Florence W. Chen

P-Type Versus n-Type Silicon Wafers: Prospects for High-Efficiency Commercial Silicon Solar Cells

Chemical and crystallographic defects are a reality of solar-grade silicon wafers and industrial production processes. Long overlooked, phosphorus as a bulk dopant in silicon wafers is an excellent way to mitigate recombination associated with these defects. This paper details the connection between defect recombination and solar cell terminal characteristics for the specific case of unequal electron and hole lifetimes. It then looks at a detailed case study of the impact of diffusion-induced dislocations on the recombination statistics in n-type and p-type silicon wafers and the terminal characteristics of high-efficiency double-sided buried contact silicon solar cells made on both types of wafers. Several additional short case studies examine the recombination associated with other industrially relevant situations-process-induced dislocations, surface passivation, and unwanted contamination. For the defects studied here, n-type silicon wafers are more tolerant to chemical and crystallographic defects, and as such, they have exceptional potential as a wafer for high-efficiency commercial silicon solar cells

Fast Photoluminescence Imaging of Silicon Wafers

Photoluminescence (PL) imaging is demonstrated as a fast characterization tool allowing variations of the minority carrier lifetime within large area silicon wafers to be measured with high spatial resolution and with a data acquisition time of only one second. PL imaging is contactless and can therefore be applied to silicon solar cells before and after every processing stage including fully processed cells and bare, unpassivated mc-Si wafers, which makes it an extremely effective process monitoring tool that is ideally suited for inline applications in the PV industry. The combination of PL imaging with electroluminescence imaging and the application of PL imaging with external bias control are demonstrated to give very quick access to additional valuable information about local series resistance variations

Application of photoluminescence characterization to the development and manufacturing of high-efficiency silicon solar cells

Characterization techniques based on quasi-steady-state photoluminescence have recently emerged as accurate, fast, and powerful tools for developing high-efficiency silicon solar cells. These techniques are contactless and provide complementary spatial and injection level dependent information about recombination. In this paper, we demonstrate the application of different photoluminescence techniques to several important aspects of high-efficiency solar cell fabrication: wafer handling, furnace contamination, process-induced defects, cell design, and cell process monitoring. The experimental results demonstrate that photoluminescence characterization techniques are excellent tools for laboratory experiments and also potentially for industrial process monitoring.

Passivation of boron emitters on n-type silicon by plasma-enhanced chemical vapor deposited silicon nitride

A well-passivated emitter is crucial to making high efficiency solar cells. With several reported potential benefits in using n-type silicon compared to p-type silicon for solar cell applications, there is a need to investigate silicon nitride passivation on boron-diffused emitters. The passivation of plasma-enhanced chemical vapor deposited silicon nitride with different refractive indices on a variety of boron doping profiles on 1Ωcm, float zoned, n-type silicon is studied. Contrary to the general perceptions that silicon nitride provides relatively poor passivation on boron-diffused surfaces, our results show that for some diffusion sheet resistances and with sufficient annealing, silicon nitride can be particularly well suited for passivating boron emitters. One-sun implied open circuit voltages of 663 and 718mV and dark saturation current densities of 25 and 13fA∕cm2 per side are achieved by silicon nitride passivation on moderately doped boron emitters (100Ω∕sq) and lightly doped boron emitters (240...

Laser-induced defects in crystalline silicon solar cells

Laser based processing has been incorporated into many successful solar cell technologies over the past 20 years; for example buried contact solar cells, laser-fired back contacts and laser texturing. However, the impact of crystal damage generated during laser processing on solar cell performance is still uncertain. This paper investigates laser-induced defects that result from a laser ablation process similar to that used to form grooves in the buried contact solar cell. The first part of this paper focuses on the formation of the defects and uses the Yang etching technique to investigate the impact of chemical etching and thermal cycles on their propagation. The second part of the paper takes a close look at the electrical properties of laser-induced defects, experimentally investigates the recombination of minority-carriers at laser-induced defects, and the potential of these defects to shunt pn-junctions.

The Influence of Parasitic Effects on Injection-Level-Dependent Lifetime Data

A circuit simulation approach is employed to investigate the influence of various parasitic effects on injection-level-dependent lifetime data of samples containing p-n junctions. Simulations of the influence of shunts, localized recombination, edge recombination, and a combination of these on lifetime data are presented. The simulation shows that the nature of the parasitic effects can be qualitatively identified due to their different lifetime behaviors at various injection levels. It is demonstrated that the parasitic effects start to dominate the lifetime data at injection levels , and the lifetime behavior can look similar to Shockley-Read-Hall recombination in some cases. A range of case studies with experimental data and data fitting are presented. The case studies show that parasitic effects can interfere with lifetime-based experiments. In some cases, the understanding of the influence of parasitic effects leads to a reinterpretation of the lifetime behavior of the test devices.

PECVD Silicon Nitride Surface Passivation for High-Efficiency N-Type Silicon Solar Cells

In this paper, we show that plasma-enhanced chemical vapor deposited silicon nitride is particularly well suited for surface passivation of n-type silicon wafers and solar cells. The surface passivation quality provided by the silicon nitride on three different important surfaces for high-efficiency n-type solar cells is studied in this work: planar, textured, and boron-diffused surfaces. Exceptional passivation quality on these surfaces is demonstrated in this work. One-sun implied open-circuit voltages of 732 mV, 719 mV, and 683 mV were achieved on 1 ohm.cm planar, 1 ohm.cm textured, and moderately doped (135 ohm/sq) boron-diffused planar surfaces, respectively. A real open-circuit voltage of 719 mV was measured at approximately 1-Sun, 25 degC condition on a voltage test structure device passivated entirely with silicon nitride

Contactless technique to quantify the edge-junction recombination in solar cells

This letter presents a method to determine the recombination properties of exposed edges of p-n-junction silicon devices, particularly, solar cells. The method is based on the extended analysis of injection level dependent carrier lifetime data of samples with various degrees of intentionally created edges. Its application to laser-cut edges is shown here. Values of per-length space-charge-region recombination current densities of approximately 2×10−8 and 2×10−9A∕cm have been determined for laser-cut- and laser-cut-and-etched edges. The technique is beneficial for both understanding edge recombination in solar cells and developing improved edge passivation techniques.

FTIR Analysis of Microwave-Excited PECVD Silicon Nitride Layers

This paper presents infrared absorption (FTIR) measurements of SiN layers and correlates them to their ability to passivate silicon wafer surfaces. The best passivation was obtained for films having a nitrogen to silicon atomic composition in the proximity of N/Si=1.2, together with a high concentration of Si-N bonds (approximately 1times1023 cm-3) and a refractive index in the vicinity of n=2. The total hydrogen concentration in these films remained practically unchanged after a high temperature firing cycle, which indicates a good thermal stability. In contrast, silicon rich layers (higher refractive index and lower Si-N bond density) suffered a large reduction in the total hydrogen content. These results support the suggestion by ECN researchers that the Si-N bond concentration can be a good indicator of the ultimate electronic impact of the SiN layers

Characterization of PECVD Silicon Nitride Passivation with Photoluminescence Imaging

In this paper, we present studies of plasma-enhanced chemical vapor deposited silicon nitride in which photoluminescence imaging was used to characterize our deposition process. A showcase of different processing issues such as equipment design, processing conditions, and manual handling is presented. We also demonstrate how photoluminescence imaging can be particularly useful for process monitoring, diagnoses, and development. An increase in implied Voc of up to 25 mV was achieved through the use of PL imaging as a diagnostic tool