Synthesis and photovoltaic properties of D–A–D type small molecules containing diketopyrrolopyrrole (DPP) acceptor central unit with different donor terminal units

Two D–A–D small molecules TDPP (FP)2 and TDPP (BuP)2 coded as CSDPP2 and CSDPP4 respectively were synthesized having same diketopyrrolopyrrole (DPP) as core acceptor and difluoro-phenyl (FP) and dibutyloxy phenyl (BuP) as different end donor units with broad absorption and suitable energy levels. We have used these small molecules as donor components along with the PC₇₀BM as electron acceptor for the preparation of bulk heterojunction (BHJ) active layer in small molecule (SM) organic photovoltaics (OPV) test cells. The optimal power conversion efficiencies obtained with CSDPP2 and CSDPP4 are 2.26% and 3.23% when the BHJ active layer was cast from CF solvent. The PCE has been further enhanced up to 3.03% and 4.65% for CSDPP2 and CSDPP4 when the BHJ active layer was cast from CN/CF solvent. The enhancement in PCE has been explained in terms of change in crystallinity and nanoscale morphology and more balanced charge transport resulting from increased hole mobility.     

Side chain effect on photovoltaic properties of D–A copolymers based on benzodithiophene and thiophene-substituted bithiazole

Two donor–acceptor (D−A) copolymers, PEHBDT-BTz and PODBDT-BTz, containing the same backbone of benzodithiophene (BDT) and bithiazole (BTz) units but different side chains were designed and synthesized. Effects of the side chains of BDT and BTz units on solubility, absorption spectra, energy levels, film morphology, and photovoltaic properties of the polymers were investigated. Results showed that the more branched side chains could increase the molecular weight and the introduction of alkylthienyl groups into BTz unit benefits to broaden the absorption and lower the bandgaps as well as deepen HOMO levels, which are propitious to improve the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of photovoltaic cells. Polymer solar cells (PSCs) were prepared with the polymers as electron donors and PCBM as an acceptor. The device fabrication conditions, including the additive, the different acceptor and blend ratio of the polymer donor and acceptor, have been optimized. PCE of PSCs based on the copolymers varied from 2.92% for PODBDT-BTz to 3.71% for PEHBDT-BTz, depending on the type and topology of the side chains on the BDT moiety. The results indicate that an appropriate choice of side chains on the backbone is an effective way to improve photovoltaic performance of the related PSCs.     

A strategy to enhance both VOC and JSC of A–D–A type small molecules based on diketopyrrolopyrrole for high efficient organic solar cells

Four different diketopyrrolopyrrole (DPP)-based small molecules (SMs) with A–D–A type structure were synthesized, where electron-donating unit (D) was systematically varied with different electron-donating power (thiophene vs. phenylene; thienothiophene vs. naphthalene) and different molecular planarity (bithiophene vs. thienothiophene; and biphenylene vs. naphthalene). The small molecules with weak donating unit (phenylene or naphthalene) have deeper HOMO energy levels than those with strong donating unit (thiophene or thienothiophene), and thus exhibit higher V_OC. When the fused aromatic ring (thienothiophene or naphthalene) with planar molecular structure is introduced in SMs, the SMs exhibit high hole mobility and thus afford high J_SC. As a result, the introduction of naphthalene (weak donating power and planar structure) enhances both V_OC and J_SC, resulting in a promising power conversion efficiency of 4.4%. This result provides a valuable guideline for rational design of conjugated small molecules for high performance organic solar cells.     

Linear and propeller-like fluoro-isoindigo based donor–acceptor small molecules for organic solar cells

Two donor–acceptor type fluoro-isoindigo based small molecule semiconductors are synthesized and their optical, electrochemical, thermal, and charge transport properties are investigated. The two molecular chromophores differ by their architecture, linear (M1) vs propeller-like (M2). Both molecules present a broad absorption in the visible range and a low optical HOMO–LUMO gap (∼1.6 eV). AFM images of solution-processed thin films show that the trigonal molecule M2 forms highly oriented fibrils after a few seconds of solvent vapor annealing. The materials are evaluated as electron donor components in bulk heterojunction organic solar cells using PC₆₁BM as the electron acceptor. The devices based on the propeller-like molecule M2 exhibit a high open-circuit voltage (around 1.0 V) and a power conversion efficiency of 2.23%.     

Synthesis,optical and electrochemical properties new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.     

Effects of electron-withdrawing group and electron-donating core combinations on physical properties and photovoltaic performance in D-π-A star-shaped small molecules

The first representatives of star-shaped molecules having 3-alkylrhodanine (alkyl-Rh) electron-withdrawing groups, linked through bithiophene π-spacer with electron-donating either triphenylamine (TPA) or tris(2-methoxyphenyl)amine (m-TPA) core were synthesized. The physical properties and photovoltaic performance of these novel molecules with 3-ethylrhodanine groups were comprehensively studied and compared to their full analogs having dicyanovinyl (DCV) units as the other type of well-known and frequently used acceptor groups. On one hand, the former demonstrate several advantages such as higher solubility and better photovoltaic performance in bulk-heterojunction (BHJ) organics solar cells (OSCs) as compared to the latter. Nevertheless, the former have slightly lower optical/electrochemical bandgaps and higher thermooxidation stability. On the other hand, molecules of both series based on m-TPA core along with higher solubility and higher position of HOMO energy levels have more pronounced tendency to crystalize as compared to the TPA-based molecules. Detailed investigation of the structure-property relationships for these series of molecules revealed that donor and acceptor unit combinations influence both charge generation and charge transport/recombination properties, as demonstrated by the ultrafast photoinduced absorption spectroscopy, space charge limited current measurements and transient photovoltage technique. These results give more insight how to fine-tune and predict physical properties and photovoltaic performance of small molecules having either alkyl-Rh or DCV units in their chemical structures and thus providing a molecular design guideline for the next generation of high-performance photovoltaic materials.     

Cyclopentadithiophene–benzothiadiazole oligomers: Synthesis via direct arylation,X-ray crystallography,optical properties,solution casted field-effect transistor and photovoltaic characteristics

This article reports the synthesis, crystallographic structure and OFET and OPV performance of the conjugated oligomer of cyclopentadithiophene (CPDT) with benzothiadiazole (BT). Synthesis of the oligomer composed of the CPDT-BT-CPDT sequence is accomplished using direct arylation reactions. Theoretical and experimental X-ray single crystallography confirms that two CPDT-BT-CPDT molecules are not entirely disordered, but are actually stacking directly across each other at the central BT units with an intermolecular distance of 3.61 Å, providing valuable insight into the polymer bulk structure. The performance of the oligomer in OFET devices is investigated by fabricating bottom gate top contact devices and demonstrates a hole mobility of 5.0 × 10⁻³ cm² V⁻¹ s⁻¹. OPV devices of the oligomer blended with PC₆₁BM and PC₇₁BM show power conversion efficiency (PCE) of 1.61%. One potential use for the oligomer could be as a sensitiser in a ternary blend with P3HT–PC₆₁BM or PCPDTBT–PC₆₁BM OPVs; the PCE can be relatively increased by 3–9% depending on concentration, primarily as a result of increased short circuit current density.     

Small molecule donors with different conjugated π linking bridges: Synthesis and photovoltaic properties

Three small molecule (SM) donors, namely B-T-CN, B-TT-CN and B-DTT-CN, with different π conjugated bridges were synthesized in this research. Interestingly, with the conjugated fused rings of the π linking bridge increasing, the SM HOMO levels exhibit a decline tendency with –5.27 eV for B-T-CN, –5.31 eV for B-TT-CN and –5.40 eV for B-DTT-CN. After blending the SM donors with the fullerene acceptor PC₇₁BM, the all SM organic solar cells (OSCs) achieved high V_ocs of 0.90 to 0.96 V. However, the phase separation morphology and molecule stacking are also unexpectedly changed together with the enhancement of conjugated degree of π bridges, resulting in a lower power conversion efficiency (PCE) for the B-DTT-CN:PC₇₁BM device. Our results demonstrate and provide a useful way to enhance OSC V_oc and the morphology needs to be further optimized.     

Side chain effect on photovoltaic properties of dibenzo[a,c]phenazine based donor–acceptor polymers

The electron–donor polymers containing dibenzo[a,c]phenazine (BPz) derivatives with 2,7-alkyl and 11,12-alkoxy substituted, PBDT-BPzC and PBDT-OBPz, respectively, were synthesized to investigate the photovoltaic effect of different side chain substitutions. The polymers exhibit similar physical properties, except the HOMO and LUMO of PBDT-BPzC are 0.18 and 0.15 eV deeper than PBDT-OBPz, resulting in the V_oc of polymer solar cells (PSCs) based on PBDT-BPzC are above 0.1 V higher than that of PBDT-OBPz. With the contribution of the superior V_oc, polymer PBDT-BPzC showed preferable photovoltaic performances, and the PCE reached 4.44%, which is 0.49% higher than PBDT-OBPz. This research reveals a preferred side chain substituted way to modify BPz unit, and gives an optimally developing the dibenzo[a,c]phenazine derivatives based electron–donor polymers.     

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor exhibited PCEs as high as 3.15%.     

Synthesis and optoelectronic properties of novel fluorene-bridged dinuclear cyclometalated iridium (III) complex with A–D–A framework in the single-emissive-layer WOLEDs

To explore the influence of push–pull chromophores on properties of emitter in organic light-emitting devices (OLEDs), an acceptor–donor–acceptor (A–D–A)-based dinuclear iridium (III) complex of (dfppy)₄Ir₂(dipic-FL) was synthesized via Suzuki coupling reaction, in which dfppy is 2-(2,4-difluorophenyl)pyridine and dipic-FL is 2,7-di(5-pyridyl-2-carboxyl)-9,9-dioctyl-9H-fluorene. An intense emission peak at about 480 nm resulting from the (dfppy)₂Ir(pic) chromophore and a weak long-wavelength emission band at 580–660 nm attributed to intramolecular charge transfer transition were exhibited for (dfppy)₄Ir₂(dipic-FL) in dichloromethane solution. But a remarkably hypsochromic photoluminescence profile with an intense characteristical emission peak at 422 nm was observed, which is attributed to the intraligand (IL) π–π^∗ excited states in its thin film. White emission with a maximum luminance of 1040 cd/m² and current efficiency of 1.2 cd/A was obtained in its single-emissive-layer (SEL) OLEDs with a configuration of ITO/PEDOT:PSS/(dfppy)₄Ir₂(dipic-FL) (10 wt%):TCTA/TPBi/LiF/Al. To our knowledge, this is one of the best examples in term of dinuclear iridium complex as single dopant in the high-performance white-emitting SEL-OLEDs.     

D–A–Ar-type small molecules with enlarged π-system of phenanthrene at terminal for high-performance solution processed organic solar cells

Two novel D–A–Ar-type small molecules of TPA–DPP–P and TPA(DPP–P)₂ were synthesized and characterized, in which triphenylamine (TPA), diketopyrrolopyrrole (DPP) and phenanthrene (P) were used as the donor (D) core, acceptor (A) arm, and enlarged π-system of polycyclic arene (Ar) terminal. Their absorptive, electro-chemical, thermal, and photovoltaic properties were preliminary investigated. Significantly improved photophysical and photovoltaic performances were observed for both small molecules containing the planar P terminal in comparison with those for their parent D–A-type molecule of TPA–DPP. The highest power conversion efficiency (PCE) of 3.42% and a maximum short-circuit current density (J_sc) of 9.2 mA/cm² were obtained in the solution-processed TPA(DPP–P)₂-based solar cells using [6,6]-phenyl-C-71-butyric acid methyl ester (PC₇₁BM) as acceptor. The PCE and J_sc values are 8.76 and 4.97 times higher than those of the TPA–DPP-based cells, respectively. It indicates that appending the enlarged π-system of the planar P terminal and incorporating the DPP–P arm into D–A-type small molecule are efficient approaches to improve photophysical and photovoltaic performances for its resulting molecules.     

Preparation,characteristics and photovoltaic properties of cuprous oxide—a review

The photovoltaic properties of cuprous oxide (Cu₂O) are reviewed and discussed. The emphasis, however, is on reviewing the characteristics of Cu₂O prepared by different techniques such as high-temperature thermal oxidation, low-temperature thermal oxidation, electro-deposition, anodic oxidation, chemical oxidation and reactive sputtering. It is shown that despite the economic prospects of efficient cells based on Cu₂O, little is known about the photovoltaic properties of this material when it is fabricated by low-temperature techniques. The areas in which further research is needed because of the lack of information in the literature are pointed out.     

Deep learning and RGB-D based human action,human–human and human–object interaction recognition: A survey

Human activity recognition is one of the most studied topics in the field of computer vision. In recent years, with the availability of RGB-D sensors and powerful deep learning techniques, research on human activity recognition has gained momentum. From simple human atomic actions, the research has advanced towards recognizing more complex human activities using RGB-D data. This paper presents a comprehensive survey of the advanced deep learning based recognition methods and categorizes them in human atomic action, human–human interaction, human–object interaction. The reviewed methods are further classified based on the individual modality used for recognition i.e. RGB based, depth based, skeleton based, and hybrid. We also review and categorize recent challenging RGB-D datasets for the same. In addition, the paper also briefly reviews RGB-D datasets and methods for online activity recognition. The paper concludes with a discussion on limitations, challenges, and recent trends for promising future directions.     

Manipulating backbone structure with various conjugated spacers to enhance photovoltaic performance of D–A-type two-dimensional copolymers

A class of low band-gap two-dimensional conjugated polymers of PBDTT-FQ, PBDTT-TQ, PBDTT-BTQ and PBDTT-TTQ was designed and synthesized, which contains the same di(alkylthiophene)-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) and 6,7-difluoroquinoxaline (Q) units, as well as various conjugated spacers of furan, thiophene, bithiophene and thieno[3,2-b]thiophene in the main chain. Significant effect of the varied spacers between the BDTT and Q units on the thermal, optical, electrochemical and photovoltaic properties was investigated and observed for these two-dimensional copolymers in the polymer solar cells. The maximum power conversion efficiency of 5.9% with a short circuit current of 13.7 mA/cm² and a fill factor of 0.56 was obtained for the PBDTT-TQ with thiophene spacer in the bulk hetero-junction PSCs using [6,6]-phenyl-C₇₁-butyric acid methyl ester as acceptor.     

Photoelectric and optical properties of Schottky-barrier photodiodes based on IrSi–Si

The basic properties in the Schottky-barrier photodiodes (SBPDs) can be reduced to photon absorption in the silicide layer and internal photoemission of charge carriers from a metal to a semiconductor. Therefore, the quantum efficiency and photoresponse of these photodiodes (PDs) are primarily determined by electronic and optical processes in the metal silicide rather than in the semiconductor. This implies that, to a first approximation, the SBPD photoresponse is independent of semiconductor parameters such as the doping level, degree of compensation of impurities, and minority-carrier lifetime. The main reasons for photoresponse nonuniformity in multielement detector arrays are thereby ruled out.     

Effect of small Sn–3.5Ag–0.5Cu additions on the structure and properties of Sn–9Zn solder in ball grid array packages

Sn–9Zn with various additions of Sn–3.5Ag–0.5Cu powder was prepared by mechanically dispersing different weight percentages (1, 3, 5 and 7) of Sn–Ag–Cu powder into Sn–9Zn solder paste. In the Sn–Zn solder, scallop-shaped AuZn₃ intermetallic compound was found at the interfaces. On the other hand, in the Sn–3.5Ag–0.5Cu content solders, an additional ε-AgZn₃ intermetallic compound layer was found to be well adhered on the top surface of the AuZn₃ layer and the ε-AgZn₃ layer thickness increased with the number of reflow cycles. In addition, fine spherical-shaped ε-AgZn₃ intermetallic compound particles as well as an acicular-shaped Zn-rich phase was clearly observed in the β-Sn matrix. On increasing the Sn–Ag–Cu content, the shear load was increased from 1.80 to 2.03 kg after one reflow cycle. In the Sn–3.5Ag–0.5Cu content solders, the fracture surfaces exhibited typical ductile behavior with very rough dimpled surfaces while the fracture surface in the Sn–Zn solder gave fractures with a brittle appearance. In the fracture surface of the Sn–3.5Ag–0.5Cu content solders, some dimples were clearly observed associated with the formation of spherical-shaped ε-AgZn₃ intermetallic compound particles.     

Synthesis and photovoltaic properties of organic dyes containing N-fluoren-2-yl dithieno[3,2-b:2′,3′-d]pyrrole and different donors

New organic dyes containing fluorene appended dithienopyrrole as electron rich linker, different arylamine/heterocyclic units as conjugating donors and cyanoacrylic acid as acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. The effect of different conjugated donors such as triarylamine, carbazole and phenothiazine on the photophysical, electrochemical and photovoltaic properties is investigated. The optical and electrochemical properties of the dyes are strongly influenced by conjugating donors. The dye containing phenothiazine donor exhibited longer wavelength absorption and low oxidation potential. The time dependent density functional calculations performed on the dye models reveal charge transfer character for the longer wavelength absorption. The dye-sensitized solar cells fabricated using a dye containing fluorenyldiphenylamine donor displayed highest power conversion efficiency (6.81%) in the series originating from the high short circuit current density (J_SC = 14.01 mA cm⁻²) and high open circuit voltage (V_OC = 738 mV).     

Efficient solution processed D1-A-D2-A-D1 small molecules bulk heterojunction solar cells based on alkoxy triphenylamine and benzo[1,2-b:4,5-b′]thiophene units

Two molecules denoted as VC96 and VC97 have been synthesized for efficient (η = 6.13% @ 100 mW/cm² sun-simulated light) small molecule solution processed organic solar cells. These molecules have been designed with the D₁-A-D₂-A-D₁ structure bearing different central donor unit, same benzothiadiazole (BT) as π-acceptor and end capping triphenylamine. Moreover, the optical and electrochemical properties (both experimental and theoretical) of these molecules have been systematically investigated. The solar cells prepared from VC96:PC₇₁BM and VC97:PC₇₁BM (1:2) processed from CF (chloroform) exhibit a PCE (power conversion efficiency) of η = 4.06% (J_sc = 8.36 mA/cm², V_oc = 0.90 V and FF = 0.54) and η = 3.12% (J_sc = 6.78 mA/cm², V_oc = 0.92 V and FF = 0.50), respectively. The higher PCE of the device with VC96 as compared to VC97 is demonstrated to be due to the higher hole mobility and broader IPCE spectra. The devices based on VC96:PC₇₁BM and VC97:PC₇₁BM processed with solvent additive (3 v% DIO, 1,8-diiodooctane) showed PCE of η = 5.44% and η = 4.72%, respectively. The PCE device of optimized VC96:PC₇₁BM processed with DIO/CF (thermal annealed) has been improved up to 6.13% (J_sc = 10.72 mA/cm², V_oc = 0.88 V and FF = 0.61). The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive plus the thermal annealing.     

High open-circuit voltage polymer solar cells based on D–A copolymer of indacenodithiophene and fluorine-substituted benzotriazole

A medium band gap D–A copolymer of indacenodithiophene (IDT) and fluorinated dithienylbenzotriazole (FBTA), PIDT-FBTA, was synthesized for the application as donor material in polymer solar cells (PSCs). PIDT-FBTA showed deeper highest occupied molecular orbital (HOMO) energy level due to the strong electron-withdrawing difluorine substitution on benzotriazole acceptor unit in the D–A copolymer. The PSCs based on PIDT-FBTA:PC₇₀BM (1:3) exhibited a high V_oc of 0.90 V and a power conversion efficiency (PCE) of 3.62% under the illumination of AM 1.5G, 100 mW cm⁻². The device performance was further improved by methanol treatment with PCE increased to 4.90% and V_oc increased to 0.92 V.