Simple-Structured Low-Cost Dopant-Free Hole-Transporting Polymers for High-Stability CsPbI2Br Perovskite Solar Cells.

Among the solution-processed devices, perovskite solar cells (PSCs) exhibit the highest power conversion efficiency (PCE) of over 25%; tremendous efforts are being undertaken to improve their stability. Recently, all-inorganic CsPbI2Br-based PSCs were reported to exhibit a significantly improved device stability, with a promising PCE of up to 16.79%. In this study, we report stable all-inorganic PSCs by incorporating novel dopant-free hole-transporting materials (HTMs). The synthesis strategy of the newly synthesized polymeric HTMs was similar to that of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), with the exception that they were designed to exhibit dopant-free characteristics. In particular, their polymeric backbone structure was significantly simpler than that of spiro-OMeTADs, and they were easily synthesized in two steps from commercially available chemicals, with an overall yield of ∼50%. The cost of synthesis at the laboratory scale was calculated to be at least 2.4 times cheaper than that of spiro-OMeTADs. The PCE of dopant-free HTM-based PSCs was 11.01%, which is 1.5 times higher than that of the dopant-free spiro-OMeTAD-based devices (7.52%) and comparable to that of the doped spiro-OMeTAD-based devices (12.22%). Notably, the stability of the device based on our dopant-free HTM to atmospheric oxygen and moisture as well as heat and light irradiation was superior to that of devices based on doped and dopant-free spiro-OMeTAD HTMs. On consideration of the synthesis cost, device efficiency, and device stability, our dopant-free HTM is highly promising for all-inorganic PSCs.

Effect of electron-withdrawing fluorine and cyano substituents on photovoltaic properties of two-dimensional quinoxaline-based polymers.

In this study, strong electron-withdrawing fluorine (F) and cyano (CN) substituents are selectively incorporated into the quinoxaline unit of two-dimensional (2D) D-A-type polymers to investigate their effects on the photovoltaic properties of the polymers. To construct the 2D polymeric structure, electron-donating benzodithiophene and methoxy-substituted triphenylamine are directly linked to the horizontal and vertical directions of the quinoxaline acceptor, respectively. After analyzing the structural, optical, and electrochemical properties of the resultant F- and CN-substituted polymers, labeled as PBCl-MTQF and PBCl-MTQCN, respectively, inverted-type polymer solar cells with a non-fullerene Y6 acceptor are fabricated to investigate the photovoltaic performances of the polymers. It is discovered that the maximum power conversion efficiency of PBCl-MTQF is 7.48%, whereas that of PBCl-MTQCN is limited to 3.52%. This significantly reduced PCE of the device based on PBCl-MTQCN is ascribed to the formation of irregular, large aggregates in the active layer, which can readily aggravate the charge recombination and charge transport kinetics of the device. Therefore, the photovoltaic performance of 2D quinoxaline-based D-A-type polymers is significantly affected by the type of electron-withdrawing substituent.

Water-resistant AgBiS2 colloidal nanocrystal solids for eco-friendly thin film photovoltaics.

Lead-free, water-resistant photovoltaic absorbers are of significant interest for use in environment-friendly and water-stable thin film solar cells. However, there are no reports on the water-resistance characteristics of such photoactive materials. Here, we demonstrate that silver bismuth sulfide (AgBiS2) nanocrystal solids exhibit inherent water resistance and can be employed as effective photovoltaic absorbers in all-solid-state thin film solar cells that show outstanding air and moisture stabilities under ambient conditions. The results of X-ray photon spectroscopy (XPS) and X-ray diffraction (XRD) analyses show that there is no change in the chemical composition and crystal structure of the AgBiS2 nanocrystal solids after a water treatment. Based on these results, AgBiS2 nanocrystal solar cells are fabricated. These devices also do not show any drop in performance after a water treatment, confirming that the AgBiS2 nanocrystal solids are indeed highly water-resistant. In contrast, lead sulfide (PbS) colloidal quantum dot (CQD) solar cells show significant decrease in performance after a similar water treatment. Using XPS analysis and density functional theory (DFT) calculations, we confirm that the iodine removal and the surface hydroxylation of the water-treated PbS CQD solids are the primary reasons for the observed decrease in the device performance of the CQD solar cells.

Morphological and Optical Engineering for High-Performance Polymer Solar Cells.

We demonstrate morphological and optical engineering by using processing additives and optical spacers for polymer solar cells. Among various processing additives, introduction of diphenyl ether (DPE) into the active layer results in the smoothest surface roughness with uniform and well-distributed donor/acceptor domains, and the device with DPE shows the highest device efficiency of 10.22% due to enhanced charge collection efficiency and minimized recombination loss. Additional ZnO optical spacers on the active layer controls the distribution of the electric field in the whole device and enhances the light absorption within the active layer, thereby improving device efficiency up to 10.81%.

Water-resistant PEDOT:PSS hole transport layers by incorporating a photo-crosslinking agent for high-performance perovskite and polymer solar cells.

We demonstrated a water-resistant PEDOT:PSS HTL by incorporating a photo-crosslinking agent into a PEDOT:PSS film. A crosslinking system was successfully formed inside the PEDOT:PSS film by simple and fast photo-polymerization of PCDSA monomers. Combination of the crosslinking system and MeOH surface treatment simultaneously improved the device efficiency and stability of both perovskite and polymer solar cells. The crosslinking system inside PEDOT:PSS changed its intrinsic water-soluble characteristic into a water-resistant property, thus preventing water penetration into the PEDOT:PSS film. In addition, MeOH treatment improved the surface conductivity and reduced the surface roughness of the PEDOT:PSS film by removing surface residues of PDAs and insulating PSS parts.

Polymerizable Supramolecular Approach to Highly Conductive PEDOT:PSS Patterns.

Owing to its high conductivity, solution processability, mechanical flexibility, and transparency, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been extensively explored for use in functional devices including solar cells, sensors, light-emitting diodes, and supercapacitors. The ability to fabricate patterned PEDOT:PSS on a solid substrate is of significant importance to develop practical applications of this conducting polymer. Herein, we describe a new approach to obtain PEDOT:PSS patterns that are based on a polymerizable supramolecular concept. Specifically, we found that UV irradiation of a photopolymerizable diacetylene containing PEDOT:PSS film followed by development in deionized water and subsequent treatment with sulfuric acid (glass and silicon wafer) or formic acid (PET) produces micron-sized PEDOT:PSS patterns on solid substrates. The newly designed photolithographic method, which can be employed to generate highly conductive (>1000 S/cm) PEDOT:PSS patterns, has many advantages including the use of aqueous process conditions, a reduced number of process steps, and no requirement for plasma etching procedures.