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The unique tunable bandgaps and straightforward synthesis of colloidal quantum dots make them promising low-cost materials for photovoltaics. High-performance colloidal quantum dot solar cells rely on good-quality electron transporting layers (ETLs) to make carrier selective contacts. Despite extensive use of n-type oxides as ETLs, a detailed understanding of their surface and interface states as well as mechanisms to improve their optical properties are still under development. Here, we report a simple procedure to produce MgCl2 passivated ZnO nanoparticles ETLs that show improved device performance. The MgCl2 treated ZnO electron transporting layers boost the PbS colloidal quantum dot cell efficiency from 6.3% to 8.2%. The cell exhibits reduced defects leading to significant improvements of both FF and J sc. This low-temperature MgCl2 treated ZnO electron transporting layer may be applied in solution processed tandem cells as a promising strategy to further increase cell efficiencies.

Multiple quantum wells have been shown significantly reduced hot carrier cooling rates compared to bulk material and are thus a promising candidate for hot carrier solar cell absorbers. However, the mechanism(s) by which hot carrier cooling is restricted is not clear. A systematic study of carrier cooling rates in GaAs/AlAs multiple quantum wells (MQWs) with either varying barrier thickness or varying well thickness is presented in this paper. These allow an investigation as to whether the mechanisms of either a modification in hot carrier diffusion or a localisation of phonons emitted by hot carriers are primarily responsible for reduced carrier cooling rates. With the conclusion that for the structures investigated the situation is rather more complex with both carrier mobility to modify hot carrier diffusion, different diffusion rates for electrons and holes and reflection and localisation of phonons to enhance phonon bottleneck all playing their parts in modulating phonon reabsorption and hot carrier behaviour.

The absorber of the hot carrier solar cell (HCSC) needs to have a considerably reduced hot carrier thermalisation rate, in order to maintain the photo-generated hot carriers for enough time such that they can be extracted. The slow carrier cooling effect is predicted in materials in which the phononic band gap is sufficiently large to block the Klemens decay. Binary compounds with a large mass ratio between the constituent elements are likely to have large phononic band gap. Titanium hydride is one of these binary compounds that has the potential to become an absorber of the HCSC. Whilst a large phononic gap has been observed in stoichiometric TiH2, it has not been experimentally confirmed for hydrogen deficient TiH x (where x < 2). In this article, we report the phonon density of states of TiH1.65 measured using inelastic neutron scattering and presented to clearly show the phononic band gap. We also present the carrier thermalisation process of a TiH x (1< x

The hot carrier (HC) solar cell is one of the most promising advanced photovoltaic concepts. It aims to minimise two major losses in single junction solar cells due to sub-band gap loss and thermalisation of above band gap photons by using a small bandgap absorber, and, importantly, collecting the photo-generated carriers before they thermalise. In this paper we will present recent development of the two critical components of the HC solar cell, i.e., the absorber and energy selective contacts (ESCs). For absorber, fabrication and carrier cooling rates in potential bulk materials — hafnium nitride, zirconium nitride, and titanium hydride are presented. Results of ESCs employing double barrier resonant tunneling structures Al2O3/Ge quantum well (QW)/Al2O3 and Al2O3/PbS quantum dots (QDs)/Al2O3 are also presented. These results are expected to guide further development of practical HC solar cell devices.

Intrinsic, boron (B)-doped, and phosphorus (P)-doped silicon nanocrystals (Si NCs) formed from an excess Si concentration of 40 at. % were investigated to study their structural, optical, and electrical properties. Atom probe tomography (APT) revealed that the size and arrangement of Si NCs were different in each sample. A strong blue shift in photoluminescence spectra for the intrinsic and B-doped Si NCs was correlated with the volume fraction of small Si NCs. The lower resistivity of the B-doped sample than the P-doped one was explained by the percolation of Si NCs through the film.

In this work, the ground-state properties of the solution processable semiconductor M2 SnBr6 (M = K, Rb, Cs) have been computed using density functional theory. Similarities in the band structures are observed among these three materials and are shown to result from minimal contributions of the cation to electronic states near the Fermi level. A fundamental bandgap of 1.2 eV is predicted for the materials, which is close to the ideal bandgap for single-junction photovoltaic applications. However, in reality, a larger bandgap is expected because DFT calculations with the PBE functional underestimate the gap. Material optical properties including dielectric constants, reflective indices, reflectance and absorption coefficients are shown to be competitive for solar-energy harvesting. The ionization energies are 6 eV below the vacuum level, while effective masses are relatively small around 0.3, with light hole masses comparable to those of electrons.

It has been proposed that the rate of hot carrier thermalisation can be slowed down if there is a sufficiently large gap in the phonon dispersion for a bulk material. This phenomenon is critical for the development of high efficiency hot carrier solar cells to minimise energy loss to thermalisation. A gap where the minimum of the optical branches is at least twice that of the maximum of the acoustic branches can prevent the primary pathway where optical phonons loses energy, the Klemens' decay mechanism. The large gap in the phonon dispersion eliminates the Klemens' decay pathway due to energy and momentum conservation laws. This enables the electron population to remain hot by allowing sufficient time for optical phonons to re-scatter its energy to electrons. Binary compounds with a large mass difference between the two constituent atoms and high level of crystal symmetry such as zirconium nitride and hafnium nitride (HfN) have such a gap in their phonon dispersion. HfN thin films have been sputtered on silicon and quartz substrates. Characterisation of hot electron lifetimes in HfN films have been performed using ultrafast transient absorption spectroscopy. Preliminary analysis of transient absorption data, both spectra and time evolution has indicated high carrier temperatures with a nanosecond long decay time. It is postulated the long hot carrier lifetime is due to the large phononic gap.

Anharmonic decay of high frequency phonons into low frequency vibrations is a significant energy loss mechanism in semiconductors. In the field of Hot Carrier Solar Cells (HCSC), preventing this decay is of great importance as it helps increase hot carriers lifetime. Phonon decay in nitride compounds as well as their consisting nano-crystals like quantum dots and multiple quantum wells (MQWs) has not been extensively studied in the literature. In this work, the decay channels of the A1 and the high-lying E2 optical phonons in an InN/GaN MQW are analysed. We find that the no Klemens decay is present in A1 mode whereas on the contrary E2 is dominated by this process. We also observe that the linewidth for A1 is enlarged a few times than the bulk counterpart while that for E2 remains similar, which is attributed to different vibration nature.

Boron (B)- and antimony (Sb)-doped Si quantum dots (QDs) in Si3N4 films were fabricated using the co-sputtering method with a post-deposition anneal. The effect of B and Sb on Si QDs films was investigated in terms of structural, optical and electrical properties. It is found that a low dopant concentration induced negligible structural changes in the Si QD films. The PL intensity decreases with increasing B or Sb content. This could result from the non-radiative recombination processes attributed to defects associated with the dopants and Auger processes due to successful doping of Si QDs. For the B-doped sample the conductivity increases about 100 times, which could be attributed to an increase in carrier concentration. For the Sb-doped sample, a significant increase (six orders of magnitude) in conductivity suggests an effective Sb doping. The charge transport mechanism in the Sb-doped Si QD films matches well with the percolation-hopping model in low temperature region. Both B- and Sb-doped samples show thermally activated hopping conduction characteristics in the range of 220–320 K.

We propose a technologically feasible concept of a hot carrier (HC) solar cell (SC) which fulfills the electronic, optical, and to some extent the phononic criteria required. The energy selective process of HCs is implemented into the hot carrier absorber (HCA). Its electronic properties are investigated by a Monte-Carlo code which simulates random deviations of structure thickness and a normal distribution of random elastic electron (e -) scattering. The structure can be grown epitaxially as a HC-SC test device.

An improved design for double-barrier resonant tunnelling structures using silicon quantum dots (QDs) was quantitatively analyzed using a multi-mode scatter matrix method. Multilayer metal-barrier-QD/matrix-barrier-metal stacks that maximize both electron transport and confined energy are sought. Si QDs grown in silicon dioxide with silicon carbide barriers were the most advantageous combination for single QD layer double-barrier structures (DBSs). Lateral SiO2 barriers provided greater confinement, especially in smaller dots and also caused increased splitting between resonant levels. These structures are excellent candidates for use as energy selective contacts (ESCs) and as layers in all-silicon tandem cells.

Highly conductive thin films composed of antimony-doped Si nanocrystals (Si-NCs) embedded in the Si3N4 matrix were prepared by co-sputtering technique. The N-type electrical behavior in the doped films as observed from Hall measurements was attributed to free carriers generation resulting from the effective Sb doping. Quantitative analysis has demonstrated that effective Sb doping at a concentration of 0.54 at.% results in an improvement on the electrical conductivity (σ) by more than six orders of magnitude, up to 2.8×10−2 S/cm. The charge transport mechanism can be explained well by the percolation-hopping model where the conductivity follows σ~exp[−(T0/T)]1/2 at temperature lower than 220 K.

A hybrid superlattice structure consisting of 30 periods of alternating amorphous Si0.7C0.3 (5 nm) layers and ultra-thin Si3N4 barrier layers (0.2–2.0 nm) has been synthesised by magnetron sputtering, with subsequent annealing by a rapid thermal annealing (RTA) process. Si nano-crystals behave well confined within individual layers when the Si3N4 layer thickness is over 0.8 nm, due to the Si very low diffusion coefficient in Si3N4 matrix. Hopping is regarded as the dominant carrier transportation mechanism in the film based on fitting the temperature-dependent I-V measurements results. In conclusion an optimum Si3N4 barrier layer thickness of approximately 0.8 nm may be appropriate as a candidate material for photovoltaic application.

Thin films composed of Ge nanocrystals embedded in amorphous SiO2 matrix (Ge-NCs TFs) were prepared using a low temperature in-situ growth method. Unexpected high p-type conductivity was observed in the intrinsic Ge-NCs TFs. Unintentional doping from shallow dopants was excluded as a candidate mechanism of hole generation. Instead, the p-type characteristic was attributed to surface state induced hole accumulation in NCs, and the hole conduction was found to be a thermally activated process involving charge hopping from one NC to its nearest neighbor. Theoretical analysis has shown that the density of surface states in Ge-NCs is sufficient to induce adequate holes for measured conductivity. The film conductivity can be improved significantly by post-growth rapid thermal annealing and this effect is explained by a simple thermodynamic model. The impact of impurities on the conduction properties was also studied. Neither compensation nor enhancement in conduction was observed in the Sb and Ga doped Ge-NCs TFs, respectively. This could be attributed to the fact that these impurities are no longer shallow dopants in NCs and are much less likely to be effectively activated. Finally, the photovoltaic effect of heterojunction diodes employing such Ge-NCs TFs was characterized in order to demonstrate its functionality in device implementation., 28 pages, 13 figures, manuscript submitted to Journal "Nanotechnology", IOP

We have evaluated the influence of practical factors on the conversion efficiency of hot carrier solar cells, from which photogenerated carriers are extracted before being completely thermalized. Equilibration and thermalization of the carriers, and energy dissipation associated with hot carrier extraction were involved in a thermodynamic modeling. Among them, thermalization has been found to have the greatest impact. Even though a 1 ns thermalization time could be realized, the conversion efficiency is close to the Shockley–Queisser limit (34%) under 1 sun irradiation, and lower than the limiting values of triple-junction cells (around 60%) at 1000 sun.

Fixed charge in metal‐insulator‐semiconductor (MIS) devices featuring silicon (Si) nanocrystals (NCs) embedded in a silicon dioxide (SiO2) insulating layer has been examined. By means of capacitance‐voltage (C-V) measurements, the density of net fixed positive oxide charge was found to decline over a 7 week period after device fabrication. Two capacitance peaks originally observed in the C-V curves were found to disappear over the same period. The disappearance of one of these peaks is attributable to reduced external inversion layer coupling. The second of these peaks disappears due to a reduced interface state response to the small signal excitation. An explanation for the decline in the small signal interface state response with the reduction in net fixed positive oxide charge is provided and supported by use of MIS tunnel diode modelling.

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si substrate by a (SiO(2)+Ge)/(SiO(2)+GeO(2)) superlattice approach, using an rf magnetron sputtering technique with a Ge+SiO(2) composite target and subsequent thermal annealing in N(2) ambient at 750 °C for 5 min. X-ray diffraction (XRD) measurements indicated the formation of Ge nanocrystals with an average size estimated to be 9.8 nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode shifted downwards to 298.8 cm(-1), which was caused by quantum confinement of phonons in the Ge nanocrystals. X-ray photoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainly Ge(0) in the (SiO(2)+Ge) layer and Ge(4+) in the (SiO(2)+GeO(2)) layer in the superlattice structure. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (SiO(2)+Ge) layers, and had good crystallinity. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the 'Z' growth direction compared with the conventional Ge-ncs fabrication method using a single and thick SiO(2) matrix film.

Silicon (Si) quantum dot (QD) materials have been proposed for 'all-silicon' tandem solar cells. In this study, solar cells consisting of phosphorus-doped Si QDs in a SiO(2) matrix deposited on p-type crystalline Si substrates (c-Si) were fabricated. The Si QDs were formed by alternate deposition of SiO(2) and silicon-rich SiO(x) with magnetron co-sputtering, followed by high-temperature annealing. Current tunnelling through the QD layer was observed from the solar cells with a dot spacing of 2 nm or less. To get the required current densities through the devices, the dot spacing in the SiO(2) matrix had to be 2 nm or less. The open-circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in Si QDs or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate. Successful fabrication of (n-type) Si QD/(p-type) c-Si photovoltaic devices is an encouraging step towards the realization of all-silicon tandem solar cells based on Si QD materials.