Applied Materials’ Baccini screen printers are the solar industry’slong-time standard for low-cost, high-volume deposition of metalcontacts on solar cells. But do not imagine that this long historyrepresents the end of the road. In fact new advances in screen printingare major contributors to the solar industry’s ongoing reductions insolar 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 beeffectively combined, depending on customers’ goals and capabilities.
Selective emitter is aimed at selectively increasing the conductance of theemitter– i.e. the n-type phosphorous-containing region on the cell front surface – so that the front metal grid contacts make a lower-losscontact to the silicon and so that more photons reach theefficient-conversion portion of the cell. A traditional uniform emitteris a compromise between making low-loss contacts by reducing emittersheet resistance and transmitting more photons by increasing the emitter sheet resistance. By selectively decreasing emitter sheet resistanceonly under the front metal contacts and increasing emitter sheetresistance elsewhere one can simultaneously minimize contact-relatedresistive losses and maximize photocurrent generation.
Applied’sselective emitter process accomplishes this by using a standard printerto deposit dopant paste in a grid-like pattern so that a standardsingle-step dopant diffusion simultaneously forms a local low-resistance emitter and an everywhere-else high-resistance emitter with anattendant increase in both photocurrent and photovoltage. The resultingcell 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 inoptimizing the selective emitter efficiency gain is a re-tuning of themetal grid pattern so that the photocurrent and photovoltage gains arenot negated by lateral “spreading resistance” losses in thenow-more-resistive emitter between the grid fingers. The re-tuning ofthe metal grid pattern generally entails decreasing the spacing betweengrid fingers, hence increasing the total number of grid fingers, whichin turn increases the total grid coverage and decreases the cellphotocurrent. The optimized balance between gains and losses is easierwhen using the second screen print advancement called double printing.
Double printing is simply printing a metal grid pattern, then over-printinganother 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 morenarrow) while still conducting well (due to their being taller).Standard one-layer printing requires a relatively wide grid to achieveadequate conductance at typical layer thicknesses. Double printingincreases the total layer thickness so that one can achieve equalconductance with narrower grids. Narrow, tall, double-printed gridscover less of the cell’s front surface so the cell has a higherphotocurrent. Double-printing can be used alone to directly substitutefor traditional single printing to provide an efficiency gain of up to0.25 % absolute, or can be combined with selective emitter technologyfor overall gains of 0.7 % absolute or higher.
In all cases –selective emitter, double printing or the combination of both – aninitial material is over-printed with another material to achievesuperior results. The printing precision required to exactly alignmultiple prints is provided by Baccini’s Esatto technology.
Applied Materials is now working with customers around the globe to implementadvanced screen printing in high-volume production by qualifyinglong-life high-precision screens, by qualifying low-cost dopant andspecialty metal pastes, and by testing recommended ‘Best Known Method’processes. Like any advanced technology, the specific benefits ofadvanced screen printing depend in no small part on the particularbaseline processes, cell designs and factory operations capabilities ofdifferent customers. The one commonality is that customers can achievelower 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.