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Varied-length conjugated polyelectrolytes (2–5 methylene units) containing CPDT-alt-BT were synthesized to study the optical, electrochemical, and morphological properties. These were used in accumulation mode organic electrochemical transistors.

A wet processing method to fabricate high-performance organic electrochemical transistors (OECTs) is reported. Wet chemical processing enables a simple and reliable patterning step, substituting several complex and expensive cleanroom procedures in the fabrication of OECTs. We fabricate depletion-mode OECTs based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and enhancement-mode OECTs based on a conjugated polyelectrolyte PCPDTBT-SO

Recently, with the development of figure-of-merit non-fullerene acceptor materials combined with a ternary strategy and layer-by-layer (LbL) processing, the efficiency of single-junction organic solar cells has exceeded 18%. However, the structural properties of LbL-processed films have not been sufficiently elucidated. Herein, we systematically investigate films fabricated via LbL processing of three different systems, including a ternary system. In particular, we focus on the structural and morphological transitions associated with the diffusion process controlled by thermal annealing and an additive solvent. Different diffusion and crystal formation mechanisms were clearly identified, which were observed to be dependent on the characteristics of the upper layer formed during the LbL process. Based on this insight, the photovoltaic properties associated with various LbL conditions are elucidated, and an ideal path toward a better device is suggested.

The 1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits different photoluminescence (PL) and aggregation-caused quenching properties due to its crystal polymorphism, which depends on the solvent evaporation process in tetrahydrofuran solution. In the slow drying process, molecules aggregated into an energetically more stable form (time-dependent density functional theory calculation), of which the PL peak maximum was 453 nm, corresponding to blue emission at the 365 nm excitation. However, the fast evaporation process induces an energetically less stable form, with a PL peak maximum of 508 nm, corresponding to green emission. The main difference between the two crystal structures is the alkyl conformation, as confirmed by X-ray single-crystal analysis. Due to the different alkyl conformations, NI groups aggregated into more obliquely aligned structures that emit blue PL, which plays a role in weakening the π-π interactions between molecules relative to green PL crystals. We found that the conformational stable molecular stacking induced instability in the electronic energy levels of the blue crystal compared to the green crystal.

Solvent vapor annealing (SVA) is an effective post-treatment process to improve the quality of crystals and grains in metal halide perovskite films. Whereas SVA has been successfully adopted for the fabrication of perovskite films via the conventional two-step method, its adaptation to the simple single-step perovskite film deposition method has been limited because of the rapid transition of the precursor to the perovskite structure. In the present study, we demonstrate highly efficient and stable single-step-based inverted perovskite solar cells fabricated with perovskite films prepared using dimethyl sulfoxide (DMSO) and water as a combined solvent in SVA treatment. The treatment with DMSO alone resulted in the growth of large grains (∼900 nm) in the lateral direction at the surface region; however, voids and defects were observed in the vertical direction at the bottom interfacial region of the CH3NH3PbI3 layer, which resulted in poor device properties. Interestingly, the combined DMSO–water induced vertical growth of almost single grains and the healing effect of water improved the grain quality. As a result, we fabricated co-SVA CH3NH3PbI3 devices with a power conversion efficiency (PCE) of 19.52%. This PCE, which is much higher than that of pristine devices, was attributed to reduced nonradiative recombination. In addition, co-SVA solar cells displayed remarkable stability and their universality in various types of perovskite materials was demonstrated. We anticipate that our proposed process and mechanism will be widely used in the future development of perovskite-based devices.

Polymers using thieno[2,3-b]indole (2-TI) with high planarity were synthesized and utilized for the photovoltaics. Conjugated polymers (PTIDTBT, PTIFDTBT and PTIDFDTBT) containing 2-TI as electron-donating unit and fluorinated DTBT as electron-accepting unit were synthesized. To enhance the effect of lowering highest occupied molecular orbital (HOMO) energy level for the high open-circuit voltages (VOC), we utilized new thieno[2,3-b]indole (2-TI) moiety as the weak electron-donating group and the fluoro-substituted electron-accepting BT unit. The polymer composed of BT with two fluorine unit coupled with 2-TI, PTIDFDTBT, shows higher VOC out of the series. The device comprising PTIDFDTBT and PC71BM (1:3) showed a VOC of 0.79 V, a JSC of 8.39 mA/cm2, and a fill factor (FF) of 0.48, giving a power conversion efficiency of 3.21%.

For the utilization in the polymer solar cells, conjugated polymers were synthesized using electron-deficient units, 2-{3-[4,6-bis-(4-hexyl-thiophen-2-yl)-pyrimidin-2-yl]-phenyl}-thiazolo[5,4-b]pyridine (mTP) or 2-{4-[4,6-bis-(4-hexylthiophen-2-yl)-pyrimidin-2-yl]-phenyl}-thiazolo[5,4-b]pyridine (pTP). To increase the electron-deficiency of the pyrimidine unit, thiazolopyridine (TP), strong electron-withdrawing group, was incorporated at the meta-/para-position of the phenylene connector which is attached to the pyrimidine. It was confirmed that the TP unit at para-position has stronger intramolecular charge transfer (ICT) effect to shift the absorption to the longer wavelength region and deepens the highest occupied molecular orbital (HOMO) energy level. The polymers with 4-(1-octylnonyl)-6-(2-thienyl)-4H-thieno[3,2,-b]indole (TTI) having the decent solubility for the morphology show the superior open-circuit voltage (V OC ) value of the mTTI/pTTI devices compared to the others. Also, the polymers with TTI show increased short-circuit current (J SC ), fill factor (FF), and power conversion efficiency (PCE) due to high face-on populations.

The development of new techniques that can be used to modify the molecular orientations and structures of organic compounds in organic thin films is in urgent demand to overcome the low levels of charge transport in such films and to ultimately achieve high-efficiency organic opto-electronics. Here, we report structural transformation and electrical characterization of a poly(3-hexylthiophene) (P3HT) semiconductor thin film under femtosecond laser irradiation at a level below the ablation threshold. A combination of grazing incidence X-ray diffraction and near edge X-ray absorption fine structure analyses reveal that the laser procedure effectively switches the P3HT orientation from edge-on to face-on, and the stability of this face-on orientation was also confirmed. Various laser fluence levels ranging up to the ablation threshold were tested, and the P3HT film with a transformed face-on orientation resulting from the optimized fluence level showed a charge current in the vertical direction three times greater than did the pristine film. This enhancement in the charge current was attributed to the vertical π–π stacking of the face-on orientation. These results also confirmed the effectiveness of our novel method for modifying organic thin films, and we expect this method to be attractive for the development of organic electronics and hybrid device applications.

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.

In this report, a series of copolymers based on 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) as an electron-rich unit and fluorinated DTBT as an electron-deficient unit were synthesized, namely PTTIF1 and PTTIF2, and applied to photovoltaic devices. TTI unit was coupled with fluorinated DTBT to utilize the merit of introduction of fluorine atom leading to the lowering of the HOMO energy level while keeping high planarity of the conjugated backbone. The synthesized copolymers show a noticeable change in HOMO energy levels as compared with non-fluorinated polymer (PTTIDTBT-h). Optimized photovoltaic devices of PTTIF2 exhibited power conversion efficiency of 4.36% with decent JSC and FF values, which can be explained by the higher charge transporting ability of PTTIF2 with preferable face-on crystallite population than PTTIF1 in grazing incident wide-angle X-ray scattering (GIWAXS).

Perovskite solar cells (PSCs) have demonstrated enormous potential for use in next-generation photovoltaics because of their promising properties. However, the poor stability of PSCs as a result of severe degradation under outdoor conditions precludes their commercialization. Therefore, ideal encapsulation is a pivotal technique for achieving good long-term stability of PSCs, which is highly demanded. Here, we first introduce a highly flexible and stable graphene encapsulant by adopting the dry transfer method based on the roll-based process. Under well-controlled moisture conditions, the optical and structural properties of encapsulated perovskite films with different numbers of layers were systematically investigated through optical spectroscopy and grazing-incidence wide-angle X-ray scattering studies. As a result, the graphene-encapsulated perovskite films exhibited outstanding stability compared with a pristine film, and the graphene device retained 95% of its initial efficiency after 100 h of exposure to 85–90% relative humidity and retained 82.4% of its initial efficiency after 3700 h in ambient air. In addition, graphene-encapsulated flexible PSCs exhibited enhanced stability even when subjected to bending stress while submerged in water. On the basis of the results, we expect the proposed graphene encapsulation technique to be compatible with diverse graphene applications and to contribute to the development of not only stable perovskites but also next-generation flexible solar cells such as organic and quantum-dot solar cells.

In this paper, the effect of NiO x thin film prepared by a radio-frequency sputtering process under different oxygen partial (Ar:O2 ratio) pressures on the photovoltaic properties of planar-type MAPbI3 perovskite solar cells (PSCs) was investigated. The electrical, optical, morphological, and structural properties of sputtered NiO x thin films deposited under varied process oxygen gas condition are scrutinized. In particular, this study demonstrates that the properties of NiO x thin film with Ni3+/Ni2+ ratio are dependent on process oxygen gas condition. The influence of Ni vacancies on the chemistry and electronic structure of NiO x thin films is investigated using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The sputtered NiO x thin film with lower Ni3+/Ni2+ ratio exhibits appropriate conductivity, hole mobility, high transmittance, and uniform surface morphology as well as a preferred in-plane orientation of the (220) plane under pure Ar gas atmosphere. When 5% (Ar:O2=19:1) or 10% (Ar:O2=18:2) oxygen is injected during the process, the conductivity of NiO x thin films with higher Ni3+/Ni2+ ratio is higher, but at the expense of optical transmittance. Additionally, this work exhibits the effect of perovskite active layer crystallinity and device performance using sputtered NiO x thin film under different oxygen partial pressures (Ar:O2 ratio) as hole transport layer for PSCs. The power conversion efficiency (PCE) of PSC obtains the highest PCE of 15.74% at the optimized NiO x thin film under pure Ar atmosphere. Compared to that, the efficiencies of the PSCs using the NiO x thin films processed at 5% and 10% oxygen partial pressure is 4.17% and 1.39%, respectively. Therefore, this study elucidate the effect of NiO x thin films deposited under different oxygen partial (Ar:O2 ratio) pressures on the performance of PSCs.

Recently, the sequential (Sq) process, which forms nanoscale network structures from quasi-solid-state inter-diffusion through swelling and annealing, is considered to be one of the most efficient methods for fabricating organic solar cells and blend films. Here, we examined the effect of the crystallinity and orientation of poly(3-hexylthiophene) (P3HT) molecules on the formation of the nanostructure by carrying out a Sq process using various solvents with different boiling points. We showed that the moderate crystallinity promoted suitable inter-diffusion between the donor (P3HT) and acceptor ([6,6]-pentadeuterophenyl C61 butyric acid methyl ester, PC60BM), and hence was important for achieving high-performance solar cells using Sq processing. Nanostructure formation by inter-diffusion was investigated and visualized by taking a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements. In addition, our Sq-processed solar cell yielded a device efficiency as high as 3.25%, and was also impressive because it was made with an eco-friendly solvent and using a short-duration annealing process, in contrast to the conventional BHJ process. The present findings provided advanced insight into the Sq process, and we anticipate this efficacious sequential process to contribute not only to the development of higher-efficiency organic solar cells but also to the fabrication of functional blend films.

Along with the development of perovskite materials, which have enormous potential for optoelectronics such as solar cells and light-emitting diode devices, numerous organic semiconductor polymers, which have been critically adopted into the hole and electron transporting layers, have been synthesized and studied. In n–i–p-structured perovskite solar cells , various outstanding polymer materials have been successfully applied. However, in p–i–n-structured solar cells, the hydrophobic nature of the polymers makes the sequential deposition of a perovskite thin film difficult. Several destructive methods have been proposed; however, a more efficacious and fundamental method is urgently needed. Here, we present a nondestructive polymer hole-transporting layer (HTL) thin-film formation process based on molecular engineering via a simple solvent process. When we used various solvents with different volatilities, perovskite film formation was achieved on polymer thin films formed from highly volatile solvents. In addition, we elucidated the structure and orientation of the molecules in the films and revealed that the molecular structure of face-on orientation for the horizontally aligned hydrophobic alkyl groups induced a lower surface energy of the film, as determined by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. Furthermore, the tilt angle of the molecules, which was calculated from the results of quantitative near-edge X-ray absorption fine structure (NEXAFS) analysis, was found to correlate with the surface energy. This result provides guidance for polymer-orientation and surface-energy studies, and perovskite solar cells fabricated using the polymer HTL demonstrated good durability and flexibility. We expect that our approach represents a new route for fabricating p–i–n-structured solar cells and that numerous valuable p-type conjugated polymers will be developed via our proposed molecular engineering process.