Advanced Screen Printing As a Reliable Pathway to Lower Costs
Applied Materials’ Baccini screen printers are the solar industry’s long-time standard for low-cost, high-volume deposition of metal contacts on solar cells. But do not imagine that this long history represents the end of the road. In fact new advances in screen printing are major contributors to the solar industry’s ongoing reductions in solar cell processing costs.
Two key advances possible with Baccini’s current generation of high-precision printers are selective emitter and double printing. These advances can be used separately or can be effectively combined, depending on customers’ goals and capabilities.
Selective emitter is aimed at selectively increasing the conductance of the emitter– i.e. the n-type phosphorous-containing region on the cell front surface – so that the front metal grid contacts make a lower-loss contact to the silicon and so that more photons reach the efficient-conversion portion of the cell. A traditional uniform emitter is a compromise between making low-loss contacts by reducing emitter sheet resistance and transmitting more photons by increasing the emitter sheet resistance. By selectively decreasing emitter sheet resistance only under the front metal contacts and increasing emitter sheet resistance elsewhere one can simultaneously minimize contact-related resistive losses and maximize photocurrent generation.
Applied’s selective emitter process accomplishes this by using a standard printer to deposit dopant paste in a grid-like pattern so that a standard single-step dopant diffusion simultaneously forms a local low-resistance emitter and an everywhere-else high-resistance emitter with an attendant increase in both photocurrent and photovoltage. The resulting cell efficiency gain varies depending on the specifics of the cell, e.g. wafer quality, baseline diffusion process, baseline grid process, etc.; but an efficiency gain of 0.5 % absolute is possible. A key factor in optimizing the selective emitter efficiency gain is a re-tuning of the metal grid pattern so that the photocurrent and photovoltage gains are not negated by lateral “spreading resistance” losses in the now-more-resistive emitter between the grid fingers. The re-tuning of the metal grid pattern generally entails decreasing the spacing between grid fingers, hence increasing the total number of grid fingers, which in turn increases the total grid coverage and decreases the cell photocurrent. The optimized balance between gains and losses is easier when using the second screen print advancement called double printing.
Double printing is simply printing a metal grid pattern, then over-printing another layer of metal exactly on top of the first to achieve a tall, narrow grid so that grid fingers shadow less (due to their being more narrow) while still conducting well (due to their being taller). Standard one-layer printing requires a relatively wide grid to achieve adequate conductance at typical layer thicknesses. Double printing increases the total layer thickness so that one can achieve equal conductance with narrower grids. Narrow, tall, double-printed grids cover less of the cell’s front surface so the cell has a higher photocurrent. Double-printing can be used alone to directly substitute for traditional single printing to provide an efficiency gain of up to 0.25 % absolute, or can be combined with selective emitter technology for overall gains of 0.7 % absolute or higher.
In all cases – selective emitter, double printing or the combination of both – an initial material is over-printed with another material to achieve superior results. The printing precision required to exactly align multiple prints is provided by Baccini’s Esatto technology.
Applied Materials is now working with customers around the globe to implement advanced screen printing in high-volume production by qualifying long-life high-precision screens, by qualifying low-cost dopant and specialty metal pastes, and by testing recommended 'Best Known Method' processes. Like any advanced technology, the specific benefits of advanced screen printing depend in no small part on the particular baseline processes, cell designs and factory operations capabilities of different customers. The one commonality is that customers can achieve lower costs and higher cell efficiencies with Applied Materials’ advanced screen printing technologies.
Learn more by visiting Applied Materials at the SNEC PV Power 2011 Conference, February 22-24 in Shanghai, China. Our booth, Hall W1 #560.