Complete Report - University of New South Wales

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ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTabsorption (a.u)40mN/m60mN/m65mN/mcollapse0.5 1.5 2.5 3.5 4.5Energy (eV)(Left) Optical absorption of Si monolayers deposited atvarious surface pressures (SP). (Right) PL of Si NP filmsat varying SP, as compared to the Si NP solution.Figure 4.5.62Conceptual Integrated DeviceFigure 4.5.63External contactSelective energy contact:double (or triple) barrier QD resonant tunnellingSlowed carrier cooling absorber layer:§ Periodic QD array tuned to block optical phonondecay by Klemens mechanism;§ Core shell QD for ease of tuning;§ Must also allow renormalization of carrier energy bye-e & h-h scattering.Proof of concept with MOVPE/MBE III-V QD array.Transfer to thin film or self-assembled QD array.Selective energy contact:selecting opposite carrier typeExternal contactMonolayer structures of Si NPs have beensuccessfully deposited using the LB method andcharacterisation has been on-going using TEM, UVvis,PL and Raman spectroscopy. Figure 4.5.61 (right)illustrates TEM image of the Si films fabricated at 50mN/m surface pressures (SP) of the water trough.As noted, high contrast is difficult to achieve due tothe small density difference of Si and the carbonpassivatingchain. As a result, to further quantifythe coverage and regularity, computer recognitionsoftware is required; on which work is on-going. It isdifficult to see for this reason, but Fig. 4.5.61 (right)appears to show a dense packing of Si NPs.UV-Vis spectroscopy was used to study the opticalabsorption of the films. As illustrated in Fig.4.5.62 (left) for single layers of Si NPs at varyingcompression surface pressures (SP), the absorptionclearly increases when SP increases, correspondingto higher packing densities of Si NPs. The PL spectraof the above films show narrow and symmetricpeaks at 1.6 ~1.7 eV, as shown in Fig. 4.5.62 (right),which are in good agreement with the estimatedbandgap using the ‘effective mass approximation’.The red shift in emission energy from Si NP filmsas compared to the solution corresponds to theelectronic energy transfer from small particles tolarger ones when they are closely packed [4.5.70].Investigation of the electronic and phononicproperties of the monolayer and multilayerstructures are on-going using low-wavenumberRaman and time resolve PL spectroscopy.4.5.3.3.3.3 Implementation ofnanostructures for Hot Carrier cellsIn order to use a nanoparticle array as a Hot Carrierabsorber, and hence utilise slowed carrier coolingdue to modified phonon dispersion, then a wayto fabricate a complete structure with ESCs mustbe established. A structure in which nanoparticlesare arranged in a uniform 3D array should give therequired phononic band gap. The degree to whichthis needs to be ordered is still under investigation.Certainly a large difference between masses isbeneficial between the nanoparticles and thematrix and this can be best achieved in an array of‘core shell QDs’ in which the core and shell have verydifferent acoustic impedance in order to promotecoherent reflection and hence confinement ofphonons [4.5.61]. The matrix in which such an arrayis embedded needs to allow transport of carriers tocontacts and also electron-electron and preferablyhole-hole scattering to renormalise carrier energies.Such a structure should also have a narrowelectronic band gap so as to absorb a wide rangeof photon energies. This combination of propertiesis challenging but not mutually exclusive becausephononic properties are largely independent ofelectronic properties. Energy selective contactsare also required for such a structure. These wouldmost likely be arranged at the top and bottom ofthe absorber. (A hole contact might not need to beselective because the hole population only containsa small fraction of the hot carrier energy and hencethermalisation of holes is less important.) TheseESCs would be QD or QW double barrier structures.(Addition of an extra layer to give double QD/QWtriple barrier structures should also give rectificationat the contact.) Such a QD array / double barrier QDESC structure is shown schematically in Fig. 4.5.63.90
Physical picture of the ND HC SolarCell (upper) with a carrier diffusionfield (core/shell thickness rationot to scale), embedded into awide band gap solar cell [4.5.71].Iso-valent impurities provide anenergy selective level for electrons.Band diagram of the ND HC SolarCell matched to the physicalstructure (lower). Direct generationof hot exciton (1), energy selectiveextraction of hot electron from HotCarrier absorber (2), thermionicemission/diffusion of holes overthe barrier (3), electron-electronscatteringwith renormalizationof carrier energies and possibleenergy loss by phonon emission(4), free carrier re-absorption (5),impact ionization (6) and carriergeneration in the wide band gapmatrix (7).ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTFigure 4.5.64Nanoparticle cell design incorporatingESCs and absorberResearcher:Dirk KönigA full Hot Carrier cell requires control of optical,electronic and phononic properties in the samestructure. One possible approach is the ‘nano-dot(ND) Hot Carrier cell’ embedded into the i-region ofa wide band gap solar cell as shown in Fig. 4.5.64.Hot Carrier solar cells must have a continuouselectronic density of states (DOS) to maximisemaximum photon absorption and elastic electronelectronscattering. The latter refills the depletedelectronic energy levels from which electrons areextracted through the ESC. On the other hand, ifthey do not have a phononic band gap themselvesthe NDs must not be much bigger than the ballisticmean free path in order to prevent inelasticscattering by which carriers cool down rapidly.However, optical phonons can be confined in thenanocrystals as mentioned in Section 4.5.3.3.3by arranging for a mismatch between opticalphonon energies in the two materials. Core shellnanocrystals can be used to realise this [4.5.61,4.5.62]. In addition the shell can be doped withiso-valent impurities which form split-off subbandswithin the electronic band gap of the material,such as boron replacing gallium in gallium nitride(GaN) [4.5.71]. Such an ESC would only work forone carrier type as co-doping of a thin nano-shellappears not to be technologically feasible. For someIII-V compounds with a very high effective massratio between holes and electrons such that most ofthe excess energy during optical generation is takenup by the electron, thermalisation of holes does notrepresent a large loss in energy. Hence thermionicemission of holes over a low barrier can be usedto transport holes into the bulk of the device to becollected by the junction with little loss in efficiency.4.5.3.3.4 Hot Carrier Absorber:Choice of MaterialsResearchers:G. Conibeer, R. Patterson, P. Aliberti, S. Huang,Y. Kamikawa, D. König, Binesh Puthen-Veettil,S.Shrestha, M.A. GreenThe properties required for a good hot carrierabsorber material are listed below in orderof priority.1. Large phononic band gap (E O(min)- E LA) - inorder to suppress Klemens decay of opticalphonons this must be at least as large as themaximum acoustic phonon energy. Hence alarge mass difference (or large force constantdifference) between constituent elements isrequired [4.5.61].2. Narrow optical phonon energy dispersion (E LO-E O(min)) – in order to minimise the loss of energyto TO phonons by Ridley decay. This requiresa high symmetry atomic or nano-structure,preferably cubic with degenerate opticalphonon energies at zone centre.3. Small electronic band gap – to allow a widerange of photon absorption. This should be lessthan 1eV. For 1 sun concentration the optimumis 0.7eV - as a band gap below this energygives no advantage in the balance betweenabsorption and emission. As the concentrationratio increases this optimum band gap decreasesto zero at maximum concentration [4.5.1].4. A small LO optical phonon energy (E LO). Thisreduces the amount of energy lost per LOphonon emission, requiring a greater numberto be emitted for a given energy loss [4.5.62].However, it is difficult to have both a small E LOand the large phononic gap required in 1. Thiscondition requires further investigation, butdoes still argue for a small E LO.5. An absolute small maximum acoustic phononenergy (E LA). This maximises the phononic gap ifE LOis also small. A small E LArequires a large massfor the heavy atom and/or small force constantfor its bonds.6. Good renormalisation rates in the material,i.e. good e-e and h-h scattering (e=electron,91
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Complete Report - University of New South Wales

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