A universal Urbach rule for disordered organic semiconductors

Drude-Lorentz classical oscillator model and Tauc's approach to study the localized density of states and energy band gap of polymer composites based on chitosan integrated with green synthesized Pb-metal complexes: Improvement of linear and non-linear optical parameters

In the current work, green-synthesized metal complexes were combined with chitosan to fabricate polymer composites with controlled optical properties. The dye extracted from green tea was enriched with ligands to capture heavy metal ions and producing metal complexes. The FTIR bands associated with chitosan reduced in intensity and shifted in position when the Pb-metal complex (Pb-MC) was inserted. The XRD pattern illustrates that chitosan transformed into an amorphous material after the insertion of Pb-MC. FESEM revealed enhanced surface roughness in CS films containing high concentration of Pb-MC particles. The optical properties indicate that the bandgap energy was reduced from 5.1 eV to 1.68 eV. Urbach energy values were correlated with the amorphous nature of the films. The Eo and Ed values estimated from W-D models were used to compute the M-1 and M-3 optical moments. The optical parameters of CS:Pb-MC, including N/m, εL, NC, μopt, ρopt, and ωp, were determined based on the Lorentz oscillator model. The Sellmeier parameters (λo and So) were successfully calculated. The determined phase (vp) and group (vg) velocities establish that the composite media are dispersive. Kirchhoff's thermal emission and surface resistance were studied for all the films. Volume and surface energy losses were determined from the data on optical dielectric properties. The susceptibilities (χ(1)), (χ(2)), and (χ(3)) correspondingly represent the linear, second-order, and third-order nonlinear optical susceptibilities.

Minimizing Energy Loss by Designing Multifunctional Solid Additives to Independent Regulation of Donor and Acceptor Layers for Efficient LBL Polymer Solar Cells

Solid additives are crucial in layer-by-layer (LBL) polymer solar cells (PSCs). Despite its importance, the simultaneous application of solid additives into both donor and acceptor layers has been largely overlooked. In this work, two multifunctional solid additives are actively designed, and investigated the synergistic effect on both donor and acceptor layers. Incorporating the multifunctional solid additives into the donor layer could effectively enhance the aggregation and molecular stacking of the donor polymer, leading to reduced energy disorder and minimizing ΔE2. When the multifunctional solid additives are introduced into the acceptor layer, they just play a role in optimizing the morphology, thereby reducing the ΔE3. Excitedly, the simultaneous addition of the multifunctional solid additives into both donor and acceptor layers produced a synergistic effect for decreasing ΔE2 and ΔE3 simultaneously, especially adding SA2, thus enabling an excellent power conversion efficiency (PCE) of 19.95% (certified as 19.68%) with an open-circuit voltage (Voc) of 0.921 V, a short circuit current density (Jsc) of 27.08 mA cm-2 and a fill factor (FF) of 79.98%. The work highlights the potential of multifunctional solid additives in independently regulating the properties of donor and acceptor layers, which is expected as a promising approach for further developing higher performance PSCs.

Annihilation-limited long-range exciton transport in high-mobility conjugated copolymer films

A combination of ultrafast, long-range, and low-loss excitation energy transfer from the photoreceptor location to a functionally active site is essential for cost-effective polymeric semiconductors. Delocalized electronic wavefunctions along π-conjugated polymer (CP) backbone can enable efficient intrachain transport, while interchain transport is generally thought slow and lossy due to weak chain-chain interactions. In contrast to the conventional strategy of mitigating structural disorder, amorphous layers of rigid CPs, exemplified by highly planar poly(indacenodithiophene-co-benzothiadiazole) (IDT-BT) donor-accepter copolymer, exhibit trap-free transistor performance and charge-carrier mobilities similar to amorphous silicon. Here, we report long-range exciton transport in HJ-aggregated IDTBT thin-film, in which the competing exciton transport and exciton-exciton annihilation (EEA) dynamics are spectroscopically separated using a phase-cycling-based scheme and shown to depart from the classical diffusion-limited and strong-coupling regime. In the thin film, we find an annihilation-limited mechanism with ≪100% per-encounter annihilation probability, facilitating the minimization of EEA-induced excitation losses. In contrast, excitons on isolated IDTBT chains diffuse over 350 nm with 0.56 cm2 s-1 diffusivity, before eventually annihilating with unit probability on first contact. We complement the pump-probe studies with temperature-dependent photocurrent and EEA measurements from 295 K to 77 K and find a remarkable correspondence of annihilation rate and photocurrent activation energies in the 140 K to 295 K temperature range.

Increase the Long-Wavelength Absorption for Carbon-Electrode Based Hole-Conductor-Free Perovskite Solar Cells by Thicker Perovskite Films

Weak reflectivity of carbon-electrode (CE) limits light harvesting of perovskite solar cells that are based on CEs (CPSCs), especially for long-wavelength region. To solve this problem, herein the crystallization of perovskite (PVSK) is regulated by tuning the concentration of PbI2 precursor during the two-step growth method. As concentration increases from 1.0 to 1.7 mol L-1, film thickness of PVSK rises from ≈360 to ≈850 nm, and the average grain/crystallite size of PVSK enlarges from 0.70 to 1.14 µm, and from 54.2 to 67.5 nm, respectively. Due to the upgraded crystallization, Urbach energy drops from 98 to 41 meV, lifetime of charge carriers increases obviously, meanwhile the light harvesting of PVSK is improved during the long-wavelength regions (600-810 nm). External quantum efficiency test on the CPSCs shows that the integrated short circuit current density (JSC) increases by 15.6% during the long-wavelength region. Accordingly, JSC improves from 20.27 (±0.36) to 22.58 (±0.27) mA cm-2. Besides, leakage is retarded, and open-circuit voltage is improved. These merits help elevate the power conversion efficiency from 15.37 (±0.42) to 16.37 (±0.68) % (optimized at 17.69%). Prolonging spin-time of PbI2 further improves PVSK crystallization and light harvest, which optimizes device efficiency to 19.17%.

Precise Manipulation on the Structural Defects of Poly (Triazine Imide) Single Crystals for Efficient Photocatalytic Overall Water Splitting

Conjugated polymers, represented by polymeric carbon nitrides (PCNs), have risen to prominence as new-generation photocatalysts for overall water splitting (OWS). Despite considerable efforts, achieving highly crystalline PCNs with minimal structural defects remains a great challenge, and it is also difficult to examine the exact impact of complex defect states on OWS process, which largely limits their quantum efficiency. Herein, we devise a 'in-situ salt flux' assisted copolymerization protocol by using nitrogen-rich and nitrogen-deficient monomers to precisely manipulate the structural defects of poly (triazine imide) (PTI) single crystals. Stoichiometric control between two comonomers enables continuous tunning of carbon- and nitrogen-vacancies within PTI, allowing the construction of a series of PTI crystals with different defect states. Theoretical and experimental results unveil the carbon vacancies are related with the radiative decay of excitons, while the nonradiative decay is mainly derived from the nitrogen vacancies. Owing to the effective suppression of both radiative and nonradiative losses, the as-synthesized PTI achieves a record apparent quantum efficiency of 37.8 % by one-step-excitation OWS. This work highlights the significance of rational control of the structural defects and describes clear structure-property-activity relationships in PTI photocatalyst, offering guidance for the development of polymer photocatalysts for solar fuel production.

Toward Low Energetic Disorder in Organic Solar Cells: The Critical Role of Polymer Donors

Compared with those of inorganic and perovskite solar cells, the power conversion efficiencies of organic solar cells (OSCs) are severely limited by a large energetic disorder. However, the origin of energetic disorder for OSCs remains poorly understood. Herein, we systematically investigate the energetic disorder in representative OSCs and the effect of both the acceptors and polymer donors by combining atomistic molecular dynamics simulations with density functional theory calculations. The results indicate that regardless of whether the OSCs are based on fullerene or acceptor-donor-acceptor (A-D-A) acceptors, the energetic disorder in the ionization potentials of the polymer donors is significantly larger than that in the electron affinities of the acceptors. Moreover, the energetic disorder of the donors matched with the fullerene acceptors is noticeably greater than that of the donors matched with the A-D-A acceptors. This implies that, different from our intuition, the reduction in the energetic disorder from the fullerene-based to A-D-A acceptor-based OSCs is primarily attributed to the change in the polymer donors rather than the acceptors. This work underscores the vital importance of optimizing polymer donors toward low energetic disorder for high-efficiency OSCs.

Effect of growth dynamics on the structural, photophysical and pseudocapacitance properties of famatinite copper antimony sulphide colloidal nanostructures (including nanosheets)

Facile phase selective synthesis of copper antimony sulphide (CAS) nanostructures is important because of their tunable photoconductive and electrochemical properties. In this study, off-stoichiometric famatinite phase CAS (fCAS) quasi-spherical and quasi-hexagonal colloidal nanostructures (including nanosheets) of sizes, 2.4-18.0 nm were grown under variable conditions of temperature (60-200 °C), time and oleylamine capping ligand concentration using copper(II) acetylacetonate and antimony(III) diethyldithiocarbamate precursors. Data from powder X-ray diffraction, Raman spectroscopy and high-resolution scanning/transmission electron microscopy confirm the tetragonal structure of the famatinite phase. X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy data suggest a correlation of particle size, morphology and composition of the off-stoichiometric fCAS nanostructures with growth temperature and time, and oleylamine concentration. The off-stoichiometric Cu3-aSb1+bS4±c (a, b, c - mole fractions) nanostructures being severely copper-deficient and antimony-rich, exhibit shallow-lying acceptor copper vacancy states, deep-lying donor states of antimony interstitials, sulphur vacancies and antimony-copper antisites and shallow-lying acceptor surface trapping states. These electronic states are likely implicated in tunable UV-visible absorption and bandgaps between 2.3 and 2.8 eV, and broad visible-NIR photoluminescence with fast recombination of radiative lifetimes between 0.2 and 6.2 ns, confirmed from absorption, steady-state and time-resolved photoluminescence spectroscopies. Additionally, cyclic voltammetry and electrochemical impedance spectroscopy confirm that electrodes of the fCAS nanostructures display slightly variable pseudocapacitance of charge-storage primarily via possible sodium ion intercalation with a high specific capacitance of ∼84 F g-1 obtained at a scan rate of 5 mV s-1. Overall, these results show the influence of composition, in particular point defects, phase quality and morphology on the optical and pseudocapacitance properties of fCAS nanostructures, suitable as solar absorbers or electrodes for energy storage devices.

Molecular-dipole oriented universal growth of conjugated polymers into semiconducting single-crystal thin films

Precise control over crystallinity and morphology of conjugated polymers (CPs) is essential for progressing organic electronics. However, manufacturing single-crystal thin films of CPs presents substantial challenges due to their complex molecular structures, distorted chain conformations, and unbalanced crystallization kinetics. In this work, we demonstrate a universal nanoconfined molecular-dipole orientating strategy to craft high-quality single-crystal thin films for a variety of CPs, spanning from traditional thiophene- and theinothiophene-based homopolymers to diketopyrrolopyrrole- (i.e., p-type) and naphthalene-based (i.e., n-type) donor-acceptor copolymers. Central to this strategy is the synergetic manipulations of molecular dipoles, π-π stackings, and alkyl-alkyl interactions of CPs within our rationally-designed spatial-electrostatic confinement capacitor, which facilitates the rotation of conjugated backbones and the alignment of π-π stackings into microscale-sized single-crystal thin films. A minimal energetic disorder of 25 meV that below the thermal fluctuation energy kBT at room temperature, as well as an excellent transistor mobility of 15.5 cm2V-1s-1 are achieved, marking a significant step towards controllable growths of conjugated-polymer single-crystal thin films that hold a cornerstone for high-performance organic electronic devices.

Modulation of Molecular Quadrupole Moments by Phenyl Side-Chain Fluorination for High-Voltage and High-Performance Organic Solar Cells

The ground-state charge generation (GSCG) in photoactive layers determines whether the photogenerated carriers occupy the deep trap energy levels, which, in turn, affects the device performance of organic solar cells (OSCs). In this work, charge-quadrupole electrostatic interactions are modulated to achieve GSCG through a molecular strategy of introducing different numbers of F atom substitutions on the BTA3 side chain. The results show that 8F substitution (BTA3-8F) and 16F substitution (BTA3-16F) lead to different patterns of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level changes. The perfluorination of the phenyl side chain endows BTA3-16F with a LUMO energy level similar to that of BTA3, ensuring a high VOC. Besides, BTA3-16F features a large charge-quadrupole moment, promoting strong electrostatic interactions between neighboring molecules along the π-π stacking direction, which then induces the GSCG in photoactive components. This efficient GSCG directly makes a significant impact on the subsequent kinetics of photogenerated exciton dissociation, recombination, and transport over longer time periods, as well as on nonradiative recombination over larger spatial scales. Benefiting from the favorable GSCG and suitable energy level arrangement, PTQ10/BTA3-16F achieves a VOC of 1.302 V and a PCE of 11.14%, setting the world record for OSCs performance with a VOC greater than 1.3 V. In addition, BTA3-16F is an effective guest molecule to improve the performance of ternary OSCs, and PM6/L8-BO/BTA3-16F-based ternary device achieves the highest PCE of 19.82%. This result emphasizes the important role of GSCG between photoactive components in OSC performance and demonstrates that modulation of molecular quadrupole moments is an effective means of designing efficient acceptors.

Improving Performance of Perovskite Solar Cells by Reducing Energetic Disorder of Hole Transport Polymer

Polymer hole transport materials offer significant efficiency and stability advantages for p-i-n perovskite solar cells. However, the energetic disorder of amorphous polymer hole transport materials not only limits carrier transport but also impedes contact between the polymer and perovskite, hindering the formation of high crystalline quality perovskites. Herein, a novel low energetic disordered polymer hole transport material, PF8ICz, featuring an indeno[3,2-b]carbazole unit with extended π-conjugation is designed and synthesized. Analyses based on both theoretical calculations and experimental validation highlight the advantages of PF8ICz as a low energetic disorder polymer hole transport material for perovskite solar cells, including improved carrier transport, enhanced perovskite affinity/passivation, and optimized energy levels. Perovskite films formed atop PF8ICz exhibit superior crystalline quality and improved exciton dynamics. PF8ICz-based perovskite solar cells achieve remarkable power efficiency (PCE > 25.4%) and outstanding stability (retaining 96.2% and 95.0% of their PCE under the ISOS-D-3 and ISOS-L-3 protocols over 1000 h, respectively). These findings underscore the importance of rational design of hole transport materials, contributing to the development of high-performance, stable perovskite solar cells for sustainable energy solutions.

Inner Side Chain Modification of Small Molecule Acceptors Enables Lower Energy Loss and High Efficiency of Organic Solar Cells Processed with Non-halogenated Solvents

Organic solar cells (OSCs) processed with non-halogenated solvents usually suffer from excessive self-aggregation of small molecule acceptors (SMAs), severe phase separation and higher energy loss (Eloss), leading to reduced open-circuit voltage (Voc) and power conversion efficiency (PCE). Regulating the intermolecular interaction to disperse the aggregation and further improve the molecular packing order of SMAs would be an effective strategy to solve this problem. Here, we designed and synthesized two SMAs L8-PhF and L8-PhMe by introducing different substituents (fluorine for L8-PhF and methyl for L8-PhMe) on the phenyl end group of the inner side chains of L8-Ph, and investigated the effect of the substituents on the intermolecular interaction of SMAs, Eloss and performance of OSCs processed with non-halogenated solvents. Through single crystal analysis and theoretical calculations, it is found that compared with L8-PhF, which possesses strong and abundant intermolecular interactions but downgraded molecular packing order, L8-PhMe with the methyl substituent possesses more effective non-covalent interactions, which improves the tightness and order of molecular packing. When blending the SMAs with polymer donor PM6, the differences in intermolecular interactions of the SMAs influenced the film formation process and phase separation of the blend films. The L8-PhMe based blend film exhibits shorten film formation and more homogeneous phase separation than those of the L8-PhF and L8-Ph based ones. Especially, the OSCs based on L8-PhMe show reduced non-radiative energy loss and enhanced Voc than the devices based on the other two SMAs. Consequently, the L8-PhMe based device processed with o-xylene (o-XY) and using 2PACz as the hole transport layer (HTL) shows an outstanding PCE of 19.27 %. This study highlights that the Eloss of OSCs processed with non-halogenated solvents could be decreased through regulating the intermolecular interactions of SMAs by inner side chain modification, and also emphasize the importance of effectivity rather than intensity of non-covalent interactions introduced in SMAs on the molecular packing, morphology and PCE of OSCs.

Insulator-donor electron wavefunction coupling in pseudo-bilayer organic solar cells achieving a certificated efficiency of 19.18

The incorporation of polymeric insulators has led to notable achievements in the field of organic semiconductors. By altering the blending concentration, polymeric insulators exhibit extensive capabilities in regulating molecular configuration, film crystallinity, and mitigation of defect states. However, current research suggests that the improvement in such physical properties is primarily attributed to the enhancement of thin film morphology, an outcome that seems to be an inevitable consequence of incorporating insulators. Herein, we report a general and completely new effect of polymeric insulators in organic semiconductors: the insulator-donor electron wavefunction coupling effect. Such insulators can couple with donor polymers to reduce the energy barrier level and facilitate intramolecular electron transport. Besides the morphological effects, we observed that this coupling effect is another mechanism that can significantly enhance electron mobility (up to 100 times) through the incorporation of polymeric insulators in a series of donor systems. With this effect, we proposed a polymeric insulator blending approach to fabricate state-of-the-art pseudo-bilayer organic solar cells, and the PM6/L8-BO device exhibits a high efficiency of 19.50% (certificated 19.18%) with an improved interfacial electron transport property. This work not only offers a novel perspective on the quantum effect of polymeric insulators in organic semiconductors, but also presents a simple yet effective method for enhancing the performance of organic solar cells.

A butterfly-shaped acceptor with rigid skeleton and unique assembly enables both efficient organic photovoltaics and high-speed organic photodetectors

It remains challenging to design efficient bifunctional semiconductor materials in organic photovoltaic and photodetector devices. Here, we report a butterfly-shaped molecule, named WD-6, which exhibits low energy disorder and small reorganization energy due to its enhanced molecular rigidity and unique assembly with strong intermolecular interaction. The binary photovoltaic device based on PM6:WD-6 achieved an efficiency of 18.41%. Notably, an efficiency of 19.42% was achieved for the ternary device based on PM6:BTP-eC9:WD-6. Moreover, the photodetection device based on WD-6 demonstrated an ultrafast response speed (205 ns response time at λ of 820 nm) and a high cutoff frequency of -3 dB (2.45 MHz), surpassing the values of most commercial Si photodiodes. Based on these findings, we showcased an application of the WD-6-based photodetection device in high-speed optical communication. These results offer valuable insights into the design of organic semiconductor materials capable of simultaneously exhibiting high photovoltaic and photodetective performance.

Designing a Highly Crystalline Polymer Donor with Alkylsilyl and Fluorine Substitution to Achieve Efficient Ternary Organic Solar Cells

Adopting a ternary strategy is an effective approach to enhance the power conversion efficiency (PCE) in organic solar cells (OSCs). Previous research on highly efficient ternary systems has predominantly focused on those based on highly crystalline dual small molecule acceptors. However, limited attention has been given to ternary systems utilizing dual polymer donors. Herein, by incorporating the fluorine and alkylsilyl substitution, a new polymer donor named PX1 is developed, which demonstrates strong crystallinity and excellent miscibility with polymer PM6. Moreover, PX1 broadens and enhances the absorption properties of the PM6:L8-BO blends, and its molecular orbital energy level is situated between those of PM6 and L8-BO, highlighting its suitability as a third component. Introducing 20% PX1 into the PM6:L8-BO system resulted in a high PCE of 18.82%. PX1 effectively suppresses charge recombination and reduces energy losses, while also serving as a morphology modulator that enhances the crystallization and improves the molecular packing order of the active layer by shortening the π-π stacking distance and extending crystalline coherence length. These factors collectively contribute to the performance improvements in ternary devices. This study demonstrates that employing a dual polymer donor strategy is a promising approach for achieving high-performance ternary OSCs.

A 0D Ge(II)-Halide-Based Perovskite with Enhanced Semiconducting Behavior for Electronic Capacitors

Perovskite materials have surged to the forefront of materials science, captivating researchers worldwide with their distinctive crystal lattice arrangement and remarkable optical, electric and dielectric attributes. The current study focuses on the development of a novel zero-dimensional (0D) Ge(II)-based hybrid perovskite, formulated as NH3(CH2)2NH3GeF6, and synthesized through a gradual evaporation process conducted at room temperature. The crystal structure is characterized by an arrangement of organic cations and isolated octahedral [GeF6]2- groups. This configuration is stabilized by relatively weak intermolecular bonds. A comprehensive analysis of the material's thermal properties using differential scanning calorimetry (DSC) revealed a distinct phase transition occurring at approximately 323 K, which was further confirmed through electrical measurements. The studied compound provided a broad absorption range across the visible spectrum and an optical band gap of 3.30 eV, indicating its potential for semiconducting applications in optoelectronic devices. Photoluminescence PL analysis displays a blueish broad-band emission with a high color rendering index CRI value of 91, when excited at 325 nm. This emission primarily originates from the self-trapped excitons (STEs) recombination in the inorganic [GeF6]2-. Herein, the temperature-dependent behavior of grain conductivity exhibited an Arrhenius-type pattern, with an activation energy (E a) of 0.46 eV, confirming the semiconductor nature of the investigated compound. In addition, a deep investigation of the alternating current conductivity, analyzed using Jonscher's law, demonstrates that the conduction mechanism is effectively described by the correlated barrier hopping (CBH) model. The dielectric performances show a significant dielectric constant (ε' ∼ 103). Thus, all these interesting physical properties of this hybrid perovskite have paved the way for advancements in various technological applications, particularly in the field of electronic capacitors.

Using Molecular Structure to Tune Intrachain and Interchain Charge Transport in Indacenodithiophene-Based Copolymers

In this work, we compare two structurally near-amorphous rigid-rod polymers─poly(indacenodithiophene-co-benzothiadiazole), p(IDT-BT), and poly(indacenodithiophene-co-benzopyrollodione), p(IDT-BPD)─with orders of magnitude different mobilities to understand the effect charge carrier intrachain delocalization has on electronic transport. Quantum chemical calculations show that p(IDT-BPD) has a barrier to torsion that is significantly lower than that of p(IDT-BT) and is thus more likely to have reduced conjugation lengths. We utilize absorption and photoluminescence spectroscopy to characterize energetic disorder and show that p(IDT-BPD) has higher energetic disorder. Charge modulation spectroscopy (CMS) and model calculations are used to show that charge carriers are substantially delocalized in p(IDT-BT) and occupy near-uniform energetic environments. We find that mobility activated hopping barriers are similar in these two materials. Electronic structure calculations show that both intrachain and interchain couplings of monomer units are poor enough in p(IDT-BPD) that charge carriers collapse to single IDT units and transport via a through-space tunneling mechanism. This work highlights the remarkable charge transport properties of p(IDT-BT) by showing that high mobilities are achievable on device-relevant length scales with only 1D carrier delocalization.

Direct bandgap quantum wells in hexagonal Silicon Germanium

Silicon is indisputably the most advanced material for scalable electronics, but it is a poor choice as a light source for photonic applications, due to its indirect band gap. The recently developed hexagonal Si1-xGex semiconductor features a direct bandgap at least for x > 0.65, and the realization of quantum heterostructures would unlock new opportunities for advanced optoelectronic devices based on the SiGe system. Here, we demonstrate the synthesis and characterization of direct bandgap quantum wells realized in the hexagonal Si1-xGex system. Photoluminescence experiments on hex-Ge/Si0.2Ge0.8 quantum wells demonstrate quantum confinement in the hex-Ge segment with type-I band alignment, showing light emission up to room temperature. Moreover, the tuning range of the quantum well emission energy can be extended using hexagonal Si1-xGex/Si1-yGey quantum wells with additional Si in the well. These experimental findings are supported with ab initio bandstructure calculations. A direct bandgap with type-I band alignment is pivotal for the development of novel low-dimensional light emitting devices based on hexagonal Si1-xGex alloys, which have been out of reach for this material system until now.

Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal-halide perovskites

The technique of alloying FA+ with Cs+ is often used to promote structural stabilization of the desirable α-FAPbI3 phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in CsxFA1-xPbI3 perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs+ tuning (Cs0.15FA0.85PbI3) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains'/domains' carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs0.15FA0.85PbI3 devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

Room Temperature Crystallized Phase-Pure α-FAPbI3 Perovskite with In-Situ Grain-Boundary Passivation

Energy loss in perovskite grain boundaries (GBs) is a primary limitation toward high-efficiency perovskite solar cells (PSCs). Two critical strategies to address this issue are high-quality crystallization and passivation of GBs. However, the established methods are generally carried out discretely due to the complicated mechanisms of grain growth and defect formation. In this study, a combined method is proposed by introducing 3,4,5-Trifluoroaniline iodide (TFAI) into the perovskite precursor. The TFAI triggers the union of nano-sized colloids into microclusters and facilitates the complete phase transition of α-FAPbI3 at room temperature. The controlled chemical reactivity and strong steric hindrance effect enable the fixed location of TFAI and suppress defects at GBs. This combination of well-crystallized perovskite grains and effectively passivated GBs leads to an improvement in the open circuit voltage (Voc) of PSCs from 1.08 V to 1.17 V, which is one of the highest recorded Voc without interface modification. The TFAI-incorporated device achieved a champion PCE of 24.81%. The device maintained a steady power output near its maximum power output point, showing almost no decay over 280 h testing without pre-processing.

Enhancement of the internal quantum efficiency in strongly coupled P3HT-C60 organic photovoltaic cells using Fabry-Perot cavities with varied cavity confinement

The short exciton diffusion length in organic semiconductors results in a strong dependence of the conversion efficiency of organic photovoltaic (OPV) cells on the morphology of the donor-acceptor bulk-heterojunction blend. Strong light-matter coupling provides a way to circumvent this dependence by combining the favorable properties of light and matter via the formation of hybrid exciton-polaritons. By strongly coupling excitons in P3HT-C60 OPV cells to Fabry-Perot optical cavity modes, exciton-polaritons are formed with increased propagation lengths. We exploit these exciton-polaritons to enhance the internal quantum efficiency of the cells, determined from the external quantum efficiency and the absorptance. Additionally, we find a consistent decrease in the Urbach energy for the strongly coupled cells, which indicates the reduction of energetic disorder due to the delocalization of exciton-polaritons in the optical cavity.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Linear and nonlinear optical parameters of biodegradable chitosan/polyvinyl alcohol/sodium montmorillonite nanocomposite films for potential optoelectronic applications

Innovations in sophisticated optoelectronic devices have increased interest in high-refractive index polymers. Herein, we report innovative nanocomposite films with high linear and nonlinear refractive indices prepared by casting chitosan (Cs) with polyvinyl alcohol (PVA) (50:50 wt%) along with different concentrations (10-50 wt%) of sodium montmorillonite (NaMMT) nanoclay. The refractive indices in addition to other optical parameters of homopolymers and hybrid materials were investigated by UV-Vis. spectroscopy and optical modeling to assess their potential applications in optics. Besides, the structure, morphology, and thermal stability of the prepared films were investigated by a multitude of experimental techniques including X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), and thermogravimetric analysis (TGA/DTG). The ATR-FTIR, XRD, SEM, and AFM measurements confirmed the complete exfoliation of NaMMT nanolayers in the Cs/PVA matrix. The TGA/DTG revealed an increase in the thermal stability of Cs/PVA film with increasing clay content. The UV-Vis. measurements revealed a decrease in the optical energy gap (Eg) and a substantial increase in the linear (nD) and nonlinear (n2) refractive indices as clay content increased. Additionally, the nanohybrids displayed low UV transmission and reflected about 80 % of UV rays, making them excellent candidates for UV protection. For the first time, the dissipation factor (tanδ) in the UV/Vis. region has been calculated and fitted with the Drude-Lorentz model to predict the plasma frequency (ωp), resonance frequency (ω0), and electron lifetime (τ) of pristine polymers and nanocomposites.

A-D-A Type Nonfullerene Acceptors Synthesized by Core Segmentation and Isomerization for Realizing Organic Solar Cells with Low Nonradiative Energy Loss

Reducing non-radiative recombination energy loss (ΔEnonrad ) in organic solar cells (OSCs) has been considered an effective method to improve device efficiency. In this study, the backbone of PTBTT-4F/4Cl is divided into D1-D2-D3 segments and reconstructed. The isomerized TPBTT-4F/4Cl obtains stronger intramolecular charge transfer (ICT), thus leading to elevated highest occupied molecular orbital (HOMO) energy level and reduced bandgap (Eg ). According to ELoss  = Eg- qVOC , the reduced Eg and enhanced open circuit voltage (VOC ) result in lower ELoss , indicating that ELoss has been effectively suppressed in the TPBTT-4F/4Cl based devices. Furthermore, compared to PTBTT derivatives, the isomeric TPBTT derivatives exhibit more planar molecular structure and closer intermolecular stacking, thus affording higher crystallinity of the neat films. Therefore, the reduced energy disorder and corresponding lower Urbach energy (Eu ) of the TPBTT-4F/4Cl blend films lead to low ELoss and high charge-carrier mobility of the devices. As a result, benefitting from synergetic control of molecular stacking and energetic offsets, a maximum power conversion efficiency (PCE) of 15.72% is realized from TPBTT-4F based devices, along with a reduced ΔEnonrad of 0.276 eV. This work demonstrates a rational method of suppressing VOC loss and improving the device performance through molecular design engineering by core segmentation and isomerization.

Propeller vs Quasi-Planar 6-Cantilever Small Molecular Platforms with Extremely Two-Dimensional Conjugated Extension

Two exotic 6-cantilever small molecular platforms, characteristic of quite different molecular configurations of propeller and quasi-plane, are established by extremely two-dimensional conjugated extension. When applied in small molecular acceptors, the only two cases of CH25 and CH26 that could contain six terminals and such broad conjugated backbones have been afforded thus far, rendering featured absorptions, small reorganization and exciton binding energies. Moreover, their distinctive but completely different molecular geometries result in sharply contrasting nanoscale film morphologies. Finally, CH26 contributes to the best device efficiency of 15.41 % among acceptors with six terminals, demonstrating two pioneered yet highly promising 6-cantilever molecular innovation platforms.

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

A Novel Density of States (DOS) for Disordered Organic Semiconductors

In this work, we proposed a novel theory of DOS for disordered organic semiconductors based on the frontier orbital theory and probability statistics. The proposed DOS has been verified by comparing with other DOS alternatives and experimental data, and the mobility calculated by the proposed DOS is closer to experimental data than traditional DOS. Moreover, we also provide a detailed method to choose the DOS parameter for better use of the proposed DOS. This paper also contains a prediction for the DOS parameters, and it has been verified by the experimental data. More importantly, the physical meaning of the proposed DOS parameter has been explained by equilibrium energy theory and transport energy theory to make this proposed model more rational. Compared with the improved DOS based on Gaussian and exponential DOS, this work is a new attempt to combine probabilistic theory with physical theory related to DOS in disordered organic semiconductors, showing great significance for the further investigation of the properties of DOS.

An n-Type All-Fused-Ring Molecule with Photoresponse to 1000 nm for Highly Sensitive Near-Infrared Photodetector

Most of all-fused-ring π-conjugated molecules have wide or medium bandgap and show photo response in the visible range. In this work, an all-fused-ring n-type molecule, which exhibits an ultrasmall optical bandgap of 1.22 eV and strong near-infrared (NIR) absorption with an onset absorption wavelength of 1013 nm is reported. The molecule consists of 14 aromatic rings and has electron donor-acceptor characteristics. It exhibits excellent n-type properties with low-lying HOMO/LUMO energy levels of -5.48 eV/-3.95 eV and high electron mobility of 7.0 × 10^-4  cm²  V^-1  s^-1 . Most importantly, its thin film exhibits a low trap density of 5.55 × 10¹⁶  cm^-3 because of the fixed molecular conformation and consequently low conformation disorder. As a result, organic photodetector (OPD) based on the compound exhibits a remarkably low dark current density (J_d ) of 2.01 × 10^-10  A cm^-2 at 0 V. The device shows a shot-noise-limited specific detectivity (D_sh *) of exceeding 10¹³  Jones at 400-1000 nm wavelength region with a peak specific detectivity of 4.65 × 10¹³  Jones at 880 nm. This performance is among the best reported for self-powered NIR OPDs.

Exploring Pyrrolo-Phenanthrolines as Semiconductors for Potential Implementation in Organic Electronics

This paper describes the synthesis and characterization of new organic semiconductors based on pyrrolo[1,2-i][1,7]phenanthrolines in the form of thin layers. The thin layers, produced via the spin coating method (with a thickness of 10-11 μm), were investigated for their electrical and optical properties. After heat treatment at temperatures ranging from 210 to 240 °C, the layers displayed consistent and reproducible properties. The layers exhibited n-type semiconductor behavior, with a thermal activation energy (E_a) in the range of 0.75-0.78 eV. Additionally, the layers showed transmittance values of 84-92% in the visible and near-infrared spectral ranges, with a direct optical band gap (E_go^d) ranging from 3.13 to 4.11 eV. These thin layers have potential applications in electronic devices such as thermistors, as well as in nanoelectronics and optoelectronics. Overall, these new organic semiconductors show promising properties for practical implementation in various electronic applications.

Subgap Absorption in Organic Semiconductors

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

Current Understanding of Band-Edge Properties of Halide Perovskites: Urbach Tail, Rashba Splitting, and Exciton Binding Energy

The band-edge structure of halide perovskites, derived from the hybridization of atomic orbitals, plays a fundamental role in determining their optical and electronic properties. Several important concepts have been frequently discussed to describe the influence of band-edge structure on their optoelectronic properties, including Urbach tail, Rashba splitting, and exciton binding energy. In this Perspective, we provide a fundamental understanding of these concepts, with the focus on their dependence on composition, structure, or dimensionality. Subsequently, the implications for material optimization and device fabrication are discussed. Furthermore, we highlight the Rashba effect on the exciton fine structure in perovskite nanocrystals (PNCs), which explains the unique emissive properties. Finally, we discuss the potential influence of band-edge properties on the light emission process. We hope that this Perspective can inspire the investigation of band-edge properties of halide perovskites for light-emitting diodes, lasers, and spin electronics.

Boosting Charge Transport in a 2D/3D Perovskite Heterostructure by Selecting an Ordered 2D Perovskite as the Passivator

We demonstrate that an ordered 2D perovskite can significantly boost the photoelectric performance of 2D/3D perovskite heterostructures. Using selective fluorination of phenyl-ethyl ammonium (PEA) lead iodide to passivate 3D FA_0.8 Cs_0.2 PbI₃ , we find that the 2D/3D perovskite heterostructures passivated by a higher ordered 2D perovskite have lower Urbach energy, yielding a remarkable increase in photoluminescence (PL) intensity, PL lifetime, charge-carrier mobilities (ϕμ), and carrier diffusion length (L_D ) for a certain 2D perovskite content. High performance with an ultralong PL lifetime of ≈1.3 μs, high ϕμ of ≈18.56 cm²  V^-1  s^-1 , and long L_D of ≈7.85 μm is achieved in the 2D/3D films when passivated by 16.67 % para-fluoro-PEA₂ PbI₄ . This carrier diffusion length is comparable to that of some perovskite single crystals (>5 μm). These findings provide key missing information on how the organic cations of 2D perovskites influence the performance of 2D/3D perovskite heterostructures.

Life on the Urbach Edge

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

Static Disorder in Lead Halide Perovskites

In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.0 ± 1.0, 13.2 ± 1.0, and 13.5 ± 1.0 meV for single, double, and triple cation perovskite. Static, temperature-independent contributions to the Urbach energy are found to be as low as 5.1 ± 0.5, 4.7 ± 0.3, and 3.3 ± 0.9 meV for the same systems. Our results suggest that, at a low temperature, the dominant static disorder in perovskites is derived from zero-point phonon energy rather than structural disorder. This is unusual for solution-processed semiconductors but broadens the potential application of perovskites further to quantum electronics and devices.

Manipulating Ion Migration and Interfacial Carrier Dynamics via Amino Acid Treatment in Planar Perovskite Solar Cells

Instability caused by the migrating ions is one of the major obstacles toward the large-scale application of metal halide perovskite optoelectronics. Inactivating mobile ions/defects via chemical passivation, e.g., amino acid treatment, is a widely accepted approach to solve that problem. To investigate the detailed interplay, L-phenylalanine (PAA), a typical amino acid, is used to modify the SnO₂/MAPbI₃ interface. The champion device with PAA treatment maintains 80% of its initial power conversion efficiency (PCE) when stored after 528 h in an ambient condition with the relative humidity exceeding 70%. By employing a wide-field photoluminescence imaging microscope to visualize the ion movement and calculate ionic mobility quantitatively, we propose a model for enhanced stability in perspective of suppressed ion migration. Besides, we reveal that the PAA dipole layer facilitates charge transfer at the interface, enhancing the PCE of devices. Our work may provide an in-depth understanding toward high-efficiency and stable perovskite optoelectronic devices.

Structural, Electrical and Optical Properties of Pyrrolo[1,2-i][1,7] Phenanthroline-Based Organic Semiconductors

This work reports a study on structural, electrical and optical properties of some recently synthesized pyrrolo[1,2-i][1,7] phenanthrolines derivatives in thin films. The thin films were deposited onto glass substrates by spin coating technique, using chloroform as solvent. The obtained films exhibited a polycrystalline structure with an n–type semiconductor behavior after heat treatment in the temperature range 293–543 K, specific to each sample. The thermal activation energy lies between 0.68 and 0.78 eV, while the direct optical band gap values were found in the range 4.17–4.24 eV. The electrical and optical properties of the investigated organic semiconductor films were discussed in relation to microstructural properties, determined by the molecular structure. The investigated organic compounds are promising for applications in organic optoelectronics and nanoelectronics.

Toward High-Efficiency Organic Photovoltaics: Perspectives on the Origin and Role of Energetic Disorder

The power conversion efficiency of organic photovoltaics is strongly limited by relatively large energy loss, which is partially due to the disordered nature of organic semiconductors. This disordered nature not only hinders the rational design of molecules with excellent photophysical properties but also prevents a more thorough understanding of the inherent link between microscopic parameters and physical phenomena. In this Perspective, we demonstrate that the injection-dependent emission line-shape in organic semiconductors is primarily associated with a state-filling effect, where the extent of spectral blue-shift can be a strong indicator for energetic disorder. Molecular geometry with rigidity and coplanarity not only promotes preferential face-on stacking that narrows the energetic distribution of subgap states but also impedes torsional deformations of the conjugated backbone away from planarity, thereby facilitating larger π-electron delocalization. These structural characteristics explain the seemingly contradictory high radiative efficiency of low-bandgap nonfullerene molecules, providing promising molecular design strategies to realize high-efficiency organic photovoltaics.

Elucidating Charge Generation in Green-Solvent Processed Organic Solar Cells

Organic solar cells have the potential to become the cheapest form of electricity. Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors. Next generation photovoltaics based upon environmentally benign “green solvent” processing of organic semiconductors promise a step-change in the adaptability and versatility of solar technologies and promote sustainable development. However, high-performing OSCs are still processed by halogenated (non-environmentally friendly) solvents, so hindering their large-scale manufacture. In this perspective, we discuss the recent progress in developing highly efficient OSCs processed from eco-compatible solvents, and highlight research challenges that should be addressed for the future development of high power conversion efficiencies devices.