High-performance Organic Solar Cells Research Articles

An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells.

Side-chain engineering on nonfullerene acceptors (NFAs) is crucial for modulating their solubility and crystallinity as well as packing behaviours in active layers to pursue high-performance organic solar cells (OSCs). High weight ratios of side chains are generally used by NFAs for the desired device efficiencies. Side-chain economy has seldom been discussed despite increased cost and difficulties in synthesis when optimizing the molecular design. Herein, we introduce 7H-dibenzo[c,g]carbazole (DCB) as an electron-donating core to design unfused-ring acceptors (UFAs) with a dramatically low weight ratio of side chains. DCB-4F has thus been designed and compared with the carbazole cored analogue (CB-4F). The unique conformation of the DCB core endows DCB-4F with higher solubility (8.2 mg mL-1 in chloroform) compared to CB-4F (2.2 mg mL-1) when using the same side chains. Featuring a lowest unoccupied molecular orbital (LUMO) level of -3.86 eV and an optical bandgap of 1.55 eV, the DCB-4F film exhibits an absorption profile (maximum 667 nm) complementary to polymer donor PM6. The PM6:DCB-4F as-cast OSCs deliver a power conversion efficiency (PCE) of 9.56% with a high open-circuit voltage (VOC) of 1.00 V. By adding 10 wt% PC71BM into the casting solutions, a greatly improved PCE of 11.17% is readily achieved, which is one of the highest PCEs for as-cast single-junction UFA-based devices. The PM6:DCB-4F based blends show homogeneous nano-fiberous morphology and higher hydrophobicity. The design of conformation-tuned NFAs using sterically hindered DCB-like cores is promising to achieve highly efficient as-cast OSCs.

Aniline-based hole transporting materials for high-performance organic solar cells with enhanced ambient stability

Our work suggests the design of new hole transport material, a protonic acid-doped oligo(aniline), for achieving both high device efficiency and improved operational stability of organic solar cells.

A recent overview of porphyrin-based π-extended small molecules as donors and acceptors for high-performance organic solar cells

Structural development of porphyrin-based small molecules as electron donors and electron acceptors leads to optimal photovoltaic performances, with PCEs of over 12% and 9.6%, respectively, in bulk-heterojunction organic solar cells.

Non-conjugated electrolytes as thickness-insensitive interfacial layers for high-performance organic solar cells

PEDETA-DBO has been developed as a thickness-insensitive ETL. The OSCs based on PEDETA-DBO retain more than 92.5% of PCE values at a thickness of 30 nm. The reason for the thickness-insensitive property of non-conjugated ETLs has been revealed.

Effects of Fluorination on Fused Ring Electron Acceptor for Active Layer Morphology, Exciton Dissociation, and Charge Recombination in Organic Solar Cells.

Fluorination is one of the effective approaches to alter the organic semiconductor properties that impact the performance of the organic solar cells (OSCs). Positive effects of fluorination are also revealed in the application of fused ring electron acceptors (FREAs). However, in comparison with the efforts allocated to the material designs and power conversion efficiency enhancement, understanding on the excitons and charge carriers' behaviors in high-performing OSCs containing FREAs is limited. Herein, the impact of fluorine substituents on the active layer morphology, and therefore exciton dissociation, charge separation, and charge carriers' recombination processes are examined by fabricating OSCs with PTO2 as the donor and two FREAs, O-IDTT-IC and its fluorinated analogue O-IDTT-4FIC, as the acceptors. With the presence of O-IDTT-4FIC in the devices, it is found that the excitons dissociate more efficiently, and the activation energy required to split the excitons to free charge carriers is much lower; the charge carriers live longer and suffer less extent of trap-assisted recombination; the trap density is 1 order of magnitude lower than that of the nonfluorinated counterpart. Overall, these findings provide information about the complex impacts of FREA fluorination on efficiently performed OSCs.

Efficient designing of triphenylamine-based hole transport materials with outstanding photovoltaic characteristics for organic solar cells

Hole transport materials (HTMs), especially dopant-free hole transport materials, are getting attention in enhancing the power conversion efficiencies and stabilities of organic solar cells (OSCs). Herein, we have designed efficient dopant-free HTMs (DM1–DM5) from an outstanding synthetic DFM molecule (having 20.6% PCE). Photo-physical, photovoltaic, optoelectronic and structural-property relationship of newly designed molecules are extensively studied and compared with DFM (R). Density functional theory (DFT) and time-dependent-density functional theory (TD-DFT) have been employed to investigate the alignment of frontier molecular orbitals (FMOs), optical properties, density of states along with transition density matrix, binding and excitation energy, reorganizational energies and for open-circuit voltages of all newly designed molecules. Red-shifting in absorption spectrum offers high power conversion efficiencies, and our tailored molecules exhibit red-shifting in absorption spectrum (λmax = 391–429 nm) as compared to R (λmax = 396 nm). In addition, our all designed molecules expressed better hole transport ability (λh = 0.0056–0.0089 eV) as compared to R (λh = 0.0101 eV). Similarly, DM1–DM5 disclosed narrow HOMO–LUMO energy gap which causes maximum charge transfer from excited HOMO to excited LUMO. The theoretical study of DM3/PC61BM and DM3/Y6 complexes is also performed in order to understand the shifting of charge between donor and acceptor molecules. Results of all analysis clearly show the efficient designing of dopant-free (DM1–DM5) molecules and their possible potential to fabricate a high performance and stable organic solar cells devices. Therefore, the theoretical proposed molecules are recommended to experimentalists for future highly efficient organic solar cells.

An A-D-A′-D-A type unfused nonfullerene acceptor for organic solar cells with approaching 14% efficiency

In recent years, power conversion efficiency (PCE) of organic solar cells (OSCs) has made significant improvement. A large number of studies were reported to achieve high PCEs through exploring new active layer materials, especially the high efficiency fused ring acceptors (FRAs). Compared with FRAs, a nother type of so-called unfused-ring acceptors (UFAs), possessing some advantages such as simple synthesis and low cost, have attracted a lot of attention. Herein, a new UFA BTzO-4F , incorporating with a benzotriazole moiety and S···O intramolecular noncovalent interactions, has been successfully synthesized. The photovoltaic device based on PBDB-T: BTzO-4F achieved a record PCE of 13.8% for UFAs, which indicates that introducing the benzotriazole moiety is an effective strategy for high quality acceptors. Thus, these findings of this work demonstrate the great potential of UFAs for high performance OSCs.

Heptacyclic S,N-Heteroacene-Based Near-Infrared Nonfullerene Acceptor Enables High-Performance Organic Solar Cells with Small Highest Occupied Molecular Orbital Offsets.

The reduction of energy offsets between donors and acceptors is a direct way to improve the open-circuit voltage (VOC) and overall performance of organic solar cells (OSCs). In this work, two nonfullerene acceptors (NFAs) (BDTBO-4F and BDTBO-4Cl) were synthesized, which were composed of a heptacyclic S,N-heteroacene core and terminal units with halogen atoms, where the latter modulates the energy level of the frontier molecular orbital. Consequently, BDTBO-4Cl exhibited a deeper highest occupied molecular orbital level (EHOMO) and lowest unoccupied molecular orbital level (ELUMO) than BDTBO-4F. Moreover, these two NFAs exhibited high electron mobility and strong absorption at 700-900 nm. The polymer donor PM6 was combined with BDTBO-4F and BDTBO-4Cl, and the resulting OSCs exhibited outstanding power conversion efficiencies of 14.83% for the PM6:BDTBO-4F device and 13.87% for the PM6:BDTBO-4Cl device. More encouragingly, these OSCs exhibited efficient hole transfer from NFAs to PM6, despite small ΔEHOMO(D-A) values (<0.10 eV). These results prove that modulation of EHOMO of acceptors to decrease ΔEHOMO(D-A) is an efficient strategy for high-performance OSCs.

V OC variation with different molecular weight fractions in highly efficient organic photovoltaic bulk heterojunctions

Tuning molecular weight fractions is one of the most important strategies for optimizing the morphology of bulk heterojunctions (BHJs) to obtain high-performance organic solar cells. Here, we investigate the effects of molecular weight fractions on the performance of state-of-the-art PM6:Y6 BHJ solar cells. A remarkable blueshift in the photoluminescence spectra can be observed with the introduction of the PM6 component. The electroluminescent and morphological analyses indicate that this observation can be attributed to increased charge-transfer state energy and an intermolecular interaction between donor and acceptor, leading to open circuit voltage (V OC) variations in the photovoltaic device. The transient absorption spectra confirm that the optimized donor:acceptor (D:A) ratio can suppress the generation of polarons and achieve high efficiency exciton dissociation. The results of atomic force microscopy and neutron reflectometry reveal that higher acceptor contents promote the formation of a desirable interpenetrating network and enable high fill factors and short-circuit currents to be obtained.

Alkylated Indacenodithiophene-Based Non-fullerene Acceptors with Extended π-Conjugation for High-Performance Large-Area Organic Solar Cells

In this work, a series of A-D-A'-D-A-type electron acceptors based on alkylated indacenodithiophene (C8IDT), dicyanated thiophene-flanked 2,1,3-benzothiadiazole (CNDTBT), and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (FINCN) are synthesized in order to investigate the effect of substituents on their photovoltaic properties. The corresponding CNDTBT-C8IDT-INCN and CNDTBT-C8IDT-FINCN acceptors vary in their optical, electrochemical, morphological, and charge transport properties. The fluorinated-INCN-based acceptor (CNDTBT-C8IDT-FINCN) exhibits lower energy levels, improved absorptivity, narrower π-π spacing, and prominent fibrillar structures when it is blended with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)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-T). CNDTBT-C8IDT-FINCN exhibits a high power conversion efficiency (PCE) of 12.33% due to its high and well-balanced charge carrier mobility and distinct face-on orientation. Furthermore, large-area organic solar cells (OSCs) (active area: 55.45 cm2) with CNDTBT-C8IDT-FINCN exhibit a high PCE of 9.21%. This result demonstrates that CNDTBT-C8IDT-FINCN is a suitable and promising electron acceptor for large-area OSCs.

First theoretical framework of Z-shaped acceptor materials with fused-chrysene core for high performance organic solar cells.

Chrysene core containing fused ring acceptor materials have remarkable efficiency for high performance organic solar cells. Therefore, present study has been carried out with the aim to design chrysene based novel Z-shaped electron acceptor molecules (Z1-Z6) from famous Z-shaped photovoltaic material FCIC (R) for organic photovoltaic applications. End-capped engineering at two electron-accepting end groups 1,1-dicyanomethylene-3-indanone of FCIC is made with highly efficient end-capped acceptor moieties and impact of end-capped modifications on structure-property relationship, photovoltaic and electronic properties of newly designed molecules (Z1-Z6) has been studied in detail through DFT and TDDFT calculations. The efficiencies of the designed molecules are evaluated through energy gaps, exciton binding energy along with transition density matrix (TDM) analysis, reorganizational energy of electron and hole, absorption maxima and open circuit voltage of investigated molecules. The designed molecules exhibit red-shift and intense absorption in near-infrared region (683-749nm) of UV-Vis-NIR absorption spectrum with narrowing of HOMO-LUMO energy gap from 2.31eV in R to 1.95 in eV in Z5. Moreover, reduction in reorganization energy of electron from 0.0071 (R) to 0.0049 (Z5), and enhancement in open circuit voltage from 1.08V in R to 1.20V in Z5 are also observed. Twisted Z-shape of designed molecules prevents self-aggregation that facilitates miscibility of donor and acceptor. Low values of binding energy, excitation energy, and reorganizational energy (electron and hole) suggest that novel designed molecules offer high charge mobilities as compared to FCIC. Our findings indicate that these novel designed molecules can display better photovoltaic parameters and are suitable candidates if used in organic solar cells.

Influence of end-capped group on structural and electronic properties of the At-π-Ac-π-At small molecule donor for high-performance organic solar cells

To achieve high efficient donors for organic solar cells, structural, electronic, and optical properties of the At-π-Ac-π-At small molecules were systematically evaluated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The At-π-Ac-π-At structure consists of a benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (BDD) unit acting as the central core acceptor (Ac) incorporated with 8 different end-capped groups (At) via trithiophene linkages. The results revealed that the coplanar structures were elucidated from A3 to A7 due to intramolecular hydrogen bonds and strong non-covalent S⋯O interactions along the molecular backbone. The HOMO-LUMO gap of a referenced DR3TBDD (A3) structure calculated from the B3LYP/6-31G(d,p) method is in very good agreement with the experimental evaluation. The end-capped units pronounced a significant influence on the modulation of molecular energy level. The HOMO-LUMO gaps obtained from A1, A4, A5, A6, and A8 are narrower than that determined from DR3TBDD (A3), implying the enchantment of the short circuit density (Jsc). For electron mobility, A1, A4, A6, and A7 structures were an easier hole transport character compared with A3, while a higher deformation energy for oxidation process was demanded for A2, A5, and A8. The dicyano terminals exhibited a larger red-shifted peak with broader absorption spectra compared with other end-capped functionals. Our results supplied theoretical descriptions of the At-π-Ac-π-At donor material for further enhancing the performance of organic solar cells.

Optimising Non-Patterned MoO3/Ag/MoO3 Anode for High-Performance Semi-Transparent Organic Solar Cells towards Window Applications.

Semi-transparent organic solar cells (ST-OSCs) have attracted significant research attention, as they have strong potential to be applied in automobiles and buildings. For ST-OSCs, the transparent top electrode is an indispensable component, where the dielectric/metal/dielectric (D/M/D) structured electrode displayed a promising future due to its simplicity in the fabrication. In this work, by using the MoO3-/Ag-/MoO3-based D/M/D transparent electrode, we fabricated ST-OSCs based on the PM6:N3 active layer for the first time. In the device fabrication, the D/M/D transparent electrode was optimised by varying the thickness of the outer MoO3 layer. As a result, we found that increasing the thickness of the outer MoO3 layer can increase the average visible transmittance (AVT) but decrease the power conversion efficiency (PCE) of the device. The outer MoO3 layer with a 10 nm thickness was found as the optimum case, where its corresponding device showed the PCE of 9.18% with a high AVT of 28.94%. Moreover, the colour perception of fabricated ST-OSCs was investigated. All semi-transparent devices exhibited a neutral colour perception with a high colour rendering index (CRI) over 90, showing great potential for the window application.

Side chain engineering investigation of non-fullerene acceptors for photovoltaic device with efficiency over 15%

Side chain engineering plays a substantial role for high-performance organic solar cells (OSCs). Herein, a series of non-fullerene acceptor (NFA) molecules with A-D-A structures, TTC n -4F, with gradient substituent lengths of terminal side chains (T-SCs) on the molecular backbones have been designed and synthesized. The effects of T-SCs length, ranging from hydrogen atom to n -dodecyl, their optoelectronic properties, thin film molecular packing, blend film morphology, and overall photovoltaic performance have been systematically studied. The results show that among this series of molecules, TTC8-4F with n -octyl substituent, showed the best photovoltaic performance when applied with PM6 as the donor due to its favorable morphology, balanced charge mobility, effective exciton dissociation and less charge recombination. Based on this, its ternary device with F-Br as the secondary acceptor achieved a high PCE of 15.34% with the simultaneously enhanced V oc of 0.938 V, J sc of 22.66 mA cm−2, and FF of 72.15%. These results indicate that the engineering of T-SCs is an effective strategy for designing high-performance NFAs.

A Non-fullerene Acceptor with Enhanced Intermolecular π-Core Interaction for High-Performance Organic Solar Cells.

Understanding the molecular structure and self-assembly of thiadiazole-derived non-fullerene acceptors (NFAs) is very critical for elucidating the origin of their extraordinary charge generation and transport properties that enable high power conversion efficiencies to be achieved in these systems. A comprehensive crystallographic study on a state-of-the-art NFA, Y6, and its selenium analog, CH1007, has been conducted which revealed that the face-to-face π-core interaction induced by benzo[2,1,3]thiadiazole S-N-containing moieties plays a significant role in governing the molecular geometries and unique packing of Y6 and CH1007 to ensure their superior charge-transport properties. Moreover, benefitting from the red-shifted optical absorption via selenium substitution, photovoltaic devices based on a PM6:CH1007:PC71BM ternary blend delivered an exceptionally high short-circuit current of 27.48 mA/cm2 and a power conversion efficiency of 17.08%.

Effects of monohalogenated terminal units of non-fullerene acceptors on molecular aggregation and photovoltaic performance

Three small molecule acceptors (SMAs), named ZF, ZC and ZB, have been designed and synthesized for organic solar cells (OSCs) by introducing single F, Cl and Br atom on each terminal unit, respectively. The effects of different halogen substituent on molecular aggregation, photophysical and photovoltaic performance have been systematically studied. Owing to its strong electronegativity of halogens, all the acceptors with monohalogenated terminal units possess red-shifted absorption spectra and deeper frontier energy levels compared to the corresponding acceptor without halogen substituents reported in the literature. Among the SMAs, ZB with brominated terminal units was found to show the higher molar absorption coefficient (2.31 × 105 M−1 cm−1), more orderly face-on π-π stacking, higher electron mobility and more favorable morphology when blended with PM6. As a result, the PM6:ZB OSCs yielded a high power conversion efficiency (PCE) of up to 15.23% with a high short-circuit current density (Jsc) of 26.38 mA cm−2, while the corresponding ZF- and ZC-based devices showed the relatively inferior PCEs of 13.36% and 14.71%, respectively. These results demonstrated that the modulation of electron-withdrawing halogen substituents on terminal group provides a promising strategy to design and synthesize efficient SMAs for fabricating high-performance OSCs.

Designing of benzodithiophene core-based small molecular acceptors for efficient non-fullerene organic solar cells.

Nowadays, organic solar cells (OSCs) with non-fullerene electron acceptors provide the highest efficiencies among all studied OSCs. To further improve the efficiencies of fullerene-free organic solar cells, end-capped acceptor modification is made with strong electron withdrawing groups. In this report, we have theoretically designed five new novel Benzodithiophene core-based acceptor molecules (H1-H5) with the aim to study the possible enhancement in photophysical, optoelectronic, and photovoltaic properties of newly designed molecules. The end-capped acceptor modification of famous and recently synthesized FBDIC molecule has been made with strong electron withdrawing groups. Density functional theory and time-dependent-density functional theory are extensively used to study the structural-property relationship, optical properties and various geometrical parameters like frontier molecular orbitals alignment, excitation and binding energy, transition density matrix along with open circuit voltage, density of states and dipole moment. Commonly, low reorganization energies (hole and electron) afford high charge mobility and our all designed systems are enriched in aspect (λe=0.0044-0.0104eV and λh=0.0060-0.0090eV). Moreover, H1-H5 molecules demonstrate red-shifting in absorption spectrum (λmax=741-812nm) as compare to R (λmax=728nm). Low excitation and binding energies with low HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap of H1-H5 suggested that designed molecules are better and suitable candidates for high performance organic solar cell. Results of all analysis indicate that this theoretical framework demonstrates that end-capped acceptors modification is a simple and effective alternative strategy to achieve the desirable optoelectronic properties. Therefore, H1-H5 are recommended to experimentalist for out-looking future developments of highly efficient solar cells.

Effect of Nitro-Substituted Ending Groups on the Photovoltaic Properties of Nonfullerene Acceptors.

Chemical modification of end groups has proved to be an effective way to design new acceptor-donor-acceptor (A-D-A)-structured nonfullerene acceptors (NFAs) for high-performance organic solar cells (OSCs). Herein, we designed and synthesized two nitro-substituted end groups, N1 and N2. Using the two end groups as A units, two A-D-A acceptors, F-N1 and F-N2, were obtained. It also has been found that the nitro substitution position on end groups affects not only the absorptions and energy levels of the resultant acceptor materials but also their molecular packing behavior and active layer morphologies. In addition, the devices based on the two acceptors showed different energy losses. Power-conversion efficiencies (PCEs) of 10.66 and 11.86% were achieved for F-N1- and F-N2-based devices, respectively. This work reveals that the nitration of end groups is one of the potential strategies for designing high-performance photovoltaic active layer materials.

Efficient Organic Ternary Solar Cells Employing Narrow Band Gap Diketopyrrolopyrrole Polymers and Nonfullerene Acceptors

Currently, high-performance organic solar cells (OSCs) are mainly composed of narrow band gap (NBG) nonfullerene acceptors and medium band gap (MBG) polymer donors, whereas the solar cells based on NBG polymer donors and MBG nonfullerene acceptors were much less successful. Herein, we report a new diketopyrrolopyrrole (DPP) polymer (PffBT-DPP) with a band gap of 1.33 eV for use in nonfullerene OSCs. When blended with a fullerene acceptor [6, 6]-phenyl C71 butyric acid methyl ester ([70]PCBM) and an MBG nonfullerene acceptor MeIC, the binary OSCs offered a power conversion efficiency (PCE) of 6.8 and 2.0%, respectively. Interestingly, a much higher PCE of 9.0% was achieved for the PffBT-DPP/[70]PCBM/MeIC ternary OSC, which is the sum of the two binary OSCs. The phase-separated morphology in this ternary OSC was characterized and correlated to the device performance. Moreover, the PCE of the ternary OSC is the best result for OSCs employing polymer donors with an optical band gap of <1.40 eV and nonfullerene acceptors. This work demonstrates the potential of constructing high-performance OSCs by employing NBG polymer donors and MBG nonfullerene acceptors.

A strategy of designing near-infrared porphyrin-based non-fullerene acceptors for panchromatic organic solar cells

Abstract Two porphyrin-based non-fullerene acceptors (NFAs), PORTIC and PORTFIC, were synthesized to enable panchromatic absorption for high performance organic solar cells (OSCs). An inverted organic solar cell architecture of ITO/ZnO/PTB7-Th:Porphyrin-based NFAs/MoO3/Ag is used. With pyridine as an additive of which further affords optimum morphology for charge separation transport, the power conversion efficiency (PCE) was improved by 40.4% and 67.8% for PORTIC and PORTFIC, respectively, relative to devices fabricated without pyridine. Two-dimensional (2D) grazing-incidence wide-angle X-ray scattering (GIWAXS) shows that the optimized active layer exhibits face-on orientation. Moreover, the photoluminescence quenching experiments reveal that more effective charge transfer between the donor and acceptor is achieved upon addition of pyridine. PTB7-Th:PORTFIC device affords a VOC of 0.74 V, a JSC of 11.27 mA cm−2, a FF of 51.29% and highest PCE of 4.28%.