N. Edwin Widjonarko

Low-temperature, solution-processed molybdenum oxide hole-collection layer for organic photovoltaics

We have utilized a commercially available metal–organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoOx) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal–organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N2 atmosphere and annealed in air yield continuous thin films of MoOx. Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoOx and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoOx thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoOx HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo5+ gap states in the MoOx HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoOx HCL.

Stoichiometric analysis of compositionally graded combinatorial amorphous thin film oxides using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) is a recently developed locally destructive elemental analysis technique that can be used to analyze solid, liquid, and gaseous samples. In the system explored here, a neodymium-doped yttrium aluminum garnet laser ablates a small amount of the sample and spectral emission from the plume is analyzed using a set of synchronized spectrometers. We explore the use of LIBS to map the stoichiometry of compositionally graded amorphous indium zinc oxide thin-film libraries. After optimization of the experimental parameters (distance between lens and samples, spot size on the samples, etc.), the LIBS system was calibrated against inductively coupled plasma atomic emission spectroscopy which resulted in a very consistent LIBS-based elemental analysis. Various parameters that need to be watched closely in order to produce consistent results are discussed. We also compare LIBS and x-ray fluorescence as techniques for the compositional mapping of libraries.

Optimization of organic photovoltaic devices using tuned mixed metal oxide contact layers

The use of oxide materials as a hole transport layers (HTL) offers the opportunity to optimize hole collection in a bulk heterojunction organic photovoltaic (OPV) device. We discuss the use of NiOx deposited by three different methods, pulsed laser deposition, sputtering and a solution precursor as an alternative to the standard OPV HTL. We also examine the ability of the HTL to improve device performance in a bulk heterojunction device utilizing a donor that has a deeper highest occupied molecular orbital (HOMO) level‥

Superimposed RF/DC magnetron sputtering of transparent Ga:ZnO with high conductivity for photovoltaic contacts applications

Transparent conducting oxides (TCOs) based on gallium-doped ZnO (GZO) are deposited using a radio-frequency (RF) superimposed direct current (DC) magnetron system. The electrical, optical, and structural properties of GZO films deposited by varying the RF/ DC power ratio are presented. Our results indicate an increase in conductivity of GZO films up to 3800 S/cm when grown in a pure argon atmosphere at an RF/DC power ratio of 1∶1. Optical transmittance for all films is ∼90% in the visible range. We find our high-conductivity samples to be highly textured wurtzite ZnO with the c-axis oriented perpendicular to the substrate. The films are relatively smooth with root-mean-square surface roughness of 2.0 nm. This high-performance GZO material deposited using this approach can pave the way to development of high-conductivity TCOs at low cost from Earth-abundant materials to enable cost-effective solar conversion technologies.