Complete Report - University of New South Wales

More documents

Recommendations

Info

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTSchematic representationof time resolvedphotoluminescencearrangement.Figure 4.5.53oa-IV results of a typical samplewith optimum barrier thicknessesand QD size at LN 2temperature(upper) and at room temperature(lower). Curves in the wave lengthintervals from 800 to 600 nm canbe compared due to their identicaloffset of V biasto absolute zero V bias.,showing the free carrier product tobe constant. Curves for 550 – 250nm and 1000 to 500 nm showmaximum optical injection limit.Both curve groups printed in bold.Figure 4.5.52600 nm with F hn≈ 50 Suns foreach interval could be directlycompared to each other. Inaddition, we evaluated the curveswith optical excitations in therange of 1000 – 500 nm (F hn≈ 600Suns) and 550 – 250 nm (F hn≈140 Suns). The former is near thepractical high concentration limitfor solar cells of 1000 Suns.As shown in Fig. 4.5.52, for thecurves in the 800 to 600 nmregion at LN 2temperature, thepotential difference between V bias(j→ 0) and the average voltage ofthe resonance (V Res,avg) shows adecrease of 48 mV with increasingphoton energy (decreasingλ range). As carrier densities(quasi-Fermi level positions) areidentical for these curves, thispotential drop corresponds to anincrease in free hole energy of48 meV. As this energy differenceis equal to the effective kT, it canbe interpreted as an increasein carrier temperature of DT = 560 K when goingfrom 800 – 750 nm (1.55 – 1.65 eV) to 650 – 600 nm(1.91 – 2.06 eV). Even at F hn≈ 50 Suns, this effectis small. This is due to Si being a very unsuitableabsorber material, further corroborated by the ratioof peak to valley current densities (quality factor ofNDR – QF NDR) dropping from 2.2 to 1.5. Si does nothave a phononic band gap so that optical phononsundergo a rapid decay which tremendouslyaccelerates carrier cooling.For the 1000 – 500 nm and 550 – 250 nm ranges,the QF NDRincreases significantly to 4.1 and 3.2,respectively. The potential difference V bias(j → 0) –V Res,avgis shifted by 365 and 200 mV, respectively;a quantitative comparison of both curves to eachother or to the curves in the spectral range from800 – 600 nm is thus not feasible. However, it showsthat the energy selectivity may increase under highconcentration F hn.For all oa-IV curves, a tunnelling feature for electronextraction can be seen at 760 mV forward bias.However, the background current density is muchhigher than the tunnelling portion of the currentdensity in this forward bias range, so no NDR canbe seen.At room temperature (T = 300 K), NDR was detectedonly for λ ranges ≤ 600 nm, while the impact ofresonant electron injection is still visible for curvesin the 800 – 600 nm range. The background currentdensity in the bias range around the tunnellingevent increases by about an order of magnitudewhen going from LN 2to room temperature.This is due to thermal activation of trap statesand increased thermal scattering during carriertransport through the QD array. It has the sameeffect on NDR suppression as explained above forelectron injection under forward bias.The QD arrays were processed by segregationanneals, comprising some 10 10 QDs as ESCsunder the contact pad. It is very encouraging tosee resonant tunnelling transport even at roomtemperature in a material system which was notgrown epitaxially. At LN 2temperature (77 K), anincrease in carrier temperature of DT = 560 K wasdetected when increasing the photon energy from1.6 ± 0.05 eV to 1.985 ± 0.075 eV under a constantphoton flux of F hn≈ 50 Suns. More suitable HotCarrier Absorber materials will be investigated.4.5.3.3 Hot Carrier Absorbers: slowing ofcarrier coolingResearchers:Robert Patterson, Gavin Conibeer, SantoshShreshtha, Dirk König, Pasquale Aliberti,Shujuan Huang, Yukio Kamikawa, Lara Treiber,Martin GreenCarrier cooling in a semiconductor proceedspredominantly by carriers scattering their energywith optical phonons. This builds up a nonequilibrium‘hot’ population of optical phononswhich, if it remains hot, will drive a reverse reaction86
to re-heat the carrier population, thus slowingfurther carrier cooling. Therefore the criticalfactor is the mechanism by which these opticalphonons decay into acoustic phonons, or heat inthe lattice. The principal mechanism by which thiscan occur is the Klemens mechanism, in which theoptical phonon decays into two acoustic phononsof half its energy and of equal and oppositemomenta [4.5.59].The build up of emitted optical phonons isstrongly peaked at zone centre both for compoundsemiconductor due to the Frölich interactionand for elemental semiconducotrs due to thedeformation potential interaction. The strongcoupling of the Frölich interaction also means thathigh energy optical phonons are also constrainedto near zone centre even if parabolicty of the bandsis no longer valid [4.5.60]. This zone centre opticalphonon population determines that the dominantoptical phonon decay mechanism is this pureKlemens decay.4.5.3.3.1 Suppression of phonon decay inbulk materialsIn some bulk semiconductors, with a largedifference in their anion and cation masses, therecan be a large gap between the highest acousticphonon energy and the lowest optical phononenergy, possibly large enough to block operationof this Klemens mechanism, which can be termeda ‘phononic band gap’. Work previously presented[4.5.61] using a simple 1D force constant model andcomplemented by high accuracy DFT computation[4.5.62] indicated that GaN, InN and InP all havelarge phononic band gaps, which are close to thosefound experimentally [4.5.63]. We are using timeresolved photoluminescence (tr-PL) to investigatethe carrier cooling rates in these materials.4.5.3.3.2 Time resolvedphotoluminescence measurements ofbulk phononic band gap materialsResearchersPasquale Aliberti, Dirk König, SantoshShrestha, Gavin Conibeer, Martin GreenCollaboration with:Raphael Clady, Murad Tayebjee, Tim Schmidt(University of Sydney), Nicholas Ekins-Daukes(Imperial College)The potential efficiency boost, which can beachieved by Hot Carrier solar cells, is directlyrelated to the possibility of extracting highenergy carriers from the absorber layer beforethermalisation, increasing the voltage and hencethe conversion efficiency. The poor conversionefficiency of photons with energies above the bandgap of the absorber is the main loss mechanismin conventional single junction solar cells. Theinvestigation of thermalisation time constants ofhot carriers is a crucial step towards the engineeringof Hot Carrier cells. The efficiency of an InN basedhot carrier solar cell has been calculated using acomplex theoretical model, see Section 4.5.3.1. Iwas found that the limiting efficiency is stronglyrelated to hot carriers relaxation velocity in theabsorber [4.5.57].A comparison of femtosecond time resolvedphotoluminescence (tr-PL) spectroscopy betweenInP and GaAs was reported in last year’s annualreport and is also now published [4.5.64]. Thisshowed a distinctly longer carrier cooling timeconstant for the wide phononic gap InP ascompared to almost zero phononic gap of GaAs.It also showed further evidence for excitationinto higher side valleys for both GaAs and InP forappropriate excitation wavelengths. In 2010 thehot carrier cooling in InN has been investigatedusing tr-PL. The wide gap between optical andacoustic branches in the InN phononic dispersionrelation (wider than that for InP) prevents theKlemens decay of optical phonon into acousticphonons. This can lead to slower carrier cooling dueto “Hot Phonon Effect” [4.5.65]. The decay of hotcarriers for different excitation wavelengths InN hasbeen investigated.Tr-PL experiments have been performedon InN samples using the measurementconfiguration shown in Fig. 4.5.53. In thistechnique a laser pulse acts as a switchinggate relating the photoluminescencesignal to the time domain. The PL signalis collected from the sample, after afemtosecond laser excitation, and focusedin a non linear crystal. The gate signalis generated from the same laser and isfocused on the same crystal after passingthrough an optical delay stage. The signalis detected using a monochromator anda PMT. Our system configuration provides150 fs pulses with tunable wavelengthover a range of 256 nm (4.84 eV) to 2.6 µm (0.48 eV).Figure 4.5.54 is a three dimensional representationof PL as a function of time for all the probedwavelengths. It can be observed that the PL sharplyrises when the carriers are photo-excited by thelaser pulse. The fast decay of the PL shows thethermalisation of carriers towards respective bandedges. The decay is faster for highly energeticcarriers compared to carriers closer to the bandgap.Thus the carrier population quickly degeneratestowards the band edges during the thermalisationprocess. In InN the thermalisation is most probablydue to interaction of highly energetic electrons andholes with LO phonons [4.5.66].To investigate the rate of the carrier coolingprocess, the effective temperature of the carrierpopulation has been calculated fitting the highenergy tail of the PL spectrum for every singletime during the cooling transient. The PL has beenfitted assuming that carriers form a Boltzmann-likedistribution in a femtosecond time scale using thefollowing equation.L(ε) represents the measured PL intensity atenergy ε, α(ε) is the measured sample absorptioncoefficient, E Gis the InN energy gap, 0.7 eV in thisARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORT3D representation of InN timeresolved PL data.Figure 4.5.5487
Page 4:
ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE
Page 8 and 9:
Page 10 and 11:
Page 12 and 13:
Page 14:
Page 17 and 18:
Efficiency and cost projections for
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
4.3 First GenerationWafer Based Pro
Page 25 and 26:
Jsc improvement afterapplying diele
Page 27 and 28:
Page 30 and 31:
Page 34 and 35:
Page 36:
Page 40 and 41: 4.4 Second Generation:Silicon, Orga
Page 42 and 43: ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE
Page 44: ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE
Page 47 and 48: I-V curves of the individual cells
Page 49 and 50: Voc [mV]2 m thick film480BSF deposi
Page 57 and 58: (a) The schematic band energystruct
Page 59 and 60: light intensity (Suns)100.0010.001.
Page 63 and 64: 4.5 Third Generation Strand -Advanc
Page 128 and 129: Z. Ouyang, X. Zhao, S. Varlamov, Y.
Page 130 and 131: J. Huang, J. Wong, M. Keevers and M
show all

Complete Report - University of New South Wales

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?