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.     

Present status and future of crystalline silicon solar cells in Japan

Crystalline silicon solar cells show promise for further improvement of cell efficiency and cost reduction by developing process technologies for large-area, thin and high-efficiency cells and manufacturing technologies for cells and modules with high yield and high productivity.In this paper, Japanese activities on crystalline Si wafers and solar cells are presented. Based on our research results from crystalline Si materials and solar cells, key issues for further development of crystalline Si materials and solar cells will be discussed together with recent progress in the field. According to the Japanese PV2030 road map, by the year 2030 we will have to realize efficiencies of 22% for module and 25% for cell technologies into industrial mass production, to reduce the wafer thickness to 50–100 μm, and to reduce electricity cost from 50 Japanese Yen/kWh to 7 Yen/kWh in order to increase the market size by another 100–1000 times.     

Self-aligned locally diffused emitter (SALDE) silicon solar cell

This paper presents, for the first time, a low-cost, high-throughput manufacturing approach for fabricating n-base dendritic web silicon solar cells with selectively doped emitters and self-aligned aluminum contacts using rapid thermal processing (RTP) and screen printing. The self-aligned locally diffused emitter (SALDE) structure is p⁺ nn⁺⁺ where aluminum is screen-printed on a boron-doped emitter and fired in a belt furnace to form a deep self-doped p⁺-layer and a self-aligned positive contact to the emitter according to the well-known aluminum-silicon (Al---Si) alloying process. The SALDE structure preserves the shallow emitter (20.2 μm) everywhere except directly beneath the emitter contact. There the junction depth is greater than 5 μm, as desired, in order to shield carriers in the bulk silicon from that part of the silicon surface covered by metal where the recombination rate is high. This structure is realized by using n-base (rather than p-base) substrates and by utilizing screen-printed aluminum (rather than silver) emitter contacts. Prototype dendritic web silicon (web) cells (25 cm² area) with efficiencies up to 13.2% have been produced.     

Monitoring of silicon solar cell technology via the surface photovoltage method

The surface photovoltage (SPV) technique adapted to thin samples was used to monitor solar cell technology. The relatively short minority carrier diffusion length from 70 to 80 μm found in p-bulk of the cells results from the presence of a layer with structural defects near the surface. The measurement of successively etched samples reveals that freshly cut off silicon wafers are already strongly destroyed to a depth of at least 35 μm. A diffusion length of about 300 μm was evaluated in the samples after removing the disturbed layer.     

Effect of design parameters on the efficiency of the solar cells fabricated using SOI structure

Thin cells demand new grid design concepts in that all the contacts (to the emitter and base) be located on the front surface. Hence, the aim of the investigation is to determine the potential and the basic limitation of the design. With this concept, an interdigitated front grid structure was realized and cells were fabricated through a set of photolithography processes. Confirmed efficiencies of up to 11.5% were achieved on bonded silicon-on-insulator wafers with a cell thickness of 50 μm in the case of finger spacing more than 1000 μm and a base width of 35 μm. It was also shown from the results that to adapt the design rules for optimizing the base fraction and the shadowing fraction was noted as an important technique to realize high-efficiency thin silicon solar cells.     

Mono- and tri-crystalline Si for PV application

Crystalline silicon wafers are by far the dominant absorber materials for today's production of solar cells and modules due to their good price/performance relation and their proven environmental stability. These wafers are mainly produced either by a solar-optimized Czochralski (Cz)-growth method yielding crystalline silicon with low defect density (c-Si) or by a directional solidification or a ribbon growth method yielding large grained multi-crystalline (mc-Si) wafers with higher defect density. To further improve the price/performance relation of Cz solar cells, tri-crystalline silicon (tri-Si) is being developed as a high-quality wafer material that combines both the high diffusion length of minority carriers of up to 1300 μm of c-Si and the productivity of mc-Si. More than 1000 μm LID free diffusion length could be reached with specially doped tri-crystals. Due to an increased mechanical stability tri-Si allows both quasi-continuous pulling and thin slicing with higher mechanical yields. This paper reviews the structural, electronic, and mechanical properties of tri-crystalline silicon wafers with respect to c-Si wafers for solar applications. Actual non-textured solar cells processed with a simple cost effective fabrication process exhibit the same cell efficiencies up to 15.9% for both tri-silicon and mono-silicon wafers. With an improved process, up to 18% cell efficiency can be obtained with textured mono-Si.     

Bow in screen-printed back-contact industrial silicon solar cells

In this paper, we present a model of the bow in thin back-contact silicon solar cells with screen-printed (SP) silver grid metallization. A modification of the bimetallic strip model is used to model the bow for the interdigitated back-contact, emitter-wrap-through (EWT) solar cell. It is proposed that the contact area fraction of the thick regions (>100 nm)of the binder glass at the Ag–Si contact interface responsible for metallization adhesion is an important parameter necessary for modeling the bow for SP back-contact solar cells with better accuracy. Techniques for reducing the bow are also proposed.     

Thin crystalline silicon solar cell bonded to sintered substrate with aluminum paste

Making thinner wafers is a simple way to reduce the production cost of silicon solar cells. However, thin wafers need to be supported mechanically in order to avoid the problem of breakage. Among the several possible supporting materials, silicon substrate made from the sintering of silicon powder, which is produced during the slicing process is the most favorable one because of its abundance and its similar thermal expansion coefficient with silicon wafers. For the bonding of the substrate and thin silicon wafers, aluminum paste is selected because of its compatibility with silicon and the possible BSF effect. Silicon solar cells of 150 μm with the sintered substrate on the back show 5.42% in solar cell conversion efficiency. Compared to commercial silicon cells, lower J_sc is obtained. This might be due to the poor conduction in the back layer of aluminum, which is absorbed into the supporting substrate during the annealing process.     

Substrate to substrate aluminized bonding of 10 cm round silicon substrates for application in solar photovoltaics

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage (V_oc), the short-circuit current density (J_sc), curve factor (CF) and the efficiency (η) were found to be in the range 572–579 mV, 16–19.2 mA cm⁻², 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm⁻² intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.     

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V_oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10⁻³ to 10³ Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.     

Optimization of front surface texturing processes for high-efficiency silicon solar cells

This paper presents the results of an experimental study regarding the increase in the efficiency of the silicon solar cells by texturing the front surface. Designing, patterning and surface etching processes led to refined structures with very low losses of the incident optical radiation. Photolithography has been used to generate patterns (disc hole) through the silicon dioxide layer grown at the beginning on silicon wafers. The holes (4 μm in diameter) have been uniformly distributed on the entire surface (2×2) cm² and the distance between the hole centres was determined to be 20 μm. Semispherical walls have been defined in holes by isotropic etching up to join together of the wells.     

Si-film growth using liquid phase epitaxy method and its application to thin-film crystalline Si solar cell

We have developed a new apparatus for the growth of liquid-phase epitaxy (LPE)-Si films on 5 in Si wafers. We have obtained high growth rates of 0.1–1.0 μm/min and minority-carrier lifetime of average value of 10 μs over the whole of wafer, whereas the thickness uniformity was degraded when rotating the wafers in the solvent. We also demonstrated to growth of LPE-Si films on porous Si layers and to separate the Si films from the porous layers. A 9.5% cell was obtained using a LPE-Si film after separation.     

Solar cells from thin silicon layers on Al2O3

The surface of the applied Al₂O₃ ceramic substrate consists of small crystals with a maximum grain size of 5 μm. A 40 μm thick layer deposited on this surface shows polycrystalline quality and grain sizes in the order of 10 μm. Silicon layers of different thickness and doping have been deposited on Al₂O₃ substrates to make thin film silicon solar cells. These structures have been processed to solar cells in a two mask process. From measurements of the spectral response a diffusion length of 8 μm can be extracted.     

Thin film GaAs solar cells with increased quantum efficiency due to light reflection

Thin film GaAs solar cells, separated from their substrate using the weight-induced epitaxial lift-off technique, were compared with conventional cells on a substrate. The thin film cells can be illuminated from both sides using a mirror. The thickness of the p-type GaAs layer, which is the base layer for front illumination and the emitter layer for rear illumination, was varied between 0.25 and 2.5 μm. For both front and rear illumination, the cell efficiency shows a maximum at a thickness of 1.5 μm. The rear illuminated cell current is only 10% lower than for front illumination. Light reflection in the thin film cell enhances the external quantum efficiency and the collection efficiency in the higher wavelength region from 0.84 to 0.90 and from 0.82 to 0.95, respectively.     

Commercial white paint as back surface reflector for thin-film solar cells

In this work, commercially available white paint is applied as a pigmented diffuse reflector (PDR) on the rear surface of thin-film crystalline silicon (c-Si) solar cells with a silicon thickness in the 1–2 μm range. We show that white paint increases the short-circuit current density of the solar cells enormously, with a boost of 41% observed for very thin planar solar cells illuminated with the global AM1.5 solar spectrum. We also show that white paint is a better back surface reflector (BSR) than aluminium, air, a transparent conductive oxide (TCO)/aluminium stack, and even a detached aluminium mirror. While previous studies have investigated the influence of PDRs on silicon solar cells with thicknesses of over 27 μm, this work closes the gap that has existed for much thinner cells.     

Development of an emitter for industrial silicon solar cells using the doped oxide solid source diffusion technique

The aim of this paper is to demonstrate for the first time the feasibility of fabricating large-area screen-printed monocrystalline silicon solar cells using the Doped Oxide Solid Source (DOSS) diffusion technique. This process was applied to form the n⁺p emitter junction from highly doped sources prepared in a POCl₃ ambient. The diffusions were performed under a pure nitrogen flow in the temperature range 900–1050°C. In this investigation attention was devoted to the influence of the source doping level on the fill factor. The solar cells were fabricated on industrial quality 4-inch Cz wafers using a simple processing sequence incorporating screen-printed contacts and a TiO₂ antireflection coating deposited by spin-on. Fill factors as high as 79% were obtained. The potential benefit of retaining for passivation purposes the thin residual oxide grown during phosphorus diffusion was evaluated. These first experiments led to a cell efficiency close to 10%.     

High efficiency ultra-thin sputtered CdTe solar cells

Large scale manufacturing of CdTe PV modules at the GW/yr level may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in the typically 3–8 μm thick CdTe absorber layer. However, we find that it is possible to reduce the CdTe layer thickness without much compromise in efficiency. The CdS/CdTe solar cells were fabricated using magnetron sputtering with ultra-thin CdTe layers in the range of 0.5–1.28 μm. The ultra-thin films and cells were characterized using X-ray diffraction (XRD), optical transmission, scanning electron microscopy (SEM), current–voltage and quantum efficiency measurements. These results were compared with those of standard 2.3 μm thick CdTe sputtered cells. Different post-deposition processing parameters were required for cells with ultra-thin and standard CdTe thicknesses to achieve high efficiency. Ultra-thin CdTe cells showed crystallographic texture and CdTe_1−xS_x alloy formation after CdCl₂ treatment very similar to standard CdTe cells. Optimization of the post-deposition CdCl₂ treatment and back-contact processing yielded cells of 11.2% efficiency with 0.7 μm CdTe compared to 13.0% obtained with standard 2.3 μm CdTe cells.     

Sliver cells in thermophotovoltaic systems

Previous modelling has indicated that silicon solar cells should be thinner than 100 μm to be optimal for use in thermophotovoltaic (TPV) systems. Sliver cells are a novel type of thin photovoltaic cells fabricated from single crystal semiconductor wafers, with their contacts at the edges of the cell. A computational model was constructed to examine and compare the performance of silicon sliver cells with silicon conventional back-contact cells in TPV systems. Within the range of parameters investigated it was found that the lateral carrier transport resistance of sliver cell geometries limits their power output relative to conventional cells in TPV systems. In practical systems, the efficiencies are comparable.     

A comparative study of the optical properties of nickel pigmented alumina films of different thicknesses exposed to elevated temperature and humidity

Commercial solar absorbers of nickel pigmented anodized aluminium are composed of an inner nickel pigmented sublayer of about 0.3 μm thickness and a 0.4–0.5 μm thick top layer of plain alumina. Thermal emittance can be reduced from 0.17 to 0.12 if the top layer is made thinner, to be about 0.1 μm. The solar absorptance is 0.96 as for the thicker coating. In this study degradation is analysed for samples with thin or thick alumina top layer after exposure to elevated temperature, 300–500°C, or humidity. The results from these tests show that a thinner aluminium oxide top layer has the same durability as a thicker top layer. The implication of making commercial nickel pigmented anodized aluminium with an oxide half as thick as today is a reduction of the anodization time to about half the time and lower manufacturing costs.     

Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock

A solution to the problem of the shortage of silicon feedstock used to grow multicrystalline ingots can be the production of a feedstock obtained by the direct purification of upgraded metallurgical silicon by means of a plasma torch. It is found that the dopant concentrations in the material manufactured following this metallurgical route are in the 10¹⁷ cm⁻³ range. Minority carrier diffusion lengths L_n are close to 35 μm in the raw wafers and increases up to 120 μm after the wafers go through the standard processing steps needed to make solar cells: phosphorus diffusion, aluminium–silicon alloying and hydrogenation by deposition of a hydrogen-rich silicon nitride layer followed by an annealing. L_n values are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.