Acceptor Combinations Research Articles

Electron Transfer Enhanced by a Minimal Energetic Driving Force at the Organic‐Semiconductor Interface

AbstractThe energetic driving force for electron transfer must be minimized to realize efficient optoelectronic devices including organic light‐emitting diodes (OLEDs) and organic photovoltaics (OPVs). Exploring the dynamics of a charge‐transfer (CT) state at an interface leads to a comprehension of the relationship between energetics, electron‐transfer efficiency, and device performance. Here, we investigate the electron transfer from the CT state to the triplet excited state (T1) in upconversion OLEDs with 45 material combinations. By analyzing the CT emission and the singlet excited‐state emission from triplet–triplet annihilation via the dark T1, their energetics and electron‐transfer efficiencies are extracted. We demonstrate that the CT→T1 electron transfer is enhanced by the stronger CT interaction and a minimal energetic driving force (<0.1 eV), which is explained using the Marcus theory with a small reorganization energy of <0.1 eV. Through our analysis, a novel donor–acceptor combination for the OLED is developed and shows an efficient blue emission with an extremely low turn‐on voltage of 1.57 V. This work provides a solution to control interfacial CT states for efficient optoelectronic devices without energy loss.

A universal hole transport layer for efficient organic solar cells processed by blade coating

Doctor-blade coating technology is a roll-to-roll compatible high-throughput thin film fabrication route. In this work, doctor-blading was applied for fabricating organic solar cells (OSCs) using a polyoxometalates material phosphomolybdic acid (PMA) as a hole transport layer (HTL). Compared to PEDOT:PSS, PMA-based devices demonstrate lower trap-assisted recombination, higher hole mobility, prolonged charge carrier lifetime and faster charge collection. With PM6:Y6 as active layer, PMA-based device delivered a high power conversion efficiency (PCE) of 17.79 % with a boosted short-circuit current density (JSC) value of 28.08 mA/cm2, which is one of the best performances for PM6:Y6-based solar cells through doctor-blading process. In addition, the performance improvement was observed in both conventional and inverted structured devices with various donor: acceptor combinations. These results indicate the high universality of PMA for printable processing and its prospect in preparation of the industrial production of OSCs.

Photodegradation of Organic Solar Cells under Visible Light and the Crucial Influence of Its Spectral Composition.

While wavelength-dependent photodegradation of organic solar cells (OSCs) under visible light is typically discussed in terms of UV/blue light-activated phenomena, we recently demonstrated wavelength-dependent degradation rates up to 660 nm for PM6:Y6. In this study, we systematically investigated this phenomenon for a broad variety of devices based on different donor:acceptor combinations. We found that the spectral composition of the light used for degradation, tuned in a spectral range from 457 to 740 nm and under high irradiances of up to 30 suns, has a crucial influence on the device stability of almost all tested semiconductors. The relevance of this phenomenon was investigated in the context of simulated AM1.5 illumination with metal halide lamps and white LEDs. It is concluded that the current stability testing protocols in OSC research have to be adjusted to account for this effect to reveal the underlying physics of this still poorly understood mechanism.

Revealing processing stability landscape of organic solar cells with automated research platforms and machine learning

AbstractWe use an automated research platform combined with machine learning to assess and understand the resilience against air and light during production of organic photovoltaic (OPV) devices from over 40 donor and acceptor combinations. The standardized protocol and high reproducibility of the platform results in a dataset of high variety and veracity to deploy machine learning models to encounter links between stability and chemical, energetic, and morphological structure. We find that the strongest predictor for air/light resilience during production is the effective gap Eg,eff which points to singlet oxygen rather than the superoxide anion being the dominant agent in degradation under processing conditions. A similarly good prediction of air/light resilience can also be achieved by considering only features from chemical structure, that is, information which is available prior to any experimentation.

Intrinsic Photoconductivity Spectral Dependence as a Tool for Prediction of Open-Circuit Voltage in Organic Solar Cells

Organic materials are known for their variety of molecules. Methods to predict the parameters of organic photovoltaic (OPV) cells are required to avoid the time- and resource-consuming processes of manufacturing and testing OPVs. Usually, the open-circuit voltage (Uoc) is estimated as the difference between the ionization energy level of the electron donor molecule (Id) and the electron affinity level of the electron acceptor molecule (EAa). Various measurement methods are used to determine the energy level values of pure materials, which, when combined with energy level shifts due to the donor:acceptor interactions, make these estimations less precise. In this work, photoconductivity measurements were applied to the donor:acceptor films. Near threshold energy, the electron can be directly transferred from the donor to the acceptor molecule. The obtained charge transfer energy (ECT) shows the difference between Id and EAa in the film. This difference was compared to the Uoc value of an OPV made of the same donor:acceptor combination. We show that this approach provides less scattered results and a higher correlation coefficient compared to the Uoc estimation using energy level values.

Morphology control by tuning electrostatic interactions for efficient polythiophene-based all-polymer solar cells

•Correlations between electrostatic force and interaction energy are established •Domain purity of the film can be modulated by tuning the donor:acceptor ESP difference •A high PCE of 15.3% is demonstrated in a PT-based all-polymer solar cell Polythiophenes are one of the most classical donors for organic solar cells, which have simple chemical structures and thus low production costs. However, their photovoltaic performance is lagging far behind the donor-acceptor-type polymers, especially when blending with polymer acceptors, where morphology control is a huge challenge. Here, we systematically study the intermolecular interaction in polythiophene:polymer acceptor (PY-IT) combinations and establish a rational relationship between chemical structure and blend miscibility. Our results suggest that the electrostatic force plays a key role in determining the donor:acceptor intermolecular interaction energy. Introducing electron-withdrawing functional groups into the backbone of P3HT can increase the electrostatic potential (ESP) and suppress the intermolecular interaction with PY-IT. Due to the proper miscibility and improved domain purity, the corresponding polymer PDCBT records a high-power conversion efficiency (PCE) of 15.3%, which is the highest value in polythiophene-based all-polymer solar cells (all-PSCs) and crucial for advancing their practical applications. Polythiophenes are one of the most classical donors for organic solar cells, which have simple chemical structures and thus low production costs. However, their photovoltaic performance is lagging far behind the donor-acceptor-type polymers, especially when blending with polymer acceptors, where morphology control is a huge challenge. Here, we systematically study the intermolecular interaction in polythiophene:polymer acceptor (PY-IT) combinations and establish a rational relationship between chemical structure and blend miscibility. Our results suggest that the electrostatic force plays a key role in determining the donor:acceptor intermolecular interaction energy. Introducing electron-withdrawing functional groups into the backbone of P3HT can increase the electrostatic potential (ESP) and suppress the intermolecular interaction with PY-IT. Due to the proper miscibility and improved domain purity, the corresponding polymer PDCBT records a high-power conversion efficiency (PCE) of 15.3%, which is the highest value in polythiophene-based all-polymer solar cells (all-PSCs) and crucial for advancing their practical applications.

Multiple donor–acceptor design for highly luminescent and stable thermally activated delayed fluorescence emitters

A considerable variety of donor–acceptor (D–A) combinations offers the potential for realizing highly efficient thermally activated delayed fluorescence (TADF) materials. Multiple D–A type compounds are one of the promising families of TADF materials in terms of stability as well as efficiencies. However, those emitters are always composed of carbazole-based donors despite a wide choice of moieties used in linearly linked single D–A molecules. Herein, we developed a multiple D–A type TADF compound with two distinct donor units of 9,10-dihydro-9,9-dimethylacridine (DMAC) and carbazole as the hetero-donor design. The new emitter exhibits high photoluminescence quantum yield (PLQY) in various conditions including polar media blend and high concentrations. Organic light-emitting diodes (OLEDs) showed a reasonably high external quantum efficiency (EQE). In addition, we revealed that the multiple-D–A type molecules showed better photostability than the single D–A type molecules, while the operational stability in OLEDs involves dominant other factors.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Interparticle energy transfer between NaNdF4 and NaYbF4 in self-assembled nanostructures

Förster resonance energy transfer (FRET) is a widely used spectroscopic technique. The development of donor–acceptor combinations is essential to advance this technique. To make the energy transfer efficiency independent of the external environment, we explored the potential of constructing donor–acceptor combinations with lanthanide ion-doped nanoparticles (LNPs), in which the 4f electrons of the lanthanide ions are shielded by the 5s and 6p electrons. Non-radiative energy transfer between LNPs was demonstrated in self-assembled nanostructures and core–shell structure of LNPs was used to control the distance between donor and acceptor in self-assembled nanostructures. The emission intensity ratio of donor and acceptor illustrates that the energy transfer efficiency decreases significantly with increasing distance and the effective energy transfer distance is 8.5 nm, showing the potential of using LNPs as donor–acceptor pairs in the construction of distance-dependent energy transfer system.

Machine-learning-guided prediction of photovoltaic performance of non-fullerene organic solar cells using novel molecular and structural descriptors

The machine learning approach was employed to explore the relationship between molecular structure and photovoltaic properties using frontier molecular orbital and RDKit descriptors, which enabled us to screen and identify potential donor and acceptor combinations for efficient organic solar cells.

Machine learning framework for the analysis and prediction of energy loss for non-fullerene organic solar cells

The efficiency of organic solar cells (OSCs) has been improved more than 19% recently with the development of non-fullerene acceptor materials. Further improvement is still attainable with the optimal combinations of donors and acceptors that provide minimal energy losses. In this work, a data-enabled machine-learning (ML) framework was employed to predict the energy losses in the polymer:non-fullerene acceptor based devices. Based on the collected experimental dataset, the prediction accuracies of various machine learning models were systematically compared by estimating mean absolute percentage errors (MAPE), root mean squared errors (RMSE), and person’s r coefficient. The Random Forest regression model showed the best performance in predicting the energy losses with a correlation coefficient of 0.83 and relative error in the range of 0 – 20%. The predictive ability of this model was further validated using the different parameters of devices with power conversion efficiency range of 6 – 18%. Three different donor–acceptor combinations were chosen for fabricating the photovoltaic devices to fit this model into practical devices and experimentally obtained energy loss values were compared with the predicted values. In addition, the device parameters with the molecular descriptors to understand the correlation and energy loss is highly correlated with the HOMO offset. This study demonstrates that the ML approach provide an effective method to predict and virtual screen of promising donor–acceptor pairs with minimal energy loss and would be useful for developing next-generation high performance solar cell materials.

Recent Advances in Single‐Junction Organic Solar Cells

Single-junction organic solar cells (OSCs) have made significant progress in recent years. Innovations in material design and device optimization have improved the power conversion efficiencies to over 19%. In this review, based on recent advances, we discuss molecular design strategies to tune the absorption spectrum, energy level, and intermolecular aggregation as well as highlight the role of molecular electrostatic potential in decreasing energy loss. Then, we introduce the latest progress in four types of OSCs composed of different donor:acceptor combinations: polymer donor:small-molecule acceptor, all-polymer, all-small-molecule, and small-molecule donor:polymer acceptor OSCs. Finally, the challenges of OSCs in practical applications, including material cost, stability, and multi-function integration, are discussed.

Recent Advances in Single‐Junction Organic Solar Cells

AbstractSingle‐junction organic solar cells (OSCs) have made significant progress in recent years. Innovations in material design and device optimization have improved the power conversion efficiencies to over 19 %. In this Minireview, based on recent advances, we discuss molecular design strategies to tune the absorption spectrum, energy level, and intermolecular aggregation as well as highlight the role of molecular electrostatic potential in decreasing energy loss. Then, we introduce the latest progress in four types of OSCs composed of different donor:acceptor combinations: polymer donor:small‐molecule acceptor, all‐polymer, all‐small‐molecule, and small‐molecule donor:polymer acceptor. Finally, the challenges of OSCs in practical applications, including material cost, stability, and multi‐function integration, are discussed.

Suppressing intermolecular interactions for enhancing the performance of exciplex‐based OLEDs

AbstractIn this study, a new triazatruxene‐based donor Tr‐iBu bearing branched alkyl groups is reported to suppress the aggregation found in a previously reported methyl‐substituted counterpart Tr‐Me. The effects of a steric factor on the propensity of exciplex formation were examined by respectively mixing these triazatruxene‐based donors with PO‐T2T and two new acceptors PO‐T2P and PO‐Pyr, all bearing phosphine oxide as the peripheral groups and N‐heteroarene cores with different numbers of nitrogen. Based on the steric hindrance of side groups and their corresponding molecular orbitals alignments of donors and acceptors, the characteristics of exciplex‐forming systems were analyzed. Among them, the exciplex‐forming blends Tr‐iBu:PO‐T2P and Tr‐iBu:PO‐Pyr were selected to serve as the emitting layer (EML) to achieve OLED devices with a moderate maximum external quantum efficiency (EQEmax) of 8.27 and 7.54%, respectively. Adopting the Förster resonance energy transfer strategy, the exciplex cohost was doped with a fluorescence emitter, DPy2CN, to realize a deep‐red OLED device with the emission peak centered at 674 nm while retaining EQEmax of 6.28%. Our results highlight the importance of suppressing the donor aggregation, adequate donor: acceptor combination, and a suitable triplet state of composing the materials for giving efficient exciplex‐based OLEDs.

Polarization‐Sensitive Photodetectors Based on Directionally Oriented Organic Bulk‐Heterojunctions

AbstractPolarized spectroscopic photodetection enables numerous applications in diverse areas such as sensing, industrial quality control, and visible light communications. Although organic photodetectors (OPDs) can offer a cost‐effective alternative to silicon‐based technology—particularly when flexibility and large‐area arrays are desired—polarized OPDs are only beginning to receive due research interest. Instead of resorting to external polarization optics, this report presents polarized OPDs based on directionally oriented blends of poly(3‐hexylthiophene) (P3HT) and benchmark polymer or nonfullerene acceptors fabricated using a versatile solution‐based method. Furthermore, a novel postprocessing scheme based on backfilling and plasma etching is advanced to ameliorate high dark‐currents that are otherwise inherent to fibrillar active layers. The resulting polarized P3HT:N2200 OPDs exhibit a broad enhancement across all principal figures of merit compared to reference isotropic devices, including peak responsivities of 70 mA W−1 and up to a threefold increase in 3 dB bandwidth to 0.75 MHz under parallel‐polarized illumination. Polarization ratios of up to 3.5 are obtained across a spectral range that is determined by the specific donor–acceptor combinations. Finally, as a proof‐of‐concept demonstration, polarized OPDs are used for photoelasticity analysis of rubber films under tensile deformation, highlighting their potential for existing and emerging applications in advanced optical sensing.

Gold(III) Porphyrin Was Used as an Electron Acceptor for Efficient Organic Solar Cells.

The widespread use of nonfullerene-based electron-accepting materials has triggered a rapid increase in the performance of organic photovoltaic devices. However, the number of efficient acceptor compounds available is rather limited, which hinders the discovery of new, high-performing donor:acceptor combinations. Here, we present a new, efficient electron-accepting compound based on a hitherto unexplored family of well-known molecules: gold porphyrins. The electronic properties of our electron-accepting gold porphyrin, named VC10, were studied by UV–Vis spectroscopy and by cyclic voltammetry (CV) , revealing two intense optical absorption bands at 500–600 and 700–920 nm and an optical bandgap of 1.39 eV. Blending VC10 with PTB7-Th, a donor polymer, which gives rise to an absorption band at 550–780 nm complementary to that of VC10, enables the fabrication of organic solar cells (OSCs) featuring a power conversion efficiency of 9.24% and an energy loss of 0.52 eV. Hence, this work establishes a new approach in the search for efficient acceptor molecules for solar cells and new guidelines for future photovoltaic material design.

Synthesis and photophysical properties of N-alkyl dithieno[3,2-b:2',3'-d]pyrrole based donor/acceptor-π-conjugated copolymers for solar-cell application.

Two kinds of donor–acceptor π-conjugated copolymer based on poly{[N-hexyl-dithieno(3,2-b:2′,3′-d)pyrrole-2,6-diyl]alt-[isoindigo]} (PDTP-IID) and poly{[N-hexyl-dithieno(3,2-b:2′,3′-d)pyrrole-2,6-diyl]alt-[thiazol-2,5-diyl]} (PDTP-Thz) were investigated. These copolymers were synthesized via a Stille coupling reaction. The results showed the structure–property relationships of different donor–acceptor (D–A) combinations. The polymer structures and photophysical properties were characterized by 1H NMR, TGA, DSC, UV-vis absorption spectroscopy, AFM, CV, and XRD measurement. Through UV-vis absorption and cyclic voltammetry (CV) measurements, it showed that the copolymers exhibit not only a low bandgap of 1.29 eV and 1.51 eV but also a deep highest occupied molecular orbital (HOMO) of −5.49 and −5.11 eV. Moreover, photovoltaic properties in combination with the fullerene derivatives were investigated. The device based on the copolymers with PC71BM exhibited higher maximum power conversion efficiency and higher maximum short-circuit current density of 0.23% with 1.64 mA cm−2 of PDTP-IID:PC71BM and 0.13% with 1.11 mA cm−2 of PDTP-Thz:PC71BM than those of the copolymers with PC61BM. Measurements performed for N-hexyl-dithieno(3,2-b:2′,3′-d)pyrrole-based copolymers proved the potential of these polymers to be applied in optoelectronic applications.

Donor-acceptor based solvent-free organic liquid hybrids with exciplex emission and room temperature phosphorescence.

Solvent-free organic liquids are well-known for their excellent luminescence features. Hence, the recent developments in this area have marked them as potential emitters with high quantum yield and enhanced processability. The support of an available liquid matrix enables doping to deliver hybrid liquids with intriguing luminescence features. In this direction, we report solvent-free liquid donor-acceptor pairs with exciplex emission and room temperature phosphorescence at very low acceptor loading. The underlying weak intermolecular interactions have been revealed by 2D NMR techniques and theoretical calculations. The formation of large-area thin films by exciplex and phosphorescent liquid hybrids will encourage the development of scalable lighting and display materials.

Prediction of non-radiative voltage losses in organic solar cells using machine learning

One of the major hurdles that are preventing organic solar cells (OSCs) from leading the efficiency chart is non-radiative voltage loss (ΔVNR). So far, however, not much effort is made to predict voltage losses and unravel the correlation of losses with electronic and structural descriptors. From the literature, we create a dataset consisting of 154 unique donor:acceptor combinations with reported ΔVNR. The dataset includes information about frontier molecular orbitals (FMO), optical bandgap (Eg), molecular descriptors, and molecular fingerprints. Four machine learning (ML) algorithms (random forest regressor, gradient boosting regressor, support vector regressor, and artificial neural network) are used to predict non-radiative voltage loss and the results obtained are compared on the basis of Pearson rs, root mean squared errors, and mean absolute percentage errors. Best results are obtained with gradient boosting regressor by using FMO + Eg + RDKit descriptors (Pearson r = 0.859) and FMO + Eg + MACCS fingerprints (Pearson r = 0.857). We have also applied these ML models by using only molecular descriptors and only molecular fingerprints and got impressive results (Pearson r = 0.78 and 0.726). These results indicate that ML models can be effectively used for the prediction of ΔVNR and virtual screening of promising donor:acceptor combinations with reduced ΔVNR.

Intramolecular Photoinduced Charge Transfer and Recombination Dynamics in Vinylarene Terminated Organosilanes.

We report on charge-transfer dynamics of newly designed acceptor-donor-acceptor organosilanes, with a specific focus on how donor-acceptor combination and local chemical environment can be used to control the lifetime for intramolecular charge-separation between silane electron donors and organic acceptors. In this work linear oligosilanes were capped with arene-vinyl end groups of variable electron-accepting strength: weak (diester vinyl), intermediate (ester,cyano vinyl), and strong (dicyanovinyl). Ultrafast transient absorption spectroscopy was used to characterize their structure-dependent charge-transfer and recombination behaviors. All structures exhibit similar photoinduced ultrafast spectral dynamics that we ascribe to relaxation of the nascent charge-separated excited state followed by a return to the ground state via charge recombination. We find that relaxation of the nascent "hot" charge-separated excited state scales with the strength of dipole-dipole interactions between solvent molecules and the polar arene-vinyl acceptor. Furthermore, electron-accepting strength governs whether electronic coupling dictates charge recombination rate: weak acceptors produce charge-separated states that exhibit relatively large electronic coupling for back-electron transfer (approaching the adiabatic limit) that result in fast recombination, whereas the strong and moderate-strength acceptors support more stable charge-separated states with weaker coupling and longer lifetimes. We find that recombination rates increase substantially for structures with weak and moderate-strength acceptors in cyclohexane (i.e., negligible solvent reorganization energy), which we attribute to an increased electronic coupling in a nonpolar solvent environment where charge pairs are weakly screened. In contrast, for structures with strong electron acceptors, the very low reorganization energy of cyclohexane places back-electron transfer even further into the Marcus inverted regime, with a resultant increase in charge-separation lifetime. Together these results provide critical insights on how to tune photoinduced charge-transfer behavior in organic-inorganic hybrids that have potential material applications in molecular electronics and optoelectronics.