Singlet/triplet exciton dissociation and charge recombination in donor-acceptor ThQs-C60/PDIxCN2 complexes

The donor-acceptor interface plays a critically important role in determining the power conversion efficiency of organic solar cells via controlling charge separation (CS) and recombination (CR) processes. Here, we combine the electronic structure calculations with electron transfer rate theory to clarify the CS and CR processes in ThQs-C₆₀ /PDIxCN₂ donor-acceptor complexes. The results reveal that in ThQs-PDIxCN₂ the CS comes from both the dissociations of photo-induced singlet exciton and singlet fission-induced triplet exciton with a high efficiency, whereas in ThQs-C₆₀ only the singlet exciton dissociation can take place because the triplet exciton lies below the charge-transfer exciton. However, very high CR rates in ThQs-PDIxCN₂ obliterate the benefit of fast CS, inversely leading to the ThQ-C₆₀ complex with a better cell efficiency. The present results are consistent with experimental observation and may furnish a possible patten to improve the overall conversion efficiency. © 2018 Wiley Periodicals, Inc.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

有机电致发光器件中载流子传输与复合的调控

在典型的多异质结器件ITO/NPB/CBP：Ir（ppy）3/Bphen/Alq3/LiF/Al的基础上,利用有机半导体掺杂技术,设计制备了单异质结-单发光层器件、单异质结-p-i-n结构器件、单异质结-双发光层器件及无异质结-混合主体结构器件,并对其光电性能进行了研究和比较.其中,单异质结-p-i-n结构器件的最大功率效率为32.1lm/W,是参考器件的3.1倍,寿命是参考器件的15倍.无异质结-混合主体结构器件的最大功率效率为37.2lm/W,是参考器件的3.5倍,其寿命是参考器件的46倍.研究结果表明,通过对载流子传输层和发光层的优化设计,构建电子、空穴传输平衡的载流子传输层和发光层,减少或取消异质结界面仍可以实现对载流子传输和复合的有效调控,从而使器件的发光效率和寿命同时得到提高.本研究将为高性能OLED的设计提供实验基础.     

Novel alkoxyanthracene donor and benzothiadiazole acceptor for organic thin film transistor and bulk heterojunction organic photovoltaic cells

Novel alkoxy anthracene (ODA)‐based polymeric semiconductors were designed for polymer solar cell applications. Alkoxyanthracene, which contains many π electrons and electron donating group, was easily synthesized. The copolymers, poly(alkoxy anthracene‐alt‐thiophene benzothiadiazole thiophene) poly(ODA‐TBT) and poly(alkoxy anthracene‐alt‐benzothiadiazole) poly(ODA‐BT), have been obtained by Suzuki coupling polymerization. Both polymers have ODA unit as a donor and benzothiadiazole as an acceptor. ODA‐TBT has thiophene linkages between ODA and benzothiadiazole. The optical, thermal, and electrochemical properties have been investigated by UV–visible absorption, thermal gravimetric analysis, differential scanning calorimetry, and CV. Organic thin‐film transistor using polymers showed that the hole mobility of poly(ODA‐alt‐TBT) was around 3.6 × 10^?3 cm²/Vs with on/off ratio of 9.91 × 10⁵ while that of poly(ODA‐alt‐BT) was around 1.21 × 10^?2 cm²/Vs with on/off ratio of 2.64 × 10⁶. Organic photovoltaic performance based on polymers were evaluated with a configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Poly(ODA‐TBT) exhibits a short circuit current (J_sc) of 3.9 mA/cm² and power conversion efficiency (PCE) of 1.4%, and poly(ODA‐BT) exhibits the J_sc of 6.4 mA/cm² and PCE of 2.2%. The better device performance of poly(ODA‐BT) is attributed to its charge transfer ability and enhanced mobility and crystallinity although poly(ODA‐BT) does not have extended π‐conjugation due to twisted structure compared with poly(ODA‐TBT). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1306–1314     

In-depth probe of researching interfacial charge transfer process for organic solar cells: A promising bisadduct fullerene derivatives acceptor

The charge transfer (CT) mechanism at the donor/acceptor (D/A) interface plays an irreplaceable role in the photoelectric conversion of efficient bulk-heterojunction (BHJ) organic solar cells (OSCs), which affects the resulting competition between charge separation and charge recombination. Extensive CT studies have preferred monoadduct fullerene derivatives ( M60 , M70 ) due to their unique spherical geometry with fewer factors to consider. However, the effect of carbon cage size, substituent group properties and the number of CT properties have not been much discussed. Here, sulfur-containing bisadducts ( B60 , B70 ) were selected to explore whether they are also suitable for CT research like classical monoadducts. Using density functional theory and time-dependent density functional theory, interface stacking configuration, key parameters relevant to CT states, charge separation, and recombination rates were determined to confirm the characteristics of B60 and B70 as a good acceptor applied to interfacial research. This work points to the CT mechanism along the route of DA → D*A → D⁺A⁻ through a theoretical analysis and also provides candidates for the theoretical interface photoelectric process in BHJ OSCs: bisadduct fullerene derivatives as good acceptor materials.     

密度泛函理论研究2种新型染料分子的分子内电荷转移现象

采用量子化学方法研究了2种新型有机染料分子P1和P4,几何优化和基态性质计算采用B3LYP密度泛函,基组为6-311G(d).由于P1和P4分子中分别存在2个对称的吸电子基团,所以2个染料分子的电子结构存在明显的特点:2个紧邻简并最低空轨道(LowestUnoccupied Molecular Orbital,LUMO)轨道.P1和P4最高占据轨道(Highest Occupied Mo-lecular Orbital,HOMO)到LUMO轨道的跃迁能级差分别为2.79和3.26eV.同时,采用含时密度泛函方法(Time-Dependent Density Functional Theory,TDDFT)研究了2个染料分子的激发态性质.通过电荷差异密度理论方法(Charge Different Density,CDD)直观的展示了分子内电荷转移的现象.对于P1,电荷转移的方向是从苯甲酸基团到2个二氰乙烯基噻吩苯基团;对于P4,电荷是由2个二氰乙烯基联苯基团基团向苯甲酸基转移.     

Benzodithiophene‐based poly(aryleneethynylene)s: Synthesis,optical properties,and applications in organic solar cells

The synthesis and characterization of building block of ethynylene‐substituted benzo[1,2‐b:4,5‐b′]dithiophene (BDT), and its application in the construction of poly(aryleneethynylene)s (PAEs) are described in this article. Alkoxy‐substituted BDT and thiazolothiazole are selected as the other copolymerized units, and polymers of PEBBDT and PEBTTZ were synthesized by Pd‐catalyzed Sonogashira coupling reaction. These polymers showed intense interchain π–π interaction and deep HOMO levels (≤ ?5.50 eV). Bulk heterojunction solar cell fabricated using PEBBDT or PEBTTZ as electron donor and PC₆₁BM as acceptor display power conversion efficiency of 0.85 and 2.40%, respectively, under the illumination of AM1.5G, 100 mW cm^?2. This study shows good prospect for the application of PAEs‐type polymers in organic solar cell. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 208–215     

Electronic Structure and Interface Properties of a Model Molecule for Organic Solar Cells

We study the electronic structure of 4,7‐bis(5‐methylthiophen‐2‐yl)benzo[c][1,2,5]thiadiazole (MTBT) and its interface properties with gold using X‐ray photoemission spectroscopy (XPS), valence‐band ultraviolet photoemission spectroscopy (UPS), X‐ray absorption spectroscopy (XAS), as well as resonant photoemission (ResPES). MTBT can be regarded as a model molecule for PCPDTBT, a promising candidate for efficient bulk heterojunction solar cells. Almost no contribution of sulfur and only a weak contribution of nitrogen to the HOMO level is found. At the interface with gold, a strong chemical interaction between the sulfur of the benzothiadiazole and gold occurs, which may have consequences for interface properties in devices.     

Slow geminate‐charge‐pair recombination dynamics at polymer: Fullerene heterojunctions in efficient organic solar cells

We explore charge recombination dynamics at electron donor‐acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC₇₀BM), by means of combined time‐resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron‐hole pairs recombines with a temperature‐independent rate distribution spanning submicrosecond timescales to produce luminescent charge‐transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge‐pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long‐lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate‐charge recombination highlights the importance of the molecular nature of specific donor–acceptor electronic interactions in defining the relaxation pathways of trapped GPP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Impact of bridging units on the dynamics of photoinduced charge generation and charge recombination in donor-acceptor dyads.

We estimate, at a full quantum-chemical level, the various molecular parameters governing the rate of photoinduced charge generation and charge recombination in model organic structures containing a donor and an acceptor unit in view of the possible use of such systems in organic solar cells. The rate of through-space excitation dissociation, as predicted in the framework of the Marcus-Levich-Jortner theory, is found to be low in comparison to intramolecular decay processes when the donor and acceptor molecules are lying in a head-to-tail arrangement and high when the donor and acceptor molecules are superimposed in a cofacial arrangement. The charge separation rates for side-by-side donor-acceptor dyads are significantly increased by promoting through-bond interactions in covalently linked donor and acceptor units. This has motivated a detailed quantitative analysis of the influence of the nature, size, and conformation of the bridging moiety on the calculated transfer rates.     

Synthesis and characterization of new selenophene‐based conjugated polymers for organic photovoltaic cells

Three new polymers poly(3,4′′′‐didodecyl) hexaselenophene) (P6S), poly(5,5′‐bis(4,4′‐didodecyl‐2,2′‐biselenophene‐5‐yl)‐2,2′‐biselenophene) (HHP6S), and poly(5,5′‐bis(3′,4‐didodecyl‐2,2′‐biselenophene‐5‐yl)‐2,2′‐biselenophene) (TTP6S) that have the same selenophene‐based polymer backbone but different side chain patterns were designed and synthesized. The weight‐averaged molecular weights (M_w) of P6S, HHP6S, and TTP6S were found to be 19,100, 24,100, and 19,700 with polydispersity indices of 2.77, 1.48, and 1.41, respectively. The UV–visible absorption maxima of P6S, HHP6S, and TTP6S are at 524, 489, and 513 nm, respectively, in solution and at 569, 517, and 606 nm, respectively, in the film state. The polymers P6S, HHP6S, and TTP6S exhibit low band gaps of 1.74, 1.95, and 1.58 eV, respectively. The field‐effect mobilities of P6S, HHP6S, and TTP6S were measured to be 1.3 × 10^?4, 3.9 × 10^?6, and 3.2 × 10^?4 cm² V^?1 s^?1, respectively. A photovoltaic device with a TTP6S/[6,6]‐phenyl C₇₁‐butyric acid methyl ester (1:3, w/w) blend film active layer was found to exhibit an open circuit voltage (V_OC) of 0.71 V, a short circuit current (J_SC) of 5.72 mA cm^?2, a fill factor of 0.41, and a power conversion efficiency (PCE) of 1.67% under AM 1.5 G (100 mW cm^?2) illumination. TTP6S has the most planar backbone of the tested polymers, which results in strong π–π interchain interactions and strong aggregation, leading to broad absorption, high mobility, a low band gap, and the highest PCE. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Charge transport,injection, and photovoltaic phenomena in oligo(phenylenevinylene) based diodes

We report on the charge transport and injection phenomena of (E,E,E,E)‐1,4‐bis[(4‐styryl)styryl]‐2‐methoxy‐5‐(2′‐ethylhexoxy)benzene (MEH‐OPV5) sandwiched between asymmetric contacts. The hole mobility of MEH‐OPV5 was determined by means of transient electroluminescence. The steady‐state current was injection‐limited. The electric field and temperature dependence of the current were quantitatively described by a phenomenological injection model of thermally assisted charge‐carrier tunneling in a one‐dimensional chain of hopping sites. Furthermore, we report on the photovoltaic properties of thin‐film photovoltaic cells on the basis of donor–acceptor heterojunctions. MEH‐OPV5 and buckminster fullerene were used as the donor and acceptor materials, respectively. The emphasis was on the role of morphology in such devices. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2665–2673, 2003     

Linkage position influences of anthracene and tricyanovinyl groups on the opto-electrical and photovoltaic properties of anthracene-based organic small molecules

Anthracene-based small molecules incorporating an electron accepting tricyanovinyl (TCV) group was prepared to investigate the linkage position influences of the anthracene and TCV groups on the opto-electrical and photovoltaic properties of the molecules. The maximum absorptions of the anthracene-based molecules incorporating the TCV group at the phenyl group of the triphenylamine unit (TCV-TpaA_9,10T, TCV-TpaTA_9,10T, and TCV-TpaA_2,6T) or at the thiophene unit (TpaA_9,10T-TCV, TpaTA_9,10T-TCV, and TpaA_2,6T-TCV) were found to be dependent on the linkage position of the anthracene unit. The HOMO energy levels of the molecules containing TCV group at the phenyl group of the triphenylamine unit were deeper than those of the molecules containing TCV group at the thiophene unit. The solution processed small molecule organic solar cells (SMOSCs) prepared with the structure of ITO/PEDOT:PSS/TCV-TpaA_9,10T or TCV-TpaA_2,6T or TpaA_2,6T-TCV:PC₇₁BM (2:1 wt %)/LiF/Al exhibited a maximum energy conversion efficiency of 1.04%, 1.67%, and 1.95%, respectively, under AM 1.5 irradiation (100 mW cm⁻²).     

Quantifying the role of interfacial width on intermolecular charge recombination in block copolymer photovoltaics

Block copolymers have the potential to control the interfacial and mesoscopic structure in the active layer of organic photovoltaics and consequently enhance device performance beyond systems which rely on physical mixtures. When utilized as the active layer, poly(3‐hexylthiophene‐2,5‐diyl)‐block‐poly((9,9‐bis‐(2‐octyldodecyl)fluorene‐2,7‐diyl)‐alt‐(4,7‐di(thiophene‐2‐yl)?2,1,3‐benzothiadiazole)?5′,5″‐diyl) donor–acceptor block copolymers have recently demonstrated 3% power conversion efficiencies in devices. Nevertheless, the role of the interfacial structure on charge transfer processes remains unclear. Using density functional theory, we examined charge transfer rate constants in model interfaces of donor–acceptor block copolymers. Our results demonstrate that intermolecular charge recombination can depend on the interfacial breadth, where sharp interfaces (ca. 1 nm) suppress intermolecular charge recombination by orders of magnitude. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1224–1230     

Polarization energies,transport gap and charge transfer states of organic molecular crystals

A self-consistent calculation of electronic polarization in organic molecular crystals and thin films is presented in terms of charge redistribution in nonoverlapping molecules in a lattice. The polarization energies P₊ and P₋ of a molecular cation and anion are found for anthracene and perelynetetracarboxylic dianhydride (PTCDA), together with binding energies of ion pairs and transport gaps of PTCDA films on metallic substrates. The 500 meV variation of P₊+P₋ with film thickness agrees with experiment, as do calculated dielectric tensors. Comparisons are made to submolecular calculations in crystals.     

The structure of 7,14-dicarbonylethoxyacenaphth[l,2-k]-fluoranthene and its tetracyanoquinodimethane charge transfer complex

A molecular mechanics simulation of the structure of 7,14-dicarbonylethoxyacenaphth[l,2-k]fluoranthene,1, indicated a preferred geometry for the hindered substituents in which the carbonyl groups were constrained to an anti conformation because of the steric hindrance associated with the in-plane buttressing hydrogen atoms. X-ray crystallographic analysis of1 verifies the correctness of the computation. Compound1 and tetracyanoquinodimethane,2, form a charge transfer complex, and a crystal structure analysis shows a slightly offset, nearly parallel arrangement of the acceptor with the cloud of the donor. The interplanar distance of 3.45 Å between acceptor and donor lies within the statistical limits of the interplanar distance of complexes formed between2 and a variety of PAH donors. The steric hindrance caused by the substituents in1 appears to offer only minimal interference to the formation of the complex.     

Synthesis,characterization, and hydrazine sensing property of two covalent organic frameworks

Sensitive and efficient detection of hydrazine is of great significance because hydrazine is a highly toxic organic molecule, which can pose great threats to human health. Herein, two covalent organic frameworks (COFs) modified with ester groups in the pores, TAPB-DHE and TAPT-DHE, have been successfully synthesized via esterification reaction. Both of the two COFs have good crystallinity, thermal stability, and fluorescence properties. TAPB-DHE can be used as a turn-on fluorescence sensor for the sensitive detection of trace hydrazine in aqueous solution with a detection limit of 0.40 μM and a wide linear range of 0–100 μM due to the fluorescence enhancement of TAPB-DHE caused by the chemical reaction between hydrazine and TAPB-DHE, in which the ester group of the COF is converted into hydroxyl group, leading to the restriction of the intramolecular charge transfer (ICT) effect. This work provides a reference for the design of COFs with hydrazine recognition function and a helpful expansion for the practical application of COFs.