Jeffrey E. Cotter

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

Experimental verification of the effect of depletion-region modulation on photoconductance lifetime measurements

Depletion-region modulation (DRM) has recently been identified as a mechanism that influences photoconductance lifetime measurements. The effect is observed in semiconductor samples containing a depletion-region (i.e., p-n junction solar cells). Experimental measurements presented within demonstrate that the DRM effect dominates the conductance measurement at low excess carrier concentrations, resulting in an overestimation of the effective lifetime by several orders of magnitude. The influence of substrate thickness on the DRM effect is experimentally verified. The previously developed analytical equation for DRM is in agreement with our experimental data and can be used to correct DRM affected photoconductance lifetime measurements. Finally, the impact on the sensitivity of a photoconductance measurement is discussed for the DRM corrected case.

Suns-photoluminescence: Contactless determination of current-voltage characteristics of silicon wafers

In good silicon solar cells, the separation of the quasi-Fermi energies Δη in the bulk is equivalent to the cell voltage. Photoluminescence is used to measure Δη in both bifacial solar cells and partly processed solar cells. The bifacial cells are used to demonstrate that simultaneous measurement of the photoluminescence signal and of the variable incident light intensity yields pseudo current-voltage characteristics, equivalent to Suns-open circuit voltage (VOC) measurements, but in contactless mode. The applicability of this method to unfinished solar cells, without the need for a solar cell structure, is demonstrated on silicon wafers after various processing steps.

OPTICAL INTENSITY OF LIGHT IN LAYERS OF SILICON WITH REAR DIFFUSE REFLECTORS

Light trapping is essential to the performance of thin-layer polycrystallinesilicon solar cells. One relatively new method of light trapping is the use of textured or pigmented dielectric layers on the rear surface which diffuse and reflect light reaching the rear surface. This article presents a one-dimensional optical model of thin silicon solar cells with such rear diffuse dielectric reflectors. Experimental front reflectance measurements and ray tracing simulations of thin, planar silicon layers with rear diffuse dielectric rear reflectors are presented and compared to the model with good agreement. Finally, an expression for the upper limit of the optical enhancement factor is developed for this type of light trapping.

Enhancing the surface passivation of TiO2 coated silicon wafers

In this letter, we demonstrate good surface passivation of lightly diffused n-type solar cell emitters using titanium dioxide (TiO2) thin films treated with a furnace oxidation process. Transient-photoconductance decay, x-ray photoelectron spectroscopy, and scanning electron microscopy measurements indicate that the silicon dioxide layer formed at the TiO2:Si interface provides excellent surface passivation. Emitter dark saturation current densities of 4.7×10−14 A/cm2 are achieved by this method, demonstrating that TiO2 films are compatible with high-efficiency solar cell structures.

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.

The influence of diffusion-induced dislocations on high efficiency silicon solar cells

Heavy boron and phosphorus diffusions are used in many high efficiency, monocrystalline silicon solar cell designs to form localized contact diffusions and back surface fields. It is important to cell performance that these diffusion processes do not increase bulk recombination by the introduction of lattice defects. This paper investigates the effect of boron and phosphorus misfit dislocation networks on the bulk recombination parameters, and performance of high efficiency silicon solar cells. It demonstrates that the formation of either a boron or phosphorus misfit dislocation network generates bulk asymmetric Shockley-Read-Hall recombination centers, and that these adversely affect the current-voltage curve, local ideality factor, and ultimately the performance of p-type silicon solar cells.

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...

Investigation of edge recombination effects in silicon solar cell structures using photoluminescence

Edge recombination can have a significant impact on the performance of small-area, high-efficiency silicon solar cells. Photoluminescence characterization techniques are applied to assess isolation trench techniques that are designed to remove edge recombination from such solar cells, thereby improving performance and allowing the true bulk properties of the solar cell to be evaluated independent of edge effects.