Power Conversion Efficiency Of Organic Photovoltaics Research Articles

Novel structural feature-descriptor platform for machine learning to accelerate the development of organic photovoltaics

Although the evolution of organic photovoltaics (OPVs) has been remarkable, the discovery of new combinations of materials for ideal bulk heterojunction (BHJ) blend systems remains challenging because of their numerous considerations. Aiming to eliminate the typically labor-intensive trial-and-error approaches, machine learning (ML) models coupled with quantitative structure-property relationships are constructed to efficiently predict the photovoltaic parameters. To this end, a unique structural-feature descriptor that fragments a molecular structure into functional group (FG) units is designed. These units are indexed and the indices are combined to translate the molecular structure into a concise matrix of integers. Our novel descriptor allows the ML models to trace the FGs with ease and assign larger weights to those FGs that significantly contribute to the photovoltaic performance. Furthermore, the descriptor enables the expansion of OPV ML to ternary or multicomponent BHJ systems because of its concise expression. The proposed ML model achieves a high correlation coefficient of 0.86 between experimental and predictive power conversion efficiency of OPVs, despite using only the molecular structure information without any additional input data. Combined with the superior prediction capabilities, our novel approach promises to perfectly screen numerous BHJ cadidates, resulting in accelerating the development of the OPV.

Control of molecular orientations by sequential deposition to enhance organic photovoltaic performance

Bulk heterojunction (BHJ) structures prepared by coating mixed solutions of a donor and an acceptor have been used for efficient organic photovoltaics (OPVs), but this approach allows little control of the molecular orientations in the films. Herein we show that a sequential deposition strategy, in which the molecular acceptor is deposited sequentially on a pristine polymer donor film, reliably controls the molecular orientations in the resulting BHJ films. Spectroscopic ellipsometry indicates that both the donor and the acceptor adopt higher degrees of orientation with the π-planes parallel to the film plane compared with mixed BHJ films. The sequentially deposited films enhance the power conversion efficiency of OPVs by 30% compared with mixed BHJ films, indicating the usefulness of this strategy for improving OPV performance.

Machine learning property prediction for organic photovoltaic devices

Organic photovoltaic (OPV) materials are promising candidates for cheap, printable solar cells. However, there are a very large number of potential donors and acceptors, making selection of the best materials difficult. Here, we show that machine-learning approaches can leverage computationally expensive DFT calculations to estimate important OPV materials properties quickly and accurately. We generate quantitative relationships between simple and interpretable chemical signature and one-hot descriptors and OPV power conversion efficiency (PCE), open circuit potential (Voc), short circuit density (Jsc), highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, and the HOMO–LUMO gap. The most robust and predictive models could predict PCE (computed by DFT) with a standard error of ±0.5 for percentage PCE for both the training and test set. This model is useful for pre-screening potential donor and acceptor materials for OPV applications, accelerating design of these devices for green energy applications.

In SituPhotoelectron Spectroscopy and Theoretical Calculation Study of Thermally Evaporated Copper Naphthalocyanine

Recently, organic photovoltaics (OPVs) have attracted attention as a next-generation energy source as their power conversion efficiency (PCE) has significantly improved. To increase the PCE of OPVs further, a fundamental understanding of material properties of a new organic semiconductor is highly important. Copper naphthalocyanine (CuNc) is composed of macrocyclic ligands with extended benzene rings of copper phthalocyanine (CuPc). Thus, it can be considered as a potential candidate for an efficient p-type organic semiconductor, similar to a well-known conventional p-type organic semiconductor CuPc. In this study, we investigated the electronic structure of thermally evaporated CuNc on indium tin oxide (ITO) wit in situ ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and density functional theory (DFT) calculations. DFT calculations predict that CuNc has a lower ionization energy (IE) and band gap than CuPc. However, the IE measured by UPS and the band gap measured by UV-vis absorption of the deposited film were much higher than CuPc. Derived from the measured UPS, XPS spectra and DFT calculation results, we concluded that CuNc could be decomposed with thermal evaporation in a vacuum and the pyrrole-based material might be deposited on ITO. The increased IE and band gap were attributed to the disappearance of the highest occupied molecular orbital originating from the macrocyclic ligands with D4h symmetry. Therefore, the thermal evaporation method would not be a suitable method to obtain the CuNc film for device application, and the alternative solution process at low temperature would be more adequate.

Understanding the Performance of Organic Photovoltaics under Indoor and Outdoor Conditions: Effects of Chlorination of Donor Polymers.

Understanding the effects of the chemical structures of donor polymers on the photovoltaic properties of their corresponding organic photovoltaic (OPV) devices under various light-intensity conditions is important for improving the performance of these devices. We synthesized a series of copolymers based on poly[(2,6-(4,8-bis(5-(2-thioethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-TS) and studied the effects of chlorine substitution of its thiophene-substituted benzodithiophene (BDT-Th) unit on its photovoltaic properties. Chlorination of the polymer resulted in a bulk heterojunction (BHJ) morphology optimized for efficient charge transport with suppressed leakage current and an increased open-circuit voltage of the OPV device; this optimization led to a remarkable enhancement of the OPV device's power conversion efficiency (PCE) not only under the condition of 1 sun illumination but also under a low light intensity mimicking indoor light; the PCE increased from 8.7% for PBDB-TS to ∼13% for the chlorinated polymers, PBDB-TS-3Cl, and PBDB-TS-4Cl under the 1 sun illumination condition and from 5.3% for PBDB-TS to 21.7% for PBDB-TS-4Cl under 500 lx fluorescence illuminance. Interestingly, although the OPV PCEs under 1 sun illumination were independent of the position of chlorine substitution onto the polymer, PBDB-TS-4Cl exhibited better performance under simulated indoor light than its derivative PBDB-TS-3Cl. Our results demonstrate that efficient light absorption and charge-carrier generation play key roles in achieving high OPV efficiency under low-light-intensity conditions.

Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells.

Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives often-but not always-exhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.

Reducing Burn‐In Loss of Organic Photovoltaics by a Robust Electron‐Transporting Layer

AbstractIt is revealed that instability of interface between photoactive layer and electron‐transporting layer (ETL) is one of the causes of the rapid degradation of organic photovoltaics (OPV) performance during initial operation (burn‐in loss) under the light soaking. The stability of OPV is greatly enhanced by applying a robust ETL composed of TiO2 nanoparticles (TNPs). The TNPs bound with π–π interactive 3‐phenylpentane‐2,4‐dione (TNP–Ph) form more robust ETLs than those bound with van der Waals interactive 3‐methyl‐2,4‐pentanedione TNP (TNP–Me). The OPV with TNP–Ph maintains 73% of its initial power conversion efficiency (PCE) after 1000 h of light soaking, whereas the PCE of OPV with TNP–Me substantially reduces to 25% of initial PCE. The impedance analysis reveals that the burn‐in loss is due to increase of resistance at the TNP ETL/photoactive layer interface during the light soaking. The transmission electron microscopy analysis shows that the TNP–Ph maintains most clear and robust interface with photoactive layer after the light‐soaking test. This is attributed to the strong π–π interaction between phenyl rings of TNP–Ph. However, the TNP–Me bound with van der Waals interactive organic ligands penetrates the photoactive layer during the light‐soaking test.

Energy level alignment at C60/DTDCTB/PEDOT:PSS interfaces in organic photovoltaics

The electronic structure of a narrow band gap small molecule ditolylaminothienyl–benzothiadiazole–dicyanovinylene (DTDCTB), possessing a donor-acceptor-acceptor configuration, was investigated with regard to its application as an efficient donor material in organic photovoltaics (OPVs). The interfacial orbital alignment of C60/DTDCTB/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was determined using in situ ultraviolet photoelectron and inverse photoelectron spectroscopic methods. The ionization energy and electron affinity values of DTDCTB were measured to be 5.27eV and 3.65eV, respectively, and thus a very small transport gap of 1.62eV was evaluated. Large band bending of DTDCTB on PEDOT:PSS was observed, resulting in a low hole extraction barrier. Additionally, the photovoltaic gap between the highest occupied molecular orbital level of the DTDCTB donor and the lowest unoccupied molecular orbital level of the C60 acceptor was estimated to be 1.30eV, which is known to be the theoretical maximum open-circuit voltage in OPVs employing the C60/DTDCTB active layer. The unique electronic structures of DTDCTB contributed toward the recently reported excellent power conversion efficiencies of OPVs containing a DTDCTB donor material.

Recent Advances in Organic Photovoltaics: Device Structure and Optical Engineering Optimization on the Nanoscale.

Organic photovoltaic (OPV) devices, which can directly convert absorbed sunlight to electricity, are stacked thin films of tens to hundreds of nanometers. They have emerged as a promising candidate for affordable, clean, and renewable energy. In the past few years, a rapid increase has been seen in the power conversion efficiency of OPV devices toward 10% and above, through comprehensive optimizations via novel photoactive donor and acceptor materials, control of thin-film morphology on the nanoscale, device structure developments, and interfacial and optical engineering. The intrinsic problems of short exciton diffusion length and low carrier mobility in organic semiconductors creates a challenge for OPV designs for achieving optically thick and electrically thin device structures to achieve sufficient light absorption and efficient electron/hole extraction. Recent advances in the field of OPV devices are reviewed, with a focus on the progress in device architecture and optical engineering approaches that lead to improved electrical and optical characteristics in OPV devices. Successful strategies are highlighted for light wave distribution, modulation, and absorption promotion inside the active layer of OPV devices by incorporating periodic nanopatterns/nanostructures or incorporating metallic nanomaterials and nanostructures.

Manipulating aggregation and molecular orientation in all-polymer photovoltaic cells.

Manipulating molecular orientation at the donor/acceptor interface is the key to boosting charge separation properties and efficiencies of anisotropic-materials-based organic photovoltaics (OPVs). By replacing the polymeric donor PBDTBDD with its 2D-conjugated polymer PBDTBDD-T, the power conversion efficiency of OPVs featuring the anisotropic polymer acceptor PNDI is drastically boosted from 2.4% up to 5.8%.

Nanostructured Electrodes Improve the Fill Factor of Organic Photovoltaics

In this study, we demonstrate that suboptical-wavelength nanostructured electrodes (NEs) improve fill factor (FF) in organic photovoltaic devices without compromising open-circuit voltage or short-circuit current. We attribute this improvement to efficient charge collection by the NEs, which reduce recombination in low-mobility organic semiconductors. NEs increase the FF of planar heterojunction devices with boron subphthalocyanine chloride (SubPc)/C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> from 28% to 40%. Optical simulations and external quantum efficiency measurements show that improved charge collection, rather than light trapping, is the mechanism for device improvement. Our findings suggest that NEs can be optimized for a given material set to improve FF performance, which is important for improving organic photovoltaic power conversion efficiency.

Solution-processable functionalized graphene oxide as an efficient hole transport layer in organic photovoltaics.

Solution-processable functionalized graphene oxide (SPFGO) thin films as the hole transport layer in organic photovoltaics (OPVs) are fabricated. The performance of the device has been improved by the incorporation of SPFGO. The best results of the open-circuit voltage (V(oc)), short-circuit current density (J(sc)), fill factor (FF) and power conversion efficiency (PCE) were 0.57 V, 7.7 mA/cm2 0.65, and 2.90%, respectively, which are achieved when the thickness of SPFGO film is 3 nm. The dramatic increase in the PCE of OPVs are comparable to those devices fabricated with PEDOT:PSS as the hole transport layers. Therefore, we suggest that SPFGO thin films could be a promising solution-processable alternative to PEDOT:PSS as the effective hole transport layers (HTLs) in OPV devices.

Correlation between ultrafast transient photomodulation spectroscopy and organic photovoltaic solar cell efficiency based on RR-P3HT/PCBM blends

Ultrafast transient spectroscopy was applied to various films of regio-regular polythiophene (RR-P3HT, donor-D) and C60 derivative (PCBM, acceptor-A) blends, in conjunction with organic photovoltaic (OPV) solar cell fabrication and evaluation based on the same blends, for investigating the existence of a correlation between the device efficiency and the transient photophysics characteristics. For our transient spectroscopy measurements we used the ps pump–probe transient photomodulation (PM) technique having a unique probe spectral range in the mid-IR (0.25–1.05eV). We found that the transient PM spectra contain photoinduced absorption bands of excitons in the donor polymer, charge transfer excitons (CTE) at the D–A interfaces, and free polarons. We compared the relative density of photogenerated CTE in D–A blends having various D–A weight ratio with the photocurrent density of fabricated solar cells based on the same blends. We found that the dissociation of CTE into free charges correlates well with the optoelectronic measurements of the corresponding solar cell. The more efficient CTE dissociation occurs in films having the optimum D–A weight ratio (which is 1.2:1 for the P3HT/PCBM system) that shows the highest OPV power conversion efficiency; this is due to the lowest CTE binding energy for this blend that results from the most suitable D- and A- grain sizes. We also show that the exciton lifetime is the shortest for the optimum blend, and this helps boosting the device efficiency by reducing energy loss.

Systematic Investigation of Organic Photovoltaic Cell Charge Injection/Performance Modulation by Dipolar Organosilane Interfacial Layers

With the goal of investigating and enhancing anode performance in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells, the glass/tin-doped indium oxide (ITO) anodes are modified with a series of robust silane-tethered bis(fluoroaryl)amines to form self-assembled interfacial layers (IFLs). The modified ITO anodes are characterized by contact angle measurements, X-ray reflectivity, ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, and cyclic voltammetry. These techniques reveal the presence of hydrophobic amorphous monolayers of 6.68 to 9.76 Å thickness, and modified anode work functions ranging from 4.66 to 5.27 eV. Two series of glass/ITO/IFL/active layer/LiF/Al BHJ OPVs are fabricated with the active layer = poly(3-hexylthiophene):phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) or poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thi-eno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM). OPV analysis under AM 1.5G conditions reveals significant performance enhancement versus unmodified glass/ITO anodes. Strong positive correlations between the electrochemically derived heterogeneous electron transport rate constants (ks) and the device open circuit voltage (Voc), short circuit current (Jsc), hence OPV power conversion efficiency (PCE), are observed for these modified anodes. Furthermore, the strong functional dependence of the device response on ks increases as greater densities of charge carriers are generated in the BHJ OPV active layer, and is attributable to enhanced anode carrier extraction in the case of high-ks IFLs.

Organic photovoltaic power conversion efficiency improved by AC electric field alignment during fabrication

Ultra-low frequency AC field was employed to align p/n polymers during organic photovoltaic device fabrication. The resulting devices show 15% increase in power conversion efficiency and four-fold increase in parallel resistance. Supported by the transmission electron microscopy and atomic force microscopy images, the performance enhancement is attributed to the optimized morphology and enlarged p/n interface by AC field, which is more effective than DC, possibly explained by the argument of better mixing via back-and-forth shaking than a single swing.

Surface Treatment of NiO Hole Transport Layers for Organic Solar Cells

Recent advances in the power-conversion efficiency of organic photovoltaics (OPVs) has largely been realized through the development of conjugated polymer absorber materials that provide for increased overlap with the solar spectrum as well as proper energy level offset with the electron acceptor. These allow for increased photocurrent and photovoltage, thus resulting in increased performance. Such systems could further be improved through the application of contact materials that have been tuned to minimize losses in carrier and potential losses at the charge-extraction interfaces. To date, these devices continue to use contacts that have not been optimized for the specific active layer components employed. Here, we demonstrate that the electrical and contact properties of NiO can be tuned through careful control of the deposition parameters as well as through surface treatments. The effects of the NiO thin-film processing and properties are investigated for application as a hole transport layer (HTL) in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester OPV devices. Devices based on the NiO HTLs demonstrate equal performance to those employing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) HTLs. Furthermore, the NiO HTLs enable the application of zinc-oxide-based materials as transparent electrodes.

Reduction of Series Resistance in Organic Photovoltaic Using Low Sheet Resistance of ITO Electrode

The series resistance of organic photovoltaic (OPV) devices was decreased by reducing the sheet resistance (R sh ) of the indium tin oxide (ITO) electrode, which leads to increasing device efficiency. The performance of bulk heterojunction OPVs was critically dependent on R sh of the ITO electrode. Upon reducing R sh of the ITO from 39 to 8.5 Ω/□, the fill factor and power conversion efficiency of OPV was improved (from 0.407 to 0.580 and from 1.63 ± 0.2 to 2.5 ± 0.1%, respectively) under an AM1.5 simulated solar intensity of 100 mW/cm 2 . The dependence of the series resistance on R sh of the ITO suggests the dominance of the bulk resistance of the ITO electrode as a limiting factor in practical cell efficiencies.

High temperature research in a solar furnace. I. On the fusion of metal oxides : Noguchi, Tetsuo; Mizuno, Masao, and Noguchi, Choji, Rept. Govt. Ind. Res. Inst. Nagoya. 6 (11): 663, Nov. 1957

The electronic structure of a narrow band gap small molecule ditolylaminothienyl–benzothiadiazole–dicyanovinylene (DTDCTB), possessing a donor-acceptor-acceptor configuration, was investigated with regard to its application as an efficient donor material in organic photovoltaics (OPVs). The interfacial orbital alignment of C60/DTDCTB/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was determined using in situ ultraviolet photoelectron and inverse photoelectron spectroscopic methods. The ionization energy and electron affinity values of DTDCTB were measured to be 5.27 eV and 3.65 eV, respectively, and thus a very small transport gap of 1.62 eV was evaluated. Large band bending of DTDCTB on PEDOT:PSS was observed, resulting in a low hole extraction barrier. Additionally, the photovoltaic gap between the highest occupied molecular orbital level of the DTDCTB donor and the lowest unoccupied molecular orbital level of the C60 acceptor was estimated to be 1.30 eV, which is known to be the theoretical maximum open-circuit voltage in OPVs employing the C60/DTDCTB active layer. The unique electronic structures of DTDCTB contributed toward the recently reported excellent power conversion efficiencies of OPVs containing a DTDCTB donor material.