Zugui Shi

Spectral Dependence of the Internal Quantum Efficiency of Organic Solar Cells: Effect of Charge Generation Pathways

The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes.

Modulating high-energy visible light absorption to attain neutral-state black electrochromic polymers

Neutral-state black diketopyrrolopyrrole (DPP)-based conjugated polymers with the colour channels a* from −3 to 0 and b* from −2 to −7 are achieved by incorporating an additional donor segment – dialkoxythiophene (DalkOT), which leads to the enhancement in blue-wavelength absorption. The polymers display black-to-transmissive grey electrochromic switching with good redox stability.

Hole mobility of 3.56 cm2 V−1 s−1 accomplished using more extended dithienothiophene with furan flanked diketopyrrolopyrrole polymer

A highly extended dithienothiophene comonomer building block was used in combination with highly fused aromatic furan substituted diketopyrrolopyrrole for the synthesis of novel donor–acceptor alternating copolymer PDPPF-DTT. Upon testing PDPPF-DTT as a channel semiconductor in top contact bottom gate organic field effect transistors (OFETs), it was found to exhibit p-channel behaviour. The highest hole mobility of 3.56 cm2 V−1 s−1 was reported for PDPPF-DTT. To our knowledge, this is the highest mobility reported so far for the furan flanked diketopyrrolopyrrole class of copolymers using conventional device geometry with straightforward processing.

Thiophene–tetrafluorophenyl–thiophene: a promising building block for ambipolar organic field effect transistors

A thiophene–tetrafluorophenyl–thiophene donor–acceptor–donor building block was used in combination with a furan-substituted diketopyrrolopyrrole for synthesizing the polymer semiconductor, PDPPF-TFPT. Due to the balance of tetrafluorophenylene/diketopyrrolopyrrole electron-withdrawing and furan/thiophene electron-donating moieties in the backbone, PDPPF-TFPT exhibits ambipolar behaviour in organic thin-film transistors, with hole and electron mobilities as high as 0.40 cm2 V−1 s−1 and 0.12 cm2 V−1 s−1.

Solution-processable low-bandgap 3-fluorothieno[3,4-b]thiophene-2-carboxylate-based conjugated polymers for electrochromic applications

In this paper, a series of low-bandgap donor–acceptor (D–A) conjugated polymers with 3-fluorothieno[3,4-b]thiophene-2-carboxylate (FTT) as an acceptor and ethylenedioxythiophene (EDOT) (P1), acyclic dioxythiophene (AcDOT) (P2) or propylenedioxythiophene (ProDOT) (P3) as donors were synthesized via Stille polymerization. The resultant polymers have good solubility in organic solvents. The polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR) and thermogravimetric analysis (TGA). Their electrochemical, morphological and electrochromic (EC) properties were investigated, and their absorption-transmission type electrochromic devices (ECDs) were fabricated and characterized. In their neutral states, the polymers displayed deep magenta (P1) to blue (P2, P3) hues, and upon electrochemical oxidation, they revealed grey tones with good optical contrasts (19–37 and 57–58% in visible and near-infrared (NIR) regions, respectively), good coloration efficiencies (158–380 and 279–378 cm2 C−1 for visible and NIR regions, respectively) and reasonable redox stability (retaining 64–80% original optical contrast after 1000 cycles) under ambient conditions and without any encapsulation of the ECDs.

Diketopyrrolopyrrole-Based Low-Bandgap Conjugated Polymers with Siloxane Side Chains for Electrochromic Applications

A series of conjugated copolymers P1–P3 were synthesized from 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(6-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)hexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione and 2,5-bis(trimethylstannyl)-3,4-dialkoxythiophene or its analogue via Stille coupling reactions, with molecular weights in the range of 13000–18000 g mol–1 and polydispersity indexes of 1.4–1.7. This new type of donor–acceptor polymers demonstrated reasonable switching speed, promising redox stability, together with high optical contrast and coloration efficiency. Although, at the current stage, the hybrid siloxane-terminated side chain did not significantly improve the overall performance of the resultant polymers, the unique chemical properties of the siloxane group would offer possibilities for crosslink after suitable post-polymerization treatment, and thus pave the way for future fine-tuning of the morphology of electrochromic films.

Influence of catalytic systems in Stille polymerization on the electrochromic performance of diketopyrrolopyrrole-based conjugated polymers

Two diketopyrrolopyrrole- (DPP) and thienothiophene-based donor–acceptor (D–A) type conjugated polymers are separately synthesized using Stille coupling reactions with Pd(PPh3)4 and PdCl2(PPh3)2 catalytic systems, respectively. The influence of the catalyst employed on the structural regularity of the resulting polymers, and their subsequent electrochromic (EC) performances are investigated. Marked differences in the optical contrasts, switching times, coloration efficiencies and redox stabilities of the polymers are uncovered. For example, polymer P1, which is prepared using Pd(PPh3)4 as a catalyst, exhibited a larger optical contrast and coloration efficiency by 40% and 79%, respectively, and a faster coloration time by 55%, at 825 nm, than its counterpart polymer P2, which is prepared using PdCl2(PPh3)2. The intrinsic optical, electrochemical and morphological properties of the polymers are characterized, revealing that the physical conformation and packing of the polymer chains are the main factors causing the discrepancy in EC performances.

Control of morphology and performance of diketopyrrolopyrrole-based electrochromic polymers using solvent vapor annealing

Post-deposition treatment including thermal and solvent annealing of polymer films in various organic electronics such as organic photovoltaics and organic thin film transistors plays a vital role in governing the film morphology and consequently their optical and electronic properties. However, such a post-treatment method has yet been used for electrochromics. This paper studied the influence of solvent vapor annealing of a diketopyrrolopyrrole-containing electrochromic conjugated polymer on its film morphology. Compared to an un-annealed film, the films exposed to acetone vapor and chloroform vapor are generally composed of polymer clusters with smaller domain sizes and more compact, aggregated structures. Subsequent evaluation of the electrochromic performances of the devices revealed the strong influence of the film morphologies on the optical contrasts, switching times and coloration efficiencies. In general, the electrochromic films treated with a poor solvent (acetone) exhibited faster switching speeds and improved coloration efficiencies. In contrast, treatment with a good solvent (chloroform) had destructive effects on the optical contrasts, switching speeds and coloration efficiency of the films. These findings showed that the electrochromic performance closely corresponded to changes in the film structure and morphology.