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Developing intrinsically self-healable semiconducting polymers is particularly important for realizing stretchable electronic devices which can spontaneously recover their mechanical damage. We synthesized a series of conjugated donor polymers (PTO2-tm series) functionalized with a thymine group at the terminal of the side chain. Bulk heterojunction (BHJ) films of PTO2-tm polymers blended with an IT-4F acceptor achieved power conversion efficiencies up to 11.9% from the BHJ-based organic photovoltaics, which are comparable with that of unfunctionalized PTO2 and, however, exhibited much improved stretchability by virtue of stress dissipation in the BHJ films during mechanical deformation. More importantly, the corresponding BHJ film prepared with a melamine additive efficiently recovered their packing structures after being mechanically damaged by stretching. As a result, reversible multiple hydrogen bonding of the melamine molecule with thymine in PTO2-tm polymers showed much improved crack onset strain and maximum recoverable strain compared with PTO2.

Understanding the effects of the chemical structures of donor polymers on the photovoltaic properties of their corresponding organic photovoltaic (OPV) devices under various light-intensity conditions is important for improving the performance of these devices. We synthesized a series of copolymers based on poly[(2,6-(4,8-bis(5-(2-thioethylhexyl)thiophen-2-yl)benzo[1,2

A new polymeric semiconductor, which can effectively extend linear dynamic ranges (LDRs) of organic photodiodes (OPDs), was developed. Copolymers based on alkylthio-substituted benzo[1,2-b:4,5-b′]d...

Random terpolymers are developed by incorporating small portions of benzodithiophene into a highly crystalline copolymer of terthiophene and difluorobenzothiadiazole, BDT-Th0. The bulk-heterojuncti...

We synthesized a donor polymer of bis(2-ethylhexyl)thiophene-substituted benzodithiophene (BDT-Th) and 1,3-bis(2-ethylhexyl)-5,7-di(thiophene-2-yl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione, for whic...

We developed donor–acceptor (D–A)-type random polymers based on 3,3′-difluoro-2,2′-bithiophene with various relative amounts of 5,6-difluoro-4,7-bis(5-bromo-(2-decyltetradecyl)thiophen-2-yl)-2,1,3-benzothiadiazole (2FBT) and 5,6-difluoro-4,7-bis(5-bromo-(2-octyldodecyl)thiophen-2-yl)-2-(3,4-dichlorobenzyloxybutyl)-2H-benzo[d][1,2,3]triazole (DCB-2FBTZ). Introducing small relative amounts of DCB-2FBTZ into the polymer was found to effectively enhance its solar cell performance, resulting in a power conversion efficiency of 9.02%, greater than the 7.29% that resulted from the PFBT-FTh copolymer. Moreover, when the active area of the BHJ film was increased to 1 cm2, the solar cell reproducibly showed a high performance, here with an efficiency of 8.01% even when the thickness of the active layer was 313 nm. Our studies revealed that including the DCB-2FBTZ group in the polymer simultaneously improved the solution processability and crystallinity of the polymer. These improvements resulted in the formation of...

A series of novel conjugated polyelectrolytes composed of two different building blocks with different composition ratios were designed and synthesized for application as a functional layer in high-performance organic photodiodes (OPDs). A homopolymer and two random copolymers were prepared using different molar ratios of dibromo 1,4-bis(4-sulfonatobutoxy)benzene (SPh) and dibromo 1,4-bis(4-tetraethylene glycol)benzene (EGPh): EG20 with SPh:EGPh ratio of 0.8:0.2 and EG40 with a ratio of 0.6:0.4. Structural analyses by two-dimensional grazing-incidence X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy studies proved that a higher EGPh content could induce more organized polymer chains with face-on orientation of EG20 and EG40. Such an orientation of EG20 and EG40 along with the ordered crystalline organization yielded effective molecular dipole moments in the thin films when applied as an interlayer between ZnO and an active layer of inverted OPDs. As confirmed by ultraviolet pho...

The photostability of donor–acceptor (D–A) polymers remains a critical issue despite recent improvements in the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. We report the synthesis of three highly photostable polymers (PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2) and their suitability for use in high-performance OPV cells. Under 1 sunlight of illumination in air for 10 h, these polymer films demonstrated remarkably high photostability compared to that of PTB7, a representative polymer in the OPV field. While the PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymer films maintained 97, 90, and 96% photostability, respectively, a PTB7 film exhibited only 38% photostability under the same conditions. We ascribed the high photostability of the polymers to both the intrinsically photostable chemical moieties and the dense packing of alkyl side chains and planar backbone polymer chains, which prevents oxygen diffusion into the PDTBDT-BZ films. This work demonstrates the high photostability of ...

We report a highly efficient p–i–n type planar perovskite solar cell with a hybrid PEDOT/NiOx hole-extraction layer. It has been found that the perovskite solar cell with a NiOx thin film as a hole-extraction layer generally exhibits lower fill factor compared to the conventionally used PEDOT:PSS thin film, whereas it shows higher photocurrent and photovoltage. The fill factor of the NiOx-based perovskite solar cell can be significantly improved by treating the NiOx surface with a dilute PEDOT solution. The photoluminescence quenching study and impedance spectroscopic (IS) analysis have revealed that the hole injection at the perovskite/NiOx interface is significantly facilitated with the PEDOT treatment, which should lead to the increased fill factor. As a result, the p–i–n type planar perovskite solar cell with the new hybrid hole-extraction layer exhibits a high conversion efficiency of 15.1% without the hysteresis effect.

We synthesized a solution-processable low band gap small molecule, Si1TDPP-EE-COC6, for use as a semiconducting channel material in organic thin film transistors (OTFTs). The Si1TDPP-EE-COC6 is composed of electron-rich thiophene–dithienosilole–thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) and carbonyl units. SiTDPP-EE-COC6-based OTFTs with Au source/drain electrodes were fabricated, and their electrical properties were systematically investigated with increasing thermal annealing temperature. The hole and electron mobilities of as-spun Si1TDPP-EE-COC6 were 3.3 × 10–4 and 1.7 × 10–4 cm2 V–1 s–1, respectively. The carrier mobilities increased significantly upon thermal annealing at 150 °C, yielding a hole mobility of 0.003 cm2 V–1 s–1 and an electron mobility of 0.002 cm2 V–1 s–1. The performance enhancement upon thermal annealing was strongly associated with the formation of a layered edge-on structure and a reduction in the π–π intermolecular spacing. Importantly, the use of at...

A series of conjugated copolymers, PDPPFT and PNDIFT, were developed using difluoroterthiophene and DPP or NDI as the cobuilding block. We obtained two different molecular weight polymers for each polymer type by changing the conditions for the Stille coupling reaction and studied their optoelectrochemical properties and charge-transport behavior in organic field-effect transistors (OFETs). Both the lower molecular weight polymers, PDPPFT(L) and PNDIFT(L), showed better long-range ordered structures in films, whereas the polymers with higher molecular weights were less long-range ordered and showed a more preferential face-on orientation. By virtue of their favorable polymer packing structures, PDPPFT(L) and PNDIFT(L) exhibited much higher hole mobilities compared with their higher molecular weight counterparts, PDPPFT(H) and PNDIFT(H). By contrast, both PDPPFT and PNDIFT maintained good n-channel properties independent of their molecular weights, thus their long-range ordering in a film. The strong elect...

It is important to develop new donor (D)–acceptor (A) type low band gap polymers for highly stable polymer solar cells (PSCs). Here, we describe the synthesis and photovoltaic properties of two D–A type low band gap polymers. The polymers consist of dithienobenzodithiophene (DTBDT) moieties with expanded conjugation side groups as donors and 2-ethyl-1-(thieno[3,4-b]thiophen-2-yl)hexan-1-one (TTEH) or 6-octyl-5H-thieno[3′,4′:4,5]thieno[2,3-c]pyrrole-5,7(6H)-dione (DTPD) as acceptors to give pDTBDT-TTEH and pDTBDT-DTPD polymers, respectively. The pDTBDT-TTEH is quite flat, resulting in a highly crystalline film. In contrast, the pDTBDT-DTPD is highly twisted to yield an amorphous film. Photovoltaic devices based on pDTBDT-TTEH and pDTBDT-DTPD exhibited power conversion efficiencies (PCEs) of 6.74% and 4.44%, respectively. The PCE difference results mainly from morphological differences between the two polymer:PC71BM blend films; the pDTBDT-TTEH polymer formed a nanoscopically networked domains in the blend ...

Bottom-contact bottom-gate organic field-effect transistors (OFETs) are fabricated using a low band gap pDTTDPP-DT polymer as a channel material and single-layer graphene (SLG) or Au source/drain electrodes. The SLG-based ambipolar OFETs significantly outperform the Au-based ambipolar OFETs, and thermal annealing effectively improves the carrier mobilities of the pDTTDPP-DT films. The difference is attributed to the following facts: (i) the thermally annealed pDTTDPP-DT chains on the SLG assume more crystalline features with an edge-on orientation as compared to the polymer chains on the Au, (ii) the morphological features of the thermally annealed pDTTDPP-DT films on the SLG electrodes are closer to the features of those on the gate dielectric layer, and (iii) the SLG electrode provides a flatter, more hydrophobic surface that is favorable for the polymer crystallization than the Au. In addition, the preferred carrier transport in each electrode-based OFET is associated with the HOMO/LUMO alignment relative to the Fermi level of the employed electrode. All of these experimental results consistently explain why the carrier mobilities of the SLG-based OFET are more than 10 times higher than those of the Au-based OTFT. This work demonstrates the strong dependence of ambipolar carrier transport on the source/drain electrode and annealing temperature.

We present controls of nanomorphology of polymer:fullerene BHJ films via synthesis of a series of push–pull-type copolymers with various molar percentages of side chains terminated with o-dichlorobenzyl (DCBZ) groups. As the molar percentage of the DCBZ-containing repeat units increases, the miscibility of the polymers in the series with PC71BM increases with respect to that of the polymer (PTBBO) that does not contain DCBZ. In the optimal film morphology, which consists of a polymer containing 25 mol % DCBZ-terminated side chains in the electron-pull unit (PTBDCB25), the polymer/PC71BM interface area is sufficiently large for efficient charge separation and percolated pathways is present for efficient charge carrier transport. In contrast, the BHJ film prepared from PTBBO has smaller interfaces and larger PC71BM aggregates. Furthermore, the intermolecular interaction between PC71BM and DCBZ induced changes in the PC71BM’s electronic structure at the polymer:PC71BM interface, resulting in an increase of t...

Kesterite Cu2ZnSnSe4 (CZTSe) thin films prepared by the selenization of mechanochemically synthe- sized Cu2ZnSnS4 (CZTS) nanocrystal films are systematically investigated as a function of the annealing time in terms of the phase purity, microstructure, composition, and device characteristics. It is shown that selenization for an extended time does not cause a noticeable amount of Sn loss or segregation of Zn-rich layers. Thus, the prolonged annealing leads to improvements (reduction) in the shunt conductance, reverse saturation current, and diode ideality factor. However, it also leads to a deterioration of the series resistance, of which influence turned out to overwhelm all of the aforementioned positive effects on the device performance. As a consequence, the CZTSe solar cell exhibits its highest efficiency (5.43%) at the shortest annealing time (10 min). Impedance spectroscopy is demonstrated to be of good use in detecting the change in the back contact of CZTSe solar cells during annealing. The impedance spectra of the CZTSe solar cells are analyzed in association with the microstructures of the back-contact electrodes, demonstrating that the increase in the series resistance is attributed to the formation of the resistive MoSe2 layer.

We synthesized a series of push–pull-type copolymers by copolymerizing an electron-deficient diketopyrrolopyrrole with three electron-donating benzodithiophene (BDT) moieties. PDPPDTT, which incorporated a dithienothiophene (DTT), showed a higher power conversion efficiency (PCE) of 6.11% compared to 3.31% for the BDT-based polymer (PDPPBDT). PDPPDTBDT, which incorporated a dithienobenzodithiophene (DTBDT), also exhibited superior performance, with a PCE of 4.75% although this value was lower than that obtained for PDPPDTT. The presence of the DTT unit in the polymer backbone lowered the energy bandgap of the polymer and induced an optimal morphology in the polymer:PC71BM blend film, resulting in higher charge carrier generation. Furthermore, the effectively delocalized frontier orbitals of PDPPDTT enhanced intermolecular interactions between the polymer chains by favoring effective π–π stacking, which facilitated charge carrier transport. By contrast, PDPPDTBDT unexpectedly showed a low-crystallinity thi...

We characterized the electrical properties of a field-effect transistor (FET) and a nonvolatile memory device based on a solution-processable low bandgap small molecule, Si1TDPP-EE-C6. The small molecule consisted of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) units. The as-spun Si1TDPP-EE-C6 FET device exhibited ambipolar transport properties with a hole mobility of 7.3×10(-5) cm2/(Vs) and an electron mobility of 1.6×10(-5) cm2/(Vs). Thermal annealing at 110 °C led to a significant increase in carrier mobility, with hole and electron mobilities of 3.7×10(-3) and 5.1×10(-4) cm2/(Vs), respectively. This improvement is strongly correlated with the increased film crystallinity and reduced π-π intermolecular stacking distance upon thermal annealing, revealed by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) measurements. In addition, nonvolatile memory devices based on Si1TDPP-EE-C6 were successfully fabricated by incorporating Au nanoparticles (AuNPs) as charge trapping sites at the interface between the silicon oxide (SiO2) and cross-linked poly(4-vinylphenol) (cPVP) dielectrics. The device exhibited reliable nonvolatile memory characteristics, including a wide memory window of 98 V, a high on/off-current ratio of 1×10(3), and good electrical reliability. Overall, we demonstrate that donor-acceptor-type small molecules are a potentially important class of materials for ambipolar FETs and nonvolatile memory applications.

We synthesized four polymers (pT3DPP-HD, pT3DPP-OD, pT2TTDPP-HD, and pT2TTDPP-OD) and characterized their photovoltaic properties as a function of the backbone planarity, alkyl side chain length, and film morphology. The polymers were donor–acceptor type low-band-gap (1.2–1.3 eV) polymers employing terthiophene (T3) or thiophene–thieno[3,2-b]thiophene–thiophene (T2TT) as the donor and 2,5-bis(2-hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (DPP-HD) or 2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (DPP-OD) as the acceptor. The T2TT moiety in the polymer backbone is more planar than the T3; the OD moiety as the alkyl side chain ensured a higher solubility than the HD moiety. Polymer solar cells (PSCs) were fabricated, and their properties were characterized. The photoactive layer consisted of one of the four polymers and one of the fullerene derivatives (PC70BM or PC60BM). For a given fullerene derivative, the PCEs prepared with each of the four polymers were ordered according to pT3DP...

We characterized the electrical properties of ambipolar polymer field-effect transistors (PFETs) based on the low-band-gap polymer, pNAPDO-DPP-EH. The polymer consisted of electron-rich 2,6-di(thienyl)naphthalene units with decyloxy chains (NAPDO) and electron-deficient diketopyrrolopyrrole units with 2-ethylhexyl chains (DPP-EH). The as-spun pNAPDO-DPP-EH PFET device exhibited ambipolar transport properties with a hole mobility of 3.64 × 10–3 cm2/(V s) and an electron mobility of 0.37 × 10–3 cm2/(V s). Thermal annealing of the polymer film resulted in a dramatic increase in the carrier mobility. Annealing at 200 °C yielded hole and electron mobilities of 0.078 and 0.002 cm2/(V s), respectively. The mechanism by which the mobility had improved was investigated via grazing incidence X-ray diffraction studies, atomic force microscopy, and temperature-dependent transport measurements. These results indicated that thermal annealing improved the polymer film crystallinity and promoted the formation of a longer...

A facile method for increasing the reaction rate of dye adsorption, which is the most time-consuming step in the production of dye-sensitized solar cells (DSSCs), was developed. Treatment of a TiO2 photoanode with aqueous nitric acid solution (pH 1) remarkably reduced the reaction time required to anchor a carboxylate anion of the dye onto the TiO2 nanoparticle surface. After optimization of the reaction conditions, the dye adsorption process became 18 times faster than that of the conventional adsorption method. We studied the influence of the nitric acid treatment on the properties of TiO2 nanostructures, binding modes of the dye, and adsorption kinetics, and found that the reaction rate improved via the synergistic effects of the following: (1) electrostatic attraction between the positively charged TiO2 surface and ruthenium anion increases the collision frequency between the adsorbent and the anchoring group of the dye; (2) the weak anchoring affinity of NO3(-) in nitric acid with metal oxides enables the rapid coordination of an anionic dye with the metal oxide; and (3) sufficient acidity of the nitric acid solution effectively increases the positive charge density on the TiO2 surface without degrading or transforming the TiO2 nanostructure. These results demonstrate the developed method is effective for reducing the overall fabrication time without sacrificing the performance and long-term stability of DSSCs.

We report the synthesis, characterization, and solar cell properties of new donor–acceptor-type low band gap polymers (POBDTPD and PEBDTPD) that incorporate dialkoxybenzodithiophene (BDT) as the donor and N-octyl-2,7-dithia-5-azacyclopenta[a]pentalene-4,6-dione (DTPD) as the acceptor. The newly developed DTPD moiety was carefully designed to lower a band gap via strong interaction between donor–acceptor moieties and keep polymer energy levels deep. Remarkably, the DTPD acceptor moiety effectively widens the light absorption range of the polymers up to ∼900 nm while positioning their HOMO and LUMO levels in the optimal range, i.e., −5.3 and −4.0 eV, respectively, for high power conversion efficiencies (PCEs) as we intended. Solar cell devices were fabricated according to the structure ITO/PEDOT:PSS/photoactive (polymer:PC70BM)/TiO2/Al. The POBDTPD devices exhibited a PCE of 4.7% with a Voc of 0.70 V, a Jsc of 10.6 mA/cm2, and a FF of 0.64. The PEBDTPD devices yielded a higher PCE of 5.3% with a Voc of 0.72...

We report synthesis and characterizations of two novel series of polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP and 2-(1-butylheptyl)thieno[3,4-d]thiazole (TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT), and BDT was also synthesized. The PBIT series of polymers showed enhanced ground and excited state dipole moments (μg and μe) when compared to the previously reported PBB3 polymer, while PBTZ1 showed the largest dipole change (1.52 D) from ground to excited state (Δμge) in respective single polymer units. It was found that the power conversion efficiencies of the polymer series were strongly correlated to Δμge. The results reported demonstrate the utility of the calculated parameter Δμge of single units of the polymers to predict the perfo...

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.