Rapid growth in the number of Earth-orbiting satellites with electric propulsion as well as plans for colonizing the Moon will dramatically increase the demand for solar photovoltaic (PV) power in space. Most of these missions will be commercially driven, heightening the need for PV systems that are more compact, lower-mass, more efficient, reliable, and affordable than ever before. In this talk, I will describe how microscale PV cells integrated with ultracompact concentrating optics offer a new opportunity to improve performance and reduce cost without sacrificing reliability. I will overview the unique constraints imposed on nonimaging concentrator design by operation in space and describe an experimental prototype <2 mm thick that achieves 26% power conversion efficiency at a geometric gain of 18x with a specific power ⪆100 W/kg and an acceptance angle of nearly ±10°.
This talk will explore a new class of organic light-emitting diode (OLED) that exhibits bistability owing to positive photonic feedback between an organic photodiode integrated in the same layer stack as a tandem OLED. These unusual devices exhibit giant hysteresis in both their current and light emission and respond sensitively to low-level external illumination, enabling optoelectronic upconversion with thousand-fold photon-to-photon gain. These devices may find use in new types of display and upconversion imaging applications as well as provide a new platform for neuromorphic optoelectronics and image recognition.
In recent years, increasing attention has been devoted to OLEDs as a promising technology for general lighting. This application requires high brightness and thus higher drive current density than displays, reducing efficiency and lifetime to levels unsuitable for general illumination. To address this limitation, we are developing OLEDs deposited on corrugated substrates that increase the effective device area within the same module size, lowering the local current density at the same level of brightness. This talk discusses the fabrication approach to making devices on non-planar substrates, and the relationship between surface topography and OLED performance.
Integrating an organic light-emitting diode (OLED) back-to-back with an organic photodiode (OPD) is a well-known path to upconvert near-infrared light for display purposes. This talk will explore a new, positive feedback regime that arises when the OLED is a tandem device with greater-than-unity quantum efficiency, enabling the OLED emission>>OPD absorption>>OLED emission loop gain to exceed one. The runaway increase in OLED light output that occurs in this situation enables low-level light detection with upconversion gain >10,000 and millisecond dynamic response.
We demonstrate graded index nanoporous fluoropolymer anti-reflection (AR) coatings for plastic optics that enable <0.5% reflectance from acrylic plastic over the 0.4-2 µm wavelength range for incidence angles up to 40º. Coatings tailored for the visible spectrum yield <0.1 % luminous reflectance, effectively rendering double-side coated plastics invisible under room lighting conditions. Adhesion to most optical plastics, pliability that prevents cracking, outstanding chemical and environmental durability, and compatibility with commercial vacuum coating systems should enable this AR technology to find widespread practical use.
Metal halide perovskites have emerged as promising gain media with the potential to realize wavelength-tunable, non-epitaxial laser diodes. This talk will focus on recent progress toward this goal, including continuous-wave optically-pumped lasing from methylammonium lead iodide (MAPbI3), the operation of MAPbI3 LEDs at ~kA/cm2 current densities, distributed feedback MAPbI3 LEDs that lase under optical pumping, and a new perovskite laser diode architecture that can be fabricated on silicon in a single, back-end solution processing step.
Polariton modes in organic semiconductor microcavities traditionally derive from singlet exciton states that possess no net charge or spin. This talk will explore the properties and prospects of charged polariton states that originate from cationic excitations in a heavily doped organic semiconductor. In addition to new electric and magnetic properties associated with their net charge and spin, charged polaritons are shown to be a useful platform for exploring cavity-modified photoinduced electron transfer and the mechanisms that underlie it.
Electrically-pumped lasing remains an elusive grand challenge for the organic and thin film electronics community. Recently, hybrid organic-inorganic perovskites have emerged as promising gain media for tunable, solution-processed semiconductor lasers, sparking interest in the use of these materials for an eventual diode laser. This talk will focus on recent progress toward this goal, including the demonstration of optically-pumped, continuous-wave lasing from methylammonium lead iodide (MAPbI3) as well as an investigation into the nature of quantum efficiency roll-off in MAPbI3 light emitting diodes operated at current densities exceeding 300 A/cm2.
Phononic crystals and acoustic metamaterials enable the precise control of elastic properties, even in ranges inaccessible to traditional materials, making them useful for applications ranging from acoustic waveguiding to thermoelectrics. In particular, surface phononic crystals (SPCs) consisting of periodic nanolines on a semi-infinite substrate can be used to generate narrow bandwidth pseudosurface acoustic waves with exquisite sensitivity to the elastic properties of the underlying substrate. Tuning the period of the surface phononic crystal tunes the penetration depth of the pseudosurface wave, and thus selectively probes different depths of layered substrates. In our experiments, we use ultrafast near infrared laser pulses to excite these waves in the hypersonic frequency range by illuminating absorbing metallic nanolines fabricated on top of complex substrates. We probe the nanoscale dynamics launched by our SPCs via pump-probe spectroscopy where we monitor the diffraction of ultrafast pulses of extreme ultraviolet light generated via tabletop high harmonic generation. We then extract the mechanical properties of the substrate by comparing our measurements to quantitative finite element analysis. Utilizing this technique, we characterize the effective elastic and thermal transport properties of 3D periodic semiconductor metalattices.
Reliability remains an ongoing challenge for organic light emitting diodes (OLEDs) as they expand in the marketplace. The ability to withstand operation and storage at elevated temperature is particularly important in this context, not only because of the inverse dependence of OLED lifetime on temperature, but also because high thermal stability is fundamentally important for high power/brightness operation as well as applications such as automotive lighting, where interior car temperatures often exceed the ambient by 50 °C or more. Here, we present a strategy to significantly increase the thermal stability of small molecule OLEDs by co-depositing an amorphous fluoropolymer, Teflon AF, to prevent catastrophic failure at elevated temperatures. Using this approach, we demonstrate that the thermal breakdown limit of common hole transport materials can be increased from typical temperatures of ∼100 °C to more than 200 °C while simultaneously improving their electrical transport properties. Similar thermal stability enhancements are demonstrated in simple bilayer OLEDs. These results point toward a general approach to engineer morphologically-stable organic electronic devices that are capable of operating or being stored in extreme thermal environments.
Dramatic improvements in white organic light emitting diode (OLED) performance and lifetime over the past decade are driving commercialization of this technology for solid-state lighting applications. As white OLEDs attempt to gain a foothold in the market, however, the biggest challenge outside of lowering their manufacturing cost arguably now lies in creating an architecturally adaptable form factor that will drive public adoption and differentiate OLED lighting from established LED products. Here, we present concepts based on kirigami (the Japanese art of paper cutting and folding) that enable intricate three-dimensional (3D) OLED lighting structures from two dimensional layouts. Using an ultraflexible, encapsulated OLED device architecture on 25 ~ 60 μm thick clear polyimide film substrate with simple cut and fold patterns, we demonstrate a series of different lighting concepts ranging from a simple ‘pop up’ structure to more complex designs such as stretchable window blind-like panel, candle flame, and multi-element globe lamp. We only find slight degradation in OLED electrical performance when these designs are shaped into 3D. Our results point to an alternate paradigm for OLED lighting that moves beyond traditional 2D panels toward 3D designs that deliver unique and creative new opportunities for lighting.
Exciton annihilation processes impact both the lifetime and efficiency roll-off of organic light emitting diodes (OLEDs), however it is notoriously difficult to identify the dominant mode of annihilation in operating devices (exciton-exciton vs. exciton-charge carrier) and subsequently to disentangle its magnitude from competing roll-off processes such as charge imbalance. Here, we introduce a simple analytical method to directly identify and extract OLED annihilation rates from standard light-current-voltage (LIV) measurement data.
The foundation of this approach lies in a frequency domain EQE analysis and is most easily understood in analogy to impedance spectroscopy, where in this case both the current (J) and electroluminescence intensity (L) are measured using a lock-in amplifier at different harmonics of the sinusoidal dither superimposed on the DC device bias. In the presence of annihilation, the relationship between recombination current and light output (proportional to exciton density) becomes nonlinear, thereby mixing the different EQE harmonics in a manner that depends uniquely on the type and magnitude of annihilation.
We derive simple expressions to extract different annihilation rate coefficients and apply this technique to a variety of OLEDs. For example, in devices dominated by triplet-triplet annihilation, the annihilation rate coefficient, K_TT, is obtained directly from the linear slope that results from plotting EQE_DC-EQE_1ω versus L_DC (2EQE_1ω-EQE_DC). We go on to show that, in certain cases it is sufficient to calculate EQE_1ω directly from the slope of the DC light versus current curve [i.e. via (dL_DC)/(dJ_DC )], thus enabling this analysis to be conducted solely from common LIV measurement data.
KEYWORDS: Solar concentrators, Nonimaging optics, Solar cells, Solar energy, Glasses, Reflectors, Optical components, Refractive index, Absorption, Thin films
Luminescent solar concentrators (LSCs) aim to deliver high concentration ratio for photovoltaic cells without tracking
the Sun, however, experimental realizations to date underperform their limiting theoretical potential by more than an
order of magnitude. Here, we pursue a new path to improve LSC performance by combining highly directional
spontaneous emission with integrated nonimaging optical elements. By minimizing the etendúe of emitted light through
use of simple and scalable photonic structure, we employ the nonimaging optical design to transform limited angular
extent into high spatial compression. We discuss the conceptual basis and theoretical potential of this approach and
show through a combination of experiment and ray-tracing simulation that dramatic increases in LSC concentration ratio
can be realized.
Luminescent solar concentrators (LSCs) were developed over three decades ago as a simple route to obtain high
concentration ratio for photovoltaic cells without tracking the sun. Despite their exciting theoretical potential, LSCs
have thus far reached only modest concentration ratio in practice. Here, we introduce two new approaches to LSC
optical design that enable significant increases in concentration ratio for any type of luminescent material. First, we
discuss the conceptual basis for nonimaging optics in luminescent concentration and then present exact ray tracing
results for the simplest implementation of compound parabolic concentrator-shaped edges that deliver ~15% increase in
concentration ratio with negligible decrease in optical efficiency. Second, we extend the recently introduced concept of
resonance-shifting to the case of shaped substrates and demonstrate that light can be channeled through an LSC with low
loss and localized at specific points, opening up entirely new directions in LSC design.
KEYWORDS: Absorption, Excitons, Heterojunctions, Data modeling, Gold, Picosecond phenomena, Interfaces, Monte Carlo methods, Solar energy, Polarizability
The dissociation of excitons at a donor-acceptor interface in organic photovoltaic devices is an important but poorly
understood process that takes place over multiple length and time scales. Here, we introduce a method to resolve
photoinduced charge movement at an organic heterojunction in both time and space through observation of the Stark
shift induced in a nearby J-aggregate probe layer. Using ultrafast transient absorption measurements, we show that this
technique provides an opportunity to quantify the recombination and separation dynamics of geminate charge pairs over
the entire sub-picosecond to millisecond range of relevant time scales.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.