Complete Report - University of New South Wales

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