Solution-processable tetrazine and oligothiophene based linear A–D–A small molecules: Synthesis,hierarchical structure and photovoltaic properties

A series of high coplanar alternative linear small molecules with acceptor–donor–acceptor (A–D–A) structure containing electron-accepting tetrazine (Tz) moiety and electron-donating oligothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moiety were designed and synthesized. The influences of varied oligothiophene length on small molecules’ optical and electrochemical properties, crystallization, self assembling morphology in blend film with (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), and photovoltaic properties for the application as donor materials in organic solar cells (OSCs) were studied. The optical and electrochemical properties of small molecules showed that the HOMO and LUMO energy levels were determined by the number of OTs moiety and electron-accepting ability of Tz in the alternative small molecules, respectively. Meanwhile, the varied OT moieties can significantly affect the hierarchical structures when mixed with PC₆₁BM. The molecule with intermediate conjugate moity length showed the highest ordering in its crystalline state, as revealed by differential scanning calorimetry (DSC) and X-ray diffraction experiments, and best photovoltaic properties when blended together with PC₆₁BM or (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as active layer in photovoltaic devices. The results indicate that hierarchical structures controlled by adjusting the conjugate moity length of small molecules is an effective way to improve the performance of OSCs. The photovoltaic device based on TT(HTTzHT)₂:PC₇₁BM with 1% DIO additives showed the best performance, with a J_sc of 7.87 mA/cm² and a PCE of 3.24%.     

Fluorenone‐Based Molecules for Bulk‐Heterojunction Solar Cells: Synthesis,Characterization, and Photovoltaic Properties

A series of four conjugated molecules consisting of a fluorenone central unit symmetrically coupled to different oligothiophene segments are conceptually designed and synthesized to provide new electroactive materials for application in photovoltaic devices. The combination of electron‐donating oligothiophene building blocks with an electron‐accepting fluorenone unit results in the emergence of a new band assigned to an intramolecular charge transfer transition that gives rise to the extension of the absorption spectral range of the resulting molecules. Detailed spectroscopic and voltammetric investigations show that all studied molecules have highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) level positions, which make them good candidates for the application as electron‐donors in bulk‐heterojunction photovoltaic cells, with (6,6)‐phenyl‐C61‐butyric acid methyl ester (PCBM)‐C₆₀ as electron acceptor component. Moderate device performances, with power conversion efficiencies (PCEs) comprised between 0.3 and 0.6%, were obtained with rigid molecules, containing either the bridging units between the thiophene rings, i.e., (2,7‐bis(4,4′‐dioctyl‐cyclopenta[2,1‐b:3,4‐b′]dithiophen‐2‐yl)‐fluoren‐9‐one (SCPTF) and 2,7‐bis(4‐(dioctylmethylene)‐cyclopenta[2,1‐b:3,4‐b′]dithiophen‐5‐yl)‐fluoren‐9‐one (MCPTF) or a vinylene unit 2,7‐bis(5‐[(E)‐1,2‐bis(3‐octylthien‐2‐yl)ethylene])‐fluoren‐9‐one (TVF), whereas with (2,7‐bis‐(3,3?‐dioctyl‐[2,2′;5′,2″;5″,2?]quaterthiophen‐5‐yl)‐fluoren‐9‐one (QTF) PCE up to 1.2% (under AM 1.5 illumination, 100 mW cm^?2, active area 0.28 cm²) was obtained. The strong π‐stacking interactions in the solid state for this oligomer leading to improved morphology could explain the good performances of QTF‐based devices, which rank among the highest recorded for non‐polymeric materials. Consequently, fluorenone‐based non‐polymeric molecules constitute highly attractive materials for solution‐processable solar cell applications.     

Structural control of bulk heterojunction films based on oligothiophene with sterically-bulky groups

Organic photovoltaic solar cells based on oligothiophene and fullerene have been investigated intensively. Until now, the morphologies of bulk heterojunction (BHJ) structures based on oligothiophene and fullerene have been difficult to control because the oligothiophene molecules have lacked substituent groups and therefore have interacted strongly with one another to the point of aggregating into microfibril structures.We succeeded in fabricating homogeneous oligothiophene:C₆₀ BHJ films by adding sterically-bulky groups to oligothiophene in order to control the extent of aggregation. We found that the aggregation onset and maximum fill factor occurred at higher oligothiophene concentrations in BHJ films with C₆₀ for the substituted molecules – the ratio that provides as much oligothiophene as possible for good photovoltaic performance but not so much as to incur aggregation-fill factor was improved. Here in, we report the morphologies and photovoltaic performances of single-component oligothiophene films and BHJ films based on oligothiophenes.     

Evolved structure of thiazolothiazole based small molecules towards enhanced efficiency in organic solar cells

Small molecule donors T0–T2 with thiazolothiazole as acceptor unit, triphenylamine as donor unit and thiophene with different number as bridge are synthesized. With increasing thiophene number and conjugation length, these molecules exhibit red-shifted absorption (300–600 nm), slightly up-shifted HOMO levels (?5.31 to ?5.28 eV), slightly down-shifted LUMO levels (?2.64 to ?2.75 eV), and reduced optical band gaps (2.55–2.11 eV). Solution processed organic solar cells based on T0–T2/PC₇₁BM (1:4, w/w) after thermal annealing exhibit power conversion efficiency up to 2.19%, 3.73% and 4.05% under AM 1.5G illumination (100 mW/cm²), respectively. Effects of thermal annealing on morphology, charge transport and photovoltaic property of blend films are investigated.     

Acceptor‐Substituted S,N‐Heteropentacenes of Different Conjugation Length: Structure–Property Relationships and Solar Cell Performance

The synthesis, optoelectronic, and photovoltaic properties of novel acceptor–donor–acceptor (A–D–A) based π‐conjugated functional molecules 1 – 3, comprising a planar S,N‐heteropentacene as central donor substituted with various terminal acceptor units, such as 1,1‐dicyanovinylene (DCV) and 1‐(1,1‐dicyanomethylene)‐cyclohex‐2‐ene (DCC), are reported. The structural variation of the end groups provides molecules 1 – 3 with gradually increased π‐conjugation due to a rising number of double bonds, which comes from the DCC unit(s). From optoelectronic investigation, structure–property relationships are deduced and the novel A–D–A heteropentacenes 1 – 3 are implemented as photoactive donor component in solution‐processed bulk heterojunction solar cells together with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester as acceptor. The structural variation in the S,N‐heteropentacenes leads to clear trends in the photovoltaic performance and power conversion efficiencies of up to 4.9% are achieved. Furthermore, due to extension of the double bonds a clear trade‐off between the open circuit voltage (V _OC) and the short circuit current density (J _SC) values is observed. The role of additives on the optimization of the nanoscale morphology and device performance is investigated. The findings presented herein demonstrate that depending on the types of materials the additive may have significantly different effects on the active layer morphology and the device performance.     

Rational Design of Small Molecular Donor for Solution‐Processed Organic Photovoltaics with 8.1% Efficiency and High Fill Factor via Multiple Fluorine Substituents and Thiophene Bridge

A series of tetrafluorine‐substituted small molecules with a D₁‐A‐D₂‐A‐D₁ linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of thiophene π‐bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open‐circuit voltage, and short‐circuit current density are dramatically improved in comparison with fluorinated‐free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very‐high FF of 0.76 for BIT‐4F‐ T/PC₇₁BM is achieved without additional additive, which is among one of the highest reported for small‐molecules‐based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.     

Difluorobenzothiadiazole‐Based Small‐Molecule Organic Solar Cells with 8.7% Efficiency by Tuning of π‐Conjugated Spacers and Solvent Vapor Annealing

The synthesis of a series of tetrafluorine‐substituted, wide‐bandgap, small molecules consisting of various π‐conjugated spacers (furan, thiophene, selenophene) between indacenodithiophene as the electron‐donating core and the electron‐deficient difluorobenzothiadiazole unit is reported and the effect of the π‐conjugated spacers on the photovoltaic properties is investigated. The alteration of the π‐conjugated spacer enables fine‐tuning of the photophysical properties and energy levels of the small molecules, and allows the adjustment of the charge‐transport properties, the morphology of the photoactive films, as well as their photovoltaic properties. Moreover, most of these devices exhibit superior device performances after CH₂Cl₂ solvent annealing than without annealing, with a high fill factor (0.70–0.75 for all cases). Notably, the devices based on the new molecule BIT4FTh (with thiophene as the spacer) show an outstanding PCE of 8.7% (with an impressive FF of 0.75), considering its wide‐bandgap (1.81 eV), which is among the highest efficiencies reported so far for small‐molecules‐based solar cells. The morphologies of the photoactive layers with/without CH₂Cl₂ solvent annealing are characterized by atomic force microscopy, transmission electron microscopy and two‐dimensional grazing incidence X‐ray diffraction analysis. The results reported here clearly indicate that highly efficient small‐molecules‐based solar cells can be achieved through rational design of their molecular structure and optimization of the phase‐separated morphology via an adapted solvent–vapor annealing process.     

Small molecules based on bithiazole for solution-processed organic solar cells

A series of donor-acceptor-donor small molecules (1-3) with bithiazole as acceptor unit, triphenylamine as donor unit and thiophene with different number (0, 1, 2) as bridge were synthesized by palladium(0)-catalyzed Suzuki or Stille coupling reactions. The thermal, optical, electrochemical, charge transport, and photovoltaic properties of these small molecules were examined. All compounds exhibit excellent thermal stability with decomposition temperatures (5% weight loss) over 390 °C in nitrogen atmosphere. As increasing the number of thiophene and π-conjugation length of molecule, the absorption maximum in film red shifts from 406 to 498 nm, the extinction coefficient increases from 1.35 × 10⁴ to 7.66 × 10⁴ M⁻¹ cm⁻¹, and the optical band gap decreases from 2.6 to 2.0 eV. The electron-donating thiophene and bithiophene in compounds 2 and 3 up-shift HOMO energy level from −5.42 (1) to −5.24 eV (2) or −5.22 eV (3), and down-shift LUMO energy level from −2.48 (1) to −2.84 eV (2) or −2.81 eV (3). The hole mobility of compound 3 is up to 3.6 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is one order of magnitude higher than that of compound 2, but compound 1 shows no field-effect transistor performance. Solution-processed bulk heterojunction organic solar cells based on 1-3:PC₇₁BM (1:4, w/w) blend films exhibit increasing power conversion efficiency (up to 2.61%) as increasing thiophene unit number.     

Enhanced photovoltaic performances of bis(pyrrolo[3,4-c]pyrrole-1,3-dione)-based wide band gap polymer via the incorporation of an appropriate spacer unit between pyrrolo[3,4-c]pyrrole-1,3-dione units

Bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (BDPPD)-based new electron deficient monomer unit (TTBDPPD) incorporating thieno[3,2-b]thiophene as a connecting spacer unit in between pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione units was prepared. The copolymerization of 2D-conjugated benzodithiophene (BDTT) and TTBDPPD derivatives afforded new alternating copolymer P(BDTT-TTBDPPD). The estimated optical band gap and highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of P(BDTT-TTBDPPD) were 2.06 eV and −5.42 eV/−3.36 eV, respectively. The organic field effect transistor made form P(BDTT-TTBDPPD) exhibited a hole mobility of 6.21 × 10⁻⁴ cm²V⁻¹s⁻¹. The polymer solar cells (PSCs) prepared using P(BDTT-TTBDPPD):PC₇₀BM (1:2 wt%)+3 vol% DIO blend offered a maximum power conversion efficiency (PCE) of 5.37% with an open-circuit voltage (V_oc) of 0.90 V, a short-circuit current (J_sc) of 8.94 mA/cm² and a fill factor (FF) of 67%. This study reveals that the photovoltaic performance of BDPPD-based reported polymer, P(BDTT-TBDPPD), incorporating thiophene spacer unit in between pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione units has been greatly improved (over 2%) when thiophene replaced with thieno[3,2-b]thiophene.     

Diketopyrrolopyrrole-based small molecules with simple structure for high VOC organic photovoltaics

A series of simple structured small molecules based on diketopyrrolopyrrole (DPP) are synthesized and their photovoltaic properties are investigated in terms of the type of electron donating unit. By introducing a donor unit with different electron-donating power such as thiophene (T) and phenylene (Ph), into A−D−A type small molecule, the frontier orbital energy levels of small molecules can effectively be tuned. The small molecule with a weak donor unit of Ph, Ph(TDPP)₂ exhibits a power conversion efficiency of 4.01% with a remarkably high open circuit voltage of 0.93 V when it is blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester as an active layer material in bulk heterojunction solar cells.     

Effects of processing additives on nanoscale phase separation,crystallization and photovoltaic performance of solar cells based on mesogenic phthalocyanine

A study on the effects of processing additives on the nanoscale phase separation, crystallization, and photovoltaic performance of bulk heterojunction (BHJ) thin films made of 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) via spin-casting for photovoltaic applications is reported. By incorporating various solvents as processing additives to a volume of a few percent, the separation of donor and acceptor phases in C6PcH₂:PCBM thin films, which discussed by taking the photoluminescence quenching, Davydov splitting at the Q-band of the absorbance spectra and the surface nanomorphology into consideration, is improved, and the crystallinity of the discotic C6PcH₂ molecules with hexagonal structures is reinforced. Photovoltaic cells with the optimum phase-separated BHJ materials and high crystallinity of the discotic C6PcH₂ molecules are demonstrated to have a power conversion efficiency of 4.2%.     

Construction of simple and low-cost acceptors for efficient non-fullerene organic solar cells

The flexibility in structural design of organic semiconductors endows organic solar cells (OSCs) not only great function-tunabilities, but also high potential toward practical application. In this work, four simple and low-cost non-fullerene acceptors with fluorene or carbazole as central cores, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c] thiophen-4-ylidene)malononitrile (TC) as terminal groups, and thiophene or furan as linkers, named DTC-T-F, DTC-F-F, DTC-T-C and DTC-F-C, are developed through twostep synthesis, and their photophysical properties, electrochemical behavior and photovoltaic performance are systematically and comparatively studied. The results revealed that fluorene-based acceptors exhibited superior photophysical properties and morphology characteristics than carbazole-based counterparts, and thiophene is more suitable as bridging groups. Combining the advantages of both, the BHJ-OSC based on PTB7-Th:DTC-T-F blend film showed a high PCE of 8.8%, with a V_oc of 0.78 V, a J_sc of 17.46 mA cm⁻², and an FF of 0.65, which is the highest value in the PTB7-Th and fluorene-based acceptors coupled devices, implying its potential application.     

Porphyrin small molecules containing furan- and selenophene-substituted diketopyrrolopyrrole for bulk heterojunction organic solar cells

Two porphyrin small molecules substituted with either furan- or selenophene-linked diketopyrrolopyrrole units are designed and synthesized. The impacts of the O and Se chalcogen atoms in the linking 5-membered ring in these new donor materials on the performance of organic solar cells are discussed and contrasted with the previously described thiophene analogue. The heavy atoms broaden the absorption and narrow the bandgap by tuning the energy levels regularly. Furthermore, the selenophene containing analogue shows better miscibility and smaller phase separation with [6,6]-phenyl C₆₁-butyric acid methyl ester (PC61BM) than the furan analogue. The optimized organic solar cells based on the furan and the selenophene containing species are achieved in the presence of pyridine and 1,8-diiodooctane additives with power conversion efficiencies of 4.3% and 5.8%, respectively, under simulated AM 1.5 illumination (100 mW cm⁻²).     

A Highly Crystalline Fused‐Ring n‐Type Small Molecule for Non‐Fullerene Acceptor Based Organic Solar Cells and Field‐Effect Transistors

N‐type organic small molecules (SMs) are attracting attention in the organic electronics field, due to their easy purification procedures with high yield. However, only a few reports show SMs that perform well in both organic field‐effect transistors (OFETs) and organic solar cells (OSCs). Here, the synthesis and characterization of an n‐type small molecule with an indacenodithieno[3,2‐b]thiophene (IDTT) core unit and linear alkylated side chain (C₁₆) (IDTTIC) are reported. Compared to the state‐of‐the‐art n‐type molecule IDTIC, IDTTIC exhibits smaller optical bandgap and higher absorption coefficient, which is due to the enhanced intramolecular effect. After mixing with the polymer donor PBDB‐T, IDTIC‐based solar cells deliver a power conversion efficiency of only 5.67%. In stark contrast, the OSC performance of IDTTIC improves significantly to 11.2%. It is found that the superior photovoltaic properties of PBDB‐T:IDTTIC blends are mainly due to reduced trap‐assisted recombination and enhanced molecular packing coherence length and higher domain purity when compared to IDTIC. Moreover, a significantly higher electron mobility of 0.50 cm² V⁻¹ s⁻¹ for IDTTIC in OFET devices than for IDTIC (0.15 cm² V⁻¹ s⁻¹) is obtained. These superior performances in OSCs and OFETs demonstrate that SMs with extended π‐conjugation of the backbone possess a great potential for application in organic electronic devices.     

Oligothiophene based small molecules with a new end group for solution processed organic photovoltaics

Two oligothiophene based small molecules (DINER5T and DINER7T) with a new end group INER were synthesized as the donors for organic solar cells, and their photovoltaic performance was studied and compared with the corresponding compounds (DRHD7T and DIN7T) with the same backbone structure but different end groups. Both of the new molecules exhibit broad and red shift absorption compared with DRHD7T and DIN7T, with very low band gaps of 1.47 eV and 1.34 eV, respectively. The devices based on DINER5T:PC₇₁BM and DINER7T:PC₇₁BM blend films gave PCEs of 4.22% and 4.02%, respectively, through a solvent vapor annealing (SVA) process with CH₂Cl₂.     

Optimization and Analysis of Conjugated Polymer Side Chains for High‐Performance Organic Photovoltaic Cells

Optimization and analysis of conjugated polymer side chains for high‐performance organic photovoltaic cells (OPVs) reveal a critical relationship between the chemical structure of the side chains and photovoltaic properties of polymer‐based bulk heterojunction OPVs. In particular, the impact of the alkyl side chain length on the π‐bridging (thienothiophene, TT) unit is considered by designing and synthesizing a series of benzodithiophene derivatives (BDT(T)) and thieno[3,2‐b]thiophene‐π‐bridged thieno[3,4‐c]pyrrole‐4,6(5H)‐dione (ttTPD) alternating copolymers, PBDT(T)‐(R₂)ttTPD, with alkyl chains of varying length on the TT unit. Using a combination of 2D X‐ray diffraction, Raman spectroscopy, and electrical device characterization, it is elucidated in detail how these subtle changes to the chemical structure affect the molecular conformation, thin film molecular packing, blend film morphology, optoelectronic properties, and hence overall photovoltaic performance. For copolymers employing both the alkoxy or alkylthienyl‐substituted BDT motifs, it is found that octyl side chains on TT unit yield the maximum degree of molecular backbone coplanarity and result in the highest quality of molecular packing and optimized hole mobility. Inverted devices fabricated using this PBDTT‐8ttTPD: polymer/[6,6]‐phenyl‐C₇₁‐butylic acid methyl ester active layer show a maximum power conversion efficiency (PCE) of 8.7% with large area cells (0.64 cm²) maintaining a PCE of 7.5%.     

Influence of oligothiophene-functionalized co-sensitizer on the electron injection efficiency for multiple dye-TiO2 interface

Co-sensitizer has been employed in dye-sensitized solar cells (DSSCs) to enhance light harvesting at organic/inorganic heterogeneous. Here, the multiple dyes@TiO₂ interface has been investigated by density functional theory simulations, to explore the role of varied oligothiophene-functionalized co-sensitizers on the electron injection efficiency. In presence of co-sensitizers, the simulated absorption spectra broaden with the increasing of the number of thiophene from 0, 1, to 2. Meanwhile, the co-sensitizer modifies the energy alignment of interface, and influences the electronic coupling between dye and TiO₂. Critically, the ratio of electron-hole recombination and electron injection rates k_rec/k_inj based on Marcus theory for both dye and co-sensitizer decrease significantly with increasing of the number of oligothiophene, resulting in the improved electron injection efficiency. Our result implies that the electron injection efficiency depends on the number of thiophene in co-sensitizer largely, and appropriate number plays an active role in tuning the electronic properties of hybrid heterostructure.     

Solution Processable Fluorenyl Hexa‐peri‐hexabenzocoronenes in Organic Field‐Effect Transistors and Solar Cells

The organization of organic semiconductor molecules in the active layer of organic electronic devices has important consequences to overall device performance. This is due to the fact that molecular organization directly affects charge carrier mobility of the material. Organic field‐effect transistor (OFET) performance is driven by high charge carrier mobility while bulk heterojunction (BHJ) solar cells require balanced hole and electron transport. By investigating the properties and device performance of three structural variations of the fluorenyl hexa‐peri‐hexabenzocoronene (FHBC) material, the importance of molecular organization to device performance was highlighted. It is clear from ¹H NMR and 2D wide‐angle X‐ray scattering (2D WAXS) experiments that the sterically demanding 9,9‐dioctylfluorene groups are preventing π–π intermolecular contact in the hexakis‐substituted FHBC 4 . For bis‐substituted FHBC compounds 5 and 6 , π–π intermolecular contact was observed in solution and hexagonal columnar ordering was observed in solid state. Furthermore, in atomic force microscopy (AFM) experiments, nanoscale phase separation was observed in thin films of FHBC and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC₆₁BM) blends. The differences in molecular and bulk structural features were found to correlate with OFET and BHJ solar cell performance. Poor OFET and BHJ solar cells devices were obtained for FHBC compound 4 while compounds 5 and 6 gave excellent devices. In particular, the field‐effect mobility of FHBC 6 , deposited by spin‐casting, reached 2.8 × 10^?3 cm² V^?1 s and a power conversion efficiency of 1.5% was recorded for the BHJ solar cell containing FHBC 6 and PC₆₁BM.     

Synthesis of 2,7‐Carbazolenevinylene‐Based Copolymers and Characterization of Their Photovoltaic Properties

New electroactive and photoactive conjugated copolymers consisting of alternating 2,7‐carbazole and oligothiophene moieties linked by vinylene groups have been developed. Different oligothiophene units have been introduced to study the relationship between the polymer structure and the electronic properties. The resulting copolymers are characterized by UV‐vis spectroscopy, size‐exclusion chromatography, and thermal and electrochemical analyses. Bulk heterojunction photovoltaic cells from different copolymers and a soluble fullerene derivative, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester, have been fabricated, and promising preliminary results are obtained. For instance, non‐optimized devices using poly(N‐(4‐octyloxyphenyl)‐2,7‐carbazolenevinylene‐alt‐3″,4″‐dihexyl‐2,2′;5′,2″;5″,2″′;5″′,2″″‐quinquethiophenevinylene 1″,1″‐dioxide) as an absorbing and hole‐carrier semiconductor exhibit power conversion efficiency up to 0.8 % under air mass (AM) 1.5 illumination. These features make 2,7‐carbazolenevinylene‐based and related polymers attractive candidates for solar‐cell applications.     

Dicyano-functionalized chlorophyll derivatives with ambipolar characteristic for organic photovoltaics

Two ambipolar chlorophyll derivatives, namely, 3²,3²-dicyano-pyropheophorbide-a (Chl-1) and methyl 13¹-deoxo-13¹-(dicyanomethylene) pyropheophorbide-a (Chl-2), were synthesized for use as either the electron acceptor or the electron donor in organic planar-heterojunction solar cells. Despite the higher electron mobilities of these chlorophyll derivatives compared with their hole mobilities, devices using them as the electron donor with fullerene C₇₀ give much better photovoltaic performance than when they are used as the electron acceptor with copper phthalocyanine. In these Chl-based solar cells, the energy gap between the LUMO levels of the donor and acceptor molecules substantially affects the charge separation and resultant photocurrent and photovoltaic performance. The highest solar energy-to-electricity conversion efficiency of up to 2.3% has been achieved using the Chl-2/C₇₀ solar cell, under AM1.5 solar illumination (100 mW/cm²) after thermal annealing of the device. It was also confirmed that the electron mobility of blend films containing Chls and fullerene derivative PC₇₀BM was determined not only by the electron mobility of PC₇₀BM but also by that of Chls.