Santosh Shrestha

Hot carrier solar cells operating under practical conditions

We theoretically investigated the features of hot carrier solar cells, from which photogenerated carriers are extracted before they are completely thermalized. There are three channels of energy dissipation from photogenerated carriers that lowers the conversion efficiency: thermalization in the absorber, emission from the absorber, and thermodynamically unavoidable heat flux to the ambient. The emission increases with increasing carrier density in the absorber, whereas the heat flux decreases. Previous calculations of the conversion efficiency have been carried out under the supposition of no thermalization of carriers. In this case, the dominant process of energy dissipation is the emission, like conventional solar cells represented by the Shockley and Queisser formula. In practice, the carriers should be extracted to external circuits immediately after photogeneration because they are partially thermalized. This restriction leads to a much smaller carrier density and consequently more significant energ...

Nitrogen-rich indium nitride

K.S.A.B. would like to acknowledge the support of an Australian Research Council Fellowship. We would also like to acknowledge the support of the Australian Research Council through a Large grant and a Discovery grant; the support of a Macquarie University Research Development Grant, and the Australian Institute of Nuclear Science and Engineering for SIMS access.

Investigation of theoretical efficiency limit of hot carriers solar cells with a bulk indium nitride absorber

Theoretical efficiencies of a hot carrier solar cell considering indium nitride as the absorber material have been calculated in this work. In a hot carrier solar cell highly energetic carriers are extracted from the device before thermalisation, allowing higher efficiencies in comparison to conventional solar cells. Previous reports on efficiency calculations approached the problem using two different theoretical frameworks, the particle conservation (PC) model or the impact ionization model, which are only valid in particular extreme conditions. In addition an ideal absorber material with the approximation of parabolic bands has always been considered in the past. Such assumptions give an overestimation of the efficiency limits and results can only be considered indicative. In this report the real properties of wurtzite bulk InN absorber have been taken into account for the calculation, including the actual dispersion relation and absorbance. A new hybrid model that considers particle balance and energy...

The nature of nitrogen related point defects in common forms of InN

The role of point defects related to the presence of excess nitrogen is elucidated for InN thin films grown by different techniques. Elastic recoil detection analysis has shown the presence of excess nitrogen in state-of-the-art InN films. Using x-ray photoelectron spectroscopy and x-ray diffraction it is shown that two distinct forms of point defects can be distinguished; one of these appears to be an interstitial form of nitrogen, common in some forms of polycrystalline InN. The other is associated with a combined biaxial and hydrostatic strain observed for molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) grown films, and may be a mixture of the nitrogen-on-metal antisite defect and lower densities of indium vacancies and interstitial nitrogen. The high density of defects present in all the InN samples examined suggests that stoichiometry related point defects dominate the electrical and optical properties of the material. The difference in the type of point defect observed for polycryst...

Applications of atomic layer deposition in solar cells

Atomic layer deposition (ALD) provides a unique tool for the growth of thin films with excellent conformity and thickness control down to atomic levels. The application of ALD in energy research has received increasing attention in recent years. In this review, the versatility of ALD in solar cells will be discussed. This is specifically focused on the fabrication of nanostructured photoelectrodes, surface passivation, surface sensitization, and band-structure engineering of solar cell materials. Challenges and future directions of ALD in the applications of solar cells are also discussed.

Efficient electron transfer in carbon nanodot–graphene oxide nanocomposites

Carbon nanodots (CNDs) have emerged as fascinating materials with exceptional electronic and optical properties, and thus they offer many promising applications in photovoltaics and photocatalysis. In this paper we investigate electron transfer in nanocomposites of CNDs–graphene oxide (GO), –multi-walled carbon nanotubes (MWNTs) and –TiO2 nanoparticles without linker molecules, using steady state and time-resolved spectroscopy. Significant fluorescence quenching was observed in the CND–GO system, and it is attributed to the ultrafast electron transfer from CNDs to GO with a time constant of 400 fs. In comparison, carbon nanotubes result in static quenching of fluorescence in CNDs. No charge transfer was observed in both CND–MWNT and CND–TiO2 nanocomposites. This finding suggests that the CND–GO nanocomposite can be an excellent candidate for hot carrier solar cells due to the effective carrier extraction, broad spectral absorption, weak electron–phonon scattering, and thus a slow cooling rate for hot carriers.

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

Ultrafast electron transfer in the nanocomposite of the graphene oxide–Au nanocluster with graphene oxide as a donor

Graphene oxide has been extensively investigated as an electron acceptor due to its exceptional electronic and optical properties. Here we report an unusual ultrafast electron transfer occurring in the nanocomposites of Au nanocluster (Au NC)–graphene oxide (GO) in which GO acts as an electron donor. An ultrafast electron transfer is corroborated from the excited states of graphene oxide into the highest occupied molecular orbital (HOMO) of Au NCs. It is found that the electron transfer rate is significantly higher in Au10–GO nanocomposites (4.17 × 1012 s−1) than that in Au25–GO (0.49 × 1012 s−1) due to a larger energy difference and smaller sized ligands. This finding suggests that graphene oxide–Au nanocluster nanocomposites can be very useful to construct novel nanostructures with enhanced visible light photovoltaic, photonic and photo-catalytic activities.

Non-ideal energy selective contacts and their effect on the performance of a hot carrier solar cell with an indium nitride absorber

The hot carrier solar cell is a third generation photovoltaic device that extracts photo-generated carriers before they thermalise. In this work, the efficiency of a hot carrier solar cell with a 50 nm indium nitride (InN) absorber layer has been calculated, taking into account the realistic transport properties of energy selective contacts. The cell performance has been modeled considering the carrier extraction through contacts as ballistic. A potential practical implementation of a hot carrier solar cell, with contacts based on an InXGa1−XN/InN/InXGa1−XN quantum well structure, has been proposed, with calculated maximum efficiency of 37.15% under 1000 suns.

Tunability Limit of Photoluminescence in Colloidal Silicon Nanocrystals

Luminescent silicon nanocrystals (Si NCs) have attracted tremendous research interest. Their size dependent photoluminescence (PL) shows great promise in various optoelectronic and biomedical applications and devices. However, it remains unclear why the exciton emission is limited to energy below 2.1 eV, no matter how small the nanocrystal is. Here we interpret a nanosecond transient yellow emission band at 590 nm (2.1 eV) as a critical limit of the wavelength tunability in colloidal silicon nanocrystals. In the “large size” regime (d > ~3 nm), quantum confinement dominantly determines the PL wavelength and thus the PL peak blue shifts upon decreasing the Si NC size. In the “small size” regime (d < ~2 nm) the effect of the yellow band overwhelms the effect of quantum confinement with distinctly increased nonradiative trapping. As a consequence, the photoluminescence peak does not exhibit any additional blue shift and the quantum yield drops abruptly with further decreasing the size of the Si NCs. This finding confirms that the PL originating from the quantum confined core states can only exist in the red/near infrared with energy below 2.1 eV; while the blue/green PL originates from surface related states and exhibits nanosecond transition.