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임경근,김학범,정재기,김진영,이태우 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.1
The work function (WF) in hole extraction layers (HELs) can be tuned by using molecular surface engineering to control the surface composition in HEL films, which depends on the surface-enriched molecules and their concentration relative to the conducting polymer. Thus we used a self-organized HEL (SOHEL) which is composed of a conducting polymer composition (e.g., PEDOT:PSS) and a perfluorinated ionomer (PFI), i.e., tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer. Here, we present solution-processed methylammonium lead iodide CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>-based perovskite solar cells with a high-WF SOHEL for good energy level alignment with the IP level of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. The SOHEL at the hole extraction interface can increase the built-in potential, the photocurrent, and thus the PCE of perovskite solar cells. We obtained high PCE of 11.7% in SP-PHJ perovskite solar cells under 100-mW/cm<sup>2</sup> illumination. We find also that Voc of the SOHEL/perovskite device is increased depends on the fluorinated molecule rates, while the Fermi-level of SOHEL becomes pinned to the midgap state of the perovskite. It is because the hole extraction offset at the SOHEL/ perovskite is decreased as function of interface energy level of SOHEL. We also demonstrated flexible perovskite solar cells on a poly(ethylene terephthalate) (PET) substrate; they had PCE as high as 8.0%.
Mechanism of conjugated polyelectrolytes for high-performance organic photovoltaics
임경근,박성민,우한영,이태우 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.1
In this research, we study for efficient charge extraction via electrode interfacial layers for high performance vertical and lateral organic photovoltaics(OPVs). We demonstrate that the photo-current and the power conversion efficiency of organic photovoltaic cells OPVs can be maximized by the oriented interfacial dipoles within the water soluble conjugated polyelectrolytes (CPE) layer. We synthesized cationic and anionic conjugated polyelectrolytes and then we employed them as an electron extraction layer in polymer photovoltaic cells. In contrast to the conventional belief, both the anionic and cationic CPE layers improved the electron injection and extraction at the cathode interface, which leads to highly efficient OPVs irrespective of the ionic groups of polyelectrolytes. Therefore, we investigated the role of the conjugated polyelectrolytes as the charge extraction layer by using surface photoelectron spectroscopy. We applied external electric field on the device which employ CPE interfacial layer to make that CPE interfacial layer has a net internal electric field without external bias.
임경근,박준모,( Hannah Mangold ),( Frédéric Laquai ),최태림,이태우 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.1
The exciton dissociation, recombination, and charge transport of bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) is strongly influenced by the nano-morphology of the blend such as the grain size and the molecular packing. Although it is well-known that amorphous poly(p-phenylenevinylene)(PPV)-based polymers have a fundamental limit to achieve high efficiency because of low carrier mobility leading to increased recombination and unbalanced charge extraction, herein, we demonstrate that the issues can be overcome by forming bimolecular cystals of amorphous PPV-based polymer: phenyl-C61-butyric acid methyl ester (PCBM) intercalated structure. We used amorphous poly (2,5-dioctyloxy-p-phenylene vinylene-alt-2’,5’-thienylene vinylene) (PPVTV) having a simple chemical structure. Then, a reasonably high power conversion efficiency (PCE ~3.5%) was obtained, although it has intrinsically an amorphous structure and a relatively large band gap (2.0 eV). We also demonstrate a correlation between a well-ordered bimolecular crystal of PPVTV:PCBM and an improved hole mobility of a PPVTV:PCBM film compared to a pristine PPVTV film by using 2D Grazing Incidence X-Ray Diffraction (GIXD) and space-charge-limited current (SCLC) measurements, respectively. Furthermore, we show that the bimolecular crystal structure in high performance OPVs is related to an optimum molecular packing, which is influenced by the PPVTV: PCBM blending ratio, side chain length and molecular weight of the PPVTV polymer. Improved charge transport in PPVTV:PCBM bimolecular crystals leads to a fast sweep out of charges and thus suppression of non-geminate recombination under operating conditions.