In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illuminati...

Small molecular weight organic thin-film photodetectors and solar cells

About Supramolecular Assemblies of π-Conjugated Systems

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Optical generation of hot electrons in metallic structures and its potential as an alternative to conventional electron–hole separation in semiconductor devices are reviewed. The possibilities for realizing high conversion efficiencies with low fabrication costs are discussed along with challenges in terms of the materials, architectures and fabrication methods

Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices

Recent Advances in Bulk Heterojunction Polymer Solar Cells

We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.

Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy

We have determined the electronic energy level alignment at the interface between 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) using ultraviolet photoelectron spectroscopy. The highest occupied molecular orbital (HOMO) of 20 nm thick HAT-CN film was located at 3.8 eV below the Fermi level. Thus the lowest unoccupied molecular orbital (LUMO) is very close to the Fermi level. The HOMO position of NPB was only about 0.3 eV below Fermi level at NPB/HAT-CN interface. This enables an easy excitation of electrons from the NPB HOMO to the HAT-CN LUMO, creating electron-hole pairs across this organic-organic interface.

Energy level alignment at a charge generation interface between 4,4'-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11 -hexaazatriphenylene-hexacarbonitrile

We report that Ca atoms severely quench the photoluminescence (PL) of an organic thin film 1,4‐bis[4‐(3,5‐di‐tert‐butylstyryl)stryyl]benzene (4PV). A submonolayer coverage (0.035 monolayer or 0.1 A) of Ca on the 4PV surface reduced the PL of a 300 A layer of 4PV by as much as 50%. Three distinct quenching rates were observed throughout the 200 A Ca deposition. An exciton diffusion length of 200 A in 4PV is estimated assuming that the PL quenching is diffusion controlled.

Dramatic photoluminescence quenching of phenylene vinylene oligomer thin films upon submonolayer Ca deposition

Highly Efficient Light‐Emitting Diodes Based on an Organic‐Soluble Poly(p‐phenylenevinylene) Derivative Carrying the Electron‐Transporting PBD Moiety

Two new poly(p-phenylenevinylene) (PPV) derivatives bearing 2-phenyl-5-(4-tert-butylphenyl)-1,3,4-oxadiazole pendants were prepared, and their photo- and electroluminescence properties were studied. The first polymer (P-1) is poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenyl}-1,4-phenylenevinylene], which is a PPV derivative having diphenyl-substituted 1,3,4-oxadiazole pendant that is known to be an excellent electron-transporting structure. The second polymer (P-2) is poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]. The only structural difference between P-1 and P-2 is the presence of additional 2-ethylhexyloxy pendant groups in P-2. Both polymers were prepared by direct polymerization of the α,α‘-dibromo-p-xylene monomers having the pendant group(s) in the presence of excess potassium tert-butoxide. Both polymers reveal much improved electroluminescence (EL) properties when compared with PPV. They emit luminescence light over the wavelength...

Yongsup Park

Papers

Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy

Energy level alignment at a charge generation interface between 4,4'-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11 -hexaazatriphenylene-hexacarbonitrile

Dramatic photoluminescence quenching of phenylene vinylene oligomer thin films upon submonolayer Ca deposition

Highly Efficient Light‐Emitting Diodes Based on an Organic‐Soluble Poly(p‐phenylenevinylene) Derivative Carrying the Electron‐Transporting PBD Moiety

Luminescence Properties of Structurally Modified PPVs: PPV Derivatives Bearing 2-(4-tert-Butylphenyl)-5-phenyl-1,3,4-oxadiazole Pendants