Complete Report - University of New South Wales

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