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.     

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.     

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.     

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。     

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.     

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.     

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.     

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.     

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.     

Buried-contact silicon solar cells

Recent technological and commercial developments for the buried-contact solar cell (BCSC) are reviwed. Four of the world's largest manufacturers have entered into manufacturing agreements, with a number of these taking advantage of the high-efficiency capabilities of the large-area BCSCs to produce cells for solar cars in the 1990 and 1993 World Solar Challenges and in the solar car race across the USA in 1993. Despite the efficiencies and commercial interest acheived by the conventional structure for the BCSC, a number of areas for improvement remain. In particular, the rear aluminium-alloyed region limits the cell performance, and dislocation generation resulting from stresses at the silicon/silicon dioxide interface can also play a significant role in reducing efficiencies. Through the use of a photolithographically defined rear metal contact, efficiencies in excess of 21% and open-circuit voltages as high as 693 mV for the hybrid BCSC have been demonstrated. the effect of the heavily diffused region beneath the metal contacts in the grooves is studied and its implications for the new generation of BCSCs with grooves on front and rear surfaces are considered. the economic and technological merits of a range of groove formation approaches are discussed, with a low-cost, high-throughput ganged dicing wheel saw with 35 wheels showing most promise.     

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.     

Amorphous silicon solar cells

Amorphous silicon solar cells have been fabricated in several different structures: heterojunctions, p-i-n junctions, and Schottky barrier devices. The procedures used in constructing the various solar cells are discussed, and their photovoltaic properties are compared. At present, the highest conversion efficiency (5.5 percent) has been obtained with a Schottky barrier cell, and this structure appears to offer the best promise of approaching the estimated efficiency limit of ∼ 15 percent.     

薄晶硅太阳电池的研究进展

薄晶硅太阳电池减少硅材料厚度不仅能降低材料消耗和电池成本,还可以赋予其一定的柔韧性,拓展其在可穿戴设备、建筑光伏一体化等领域的潜在应用,成为目前太阳电池领域的研究热点。近年来的研究工作多集中在通过纳米图案化结构、等离激元效应等途径增强薄晶硅对太阳光,尤其是长波长太阳光的吸收,以弥补硅吸收层薄化后引起的吸光能力不足的问题。本文将侧重从图案化纳米结构、等离激元效应增强薄晶硅电池的光吸收性能、薄晶硅太阳电池电学性能的优化、新型薄晶硅太阳电池等方面,对薄晶硅太阳电池的发展现状进行阐述。     

Very thin film crystalline silicon solar cells on glass substratefabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p-type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si laser was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), the open-circuit voltage of 0.539 V and the short current density of 25.8 mA/cm² as independently confirmed by Japan Quality Assurance, which shows the no clear light-induced degradation. The optical and transport properties of poly Si cells are summarized     

Impurities in silicon solar cells

The effects of various metallic impurities, both singly and in combinations, on the performance of silicon solar cells have been studied. Czochralski crystals were grown with controlled additions of secondary impurities. The primary dopants were boron and phosphorus while the secondaires were: A1, B, C, Ca, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, P, Pd, Ta, Ti, V, W, Zn, and Zr. Impurity concentrations ranged from 10¹⁰to 10¹⁷/cm³. Solar cells were made using a conventional diffusion process and were characterized by computer reduction ofI-Vdata. The collected data indicated that impurity-induced performance loss was primarily due to reduction of the base diffusion length. Based on this observation, an analytic model was developed which predicts cell performance as a function of the secondary impurity concentrations. The calculated performance parameters are in good agreement with measured values except for Cu, Ni, and Fe, which at higher concentrations, degrade the cell substantially by means of junction mechanisms. This behavior can be distinguished from base diffusion length effects by careful analysis of theI-Vdata. The effects of impurities in n-base and p-base devices differ in degree but submit to the same modeling analysis. A comparison of calculated and measured performance for multiple impurities indicates a limited interaction between impurities, e.g., copper appears to improve titanium-doped cells.     

Radiation damage in silicon solar cells from low-energy protons

Recent evidence has indicated that kilovolt-energy protons are a probable cause for solar-cell array degradation in synchronous-orbit satellites. This damage can occur in the small areas of the cell that have been unprotected by the coverslip. This paper reports on solar-cell current degradation at fixed voltages in nominal 10-Ω.cm silicon solar cells with coverslips which are irradiated by 150- and 270-keV protons. The damage, which is shown to be dependent on exposed area and proton energy, can be minimized by putting adhesive on uncovered cell areas or by reducing the open area with close-fitting coverslips.