As PERC is becoming the new standard cell technology, the share of mono increases over longtime market leader multi. Nevertheless, the lower cost of multi wafers defends the position of multi PERC in the PV market, as long as the efficiency gap against mono remains moderate. Black silicon (b-Si) is the key to enable high-efficiency multi PERC cells. A team of researchers at Aalto University in Espoo, Finland recently demonstrated that b-Si not only improves the optics of mc-Si PERC but also makes the cells more stable.
In their study, the researchers applied a b-Si nanostructure fabricated by reactive ion etching (RIE) to mc-Si PERC cells, which were fabricated at an industrial production line of a European PV manufacturer. Since the nanotextured surface appears black even without any anti-reflection coatings, the front surface of the b-Si cells was passivated with atomic-layer-deposited (ALD) aluminum oxide (AlOx), instead of standard chemical-vapor-deposited (CVD) silicon nitride (SiNx) that was used in the acidic-textured reference cells. The completed cells were degraded under 0.5 sun illumination at 75 °C for one week, during which the current-voltage (I-V) characteristics were measured under the standard test conditions (25 °C, 1000 W/m2, AM1.5 spectrum) at regular intervals.
The acidic-textured reference cells degraded by nearly 4 %relunder illumination at elevated temperature (Fig. 1). On the contrary, the b-Si cells not only retained their performance, but their efficiency even improved by ~1 %rel, which was most likely due to enhanced field-effect passivationinduced by increased charge density in the AlOxthin film.
Figure 1. Power conversion efficiency of b-Si and acidic-textured PERC cells as a function of degradation time normalized to the initial efficiency. The dotted lines act as a guide for the eye.
To get a better understanding on the observed phenomenon, the researchers investigated the properties of the wafer bulk by measuring spatially-resolved photoluminescence (PL) maps both before and after the degradation treatment. The map in Fig. 2 represents the PL intensity decrease after a one-week degradation normalized to the initial PL intensity, i.e., blue color represents no change in PL intensity, indicating no degradation in the wafer bulk, whereas red color represents the highest decrease in the PL intensity during degradation. The map evidently shows that the b-Si area in the center completely retains its performance under illumination at elevated temperature, while the edges, which were not subjected to b-Si etching, show a significant degradation. A similar trend was observed also for the internal quantum efficiency (IQE) of the cells. The b-Si cells showed no degradation, while light-induced degradation (LID) was clearly visible in the acidic-textured cells as a reduction in IQE under illumination, especially in the ~850–1100 nm wavelength range, which suggests that the degradation occurred in the bulk.
Figure 2. Left: Photograph of a 156 x 156 mm multicrystalline b-Si PERC solar cell with AlOx surface passivation. Approximately 5 mm from wafer edges remained without b-Si due to clamping during the b-Si etching process. Right: PL intensity change during degradation normalized to the initial intensity: red color represents a large decrease in the PL intensity indicating strong degradation, while blue color represents stable PL intensity and no degradation.
The results demonstrate that the amount of LID is heavily reduced by the application of b-Si, indicating that the defect responsible for the phenomenon is in an inactive state or removed in the b-Si cells. One widely-suspected cause for LID is hydrogen. Hence, the possibility to replace the hydrogen-rich SiNxanti-reflection coating by ALD AlOxdue to the negligible reflectance of b-Si promotes efficient suppression of LID. Indeed, b-Si PERC cells with SiNxpassivation on the front showed some degradation, although the magnitude was significantly lower compared to the acidic-textured equivalents. Although hydrogen clearly has a role in the observed degradation, the lower amount of hydrogen in the surface passivation layer does not alone explain the reduced degradation in the b-Si cells, indicated by the drastically different behavior of b-Si and planar areas in the PL map and IV characteristics.
The same research group earlier demonstrated that heavy phosphorus doping in the b-Si nanostructures after a POCl3diffusion process, which is due to the large surface area of b-Si, enhances segregation of metal impurities to the emitter and substantially improves minority carrier lifetime in contaminated substrates. Thus, the reduced LID could be at least partly explained by a heavier phosphorus emitter, and hence, enhanced gettering in the b-Si cells.
Although the exact mechanism for LID mitigation by b-Si remains unresolved, the results demonstrate that benefits of b-Si are not limited to the excellent optical properties, as commonly understood.
More details: P. Pasanen et al.” Impact of black silicon on light‐and elevated temperature-induced degradation in industrial passivated emitter and rear cells”, Prog. Photovolt: Res. Appl.2018;1–8. DOI: 10.1002/pip.3088.
The author of this text, Toni Pasanen, won a Student Award at the European PV Solar Energy Conference and Exhibition (EU PVSEC) 2018. With the support of LONGi Solar and Prof. Wen Zhong Shen from Shanghai Jiatong University, TaiyangNews organized a trip for the EU PVSEC Student Award winners to present their research at the CNPV 2018 Conference in X’ian, China and invited them to publish an article on their results on our website.
