Small molecules based on benzo[1,2-b:4,5-b']dithiophene unit for high-performance solution-processed organic solar cells

Computational method on highly efficient D-π-A-π-D-based different molecular acceptors for organic solar cells applications and non-linear optical behaviour

Eight molecular structures (BT-A1 to BT-A8) with high-performance non-fullerene acceptor (NFA) were selected for organic solar cells (OSCs) and non-linear optical (NLO) applications. Their electronic, photovoltaic (PV) and optoelectronic properties were tuned by adding powerful electron-withdrawing groups to the acceptor (A) of the D-π-A-π-D structure. Using time-dependent density functional theory (TD-DFT) techniques, based on the laws of quantum chemical calculations, the absorption spectra, stability of the highest and lowest-energy molecular orbitals (HOMO/LUMOs), electron density, intramolecular charge transfer (ICT), transition density matrix (TDM), were examined. The binding energy (Eb) and density of states (DOS) were probed to realize the optoelectronic analysis of the structures BT-A1 to BT-A8. Noncovalent interactions (NCIs) based on a reduced density gradient (RDG) were used to describe the nature and strength of D-A interactions in the molecules BT-A1 to BT-A8. The new refined molecules BT-A1 to BT-A8 exhibited strong absorbance bands between 408-721 nm and high electron transfer contribution (ETC) ranges between 87-96 %, along with the smallest excitation energies (Ex) between 1.71-3.55 eV in the solvent dichloromethane. Dipolar moment strengths ranging from 0.38 to 4.72 Debye in both the excited and ground states have determined with good solubility properties of BT-A1 to BT-A8 in polar solvent. Highly effective charge mobilities and prevention of charge recombination have been demonstrated by the electron (0.18-0.41 eV) and hole RE values (0.13-0.89 eV) for the new compounds. Power conversion efficiencies (PCE) of BT-A1 to BT-A8 were nearly the same because of better outcomes compared to the molecules in the BT. Compared to poly[4.8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b']dithiophene-2,6- diyl-alt-(4-2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th), the open circuit voltages (Voc) of compounds BT-A1 to BT-A8 were ranged from 1.52 to 2.13 eV. The polarizability (α) and hyperpolarizability (β) of the molecules BT-A1 to BT-A8 were used to determine the non-linear optical (NLO) properties. The results showed that BT-A2, BT-A6 and BT-A7 have good NLO activity. This computational analysis demonstrates the superiority of the molecules with NFA. Hence the compounds are advised for the use in production of high-performance OSCs and NLO activity.

Ethylenedioxythiophene-Based Small Molecular Donor with Multiple Conformation Locks for Organic Solar Cells with Efficiency of 19.3

Ternary organic solar cells (T-OSCs) represent an efficient strategy for enhancing the performance of OSCs. Presently, the majority of high-performance T-OSCs incorporates well-established Y-acceptors or donor polymers as the third component. In this study, a novel class of conjugated small molecules has been introduced as the third component, demonstrating exceptional photovoltaic performance in T-OSCs. This innovative molecule comprises ethylenedioxythiophene (EDOT) bridge and 3-ethylrhodanine as the end group, with the EDOT unit facilitating the creation of multiple conformation locks. Consequently, the EDOT-based molecule exhibits two-dimensional charge transport, distinguishing it from the thiophene-bridged small molecule, which displays fewer conformation locks and provides one-dimensional charge transport. Furthermore, the robust electron-donating nature of EDOT imparts the small molecule with cascade energy levels relative to the electron donor and acceptor. As a result, OSCs incorporating the EDOT-based small molecule as the third component demonstrate enhanced mobilities, yielding a remarkable efficiency of 19.3 %, surpassing the efficiency of 18.7 % observed for OSCs incorporating thiophene-based small molecule as the third component. The investigations in this study underscore the excellence of EDOT as a building block for constructing conjugated materials with multiple conformation locks and high charge carrier mobilities, thereby contributing to elevated photovoltaic performance in OSCs.

Design of Isoindigo-Based Small-Molecule Donors for Bulk Heterojunction Organic Solar Cell Applications in Combination with Nonfullerene Acceptors

The development of small-molecule organic solar cells with the required efficiency depends on the information obtained from molecular-level studies. In this context, 39 small-molecule donors featuring isoindigo as an acceptor moiety have been meticulously crafted for potential applications in bulk heterojunction organic solar cells. These molecules follow the D2-A-D1-A-D2 and D2-A-π-D1-π-A-D2 framework. Similar molecules considered in the previous experimental study (molecules R1 ((3E,3″E)-6,6″-(benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione)) and R2 ((3E,3″E)-6,6″-(4,8-dimethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione))) have been chosen as reference molecules. Molecules with and without π-spacers have been considered to understand the impact of the length of the π-spacer on intramolecular charge-transfer transitions and absorption properties. A detailed investigation is carried out to establish the relationship between the structure and photovoltaic parameters using density functional theory and time-dependent density functional theory methods. The newly developed molecules exhibit better electronic, excited-state, and charge transport properties than the reference molecules. Additionally, model donor-acceptor interfaces are constructed by integrating the designed donor molecules with fullerene/nonfullerene acceptors. The electronic and excited-state properties of these interfaces are rigorously evaluated. Results elucidate that the donor comprising of isoindigo-bithiophene-pyrroloindacenodithiophene (IIG-T2-PIDT) emerges as a promising candidate for bulk heterojunction solar cells based on nonfullerene acceptors. This research provides systematic design strategies for the development of small-molecule donors for organic solar cells.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Designing of banana shaped chromophores via molecular engineering of terminal groups to probe photovoltaic behavior of organic solar cell materials

To meet the rising requirement of photovoltaic compounds for modernized hi-tech purpose, we designed six new molecules (DTPD1-DTPD6) from banana shaped small fullerene free chromophore (DTPR) by structural tailoring at terminal acceptors. Frontier molecular orbitals (FMOs), density of states (DOS), open circuit voltage (Voc), transition density matrix (TDM) analysis, optical properties, reorganization energy value of hole and electron were determined utilizing density function theory (DFT) and time-dependent density function theory (TD-DFT) approaches, to analyze photovoltaic properties of said compounds. Band gap contraction (∆E = 2.717-2.167 eV) accompanied by larger bathochromic shift (λmax = 585.490-709.693 nm) was observed in derivatives contrary to DTPR. The FMOs, DOS and TDMs investigations explored that central acceptor moiety played significant role for charge transformation. The minimum binding energy values for DTPD1-DTPD6 demonstrated the higher exciton dissociation rate with greater charge transferal rate than DTPR, which was further endorsed by TDM and DOS analyses. A comparable Voc (1.49-2.535 V) with respect to the HOMOPBDBT-LUMOacceptor for entitled compounds was investigated. In a nutshell, all the tailored chromophores can be considered as highly efficient compounds for promising OSCs with a good Voc response.

Quantum Modification of Indacenodithieno[3,2-b]thiophene-Based Non-fullerene Acceptor Molecules for Organic Solar Cells of High Efficiency

In order to enhance the efficacy of organic solar cells, six new three-dimensional small donor molecules (IT-SM1 to IT-SM6) have been computationally designed by modifying the peripheral acceptors of the reference molecule (IT-SMR). The frontier molecular orbitals revealed that IT-SM2 to IT-SM5 had a smaller band gap (E_gap) than IT-SMR. They also had smaller excitation energies (E_x) and exhibited a bathochromic shift in their absorption maxima (λ_max) when compared to IT-SMR. In both the gas and chloroform phases, IT-SM2 had the largest dipole moment. IT-SM2 also had the best electron mobility, while IT-SM6 had the best hole mobility owing to their smallest reorganization energy for electron (0.1127 eV) and hole (0.0907 eV) mobility, respectively. The analyzed donor molecules' open-circuit voltage (V_OC) indicated that all of these proposed molecules had greater V_OC and fill factor (FF) values than the IT-SMR molecule. In accordance with the evidence of this work, the altered molecules can seem to be quite proficient for usage by experimentalists and have prospective use in future in the manufacture of organic solar cells with improved photovoltaic properties.

Progress and Future Potential of All-Small-Molecule Organic Solar Cells Based on the Benzodithiophene Donor Material

Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future.

Quantum modeling of dimethoxyl-indaceno dithiophene based acceptors for the development of semiconducting acceptors with outstanding photovoltaic potential

In the current DFT study, seven dimethoxyl-indaceno dithiophene based semiconducting acceptor molecules (ID1-ID7) are designed computationally by modifying the parent molecule (IDR). Here, based on a DFT exploration at a carefully selected level of theory, we have compiled a list of the optoelectronic properties of ID1-ID7 and IDR. In light of these results, all newly designed molecules, except ID5 have shown a bathochromic shift in their highest absorbance (λ _max). ID1-ID4, ID6 and ID7 molecules have smaller band gap (E _gap) and excitation energy (E _x). IP of ID5 is the smallest and EA of ID1 is the largest among all others. Compared to the parent molecule, ID1-ID3 have increased electron mobility, with ID1 being the most improved in hole mobility. ID4 had the best light harvesting efficiency in this investigation, due to its strongest oscillator. The acceptor molecules' open-circuit voltages (V _OC) were computed after being linked to the PTB7-Th donor molecule. Fill factor (FF) and normalized V _OC of ID1-ID7 were calculated and compared to the parent molecule. Based on the outcomes of this study, the modified acceptors may be further scrutinised for empirical usage in the production of organic solar cells with enhanced photovoltaic capabilities.

End-Group Engineering of Chlorine-Trialkylsiylthienyl Chain-Substituted Small-Molecule Donors for High-Efficiency Ternary Solar Cells

Ternary architecture has been widely demonstrated as a facile and efficient strategy to boost the performance of organic solar cells (OSCs). However, the rational design of the third component with suitable core and end-group modification is still a challenge. Herein, two new small-molecule (SM) donors BT-CN and BT-ER, featuring the identical conjugated backbone with distinct end group, have been designed, synthesized, and introduced into the PM6:Y6 binary system as the second donor. Both molecules exhibit complementary absorption and good miscibility with PM6, contributing to the nanofibrous phases and strong face-on molecular packing. Importantly, the incorporation of BT-CN/BT-ER has significantly facilitated charge collection and transportation with remarkable suppression of carrier recombination. As a result, ternary OSCs with 20 wt% BT-CN/BT-ER achieved a PCE of 16.8%/17.22% with synchronously increased open-circuit voltage (V_OC ), short-circuit current density (J_SC ) and fill factor (FF). Moreover, replacing Y6 with L8-BO further improves the PCE to 18.05%/18.11%, indicating the universality of both molecules as the third component. This work demonstrates not only two efficient SM donors with 4,8-bis(4-chloro-5-(tripropylsilyl)thiophen-2-yl) benzo[1,2-b:4,5-b']dithiophene (BDTT-SiCl) as the core but also end group modification strategy to fine-tune the absorption spectrum, molecular packing, and energy levels of SM donors to construct high-performance ternary OSCs.

Synthesis, characterization and application of a novel polyazo dye as a universal acid-base indicator

A novel organic polyazo dye is synthesized by the diazotization of aromatic aniline, followed by coupling it with sulfanilic acid and N,N-dimethylaniline. Characterization was done by ¹H-NMR, ¹³C-NMR, and FTIR spectroscopy. Differential scanning calorimetry (DSC) reveals that phase transition for this molecule is exothermic. The optical band gap is estimated from the absorption cutoff point using UV-Visible spectroscopy. Thermal gravimetric analysis (TGA) addresses the thermal stability of the molecule and is found to be at ∼250 °C. The structure of the synthesized molecule is analogous to that of methyl orange and contains three azo groups. These three azo groups help accept more than two protons and provide two pK _a values when diprotic acid or a mixture of acids is used in different titrations. Specifically, when a polybasic acid is in strong base titration, the pK _a values were found to be 3.5 and 9.1. Moreover, for strong base and (strong + weak) acid mixture titration, the pK _a values are found to be 9.2 and 3.3. Furthermore, the pK _a values are found to be 8.6 and 2.8 for (strong and weak) base mixture and (strong and weak) acid mixture titration, respectively. Owing to its increased proton accepting capacity, it can be found in the two pH ranges of 2.1-3.8 for orange color and 8.2-9.8 for yellow color, thus indicating a unique property as a universal indicator for acid-base titration. The dissociation constant of this dye is found to be 3.4 × 10^-6, determined in a mixed aqueous solution of 10 wt% ethanol, and a linear relationship between pK _a and pH is observed in this solvent system.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Effects of Side-Chain Engineering with the S Atom in Thieno[3,2-b]thiophene-porphyrin to Obtain Small-Molecule Donor Materials for Organic Solar Cells

To explore the effect of the introduction of heteroatoms on the properties of porphyrin materials, a new porphyrin-based derivative small-molecule donor named as PorTT-T was designed and synthesized based on alkyl-thieno[3,2-b]thiophene(TT)-substituted porphyrins. The linker bridge and end groups of PorTT-T were the same as those of XLP-II small-molecule donor materials, while the side-chain attached to the core of thieno[3,2-b]thiophene(TT)-substituted porphyrin was different. Measurements of intrinsic properties showed that PorTT-T has wide absorption and appropriate energy levels in the UV-visible range. A comparison of the morphologies of the two materials using atomic force microscopy showed that PorTT-T has a better surface morphology with a smaller root-mean-square roughness, and can present closer intermolecular stacking as compared to XLP-II. The device characterization results showed that PorTT-T with the introduced S atom has a higher open circuit voltage of 0.886 eV, a higher short circuit current of 12.03 mAcm⁻², a fill factor of 0.499, a high photovoltaic conversion efficiency of 5.32%, better external quantum efficiency in the UV-visible range, and higher hole mobility.

An Electron Acceptor Analogue for Lowering Trap Density in Organic Solar Cells

Typical organic semiconductor materials exhibit a high trap density of states, ranging from 10¹⁶ to 10¹⁸  cm^-3 , which is one of the important factors in limiting the improvement of power conversion efficiencies (PCEs) of organic solar cells (OSCs). In order to reduce the trap density within OSCs, a new strategy to design and synthesize an electron acceptor analogue, BTPR, is developed, which is introduced into OSCs as a third component to enhance the molecular packing order of electron acceptor with and without blending a polymer donor. Finally, the as-cast ternary OSC devices employing BTPR show a notable PCE of 17.8%, with a low trap density (10¹⁵  cm^-3 ) and a low energy loss (0.217 eV) caused by non-radiative recombination. This PCE is among the highest values for single-junction OSCs. The trap density of OSCs with the BTPR additives, as low as 10¹⁵  cm^-3 , is comparable to and even lower than those of several typical high-performance inorganic/hybrid counterparts, like 10¹⁶  cm^-3 for amorphous silicon, 10¹⁶  cm^-3 for metal oxides, and 10¹⁴ to 10¹⁵  cm^-3 for halide perovskite thin film, and makes it promising for OSCs to obtain a PCE of up to 20%.

15.3% Efficiency All-Small-Molecule Organic Solar Cells Achieved by a Locally Asymmetric F, Cl Disubstitution Strategy

Single junction binary all-small-molecule (ASM) organic solar cells (OSCs) with power conversion efficiency (PCE) beyond 14% are achieved by using non-fullerene acceptor Y6 as the electron acceptor, but still lag behind that of polymer OSCs. Herein, an asymmetric Y6-like acceptor, BTP-FCl-FCl, is designed and synthesized to match the recently reported high performance small molecule donor BTR-Cl, and a record efficiency of 15.3% for single-junction binary ASM OSCs is achieved. BTP-FCl-FCl features a F,Cl disubstitution on the same end group affording locally asymmetric structures, and so has a lower total dipole moment, larger average electronic static potential, and lower distribution disorder than those of the globally asymmetric isomer BTP-2F-2Cl, resulting in improved charge generation and extraction. In addition, BTP-FCl-FCl based active layer presents more favorable domain size and finer phase separation contributing to the faster charge extraction, longer charge carrier lifetime, and much lower recombination rate. Therefore, compared with BTP-2F-2Cl, BTP-FCl-FCl based devices provide better performance with FF enhanced from 71.41% to 75.36% and J sc increased from 22.35 to 24.58 mA cm-2, leading to a higher PCE of 15.3%. The locally asymmetric F, Cl disubstitution on the same end group is a new strategy to achieve high performance ASM OSCs.

End-capped group manipulation of non-fullerene acceptors for efficient organic photovoltaic solar cells: a DFT study

A series of acceptors, S1-S5, has been designed based on the acceptor-π-donor-π-acceptor (A-π-D-π-A) architecture by incorporating a phenothiazine unit as the central donor unit. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods have been employed to study the effect of various end-capped groups on the geometric, electronic, optical and charge transport properties of the designed acceptor molecules. The results reveal that on increasing the electron-withdrawing nature of the end-capped groups, the performance of the acceptor molecules increases. It is also observed that on increasing the flexibility of the end-capped groups, the planarity of the molecules gets destroyed and, as a result, the performance of the acceptor molecules decreases. The investigated molecules exhibit high electron affinity (EA) and low reorganization energy for electrons (λ-), indicating the electron acceptor nature of the designed molecules. The absorption properties of the molecules manifest that compounds S2-S4 possess high values of the maximum wavelength (λmax) of absorption. We have also studied the properties of a D/A active layer by considering PffBT4T-2OD as the electron donor and arranging PffBT4T-2OD/S1-S5 molecules in a face to face manner. Properties of the D/A blend indicate that molecules S2-S4 have capacity to promote charge carrier separation at the D/A active layer. Our results provide guidelines for further designing of acceptors to enhance the performance of organic solar cells (OSCs).

Differently Linked Perylene Bisimide Dimers with Various Twisting and Phase Structures for Nonfullerene All-Small-Molecule Organic Solar Cells

Nonfullerene all-small-molecule organic solar cells (NF all-SMSCs) are an important classification in the organic solar cell system. However, the application and research of NF all-SMSCs are limited due to the easy aggregation of small molecules to form large-phase domains. Perylene bisimides (PBIs) have been widely used as nonfullerene acceptors. Simply changing the link position of the PBI dimer can control the accumulation of molecules to regulate the size of the phase domain. Herein, the bay-linked, ortho-linked, and hydrazine-linked PBI dimers as nonfullerene acceptors, named as B-SdiPBI, O-SdiPBI, and H-SdiPBI, respectively, were chosen. The link position of the PBI dimer can lead to diverse molecular torsion and planarity, which affects the film-forming ability, phase separation, and thus optoelectronic properties. NF all-SMSCs based on B-SdiPBI, O-SdiPBI, and H-SdiPBI as nonfullerene acceptors and a small molecule DR3TBDTT as the donor achieve the best power conversion efficiencies of 1.93, 3.30, and 4.05%, respectively. The result is consistent with the sequence of inter-PBI twist and phase domain size of the corresponding blend films in the device. The DR3TBDTT:H-SdiPBI system has the best efficiency with the largest dihedral angle of H-SdiPBI (ψ = 90°) and an appropriate phase size (10-40 nm), whereas the smaller dihedral angle of O-SdiPBI (ψ = 86°) produces a larger phase size (20-50 nm) and the smallest dihedral angle of B-SdiPBI (ψ = 71°) gives the largest phase size (30-80 nm). This illustrates that the twist angle can effectively increase the phase separation between the acceptor and donor to obtain an effective phase size in this system. The work provides a guide for designing the acceptors and controlling phase domains of high-performance NF all-SMSCs.

Rhodanine-based Knoevenagel reaction and ring-opening polymerization for efficiently constructing multicyclic polymers

Cyclic polymers have a number of unique physical properties compared with those of their linear counterparts. However, the methods for the synthesis of cyclic polymers are very limited, and some multicyclic polymers are still not accessible now. Here, we found that the five-membered cyclic structure and electron withdrawing groups make methylene in rhodanine highly active to aldehyde via highly efficient Knoevenagel reaction. Also, rhodanine can act as an initiator for anionic ring-opening polymerization of thiirane to produce cyclic polythioethers. Therefore, rhodanine can serve as both an initiator for ring-opening polymerization and a monomer in Knoevenagel polymerization. Via rhodanine-based Knoevenagel reaction, we can easily incorporate rhodanine moieties in the backbone, side chain, branched chain, etc, and correspondingly could produce cyclic structures in the backbone, side chain, branched chain, etc, via rhodanine-based anionic ring-opening polymerization. This rhodanine chemistry would provide easy access to a wide variety of complex multicyclic polymers.

Characterization and simulation study of organic solar cells based on donor-acceptor (D-π-A) molecular materials

In this study, the analysis of microelectronic and photonic structure in a one dimension program [AMPS-1D] has been successfully used to study organic solar cells. The program was used to optimize the performance of organic solar cells based on (carbazole-methylthiophene), benzothiadiazole and thiophene [(Cbz-Mth)-B-T]2 as electron donors, and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as an electron acceptor. The optoelectronic properties of these dyes were investigated by using the Density Functional Theory DFT/B3LYP/6-31G(d,p) method. We studied the influence of the variation of the thickness of the active layer, the temperature, and the density of the effective states of the electrons and the holes in the conduction and valence bands respectively on the performance of the solar cells based on [(Cbz-Mth)-BT]2–PCBM as a photoactive material, sandwiched between a transparent indium tin oxide (ITO) and an aluminum (Al) electrode. The addition of other thiophene units in the copolymer or the deposition of a layer of PEDOT between the anode (ITO) and the active layer, improves the performances of the cell, especially resulting in a remarkable increase in the value of the power conversion efficiency (PCE).

The solar cell ITO/PEDOT/[(Cbz-Mth)-B-DT]2-A:PCBM/Al under study and the results obtained, including a power conversion efficiency of 11%. The impact of several parameters on the performance has been studied to obtain the optimal device architecture.

Constructive Quantum Interference in Single-Molecule Benzodichalcogenophene Junctions

Heteroatom substitution into the cores of alternant, aromatic hydrocarbons containing only even-membered rings is attracting increasing interest as a method of tuning their electrical conductance. Here, the effect of heteroatom substitution into molecular cores of non-alternant hydrocarbons, containing odd-membered rings, is examined. Benzodichalcogenophene (BDC) compounds are rigid, planar π-conjugated structures, with molecular cores containing five-membered rings fused to a six-membered aryl ring. To probe the sensitivity or resilience of constructive quantum interference (CQI) in these non-bipartite molecular cores, two C₂ -symmetric molecules (I and II) and one asymmetric molecule (III) were investigated. I (II) contains S (O) heteroatoms in each of the five-membered rings, while III contains an S in one five-membered ring and an O in the other. Differences in their conductances arise primarily from the longer S-C and shorter O-C bond lengths compared with the C-C bond and the associated changes in their resonance integrals. Although the conductance of III is significantly lower than the conductances of the others, CQI was found to be resilient and persist in all molecules.

Design of single-porphyrin donors toward high open-circuit voltage for organic solar cells via an energy level gradient-distribution screening strategy of fragments: a theoretical study

Open-circuit voltage (V_OC) is a key factor for improving the power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic solar cells (OSCs). At present, increasing attention has been devoted towards modifying π bridges in single-porphyrin small molecule donors with an A-π-D-π-A configuration to reduce the highest occupied molecular orbital (HOMO) levels and improve the V_OC of devices. However, how to screen the π bridges is a key issue. In this work, nine π bridges were screened by the HOMO level gradient-distribution strategy of fragments (electron-donating donor (D), π bridges, and electron-withdrawing acceptor (A)), where fragments meeting the requirements were combined into five novel small molecule donors. Meanwhile, in order to test whether the strategy is beneficial to increasing V_OC, [6,6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM) was selected as the acceptor material. The energy levels of all molecules were compared and the photoelectric properties (i.e., energy gap, energy driving force, reorganization energy, intermolecular charge transfer rate, charge recombination rate, and V_OC) of the five small molecules were studied. The results showed that the HOMO levels of porphyrin donors could be significantly lowered via this strategy, and V_OC was raised without losing the short-circuit current (J_SC) and fill factor (FF) of the devices. Meanwhile, the designed five small molecules could be used as donor candidates to improve the performance of OSCs.

Donor Derivative Incorporation: An Effective Strategy toward High Performance All-Small-Molecule Ternary Organic Solar Cells

Thick-film all-small-molecule (ASM) organic solar cells (OSCs) are preferred for large-scale fabrication with printing techniques due to the distinct advantages of monodispersion, easy purification, and negligible batch-to-batch variation. However, ASM OSCs are typically constrained by the morphology aspect to achieve high efficiency and maintain thick film simultaneously. Specifically, synchronously manipulating crystallinity, domain size, and phase segregation to a suitable level are extremely challenging. Herein, a derivative of benzodithiophene terthiophene rhodanine (BTR) (a successful small molecule donor for thick-film OSCs), namely, BTR-OH, is synthesized with similar chemical structure and absorption but less crystallinity relative to BTR, and is employed as a third component to construct BTR:BTR-OH:PC71BM ternary devices. The power conversion efficiency (PCE) of 10.14% and fill factor (FF) of 74.2% are successfully obtained in ≈300 nm OSC, which outperforms BTR:PC71BM (9.05% and 69.6%) and BTR-OH:PC71BM (8.00% and 65.3%) counterparts, and stands among the top values for thick-film ASM OSCs. The performance enhancement results from the enhanced absorption, suppressed bimolecular/trap-assisted recombination, improved charge extraction, optimized domain size, and suitable crystallinity. These findings demonstrate that the donor derivative featuring similar chemical structure but different crystallinity provides a promising third component guideline for high-performance ternary ASM OSCs.

Design rules for high mobility xanthene-based hole transport materials

Tunable and highly conductive hole transport materials are crucial for the performance of organic electronics applications such as organic light emitting diodes and perovskite solar cells. For commercial applications, these materials' requirements include easy synthesis, high hole mobility, and highly tuned and compatible electronic energy levels. Here, we present a systematic study of a recently discovered, easy-to-synthesize class of spiro[fluorene-9,9'-xanthene]-based organic hole transport materials. Systematic side group functionalization allows us to control the HOMO energy and charge carrier mobility. Analysis of the bulk simulations enables us to derive design rules for mobility enhancement. We show that larger functional groups (e.g. methyl) decrease the conformational disorder due to steric effects and thus increase the hole mobility. Highly asymmetric or polar side groups (e.g. fluorine), however, increase the electrostatic disorder and thus reduce the hole mobility. These generally applicable design rules will help in the future to further optimize organic hole transport materials.

Metal-Free Organic Optoelectronic Molecule as a Highly Efficient Photocatalyst for the Degradation of Organic Pollutants

With the increasing consumption of natural resources, photocatalysis converting solar energy to chemical energy has attracted extensive attention of researchers owing to the advantages of developing energy-saving and environmentally benign processes. In this work, a facile and simple method was developed to synthesize a metal-free organic optoelectronic molecule (denoted as DPPRD), which is composed of a central diketopyrrolopyrrole moiety and two terminal units of a rhodanine (RD) moiety. This is a first green strategy toward the synthesis of DPPRD. Because of good thermal stability, narrow band gap, and excellent visible light absorption of solar spectrum, DPPRD exhibited to be an efficient and chemically stable photocatalyst for visible light degradation of organic pollutants such as bisphenol A (BPA) and methyl orange (MO) in aqueous solution. The control experiments with different types of radical scavengers implied that the hole (h⁺) and hydroxyl radicals (^•OH) were the key reactive species during the photodegradation processes. The photodegradation pathways of BPA and MO were thus proposed based on the identified intermediates. This improved method for DPPRD synthesis is expected to widen its applications to industrial production, whereas its excellent visible light photocatalytic activity would be utilized potentially in the field of environmental and industrial applications.

Revisiting the Optimal Nano-Morphology: Towards Amorphous Organic Photovoltaics

Organic photovoltaic cells commonly use an active layer with a polycrystalline bulk heterojunction. However, for simplifying the fabrication process, it may be worthwhile to use an amorphous active layer to lessen the burden on processing to achieve optimal performance. While polymers can adopt amorphous phases, molecular glasses, small molecules that can readily form glassy phases and do not crystallize over time, offer an appealing alternative, being monodisperse species. Our group has developed a series of reactive molecular glasses that can be covalently bonded to chromophores to form glass-forming adducts, and this strategy has been used to synthesize glass-forming donor and acceptor materials. Herein, the results of devices incorporating these materials in either partially or fully amorphous active layers are summarized. Additionally, these molecular glasses can be used as ternary components in crystalline systems to enhance efficiency without perturbing the morphology.

High-Crystallinity π-Conjugated Small Molecules Based on Thienylene-Vinylene-Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties

Narrow-band-gap small molecules with π-extended backbones are promising donor materials for solution-processed bulk-heterojunction (BHJ) organic solar cells (OSCs). Herein, a series of acceptor-donor-acceptor (A-D-A) photovoltaic small molecules incorporating thienylene-vinylene-thienylene (TVT) as a central D unit and alkyl-substituted rhodanine or 2-(1,1-dicyanomethylene)rhodanine as terminal A units are designed and synthesized. Their physical properties including photoabsorption, electronic energy levels, hole mobility, and morphological characteristics are systematically investigated. Using solvent vapor annealing (SVA), the morphologies of the BHJ photoactive layers composed of these small-molecule donors and a [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor can be properly modulated. As a result of increased crystallinity of the donors and desired phase segregation between the donors and PC₇₁BM upon rapid SVA treatment, the photovoltaic performances of the resultant OSC devices undergo drastic enhancement. The results reported here indicate that high-efficiency small-molecule OSCs can be achieved through rational design of the TVT-based molecular framework and optimization of the nanoscale phase-segregated morphology via proper SVA treatment.

Impact of Terminal End-Group of Acceptor-Donor-Acceptor-type Small Molecules on Molecular Packing and Photovoltaic Properties

In this study, we synthesized two acceptor-donor-acceptor (A-D-A)-type small molecules (SMs) (P3T4-VCN and P3T4-INCN) with different terminal end-groups (dicyanovinyl (VCN) and 2-methylene-3-(1,1-dicyanomethylene)indanone (INCN)) based on the 1,4-bis(thiophenylphenylthiophene)-2,5-difluorophenylene (P3T4) core that possesses high coplanarity because of intrachain noncovalent Coulombic interactions. We investigated the influence of terminal end-groups on intermolecular packing and the resulting electrical and photovoltaic characteristics. A small change in the end-group structure of the SMs induces a significant variation in the torsional structures, molecular packing, and pristine/blend film morphology. It is noteworthy that the less crystalline P3T4-INCN with tilted conformation is highly sensitive to post-treatments (i.e., additives and annealing) such that it permits facile morphological modulation. However, the highly planar and crystalline P3T4-VCN exhibits a strong tolerance toward processing treatments. After morphology optimization, the fullerene-based bulk-heterojunction solar cell of tilted P3T4-INCN exhibits a power conversion efficiency (PCE) of 5.68%, which is significantly superior to that of P3T4-VCN:PC₇₁BM (PCE = 1.29%). Our results demonstrate the importance of the terminal end-group for the design of A-D-A-type SMs and their sensitivity toward the postprocessing treatments in optimizing their performance.

Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV-vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.