Broad Absorption Region Research Articles

Recent advances in electron acceptors with ladder-type backbone for organic solar cells

Ladder-type non-fullerene acceptors have developed very rapidly since 2015 as they have tunable structure, broad absorption region and good morphology control. The design guidelines and material progress as well as the device understanding are summarized in this review.

Bimetallic Ag‒Au‒Ag nanorods used to enhance efficiency of polymer solar cells

In this work, various sizes of bimetallic Ag–Au–Ag nanorods (NR), designated NR1, NR2, and NR3, were successfully synthesized and introduced into the hole-transporting layer (HTL, PEDOT:PSS) of polymer solar cells (PSCs) to enhance the power conversion efficiencies (PCEs) by taking advantage of the broadband cooperative plasmonic effect. With an increase in the length of Ag in the bimetallic NRs, a wider range of wavelengths, from the visible to the near-infrared region, could be absorbed. This was attributed to the transverse and longitudinal plasmon excitations of the NRs. As a result, P3HT:PC61BM-based PSCs with NR1 or NR2 embedded in the HTL showed a 9% and 10% enhancement in the PCE, respectively, relative to the reference device. Notably, NR3, with broad localized surface plasmon resonance excitations, showed a major improvement in the PCE of up to 18% with the same P3HT:PC61BM-based device structure. High-performance PSCs were achieved when NR3 was embedded into the HTL of PTB7:PC71BM-based PSCs. A device with the optimized doping content of 7 vol% NR3 can significantly enhance the short-circuit current density (Jsc) by 12%, and achieves an excellent level of performance with a PCE of 7.36%, accounting for a 19% enhancement contrast, relative to the control device. The origin of the performance improvement of PSCs with NRs was analyzed using TEM, SEM, and UV–Vis characterizations, along with theoretical simulations. It was found that the enhancement could be ascribed to the broad absorption region, appropriate doping concentration, and the scattering-induced absorption enhancement of the active layer. The findings herein successfully demonstrate a general and efficient approach toward the enhancement of the performance of PSCs by incorporating the Ag–Au–Ag NRs into the HTL, and indicate that an appropriate doping content and NR size can significantly influence Jsc and the fill factor.

Alternating polymers based on fluorinated alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene and isoindigo derivatives for polymer solar cells

To effectively modulate the molecular energy levels of isoindigo-based polymers through side-chain modification on the donor segments, three alkoxyphenyl-modified benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized and used as electronic-donor units, which were copolymerized with the electron-withdrawing segment 2-ethylhexyl-substituted isoindigo (IID) to construct donor-acceptor (D-A) polymers. The obtained three polymers are named as P-IID, mF-IID and oF-IID, which possess 4-(2-octyldodecaneoxy)-phenyl, 2-fluoro-4-(2-octyldodecaneoxy)-phenyl and 3-fluoro-4-(2-octyldodecaneoxy)-phenyl side-chains in the BDT unit, respectively. All polymers display broad absorption regions with the film absorption edges of 759 nm (P-IID), 742 nm (mF-IID) and 726 nm (oF-IID). The fluorination on the phenyl side-chains of BDT unit results in obvious blue-shift in the absorption spectra, especially for the ortho-position fluorinated polymer oF-IID because of the enlarged dihedral angles between the phenyl rings and the BDT backbones. Gradient reduction in the highest occupied molecular orbital (HOMO) energy levels are observed for the three polymers, with HOMO values of −5.35 eV for P-IID, −5.48 eV for mF-IID and −5.53 eV for oF-IID, separately. As the result of the HOMO energy levels, polymer solar cells (PSCs) with inverted device configuration by blending the polymers with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) show continually increased open circuit voltage (Voc) from 0.84 V for P-IID to 0.88 V (mF-IID) and then a notable value of 0.96 V for oF-IID. However, the fluorination leads to intensively reduced short-circuit current density (Jsc) of the PSCs, 9.68 mA cm−2 for P-IID, 5.61 mA cm−2 for mF-IID and a very low value of 1.61 mA cm−2 for oF-IID, respectively. Consequently, moderate power conversion efficiency (PCE) of 5.23% is achieved for the non-fluorinated polymer P-IID as well as a decreased value of 2.50% for the meta-fluorinated polymer and a very low PCE of 0.93% for the ortho-fluorinated polymer. These results imply that although fluorine substituted side-chain in the donor unit can effectively deepen the HOMO energy levels of the polymers to obtain enlarged Voc, the absorption losses of the fluorinations caused low Jsc should be taken into consideration to design high efficient electron-donating alternating polymers for polymer solar cells.

Perovskite—Cavity Complex: the Modulated Optical Characteristics of Organometallic Halide Perovskite CH3NH3PbIxCl3–x Doped in Length Tunable Optical Microcavities

When organometallic halide perovskite semiconductors with continuous transition band were placed into an optical microcavity, interaction/modulation effect between confined photonic mode supported by cavity and free charge carriers mode triggered in perovskite occurred. In this metal–insulator–metal (MIM) microcavity, given the broad absorption region of perovskite CH3NH3PbIxCl3–x, the cavity resonant energy can be selectively designed to modulate the photophysical characteristics of perovskite charge carriers at different levels. Another advantage of using perovskite is that its band filling effect (charge carrier’s accumulation process) and relaxation process could last for few and hundreds picoseconds respectively, allowing us to accurately examine the time-resolved couple modulating effect by microcavities. Steady-state and transient absorption (TA) spectra were carried out to experimentally confirm the modulation effect of microcavity on optical characteristics of perovskite free charge carriers. It ...

Development of Tin-Rich Methylammonium Halide Perovskites for Carbon-Based Solar Cells Free of Hole-Transport Materials

Methylammonium halide perovskite solar cells have received a tremendous amount of attention in recent years due to their skyrocketing efficiency improvement starting from 4% to 20% within six years. However, the employment of metal electrode, expensive hole-transporting materials (HTM), and particularly, environmentally hazardous material of lead presents a barrier to successful commercialization. In this study, we report on preparation and characterization of perovskite material with PbI2 partially replaced by SnI2 or SnCl2. Moreover, the performances of mixed perovskite solar cells are based on a HTM-free carbon electrode configuration. Noticeably, the case of employing SnCl2, CH3NH3SnyPb(1−y)I3-xClx, which shows completely different energy level trend to CH3NH3SnyPb(1−y)I3 and better photovoltaic performance when more SnCl2 component in the devices and stability of devices are 70 times more than SnI2 one. Moreover, the device has the highest powder conversion efficiency (PCE) of 2.15% (CH3NH3Sn0.75Pb0.25I3-xClx), which shows broad light absorption region up to 1000 nm and highest current density of 15.66 mA cm-2. In order to further improving device performance, a 20 mol% SnF2 additive was introduced into CH3NH3Sn0.75Pb0.25I3-xClx, after that current density can be improved to 23 mA cm-2, V OCis 0.35V, FF is 0.52 and PCE will beyond 4.0% with a small effect of hysteresis and excellent long-term stability over 1000 h (storing at glove box). Figure 1

Synthesis of new conjugated copolymers containing 4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene/5,7-bis(3,4-diethylthien-2-yl)-2,3-diphenylthieno[3,4-b]pyrazine and 4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene/4,6-di(3,4-diethylthien-2-yl)-thieno[3,4-c][1,2,5]thiadiazole derivatives for photovoltaic applications

Two new donor-acceptor narrow-band-gap photovoltaic copolymers containing various electron-acceptor functional groups, such as thienopyrazine and thienodithiazole derivatives, are synthesized under the conditions of the Suzuki polycondensation reaction and characterized via 1H NMR spectroscopy and GPC. The optical, electrochemical, and photovoltaic properties of the polymers are examined in detail. Both copolymers have good solubilities, high thermal stabilities, broad absorption regions (350–950 nm), relatively low levels of the highest occupied molecular orbital (−5.01 and −5.02 eV for polymers I and II, respectively), and narrow band gaps (1.31 and 1.41 eV, respectively). Photovoltaic devices based on a polymer-II-PC60BM (1: 3, wt/wt) mixture under AM 1.5 irradiation of 100 mV/cm2 exhibit a power-conversion efficiency of ∼0.29%, a short-circuit current of 1.45 mA/cm2, an open-circuit voltage of Voc = 0.67 V, and a fill factor of 0.30. These results are evidence that thienopyrazine functional groups are attractive as electron-acceptor building blocks in organic electronics.

Synthesis and photovoltaic properties of new donor-acceptor benzodithiophene-containing copolymers

Four new alternating narrow band-gap copolymers containing benzodithiophene, 4,8-dithiophen-2-yl-benzo[1,2-c;4,5-c′-bis[1,2,5]thiadiazole, 4,9-bis(thiophen-2-yl)-6,7-di(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline, 5,8-dibromo-2,3-bis(5-octylthiophen-2-yl)quinoxaline, and 4,7-bis(5-bromothiophen-2-yl)benzo[1,2,5] thiadiazole units are synthesized under Stille reaction conditions. The structures, molecular masses, and physical properties of the copolymers are studied via 1H NMR spectroscopy, GPC, cyclic voltammetry, and thermomechanical and thermogravimetric analyses. The polymers show solubility and a broad absorption region (with the band gap in the range from 0.81 to 1.53 eV). All of the polymers are photostable in air, and their levels of the highest occupied molecular orbital vary from −4.98 to −5.30 eV. Polymer solar cells based on these copolymers as donors and fullerene PC60BM as an acceptor show open-circuit voltages in the range 0.16–0.61 V, and the efficiencies of the devices are in the range 0.02–0.49%.

New donor–acceptor conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene for photovoltaic cells

Two new donor–acceptor (D–A) low band gap conjugated copolymers P1 and P2 containing different electron acceptor functional groups i.e. thienodithiadiazole (P1) and thienopyrazine (P2) derivatives were synthesized by Suzuki polycondensation reaction and characterized using 1H NMR and gel-permeation chromatography. The optical and electrochemical properties of these copolymers were investigated. Both copolymers have a good solubility, high thermal stability, broad absorption region from 350 to 950nm, a relatively low HOMO energy in the range of −5.28eV for P1 and −5.25eV for P2 and a narrow optical band gap 1.32 and 1.46eV, respectively. Photovoltaic devices based on a mixture of the P2: PC60BM (1:3 (w/w)) at AM1.5 illumination of 100mWcm−2 show the over all power conversion efficiency (PCE) of 0.74% with a short-circuit current (Jsc) of 2.28mAcm−2, open circuit voltage (Voc) of 0.72V and fill factor (FF) of 0.45, whereas P1:PC60BM showed PCE of 0.43% with a Jsc of 1.54mAcm−2, Voc of 0.70V and FF of 0.40. These results suggest that the thienopyrazine functional groups are attractive electron-withdrawing building blocks in conjugated D–A copolymers and can be used as electron donors for bulk heterojunction solar cells along with fullerene derivatives as electron acceptors.

Synthesis and characterization of naphthalene diimide polymers based on donor-acceptor system for polymer solar cells

Three donor-acceptor (D-A) naphthalene diimide copolymers, poly{thieno(3,2-b)thiophene-diyl-alt-N,N!-bis(2- octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)}(P1), poly{4,8-dioctyloxybenzo(1,2-b;3,4-b!)dithiophene-diyl-alt- N,N!-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)}(P2), and poly{4,8-bis(2-ethylhexyloxy)benzo(1,2- b:4,5-b!)dithiophene-diyl-alt-N,N!-bis(2-octyldodecyl) -naphthalene-1,4,5,8-bis(dicarboximide)}(P3), were synthesized by Stille coupling reaction. All copolymers showed good solubility in common organic solvents with broad absorption region and narrow optical band gap. The electrochemical properties of the polymers can be adjusted by changing the donor seg- ment. P1 showed n-type characteristic while P2 and P3 showed p-type characteristic. All-polymer solar cells using P3HT as the donor and P1 as the acceptor were fabricated and the highest power conversion efficiencies (PCEs) of 0.068% were obtained under the preliminary condition. Moreover, p-type naphthalene diimide copolymers (P2 and P3) based on D-A system were used as donors to fabricate bulk heterojunction polymer solar cells (BHJ, PSCs) for the first time, and the max- imum power conversion efficiencies PCEs were about 0.021 and 0.017%, respectively.

Attenuation of vacuum ultraviolet light in liquid argon

The transmission of liquid argon has been measured, wavelength resolved, for a wavelength interval from 118 to 250 nm. The wavelength dependent attenuation length is presented for pure argon. It is shown that no universal wavelength independent attenuation length can be assigned to liquid argon for its own fluorescence light due to the interplay between the wavelength dependent emission and absorption. A decreasing transmission is observed below 130 nm in both chemically cleaned and distilled liquid argon and assigned to absorption by the analogue of the first argon excimer continuum. For not perfectly cleaned argon a strong influence of impurities on the transmission is observed. Two strong absorption bands at 126.5 and 141.0 nm with approximately 2 and 4 nm width, respectively, are assigned to traces of xenon in argon. A broad absorption region below 180 nm is found for unpurified argon and tentatively attributed to the presence of water in the argon sample.

Design of donors with broad absorption regions and suitable frontier molecular orbitals to match typical acceptors via substitution on oligo(thienylenevinylene) toward solar cells

A series of oligo(thienylenevinylene) derivatives with or without thieno[3,2-b]thiophene analogs as cores and three types substituent has been investigated at the PBE0/6-31G(d) and the TD-PBE0/6-31+G(d,p) levels to design materials with high performance such as suitable frontier molecular orbital (FMO) energies to match those of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) and its derivatives, broad absorption spectra, higher and balance transfer property, and better stability. The results reveal that fused cores have slight effects on photophysical properties of investigated derivatives. The electron-withdrawing or push-pull substituents result in red shifts of absorption spectra and better stabilities for investigated derivatives. The calculated reorganization energies of designed derivatives suggest them to be good potential ambipolar transport materials under the proper operating conditions. The promising donors for PCBM, bisPCBM, PC70BM, and indene-C(60) bisadduct (ICBA) as acceptors are recommended theoretically for solar cells based on the proper match for FMOs between donors and acceptors. Moreover, we have also predicted the mobility of designed molecule with better performance.

Properties and H 2 production ability of Pt photodeposited on the anatase phase transition of nitrogen-doped titanium dioxide

The effect of calcination temperature on the properties and H 2 production ability of nitrogen-doped (N-doped) titanium dioxide (TiO 2) photodeposited with 0.2 wt% Pt (platinum) was studied. The increase in crystallinity of pre-calcinated N-doped TiO 2 initiated at temperatures higher than 131 °C transformed the morphology from anomalous nanostructure to texture composed of nanoparticles and enhanced the specific surface areas. At 200–400 °C, the anatase peaks gradually became sharper and the visible light absorption region decreased due to the growth of crystallites and the decrease of N-doping content, respectively. Maximum H 2 production was reached when N-doped TiO 2 was calcined at 200 °C followed by Pt photodeposition. The maximum condition is brought about by the formation of textures consisting of nanoparticles and a broad absorption region, thus creating superior active sites for photocatalytic H 2 production.

Rational Design of Organic Asymmetric Donors D1–A–D2 Possessing Broad Absorption Regions and Suitable Frontier Molecular Orbitals to Match Typical Acceptors toward Solar Cells

A series of donors have been designed based on three building blocks, i.e., triphenylamine (TPA), 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DBT), and oligothiophenes (5Th), to investigate suitable molecules toward solar cell materials. The donors are built by two donor fragments (TPA as D1 and 5Th as D2) and one acceptor fragment (DBT as A) in three topologies with or without linkages and electron-withdrawing groups. Their electronic and optical properties have been characterized by the PBE0/6-31G(d,p) and the TD-PBE0/6-31+G(d,p) approaches. The calculated results reveal that the asymmetric donor in the topology of D1-A-D2 with vinyl linkage and -NO(2) groups is a rational strategy to design novel material possessing a small HOMO-LUMO gap, broad and intense absorption spectra, and higher charge transfer ability. The promising donors for [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), bisPCBM, and PC70BM as acceptors are recommended theoretically for the first time for solar cells based on the proper match for frontier molecular orbitals between donors and acceptors.

First principles investigation on the key factors of broad absorption spectra and electronic properties for oligothiophene and its derivatives for solar cells

On the basis of optimized structures of two kinds thiophene oligomers, we investigated the configuration and conformation dependent optical and electronic properties of α or β substituted oligothiophene (PT) and oligo(thienylenevinylene) (PTV) isomers, respectively, at the TD-DFT/6-31+G(d,p)//PBE0/6-31G(d) level. The corresponding properties of polymers were predicted by extrapolation method. The broad absorption regions for polythiophene and/or poly(thienylenevinylene) can be ascribed to the coexistence of various conformational and/or configurational isomers with similar energies and each corresponding excited state possessing different absorption spectrum and overlapping each other. Furthermore, the reorganization energies for electron (λe) and hole (λh) of α or β substituted PTV and PT indicate them to be potential ambipolar charge transport materials under the proper operating conditions. Therefore, these results are important because they can provide a rational way for the design of multifunctional materials with broad absorption and ambipolar charge transport properties for solar cells devices. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Solution‐Processed Bulk Heterojunction Photovoltaic Cells from Gradient π‐Conjugated Thienylene Vinylene Dendrimers

A series of gradient pi-conjugated dendrimers and their corresponding models based on 5,5,10,10,15,15-hexahexyltruxene moieties as nodes and oligo(thienylene vinylene) (OTVs) units with different lengths as branching arms are synthesized in good yields through Wittig-Horner reactions. All new compounds are fully characterized by (1)H and (13)C NMR spectroscopy, elemental analysis, and MALDI-TOF MS or ESI-MS. Investigation of their photophysical properties reveals that the gradient dendritic scaffold not only results in a higher molar absorption coefficient and broader absorption region than those of their corresponding model compounds, but also improves the PL quantum yields relative to the corresponding OTVs. The suitable HOMO and LUMO levels as well as excellent film forming properties make these molecules potential candidates for organic solar cells. Solution-processed bulk heterojunction solar cells using these dendrimers as donor and [6,6]-phenyl-C(61) butyric acid methyl ester as acceptor are prepared and tested. The power conversion efficiency of the devices based on G0-4-2 is 0.40 % under illumination of air mass 1.5 and 100 mW cm(-2). This is the highest record value for OTV-based materials to date. Although the absorption band of dendrimer G0-4-2 is much narrower than that of poly(3-hexylthienylene vinylene) (P3HTV), the efficiency of its solar cell device is almost twice that of the device based on P3HTV. This result shows clearly the advantage of gradient dendritic structures as active materials for photovoltaic cells.

Methylene blue doped polymers: efficient media for optical recording

Abstract Polymer materials find application in optical storage technology, namely in the development of high information density and fast access type memories. A new polymer blend of methylene blue sensitized polyvinyl alcohol (PVA) and polyacrylic acid (PAA) in methanol is prepared and characterized and its comparison with methylene blue sensitized PVA in methanol and complexed methylene blue sensitized polyvinyl chloride (CMBPVC) is presented. The optical absorption spectra of the thin films of these polymers showed a strong and broad absorption region at 670–650 nm, matching the wavelength of the laser used. A very slow recovery of the dye on irradiation was observed when a 7:3 blend of polyvinyl alcohol/polyacrylic acid at a pH of 3.8 and a sensitizer concentration of 4.67 · 10−5 g/ml were used. A diffraction efficiency of up to 20% was observed for the MBPVA/alcohol system and an energetic sensitivity of 2000 mj/cm2 was obtained in the photosensitive films with a spatial frequency of 588 lines/mm.

Methylene blue doped polymers: Efficient media for optical recording

Polymer materials find application in optical storage technology, namely in the development of high information density and fast access type memories. A new polymer blend of methylene blue sensitized polyvinyl alcohol (PVA) and polyacrylic acid (PAA) in methanol is prepared and characterized and its comparison with methylene blue sensitized PVA in methanol and complexed methylene blue sensitized polyvinyl chloride (CMBPVC) is presented. The optical absorption spectra of the thin films of these polymers showed a strong and broad absorption region at 670–650 nm, matching the wavelength of the laser used. A very slow recovery of the dye on irradiation was observed when a 7:3 blend of polyvinyl alcohol/polyacrylic acid at a pH of 3.8 and a sensitizer concentration of 4.67 · 10−5 g/ml were used. A diffraction efficiency of up to 20% was observed for the MBPVA/alcohol system and an energetic sensitivity of 2000 mj/cm2 was obtained in the photosensitive films with a spatial frequency of 588 lines/mm.

Synthesis and optical properties of axially bromo-substituted subphthalocyanines

Subphthalocyanines (SubPc) are composed of three iso-indole units containing boron inside, which defines a molecular axis and forms a cone-shaped structure. Despite being non-planar, they show a delocalized 14π-electron system similar to that present in phthalocyanines. The attractive characteristics of SubPc are their chemical stability and thermal stability. In this paper, we report here the synthesis of five SubPc compounds, in which the axial substituent is a bromide. The peripheral substituent in the compounds leads to an improved solubility in common organic solvents such as ethanol and chloroform etc. The spectral and optical properties of these compounds were studied. The spectra and optical constants (refractive index, extinction coefficient) of the sublimed films of these SubPcs were measured by a spectrophotometer and an ellipsometer respectively. We also measured the static optical recording performance of these films with a thin Al (or Ag) reflective layer on them. These results demonstrated that SubPc showed a strong and broad absorption region near 600 nm and it will be a promising candidate for short-wavelength optical recording (such as DVD-R), if high-quality films can be got.

Raman and absorption spectrum of mass-selected lutetium dimers in argon matrices

We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass selected lutetium dimers in argon matrices. Two broad absorption regions were found between 415 and 545 nm with peaks at 418, 433, 452, 472, 505, 540 nm, and between 635 and 660 nm with peaks at 640 and 652 nm. Resonance Raman spectra were obtained by excitation into one of these regions with the visible light of an Ar ion laser between 458 and 514 nm. Two progressions were assigned to lutetium dimers. We interpret them to represent two distinct electronic states: A ground-state X and an excited state A. For the ground (X) state, we obtain ωe=121.6±0.8 cm−1 with ωexe=0.16±0.10 cm−1, leading to a spectroscopic dissociation energy of 2.9±1.8 eV, and force constant ke=0.76±0.01 mdyne/Å. The lowest excited electronic state (A) has an origin at 210.7 cm−1 and almost identical vibrational parameters. Comparison of spectroscopic properties of lanthanide dimers is discussed, and evidence is presented that lutetium should occupy the position normally given to lanthanum in the periodic table.

Vibrational modes in amorphous silicon dioxide

The optical properties of the broad absorption region due to asymmetric stretching vibration of bridging oxygen in the amorphous structure of bulk SiO 2 were studied. The complex refractive index of thick densified chemical-vapour-deposited silicon oxide film was calculated and used further to obtain the dielectric response function of the sample. Vibrational modes were analysed by energy-loss functions for transverse and longitudinal optical phonons. In the 900–1300 cm −1 spectral region, three vibrational modes with well-resolved TO–LO phonon splitting were obtained. The phonon spectra appears as the smeared-out version of the crystalline counterpart, i.e. the α-quartz. The most intense phonon couple at 1064 (TO), and 1248 cm −1 (LO) has the smallest damping and appears to be more crystalline-like than the remaining two. The broad frequency distribution of lattice-like phonon lines gives rise to frequency-dependent damping where the phonons only purely feel each other and have different phonon decays.