Catherine Chan

Advanced Bulk Defect Passivation for Silicon Solar Cells

Through an advanced hydrogenation process that involves controlling and manipulating the hydrogen charge state, substantial increases in the bulk minority carrier lifetime are observed for standard commercial grade boron-doped Czochralski grown silicon wafers from 250-500 μs to 1.3-1.4 ms and from 8 to 550 μs on p-type Czochralski wafers grown from upgraded metallurgical grade silicon. However, the passivation is reversible, whereby the passivated defects can be reactivated during subsequent processes. With appropriate processing that involves controlling the charge state of hydrogen, the passivation can be retained on finished devices yielding independently confirmed voltages on cells fabricated using standard commercial grade boron-doped Czochralski grown silicon over 680 mV. Hence, it appears that the charge state of hydrogen plays an important role in determining the reactivity of the atomic hydrogen and, therefore, ability to passivate defects.

Evidence for the role of hydrogen in the stabilization of minority carrier lifetime in boron-doped Czochralski silicon

This study demonstrates that the presence of a hydrogen source during fast-firing is critical to the regeneration of B-O defects and that is it not a pure thermally based mechanism or due to plasma exposure. Boron-doped p-type wafers were fired with and without hydrogen-rich silicon nitride (SiNx:H) films present during the fast-firing process. After an initial light-induced degradation step, only wafers fired with the SiNx:H films present were found to undergo permanent and complete recovery of lifetime during subsequent illuminated annealing. In comparison, wafers fired bare, i.e., without SiNx:H films present during firing, were found to demonstrate no permanent recovery in lifetime. Further, prior exposure to hydrogen-rich plasma processing was found to have no impact on permanent lifetime recovery in bare-fired wafers. This lends weight to a hydrogen-based model for B-O defect passivation and casts doubt on the role of non-hydrogen species in the permanent passivation of B-O defects in commercial-gra...

Rapid Stabilization of High-Performance Multicrystalline P-type Silicon PERC Cells

Light-induced or, more broadly, carrier-induced degradation (CID) in high-performance multicrystalline silicon (TIP mc-Si) solar cells remains a serious issue for many manufacturers, and the root cause of the degradation is still unknown. In this paper, the impact of firing temperature on the stability of lifetime test structures is investigated, and it is found that substantial CID can be triggered if peak temperatures exceed approximately 700 °C. We then investigate two pathways to stabilize the performance of industrially produced TIP mc-Si passivated emitter rear contact cells which have been fired at CID-activating temperatures (~740 °C-800 °C) currently required for silver contact formation. The first is a fast-firing approach, whereby it is demonstrated that an additional firing step at a reduced temperature after cell metallization can suppress the extent of Voc degradation by up to 80%. The second approach is the accelerated degradation and subsequent recovery of carrier lifetime through the use of high-intensity illumination during annealing at elevated temperatures. A 30 s process is found to suppress the maximum extent of degradation in Voc by up to 60% and up to 80% for longer processes. Ultimately, the results suggest that a combined approach of fast-firing and a high-intensity-illuminated anneal could achieve the best results in terms of Voc, stability.

Recombination parameters of lifetime-limiting carrier-induced defects in multicrystalline silicon for solar cells

In p-type multicrystalline silicon solar cells, carrier-induced degradation (CID) can cause up to 10% relative reduction in conversion efficiency. Although, a great concern has been drawn on this degradation in the photovoltaic community, the nature of this degradation is still yet unknown. In this contribution, the recombination parameters of the responsible defect causing this degradation are extracted via temperature and injection dependent lifetime spectroscopy. Three wafers from three different ingots were processed into cell precursor and lifetime structures for the study. Similar defect recombination parameters were obtained for all samples. Two candidates for the defect energy level were identified: Et − Ei = −(0.32 ± 0.05) eV or Et − Ei = (0.21 ± 0.05) eV in the lower and upper bandgap halves, respectively. The capture cross section ratios were found to be k = 56 ± 23 or k = 49 ± 21 for the lower and upper bandgap halves, respectively. Contrary to previous studies, these parameters have been extr...

Influence of Hydrogen on the Mechanism of Permanent Passivation of Boron–Oxygen Defects in p-Type Czochralski Silicon

Strong evidence is provided for the critical role of hydrogen in the permanent passivation of boron-oxygen (B-O) defects in p-type Czochralski silicon. In particular, the impact of rapid thermal processing (firing), plasma exposure, and hydrogen-containing dielectrics on B-O defect passivation is explored. Importantly, no permanent passivation of B-O defects is observed in samples fired bare (both with and without exposure to a hydrogen-rich plasma prior to firing) and in nonfired samples coated with hydrogenated silicon nitride (SiNx:H). In contrast, samples with SiNx:H layers present during firing resulted in significant levels of B-O passivation, even at firing temperatures as low as ~500 °C. Increasing peak firing temperatures (Tpeak) appeared to correlate to increased B-O passivation ability; however, increasing Tpeak above a value of 670 °C resulted in suboptimal levels of surface and bulk passivation. These observations are explained within a hydrogen-based model for permanent passivation of B-O defects. Implications for nonhydrogen-based models are also discussed.

Assessing the Performance of Surface Passivation Using Low-Intensity Photoluminescence Characterization Techniques

This paper applies quasi-steady-state photoluminescence (QSS-PL) and photoluminescence imaging to characterize the recombination properties of various surface passivation techniques. Particular interest is given to the performance at low excess carrier densities where many types of surface passivation show a strong increase in surface recombination velocity. These techniques are then used to further understand the ability of parasitic effects such as nonuniform illumination, edge recombination and areas of high recombination to affect these measurements. Furthermore, a new technique for edge isolation using laser doping is shown to be effective against the effect of edge recombination. This technique is useful to implement when using QSS-PL to analyze small samples as carriers conducted to the edge regions can dramatically alter the effective lifetime in low injection.

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.

Carrier-Induced Degradation in Multicrystalline Silicon: Dependence on the Silicon Nitride Passivation Layer and Hydrogen Released During Firing

Carrier-induced degradation (CID) of multicrystalline silicon (mc-Si) solar cells has been receiving significant attention; however, despite this increasing interest, the defect (or defects) responsible for this degradation has not been determined yet. Previous studies have shown that the surface passivation layer and the firing temperature have a significant impact on the rate and extent of this degradation. In this paper, we further study this impact through an investigation of the CID behavior of the mc-Si wafers passivated with six different silicon nitride layers, each fired at four different peak temperatures. At low firing temperatures, no significant difference in the CID was identified between the samples with different passivation layers; however, a large range of degradation extents was observed at higher firing temperatures. Using Fourier transform infrared spectroscopy, a correlation was found between the degradation extent and the amount of hydrogen released from the dielectric during firing. We verified that no degradation of the surface passivation quality occurred, indicating that the degradation is primarily associated with a bulk defect.

Anomalously high lifetimes measured by quasi-steady-state photoconductance in advanced solar cell structures

Quasi-Steady-State Photoconductance is widely used in photovoltaics industry to measure the effective minority carrier lifetime of silicon wafers, a key material parameter affecting final solar cell efficiency. When interpreting photoconductance based lifetime measurements, it is important to account for various artefacts that can cause an over-estimation of the carrier lifetime, such as minority carrier trapping. This paper provides experimental evidence for another artefact in photoconductance lifetime measurements, affecting samples that have a conductive layer that is interrupted by lines of the opposite polarity doping, forming laterally alternating regions of p/n doping. This structure often appears in the emitter region of samples used to monitor the lifetime of interdigitated back contact cells. The cause of this artefact is linked to a reduction in the measured dark conductance. Experimental data are presented that suggest this is due to the formation of a phototransistor type structure on the sa...

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.