High-performance Organic Solar Cells Research Articles

Designing of small organic non-fullerene (NFAs) acceptor molecules with an A−D−A framework for high-performance organic solar cells: A DFT and TD-DFT method

Abstract The main objective of this research was to design non-fullerene acceptors (NFAs) A–D–A framework, using carbazole and benzothiazole derivatives. Density functional theory (DFT) was used to calculate the geometry optimized structures and electronic properties at B3LYP functional with a 6-311G basis set in the gas and solvent phase. The frontier molecular orbitals (FMO), bandgap, open-circuit voltage (VOC) and dipole moments of these developed acceptors have been calculated. The theoretical UV absorption spectra were calculated from time-dependent DFT with the same level of theory used DFT method. They show a suitable bandgap (2.24–2.93 eV) and dipole moment (1.8–10.8 Debye). The maximum wavelength (λmax) for all studied molecules in the range is 665.17–679.97 in both gas and solvent. A slight redshift was observed in all acceptors selected for chlorobenzene compared with gas phase absorption. The NFA A11 has the lowest bandgap energy (2.24 eV), gas-phase excitation energy (1.86 eV) and chlorobenzene excitation energy (1.86 eV). As a result, A11 is predicted to be a good contender for organic NFAs in the future. The open-circuit voltage (VOC) values range from 1.53 to 2.56 eV. Consequently, the optoelectronic, molecular orbital distribution and A11 and A12 molecules were suitable acceptors for NFAs.

TiO2 nanoparticles via simple surface modification as cathode interlayer for efficient organic solar cells

Cathode interlayers (CILs) play important roles in promoting high-performance organic solar cells (OSCs). Among the commonly used CILs, metal oxides, such as ZnO, TiO 2 and SnO 2 , display suitable work function and high conductivity. However, they are usually processed under high temperature and in most cases, they are adopted in the inverted OSCs. Since high-performing non-fullerene OSCs are usually fabricated with conventional configuration, it is interesting to explore the potential application of these metal oxide materials as CIL in these cells. An efficient strategy is to use metal oxide nanocrystals as CIL to deposit onto photoactive layers. However, the as-synthesized nanocrystals have the difficulty to form smooth and dense films due to the serious aggregation. In this work, we developed a method via the surface modification of TiO 2 nanoparticles by using 4-dimethylamino benzoic acid to reduce the aggregation, so that smooth TiO 2 –N film can be formed on top of the photoactive layer. Besides, TiO 2 –N displays lower work function, which is helpful for better energy levels alignment. All these factors contribute to the improved charge generation and transport, leading to the power conversion efficiencies enhanced from 13.68% to 15.90% for PM6:BTP-4Cl solar cells. • Metal oxide nanocrystals are applied as cathode interlayer deposited onto the photoactive layer in organic solar cells. • The method via the surface modification on the TiO 2 nanoparticles by using polar acid ligand is facile. • TiO 2 –N displays lower work function, leading to better energy level alignment. • The TiO2–N based interlayer results in the PCE enhancement of PM6:BTP-4Cl type OSC from 13.68% to 15.90%.

Novel Oligomer Enables Green Solvent Processed 17.5% Ternary Organic Solar Cells: Synergistic Energy Loss Reduction and Morphology Fine-Tuning.

The large non-radiative recombination is the main factor that limits state-of-the-art organic solar cells (OSCs). In this work, two novel structurally similar oligomers (named 5BDTBDD and 5BDDBDT) with D-A-D-A-D and A-D-A-D-A configuration are synthesized for high-performance ternary OSCs with low energy loss. As third components, these PM6 analogue oligomers effectively suppress the non-radiative recombination in OSCs. Although the highest occupied molecular orbital (HOMO) levels of 5BDTBDD and 5BDDBDT are higher than that of PM6, the oligomers enabled ultra-high electroluminescence quantum efficiency (EQEEL ) of 0.05% and improved VOC , indicating suppressing non-radiative recombination overweighs the common belief of deeper HOMO requirement in third component selection. Moreover, the different compatibility of 5BDTBDD and 5BDDBDT with PM6 and BTP-BO4Cl fine-tunes the active layer morphology with synergistic effects. The ternary devices based on PM6:5BDTBDD:BTPBO4Cl and PM6:5BDDBDT:BTP-BO4Cl achieve a significantly improved PCEs of 17.54% and 17.32%, representing the state-of-the art OSCs processed by green solvent of o-xylene. The strategy using novel oligomer as third component also has very wide composition tolerance in ternary OSCs. This is the first work that demonstrates novel structurally compatible D-A type oligomers are effective third components, and provides new understanding of synergetic energy loss mechanisms towards high performance OSCs.

Exploiting Novel Unfused-Ring Acceptor for Efficient Organic Solar Cells with Record Open-Circuit Voltage and Fill Factor.

Unfused-ring acceptors (UFAs) show bright application prospects in organic solar cells (OSCs) thanks to their easy synthesis, low cost, and good device performance. The selection of central-core building block and suitable side chain are the key factors to achieve high-performance UFAs. Current tremendous endeavors for the development of UFAs mainly concentrate on obtaining higher short-circuit current density (Jsc ), albeit accompanied by low open-circuit voltage (Voc ) and modest fill factor (FF). Herein, two novel A-D-A'-D-A type UFAs (BTCD-IC and BTCD-2FIC), which have the same new electron-withdrawing central-core dithieno[3',2':3,4;2'',3'':5,6]-benzo[1,2-c][1,2,5]thiadia-zole (DTBT) and cyclopentadithiophene unit (CPDT, substituted by 2-butyl-1-octyl alkyl chain) coupling with different terminals, were designed and synthesized. Two UFAs showed strong and broad light absorption in the wavelength range of 300-850 nm owing to the strong intramolecular charge transfer effect favorable by DTBT core. Compared with BTCD-IC, BTCD-2FIC with F-containing terminal group exhibited higher molar extinction coefficient, lower energy level, higher charge mobility, stronger crystallinity, more ordered molecular stacking, and better film morphology. As a result, when blended with donor polymer PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']-dithiophene-4,8-dione)]), the BTCD-2FIC-based OSC achieved a superior power conversion efficiency (PCE) of 11.32 %, with a high Voc of 0.85 V, a Jsc of 18.24 mA cm-2 , and a FF of 73 %, than BTCD-IC-based OSC (PCE=8.96 %). Impressively, the simultaneously enhanced Voc and FF values of the PBDB-T:BTCD-2FIC device were the highest values of the A-D-A'-D-A-type UFAs. The results demonstrate the application of electron-withdrawing DTBT central-core unit in efficient UFAs provides meaningful molecular design guidance for high-performance OSCs.

Nanometer-scaled landscape of polymer: fullerene blends mapped with visible s-SNOM

Bulk heterojunction is one key concept leading to breakthrough in organic photovoltaics. The active layer is expectantly formed of distinct morphologies that carry out their respective roles in photovoltaic performance. The morphology-performance relationship however remains stymied, because unequivocal morphology at the nanoscale is not available. We used scattering-type scanning near-field optical microscopy operating with a visible light source (visible s-SNOM) to disclose the nanomorphology of P3HT:PCBM and pBCN:PCBM blends. Donor and acceptor domain as well as intermixed phase were identified and their intertwined distributions were mapped. We proposed energy landscapes of the BHJ active layer to shed light on the roles played by these morphologies in charge separation, transport and recombination. This study shows that visible s-SNOM is capable of profiling the morphological backdrop pertaining to the operation of high performance organic solar cells.

Volatile Solid Additive-Assisted Sequential Deposition Enables 18.42% Efficiency in Organic Solar Cells.

Morphology optimization of active layer plays a critical role in improving the performance of organic solar cells (OSCs). In this work, a volatile solid additive‐assisted sequential deposition (SD) strategy is reported to regulate the molecular order and phase separation in solid state. The OSC adopts polymer donor D18‐Cl and acceptor N3 as active layer, as well as 1,4‐diiodobenzene (DIB) as volatile additive. Compared to the D18‐Cl:N3 (one‐time deposition of mixture) and D18‐Cl/N3 (SD) platforms, the D18‐Cl/N3(DIB) device based on DIB‐assisted SD method exhibits a finer phase separation with greatly enhanced molecular crystallinity. The optimal morphology delivers superior charge transport and extraction, offering a champion power conversion efficiency of 18.42% with significantly enhanced short‐circuit current density (J sc) of 27.18 mA cm−2 and fill factor of 78.8%. This is one of the best performances in binary SD OSCs to date. Angle‐dependent grazing‐incidence wide‐angle X‐ray scattering technique effectively reveals the vertical phase separation and molecular crystallinity of the active layer. This work demonstrates the combination of volatile solid additive and sequential deposition is an effective method to develop high‐performance OSCs.

1-Chloronaphthalene-Induced Donor/Acceptor Vertical Distribution and Carrier Dynamics Changes in Nonfullerene Organic Solar Cells and the Governed Mechanism.

Electron donors and acceptors in organic solar cells (OSCs) shall strike a favorable vertical phase separation that acceptors and donors have sufficient contact and gradient accumulation near the cathodes and anodes, respectively. Random mixing of donors/acceptors at surface will result in charge accumulation and severe recombination for low carrier-mobility organic materials. However, it is challenging to tune the vertical distribution in bulk-heterojunction films as they are usually made from a well-mixed donor/acceptor solution. Here, for the first time, it presents with solid evidence that the commonly used 1-chloronaphthalene (CN) additive can tune the donor/acceptor vertical distribution and establish the mechanism. Different from the previous understanding that ascribed the efficiency enhancement brought by CN to the improved molecular stacking/crystallization, it is revealed that the induced vertical distribution is the dominant factor leading to the significantly increased performance. Importantly, the vertical distribution tunability is effective in various hot nonfullerene OSC systems and creates more channels for the collection of dissociated carriers at corresponding organic/electrode interfaces, which contributes the high efficiency of 18.29%. This study of the material vertical distribution and its correlation with molecular stacking offers methods for additives selection and provides insights for the understanding and construction of high-performance OSCs.

Theoretical Study of Non-Fullerene Acceptors Using End-Capped Groups with Different Electron-Withdrawing Abilities toward Efficient Organic Solar Cells.

Acceptors in organic solar cells (OSCs) are of paramount importance. On the basis of the well-known non-fullerene acceptor Y6, six acceptors (Y6-COH, Y6-COOH, Y6-CN, Y6-SO2H, Y6-CF3, and Y6-NO2) were designed by end-capped manipulation. The effects of end-capped engineering on electronic properties, optical properties, and interfacial charge-transfer states were systematically studied by density functional theory, time-dependent density functional theory, and molecular dynamics. The designed acceptors possess suitable energy levels and improved optical properties. More importantly, the electron mobility of the new acceptors was greatly enhanced, even more than 20 times that of the parent molecule. Among them, Y6-NO2 with the lowest-lying frontier molecular orbitals and the largest red-shifted absorption was selected to construct interfaces with the donor PM6. PM6/Y6-NO2 exhibits stronger interfacial interactions and enhanced charge-transfer characteristics compared with PM6/Y6. This work not only enhances the understanding of the structure-property relationship for acceptors but also offers a set of promising acceptors for high-performance OSCs.

Amplifying the photovoltaic properties of azaBODIPY core based small molecules by terminal acceptors modification for high performance organic solar cells: A DFT approach

Molecular engineering of azaBODIPY based molecule was done to construct four novel small photoactive materials (SM1-SM4) by varying terminal thiophene spaced acceptors for theoretical investigation of photovoltaic properties. In the present study, DFT and TD-DFT methods using B3LYP functionalwith a basis set of 6-31G were employed to systematically evaluate wavelength, determine parameters (absorption values, first excitonic energy, LHE), charge transfer elements (fill factor FF, open circuit voltage Voc), reactivity descriptors (IP and EA) and electronic processes (FMO, DOS, TDM) of modelled molecules (SM1-SM4). All studied molecules showed intense optical absorption in the visible region (710–751 nm), strong light harvesting capability, clearly assigned HOMO-LUMO placement, and lessened exciton binding energies due to highly electron withdrawing acceptor moieties. Greater dipole moment in solution phase than in the gas phase as compared to gaseous medium indicated that these small molecules possessed good solubility in Benzonitrile solvent.The internal reorganization energy values of novel molecules suggested their reliability as efficient hole transport materials. Additionally, results interpreted through analysis of values of Voc (with PTB7-Th), and FF, anticipated an escalation of power conversion efficiency. Among all considered molecules, SM2 and SM3 emerged as promising photovoltaic materials owing to their significant optical properties, reduced band gaps, rapid exciton dissociation, and high estimated PCE. Conclusively, these supportive outcomes favor the structural amendment of small molecules through effective terminal acceptor entities which could enhance the efficiency of photovoltaic materials.

Design of Near-Infrared Nonfullerene Acceptor with Ultralow Nonradiative Voltage Loss for High-Performance Semitransparent Ternary Organic Solar Cells.

Semitransparent organic solar cells (ST-OSCs) are considered as one of the most valuable applications of OSCs and a strong contender in the market. However, the optical band gap of current high-performance ST-OSCs is still not low enough to achieve the optimal balance between power conversion efficiency (PCE) and average visible transmittance (AVT). An N-substituted asymmetric nonfullerene acceptor SN with over 40 nm bathochromically shifted absorption compared to Y6 was designed and synthesized, based on which the device with PM6 as donor obtained a PCE of 14.3 %, accompanied with a nonradiative voltage loss as low as 0.15 eV. Meanwhile, ternary devices with the addition of SN into PM6 : Y6 can achieve a PCE of 17.5 % with an unchanged open-circuit voltage and improved short-circuit current. Benefiting from extended NIR absorption and lowered voltage loss, ST-OSCs based on PM6 : SN : Y6 were fabricated and the optimized device demonstrated a PCE of 14.0 % at an AVT of 20.2 %, which is the highest PCE at an AVT over 20 %.

Rationalizing charge carrier transport in ternary organic solar cells

Ternary bulk heterojunction (BHJ) organic solar cells have energy offsets between multiple donors and acceptors. In such bi-continuous percolating films, electron carriers mainly transport in acceptor materials, and hole carriers typically transport in donor materials. Changing the third component of additional donors or acceptors is a common method to fine-tune the transport properties in ternary BHJs. Experimentally, although there are some empirical guidelines for the mobility evaluation, a clear charge transporting model has still not been fully established in multi-component BHJ films. Herein, we observed a general regularity about charge transport that the charge carriers have higher possibilities to transport in polymeric materials even if only low dosage (<10% in weight) polymeric materials are introduced for both the hole and electron cases. From the extended Su–Schrieffer–Heeger model, a polymer “bridge” assisted charge transport mode in small molecules is proposed after the energy offset exceeds the threshold (ΔEe > 0.15 eV). This work provides a perspective for fine tuning the charge transport property in high-performance ternary organic solar cells.

Achieving high-performance ternary organic solar cells by adding a high hole-mobility non-fullerene acceptor

The addition of a functional third component in PM6:Y6 system has demonstrated great potential in achieving highly power conversion efficiency (PCE) for organic solar cells (OSCs). However, photocurrent generation is still limited by slow hole transfer from Y6 to PM6. Herein, a non-fullerene acceptor BTP-eC9, with high hole-mobility property, is incorporated into the PM6:Y6 blend to construct a ternary system. The good compatibility of acceptors with similar chemical structure is conducive to simultaneously fine-tune the morphology, energy level and spectra absorption. Moreover, the introduction of BTP-eC9 endows the active layer with high hole mobility and balanced electron and hole transfer, further achieving the increased short-circuit current density (JSC) and fill factor (FF). The optimized ternary OSCs give an improved PCE of 16.39%, which makes a significant enhancement compared to binary devices based on PM6:Y6 and PM6:BTP-eC9 (14.87% and 13.27%, respectively).

Insights into end‐capped modifications effect on the photovoltaic and optoelectronic properties of S‐shaped fullerene‐free acceptor molecules: A density functional theory computational study for organic solar cells

AbstractNon‐fused ring electron acceptors with different end‐capped substituents were designed to construct efficient organic solar cells (OSCs). End‐capped modification of acceptor materials is an efficient approach for developing high performance OSCs. In this report, five new S‐shaped fullerene‐free acceptor molecules (3P1 to 3P5) have been designed and examined through density function and time‐dependent density functional theories (TD‐DFTs). Designed molecules have twisted backbone with different out‐of‐plane rings that cause high solubility in dilute solvent. Different optoelectronic and photovoltaic properties of newly designed S‐shape molecules have been examined and explored through DFT tools. Orientation of frontier molecular orbitals (FMOs), light absorption range, exciton binding energy (Eb), oscillating strength (f), open circuit voltage (Voc), and transition density matrix (TDM) analysis have been performed for 3P1 to 3P5 molecules. Further, electron and hole reorganization energies are also estimated with excitation energy. Results of different computed parameters have recommended that these designed molecules are effective contributors for possible applications in the field of OSCs.

Exploring the effect of end-capped modifications of carbazole-based fullerene-free acceptor molecules for high-performance indoor organic solar cell applications

Exploring the effect of end-capped modifications of carbazole-based fullerene-free acceptor molecules for high-performance indoor organic solar cell applications

Revival of Insulating Polyethylenimine by Creatively Carbonizing with Perylene into Highly Crystallized Carbon Dots as the Cathode Interlayer for High-Performance Organic Solar Cells.

The development of new electron transporting layer (ETL) materials to improve the charge carrier extraction and collection ability between cathode and the active layer has been demonstrated to be an effective approach to enhance the photovoltaic performance of organic solar cells (OSCs). Herein, water-soluble carbon dots (CDs) as ETL material have been creatively synthesized by a vigorous chemical reaction between polyethylenimine (PEI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) via a simple one-step hydrothermal method. Taking full advantage of the high electron transfer property of PTCDA and the work function (WF) reduction ability of PEI, CD gained high electron mobility due to its large π-conjugated area and reduced the WF of indium tin oxide (ITO) by 0.75 eV. As for the photovoltaic performance of devices, inverted OSCs based on CDs have achieved a high power conversion efficiency (PCE) of 17.35%, exhibiting no burn-in effect with no reduction in PCE after more than 4000 h of storage. The successful application of CDs in OPV has developed a new avenue for designing efficient ETL materials that benefits the photovoltaic performance of OSCs.

High-performance and low-cost organic solar cells based on pentacyclic A–DA′D–A acceptors with efficiency over 16%

Developing high-performance and low-cost donor/acceptor materials is crucial for the industrialization of organic solar cells (OSCs).

High-performance pseudo-bilayer ternary organic solar cells with PC71BM as the third component

PM6/Y6:PC71BM pseudo-bilayer ternary OSCs were prepared. PC71BM enhances light absorption and boosts VOC, and improves the contact between the donor layer and acceptor layer, which improves photovoltaic performance and device stability significantly.

Regioselectivity control of block copolymers for high-performance single-material organic solar cells

Narrow bandgap (NBG) block copolymers are promising materials to realize single-material organic solar cells (SMOSCs) that combine high performance with minimized fabrication procedures.

New wide-bandgap D–A polymer based on pyrrolo[3,4-b] dithieno[2,3-f:3′,2′-h]quinoxalindione and thiazole functionalized benzo[1,2-b:4,5-b′]dithiophene units for high-performance ternary organic solar cells with over 16% efficiency

We have achieved a power conversion efficiency of 16.44% for the ternary polymer solar cell using a wide bandgap copolymer and two non-fullerene acceptors.

Multi-site functional cathode interlayers for high-performance binary organic solar cells

Bis(2-hydroxyethyl) amino derivatives are used as multi-site functional interlayers, whose chemical stability with non-fullerene acceptors is clarified, affording a remarkable efficiency of 18.50%, among the highest in binary organic photovoltaics.