Selective emitters in Si by single step rapid thermal diffusion forphotovoltaic devices

Selective phosphorous diffusion is performed in Si to simultaneously form shallow n⁺p junctions of different depths in the submicron range by rapid thermal annealing (RTA). Low temperature (400°C) atmospheric pressure chemical vapor deposited (APCVD) phosphosilicate glass (PSG) is used as diffusion source. A wide range of n⁺p junctions could be tailored with the same thermal budget by changing only the APCVD-PSG composition. This allows the formation of selectively diffused emitters in different regions of the wafer in one RTA step. 10 cm×10 cm Cz-Si selective emitter photovoltaic (PV) devices are fabricated this way with high energy conversion efficiencies in the range of 17% to 18%     

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.     

A Novel Silicon Photovoltaic Cell Using a Low-Temperature Quasi-Epitaxial Silicon Emitter

A new silicon solar cell fabricated using a low-temperature process is demonstrated with a highly conductive (n+) quasi-epitaxial (qEpi-Si) silicon emitter deposited on silicon substrates, without using transparent conductive oxides. The emitter was formed by a plasma-enhanced chemical vapor deposition process on granular multicrystalline silicon (mc-Si) substrates at a substrate temperature of 250 . The new qEpi-Si/(p)mc-Si junction was found to be of good quality for photovoltaic applications. Solar cells of 1- area and conversion efficiencies exceeding 10% have been fabricated in a simple fabrication process and device structure.     

Advanced manufacturing concepts for crystalline silicon solar cells

An overview is given concerning current industrial technologies, near future improvements and medium term developments in the field of industrially implementable crystalline silicon solar cell fabrication. The paper proves that considerable improvements are still possible, both in efficiency and in production cost. The paper also proves that a lot of effort is being put worldwide on thinner substrates and on thin-film crystalline silicon cells deposited on cheap carriers, in order to save in substrate cost and in order to gain more independence from availability problems of silicon feedback     

Recent results obtained at IMEC on multicrystalline silicon solar cells

In order to optimize the efficiency of multicrystalline silicon solar cells, the influence of specific process steps and sequences were studied. Therefore clean-room high efficiency as well as industrial screen-printed cells were fabricated. Benefits are found in choosing a substrate with lower base resistivity, using front and rear oxide passivation, using hydrogen passivation for bulk and surfaces, the use of Si₃N₄ with a double function i.e. as an anti-reflection and passivation layer and the use of mechanical V-grooving. Efficiencies of 17% are found on 4 cm² clean-room fabricated cells and 15.2% has been obtained on 100 cm² V-grooved screenprinted industrial cells.     

Improving low-temperature APCVD SiO2 passivation byrapid thermal annealing for Si devices

The quality of low-temperature (≈400°C) atmospheric pressure chemical vapor deposited (APCVD) silicon dioxide (SiO₂ ) films has been improved by a short time rapid thermal annealing (RTA) step. The RTA step followed by a low temperature (400°C) forming gas anneal (FGA) results in a well-passivated Si-SiO₂ interface, comparable to thermally grown conventional oxides. Efficient and stable surface passivation is obtained by this technique on virgin silicon as well as on photovoltaic devices with diffused (n⁺p) emitter surface while maintaining a very low thermal budget. Device parameters are improved by this APCVD/RTA/FGA passivation process     

Three-dimensional modeling and simulation of p-n junction spherical silicon solar cells

A three-dimensional numerical model is presented to simulate spherical p-n junction silicon solar cells, which is a promising new technology for photovoltaic (PV) energy conversion for terrestrial applications. Material properties imposed by the sphere formation method, geometry of the device, and the specific device structure stemming from the fabrication technology are taken into account in the optical and electrical models of the device. The spherical device is numerically simulated based on these models using finite-difference method in a spherical system of coordinates, generating the internal quantum efficiency and current-voltage (I-V) characteristics of the device. It has been shown that the efficiency of a spherical solar cell is slightly lower than a conventional device; however, the slightly inferior performance does not outweigh the cost advantage. It has been also found that subsurface diffusion length from effective impurity segregation and the depth of the denuded zone in spherical devices are parameters that mainly affect the device efficiency. Based on the simulation and analysis, design guidelines have been presented for spherical PV devices.     

Development of RTP for industrial solar cell processing

Rapid Thermal Processing (RTP), originally developed for processing microelectronic devices has been investigated in the recent decade for its potential in the production of Si solar cells. This paper will discuss the use of RTP for industrial Si solar cells with screen-printed contacts. Printed metal contacts require adapted emitters when good fill factors should be achieved. Multi-crystalline Si substrates require to adapt the temperature ramps of RTP to avoid minority carrier lifetime degradation from activated defect centres. Finally, industrial processing requires high throughput that cannot be achieved with conventional RTP equipment. This paper will present an advanced selective emitter process and a recently developed continuous RTP system that meet for the first time the requirements to make RTP compatible with industrial solar cell processing. The limits of industrial RTP solar cell processing will be discussed.     

Trends in industrial silicon solar cell processes

The performance and technology of industrial silicon solar cells have improved considerably in recent years. Conversion efficiencies exceeding 18% are reproducibly obtained by cost-effective technologies on large area Cz-silicon. The performance of multicrystalline silicon cells is closing-in at 17.2%. Improved material casting techniques, a refined technology, and efficient in-process material improvement techniques are found to be the major causes behind such advancement. The trend to towards thinner substrates leads to considerable material cost reduction while yielding better performance. The major processing technologies and steps are critically discussed in this article, keeping in mind the priorities of today's PV industry: cost, and environmental issues. The future trends of the technology are outlined.     

Low-cost industrial technologies of crystalline silicon solar cells

Approximately 2 billion people, mainly in Third World countries, are not connected to an electric grid. The standard, centralized grid development is too expensive and time consuming to solve the energy demand problem. Therefore, there is a need for decentralized renewable energy sources. The main attractiveness of solar cells is that they generate electricity directly from sunlight and can be mounted in modular, stand-alone photovoltaic (PV) systems. Particular attention is paid in this paper to crystalline silicon solar cells, since bulk silicon solar-cell (mono and multi) modules comprise approximately 85% of all worldwide PV module shipments. Energy conversion efficiency as high as 24% has been achieved on laboratory, small-area monocrystalline silicon cells, whereas the typical efficiency of industrial crystalline silicon solar cells is in the range of 13-16%. The market price of PV modules remains for the last few years in the range of $3.5-4.5/watt peak (Wp). For the photovoltaic industry, the biggest concern is to improve the efficiency and decrease the price of the commercial PV modules. Efficiency-enhancement techniques of commercial cells are described in detail. Adaptation of many high-efficiency features to industrially fabricated solar cells. The latest study shows that increasing the PV market size toward 500 MWp/y and accounting for realistic industrial improvements can lead to a drastic PV module price reduction down to $1/Wp