Przemyslaw Rupnowski

Studies on Backside Al-Contact Formation in Si Solar Cells: Fundamental Mechanisms

We have studied mechanisms of back-contact formation in screen-printed Si solar cells by a fire-through process. An optimum firing temperature profile leads to the formation of a P-Si/P + - Si/ Si-Al eutectic/agglomerated Al at the back contact of a Si solar cell. Variations in the interface properties were found to arise from Al-Si melt instabilities. Experiments were performed to study melt formation. We show that this process is strongly controlled by diffusion of Si into Al. During the ramp-up, a melt is initiated at the Si-Al interface, which subsequently expands into Al and Si. During the ramp-down, the melt freezes, which causes the doped region to grow epitaxially on Si, followed by solidification of the Si-Al eutectic. Any agglomerated (or sintered) Al particles are dispersed with Si. Implications on the performance of the cell are described.

Wafer preparation and iodine-ethanol passivation procedure for reproducible minority-carrier lifetime measurement

We have determined that mechanisms of irreproducibility in measurement of lifetime arise from two sources: (i) improper wafer cleaning, and (ii) instability of I-E solution when in contact with a Si wafer. This paper describes a sequential optical oxidation and chemical cleaning procedure that can reproducibly yield the correct values of bulk lifetime. We have observed that surface passivation is optically activated, which often causes an initial increase in the measured lifetime. This initial activation time can be greatly reduced by exposing the wafer in the bag to higher intensity light such as a solar simulator for 5–10 minutes. This procedure yields reproducible, very low recombination surfaces, suitable for measuring tb as high as 2 ms. We will discuss why this cleaning procedure is necessary and propose mechanism of instability in the passivation of I-E/Si.

A reflectance spectroscopy-based tool for high-speed characterization of silicon wafers and solar cells in commercial production

Some new applications of reflectance spectroscopy using the GT FabScan are described, which make this system highly desirable for process monitoring in commercial Si solar cell fabrication. These applications include grain orientation, grain size distribution, dislocation density distribution, and antireflection coating thickness on a finished solar cell. These measurements are performed very fast, typically in less than 10 ms over the entire wafer.

Defect clusters in multicrystalline silicon: Their nature and influence on the solar cell performance

This paper presents theoretical and experimental results of our studies to determine the performance limitations set by defect clusters (DCs) in multicrystalline silicon (mc-Si) solar cells. DCs in mc-Si wafers are manifested as spatially localized, large regions of high dislocation density, around grains of certain preferred orientations. The presence of DCs strongly influences material properties and cell performance. The objective of our comprehensive study is to understand their nature, influence on cell performance, and to assess the performance improvement that can be realized if the influence of DCs can be eliminated. The effect of grain orientation on the defect density is also investigated. Finally, we explore possibilities of eliminating the formation of DCs during crystal growth or treating them during cell processing to minimize their electronic influence.

A new method for rapid measurement of orientations and sizes of grains in multicrystalline silicon wafers

We describe a new technique for rapid measurement of orientations and sizes of various grains in a multicrystalline silicon (mc-Si) wafer. The wafer is texture etched to expose (111) faces nearest to each surface. Because grains of different orientations result in uniquely different texture shapes, they also have well-defined reflectance values. Hence, the process of determining the grain orientations is brought down to making reflectance maps. Reflectance maps are produced by PVSCAN or reflectometer (GT FabScan), and then transformed into orientation maps. Because the grain boundaries are very well delineated in the reflectance maps, they are also excellent for making measurements of size and distribution of grains. We will compare the results of this technique with other standard techniques.

Light-Induced Passivation of Si by Iodine Ethanol Solution: Preprint

We report on our observations of light-activated passivation of silicon surfaces by iodine-ethanol solution.

Influence of defects and defect distributions in multicrystalline silicon on solar cell performance

We describe results of our theoretical and experimental studies performed to investigate the influence of defects and defect distributions in multicrystalline silicon (mc-Si) wafers on the solar cells performance. Dislocation distributions were measured on wafers from various bricks of a mc-Si ingot. Solar cells were fabricated on sister wafers and characterized by a variety of methods. Cell performance can be accurately predicted from dislocation distribution of a mc-Si wafer using local N/P junction characteristics in a distributed network model. This analysis is applied to investigate changes in cell performance caused by dislocation propagation within a brick of mc-Si ingot. The theoretical results agree well with the measured performance of cells fabricated on wafers taken from different places in a brick.

Studies on the use of liquid surface passivation for lifetime measurements on good-quality silicon wafers

We evaluated several liquid passivants, viz. solutions of iodine ethanol (IE), quinhydrone methanol (QHM), and potassium cyanide (KCN), for measuring minority-carrier lifetime. Lifetime was measured by the WCT-100 (Sinton Instruments) and WT-2000 (Semilab). Our results show that both IE and QHM passivation are reliable mechanisms. We also find that the KCN solution is moderately passivating on oxidized surfaces, but is only minimally effective on bare Si surfaces. This paper presents details of our studies. In particular, the effect of illumination on IE-passivated surfaces and possible reasons for variations in lifetime measurement are discussed.

A high throughput, noncontact system for screening silicon wafers predisposed to breakage during solar cell production

We describe a non-contact, on-line system for screening wafers that are likely to break during solar cell/module fabrication. The wafers are transported on a conveyor belt under a light source, which illuminates the wafers with a specific light distribution. Each wafer undergoes a dynamic thermal stress whose magnitude mimics the highest stress the wafer will experience during cell/module fabrication. As a result of the stress, the weak wafers break, leaving only the wafers that are strong enough to survive the production processes. We will describe the mechanism of wafer breakage, introduce the wafer system, and discuss the results of the time-temperature (t-T) profile of wafers with and without microcracks.

Defect generation and propagation in mc-Si ingots: Influence on cell-to-cell performance variation

This paper describes results of our study aimed at understanding mechanism(s) of dislocation generation and propagation in multicrystalline silicon (mc-Si) ingots, and evaluating their influence on the solar cell performance. This work was done in two parts: (i) Measurement of dislocation distributions along various bricks, selected from strategic locations within several ingots; and (ii) Theoretical modeling of the cell performance corresponding to the measured dislocation distributions. Solar cells were fabricated on wafers of known dislocation distribution, and the results were compared with the theory. These results show that cell performance can be accurately predicted from the dislocation distribution, and the changes in the dislocation distribution are the primary cause for variations in the cell-to-cell performance. The dislocation generation and propagation mechanisms, suggested by our results, are described in this paper.