Organic photovoltaics (OPV) can lead to a low cost and short energy payback time alternative to existing photovoltaic technologies. However, to fulfill this promise, power conversion efficiencies must be improved and simultaneously the architecture of the devices and their processing steps need to be further simplified. In the most efficient devices to date, the functions of photocurrent generation, and hole/electron collection are achieved in different layers adding complexity to the device fabrication. In this talk, we present a novel approach that yields devices in which all these functions are combined in a single layer.
Specifically, we report on bulk heterojunction devices in which amine-containing polymers are first mixed in the solution together with the donor and acceptor materials that form the active layer. A single-layer coating yields a self-forming bottom electron-collection layer comprised of the amine-containing polymer (e.g. PEIE). Hole-collection is achieved by subsequent immersion of this single layer in a solution of a polyoxometalate (e.g. phosphomolybdic acid (PMA)) leading to an electrically p-doped region formed by the diffusion of the dopant molecules into the bulk. The depth of this doped region can be controlled with values up to tens of nm by varying the immersion time. Devices with a single 500 nm-thick active layer of P3HT:ICBA processed using this method yield power conversion efficiency (PCE) values of 4.8 ± 0.3% at 1 sun and demonstrate a performance level superior to that of benchmark three-layer devices with separate layers of PEIE/P3HT:ICBA/MoOx (4.1 ± 0.4%). Devices remain stable after shelf lifetime experiments carried-out at 60 °C over 280 h.
Organic field-effect transistors (OFETs) have the potential to lead to low-cost flexible displays, wearable electronics, and sensors. While recent efforts have focused greatly on improving the maximum charge mobility that can be achieved in such devices, studies about the stability and reliability of such high performance devices are relatively scarce. In this talk, we will discuss the results of recent studies aimed at improving the stability of OFETs under operation and their shelf lifetime. In particular, we will focus on device architectures where the gate dielectric is engineered to act simultaneously as an environmental barrier layer.
In the past, our group had demonstrated solution-processed top-gate OFETs using TIPS-pentacene and PTAA blends as a semiconductor layer with a bilayer gate dielectric layer of CYTOP/Al2O3, where the oxide layer was fabricated by atomic layer deposition, ALD. Such devices displayed high operational stability with little degradation after 20,000 on/off scan cycles or continuous operation (24 h), and high environmental stability when kept in air for more than 2 years, with unchanged carrier mobility. Using this stable device geometry, simple circuits and sensors operating in aqueous conditions were demonstrated. However, the Al2O3 layer was found to degrade due to corrosion under prolonged exposure in aqueous solutions. In this talk, we will report on the use of a nanolaminate (NL) composed of Al2O3 and HfO2 by ALD to replace the Al2O3 single layer in the bilayer gate dielectric use in top-gate OFETs. Such OFETs were found to operate under harsh condition such as immersion in water at 95 °C.
This work was funded by the Department of Energy (DOE) through the Bay Area Photovoltaics Consortium (BAPVC) under Award Number DE-EE0004946.
The package and system level temperature distributions of a high power (>1W) light emitting diode (LED) array has
been investigated using numerical heat flow models. For this analysis, a thermal resistor network model was
combined with a 3D finite element submodel of an LED structure to predict system and die level temperatures. The
impact of LED array density, LED power density, and active versus passive cooling methods on device operation
were calculated. In order to help understand the role of various thermal resistances in cooling such compact arrays,
the thermal resistance network was analyzed in order to estimate the contributions from materials as well as active
and passive cooling schemes. An analysis of thermal stresses and residual stresses in the die are also calculated
based on power dissipation and convection heat transfer coefficients. Results show that the thermal stress in the
GaN layer are compressive which can impact the band gap and performance of the LEDs.
The temperature distribution of a dual Multi-Quantum Well (MQW) light emitting diode (LED) has been investigated using both infrared imaging and micro-Raman Spectroscopy; mean values over the device yielded temperatures ranging from 30-75°C. The InGaN/GaN based LED, grown by Metal Organic Chemical Vapor Deposition (MOCVD), was also studied using the 3ω method in order to determine an effective thermal conductivity of the MQW stack in the temperature range from 300-540K. The LED structure under investigation showed effective thermal conductivities in the range from 82-140 W/mK with the peak conductivity occurring at 440K, well above room temperature. Using temperature dependent properties determined experimentally, a numerical model of the LED structure was developed in order to study the effect that the package thermal resistance and input power has on the temperature of the device.
Conference Committee Involvement (4)
LED-based Illumination Systems
26 August 2013 | San Diego, California, United States
Twelfth International Conference on Solid State Lighting and Fourth International Conference on White LEDs and Solid State Lighting
13 August 2012 | San Diego, California, United States
Eleventh International Conference on Solid State Lighting
22 August 2011 | San Diego, California, United States
Tenth International Conference on Solid State Lighting
2 August 2010 | San Diego, California, United States