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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTRear reflectance10.950.90.850.80.75Rb and IQE under perpendicular incident lightR (n=1.5)R (n=2.0)R (n=2.6)R (n=3.0)R (n=3.2)IQE (n=1.5)IQE (n=2.0)IQE (n=2.6)IQE (n=3.0)IQE (n=3.2)1.210.80.60.40.2IQERear reflectance0.990.970.950.930.910.890.87Rb and IQE under Lambertian light distributionR (SiO2 / Al)R (SiO2 / Ag)R (optimised DS / Al)R (optimised DS / Ag)IQE (SiO2 / Al)IQE (SiO2 / Ag)IQE (optimised DS / Al)IQE (optimised DS / Ag)1.210.80.60.40.2IQE0.70900 950 1000 1050 1100 1150 1200 1250Wavelength (nm)0.850900 950 1000 1050 1100 1150 1200 1250Wavelength (nm)R (left axis) and IQE (right axis) under normallyincident light for a planar silicon solar cell withdielectric stack plus metal rear reflector. Theeffect of varying the refractive index of themiddle layer of the dielectric stack (DS) is shown.Figure 4.3.1.2R and IQE under a Lambertian distribution of light fora silicon solar cell with various types of rear reflectorsincluding the standard SiO 2/metal and the optimiseddielectric stack (DS)/metal rear reflectors.Figure 4.3.1.31.00E+029.00E+01Measured EQE of un-textured solar cell(4.3.1.1)8.00E+01EQE (%)7.00E+016.00E+015.00E+014.00E+01Original solar cellEQE enhancement(4.3.1.2)3.00E+01Solar cell only with passivation layer2.00E+01Solar cell with passivation layer and Ag plasmon1.00E+010.00E+00300 400 500 600 700 800 900 1000 1100 1200wavelength (nm)EQE of a planar solar cell withstandard rear reflector (SiO 2/Al)structure and after the applicationof Ag nanoparticles (prior tothe application of padding andmetal layers).Figure 4.3.1.41.5 to 3.2. Such stacks with a rear contacting metallayer applied onto a planar cell result in internalreflectance (R) and internal quantum efficiencies(IQE) shown in Fig. 4.3.1.2. Figure 4.3.1.3 comparesthe R and IQE of standard SiO 2/metal reflectors tooptimised DS/metal reflectors when applied to cellswith textured front surface.(4.3.1.3)Current density Jsc (mA/cm 2 ) was calculatedby integrating the product of IQE and photonflux, from 300nm to 1300 nm (assuming zerofront surface reflection). The standard AM1.5Gspectrum has been used. Table 4.3.1.1 shows theimprovement of Jsc as a result of applying DS/metalrear reflectors (over the standard SiO 2/Al reflector)on both planar and textured solar cells.SunlightSpectrally sensitive mirrorsSolar cells ofdifferent band gapsMulti-junction cell concept usingspectral splitting.Figure 4.3.1.5The calculation of R and the IQE for Figures 4.3.1.2and 4.3.1.3 is carried out as follows. All photogeneratedcarries are assumed to be collectedby the solar cell (which is roughly the case inthe Centre’s PERL cells). A Bis the absorptionin the bulk region, A Trefers to the cell’s totalabsorption (A T=A B+A R, with A Rdefined as rearreflector absorption). IQE equals the ratio ofA Band A Tas indicated in Eq. 4.3.1.1. Formulaefor A Band A Tare given elsewhere [4.3.1.1].R is the wavelength dependent internalrear reflectance. It can be simply calculatedfor perpendicularly incident light, or be aweighted value of a Lambertian angular lightdistribution as shown in Eq. 4.3.1.3. T + is thefraction of light transmitted to the rear surfaceand T - is the fraction of light reflected from therear side transmitted to the front surface. Anapproximation for T + T - is used to simplified formulain Eq. 4.3.1.2, where a is the absorption coefficientwhich varies with wavelength and W is the waferthickness.(4.3.1.4)Another promising scattering rear reflectorstructure involves plasmonic scattering from Agnanoparticles. The idea is to scatter the unabsorbedlong wavelength light back into the silicon bulk,thereby randomising the reflected light directionsto enhance light trapping. This can be particularlyimportant from the first double-pass of light acrossa cell with pyramidal texture since a large fractionis lost when internally striking the surface of apyramid of opposite slope to that coupling it in.Light travelling past the Ag nanoparticles will bereflected back by the padding layer/metal interfaceas shown in Fig.4.3.1.1 (b), and be scattered forthe second time. Fig. 4.3.1.4 shows the preliminaryresults of applying such structure on a silicon solarcell prior to the application of the padding layer.The application of such a nanoparticle reflectorinvolves the removal of existing back metal layersand the thinning of the rear passivating SiO 2layer followed by the growth of Ag plasmonic22
Jsc improvement afterapplying dielectric stack (DS)/metal rear reflectors comparedto the standard SiO 2/Alreflector for both planar andtextured solar cells.Refractive index of middle layer in DSJsc improvement n=2.0 n=2.6 n=3.0 n=3.2Under normal incident light 1.08% 1.97% 2.50% 2.65%Under Lambertian incident light -0.03% 0.06% 0.15% 0.15%ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTTable 4.3.1.140.7% III-V cell stack on Ge showingthe I-V curves of constituentcells [4.3.1.2].Figure 4.3.1.6particles onto the thinned passivating layer. Whilethe EQE enhancement as shown in Fig. 4.3.1.4is encouraging, it is anticipated that more EQEgain is possible by adding the padding and metalreflector layers.Spectrum SplittingDuring 2010, the group continued its workon improving the efficiency of solar cells bysubdividing the broad solar spectrum into smallerenergy ranges and to convert each range witha cell of appropriately matched bandgap (seeFigure 4.3.1.5).The Centre is involved in a collaborative researchproject involving Solar Systems and Spectrolab infully exploring the potential of spectrum splittingin combination with the power tower concept,for higher efficiency conversion of concentratedsolar power (CSP). A working prototype CSP powertower system of 140kW rating at Waterbridge,Victoria has demonstrated the highest sustainedconversion efficiency to date for any solar to electricconversion system without yet the benefit of suchsplitting [4.3.1.3]. We plan to demonstrate theadditional potential by the construction of a small“power-cube” prototype capable of independentconfirmation of energy conversion efficiency above40%, including all optical losses.The work is based on combining the only twocommercial solar cell technologies for concentratedsunlight, silicon and III-V semiconductor stacks onGe substrates, with the latter shown in Fig. 4.3.1.6.The Ge substrate provides a good crystallographictemplate for subsequent layers but Ge has too low abandgap to be a good opto-electronic match to thetwo cells on top. From the current-voltage curvesshown, the Ge cell can be seen to be generatingmuch more current than the other two cells. Theextra current is essentially wasted as heat in theGe cell with the situation becoming worse as theairmass (AM) increases from the standard AM1.5test condition shown (particularly in the morningsand afternoons).The initial idea was to reflect some of the excessphotons reaching the Ge cell onto a separate andrelatively inexpensive silicon cell array using aninnovative “power cube” receiver as in Fig. 4.3.1.7(a).For the cell stack shown in Fig. 4.3.1.6, up to 0.085A/W of photon current could be reflected onto asilicon cell where it would be converted at about0.7V under concentration. This would add an extra6% absolute to the combined cell performancetaking it from 40.7% efficiency to above 46%.Additional advantages would be that theessentially lossless dicroic reflector used to give thiswavelength selective reflection could also be usedto control the angular emission from the III-V cells,increasing their voltage output (by 2-20 mV/cell forradiative efficiencies above 10%). Moreover, bycontrolling the precise wavelengths reflectedduring the day, spectral mismatch losses couldbe greatly reduced compared to the use of aIII-V array alone. An additional advantage is thereduced amount of waste heat to be removedfrom the receiver and the larger area availablefor its extraction.More detailed analysis of the receiver hasshown that the “demi-cube” approach of Fig.4.3.1.7(b) is even more attractive. By integratingthe array and reflector and switching theposition of the two arrays, the design of thereflector is greatly simplified. Low UV and far-IRabsorption in the filter to avoid excessive heatingR1.00(a) Original “Power Cube” receiverconcept. Light in approximately the890-1100nm wavelength range isreflected by a band-pass dielectricreflector onto a silicon array ratherthan being dissipated as heat inthe Ge cell; (b) Improved “Demi-Cube” concept. Thermal design isimproved by integrating one arrayand the reflector. By switching theposition of the silicon and III-V array,the filter design becomes far lesscritical as shown in Fig. 4.3.1.8.Figure 4.3.1.7Fig. 4.3.1.7 (b)Fig. 4.3.1.7 (a)900Simplification of filter design withnew configuration.Figure 4.3.1.8110023
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