Franco Cacialli

Molecular-scale interface engineering for polymer light-emitting diodes

Achieving balanced electron–hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (<100 Å) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15–20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.

Indium–tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance

We report combined studies of the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes (LEDs) incorporating poly(p-phenylene vinylene) (PPV) as the emitter material, with, or without, doped poly(3,4-ethylene dioxythiophene) (PEDOT) as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly(4,4′-diphenylene diphenylvinylene) were also tested. We studied chemical (aquaregia, degreasing, RCA protocol) and physical (oxygen and argon plasmas, Teflon, and paper rubbing) treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. I...

BUILT-IN FIELD ELECTROABSORPTION SPECTROSCOPY OF POLYMER LIGHT-EMITTING DIODES INCORPORATING A DOPED POLY(3,4-ETHYLENE DIOXYTHIOPHENE) HOLE INJECTION LAYER

We report electroabsorption measurements of polymer light-emitting diodes, (LEDs), fabricated with poly(4-4′-diphenylene diphenylvinylene), PDPV, as the emissive layer, Ca–Al cathodes, and indium tin oxide (ITO) anodes, with and without a doped conducting polymer hole injection/transport layer, namely poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS−. In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes’ work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: (a) the increase of luminescence efficiency, (b) the reduction of the turn-on voltage, and (c) the increase of the device lifetime.

Inorganic caesium lead iodide perovskite solar cells

The vast majority of perovskite solar cell research has focused on organic–inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current–voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current–voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current–voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices.

Improved operational stability of polyfluorene-based organic light-emitting diodes with plasma-treated indium–tin–oxide anodes

We report the influence of various surface treatments of indium–tin–oxide (ITO) anodes on the operational stability of high-efficiency (up to 8.2 lm/W) green-emitting polymer light-emitting diodes (PLEDs), employing a doped poly(3,4-ethylene dioxythiophene), hole transport layer, a polyfluorene based emissive layer, and Ca–Al cathodes. The anodes were modified by physical (oxygen-plasma), chemical (aquaregia), and combined treatments. Oxygen plasma improves the stability under constant current with respect to all other anodes, with half-brightness (100 cd/m2) lifetimes two to five times longer than for untreated samples, and 1000 times longer than for aquaregia samples. We derive two major indications for optimization of PLEDs. First, thermal management of the diode is of the uppermost importance and there is significant scope for improvement. Second, the ITO anode and in general the electrical properties of the hole-injecting contact are crucial to device operation, even in the presence of a hole transpo...

Surface energy and polarity of treated indium–tin–oxide anodes for polymer light-emitting diodes studied by contact-angle measurements

We present contact-angle hysteresis and surface energy of differently treated indium–tin–oxide (ITO) thin films obtained from contact angles for liquids with different polar character. We find that the hysteresis and the polar and dispersion component of the surface energy depend strongly on the surface treatments. Oxygen-plasma treatments induce the highest polarity and the highest total surface energy, and we suggest that this improves the interface formation with polymers, and therefore, the performance of light-emitting diodes. We discuss the results in relation to the ITO surface roughness and chemical heterogeneity modified by the different treatments.

Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: a comparison

Abstract We report a comparison of the work functions of thin films of indium tin oxide (ITO), carried out by means of ultraviolet photoelectron spectroscopy (UPS) and by measurements of the contact potential difference with respect to a gold reference electrode (Kelvin probe (KP) method). We investigated commercially available ITOs both “as-received”, and after certain surface treatments, such as oxygen plasma. First, we find measurable discrepancies between KP values measured with three different instruments, and between the KP and the UPS values. Secondly, and unexpectedly, we find that the KP, although more sensitive than UPS, does not detect certain differences between ITOs with different surface treatments. We discuss the results in view of the different environments in which the measurements are carried out (UHV for the UPS and air/Ar for the Kelvin method), of the effects which may be induced by the high-energy photon irradiation in the UPS measurement, and of the stability of the gold probe work function in gas ambient. We conclude that UPS is better-suited for absolute work function determination, although KP remains a convenient and inexpensive tool for fast screening of contact potential differences.

Efficient electron injection in blue-emitting polymer light-emitting diodes with LiF/Ca/Al cathodes

We report electroabsorption and electroluminescence investigations of polymer light-emitting diodes featuring a LiF/Ca/Al cathode, for efficient electron injection into the electroluminescent polymer layer. Our measurement of the built-in potential gives direct evidence of a sizeable reduction of the cathodic barrier height not only with respect to Ca, but also versus LiF/Al or CsF/Al bilayer cathodes, currently amongst the most efficient electron injectors for low electron affinity polymers. In blue-emitting (∼2.7 at peak) polyfluorene-based LEDs, with poly(ethylenedioxythiophene)/poly(styrene sulphonic acid) anodes and LiF/Ca/Al cathodes, we measure a built-in potential of 2.7 V, a luminance of ∼1600 cd/m2 (the highest among the devices studied here) at a driving voltage of 5 V, and efficiencies as high as ∼3 lm/W. We also find that the turn-on voltage essentially coincides with the built-in potential within the experimental error.

LiF/Al cathodes and the effect of LiF thickness on the device characteristics and built-in potential of polymer light-emitting diodes

We report the characteristics of a series of polymer light-emitting diodes, fabricated with LiF/Al cathodes and differing only by the thickness of the LiF interlayer (0 nm⩽d⩽11 nm). Electroabsorption studies of the internal electrostatic potential give direct evidence of a sizable reduction of the cathodic barrier height brought about by the LiF films. These results also correlate with photoemission experiments [S. E. Shaheen, G. E. Jabbour, M. M. Morrell, Y. Kawabe, B. Kippelen, N. Peyghambarian, M. F. Nabor, R. Schlaf, E. A. Mash, and N. R. Armstrong, J. Appl. Phys. 84, 2324 (1998)] and with the electroluminescence performance of the diodes.

Electronic line-up in light-emitting diodes with alkali-halide/metal cathodes

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor device physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the noninvasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky–Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g., Bardeen model), except for electrodes that extensively react with the polymer, and introduce deep gap states. In addition, we show results that validate the approximation of rigid tilting of polymer energy levels with bias (for biases for which no significant injection of carriers occurs). To investigate further the consequences of the electronic line-up on device operation, we complemented the electroabsorption measurements with characterization of the emissive and transport properties of the light-emitting diodes, and confirmed that the cathodic barrier lowering in CsF/Ca/Al and LiF/Ca/Al electrodes leads to the best improvements in electron injection. We found that luminance and overall current are greatly affected by the barrier-reducing cathodes, indicating a truly bipolar transport, with comparable electron and hole currents. We also found significant indications of CsF/Ca/Al cathodes strongly reacting with the polymer, which is suggestive of CsF dissociation and diffusion in the bulk of the polymer.