Organic Charge Transport Layers Research Articles

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer.

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C61-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene-C60-epoxide, IBF-Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH3NH3PbI(3-x)Cl(x)) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF-Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF-Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF-Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF-Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air.

ペロブスカイト太陽電池へのITO透明電極スパッタリング直接堆積の影響

Thin film solar cells using perovskite materials are very intensively studied since the perovskite materials such as CH3NH3PbI3 crystal have been found to show sensitizing effects on a TiO2 electron transport layer. This class of solar cell has made tremendous progress during the last few years, leading to a recently certified record power conversion efficiency (PCE) of 21.02%. In the perovskite /crystalline-Si (c-Si) tandem solar cell, furthermore, the theoretical value of PCE is expected as large as 35 %. Toward this application, the perovskite solar cell must be highly transparent at near-infrared wavelengths so that sufficient light is transmitted to the narrow-bandgap bottom cell. In this study, we fabricated the organic-lead halide perovskite solar cells comprising a transparent sputtered indium tin oxide (ITO) top electrode. We observed the PCE of 1.5% in transparent perovskite solar cells with a thin molybdenum oxide buffer layer and ITO electrode. Moreover, we obtained the PCE of 2% even in solar cells with ITO electrode sputtered directly on the organic charge transport layer. The photovoltaic property could be confirmed under the light irradiation from the ITO top electrode side.

Efficient vacuum deposited p-i-n and n-i-p perovskite solar cells employing doped charge transport layers

Methylammonium lead halide perovskites have emerged as high performance photovoltaic materials. Most of these solar cells are prepared via solution-processing and record efficiencies (>20%) have been obtained employing perovskites with mixed halides and organic cations on (mesoscopic) metal oxides. Here, we demonstrate fully vacuum deposited planar perovskite solar cells by depositing methylammonium lead iodide in between intrinsic and doped organic charge transport molecules. Two configurations, one inverted with respect to the other, p-i-n and n-i-p, are prepared and optimized leading to planar solar cells without hysteresis and very high efficiencies, 16.5% and 20%, respectively. It is the first time that a direct comparison between these two opposite device configurations has been reported. These fully vacuum deposited solar cells, employing doped organic charge transport layers, validate for the first time vacuum based processing as a real alternative for perovskite solar cell preparation.

Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers.

Lead halide perovskite solar cells have recently attracted tremendous attention because of their excellent photovoltaic efficiencies. However, the poor stability of both the perovskite material and the charge transport layers has so far prevented the fabrication of devices that can withstand sustained operation under normal conditions. Here, we report a solution-processed lead halide perovskite solar cell that has p-type NiO(x) and n-type ZnO nanoparticles as hole and electron transport layers, respectively, and shows improved stability against water and oxygen degradation when compared with devices with organic charge transport layers. Our cells have a p-i-n structure (glass/indium tin oxide/NiO(x)/perovskite/ZnO/Al), in which the ZnO layer isolates the perovskite and Al layers, thus preventing degradation. After 60 days storage in air at room temperature, our all-metal-oxide devices retain about 90% of their original efficiency, unlike control devices made with organic transport layers, which undergo a complete degradation after just 5 days. The initial power conversion efficiency of our devices is 14.6 ± 1.5%, with an uncertified maximum value of 16.1%.

Paper No S2.2: Ecofriendly Quantum Dot Light‐Emitting Diode With Inorganic Charge Transport Layer

AbstractRecently, the display technologies using colloidal quantum dot (QD) materials have become one of the most important technologies in the display industry. But, the organic charge transport layer for quantum dot‐light‐emitting diode (QD‐LED) can cause the device stability issues. In this article, we describe the QD‐LED devices with inorganic charge transport layer and the solution‐based oxide thin‐film transistor (TFT) using non‐thermal treatment. And we also show the active matrix driving using both technologies simultaneously.

Sputtered rear electrode with broadband transparency for perovskite solar cells

Due to their high efficiencies combined with simple and cost-effective device fabrication, perovskite solar cells are promising candidates as top cells in tandem devices. For this application, the perovskite solar cell must be highly transparent at near-infrared wavelengths such that sufficient light is transmitted to the narrow-bandgap bottom cell. We demonstrate perovskite solar cells featuring a sputtered amorphous indium zinc oxide (IZO) layer as broadband transparent rear electrode. This electrode absorbs less than 3% in the 400–1200nm wavelength range, while having a sheet resistance of 35Ω/sq. We show over 9% efficient semitransparent perovskite solar cells with IZO sputtered directly on the sensitive organic charge transport layer. The efficiency can be raised up to 10.3% by inserting a thin molybdenum oxide buffer layer, mitigating sputter damage to the underlying organic layer. These cells show more than 60% average transmittance in the 800–1200nm wavelength range, which makes them suitable top-cell candidates for tandem devices. Finally, we discuss the performance potential of this highly transparent rear electrode for four-terminal tandem devices.

Identification of Trap States in Perovskite Solar Cells.

Thermally stimulated current (TSC) measurements are used to characterize electronic trap states in methylammonium lead iodide perovsite solar cells. Several TSC peaks were observed over the temperature range from 20 K to room temperature. To elucidate the origins of these peaks, devices with various organic charge transport layers and devices without transport layers were tested. Two peaks appear at very low temperatures, indicating shallow trap states that are mainly attributed to the PCBM/C60 electron transport bilayer. However, two additional peaks appear at higher temperatures, that is, they are deeper in energy, and are assigned to the perovskite layer. At around T = 163 K, a sharp peak, also present in the dark TSC measurements, is assigned to the orthorhombic-tetragonal phase transition in the perovskite. However, a peak at around T = 191 K is assigned to trap states with activation energies of around 500 meV but with a rather low concentration of 1 × 10(21) m(-3).

Simplified phosphorescent organic light-emitting diodes with a WO3-doped wide bandgap organic charge transport layer

The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO3 were investigated through detailed electrical device characterization. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20mol% and the doped layer should be at least 10nm thick. A large downward shift of the Fermi level in WO3-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO3-doped hole transport layer, the PhOLEDs attained brightness of 11,163cd/m2 at 20mA/cm2, and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.

Effects of substrate topography on current injection and light emission properties of organic light emitting devices

Substrate topography plays a critical role in the function of nano-scale materials and devices. We study small molecular organic light emitting devices (OLEDs) deposited onto non-planar substrates, where the substrate’s radius of curvature in some regions approaches the thickness of the active device layers. As a result, the electric field profile inside the organic charge transport layers is modified, influencing carrier injection, transport, and light emission properties. Experiments and numerical modeling suggest that charge balance and electroluminescence efficiency potentially can be improved in electron injection-limited OLED architectures via substrate geometry. These findings elucidate the optoelectronic behavior (and degradation) of OLEDs on imperfect substrates, and suggest a strategy based on substrate topography for controlling device behavior.

Metal‐Oxide‐Free Methylammonium Lead Iodide Perovskite‐Based Solar Cells: the Influence of Organic Charge Transport Layers

Metal‐oxide‐free methylammonium lead iodide perovskite‐based solar cells are prepared using a dual‐source thermal evaporation method. This method leads to high quality reproducible films with large crystal domain sizes allowing for an in depth study of the effect of perovskite film thickness and the nature of the electron and hole blocking layers on the device performance. The power conversion efficiency increases from 4.7% for a device with only an organic electron blocking layer to almost 15% when an organic hole blocking layer is also employed. In addition to the in depth study on small area cells, larger area cells (approx. 1 cm−2) are prepared and exhibit efficiencies in excess of 10%.

Perovskite solar cells employing organic charge-transport layers

Highly efficient perovskite solar cells have been fabricated by using room-temperature deposition processes. The cells are based on a layer of methylammonium lead iodide perovskite that is prepared by sublimation in a high-vacuum chamber and sandwiched between two thin organic charge-transport layers.

Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing

We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m2 and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer.

Electroluminescence from a Mixed Red−Green−Blue Colloidal Quantum Dot Monolayer

We demonstrate light emitting devices (LEDs) with a broad spectral emission generated by electroluminescence from a mixed-monolayer of red, green, and blue emitting colloidal quantum dots (QDs) in a hybrid organic/inorganic structure. The colloidal QDs are reproducibly synthesized and yield high luminescence efficiency materials suitable for LED applications. Independent processing of the organic charge transport layers and the QD luminescent layer allows for precise tuning of the emission spectrum without changing the device structure, simply by changing the ratio of different color QDs in the active layer. Spectral tuning is demonstrated through fabrication of white QD-LEDs that exhibit external quantum efficiencies of 0.36% (Commission Internationale de l'Eclairage) coordinates of (0.35, 0.41) at video brightness, and color rendering index of 86 as compared to a 5500 K blackbody reference.

Electrophotographic and Mechanical Properties of OPC doped Polycarbonate Layers

Charge carrier mobility and mechanical properties of organic charge transport layers (CTLs) for xerography have been investigated. The CTLs were based on molecularly doped polycarbonate (PC) of different chemical structure (PC-A and PC-Z) and molecular mass. Low weight organic photoconductors (OPCs), in particular, DEH and phenyl-4-tolyl-2-naphthylamine (PTNA) were used as a dopant.In the CTLs, it was found out that the hole drift mobility tends to decrease as the molecular mass of the PC-A increases from 3x104 to (1.0-1.3)x105. In particular, a two fold and ten fold decrease in mobility was observed for the PC/PTNA and PC/DEH layers, respectively.Indentation hardness, density and durability of the CTLs were studied and some structural characteristics of the CTL components were also calculated. The structure and composition of the CTLs were shown to influence slightly on the mechanical properties of the layers under study. Characteristics of the xerographic drums layered with the CTL are presented and discussed.

Short-term evolution of injection efficiency at metal/organic interfaces

A contact forming phenomenon between a variety of metal contacts and organic charge transport layers has been previously reported whereby the hole injection efficiency of the contacts evolves from blocking to ohmic over time periods of up to 1 month. Two main processes, one slow and one rapid, were found to govern the injection evolution by using an experimental technique that combines field-dependent injection current measurements with time-of-flight drift mobility measurements on the same films. Results regarding the rapid process (1-3 hours) are now presented for a variety of organic interfaces, including a molecularly doped polymer of triarylamine (TPD) in polycarbonate, evaporated TPD films and films of the electroluminescent polymer MEH-PPV. The process occurs with evaporated metal contacts, with metal substrates and with liquid Hg, ie. independantly of the contact fabrication method. Variations of the intermolecular structure by organic surface treatment and by the use of sterically hindered TPD isomers suggest a reorganization at the organic surface on interface formation.