Activation of dopant in the p-layer of amorphous silicon solar cells under illumination

We have investigated the effect of light-soaking on the p-doped layer of amorphous silicon (a-Si:H) solar cells by low temperature (50–300 K) AC conductance measurements. The experimental results are interpreted on the basis of an equilibration model of the doped material. The model takes into account the finite dimension of the layer and its presence inside a complex structure. It is shown that the Fermi level shifts after light soaking, which can result in activation of the doping impurities.     

Effect of controlled dopant distribution in thin silicon solar cell

Dopant incorporation leading to drift field in a thin silicon solar cell may be controlled and compensated suitably to produce positive or negative drift field. A positive drift field at the base increases short-circuit current density simultaneously decreasing open-circuit voltage, resulting in some net benefit in efficiency for unpassivated front and back surface. In case of well-passivated front and back, however, positive drift field may result in a marginal drop in efficiency. If the dopant incorporation is compensated to produce a small negative drift field, open-circuit voltage increases to such an extent that the overall effect is an increase in efficiency for moderately passivated front and back. Moderate passivation and small negative field seems to produce the best results. The effect may yield 10% gain in overall efficiency for thin cells. Methods of incorporating small negative drift field in LPE grown thin silicon cells are also suggested.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Flexible silicon solar cells

In order to be useful for certain niche applications, crystalline silicon solar cells must be able to sustain either one-time flexure or multiple non-critical flexures without significant loss of strength or efficiency. This paper describes experimental characterisation of the behaviour of thin crystalline silicon solar cells, under either static or repeated flexure, by flexing samples and recording any resulting changes in performance. Thin SLIVER cells were used for the experiment. Mechanical strength was found to be unaffected after 100,000 flexures. Solar conversion efficiency remained at greater than 95% of the initial value after 100,000 flexures. Prolonged one-time flexure close to, but not below, the fracture radius resulted in no significant change of properties. For every sample, fracture occurred either on the first flexure to a given radius of curvature, or not at all when using that radius. In summary, for a given radius of curvature, either the flexed solar cells broke immediately, or they were essentially unaffected by prolonged or multiple flexing.     

Substrates for thin crystalline silicon solar cells

Choice of substrate for thin crystalline silicon solar cells requires a compromise between cost and quality. There are three generic substrate types, namely a transparent substrate (such as glass), an opaque substrate (such as a ceramic or metal) and low-cost multicrystalline silicon. Glass has the advantage of eliminating absorption within the substrate. However, the larger effective diffusion length, the improved surface passivation and the increased process flexibility obtainable with an opaque substrate, particularly low-cost multicrystalline silicon, may considerably outweigh the modest optical benefits of a transparent substrate. In this paper it is shown that the advantage in effective diffusion length that is required of a cell grown on an opaque substrate in order to offset the light-trapping advantages of a glass substrate is about a factor of two.     

Selective emitter using porous silicon for crystalline silicon solar cells

This study is devoted to the formation of high–low-level-doped selective emitter for crystalline silicon solar cells for photovoltaic application. We report here the formation of porous silicon under chemical reaction condition. The chemical mixture containing hydrofluoric and nitric acid, with de-ionized water, was used to make porous on the half of the silicon surface of size 125×125 cm. Porous and non-porous areas each share half of the whole silicon surface. H₃PO₄:methanol gives the best deposited layer with acceptable adherence and uniformity on the non-porous and porous areas of the silicon surface to get high- and low-level-doped regions. The volume concentration of H₃PO₄ does not exceed 10% of the total volume emulsion. Phosphoric acid was used as an n-type doping source to make emitter for silicon solar cells. The measured emitter sheet resistances at the high- and low-level-doped regions were 30–35 and 97–474 Ω/□ respectively. A simple process for low- and high-level doping has been achieved by forming porous and porous-free silicon surface, in this study, which could be applied for solar cells selective emitter doping.     

Crystallized amorphous silicon for low-cost solar cells

Hydrogenated amorphous silicon (a-Si:H), 1–10 μm thick, was deposited onto stainless steel and molybdenum sheets using catholic d.c. glow discharge in a gradient field and by plasma-enhanced chemical vapor deposition. The films were subsequently crystallized by isothermal heating in N₂, rapid thermal processing, isothermal annealing in vacuum (IAV) or isothermal annealing after vycor encapsulation (IAE). All techniques led to crystallization as revealed by X-ray diffraction. Annealing by IAV at 1000 °C for 7 h or IAE at 700 °C for 8 h gave the most intense (111) silicon diffraction peaks. Auger electron spectroscopy showed significant diffusion of iron into the silicon for stainless steel substrates. Energy recoil detection of as-deposited a-Si:H showed good uniformity of both silicon and hydrogen.     

Performance simulation for bifacial silicon solar cells

An exact analytical performance simulation program for designing recent advanced bifacial silicon solar cells has been developed. The simulated performances showed an exact consistency with that of PC-1D for conventional cell structures. The simulated internal quantum efficiency for the bifacial cell structure showed high values in longer wavelength for the cell with shorter carrier lifetime and the performance with additional rear irradiation revealed bifacial cell structures gives a higher output power for the cells with short diffusion lengths.     

Optimum design for bifacial silicon solar cells

In this paper, an optimization technique is used to achieve maximum efficiency for the bifacial buried emitter silicon solar cell (BBESC). The optimum thicknesses of the sub-cells of the BBESC are function of the level of back illumination through its dependence on the degree of the electrical mismatching between the different sub-cells.     

New concepts for high-efficiency silicon solar cells

This paper gives an overview about recent activities in the industrial application of high-efficiency monocrystalline silicon solar cells. It also presents the latest results achieved at Fraunhofer ISE, especially a new patented process for the formation of back-contact points on dielectrically passivated cells called laser-fired contacts and its application to thin wafers.     

High-efficiency silicon space solar cells

SHARP's activities on Si solar cells developments and features of Si solar cells for space use in comparison with GaAs solar cells are presented. Two types of high-efficiency silicon solar cells and the same kinds of high-efficiency solar cells with integrated bypass function (IBF cells) were developed and qualified for space applications. The NRS/LBSF cells and NRS/BSF cells showed an average of 18% and 17% efficiencies, respectively, at AMO and 28°C conditions. The IBF cells have P⁺N⁺ diodes on the front surface to protect itself from reverse voltage due to shadowing. The designs and features of these solar cells are presented. The radiation tests results of these solar cells are also presented. The NRS/BSF cells showed lower degradation rate compared to conventional BSFR cells with the same thickness (100 μm). But the NRS/LBSF cells showed a higher degradation rate than the BSFR cells. The IBF cells showed almost the same radiation characteristics as the same kinds of cells without IBF. The results of radiation tests on these high-efficiency solar cells and the discussions about the radiation characteristics of them are presented. In the last section, the future silicon solar cell development plan is discussed.     

Thin crystalline silicon solar cells

A 50 μm thin layer of high quality crystalline silicon together with efficient light trapping and well passivated surfaces is in principle all that is required to achieve stable solar cell efficiencies in the 20% range. In the present work, we propose to obtain these layers by directly cutting 50 μm thin wafers from an ingot with novel cutting techniques. This development is discussed in the frame of a defect tolerant mass production scenario and aims at obtaining twice the amount of wafers as compared to present wire/slurry technology. The ability to process such mechanically flexible wafers into solar cells with standard laboratory equipment is experimentally verified.     

Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

Concentrator silicon solar cells from silicon industry rejects

Two types of silicon (Si) substrates (40 n-type with uniform base doping and 40 n/n⁺ epitaxial wafers) from the silicon industry rejects were chosen as the starting material for low-cost concentrator solar cells. They were divided into four groups, each consisting of 20 substrates: 10 are n/n⁺ and 10 are n substrates, and the solar cells were prepared for different diffusion times (45, 60, 75 and 90 min). The fabricated solar cells on n/n⁺ substrates (prepared with a diffusion time of 75 min) showed better parameters. In order to improve their performances, particularly the fill factor, 20 new solar cells on n/n⁺ substrates were fabricated using the same procedure (the diffusion time was 75 min)—but with four new front contact patterns. Investigation of current–voltage (I–V) characteristics under AM 1.5 showed that the parameters of these 20 new solar cells have improved in comparison to previous solar cells' parameters, and were as follows: open-circuit voltage (V_OC=0.57 V); short circuit current (I_SC=910 mA), and efficiency (η=9.1%). Their fill factor has increased about 33%. The I–V characteristics of these solar cells were also investigated under different concentration ratios (X), and they exhibited the following parameters (under X=100 suns): V_OC=0.62 V and I_SC=36 A.     

The doping of amorphous silicon for solar cells

Schottky barrier diodes of gold on n-type amorphous silicon give photovoltages dependent on the silicon doping. The best voltages require an undoped layer of silicon for the junction region and approximately 100 nm of heavily doped silicon adjacent to the ohmic substrate contact. There is virtually no carrier collection outside the space charge region, as is shown by the long-wave photocurrent spectral response, and a drift field is desirable. High substrate temperatures during silicon deposition increase the long-wave wave response of these cells but give poorer diodes.     

Rapid thermal technologies for high-efficiency silicon solar cells

This paper shows that rapidly formed emitters in less than 6 min in the hot zone of a conveyor belt furnace or in 3 min in an rapid thermal processing (RTP) system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 min can produce very simple high-efficiency n⁺-p-p⁺ cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/□ emitter and SP Al-back surface field (BSF) formed in a high throughput belt furnace produced 19% FZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and 17% boron-doped CZ by emitter and SP Al-BSF formation in <10 min in a single wafer RTP system. Finally, manufacturable cells with 45 Ω/□ emitter and SP Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the PL cells, the SP cell gave 2% lower efficiency along with a decrease in J_sc and fill factor. This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices     

Porous silicon layer transfer processes for solar cells

A promising cost-effective way of converting sun light into electricity could be a solar cell realized in a thin monocrystalline silicon film, due to its potential to achieve cell efficiencies of more than 20% in a 20 μm thick film. A porous silicon layer transfer technique provides an opportunity to get monocrystalline films on low-cost substrates such as glass. This paper reviews various processes, which are being developed for the layer transfer using porous silicon as a sacrificial layer while reusing of starting silicon substrate. The four basic steps—porous silicon formation, active layer deposition, layer separation and transfer, and device fabrication—have been identified in layer transfer process. The processes have been categorized and compared on the basis of the sequence of steps used in individual processes.     

Oxygen in Czochralski silicon used for solar cells

Compared to the Czochralski (CZ) silicon used in microelectronic industry (M-CZ Si), the annealing behavior of oxygen in the CZ silicon used for solar cells (S-CZ Si) was investigated by means of FTIR and SEM. It was found that the oxygen concentration in S-CZ Si crystal was lower than in the M-CZ Si crystal. During single-step annealing in the temperature range of 800–1100°C, the oxygen in S-CZ Si was hard to precipitate, even if the material contained higher carbon concentrations. After pre-annealing at 750°C, many more oxygen precipitates were formed. The amount and density of the oxygen precipitates were almost the same as in M-CZ Si annealed in single step. It is considered that oxygen has no significant influence on the efficiency of solar cells made from Cz silicon if it is annealed only by a single step in the range of 800–1100°C.