Passivating thin bifacial silicon solar cells for industrial production

A scheme for passivating thin multi‐crystalline silicon solar cells compatible to mass production is presented. Wafers with a thickness of 180 µm were processed into solar cells. The otherwise severe bowing has been avoided by reduced aluminium coverage on the rear surface. The process scheme includes a silicon nitride firing through step for conventional screen printed contacts, where a silicon nitride layer on the rear surface acts as surface passivation layer and enables a gain in efficiency of 0.6% [abs.]. Copyright © 2007 John Wiley & Sons, Ltd.     

New insights gained through pilot production of high-eficiency silicon solar cells

Examination of results from the pilot production of buried-contact solar cells (BCSC) allows several new insights into the effects of the substrate resistivity, the differences between upright and inverted pyramid texturing, the reflection after encapsulation and the doping level at which the emitter begins to dominate the overall recombination. A lower substrate resistivity in conjunction with thicker wafers reduces the effects of a high back surface recombination velocity and allows both higher voltages and efficiencies. In BCSCs with low substrate resistivities, the voltage is not limited by the back but by the emitter diffusion and the dislocation formation at the surface. Contrary to previous reports, best results have been realized with upright pyramids rather than inverted pyramids. In addition, the relative performance of the upright pyramids improves after encapsulation owing to the less than optimal unencapsulated reflection of these surfaces in the regions of the pyramid peaks where oxide layers are too thin to gain benefits as an antireflection layer. Recent results also indicate that the contributions to the dark saturation current from both the heavily phosphorus-diffused region beneath the metal contact and the more lightly diffused top surface emitter are less than indicated previously. Finally, comparison between experimentally obtained voltages and those predicted through modelling with PC-1D provides an estimate of the bulk material lifetimes in the pilot line cells.     

Polycrystalline silicon solar cells from recrystallized plasma deposited thin films

Large grain polycrystalline silicon films are produced by a two step process involving plasma deposition of microcrystalline silicon films on a substrate, separation from the substrate, and subsequent grain enhancement of the silicon films. The effects of doping and substrate temperature during deposition on the solar cell conversion efficiency are investigated. Effects of ppm level molybdenum contamination from the substrate, and silicon microstructure after grain enhancement, on solar cell efficiency parameters are also investigated. Solar cells with efficiencies of up to 10.1% under AM1 illumination, were fabricated on these silicon films.     

Schottky solar cells on thin epitaxial silicon

Schottky solar cells fabricated on 10, 20 and 30 μm epitaxial silicon produce a current density ranging from about 10–22 mA/cm², depending on Si thickness and orientation, in close agreement with theoretically predicted data. These results are also in close agreement with recent data on p-n solar cells, using thin epitaxial silicon. Data reported herein predict that 10% efficient Schottky solar cells could be produced using about 20 μ of silicon on a suitable substrate. A 7.6% efficient Schottky solar cell on epitaxial silicon has been recently fabricated and tested using AM1 sunlight (100 mW/cm²).     

Increased photogeneration in thin silicon concentrator solar cells

Recent progress in silicon concentrator solar cells has resulted in several designs capable of 25-percent efficiency with one group reporting 28 percent under 14 W/cm²of incident power at 25°C. It has been shown that further improvement is possible by restricting the sunlight acceptance angle of the cell. In this letter, a practical implementation which is equivalent in its effect is proposed which results in an increased utilization of weakly absorbed near-bandgap light. This increased absorption is obtained by placing the cells in a cavity with a small entrance aperture. An analysis is given based upon work on the acceptance angle enhancements by Campbell and Green. The design is expected to improve the efficiencies of existing solar cells to 30 percent. If used in conjunction with previously proposed cell improvements, the efficiencies will be improved towards 33 percent, very near the limit efficiency of 36 percent. This design also has the effect of decreasing the differences in performance between the leading candidate concentrator cell designs and diminishing the dependence of the efficiencies on the cell texturization and bulk carrier lifetimes.     

TCOs for nip thin film silicon solar cells

Substrate configuration allows for the deposition of thin film silicon (Si) solar cells on non‐transparent substrates such as plastic sheets or metallic foils. In this work, we develop processes compatible with low T_g plastics. The amorphous Si (a‐Si:H) and microcrystalline Si (µc‐Si:H) films are deposited by plasma enhanced chemical vapour deposition, at very high excitation frequencies (VHF‐PECVD). We investigate the optical behaviour of single and triple junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of low pressure CVD (LP‐CVD) ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 µm thick LP‐CVD ZnO. We observe that, for a‐Si:H, the cell performance depends critically on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a‐Si:H, a thick LP‐CVD ZnO front contact provides more light trapping on the 2D periodic substrate. Then, we investigate the influence of the thick and thin TCOs in conjunction with thick absorbers (µc‐Si:H). Because of the different nature of the optical systems (thick against thin absorber layer), the antireflection effect of ITO becomes more effective and the structure with the flat TCO provides as much light trapping as the rough LP‐CVD ZnO. Finally, the conformality of the layers is investigated and guidelines are given to understand the effectiveness of the light trapping in devices deposited on periodic gratings. Copyright © 2008 John Wiley & Sons, Ltd.     

Novel porous silicon backside light reflector for thin silicon solar cells

High efficiencies of thin crystalline Si solar cells grown on highly doped substrates have been reported. We propose porous Si layers located near the interface of the active layer and the substrate to introduce an optical confinement into these cells. We report on the experimental proof of the principle for this novel type of back-surface reflector. Spectral reflectance measurements agree well with computer simulations. On the basis of this agreement, we calculate the enhancement of short-circuit current densities due to porous reflectors for textured and non-textured cells. These simulations are of particular relevance for multicrystalline Si cells on foreign substrates and for space cells. Copyright © 1998 John Wiley & Sons, Ltd.     

Conformal films for light-trapping in thin silicon solar cells

The anti-reflection and light-trapping performance of thin silicon films deposited conformally on V-grooved substrates is investigated by ray-tracing for a wide range of groove angles and period widths. The tracing is done with incident light rays at angles representative of typical Sydney yearly illumination. Using a double-layer antireflection coating and a SiO₂/Ag reflector, the best result predicted was for asymmetric grooves, with a short-circuit current density of 35.2 mA/cm² for a deposited film thickness of 6μm, which is 96% of what could be achieved with a perfectly randomising cell of the same silicon volume per unit module area. For best results, the period of the texture should not be very much greater than the thickness of the film. Larger periods are useful if combined with partially randomising surfaces. In both cases, a mechanism that gives escaping rays a good chance of re-entering the silicon is responsible for the good performance. Performance is limited by glass reflection and the restriction of scatter to only two dimensions. New structures are proposed to overcome these limitations.     

Alternative contact designs for thin epitaxial silicon solar cells

Two major opportunities for increasing the performance of crystalline silicon solar cells involve reducing their thickness and reducing the losses associated with their front metallic grid contacts. Front grid contacted thin epitaxial silicon solar cells based on the growth of crystalline silicon films on a substrate or superstrate have been reported for many years, as have wafer‐based solar cells with alternative contact approaches. Integrating these two concepts into a single device presents an opportunity for simultaneously reducing two major loss mechanisms associated with crystalline silicon solar cells. The opportunities that exist and challenges that must be overcome in order to realize such a device are described in this paper. The design space is defined and explored by considering a wide range of possible approaches. A specific approach was chosen and used to design, grow, and fabricate a proof‐of‐concept thin epitaxial silicon solar cell with an embedded semiconductor grid as an alternative to a conventional front metallic grid. The work presented here has resulted in a thin epitaxial silicon solar cell with a 7·8% designated area conversion efficiency, well isolated contacts, negligible series resistive power loss, and less than 1% shading of the designated area. Copyright © 1999 John Wiley & Sons, Ltd.     

Theoretical comparison of conventional and multilayer thin silicon solar cells

Thin solar cells based on low-quality silicon are assessed for a range of possible material parameter values and device structures. Device thickness is freely optimized for maximum efficiency for a range of doping densities and numbers of junctions, le ading to results differing markedly from previous investigations. Modelling of conventional and multilayer structures in this paper indicates little difference in efficiency potential on low-lifetime (<50 ns) crystalline silicon layers. Moderate effici encies (>15%) are possible given adequate light trapping. Conventional structures (single and double junction cells) are superior if excellent light trapping is assumed. Thicker multilayer structures are advantageous in the case of poor light trapping or surface passivation. In an optimized cell in low-quality silicon, increasing the number of junctions allows a high current to be maintained, but at the cost of a reduced voltage and fill factor caused by increased junction recombination. Formidable pra ctical difficulties are likely to be encountered to realize the theoretical performances discussed.     

非晶硅薄膜太阳能电池应用开发新进展

非晶硅（ａ－Ｓｉ）薄膜太阳能电池是取之不尽的洁净能源－太阳能的光电元（组）件。文章详述了ａ－Ｓｉ薄膜太阳能电池的工艺优势，市场开发状况，可能应用领域，存在问题和展望。     

p-Layer bandgap engineering for high efficiency thin film silicon solar cells

Spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and optical transmittance measurements were used to study and establish a correlation between the open-circuit voltage (V_oc) of solar cells and the p-layer optical band gap (E_p). It is found that the ellipsometry measurement can be used as an inline non-destructive diagnostic tool for p-layer deposition in commercial operation. The analysis of ellipsometric spectra, together with the optical transmittance data, shows that the best p-layer appears to be very fine nanocrystallites with an E_p 1.95 eV. HRTEM measurements reveal that the best p-layer is composed of nanocrystallites ~9 nm in size. It is also found that the p-layer exhibits very good transmittance, as high as ~91.6% at ~650 nm. These results have guided us to achieve high V_oc value 1.03 V for thin film silicon based single junction solar cell.     

工业化生产硅太阳电池的特性分析

分析了工业化生产的硅太阳电池中，影响其短路电流、开路电压和填充因子等充电特性的因素及改善方法。     

Cr-MIS solar cells using thin epitaxial silicon grown on poly-silicon substrates

Cr-MIS solar cells were fabricated on 18-30 µm epitaxial-Si layers grown on poly-Si substrates. Solar conversion efficiency values ranged from an average of 8.8% to 4.0% depending on choice of substrate. Nonuniformity of certain substrates led to low efficiency values. Interface state density > 5 × 10¹²/cm²-eV contributed to low Vocand high n-factor. Low minority carrier diffusion length caused Jscto drop to 60% of the optimum value. Substrates with imperfections caused an increase in dark current density by three orders of magnitude, which served to decrease photovoltaic response. The procedures given herein could lead to a low-cost solar cell for terrestrial applications.     

Angular behavior of the absorption limit in thin film silicon solar cells

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n² limit can be potentially exceeded in a wide angular and wavelength range using two‐dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure, but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure that is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances that can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one‐dimensional gratings, and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to obtain more practical values for the absorption enhancement, we consider the effect of parasitic loss that can significantly reduce the enhancement factor. Copyright © 2013 John Wiley & Sons, Ltd.     

Survey of material options and issues for thin film silicon solar cells

The high production cost of thick high-efficiency crystalline silicon solar cells inhibits widespread application of photovoltaic devices whereas the most developed of thin film cell technologies, that based on amorphous silicon, suffers inherent instability and low efficiency. Crystalline thin-film silicon solar cells offer the potential for a long-term solution for low cost but high-efficiency modules for most applications. This paper reviews the progress in thin-film silicon solar cell development over the last two decades, including progress in thin-film crystal growth, device fabrication, novel cell design, new material development, light trapping and both bulk and surface passivation. Quite promising results have been obtained for both large-grain (>100 μm) polycrystalline silicon material and the recently developed microcrystalline silicon materials. A novel multijunction solar cell design provides a new approach to achieving high-efficiency solar cells from very modest quality and hence low-cost material. Light trapping is essential for high performance from thin-film silicon solar cells. This can be realized by incorporating an appropriate texture on the substrate surface. Both bulk and surface passivation is also important to ensure that the photogenerated carriers can be collected effectively within the thin-film device. © 1998 John Wiley & Sons, Ltd.     

A life cycle assessment of perovskite/silicon tandem solar cells

Given the rapid progress in perovskite solar cells in recent years, perovskite/silicon (Si) tandem structure has been proposed to be a potentially cost‐effective improvement on Si solar cells because of its higher efficiency at a minimal additional cost. As part of the evaluation, it is important to conduct a life cycle assessment on such technology in order to guide research efforts towards cell designs with minimum environmental impacts. Here, we carry out a life cycle assessment to assess global warming, human toxicity, freshwater eutrophication and ecotoxicity and abiotic depletion potential impacts and energy payback time associated with three perovskite/Si tandem cell structures using silver (Ag), gold (Au) and aluminium (Al) as top electrodes compared with p–n junction and hetero‐junction with intrinsic inverted layer Si solar cells. It was found that the replacement of the metal electrode with indium tin oxide/metal grid in the tandem cell reduces the environmental impacts significantly compared with the perovskite cell. For all the impacts assessed, we conclude that the perovskite/Si tandem using Al as top electrode has better environmental outcomes, including energy payback time, when compared with the other tandem structures studied. Use of Al in preference to noble metals for contacts, Si p–n junction in preference to intrinsic inverted layer and the avoidance of 2,20,7,70‐tetrakis(N ,N‐di‐p‐methoxyphenylamine)9,90‐spirobifluorene (Spiro‐OMeTAD) are environmentally beneficial. The key result found of this work is that the most important factor for the better environmental impacts of these tandem solar cells is the transparency and electrical conductivity of the perovskite layer after it fails. Copyright © 2017 John Wiley & Sons, Ltd.     

21.5% Efficient thin silicon solar cell

Although many calculations since the early 1980s have predicted that high performance in thin crystalline silicon cells is feasible, performance levels demonstrated in the past have been quite modest. Using a self-supporting silicon membrane, experimen tal energy conversion efficiency above 20% is described for the first time for a silicon cell of less than 50 μm thickness, with efficiency up to 21.5% independently confirmed for a 47-μm thick device. The cells demonstrate a better ability to tra p light internally within their structure than any previously measured device. They also demonstrate the surface passivation benefits of the recently described parallel multijunction thin-film silicon cell approach.     

Photocurrent enhancement in thin film amorphous silicon solar cells with silver nanoparticles

Silver nanoparticles embedded in a dielectric material have strong scattering properties under light illumination, due to localized surface plasmons. This effect is a potential way to achieve light trapping in thin‐film solar cells. In this paper we study light scattering properties of nanoparticles on glass and ZnO, and on silver coated with ZnO, which represent the back reflector of a solar cell. We find that large nanoparticles embedded in the dielectric at the back contact of amorphous silicon solar cells lead to a remarkable increase in short circuit current of 20% compared to co‐deposited cells without nanoparticles. This increase is strongly correlated with the enhanced cell absorption in the long wavelengths and is attributed to localized surface plasmons. We also discuss the electrical properties of the cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Point-contact silicon solar cells

A new type of silicon photovoltaic cell designed for high-concentration applications is presented. The device is called the point-contact-cell and shows potential for achieving energy conversion efficiencies in the neighborhood of 28 percent at the design operating point of 500× geometric concentration and 60°C cell temperature. This cell has alternating n and p regions that form a polkadot array on the bottom surface. A two-layermetallization on the bottom provides contact. Initial experimental results have yielded a cell with 20-percent efficiency at a concentration of 88.