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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTIIA IIIA IB IIB IIIB IVB VB VIBBe B C N OMg Al Si P SCa Sc Cu Zn Ga Ge As SeSr Y Ag Cd In Sn Sb TeBa La Au Hg Tl Pb Bi PoErUse of the periodic table to analyse possible analogue compounds of InNbased on atomic mass combination and electro-negativity.Figure 4.5.65h=hole). This requires a reasonable DOS atall energies above E g, or at least only verynarrow gaps between energy levels, ≤kT 300K. This in turn requires a good overlapof wavefunction for carriers through thematerial, i.e. poor electronic confinementor a reasonable conductor. This condition ismet in all inorganic semiconducotrs quiteeasily, with e-e scattering rates of less than100fs for reasonable carrier concentrations.It may not be met in organicsemiconductors or in nanostructures withlarge barrier heights.7. Good carrier transport in order to allowtransport of hot carriers to the contacts.This is similar to 6, except that it onlyneed be in the direction of the contacts,probably the z growth direction. Areasonably low resistance is probably good.8. It should be possible to make good quality,highly ordered, low defect material.Preferably it should be easy and cheap todo this.9. Earth abundant and readily available andprocessable constituent elements andprocesses.10. No or low toxicity of elements, compoundsand processes.[(E LO, E O(min)& E LAare the maximum optical,minimum optical and maximum acousticphonon energies respectively.]InN has most of these properties, except 4, 8 &9, and is therefore a good model material for ahot carrier cell absorber.Analogues of InNAs InN is a model material, but has theproblems of abundance and bad materialquality, another approach is to use analoguesof InN to attempt to emulate its near idealproperties. These analogues can be II-IV-nitridecompounds, large mass anion III-Vs, group IVcompounds/alloys or nanostructures.II-IV-Vs: ZnSnN; ZnPbN; HgSnN; HgPbNWith reference to Fig 4.5.65, it can be seen thatreplacement of In on the III sub-lattice withII-IV compounds is analogous and is now quitewidely being investigated in the Cu 2ZnSnS 4analogue to CuInS 2[4.5.72].ZnGeN can be fabricated [4.5.73] and is mostdirectly analogous with Si and GaAs. However,its band gap is large at 1.9eV. It also has a smallcalculated phononic band gap [4.5.74]. ZnSnNhas a smaller electronic gap (1 eV) and largercalculated phononic gap [4.5.74]. It is howeverdifficult to fabricate, and also its phononic gapis not as large as the acoustic phonon energymaking it difficult to block Klemens decaycompletely. HgSnN or HgPbN should bothhave smaller E gand larger phononic gaps.These materials have not yet been fabricated[4.5.75].Large mass cation:The Bi and Sb compounds have largepredicted phononic gaps and Bi is a relativelyabundant material, with only low toxicity[4.5.75]. BiB has the largest phononic gapbut AlBi, Bi 2S 5, Bi 2O 3(bismuthine) are alsoattractive. Similarly SbB has a large predictedphononic gap. That for AlSb is the same sizeas the acoustic phonon energy and its bandgap is 1.5eV, making it marginal as an absorbermaterial and similar to InP.Group IIIA III-VsLaN and YN both have large phononic gapswhilst that for ScN is too small.The Lanthanides can also form III-Vs. ErN andother RE nitrides can be grown by MBE. Thephononic band gaps of the Er compounds arepredicted to be large, because of the heavyEr cation, but its discrete energy levels makeit not useful as an absorber, although thecombination of properties in a nanostructurecould be advantageous.Group IV alloy/compounds:All of the combinations Si/Sn, Ge/C or Sn/Clook attractive with large gaps predicted in1D models. However being all group IVs theyonly form weak compounds. UnfortunatelySiC, whether 3C, 4H or 6H, has too narrowa phononic gap. Nonetheless GeC doesform a compound and is of significantinterest [4.5.76].There are also several other inherentadvantages of group IV compounds/alloys allof which are associated with the four valenceelectrons of the group IVs which result inpredominantly covalent bonding:a) The elements form completely covalentlybonded crystals primarily in a diamondstructure (tetragonal is also possible as inβSn). However for group IV compoundsthe decreasing electronegativity down thegroup results in partially ionic bonding.This is not strong in SiC and whilst it tendsto give co-ordination numbers of 4, cannonetheless result in several allotropes ofdecreasing symmetry: 4c, 4h, 6h. However,as the difference in period increases for theas yet theoretical group IV compounds, sotoo does the difference in electronegativityand hence also the bond ionicity and thedegree of order. For a hot carrier absorberthis is ideal because it is just such a largedifference in the period which is neededto give the large mass difference andhence large phononic gaps. All of GeC,SnSi, SnC (and the Pb compounds) havecomputed phononic gaps large enoughto block Klemens decay, and should alsotend to form ordered diamond structurecompounds.b) Because of their covalent bonding, thegroup IV elements have relatively smallelectronic band gaps as compared to theirmore ionic III-V and much more ionic II-VIanalogues in the same period: e.g. Sn0.15eV, InSb 0.4eV, CdTe 1.5eV. In fact toachieve approximately the same electronicband gap one must go down one periodfrom group IV to III-V and down anotherperiod from III-V to II-VI: e.g. Si 1.1eV, GaAs1.45eV, CdTe 1.5eV. This means that forgroup IV compounds there is greater scopefor large mass difference compounds92
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTCross-sectional electron micrograph of a 30-bilayerporous silicon distributed Bragg reflector.Figure 4.5.66whilst still maintaining small electronicband gaps. A small band gap of coursebeing important for broadband absorptionin an absorber - property 3 in the desirableproperties for hot carrier absorbers listedabove.c) The smaller E gwould tend to be for thelarger mass compounds of Pb or Sn. Which,to give large mass difference, would becompounded with Si or Ge. This trendtowards the lower periods of group IV alsomeans that the maximum optical phononand maximum acoustic phonon energieswill be smaller for a given mass ratio - thedesirable properties 4 and 5.d) Furthermore, unlike most groups, thegroup IV elements remain abundant for thehigher mass number elements – desirableproperty 9. Property 10 is also satisfiedbecause the group IVs have low toxicity.Nanostructures:As discussed in section 4.5.3.3.3 QDnanostructures can be viewed in the sameway as compounds. Their phononic propertiescan be estimated from consideration of theircombination force constants. Hence it ispossible to ‘engineer’ phononic properties ina wider range of nanostructure combinations.Of the materials discussed above the Group IVslend themselves most readily to formation ofnanostructures instead of compounds due totheir predominantly covalent bonding, whichallows variation in the coordination number.Therefore the nanostructure approaches ofsection 4.5.3.3.3 are consistent with a similardescription as analogues of InN, whether it beIII-V QDs, colloidally dispersed QDs or for coreshell QDs.4.5.3.3.5 Summary of Hot Carriersolar cell research2010 has seen significant development inmost areas of Hot Carrier solar cell work.The modelling of efficiencies not only nowincludes real material parameters for highlypromising absorber materials such as InN,but also is now extended to Energy Selectivecontacts of finite width. The transport acrosssuch contacts is further modelled for a rangeof QD matrix combinations. Work on ESCs hasseen further more detailed demonstrationof the necessary resonance in double barrierresonant tunnelling structures, with additionalevidence for hot carrier populations, albeitvery small, from illuminated I-V measurementswith the ‘optically assisted I-V’ technique.Measurement of InN with time-resolved PLhas indicated some evidence for slowedcarrier cooling, further corroborating theimportance of a large phonon band gapto block optical phonon decay, but alsohighlights the importance of materialquality. Modelling of nanostructures forabsorber materials has focussed on real III-VQD structures, showing phonon bandgapswhich will soon be measured in phonondispersion measurements. Progress on theLangmuir-Blodgett approach to orderednanoparticle arrays has seen development ofordered single layer arrays of Si nanoparticles.The potential application of nanostructuresto fully integrated devices has started tobe investigated conceptually, with variousdesigns considered. Similarly the possibilityof absorber materials which are analogousto InN is also being investigated. These manyaspects of Hot Carrier cells will see furtherdevelopment and consolidation in 2011with recent success in significant additionalfunding.4.5.4 Up-conversionResearchers:Craig Johnson, Gavin ConibeerCollaboration with:Peter Reece (Physics, UNSW)Up-conversion in novel silicon-basedmaterialsUp-conversion (UC) in erbium-dopedphosphor compounds (particularly NaYF 4:Er)has been shown to be a promising meansof enhancing the sub-band-gap spectralresponse of conventional Si solar cells withoutmodification of the electrical properties of thecell [4.5.77]. In this scheme, a layer containingthe phosphor is applied to the rear of a highefficiencybifacial cell. After absorbing twolong-wavelength (~1500nm) photons - whichare transmitted by the cell - the excited Erions can relax by emitting a photon with anenergy greater than the Si band gap, therebyincreasing the current that can be extractedfrom the cell.While phosphors have demonstratedhigh-efficiency UC behaviour, their usepresents particular challenges with regardto fabrication and cost. Our work in the lastyear has focused on the development ofEr-doped porous Si (PSi:Er) as an alternativeUC material. PSi is unique in that its porosity -and hence the material refractive index - canbe varied as a function of depth, allowing forthe elaboration of high-quality monolithic Sioptical structures such as distributed Braggreflectors (DBRs). A cross-sectional electronmicrograph of such a structure is shown in Fig.4.5.66. Its porous substructure also allows fordeep infusion of dopant atoms via techniquessuch as electroplating.93
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