Diketopyrrolopyrrole-based Copolymer Research Articles

Thermal-induced bandgap broadening phenomenon in copolymer organic semiconductors

This study elucidates the thermal-induced bandgap broadening (TBB) phenomenon in copolymer organic semiconductors (OSCs). From the perspective of device physics, the diketopyrrolopyrrole-based copolymer was employed as the semiconductor layer to fabricate a back-to-back Schottky junction structure. Characterization through the analysis of I–V curves enables the qualitative exploration of the correlation between bandgap and temperature in copolymer OSCs. Then, technology computer-aided design was utilized to explore the influence of thermal-induced bandgap broadening on the back-to-back Schottky I–V curves. Subsequently, the variable temperature UV-VIS-NIR absorption spectra of the copolymer OSCs were analyzed, providing quantitative evidence of the thermal-induced bandgap broadening phenomenon and confirming its recoverability. Through the research of the Schottky junction and absorption spectra, we verified the consistency of the TBB phenomenon in both the electrical and optical bandgaps. From the scanning electron microscope images of the copolymer OSC films, it is found that the thermal-induced bandgap broadening phenomenon is mainly caused by thermal expansion and increased disorder of copolymer molecules. This study highlights a physical phenomenon of copolymer OSCs that is different from most inorganic semiconductors, and such insight offers a theoretical perspective for the application and thermal stability investigation of copolymer devices under high temperature conditions.

Crystalline Domain Formation to Enable High-Performance Polymer Thermoelectrics Inspired by Thermocleavable Materials.

Improving the electrical conductivity is an important role in realizing high thermoelectric performance of solution-processable polymers. Herein, a simple and robust approach to boost the mobility and doping efficiency of a diketopyrrolopyrrole-based copolymer with the introduction of thermocleavable side chains (PDPPS-X, where X is the molar ratio of the thermocleavable side chains and alkyl chains) is first provided. Notably, the incorporated thermocleavable groups can be effectively removed after thermal treatment and therefore contribute to the crystalline domain formation via hydrogen-bonded networks, which is critical for conductivity enhancements. Grazing incidence wide-angle X-ray scattering (GIWAXS) patterns give a clear indication that the thermal treatment of PDPPS-5 can greatly improve the structural arrangement, resulting in a significantly enhanced hole mobility (5.4 times that of PDPPS-0 without thermocleavable chains). Compared to PDPPS-0, a larger Fermi level shift is observed after doping PDPPS-5 with FeCl3, reflecting a better doping efficiency. Consequently, remarkably improved conductivity and power factor are achieved by PDPPS-5 after doping with 0.03 M FeCl3 at room temperature, which are about 2.2 and 3.5 times higher than that of PDPPS-0 at the same testing condition, respectively. Moreover, PDPPS-5 achieved a maximum power factor of 57.5 μW m-1 K-2 at 404 K.

Organic Transistors: Enhancing the Electrochemical Doping Efficiency in Diketopyrrolopyrrole‐Based Polymer for Organic Electrochemical Transistors (Adv. Electron. Mater. 1/2021)

Organic electrochemical transistors (OECTs) are a promising candidate for next-generation bioelectronic applications. In article number 2000701, Prashant Sonar, Wei Lin Leong, and co-workers report the OECT performance of a diketopyrrolopyrrole-based copolymer. High transconductance and excellent device stability demonstrate this copolymer's great potential for use in future electrophysiology, for example, electrocardiography and electroencephalography.

Electrohydrodynamic-Jet (EHD)-Printed Diketopyrrolopyroole-Based Copolymer for OFETs and Circuit Applications.

We report the employment of an electrohydrodynamic-jet (EHD)-printed diketopyrrolopyrrole-based copolymer (P-29-DPPDTSE) as the active layer of fabricated organic field-effect transistors (OFETs) and circuits. The device produced at optimal conditions showed a field-effect mobility value of 0.45 cm2/(Vs). The morphologies of the printed P-29-DPPDTSE samples were determined by performing optical microscopy, X-ray diffraction, and atomic force microscopy experiments. In addition, numerical circuit simulations of the optimal printed P-29-DPPDTSE OFETs were done in order to observe how well they would perform in a high-voltage logic circuit application. The optimal printed P-29-DPPDTSE OFET showed a 0.5 kHz inverter frequency and 1.2 kHz ring oscillator frequency at a 40 V supply condition, indicating the feasibility of its use in a logic circuit application at high voltage.

Side-chain engineering of diketopyrrolopyrrole-based copolymer using alkyl ester group for efficient polymer solar cell

The new diketopyrrolopyrrole-based copolymer, referred to as PDPPBDTE, bearing linear alkyl ester groups, was synthesized with the objective of improving the miscibility between the donor polymer and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). PDPPBDTA bearing ethylhexyl side-chains was also synthesized as a control polymer. The PDPPBDTE:PC71BM blend film showed a relatively small crystalline domain size compared to the PDPPBDTA:PC71BM blend film. A polymer solar cell fabricated with PDPPBDTE and PC71BM exhibited twofold higher power conversion efficiency than that employing PDPPBDTA.

Flexible ambipolar organic field-effect transistors with reverse-offset-printed silver electrodes for a complementary inverter

We report ambipolar organic field-effect transistors and complementary inverter circuits with reverse-offset-printed (ROP) Ag electrodes fabricated on a flexible substrate. A diketopyrrolopyrrole-based co-polymer (PDPP-TAT) was used as the semiconductor and poly(methyl methacrylate) was used as the gate insulator. Considerable improvement is observed in the n-channel electrical characteristics by inserting a cesium carbonate (Cs2CO3) as the electron-injection/hole-blocking layer at the interface between the semiconductors and the electrodes. The saturation mobility values are 0.35 cm2 V−1 s−1 for the p-channel and 0.027 cm2 V−1 s−1 for the n-channel. A complementary inverter is demonstrated based on the ROP process, and it is selectively controlled by the insertion of Cs2CO3 onto the n-channel region via thermal evaporation. Moreover, the devices show stable operation during the mechanical bending test using tensile strains ranging from 0.05% to 0.5%. The results confirm that these devices have great potential for use in flexible and inexpensive integrated circuits over a large area.

Compact Layer Free Perovskite Solar Cells with a High-Mobility Hole-Transporting Layer

A high-mobility diketopyrrolopyrrole-based copolymer (P) was employed in compact layer free CH3NH3PbI3 perovskite solar cells as a hole-transporting layer (HTL). By using the P-HTL, the 6.62% device efficiency with conventional poly-3-hexylthiophene was increased to 10.80% in the simple device configuration (ITO/CH3NH3PbI3/HTL/MoO3/Ag). With improved short circuit current density, open circuit voltage, and fill factor, the higher power conversion efficiency of P-HTL device is ascribed to the higher carrier mobility, more suitable energy level, and lower interfacial charge recombination. Advantages of applying P-HTL to perovskite solar cells, such as low cost, low-temperature processing, and excellent performance with simple cell structure, exhibit a possibility for commercial applications.

Molecular-weight engineering of high-performing diketopyrrolopyrrole-based copolymer bearing high π-extended long donating units

Two D–A alternating π-conjugated copolymers bearing diketopyrrolopyrrole (DPP) and sextetthiophene with different molecular weights were synthesized to investigate their physical properties and performance in thin-film transistors and photovoltaic cells. The solubility and molecular weight (MW) of these copolymers with a highly rigid backbone were improved by controlling the feeding ratio of the monomer in the polymerization mixture. These polymers exhibited high crystallinity with a predominant edge-on chain orientation on the substrate. The higher-MW P(DPP-6T)-2 copolymer displayed a higher hole mobility of 3.26 cm2 V−1 s−1 than the low-MW P(DPP-6T)-1 copolymer. Bulk heterojunction photovoltaic cells made of a blend of P(DPP-6T)-2 and (6,6)–phenyl C71–butyric acid methyl ester displayed good device performance with a power conversion efficiency of 4.68%.

(Invited) Photoexcited-State Dynamics in Organic Solar Cells Utilizing Diketopyrrolopyrrole-Based Copolymer Investigated By Transient Optical Spectroscopy

Recently, donor-acceptor (D-A) copolymers have emerged a significant class of the materials for bulk-heterojunction (BHJ) solar cells. Among the possible D-A copolymers, diketopyrrolopyrrole (DPP)-based copolymers have been studied intensively as attractive materials for solar cells because of their low band gaps with wide-range absorption and high carrier mobility. In fact, DPP-based copolymers exhibited promising power conversion efficiency of ~8% [1]. In this study, we investigate the photoexcited-state dynamics in BHJ solar cells composed of the DPP-naphthalene copolymer (PDPP-TNT) [the inset of Fig. (a)] and PC71BM, using picosecond fluorescence and femtosecond transient absorption (TA) spectroscopies [2]. PDPP-TNT/PC71BM (1:2 by weight) blend films were prepared on glass substrates by spin-coating process from solutions in chloroform alone or a mixture of chloroform and o-dichlorobenzene (o-DCB) (4:1 by volume). The absorption spectrum of a blend film from chloroform alone is shown in Fig. (a). For TA measurements, 150-fs light pulse at 650 nm was used as a pump pulse. To detect the ground-state bleaching in PDPP-TNT, we used light pulse at 740 nm as a probe pulse. For the picosecond fluorescence measurements, 150-fs light pulse at 660 nm was used as an excitation light. A streak camera equipped with a monochromator was employed to monitor the fluorescence spectra [Fig. (a)] and kinetics of the PDPP-TNT polymer. Figure (b) shows the transient absorption kinetics of a neat PDPP-TNT film and the PDPP-TNT/PC71BM blend films (chloroform alone and chloroform & o-DCB) at a low excitation photon density (~ 100 nJ/cm2). The time profile for a neat PDPP-TNT film can be reproduced by single exponential decay (time constant: 175 ps), which is in good agreement with that of fluorescence. This result indicates that the observed signal originates from singlet excitons, and the absence of a long-lived component implies that charge carriers such as polaron pairs and triplet excitons are not efficiently photogenerated in the neat film. On the other hand, time profile for blend films can be reproduced by sum of exponential decay (time constant: 125 ps) and constant component. Therefore, in the blend films, the fast decay component can be interpreted as recombination of singlet excitons, and the long-lived one is attributable to the initially separated charge carriers. The signal due to charge carriers is more dominant in the blend film from a mix of chloroform and o-DCB than that from chloroform alone. This is consistent with the result from the photovoltaic measurement that more photocurrent is generated in the film from a mix of chloroform and o-DCB than in that from chloroform alone. Discussion on the diffusion length of singlet exciton in neat PDPP-TNT film will be also presented from the results of singlet–singlet annihilation analysis. Acknowledgement: The work was partly supported by the Strategic International Research Cooperative Program (SICP), Japan Science and Technology Agency (JST). Figure: (a) Molecular structure of PDPP-TNT and absorption spectrum of PDPP-TNT/PC71BM blend film from chloroform alone. Fluorescence spectra of the PDPP-TNT polymer. (b) Transient absorption kinetics of the neat and blend films. Reference [1] K. H. Hendriks, G. H. L. Heintges, V. S. Gevaerts, . M. Wienk, and R. A. J. Janssen, Angew. Chem. Int. Ed., 52, 8341 (2013). [2] H. Matsuzaki, A. Furube, R. Katoh, S. P. Singh, P. Sonar, E. L.Williams, C. Vijila, G. S. Subramanian, S. Gorelik, and J. Hobley, Jpn. J. Appl. Phys., 53, 01AB11 (2014). Figure 1

Efficiency limitations in a low band-gap diketopyrrolopyrrole-based polymer solar cell

We report the performance and photophysics of a low band-gap diketopyrrolopyrrole-based copolymer used in bulk heterojunction devices in combination with PC71BM. We show that the short lifetime of photogenerated excitons in the polymer constitutes an obstacle towards device efficiency by limiting the diffusion range of the exciton to the donor-acceptor heterojunction. We employ ultrafast transient-probe and fluorescence spectroscopy techniques to examine the excited state loss channels inside the devices. We use the high boiling point solvent additive 1,8-diiodooctane (DIO) to study the photoexcited state losses in different blend morphologies. The solvent additive acts as a compatibiliser between the donor and the acceptor material and leads to smaller domain sizes, higher charge formation yields and increased device efficiency.

A high molecular weight triisopropylsilylethynyl (TIPS)-benzodithiophene and diketopyrrolopyrrole-based copolymer for high performance organic photovoltaic cells

A high-molecular-weight conjugated polymer consisting of alternating TIPS-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) and diketopyrrolo[3,4-c]pyrrole (DPP) was synthesized by Suzuki cross-coupling and characterized for use in high performance OPVs. Good structural order, self-assembly in spun-cast films, and carrier mobility were observed. PTIPSBDT–DPP showed an OFET hole mobility of up to 0.12 cm2 V−1 s−1. Power conversion efficiencies of up to 8.0% were demonstrated for OPV devices.

Excited-state dynamics in diketopyrrolopyrrole-based copolymer for organic photovoltaics investigated by transient optical spectroscopy

We investigate the photoexcited state dynamics in a donor–acceptor copolymer, poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene} (pDPP-TNT), by picosecond fluorescence and femtosecond transient absorption spectroscopies. Time-resolved fluorescence lifetime measurements of pDPP-TNT thin films reveal that the lifetime of the singlet excited state is 185 ± 5 ps and that singlet–singlet annihilation occurs at excitation photon densities above 6 × 1017 photons/cm3. From the results of singlet–singlet annihilation analysis, we estimate that the single-singlet annihilation rate constant is (6.0 ± 0.2) × 10−9 cm3 s−1 and the singlet diffusion length is ~7 nm. From the comparison of femtosecond transient absorption measurements and picosecond fluorescence measurements, it is found that the time profile of the photobleaching signal in the charge-transfer (CT) absorption band coincides with that of the fluorescence intensity and there is no indication of long-lived species, which clearly suggests that charged species, such as polaron pairs and triplet excitons, are not effectively photogenerated in the neat pDPP-TNT polymer.

Novel Polymer Nanowire Crystals of Diketopyrrolopyrrole‐Based Copolymer with Excellent Charge Transport Properties

The first demonstration of polymer nanowire (PNW) crystals based on a diketopyrrolopyrrole-based copolymer (i.e., PDTTDPP), and their application to field-effect transistors (FETs) is reported. Remarkably, transmission electron microscopy and selected area electron diffraction analyses of the PNW reveal its single-crystalline (SC) nature. FETs fabricated of a SC PNW exhibit a maximal charge carrier mobility of ≈7.00 cm(2) V(-1) s(-1) , which is almost one order of magnitude higher than that of the thin-film transistors made of the same polymer (PDTTDPP).

High performance ink-jet printed diketopyrrolopyrrole-based copolymer thin-film transistors using a solution-processed aluminium oxide dielectric on a flexible substrate

We report high performance, ink-jet printed diketopyrrolopyrrole-based copolymer devices consisting of a solution-processed AlOX gate dielectric on a polyimide (PI) substrate. The ink-jet printed thin-film transistors (TFTs) exhibited a field-effect mobility of 2.41 cm2 V−1 s−1 and a subthreshold swing of 65 mV dec−1. We also demonstrated a flexible, low voltage-driven inverter, exhibiting a gain of −1.81 V V−1 at a driving voltage of −1.0 V.

A new diketopyrrolopyrrole-based co-polymer for ambipolar field-effect transistors and solar cells

A new thieno[3,2-b]thiophenediketopyrrolopyrrole-benzo[1,2-b:4,5-b′]dithiophene based narrow optical gap co-polymer (PTTDPP-BDT) has been synthesized and characterized for field-effect transistors and solar cells. In field-effect transistors the polymer exhibited ambipolar charge transport behaviour with maximum hole and electron mobilities of 10−3cm2V−1s−1 and 10−5cm2V−1s−1, respectively. The respectable charge transporting properties of the polymer were consistent with X-ray diffraction measurements that showed close molecular packing in the solid state. The difference in hole and electron mobilities was explained by density functional theory calculations, which showed that the highest occupied molecular orbital was delocalized along the polymer backbone with the lowest unoccupied molecular orbital localized on the bis(thieno[3,2-b]thiophene)diketopyrrolopyrrole units. Bulk heterojunction photovoltaic devices with the fullerene acceptor PC70BM were fabricated and delivered a maximum conversion efficiency of 3.3% under AM1.5G illumination.

Electrical and Optical Properties of Diketopyrrolopyrrole-Based Copolymer Interfaces in Thin Film Devices

Two donor-acceptor diketopyrrolopyrrole (DPP)-based copolymers (PDPP-BBT and TDPP-BBT) have been synthesized for their application in organic devices such as metal-insulator semiconductor (MIS) diodes and field-effect transistors (FETs). The semiconductor-dielectric interface was characterized by capacitance-voltage and conductance-voltage methods. These measurements yield an interface trap density of 4.2 × 10(12) eV⁻¹ cm⁻² in TDPP-BBT and 3.5 × 10¹² eV⁻¹ cm⁻² in PDPP-BBT at the flat-band voltage. The FETs based on these spincoated DPP copolymers display p-channel behavior with hole mobilities of the order 10⁻³ cm²/(Vs). Light scattering studies from PDPP-BBT FETs show almost no change in the Raman spectrum after the devices are allowed to operate at a gate voltage, indicating that the FETs suffer minimal damage due to the metal-polymer contact or the application of an electric field. As a comparison Raman intensity profile from the channel-Au contact layer in pentacene FETs are presented, which show a distinct change before and after biasing.

Observation of negative differential resistance in diketopyrrolopyrrole-based copolymer films

Remarkable and repeatable negative differential resistance (NDR) phenomenon was observed in a metal-polymer-metal structure diode based on bishexyloxy-divinyl-benzene-alt-diketopyrrolopyrrole (C6DPPPPV), a type of donor-acceptor (D-A) conjugated copolymer. Thickness dependence of the devices implied that the observed NDR characteristics were bulk-controlled. The device performance was considered to depend on the slow trapping and releasing processes related to the local deep states, which was enhanced by the growth and thermal rupture of conducting filaments through the organic layer. The results suggest that the D-A conjugated copolymer is a promising memory material based on NDR effect.