Vishal Mehta

Understanding light-induced degradation of c-Si solar cells

We discuss results of our investigations toward understanding bulk and surface components of light-induced degradation (LID) in low-Fe c-Si solar cells. The bulk effects, arising from boron-oxygen defects, are determined by comparing degradation of cell parameters and their thermal recovery, with that of the minority-carrier lifetime (τ) in sister wafers. We found that the recovery of t in wafers takes a much longer annealing time compared to that of the cell. We also show that cells having SiN:H coating experience a surface degradation (ascribed to surface recombination). The surface LID is seen as an increase in the q/2kT component of the dark saturation current (J02). The surface LID does not recover fully upon annealing and is attributed to degradation of the SiN:H-Si interface. This behavior is also exhibited by mc-Si cells that have very low oxygen content and do not show any bulk degradation.

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.

Using silicon injection phenomenon during fire-through contact formation to improve process control and performance of screen-printed multicrystalline-silicon solar cells

High-performing screen printed, fire-through Si solar cells require a uniform, about 10-µm-thick, back surface field (BSF), with a peak Al concentration of about 1018 cm−3. This entails forming an ∼30-µm-thick Si-Al melt, which initiates sporadically and tends to agglomerate and ball up, thus producing a non-uniform BSF. We have developed a method to stabilize the Si-Al melt by deploying Si injection (at about 550°C for 2–5 s) during ramp-up, which initiates a thin, uniform melt along the entire Si-Al interface. This promotes adhesion between the Si surface and the growing Al-Si melt. We have previously shown that this process produces an excellent back contact, but the quality of the front contact and overall cell efficiency were not evaluated. The primary concern was whether molten glass frit would rapidly react with SiN:H and etch away a significant thickness of N+ silicon. Here, we report that high-efficiency cells with a uniform BSF, an open-circuit voltage >620 mV, and short-circuit current density >32 mA/cm2 can be produced by a T-t firing profile that includes Si injection.

Defect Generation and Propagation in Mc-Si Ingots: Influence on the Performance of Solar Cells

This paper describes results of our study aimed at understanding mechanism (s) of dislocation generation and propagation in multi-crystalline 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.

Formation of a back contact by fire-through process of screen-printed silicon solar cells

We have investigated various mechanisms that participate in formation of a good, screen-printed, Si-Al contact on the back side of a crystalline-Si solar cell. We observed a rapid diffusion of Si into Al during the temperature ramp-up. The Si diffusion produces a graded composition that causes an Al-Si melt to initiate from the interface. The interface melt of eutectic composition can be used to promote a uniform, dimple-free Al melt. We have also investigated the kinetics of stratification of the back-contacts into various regions: P+, eutectic, and molten (but unconnected) Al particles. The properties of these regions are primarily dictated by the temperature ramp-down profile, and they strongly depend on incident light flux that heats the wafer, on thickness of Al, and on emissivity of the Al-Air interface. We will discuss methods to improve back-contact properties of screen-printed multi-crystalline Si solar cells.

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.

A high-quality cross-sectioning method: Examples of applications in optimizing solar cell contact firing

A damage-free polishing method is developed to prepare a high-quality cross-section of a large length of a solar cell. A 1-inch-long sample is diced from the solar cell and embedded in wax using a specially designed chuck. The sample edge is sequentially polished by progressively reducing the grit sizes. The final polishing is done by Chemical Mechanical Polishing (CMP). This polishing procedure produces a highly flat edge, with excellent interfaces between metal contacts and the Si cell. The planarity of the wafer edge makes it possible to perform a variety of analyses of various regions and the interfaces of the cell, using optical microscopy, EDX, scanning electron microscopy (SEM), and conductive AFM (C-AFM). Here, we will discuss some details of the chuck and the polishing procedure, and present some applications for optimizing the contact firing process. This method has an added advantage of delineating the back surface field for optical observation.

Cheaper, improved solar cell fabrication

The photovoltaic industry routinely uses infrared furnaces to make solar cells from a thin wafer of silicon. Although modern furnaces can handle high throughput, they are typically not energy efficient. Generally, they produce a uniform energy flux over the wafers, but the wafer edges radiate more heat than their centers, and so the resulting temperature is not uniform throughout the wafer. This results in compromised performance: for example, a large-area solar cell’s edges will have poor electrical performance and degrade its overall output power. To rectify this, we have developed an optical cavity furnace as a single-wafer processing system, akin to a rapid thermal processing system. It uses unique geometry and multiple reflections to efficiently produce an energy distribution on the wafer that exactly compensates for the heat loss at the edges. The result is a highly uniform temperature distribution across the wafer. With this furnace, we have also established that an uncooled system can be run in a steady state to minimize energy loss. In addition, new processes that take advantage of photonic effects can enhance the cell efficiency. The furnace takes advantage of changes in the optical properties of semiconductormetal interfaces—for instance, between silicon and aluminum— due to interdiffusion and alloying. We are confident that the principles of our single-wafer processor can easily be scaled to a high-throughput system (e.g., 2000 wafers per hour) using conveyor-belt technology.1 The optical cavity furnace uses banks of lights, segmented into different lateral zones and heights, and dispersed inside an insulating ceramic cavity (see Figure 1). The geometry of the cavity reflects the visible and infrared light directly onto the solar cells. Ports for insertion of wafers and exhaust perturb the radiation flux very little. This design reduces unnecessary heating of the furnace chamber, saving energy and shortening the energy payback time (the amount of time a device must be in operation to recover the energy used in its own construction). This Figure 1. Photo of commercial version of optical cavity furnace manufactured by AOS Solar.