ALLPCB
Overview
Solar cells are a key energy-conversion technology that use sunlight to generate electricity for a wide range of devices. This article summarizes the characteristics and manufacturing approaches of monocrystalline silicon solar cells and polycrystalline silicon thin-film solar cells, with emphasis on materials, processes, and reported performance.
Monocrystalline Silicon Solar Cells
Among silicon-based solar cells, monocrystalline silicon cells offer the highest conversion efficiency and the most mature technology. High-performance monocrystalline cells rely on high-quality monocrystalline silicon and established thermal processing and fabrication techniques. Current monocrystalline cell production typically uses surface texturing, emitter passivation, and selective or local doping. The main cell types are planar monocrystalline silicon cells and grooved buried-contact monocrystalline silicon cells.
Improvements in conversion efficiency mainly come from surface microstructure treatments and selective doping processes. The Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany, has been a leader in these areas. The institute used photolithography to texture the cell surface into an inverted pyramid structure, and combined a 13 nm thick oxide passivation layer with two anti-reflection coating layers on the surface. By optimizing the electroplating process to increase the width-to-height ratio of the metal grid lines, the resulting cells achieved conversion efficiencies above 23%, with a maximum reported value of 23.3%.
Kyocera produced large-area (225 cm2) monocrystalline cells achieving 19.44% conversion efficiency. China’s Beijing Solar Research Institute has also pursued high-efficiency crystalline silicon cell research; their planar high-efficiency monocrystalline cell (2 cm x 2 cm) reached 19.79% conversion efficiency, while a grooved buried-grid crystalline silicon cell (5 cm x 5 cm) was reported with 8.6% conversion efficiency.
Monocrystalline silicon cells remain the dominant choice for large-scale applications and industrial production due to their high efficiency. However, their cost remains relatively high because of the price of monocrystalline silicon material and the complexity of the associated fabrication processes, limiting the potential for large cost reductions.
Polycrystalline Silicon Thin-Film Solar Cells
Conventional crystalline silicon solar cells are fabricated on wafers with thicknesses of about 350-450 μm, which are sawn from grown or cast ingots and therefore consume substantial silicon material. To reduce material use, research since the mid-1970s has focused on depositing polycrystalline silicon thin films on low-cost substrates. Early efforts failed to produce useful cells due to small grain sizes in the deposited films. To obtain larger grains, many methods have been developed and studied.
Today, polycrystalline silicon thin films are commonly prepared by chemical vapor deposition (CVD), including low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD). Liquid-phase epitaxy (LPE) and sputter deposition can also be used to produce polycrystalline silicon thin films. Typical CVD processes use SiH2Cl2, SiHCl3, SiCl4, or SiH4 as precursor gases, depositing silicon atoms onto heated substrates in a protective ambient. Substrate materials usually include Si, SiO2, or Si3N4.
It is difficult to form large grains on non-silicon substrates and voids can occur at grain boundaries. A common solution is to first deposit a thin amorphous silicon seed layer by LPCVD on the substrate, anneal this layer to produce larger grains, and then deposit a thicker polycrystalline silicon film on the recrystallized seed. Recrystallization is therefore a critical step. The main recrystallization techniques include solid-phase crystallization and zone-melting recrystallization. In addition to recrystallization, many of the techniques used for monocrystalline silicon cells are applied to polycrystalline thin-film cells, which has led to improved cell efficiencies. Using zone-melting recrystallization on FZ Si substrates, the Fraunhofer ISE reported polycrystalline silicon cell efficiencies of 19%. Mitsubishi reported efficiencies of 16.42% using similar methods.
Liquid-phase epitaxy (LPE) grows silicon films by dissolving silicon in a melt and precipitating the film by lowering the temperature. Astropower in the U.S. reported cells produced by LPE with 12.2% efficiency. Chen Zheliang of the China Photovoltaic Development Technology Center used liquid-phase epitaxy on metallurgical-grade silicon wafers to grow silicon grains and proposed a novel cell concept called the "silicon-grain" solar cell, although detailed performance reports are limited.
Polycrystalline silicon thin-film cells use far less silicon than monocrystalline cells, do not have the same efficiency-degradation issues observed in certain other thin-film materials, and can potentially be produced on low-cost substrates. Their manufacturing cost is therefore much lower than that of monocrystalline silicon cells, while their efficiency is higher than that of amorphous silicon thin-film cells. As a result, polycrystalline silicon thin-film technology is expected to play an important role in the solar cell market.
