Photoconductance of ITO/Conductive Polymer Junctions in the UV and Visible Ranges

Controlling charge transfer at indium-doped tin oxide (ITO)/conductive polymer junctions is of special importance for organic photovoltaic (OPV) devices and organic light emitting diodes (OLEDs), where ITO is often the transparent electrode of choice. Light induced conductance enhancement, i.e., photoconductance, can allow such control. ITO/conductive polymer junctions are shown herein to exhibit photoconductance under UV illumination mostly due to photoinduced decrease of an electron barrier at the ITO–polymer interface by discharging of ITO extrinsic surface states, related to the adsorption of oxygen species. Furthermore, we show that ITO surface modification by photoactive porphyrin adsorption can sensitize the ITO/conductive polymer junctions, extending the photoconductance to the visible range, to which ITO is transparent. This process is ascribed mostly to discharging of ITO adsorbate states by recombination with photogenerated holes in the photoexcited molecules. Such sensitization is highly relevant for organic optoelectronic devices utilizing ITO interfaced with photoactive organic species and operating in the visible range, such as OPV and OLED devices, and might be applicable also to other UV-photoconductive metal oxide electrodes

Microscopic Investigation of Degradation Processes in a Polyfluorene Blend by Near-Field Scanning Optical Microscopy

We have studied the degradation of the photoluminescence (PL) of a phase-separated film of a polyfluorene blend, F8BT/PFO, on the submicron length scale using near-field scanning optical microscopy, visualizing the PL of blend compositions that do not exist macroscopically in equilibrium. In the initial scans, the topography and the PL were anticorrelated, as the emission was dominated by the PFO-rich phase. This behavior changed at longer illumination times, where the emission was dominated by the F8BT-rich phase; i.e., the topography and PL were correlated. Using macroscopic investigation of the mechanisms that govern the PL, we could explain the time dependence of the PL spatial distribution: while the degradation of F8BT was driven by photobleaching, both faster absorption degradation and photobleaching processes dominate the degradation of PFO. In addition, we found that energy transfer does not protect the PFO from degradation and does not improve its resistance to oxidation

Models of Surface Morphology and Electronic Structure of Indium Oxide and Indium Tin Oxide for Several Surface Hydroxylation Levels

Indium oxide (IO) and indium tin oxide (ITO) are important metal oxide materials with a wide array of applications. Particularly, ITO is employed as a transparent conductive electrode in photovoltaic systems. While bulk metal oxides are typically well characterized, their surfaces, especially in real-life applications, can be hydroxylated and intrinsically disordered to a level that a structure–function prediction becomes a daunting task. We tackle this problem by carrying out simulations based on Density Functional Theory. We propose IO and ITO hydroxylated surfaces derived from the bcc and rombohedral IO polymorphs (100%, 66%, 33%, and 0% hydroxylation coverages were considered). By correlating computed quantities such as surface partial density of states, work functions, and surface dipole strength, a clear picture of the structure–function relationships in these model systems emerges. In line with conclusions drawn from experiments, we find that the density of states of 100% hydroxylated surfaces and bulk models are unaltered by Sn doping, with the only difference being the position of the Fermi level. The partially hydroxylated surfaces, instead show a rich array of behaviors, including appearance of surface states in the gap and appearance of interesting morphologies, such as chemisorbed molecular oxygen. We also find that the hydroxylation level affects surface dipoles in a systematic way, that is, the higher the hydroxylation level, the higher the surface dipole (screening/reducing the work function). Furthermore, models with In-atom vacancies show a relatively small decrease in surface dipole with hydroxyl coverage due to surface distortions

Effect of Orientation on Bulk and Surface Properties of Sn-doped Hematite (α-Fe₂O₃) Heteroepitaxial Thin Film Photoanodes

The orientation dependence on the photoelectrochemical properties of Sn-doped hematite photoanodes was studied by means of heteroepitaxial film growth. Nb-doped SnO₂ (NTO) was first grown heteroepitaxially on <i>c</i>, <i>a</i>, <i>r</i>, and <i>m</i> plane single crystal sapphire substrates in three different orientations. Hematite was then grown in the (001), (110), and (100) orientations on the NTO films. The structural, morphological, optical, and photoelectrochemical properties of the photoelectrodes were studied. The hematite photoanodes possessed high crystallinity and smooth surfaces. Hole scavenger measurements made in H₂O₂-containing electrolyte revealed that the flux of photogenerated holes arriving at the surface was not significantly affected by orientation. Cathodic shifts in the onset potential for water photo-oxidation of up to 170 mV were observed for (110) and (100) oriented hematite photoanodes as compared to (001) oriented films. These results suggest that varying the orientation of heteroepitaxial thin film Sn-doped hematite photoelectrodes primarily affects charge transfer into the electrolyte arising from the surface properties of the different crystal faces rather than affecting hole transport through the bulk under illumination. Electrochemical techniques were then used to probe the existence of surface states which were found to vary with both exposed crystal face as well as foreign dopant inclusion. Kelvin probe force microscopy (KPFM) measurements revealed correlation between the work function of the hematite films (measured in air) and the flat-band and onset potentials for water photo-oxidation (in alkaline aqueous solution)

Oriented Attachment: A Path to Columnar Morphology in Chemical Bath Deposited PbSe Thin Films

We have studied columnar PbSe thin films obtained using chemical bath deposition. The columnar microstructure resulted from an oriented attachment growth mechanism, in which nuclei precipitating from solution attached along preferred crystallographic facets to form highly oriented, size-quantized columnar grains. This is shown to be an intermediate growth mechanism between the ion-by-ion and cluster growth mechanisms. A structural zone model depicting the active growth mechanisms is presented for the first time for semiconductor thin films deposited from solution. The columnar films showed well-defined twinning relations between neighboring columns, which exhibited 2D quantum confinement, as established by photoluminescence spectroscopy. In addition, anisotropic nanoscale electrical properties were investigated using current sensing AFM, which indicated vertical conductivity, while maintaining quantum confinement

Dynamics of Photoinduced Degradation of Perovskite Photovoltaics: From Reversible to Irreversible Processes

The operational stability of perovskite solar cells (PSCs) remains a limiting factor in their commercial implementation. We studied the long-term outdoor stability of ITO/SnO₂/Cs_0.05((CH₃NH₃)_0.15(CH­(NH₂)₂)_0.85)_0.95PbI_2.55Br_0.45/spiro-OMeTAD/Au cells, as well as the dynamics of their degradation, under simulated sunlight indoors and their recovery in the dark. The extent of overall degradation was found to depend on processes occurring both under illumination and in the dark, i.e., during the daytime and nighttime, with the dynamics varying with cell aging. Full recovery of efficiency in the dark was observed for cells at early degradation stages. Further cell degradation resulted in recovery times much longer than one night, appearing as irreversible degradation under real operational conditions. At later degradation stages, very different dynamics were observed: short-circuit current density and fill factor exhibited a pronounced drop upon light turn-off but strong improvement under subsequent illumination. The interplay of reversible and irreversible degradation processes with different recovery dynamics was demonstrated to result in changes in the cell’s diurnal PCE dependence during its operational lifespan under real sunlight conditions