The field of organic solar cells (OSCs) has developed rapidly over the last two decades, and polymer solar cells with power conversion efficiencies of 13.1% have been reported. The understanding of the photophysical mechanisms in photovoltaic devices plays a significant role in determining the optoelectronic properties of high-performance OSCs. This review gives a fundamental description of the photophysical mechanisms in OSCs, with the main emphasis on the exciton transfer and charge-transfer mechanisms. In addition, it highlights the capabilities of various experimental techniques for the characterization of energy transfer and charge transport, as well as their roles in dictating the architecture of photovoltaic devices.

This guest editorial introduces the Special Section on Spectral Management for Renewable Energy Conversion in JPE, Vol. 8, Issue 2.

Solar energy storage and conversion is a key technology for the future realization of a sustainable energy production worldwide. The utilization of low-energy photons by means of upconversion (UC) processes has the main advantage of the upconversion process and solar energy-harvesting devices being considered and optimized independently, without affecting the particular physical characteristics of the operating sunlight excited material or device architectures. The UC-device must function efficiently with noncoherent light excitation, under light intensities comparable with those obtainable from moderate concentrated sunlight (1 to 20 suns, AM1.5); such a low degree of concentration allows usage of Fresnel-type focusing systems. We report chemical synthesis and efficient functioning of a triplet–triplet annihilation upconversion-based device, harvesting the entire deep-red spectral range of the sun irradiation ( Δλ∼142  nm) in a gap-free manner.

Improving efficiency of triplet–triplet annihilation-based photon upconversion (TTA-UC) in crystalline media is challenging because it usually suffers from the severe aggregation of the donor (sensitizer) molecules in acceptor (emitter) crystals. We show a kinetically controlled crystal growth approach to improve donor dispersibility in acceptor crystals. As the donor–acceptor combination, a benchmark pair of platinum(II) octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA) is employed. A surfactant-assisted reprecipitation technique is employed, where the concentration of the injected PtOEP–DPA solution holds the key to control dispersibility; at a higher PtOEP–DPA concentration, a rapid crystal growth results in better dispersibility of PtOEP molecules in DPA crystals. The improvement of donor dispersibility significantly enhances the TTA-UC quantum yield. Thus, the inherent function of donor-doped acceptor crystals can be maximized by controlling the crystallization kinetics.

It is well known that the rate and efficiency of photon upconversion via triplet–triplet annihilation (TTA-UC) are strongly dependent on the energetics of the sensitizer and acceptor molecules. In rigid scaffoldings, where the dyes are fixed in position, the structure and orientation of the molecules also presumably play an important role. We investigate how the variation in the position of the phosphonate surface binding group on 9,10-diphenylanthracene influences TTA-UC in self-assembled bilayers on ZrO2. Interestingly, meta- or para-substitution of the anthracene dye with phosphonate groups had minimal influence on their energetics, surface loadings, or sensitizer to acceptor triplet energy transfer efficiency. However, the TTA-UC efficiency is three times lower with the meta-substituted dye, which is attributed to its threefold decrease in its triplet excited state lifetime relative to the para-substituted dye.

Upconversion (UC) of low-energy photons into high-energy light can in principle increase the efficiency of solar devices by converting photons with energies below the energy absorption threshold into radiation that can be utilized. Among UC mechanisms, the sensitized triplet–triplet annihilation-based upconversion (sTTA-UC) is the most recent that would be applied in solar technologies. sTTA-UC was demonstrated using sunlight in 2006, and due to its high efficiency at low excitation intensities with noncoherent light, it is considered a promising strategy for the recovering of subbandgap photons. Here, we describe briefly the working principle of the sTTA-UC, and we review the most recent advances of its use in solar applications.

Triisopropylsilylethynyl-substituted acenes (TIPS-acenes) have received prominent attention in the field of singlet fission, the pentacene derivative being an exothermic singlet fission material, and the tetracene being a prototypical endothermic material. Little attention has been given to TIPS-anthracene, which is expected to exhibit exothermic triplet–triplet annihilation, despite literature reports to the contrary. We show that there is some evidence for singlet fission in TIPS-anthracene solutions, and that it does exhibit triplet–triplet annihilation. We apply anti-Stokes action spectroscopy to determine the upconversion efficiency of a composition of TIPS-anthracene and platinum octaethylporphyrin. At a bias equivalent to 0.86 suns, the composition exhibited an annihilation efficiency of 3.2%, which may be compared to diphenylanthracene, which yielded 9.2% under the same conditions. We attribute the low efficiency to a combination of a shorter triplet lifetime and low lying T₂ and T₃ states. The results are supported by ab initio quantum chemical calculations.

The limiting efficiency of a solar cell enhanced by both up- and downconversion is calculated to be 44.6% under one sun illumination using a detailed balance formalism. We show that solar cells are enhanced by both spectral conversion and photon recycling. This device architecture has the advantage of being able to enhance existing technologies after relatively few modifications. Importantly, we show that this device architecture’s limiting efficiency peaks close to the band gap of silicon, unlike other spectral converters with three absorbing thresholds.

One way for solar cell efficiencies to overcome the Shockley–Queisser limit is downconversion of high-energy photons using singlet fission (SF) in polyacenes like tetracene (Tc). SF enables generation of multiple excitons from the high-energy photons, which can be harvested in combination with Si. In this work, we investigate the use of lead sulfide quantum dots (PbS QDs) with a band gap close to Si as an interlayer that allows Förster resonant energy transfer (FRET) from Tc to Si, a process that would be spin-forbidden without the intermediate QD step. We investigate how the conventional FRET model, most commonly applied to the description of molecular interactions, can be modified to describe the geometry of QDs between Tc and Si and how the distance between QD and Si, and the QD bandgap affects the FRET efficiency. By extending the acceptor dipole in the FRET model to a 2-D plane, and to the bulk, we see a relaxation of the distance dependence of transfer. Our results indicate that FRET efficiencies from PbS QDs to Si well above 50% are possible at very short but possibly realistic distances of around 1 nm, even for QDs with relatively low photoluminescence quantum yield.

This guest editorial introduces the special series on Perovskite-Based Solar Cells.

An approach to enhance the ultimate efficiency of the silicon nanowires (Si NWs) solar cell is proposed based on a hybrid population-based algorithm. The suggested technique integrates the ability of exploration in a gravitational search algorithm (GSA) with the exploitation capability of particle swarm optimization (PSO) to synthesize both algorithms’ strengths. The hybrid GSA-PSO algorithm in MATLAB^® code is linked to finite-difference time-domain solution technique based on Lumerical-software to simulate and optimize the Si NWs’ geometrical parameters. The suggested GSA-PSO algorithm has advantages in terms of better convergence and final fitness values than that of the PSO algorithm. Further, the Si NWs lattice with optimized diameters and heights shows a high ultimate efficiency of 42.5% with an improvement of 42.8% over the Si NWs lattice with the same diameters and heights. This enhancement is attributed to the different generated optical modes combined with multiple scattering and reduced reflection due to the different heights and different diameters, respectively.

An innovative approach to improve the performance of photovoltaic solar cells is presented. Until recently, the fabrication of grating layers has been well proven using bulk micromachining techniques, but lately low-cost dip-pen nanolithography (DPN) has been proposed as a method for printing nanostructures on different substrates and has matured to become one of the most versatile patterning techniques available at the nanoscale. However, this technique has scarcely been studied and tested for fabricating grating layers. In this research, submicron grating patterns from high refractive index polymers are fabricated on a few types of solar cells, significantly improving their efficiency. The appropriate geometries and materials for the grating patterns are obtained via numerical optimization using rigorous coupled wave analysis for electromagnetic simulations of the grating multilayer. Possible light-confinement schemes are analyzed, and their figures of merit are assessed. The simulation of the electrical characteristics is integrated with postdesign electromagnetic simulation. The corresponding theoretical and experimental studies shed light on the impact of the merger of the grating structure with the light harvester on the device’s optical and electrical properties. Success in using DPN paves pathways to low-cost fabrication of light harvesting devices with improved performance.

This editorial introduces the special section on Tandem Junction Solar Cells.

Currently, perovskite–silicon tandem solar cells are one of the most investigated concepts for overcoming the theoretical limit for the power conversion efficiency of silicon solar cells. For monolithic tandem solar cells, the available light must be distributed equally between the two subcells, which is known as current matching. For a planar device design, a global optimization of the layer thicknesses in the perovskite top cell allows current matching to be reached and reflective losses of the solar cell to be minimized at the same time. However, even after this optimization, the reflection and parasitic absorption losses add up to 7  mA  /  cm². In this contribution, we use numerical simulations to study how well hexagonal sinusoidal nanotextures in the perovskite top-cell can reduce the reflective losses of the combined tandem device. We investigate three configurations. The current density utilization can be increased from 91% for the optimized planar reference to 98% for the best nanotextured device (period 500 nm and peak-to-valley height 500 nm), where 100% refers to the Tiedje–Yablonovitch limit. In a first attempt to experimentally realize such nanophotonically structured perovskite solar cells for monolithic tandems, we investigate the morphology of perovskite layers deposited onto sinusoidally structured substrates.

Optimizing the luminescence coupling (LC) effect can reduce the current mismatch in multijunction solar cells (MJSCs), thereby enhancing their performance. In a previous work, the LC current collection was measured at room temperature. The temperature dependence of LC effect in InGaP/GaAs/Ge triple junction solar cells was observed by light J–V characteristics measurements and by laser beam-induced current mapping at operating temperatures between 18°C and 72°C. Results were analyzed using quasi-two-dimensional electro-optical model. At low operating temperatures, the photocurrent collection by LC effect has been found to be more dominant than the current collection due to bandgap reduction and thermal excitation of carriers. At 18°C and 72°C, when Ge bottom cell was made current limiting, the conversion efficiencies calculated were 6.68% and 0.01%, respectively. These findings show that the LC effect can contribute best to the MJSC performance when operated at the lowest temperature possible.

The development of perovskite solar cells has approached the theoretical efficiency limitation for single junction device geometry. To break this single junction efficiency limitation, it is highly desirable to construct a tandem structure in which the high bandgap perovskite is stacked with a current commercial prevailing low bandgap CuInGaSe2 (CIGS) solar cell. Improving the bottom CIGS subcell absorption with a reduced thickness for photocurrent matching holds the key to developing a high-performance perovskite/CIGS tandem device and reducing the overall manufacturing cost. We propose a monolithic perovskite/CIGS tandem solar cell integrated with optimized nanophotonic structures: dielectric nanocones on the top surface and Ag nanoparticles at the rear surface. Significant light absorption enhancement as large as 12.5% is achieved with a reduced bottom cell thickness by 58.7% through simulation. This approach opens up a viable pathway for achieving high-performance and cost-effective perovskite/CIGS tandem solar cells.

Molybdenum sulfide (MoS2) has been suggested as a light-absorbing material to enhance solar cell efficiency because of its suitable electrical and optical properties. However, very few experimental results have been reported with efficiencies below 10%. In this work, a solar cell device has been studied numerically using MoS2 absorber layer sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL). Numerical simulations provide a powerful tool to assess the potential of various device configurations and materials to achieve high performance. Various HTLs are analyzed, including Cu2O, CuSCN, CuI, NiO, and Spiro-OMETAD, whereas ZnO is used as an ETL. The key parameters that determine the power conversion efficiency of the device were analyzed, namely the short circuit current   (  Jsc  )  , the open circuit voltage   (  Voc  )  , and the fill factor (FF). Both p-type and n-type MoS2 were considered. As for losses, they are summed in the band-to-band recombination in the bulk of MoS2. The results demonstrate that power conversion efficiencies exceeding 20% can be obtained by optimizing the cell design.

Conventional Cu  (  In  ,  Ga  )  Se₂ (CIGS) solar cells suffer poor response in the short-wavelength region (280 to 520 nm) due to the parasitic absorption in the ZnO transparent conductive oxide and inefficient collection of generated electron–hole pairs in the CdS layer. The short-wavelength response can be enhanced using a downshifting (DS) layer mounted on the top surface of the solar cell. The performance effects of coupling silicon nanocrystals DS layer to a CIGS solar cell are analyzed using numerical simulations. Measured photoluminescence of fabricated Si-nC using plasma-enhanced chemical vapor deposition is used to calibrate the model. A 13.9% relative enhancement of the conversion efficiency is observed when a DS layer with a photoluminescence quantum yield (PLQY) of 20% is added. The relative enhancement increases to 17.8% when the DS layer has a PLQY of 100%. The results are further analyzed to decouple and quantify the surface reflectance effect and the DS effect using a passive cell. The surface reflectance effect is dominant for DS layers with PLQY of 20% and accounts for 13.6% relative enhancement. Whereas the DS effect dominates when the PLQY is >70  %   and reaches 17.1% relative enhancement for a DS layer with PLQY of 100%. Overall, cells with poor UV spectral responses are found to benefit the most from the coupling of a DS layer, albeit only for a very high PLQY.

Perovskite solar cells based on methylammonium lead halide perovskites (abbreviated as perovskites) maintain great promise for low-cost solar-to-electricity energy conversion, due to the excellent photoelectric properties of perovskites, abundance of raw materials to synthesize perovskites, and the capability to deposit perovskites by a solution process. Significant effort has been taken to deposit high-quality perovskite thin films and, thereafter, to fabricate highly efficient perovskite solar cells. Nevertheless, the degradation of perovskite solar cells greatly limits their chance for mass production and deployment. Here, we present a device structure that can significantly improve the stability of perovskite solar cells using all inorganic charge transport layers. Specifically, solution-processed inorganic thin films, lithium (Li)-doped NiO_x (Li  :  NiO_x), and ZnO nanoparticles are used as a hole transport layer and an electron transport layer, respectively. The mechanisms behind the enhanced stability are analyzed. Solutions for further improvement in the stability and the performance of perovskite solar cells are suggested.

The phosphor excitation by blue laser diode and an influence of its irradiation on the material were investigated. Two types of materials were elaborated to perform these experiments—yellow phosphor (YAG) and green phosphor mixed with nitride phosphor (GYAG). These phosphors were packaged into silicone plates, having different thickness and concentration in silicon resin. The results from exciting the phosphor by blue laser present that emission of converted light increases when concentration and thickness increase. Also, the dependency of these two parameters on optical power shows certain behavior. Subsequently, the concentration and thickness were replaced by particle number in sample. It revealed that the dependency of the particle numbers on the optical power can replace the conventional parameters as thickness or concentration. Results show that correlated color temperature finds it dependency on number of particles also. In addition, it turned out, that for each of the material, there might exist an optimal particle number for the maximum luminous power. Finally, the influence of the irradiation of blue laser diode on the materials was investigated. After intensive irradiation for 30 min, we observed that the efficiency of light conversion decreases. It can be caused by damages made by laser on particles. Results show parts of nitride (650 nm) conversion decrease less than yellow or green phosphor. Less affection of irradiation can be explained by nitride being very covalent material, more resistant to modification. Also, we found that there is no fundamental modification of material structure because of no changing in spectrum shape of converted light.

The electronic and optical properties of niobium oxynitride (NbON) complexes as a function of the N/O ratio were examined using the Perdew–Burke–Ernzerhof exchange–correlation functional of density functional theory. The investigated NbON complexes (i.e., pristine NbON crystal and Nb16O16N16, Nb16O15N17, Nb16O14N18, Nb16O17N15, and Nb16O18N14 super crystals) were found to be indirect bandgap semiconductors. Varying the N/O ratio reallocates the band edges, giving rise to redshifts in the absorption spectra at higher N/O ratios. Partial density of states calculations showed valence band dependency on the N 2p orbital energy, with the conduction band predominantly composed of Nb 4d states. The dielectric constant and refractive index of pristine NbON are found to be comparable to those of isostructural TaON, and varying the N/O ratio leads to a significant improvement in the optical properties of the studied systems.

We demonstrated a solar-pumped laser with a heliostat–parabolic mirror system. A designed conical pump cavity with a water-filled quartz tube lens was used to couple efficiently the concentrated solar radiation from the focal zone of the primary concentrator into a 5.5-mm-diameter, 95-mm-length grooved Nd:YAG crystal rod within a gold-plated conical pump cavity. For 1.0-m² effective solar radiation collection area, 20.1-W continuous-wave (CW) output laser power was obtained, corresponding to a total system slope efficiency of 5.04%. This value was 1.31 times higher than the previous with the same solar facility. For 1.5-m² effective collection area, 34.6-W CW output laser power was achieved at the total system slope efficiency of 4.51%. A strong dependency of laser power on laser rod mounting position was also found.