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.

Tailoring Light-Harvesting in Zn-Porphyrin and Carbon Fullerene based Donor-Acceptor Complex through Ethynyl-Extended Donor π-Conjugation

Organic photovoltaic efficiency though currently limited for practical applications, can be improved by means of various molecular-level modifications. Herein the role of extended donor π ${\pi }$ -conjugation through ethynyl-bridged meso-phenyl/pyridyl on the photoinduced charge-transfer kinetics is studied in noncovalently bound Zn-Porphyrin and carbon-fullerene based donor-acceptor complex using time-dependent optimally tuned range-separated hybrid combined with the kinetic rate theory in polar solvent. Noncovalent dispersive interaction is identified to primarily govern the complex stability. Ethynyl-extended π ${\pi }$ -conjugation results in red-shifted donor-localized Q-band with substantially increased dipole oscillator strength and smaller exciton binding energy, suggesting greater light-harvesting efficiency. However, the low-lying charge-transfer state below to the Q-band is relatively less affected by the ethynyl-extended π ${\pi }$ -conjugation, yielding reduced driving forces for the charge-transfer. Detailed kinetics analysis reveals similar order of charge-transfer rate constants (~1012 s-1) for all donor-acceptor composites studied. Importantly, enhanced light-absorption, smaller exciton binding energy and similar charge-transfer rates together with reduced charge-recombination make these complexes suitable for efficient photoinduced charge-separation. These findings will be helpful to molecularly design the advanced organic donor-acceptor blends for energy efficient photovoltaic applications.

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.

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.

Panchromatic ternary organic solar cells with 9.44% efficiency incorporating porphyrin-based donors

In an effort to improve the short-circuit current and fill factor, organic solar cells have been developed with ternary blending in a single bulk heterojunction active layer. We report here several all small molecule organic solar cells based on ternary bulk heterojunction active layers. These layers consist of two small molecule porphyrin donors (MV71 and MV72), which have the same backbone but different end-capping acceptor units, and PC71BM as the acceptor. The organic solar cells showed overall power conversion efficiencies of 3.21% and 4.03% for the as-cast MV71:PC71BM and MV72:PC71BM binary active layers, respectively. However, the power conversion efficiency of the ternary active layer, i.e., MV71:MV72:PC71BM (0.2 : 0.8 : 2), was 6.72% and this is higher than the two binary active layer counterparts. The enhancement in the PCE of the ternary active layer is mainly related to the improvement in both the short-circuit current and fill factor and is related to the synergistic effect of the good miscibility of the two donors and improved hole transportation due to the slightly deeper highest occupied molecular orbital energy level of MV72 than MV71. The PCE was further improved to 9.44% with an enhanced short-circuit current and fill factor when the ternary active layer was subjected to solvent vapour annealing for 40 seconds. The ternary organic solar cells showed higher values of the incident photon to current conversion efficiency across the entire wavelength region when compared to the binary counterparts. The same donor backbone facilitates miscibility at the molecular level and the different HOMO and LUMO energy levels of the donors enable charge transport in the devices based on the ternary active layers. The increase in the power conversion efficiency after SVA treatment may be attributed to the migration of MV71 from the mixed region to the donor-acceptor (D-A) interfaces, which in turn affects the charge transfer and recombination processes and is confirmed by the impedance spectroscopy and dark current-voltage measurements.

Controlling the excited-state dynamics of low band gap, near-infrared absorbers via proquinoidal unit electronic structural modulation

Modulating the extent of configuration interaction steers the excited-state relaxation pathways and dynamics of high oscillator strength NIR absorbers that exploit proquinoidal conjugation.

While the influence of proquinoidal character upon the linear absorption spectrum of low optical bandgap π-conjugated polymers and molecules is well understood, its impact upon excited-state relaxation pathways and dynamics remains obscure. We report the syntheses, electronic structural properties, and excited-state dynamics of a series of model highly conjugated near-infrared (NIR)-absorbing chromophores based on a (porphinato)metal(ii)-proquinoidal spacer-(porphinato)metal(ii) (PM-Sp-PM) structural motif. A combination of excited-state dynamical studies and time-dependent density functional theory calculations: (i) points to the cardinal role that excited-state configuration interaction (CI) plays in determining the magnitudes of S₁ → S₀ radiative (k_r), S₁ → T₁ intersystem crossing (k_ISC), and S₁ → S₀ internal conversion (k_IC) rate constants in these PM-Sp-PM chromophores, and (ii) suggests that a primary determinant of CI magnitude derives from the energetic alignment of the PM and Sp fragment LUMOs (ΔE_L). These insights not only enable steering of excited-state relaxation dynamics of high oscillator strength NIR absorbers to realize either substantial fluorescence or long-lived triplets (τ_T1 > μs) generated at unit quantum yield (Φ_ISC = 100%), but also crafting of those having counter-intuitive properties: for example, while (porphinato)platinum compounds are well known to generate non-emissive triplet states (Φ_ISC = 100%) upon optical excitation at ambient temperature, diminishing the extent of excited-state CI in these systems realizes long-wavelength absorbing heavy-metal fluorophores. This work highlights approaches to: (i) modulate low-lying singlet excited-state lifetime over the picosecond-to-nanosecond time domain, (ii) achieve NIR fluorescence with quantum yields up to 25%, (iii) tune the magnitude of S₁–T₁ ISC rate constant from 10⁹ to 10¹² s^–1 and (iv) realize T₁-state lifetimes that range from ∼0.1 to several μs, for these model PM-Sp-PM chromophores, and renders new insights to evolve bespoke photophysical properties for low optical bandgap π-conjugated polymers and molecules based on proquinoidal conjugation motifs.

Artificial light-harvesting n-type porphyrin for panchromatic organic photovoltaic devices

We developed a novel NIR-harvesting n-type porphyrin derivative, PDI–P_Zn–PDI, that shows a low bandgap of 1.27 eV. Panchromatic absorption was extended to the NIR area with a significantly low energy loss of 0.54 eV which led to promising photovoltaic performance.

A near-infrared-harvesting n-type porphyrin-based acceptor for organic photovoltaics (OPVs) was developed. The n-type acceptor, PDI–P_Zn–PDI, was designed by connecting a zinc porphyrin (P_Zn) core to two perylenediimide (PDI) wings through ethyne bridges. A narrow bandgap of 1.27 eV was achieved through the extended π-conjugation and intramolecular charge transfer between the strongly electron-donating P_Zn core and the electron-accepting PDI wings. A bulk heterojunction (BHJ) structured photovoltaic device fabricated from PDI–P_Zn–PDI with PTB7-Th exhibited panchromatic photon-to-current conversion from 350 to 900 nm. A power conversion efficiency of 5.25% with a remarkably low E_loss of 0.54 eV was achieved by optimizing the nanomorphology of the BHJ films by adding pyridine and by controlling the ZnO/BHJ interfacial properties.

Efficient Polymer Solar Cells with High Open-Circuit Voltage Containing Diketopyrrolopyrrole-Based Non-Fullerene Acceptor Core End-Capped with Rhodanine Units

Herein we report the synthesis of a novel A-D-A-D-A non-fullerene small-molecule acceptor (NFSMA) bearing a diketopyrrolopyrrole (DPP) acceptor central core coupled to terminal rhodanine acceptors via a thiophene donor linker (denoted as MPU1) for use in non-fullerene polymer solar cells (PSCs). This NFSMA exhibits a narrow optical band gap (1.48 eV), strong absorption in the 600-800 nm wavelength region of the solar spectrum, and a lowest unoccupied energy level of -3.99 eV. When the mixture of a medium band gap D-A copolymer P (1.75 eV) was used as donor and MPU1 as acceptor, the blend film showed a broad absorption profile from 400 to 850 nm, beneficial for light harvesting efficiency of the resulted polymer solar cell. After optimization of the donor-to-acceptor weight ratios and concentration of solvent additive, the P-MPU1-based PSC exhibited a power conversion efficiency of 7.52% (J_sc= 12.37 mA/cm², V_oc = 0.98 V, and fill factor = 0.62), which is much higher than that for a P3HT-MPU1-based device (2.16%) prepared under identical conditions. The higher value for the P-MPU1-based device relative to the P3HT-MPU1-based one is related to the low energy loss and more balanced charge transport in the device based on the P donor. These results indicate that alteration of the absorption spectra and electrochemical energy levels of non-fullerene acceptors, and appropriate selection of the polymer donor with complementary absorption profile, is a promising means to further boost the performance of PSCs.

New Terthiophene-Conjugated Porphyrin Donors for Highly Efficient Organic Solar Cells

To mimic the natural photosynthetic systems utilizing chlorophylls to absorb light and store light energy, two new porphyrin-based small molecules of PTTR and PTTCNR have been developed for photovoltaic applications. The highest power conversion efficiency of 8.21% is achieved, corresponding to a short-circuit current of 14.30 mA cm^-2, open-circuit voltage of 0.82 V, and fill factor of 70.01%. The excellent device performances can be ascribed to the engineering of molecule structure and film morphology. The horizontal conjugation of 3,3″-dihexyl-terthiophene to porphyrin-core with the vertical aliphatic 2-octylundecyl peripheral substitutions, can not only effectively increase the solar flux coverage between the conventional Soret and Q bands of porphyrin unit, but also optimize molecular packing through polymorphism associated with side-chains and the linear π-conjugated backbones. And the additive of 1,8-diiodooctane and subsequent chloroform solvent vapor annealing facilitate the formation of the blend films with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) characteristics of bicontinuous, interpenetrating networks required for efficient charge separation and transportation.

Efficiency improvement using bis(trifluoromethane) sulfonamide lithium salt as a chemical additive in porphyrin based organic solar cells

Two new conjugated acceptor-π-donor-π-acceptor (A-π-D-π-A) porphyrins have been synthesised using 3-ethylrhodanine (1a) or dicyanovinylene (1b) groups as acceptor units. Their optical and electrochemical properties made these materials excellent electron donors along with PC₇₁BM as the acceptor for solution-processed bulk heterojunction organic solar cells. The devices based on 1a:PC₇₁BM (1 : 2) and 1b:PC₇₁BM (1 : 2) processed with CB showed low power conversion efficiencies (PCE) of 2.30% and 2.80%, respectively. Nonetheless, after processing the active layer using a mixture of 3 vol% of a pyridine additive in THF, the PCE was enhanced up to 5.14% and 6.06% for 1a:PC₇₁BM and 1b:PC₇₁BM, respectively. Moreover, when we used LiTFSI as the chemical additive in pyridine/CB-processed 1b:PC₇₁BM an excellent PCE of 7.63% was recorded. The effects over the film morphology and the device characteristics (J_sc, V_oc and FF) due to the introduction of LiTFSI are discussed.

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

A new category of deep-absorbing small molecules is developed. Optimized devices driven by mixed additives show a remarkable short-circuit current of ≈20 mA cm(-2) and a highest power conversion efficiency of 9.06%. A multi-length-scale morphology is formed, which is fully characterized by resonant soft X-ray scattering, high-angle annular dark film image transmission electron microscopy, etc.

Porphyrin dimers as donors for solution-processed bulk heterojunction organic solar cells

We synthesize a novel class of porphyrin dimers consisting of two zinc-metalated porphyrin units covalently linked through ethynyl or butadiyne group as electron donors for the fabrication of organic bulk heterojunction (BHJ) solar cells.

Artificial hemes for DSSC and/or BHJ applications

In this review paper a summary of our studies is presented concerning the power conversion efficiency of DSSC and BHJ based on porphyrin hybrid materials.

New low bandgap near-IR conjugated D–A copolymers for BHJ polymer solar cell applications

Two ultra low bandgap (E_g ≤ 1.2 eV) D–A copolymers, with UV to near-IR absorption, are synthesized and used as donors for polymer solar cells, obtaining PCEs of 7.27% and 6.68%.

High-Efficiency Small Molecule-Based Bulk-Heterojunction Solar Cells Enhanced by Additive Annealing

Solvent additive processing is important in optimizing an active layer's morphology and thus improving the performance of organic solar cells (OSCs). In this study, we find that how 1,8-diiodooctane (DIO) additive is removed plays a critical role in determining the film morphology of the bulk heterojunction OSCs in inverted structure based on a porphyrin small molecule. Different from the cases reported for polymer-based OSCs in conventional structures, the inverted OSCs upon the quick removal of the additive either by quick vacuuming or methanol washing exhibit poorer performance. In contrast, the devices after keeping the active layers in ambient pressure with additive dwelling for about 1 h (namely, additive annealing) show an enhanced power conversion efficiency up to 7.78% with a large short circuit current of 19.25 mA/cm(2), which are among the best in small molecule-based solar cells. The detailed morphology analyses using UV-vis absorption spectroscopy, grazing incidence X-ray diffraction, resonant soft X-ray scattering, and atomic force microscopy demonstrate that the active layer shows smaller-sized phase separation but improved structure order upon additive annealing. On the contrary, the quick removal of the additive either by quick vacuuming or methanol washing keeps the active layers in an earlier stage of large scaled phase separation.

Synthesis, optical and electrochemical properties of the A-π-D-π-A porphyrin and its application as an electron donor in efficient solution processed bulk heterojunction solar cells

A conjugated acceptor-donor-acceptor (A-π-D-π-A) with the Zn-porphyrin core and the di-cyanovinyl substituted thiophene (A) connected at meso positions denoted as was designed and synthesized. The optical and electrochemical properties of were investigated. This new porphyrin exhibits a broad and intense absorption in the visible and near infrared regions. Bulk-heterojunction (BHJ) solution processed organic solar cells based on this porphyrin, as electron donor material, and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), as electron acceptor material, were fabricated using THF and a pyridine-THF solvent exhibiting a power conversion efficiency of 3.65% and 5.24%, respectively. The difference in efficiencies is due to the enhancement of the short circuit current J(sc) and FF of the solar cell, which is ascribed to a stronger and broader incident photon to current efficiency (IPCE) response and a better balanced charge transport in the device processed with the pyridine-THF solvent.

Highly efficient small molecule solar cells fabricated with non-halogenated solvents

Fabricated with non-halogenated solvents toluene and o-xylene, the bulk-heterojunction organic solar cells based on a porphyrin small molecule show high power conversion efficiencies up to 5.46% and 5.85%, respectively.

Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers

Dye-sensitized solar cells have gained widespread attention in recent years because of their low production costs, ease of fabrication and tunable optical properties, such as colour and transparency. Here, we report a molecularly engineered porphyrin dye, coded SM315, which features the prototypical structure of a donor-π-bridge-acceptor and both maximizes electrolyte compatibility and improves light-harvesting properties. Linear-response, time-dependent density functional theory was used to investigate the perturbations in the electronic structure that lead to improved light harvesting. Using SM315 with the cobalt(II/III) redox shuttle resulted in dye-sensitized solar cells that exhibit a high open-circuit voltage VOC of 0.91 V, short-circuit current density JSC of 18.1 mA cm(-2), fill factor of 0.78 and a power conversion efficiency of 13%.

New solution processed bulk-heterojunction organic solar cells based on a triazine-bridged porphyrin dyad as electron donor

An unsymmetrical porphyrin dyad (ZnP)-[triazine-Npip]-(ZnPCOOH) consisting of two zinc-porphyrin units covalently linked to a peridine-containing triazine group has been used with PC₇₁BM as electron donor and acceptor, respectively, for the active layer of solution-processed BHJ organic solar cells.