Ly Mai

18.7% efficient laser-doped solar cell on p-type Czochralski silicon

The use of laser doping in solar cell fabrication has received increased attention in recent years, especially due to its ability to form a selective emitter without subjecting the wafer to prolonged high-temperature processes. At the University of New South Wales, a laser doping method was developed that combines the formation of the selective emitter with a self-aligned metallization pattern. This letter reports 18.7% efficient laser-doped solar cells, fabricated on large area commercial-grade p-type Czochralski silicon, and analyzes the loss mechanisms.

Rear junction laser doped solar cells on CZ n-type silicon

N-type silicon (Si) has been shown to have generally higher bulk lifetimes and far better post illumination performance stability compared to boron doped p-type materials of similar crystallographic quality. In particular, the high minority carrier diffusion lengths in n-type wafers makes the rear emitter n+np+ structure an attractive option, especially when incorporated with screen printing as a simple and cost effective way to create an Al-alloyed junction on the back surface. However, when screen printing is used to apply the front contacts, its wide metal lines and its requirement for a heavy Phosphorus-doped front surface significantly reduces the performance of this simple device with the latter limiting the blue wavelength response and surface passivation quality. Recently, the laser doping process has been shown capable of overcoming these major drawbacks due to its ability to produce a selective emitter. In this present work, an innovative application of the laser doping process in the fabrication of such rear Al-alloyed emitter n+np+ device enables an excellent energy conversion efficiency of 18.2% to be achieved on commercial grade n-type CZ wafers (148.6cm2).

Laser Enhanced Hydrogen Passivation of Silicon Wafers

The application of lasers to enable advanced hydrogenation processes with charge state control is explored. Localised hydrogenation is realised through the use of lasers to achieve localised illumination and heating of the silicon material and hence spatially control the hydrogenation process. Improvements in minority carrier lifetime are confirmed in the laser hydrogenated regions using photoluminescence (PL) imaging. However with inappropriate laser settings a localised reduction in minority carrier lifetime can result. It is observed that high illumination intensities and rapid cooling are beneficial for achieving improvements in minority carrier lifetimes through laser hydrogenation. The laser hydrogenation process is then applied to finished screen-printed solar cells fabricated on seeded-cast quasi monocrystalline silicon wafers. The passivation of dislocation clusters is observed with clear improvements in quantum efficiency, open circuit voltage, and short circuit current density, leading to an improvement in efficiency of 0.6% absolute.

New Emitter Design and Metal Contact for Screen-Printed Solar Cell Front Surfaces

A new emitter design incorporating semiconductor fingers has been developed for use with conventional screen-printing technology. The primary advantage of this new design is that it alleviates the need for a heavy top surface diffusion and therefore overcomes the major weaknesses traditionally associated with screen-printed solar cells, namely a poor response to short wavelengths of light and a high top surface metal shading loss. Importantly, the new technology is compatible with all existing screen-printing equipment and infrastructure. Direct comparison between conventional screen-printed cells and those incorporating semiconductor fingers shows the latter to have about a 10% performance advantage when fabricated on a large scale manufacturers production line. Efficiencies about 18% have been demonstrated on 150cm2 devices with the new technology planned for large scale manufacturing by the end of 2006

Investigation of Al-Doped Emitter on N-Type Rear Junction Solar Cells

This brief models the junction discontinuities of a rear Al-doped p⁺ emitter (np⁺) formed by screen printing and firing. Theoretical fitting of the suns-<i>V</i> _oc data to the circuit model shows that not only do the junction discontinuities deteriorate cell <i>V</i> _oc, for the case of p-type cells, but they also reduce cell fill factor on n-type cells through increased junction recombination and nonlinear shunts.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm2 of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Rapid mitigation of carrier-induced degradation in commercial silicon solar cells

We report on the progress for the understanding of carrier-induced degradation (CID) in p-type mono and multi-crystalline silicon (mc-Si) solar cells, and methods of mitigation. Defect formation is a key aspect to mitigating CID. Illuminated annealing can be used for both mono and mc-Si solar cells to reduce CID. The latest results of an 8-s UNSW advanced hydrogenation process applied to industrial p-type Czochralski PERC solar cells are shown with average efficiency enhancements of 1.1% absolute from eight different solar cell manufacturers. Results from three new industrial CID mitigation tools are presented, reducing CID to 0.8–1.1% relative, compared to 4.2% relative on control cells. Similar advanced hydrogenation processes can also be applied to multi-crystalline silicon passivated emitter with rear local contact (PERC) cells, however to date, the processes take longer and are less effective. Modifications to the firing processes can also suppress CID in multi-crystalline cells during subsequent illumination. The most stable results are achieved with a multi-stage process consisting of a second firing process at a reduced firing temperature, followed by extended illuminated annealing.

Low-Absorbing and Thermally Stable Industrial Silicon Nitride Films With Very Low Surface Recombination

Amorphous silicon nitride has become the state-of-the-art antireflection coating for silicon solar cells. Optimization of silicon nitride films requires consideration of both the films optical and electrical properties. It is commonly assumed that silicon-rich silicon nitride films (films with high refractive index) provide better surface passivation, compared to that obtained by films with lower indices. However, silicon-rich films are usually very absorptive in the short (and even medium) wavelength range. Development of low absorption silicon nitride films, that provide good surface passivation, is therefore highly valuable. In this study we compare nine different industrial silicon nitride films, all with similarly low refractive index of 2.09 ± 0.01 measured at 633 nm. We demonstrate that these films exhibit very different electrical, chemical, and optical properties despite their similar refractive index values and correlate these differences with the specific deposition conditions. As a result of this investigation, we have developed industrial thermally stable low-absorbing silicon nitride films that provide excellent surface passivation, with surface saturation current density of 7 fA/cm2 on both n- and p-type wafers. We demonstrate that the developed low absorption films provide surface passivation with equal quality to that obtained by industrial silicon-rich silicon nitride films.

Should the refractive index at 633 nm be used to characterize silicon nitride films

The refractive index at 633 nm is often used to characterize silicon nitride films. Besides providing information about the reflection at this particular wavelength, it is frequently used to indicate additional information regarding the films absorption and even regarding its surface passivation quality. In this study, we compare nine different silicon nitride films, all with a similar refractive index at 633 nm (2.09±0.01). We demonstrate that these films exhibit very different electrical, chemical and optical properties despite their similar refractive index values. As a result of this investigation, we have developed industrial low-absorption silicon nitride films that provide excellent surface passivation, with saturation current density of 7 fA/cm2 on both n- and p-type wafers. This surface passivation quality is equal to that obtained by industrial silicon-rich silicon nitride films. All the films developed in this study were fabricated using industrial equipment and are thermally stable.

The influence of silicon nitride layer parameters on the implied Voc of CZ silicon wafers after annealing

The passivation potential of PECVD SiNx deposited on undiffused p-type Si surfaces is investigated. The influence of post-deposition annealing temperature and the film parameters (refractive index and thickness) on the implied Voc of textured, commercial grade, p-type CZ wafers was studied. Improvement in the implied Voc values of SiNx passivated CZ wafers was observed for two different SiNx films for all annealing temperatures in the range of 600–820°C. Excellent implied VOC values above 700 mV achieved on these wafers indicate that this process can be potentially used as rear passivation for various commercial cell technologies. Initial results on the use of this process in the fabrication of the new Double Sided Laser Doped solar cells structure demonstrate this potential.