Tae Kyu Ahn

Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency

Hysteresis-less inverted ITO/PEDOT:PSS/CH3NH3PbI3 (MAPbI3)/PCBM/Au planar hybrid solar cells with 18.1% average power conversion efficiency irrespective of the scan rate were fabricated by depositing dense pinhole-free MAPbI3 perovskite on a PEDOT:PSS/ITO substrate via a single-step spin-coating of solubility controlled MAPbI3 solution. The conductivities of PEDOT:PSS, PCBM, poly(triaryl amine) (PTAA):tert-butylpyrridne (tBP) + Li-bis(trifluoromethanesulfonyl)imide (Li–TFSI), MAPbI3, and TiO2 were 0.014, 0.016, 0.034, 0.015, and 0.00006 mS cm−1, respectively. The average PL lifetimes (τav) of the inverted and normal cell were 1.277 and 1.94 ns, respectively. The diffusion coefficient (Dn) and charge carrier lifetime (τn) for the inverted MAPbI3 planar hybrid solar cells were increased by 1.14-fold and 1.1-fold, respectively, compared with the conventional FTO/TiO2/MAPbI3/PTAA:tBP + Li–TFSI/Au planar hybrid cells. Hence, the inverted MAPbI3 planar hybrid solar cells exhibited better power conversion efficiency and stability than the conventional MAPbI3 cells because (i) the electron extraction from MAPbI3 to the electron conductor was improved because the electron conductivity of PCBM is higher than that of TiO2; (ii) the EQE value was increased by the better charge injection/separation efficiency between MAPbI3 and PCBM, and by the higher charge collection efficiency than the conventional cell; (iii) the fill factor is improved by the increased Dn and τn; and (iv) the air and humidity stability is improved by the absence of corrosive additives in the device architecture and the hydrophobicity of the PCBM top layer. The reduced current density–voltage (J–V) hysteresis with respect to the scan rate and scan direction in the inverted planar hybrid solar cells could be attributed to a more balanced electron flux (Je) and hole flux (Jh), and a reduced number of surface traps.

Planar CH3NH3PbI3 Perovskite Solar Cells with Constant 17.2% Average Power Conversion Efficiency Irrespective of the Scan Rate

J. H. Heo, D. H. Song, H. J. Han, Prof. S. H. Im Functional Crystallization Center (FCC) Department of Chemical Engineering Kyung Hee University 1732 Deogyeong-daero , Giheung-gu, Yongin-si , Gyeonggi-do 446-701 , Republic of Korea E-mail: imromy@khu.ac.kr S. Y. Kim, Prof. J. H. Kim Department of Physics Incheon National University 119 Academy-ro , Yeonsu-gu , Incheon 406-772 , Republic of Korea D. Kim, Dr. H. W. Shin, Prof. T. K. Ahn Department of Energy Science Sungkyunkwan University Seobu-ro 2066 , Jangan-gu , Suwon 440-746 , Republic of Korea C. Wolf, Prof. T.-W. Lee Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH) 77 Cheongam-Ro , Nam-Gu, Pohang , Gyungbuk 790-784, Republic of Korea

Highly efficient and bending durable perovskite solar cells: toward a wearable power source

Perovskite solar cells are promising candidates for realizing an efficient, flexible, and lightweight energy supply system for wearable electronic devices. For flexible perovskite solar cells, achieving high power conversion efficiency (PCE) while using a low-temperature technology for the fabrication of a compact charge collection layer is a critical issue. Herein, we report on a flexible perovskite solar cell exhibiting 12.2% PCE as a result of the employment of an annealing-free, 20 nm thick, amorphous, compact TiOx layer deposited by atomic layer deposition. The excellent performance of the cell was attributed to fast electron transport, verified by time-resolved photoluminescence and impedance studies. The PCE remained the same down to 0.4 sun illumination, as well as to a 45° tilt to incident light. Mechanical bending of the devices worsened device performance by only 7% when a bending radius of 1 mm was used. The devices maintained 95% of the initial PCE after 1000 bending cycles for a bending radius of 10 mm. Degradation of the device performance by the bending was the result of crack formation from the transparent conducting oxide layer, demonstrating the potential of the low-temperature-processed TiOx layer to achieve more efficient and bendable perovskite solar cells, which becomes closer to a practical wearable power source.

Efficient CH3NH3PbI3 Perovskite Solar Cells Employing Nanostructured p‐Type NiO Electrode Formed by a Pulsed Laser Deposition

Highly transparent and nanostructured nickel oxide (NiO) films through pulsed laser deposition are introduced for efficient CH3 NH3 PbI3 perovskite solar cells. The (111)-oriented nanostructured NiO film plays a key role in extracting holes and preventing electron leakage as hole transporting material. The champion device exhibits a power conversion efficiency of 17.3% with a very high fill factor of 0.813.

Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent

The reported photocurrent density (JSC) of PbS quantum dot (QD)-sensitized solar cell was less than 19 mA/cm2 despite the capability to generate 38 mA/cm2, which results from inefficient electron injection and fast charge recombination. Here, we report on a PbS:Hg QD-sensitized solar cell with an unprecedentedly high JSC of 30 mA/cm2. By Hg2+ doping into PbS, JSC is almost doubled with improved stability. Femtosecond transient study confirms that the improved JSC is due to enhanced electron injection and suppressed charge recombination. EXAFS reveals that Pb-S bond is reinforced and structural disorder is reduced by interstitially incorporated Hg2+, which is responsible for the enhanced electron injection, suppressed recombination and stability. Thanks to the extremely high JSC, power conversion efficiency of 5.6% is demonstrated at one sun illumination.

Fabrication of Efficient Formamidinium Tin Iodide Perovskite Solar Cells through SnF2–Pyrazine Complex

To fabricate efficient formamidinium tin iodide (FASnI3) perovskite solar cells (PSCs), it is essential to deposit uniform and dense perovskite layers and reduce Sn(4+) content. Here we used solvent-engineering and nonsolvent dripping process with SnF2 as an inhibitor of Sn(4+). However, excess SnF2 induces phase separation on the surface of the perovskite film. In this work, we report the homogeneous dispersion of SnF2 via the formation of the SnF2-pyrazine complex. Consequently, we fabricated FASnI3 PSCs with high reproducibility, achieving a high power conversion efficiency of 4.8%. Furthermore, the encapsulated device showed a stable performance for over 100 days, maintaining 98% of its initial efficiency.

Tandem Synthesis of Photoactive Benzodifuran Moieties in the Formation of Microporous Organic Networks

Over the last decade, microporous organic materials have been extensively prepared through various coupling reactions. In the early stages, relatively simple aromatic building blocks were used to prepare microporous organic networks (MONs) and relevant studies have focused on their physisorption behavior toward gas guests. Recently, more specific functionalities were achieved by the introduction of designed active sites into MONs. Usually, the active sites could be introduced by using the predesigned building blocks or by postmodification of the porous materials. If the active sites could be concomitantly formed in the network formation process for porous materials, this synthetic process would be very efficient and ideal for functional materials. For example, we have demonstrated the successful incorporation of active N-heterocyclic carbene metal species into metal– organic frameworks (MOFs) during self-assembly processes. However, this kind of synthetic approach is relatively rare, especially in the synthesis of MONs. Benzodifurans (BDFs) are very interesting materials owing to their unique optical and electrical properties. Their electron-rich nature has enabled them to be applied as redox-active hole transfer materials in organic lightemitting devices. Moreover, very recently, anti-benzodifuran-based organic materials have attracted significant attention as photoand redox-active materials in solar cells and organic field-effect transistors. anti-Benzodifurans can be prepared in the intramolecular cyclization reaction of 1,4hydroquinone with two alkyne groups at the 2,5-positions. Generally, tandem reactions in organic synthesis can be defined as a consecutive series of intramolecular reactions. In well-designed tandem processes, the functional groups for the following successive reactions can be generated in situ as a result of the previous reaction. Through the introduction of tandem processes to organic synthesis, the synthetic strategies become more atom-economical, because the work-up and isolation processes for intermediates can be reduced. Thus, much effort has been made for the development of smart tandem processes for complicated target organic materials. The Cooper research group and others have shown that MONs can be prepared by Sonogashira coupling between multialkyne connectors and multihalo arene building blocks. 7, 8] We have continued to develop functional MONs. We speculated that the generation of benzodifuran species can be induced in a tandem manner during the formation of the MON through a Sonogashira coupling. As far as we are aware, tandem synthetic strategies for the preparation of functional MONs have not been reported. Herein, we report the preparation of photoactive MONs with benzodifuran moieties through tandem synthetic processes, and their applications to photocatalytic coupling of primary amines. Figure 1 shows the synthetic strategy for the synthesis of a MON containing benzodifuran moieties (BDF-MON).

Photocatalysis by phenothiazine dyes: visible-light-driven oxidative coupling of primary amines at ambient temperature.

New phenothiazine based organic dyes were prepared for visible-light-driven organic transformations. The 3,7-disubstituted phenothiazine derivatives showed visible light absorption and reversible one-electron oxidation behavior. In the presence of 0.5 mol % of 3,7-disubstituted phenothiazines, primary benzylamines showed oxidative coupling under visible light irradiation from a blue LED. The electronic effect of substituents in phenothiazine dyes was observed in catalytic activities. The mechanistic pathway of oxidative coupling was discussed based on the detection of H(2)O(2) after the reaction.

Photoresponse of CsPbBr3 and Cs4PbBr6 Perovskite Single Crystals

High-quality and millimeter-sized perovskite single crystals of CsPbBr3 and Cs4PbBr6 were prepared in organic solvents and studied for correlation between photocurrent generation and photoluminescence (PL) emission. The CsPbBr3 crystals, which have a 3D perovskite structure, showed a highly sensitive photoresponse and poor PL signal. In contrast, Cs4PbBr6 crystals, which have a 0D perovskite structure, exhibited more than 1 order of magnitude higher PL intensity than CsPbBr3, which generated an ultralow photoresponse under illumination. Their contrasting optoelectrical characteristics were attributed to different exciton binding energies, induced by coordination geometry of the [PbBr6]4- octahedron sublattices. This work correlated the local structures of lead in the primitive perovskite and its derivatives to PL spectra as well as photoconductivity.

Enhancing the Performance of Sensitized Solar Cells with PbS/CH3NH3PbI3 Core/Shell Quantum Dots.

UNLABELLED We report on the fabrication of PbS/CH3NH3PbI3 (=MAP) core/shell quantum dot (QD)-sensitized inorganic-organic heterojunction solar cells on top of mesoporous (mp) TiO2 electrodes with hole transporting polymers (P3HT and PEDOT PSS). The PbS/MAP core/shell QDs were in situ-deposited by a modified successive ionic layer adsorption and reaction (SILAR) process using PbI2 and Na2S solutions with repeated spin-coating and subsequent dipping into CH3NH3I (=MAI) solution in the final stage. The resulting device showed much higher efficiency as compared to PbS QD-sensitized solar cells without a MAP shell layer, reaching an overall efficiency of 3.2% under simulated solar illumination (AM1.5, 100 mW·cm(-2)). From the measurement of the impedance spectroscopy and the time-resolved photoluminescence (PL) decay, the significantly enhanced performance is mainly attributed to both reduced charge recombination and better charge extraction by MAP shell layer. In addition, we demonstrate that the MAP shell effectively prevented the photocorrosion of PbS, resulting in highly improved stability in the cell efficiency with time. Therefore, our approach provides method for developing high performance QD-sensitized solar cells.