Organic Photovoltaic Materials Research Articles

Near-Infrared Materials: The Turning Point of Organic Photovoltaics.

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by ourgroup, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Effect of Quencher, Geometry, and Light Outcoupling on the Determination of Exciton Diffusion Length in Nonfullerene Acceptors

The correct determination of the exciton diffusion length (LD) in novel organic photovoltaics (OPV) materials is an important, albeit challenging, task required to understand these systems. Herein, a high‐throughput approach to probe LD in nonfullerene acceptors (NFAs) is reported, that builds upon the conventional photoluminescence (PL) surface quenching method using NFA layers with a graded thickness variation in combination with spectroscopic PL mapping. The method is explored for two archetypal NFAs, namely, ITIC and IT‐4F, using PEDOT:PSS and the donor polymer PM6 as two distinct and practically relevant quencher materials. Interestingly, conventional analysis of quenching efficiency as a function of acceptor layer thickness results in a threefold difference in LD values depending on the specific quencher. This discrepancy can be reconciled by accounting for the differences in light in‐ and outcoupling efficiency for different multilayer architectures. In particular, it is shown that the analysis of glass/acceptor/PM6 structures results in a major overestimation of LD, whereas glass/acceptor/PEDOT:PSS structures give no significant contribution to outcoupling, yielding LD values of 6−12 and 8−18 nm for ITIC and IT‐4F, respectively. Hence, practical guidelines for quencher choice, sample geometries, and analysis approach for the accurate assessment of LD are provided.

High-Efficiency Non-Fullerene Acceptors Developed by Machine Learning and Quantum Chemistry.

Y6 and its derivatives have greatly improved the power conversion efficiency (PCE) of organic photovoltaics (OPVs). Further developing high‐performance Y6 derivative acceptor materials through the relationship between the chemical structures and properties of these materials will help accelerate the development of OPV. Here, machine learning and quantum chemistry are used to understand the structure–property relationships and develop new OPV acceptor materials. By encoding the molecules with an improved one‐hot code, the trained machine learning model shows good predictive performance, and 22 new acceptors with predicted PCE values greater than 17% within the virtual chemical space are screened out. Trends associated with the discovered high‐performing molecules suggest that Y6 derivatives with medium‐length side chains have higher performance. Further quantum chemistry calculations reveal that the end acceptor units mainly affect the frontier molecular orbital energy levels and the electrostatic potential on molecular surface, which in turn influence the performance of OPV devices. A series of promising Y6 derivative candidates is screened out and a rational design guide for developing high‐performance OPV acceptors is provided. The approach in this work can be extended to other material systems for rapid materials discovery and can provide a framework for designing novel and promising OPV materials.

Exciton Binding Energies in Organic Photovoltaic Materials: A Theoretical Perspective

Exciton Binding Energies in Organic Photovoltaic Materials: A Theoretical Perspective

Perylene-diimide derived organic photovoltaic materials

Perylene-diimide derived organic photovoltaic materials

Fully printed organic solar modules with bottom and top silver nanowire electrodes

AbstractOne of the advantages of organic photovoltaics (OPV) over other contemporary technologies is its relative ease of processing. There are, however, very few works that have realized fully printed devices, including the bottom electrode, let alone with a scalable process in a reasonable device size (>1 cm2). In this work, design steps and optimization processes towards fully printed OPV modules with scalable processes are demonstrated for the first time. An overview on issues related to upscaling with printed electrodes is first provided. The various issues are then addressed by a rational design process supported by measurements and calculations. Finally, a set of fully printed OPV modules are fabricated using these optimized parameters that have over 3.5‐cm2 active area with 5% efficiency. For the first time, this work has also demonstrated the process compatibility of fully printed device structures with non‐fullerene acceptor systems, which enables more design opportunities for the current generation of high‐performance OPV materials.

The Halogenation Effects of Electron Acceptor ITIC for Organic Photovoltaic Nano-Heterojunctions.

Molecular engineering plays a critical role in the development of electron donor and acceptor materials for improving power conversion efficiency (PCE) of organic photovoltaics (OPVs). The halogenated acceptor materials in OPVs have shown high PCE. Here, to investigate the halogenation mechanism and the effects on OPV performances, based on the density functional theory calculations with the optimally tuned screened range-separated hybrid functional and the consideration of solid polarization effects, we addressed the halogenation effects of acceptor ITIC, which were modeled by bis-substituted ITIC with halogen and coded as IT-2X (X = F, Cl, Br), and PBDB-T:ITIC, PBDB-T:IT-2X (X = F, Cl, Br) complexes on their geometries, electronic structures, excitations, electrostatic potentials, and the rate constants of charge transfer, exciton dissociation (ED), and charge recombination processes at the heterojunction interface. The results indicated that halogenation of ITIC slightly affects molecular geometric structures, energy levels, optical absorption spectra, exciton binding energies, and excitation properties. However, the halogenation of ITIC significantly enlarges the electrostatic potential difference between the electron acceptor and donor PBDB-T with the order from fluorination and chlorination to bromination. The halogenation also increases the transferred charges of CT states for the complexes. Meanwhile, the halogenation effects on CT energies and electron process rates depend on different haloid elements. No matter which kinds of haloid elements were introduced in the halogenation of acceptors, the ED is always efficient in these OPV devices. This work provides an understanding of the halogenation mechanism, and is also conducive to the designing of novel materials with the aid of the halogenation strategy.

Beyond single crystals: Imaging rubrene polymorphism across crystalline batches through lattice phonon Raman microscopy.

Polymorphism is an issue troubling numerous scientific fields. A phenomenon where molecules can arrange in different orientations in a crystal lattice, polymorphism in the field of organic photovoltaic materials can dramatically change electronic properties of these materials. Rubrene is a benchmark photovoltaic material showing high carrier mobility in only one of its three polymorphs. To use rubrene in devices, it is important to quantify the polymorph distribution arising from a particular crystal growth method. However, current methods for characterizing polymorphism are either destructive or inefficient for batch scale characterization. Lattice phonon Raman spectroscopy has the ability to distinguish between polymorphs based on low frequency intermolecular vibrations. We present here the addition of microscopy to lattice phonon Raman spectroscopy, which allows us to not only characterize polymorphs efficiently and nondestructively through Raman spectroscopy but also concurrently gain information on the size and morphology of the polymorphs. We provide examples for how this technique can be used to perform large, batch scale polymorph characterization for crystals grown from solution and physical vapor transport. We end with a case study showing how Raman microscopy can be used to efficiently optimize a green crystal growth method, selecting for large orthorhombic crystals desired for rubrene electronic device applications.

DFT analysis of different substitutions on optoelectronic properties of carbazole-based small acceptor materials for Organic Photovoltaics

In this present study, we have investigated a series of six new novels A-π-D-π-A type small non-fullerene acceptor molecules to enhance the efficiency of organic solar cells. The designed acceptors consist of carbazole moiety as donor (central core) and 3-ethylrhodanine moiety as end-capped acceptor group which is linked via different conjugated π-linkers namely benzo[c][1,2,5]thiadiazole, benzo[c][1,2,5]oxadiazole, thiophene, furan, and [1,2,5]oxadiazolo[3,4-c]pyridine. DFT (HSE06) and TD-DFT (M05) calculations have been carried out with 6-311G(d,p) basis set to investigate the effect of different π-linkers on the charge transfer rate, stability, and optoelectronic properties. Different π-linkers have resulted in smaller HOMO-LUMO energy gap as compared to reference. All investigated molecules M1, M2, M4, M5 and M6 have shown significant bathochromic shift of 31, 12, 92, 78 and 47 nm compared to R except M3 which has shown slightly blue shift of 22 nm, respectively. The reorganization energy analysis predicted the ambipolar behavior of designed molecules. Molecular electrostatic potential maps (MEP), the density of state (DOS), and electron density difference maps (EDDM) analysis have shown the stability parameters and confirmed the higher charge transfer between different fragments within the designed molecules. Thusly, energy level analysis indicates that designed molecules M1, M2, M4 and M5 could act as suitable non-fullerenes acceptors with previously designed PffBT4T-2OD donor for practical solar cells applications.

15.71% Efficiency All‐Small‐Molecule Organic Solar Cells Based on Low‐Cost Synthesized Donor Molecules

AbstractLow cost, high efficiency, and high stability are the three key issues of organic solar cells (OSCs) that should be carefully considered to meet the requirement of future commercial applications. Therefore, the development of high‐performance organic photovoltaic materials with low synthetic cost has been becoming a crucial challenge in the field of OSCs. Herein, two new low‐cost small‐molecule donors (SM‐BF1 and SM‐BF2) are designed and synthesized with a facile synthetic route by replacing 4‐bromo‐2‐fluorobenzenethiol and 4‐bromo‐3‐fluorobenzenethiol with low‐cost 4‐bromo‐2‐fluoro‐1‐iodobenzene and 4‐bromo‐3‐fluoro‐1‐iodobenzene as key raw materials. Besides, the influence of the chemical steric effect of the phenyl conjugated side chains of the benzodithiophene (BDT) unit on photophysical properties, charge transfer, and photovoltaic properties are deeply investigated by the modulation of fluorine atom substituted position. As a result, SM‐BF1 with ortho‐fluorinated substituent has outstanding crystallization properties and better miscibility with acceptor Y6 and exhibits more desirable morphology and more balanced charge‐carrier transport properties, leading to a superior power conversion efficiency (PCE) to 15.71%. More encouragingly, according to the figure of merit (FOM) and the industrial figure of merit (i‐FOM) to evaluate the small‐molecule donors, the SM‐BF1‐based device has excellent potential for future commercial applications.

Effect of Chlorine or Ester Incorporation on Diketopyrrolopyrrole -Based Polymeric Semiconductors

Organic photovoltaic materials incorporating either chlorine atoms or ester groups have been studied as replacement strategy for fluorinated compounds. However, the characteristics of such materials when used in organic field-effect transistors (OFETs) remain unclear. The present work synthesized three polymers via the copolymerization of diketopyrrolopyrrole (DPP) with monothiophene units having no substituents or incorporating either chlorine atoms or ester groups. All three materials were found to have similar photophysical and electrochemical properties even though their repeating units had different substituents. The maximum hole mobility values for the polymer specimens were determined to be 0.37 cm 2 V −1 s −1 . The introduction of chlorine into the DPP-based polymer generated an amorphous semiconductor material that makes it a very promising candidate for commercial OFETs since its properties are independent of fabrication conditions. The new polymeric semiconductors demonstrated herein suggest the viability of such materials with regard to incorporation in organic field-effect transistors. •Both ester groups and chlorine atoms were successfully introduced into DPP-based polymers. •Chlorination and esterification applied to polymeric OFET materials •The introduction of chlorine into the DPP-based polymer generated an amorphous semiconductor material.

Asymmetric Non-Fullerene Small-Molecule Acceptors toward High-Performance Organic Solar Cells.

Applying an asymmetric strategy to construct non-fullerene small-molecule acceptors (NFSMAs) in organic solar cells (OSCs) plays a vital role in the development of organic photovoltaic materials. In the past several years, taking advantage of the larger dipole moment and stronger intermolecular interactions, asymmetric NFSMAs have witnessed tremendous progress in OSCs with a power conversion efficiency of over 18%. From a structural point of view, besides the possible changes in the conformation effect on molecular packing, asymmetric acceptors can also achieve a balance between the solubility and the crystallinity. Herein, we systematically investigate the structure–property–performance relationships of asymmetric NFSMAs that have recently emerged and try to clarify the feasibility and practicality of an asymmetric strategy for the design of higher-performance NFSMAs. Finally, we put forward our views and a concise outlook on the asymmetric strategy.

Effects of Alkyl Side Chains of Small Molecule Donors on Morphology and the Photovoltaic Property of All-Small-Molecule Solar Cells.

Unraveling the relationship between nanoscale morphology of active layers and chemical structures of organic semiconductor photovoltaic materials is crucially important for further advancing the development of all-small-molecule organic solar cells (SM-OSCs). Here, in order to delve into the effect of flexible side chains of small molecule donors on the photovoltaic properties of SM-OSCs, we synthesized two new small molecule donors substituted by different flexible alkyl chains (iso-octyl chains for SM1-EH and n-octyl chains for SM1-Oct). As a result, the two small molecules present different absorption properties, energy levels, and stacking characteristics. When blending with Y6 as an acceptor, the SM1-Oct-based SM-OSC demonstrated a higher PCE value of 11.73%, while the SM1-EH-based device presents a relatively poorer PCE value of 8.42%. In addition, the morphology analysis demonstrated that, compared with the SM1-EH:Y6 blend, the SM1-Oct:Y6 blend film displayed better molecular stacking properties with stronger multilevel diffraction and preferable phase separation, resulting in the higher hole mobility, more efficient charge separation efficiency, and better device performance. These results underline that reasonably adjusting the flexible alkyl chains of small molecule donors can be an effective approach to further advance the development of the SM-OSCs field.

Utilizing the unique charge extraction properties of antimony tin oxide nanoparticles for efficient and stable organic photovoltaics

Simultaneously enhancing device performance and longevity, as well as balancing the requirements on cost, scalability, and simplification of processing, is the goal of interface engineering of organic solar cells (OSCs). In our work, we strategically introduce antimony (Sb 3+ ) cations into an efficient and generic n-type SnO 2 nanoparticles (NPs) host during the scalable flame spray pyrolysis synthesis. Accordingly, a significant switch of conduction property from an n-type character to a p-type character is observed, with a corresponding shift in the work function (WF) from 4.01 ± 0.02 eV for pristine SnO 2 NPs to 5.28 ± 0.02 eV for SnO 2 NPs with 20 mol. % Sb content (ATO). Both pristine SnO 2 and ATO NPs with fine-tuned optoelectronic properties exhibit remarkable charge carrier extraction properties, excellent UV resistance and photo-stability being compatible with various state-of-the-art OSCs systems. The reliable and scalable pristine SnO 2 and ATO NPs processed by doctor-blading in air demand no complex post-treatment. Our work offers a simple but unique approach to accelerate the development of advanced interfacial materials, which could circumvent the major existing interfacial problems in solution-processed OSCs. • Introducing antimony cations to effectively dope the n-type SnO 2 nanoparticles. • In-depth characterization of the cation doping effect and mechanisms. • Validation of the generic applicability of the p-type doped nanoparticles for organic photovoltaic applications. • Demonstrating efficient and stable organic solar cells based on state-of-the-art organic photovoltaic materials.

Alkyl Chain Tuning of Non-fullerene Electron Acceptors toward 18.2% Efficiency Binary Organic Solar Cells

Tailoring of the chemical structure is an effective method to tune the aggregation and optoelectronic properties of organic photovoltaic materials to boost the performance of organic solar cells (O...

Engineering highways for excitons

<h2>Summary</h2> The distance that excitons can travel is a key parameter for organic photovoltaic materials. In the August issue of <i>Science Advances</i>, Sneyd and colleagues report a breakthrough in increasing exciton diffusion length to 300 nm by using highly ordered nanofibers. This approach can enable simpler and more stable solar cells.

Effects of Heteroatom Substitution on the Photovoltaic Performance of Donor Materials in Organic Solar Cells

ConspectusOver the past few years, the innovation of narrow bandgap acceptor combined with wide bandgap donor materials significantly promotes the power conversion efficiencies (PCEs) of organic solar cells (OSCs) to exceed 18%. To build a state-of-the-art OSC, absorption spectra, frontier molecular orbital energy levels, molecular packing and crystallinity, and charge carrier mobilities of the photovoltaic materials should be considered in their molecular design. The donor and acceptor materials are the key components determining the photovoltaic performance of the OSCs. The side chain engineering on the conjugated backbone is a critical strategy to optimize the photovoltaic properties of the donor materials. In this Account, we focus on the topic of heteroatom substitution on the molecular backbone of the donor materials for improving their photovoltaic performance, aiming to provide in-depth understanding of the molecular structure optimization for the design of state-of-the-art photovoltaic materials.First, we highlight the halogen (fluorine and chlorine) atom substitution strategies applied on the conjugated molecular backbone of the organic photovoltaic materials. The strong electronegativity of halogen atoms can downshift the highest occupied molecular orbital (HOMO) energy levels of the donor materials, which could increase open-circuit voltages of the resulting OSCs. In addition, the hydrogen bonding aroused by the halogen atoms is beneficial to improve their charge transport property and crystallinity of the organic semiconductors. On the other hand, flexible side chains are critical components for improving the solubility of the photovoltaic materials. Using the flexible side chains of alkylthio and alkylsilyl is an easy and effective approach to tune the electronic energy levels and absorption spectra of the photovoltaic materials. Due to the formation of pπ(C)–dπ(S) orbital overlap, the empty 3d-orbitals of the sulfur atom in the alkylthio substituents can accept the π-electron of conjugated skeleton to modulate the optical and electrical properties of the photovoltaic materials. Similarly, the silicon atom in the alkylsilyl side chains can stabilize the lowest unoccupied molecular orbital (LUMO) level and downshift the HOMO level of the organic semiconductor, which leads to the improved photovoltaic performance of the organic donor materials. Finally, we briefly discussed the challenges for the photovoltaic materials toward performance optimization and practical application of the OSCs.

Resist or Oxidize: Identifying Molecular Structure-Photostability Relationships for Conjugated Polymers Used in Organic Solar Cells.

Although organic solar cells have started to demonstrate competitive power conversion efficiencies of >18 %, their operational lifetimes remain insufficient for wide practical use and the factors influencing the photostability of absorber materials and completed devices are still not completely understood. A systematic study of two series of structurally similar [XTBT]n and [XTTBTBTT]n polymers (16 structures in total) reveals the building blocks that enable the highest material stability towards photooxidation: fluorene, silafluorene, carbazole, diketopyrrolopyrrole, and isoindigo. Furthermore, a direct correlation is evident between the electronic properties of the conjugated polymers and their reactivity towards oxygen. The structures with the lowest highest occupied molecular orbital (HOMO) energies show the highest electrochemical oxidation potentials and appear to be the most resistant towards chemical oxidation. These relationships set important guidelines for the further rational design of new absorber materials for efficient and stable organic photovoltaics.

Alkylsilyl Fused Ring-Based Polymer Donor for Non-Fullerene Solar Cells with Record Open Circuit Voltage and Energy Loss.

The energy loss (Eloss ), especially the nonradiative recombination loss and energetic disorder, needs to be minimized to improve the device performance with a small voltage (VOC ) loss. Urbach energy (EU ) of organic photovoltaic materials is related to energetic disorder, which can predict the Eloss of the corresponding device. Herein, a polymer donor (PBDS-TCl) with Si and Cl functional atoms for organic solar cells (OSCs) is synthesized. It can be found that the VOC and Eloss can be well manipulated by regulation of the energy level of the polymer donor and EU , which is dominated by the morphology. A low energetic disorder with an EU of 23.7 meV, a low driving force of 0.08eV, and a low Eloss of 0.41eV are achieved for the PBDS-TCl:Y6-based OSCs. Consequently, an impressive open circuit voltage (VOC ) of 0.92V is obtained. To the best of knowledge, the VOC value and Eloss are both the record values for the Y6-based device. These results demonstrate that fine-tuning the polymer donor by functional atom modification on the side chain is a promising way to reduce EU and energy loss, as well as obtain small driving force and high VOC for highly efficient OSCs.

Contrasting Effect of Side-Chain Placement on Photovoltaic Performance of Binary and Ternary Blend Organic Solar Cells in Benzodithiophene-Thiazolothiazole Polymers.

π-Conjugated polymers are important materials for organic photovoltaics. While search for new backbone systems is central to the development of π-conjugated polymers, side-chain engineering is also imperative. Here, two benzodithiophene-thiazolothiazole copolymers, PSTz1 and POTz1, were synthesized, for which the side-chain placement was different. Due to less steric hindrance between the side chains, PSTz1 had a more coplanar backbone than POTz1. This led to significant differences in trend of the performance for the binary and ternary blend cells that used a fullerene (PC71 BM) and/or non-fullerene (ITIC) as the acceptor materials. Whereas PSTz1 showed higher photovoltaic performance in the PC71 BM-based cell, POTz1 showed higher performance in the ITIC-based cell. Furthermore, in the ternary blend cell, whereas increase in the PC71 BM content improved the photovoltaic performance for the PSTz1 system, it was detrimental to the performance for the POTz1 system. These results could be a good guideline for maximizing the performance of organic photovoltaics.