Some of the significant steps in technological development for large-scale commercialization of amorphous silicon (a-Si:H) based multijunction photovoltaic modules are presented. These developments are establishing a high quality baseline process for manufacturing large-area ( ∼ 8 ft2) a-Si:H/a-SiGe:H tandem junction modules with improved stabilized conversion efficiency, throughput, yield, and reduced materials usage.

A systematic investigation has been made on the barrier properties of ZnO layer between ndoped a-Si:H and Ag metallization films in the structures (001)Si/SiO2/Ag/ZnO/a-Si:H:P and (001)Si/SiO2/a-Si:H:P/ZnO/Ag. Plasma assisted CVD deposition was used to produce a-Si:H (2500Å thick) highly P-doped films over thermally oxidized Si-wafers at 190 and 270°C. Transparent conductive ZnO:AI layers, 1000Å and lμm thickness, and Ag films (1000Å thick) were deposited by sputtering. The polycrystalline ZnO layers were textured along the <0001> axis in the as-deposited state. The structures were annealed in vacuum in the temperature range from 300 to 700°C for 1/4 to 16h. X-ray diffraction and transmission electron microscopy studies demonstrated the a-Si:H:P stability against crystallization under ZnO buffer protection up to 700°C (when free a-Si crystallizes itself). The (111) peak position of the Ag reflection was used to show that while the Ag was always strained, the strain was partially relaxed when in contact with the 0. 1μm ZnO film, it developed additional strain when in contact with the 1μm ZnO film.

In contrast to the successful application of analytic equations to the current-voltage behaviour of crystalline silicon solar cells in the dark and under AM1.5 illumination, the description of a-Si:H solar cells parameters requires device modelling concepts taking the full set of semiconductor equations into account. This in particular holds for the explanation of the temperature dependence (225–400K) of experimentally determined a-Si:H p-i-n solar cell parameters. Device modelling calculations show that the observed decrease of the short circuit current at AM 1.5 with lower T is much more effected by the additional charge trapped in the tail states and recharging of defect states than by the broadening of the gap. The induced electric field distortion blocks the extraction of photo generated holes. The open circuit voltage Voc increases with lower T which is caused by the same trapping effect.

The use of ITO thin films on glass/ITO/p-i-n/metal amorphous silicon solar cells is reviewed. It is suggested a new application for silicon monoxide thin films on the ITO-p interface, as an intermediate layer, to minimize the fTO thin film deterioration process, during the early stage of exposure to a silane plasma rich in hydrogen. The thickness of the silicon monoxide thin films is chosen not to worsen the optical and electrical properties of the ITO thin films. The TO-p interface is optimized (due to impurities diffusion decrease), leading to an overall improvement of the device performance.

The origin of metastability in a-Si:H films deposited at different deposition conditions has been investigated by joint analysis of light-induced defect generation kinetics (Staebler-Wronski Effect) and differential scanning calorimetry (DSC) measurements. Light-induced defect generation kinetics were measured at elevated temperatures of 80–150 °C. DSC measurements were performed in a temperature range of 20 °C – 570°C. The films microstructure were studied by IR spectroscopy analysis.

The simulation of DSC curves has been used to deduce the kinetics of the low-temperature exothermic peak (LTEP) observed at 80–150°C. It was found that kinetic parameters of LTEP and SW effects has the same dependence on microstructure parameters, γ, which suggests their common nature. The analysis of a-Si:H stability improvement concerning the influence of films microstructure on the kinetics of relaxation processes is presented.

The group VI element, sulfur, is an inefficient donor in hydrogenated amorphous silicon (a-Si:H). The compensation by sulfur donors of the p-type conductivity obtained with diborane in a-Si:H provides additional evidence of the role of sulfur as a donor. By adding equal amounts of diborane and hydrogen sulfide in the plasma the dark conductivity at room temperature can be reduced by one to two orders of magnitude compared to the corresponding p-type a-Si:H with the same boron concentration. Unlike phosphorus doping, a portion of the sulfur-related donors is passivated by hydrogen in the annealed state. This passivated portion can be rendered electrically active by optical excitation. This effect is similar to that which has been called persistent photoconductivity (PPC) and occurs in some compensated samples of a-Si:H and in some multilayer structures. The PPC effect has the opposite effect on both the photo- and dark conductivities from the Staebler-Wronski effect. For this reason it is possible to find an appropriate S/Si ratio where the two effects cancel as far as the conductivity is concerned. For an appropriate concentration of S in “intrinsic” a-Si:H one can obtain samples with high photoconductivity and essentially no degradation in either the dark or the photo-conductivities upon prolonged optical excitation (light soaking). These results suggest that at least the majority carriers are unaffected; however, it remains unclear what effect this second metastability will have on the minority carriers, and hence on PV device applications. The general idea that the addition of a second metastability to hydrogenated amorphous silicon (a-Si:H) might counteract the deleterious consequences of the Staebler-Wronski effect is presented.

We have deposited amorphous silicon films from mixtures of dichlorosilane (SiH2CI2, DCS), and silane (SiH4) and made the first p-i-n solar cells using i-layers of this material. We measured optical and electronic transport properties of the DCS-derived films and relate them to the solar cell performance. The DCS cells are compared to standard cells made with SiH4.

The growth mode of a-Si:H plasma CVD film was observed by AFM to range from homogeneous deposition to heterogeneous island formation depending on the substrate material and deposition temperature. The correlation between the grain-like structure and electric property of the film deposited on transparent conductive SnO2 substrates has been elucidated by using an AFM with a conductive cantilever. The local distribution of photo-current in the a-Si:H films was found to agree well with the topographic image; the photo-current around the grain boundaries was lower than that on the grains. The effect of surface treatment with an aqueous HF solution indicates that the imhomogeneity in the photo-current originates from the preferential oxidation around the grain boundaries rather than intrinsic electric property.

Films of a-SiNx:H with x = 0.0..0.62 were deposited by glow discharge decomposition of (10% SiH4+90%/H2 )+ NH3 mixture. The chemical bonding and composition of films were investigated with using of infrared spectroscopy. The deformation energy per Si atom connected to bond bending Vkθ was calculated from data of Raman scattering. Characteristic energy of valence band tail (VBT) states distribution, E0v, were determined fromrsubgap absorption spectra. The dependencies of E0v and Vkθ on film composition, x, were considered in order to estimate the influence of the bond angle disorder on the distribution of VBT states.

The essential difference in behavior of E0v and Vkθ dependencies on x was found for Si-rich (x<0. 15) alloys. When Vkθ value increase with x, the E0v parameter stays almost constant; while at x>0. 15 the E0v increase with x as well as the VKO. It means that bond angle disorder in the bulk of the material contributes to VBT characteristic energy, but it is not the only source. Another factors as it was shown may be connected with valence states of Si atoms, localized near inner boundaries.

Photosensitivity spectral dependencies of the a-Si(n-type)/c-Si(p-type) heterostructure for the different reverse biases, Vb, amorphous Si film thickness, substrate predeposition temperatures, Ts, and annealing conditions, Ta, were investigated in the wavelengths range of 500–1200 nm It was found that the position of the relative photosensitivity maximum depends on Ts, and Vb and can be varied in the wavelengths range of 840–1080 nm. The energy band diagram of the heterostructure was analyzed to explain the observed results. It was shown that the photosensitivity properties of the a-Si/c-Si heterostructure depend on the interfacial condition. The perspective application of the structures investigated is IR detector fabrication.

This work presents essential differences in the fill factors (FF) of pin and nip solar cell structures based on hydrogenated amorphous silicon (a-Si:H). The nomenclature “pin’ (‘nip’ respectively) determines the deposition sequence of the single layers. Fill factor measurements are carried out with illumination through p- and n-layers at different wavelengths. The use of laser light provides a wide range of illumination levels and photo current densities of up to 14mA/cm2. The spectrally resolved FF measurements indicate an incorporation of dopants in the i-layer depending on the layer deposited first. Nip and pin structures generally show opposite FF dispersion when illuminated through the same layer. However, due to the slight n-conductivity of intrinsic a-Si:H material, a weak boron incorporation leads to a net charge compensation in the ilayer. In contrast to other investigations we do not find a significant deviation in the open circuit voltages of the pin and nip devices as long as the deposition parameters of the single layers are identical.

The photovoltaic (PV) properties of undoped amorphous silicon (a-Si)/ p-type crystalline silicon (c-Si) solar cells were found to improve by a hydrofluoric acid treatment of c-Si just prior to glow discharge deposition of a-Si. The short circuit current density (Jsc) improved from 2.7 to 23.5 mA/cm2 for an 0.1 μm thick a-Si layer. This also resulted in an improved spectral response of the solar cell in the violet region of the spectrum. The enhanced properties have been attributed to the improved carrier transport across the interface, as seen in the current-voltage-temperature relationships, and also PC-1D simulation of the devices. Solar cells of a similar type were also fabricated by dc magnetron sputtering of the a-Si layer. HF passivated cells (area ∼ 0.24 cm2) yielded about 9.5 % efficiency with Jsc of 30 mA/cm2 and a FF of 0.6, without use of an A/R coating. The variation of the PV properties of these cells was investigated as a function of a-Si thickness and c-Si doping.

As-deposited and wet oxidized a-Si:B alloy deposited by low pressure chemical vapor deposition (LPCVD) with various boron content was studied by UV Spectroscopy and Fourier Transform Infrared Spectroscopy. It is found that the optical band gap of a-Si:B varies with respect to the boron content, which provides potential application probability in narrow-gap photovoltaic devices. This effect is associated with the structure change induced by B content. The oxidation behaviors of a-Si:B films with boron content 3 at.% – 25 at.% are different from those with boron content exceeded 30 at.%. Possible oxidation mechanism for different boron content a-Si:B film are proposed.

Commercial aluminium and stainless steel sheets were used as substrates for thin film silicon solar cell deposition. The influence of elemental contamination and disturbed film growth on solar cell performance and yield were studied. Diffusion during film growth was found to be more pronounced than thermally activated interdiffusion of existing films. Surface irregularities of the substrate are shown to reduce the fill factor of the solar cell. Initial efficiencies of 8.5% with single-junction amorphous silicon solar cells on both types of metal sheets were reached.

A p-i-n a-Si:H solar cell structure which can eliminate detrimental effect of TCO and a heavily doped window layer has been investigated in detail using a two-dimensional device simulator. The cell is designed to use an inversion layer induced by field effect instead of the heavily doped window layer while maintaining p-i-n junction locally to keep the built-in potential high and stable. Device simulation has revealed that the conversion efficiency of p-in a-Si:H solar cells can be improved by 30% with the use of this cell structure. This improvement is mainly due to the increase in the photo-currents, which can be explained by the increased quantum efficiency for light with short wavelength.

A new silicon parallel multilayer solar cell structure has recently been reported which can give high solar cell energy conversion efficiency from low quality silicon material. Advantages of this structure are described as is recent characterization work which compares the properties of grain boundaries in experimental devices to those predicted by earlier calculations.

We compare the suitability of vapor phase and liquid phase epitaxy in a two step deposition process for the formation of thin crystalline Si films on glass substrates. In a first deposition step, we form polycrystalline Si seeding layers on glass. In a second step, we increase the thickness either by vapor phase or liquid phase epitaxy. Liquid phase epitaxy leads to growth of faceted grains of more than 100 μm in diameter but the films are not continuous. In contrast, chemical vapor deposition results in continuous, smooth films with grain sizes up to 7 μm. This difference of morphology originates from the influence of supersaturation and growth rate anisotropy. Chemical vapor deposited films exhibit surface morphologies and electrical properties that are promising for the preparation of crystalline thin film Si solar cells.

Thin, polycrystalline silicon solar cells have the potential for the realization of a 15%, lowcost photovoltaic product. As a photovoltaic material, polycrystalline material is abundant, benign, and electrically stable. The thin-film polycrystalline silicon solar cell design achieves high efficiency by incorporating techniques to enhance optical absorption, ensure electrical confinement, and minimize bulk recombination currents. AstroPower's approach to a thin-film polycrystalline silicon solar cell technology is based on the Silicon-Film™ process, a continuous sheet manufacturing process for the growth of thin films of polycrystalline silicon on low-cost substrates. A new barrier layer and substrate have been developed for advanced solar cell designs. External gettering with phosphorus has been employed to effect significant improvements leading to effective minority carrier diffusion lengths greater than 250 micrometers in the active silicon layer. Light trapping has been observed in 60-micrometer thick films of silicon grown on the new barrier-coated substrate. An efficiency of 12.2% in a 0.659 cm2 solar cell has been achieved with the advanced structure.

P type silicon layers for photovoltaic applications have been deposited on low cost multicrystalline silicon substrates prepared by directional solidification or by electromagnetic casting with solar grade charges. The thickness is between 4 to 20 μm. The layers were characterized by Light Beam Induced Current (LBIC) mappings at different wavelengthes. Collecting structures are p-n junctions or Al-Si semitransparent diodes. The layer and the substrate can be investigated separately by changing the wavelength of the LBIC maps, or/and by realizing a collecting structure on the free surface of the substrate.

It is found that the same extended crystallographic defects are detected by LBIC in the layer and in the substrate. However their recombination strength in the layer is less marked probably due to a higher purity of the layer. The computed values of minority carrier diffusion length are close or larger than the film thickness and the investigated layers could be used to make solar cells.

Silicon nitride films on crystalline silicon were deposited in a low-temperature (< 400 °C ) Plasma Enhanced Chemical Vapour Deposition process. The deposition process is monitored with in situ Time Resolved Microwave Conductivity measurements leading to an on-line quality control of the deposited films. It is shown that at the start of the deposition there is a strong decrease of the lifetime of the measured transient signal due to plasma induced damage at the silicon surface. Afterwards an increase of the lifetime is observed due to passivation of the interface. For thin films (< 30 nm), the lifetime and the film composition depend on the film thickness. Furthermore, the film composition has a strong impact on the passivation of thick (100 nm ) silicon nitride films. The best passivation is obtained for almost stoichiometric films characterized by a refractive index of 1.95.