S.R. Wenham

High-efficiency silicon solar cells

Silicon solar cells are described which operate at energy conversion efficiencies independently measured at 18.7 percent under standard terrestrial test conditions (AM1.5, 100 mW/cm2, 28°C). These are apparently the most efficient silicon cells fabricated to date. The high-efficiency results from a combination of high open-circuit voltage due to the careful attention paid to the passivation of the top surface of the cell; high fill factor due to the high open-circuit voltage and low parasitic resistance losses; and high short-circuit current density due to the use of shallow diffusions, a low grid coverage, and an optimized double layer antireflection coating.

Progress and outlook for high-efficiency crystalline silicon solar cells

The last 15 years have seen large improvements in crystalline silicon solar cells, with efficiencies improved by over 50%. The main drivers have been improved electrical and optical design. Electrical improvements include improved passivation of contact and surface regions and a reduction in the volume of heavily doped cell material. Optically, reduced reflection and improved light trapping within the cell have had a large impact. Such features have increased silicon cell efficiency to a recently confirmed 24.7%. Recently, progress has been made in transferring some of the corresponding design improvements into production with commercial cells of 17–18% efficiency now available, world record values of a mere 15 years ago. The theory supporting these improvements in bulk cell efficiency shows that thin silicon layers, only a micron or so in thickness, are capable of comparably high efficiency.

45% efficient silicon photovoltaic cell under monochromatic light

Improvements in the performance of silicon photovoltaic cells for solar applications are adapted for nonsolar photovoltaic applications. Improved monochromatic light efficiencies above 45% are reported including efficiencies close to 40% for relatively long-wavelength (1.064 mu m) light as produced by neodymium-doped yttrium-aluminum garnet (Nd:YAG) lasers.<<ETX>>

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.

Mass production of the innovative PLUTO solar cell technology

Following 6 years of intensive R&D at Suntech Power, the world record holding PERL cell design from The University of New South Wales (UNSW) has been successfully commercialized with product sales of the PLUTO technology to commence in mid 2009. This innovative new technology is having a major impact on production activities and plans for Suntech Power, the worlds largest silicon solar cell manufacturer, with more than 100MW production capacity already installed, increasing to 300MW by the end of 2009. GW levels of PLUTO production capacity are planned by the end of 2011. Successful commercialization has required the simplification or elimination of all the expensive and sophisticated processes and materials from the record PERL cells, while simultaneously achieving high device efficiencies, production yields and throughput. A simplified version of the PLUTO technology can be easily retrofitted onto existing screen-printing production lines, with similar costs per unit area to conventional screen printed solar cells but with a 10–15% performance advantage over the latter fabricated on juxtaposed production lines using essentially the same equipment, wafers and materials.

Buried contact silicon solar cells

Abstract Despite the growing commercial success of the conventional buried contact solar cell (BCSC), significant improvements in cell performance, simplicity of fabrication, and costs are being made. Five strands to this work with five corresponding variations of the cell structure are responsible for the developments. Hybrid solar cells (standard BCSC front surface with photolithographically defined rear metal contact scheme) have demonstrated open-circuit voltages approaching 700 mV while a simpler cell design requiring no photolithography has demonstrated open-circuit voltages as high as 685 mV. Large area, 20 sun BCSC concentrator cells have been developed with very low metal shading losses (below 3%) due to the redesigning of the groove structure to recess the metal to below the top surface. The resulting record efficiency of 21.5% has been independently confirmed (Sandia). The most recent BCSC structure, where the emphasis is on simplicity and low cost, has the number of high temperature processing steps reduced to one, while efficiencies in the vicinity of those achieved by the conventional BCSC are anticipated. The highest efficiencies demonstrated to date with any of the BCSC structures are well above 21% (Sandia) although all five variations of the BCSC structure appear capable of achieving similar performance levels in the future.

Recombination rate saturation mechanisms at oxidized surfaces of high‐efficiency silicon solar cells

Shoulders have been observed in the measured semilogarithmic current‐voltage (I–V) characteristics of high‐efficiency passivated emitter and rear locally diffused silicon (Si) solar cells. An improved understanding is given of the mechanism proposed to cause these nonideal I–V curves. It is shown that this mechanism is due to the electrostatic behavior of free carriers at the Si/SiO2 interface of oxidized Si devices in which the Si adjacent to the oxide is depleted (or in some cases, inverted) at equilibrium, and results in saturation of the surface recombination rate. Two‐dimensional numerical computer simulations are used to investigate this mechanism and its effect on cell performance. In addition, the simulations provide a means of estimating the extent to which lateral conduction in the rear surface channel also contributes to the observed recombination saturation in these cells. It is shown that ohmic limitation of lateral conduction occurs, however, the lateral current flows are negligible in compa...

Light trapping and reflection control in solar cells using tilted crystallographic surface textures

Abstract Surface textures made of crystallographically oriented facets are examined for their capacity to enhance absorption and reduce reflection in solar cells. Some that employ microstructures with their axes of symmetry tilted from the surface normal can almost eliminate broadband top surface reflection and enhance the pathlength of weakly absorbed radiation as well as any schemes previously suggested. For example, tilting the axes of upright pyramids by about 24° towards the same base corner is estimated to enhance absorption of the solar spectrum in a 280 μm cell by up to 4.5%. Practical issues concerning the fabrication of “tilted” structures and suitability for reducing back surface losses are also discussed.

685 mV open-circuit voltage laser grooved silicon solar cell

Abstract The recombination limiting the voltage of the present buried contact solar cell (BCSC) can be reduced by replacing the present high recombination sintered aluminium back with a floating rear junction for passivation, heavy boron diffusion below the rear contact, and by limiting the rear surface contact area. Analysis of these implementations in the double sided laser grooved (DSLG) structure shows that the floating junction passivation is effective in reducing the recombination component at the rear surface and that the boron diffusion in the rear groove comprises up to half of the total saturation current. Limiting the area of the heavily diffused boron grooves allows open-circuit voltages of 685 mV while maintaining the simplicity of the BCSC processing sequence. An open-circuit voltage of 685 mV represents nearly a 50 mV increase over the conventional BCSC.

Anodic Aluminum Oxide Passivation For Silicon Solar Cells

The requirement to form localized rear metal contact regions for higher silicon solar cell efficiencies places demand on patterning techniques in terms of the small size of the openings and the ability to perform the patterning at commercial wafer processing rates. We suggest here the possibility of using a self-patterning approach which offers the potential of enhanced surface passivation and nanoscale patterning achieved using a single electrochemical anodization process. It is shown that when nanoporous anodic aluminum oxide (AAO) layers are formed by anodizing an aluminum layer over an intervening SiO2 or SiN x dielectric layer, the implied open-circuit voltages of p-type silicon test structures can be increased by an average of 40 and 47 mV, respectively. Capacitance-voltage measurements show that these passivating AAO dielectric stack layers store positive charges, which differs from what is observed for Al2O3 layers deposited by plasma-enhanced chemical vapor deposition or atomic layer deposition. Furthermore, we show that the magnitude of the stored charge in the dielectric stacks depends on the anodization conditions, highlighting the possibility of controlling the charge storage properties of these layers for specific cell requirements. Although the passivating properties of the anodized aluminum layer appear to be strongly influenced by charge effects, it is also possible that hydrogenation effects may play a role as has been previously observed for other electrochemical processes, such as metal plating.