Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTPL images illustrate the improvement in uniformityand quality of the p + layer achieved by the newfiring process. (a) before and (b) after the lowtemperature treatment.Figure 4.3.2.6Inverted form of the PERL (Passivated Emitter and RearLocally diffused) solar cell developed at UNSW basedon the use of N-type silicon.Figure 4.3.2.7field in conventional p-type cells. However, they cansignificantly degrade the quality of the Al-alloyedemitter in n-type wafers by allowing Al to locallybypass the p + region and directly contact the n-typebulk via a Schottky barrier causing a non-linearshunt. A new and modified firing process hastherefore been developed to avoid the damagefrom such non-uniformities. In this method, apatented low temperature solid phase epitaxialgrowth process is employed after the conventionalstandard spike firing to minimize the impacts ofthese junction discontinuities so that a uniform andgood quality junction as illustrated in Fig. 4.3.2.5 (b)can be achieved.These improvements have facilitated a 15-20mVincrease in Voc relative to those reported in theliterature and efficiencies over 17% for standardscreen-printed solar cell technology withhomogeneous n-type emitter (front surface field)applied to n-type wafers. Figure 4.3.2.6 showsthe improvement in the photoluminescent (PL)response resulting from the improved firing andformation of the rear contact and junction.A common problem with the manufacture ofconventional screen-printed solar cells is minuteamounts of aluminium paste accidently cominginto contact with the cell front surface. This canhappen when wafers are face down during theprinting of the rear or during wafer transfer/handling. Aluminium paste contamination of suchsurfaces and transfer belts happens relatively easilyin a production environment such as through abroken wafer that has been Al printed, operators’contaminated gloves, tiny holes in the screenprintingscreen etc. However such contaminationwith the current rear junction n-type technologycreates unusual photoactivated shunts that are notpresent in the dark.This is because a p-n-p transistor structure is formedwhen the unwanted aluminium on the top surfaceis fired into the n-type surface (with the second p-njunction of course being at the rear of the device).In this phototransistor, the lightly doped waferforms the base of the transistor, and despite thebase being very thick, approaching200 microns, the high lifetime of theCZ n-type material allows the transistorto achieve moderate gain levels andtherefore conduct large currents whenilluminated by light that generatesthe necessary base current for thetransistor. The unusual consequenceis that the apparent shunt resistanceof the cell is very poor (low) whenilluminated brightly, increasing tohigh values in the dark or even lowillumination levels. The ramificationsof this for cell efficiency and fill-factorare that values fall with reducing lightintensity as is normally the case forshunted cells, but with values increasing againfor low illumination levels as the shunt problemsdisappear as the phototransistors are deactivated.Even without such phototransistor shunting,analysis of such homogeneous emitter n-typedevices indicates that even higher voltagesare potentially achievable if not for the largedark saturation current contribution from theheavily doped phosphorus diffused top surface.This emphasises the importance of moving tothe equivalent of a selective emitter design forthe front surface to facilitate both improvedshort wavelength response as well as increaseddevice voltages.4.3.2.3.4.2 N-type Screen-Printed Cellswith the Equivalent of a Selective EmitterUntil recently, the Centre held the world record(jointly with Stanford University) for the mostefficient n-type silicon devices with 22.7% efficiency.The cell design was based on the inverted formof the PERL (Passivated Emitter and Rear Locallydiffused) solar cell developed at UNSW and isshown below. In this work, the cell design hasbeen adapted to accommodate the use of low costscreen-printed solar cell processes involving thealignment of the screen-printed front metal lines tothe heavily doped n + regions to form the equivalentof a selective emitter on the front surface and theSchematic of a laser dopedAl-alloyed rear junction n-typesolar cell.Figure 4.3.2.829