Complete Report - University of New South Wales

More documents

Recommendations

Info

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTlifetime (s)1.E-081.E-091.E-10PECVD 2.2e15PECVD 5.1e15e-beam 6.4e15ALICE 2e16ALICE 4.5e16ALICE 5e16lifetime (s)1.E-081.E-091.E-10PECVD 3e15PECVD 1e16PECVD 2.4e16PECVD 1e16e-beam 1.56e17e-beam 7.2e15e-beam 6.9e16e-beam 2.5e161.E-111.E-111.E-1230 40 50 60 70 80q/kT1.E-1230 40 50 60 70 80q/kTLifetime vs q/kT plots for a) p-type, and b) n-typepoly-Si thin-film cells.Figure 4.4.1.35Arrhenius plots for the diodesaturation current J 01/(J L/I) (a) andfor short-circuit current Jsc (c) forpoly-Si and c-Si solar cells; b) Vocplots extrapolated to 0K for poly-Siand c-Si solar cells.Figure 4.4.1.33Typical sequence of EQE curvesat different temperatures rangingfrom 120K to 320KFigure 4.4.1.34In the ideal sc-Si the diode saturation current J 01follows the Arrhenius law with the activation energyEa equal to the Si bandgap when extrapolated to0K. The short-circuit current density, Jsc, in sc-Si isonly very weakly dependent on the temperature(due to the effect of band-gap variation). However,poly-Si solar cells exhibit different characteristics[4.4.1.31]. The Arrhenius plots for J 01of poly-Sisolar cells have distinctively a smaller slope, whichreflects a lower Ea associated with J 01by 0.15-0.18 eV. As a consequence, the Voc plots versus Textrapolated to 0K have a lower interceptin the range 1.0~1.1 eV as comparedto 1.27 eV for sc-Si solar cells (Figure4.4.1.33a, b). The Arrhenius plots for Jscof poly-Si cells whose absorber diffusionlength is shorter than the absorberthickness have steeper slopes indicatingstronger temperature dependence of thepoly-Si cell current (Figure 4.4.1.33c).Although the smaller Ea and theintercept could in principle be due to thenarrower band gap in poly-Si material ascompared to c-Si, it cannot explain thetemperature sensitivity of Jsc. On theother hand, both effects are consistentwith existence of shallow sub-bandgapstates acting as either minority carrier traps orrecombination centres.Further confirmation of the shallow-level relatedrecombination in poly-Si solar cells comes fromthe study of the bulk lifetimes as a functionof temperature which allows a more detailedunderstanding of the behaviour of these shallowlevels in the device quasi-neutral, bulk regions.Minority carrier diffusion lengths in the absorberlayers of various poly-Si thin-film cells wereextracted by fitting the device EQE simultaneouslywith its optical reflectance, a procedure whichhas become a routine for poly-Si thin-film cellswhose short lifetimes precludes quantificationby the standard techniques such as the quasisteady state photoconductance (QSSPC). The EQEmeasurement and fitting procedure were repeatedfor each sample at different temperatures rangingfrom 120K to 320K. Figure 4.4.1.34 superposes thefront-side EQE curves of the same cell at differenttemperatures (note that the heights of the curvesare reduced slightly due to additional reflectanceby a quartz window on the cryostat in which thesample is placed). Clearly the quantum efficiency ofthe cell varies strongly with temperature.Figures 4.4.1.35 a) and b) plot the extracted lifetimeτ against q/kT for p- and n-type cells respectively.Although the actual lifetime values at a giventemperature differ greatly from cell to cell, all thelifetime curves exhibit Arrhenius law with theactivation energy (Ea) of 0.17-0.21 eV near roomtemperature [4.4.1.32]. These activation energies50
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTTypical TEM DFWB image of a poly-Si film on glass.0.5 μmFigure 4.4.1.37are much larger than those reported for plasticallydeformed Si and multicrystalline Si [4.4.1.35],where the dominant recombination is thoughtto be via transitions between the shallow anddeep levels. Assuming direct transitions betweenshallow levels, the Ea is expected to be (Ec-E de)+(E dh-Ev)+2.5kT=180~120 meV (where E deand E dhareshallow band energies near the conduction and thevalence bands respectively), which is in remarkableagreement with the experimental values.Furthermore, as a consequence of electrontransitions between the shallow bands, therecombination rate becomes proportional to theconcentrations of both minority and majoritycarriers, as in the case of radiative recombination,in contrast to recombination via deep levels,where the rate is proportional to the minoritycarrier concentration only. It leads to minoritycarrier lifetime inversely proportional to themajority carrier concentration, or roughly to thedopant concentration [4.4.1.32]. The experimentalconfirmation of this effect is shown in Figure4.4.1.36 which plots the minority carrier lifetimesin a number of poly-Si solar cells versus the dopantdensity. There is a clear inverse relationshipbetween the lifetime and dopant density.Amongst the n-type cells in Figure 4.4.1.35b, threeare made from small grained (~1 μm) poly-Si (circleor square data points), and three are made fromlarge grained (~5 μm) poly-Si epitaxially grown onan Al induced crystallised (AIC) seed layer (triangledata points). The similarity in the temperaturebehaviour of the lifetime, regardless of the grainsize, indicates that large and small grain materialshave a common lifetime limiting mechanism. Also,in Figure 4.4.1.36 one readily sees that the AIC cells(triangles) can take on the lifetime values eitherabove or below the power law trend line, indicatingthat they do not have consistently superior lifetimesjust by virtue of their larger grain size. It was foundthat the predominant grain orientation plays agreater role in determining the Voc of AICcells in fact, of the two rightmost triangledata points representing AIC cells atabout 5E16 cm -3 doping in Figure 4.1.1.36,the higher lifetime point originates from(100) preferentially oriented material andthe lower lifetime point comes from (111)preferentially oriented material.1.E-091.E-10All the above evidence points atintragrain defects, as a likely limiton poly-Si thin-film cell lifetimes. In1.E-11particular, dislocations are the most likelycandidates because they provide thestrain fields necessary to create split-offstates from the band edges that can actas shallow levels [4.4.1.36]. To estimate dislocationdensity in poly-Si solar cells a TEM study wasconducted using the Weak-Beam-Dark-Field (DFWB)technique. An example image is shown in Figure4.4.1.37 and the dislocation density deduced by theimage analysis is in the order of 1E10 cm -2 . Thereis very rough correlation between the dislocationdensity found in poly-Si solar cells and the cell Voc.The best cells with Voc > 500 mV have dislocationdensity of 6~8x10 9 cm -2 , while for the cell with verypoor Voc < 450 mV the dislocation density is about1.5x10 10 cm -2 [4.4.1.37].As the dopant density decreases, the shallowband recombination gradually becomes less andless significant and eventually, at sufficiently lowdopant densities, less than about 5x10 15 cm -3 therecombination involving deep levels starts todominate (process 2 in Figure 4.4.1.32a). Such deeplevels are associated with defects concentrating indislocations and grain boundaries. The defects areusually charged to the same polarity as the majoritycarriers thus creating potential barriers whichattract the minority carriers. The lifetime in this casedepends on the potential barrier heights, whichbecome greater with lowering dopant density.1.00E+15 1.00E+16 1.00E+17 1.00E+18dopant concentration (cmlifetimeat 232K (s)-0.9619y = 462576xR 2 = 0.8161Series1AIC ALICE (n-type)e-beam (n-type)PECVD (ne-beam(p type)PECVD (p-type)Power (Series1)Lifetimes at 232K for various poly-Sithin-film cells plotted againstdopant concentration.Figure 4.4.1.3651
Page 4: ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE
Page 8 and 9: ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE
Page 17 and 18: Efficiency and cost projections for
Page 23 and 24: 4.3 First GenerationWafer Based Pro
Page 25 and 26: Jsc improvement afterapplying diele
Page 40 and 41: 4.4 Second Generation:Silicon, Orga
Page 47 and 48: I-V curves of the individual cells
Page 49 and 50: Voc [mV]2 m thick film480BSF deposi
Page 57 and 58: (a) The schematic band energystruct
Page 59 and 60: light intensity (Suns)100.0010.001.
Page 63 and 64: 4.5 Third Generation Strand -Advanc
Page 102 and 103:
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Z. Ouyang, X. Zhao, S. Varlamov, Y.
Page 130 and 131:
J. Huang, J. Wong, M. Keevers and M
Page 132:
show all

Complete Report - University of New South Wales

Create successful ePaper yourself

Delete template?

Save as template?