Optical confinement in high-efficiency a-Si solar cells with textured surfaces

The experimental spectral response and reflectance of high-efficiency a-Si solar cells are systematically investigated by using an optical simulation based on realistic optical properties of the transparent conducting oxide (TCO), a-Si, and metal electrode, in order to improve the spectral response. It is shown that a practically important optical loss results from absorption by the TCO, which is enhanced by the optical confinement effect. This suggests that improvement in the spectral response is possible by suppressing the optical confinement in the TCO.     

Experimental model and long-term prediction of photovoltaic conversion efficiency of a-Si solar cells

Degradation and recovery tests have been conducted on recent single-junction a-Si solar cells under various light intensities and temperature conditions to predict long-term stability. Saturation phenomena of the degradation of efficiency have been shown experimentally. The degradation characteristics are expressed as the second extreme distribution function of the largest value with saturation introduced. The dependence of saturation on the cell temperature and the light intensity are investigated. The worst efficiency of seasonal changes over a long term can be predicted from the saturation characteristics.     

The evaluation of accelerated test for degradation a stacked a-Si solar cell and EVA films

We studied the photothermal stability of ethylene-vinyl acetate (EVA) laminates using accelerated testing in order to identify the reasons for its performance degradation on stacked amorphous silicon (a-Si) solar cells and EVA films. Stacked a-Si solar cells and EVA films were aged in accelerated tests under conditions of 1-SUN and 4-SUN. The results evaluated the deterioration of each separate level for solar cells made of three-layer stacked structure and EVA films. Spectral sensitivity and conversion efficiency were used as evaluation parameters. Spectral sensitivity could be used as an evaluation parameter after the accelerated test caused degradation on the EVA films. EVA films under 4-SUN accelerated testing revealed a phenomenon in which light transmission increased on the short-wavelength side of the spectrum.     

Low-temperature a-Si:H/ZnO/Al back contacts for high-efficiency silicon solar cells

The paper analyses the electronic transport of high-efficiency silicon solar cells with high-quality back contacts that use a sequence of amorphous (a-Si) and microcrystalline (μc-Si) silicon layers prepared at a maximum temperature of 220 °C. Our best solar cells having diffused emitters with random texture and full-area a-Si/μc-Si contacts have an independently confirmed efficiency of 21.0%. An alternative concept uses a simplified a-Si layer sequence combined with Al-point contacts and yields a confirmed efficiency of 19.3%. Analysis of the internal quantum efficiency (IQE) shows that both types of back contacts lead to effective diffusion lengths L_eff exceeding the wafer thickness considerably. Fill factor limitations for the full area contacts result from non-ideal diode behavior, possibly due to the injection dependence of the interface recombination velocity.     

Mass-production of large size a-Si modules and future plan

We have achieved stable 8% module efficiency with large area monolithic amorphous silicon pin single-junction cell on 910 mm×455 mm glass substrate. Utilizing this type of modules, we have designed, developed and tested photovoltaic modules integrated with roofing material for personal house. We proved that the modules are suitable for roofing purpose, through the detailed designing, actual installation and the performance evaluation. We will start to produce mass-production of amorphous modules with 20 MW/yr capacity in October 1999 in Toyooka-city, Hyogo-prefecture where we establish Kaneka Solartech Corporation as the production subsidiary. We also achieved 11.3% stable efficiency of hybrid solar cell (25 cm²) with amorphous Si and poly-Si thin film. We will introduce this hybrid cell technologies to Toyooka Plant by adding the CVD for thin-film poly-Si till 2003.     

Spectral response of a-Si:H p–i–n solar cells

The spectral response of a typical thin-film a-Si:H p–i–n solar cell has been investigated using the simulation RAUPV2. The peak in the external quantum efficiency has been observed to shift towards the violet part of the spectrum on decreasing the cell thickness. Moreover, the height of the peak increases as cell thickness is decreased. This is correlated with an enhancement in cell performance for thinner cells, due to a general increase in the drift field within the cell. The external quantum efficiency of a cell with an optimal concentration of phosphorous in the intrinsic layer has also been investigated. The external quantum efficiency for this cell is similar to that of the thinner cell, and is associated with the enhancement of the drift field near the p/i interface that is brought about by the phosphorous doping of the intrinsic layer. However, the integrated recombination for the thinner cell and the phosphorous-profiled cell differ significantly at long wavelengths, despite their similarity at shorter wavelengths. This effect is due to the weakening of the drift field near the n/i interface in the phosphorous-profiled cell.     

Development of an ultralight, flexible a-Si solar cell submodule

An ultralight, flexible a-Si solar cell fabricated on a polyimide film substrate has been developed. It was found that prebaking the polyimide film substrate improved the output performance of the cell by reducing the amount of H₂0 and polymers released from the substrate. Also, using an i-layer fabricated at high temperature (250°C) and a p-layer fabricated at low temperature (80°C) improved the cell's output performance to a level of over 10%. A maximum output of 782.4 mW and a power-to-weight ratio of 340 mW/g were obtained for an integrated-type flexible a-Si solar cell submodule with a size of 113 mm × 120 mm.     

Recent progress of amorphous silicon solar cell applications and systems

Fifteen years have passed since the first industrial use of amorphous silicon (a-Si) solar cells for consumer products. At present, a-Si solar cells are entering a new age of use in power generating systems at private residences and other outdoor applications. This paper reviews recent advances in amorphous silicon (a-Si) solar cells and their applications. Technological developments in the field of a-Si solar cells are discussed. Various applications and systems that take advantage of the a-Si solar cell are then introduced. Finally, future prospects are discussed, including a new concept of GENESIS system for worldwide energy generation and transmission.     

a-Si/mc-Si hybrid solar cell using silicon sheet substrate

A hybrid junction solar cell with amorphous silicon (a-Si) and multicrystalline silicon (mc-Si) was fabricated using a mc-Si sheet substrate, which is produced directly from molten silicon using a novel rotational solidification method. The efficiency of 11.6% was obtained for the hybrid junction cell, while 10.2% for the single junction cell made of a mc-Si sheet substrate, which confirmed that the hybrid structure is effective to improve the solar cell property made of a mc-Si substrate. With introducing light trapping structure, the efficiency was improved to be 12.0%. Moreover, the possibility of Jsc improvement was investigated using the advanced light trapping structure. Jsc of 15.6 mA/cm² was obtained and it was confirmed that the hybrid junction is a promising structure.     

Development status of high-efficiency HIT solar cells

This paper describes the development status of high-efficiency heterojunction with intrinsic thin-layer (HIT) solar cells at SANYO Electric. Presently, the conversion efficiency of our standard HIT solar cell has reached a level of 23.0% for a practical size of (100.4 cm²) substrate. On the other hand, we have developed special technologies for effectively using thinner substrates for HIT solar cells. Surprisingly, we have achieved a quite high open circuit voltage (V_oc) of 743 mV, and a high conversion efficiency of 22.8% using only a 98-μm-thick substrate. A 98-μm-thick cell also exhibits a good temperature coefficient, and allows the thickness of the substrate to be reduced by more than 50% while maintaining its efficiency. These results suggest that the HIT solar cell has the potential to further improve cost-performance.     

Influence of the intrinsic layer characteristics on a-Si:H p–i–n solar cell performance analysed by means of a computer simulation

In this paper a set of one-dimensional simulations of a-Si:H p–i–n junctions under different illumination conditions and with different intrinsic layer are presented. The simulation program ASCA permits the analysis of the internal electrical behaviour of the cell allowing a comparison among the different internal configurations determined by a change in the input set. Results about the internal electric configuration will be presented and discussed outlining their influence on the current tension characteristic curve. Considerations about the drift–diffusion and the generation–recombination balance distributions, outlined by the simulation, can be used to explain the correlation between the basic device output, the i-layer characteristics (thickness and DOS), the incident radiation intensity and photon energy.     

Role of buffer layer at the p/i interface on the stabilized efficiency of a-Si solar cells

Light induced degradation of single junction and double junction a-Si:H solar cells has been studied. Cells with and without buffer layers at the p/i interfaces have been fabricated. It is found that light induced degradation is faster in the cells with buffer layers. Defect density increases faster and degraded efficiency with respect to the initial efficiency decreases at a higher rate for the cells with buffer layers. Spectral response study for double junction cells shows that collection efficiency decreases for the bottom cell only. So it is found that the absolute stabilized efficiency is highest for a double junction cell with buffer layer at the top cell only.     

A computer analysis of double junction solar cells with a-Si : H absorber layers

An integrated electrical-optical model has been used to examine the design of double junction solar cells, where the component cells have a-Si : H absorber layers of identical material quality in the initial state. The model takes into account both specular interference effects; and diffused reflectances and transmittances due to interface roughness. The carrier transport at the junction between the two p–i–n subcells is simulated with the help of a thin heavily defective “recombination” layer with a reduced mobility gap.Analysis of the transport properties as a function of position in the device indicates that for the highest double junction cell efficiency the thicknesses of the absorber layers of the component subcells are such that the electric fields over these absorber layers are high simultaneously. Our results also show that whereas in the initial state, the open-circuit voltage and the fill factor of the double junction cell are heavily dependent on the electric field in the thicker bottom subcell; in the light-stabilised state, the more degraded top subcell plays an important role in limiting double junction cell performance. The quantum efficiency under AM 1.5 bias light has been shown to be very sensitive to thickness variations of the component subcells. Using this tool we have arrived at a simplified procedure for designing the double junction structure likely to exhibit the highest efficiency in the stabilised state.     

Performance of double junction a-Si solar cells by using ZnO:Al films with different electrical and optical properties at the n/metal interface

High-quality ZnO:Al films have been prepared by using RF-magnetron-sputtering method with resistivity ranging from 10⁻¹ to 10⁻⁴ Ω cm and transmittance above 90% in visible region. We have fabricated small area (1 cm²) double junction (a-Si/a-Si) solar cells using ZnO/Al and ZnO/Ag as back contact. The conversion efficiency of double junction a-Si solar cell increases from 9.9% to 10.9% by using ZnO/Al back contact and to 11.4% by using ZnO/Ag as back contact. Effect of variation of thickness of i-layer on performance of the cell has also been studied.     

Spray-coated organic solar cells with large-area of 12.25 cm

This study evaluated the possibility of utilizing a spray-coating process for large-area organic solar cells (OSCs) combined with a metal electrode geometry. The effects of the cell area in spray-coated OSCs were investigated systematically by introducing a metal sub-electrode and grid-electrode to realize large-area cells of up to 12.25 cm². The series resistance could be reduced significantly by inserting a metal grid-electrode into the indium tin oxide (ITO) anode, yielding a power conversion efficiency of 2.11% at a cell area of 12.25 cm² and 2.49% at an effective photocurrent generated area of 11.23 cm² under AM.1.5 simulated illumination. This is comparable to the 3.13% obtained in the cell produced by spray-coating at a cell area of 0.38 cm².     

Impact of lateral junction on selective emitter solar cell performance

Investigation of selective emitter solar cells has been undertaken using both device fabrication and accurate two-dimensional simulation program. Our results show that selective emitter solar cells exhibit a relatively low fill factor because of minority carriers crowding at the lateral junction. It is also found that carrier recombination in the space-charge region of the lateral junction limits open-circuit voltage improvements.     

Computer simulation of the effect of phosphorous doping of the i-layer in a thin-film a-Si:H p–i–n solar cell

The simulation RAUPV2 has been used to model a thin-film a-Si:H p–i–n solar cell, fabricated at the Rand Afrikaans University. For a physically acceptable set of input parameters, the simulated J–V curve agrees very well with the empirical J–V curve, under AM1.5 g illumination. The effect of boron- and phosphorous doping of the i-layer (B- and P-profiling) was studied. It was found that boron doping of the i-layer greatly reduced cell performance. On the other hand, there seemed to be an optimal phosphorous concentration in the i-layer, P_opt, for which cell performance, measured in terms of maximum power output, was a maximum. It was observed that as the P concentration in the i-layer was increased towards P_opt, the recombination rate in the front of the i-layer decreased, whilst that in the back part of the i-layer increased. The short-circuit current was seen to decrease under P-profiling. It was seen that as a consequence of P-profiling, the drift field in the back part of the i-layer was relatively insensitive to the effect of an applied voltage, for applied voltages up to about 0.55 V.     

Optimal optical generation profiles in a-Si:H p–i–n solar cells

The dependence of the maximum power output P_M and short-circuit current J_SC on the form (relative variation with position) of the optical generation rate profile in an a-Si:H p–i–n solar cell has been investigated computationally. It was found that there was an optimal form for the generation profile, and that P_M increased from 4.64 to 5.29 mWcm⁻², an increase of about 14%, when this optimal generation profile was used in the simulation. Optimal doping of the i-layer of the cell with phosphorous led to a P_M of 5.60 mWcm⁻², and when the optimal generation profile for this P-profiled cell was found, it yielded a P_M of 7.86 mWcm⁻², an increase of about 40%. This suggests that the combination of P-profiling and optimal generation could lead to significant improvements in cell performance. Moreover, it was found that for both cells the form of the optimal generation profile could be associated with the position of the peak in the external quantum efficiency, obtained from the spectral response. The possibility of using band-gap grading to achieve an optimal generation rate profile has been suggested.     

Performance variation of carbon counter electrode based dye-sensitized solar cell

The effect of dark and room temperature aging on the performance of carbon counter electrode based dye-sensitized solar cell (DSSC) has been investigated. Using nano size carbon as a counter electrode material, DSSC with power conversion efficiency of 7.56% was fabricated. Storing the devices in the dark at room temperature enhanced both the open-circuit voltage (V_OC) and fill-factor (FF) but reduced the short-circuit current density (J_SC). After 60 days of aging, carbon counter electrode DSSC retains 84% of its initial day efficiency (η). The variation in the current–voltage parameters was explained on the basis of electrochemical impedance spectroscopic (EIS) analysis.     

16.0% Efficiency of large area (10 cm×10 cm) thin film polycrystalline silicon solar cell

High efficient large area thin film polycrystalline Si solar cell based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. Fabrication process of the via-hole etching for the separation of thin films (VEST) is newly developed. It is found that phosphorus treatment and back surface field (BSF) are quite effective for the VEST structure and the ZMR thin film polycrystalline silicon. The conversion efficiency as high as 16.0% for a practical size (10 cm×10 cm) is achieved. This is the highest for large area thin film polycrystalline Si solar cells ever reported.