficiencies of 12% have been achieved using this method. When atmospheric pressure deposition of inks is utilized for the precursor films, the approach additionally provides further reduced capital equipment cost, lower thermal budget, and higher throughput.

Stabilized electrical performance data are necessary to compare different types of modules in the emerging thin-film-PV market and as basic information for energy yield calculations. The problems with accurate power measurements of thin film modules are well known. The module test-standard IEC 61646 ed. 2 tries to take this into account by demanding light-soaking and repeated STC-measurements which leads to time analysed procedures. The stabilisation behaviour over time of short circuit current, open-circuit voltage, efficiency and filling-factor is compared under the influence of different illumination conditions for various types of CdTe, CI(G)S and a-Si modules. Therefore, I-V-curves are measured with high frequency during outdoor exposition and indoor exposition to 1000 W/m² irradiation from a class B solar simulator in a climatic cabinet under temperature-controlled conditions. The different modules are held in MPP conditions between the measurements. The results are compared with STC measurements according to IEC 61646 procedures. To describe the development of the performance of the different types of thin film modules, suitable mathematical approaches are taken to describe the different developments during the process of light soaking. It turns out that the different module types behave very differently and some types need very long times until a stabilised state is reached.

Individual shunts and "weak diodes" can have a significant effect, one much larger than implied by their physical area, on the performance of laboratory-sized (~ 1cm²) solar cells. For larger areas typical of thin-film modules, the sheet resistance of the transparent contact minimizes the impact of a single, small-area non-uniformity. If there are significant numbers of shunts or weak diodes throughout a module, however, its performance may also be reduced. In this case, the number, the magnitude, the nature, and the distribution of the non-uniformities combine to affect the degree of reduction. In particular, a concentration of most shunts or weak diodes in a small number of module cells will be less destructive than if they are distributed among a greater number of cells. In the case of non-uniform illumination, however, module performance is less degraded if the shadowing is spread relatively uniformly over all or most of the cells.

We proposed a method to measure the optical constants of thin film through polarizing phase shifting interferometer based on Twyman-Green interferometer structure. A broadband light source coming with a narrow band-pass filter was used to generate a low coherence light and the wavelength is tunable by changing the filter. A pixelated micro-polarizer mask on the detection camera made phase shifting array to make different phase shifts at once. Therefore, we can use one single interferogram to extract phase information, and it is effective in reducing environmental vibration. The refractive index and thickness of thin film can be derived from the obtained reflection coefficient's magnitude and phase. The measurement results were compared with the results obtained by an ellipsometer.

Thin films of the ternary material Cd_1-xMg_xTe were developed by the co-evaporation of CdTe and Mg at substrate temperatures ranging from 300 to 400 °C. Films with band gap close to 1.6 eV were deposited on both corning glass as well as CdS/Tec7 substrates depending on the end use. Films were characterized for structure, morphology, composition, optical and optoelectronic properties. Similar to CdTe based solar cells, Cd_1-xMg_xTe/CdS devices also showed dependence on the vapor chloride annealing temperature and Cu diffusion process. Stability of the material during chloride annealing hampers high temperature annealing and recrystallization. With improved contact processing and post deposition treatments we were able to fabricate Cd_1-xMg_xTe/CdS devices with efficiency over 7.5%.

The well-known indium-tin-oxide is not suitable for solar cell, because of the chemical reduction, even without any hydrogen dilution. The inexpensive and non-toxic of transparent conducting Aluminum and Gallium doped ZnO (AZO and GZO) thin films have been investigated for the substitutes for the indium-tin-oxide thin films. AZO performs high transmittance at visible region, however, higher resistance than GZO. In this study, AZO and GZO composed film (GAZO) will be fabricated using DC magnetron co-sputtering deposition system to achieve lower resistance than AZO and higher transmittance than GZO. The optical and electric properties of different thickness of GAZO such as transmittance, reflection, carrier concentration, Hall mobility, and resistivity will be measured and analyzed.

We report on the optical properties of erbium oxide thin films prepared by physical vapor deposition. The films were subjected to various rapid thermal annealing (RTA) treatments. The best result was obtained for samples annealed at 500 °C, where the ramp rate was 200 °C/s, zero soak time, and a cooling rate of 25 °C/s. The average reflection from this erbium oxide coated c-Si substrate, measured over a wavelength range of 300nm to 1100nm, is around 18% and 8% before and after annealing, respectively. The average transmission of erbium oxide on glass is 50 % and 90 % before and after annealing, respectively. Using this antireflection coating the short circuit current of a silicon base photovoltaic device increases by more than 40 %.

In this paper, we designed series of In_xGa_1-xN/Si hetero-junction solar cells. Key properties of In_xGa_1-xN/Si solar cells (single junction, double junctions) are simulated by using AMPS-1D software, including I-V characteristic, conversion efficiency, band structure etc. The In_xGa_1-xN/Si hetero-junction solar cells are compared with the performances of Si homo-junction solar cells. We also discuss some sensitive performance-related parameters in the preparation of InGaN/Si hetero-junction solar cells.

Sodalime glass is the most commonly used substrate in fabrication of CIGS thin film solar cells. Small amount of sodium is known to have a favorable effect on device performance; however, an excess amount degrades device performance. The sodium out-diffusion from the sodalime glass is not well-controlled. To obtain the advantages of sodium without losing process control, an alkali barrier layer such as Si_xN_y is deposited before molybdenum layer followed by a sodium precursor. It is essential to optimize thickness of the barrier layer in terms of the ability to effectively avoid the diffusion of sodium from the sodalime glass into the absorber as well as in terms of the adhesion properties of the alkali barrier layer with the sodalime glass substrate. This paper presents effect of Si_xN_y barrier layer thickness on device performance and consequent optimization of thickness.

Amorphous silicon carbide alloys are being investigated as a possible top photovoltaic layer in photoelectochemical (PEC) cells used for water splitting. In order to be used as such, it is important that the effect that varying carbon concentration has on bonding, and thus the electronic and optical properties, is well understood. The samples being studied are silicon rich films with between 6 and 11 atomic percent of carbon. Electron spin resonance (ESR) experiments, including light-induced ESR (LESR), were performed to study defects from dangling bonds which occur dominantly at the silicon atoms in these films. Spin densities resulting from silicon dangling bonds varied between 10¹⁶ and 10¹⁷ spins/cm³. Lastly, to test the validity of these materials being used for devices we prepared pin structured solar cells with the films being studied used as the absorber layer.

Thinner CuIn_1-xGa_xS₂ (CIGS2) solar cells are being prepared with an aim to reduce the consumption of indium and gallium. Post-sulfurization annealing is being used to enhance the grain size in order to overcome the problem of very small grains that tend to form in thinner films that are not desirable for device quality solar cells. Based on the fact that gallium gradient that is typically found in CIGS and CIGS2 solar cells has beneficial effect on preventing back contact recombination of minority carriers, an attempt to determine the optimum regime for post-sulfurization annealing is made to derive the benefits from larger grains and gallium gradient. An initial set of experiments carried out at PV materials laboratory at FSEC has shown encouraging results with cell efficiencies of 9-10% for thinner (1.2-1.6 μm) films.

Tin sulfide (SnS) thin films of different thicknesses were deposited on TCO-coated glass substrates by pulse electrodeposition. The applied potential pulses were -0.95V (V_on) and +0.1V (V_off). Crystal structure of the deposited films was orthorhombic with lattice parameters similar to that of the mineral herzenbergite. A systematic increase in the band gap value was observed with increase in the film thickness. The dark conductivities of 60 and 510 nm thick films were 3.8 x 10^-8 Ω cm^-1 and 6.72 x10^-7 Ω cm^-1 respectively. The structural parameters such as lattice constants and grain size showed a systematic change with film thickness.

In this work InGa_0.85N p-n homojunction solar cells were grown by MOCVD on GaN/sapphire substrates and fabricated using standard techniques. When illuminated from the backside, these devices showed 65.9% improvement in J_SC and 4.4% improvement in V_OC as compared to identical illumination from the front. These improvements arise from removal of the losses from electrical contact shading on the front of the devices (11.7% of active area), as well as significant optical absorption by the top current spreading layer. These improvements can likely be further enhanced by utilizing double-side polished wafers, which would eliminate scattering losses on the back surface. In addition to improving electrical characteristics of single cells, backside illumination is necessary for the realization of monolithic tandem InGaN solar cells.