Semiconductor quantum dots (QDs) have a potential to increase the power conversion efficiency in photovoltaic operation because of the enhancement of photoexcitation. Recent advances in self-assembled QD solar cells (QDSCs) and colloidal QDSCs are reviewed, with a focus on understanding carrier dynamics. For intermediate-band solar cells using self-assembled QDs, suppression of a reduction of open circuit voltage presents challenges for further efficiency improvement. This reduction mechanism is discussed based on recent reports. In QD sensitized cells and QD heterojunction cells using colloidal QDs well-controlled heterointerface and surface passivation are key issues for enhancement of photovoltaic performances. The improved performances of colloidal QDSCs are presented.

Metal halide-based perovskite quantum dots (QDs) have emerged as promising materials for optoelectronics and future energy applications. Among them, cesium lead halide-based perovskite quantum dots (Cs-LHQDs) have been found to be potential luminescent candidates and alternatives for the II–VI and I−III−VI2 groups semiconductor nanoparticles. These perovskites provide an excellent quantum yield (90%) larger than any other semiconductor QDs. At present, synthesis of Cs-LHQDs has been successfully achieved through a traditional colloidal-based hot-injection method and a room temperature precipitation method. Some of the interesting results in their structural, optical, and morphological properties are being analyzed to understand their energy-transfer mechanism in the colloidal state. Morphology of nanoplates, nanowires, nanocube, and nanosheets in these materials confirms their physical parameters-dependent self-assembly nature in a colloidal medium. Their potential use for light emitting diodes, photodetectors, and lasers is also highly motivated. This review provides a collective view of recent developments made in the synthesis of Cs-LHQDs and their properties.

This PDF file contains the editorial “Special Section Guest Editorial:Hot Carrier Energy Harvesting and Conversion” for JPE Vol. 6 Issue 04

The use of hot electrons arising from the nonradiative decay of surface plasmons (SPs) is increasingly attracting interests in photodetection, photovoltaics, photocatalysis, and surface imaging. Nevertheless, the quantum efficiency of the hot-electron devices has to be improved to promote the practical applications. We propose an architecture of conformal TCO/semiconductor/metal nanowire (NW) array for hot-electron photodetection with a tunable optical response across the visible and near-infrared bands. The wavelength, strength, and bandwidth of the plasmonic resonance are tailored by controlling the lattice periodicity and topology. Finite-element simulation demonstrates that the near-perfect, polarization-insensitive, and ultranarrow-band optical absorption can be achieved in the conformal NW system. By the excitation of localized SPs, a strong field concentrates at the top corner of the NWs with a high hot-electrons generation rate. The analytical probability-based electrical calculation further shows that the SPs-enhanced photoresponsivity can be more than five times larger than that of the flat reference.

A broadband perfect light absorption scheme relying on planar layers of lossy metallic and semiconductor films without structure pattering for hot electron photodetection is proposed and demonstrated. The nontrivial reflection phase shift occurring in the ultrathin lossy metal film is responsible for the strong resonance effects of the metal–semiconductor–metal (M-S-M) structures. Broadband and perfect light absorption is demonstrated experimentally for the platinum (Pt)—titanium dioxide (TiO₂)—aluminum (Al) absorber, in which the ultrathin (10 nm) Pt layer electrically serves as the hot electron emitter. Under the monochromatic light illumination, the proposed M-S-M diode shows a clear hot-electron-based photoresponse.

The tunable and amazing properties of plasmonic nanostructures have received significant attentions in the fields of solar energy conversion. Plasmonic nanostructures provide pathways to directly convert solar energy into electric energy by hot-carrier generation. They can also serve as economical electrodes for high-efficient carrier collection. Both have promising potential for manufacturing new generation solar cells. Here, we review recent advances in plasmonic nanostructures for electronic designs of photovoltaic devices and specially focus on plasmonic hot-carrier photovoltaic architectures and plasmonic electrode structures. Technical challenges toward low-cost and high-performance plasmonics-based solar cells are also discussed.

Plasmon-induced charge separation (PICS), in which an energetic electron is injected from a plasmonic nanoparticle (NP) to a semiconductor on contact, is often inhibited by a protecting agent adsorbed on the NP. We addressed this issue for an Ag nanocube-TiO₂ system by coating it with a thin Au layer or by inserting the Au layer between the nanocubes (NCs) and TiO₂. Both of the electrodes exhibit much higher photocurrents due to PICS than the electrodes without the Au film or the Ag NCs. These photocurrent enhancements can be explained in terms of PICS with accelerated electron transfer, in which electron injection from the Ag NCs or Ag@Au core-shell NCs to TiO₂ is promoted by the Au film, or PICS enhanced by a nanoantenna effect, in which the electron injection from the Au film to TiO₂ is enhanced by optical near field generated by the Ag NC.

Electromagnetic coupling between resonant plasmonic oscillations of two closely spaced noble metal particles can lead to a strongly enhanced optical near field in the cavity formed by the gap between the metal particles. However, discoveries in quantum plasmonics show that an upper limit is imposed to the field enhancement by the intrinsic nonlocality of the dielectric response of the metal and the tunneling of the coherently oscillating conduction electrons through the gap. Here, we introduce and experimentally demonstrate optical amplification by radiative relaxation of hot electrons in a tunneling junction of a scanning tunneling microscope forming an extremely small point light source. When electrons tunnel from the sample to the tip, holes are left behind. These can be repopulated by hot electrons induced by the laser-driven plasmon oscillation on the metal surfaces enclosing the cavity and lead to a much higher electron to photon conversion efficiency. The dynamics of this system can be described by rate equations similar to laser equations. They show that the repopulation process can be efficiently stimulated by the gap mode’s near field. Our results demonstrate how optical enhancement inside the plasmonic cavity can be further increased by a stronger localization via tunneling through molecules.

Hybrid white organic light-emitting diodes (WOLEDs) were fabricated by applying triplet harvesting (TH) using a green thermally activated delayed fluorescence (TADF) emitter. The triplet exciton of the green TADF emitter can be upconverted to its singlet state. The TH involved energy transfer of triplet exciton from a blue fluorescent emitter to a green TADF and red phosphorescent emitters, where they can decay radiatively. In addition, the triplet exciton of the green TADF emitter was energy transferred to its singlet state for a reverse intersystem crossing by green emission. Enhanced hybrid WOLEDs were demonstrated using an efficient green TADF emitter combined with red phosphorescent and blue fluorescent emitters. Hybrid WOLEDs were fabricated with various hole-electron recombination zones as changing blue emitting layer thicknesses. Among these, hybrid WOLEDs showed a maximum external quantum efficiency of 11.23%, luminous efficiency of 29.20  cd/A, and a power efficiency of 26.21  lm/W. Moreover, the WOLED exhibited electroluminescence spectra with Commission International de L’Éclairage chromaticity of (0.38, 0.36) at 1000  cd/m² and a color rendering index of 82 at a practical brightness of 20,000  cd/m².

In this work, we present the enhancement of ultraviolet (UV) photodetection of Ag-ZnO thin film deposited by radio frequency magnetron sputtering. The surface morphological, optical, structural, and electrical properties of the deposited thin films were investigated by various characterization techniques. With this Ag-ZnO thin film structure and proper geometry of metal–semiconductor–metal (MSM) interdigitated structure design, photocurrent enhancement has been accomplished. MSM-photodetectors (PDs) using structures of Ag-ZnO gave a 30 times higher magnitude photocurrent at 340 nm of the wavelength. Plasmon-induced hot electrons contributed to improved spectral response to the UV region, while absorption and scattering effect enhanced broadband improvement to a response in the VIS–IR spectrum range. The improvement of Ag-ZnO PD in comparison with ZnO is attributed to the surface plasmon effect using Ag nanodisks. These results indicate that Ag-ZnO thin films can serve as excellent ultraviolet-PD and a very promising candidate for practical applications.

Plasmon field-effect transistor is a hybrid device using nanostructures to detect the plasmonic energy. This device efficiently transfers plasmonic hot electrons from the metal nanostructures to the semiconductor. The transported hot electrons to the electron channel increases transistor drain current. We investigate the efficiency of plasmonic hot carrier harvesting between metal and semiconductor. We analyzed the effect of gold nanoparticle (NP) density and distribution on plasmon FET spectral response. Then, we studied electric field-assisted hot electron transfer and transport using different device structures. The position of plasmonic structures plays an important role in plasmonic energy detection efficiency because the gradient of electric field seen by induced hot electrons varies depending on the distance between drain and source. Both the experimental and simulation results confirm that by fabricating the gold NPs close to source the spectral response increases by 31% in comparison with having gold NPs close to the drain. Our simulation and experimental data suggest important design considerations to improve hot electron collection and conversion using metallic nanostructures for plasmonic energy harvesting.

Semiconductor materials are well suited for power conversion when the incident photon energy is slightly larger than the bandgap energy of the semiconductor. However, for photons with energy significantly greater than the bandgap energy, power conversion efficiencies are low. Further, for photons with energy below the bandgap energy, the absence of absorption results in no power generation. Here, we describe photon detection and power conversion of both high- and low-energy photons using hot carrier effects. For the absorption of high-energy photons, excited electrons and holes have excess kinetic energy that is typically lost through thermalization processes between the carriers and the lattice. However, collection of hot carriers before thermalization allows for reduced power loss. Devices utilizing plasmonic nanostructures or simple three-layer stacks (transparent conductor–insulator–metal) can be used to generate and collect these hot carriers. Alternatively, hot carrier collection from sub-bandgap photons can be possible by forming a Schottky junction with an absorbing metal so that hot carriers generated in the metal can be injected across the semiconductor–metal interface. Such structures enable near-IR detection based on sub-bandgap photon absorption. Further, utilization and optimization of localized surface plasmon resonances can increase optical absorption and hot carrier generation (through plasmon decay). Combining these concepts, hot carrier generation and collection can be exploited over a large range of incident wavelengths spanning the UV, visible, and IR.

Surface plasmon photodetectors are of broad interest. They are promising for several applications including telecommunications, photovoltaic solar cells, photocatalysis, color-sensitive detection, and sensing, as they can provide highly enhanced fields and strong confinement (to subwavelength scales). Such photodetectors typically combine a nanometallic structure that supports surface plasmons with a photodetection structure based on internal photoemission or electron–hole pair creation. Photodetector architectures are highly varied, including waveguides, gratings, nanoparticles, nanoislands, or nanoantennas. We review the operating principles behind surface plasmon photodetectors based on the internal photoelectric effect, and we survey and compare the most recent and leading edge concepts reported in the literature.

Copper phthalocyanine (CuPc) is an important hole transport layer for organic photovoltaics (OPVs), but interaction with ambient gas/vapor may lead to changes in its electronic properties and limit OPV device lifetimes. CuPc films of thickness 25 and 100 nm were grown by thermal sublimation at 25°C, 150°C, and 250°C in order to vary morphology. We measured electrical resistance and film mass in situ during exposure to controlled pulses of O₂ and H₂O vapor. CuPc films deposited at 250°C showed a factor of 5 higher uptake of O₂ as detected by a quartz crystal microbalance (QCM), possibly due to the formation of β-CuPc at T>200°C which allows higher O₂ mobility between stacked molecules. While weight-based measurements stabilize after ∼10  min of gas exposure, resistance response stabilizes over times >1  h, suggesting that mass change occurs by rapid adsorption at active surface sites whereas resistive response is dominated by slow diffusion of adsorbates into the bulk film. The 25 nm films exhibit higher resistive response than 100 nm films after an hour of O₂/H₂O exposure due to fast analyte diffusion down to the film/electrode interface. We found evidence of decoupling of CuPc from the gold-coated QCM crystal due to preferential adsorption of O₂/H₂O molecules on gold.

Organic solar cells are usually nonreproducible due to the presence of defects in the structure of their constituting thin films. To minimize the density of pinholes and defects in PEDOT:PSS, which is the hole transporting layer of a standard polymer solar cell, i.e., glass/ITO/PEDOT:PSS/P3HT:PCBM/Al, and to reduce scattering in device performance, wet spun-on PEDOT:PSS films are subjected to imposed ultrasonic substrate vibration posttreatment (SVPT). The imposed vibration improves the mixing and homogeneity of the wet spun-on films, and consequently the nanostructure of the ensuing thin solid films. For instance, our results show that by using the SVPT, which is a mechanical, single-step and low-cost process, the average power conversion efficiency of 14 identical cells increases by 25% and the standard deviation decreases by 22% indicating that the device photovoltaic performance becomes more consistent and significantly improved. This eliminates several tedious and expensive chemical and thermal treatments currently performed to improve the cell reproducibility.

In the last years, hybrid organic/inorganic solar cells have attracted great interest in photovoltaic research due to their expected potential to combine the advantages of both material classes, the excellent electrical properties and stability of the inorganic and the low-cost processability of the organic semiconductors. This work is focused on hybrid solar cells based on n-doped crystalline Si as the inorganic and the polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) as the organic part of the device. The hole-conducting organic semiconductors poly(3-hexylthiophene-2,5-diyl) (P3HT) and 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-MeOTAD) are investigated as electron blocking interlayers to reduce the parasitic electron current into the metal top contact and thereby increase the efficiency of the solar cell. In this context, P3HT is identified to be insufficient as an interlayer material due to unfavorable hysteresis effects. On the other hand, for solar cells with a Spiro-MeOTAD interlayer, the power conversion efficiency (PCE) is significantly increased. This is mainly attributed to an increased short-circuit current density. For the best performing device, a PCE of 14.3% is achieved, which is one of the highest values reported for this type of hybrid solar cells so far.

We present an optimized luminescent solar concentrator (LSC) based on the combination of different organic dyes as luminescent species and silicon solar cells. As a first part of this work, a deep screening of organic dyes used since the 1980s in LSC research has been performed and 14 of them have been chosen and characterized in a polymethyl methacrylate (PMMA) host at a fixed concentration (1%). Departing from this initial study, an empirical optimization procedure has been implemented, aiming the highest photon collection at the edges of the LSC plate under a solar AM1.5G spectrum. An optimal three-dye system has been selected in order to achieve a high absorption range over the uv–vis spectrum. The incorporation of these molecules in the PMMA host has been made following two different strategies: (i) the stacking of three individual LSC plates (one dye in each) and (ii) the merging of three organic dyes in a single LSC plate allowing Förster resonance energy transfer (FRET) to occur. In addition, PMMA bulk and thin film on glass plates have been manufactured considering stacking and FRET strategies. Finally, a bulk FRET-based LSC has been manufactured and its optical and electrical performance evaluated and compared to the best reported values used for LSC characterization. To the best of our knowledge, our LSC possesses the highest reported concentration (C=0.80), surpassing Slooff et al. (C=0.73).

A visible-light-driven hydrogen evolution system was constructed in one step with xanthene dye as photosensitizer, triethanolamine (TEOA) as electron donor, Pt as cocatalyst, and Keggin-type silicon-tungsten polyoxometalate (POM) as electron relay. The average rate of hydrogen evolution from the eosin Y−POM−TEOA−PtCl^2-6 systems is 51.8  μ mol h⁻¹, and the apparent quantum yield is 8.9% during 20 h irradiation (λ>420  nm;, a high-pressure Hg lamp was used as the light source). Ultraviolet–visible absorption spectra and fluorescence spectra were used to study the mechanism of charge transfer. A radical anion Dye−, which generates from quenching of the triplet-excited state ³Dye* by sacrificial electron donor, was considered the main species for transmitting electrons to POM and then to Pt. SiW₉O¹⁰⁻³⁴, derived from SiW₁₂O⁴⁻⁴⁰, is the main form of POM in our reaction system, which can mediate the electron transfer from Dye− to Pt, so an enhanced hydrogen evolution was realized. The present study highlights POM as electron relay in the photocatalytic hydrogen evolution process, which is expected to contribute to the development of functional and efficient artificial photosynthetic systems.

Ultrathin crystalline silicon wafers for photovoltaic applications have attracted intensive attention because of potential benefits in cost-effectiveness. Structural design with high light absorption is important for photovoltaics because planar ultrathin silicon is poor in absorption. We conduct a comparative investigation on designs of light absorption enhancement for 2-μm-thick ultrathin crystalline silicon, where the front texture is a nanopyramidal structure and the rear adopts several designs. Our calculation results show that both of the ultrathin silicon with front nanopyramids and rear silver nanoarrays and the ultrathin silicon with two-sided nanopyramids are promising for photovoltaic applications. For the latter design, the calculated photocurrent achieves the highest value of 35.1  mA/cm² when a perfect electric conductor layer is applied at the bottom. In contrast, the former design has a lower photocurrent value of 31.2  mA/cm². But, this design is of practical significance because the majority of experimental reports on ultrathin crystalline silicon solar cells are single-sided front-textured at present and the fabrication techniques of plasmonic Ag nanoarrays are matured. Compared with previous reports, the present work offers a multiple option of structural designs for ultrathin crystalline silicon to enhance the light absorption for photovoltaic applications.

The effect of Fresnel reflection and total internal reflection (TIR) losses on the performance parameters in refractive solar concentrators has often been downplayed because most refractive solar concentrators are traditionally the imaging type, yielding a line or point image on the absorber surface when solely interacted with paraxial etendue ensured by solar tracking. Whereas, with refractive-type nonimaging solar concentrators that achieve two-dimensional (rectangular strip) focus or three-dimensional (circular or elliptical) focus through interaction with both paraxial and nonparaxial etendue within the acceptance angle, the Fresnel reflection and TIR losses are significant as they will affect the performance parameters and, thereby, energy collection. A raytracing analysis has been carried out to illustrate the effects of Fresnel reflection and TIR losses on four different types of stationary dielectric-filled nonimaging concentrators, namely V-trough, compound parabolic concentrator, compound elliptical concentrator, and compound hyperbolic concentrator. The refractive index (RI) of a dielectric fill material determines the acceptance angle of a solid nonimaging collector. Larger refractive indices yield larger acceptance angles and, thereby, larger energy collection. However, they also increase the Fresnel reflection losses. This paper also assesses the relative benefit of increasing RI from an energy collection standpoint.

Nonimaging optics is the theory of thermodynamically efficient optics and as such depends more on thermodynamics than on optics. Hence, in this paper, a condition for the “best” design is proposed based on purely thermodynamic arguments, which we believe has profound consequences for the designs of thermal and even photovoltaic systems. This way of looking at the problem of efficient concentration depends on probabilities, the ingredients of entropy and information theory, while “optics” in the conventional sense recedes into the background. Much of the paper is pedagogical and retrospective. Some of the development of flowline designs will be introduced at the end and the connection between the thermodynamics and flowline design will be graphically presented. We will conclude with some speculative directions of where the ideas might lead.