Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTScreen-printed fingers runningperpendicular to the heavilydiffused semiconductor fingerswhere electrical contact ismade. A dielectric/AR coatingpassivates the top surface andisolates the metal from thelightly diffused top surface.Figure 4.3.2.3sheet resistivity to above 100 ohms per square, asrequired for near unity internal quantum efficienciesfor short wavelength light, serious resistive lossesare introduced, both in the emitter and throughthe contact resistance at the metal to n-typesilicon interface.Furthermore, the conventional design for screenprintedsolar cells has quite poor surface passivationin both the metallised and non-metallisedregions. Even if good ohmic contacts could bemade to more lightly doped emitters, the largemetal/silicon interface area would significantlylimit the voltages achievable due to the highlevels of recombination in these regions and thecorresponding contribution to the device darksaturation current. These voltage limitations arenot of major significance at the moment due to thelimitations imposed by the substrates. However,in the future as wafer thicknesses are reduced toimprove the device economics and improved rearsurface passivation is introduced, the cells will havethe potential for improved open circuit voltages,but only provided the surfaces, including under themetal, are well passivated.4.3.2.3.2 Semiconductor FingerSolar CellsTo accommodate a top surface emitter sheetresistivity of at least 100 ohms per square, metalfingers need to be spaced no more than 1mm apartto avoid excessive sheet resistivity losses. Due tothe large width of screen printed metal lines or100 microns or more, such a close spacing is notpossible without shading well over 10% of the cellsurface. The concept of semiconductor fingers istherefore introduced as shown in Figure 4.3.2.3.These semiconductor fingers are formed by laserdoping the silicon surface while simultaneouslypatterning the dielectric layer to expose theheavily doped silicon surface. Developmentalwork with the laser doping process allows sheetresistivities as low as 1 ohm/square to be achievedwhile simultaneously forming laser doped lines ofonly 8 microns width. Spacing such lines 0.8mmapart avoids significant resistive losses within thelightly doped emitter which can be diffused to100 ohms/square, ensuring excellent response tothe short wavelengths of light. In the laser dopedregions, approximately half of the incident light islost due to absorption within the heavily dopedsemiconductor fingers.This is defined as a 50% effective shading lossof the semiconductor fingers. Using this design,the effective shading loss of the semiconductorfingers is only 0.5%, while the effective emittersheet resistivity of the emitter is 50 ohms/square.This latter figure results from the emitter sheetresistance of 100 ohms/square in parallel with thelaser doped lines which cover 1% of the area with asheet resistivity of 1 ohm/square and therefore alsoeffectively contribute 100 ohms/square. This allowsthe screen-printed lines to retain their normalspacing, but with 99% of the emitter being lightlydoped and therefore able to achieve near unityinternal quantum efficiencies for short wavelengthsof light. In addition, the passivating dielectric layernot only passivates the lightly diffused surface soas to give near unity internal quantum efficienciesfor short wavelength light, but it also isolates themetal from these same regions to minimise thedevice dark saturation current. Importantly, thesilicon is only exposed at the semiconductor fingers,with the screen-printed metal having been shownto make excellent ohmic contact to the heavilyphosphorus diffused silicon in these regions.Both thick oxides and silicon nitride layers, whenused with appropriate pastes, appear to provideadequate protection to the lightly diffused surfaceregions, preventing the screen-printed metal fromcontacting the silicon.A typical cell design based on this concept usessemiconductor fingers 8 microns wide of sheetresistivity 1-2 ohms/square and spacing of 0.8mm.The top surface sheet resistivity is typically 100ohms/square, with the effective sheet resistivityin the direction parallel to the semiconductorfingers typically 50-60 ohms per square. The screenprinted metal lines are printed perpendicularto the semiconductor fingers with a width of110 microns and spacing of 2.2mm. The overall26