Two types of amorphous silicon (a-Si:H)/silicon nitride (a-Si3 N4 :H) multilayers have been prepared either by turning-off the plasma at each step of individual layer growth (step by step deposition) or by quick gas switching without interrupting the plasma (continuous deposition). It is found that the continuous deposition causes a significant mixing of nitrogen into the a-Si:H well layers and deteriorates the compositional abruptness in the a-Si:H/a-Si3N4 :H interface. Also, undesirable incorporation of nitrogen into the a-Si:H well layers tends to induce nonradiative recombination centers near the interface.

The optoelectronic properties of multilayer a-Ge:H/a-Si:H superlattices with bandgaps between ˜1.4 and 1.1eV are presented. The dependence of the electronic properties on the band alignment and the layer thicknesses is established and quantified. Particular emphasis is given to properties relevant to practical device applications and which involve carrier transport perpendicular to the layers.

The conductivity of multilayer P-doped amorphous hydrogenated silicon (a-Si:H) thin films is measured for films prepared with different, deposition procedures. Multilayer films are deposited by plasma enhanced CVD following a procedure in which the plasma is extinguished and the deposition chamber is filled with air or argon after the deposition of each layer. These films are compared to films grown in continuous deposition runs. The technique provides a direct means to determine the effects of continuous versus interrupted deposition and to analyze oxide interface and bulk gap state densities. Exposing the layers to air between depositions produces deleterious effects whereas the effect of argon exposure are slight. Literature values for the density of states in oxidized a-Si:H are used to provide evidence for a defective layer in very thin P-doped a-Si:H having a defect density of over 1013 cm−2 eV−1 approximately 0.3 eV below the transport level.

Drift mobilities, drift mobility-lifetime products and minority carrier diffusion lengths perpendicular to the planes of a-Si:H, F/a-Si0.35,Ge0.65:H, F superlattices have been measured over a range of well sublayer widths. The transport data suggest a transition from scattering dominated transport in states at the top of the barriers for short superlattice periods to recombination dominated transport at the bottom of the wells for large periods. The characteristic period for this transition is ˜3 nm.

Doping modulated amorphous silicon films have been synthesized which exhibit photoconductivity decay times in excess of 1000 seconds at 78 K. The time and intensity dependence of the photoconductivity is described by a model in which photo-excited charge carriers are spatially separated to separate layers by the built-in pn junction fields and reside in band tail states.

We show that persistent photoconductivity (PPC) observed in npn … multilayers of hydrogenated amorphous silicon (a-Si:H) and compensated thin a-Si:H films have a common origin. It is caused by special centers lying in the upper half of the mobility gap. The centers are not related to the presence of argon during deposition. Similarities of PPC between the layered and compensated films are shown. We also find that PPC in compensated samples, is not entirely a surface effect. Experiments are suggested to decide whether these centers are P-B complexes or can arise from the presence of boron alone.

The phenomenon of light-induced excess conductivity (LEC) which occurs in a-Si:H npnp doping-modulated multilayers is found experimentally to be dependent on several different factors. The concentrations of the dopants in n-type and p-type layers affect the Fermi level position, the height of the barriers, and also the density of defects. These parameters are altered by different choices of inert gas diluent (Ar or He) and substrate temperature T. For a given set of deposition conditions, the LEC effect can be maximized by varying the layer thickness. With undiluted silane at T = 250 °C, the effect was relatively small, reaching a maximum in relatively thick layers (540 Å). The largest effects were obtained for films deposited from silane diluted in helium, using thinner (330 Å) layers. However, for films deposited from silane diluted in argon, thS magnitude of the effect and optimum layer thickness was intermediate (440 Å). When Ts was varied, a minimum in LEC was found near 200–250° C. The influence of Sinternal field was examined by using nini, pipi and npnp multilayers. The internal field is a necessary factor to observe a large LEC effect. A compensated film shows a small LEC effect.

Recent advances in a-Si solar cells in Japan are reviewed. Improvements in single-junction and multi-junction solar cells are described in three main points, namely, fabrication methods, materials, and cell structures. Recently, a conversion efficiency of 11.7% was obtained for a single-junction structure. For an a-Si/poly-Si stacked structure and an a-Si/(CdS/CdTe) 4 terminal structure, conversion efficiencies of more than 13% were achieved.

Then recent advances in the prevention of the light induced degradation of a-Si solar cells is mentioned. Several methods which can improve the a-Si solar cell stability are described.

Finally, the present status of the industrialization of a-Si solar cells and some of the latest applications are described together with their propects.

Amorphous silicon solar cells are adversely affected by impurities through the creation of traps, recombination centers and metastable centers. The microstructure of discharged-produced, hydrogenated amorphous silicon (a-Si:H) appears to be strongly affected by the presence of impurities in the discharge atmosphere. A model is developed in which impurities create microvoids in a-Si:H, and traps, recombination centers and metastable centers are associated with the internal surfaces of the microvoids. In this model, hydrogen plays an important role in determining the electronic activity and diffusivity of impurities, and metastable centers are created by the trapping of holes near microvoids and the induced motion of hydrogen on the internal surfaces of the microvoids.

We present in this paper updated solar cell performance data on fluorinated amorphous silicon and fluorinated amorphous silicon-germanium alloys. We have achieved a 9.7% conversion efficiency for a singlejunction device using a-Si:Ge:F:H alloy having an optical band gap of 1.5 eV. We have also achieved 12.5% and 13.0% efficiencies, respectively, for dual-band-gap tandem and triple devices using a-Si:F:H and a-Si:Ge:F:H alloys. These represent the highest values reported to date for their respective configurations. Multi-junction solar cells exhibit excellent stability not only in terms of light-induced effect but also in terms of prolonged heating at elevated temperatures.

A systematic study has been made on a-Si basis heterojunction tandem structure solar cells. Design rule and optimizations of cell parameters on two and three stacked heterojunction solar cells are discussed on various material bottom junctions stacked with a-Si. A series of experimental trials has been made on some candidate materials. Technical data on cell fabrication processes, device performance are presented. In the -present stage of experiments, 13.3% efficiency on a-Si/poly-Si two terminals tandem solar cell and also on a-Si//poly- CdS/CdTe four terminals cell have been obtained. Some key technologies for future low cost massproduction processes such as growth of polycrystalline thin film for bottom cell are also discussed.

PIN-structure small-area solar cells using a-Si have been frequently reported on, but only a few reports are available on the study of solar cells using a large-area (10-cm square) substrate, all with a resultant conversion efficiency of above 9.0 %[1,2]. Our study has been concentrated on solar cells using a batch of ten 10-cm square substrates with an average conversion efficiency of 9.5 % or more.

As a result, without an anti-reflection coating on the surface of the glass substrate, the following values have been obtained: average conversion efficiency (EFF)=9.63 % (standard deviation of 0.195 %) -Open-circuit voltage (Voc)=12.668 V (standard deviation of 0.215 V) -Short-circuit current (Isc)=78.467 mA (standard deviation of 1.619 mA) -Fill factor (FF)=0.6672 (standard deviation of 0.009)

The process, equipment and methods for measurements through which these results were obtained are described below.

Amorphous SiGe:H alloy is the key material in achieving high conversion efficiency with tandem-type amorphous silicon alloy solar cells. Status and issues for this key material are discussed, and efforts made to irprove it are reviewed to obtain directions for higher quality a-SiGe:H alloys. An application of the improved alloy to tandem-type solar cell to achieve 9.6% efficiency for the cell size of 100 cm2 is reported.

We tested several methods of patterning amorphous-silicon (a-Si:H) solar cells to form stripe cells and to connect these stripe cells integrated with cell deposition process to a series-connected module. The patterning techniques used included: laser cut, screen print lift-off techniques, adhesive tapes with lift-off, and a special mechanical milling method. The three layers of a cell which must be patterned are 1. SnO2 :F on glass, 2. a-Si (p(C)in), 3. metal-electrode.

We developed a convenient patterning method for a laboratory size with a loss of area of 13 %: 1. laser cut, 2. milling, 3. adhesive tape lift-off. For a higher level of production we tested the following sequence with a loss of area of 4 %: 1. screenprint and lift-off, 2. milling/slotting, 3. screen print and lift-off. The last method has the advantage that without any laser treatment the costs are only third of those of the other methods with a laser cut; so we will eliminate the laser cut in future for manufacturing modules.

As a new preparation method for high-quality a-Si films, we have developed the super chamber, a separated UHV reaction chamber system. A low impurity concentration and excellent film properties were obtained by the super chamber. A conversion efficiency of 11.7% was obtained for an a-Si solar cell using a high-quality i-layer deposited by the super chamber, and a p-layer fabricated by a photo-CVD method.

As a new material, amorphous superlattice structure films were fabricated by the photo-CVD method for the first time. Quantization effects and low damage to the interfaces were observed. Superlattice structure p-layer a-Si solar cells were fabricated for the first time, and a conversion efficiency of 10.5% was obtained.

Superlattice doped layers of the type ABABAB … have been prepared where A is the wide bandgap a-Si:C:H doped layer and B is the narrow bandgap a-Si:C:H or a-Si:H doped layer. The bandgaps of the individual layers were modulated by changing the gas phase composition of methane during the plasma deposition. By varying the structure of the films, superlattice p-layers with resistivities in the range of 106 - 107 ohm-cm with optical bandgaps of 2.0 - 2.4 eV, and activation energies of 0.35 - 0.48 eV and superlattice n-layers with resistivities in the range of 104 - 105 ohm-cm with optical bandgaps of 1.86 - 2.03 eV and activation energies of 0.38 - 0.47 eV have been obtained.

P-I-N solar cells have been prepared with both p and n layers comprised of superlattice structures. Conversion efficiencies as high as 10.86% have been achieved under simulated AM1.5 Global conditions. Measurements reveal a marked improvement in both built-in voltage and carrier collection length.

The consequences of the asymmetries in the density of electronic states of hydrogenated amorphous silicon on the behavior of electronic devices are discussed. Asymmetries in the relative widths of valence and conduction bandtails, the position of the dangling-bond states within the gap, and the occupation statistics of non-correlated defects are shown to affect the performance of p-i-n solar cells, and explain their superior performance and stability when compared with such devices illuminated through the n-layer (n-i-p). The device-modeling concepts which emerge help explain the differences between cell degradation via light exposure under various bias conditions, and cell degradation by dark forward bias.

we have studied the effects of intrinsic layer dopant profiles on the open circuit voltage and fill factor of p-i-n and n-i-p solar cells. Data is presented showing that boron profiling into the i-layer is responsible for the commonly observed difference in open circuit voltage between p-i-n and n-i-p devices. As only a shallow profile is required to produce this effect, it is proposed that the lower Voc samples are being limited by surface recombination at the p/i interface and that the boron profile reduces this recombination current. The fill factor for devices illuminated through the n+ layer is shown to be very sensitive to the depth of a boron profile, yet not so sensitive to the profile concentration. A maximum occurs for a profile penetrating slightly more than half way through the cell. Spectral response data clearly shows a shifting of high collection from the back to the front of the cell.

The spectral dependence of Voc is studied in a-Si:H based p-i-n junctions by using chopped monochromatic light. A dispersion ΔVoc = Voc(blue) - Voc(red) is observed which depends systematically on a low level doping of the central layer. While ΔVoc is largest and positive in phosphorous doped cells it is negative in boron doped cells. Voc is found to be enhanced by an additional white bias light.

The dispersion data are discussed by considering the contributions to Voc arising from drift in an inhomogeneous field and from carrier diffusion.

The temperature and incident light intensity dependences of open circuit voltage for hetero-and homojunction a-Si:H pin solar cells have been studied. The temperature coefficient of the open circuit voltage decreases from -2.40 to -2.67 mV/°C as the illumination intensity is decreased from 150 to 7mW/cm2 This is consistent with the empirically calculated open circuit voltages. The diode quality factor obtained under dark condition shows high values(l.6–3.0) and strong temperature dependence. On the other hand, the diode quality factor under illumination is less than 1.5 and shows slight temperature dependence. The results appear to be due to the long dielectric relaxation time compared with the carrier lifetime in the i-layer.