Multi-Length-Scale Morphologies Driven by Mixed Additives in Porphyrin-Based Organic Photovoltaics

A unified evaluation descriptor for π-bridges applied to metalloporphyrin derivatives

Establishing the structure of porphyrins with a A-π-D-π-A configuration is one of the effective strategies to maintain their dominance and compensate shortcomings through flexible changes in fragments. In this regard, π-bridges have attracted wide attention as a parameter affecting molecular backbones, electron transfer, energy levels, absorption, and other properties. However, the essence and influence of π-bridges have not yet been confirmed. In order to satisfy the requirements of intelligent application in molecular design, this study aimed to investigate the control effect of differences in π-bridge composition (thiophene and selenophene) and connection type (single bonds, ethylenic bonds and fused) on photoelectric performance. Y6 and PC61BM were used as acceptors to build donor/acceptor (D/A) interfaces and characterize the film morphology in three dimensions. Results showed that the essence of π-bridges involves a strong bridging effect (adjusting ability) between A and D fragments rather than highlighting its own nature. The large value could obtain high open circuit voltages (VOC), large separation and small recombination rates as well as stable and tight morphology. Therefore, adjusting ability is a unified descriptor for evaluating π-bridges, and it is an effective strategy to adjust material properties and morphology. This insight and discovery may provide a new evaluation descriptor for the screening and design of π-bridges.

Optimization of Charge Management and Energy Loss in All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) have received tremendous attention in recent decades because of their advantages over their polymer counterparts. These advantages include well-defined chemical structures, easy purification, and negligible batch-to-batch variation. Remarkable progress with a power conversion efficiency (PCE) of over 17% has recently been achieved with improved charge management (FF × JSC) and reduced energy loss (Eloss). Morphology control is the key factor in the progress of ASM-OSCs, which remains a significant challenge because of the similarities in the molecular structures of the donors and acceptors. In this review, the effective strategies for charge management and/or Eloss reduction from the perspective of effective morphology control are summarized. The aim is to provide practical insights and guidance for material design and device optimization to promote further development of ASM-OSCs to a level where they can compete with or even surpass the efficiency of polymer solar cells.

Novel Beta-Functionalized Porphyrins Approaching 11% Efficiency for Organic Solar Cells

Porphyrins and their derivatives possess high molar extinction coefficients and strong electron-donating abilities and have been widely used in organic solar cells (OSCs). Though porphyrins can be easily functionalized at the four meso-positions and the eight β-positions, nearly all porphyrin photovoltaic materials are reported to be functionalized at the meso-positions, and the porphyrin photovoltaic materials functionalized at the β-positions are to be explored. Herein, the regioselective β-positions of a porphyrin are first brominated without using rare metal iridium catalysts, and then, after two more reactions, two antipodal β-substituted porphyrin donors EHDPP-Por and BODPP-Por are synthesized, in which four DPP (diketopyrrolopyrrole) units are connected symmetrically with acetylene at four of the β-positions, for OSCs. The all-small-molecule organic solar cells based on EHDPP-Por:Y6 and BODPP-Por:Y6 active layers achieved power conversion efficiencies of 10.19 and 10.99%, respectively, which are higher than most of the binary OSCs based on the porphyrins functionalized at the meso-positions, demonstrating that β-functionalized porphyrins are very promising for OSCs.

Nonfullerene Acceptor Featuring Unique Self-Regulation Effect for Organic Solar Cells with 19 % Efficiency

The blend nanomorphology of electron-donor (D) and -acceptor (A) materials is of vital importance to achieving highly efficient organic solar cells. Exogenous additives especially aromatic additives are always needed to further optimize the nanomorphology of blend films, which is hardly compatible with industrial manufacture. Herein, we proposed a unique approach to meticulously modulate the aggregation behavior of NFAs in both crystal and thin film nanomorphology via self-regulation effect. Nonfullerene acceptor Z9 was designed and synthesized by tethering phenyl groups on the inner side chains of the Y6 backbone. Compared with Y6, the tethered phenyl groups participated in the molecular aggregation via the π-π stacking of phenyl-phenyl and phenyl-2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) groups, which induced 3D charge transport with phenyl-mediated super-exchange electron coupling. Moreover, ordered molecular packing with suitable phase separation was observed in Z9-based blend films. High power conversion efficiencies (PCEs) of 19.0 % (certified PCE of 18.6 %) for Z9-based devices were achieved without additives, indicating the great potential of the self-regulation strategy in NFA design.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Recent Progress in All-Small-Molecule Organic Solar Cells

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed.

Ternary Blend Organic Solar Cells: Understanding the Morphology from Recent Progress

Ternary blend organic solar cells (TB-OSCs) incorporating multiple donor and/or acceptor materials into the active layer have emerged as a promising strategy to simultaneously improve the overall device parameters for realizing higher performances than binary devices. Whereas introducing multiple materials also results in a more complicated morphology than their binary blend counterparts. Understanding the morphology is crucially important for further improving the device performance of TB-OSC. This review introduces the solubility and miscibility parameters that affect the morphology of ternary blends. Then, this review summarizes the recent processes of morphology study on ternary blends from the aspects of molecular crystallinity, molecular packing orientation, domain size and purity, directly observation of morphology, vertical phase separation as well as morphological stability. Finally, summary and prospects of TB-OSCs are concluded.

Multiple-cation wide-bandgap perovskite solar cells grown using cesium formate as the Cs precursor with high efficiency under sunlight and indoor illumination

Owing to the advantages of adjustable bandgap, low-cost fabrication and superior photovoltaic performance, wide-bandgap (WBG) perovskite solar cells (PSCs) are considered as the promising top-cell for multi-junction solar cells. At the same time, WBG PSCs have also shown great potential for indoor photovoltaic applications. To further improve the performance of WBG PSCs, in this work, we fabricated efficient WBG PSCs via introducing cesium formate (CsFa) as the Cs precursor. Due to the HCOO·Pb⁺ and HCOOH·Cs⁺ complex formation and HCOOH volatilization accompanying the crystallization process, the crystallization of the perovskite using the CsFa precursor (CsFa-perovskite) is promoted. Compared to the perovskite prepared using the CsBr precursor (CsBr-perovskite), the WBG CsFa-perovskite shows better perovskite crystallization, reduced trap-state density, and better phase stability under light illumination. Finally, the 1.63 eV WBG PSCs based on the CsFa-perovskite achieve a significant PCE of 20.01% under one sun illumination (AM 1.5G, 100 mW cm^-2), which is higher than that of PSCs based on the CsBr-perovskite (18.27%). Moreover, the PCE of CsFa-perovskite PSCs also under indoor warm-white 2700 K LED light illumination (1000 lux) is as high as 38.52%. Our results demonstrate that CsFa as the Cs precursor is a promising candidate to promote the device performance of WBG PSCs.

Enhanced Thermal Stability of Planar Perovskite Solar Cells Through Triphenylphosphine Interface Passivation

While extensive research has driven the rapid efficiency trajectory noted to date for organic-inorganic perovskite solar cells (PSCs), their thermal stability remains one of the key issues hindering their commercialization. Herein, a significant reduction in surface defects (a precursor to perovskite instability) could be attained by introducing triphenylphosphine (TPP), an effective Lewis base passivator, to the vulnerable perovskite/spiro-OMeTAD interface. Not only did TPP passivation enable a high power conversion efficiency (PCE) of 20.22 % to be achieved, these devices also exhibited superior ambient and thermal stability. Unlike the pristine device, which exhibited a sharp descend to 16 % of its initial PCE on storing in relative humidity of 10 %, at 85 °C for more than 720 h, the TPP-passivated devices retained 71 % of its initial PCE. Hence, this study presents a facile yet excellent approach to attain high-performing yet thermally stable PSCs.

Impact of Aryl End Group Engineering of Donor Polymers on the Morphology and Efficiency of Halogen-Free Solvent-Processed Nonfullerene Organic Solar Cells

End group engineering on the side chain of π-conjugated donor polymers is explored as an effective way to develop efficient photovoltaic devices. In this work, we designed and synthesized three new π-conjugated polymers (PBDT-BZ-1, PBDT-S-BZ, and PBDT-BZ-F) with terminal aryl end groups on the side chain of chlorine-substituted benzo[1,2-b:4,5b']dithiophene (BDT). End group modifications showed notable changes in energy levels, dipole moments, exciton lifetimes, energy losses, and charge transport properties. Remarkably, the three new polymers paired with IT-4F (halogen-free solvent processed/toluene:DPE) displayed high power conversion efficiencies (PCEs) compared to a polymer (PBDT-Al-5) without a terminal end group (PCE of 7.32%). Interestingly, PBDT-S-BZ:IT-4F (PCE of 13.73%) showed a higher PCE than the benchmark PM7:IT-4F. The improved performance of PBDT-S-BZ well correlates with its improved charge mobility, well-interdigitated surface morphology, and high miscibility with a low Flory-Huggins interaction parameter (1.253). Thus, we successfully established a correlation between the end group engineering and bulk properties of the new polymers for realizing the high performance of halogen-free nonfullerene organic solar cells.

Influence of Fluorine Substitution on the Photovoltaic Performance of Wide Band Gap Polymer Donors for Polymer Solar Cells

The design and development of wide band gap (WBG) polymer donors are critical for achieving high power conversion efficiencies (PCEs) in polymer solar cells. In this work, four WBG polymer donors, Q4, Q5, Q6, and Q7, with different numbers and positions of fluorine substitution (n = 0, 2, 2, and 4, respectively) were prepared, and the effect of fluorination on their photovoltaic performance was systematically investigated. When blended with a small-molecule electron acceptor MeIC, the devices based on Q4, Q5, Q6, and Q7 showed PCEs of 10.34, 11.06, 5.26, and 0.48%, respectively. When coupled with a low band gap polymer acceptor PYIT to fabricate all-polymer solar cells (all-PSCs), while the other three polymers (Q5-Q7) exhibited much lower PCEs in the range of 0.12-6.71%, the Q4 polymer-based all-PSCs showed the highest PCE of 15.06%, comparable to that of the devices fabricated with the star polymer donor PM6 (PCE = 15.00%). Detailed physicochemical and morphological studies revealed that an over-substitution of F in Q7 results in undesired low-lying HOMO levels and phase separation with the acceptors, thus resulting in its inferior PCEs. Moreover, the less F-substitution and controlling of the positions of F-substitution position in Q4 and Q5 can improve the HOMO energy level matching as well as morphologies between these two polymers with the acceptors, which in turn gives rise to higher performances. Clearly, our results indicate that Q4 is a promising donor candidate for high-performance all-PSCs, and the fine-tuning of both the number and positions of F-substitution in the polymer backbone is essential in developing high-performance WBG polymer donors.

Ultrafast charge transfer in a nonfullerene all-small-molecule organic solar cell: a nonadiabatic dynamics simulation with optimally tuned range-separated functional

The combination of nonadiabatic dynamics simulation and optimally tuned range-separated functional might be a powerful tool for elucidating the ultrafast charge transfer in nonfullerene all-small-molecule organic solar cells.

Efficient all-small-molecule organic solar cells based on a fluorinated small-molecule donor

A fluorinated donor with a deep HOMO energy level enables efficient all-small-molecule organic solar cells.

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.

High-quality WS2 film as a hole transport layer in high-efficiency non-fullerene organic solar cells

Liquid-exfoliated 2D transition metal disulfides (TMDs) are potential substitutes for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layers (HTLs) in Organic Solar Cells (OSCs). Herein, high-yield and high-quality WS₂ flake layers are prepared by comprehensively controlling the initial concentration, sonication processing time and centrifugal speed. The WS₂ layers deposited on in situ transparent indium tin oxide (ITO) without plasma treatment show higher uniformity and conductivity than that formed on ITO after plasma treatment. With a significant increase in the short-circuit current density (J_SC), the power conversion efficiency (PCE) of PM6:Y6-based non-fullerene OSCs using optimized WS₂ as the HTL is higher than that using PEDOT:PSS as the HTL(15.75% vs. 15.31%). Combining the morphology characteristics with carrier recombination characteristics, the higher quality of the ITO/WS₂ composite substrate leads to better charge transport and a lower bimolecular recombination rate in OSCs, thereby improving the device performance.

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.

Reducing Energy Loss in Organic Solar Cells by Changing the Central Metal in Metalloporphyrins

The effect of central donor core on the properties of A-π-D-π-A donors, where D is a porphyrin macrocycle, cyclopenta[2,1-b:3,4-b']dithiophene is the π bridge, and A is a dicyanorhodanine terminal unit, was investigated for the fabrication of the organic solar cells (OSCs), along [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁ BM) as electron acceptor. A new molecule consisting of Ni-porphyrin central donor core (VC9) showed deep HOMO energy level and OSCs based on optimized VC9:PC₇₁ BM realized overall power conversion efficiency (PCE) of 10.66 % [short-circuit current density (J_SC )=15.48 mA/cm² , fill factor (FF)=0.65] with high open circuit voltage (V_OC ) of 1.06 V and very low energy loss of 0.49 eV, whereas the Zn-porphyrin analogue VC8:PC₇₁ BM showed PCE of 9.69 % with V_OC of 0.89 V, J_SC of 16.25 mA/cm² and FF of 0.67. Although the OSCs based on VC8 showed higher J_SC in comparison to VC9, originating from the broader absorption profile of VC8 that led to more exciton generation, the higher value of PCE of VC9 is owing to the higher V_OC and reduced energy loss.

Porphyrin Acceptors with Two Perylene Diimide Dimers for Organic Solar Cells

Three small-molecule acceptors (Por-PDI, TEHPor-PDI, and BBOPor-PDI) with different side chains were synthesized by using a porphyrin core as the electron-donating unit and connecting electron-withdrawing perylene diimide dimers via acetylene bridges. The bulk heterojunction organic solar cells based on the three acceptors and a polymer donor provided power conversion efficiencies (PCEs) of 3.68-5.21 % when the active layers were fabricated with pyridine additives. Though the synthesis of Por-PDI is easier with fewer reaction steps and higher yields, the devices based on Por-PDI showed the best performance with a PCE of 5.21 %. The more ordered intermolecular packing due to the reduced steric hindrance at the porphyrin core of Por-PDI could contribute to the more balanced hole/electron mobilities, higher maximum charge generation rate, and less bimolecular recombination in Por-PDI devices, which are beneficial for the higher PCE.

Natural biomaterial sarcosine as an interfacial layer enables inverted organic solar cells to exhibit over 16.4% efficiency

The natural biomaterial sarcosine as an electron transport layer (ETL) to modify ITO or ITO/ZnO was successfully introduced into inverted organic solar cells (OSCs) with PM6:BTP-BO-4Cl as the active layer. The introduction of sarcosine on the surface of ITO or ITO/ZnO resulted in lower work function (WF) and higher surface energy. The active layers processed on the surfaces of ITO or ITO/ZnO presented a more optimized morphology and a more ordered molecular arrangement after their modification with sarcosine. The introduction of sarcosine as an ETL promoted charge transport and collection in the OSCs. Therefore, the power conversion efficiency (PCE) of the OSCs increased to 13.53% from 3.86% by modifying ITO with sarcosine. The PCE of the OSCs with ZnO as ETLs improved to 16.45% from 14.85% by modifying ZnO with sarcosine. The improved PCEs benefited from the simultaneously improved short-circuit current density (J_SC), fill factor (FF), and open-circuit voltage (V_OC). Therefore, this work demonstrates that sarcosine has great potential as an ETL to improve the performance of OSCs.

Fluorination Enables Tunable Molecular Interaction and Photovoltaic Performance in Non-Fullerene Solar Cells Based on Ester-Substituted Polythiophene

Owing to the advantages of low synthetic cost and high scalability of synthesis, polythiophene and its derivatives (PTs) have been of interest in the community of organic photovoltaics (OPVs). Nevertheless, the typical efficiency of PT based photovoltaic devices reported so far is much lower than those of the prevailing push-pull type conjugated polymer donors. Recent studies have underscored that the excessively low miscibility between PT and nonfullerene acceptor is the major reason accounting for the unfavorable active layer morphology and the inferior performance of OPVs based on a well-known PT, namely PDCBT-Cl and a non-halogenated nonfullerene acceptor IDIC. How to manipulate the miscibility between PT and acceptor molecule is important for further improving the device efficiency of this class of potentially low-cost blend systems. In this study, we introduced different numbers of F atoms to the end groups of IDIC to tune the intermolecular interaction of the hypo-miscible blend system (PDCBT-Cl:IDIC). Based on calorimetric, microscopic, and scattering characterizations, a clear relationship between the number of F atoms, miscibility, and device performance was established. With the increased number of F atoms in IDIC, the resulting acceptors exhibited enhanced miscibility with PDCBT-Cl, and the domain sizes of the blend films were reduced substantially. As a result, distinctively different photovoltaic performances were achieved for these blend systems. This study demonstrates that varying the number of F atoms in the acceptors is a feasible way to manipulate the molecular interaction and the film morphology toward high-performance polythiophene:nonfullerene based OPVs.

Significantly Boosting Efficiency of Polymer Solar Cells by Employing a Nontoxic Halogen-Free Additive

Traditional additives like 1,8-diiodooctane and 1-chloronaphthalene were successfully utilized morphology optimization of various polymer solar cells (PSCs) in an active layer, but their toxicity brought by halogen atoms limits their corresponding large-scale manufacturing. Herein, a new nontoxic halogen-free additive named benzyl benzoate (BB) was introduced into the classic PSCs (PTB7-Th:PC₇₁BM), and an optimal power conversion efficiency (PCE) of 9.43% was realized, while there was a poor PCE for additive free devices (4.83%). It was shown that BB additives could inhibit PC₇₁BM's overaggregation, which increased the interface contact area and formed a better penetration path of an active layer. In addition, BB additives could not only boost the distribution of a PTB7-Th donor at the surface, beneficial to suppressing exciton recombination in inverted devices but also boost the crystallinity of a blend layer, which is conducive to exciton dissociation and charge transport. Our work effectively improved a device performance by using a halogen-free additive, which can be referential for industrialization.

The coupling and competition of crystallization and phase separation, correlating thermodynamics and kinetics in OPV morphology and performances

The active layer morphology transition of organic photovoltaics under non-equilibrium conditions are of vital importance in determining the device power conversion efficiency and stability; however, a general and unified picture on this issue has not been well addressed. Using combined in situ and ex situ morphology characterizations, morphological parameters relating to kinetics and thermodynamics of morphology evolution are extracted and studied in model systems under thermal annealing. The coupling and competition of crystallization and demixing are found to be critical in morphology evolution, phase purification and interfacial orientation. A unified model summarizing different phase diagrams and all possible kinetic routes is proposed. The current observations address the fundamental issues underlying the formation of the complex multi-length scale morphology in bulk heterojunction blends and provide useful morphology optimization guidelines for processing devices with higher efficiency and stability.

Cu(ii)-Porphyrin based near-infrared molecules: synthesis, characterization and photovoltaic application

Three novel Cu(ii)-porphyrin-based near-infrared non-fullerene acceptors were developed, which show strong intramolecular charge transfer absorption spectra.

A New Ester-Substituted Quinoxaline-Based Narrow Bandgap Polymer Donor for Organic Solar Cells

The electron-deficient ester group substitution in the sidechain of the commonly used electron-withdrawing quinoxaline (Qx) unit is seldom studied, while ester-substituted Qx units possess easy syntheses and facile modulation of the polymer solubility, and the enhanced electron-withdrawing property of ester substituted Qx unit can theoretically broaden the optical absorption of the resulting polymers and improve the open circuit voltage in the corresponding organic solar cells (OSCs). In this work, a novel ester-substituted Qx-based narrow bandgap polymer (NBG) donor material PBDTT-EFQx, which exhibits an absorption edge of 790 nm (bandgap < 1.6 eV), is designed and synthesized. Results show that the OSCs composed of PBDTT-EFQx and PC₇₁ BM present the highest power conversion efficiency (PCE) of 6.8%, compared to PCEs of 5.0% for PBDTT-EFQx:ITIC based devices and 4.1% for PBDTT-EFQx:N2200 based devices, respectively. Characterizations and analyses indicate that the PC₇₁ BM-based OSCs have well-matched energy levels, better complementary light absorption, the highest and most balanced carrier mobilities, as well as the lowest degree of recombination losses, and therefore, leading to the highest PCE among the three types of OSCs. This work reveals that the ester-substituted quinoxaline unit is one of the potential building blocks for NBG polymer donors.

Isomeric Effect of Wide Bandgap Polymer Donors with High Crystallinity to Achieve Efficient Polymer Solar Cells

Two highly crystalline polymer donors (PBTz4T2C-a, PBTz4T2C-b) with isomers (4T2C-a, 4T2C-b) are synthesized and applied in polymer solar cells. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor IT2F. It is interesting that the photophysical properties, crystallinity, and active layer morphology characteristic can be significantly changed by just slightly regulating the substitution position of the carboxylate groups. A series of simulation calculations of the two isomers are conducted in the geometry and electronic properties to explore the difference induced by the position adjustment of carboxylate groups. The results decipher that 4T2C-b moiety features much stronger intramolecular noncovalent S⋯O interactions compared to that of 4T2C-a, implying a higher coplanarity and much stronger crystallinity, and leading to excessive phase separation in PBTz4T2C-b:IT2F blend film. In contrast, PBTz4T2C-a with 4T2C-a moiety exhibits suitable crystallinity with a lower the highest occupied molecular orbital level, higher film absorption coefficient, and charge mobilities, resulting in a much higher power conversion efficiency of 11.02%. This research demonstrates that the molecular conformation is of great importance to be considered for developing high-performance polymer donors.

Low-Bandgap Porphyrins for Highly Efficient Organic Solar Cells: Materials, Morphology, and Applications

With developments in materials, thin-film processing, fine-tuning of morphology, and optimization of device fabrication, the performance of organic solar cells (OSCs) has improved markedly in recent years. Designing low-bandgap materials has been a focus in order to maximize solar energy conversion. However, there are only a few successful low-bandgap donor materials developed with near-infrared (NIR) absorption that are well matched to the existing efficient acceptors. Porphyrin has shown great potential as a useful building block for constructing low-bandgap donor materials due to its large conjugated plane and strong absorption. Porphyrin-based donor materials have been shown to contribute to many record-high device efficiencies in small molecule, tandem, ternary, flexible, and OSC/perovskite hybrid solar cells. Specifically, non-fullerene small-molecule solar cells have recently shown a high power conversion efficiency of 12% using low-bandgap porphyrin. All these have validated the great potential of porphyrin derivatives as effective donor materials and made DPPEZnP-TRs a family of best low-bandgap donor materials in the OSC field so far. Here, recent progress in the rational design, morphology, dynamics, and multi-functional applications starting from 2015 will be highlighted to deepen understanding of the structure-property relationship. Finally, some future directions of porphyrin-based OSCs are presented.

Alloy-like ternary polymer solar cells with over 17.2% efficiency

Ternary strategy has been considered as an efficient method to achieve high performance polymer solar cells (PSCs). A power conversion efficiency (PCE) of 17.22% is achieved in the optimized ternary PSCs with 10 wt% MF1 in acceptors. The over 8% PCE improvement by employing ternary strategy is attributed to the simultaneously increased J_SC of 25.68 mA cm^-2, V_OC of 0.853 V and FF of 78.61% compared with Y6 based binary PSCs. The good compatibility of MF1 and Y6 can be confirmed from Raman mapping, contact angle, cyclic voltammetry and morphology, which is the prerequisite to form alloy-like state. Electron mobility in ternary active layers strongly depends on MF1 content in acceptors due to the different lowest unoccupied molecular orbital (LUMO) levels of Y6 and MF1, which can well explain the wave-like varied FF of ternary PSCs. The third-party certified PCE of 16.8% should be one of the highest values for single bulk heterojunction PSCs. This work provides sufficient references for selecting materials to achieve efficient ternary PSCs.

Insights into Excitonic Dynamics of Terpolymer-Based High-Efficiency Nonfullerene Polymer Solar Cells: Enhancing the Yield of Charge Separation States

Ternary copolymerization strategy is considered an effective method to achieve high-performance photovoltaic conjugated polymers. Herein, a donor-acceptor1-donor-acceptor2-type random copolymer, named PBDTNS-TZ-BDD (T1), containing one electron-rich unit alkylthionaphthyl-flanked benzo[1,2-b/4,5-b'] di-thiophene (BDTNS) as D and two electron-deficient moieties benzo[1,2-c/4,5-c']dithiophene-4,8-dione (BDD) and fluorinated benzotriazole as A, was synthesized to investigate the excitonic dynamic effect. Also, the D-A-type alternating copolymer PBDTNS-BDD (P1) was also prepared for a clear comparison. Although the UV-Vis spectra and energy levels of P1 and T1 are similar, the power conversion efficiencies (PCEs) of the related devices are 11.50% (T1/ITIC) and 8.89% (P1/ITIC), respectively. The reason for this is systematically investigated and analyzed by theoretical calculation, photoluminescence, and pump-probe transient absorption spectroscopy. The density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculation results show that the terpolymer T1 with a lower exciton binding energy and a longer lifetime of spontaneous luminescence can synergistically increase the number of excitons reaching the donor/acceptor interface. The results of the pump-probe transient absorption spectroscopy show that the yield of charge separation of T1/ITIC is higher than that of the P1/ITIC blend film, and improved PCE could be achieved via copolymerization strategies. Moreover, the fabrication of the T1-based device is also simple without any additive or postprocessing. Therefore, it provides a promising and innovative method to design high-performance terpolymer materials.

Porphyrin–diketopyrrolopyrrole conjugates and related structures: Synthesis, properties and applications

A large diversity of porphyrin–diketopyrrolopyrrole conjugates and related structures formed by diketopyrrolopyrrole units and pyrrole-based moieties such as phthalocyanine, porphycene, calix[4]pyrrole or BODIPY have been reported since 2010. The new compounds, whether small molecules or polymeric materials, exhibit very interesting photophysical properties and have been tested in a range of technical or biological applications. This review summarizes the advances in the synthesis of such compounds. Their photophysical properties and potential applications are also briefly discussed.

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.

End Group Engineering on the Side Chains of Conjugated Polymers toward Efficient Non-Fullerene Organic Solar Cells

Side chains properties of conjugated polymers, such as the length, branching point, and heteroatom, have been widely studied for application in organic solar cells (OSCs), but the end groups of side chains have been rarely reported. In this work, we systematically explored a series of new conjugated polymers with distinct side-chain end groups for high performance non-fullerene OSCs. The key components for the polymers contained functionalized units as the end groups of side chains, such as Br, alkyloxy (OMe), and alkylthienyl (T) groups. We found that the new conjugated polymers have similar absorption spectra and crystallinity with the polymer without substitution, but they showed distinct photovoltaic performance in solar cells. When the polymer without functionalized units had a power conversion efficiency (PCE) of 9.94%, the modified conjugated polymers provided high PCEs of over 13% with significantly enhanced photocurrent and fill factors. In addition, they also show additive-free and highly stable characteristics. These results demonstrate that end group engineering on side chains is a promising strategy to design new conjugated polymers toward efficient OSCs.

Developments of Diketopyrrolopyrrole-Dye-Based Organic Semiconductors for a Wide Range of Applications in Electronics

In recent times, fused aromatic diketopyrrolopyrrole (DPP)-based functional semiconductors have attracted considerable attention in the developing field of organic electronics. Over the past few years, DPP-based semiconductors have demonstrated remarkable improvements in the performance of both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices due to the favorable features of the DPP unit, such as excellent planarity and better electron-withdrawing ability. Driven by this success, DPP-based materials are now being exploited in various other electronic devices including complementary circuits, memory devices, chemical sensors, photodetectors, perovskite solar cells, organic light-emitting diodes, and more. Recent developments in the use of DPP-based materials for a wide range of electronic devices are summarized, focusing on OFET, OPV, and newly developed devices with a discussion of device performance in terms of molecular engineering. Useful guidance for the design of future DPP-based materials and the exploration of more advanced applications is provided.

Integrated Perovskite/Bulk-Heterojunction Organic Solar Cells

The recently emerged integrated perovskite/bulk-heterojunction (BHJ) organic solar cells (IPOSCs) without any recombination layers have generated wide attention. This type of device structure can take the advantages of tandem cells using both perovskite solar and near-infrared (NIR) BHJ organic solar materials for wide-range sunlight absorption and the simple fabrication of single junction cells, as the low bandgap BHJ layer can provide additional light harvesting in the NIR region and the high open-circuit voltage can be maintained at the same time. This progress report highlights the recent developments in such IPOSCs and the possible challenges ahead. In addition, the recent development of perovskite solar cells and NIR organic solar cells is also covered to fully underline the importance and potential of IPOSCs.

Ionic Conductive Gels for Optically Manipulatable Microwave Stealth Structures

Smart structures with manipulatable properties are highly demanded in many fields. However, there is a critical challenge in the pursuit of transparent windows that allow optical waves (wavelength of µm-nm) for transmitting while blocking microwave (wavelength of cm) in terms of absorbing electromagnetic energy, specifically for meeting the frequency requirement for the 5th generation (5G) mobile networks. For fundamentally establishing novel manipulatable microwave absorbing structures, here, new polymeric aqueous gels as both optically transparent materials and microwave absorbing materials are demonstrated, in which polar networks play significant roles in attenuating electromagnetic energy. By manipulating the hydrogen bonding networks, the resulting optically transparent solid-state gels are able to offer the capabilities for absorbing microwaves. Interestingly, such gels can be switched into an optically opaque state via converting the amorphous state into a polycrystal state when the temperature is decreased. Such ionic conductive gels can endow the assembled sandwich windows with effective microwave absorbing capability in the range of 15-40 GHz, covering a branch of 5G frequency bands. The results highlight a new strategy for using ionic conductive gels to design and fabricate manipulatable microwave stealth structures for various applications.

Design of New n-Type Porphyrin Acceptors with Subtle Side-Chain Engineering for Efficient Nonfullerene Solar Cells with Low Energy Loss and Optoelectronic Response Covering the Near-Infrared Region

A series of tailor-made highly efficient and near-infrared (NIR) porphyrin-based acceptors is designed and synthesized for fullerene-free bulk-heterojunction (BHJ) organic solar cells. Constructing BHJ active layers using a PTB7-Th donor and porphyrin acceptors (P-x), which have complementary absorption, accomplishes panchromatic photon-to-current conversion from 300 to 950 nm. Our study shows that side chains of the porphyrin acceptors fairly influence the molecular ordering and nanomorphology of the BHJ active layers. Significantly, the porphyrin acceptor with four dodecoxyl side chains (P-2) achieves an open-circuit voltage (V_OC) of 0.80 V, short-circuit current density (J_SC) of 13.94 mA cm^-2, fill factor of 64.8%, and overall power conversion efficiency of 7.23%. This great performance is attributable to the ascendant light-harvesting capability in the visible and near-infrared region, a high-lying LUMO energy level, a relatively high and more balanced carrier mobilities, and more ordered face-on molecular packing, which is beneficial for obtaining high V_OC and J_SC.

Unraveling Correlations between Molecular Properties and Device Parameters of Organic Solar Cells Using Machine Learning

Understanding the relationships between molecular properties and device parameters is highly desired not only to improve the overall performance of an organic solar cell but also to fulfill the requirements of a device for a particular application such as solar-to-fuel energy conversion (high open-circuit voltage (V_OC)) or solar window applications (high short circuit current (J_SC)). In this work, a series of machine learning models are built for three important device characteristics (V_OC, J_SC, and fill factor) using 13 crucial molecular properties as descriptors, resulting in an impressive predictive performance (r = 0.7). These models may play a vital role in designing promising organic materials for a specific photovoltaic application with high V_OC/J_SC. The importance of descriptors for each device parameter is unraveled, which may assist in tuning them and improve understanding of the energy conversion process.

Selective Adsorption of C60 in the Supramolecular Nanopatterns of Donor-Acceptor Porphyrin Derivatives

The nanostructure of active layers consisting of donor and acceptor molecules is responsible for the separation and transfer processes of charge carriers, which may result in different photoelectric conversion efficiencies of organic photovoltaic cells (OPVCs). Therefore, intensive study on the relationships among nanostructures, intermolecular interactions, and molecular chemical skeletons is necessary for preparing controlled nanostructures of active layers by designing photovoltaic molecules. In this research, the self-assembled nanopatterns of three (DPP-ZnP-E)2-based molecules on highly oriented pyrolytic graphite surface were probed by scanning tunneling microscopy and analyzed by density functional theory calculations. The results indicated that different bridges, diethynylene, diethynylene-dithienyl, and diethynylene-phenylene, in (DPP-ZnP-E)2-based molecules not only made a difference to intermolecular interactions and cooperated with molecule-substrate interactions, consequently affecting the packed nanopattern, but also influenced the adsorption of fullerene acceptors in the nanopatterns of (DPP-ZnP-E)2-based molecules. C₆₀ molecules were found to be selectively adsorbed atop the dithienyl groups of (DPP-ZnP-E)2-2T donor molecules probably by S···π interactions compared with (DPP-ZnP-E)2 or (DPP-ZnP-E)2-Ph molecules. This study on the assembled nanopatterns of the three (DPP-ZnP-E)2-based molecules would be conductive to (DPP-ZnP-E)2-based optoelectronic materials design in OPVCs.

Conjugation-Curtailing of Benzodithionopyran-Cored Molecular Acceptor Enables Efficient Air-Processed Small Molecule Solar Cells

Small molecule solar cells (SMSCs) lag a long way behind polymer solar cells. A key limit is the less controllable morphology of small molecule materials, which can be aggravated when incorporating anisotropic nonfullerene acceptors. To fine-tune the blending morphology within SMSCs, a π-conjunction curtailing design is applied, which produces a efficient benzodithionopyran-cored molecular acceptor for nonfullerene SMSCs (NF-SMSCs). When blended with a molecular donor BDT3TR-SF to fabricate NF-SMSCs, the π-conjunction curtailed molecular acceptor NBDTP-M obtains an optimal power conversion efficiency (PCE) of up to 10.23%, which is much higher than that of NBDTTP-M of longer π-conjunction. It retains 93% of the PCE of devices fabricated in a glove box when all spin-coating and post-treating procedures are conducted in ambient air with relative humidity of 25%, which suggests the good air-processing capability of π-conjunction curtailed molecules. Detailed X-ray scattering investigations indicate that the BDT3TR-SF:NBDTP-M blend exhibits a blend morphology featuring fine interpenetrating networks with smaller domains and higher phase purity, which results in more efficient charge generation, more balanced charge transport, and less recombination compared to the low-performance BDT3TR-SF:NBDTTP-M blend. This work provides a guideline for molecular acceptors' design toward efficient, low-cost, air-processed NF-SMSCs.

Synthesis and Photovoltaic Effect of Electron-Withdrawing Units for Low Band Gap Conjugated Polymers Bearing Bi(thienylenevinylene) Side Chains

A novel (E)-5-(2-(5-alkylthiothiophen-2-yl)vinyl)thien-2-yl (TVT)-comprising benzo[1,2-b:4,5-b']dithiophene (BDT) derivative (BDT-TVT) was designed and synthetized to compose two donor-acceptor (D-A) typed copolymers (PBDT-TVT-ID and PBDT-TVT-DTNT) with the electron-withdrawing unit isoindigo (ID) and naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NT), respectively. PBDT-TVT-ID and PBDT-TVT-DTNT showed good thermal stability (360 °C), an absorption spectrum from 300 nm to 760 nm and a relatively low lying energy level of Highest Occupied Molecular Orbital (E_HOMO) (-5.36 to -5.45 eV), which could obtain a large open-circuit voltage (V_oc) from photovoltaic devices with PBDT-TVT-ID or PBDT-TVT-DTNT. The photovoltaic devices with ITO/PFN/polymers: PC₇₁BM/MoO₃/Ag structure were assembled and exhibited a good photovoltaic performance with a power conversion efficiency (PCE) of 4.09% (PBDT-TVT-ID) and 5.44% (PBDT-TVT-DTNT), respectively. The best PCE of a PBDT-TVT-DTNT/PC₇₁BM-based device mainly originated from its wider absorption, higher hole mobility and favorable photoactive layer morphology.

Improving optoelectronic and charge transport properties of D-π-D type diketopyrrolopyrrole-pyrene derivatives as multifunctional materials for organic solar cell applications

A series of novel diketopyrrolopyrrole-pyrene-based molecules were designed for small molecule based organic solar cell (SMOSC) applications. Their electronic and charge transfer properties were investigated by applying the PBE0/6-31G(d,p) method. The absorption spectra were simulated using the TD-PBE0/6-31G(d,p) method. The results showed that the frontier molecular orbital (FMO) energy levels, reorganization energy, the energetic driving force, and absorption spectra can be tuned by the introduction of different aromatic heterocyclic groups to the side of diketopyrrolopyrrole fragments' backbones. Additionally, the designed molecules possess suitable FMOs to match those of typical acceptors PC₆₁BM and PC₇₁BM. Meanwhile, the designed molecules can act as good ambipolar charge transport materials in SMOSC applications. Meanwhile, the electron and hole reorganization energies of the designed molecules are smaller than those of the typical electron and hole transport materials, respectively. Moreover, the differences between electron and hole reorganization energies do not exceed 0.046 eV. Our results suggest that the designed molecules can act as promising candidates for donor and ambipolar charge transport materials in SMOSC applications.