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The thermal stability of inverted, halogen-rich non-fullerene acceptor (NFA)-based organic photovoltaics with MoO

Recent advances in thermophotovoltaic (TPV) power generation have produced notable gains in efficiency, particularly at very high emitter temperatures. However, there remains substantial room for improving TPV conversion of waste, solar, and nuclear heat streams at temperatures below 1,100°C. Here, we demonstrate the concept of transmissive spectral control that enables efficient recuperation of below-bandgap photons by allowing them to transmit through the cell to be absorbed by a secondary emitter. We fabricate a semitransparent TPV cell consisting of a thin InGaAs–InP heterojunction membrane supported by an infrared-transparent heat-conducting substrate. The device absorbs less than 1% of below-bandgap radiation, resulting in a TPV efficiency of 32.5% at an emitter temperature of 1,036°C. To our knowledge, this represents an 8% absolute improvement (~33% relative) in efficiency relative to the best TPV devices at such low temperatures. By enabling near-zero photon loss, the semitransparent architecture facilitates high TPV efficiencies over a wide range of applications.

We report a method to neutralize the mid-gap defect states in MoS2 monolayers using laser soaking of an organic/transition metal oxide (TMO) blend thin film. The treated MoS2 monolayer shows negligible emission from defect states as compared to the as-exfoliated MoS2, accompanied by a photoluminescence quantum yield improvement from 0.018 to 4.5% at excitation power densities of 10 W/cm2. The effectiveness of the method toward defect neutralization is governed by the polaron pair generated at the organic/TMO interface, the diffusion of free electrons, and the subsequent formation of TMO radicals at the MoS2 monolayer. The treated monolayers are stable in air, vacuum, and acetone environments, potentially enabling the fabrication of defect-free optoelectronic devices based on 2D materials and 2D/organic heterojunctions.

We demonstrate a hybrid light-emitting device (LED) employing a chemical-vapor-deposition grown, centimeter-scale monolayer of WS2 (mWS2) as the active luminescent material embedded within conducti...

Controlling matter-light interactions with cavities is of fundamental importance in modern science and technology. It is exemplified in the strong-coupling regime, where matter-light hybrid modes form, with properties controllable via the photon component on the optical-wavelength scale. In contrast, matter excitations on the nanometer scale are harder to access. In two-dimensional van der Waals heterostructures, a tunable moir\'e lattice potential for electronic excitations may form, enabling correlated electron gases in lattice potentials. Excitons confined in moir\'e lattices have also been reported, but cooperative effects have been elusive and interactions with light have remained perturbative. Here, integrating MoSe$_{2}$-WS$_{2}$ heterobilayers in a microcavity, we establish cooperative coupling between moir\'e-lattice excitons and microcavity photons up to liquid-nitrogen temperature, thereby integrating into one platform versatile control over both matter and light. The density dependence of the moir\'e polaritons reveals strong nonlinearity due to exciton blockade, suppressed exciton energy shift, and suppressed excitation-induced dephasing, all of which are consistent with the quantum-confined nature of the moir\'e excitons. Such a moir\'e polariton system combines strong nonlinearity and microscopic-scale tuning of matter excitations with the power of cavity engineering and long range coherence of light, providing a new platform for collective phenomena from tunable arrays of quantum emitters.

We introduce a method to analyze the performance of bulk heterojunction (BHJ) organic photovoltaics (OPVs) by calculating its “bulk quantum efficiency” (BQE), a quantity related to the recombination losses within the BHJ, but not in the surrounding device layers. By applying the method to both vacuum- and solution-processed OPVs with various BHJ, buffer layers and interface layer compositions, we show that measurements of the BQE isolates the properties of the BHJ from other device layers and interfaces. We use measurements of the BQE to study various mechanisms in OPV degradation and find that for solution-processed OPVs with a ZnO cathode buffer layer, the BHJ undergoes degradation due primarily to the ZnO. By inserting a self-assembled monolayer at the interface between the buffer and the BHJ, the stability of the OPV is significantly improved.

Tension control is critical for maintaining good product quality in most roll-to-roll (R2R) production systems. Previous work has primarily focused on improving the disturbance rejection performance of tension controllers. Here, a robust linear parameter-varying model predictive control (LPV-MPC) scheme is designed to enhance the tension tracking performance of a pilot R2R system for deposition of materials used in flexible thin film applications. The performance of a tension controller may degrade due to disturbances associated with model uncertainties and the slowly-changing dynamics in R2R systems. We introduce a method that separately treats these two sources of disturbance. The controller utilizes an incremental model to eliminate the errors caused by the mismatch between the nominal model and the actual system. A tube-based MPC formulation combined with scheduled parameters adequately updates models and corrects for the time-varying dynamics. Constraints on the rated motor torque are incorporated in the MPC to maintain the controller reliability and avoid machine failures. We illustrate the operation of our control algorithm through simulation of an actual R2R system. The controller outperforms the benchmarks in terms of fast transient response and offset-free tension tracking. It also demonstrates immunity from variations due to parametric uncertainties.

We determine precise nanoscale information about the morphologies of several organic thin film structures using Fourier plane imaging microscopy (FIM). We used FIM microscopy to detect the orientation of molecular transition dipole moments from an extremely low density of luminescent dye molecules, which we call "morphology sensors". The orientation of the sensor molecules is driven by the local film structure and thus can be used to determine details of the host morphology without influencing it. We use symmetric planar phosphorescent dye molecules as the sensors that are deposited into the bulk of organic film hosts during the growth. We demonstrate morphological mapping with a depth resolution to a few Ångstroms that is limited by the ability to determine thickness during deposition, along with an in-plane resolution limited by optical diffraction. Furthermore, we monitor morphological changes arising from thermal annealing of metastable organic films that are commonly employed in photonic devices.

Moire lattices formed in twisted van der Waals bilayers provide a unique, tunable platform to realize coupled electron or exciton lattices unavailable before. While twist angle between the bilayer has been shown to be a critical parameter in engineering the moire potential and enabling novel phenomena in electronic moire systems, a systematic experimental study as a function of twist angle is still missing. Here we show that not only are moire excitons robust in bilayers of even large twist angles, but also properties of the moire excitons are dependant on, and controllable by, the moire reciprocal lattice period via twist-angle tuning. From the twist-angle dependence, we furthermore obtain the effective mass of the interlayer excitons and the electron inter-layer tunneling strength, which are difficult to measure experimentally otherwise. These findings pave the way for understanding and engineering rich moire-lattice induced phenomena in angle-twisted semiconductor van der Waals heterostructures. Here, the authors show that the properties of the moire excitons in twisted van der Waals bilayers of transition metal dichalcogenides are determined by the moire reciprocal lattice period, and can be controlled via twist-angle tuning.

Organic light-emitting diode (OLED) displays are now poised to be the dominant mobile display technology and are at the heart of the most attractive televisions and electronic tablets on the market today. But this begs the question: what is the next big opportunity that will be addressed by organic electronics? We attempt to answer this question based on the unique attributes of organic electronic devices: their efficient optical absorption and emission properties, their ability to be deposited on ultrathin foldable, moldable and bendable substrates, the diversity of function due to the limitless palette of organic materials and the low environmental impact of the materials and their means of fabrication. With these unique qualities, organic electronics presents opportunities that range from lighting to solar cells to medical sensing. In this paper, we consider the transformative changes to electronic and photonic technologies that might yet be realized using these unconventional, soft semiconductor thin films.

Semitransparent organic photovoltaic cells (ST-OPVs) are emerging as a solution for solar energy harvesting on building facades, rooftops, and windows. However, the trade-off between power-conversion efficiency (PCE) and the average photopic transmission (APT) in color-neutral devices limits their utility as attractive, power-generating windows. A color-neutral ST-OPV is demonstrated by using a transparent indium tin oxide (ITO) anode along with a narrow energy gap nonfullerene acceptor near-infrared (NIR) absorbing cell and outcoupling (OC) coatings on the exit surface. The device exhibits PCE = 8.1 ± 0.3% and APT = 43.3 ± 1.2% that combine to achieve a light-utilization efficiency of LUE = 3.5 ± 0.1%. Commission Internationale d’eclairage chromaticity coordinates of (0.38, 0.39), a color-rendering index of 86, and a correlated color temperature of 4,143 K are obtained for simulated AM1.5 illumination transmitted through the cell. Using an ultrathin metal anode in place of ITO, we demonstrate a slightly green-tinted ST-OPV with PCE = 10.8 ± 0.5% and APT = 45.7 ± 2.1% yielding LUE = 5.0 ± 0.3% These results indicate that ST-OPVs can combine both efficiency and color neutrality in a single device.

Four new donor-acceptor-acceptor' (D-A-A')-configured donors, CPNT, DCPNT, CPNBT, and DCPNBT equipped with naphtho[1,2-c:5,6-c']bis([1,2,5]-thiadiazole) (NT) or naphtho[2,3-c][1,2,5]thiadiazole (NBT) as the central acceptor (A) unit bridging triarylamine donor (D) and cyano or dicyanovinylene acceptor (A'), were synthesized and characterized. All molecules exhibit bathochromic absorption shifts as compared to those of the benzothiadiazole (BT)-based analogues owing to improved electron-withdrawing and quinoidal character of NT and NBT cores that lead to stronger intramolecular charge transfer. Favorable energy level alignments with C70 , together with the good thermal stability and the antiparallel dimeric packing render CPNT and DCPNT suitable donors for vacuum-processed organic photovoltaics (OPV)s. OPVs based on DCPNT : C70 active layers displayed the best power conversion efficiency (PCE)=8.3%, along with an open circuit voltage of 0.92 V, a short circuit current of 14.5 mA cm-2 and a fill factor of 62% under 1 sun intensity, simulated AM1.5G illumination. Importantly, continuous light-soaking with AM 1.5G illumination has verified the durability of the devices based on CPNT:C70 and DCPNT : C70 as the active blends. The devices were examined for their feasibility of indoor light harvesting under 500 lux illumination by a TLD-840 fluorescent lamp, giving PCE=12.8% and 12.6%, respectively. These results indicate that the NT-based D-A-A'-type donors CPNT and DCPNT are potential candidates for high-stability vacuum-processed OPVs suitable for indoor energy harvesting.

Since its first observation, the polariton lasing threshold in organic films has been anomalously high, given that Bosonic condensation should occur at vanishingly small pumping powers. Here, we in...

Building integrated photovoltaics (BIPVs) are attached to commercial and residential structures to enable solar energy harvesting. While conventional Si photovoltaics (PVs) are dominant in the current market, second and third generation thin film solar cells based on amorphous Si, CdTe, CIGS, perovskites or organic photovoltaics (OPVs) are often considered as an alternative for BIPV applications since they may offer reduced costs compared to Si PVs. Indeed, recent advances in performance suggest that lightweight, flexible and visibly transparent OPVs can potentially be integrated into windows or other applications to which Si PVs are less well suited. Here, we estimate the cost of high efficiency, semitransparent OPVs (ST-OPVs) based on solution processing in a roll-to-roll (R2R) manufacturing line. Assuming modules with 10% power conversion efficiency (PCE), a 70% geometric fill factor (GFF), and 95% inverter efficiency, we anticipate a %1.6 per Wp module manufacturing cost that includes the cost of the microinverter to condition the OPV dc output to be compatible with the ac line voltage of the building. The materials and inverter cost comprise ∼90% of the total module cost. Hence, with simplified material synthesis and a lower inverter cost, including marginally improved PCE and GFF, we expect the cost can be as low as $0.47 per Wp. While the module costs ∼60% of the average (uninstalled) double-pane window, we expect the payback period can be as short as 2 to 6 years, suggesting that OPVs can be an economic and attractive candidate for BIPV applications.

In the light-matter strong coupling regime, we observe long-range exciton-polariton propagation in monolayer transition metal dichalcogenides at room temperature, where the propagation distance depends on the Bloch surface wave supported by all-dielectric photonic structures.

In light-matter strong coupling regime, we observe long-range photodetection response at room temperature mediated by organic exciton-polaritons, which results from strong interactions between organic excitons and low-loss Bloch surface wave (BSW) modes.

Fair and meaningful device per- formance comparison among luminescent solar concentrator- photovoltaic (LSC-PV) reports cannot be realized without a gen- eral consensus on reporting stan- dards in LSC-PV research. There- fore, it is imperative to adopt standardized characterization protocols for these emerging types of PV devices that are consistent with other PV devices. This commentary highlights several common limitations in LSC literature and summarizes the best practices moving for- ward to harmonize with standard PV reporting, considering the greater nuances present with LSC-PV. Based on these prac- tices, a checklist of actionable items is provided to help stan- dardize the characterization/re- porting protocols and offer a set of baseline expectations for au- thors, reviewers, and editors. The general consensus combined with the checklist will ultimately guide LSC-PV research towards reliable and meaningful ad- vances.

Strong light–matter coupling results in the formation of hybrid half-light half-matter excitations with modified energy levels. The strong coupling of excitons with photons in organic molecular sys...

Charge transfer (CT) between donors and acceptors following photoexcitation of organic photovoltaics (OPVs) gives rise to bound electron–hole pairs across the donor–acceptor interface, known as CT states. While these states are essential to charge separation, they are also a source of energy loss. As a result of reduced overlap between electron and hole wavefunctions, CT states are influenced by details of the film morphology and molecular structure. Here, we describe several important strategies for tuning the energy level and dynamics of the CT state and approaches that can enhance their dissociation efficiency into free charges. Furthermore, we provide an overview of recent physical insights into the key parameters that significantly reduce the Frenkel-to-CT energy offset and recombination energy losses while preserving a high charge-generation yield. Our analysis leads to critical morphological and molecular design strategies for achieving efficient, low-energy-loss OPVs.

Natural photosynthesis exploited by both plants and bacteria provides essential energy and chemicals for sustaining life. A photosynthetic system often comprises antenna complexes (ACs) that absorb incident sunlight and deliver the molecular excited state energy to a central reaction center (RC) that converts the energy to charge. Numerous efforts have been made to mimic natural photosynthesis using organic materials. However, short-range energy transport due to material disorder has hindered their success. Here, we demonstrate a photodetector that mimics this natural architecture. It employs an AC that exploits exceptionally long-range ( ∼ 100 µ m ) Bloch surface wave polariton propagation that directs the excited states to an organic heterojunction detector serving as the RC. Exciton–polaritons are largely immune to localization due to the partial photonic character of the polariton, which consequently transports energy over extraordinary distances from its point of origin. Exciton–polaritons are edge-coupled into an RC that harvests excitation energy by dissociation into an electron and hole. This device relies on polariton-to-charge conversion combined with long-range energy transport for the direct detection of excited states in organic semiconductors. Our work paves the way for the realization of large-scale artificial photosynthetic systems comprising integrated organic photonic and optoelectronic devices with efficient energy transport and harvesting under ambient conditions.

We report a method to neutralize the mid-gap defect states in MoS

A thermophotovoltaic (TPV) cell is specially designed to minimize the absorption of radiation below its lowest bandgap. This ensures that the unused power is returned to the hot thermal emitter, which keeps it from being wasted. This approach is termed photon recycling because the energy is recycled until it is emitted at a high enough frequency to be efficiently converted. To facilitate this process, we recently created a cell architecture that has a thin air layer behind the light-absorbing semiconductor. The resulting air-bridge cell (ABC) reflects back almost all of the low-energy photons. In this talk, I will discuss the development of an InGaAs ABC that achieved a record-high peak conversion efficiency of 32% and our recent efforts to improve performance.

Thermophotovoltaic approaches that take advantage of near-field evanescent modes are being actively explored due to their potential for high-power density and high-efficiency energy conversion. However, progress towards functional near-field thermophotovoltaic devices has been limited by challenges in creating thermally robust planar emitters and photovoltaic cells designed for near-field thermal radiation. Here, we demonstrate record power densities of ~5 kW/m2 at an efficiency of 6.8%, where the efficiency of the system is defined as the ratio of the electrical power output of the PV cell to the radiative heat transfer from the emitter to the PV cell. This was accomplished by developing novel emitter devices that can sustain temperatures as high as 1270 K and positioning them into the near-field (, Near-field thermophotovoltaic holds the potential for achieving high-power density and energy conversion efficiency by utilizing evanescent modes of heat transfer, yet the performance still lags behind the far-field counterpart. Here, the authors combine thermally robust planar emitter with InGaAs PV to push the limit of near-field device further.

Charge coupled devices (CCDs), often used for scientific imaging and photography, conventionally comprise an active inorganic semiconductor such as silicon. Organic semiconductors, however, are typ...

Optoelectronic circuits in 3D shapes with large deformations can offer additional functionalities inaccessible to conventional planar electronics based on 2D geometries constrained by conventional photolithographic patterning processes. A light-sensing focal plane array (FPA) used in imagers is one example of a system that can benefit from fabrication on curved surfaces. By mimicking the hemispherical shape of the retina in the human eye, a hemispherical FPA provides a low-aberration image with a wide field of view. Due to the inherently high value of such applications, intensive efforts have been devoted to solving the problem of transforming a circuit fabricated on a flat wafer surface to an arbitrary shape without loss of performance or distorting the linear layouts that are the natural product of this fabrication paradigm. Here we report a general approach for fabricating electronic circuits and optoelectronic devices on nondevelopable surfaces by introducing shear slip of thin-film circuit components relative to the distorting substrate. In particular, we demonstrate retina-like imagers that allow for a topological transformation from a plane to a hemisphere without changing the relative positions of the pixels from that initially laid out on a planar surface. As a result, the resolution of the imager, particularly in the foveal region, is not compromised by stretching or creasing that inevitably results in transforming a 2D plane into a 3D geometry. The demonstration provides a general strategy for realizing high-density integrated circuits on randomly shaped, nondevelopable surfaces.

We investigate molecular fragmentation in the long lived archetype blue PHOLED guest/host system based on iridium (III) tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f] phenanthridine] (Ir(dmp)3) and 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) as a function of luminance loss using laser desorption ionization time-of-flight mass spectrometry and X-ray photoelectron spectroscopy (XPS). The blue phosphor experiences significant ligand loss due to its labile Ir-ligand bonds, whereas the mCBP host is relatively stable as the luminance decays to 40% of its initial value at 3000 cd/m2. Density functional theory calculations indicate that fragments from mCBP are less likely to cause luminance loss via exciton quenching compared to those of the Ir-complex. Phosphor fragments are observed by XPS only in aged samples, and the concentration of fragments is found to increase with aging time. These findings suggest that the lifetime of blue PHOLEDs can be improved by using phosphors without labile Ir-ligand and intra-ligand bonds, and a host material that does not form large mass fragments or complexes with the degraded phosphor molecules. Insights into the origin of the loss in luminance due to trap formation from molecular fragmentation of this guest/host system during use are discussed.

A class of wide bandgap host materials is introduced as an alternative to carbazole-based hosts to enhance the efficiency and transport properties of organic light emitting diodes (OLEDs). We have synthesized and investigate the photophysical and electrochemical properties of a series of phenanthrene derivatives incorporating fused triazole or imidazole rings. The resulting phenanthro[9,10-d]triazoles and phenanthro[9,10-d]imidazoles are suitable host materials for blue phosphors due to their high triplet energies and conductivities. Incorporation of bulky substituent groups leads to retention of the high triplet energies in the solid state. The most promising materials are incorporated in blue phosphorescent OLEDs that achieve external quantum efficiencies >20%.

We demonstrate the surface passivation of InP using thin layers of a perylenetetracarboxylic diimide derivative (PTCDI-C9) applied via organic vapor phase deposition (OVPD). The organic layer forms a conformal crystalline film on the InP surface, which is confirmed by atomic force microscopy and X-ray diffraction. Area-dependent photoluminescence measurements indicate that the coating reduces surface recombination. The organic thin film deposited by OVPD exhibits improved photoconductivity compared to an unpassivated InP sample, and to a layer deposited via vacuum thermal evaporation. Our results suggest that semiconductor surface passivation using organic thin films deposited by OVPD has applications to a variety of optoelectronic devices, particularly with structures requiring sidewall or conformal coatings.

The orientation of facial (fac) tris-cyclometalated iridium complexes in doped films prepared by vacuum deposition is investigated by altering the physical shape and electronic asymmetry in the molecular structure. Angle-dependent photoluminescence spectroscopy and Fourier-plane imaging microscopy show that the orientation of roughly spherical fac-tris(2-phenylpyridyl)iridium (Ir(ppy)3 ) is isotropic, whereas complexes that are oblate spheroids, fac-tris(mesityl-2-phenyl-1H-imidazole)iridium (Ir(mi)3 ) and fac-tris((3,5-dimethyl-[1,1'-biphenyl]-4-yl)-2-phenyl-1H-imidazole)iridium (Ir(mip)3 ), have a net horizontal alignment of their transition dipole moments. Optical anisotropy factors of 0.26 and 0.15, respectively, are obtained from the latter complexes when doped into tris(4-(9H-carbazol-9-yl)phenyl)amine host thin films. The horizontal alignment is attributed to the favorable van der Waals interaction between the oblate Ir complexes and host material. Trifluoromethyl groups substituted on one polar face of the Ir(ppy)3 and Ir(mi)3 complexes introduce chemical asymmetries in the molecules at the expense of their oblate shapes. The anisotropy factors of films doped with these substituted derivatives are lower relative to the parent complexes, indicating that the fluorinated patches reinforce horizontal alignment during deposition. High efficiencies obtained from organic light emitting diodes prepared using the Ir dopants is attributed, in part, to improved outcoupling of electroluminescence brought about by molecular alignment.

Organic photovoltaic cells (OPVs) have the potential of becoming a productive renewable energy technology if the requirements of low cost, high efficiency and prolonged lifetime are simultaneously fulfilled. So far, the remaining unfulfilled promise of this technology is its inadequate operational lifetime. Here, we demonstrate that the instability of NFA solar cells arises primarily from chemical changes at organic/inorganic interfaces bounding the bulk heterojunction active region. Encapsulated devices stabilized by additional protective buffer layers as well as the integration of a simple solution processed ultraviolet filtering layer, maintain 94% of their initial efficiency under simulated, 1 sun intensity, AM1.5 G irradiation for 1900 hours at 55 °C. Accelerated aging is also induced by exposure of light illumination intensities up to 27 suns, and operation temperatures as high as 65 °C. An extrapolated intrinsic lifetime of > 5.6 × 104 h is obtained, which is equivalent to 30 years outdoor exposure., Through development of non-fullerene acceptors, OPVs have reached efficiencies of 18%, yet the inadequate operational lifetime still poses a challenge for the commercialisation. Here, the authors investigate the origin of instability of NFA solar cells, and propose some strategies to mitigate this issue.

State-of-the-art TPV converters use cells with high out-of-band reflectance to facilitate a photon recycle process, in which sub-bandgap photons are reflected by the cell and subsequently re-absorbed at the emitter. However, cells relying on metallic back surface reflectors, Bragg/plasma filters, and photonic crystals for spectral control suffer from undesired out-of-band absorptance and have yet to surpass 95% out-of-band reflectance. Here we describe the fabrication and characterization of a thin-film In0.53Ga0.47As thermophotovoltaic cell with an air-bridge architecture, in which the absorber material is suspended over an air gap, supported by Au grid lines. The average out-of-band reflectance of the cell exceeds 98% due to lossless Fresnel reflectance at the In0.53Ga0.47As-air interface and < 2% loss at the air-Au interface. The result is a record-high TPV conversion efficiency of 32%, characterized under illumination by a 1455K SiC globar.

Organic photovoltaics (OPVs) based on nonfullerene acceptors are now approaching commercially viable efficiencies. One key to their success is efficient charge separation with low potential loss at the donor-acceptor heterojunction. Due to the lack of spectroscopic probes, open questions remain about the mechanisms of charge separation. Here, we study charge separation of a model system composed of the donor, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))