wileyonlinelibrary.comAdv. Funct. Mater. 2011, 21, 897-910 Photovoltaic properties of DCV n T-Bu (n = 4, 5 and 6) with various lengths of the oligothiophene unit were also investigated. The V OC decreased from 1.13 V for DCV4T-Bu to 1.0 V for DCV5T-Bu and to 0.93 V for DCV6T-Bu, which is ascribed to the decrease in their IP and an increase in energy of the highest occupied molecular orbital (HOMO). On the other hand, no clear trend was observed for short-circuit current density ( J SC ) and fi ll factor ( FF ) values. The power conversion effi ciency of DCV4T-Bu was lower compared to DCV5T-Bu because of its lower J SC and FF, while the small differences in J SC and FF values between DCV5T-Bu and DCV6T-Bu do not represent a general trend. [ 34 ] One advantage of the vacuum-processable SMOSCs is that tandem cells can easily be fabricated. Very recently, by combining in a tandem device two bulk heterojunctions of C 60 in blend layers with an optimized DCV n T-derivative and a novel pigment with absorption range between 600 and 750 nm developed by BASF, respectively, a certifi ed effi ciency of 6.1% on 2 cm 2 active area [ 36 ] and shortly after a certifi ed record effi ciency of 8.3% on 1.1 cm 2 device have been released. [ 37 ] Herein, we investigate structure-property relationships in a series of DCV n Ts with conjugated chain lengths of one to six thiophene units (n = 1-6) and without solubilizing alkyl side chains, well being aware that synthesis and characterization of longer derivatives might be problematic due to low solubility. However, we expect that these oligomers may show good packing and thermal stability which might infl uence the solar cell performance. Synthesis of Dicyanovinyl-FunctionalizedOligothiophenes DCV n T Linear SynthesesSynthesis of the smaller members of the series, DCV1T-DCV3T 1-3 , were already described in literature and were prepared for non-linear optical properties. [ 38 , 39 ] Based on our experience with alkylated DCV n Ts, we fi rstly synthesized DCV1T-DCV4T 1-4 by a linear approach ( Scheme 1 ). Thus, bi-, ter-, and quaterthiophene dialdehydes 8-10 , [40][41][42][43] which were prepared by Vilsmeier-Haack formylation of the parent oligothiophenes [44][45][46] were reacted with malononitrile and ß-alanine as catalyst to form target DCV n Ts 2 and 3 (n = 2, 3) in nearly quantitative yield and in high purity. DCV1T 1 was obtained from commercially available thiophene-2,5-dicarbaldehyde 7 and malononitrile without the use of catalyst. However, due to a dramatic decrease in solubility, reaction of quaterthiophene-dialdehyde 10 resulted in an inseparable mixture of target DCV4T 4 and the singly reacted intermediate with a DCV-group at one chain end and an unreacted formyl group at the other. Convergent SynthesesDue to solubility problems of the longer homologues, only a series up to the trimer could be reliably synthesized by the usual linear synthetic approach. Thus, we developed a convergent route to DCV-capped oligothiophenes DCV3T 3 to DCV6T 6 ( Scheme 2 ). Terminal buildin...
Organic solar cells based on a new oligothiophene derivative (see figure) and fullerene C60 exhibit power efficiencies of up to 3.4 %. α,α′‐Bis(2,2‐dicyanovinyl)‐quinquethiophene (DCV5T) features a reduced optical gap with high absorption coefficients between 450 and 650 nm. Dicyanovinyl substituents on the oligothiophene rings lower the bandgap and increase the ionization energy of these oligomers, enabling the formation of photoactive heterojunctions with C60 and yielding open‐circuit photovoltages as high as 1 V.
The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.
X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.
A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C 60 as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor ͓K. Schulze et al., Adv. Mater. ͑Weinheim, Ger.͒ 18, 2872 ͑2006͔͒. This prompted us to systematically study the energy-and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups ͑dicyanovinyl͒ attached to the oligothiophene. Use of photoinduced absorption ͑PA͒ allows the study of the electron transfer process at the heterojunction to C 60. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0-1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.