Yung Jin Yoon

High-Efficiency Colloidal Quantum Dot Photovoltaics via Robust Self-Assembled Monolayers

The optoelectronic tunability offered by colloidal quantum dots (CQDs) is attractive for photovoltaic applications but demands proper band alignment at electrodes for efficient charge extraction at minimal cost to voltage. With this goal in mind, self-assembled monolayers (SAMs) can be used to modify interface energy levels locally. However, to be effective SAMs must be made robust to treatment using the various solvents and ligands required for to fabricate high quality CQD solids. We report robust self-assembled monolayers (R-SAMs) that enable us to increase the efficiency of CQD photovoltaics. Only by developing a process for secure anchoring of aromatic SAMs, aided by deposition of the SAMs in a water-free deposition environment, were we able to provide an interface modification that was robust against the ensuing chemical treatments needed in the fabrication of CQD solids. The energy alignment at the rectifying interface was tailored by tuning the R-SAM for optimal alignment relative to the CQD quantum-confined electron energy levels. This resulted in a CQD PV record power conversion efficiency (PCE) of 10.7% with enhanced reproducibility relative to controls.

Fluorine Functionalized Graphene Nano Platelets for Highly Stable Inverted Perovskite Solar Cells

Edged-selectively fluorine (F) functionalized graphene nanoplatelets (EFGnPs-F) with a p-i-n structure of perovskite solar cells achieved 82% stability relative to initial performance over 30 days of air exposure without encapsulation. The enhanced stability stems from F-substitution on EFGnPs; fluorocarbons such as polytetrafluoroethylene are well-known for their superhydrophobic properties and being impervious to chemical degradation. These hydrophobic moieties tightly protect perovskite layers from air degradation. To directly compare the effect of similar hydrophilic graphene layers, edge-selectively hydrogen functionalized graphene nanoplatelet (EFGnPs-H) treated devices were tested under the same conditions. Like the pristine MAPbI3 perovskite devices, EFGnPs-H treated devices were completely degraded after 10 days. The hydrophobic properties of EFGnPs-F were characterized by contact angle measurement. The test results showed great water repellency compared to pristine perovskite films or EFGnPs-H coated films. This resulted in highly air-stable p-i-n perovskite solar cells.

Correction to High-Efficiency Colloidal Quantum Dot Photovoltaics via Robust Self-Assembled Monolayers

The optoelectronic tunability offered by colloidal quantum dots (CQDs) is attractive for photovoltaic applications but demands proper band alignment at electrodes for efficient charge extraction at minimal cost to voltage. With this goal in mind, self-assembled monolayers (SAMs) can be used to modify interface energy levels locally. However, to be effective SAMs must be made robust to treatment using the various solvents and ligands required for to fabricate high quality CQD solids. We report robust self-assembled monolayers (R-SAMs) that enable us to increase the efficiency of CQD photovoltaics. Only by developing a process for secure anchoring of aromatic SAMs, aided by deposition of the SAMs in a water-free deposition environment, were we able to provide an interface modification that was robust against the ensuing chemical treatments needed in the fabrication of CQD solids. The energy alignment at the rectifying interface was tailored by tuning the R-SAM for optimal alignment relative to the CQD quantum-confined electron energy levels. This resulted in a CQD PV record power conversion efficiency (PCE) of 10.7% with enhanced reproducibility relative to controls.

Clean thermal decomposition of tertiary-alkyl metal thiolates to metal sulfides: environmentally-benign, non-polar inks for solution-processed chalcopyrite solar cells

We report the preparation of Cu2S, In2S3, CuInS2 and Cu(In,Ga)S2 semiconducting films via the spin coating and annealing of soluble tertiary-alkyl thiolate complexes. The thiolate compounds are readily prepared via the reaction of metal bases and tertiary-alkyl thiols. The thiolate complexes are soluble in common organic solvents and can be solution processed by spin coating to yield thin films. Upon thermal annealing in the range of 200–400 °C, the tertiary-alkyl thiolates decompose cleanly to yield volatile dialkyl sulfides and metal sulfide films which are free of organic residue. Analysis of the reaction byproducts strongly suggests that the decomposition proceeds via an SN1 mechanism. The composition of the films can be controlled by adjusting the amount of each metal thiolate used in the precursor solution yielding bandgaps in the range of 1.2 to 3.3 eV. The films form functioning p-n junctions when deposited in contact with CdS films prepared by the same method. Functioning solar cells are observed when such p-n junctions are prepared on transparent conducting substrates and finished by depositing electrodes with appropriate work functions. This method enables the fabrication of metal chalcogenide films on a large scale via a simple and chemically clear process.

Peroptronic devices: perovskite-based light-emitting solar cells

Solution processed perovskite semiconductors have developed rapidly over the past decade to yield excellent performance in both solar cell and light emitting diode devices. Both of these device types are prepared using similar materials and architectures, raising the possibility of perovskite based light emitting solar cells. Recent reports have indicated that some low band gap perovskite solar cells are able to emit infrared light efficiently, however, intermediate band gap perovskite solar cells which emit visible light have not, to the best of our knowledge been deliberately designed or extensively characterized. In this work, we have investigated the use of different electron transport layers in order to minimize energetic barriers to electron injection and extraction in methylammonium lead bromide (MAPbBr3) films. We demonstrate that through appropriate band structure engineering, MAPbBr3 can be used to make such “peroptronic” light-emitting solar cells, which simultaneously exhibit efficient solar cell power conversion efficiencies over 1% and 0.43 lm W−1 green light emission.

Formamidinium-based planar heterojunction perovskite solar cells with alkali carbonate-doped zinc oxide layer

We herein demonstrate n-i-p-type planar heterojunction perovskite solar cells employing spin-coated ZnO nanoparticles modified with various alkali metal carbonates including Li2CO3, Na2CO3, K2CO3 and Cs2CO3, which can tune the energy band structure of ZnO ETLs. Since these metal carbonates doped on ZnO ETLs lead to deeper conduction bands in the ZnO ETLs, electrons are easily transported from the perovskite active layer to the cathode electrode. The power conversion efficiency of about 27% is improved due to the incorporation of alkali carbonates in ETLs. As alternatives to TiO2 and n-type metal oxides, electron transport materials consisting of doped ZnO nanoparticles are viable ETLs for efficient n-i-p planar heterojunction solar cells, and they can be used on flexible substrates via roll-to-roll processing.

The introduction of a perovskite seed layer for high performance perovskite solar cells

Processing for obtaining compact and uniform perovskite photoactive layers has been intensively studied over the last few years to achieve high power conversion efficiencies (PCEs) in solar cells. Particularly, high quality crystal growth of perovskite layers is critical to enhance device performance. We demonstrate an easy and effective new process for high efficiency p–i–n planar heterojunction structures of perovskite solar cells (PeSCs) by using a compact seed perovskite layer (CSPL). The CSPL assists vertical growth of perovskite crystals and obtains the highly crystalline perovskite photoactive layer, which leads to the reduction in the charge transfer resistance and a longer photoluminescence lifetime. The PeSC device with a CSPL shows a remarkably improved PCE, from 15.07% to 19.25%, with a record open circuit voltage (VOC) of 1.16 V in the p–i–n structure with pure crystal perovskite and negligible current density–voltage hysteresis. Additionally, a PCE of 20.37% was achieved in CSPL assisted n–i–p structure PeSCs.

Conjugated Polyelectrolytes Bearing Various Ion Densities: Spontaneous Dipole Generation, Poling‐Induced Dipole Alignment, and Interfacial Energy Barrier Control for Optoelectronic Device Applications

Conjugated polyelectrolytes (CPEs) with π-delocalized main backbones and ionic pendant groups are intensively studied as interfacial layers for efficient polymer-based optoelectronic devices (POEDs) because they facilitate facile control of charge injection/extraction barriers. Here, a simple and effective method of performing precise interfacial energy level adjustment is presented by employing CPEs with different thicknesses and various ion densities under electric poling to realize efficient charge injection/extraction of POEDs. The effects of the CPE ion densities and electric (positive or negative) poling on the energy level tuning process are investigated by measuring the open-circuit voltages and current densities of devices with the structure indium tin oxide/zinc oxide/CPE/organic active layer/molybdenum oxide/gold while changing the CPE film thickness. The performances of inverted polymer light-emitting diodes and inverted polymer solar cells are remarkably improved by precisely controlling the interfacial energy level matching using optimum CPE conditions.