Research

Ongoing research activities are focused on the application of organic and hybrid organic-inorganic semiconductor materials in optoelectronic devices. This involves characterizing all aspects of charge and excited state behavior, device design, fabrication and testing, and supporting activities in modeling and simulation. Current activities are focused on organic and perovskite solar cells and organic light-emitting devices.  Abstracts and summaries from some recently published works by the group are included below: 

 

CGAVD

Carrier-gas Assisted Vapor Deposition for Highly Tunable Morphology of Halide Perovskite Thin Films - We have demonstrated carrier-gas assisted vapor deposition (CGAVD) as a promising synthesis technique for metal halide perovskite thin films. Wide tunability of film microstructure and morphology are accesible with CGAVD via the combination of several independently controllable experimental variables. We have examined the material transport mechanisms in CGAVD and developed analytical expressions for the deposition rates of the precursors MABr, MAI, SnBr2, and SnI2 as a function of experimentally tunable temperatures, pressures, and flow rates. The method is applied to systematically control the growth of MASnBr3 thin films via co-deposition across a range of stoichiometries and morphologies. In varying processing conditions, changes are realized in the degree of crystallinity, grain orientation, and average grain size (from ~0.001 to > 0.7 μm2).

Formation of aligned periodic patterns during the crystallization of organic semiconductor thin films - Self-organizing patterns with micrometre-scale features are promising for the large-area fabrication of photonic devices and scattering layers in optoelectronics. Pattern formation would ideally occur in the active semiconductor to avoid the need for further processing steps. We have reported an approach to form periodic patterns in single layers of organic semiconductors by a simple annealing process. When heated, a crystallization front propagates across the film, producing a sinusoidal surface structure with wavelengths comparable to that of near-infrared light. Features initially form in the amorphous region within a micrometre of the crystal growth front, probably due to competition between crystal growth and surface mass transport. The pattern wavelength can be tuned from 800 nm to 2,400 nm by varying the film thickness and annealing temperature, and millimetre-scale domain sizes are obtained. This phenomenon could be exploited for the self-assembly of microstructured organic optoelectronic devices.

Grating
IQE

Intrinsic measurements of exciton transport in photovoltaic cells - Organic photovoltaic cells are partiuclarly sensitive to exciton harvesting and are thus, a useful platform for the characterization of exciton diffusion. While device photocurrent spectroscopy can be used to extract the exciton diffusion length, this method is frequently limited by unknown interfacial recombination losses. We have resolved this limitation and demonstrated a general, device-based photocurrent-ratio measurement to extract the intrinsic diffusion length. Since interfacial losses are not active layer specific, a ratio of the donor- and acceptor-material internal quantum efficiencies cancels this quantity. We further show that this measurement permits extraction of additional device-relevant information regarding exciton relaxation and charge separation processes. The generality of this method is demonstrated by measuring exciton transport for both luminescent and dark materials, as well as for small molecule and polymer active materials and semiconductor quantum dots. Thus, we demonstrated a broadly applicable device-based methodology to probe the intrinsic active material exciton diffusion length.

Measurement of the triplet exciton diffusion length in organic semiconductors - We demonstrated photoluminescence-based method to measure the exciton diffusion length (LD) of optically dark triplet excitons in organic semiconductor thin films. In order to directly probe only these states, triplets are optically injected into the material of interest via energy transfer from an adjacent phosphorescent thin film. Injected triplets migrate through the full thickness of the material before undergoing energy transfer to a phosphorescent sensitizer. By measuring photoluminescence from the sensitizer as a function of active layer thickness and sensitizer layer concentration, we are able to extract both LD and the transfer rate to the sensitizer. Extraction of the transfer rate is critical, as the assumption of unity quenching can lead to incorrect measurements of LD. The triplet LD is extracted for a series of archetypical fluorescent organic semiconductors with values falling in the range of 15–30 nm. In addition to probing the diffusion of dark triplets, this method also offers the ability to measure the singlet and triplet LD with only a change in injection layer.

Triplet LD

Mixed host

 

Improved stability in organic light‐emitting devices by mixing ambipolar and wide energy gap hosts - We demonstrated improved stability in phosphorescent organic light‐emitting devices (OLEDs) by incorporating a wide energy gap host material into an ambipolar emissive layer. Unlike conventional mixed‐host OLEDs that combine hole‐ and electron‐transporting hosts, charge transport in this device occurs primarily along the ambipolar host and the emitter, while the wide energy gap host serves to modify the charge injection and transport characteristics of the emissive layer. This approach allows both the width and position of the exciton recombination zone to be tuned without introducing exciplex states. Whereas overall device stability improves with increasing recombination zone width in conventional mixed‐host OLEDs, mixing in this system reduces the recombination zone extent yet still increases device lifetime. By decoupling luminance losses into the photostability of the emitter and the exciton formation efficiency, we show that this enhancement arises from a trade‐off between bulk and interfacial degradation. The addition of the wide energy gap host moves the recombination zone away from the interface between the hole‐transport layer and the emissive layer, sacrificing a modest increase in bulk degradation to substantially reduce interfacial degradation. We found that the lifetime can be improved by 50% by balancing these competing degradation pathways.

Overcoming the trade-off between exciton dissociation and charge recombination in organic photovoltaic cells - The electron donor-acceptor (D-A) interface is an essential component for realizing efficient exciton dissociation and charge generation in organic photovoltaic cells (OPVs). It can also however enable rapid charge recombination due to the close spatial proximity of electrons and holes. To frustrate recombination losses, attempts have been made to separate charge carriers by introducing an insulating blocking interlayer at the D-A interface. It is challenging to realize increased efficiency using this approach as the relative similarity of interlayer optical and transport energy gaps may also frustrate exciton harvesting and charge generation. To overcome this trade-off, the interlayer must block charge carriers while continuing to permit exciton migration to the dissociating interface. We have demonstrated this configuration in archetypical copper phthalocyanine (CuPc)-C60 planar OPVs containing a rubrene interlayer to frustrate charge recombination. Critically, the similarity in triplet exciton energy levels between rubrene and CuPc allows the interlayer to be permeable to excitons. Devices containing an interlayer show a reduction in the charge transfer state binding energy and non-geminate recombination rate with increasing interlayer thickness. For thin interlayers, geminate recombination is also suppressed. Thus, devices containing an exciton permeable interlayer show a simultaneous increase in open-circuit voltage, short-circuit current, and power efficiency.

Triplet spacer