A Solution-Processable Star-Shaped Molecule for High-Performance Organic Solar Cells (original) (raw)
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2012
Two solution-processable star-shaped D-p-A organic molecules with triphenylamine (TPA) as donor unit, benzothiadiazole (BT) as acceptor unit and 4-hexyl-thienylenevinylene as pi conjugated bridge, S(TPA-TBTT) and S(TPA-TBTT-TPA), have been designed and synthesized for the application as donor materials in bulk-heterojunction organic solar cells (OSCs). The two molecules possess broader absorption from 350 to 700 nm benefitted from the longer pibridge in the molecules but weaker absorbance and poorer solubility in comparison with their corresponding organic molecules with shorter vinylene pi-bridge. The OSC based on S(TPA-TBTT): PC 70 BM (1:3, w/w) exhibited J sc of 6.41 mA/cm 2 , V oc of 0.75 V, FF of 39.0% and power conversion efficiency of 1.90%, under the illumination of AM 1.5, 100 mW/cm 2 .
Cyclopentadithiophene-based co-oligomers for solution-processed organic solar cells
Dyes and Pigments
A new family of low band-gap co-oligomers based on 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) and thieno[3,2-b]thiophene (TT) units as central electron-donor cores has been synthesized and characterized for use as electron-donor materials in solution-processed bulkheterojunction organic solar cells. An in-depth study into the role played by the hexyl chains linked to the thienylenevinylene-based π-bridge has been carried out. Power conversion efficiencies (PCE) of up to 4% and external quantum efficiencies as high as 50% have been achieved. Experiments carried out after solvent vapor annealing (SVA) as a post-treatment led to a doubling of the fill factor (FF) and PCE. 1. Introduction Bulk heterojunction (BHJ) organic solar cells (OSCs) based on oligomers as electron-donor along with fullerene derivatives as electron-acceptor materials have been widely investigated in recent years owing to their excellent advantages, which include light weight, less marked batch to batch variations, high reproducibility, low-cost, flexibility and large-area applications [1]. Until now, BHJ OSCs based on π-conjugated oligomers, typically denoted as 'small molecules', have reached power conversion efficiencies (PCE) over 10% [2], which is comparable to those of polymer-based OSCs [3]. Π-Conjugated co-oligomers based on alternating electron-donor (D) and electron-acceptor (A) moieties lead to materials with outstanding properties, such as 2 absorption in the visible to near-infrared region, strong charge transfer character, low highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap, and good charge carrier mobility [4]. Of these materials, acceptor-donor-acceptor (A-D-A) oligomers are considered to be among the most promising molecular structures owing to their broad absorption, high hole mobility, suitable electronic levels, controlled solubility, and high open circuit voltage (VOC) of their resulting devices [5], and these have been extensively studied as donor materials in BHJ OSCs [2c,6]. Most of the research into the design of new AD A systems has been focused on optimization of the molecular structure of donors and acceptors. Regarding the donor fragments, fused aromatic rings have been widely used due to their extended π-conjugation, high charge carrier mobility, and reduced bandgap according to the enhancement of the π-electron delocalization [7]. The fused ring aromatic structures tend to form π-π stacks with a large overlapping area and this leads to high charge carrier transport through intermolecular hopping, large crystalline domains, and more ordered domain boundaries. Additionally, the position and length of alkyl side chains on the central electron-donor building block play an important role for solubility, ππ stacking, energy levels, and charge transport properties of the oligomers [8]. However, despite the fact that a great deal of effort has been focused on identifying structure-property relationships for these materials, their correlation with device performance parameters remains very difficult and several issues remain unresolved. 4H-Cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) derivatives have recently been used in organic electronics and optical materials owing to their unique semiconducting and electronic properties, which include high electrical conductivity, low band-gap, and extended πconjugation [9]. The fused-ring structure of CPDT, which is regarded as a fused-ring analogue of 3-alkylthiophene and a structural analogue of fluorene, has extended π-conjugation in the ground state due to the highly planar molecular geometry. This fused structure leads to a low
ACS Applied Materials & Interfaces, 2013
In this study, we have strategically designed and convergently synthesized two novel, symmetrical, and linear A− D−A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-ylsubstituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. We thoroughly characterized both of these materials and investigated the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties. We then fabricated solution-processed bulk heterojunction organic solar cells incorporating TBDTCNR and TBDTCN. Among our tested devices, the one containing TBDTCNR and [6,6]-phenyl-C 61-butyric acid methyl ester in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (J sc) of 9.08 mA cm −2 , an open circuit voltage (V oc) of 0.90 V, and an impressive fill factor (FF) of 0.66 under AM 1.5G irradiation (100 mW cm −2). The FFs of these solutionprocessed small-molecule organic solar cells (SMOSCs) are outstanding when compared with those recently reported for benzodithiophene (BDT)-based SMOSCs, because of the high crystallinity and excellent stacking properties of the TBDT-based compounds.
2014
The preparation of four different star-shaped donor (D)-p-acceptor (A) small molecules (N(Ph-1T-DCN-Me) 3 , N(Ph-2T-DCN-Me) 3 , N(Ph-2T-DCN-Hex) 3 and N(Ph-3T-DCN-Hex) 3 ) possessing various oligothiophene p-bridge lengths and their use in solution-processed bulk heterojunction small molecule solar cells is reported. Optical and electrochemical data show that increasing oligothiophene p-bridge length leads to a decrease of the optical band gap due to a parallel increase of the highest occupied molecular orbital (HOMO) level. Furthermore, subtle modifications of a molecular p-bridge length strongly affect the thermal behavior, solubility, crystallization, film morphology and charge carrier mobility, which in turn significantly change the device performance. Although the moderately increasing oligothiophene p-bridge length uplifts the HOMO level, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the short circuit current (J sc )
Structure engineering of small molecules for organic solar cells
Molecular Crystals and Liquid Crystals, 2020
Two 1,3-bis(thiophen-2-yl)À5,7-bis(2-ehtylhexyl)benzo-[1,2-c:4,5-c]dithiophene-4,8-dione (BDD) based small molecules, SM1 and SM2 are designed and synthesized by incorporating benzodithiophene (BDT) central core, BDD dual accepting units and 3-ethyl rhodamine as endcap group with various number of BDT units. We systematically investigated the synthesis, optical and electrochemical properties, and photovoltaic characteristics of these donor materials. The number of BDT units have a significant influence on Jsc due to interconnected structure and results in a broader absorption on thin film. The inverted devices employed for both small molecules exhibited power conversion efficiencies of 0.41% for SM1 and 0.82% for SM2.
Advanced Materials
Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.