Small Molecule Organic Photovoltaics Research Articles

Interpretable machine learning for developing high-performance organic solar cells

Rapidly screening the underlying relationships between organic photovoltaics (OPVs) and their chemical structures remains an open challenge due to their complex interconnectivity. In this study, a new methodology for structure-property mappings of OPVs and device performances prediction is designed by combining the machine learning (ML) approach with the Taguchi Design of Experiments (TDOE). The established structure-property relationships are built up with the ML models from 240 data points of small molecule OPV systems and ten important microscopic features of OPVs. The quite remarkable performance of the ML model (Pearson's coefficient = 0.79) depicts its ability to extract hidden physical principles of OPVs. The TDOE model shows that molecular orbitals other than the highest and the lowest ones that are not frequently considered in the designing process of OPVs play quite essential roles in developing promising OPV materials. Moreover, strategies to boost the design of high-performing devices with different values of the considered features are also extracted from the model with the DOE approach. These results reveal that ML combined with DOE is an impressive package for guiding the design process effectively and efficiently.

Elucidation of hole transport mechanism in efficient energy cascade organic photovoltaics using triple donor system

Recently, the energy cascade structure using multiple light-absorbing layers has been considered as a novel device architecture to overcome the inherent limitations of small-molecule organic photovoltaics (OPVs). The light-absorbing layers in the energy cascade OPVs should have a large overlap between the emission spectrum of a Förster resonance energy transfer (FRET) donor and absorption spectrum of a FRET acceptor. An equally important requirement is efficient charge transport across the organic layers by the formation of a suitable energy-level alignment. 5,10,15,20-tetraphenylbisbenz[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene (DBP)/5,6,11,12-tetraphenyltetracene (RUB)/5,12-diphenyltetracene (DPT) layers are representative donor systems, which exhibited a high power conversion efficiency in energy cascade OPVs. However, the detailed hole-transport mechanism has not been understood. In this study, the energy level alignment of C60/DBP/RUB/DPT/poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) layers was investigated via in-situ ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The highest-occupied molecular orbital levels of DBP/RUB/DPT were almost perfectly aligned after their contact, indicating an efficient hole transport toward the anode without hindrance of energy barrier after exciton dissociation. This is owing to the formation of an interface dipole that aligns the charge neutrality levels of the organic layers.

Ionically Gated Small-Molecule OPV: Interfacial Doping of Charge Collector and Transport Layer.

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotubes (MWCNTs) coated on fullerene electron transport layers (ETLs): C60 and C70. Such electric double-layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning of the electronic concentration in MWCNTs and the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to the decrease of the series and increase of the shunt resistances of OPVs and allows use of thick (up to 200 nm) ETLs, increasing the durability of OPVs. Two stages of OPV enhancement are described upon the increase of gating bias Vg: at small (or even zero) Vg, the extended interface of ILs and porous transparent MWCNTs is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped J-V curve via improving the electron collection by an n-doped MWCNT cathode with an ohmic interfacial contact. The J-V curves further improve at higher gating bias Vg due to the increase of the Fermi level and decrease of the MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNTs while ions of ILs penetrate into the fullerene. At this step, the internal built-in field is created within OPV, which helps in exciton dissociation and charge separation/transport, increasing further the Jsc and the fill factor. The ionic gating concept demonstrated here for most simple classical planar small-molecule OPV cells can be potentially applied to more complex highly efficient hybrid devices, such as perovskite photovoltaic with an ETL or a hole transport layer, providing a new way to tune their properties via controllable and reversible interfacial doping of charge collectors and transport layers.

Novel thieno[3,2-b]thiophene-embedded small-molecule donors for highly efficient and photostable vacuum-processed organic photovoltaics

Four thieno[3,2-b]thiophene (TT)-based donor-π-acceptor–type small-molecule donors DTCPTT, DTCPTT-2CN, DTDCPTT, and DTDCPTT-2CN were synthesized and characterized. A clear structure-property relationship correlating the effects of introducing cyano groups onto the TT and the end-capping groups has been successfully established. DTDCPTT and DTDCPTT-2CN were adopted as electron donors for organic photovoltaics (OPVs) mainly due to the strong intramolecular charge transfer absorption in the visible light region, sufficiently high thermal stability, and appropriate energy level alignment with C70. Moreover, the analyses of crystal structures reveal that DTDCPTT and DTDCPTT-2CN form antiparallel dimer pairs through strong dipolar and non-covalent intermolecular interactions. X-ray structures also indicate that introducing cyano groups onto the TT moiety does not affect the backbone planarity significantly. Compared with DTDCPTT-2CN, the more compact and uniform crystal packing renders DTDCPTT advantageous for exciton dissociation and charge carrier transport. DTDCPTT-based device achieves power conversion efficiency as high as 7.81% and 16.89%, respectively, under simulated AM 1.5G illumination and 500 lux TLD-840 fluorescent lamp illumination, which could be attributed to stronger absorption and better photocurrent extraction. In addition, both devices exhibit high stability under continuous simulated 1 sun illumination for more than 300 h. As far as we know, this is the best performance for vacuum-processed small-molecule indoor OPVs to date.

Efficient ternary all small molecule organic photovoltaics with NC70BA as third component materials

Inverted small molecule organic photovoltaics (SMPVs) were fabricated with BTR as a donor, the narrow band gap material Y11 as an acceptor and NC70BA fullerene derivatives with a broad band gap as the third component materials. When 20 wt% Y11 was replaced by NC70BA, a higher PCE of 13.92% was achieved, which is attributed to simultaneous enhancement of the JSC of 22.16 mA cm−2, the VOC of 0.905 V and the FF of 69.4%. The hollow spherical structure of NC70BA may lead to the connection of Y11 and BTR molecules to form more continuous electron transport channels. Shallow lowest unoccupied molecular orbital (LUMO) energy levels form a cascade of LUMO energy levels among the used materials. In addition, the molecular arrangement and crystallinity in the photoactive layer can be markedly adjusted by incorporating NC70BA into BTR:Y11 binary films. The well-optimized phase separation and molecular arrangement in ternary photoactive layers can well support an enhanced JSC and FF for ternary SMPVs. Thus, the photovoltaic performance of SMPVs can be well improved by employing a ternary strategy with NC70BA as the third component materials.

Benzodithiophene-based wide-bandgap small-molecule donors for organic photovoltaics with large open-circuit voltages

Abstract We have developed new small-molecule electron donors, SMBDT-S and SMBDT-SF, based on benzo[1,2-b:4,5-b’]dithiophene (BDT) with thiophene side chains as a core unit and rhodanine end-capping units to enhance the open-circuit voltage (Voc), which is one of the factors limiting performance of small-molecule organic photovoltaics (OPVs). The SMBDT-S and SMBDT-SF donor molecules exhibited excellent thermal stability and high crystallinity. SMBDT-S was found to have a wide bandgap of 1.85 eV with the highest occupied molecular orbital (HOMO) energy level of − 5.56 eV. The fluorination in SMBDT-SF led to the same bandgap of 1.86 eV with an even lower-lying HOMO energy level of − 5.72 eV. Both SMBDT-S and SMBDT-SF have large thin-film absorption coefficients of (1.4–1.5) × 105 cm−1. Bulk heterojunction OPV devices based on pairing SMBDT-S and SMBDT-SF respectively with PC71BM electron acceptor showed Voc as high as 1.18 eV, which is among the highest Voc reported for all-small-molecule solar cells. These results demonstrate that introducing fluorine atoms and alkylthio side chains are effective strategies to downshift the HOMO energy level and enhance the Voc in OPVs.

14.46% Efficiency small molecule organic photovoltaics enabled by the well trade-off between phase separation and photon harvesting

Small molecule organic photovoltaics (SMPVs) were prepared by utilizing liquid crystalline donor material BTR-Cl and two similar optical bandgap non-fullerene acceptor materials BTP-BO-4F and Y6. The BTP-BO-4F and Y6 have the similar optical bandgap and different absorption coefficients. The corresponding binary SMPVs exhibit different short circuit current density (JSC) (20.38 vs. 23.24 mA cm−2), and fill factor (FF) (70.77% vs. 67.21%). A 14.46% power conversion efficiency (PCE) is acquired in ternary SMPVs with 30 wt% Y6, companied with a JSC of 24.17 mA cm−2, a FF of 68.78% and an open circuit voltage (VOC) of 0.87 V. The improvement on PCE of ternary SMPVs should originate from the well trade-off between phase separation and photon harvesting of ternary active layers by incorporating 30 wt% Y6 in acceptors. This work may deliver insight onto the improved performance of SMPVs by superposing the superiorities of binary SMPVs with similar optical bandgap acceptors into one ternary cell.

Crystal-structure of active layers of small molecule organic photovoltaics before and after solvent vapor annealing

Abstract It is demonstrated by a detailed structural analysis that the crystallinity and the efficiency of small molecule based organic photovoltaics can be tuned by solvent vapor annealing (SVA). Blends made of the small molecule donor 2,2′-[(3,3′″,3″″,4′-tetraoctyl[2,2′:5′,2″:5″,2′″:5′″,2″″-quinquethiophene]-5,5″″-diyl)bis[(Z)-methylidyne(3-ethyl-4-oxo-5,2-thiazolidinediylidene)]]bis-propanedinitrile (DRCN5T) and the acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were annealed using solvent vapors with either a high solubility for the donor (tetrahydrofuran), the acceptor (carbon disulfide) or both (chloroform). The samples were analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS), electron diffraction, X-ray pole figures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC71BM is induced by SVA leading to a crystallization of DRCN5T and the formation of a DRCN5T enriched layer. The DRCN5T crystallites possess the two dimensional oblique crystal system with the lattice parameters a = 19.2 Å, c = 27.1 Å, and β = 111.1° for the chloroform case. No major differences in the crystal structure for the other solvent vapors were observed. However, the solvent choice strongly influences the size of the DRCN5T enriched layer. Missing periodicity in the [010]-direction leads to the extinction of all Bragg reflections with k ≠ 0. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture).

Printed Large-Area Photovoltaic Modules Based on Small Molecules with Different Alkyl Terminal Chains

Despite significant progress in organic photovoltaics (OPVs), efforts to fabricate the small-molecule OPVs by scalable printing methods have hardly been devoted. Herein, we demonstrate large-area p...

An universal electron transport layer involving hydrogen plasma–treated tungsten disulfide nanosheets doped zinc oxide layers for polymer donors with fullerene or small molecule acceptor photovoltaics

Abstract A new universal electron transport layer that involves doping hydrogen-plasma-treated tungsten disulfide (WS2) nanosheets into ZnO for polymer/fullerene or small molecule organic photovoltaics (OPVs) was prepared. A hydrogen plasma treatment was used to alter the structures of WS2 nanosheets such that the W6+ content was converted into W4+; then ZnO:WS2 nanosheets composites were prepared to form electron transport layers (ETLs). The energy band of the ZnO:WS2 films could be tuned from 5.15 to 4.60 eV by varying the concentration of the WS2 nanosheets up to 0.5 wt%. It was found that ZnO:WS2 ETLs exhibited superior charge transport properties than those of the pristine ZnO layer because of the structure changes, as determined from the X-ray scattering characterizations. OPVs incorporating active layers of PTB7-TH/PC71BM and PTB7-TH/IDIC blends exhibited their power conversion efficiencies of 10.3% and 6.7%, respectively, with the incorporation of 0.3 wt% of the WS2 nanosheets, up from 8.9% to 5.4% for the corresponding devices featuring pristine ZnO—relative increases of 16% and 24%, respectively. This study demonstrates the effectiveness of hydrogen plasma treatment for altering the surface structures of two-dimensional transition-metal-dichalcogenide nanosheets, and paves a way for the composite electron transport layers for use in organic photovoltaics.

High-Performance Organic Energy-Harvesting Devices and Modules for Self-Sustainable Power Generation under Ambient Indoor Lighting Environments.

Organic photovoltaics (OPVs) that perform more efficiently under artificial indoor lighting conditions than they do under sunlight are attracting growing interest as they can potentially serve as ambient energy harvesters for powering low-power electronics and portable devices for the Internet of Things. Herein, solution-processed small-molecule OPVs are demonstrated to exhibit high power conversion efficiencies exceeding 16% under white LED illumination, delivering high output power densities of up to 12.4 and 65.3 μW cm-2 at 200 and 1000 lx, respectively. Increasing the open-circuit voltage ( Voc) of OPVs is a critical factor for achieving higher indoor photovoltaic performance. Toward real applications, this small-molecule OPV system is adopted to fabricate six series-connected modules with an active area of ∼10 cm2 that are capable of generating a high output power surpassing 100 μW and a high Voc of over 4.2 V even under dimly lit indoor conditions of 200 lx. These results indicate that OPVs are promising as indoor electric power sources for self-sustainable electronic devices.

Predictive Simulations for Tuning Electronic and Optical Properties of SubPc Derivatives

Boron subphthalocyanine chloride is an electron donor material used in small-molecule organic photovoltaics with an unusually large molecular dipole moment. Using first-principles calculations, we investigate how to control the electronic and optical properties of boron subphthalocyanine chloride by substituting the boron and chlorine atoms with other trivalent and halogen atoms and thereby modifying the molecular dipole moment. Gas-phase molecular structures and properties are predicted using hybrid functionals. Using positions and orientations of the known compounds as the starting coordinates for these molecules, stable crystalline structures are derived following a procedure that involves perturbation and accurate total energy minimization. Electronic structure and photonic properties of the predicted crystals are computed using the GW method and the Bethe–Salpeter equation, respectively. Finally, a simple transport model is used to quantitatively demonstrate the effects of the strength and orientation of molecular dipole moments at interfaces on device performance.

Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics: Improved electron transport by charged state of C60

Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH)2 is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH)2 layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH)2 interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH)2 layer. On implementing the optimum thickness of the Ba(OH)2 layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C60/indium tin oxide (ITO) and C60/Ba(OH)2/ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH)2 was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH)2, the reduced WF of the ITO is lower than the electron affinity of C60. Thus, a charge transfer is induced from Ba(OH)2/ITO to C60, and the charged states of C60 are observed within the monolayer. These charged states result in significant band bending in the C60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (EF) of the Ba(OH)2/ITO. As a result, the energy offset between the C60 LUMO level and the cathode EF is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH)2 layer. This is the origin of the enhanced device performance of OPVs.

Toward Predicting Efficiency of Organic Solar Cells via Machine Learning and Improved Descriptors

AbstractTo design efficient materials for organic photovoltaics (OPVs), it is essential to identify the largest number of parameters that control their properties and build a model using these parameters (known as descriptors) for the prediction of the power conversion efficiency (PCE). By constructing a dataset for 280 small molecule OPV systems, it is found that for all high‐performing devices, frontier molecular orbitals of donor molecules are nearly degenerated and in such cases, orbitals other than just highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are involved in exciton formation, exciton dissociation, and hole transport processes influencing the macroscopic properties of OPVs. Machine learning approaches, including random forest, gradient boosting, deep neural network are used to build models for the prediction of PCE using 13 important microscopic properties of organic materials as descriptors. Quite impressive performance of the gradient boosting model (Pearson's coefficient = 0.79) indicates that it can certainly be applied to high‐throughput virtual screening of promising new donor molecules for high‐efficiency OPVs.

Efficiency enhancement in DIBSQ:PC71BM organic photovoltaic cells by using Liq-doped Bphen as a cathode buffer layer

We have improved the photovoltaic performance of 2,4-bis[4-(N,Ndiisobutylamino)- 2,6-dihydroxyphenyl] squaraine:[6,6]-phenyl C71-butyric acid methyl ester (DIBSQ:PC71BM) organic photovoltaic (OPV) cells via incorporating Liq-doped Bphen (Bphen-Liq) as a cathode buffer layer (CBL). Based on the Bphen-Liq CBL, a DIBSQ:PC71BM OPV cell possessed an optimal power conversion efficiency of 4.90%, which was 13% and 60% higher than those of the devices with neat Bphen as CBL and without CBL, respectively. The enhancement of the device performance could be attributed to the enhanced electron mobility and improved electrode/active layer contact and thus the improved photocurrent extraction by incorporating the Bphen-Liq CBL. Light-intensity dependent device performance analysis indicates that the incorporating of the Bphen-Liq CBL can remarkably improve the charge transport of the DIBSQ:PC71BM OPV cell and thus decrease the recombination losses of the device, resulting in enhanced device performance. Our finding indicates that the doped Bphen-Liq CBL has great potential for high-performance solution-processed small-molecule OPVs.

Electrochemical studies on triarylamines featuring an azobenzene substituent and new application for small-molecule organic photovoltaics

Three N,N′-diaryl-4-amino-azobenzene derivatives containing donor (D) and acceptor (A) moieties have been synthesized for light harvesting. Their optical and electronic characteristics have been analyzed to determine the optical band gap between highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO). The electrochemical properties and electrochemical redox potential of the triarylamines exhibited linear relationship with respect to the Hammett equation. The stability of the oxidation products and spectral changes with different applied potentials were monitored with UV–vis spectroelectrochemical investigations and DFT calculations. A solution-processed organic photovoltaic cell employing N,N′-ditolyl-4-aminoazobenzene (DTAA) mixed with PC60BM produced a power conversion efficiency (PCE) of 0.28% under 1 sun illumination.

Effects of morphology evolution on solution-processed small molecule photovoltaics via a solvent additive

A newly synthesized small molecule donor, BDTT-2DPPBFu, is designed for incorporation into the photo-active layer in solution-processed small molecule organic photovoltaics (SM-OPVs).

Outdoor operation of small-molecule organic photovoltaics

We measure the diurnal dependence of the operating characteristics of tetraphenyldibenzoperiflanthene (DBP):C70 planar-mixed heterojunction small-molecule organic photovoltaic (OPV) cells with 2,2′,2″-(1,3,5-benzenitryl tris-[1-phenyl-1H-benzimidazole] (TPBi):C70 electron-filtering cathode buffer layers. Over the course of a day, efficiency gradually increases as a result of a concomitant increase in short-circuit current, while the fill factor and open-circuit voltage remain constant. The results are analyzed on the basis of independent measurements of temperature- and intensity-dependent OPV performance. The power conversion efficiency is maximized slightly below 1 sun intensity and at 40 °C, which is beneficial for practical outdoor operation. We attribute the increased short circuit current with temperature to broadening of the absorption spectrum due to population of phonon states along with increased charge mobility, which also results in an increase in fill factor.

Impact of Fullerene Structure on Nanoscale Morphology and Miscibility and Correlation of Performance on Small Molecules: Fullerene Solar Cell

This manuscript reports the impact of fullerene structure on the morphology and miscibility of small molecules via a fullerene bulk heterojunction solar cell. The small angle neutron scattering and neutron reflectometry measurements were analyzed to provide quantifiable measures of the morphology of the resultant mixtures, offering miscibility, domain sizes, interfacial area between the small molecule and fullerene, and depth profiles in the mixtures. These results indicate that the bis-adduct fullerenes exhibit lower miscibility in small molecules. Correlation of miscibility and morphology to photovoltaic properties indicates that small molecule/fullerene miscibility is crucial to rationally optimize the design of fullerenes for use in small molecule organic photovoltaics. A higher open circuit voltage was obtained for bis-adduct fullerene devices which, however, does not translate to an increased power conversion efficiency. This decrease in performance is associated with the lower miscibility of bis-fu...

Reliability of Small Molecule Organic Photovoltaics with Electron‐Filtering Compound Buffer Layers

Electron‐filtering compound buffer layers (EF‐CBLs) improve charge extraction in organic photovoltaic cells (OPVs) by blending an electron‐conducting fullerene with a wide energy gap exciton‐blocking molecule. It is found that devices with EF‐CBLs with high glass transition temperatures and a low crystallization rate produce highly stable morphologies and devices. The most stable OPVs employ 1:1 2,2′,2″‐(1,3,5‐benzenetriyl tris‐[1‐phenyl‐1H‐benzimidazole] TPBi:C70 buffers that lose <20% of their initial power conversion efficiency of 6.6 ± 0.6% after 2700 h under continuous simulated AM1.5G illumination, and show no significant degradation after 100 days of outdoor aging. When exposed to 100‐sun (100 kW m−2) concentrated solar illumination for 5 h, their power conversion efficiencies decrease by <8%. Moreover, it is found that the reliability of the devices employing stable EF‐CBLs has either reduced or no dependence on operating temperature up to 130 °C compared with BPhen:C60 devices whose fill factors show thermally activated degradation. The robustness of TPBi:C70 devices under extreme aging conditions including outdoor exposure, high temperature, and concentrated illumination is promising for the future of OPV as a stable solar cell technology.