Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTLoss processes in a standard solarcell: (1) non-absorption of belowband gap photons; (2) latticethermalisation loss; (3) and (4)junction and contact voltage losses;(5) recombination loss.Figure 4.5.1colloidal dispersion of Si nanoparticles, whichhas seen coherent arrays successfully grown,and growth of III-V quantum dot structures withcollaborators, with modelling and characterisationindicating their usefulness as absorbers. Theparameters needed for a real hot carrier solar cellare now better defined, and candidates for itsstructure are now being modelled in more detail.This will feed into fabrication and characterisationof some candidate structures in the near future.The up-conversion project in 2009 reported theapproach of use of porous-Si as a host material foran Erbium upconverter. This has the advantageof much greater versatility in fabrication andelectrochemical doping with Er, compared toother host phosphors. Work in 2010 has furtherdemonstrated up-conversion of below band gapphotons. The ease of control of porous-Si growth atdifferent refractive indices has also been exploitedto create fully integrated multiple Distributed BraggReflectors. These allow a tuning of the forbiddenphotonic band such that regions just outsidethe bandgap (which have enhanced density ofphotonic states) lie at the 1500nm absorptionwindow of Er. This effectively concentrates lightinto this absorbance window and boosts upconversionquadratically. Enhancements of upto 80 are possible with multiple levels, with aconsequent narrowing of the wavelength rangeenhanced. Enhancements of 40 are very practical.This represents a big potential improvement for upconversionefficiency.Plasmonics has previously been applied to bothfirst generation and second generation Si cells withvery significant enhancements in absorbance ofnear band gap photons. Work in 2010 has appliedplasmonic silver nanoparticles to nanostructuredSi QD layers. Again significant enhancements ofphotoluminescence (reciprocal with absorption)have been absorbed, up to 16x for wavelengthsjust shorter than the effective band gap. Alsoapplication to the rear of the layer is seen to bebetter than the front because the reflection of thesilver particles themselves is beneficial rather thanparasitic. Just as with thin film second generationcells, light trapping by these non-texturing methodsis very important for third generation devices, whichconsist of very thin layers of material.This progress in all the main Third Generationproject areas is improving understanding andallowing optimisation of modelling, structuresand devices. Developments to come in thenext year will see significant advancement inthese areas, with excellent prospects for gooddemonstration devices.4.5.1 Third Generation PhotovoltaicsThe “Third Generation” photovoltaic approach isto achieve high efficiency whilst still using “thinfilm” second generation deposition methods. Theconcept is to do this with only a small increasein areal costs and to use abundant and non-toxicmaterials and hence reduce the cost per Watt peak[4.5.1]. Thus these “third generation” technologieswill be compatible with large scale implementationof photovoltaics. The aim is to decrease costs to wellbelow US$0.50/W, towards US$0.20/W or better, bydramatically increasing efficiencies but maintainingthe economic and environmental cost advantagesof thin film deposition techniques (see Fig. 4.1.3showing the three PV generations) [4.5.1, 4.5.2]. Toachieve such efficiency improvements such devicesaim to circumvent the Shockley-Queisser limit forsingle band gap devices that limits efficiencies tothe “Present limit” indicated in Fig. 4.1.3 of either31% or 41% (depending on concentration ratio).This requires multiple energy threshold devicessuch as the tandem or multi-colour solar cell. TheThird Generation Strand is investigating severalapproaches to achieve such multiple energythreshold device [4.5.1, 4.5.3].The two most important power loss mechanismsin single-band gap cells are the inability to absorbphotons with energy less than the band gap (1 inFig. 4.5.1), and thermalisation of photon energyexceeding the band gap, (2 in Fig. 4.5.1). Thesetwo mechanisms alone amount to the loss ofabout half of the incident solar energy in solar cellconversion to electricity. Multiple energy threshold62