Nonlinear Transient Photovoltaic Response in Al/C60/Au Devices: Control of Polarity with Optical Bias

We report fast (nanosecond) transient photovoltaic response and steady-state photovoltage of rectifying Al/C60/Au sandwich devices. The transient photovoltage changes polarity at a critical light intensity, which can be varied and controlled by applying steady-state light as bias. This ability to change the polarity of the transient photovoltaic response and to control the intensity at which the polarity changes sign offers potential for use in applications in the area of fast nonlinear optoelectronic detectors. Applied Physics Letters is copyrighted by The American Institute of Physics

Altering Glass Transition of TPD thin Films with UV Light

N,N´-Bis(3-methylphenyl)-N,N’dyphenilbenzidine (TPD) is a hole-transport material used in electroluminescent devices whose glass transition temperature, Tg, depends on the film thickness.[1] For sufficiently thin films (d<30 nm), dewetting of amorphous TPD films deposited on a on fused silica or an ITO substrate occurs even at room temperature.[2] Following a brief report on increased thermal stability of UV irradiated TPD films,[3] we investigated the underlying mechanism responsible for it. From proton NMR and mass spectrometry measurements, coupled with morphology (AFM) and spectroscopy (UV-VIS) studies, we find that photo-excited TPD species react with oxygen in air. This leads to partially oxidized TPD films whose increased thermal stability we ascribe to stronger hydrogen bonding of photo-oxidized TPD species with hydrophilic substrates

Towards the mechanisam of stabilization of TPD thin films with UV light

Triphenyldiamine (TPD) or N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)benzdine is a well known hole-transporting material often used in electroluminescent devices. In bulk material glass transition temperature TTPDg ~ 60°C [1] is rather low and for sufficiently thin films (thickness d ~ 30 nm) deposited on a fused-silica substrate, dewetting occurs even at room temperature [2]. Morphological changes, which are often related to low Tg, lead to degradation of device performance in which thin film s are incorporated. That is why it is interesting to find a way to stabilize thin films. Following a brief report [3] on increased stability of UV irradiated TPD films, we focused on elucidating the underlying mechanism, since an explanation of chemical changes on molecular level has not yet been given. Thin amorphous TPD films were produced in physical vapor deposition (PVO) process on a fused silica or glass substrates. Immediately after evaporation one half of each sample was exposed to UV light under ambient conditions in order to compare effects of irradiation on a single film. Illuminated and non-illuminated areas of films are characterized using UV-visible spectroscopy and atomic force microscopy (AFM). Decrease in absorption bands intensity was observed after irradiation, indicating a chemical change in the sample. AFM study clearly shows that dewetting process at room temperature is stopped for irradiated samples thinner than 30nm. Illuminated samples remained stable even after few weeks of storage under ambient conditions and after 24h exposure to temperatures T > TTPDg. From proton nuclear magnetic resonance and mass spectrometry measurements, we find that photo-excited TPD reacts with oxygen from air, which leads to oxidation and hydroxylation of small amount of TPD molecules. W e conclude that increased thermal stability of irradiated films is due to hydrogen bonding among TPD molecules and molecules formed in hydroxylation process

Spectral and Photocarrier Dynamics in Thin-Films of Pristine and Alkali-Doped C60

This article summarizes our contribution to three distinct areas of research on thin films of fullerenes. In the first part we present results of an in-situ optical absorption study on K-doped thin films of C60 from which an unambiguous assignment of certain low energy electronic transitions in C60 and KxC60 was deduced. The second part of the article concerns transient and steady-state photoconductivity (PC) of pristine and oxygen-exposed thin films of C60 examined as a function of temperature and photon energy. We find that the presence of oxygen is responsible for the creation of deep traps which reduce the steady-state and transient PC by several orders of magnitude, particularly in the 1.6–2.3 eV excitation region. The thermally activated transport in an oxygen-free C60 sample is quenched upon exposure to oxygen so that both steady-state and transient PC become nearly temperature independent. In the last part we present two examples of utilizing C60 thin films for application in opto-electronics. The mechanism of light-induced charge transfer at the conducting polymer-C60 interface is described and the current versus voltage rectifying characteristics (> 104) of the MEH-PPV/C60 thin film heterojunction is presented. In addition, a non-linear transient photovoltaic response in a AlC60/Au sandwich-type device featuring control of polarity with optical bias is described

Dynamics of Photoexcited Carriers in Pristine Oxygen-Free C60 Films: Study of the Time-Resolved Transient Photoconductivity

We study the dynamics of photoexcited carriers in pristine oxygen-free C60 films using the measurement of the transient photoconductivity (PC). The nonexponential relaxation of photocarriers in oxygen-free C60 can be described by the multiple trapping model. The carrier recombination kinetics is characteristic of bimolecular recombination at high temperature. As the thermally-activated long-lived PC component freezes out at the low temperature regime, a monomolecular recombination prevails and the carrier relaxation approaches an exponential decay. The peak transient PC exhibits a local maximum at T ≈ 240 K, near a first-order structural phase transition temperature, indicating that evaporated film of oxygen-free C60 contains regions of structurally ordered C60. The disorder in a pristine C60 film is manifested by the existence of a large, thermally-activated, long-lived PC component

Effects of Oxygen on the Photocarrier Dynamics in a C60 Film: Studies of Transient and Steady-State Photoconductivity

The intrinsic dynamics of photoexcited carriers in an oxygen-free C60 film and their remarkable evolution as the film is exposed to oxygen are revealed by transient and steady-state photoconductivity (PC) measurements at various temperatures, light intensities, and photon energies. Exposure of C60 film to oxygen creates deep traps that reduce drastically the carrier lifetime and, consequently, the room-temperature steady-state photoconductivity by three to six orders of magnitude. Oxygen affects the steady-state photoconductivity excitation spectrum in a qualitatively similar way, as does decreasing the ambient temperature; in both cases, the photoconductivity decreases faster when carriers are photoexcited into the band edges. The transient and steady-state PC of a C60 film fully exposed to oxygen becomes temperature independent

Photocarrier Dynamics in C60: Studies of Transient Photoconductivity and Transient Photoinduced Absorption

The intrinsic dynamics of photoexcited carriers in oxygen-free C60 film and their remarkable evolution as the film is exposed to oxygen are revealed by transient photoconductivity (PC) and transient photoinduced absorption (PA) measurements at various temperatures, light intensities and photon energies. These studies reveal the carrier transport and carrier recombination mechanisms, and provide an estimate for the initial mobility in pristine C60 film; these measurements also demonstrate that exposure C60 film to oxygen creates deep traps that localize the photocarriers and effectively quenches the long-lived multiple trapping transport mechanism