Polycrystalline silicon solar cells on metallurgical silicon substrates

The use of a polycrystalline silicon p-n junction structure deposited on low-cost substrates is a promising approach for the fabrication of low-cost solar cells. Metallurgical-grade silicon, with a purity of about 98% and a cost of about $1/kg, was cast into plates in a boron nitride container and used as substrates for the deposition of solar cell structures. The substrates were polycrystalline with millimeter size crystallites. Solar cells of the configurations n>⁺-silicon/p-silicon/metallurgical silicon and n⁺-silicon/p⁺-silicon/metallurgical silicon were prepared by the thermal decomposition of silane and the thermal reduction of trichlorosilane containing appropriate dopants. The AMO efficiencies of n⁺-silicon/p-silicon/metallurgical silicon solar cells were up to 2.8% (with no anti-reflection coatings) and were limited by the grain boundaries in the p-layer. The grain boundary effects were reduced by increasing the dopant concentration in the p-layer, and AMO efficiencies of about 3.5% were obtained from n⁺-silicon/p⁺-silicon/metallurgi silicon solar 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.     

Polycrystalline silicon thin-film transistors fabricated by rapid Joule heating method

We report n- and p-channel polycrystalline silicon thin film transistors (poly-Si TFTs) fabricated with a rapid joule heating method. Crystallization of 50-nm-thick silicon films and activation of phosphorus and boron atoms were successfully achieved by rapid heat diffusion via 300-nm-thick SiO/sub 2/ intermediate layers from joule heating induced by electrical current flowing in chromium strips. The effective carrier mobility and the threshold voltage were 570 cm/sup 2//Vs and 1.8 V for n-channel TFTs, and 270 cm/sup 2//Vs and -2.8 V for p-channel TFTs, respectively.     

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.     

Intensity enhancement in textured optical sheets for solar cells

We adopt a statistical mechanical approach toward the optics of textured and inhomogeneous optical sheets. As a general rule, the local light intensity in such a medium will tend to be2 n^{2}(x)times greater than the externally incident light intensity, wheren(x)is the local index of refraction in the sheet. This enhancement can contribute toward a4 n^{2}(x)increase in the effective absorption of indirect-gap semiconductors like crystalline silicon.     

Electrochemically passivated contacts for silicon solar cells

Electrochemically passivated Ti(Pd)Ag contacts for Si solar cells have been developed. The passivation is accomplished by shifting the electrochemical exchange potential of the Ti/Ag couple into positive direction by the addition of a layer of Pd between the Ti and the Ag layers. The new contacts do not degrade during humidity stress tests. They withstand temperature cycling from -- 196°C to +150°C, can be subjected to high temperatures, and are compatible with solderless interconnection schemes.     

Double-layered silicon nitride antireflection coatings for multicrystalline silicon solar cells

Silicon nitride coating possesses both optical antireflection and electrical passivation effects for crystalline silicon solar cells. In this work, we employed a double-layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. Double-layered silicon nitride coating provides a lower optical reflection and better surface passivation than those of single-layered silicon nitride. Details for optimizing the double-layered silicon nitride coating are presented. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the production line for both the double-layered and single-layered cell types. It was statistically demonstrated that the double-layered silicon nitride coating provided a consistent enhancement in the photovoltaic performance of multicrystalline silicon solar cells over those of the single-layered silicon nitride coating.     

Polycrystalline silicon resistors for integrated circuits

The suitability of thin films of doped polycrystalline silicon on SiO₂ substrates for the production of high value resistors for monolithic integrated circuits is considered. Resistors fabricated from this material posses the advantages of high sheet resistivity and dielectric isolation while still preserving an all silicon technology compatible with conventional production techniques.Relevant structural and electrical properties of doped polycrystalline films produced by both vacuum evaporation onto hot substrates with gas-doping and by diffusion-annealing of amorphous films have been investigated. Sheet resistivities and TCR values measured on 2500 Å polycrystalline films have proved superior to those encountered with conventional diffused resistors. Typically films with sheet resistivities of 1 kΩ/□ had TCR's of ?1000 ppm/°C while conventional diffused resistors are generally made from material of 200 Ω/□ and +2000 ppm/°C TCR. Etched resistor line widths of 0·25 mil. have been obtained in the polycrystalline material employing conventional photolithographic techniques. The temperature stability and linearity of doped polycrystalline resistors have been investigated.     

Screening solar cells for improved array performance

Solar cells are produced in batches and undergo a screening process, usually based on the maximum power point. The manufacturer or consumer aggregates the solar cells randomly to form an array. By screening the solar cells and aggregating in a ranking order we may obtain improved array performance. In the present study we investigate several screening criteria of a batch of solar cells and rank them in an order. We also investigate several array topologies constructed by the screened solar cells and compare the results with arrays of randomly selected cells. In terrestrial applications, panels usually form the array. In this case the panel may be viewed as the basic unit for the screening. The results of the study show that improved array performance may be obtained for screened solar cells as opposed to randomly selected cells if part of the solar cells in the batch is used to form the array. If allthe cells are used, no noticeable improvement is seen. For a sufficiently tight manufacturing tolerance there is no need to screen the solar cells. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

High-efficiency ion-implanted silicon solar cells

The development of solar cells with AM1 coversion efficiency of 18 percent is reported. The cells comprise an n⁺-p-p⁺structure fabricated from float zone silicon having resistivity of 0.3 Ω . cm. The n⁺and p⁺regions are formed by low energy ion implantation and thermal annealing. An important feature of cell fabrication is the growth of SiO2passivation for reduction of surface recombination velocity. Details of both cell fabrication and testing are reported.     

Thin monocrystalline silicon solar cells

One of the most effective approaches for a cost reduction of crystalline silicon solar cells is the better utilization of the crystals by cutting thinner wafers. However, such thin silicon wafers must have sufficient mechanical strength to maintain a high mechanical yield in cell and module manufacturing. The electrical performance of thin cells drops strongly with decreasing cell thickness if solar cell manufacturing technologies without a backside passivation or a back-surface-field (BSF) are applied. However, with the application of a BSF, stable efficiencies of over 17%, even with decreasing cell thickness, have been reached. Thin solar cells show lower photodegradation, as is normally observed for Cz-silicon cells with today's standard thickness (about 300 μm) because of a higher ratio of the diffusion length compared to the cell thickness. Cells of about 100-150 μm thickness fabricated with the production Cz-silicon show almost no photodegradation. Furthermore, thin boron BSF cells have a pronounced efficiency response under backside illumination. The backside efficiency increases with decreasing cell thickness and reaches 60% of the frontside cell efficiency for 150 μm solar cells and also for solar modules assembled of 36 cells of a thickness of 150 μm. Assuming, for example, a rearside illumination of 150 W/m², this results in an increased module power output of about 10% relatively     

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.     

An evaluation by solar simulation of radiation damage in silicon solar cells

From the viewpoint of the space power-systems designer, the most useful data for radiation-damaged solar cells is that of output power as a function of cell voltage, temperature, and radiation. This paper reviews the available results from laboratory radiation experiments where solar simulators were used. The solar cells studied were 1 and 10 ohm-cm n-on-p boron-doped cells, 5 and 10 ohm--cm aluminum-doped cells, and dendritic drift-field cells. Most of the experiments use 1 MeV electrons with some data for 0.5 to 2 MeV electrons and 0.5 to 2.7 MeV protons. Comparisons are made between types of cells on the basis of maximum power output and power at a fixed voltage. A fixed voltage is determined for each cell type using the value of cell voltage at maximum power after a 1 MeV electron fluence of 10¹⁶e/cm². There is an apparent lack of agreement among experimental results in the order of 3 or 4 percent, due to spectral variations between simulators. Another reason for the spread in data is attributed to differences that may occur from one group of cells to another, even from the same manufacturer. However, taking this into account, the average power at fixed voltage for the 1 ohm-cm cells is greater than the average for 10 ohm-cm up to a fluence of 5 × 10¹⁵e/cm², where a crossover occurs, and the 10 ohm-cm cells became superior.