Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTBand diagram of the Hot Carrier cell. The device hasfour stringent requirements:Figure 4.5.44(a) Schematic representationof energy and particle fluxesinteractions used in the model(Particle fluxes - full line arrow,energy fluxes- dotted line arrow).(b) HCSC efficiency as a functionof carrier extraction energy level.Parameters used are: thermalisationtime = 100 ps, concentration =1000, lattice temperature = 300Kand absorber layer thickness = 50nm. [J, F and E are current densityand particle and energy fluxesas denoted by subscripts for Aabsorption, E emission and IAAuger processes.]Figure 4.5.45Carriers transmission probabilityversus energy for (a) ideal ESC, (b)non-ideal ESC.Figure 4.5.46ESCs). This is necessary in order to prevent coldcarriers in the contact from cooling the hot carriers,i.e. the increase in entropy on carrier extraction isminimized [4.5.50]. The limiting efficiency for thehot carrier cell is over 65% at 1 sun and 85% atmaximum concentration – very close to the limitsfor an infinite number of energy levels [4.5.1, 4.5.51,4.5.52]. Fig. 4.5.44 is a schematic band diagram of aHot Carrier cell illustrating these two requirements.a) To absorb a wide range of photon energies;b) To slow the rate of photogenerated carriercooling in the absorber;c) To extract these ‘hot carriers’ over a narrow rangeof energies, such that excess carrier energy is notlost to the cold contacts;d) To allow efficient renormalisation of carrierenergy via carrier-carrier scattering.In 2010 modelling of Hot Carrier efficiencies hasprogressed with implementation of realmaterial properties to give more realisticefficiencies for InN which include Augerprocesses and more realistic contactstructures. Significant progress has beenmade on demonstrating resonance indouble barrier selective energy structures.Further work on triple barrier double SiQW structures has been carried out forrectifying ESCs. This is complemented byimprovements in 2/3D modelling of transport inthese ESC structures. For absorbers, modelling ofnanocrystals superlattice arrays hasbee applied to real material systems.The growth of such systems in bothIII-V QD superlattices with collaboratorsand with colloidal Langmuir-Blodgettdispersion of Si nanocrsystals hasproduced structures which arenow being characterised for theirmodulation of phononic properties.Also meausurement of carrier coolingrates has been extended to other largephononic gap bulk materials includingInN, demonstrating the importance ofmaterial quality. Design of structuresfor hot carrier cells which should bepractical and realisable has developed, with thedevice properties more carefully specified and plansfor fabricating such structures in real devices.4.5.3.1 Modelling of Hot Carrier Solar CellEfficiencyResearchers:Pasquale Aliberti, Yu Feng, Santosh Shrestha, GavinConibeer, Martin GreenCollaboration with:Yasuhiko Takeda (Toyota Central ResearchLaboratories, Nagoya)Previous work was focused on developing a newmodel to calculate limiting efficiency of a real HCSCbased on an Indium Nitride (InN) absorber layer. InNhas been chosen as a potential material because ofits narrow electronic band gap for absorption of awide range of photon energies, whilst also having awide phonon band gap. This is good for suppressionof phonon decay, and hence for slowing carriercooling [4.5.53, 4.5.54].Calculation of limiting efficiency was performedtaking into account real optical and electronicproperties of InN, removing most of the idealityassumptions used in other models [4.5.51, 4.5.52,4.5.55, 4.5.56]. The detailed band structure82