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Quantum dots (QDs) have a wide range of applications in the field of optoelectronics. They have been playing multiple roles within the configuration of a device, by serving as the building blocks for both the active layer and the carrier transport layer. While the performance of various optoelectronic devices has been steadily improving via developments in passivating the QD active layer, the possible improvement via passivation of the QD-based carrier transport layer has been largely overlooked. Here, with lead sulfide QD photovoltaics as the platform of study, we demonstrate that the device performance can be significantly improved by passivating the QD hole transport layer (HTL) with zinc salt post-treatments. The power conversion efficiency is improved from 8.7% of the reference device to 10.2% and 9.5% for devices with zinc acetate (ZnAc)- and zinc iodide (ZnI2)-treated HTLs, respectively. Transient absorption spectroscopy confirms that both treatments effectively reduce band-tail states and increase carrier lifetime of the HTLs. Further elemental analysis shows that ZnAc provides a higher amount of Zn2+ for passivation while maintaining the function of HTL by allowing essential p-doping oxidation. In contrast, the additional I- passivation from ZnI2 inhibits p-doping oxidation and limits the function of HTL. This work demonstrates the potential of improving device performance by passivating the QD-based HTLs, and the method developed is likely applicable to other optoelectronic devices.

Thermal annealing temperature and time dictate the microstructure of semiconductor materials such as silicon nanocrystals (Si NCs). Herein, atom probe tomography (APT) and density functional theory (DFT) calculations are used to understand the thermal annealing temperature effects on Si NCs grown in a SiO2 matrix and the distribution behaviour of boron (B) and phosphorus (P) dopant atoms. The APT results demonstrate that raising the annealing temperature promotes growth and increased P concentration of the Si NCs. The data also shows that the thermal annealing does not promote the incorporation of B atoms into Si NCs. Instead, B atoms tend to locate at the interface between the Si NCs and SiO2 matrix. The DFT calculations support the APT data and reveal that oxygen vacancies regulate Si NC growth and dopant distribution. This study provides the detailed microstructure of p-type, intrinsic, and n-type Si NCs with changing annealing temperature and highlights how B and P dopants preferentially locate with respect to the Si NCs embedded in the SiO2 matrix with the aid of oxygen vacancies. These findings will be useful towards future optoelectronic applications.

We demonstrated an organic-inorganic composite as hole transport layer consisting of water-free PEDOT:PSS and tungsten oxide (WO3) nanoparticles, which enhanced the power conversion efficiency of normal-structure perovskite solar cells (PSCs). The PSC employing the WO3/PEDOT:PSS film as a hole transport layer exhibited better device stability and improved power conversion efficiency (PCE) with approximately 20% performance enhancement. The highest PCE obtained from WO3/PEDOT:PSS device was 15.1%, owing to simultaneous increments on short circuit current, open circuit voltage and fill factor. WO3 nanoparticles can fill out the pinholes of porous PEDOT:PSS layer, demonstrating a void-free and homogenous HTL surface. WO3/PEDOT:PSS composite demonstrated much higher charge conductivity and hole mobility than pristine PEDOT:PSS layer. The interface of perovskite absorber and WO3/PEDOT:PSS HTL also contained less defect density. The superior performance of WO3/PEDOT:PSS composite was attributed to the reduced current leakage, enhanced hole extraction characteristics, and less trap-assisted interfacial recombination via current density - voltage, electron microscopy (SEM, AFM and c-AFM), electrochemical impedance spectroscopy (EIS) and space-charge limited current–voltage (SCLC) measurements and analysis.

Photovoltaic devices directly convert incident solar energy to electricity. Above bandgap incident photons excite carriers (electron and holes) deep into their respective bands. These excited carriers are called hot carriers and they undergo several physical loss processes to lose their excess energy to the lattice and thermalize to the band edge. In conventional photovoltaics, these carriers are collected from the band-edge before they recombine. The Hot Carrier Solar Cell (HCSC) is a promising third generation photovoltaics concept for energy loss reduction in solar cells by inhibiting the thermalization losses in the absorber. It has a limiting efficiency of 85%, well beyond the Shockley–Queisser limit of 33% which is the upper threshold for conventional photovoltaic cells e.g., Si solar cells. The primary mission of the hot carrier absorber engineering is to achieve a cooling rate of the carriers from 100s of picoseconds to nanoseconds, comparable to radiative recombination. One way to achieve a slower hot carrier cooling rate in the absorber is to acquire a phonon bottleneck effect. This would impede phonon assisted hot carrier relaxation. We have investigated the use of transition metal hydrides as HCSC absorber materials because of several potential advantages. The large atomic mass difference between the transition metal and hydrogen atoms leads to a large phononic band gap and a separation of the optical and acoustic phonon branches. This large phononic gap can inhibit the Klemens mechanism whereby longitudinal optical phonon decays into two acoustic phonons – the main route by which optical phonons decay - hence slowing down the hot carrier thermalization. In this work, we report on the fabrication and preliminary results of non-stoichiometric titanium hydride, vanadium hydride and zirconium hydride thin films for their quality and useability as HCSC absorbers. The thin films were prepared using electron beam evaporation and characterized using XRD, UV-VIS/IR, Ellipsometry and Raman spectroscopy. The results show that the amount of hydrogen in TiH x is such that x=1, for VH x x= 0.81 and for ZrH x x=1. From UV/VIS/IR, the reflection from all samples is dominated by a Pd layer used to prevent oxidation. The suitability of these materials as hot carrier absorbers is yet to be fully assessed, but the data presented here are important initial parameters in making that determination.

Breakdown and utilization of cellulose are critical for the bioenergy sector; however, current cellulose-to-energy conversion schemes often consume large quantities of unrecoverable chemicals, or are expensive, due to the need for enzymes or high temperatures. In this paper, we demonstrate a new method for converting cellulose into soluble compounds using a mixture of Fe2+ and Fe3+ as catalytic centers for the breakdown, yielding Fe3O4 nanoparticles during the hydrothermal process. Iron precursors transformed more than 61% of microcrystalline cellulose into solutes, with the composition of the solute changing with the initial Fe3+ concentration. The primary products of the breakdown of cellulose were a range of aldaric acids with different molecular weights. The nanoparticles have concentration-dependent tuneable sizes between 6.7 and 15.8 nm in diameter. The production of value-added nanomaterials at low temperatures improves upon the economics of traditional cellulose-to-energy conversion schemes with the precursor value increasing rather than deteriorating over time.

The Scolar Programme for Photovoltaics is installing 100 grid-connected photovoltaic systems in schools and colleges in the UK. Participating schools gain an architecturally integrated PV array (average 650Wp), an invertor, a dedicated computer and an Internet link via the Scolar website. Here all the performance data for the 100 schools will be available to all the others. Access is also provided to a comprehensive educational package; which covers a wide range of information on renewable and solar energy as well as project based material aimed at various key stages of the UK national curriculum. NB: Bob Hill has been an invaluable asset to the initialisation and the implementation of this programme. His death is a severe blow to it and to the PV community as a whole.

Hot carrier solar cells promise theoretical power conversion efficiencies far beyond the single junction limit. However, practical implementations of hot carrier solar cells have lagged far behind those theoretical predictions. Reciprocity relations for electro-luminescence from conventional single junction solar cells have been extremely successful in driving their efficiency ever closer to the theoretical limits. In this work, we discuss how the signatures of a functioning hot carrier device should manifest experimentally in electro-luminescence and dark $I-V$ characteristics. Hot carrier properties lead to deviations from the Shockley diode equation that is typical for conventional single junction solar cells. These deviations are directly linked to an increase in temperature of the carriers and therefore the temperature measured from electro-luminescence spectra. We also elucidate how the behaviour of hot carrier solar cells in the dark depends on whether Auger processes play a significant role, revealing a stark contrast between the regime of negligible Auger recombination (carrier conservation model) and dominant Auger recombination (Impact Ionization model) for hot carrier solar cells., 11 pages, 8 figures

Hafnium nitride with rock – salt structure (HfN) is a suitable material for the hot carrier solar cell (HCSC) absorber, primarily due to wide phonon band gap. Nitrogen – rich phase of hafnium nitride crystallizes in cubic Th 3 P 4 structure and is a semiconductor. Here, we have synthesized nitrogen – rich c-Hf 3 N 4 using thermal atomic layer deposition and characterized the hot carrier dynamics with transient absorption (TA) spectroscopy. Fitting of the TA kinetics result in lifetime of the carriers in the order of hundreds of picoseconds. This opens feasibility of nitrogen – rich hafnium nitride to be used in photovoltaic device as an HCSC absorber.

Hot carrier cells offer the potential for very high efficiencies if slowed carrier cooling can be demonstrated effectively. Various mechanisms of phonon interaction have been identified that can lead to such slowed cooling with classes of materials exhibiting long hot carrier lifetimes. The exact reasons for these are not completely clear, but a study and comparison of the materials and mechanisms will give greater insight into constructed robust hot carrier absorber materials.

Due to excellent performance and low fabrication costs, perovskite solar cells have become an attractive choice for building integrated photovoltaics (BIPV), especially semitransparent windows. Semitransparent perovskite solar cells are also useful as the upper sub-cell in tandem applications. In this study, we demonstrate efficient perovskite solar cells with good transparency in the visible spectrum (400–800 nm). The device efficiency and transparency were controlled by varying the perovskite layer thickness, including devices with a perovskite thickness as low as 40 nm. A dielectric/metal/dielectric (D/M/D) electrode was used as the transparent back electrode. The best performing device exhibited >9% efficiency at 12.4% average visible transparency. By reducing the perovskite layer thickness, the transparency further increased to 20.5% at an efficiency of 3.5%. Furthermore, perovskite layer thickness-dependent hysteresis behaviour, colour perception in human eye and month-long systematic degradation patterns are investigated and discussed comprehensively. The resulting solar cells are one of the highest performing semitransparent perovskite solar cells using D/M/D back electrode and a significant step forward to BIPV and tandem applications.

Wastewater was sourced directly from an industrial anaerobic biodigestion system to assess its potential to serve as a growth medium and improve the digestibility of microalgae in a closed-loop bioenergy system. Cyanobacteria (Oscillatoria sp.) were cultured in the wastewater and further explored as a feedstock for biomethane production via methanogenesis. Promisingly high levels of nitrate (1180 mg L−1) and phosphate (61.5 mg L−1) were found in the nitrification-treated, NO3-rich wastewater stream. While cyanobacteria were not observed to grow directly in this NO3-rich wastewater stream, a modest dilution of the wastewater by a factor of two produced denser cultures than could be grown in standard f/2 media, which was made by enriching seawater. Growth in wastewater to improve the digestibility of the cyanobacteria resulted in significantly more methane from cultures in both the NO3-rich and the untreated, NH4-rich wastewater streams.

Hybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed-halide hybrid perovskite single crystals of MAPbI3-xBrx (MA = CH3NH3+ and x = 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed-halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed-halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties.

Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination-well known to detrimentally affect the performance of devices-is still not well understood for thin samples where the thickness is comparable to or less than the carrier diffusion length. Here, using time-resolved microspectroscopy along with modeling, we investigate charge-carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ∼20 to 200 nm, ranging from much lesser than to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photostability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model, we determine the surface recombination velocity. This work provides a comprehensive understanding of the role of surface recombination and charge-carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.

Although predicted to be stable under ambient conditions, the experimental synthesis of silicene—the two-dimensional silicon analogue of graphene—has been a great challenge. Here, we report the pre...

PbS quantum dot solar cells are promising candidates for low-cost and highly efficient light harvesting devices owing to their solution processability and bandgap tunability. The p-type ethanedithiol (EDT) treated PbS quantum dot film plays an important role in PbS quantum dot solar cells with an n-i-p junction device structure. However, despite their sulphur-rich surface the EDT-treated PbS quantum dot film still have a relatively low carrier concentration. Higher carrier concentrations in this layer are desirable to extend depletion regions and improve hole extraction. Also imbalances in the charge mobility between the intrinsic layer and the p-type layer may lead to charge build-up at this interface. These obstacles limit further improvement of the device performance. Herein, we utilize EDT-treated Ag-doped PbS quantum dots as a p-type layer to fabricate PbS quantum dot photovoltaic cells. The carrier carrier concentration, mobility and band extrema as well as Fermi energy levels of Ag doped PbS quantum dot film can be tailored by tuning the Ag/Pb mole ratio from 0.0% to 2.0% during fabrication. The device performance has been significantly improved from 9.1% to 10.6% power conversion efficiency largely due to improvements in carrier concentration in the PbS-EDT layer through the incorporation of silver impurities.

Hybrid halide perovskite solar cells (PSCs) have emerged as a strong candidate for low cost photovoltaics, owing to ease of processing and material abundance. The stability and performance of these devices are contingent on the quality of the interfaces. In this work, we report the novel interface engineered hole transport layer (HTL), incorporating Vanadium Pentoxide (V2O5) and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) bilayer (PVO) for efficient charge transport in the devices. The devices incorporating the PVO bilayer HTL exhibits 20% higher power conversion efficiency (PCE) than conventional PEDOT only devices. The PSCs incorporating the PVO bilayer HTL demonstrated superior electronic properties as evaluated using impedance spectroscopy measurements. The recombination resistance (RRec) of the bilayer based devices are 57% higher than the reference cells. In addition to high charge selectivity, the bilayer PSCs exhibit low interfacial capacitance originating from electrode polarization and almost zero hysteresis. Furthermore, the bilayer based devices are more stable than PEDOT only devices; retaining 95% of their initial PCE even after 18 days of testing. The mechanism behind superior charge transport in PVO bilayer HTL and its role in stability enhancement are also discussed.

AgBiS2 nanocrystals are good candidates in optoelectronic applications due to their solution-processability, earth abundance and non-toxic properties. AgBiS2 thin film solar cells have demonstrated promising performance due to their high absorption coefficients and suitable bandgaps for light harvesting. However, their physical properties such as mobility, carrier concentration and photosensitivity, which are very important for photovoltaics, remain unreported. Herein, we develop an improved amine-based synthesis route and systematically investigate these properties by characterizing field effect transistors and photodetectors fabricated from AgBiS2 nanocrystals. We found that the amine-based synthesis improved their measurable semiconducting properties and their solar cell performance. The optimal thickness of a champion solar cell was found to increase to 65 nm, which achieved a power conversion efficiency of 4.3% based on a ZnO/AgBiS2/P3HT/Au device structure, compared with 35 nm of a champion solar cell with a power conversion efficiency of 3.9% reported previously. This modified approach could be applied in the preparation of other high quality nanocrystals that uses silver, where single valence Ag cations are of increasing importance in stable solution processed nanomaterials.

Solution-processable lead selenide (PbSe) colloidal quantum dots (QDs) are promising candidates for photovoltaics due to their efficient multiple exciton generation and carrier transport. However, despite these advantages, currently the best PbSe QD solar cells (QDSCs) still have short-circuit current densities (JSC) of about 25 mA cm−2. Here, we report the introduction a solution-processed trivalent antimony-doped tin oxide buffer layer at the interfaces in the device, which led to significant improvement in the JSC. Consistent with the optical simulations, the external quantum efficiency of the devices was improved in a region corresponding to the PbSe QD/buffer layer interfaces (400–600 nm), implying enhanced electron extraction. The improved performance is attributed to optimized gradient energy level alignment and shunt blocking at the interfaces. With this simple interfacial treatment, the JSC of the champion device was increased significantly to 26.7 mA cm−2, a more than 8% improvement compared to the control device. A further increase in the fill factor was also observed, leading to an over 11% improvement in the champion power conversion efficiency, from 7.1% to 7.9%. This work offers a simple method for interfacial engineering that led to PbSe QDSCs with efficiencies that are among the highest reported in the literature.

A major factor limiting the performance of nanostructured CuInS2 photovoltaic devices is the current density, pointing to poor charge carrier transport in CuInS2 nanoparticle films. In a typical CuInS2 core/shell structure synthesis, ZnS is typically chosen as a shell material for CuInS2 core passivation, which leads to a significant enhancement of the photoluminescence quantum yield from the CuInS2 nanoparticles. While typically a marker for excellent photovoltaic performance, in this case the increased photoluminescence likely signals increased charge carrier confinement and reduced transport through any thin films fabricated from the nanoparticles. Here we show that replacing the typical divalent Zn cation surface termination with a monovalent Ag cation leads to small improvements in charge carrier transport through nanostructured films. This surface termination intentionally introduces lower energy electronic states directly at the surface of the CuInS2 nanoparticles, reducing charge carrier confinement and thus increasing charge carrier mobility between nanoparticles. The study assessed appropriate Ag molar ratios to be used in synthesis, with an 8% Ag : Cu ratio found to be optimal. The passivation offered by Ag surface termination appears comparable to that from Zn with strong photoluminescence observed in both cases. Slight improvements in the performance of all-solid-state nanoparticle CuInS2 photovoltaic devices are obtained, with current densities in the Ag surface terminated case being increased by just under 10%. These findings outline a potential strategy for the synthesis of type II core–shell CuInS2 quantum dot thin film devices with improved charge transport.

Slowed carrier intervalley scattering (IVS) may trap some photo-excited carriers within side valleys and reduce the overall thermalization rate. On the other hand, the thermalization rate could also be significantly retarded within the gamma valley by slowing the Klemens decay due to a large phononic bandgap via the phonon bottleneck effect (PBE). However, the correlation between IVS and PBE on slowing the carrier relaxation rate is still unclear. Herein, high-quality bulk InN and GaSb with similar electronic bandgap but remarkably different phononic structures are studied to clarify the above correlation through picosecond time-resolved photoluminescence. The occurrence of IVS is controlled by using different pump energies at high carrier density (∼1019 cm−3). The relaxations of carriers excited at the band edge just above and far above the side valley threshold are, therefore, studied in GaSb. It shows that the effective hot carrier lifetime (τ1) of GaSb gets significantly extended with higher pump energy mainly due to more carriers being trapped in side valleys through IVS. Nevertheless, the longest τ1 of GaSb is still shorter than the shortest τ1 of InN with a more polar nature and larger phononic bandgap. This implies that PBE plays a more decisive role in retarding Γ valley carrier relaxation than the slowed IVS. It indicates that the large phononic bandgap and Frohlich coupling constant are keys to the slow thermalization rate if the Frohlich interaction dominates. If the deformation potential interaction dominates this process, then low side valley threshold energy and the Frohlich constant determine the reduction of the overall thermalization rate.

A novel light-to-bioenergy system produced 3.5 times the baseline methane output using a co-culture of cyanobacteria (Oscillatoria sp.) and a methanogenic microbial community. Analysis of micronutrients in the system during the growth phase indicated that cobalt, iron, nickel and zinc were not appreciably consumed. The stable consumption and return of macronutrients calcium and magnesium were also observed. Essential macronutrients nitrogen, in the form of nitrate, and phosphorus showed no cycling during the growth phase and were depleted at rates of 0.35mg/L/day and 0.40µg/L/day, respectively. Biofilm formation increased the resilience of biomass to bacterial degradation in an anaerobic digester, as shown by viability assays of cyanobacterial biofilms in the co-culture.

Increasing photovoltaic conversion efficiency by overcoming the fundamental loss mechanisms due to hot carrier thermalisation remains one of the challenges in solar energy research. A large proportion of dynamic energy is lost due to thermalisation where hot carriers lose their energy to lattice vibrations within picoseconds. Extending hot carrier lifetimes will potentially improve the efficiency of photovoltaic devices. In this study, the bulk hafnium nitride film is proposed as a potential hot carrier absorber from its phononic properties. Hafnium nitride thin films were deposited by sputtering on quartz substrates. The films are polycrystalline as determined from X-ray diffraction and from X-ray photoelectron spectroscopy the chemical composition of the films are estimated to be HfN 0.9 . The absorption coefficient of the film exhibits strong amplitude below 500 nm and free electron absorption at longer wavelengths. Transient absorption spectroscopy was used to investigate carrier dynamics in the visible and near-infrared spectral regions. For the first time we present evidence of up to nanosecond long lifetimes of photo-excited carriers observed in hafnium nitride. We propose that the long lived excited states are due to restricted phonon decay pathways in hafnium nitride. The long lived excited states in the bulk hafnium nitride film provide a simple route to devices designed to utilise hot carriers efficiently.

Molybdenum (Mo) thin films are still a dominant choice for the back contact layer of Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) solar cells. This paper presents a review of Mo back contacts for CIGS and CZTS solar cells, including the requirements for a good back contact, the reason for the choice of Mo, and post-treatment. Additionally, a Mo bilayer back contact was fabricated by varying the argon (Ar) pressure during sputtering to provide both low resistivity and good adhesion to the soda-lime glass substrate. The effects of vacuum thermal annealing on the electrical, morphological and structural properties of the Mo bilayer were also investigated. Vacuum thermal annealing was seen to densify the Mo bilayer, reduce the sheet resistance, and improve the bilayer’s adhesion to the soda-lime glass. The Mo bilayer back contact with a low sheet resistance of 0.132 Ω/□ and strong adhesion was made for chalcogenide- and kesterite-based solar cells.

Hot carrier solar cells require mechanisms to dramatically reduce the rate at which carriers thermalize in semiconductors. Now, side-valley trapping of hot carriers with long decay lifetimes is shown to increase the chance of extraction of carriers while they are still hot.

Colloidal quantum dot solids are attractive candidates for tandem solar cells because of their widely tunable bandgaps. However, the development of the quantum dot tandem solar cell has lagged far behind that of its single-junction counterpart. One of the fundamental problems with colloidal quantum dot solar cells is the relatively small diffusion length, which limits the quantum dot absorbing layer thickness and hence the power conversion efficiency. In this research, guided by optical modeling and utilizing a graded band alignment strategy, a two-terminal monolithic solution-processed quantum dot tandem solar cell has been successfully fabricated and a power conversion efficiency of 6.8% has been achieved. The band grading approach utilized the complementary tuning of work functions and band alignment through judicious choices of the nanoparticle surface chemistry and quantum dot confined size. This work demonstrates a general approach to improving the efficiency for tandem thin-film solar cells.

Structural and optical properties of atomic layer deposited hafnium nitride films have been investigated in terms of hot carrier solar cell (HCSC) application. To analyze the suitability of hafnium nitride as a hot carrier absorber (HCA) material, phonon band structure is also calculated based on density functional theory (DFT) which shows a large phonon gap between optical and acoustic phonons. It is observed that rapid thermal annealing (RTA) enhances crystallinity and absorption coefficient of the material, however, with heat treatment several oxynitride phases are evident. As the films contains oxygen, potential of oxynitride as HCA should be tested.

PEDOT:PSS has been widely used as a hole conductive polymer due to its high conductivity and transparency, which is ideal for thin film solar cell with large-scale printing processability. We demonstrated a simple way to enhance the efficiency and reproducibility of PEDOT:PSS-based normalstructure perovskite solar cells. Metal oxide MoO 3 layer has been thermal-evaporated onto PEDOT:PSS as a surface passivation layer. The PCE of perovskite solar cell has been enhanced by 18% after MoO 3 passivation and the short circuit current density (J sc ), open circuit voltage (V oc) and fill factor (FF) has been improved simultaneously. The performance enhancement of MoO 3 modified PSCs is attributed to the reduced current leakage, enhanced hole extraction characteristics, and mitigated the trap-assisted interfacial recombination.

Hot carrier cells offer the potential for very high efficiencies if slowed carrier cooling can be demonstrated effectively. Various mechanisms of phonon interaction have been identified that can lead to such slowed cooling with classes of materials exhibiting long hot carrier lifetimes. The exact reasons for these are not completely clear, but a study and comparison of the materials and mechanisms will give greater insight into constructed robust hot carrier absorber materials.

The hot carrier cell (HCSCs) is a promising advanced photovoltaic concept. It aims to minimize major losses in p-n junction solar cells and is predicted to have energy conversion efficiency over 65% at one sun. Energy selective contacts (ESCs) is a critical component of the HCSC. In this paper, potential of ESCs based on quantum dot and quantum well double barrier resonant tunneling structures is presented. Results of I-V measurements these show negative differential resistance (NDR) characteristics both at low temperature and room temperature, thus indicating that these structures can be suitable for ESC application.

The hot carrier solar cells (HCSCs) is one of the most promising advanced concept solar cells. It aims to prevent or reduce the dominant energy loss from hot carrier thermalization, so that its theoretical efficiency limit is no longer limited by the Shockley-Queisser efficiency limit of 31% and could in principle reach 66% under one sun conditions. Such cells consist of two essential components: a hot carrier absorber (HCA) and energy selective contacts (ESCs), which play the role of reduction of thermalization rate and extraction of hot carriers, respectively. However, no operational device of such cells with the expected high efficiency have yet been realized at present. In order to attain a significant breakthrough on HCSCs, it is vital to clarify the interplay between the mechanisms underlying the thermalization in both bulk and low dimensional semiconductor candidates for HCA. This paper reviews mechanisms affecting the thermalization processes including quantum confinement effect, phonon bottleneck effect (or hot phonon effect) and carrier screening in terms of electronic and phononic band structures via different characterization techniques. Several promising III-V semiconductors and perovskite material systems in bulk and quantum well structures are systematically reviewed. It has been found that phonon bottlenecks can play a key role in interrupting thermalization processes by restricting phonon interactions and their ability to dissipate hot carrier energy into the lattice. Several other mechanisms affecting thermalization could eventually be attributed to different types of phonon bottleneck effect.

Indigo-carmine-mediated direct alkaline fuel cells have demonstrated superior power outputs over their non-mediated counterparts. Currently, the mechanism of mediation and stability of mediators are poorly understood. Upon exposure to highly alkaline solutions, we observed that the redox action of indigo carmine diminished, and significant currents were produced, indicating substantial degradation of the dye. The decomposition of indigo carmine at high pH and a poor thermal stability suggest that alternative mediators with low toxicity may be required to enable the widespread application of this device type.

A narrow size distribution of quantum dots (QDs) is needed for their application in photovoltaics but collection of such information is difficult. This paper demonstrates the application of Raman spectroscopy as a characterisation tool to extract the size distribution and crystalline fraction of Si QD samples fabricated through the sputter-anneal method. Measured Raman spectra of Si QD materials are de-convoluted into four components according to their origins and Raman scattering by Si QD cores is described by a modified one phonon confinement model, while other components are reproduced with Gaussian functions. Through fitting of Raman spectra, Si QD size distributions and Si crystalline fractions are obtained. The results are compared with the values extracted from PL modelling on a series of B doped Si QD samples. The good consistency between the values extracted by these two methods confirms the validity of the Raman model. The result confirms that Si crystallization has been suppressed by B doping as the average Si QD size and Si crystalline fraction are reduced with increased B doping level.

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., Slow cooling of hot charge carriers in lead halide perovskite could be used in photovoltaics devices. Here, Yang et al. study hot carrier dynamics by transient absorption spectroscopy. They relate the phonon bottleneck to the up-conversion of low-energy phonons, facilitated by the presence of organic cations.

Using advanced two-photon excitation confocal microscopy, associated with time-resolved spectroscopy, we characterize InGaN/GaN multiple quantum wells on nanorod heterostructures and demonstrate the passivation effect of a KOH treatment. High-quality InGaN/GaN nanorods were fabricated using nanosphere lithography as a candidate material for light-emitting diode devices. The depth- and time-resolved characterization at the nanoscale provides detailed carrier dynamic analysis helpful for understanding the optical properties. The nanoscale spatially resolved images of InGaN quantum well and defects were acquired simultaneously. We demonstrate that nanorod etching improves light extraction efficiency, and a proper KOH treatment has been found to reduce the surface defects efficiently and enhance the luminescence. The optical characterization techniques provide depth-resolved and time-resolved carrier dynamics with nanoscale spatially resolved mapping, which is crucial for a comprehensive and thorough understanding of nanostructured materials and provides novel insight into the improvement of materials fabrication and applications.

We analyze phosphorus (P)- and boron (B)-doped silicon nanocrystals (Si NCs) with various compositions of silicon-rich oxide using atom probe tomography. By creating Si iso-concentration surfaces, it is confirmed that there are two types of Si NC networks depending on the amount of excess Si. A proximity histogram shows that P prefers to locate inside the Si NCs, whereas B is more likely to reside outside the Si NCs. We discuss the difference in a preferential location between P and B by a segregation coefficient.

Silicon nanocrystals (Si NCs) are intensively studied for optoelectronic and biological applications due to having highly attractive features such as band engineering. Although doping is often used to control the optical and electrical properties, the related structural properties of solely doped and codoped Si NCs are not well-understood. In this study, we report the boron (B) and/or phosphorus (P) distribution in Si NCs embedded in borosilicate glass (BSG), phosphosilicate glass (PSG), and borophosphosilicate glass (BPSG) using atom probe tomography (APT). We compared solely and codoped Si NCs grown at different temperatures so that we may compare the effects of codoping and temperature on the B and/or P distribution. Proximity histograms and cluster analyses reveal that there exist boron-rich layers surrounding Si NCs and also B–P clusters within the Si NCs. Raman spectra also show a structural change between codoped Si NCs in solids and free-standing codoped Si NCs. These results lead us to understand...

Fabrication of GexC1 − x has been a big challenge because of the solubility of C in Ge. Only a small percentage of Ge C bonds (11.6%) have been introduced so far. In this work, a-GexC1 − x with GeC content up to 21% has been fabricated with 50 W RF power at 250 °C by reactive sputtering methods. The effects of the radio frequency power and substrate temperature on the yield of GeC were analysed in detail. The GeC percentage by volume was found to first increase and then decrease with increasing substrate temperature. Introduction of C into the Ge matrix seems to tune the optical bandgap over a range of 2.7 eV to 1.0 eV depending on the combination of substrate temperature and radio frequency power.

The hot carrier solar cell is a promising concept for high efficiency photovoltaics. Obtaining the dynamic temperature information for the hot carriers in an absorber is of crucial importance to this concept. In this paper, we present a dynamical hot carrier theory that extracts the carrier temperature and the instantaneous carrier occupation from transient absorption spectra. We have applied this model to two transition metal nitride materials, HfN and ZrN. The analysis showed that for HfN the initial carrier temperature rose to 360 K immediately after a femtosecond pulse excitation and then dropped rapidly to 330 K within 0.13 ps. After this initial process, the carrier temperature still remained relatively elevated at above 320 K for as long as 2 ns. The initial fast process was attributed to the strong carrier–carrier scattering in the material while the slow decay of temperature might be due to unusually weak electron–phonon interactions. A shorter cooling time and overall lower hot carrier temperatures were observed in ZrN.

Silver sulfide (Ag2S) nanoparticle (NP) is a promising material for quantum-dot-based hot-carrier solar cell, yet the hot carrier dynamics of the material has not been thoroughly studied for this purpose. In this work, carrier cooling dynamics of Ag2S NP thin film were measured and analyzed using the transient absorption (TA) spectrometer. The NPs exhibit a relatively high carrier temperature and long carrier cooling time. This result was further verified by the carrier temperature extraction from steady-state photoluminescence (ssPL) spectrum. The long carrier cooling time in Ag2S NPs can potentially lead to efficiency of 33.6%, demonstrating potential application for the high-efficiency hot-carrier solar cell.