Ever since the first publications by R.J. Schwartz in 1975, research into back-contact cells as an alternative to cells with a front and rear contact has remained a research topic. In the last decade, interest in back-contact cells has been growing and a gradual introduction to industrial applications is emerging. The goal of this review is to present a comprehensive summary of results obtained throughout the years. Back-contact cells are divided into three main classes: back-junction (BJ), emitter wrap-through (EWT) and metallisation wrap-through (MWT), each introduced as logical descendents from conventional solar cells. This deviation from the chronology of the developments is maintained during the discussion of technological results. In addition to progress on manufacturing these cells, aspects of cell modelling and module manufacturing are discussed and an outlook towards industrial implementation is presented. Copyright © 2005 John Wiley & Sons, Ltd.

Back-contact solar cells : A review

Substituting the doped amorphous silicon films at the front of silicon heterojunction solar cells with wide-bandgap transition metal oxides can mitigate parasitic light absorption losses. This was recently proven by replacing p-type amorphous silicon with molybdenum oxide films. In this article, we evidence that annealing above 130 °C—often needed for the curing of printed metal contacts—detrimentally impacts hole collection of such devices. We circumvent this issue by using electrodeposited copper front metallization and demonstrate a silicon heterojunction solar cell with molybdenum oxide hole collector, featuring a fill factor value higher than 80% and certified energy conversion efficiency of 22.5%.

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22.5% efficient silicon heterojunction solar cell with molybdenum oxide hole collector

A wafer-based monocrystalline silicon photovoltaics road map: Utilizing known technology improvement opportunities for further reductions in manufacturing costs

In a solar cell having p doped regions and n doped regions alternately formed in a surface of a semiconductor wafer in offset levels through use of masking and etching techniques, metal contacts are made to the p regions and n regions by first forming a base layer contacting the p doped regions and n doped regions which functions as an antireflection layer, and then forming a barrier layer, such as titanium tungsten or chromium, and a conductive layer such as copper over the barrier layer. Preferably the conductive layer is a plating layer and the thickness thereof can be increased by plating.

Metal contact structure for solar cell and method of manufacture

A precursor composition for the deposition and formation of an electrical feature such as a conductive feature. The precursor composition advantageously has a low viscosity enabling deposition using direct-write tools. The precursor composition also has a low conversion temperature, enabling the deposition and conversion to an electrical feature on low temperature substrates. A particularly preferred precursor composition includes silver metal for the formation of highly conductive silver features.

Low viscosity precursor compositions and methods for the deposition of conductive electronic features

A solar cell that is readily manufactured using processing techniques which are less expensive than microelectronic circuit processing. In preferred embodiments, printing techniques are utilized in selectively forming masks for use in etching of silicon oxide and diffusing dopants and in forming metal contacts to diffused regions. In a preferred embodiment, p-doped regions and n-doped regions are alternately formed in a surface of the wafer through use of masking and etching techniques. Metal contacts are made to the p-regions and n-regions by first forming a seed layer stack that comprises a first layer such as aluminum that contacts silicon and functions as an infrared reflector, second layer such titanium tungsten that acts as diffusion barrier, and a third layer functions as a plating base. A thick conductive layer such as copper is then plated over the seed layer, and the seed layer between plated lines is removed. A front surface of the wafer is preferably textured by etching or mechanical abrasion with an IR reflection layer provided over the textured surface. A field layer can be provided in the textured surface with the combined effect being a very low surface recombination velocity.

Solar cell and method of manufacture

In one embodiment, a method of forming doped regions in a substrate of a back side contact solar cell includes the steps of depositing a first doped oxide layer on a back side of a substrate, depositing a first undoped oxide layer over the first doped oxide layer, diffusing a first dopant from the first doped oxide layer into the substrate to form a first doped region in the substrate, and diffusing a second dopant into the substrate by way of a front side of the substrate, wherein the diffusion of the first dopant and the second dopant into the substrate are performed in-situ The method may further include the steps of patterning the first doped and undoped oxide layers to expose portions of the back side of the substrate and depositing a second doped and undoped oxide layers on the back side of the substrate

Use of doped silicon dioxide in the fabrication of solar cells

SunPower manufactures high-efficiency rear-contact solar cells. To offer these cells at a competitive price, SunPower requires a source of low-cost wafers with the necessary lifetime, thickness and resistivity to attain a cell efficiency of at least 20%. With simulation and experiment, this paper investigates how each of these parameters affects the efficiency of SunPower's low-cost, rear-contact solar cell. It concludes that the cell efficiency approaches an optimum when the wafers have (i) a lifetime in excess o 1 ms, (ii) a thickness of 160-280 /spl mu/m, and (iii) a resistivity of 2-10 /spl Omega/cm (n-type). The requirement of the high lifetime restricts the wafer choice to FZ, PV-FZ or n-type Cz; but the wide tolerance on wafer thickness and resistivity help lower the wafer cost by maximizing the yield from a silicon ingot.

The choice of silicon wafer for the production of low-cost rear-contact solar cells

SunPower is developing a novel 300/spl times/ concentrating photovoltaic (CPV) module. Tiny silicon solar cells and a compact nonimaging optics design enable a very low profile "micro-concentrator" design. The module is only 3 cm thick, but has a large acceptance angle (/spl plusmn/2/spl deg/) and looks much like a conventional flat plate module. Module photovoltaic conversion efficiency is projected to be about 18% (AM1.5d, 60/spl deg/C). This paper presents an overview of the solar cell design, concentrating optics and module design issues, as well as assembly details, cost projections and preliminary test results.

Michael J. Cudzinovic

Papers

Metal contact structure for solar cell and method of manufacture

Solar cell and method of manufacture

Use of doped silicon dioxide in the fabrication of solar cells

The choice of silicon wafer for the production of low-cost rear-contact solar cells

A flat-plate concentrator: micro-concentrator design overview