Low Highest Occupied Molecular Orbital Energy Research Articles

Harnessing the Inductive Effect To Design New Donor–Acceptor–Acceptor′-Configured Small-Molecule Donors for Vacuum-Processed Organic Photovoltaics

Three new donor–acceptor–acceptor′ (D–A–A′)-configured molecules, DTCFiBT, DTCFoBT, and DTCF2BT, with F-substituted benzothiadiazole (BT) as the A group, and two molecules, DTCPiTD and DTCPoTD, adopting pyridal[2,1,3]-thiadiazole (PTD) as the A group, were synthesized and characterized. The effects of the F-substitution number and the orientation of mono F-substituted BT and PTD relative to the D group on the physical properties and intermolecular interactions were examined, together with theoretical calculations to establish the structure–property relationship. In comparison to the parent molecule DTCPB, the inductive effects of F-substituted BTs lower both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of DTCFiBT, DTCFoBT, and DTCF2BT and, thus, similar optical energy gaps (Egopt), while DTCPiTD and DTCPoTD exhibit much lower LUMO energy levels and reduced Egopt, ascribing to the electron-deficient character of PTD. The strong dipolar features of the D–A–A′-configured structure resulting in the antiparallel dimeric packing with different intermolecular interactions in their crystal structures were observed by X-ray analyses. Small-molecule organic solar cells (SMOSCs) with a bulk heterojunction active layer comprising new D–A–A′ donors and C70 were fabricated and characterized. The deeper LUMO levels and reduced Egopt benefit the DTCPiTD- and DTCPoTD-based devices to have higher short current density (JSC), while the DTCFiBT-, DTCFoBT-, and DTCF2BT-based devices benefited from the lower HOMO energy levels that lead to the higher open circuit voltage (VOC). Transient photoluminescence, atomic force microscopy, and incident-light-intensity-dependent device characteristics were examined to reveal the recombination issue for the inferior DTCFoBT-based device. Among these new donors, the DTCFiBT-based device shows the best performance, with VOC of 0.94, JSC of 11.3 mA/cm2, fill factor (FF) of 0.65, and power conversion efficiency of 6.8%, which are attributed to the high VOC as a result of the deeper HOMO level and the superior FF as a result of good exciton separation and charge carrier transport.

Computational study of organic small molecules based on imidazolinone for photovoltaic applications

ABSTRACT Organic photovoltaic cells have received significant interest due to the low-cost manufacturing and compatibility to deposit on flexible substrate. However, this technology suffers from certain problems including lowest solar spectrum absorption by reason of the large band gap of donor materials. This could return to the lack of extensive studies concerning the effects of geometric structures and the energy levels associated with the synthesis of molecules. In this context, six organic π-conjugated molecules based on imidazolinone with low band gap have been studied theoretically using quantum methods namely density functional theory (DFT) and time dependent-density functional theory (TD-DFT). The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and chemicals descriptors of these molecules have been examined in this work. Spectroscopic and electronic properties have been reported in order to study the parameters which affect photovoltaic efficiency. The calculated results of these compounds show a lower HOMO energy, large absorption spectra, and good theoretical open circuit voltage. The findings of this work are helpful to enhance the performance of bulk hetero junction organic solar cells (BHJ).

Theoretical design of new small molecules with a low band-gap for organic solar cell applications: DFT and TD-DFT study

In search of novel high-performance materials for use in organic solar cells, we used density-functional theory and time-dependent density functional theory to design a series of organic small molecules derived from the recently synthesized BT(-2T-DCV-Hex)2 donor molecule. In this work, we replaced the BT unit by different acceptors in order to improve their electronic properties, optical absorption and performance in organic solar cell applications. We found that the hybrid functional B3PW91 with the 6-31G(d) basis set gave highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the BT(-2T-DCV-Hex)2 in better agreement with the experimental oxidation and reduction potentials. However, the range-separated hybrid functional WB97XD was the most appropriate functional for describing the maximum absorption wavelength. Our calculation indicate that the designed small molecules donor proposed here are expected to offer better performances compared to the BT(-2T-DCV-Hex)2, such as a lower HOMO energy, a narrower HOMO-LUMO energy gap, a larger absorption range and may lead to power conversion efficiencies reaching the (7–9)% range.

New D-A1-D-A2-Type Regular Terpolymers Containing Benzothiadiazole and Benzotrithiophene Acceptor Units for Photovoltaic Application.

Two novel regular terpolymers that are of D-A1-D-A2 type and contain benzothiadiazole and 2,5-dibromo-8-dodecanoylbenzo[1,2-b:3,4-b':5,6-d″]trithiophene (P1) or 2,8-dibromo-5-dodecanoylbenzene[1,2-b:3,4-b':5,6-d″]trithiophene (P2) acceptor units with the same thiophene donor were synthesized through Stille coupling, and their optical and electrochemical properties were investigated. The highest occupied molecular orbital (HOMO) and lowest unoccupied (LUMO) molecular orbital energy levels of these terpolymers indicate that there is sufficient LUMO offset with PCBM for efficient exciton dissociation, and their deeper HOMO levels ensure the high open-circuit voltage for the resultant bulk heterojunction solar cells. Measurements on the solar cell devices also confirm that compared to those based on P2 the devices based on P1 possess a higher short-circuit photocurrent (Jsc) as well as a higher fill factor (FF), which is attributed to the lower bandgap and higher hole mobility for P1, whereas the Voc is higher for the devices that are based on P2, which may be a result of P2 having a lower HOMO energy level than P1. The optimized polymer solar cells fabricated using P1:PC71BM (DIO/CF) and P2:PC71BM (CF/DIO) for the active layers showed a PCE of 7.19% and 6.34%, respectively. Atomic force microscopy (AFM) images of P1:PC71BM blend films show that they exhibit more suitable morphology with favorable interpenetrating networks, which favors high Jsc and FF. Moreover, P1 exhibits a more crystalline nature than P2 that also favors the charge transport. This may be a result of better molecular packing, more distinct phase separation of the blended films, as well as a reduction of charge recombination.

Theoretical Investigation of Hole Transporter Materials for Energy Devices

We present combined MD-DFT calculations to investigate the electronic, optical, and charge-transport properties of six triphenylamine-based hole-transporter materials (HTMs) used in solid-state dye-sensitized solar cells (ssDSSC), including the state-of-the-art material in this field, 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD). We find that all of the studied materials present typical features of a HTM: (1) delocalized highest occupied molecular orbital (HOMO); (2) hole reorganization energies higher than the electronic ones; and (3) transparency in the visible region of the electromagnetic spectrum. Among the investigated compounds, 4-(4-phenyl-4-α-naphthylbutadienyl)-N,N-di(4-tolyl)-phenylamine (HTM1) shows the most promising features: a low HOMO energy level that favors high open-circuit voltage in solar cells, high adiabatic ionization potential ensuring great stability in terms of resistance to ionization, and small exciton binding energy. Our results also indicate that the presence of a butadiene moiety in HTM1 is somehow responsible for a higher molecular flexibility, thus favoring the pore filling of the semiconductor in ssDSSC. Finally, its optimal charge delocalization favors the overlap of the atomic orbitals, thus enhancing the electronic couplings, while its low energetic disorder causes a high hole mobility in the amorphous phase. The obtained results are in qualitative and quantitative agreement with the experimental data and suggest that the current computational approach could be further employed to obtain valuable insights for the design of new HTMs aiming at improving the performances of presently available ssDSSC.

Highly Efficient and Low Turn-On Voltage Quantum Dot Light-Emitting Diodes by Using a Stepwise Hole-Transport Layer.

Highly efficient red quantum dot light-emitting diodes (QD-LEDs) with a very high current efficiency of 16 cd/A were demonstrated by adopting stepwise hole-transport layers (HTLs) consisting of 4,4'-N,N'-dicarbazole-biphenyl (CBP) combined with N,N'-dicarbazolyl-3,5-benzene (mCP). The mCP layer plays two important roles in this kind of QD-LEDs. One is that it can block the electron to leak into the HTL due to its higher LUMO (LUMO = the lowest unoccupied molecular orbital) energy level than that of CBP; and the other is it can separate the carrier accumulation zone from the exciton formation interface, which is attributed to the stepwise hole-transport layer structure. Moreover, the lower HOMO (HOMO = the highest occupied molecular orbital) energy level of mCP decreases the hole-injection barrier from the HTL to the QD emitting layer, which improves the charge carrier balance injected into the QD layer, reducing the turn-on voltage of QD-LEDs fabricated with the stepwise HTL structure.

Synthesis and Characterization of Soluble Silyl End‐capped Arene‐Thiophene Co‐Oligomers

Soluble dimethyl‐n‐octylsilyl end‐capped arene‐thiophene oligomers with the central unit including phenyl, biphenyl, fluorine, and naphthalene were synthesized by Stille reaction. The optical, electrochemical, and thermal properties of the oligomers were then measured. The absorption maxima of the arene‐thiophene oligomers in dilute solutions were close to that obtained for DSi‐4T (λabs = 403 nm). They exhibited well‐resolved fluorescence spectra in solution with emission maxima at 445, 446, 448, and 450 nm for DSi‐TTPTT , DSi‐TTPPTT , DSi‐TTFTT , and DSi‐TTNTT , respectively. The highest occupied molecular orbital (HOMO) energy levels were determined to be −5.29, −5.33, −5.27 and −5.29 eV, respectively; therefore all co‐oligomers displayed a lower HOMO energy level than the −5.14 eV of DSi‐6T . The thermal stability of the arene‐thiophene oligomers was investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Except for DSi‐TTPTT , the TGA results showed a 5% weight loss at around 400 °C, which indicates high thermal stability. Dimethyl‐n‐octylsilyl end substituents allowed solution processability due to their high solubility. Overall, the electrochemical analysis of arene‐thiophene oligomers indicated superior oxidative stability to DSi‐6T .

Linked-Acceptor Type Conjugated Polymer for High Performance Organic Photovoltaics with an Open-Circuit Voltage Exceeding 1 V.

A linked‐acceptor type conjugated polymer is designed and sythesized based on 4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene (BDTT) and linked‐thieno[3,4‐c]pyrrole‐4,6‐dione (LTPD). This polymer uses alkyl‐substituted thiophene as a bridge. The PBDTT‐LTPD includes two TPD units in one repeating unit, which can enhance acceptor density in the polymer backbone and lower the highest occupied molecular orbital (HOMO) level. By contrast, variable alkyl substitutions in the thiophene‐bridges ensure the subtle regulation of polymer properties. The solar cells based on PBDTT‐LTPD display an open‐circuit voltage (V oc) that exceeds 1 V, and a maximum power conversion efficiency (PCE) of 7.59% is obtained. This PCE value is the highest for conventional single‐junction bulk heterojunction solar cells with V oc values of up to 1 V. Given that PBDTT‐LTPD exhibits a low HOMO energy level and a band gap equivalent to that of poly(3‐hexylthiophene), PBDTT‐LTPD/phenyl‐C61‐butyric acid methyl ester may be a promising candidate for the front cell in tandem polymer solar cells.

Fine tuning of terpolymer properties by incorporating electron-accepting difluorobenzene and diketopyrrolopyrrole units

A series of low band gap terpolymers (PBDT–DFBx–DPPy) incorporating various ratios of two acceptor units, difluorobenzene (DFB) and diketopyrrolo[3,4-c]pyrrole (DPP), were synthesized by Pd-catalyzed Stille polymerization. The effects of the DFB and DPP units on UV–Vis absorption, highest occupied molecular orbital (HOMO) energy levels, and terpolymer: PC60BM solar cell performance, especially the short-circuit current (J sc), were investigated. The polymer films exhibited broad and intense absorption spectra, covering the spectral range from 300 nm up to 900 nm. The optical properties of these random terpolymers can be easily controlled by tuning the ratio between DFB and DPP; an increase in DFB induced increased absorption between 300 and 550 nm and a lower HOMO energy level. The terpolymer PBDT–DFB50–DPP50 with the percentage of DFB up to 50 % in the bulk heterojunction polymer solar cell showed a high open-circuit voltage of 0.74 V and a J sc of 7.83 mA cm−2. This study demonstrated that significant changes in polymer electronic properties can be tuned by the strategic combination of simple electron-deficient monomers and their composition in the random terpolymers.

Developing conjugated polymers with high electron affinity by replacing a C-C unit with a B←N unit.

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a C-C unit by a B←N unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices.

New –(D–A1–D–A2)n– type conjugated polymers for photovoltaic applications: consensus between low band-gap and low HOMO energy level

To gain ideal conjugated polymers with a low band-gap (Eg) and a deep HOMO (highest occupied molecular orbital) energy level for photovoltaic application, both strong and weak electron withdrawing units were introduced into polymer backbone. A series of –(D–A1–D–A2)n– type diketopyrrolopyrrole (DPP) containing polymers P1–P7 were designed and synthesized. The resulting copolymers exhibited both low Eg of 1.27–1.48 eV and deep HOMO energy levels of −5.25to −5.44 eV. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated. All photovoltaic devices featured high open circuit voltages (Voc, 0.72–0.91 V). Among the seven copolymers, P5 exhibited the best performance with a Voc of 0.80 V, a short circuit current density (Isc) of 3.48 mA/cm2, a fill factor (FF) of 43.2%, and a power conversion efficiency (PCE) of 1.33%.

Development of naphthalene and quinoxaline‐based donor–acceptor conjugated copolymers for delivering high open‐circuit voltage in photovoltaic devices

AbstractTwo new quinoxaline‐based polymers, poly[1,5‐didecyloxynaphthalene‐alt‐5,5′‐(5,8‐dithiophen‐2‐yl)‐2,3‐bis(4‐octyloxyphenyl)quinoxaline (PNQx‐p) and poly[1,5‐didecyloxynaphthalene‐alt‐5,5′‐(5,8‐dithiophen‐2‐yl)‐2,3‐bis(3‐octyloxyphenyl)quinoxaline (PNQx‐m), were synthesized by Suzuki coupling reaction and characterized. Thermogravimetric analysis revealed that these polymers are thermally stable with degradation temperature up to 320 °C. As evident from the electrochemical and optical studies, the copolymers have comparable optical band gap (∼2 eV) and nearly similar deep highest occupied molecular orbital (HOMO) energy levels of −5.59 (PNQx‐p) and −5.61 eV (PNQx‐p). The resulting copolymers possessed relatively low HOMO energy levels promising good air stability and high open circuit voltage (Voc) for photovoltaic applications. The optimized photovoltaic device with a structure of ITO/PEDOT:PSS/PNQx‐m:PC71BM (1:2, w/w)/LiF/Al shows a power conversion efficiency up to 2.29% with a short circuit current density of 5.61 mA/cm2, an Voc of 0.93 V and a fill factor of 43.73% under an illumination of AM 1.5, 100 mW/cm2. The efficiency of the PNQx‐m polymer improved from 2.29 to 2.95% using 1,8‐diiodoocane as an additive (0.25%). © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Synthesis and photovoltaic properties of benzotriazole-based donor–acceptor copolymers

Two new D–A copolymers containing benzotriazole (BTz) acceptor unit and different donor units of benzodithiophene (BDT) and 2,7-carbazole, PBDT-DTBTz and PC-DTBTz, were synthesized for the application as donor materials in polymer solar cells (PSCs). By changing the donor units, the band gaps and the highest occupied molecular orbital (HOMO) energy levels of the copolymers could be finely tuned. PC-DTBTz exhibited the lower HOMO energy level of −5.34 eV, and the lower HOMO energy level can lead to a higher open-circuit voltage (Voc) of 0.73 V in PC-DTBTz-based devices. The PCEs of the PSCs based on PBDT-DTBTz and PC-DTBTz blends with [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) were 1.55 and 1.33 %, respectively, under the illumination of AM1.5, 100 mW/cm2.

Preparation of new FI-type catalyst for polymerization of ethylene

AbstractA new FI-type Zr-based catalyst of bis[N-(3,5-dicumylsalicylidene)- anthracylaminato]zirconium(IV) dichloride was found to display a moderate ethylene polymerization activity of 42×104 g PE/mmol Zr. h with a viscosity average molecular weight (Mv) of 56×104 at the monomer pressure of 4 bars and temperature of 30 °C. HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) energy level of the anionic phenoxyimine ligands, were calculated using HF/6-31G method. Plurality of the fused aromatic rings on the catalyst structure decreased the energy gap between the HOMO and LUMO, but could not increase the activity. The highest activity of the catalyst was obtained at about 30 °C following a sharp decrease at higher temperature. The catalyst showed the high activity at about [Al]:[Zr]=80000:1 molar ratio and further addition did not affect the activity of the catalyst. Experimental results revealed that low HOMO energy level is not a necessity to reach high activity of the catalyst. Despite low HOMO energy level, the prepared catalyst displayed moderate polymerization activity. Either the activity of the catalyst or the viscosity average molecular weight of the obtained polymer was affected by the hydrogen concentration slightly. However, activity of the catalyst showed higher sensitivity to amount of hydrogen and increased with increasing the hydrogen concentration. Higher monomer pressure increased productivity, crystallinity and the Mv values of the resulting polymer. Some specification of the obtained polymer was studied.

A low bandgap polymer based on isoindigo and bis(dialkylthienyl)benzodithiophene for organic photovoltaic applications

AbstractA new conjugated polymer PBDTT‐ID based on N‐alkylated isoindigo (ID) and bis(2,3‐dialkylthienyl)‐substituted benzo[1,2‐b:4,5‐b′]dithiophene (BDTT) as repeating units was synthesized. It had an optical bandgap of 1.56 eV and a highest occupied molecular orbital (HOMO) energy level of −5.71 eV. The optical, electrochemical, and photovoltaic properties of new polymer were compared with previous reported polymer PBDT‐ID, which was based on bis(alkoxy)‐substituted benzo[1,2‐b:4,5‐b′]dithiophene. The new polymer displayed lower HOMO energy level and better absorption properties than polymer PBDT‐ID. The solar cells fabricated with PBDTT‐ID/PC61BM (1:2, w/w) blends as active layers exhibited photoresponse in the range of 300–800 nm. A power conversion efficiency of 4.02% and an open circuit voltage (Voc) of 0.94 V were achieved in polymer solar cell device based on the new polymer. This was the highest Voc realized among the isoindigo‐based polymers. The relatively high performances of new polymer in solar cell devices were interpreted in terms of material properties and morphologies of polymer/PCBM blends. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Synthesis and Photovoltaic Properties of Thieno[3,4-c ]pyrrole-4,6-dione-based donor-acceptor Copolymers

Three donor–acceptor (D–A) 1,3-di(thien-2-yl)thieno [3,4-c]pyrrole-4,6-dione-based copolymers, poly{9,9-dioctylfluorene-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}, poly{N-(1-octylnonyl)carbazole-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}, and poly {4,8-bis(2-ethylhexyloxyl) benzo[1,2-b:3,4-b′]dithiophene-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c] pyrrole-4,6-dione} were synthesized by Suzuki or Stille coupling reaction. By changing the donor segment, the bandgaps and energy levels of these copolymers could be finely tuned. Cyclic voltammetric study shows that the highest occupied molecular orbital (HOMO) energy levels of the three copolymers are deep-lying, which implies that these copolymers have good stability in the air and the relatively low HOMO energy level assures a higher open-circuit potential when they are used in photovoltaic cells. Bulk-heterojunction photovoltaic cells were fabricated with these polymers as the donors and PC71BM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.22, 0.74, and 3.11% with large open-circuit potential of 1.01, 0.99, and 0.90 V under one sun of AM 1.5 solar simulator illumination (100 mW/cm2). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Alternating Copolymers of Cyclopenta[2,1‐b;3,4‐b′]dithiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione for High‐Performance Polymer Solar Cells

AbstractA series of alternating copolymers of cyclopenta[2,1‐b;3,4‐b′]dithiophene (CPDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) have been prepared and characterized for polymer solar cell (PSC) applications. Different alkyl side chains, including butyl (Bu), hexyl (He), octyl (Oc), and 2‐ethylhexyl (EH), are introduced to the TPD unit in order to adjust the packing of the polymer chain in the solid state, while the hexyl side chain on the CPDT unit remains unchanged to simplify discussion. The polymers in this series have a simple main chain structure and can be synthesized easily, have a narrow band gap and a broad light absorption. The different alkyl chains on the TPD unit not only significantly influence the solubility and chain packing, but also fine tune the energy levels of the polymers. The polymers with Oc or EH group have lower HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, resulting higher open circuit voltages (Voc) of the PSC devices. Power conversion efficiencies (PCEs) up to 5.5% and 6.4% are obtained from the devices of the Oc substituted polymer (PCPDTTPD‐Oc) with PC61BM and PC71BM, respectively. This side chain effect on the PSC performance is related to the formation of a fine bulk heterojunction structure of polymer and PCBM domains, as observed with atomic force microscopy.

Synthesis, characterization, and photovoltaic property of a low band gap polymer alternating dithienopyrrole and thienopyrroledione units

The lower the highest occupied molecular orbital (HOMO) energy level of the conjugated polymer is, the higher the open-circuit voltage ( V OC ) of the obtained polymer solar cell (PSC) is. To achieve this goal, a new conjugated polymer (PDTPTPD) alternating dithienopyrrole (DTP) and thienopyrroledione (TPD) units was designed and synthesized by Stille coupling reaction. Through UV–vis absorption and cyclic voltammetry (CV) measurements, it was found that the resulting copolymer exhibited both a low optical band gap of 1.62 eV and a low HOMO energy level of −5.09 eV owing to the electronegativity of TPD moiety. Preliminary photovoltaic study disclosed that the PSC based on PDTPTPD:PCBM ([6,6]-phenyl-C 61 butyric acid methyl ester) blend showed a power conversion efficiency (PCE) of 1.9%, with a V OC of 0.70 V, and a short circuit current ( I SC ) of 6.97 mA/cm 2, suggesting that PDTPTPD is a promising photovoltaic polymer.

Substituent effects on the photofading of disperse azo dyes on poly(ethylene terephthalate) substrate

The relationships between the chemical structures and oxidative fading of the disperse azo dyes, p‐nitrophenylazo‐ and benzothiazoleazo‐anilines, on poly(ethylene terephthalate) substrate are discussed in terms of the parameters k0,i (rate constants of reaction towards 1O2) and fi (photosensitivity), the molecular parameters of molecular orbital theory and substituents in the diazo and coupling components, on the assumption that the initial rates of oxidative fading are proportional to the product of k0,i and fi. 2‐Methoxy‐5‐acetylamino‐N‐substituted aniline couplers exhibited large fi values. 2‐Chloro and 4‐nitro substituents of aniline diazo components exhibited small fi values or high quantum yields of internal conversion, while 4‐nitro substituent did not. A close correlation between N‐substituents and light fastness, proposed by Müller and supplemented by Dawson, demonstrates the applicability of frontier orbital theory, through the highest occupied molecular orbital (HOMO) energy of the dyes, to the analysis of oxidative fading. Dyes with N‐2‐cyanoethyl substituents, which gave a lower HOMO energy, also exhibited superior light fastness compared with N‐2‐hydroxyethyl substituents.

A Weak Donor−Strong Acceptor Strategy to Design Ideal Polymers for Organic Solar Cells

Polymers to be used in bulk heterojunction (BHJ) solar cells should maintain a low highest occupied molecular orbital (HOMO) energy level as well as a narrow band gap in order to maximize the open circuit voltage (V(oc)) and the short circuit current (J(sc)). To concurrently lower the HOMO energy level and the band gap, we propose to modify the donor-acceptor low band gap polymer strategy by constructing alternating copolymers incorporating a "weak donor" and a "strong acceptor". As a result, the "weak donor" should help maintain a low HOMO energy level while the "strong acceptor" should reduce the band gap via internal charge transfer (ICT). This concept was examined by constructing a library of polymers employing the naphtho[2,1-b:3,4-b']dithiophene (NDT) unit as the weak donor, and benzothiadiazole (BT) as the strong acceptor. PNDT-BT, designed under the "weak donor-strong acceptor" strategy, demonstrated both a low HOMO energy level of -5.35 eV and a narrow band gap of 1.59 eV. As expected, a noticeably high V(oc) of 0.83 V was obtained from the BHJ device of PNDT-BT blended with PCBM. However, the J(sc) ( approximately 3 mA/cm(2)) was significantly lower than the maximum expected current from such a low band gap material, which limited the observed efficiency to 1.27% (with a 70 nm thin film). Further improvements in the efficiency are expected from these materials if new strategies can be identified to (a) increase the molecular weight and (b) improve the hole mobility while still maintaining a low HOMO energy level and a narrow band gap.