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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.

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.