Alkali acetate-assisted enhanced electronic coupling in CsPbI₃ perovskite quantum dot solids for improved photovoltaics

Kim, Jigeon,Koo, Bonkee,Kim, Wook Hyun,Choi, Jongmin,Choi, Changsoon,Lim, Sung Jun,Lee, Jong-Soo,Kim, Dae-Hwan,Ko, Min Jae,Kim, Younghoon Elsevier 2019 Nano energy Vol.66 No.-

<P><B>Abstract</B></P> <P>Fully inorganic CsPbI<SUB>3</SUB> perovskite quantum dots (CsPbI<SUB>3</SUB>-PQDs) are known as the best-performing photovoltaic absorber in colloidal quantum dot solar cells. This is achieved by improving the cubic-phase-stabilization and electronic-coupling in CsPbI<SUB>3</SUB>-PQD solids. In conventional approaches, the hydrolysis of methyl acetate (MeOAc) resulting in acetic acid and methanol as intermediate substances plays a key role in replacing long-chain hydrocarbons with short-chain ligands, which improves charge transport in the CsPbI<SUB>3</SUB>-PQD solids. However, CsPbI<SUB>3</SUB>-PQDs suffer from lattice distortion and instability under acidic conditions including protons and polar media, leading to CsPbI<SUB>3</SUB>-PQD fusion and poor photovoltaic performance. Herein, we report that electronic coupling and photovoltaic performance of CsPbI<SUB>3</SUB>-PQD solids are improved by efficient removal of long-chain oleate ligands using a solution of sodium acetate (NaOAc) in MeOAc, which results in the direct generation of OAc ions without forming protons and methanol. NaOAc-based ligand exchange of CsPbI<SUB>3</SUB>-PQDs enables preservation of their nanocrystal size without fusion and minimization of surface trap states originating from metal hydroxide formation on their surfaces. Consequently, the best solar cell comprising NaOAc-treated CsPbI<SUB>3</SUB>-PQDs shows an improved device performance with a power conversion efficiency (<I>PCE</I>) of 13.3%, as compared with a lead nitrate-treated control device (12.4% <I>PCE</I>).</P> <P><B>Highlights</B></P> <P> <UL> <LI> NaOAc directly generates short-chain OAc ions to exchange the oleate ligands of CsPbI<SUB>3</SUB>-PQDs. </LI> <LI> Our strategy enables minimizing the formation of protons and methanol during the ligand exchange. </LI> <LI> NaOAc-based ligand exchange enables preserving nanocrystal size and minimizing surface traps. </LI> <LI> Resultant CsPbI<SUB>3</SUB>-PQD solids show enhanced electronic coupling with improved charge transport. </LI> <LI> NaOAc-treated CsPbI<SUB>3</SUB>-PQD solar cells show improved photovoltaic performance up to 13.33% PCE. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>We demonstrate that sodium acetate (NaOAc) directly generates short-chain OAc ions to exchange the long-chain oleate ligands of CsPbI<SUB>3</SUB> perovskite quantum dots (CsPbI<SUB>3</SUB>-PQDs). NaOAc-based ligand exchange enables preservation of CsPbI<SUB>3</SUB>-PQD size, minimization of surface trap states, and enhancement of electronic coupling in the resultant CsPbI<SUB>3</SUB>-PQD solids. Consequently, NaOAc-treated CsPbI<SUB>3</SUB>-PQD solar cells show improved device performance with 12.4% power conversion efficiency.</P> <P>[DISPLAY OMISSION]</P>

Enhanced photovoltaic performance of solution-processed Sb2Se3 thin film solar cells by optimizing device structure

Taifeng Ju,Bonkee Koo,Jea Woong Jo,Min Jae Ko 한국물리학회 2020 Current Applied Physics Vol.20 No.2

Thin-film solar cells have attracted worldwide attention due to their high efficiency and low cost. Antimony selenide (Sb2Se3) is a promising light absorption material candidate for thin-film solar cells due to its suitable band gap, abundance, low toxicity, and high chemical stability. Herein, we fabricate an Sb2Se3 thin film solar cell using a simple hydrazine solution process. By controlling the thickness of the photoactive layer and inserting a poly(3-hexylthiophene) hole-transporting layer, an Sb2Se3 solar cell with a power conversion efficiency of 2.45% was achieved.

Ultrathin supercapacitor electrodes with high volumetric capacitance and stability using direct covalent-bonding between pseudocapacitive nanoparticles and conducting materials

Ko, Yongmin,Shin, Dongyeeb,Koo, Bonkee,Woo Lee, Seung,Yoon, Won-Sub,Cho, Jinhan Elsevier 2015 Nano energy Vol.12 No.-

<P><B>Abstract</B></P> <P>We introduce high-performance ultrathin supercapacitor electrodes obtained through the direct covalent-bonding layer-by-layer (LbL) assembly (or ligand-exchange LbL assembly) of amine-functionalized multiwalled carbon nanotubes (CNTs) and transition metal oxide nanoparticles (TMO NPs). The main characteristic of our approach is that the internal interfacial resistance of the electrodes can be minimized through the direct covalent-bonding adsorption of densely packed, high-quality TMO NPs onto CNTs without the aid of nonactive binders or insulating NP ligands, and the resulting volumetric capacitance and cycling stability of the electrodes can be significantly enhanced. For this study, well-defined oleic acid-stabilized pseudocapacitive metal oxide nanoparticles (i.e., OA–Fe<SUB>3</SUB>O<SUB>4</SUB> and OA–MnO NPs) prepared in toluene were densely adsorbed onto the CNT layer due to the high affinity between the surface of the TMO NPs and the NH<SUB>2</SUB> moieties of the CNTs. The (CNT/OA–Fe<SUB>3</SUB>O<SUB>4</SUB> NP)<SUB>20</SUB> multilayer electrode exhibited a high volumetric capacitance of 248±15Fcm<SUP>−3</SUP> (128±7Fg<SUP>−1</SUP>) at 5mVs<SUP>−1</SUP> despite the intrinsically low specific capacitance of the Fe<SUB>3</SUB>O<SUB>4</SUB> NPs. Additionally, these film electrodes exhibited high performance stability, maintaining 99.2% of their initial capacitance after 1000 cycles. Furthermore, upon the insertion of OA–MnO NPs with high crystallinity and a high theoretical pseudocapacitance value within multilayers instead of OA–Fe<SUB>3</SUB>O<SUB>4</SUB> NPs, the formed electrodes (i.e., (CNT/OA–MnO NP)<SUB>20</SUB> multilayers) exhibited a higher volumetric capacitance of 305±10Fcm<SUP>−3</SUP> (183±5Fg<SUP>−1</SUP>) (at a scan rate of 5mVs<SUP>−1</SUP>) than other conventional ultrathin supercapacitor electrodes, including manganese oxide or iron oxide NPs.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Various well-defined NPs could be incorporated into carbon materials in organic media. </LI> <LI> Internal interfacial resistance of electrode was minimized by direct covalent bonding. </LI> <LI> Remarkable volumetric capacitance was achieved by high packing density of NPs. </LI> <LI> High operation stability of electrode was caused by covalent bonding. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Hybrid supercapacitor electrodes for electrochemical energy storage devices were prepared from covalent-bonding LbL-assembled (CNT/OA–Fe<SUB>3</SUB>O<SUB>4</SUB> or OA–MnO NP)<SUB>n</SUB> multilayers. The resulting (CNT/OA–Fe<SUB>3</SUB>O<SUB>4</SUB> NP)<SUB>n</SUB> and (CNT/OA–MnO NP)<SUB>n</SUB> electrodes exhibited the high volumetric capacitances of 248±15Fcm<SUP>−3</SUP> (128±7Fg<SUP>−1</SUP>) and 305±10Fcm<SUP>−3</SUP> (183±5Fg<SUP>−1</SUP>) at 5mVs<SUP>−1</SUP>, respectively and the excellent operation stability due to a facile electrolyte access, a dense adsorption of pseudocapacitive TMO NPs, and high surface area for fast electrochemical reactions.</P> <P>[DISPLAY OMISSION]</P>

Transparent 3 nm-thick MoS₂ counter electrodes for bifacial dye-sensitized solar cells

Jeong, Taehee,Ham, So-Yeon,Koo, Bonkee,Lee, Phillip,Min, Yo-Sep,Kim, Jae-Yup,Ko, Min Jae Elsevier 2019 Journal of industrial and engineering chemistry Vol.80 No.-

<P><B>Abstract</B></P> <P>Molybdenum disulfide (MoS<SUB>2</SUB>) counter electrode (CE) is considered one of the most viable alternatives to Pt CE in dye-sensitized solar cells (DSSCs) owing to its abundance, low cost, and superior electrocatalytic activity. However, mostly, MoS<SUB>2</SUB> CEs for DSSCs are prepared by conventional chemical reactions and annealing at a high temperature. By these conventional processes, deposition of sufficiently thin and transparent MoS<SUB>2</SUB> layers is challenging; therefore, bifacial DSSCs employing transparent MoS<SUB>2</SUB> CEs have not been studied. Here, we report transparent few-nanometer-thick MoS<SUB>2</SUB> CEs prepared by atomic layer deposition at a relatively low temperature (98°C) for bifacial DSSC applications. MoS<SUB>2</SUB> nanofilms with precisely controlled thicknesses of 3–16nm are conformally coated on transparent conducting oxide glass substrates. With increase in the MoS<SUB>2</SUB> nanofilm thickness, the MoS<SUB>2</SUB> CE electrocatalytic activity for the iodide/triiodide redox couple enhances, but its transparency decreases. Notably, the application of a thinner MoS<SUB>2</SUB> nanofilm in a bifacial DSSC leads to lower conversion efficiency under front-illumination, but higher conversion efficiency under back-illumination. In particular, only the 3nm-thick MoS<SUB>2</SUB> nanofilm shows reasonable photovoltaic performances under both front- and back-illumination conditions.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Layer-by-layer assembled enzyme multilayers with adjustable memory performance and low power consumption <i>via</i> molecular-level control

Baek, Hyunhee,Lee, Chanwoo,Park, Jeongju,Kim, Younghoon,Koo, Bonkee,Shin, Hyunjung,Wang, Dayang,Cho, Jinhan The Royal Society of Chemistry 2012 Journal of materials chemistry Vol.22 No.11

<P>Electrochemical properties of enzymes are of fundamental and practical importance in bio-electrochemical applications. These redox properties, which can cause the reversible changes in the current according to their redox reactions in solution, often depend on the chemical activity of transition metal ions as cofactors within the active sites of enzymes. Here, we demonstrate that the reversible resistance changes in enzyme-based multilayer films can be caused by the externally applied voltage as a result of charge trap/release of haem Fe<SUP>III</SUP>/Fe<SUP>II</SUP> redox couples in dry form. It is also demonstrated that the electrically bistable switching properties of redox enzymes can be applied to nonvolatile memory devices requiring low power consumption. For this study, cationic poly(allylamine hydrochloride) (PAH) was alternately layer-by-layer assembled with anionic catalase enzyme onto Pt-coated substrates until the desired number of layers was deposited. A top electrode was deposited onto (PAH/catalase)<SUB><I>n</I></SUB> multilayer films to complete device fabrication. When an external bias was applied to the devices, a switching phenomenon depending on the voltage polarity (<I>i.e.</I>, bipolar switching) was observed at low operating voltages (RESET at 1.8 V and SET voltage at −1.5 V), fast switching speed at the nanosecond level, and an ON/OFF current ratio of ∼10<SUP>2</SUP>. In the case of inserting insulating layers of about 2 nm thickness between adjacent catalase (CAT) layers, these devices exhibited the higher memory performance (ON/OFF current ratio of ∼10<SUP>6</SUP>) and the lower power consumption than those of (PAH/CAT)<SUB>15</SUB> multilayer devices.</P> <P>Graphic Abstract</P><P>We demonstrate that the redox enzymes can be used as electrically active materials for nonvolatile memory devices and that, furthermore their switching behavior originates from redox sites within enzymes. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2jm16231h'> </P>