Symposium EP5 : Metal Oxide Hetero-Interfaces in Hybrid Electronic Platforms

Solution-manufacturing of emerging solar cell technologies at low cost requires that all layers be solution processable without compromising the efficiency of solar cells. While tremendous progress has been made in solution processing the organic or hybrid photoactive layer (e.g., organic bulk heterojunction, quantum dot solid, or organohalide perovskite), much less is known about how to formulate and process high quality metal oxide interlayers that can be solution processed at low temperature and yield performance on par or superior to their vacuum-evaporated counterparts. In this talk, I will begin by discussing zinc oxide-based sol-gel and nanocrystalline electron transporting layers (ETL) which can yield organic and colloidal quantum dot solar cells with PCE > 10%. In the second part, I will present our recent results in replacing vacuum-evaporated molybdenum oxide hole transporting layer (HTL) with a solution-processed MoOx nanocrystalline layer. By compositing MoOx with silver nanowires, we demonstrate state-of-the-art semitransparent organic solar cells with PCE = 7% and fill factor > 60%. In the last part, I will discuss our recent efforts to develop a multidoped ZnO-based ETL which can be prepared at low temperature and yields highly efficient perovskite solar cells with PCE >17%.

We characterize, using photoemission spectroscopies, new electron-selective interlayers that significantly enhance the performance of inverted bulk-heterojunction organic photovoltaic (OPV) cells. Insertion of thin metal hydroxide (e.g., LiOH, NaOH, KOH and Ba(OH)₂) interlayers between Al-doped zinc oxide (AZO) electron selective contacts and the organic semiconductor blend produces dramatic effects in device performance. These solution-processed metal hydroxides (MOHs) dramatically reduce the work function (in some cases, up to almost 1 eV) of the aluminum-doped zinc oxide (AZO) bottom contact and are further proven to effectively passivate defect states at the contact interface, removing the undesirable light-soaking effect often observed for metal oxide contacts in OPV devices. We use photoelectron spectroscopy (X-ray and UV) to determine the chemical and energetic aspects of the contacts and their energetic alignment with C₆₀, which is used as a prototype for PCBM used in the OPV active layer blend. C₆₀ molecules appear to be negatively charged (n-doped) and may in fact be hybridized with the contact at the near surface region due to Fermi level equilibration with the low work function metal hydroxide contacts, providing for beneficial energy level alignment for electron extraction in solar cell devices. These air-stable, solution-processable and inexpensive alkali-hydroxide interface modifiers presented here provide an enabling technology to afford environmentally stable, inexpensive low work function contacts for high-throughput production of next generation organic solar cells.

Recently, we developed a comprehensive theoretical framework to understand optoelectronic processes at the OI-HJ, predicting the existence of a hybrid charge transfer exciton (HCTE) at the HJ, analogous to the polaron pair state at an excitonic HJ[1], [2]. The HCTE is at the heart of exciton-to-charge conversion and charge recombination processes at the junction. In this work, we study the temperature dependent electrical and optical properties of HCTEs at a ZnO/4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) organic/inorganic semiconductor heterojunction (OI-HJ). We observe that the electroluminescence (EL) spectrum of the HCTE formed by recombination of injected charge carriers at the OI-HJ blue-shifts with decreasing temperature from T = 300 K to 25 K at a constant current density 390 mA/cm² and with increasing current density at fixed temperature, while the full width at half maximum of the spectrum does not systematically change. We describe these phenomena by modelling the change in Fermi level on the organic side of the HJ under space charge injection. Furthermore, an external quantum efficiency of 2.7 ± 0.3 % at λ = 330 nm for excitons generated in the CBP is observed by optical pumping of the junction, which confirms that the exciton-to-free-charge conversion at the OI-HJ is mediated by an intermediate HCTE. However, we do not directly observe HCTE photoluminescence by optically pumping the CBP. This suggests that the HCTE EL results from electrically populated localized defect states at the ZnO surface that radiatively recombine with holes in the CBP layer. The trapped-HCTE states do not play a role in the conversion of excitons generated in CBP molecules into free carriers, which form a free-HCTE at the OI-HJ. We understand these two distinct states based on a quantum mechanical model of both trapped- and free-HCTEs.

[1] C. K. Renshaw, and S. R. Forrest. Phys. Rev. B 90, 045302 (2014).
[2] A. Panda and S. R. Forrest et al. Phys. Rev. B 90, 045303 (2014).

Solution processing is a scalable means of depositing large-area electronics for applications in displays, sensors, smart windows, and photovoltaics. Yet solution-based deposition routes to inorganics typically yield films with electronic quality inferior to traditional vacuum deposition – most solution precursors contain excess organic ligands, counter ions, and/or solvent that leads to porosity in the final film. We discover, study, and apply precise nanoscale inorganic cluster precursors to functional thin films. These all-inorganic nanoclusters are, in general, excellent precursors with high solubility (≥ 1 M) in water, minimal counter ions, and no organic ligands that ultimately must be removed. They also do not readily crystallize upon solvent removal (minimizing film roughness) are stable in solution, and readily crosslink-able after spin coating to yield dense films with mild heating. By tuning solution and film chemistry we have developed inks for low-temperature deposition of F:SnO₂ (a conducting oxide),¹ In-Ga-Zn-O with high electron mobilities > 30 cm² V s (for large-area thin film transistors),² and protective layers for water splitting photoelectrodes.³ By tuning composition precisely we also target amorphous mixed-metal oxides with tunable electronic density of states profiles that may have applications in energy/carrier-selective contacts to solar photovoltaic and photoelectrochemical devices.

1) Nadarajah, A.; Carnes, M. E.; Kast, M. G.; Johnson, D. W.; Boettcher, S. W. Aqueous Solution Processing of F-Doped SnO₂ Transparent Conducting Oxide Films Using a Reactive Tin(II) Hydroxide Nitrate Nanoscale Cluster. Chem. Mater. 2013, 25, 4080.
2) Nadarajah, A.; Wu, M. Z. B.; Archila, K.; Kast, M. G.; Smith, A. M.; Chiang, T. H.; Keszler, D. A.; Wager, J. F.; Boettcher, S. W. Amorphous In–Ga–Zn Oxide Semiconducting Thin Films with High Mobility from Electrochemically Generated Aqueous Nanocluster Inks. Chem. Mater. 2015, 27, 5587.
3) Kast, M. G.; Enman, L. J.; Gurnon, N. J.; Nadarajah, A.; Boettcher, S. W. Solution-deposited F:SnO₂/TiO₂ as a base stable protective layer and anti-reflective coating for micro-textured buried-junction H₂-evolving Si photocathodes. ACS Appl. Mater. Interfaces 2014, 6, 22830.

With the intensifying interest in nanoscale materials for a variety of energy applications, methods for fabricating materials with atomic-level control are becoming increasingly important. Atomic layer deposition (ALD) is a method that provides excellent capabilities for depositing thin films and other nanoscale materials, and it has been widely applied to metal oxides. For example, deposition of binary, ternary, and doped metal oxides at a range of important interfaces has already been demonstrated using ALD. However, a fundamental understanding of the growth process for metal oxides is still lacking. Moreover, it is often difficult to correlate the material properties and growth characteristics with the process parameters due to the limited understanding of the underlying surface chemistry. In this talk, we will describe the results of both in situ and ex situ studies investigating nucleation in metal oxide deposition, using zinc tin oxide (ZTO) as a model system. We will show that the growth rate of ZTO is strongly reduced as compared to the growth rates of the binary materials. Based on a combination of quadrupole mass spectrometry, infrared spectroscopy, low energy ion scattering, and density functional theory, we propose mechanisms explaining the reduction in nucleation delay that occurs when the metal oxide processes are mixed. The potential of ALD metal oxide layers in solar cell applications will also be discussed.

Zinc oxide thin films are a commonly used electron selective interlayer material in numerous photovoltaic devices due to the ease of processing and high earth abundance of Zn (low cost). The deep valence band energy of ZnO (relative to the surface vacuum level) blocks hole extraction, while the conduction band aligns with the electron transport band of many n-type semiconductors. Intrinsically ZnO is an n-type semiconductor with a large band gap but typically also contains a significant number of mid-band-gap energy states owing to native defects in the microstructure. These sites can act as recombination centers for photo-generated carriers and result in parasitic absorbances that decrease overall solar cell efficiency. Hence, there is a great demand to both understand and control the band gap of ZnO to improve electron selectivity.
MgO-doped ZnO has been shown to facilitate band gap engineering in addition to enhancing transparency while also decreasing the intrinsic carrier density for interface passivation. It is also hypothesized that MgO influences the microstructural defects in ZnO that contribute to mid-gap states, reducing the overall density of states in this energy region. In order to retain the low-cost nature of ZnO, we employ solution-based processing techniques, specifically sol-gel based, to fabricate a series of ZnO films with different concentrations of MgO (0-30 mol%). We evaluate the relative change in the carrier density using Mott-Schottky analysis, as a function of MgO doping concentration. Likewise, these measurements will be coupled with electrochemical impedance spectroscopy (EIS) to evaluate the change in the interfacial capacitance of the ZnO(Mg) films. These results will then be compared with photoelectron spectroscopy measurements (XPS and UPS) to correlate changes in carrier density with near surface composition and band tail states. Finally, we demonstrate via a combination of SEM and AFM measurements the role of MgO on the grain structure of the ZnO films. Our results present new insights into the possible contributions to charge carrier selectivity in optoelectronic platforms.

Amorphous oxide semiconductor thin film transistors (TFTs) have attracted much attention with distinguished features of high optical transparency, high mobility, good compatibility, and solution process ability compared to traditional a-Si TFT process. Solution-process fabricated TFTs are attracting increased interest due to the advantages of low cost and high throughput compared to conventional vacuum techniques. High-k gate dielectrics such as aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), hafnium oxide (HfO₂), yttrium oxide (Y₂O₃) and titanium dioxide (TiO₂) can substitute the traditional gate dielectric silicon dioxide (SiO₂) because of the higher dielectric constant and the lower leakage current in nanometer scale.
The solution-processed zinc tin oxide (ZTO) TFTs based on SiO₂ gate dielectric that are easily inclined to make oxygen vacancies do not show good device performances. Also, the leakage current of SiO₂ of nanometer scale increased dramatically owing to the tunneling effect. Thus, if a mixed system of ZrO₂ and Al₂O₃ (ZAO) is used as the gate dielectric, electrical properties of TFTs will be improved because of the good interface state and oxygen vacancy suppression in the nearby ZTO system through aluminum and zirconium. There have been reports on double or stacked semiconductors, however, almost no report was found on mixed or stacked soluble gate dielectric system so far.
We fabricated and evaluated solution-processed mixed and stacked ZAO dielectrics, and the electrical characteristics of solution-processed ZTO TFT devices are also characterized. The dielectric properties including capacitance were analyzed with different soluble dielectric configurations, aluminum and zirconium oxides. A mobility, on/off ratio, threshold voltage and sub-threshold voltage of a solution-processed ZTO TFT with ZAO dielectric having high dielectric constant (~7.1) are 33.96 cm²/Vs, 1.46 x 10⁵, 1.98 V, and 100 mV at 500^oC, respectively, at low operating voltage (5 V).

Perovskite cobaltite thin films are considerably interesting in terms of their remarkable magnetic and relevant transport properties. Fundamental understanding of the strain relaxation mechanisms of the cobaltite thin films is essential for manipulating and tuning their magnetic properties.
The presence of dark stripes in cobaltite thin films related to the strain relaxation is known to be responsible for their magnetic properties[1][2]. So far, various strain relaxation mechanisms, including the spin-state ordering of Co cations[1] and the ordering of oxygen vacancies in the film[2], have been proposed to account for the origin of dark stripes. Nevertheless, the exact process of strain relaxation in La/Sr cobaltite thin films is still under debate.
Here, we study the relation of oxygen vacancies in the cobaltite thin films. We prepared various (0
[1] Ji-Hwan Kwon et al., Chem Mater. 2014, 26, 2496-2501
[2] J. Gazquez et al., Nano Lett 2011, 11, 973-976

Over the last decade or so, wide band gap semiconductor based structures have been extensively studied for UV light emission applications. Zinc oxide is one of the materials that has seen significant interest because of the earth abundance of its precursors, its large direct band gap (3.6eV), and its favorable exciton energy (60meV). The main challenge to ZnO based UV LEDs development is the fabrication of p-type doped ZnO due to the absence of shallow acceptors. Alternative approaches include combining other p-type doped materials (e.g., Si, GaN, or NiO) with n-type ZnO or using metal-insulator-semiconductor (MIS) structures. In the latter, the interface between the insulator and the semiconductor can confine carriers for radiative recombination as long as specific conditions (optimal barrier thickness and forward voltage) are satisfied.
In this work, an Al/ZnO/NiO/Ni stack based structure was studied and it showed the expected LED behavior. Although n-ZnO based MIS structures have been explored by a number of other groups, to our knowledge, MIS structures with p-NiO have not been studied for UV light emission applications. In this work we also examined a new MIS stack consisting of Au/HfO₂/NiO/Ni for in terms of its current-voltage characteristics and light emission. HfO₂ and NiO were fabricated by reactive RF sputtering. During the HfO₂ deposition the Ar/O₂ ratio and the growth temperature was controlled to insure good insulator quality. P-type trap assisted doping of NiO was also achieved by controlling the O/Ar ratio, the film thickness and the growth temperature.
The thickness of the HfO2 layer of this MIS structure influences tunneling processes and thus affects the current through the devices and light emission. If the thickness of HfO₂ is too small, the MIS structure behaves as conductor since all the holes tunnel through the barrier. If the thickness is too large, the MIS structure behaves as an insulator because none of the holes can tunnel through the barrier even with any voltage bias. At an optimal thickness, when the energy bands bend downward, holes are accumulated at the interface under a relatively low voltage bias, and the radiative recombination of electron and holes is enhanced.
Other characterization methods used in this work include Hall-effect measurements, four-point probe resistivity measurements, EDS, XRD, as well as C-V electrical measurements. I-V characteristics showed that Au/HfO₂ and NiO/Ni form good ohmic contacts, and the MIS structure displayed good diode behavior for HfO₂ with 100nm or less. The turn on voltage for this stack is closer to ZnO based MIS LEDs. Spectroscopic ellipsometry and light emission (electroluminescence) studies on these structures are in progress and the results will be reported as well.

The talk presents several examples arising from studies into the influence of interfaces on photophysical properties, primarily from conjugated polymers. Optical modal descriptions are shown to be particularly effective at describing the various energy transfer processes between the excited states and various planar (multi-)layer structures. In the case of highly conducting layers, surface waves (e.g. plasmons in the case of metals) are particularly interesting given their ability also to exert a force on conduction electrons, thereby generating an electrical current. Simultaneous measurements of the angular spectrum (reflectivity) and the electrical current generated through exciting surface (plasmon) waves will also be presented and discussed in the context of efficiency and overall response. The induced electron drift, studied for various structures, is shown to be a highly sensitive measure of the quality of the local interface – far more so than the optical response.

Transparent electrode (TE) is a key component in many modern electronic devices. While it has been heavily dominated by indium tin oxides (ITOs), various process- or material-related constraints in next-generation devices such as flexible displays and see-through electronic devices often require an alternative TE technology to be developed. In general, transparent electrodes to be used with these emerging devices should fulfill the following requirements: (i) their sheet resistance should be as low as possible; (ii) TEs themselves should be compatible with pre- or post-processing used during the fabrication of devices of interest; (iii) their deposition should be mild enough not to damage layers underneath; and (iv) for flexible devices, TEs should also possess an excellent flexibility with sufficiently high onset strain for bending-induced crack formation. In fact, one of the major reasons to develop ITO-alternatives is because ITO forms crack at a relatively low strain (typically around 1.5% or less). In addition, TEs should be able to provide optical properties required by a given application. In many cases, high transmittance is generally desired; in case of light-emitting or photovoltaic devices, however, a certain range of reflectance (at the expense of transmittance) may be preferred for a cavity resonance effect for wide color gamut or for efficiency enhancement. Therefore, ability to tune transmittance (reflectance) in a wide range could be highly useful to cope with desired specifications of various applications.
In this talk, oxide-metal-oxide (OMO) or dielectric-metal-dielectric (DMD) layers are introduced as a versatile TE that can fulfill all the requirements mentioned above and, moreover, can easily tune its transmittance/ reflectance for the required optical characteristics. Highly flexible organic light-emitting diodes (OLEDs), transparent thin-film transistors, efficient see-through solar cells are introduced as representative examples. Subtle but significant effect of the refractive index of oxide layers is also discussed to reveal its versatile potential.

With the application of metal oxide contacts to high efficiency silicon solar cells and continued interest in perovskite absorbers (utilizing selective contact device architectures) there is increased need for fundamental research into next generation selective contact material systems. A designer mixed metal oxide consisting of a matrix material with proven passivation properties, Al₂O₃, mixed with a small band gap transition metal of choice with d-states near the valence band or conduction band of common absorbers is one such material system. Critical to the integration of such a material is understanding its fabrication, physical and electronic structure.
Transition metal (V, Cr, Mn, Fe, Co, Ni, Cu, Zn) aluminum oxide alloys and nano-composites have been fabricated via solution deposition from aqueous precursors. The identity of the transition metal appears to effect the critical concentration of transition metal which can be added to the film before transition metal oxide nanoparticles phase segregate form the alumina matrix. Trends in the critical concentrations for transition metals have been correlated to the glass forming capability of the transition metal. Ligand field stabilization energies are also shown to contribute a second order effect which allows for the very high critical concentration of iron in an amorphous Fe_xAl_(1-x)O_1.5 alloy and the seemingly total phase segregation (low critical concentration) of crystalline CuO from amorphous Al₂O₃.
Furthermore, the effect of the matrix on the structure of the film as a function of transition metal concentration has been investigated. We show that as the glass forming capability of the matrix (e.g. P₂O₅) is increased the critical concentration for a given transition metal is also increased. This solidifies that the weighted average glass forming ability between the d0 matrix element and the transition metal is a good indicator of whether a mixed film will form an amorphous alloy or a transition metal nanoparticle amorphous-dielectric composite.
Kinetic factors in the processing of the thin films were investigated to determine if there were other explanations for which transition metals readily phase segregated. It was determined that differences in solubility did not track with the critical concentration for different transition metals but that the dopant precursor (metal nitrate solutions) decomposition temperature relative to that of Al(NO₃)₃ correlated with decent agreement to the transition metals that phase segregated. Thus a combined processing-kinetic and thermodynamic glass forming argument can explain the trends observed.
Finally the electronic structure of transition metal aluminum oxide amorphous alloys has been investigated. Filled d-states within the bandgap of alumina have been observed in Fe_xAl_(1-x)O_1.5 films with UV and X-Ray photoelectron spectroscopies.

There is considerable interest in titanium oxide/dioxide (TiO_X/TiO₂) based electron-selective, hole-blocking heterojunctions due to their applications in organic ¹ and inorganic photovoltaics ², and organic light-emitting diodes. In this work, a low temperature (< 100 °C) chemical vapor deposition (CVD) technique is used to deposit ultra-thin (n-type) TiO₂ layers onto hydrogen-passivated surfaces of crystalline silicon (c-Si). Energy level alignment and chemical composition at these abrupt, interfacial layer-free TiO₂/Si heterojunctions are investigated via ultra-violet, X-ray and inverse photoemission spectroscopy (UPS, XPS, IPES), for c-Si doping ranging from p⁺⁺(10¹⁹) to n⁺⁺(10¹⁹). The interface Fermi level position and device-relevant TiO₂/Si band offsets are found to shift monotonically as a function of the Si doping, revealing the absence of Fermi level pinning at the c-Si interface and pointing to simple Fermi level equilibration as the driving mechanism behind the interface energy level alignment. Electrical transport measurements performed on TiO₂/Si-based diodes confirm the energy level alignment yielded by spectroscopic measurements and the hole-blocking properties of the TiO₂/Si heterojunction, exclude hole conduction in the TiO₂ as a transport mechanism, and show carrier recombination at the TiO₂/p-Si heterojunction.

1. Kim, H. et al. Investigation of ultra-thin titania films as hole-blocking contacts for organic photovoltaics. J. Mater. Chem. A 18–28 (2015).
2. Nagamatsu, K. A. et al. Titanium dioxide/silicon hole-blocking selective contact to enable double-heterojunction crystalline silicon-based solar cell. Appl. Phys. Lett. 106, 123906 (2015).

The capability to dynamically modify the properties of interfaces between organic and inorganic electronic materials has the potential to provide access to a wide range of new phenomena with both fundamental and practical importance. Monolayers that are dynamically reconfigurable through the use of external stimuli would enable precise control over charge transfer dynamics, electronic energy levels, dipole moments or polarizations, and vibrational phenomena interfaces. Mechanisms through which these properties can be modified include molecules with multiple stable configurations and dynamic manipulation of the local chemical environment. The development of precise time-resolved structural x-ray probes now facilitates not only the characterization of the static structure of interfaces but also control of the dynamic structure and electronic properties using external stimuli.

Azobenzene compounds can be reversibly driven through a photoisomerization process by optical illumination. Electronic and optical signatures of isomerization phenomena demonstrate the electronic importance of the change in conformation but do not provide insight into how the structure of the molecular layer changes during illumination, or over what timescale the changes occur. Time-resolved x-ray reflectivity measurements were performed at the Advanced Photon Source on organic monolayers of azobenzene containing molecules deposited on silicon and quartz by Langmuir-Blodgett deposition. The molecular monolayers consisted of a carboxylic acid group attaching the molecule to the substrate, an azobenzene group and Re-complex group with the electron rich Re-complex providing strong contrast in x-ray reflectivity. The packing density of the molecular monolayer varies on the length scale of 200 mm within an individual film and depends on the substrate and deposition conditions. Time-dependent structural changes are observed as a result external applying optical stimuli to photoisomerize the azobenzene groups within the monolayer. The dynamics depend on the incident optical intensity and the total number of photons delivered to the sample on the timescale of seconds to hundreds of seconds. The results of this x-ray reflectivity study demonstrate the importance of understanding the dynamic structural contribution to the interfacial properties. Additionally, the results underscore the importance of understanding and controlling the heterogeneity in these interfacial systems, as for example in field effect transistors incorporating an azobenzene monolayer [1].

[1] P. Paoprasert, B. Park, H. Kim, P. Colavita, R.J. Hamers, P.G. Evans, et al., Dipolar Chromophore Functional Layers in Organic Field Effect Transistors, Adv. Mater. 20 (2008) 4180–4184.

The microphysics of many localized inhomogeneous phenomena, especially in transition metal oxides, like memristive switching, Mott-Peierls transitions and interfacial charge storage, has been elusive to complete understanding because of the small scale, interfacial localization, and subtle physical changes involved. Here we developed a soft x-ray technique to enhance the weak and localized elemental, chemical and electronic changes in the oxide with spatial and spectral resolutions of <30 nm and 70 meV, along with a tunable temporal resolution. This technique works best on devices that exhibit multiple states accessible by an electrical stimuli, like bistable resistive switching in memristors.
We used a synchrotron-based soft x-ray (200 – 2000 eV) absorption spectromicroscopy technique and built a system around it that combined the following attributes: (1) in-operando switching of oxide states (e.g.: resistance), (2) synchronously gating the x-ray detector to record signals from distinct states, (3) repeated electrical switching (at speeds ~1-100 kHz) over which the signals are accumulated and averaged, (4) x-rays are measured only after desired state is reached (e.g.: threshold resistance level), (5) adaptive control circuit corrects for drift in device behavior over time.
We employed this technique on functioning tantalum oxide memristors with platinum electrodes to study the local changes during its operation. We found that the technique described above provides remarkable advantages in the following ways: (1) spatial drift between reading multiple states is reduced from >minutes to ~µs-ms (5-7 orders of magnitude improvement), hence allowing for the maximum spatial resolution, (2) background changes over time are negligible when comparing simultaneously acquired signals, (3) repeated switching averages out stochastic effects, (4) very small signals (~1%) over a large background can be distinguished, (5) drift in device behavior with time is corrected by the control circuit. However, one of the key requirements of this technique is the ability of the device to undergo thousands to millions of operational switching events. We were able to distinguish changes to the material during application of different electrical stimuli, namely electric field and joule heating, and also distinguish changes in the material originating from these stimuli.
As an example of the working of this technique, using O K-edge, we describe a direct observation of nanoscale atomic oxygen migration at the interface of tantalum oxide in micrometer sized memristors during electrical operation. We show how the technique was able to isolate local elemental, chemical and electronic information that were inaccessible without using the synchronous in-operando technique described here.
Reference: Journal of Applied Physics 118, 034502 (2015)

Tungsten trioxide (WO₃) is an n-type metal oxide semiconductor well-investigated for applications in electrochromism, sensing, photo-catalysis and photo-electrochemistry [1-3]. Such applications involve the modulation of the charge carrier density in the metal oxide. The study of the charge carrier density can be carried out using different approaches, e.g. electrolyte-gating or illumination with suitable light source (e.g. WO₃ has a bandgap of ca 2.5 eV). In the former case, the material can be doped by mechanisms such as electrostatic or electrochemical through upon application of a gate electrical bias [4]. In the latter case, the density of the charge carrier is changed by photo-generation and is controlled by factors such as the light power density. It is possible to conceive a synergy among electrolyte-gating and illumination, to modulate the charge carrier density. From the technological point of view, this synergy could lead to further lowering the operation voltage of electrolyte-gated transistors. For operation under illumination conditions, an electrical bias applied to the gate electrode can increase the lifetime of the photo-excited charge carriers in the transistor photo-conducting channel [5].
Here, we report an extended optical absorption spectroscopy, cyclic voltammetry (CV) and transistor investigation (under dark and illumination conditions) for electrolyte-gated photo-transistors based on sol-gel synthesized WO₃ thin films thermally treated at different temperatures (from 250 °C, compatible to plastic substrates to 550 °C). Absorption spectra show that WO₃ films are mainly sensitive to the 400-500 nm portion of visible light. CV measurements were also conducted to determine the device operation voltage and charge carrier density accumulated. An I_ON/I_OFF of ca. 5×10³ and photo-responsivity of ca. 4.5 A/W were extracted from transistor measurements under 1 kW/m² illumination.
The combined electrical and photo response of WO₃ thin films in an electrolyte-gated transistor configuration will permit to shed light on the working principle of electrolyte-gated transistors and photo-transistors, in particular the photo-responsivity when the transistor is ON. In turn the dual optical/electrical effect of such transparent transistors (90% transmittance at 600 nm) can be relevant for numerous applications, such as active matrix displays and photo-detecting devices.
1. Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201−262.
2. Deb, S. K. Sol.Energy Mater. Sol. Cells 2008, 92, 245−258.
3. Baeck, S. H., et al. Adv. Mater. 2003, 15, 1269−1273.
4. Meng, X., et al. J. Phys. Chem. C 2015, 119, 21732–21738.
5. Lhuillier, E., et al. Nano letters 2014, 14, 2715-2719.

Copper(I) thiocyanate (CuSCN) has recently been reported as a promising hole-transporting material due to its unique physical properties including high optical transparency, appropriate energy band levels, high hole mobility (0.01 to 0.1 cm² V^-1 s^-1), and excellent solution-processability. Following on our recent studies on p-channel thin-film transistors (TFTs), organic photovoltaics (OPVs), and organic light-emitting diodes (OLEDs) employing CuSCN as the hole-transporting layer, we report on a comprehensive study of the electronic properties of CuSCN with particular emphasis on the hole transport processes. Metal-insulator-semiconductor (MIS) capacitors were employed to characterize important parameters, i.e., dielectric constant (5.1 ±1.0), flat-band voltage (–0.7 ±0.1 V), and unintentional hole doping concentration (7.2 ±1.4×10¹⁷ cm^-3). Further, the analysis of field-effect measurements of CuSCN-based TFTs yields insight into the localized hole density of states in the mobility gap which can be approximated with an exponential function with a characteristic energy of 42.4 (±0.1) meV. The temperature-dependence of the field-effect hole mobility also reveals three different transport regimes at different temperatures. The first regime at high temperatures (303–228 K) is ascribed to the multiple trapping and release mechanism while the second regime at intermediate temperatures (228–123 K) is described with the variable range hopping process. The third regime at low temperatures (123–78 K) is weakly temperature-dependent and attributed to a field-assisted hopping process. The different hole transport mechanisms can be reconciled with the transport energy concept, and the transition from one process to another is discussed based on the temperature dependence of the transport energy. Also, as the field-effect measurements are sensitive to the dielectric/semiconductor interface, two different polymeric dielectrics, i.e., one low-k and one high-k material, were employed in this study to ascertain that the observed phenomena are due to the intrinsic properties of CuSCN. The localized hole density of states and transport processes are found to be similar in both dielectric/CuSCN systems; however, there is a discrepancy in the mobility values, which is speculated to be a result of the Fermi level pinning at the semiconductor/dielectric interface. Due to the promising potential of CuSCN as a wide band gap p-type semiconductor for transparent microelectronics and as a candidate to replace the commonly used hole-transporting/extracting polymer poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) in OPVs and OLEDs, this work timely provides the foundation for further studies to gain deeper understanding of CuSCN as well as for further technological advancements.

We present a review of our recent work on using functional polymers, particularly ferroelectric polymers, for flexible electronic memory applications. Devices based on thin film structures and blends have been fabricated. In one case, a ferroelectric diode device based on phase separated blend films of PCBM and P(VDF-TrFE) will be described. This unique memory device relies on ferroelectric polarization to change barrier height at the active material-electrode interface, and can also be used with conducting polymer electrodes. The device shows bi-stable resistive switching operation at low voltage. In addition, we show that blends of P(VDF-TrFE) and selected insulating polymers result in improving the reliability of the P(VDF-TrFE) based flexible memories. Both polarization fatigue, retention, and dielectric breakdown performance is improved. Hybrid devices in which p-type oxide semiconductors (SnO) are combined with P(VDF-TrFE) are shown to give the largest reported mobility of ferroelectric field effect transistors with p-type channel layer. Multistate memory devices are demonstrated using the hybrid ferroelectric/oxide structure with excellent memory window and retention characteristics. Finally, a dual-transistor ferroelectric field effect device is demonstrated in which identical transistors are used for both access and memory transistors. The function of the transistor (access or memory) is tuned by operating the transistor below or above the coercive voltage of the ferroelectric layer.

We present an overview of the structural evolution of silicon nanocrystals (Si-nc’s), formed from a supersaturated SiO_x matrix prepared by Si⁺ implantation of SiO₂ and rapid thermal processing. Probing the early stage formation using electron microscopy correlated with Raman scattering, we reveal a size dependent Si-nc surface energy, which is partly responsible for driving their growth. Red shifted photoluminescence (PL) spectra and an increased PL lifetime with Si-nc size, results from charge de-localisation.
We will examine the effects of co-doping with erbium (Er), in particular the effect of the Si-nc size on the Er³⁺ intra-4f related PL at the technologically important wavelength of 1534nm. The Er PL transients reveal a multi-exponential character indicative of the local environment of the emitting centres. Detailed analysis reveals two distinct classes of luminescent Er; one exhibiting a relatively short lifetime (few ms) and the second, exhibiting a much longer lifetime (between 10 and 15ms). The latter is characteristic of that of Er in stoichiometric SiO₂, i.e. far from any Si-nc’s, whilst the former may be attributed to a Purcell-like enhancement of the radiative rate induced by local changes in the refractive index for Er ions close to a spherical dielectric interface. These results have implications for the design of future Er-doped fiber (and waveguide) based optical amplifiers.
Finally, we will discuss the optoelectronic properties of phosphorus (P) doped Si-nc’s. Whilst it is known that the PL is quenched with increasing P concentration, the mechanism is not entirely understood. Our studies suggest that this is due to an efficient Auger assisted non-radiative recombination process where the energy of photo-excited carriers is released by electron collisions with activated P donors in Si-nc’s. We present our most recent results in which the effects of this Auger mechanism are directly probed using an electrical frequency response analysis.

High-performanced solar-blind (200-280 nm) avalanche photodetectors (APDs) were fabricated based on highly-crystallized ZnO-Ga₂O₃ core-shell microwires. The responsivity can reach up to 1.3×10³ A/W under -6 V. Besides, the corresponding detectivity was as high as 9.91×10¹⁴ cm Hz^1/2/W. The device also had a fast response with the rise time shorter than 20 μs and the decay time of 42 μs. The quality of the detectors can be comparable or even higher than the commercial Si APD (APD120A2 in Thorlabs company) in solar-blind waveband (respectively as ~8 A/W, ~10¹² cm Hz^1/2/W, ~20 ns). The high performance of this APD make it highly suitable for practical applications as solar-blind photodetection, and this core-shell microstructure heterojunction design method would also provide new approach to realize the APD device.

Alloying Mg with ZnO has been described as a way toward engineering the bandgap of MgZnO to as high as a 5.8 eV for various electronic and optoelectronic applications, including solar blind detectors for communication systems. Mg-ZnO alloy systems have also shown to phase segregate for large Mg content. These factors, enhanced optical combined have made nanostructred MgZnO a promising solution for achieving the stated goals of such alloyed systems. In this study, MgZnO nanowires were synthesized via a high temperature vapor transport technique, in which Mg was incorporated by the tandem disassociation of Mg₃N₂ powder, along with ZnO powder in the same reaction tube furnace. These structures were grown on a Si substrate with a thin, sputtered ZnO seed to catalyze growth. The resulting nanowires were highly ordered and tended toward vertical orientation [1,2]. The nanowire arrays were then subject to a combination of annealing conditions, varying temperature and ambient gas environments. Scanning electron microscopy, as well as EDX mapping confirmed the presence of Mg clustered formations along the ZnO nanowires. Photoluminescence measurements revealed the presence of a small tail peak at a higher energy than the band edge of ZnO. Additionally, it was found that the peaks consistent with the 1LO and 2LO (longitudinal optical) phonon modes had blue-shifted from approximately 585 cm-1 to 605 cm-1 averaged across several positions among the samples. These results indicate a greater composition and better registration of Mg in the ZnO nanowire lattice, as compared to the unnannealed case [3]. Additional electrical and optoelectronic measurements were conducted, and enhanced conduction in the presence of UV light was confirmed. The relative photodetection metrics (responsivity, photoconductive gain, detectivity, noise equivilant product) were also compared in all cases.
References
[1]H. Yu, E. A. Azhar, T. Belagodu, S. Lim, and S. Dey, “ZnO nanowire based visible-transparent ultraviolet detectors on polymer substrates,” Journal of Applied Physics, vol. 111, no. 10, p. 102806, May 2012.
[2]T. Belagodu, E. A. Azhar, and H. Yu, “Modulation of charge conduction in ZnO nanowires through selective surface molecular functionalization,” Nanoscale, vol. 4, no. 23, pp. 7330–7333, Nov. 2012.
[3]J. Huso, H. Che, D. Thapa, A. Canul, M. D. McCluskey, and L. Bergman, “Phonon dynamics and Urbach energy studies of MgZnO alloys,” Journal of Applied Physics, vol. 117, no. 12, p. 125702, Mar. 2015.

In the past few decades, gate insulators with a high dielectric constant (high-k dielectric) enabling a physically thick but dielectrically thin insulating layer, have been used to replace traditional SiO_x insulator and to ensure continuous downscaling of Si-based transistor technology. However, due to the non-silicon derivative natures of the high-k metal oxides, transport properties in these dielectrics are still limited by various structural defects on the hetero-interfaces and inside the dielectrics. Here, we show that another insulating silicone compound, amorphous silicon nitride (a-Si₃N₄), is a promising candidate effective electrical insulator for use as a high-k dielectric. We have examined a-Si₃N₄ deposited using the plasma-assisted atomic beam deposition (PA-ABD) technique in an ultra-high vacuum (UHV) environment and demonstrated the absence of defect-related luminescence; it was also found that the electronic structure across the a-Si₃N₄/Si heterojunction approaches the intrinsic limit, which exhibits large band gap energy and valence band offset. We demonstrate that charge transport properties in the metal/a-Si₃N₄/Si (MNS) structures approach defect-free limits with a large breakdown field and a low leakage current. Our results obtained using PA-ABD suggest a general strategy to markedly improve the performance of Si devices with a nearly defect-free dielectric.

High performance inverted bulk heterojunction organic solar cells (OSCs), based on the blend of PTB7: PC₇₀BM, were achieved using an aluminum-doped zinc oxide (AZO) front transparent cathode. A structurally identical PTB7:PC₇₀BM-based OSC having an indium tin oxide (ITO) front cathode was also made for comparison studies. The surface of AZO and ITO was modified with a 10 nm thick solution-processed ZnO interlayer to facilitate the efficient electron extraction. This work yielded AZO-based OSCs with a promising power conversion efficiency of 6.15%, slightly lower than 6.57% of a control ITO-based OSC, however, a significant enhancement in the stability of AZO-based OSCs was observed under an ultraviolet (UV)-assisted acceleration aging test. The distinctive enhancement in the lifetime of AZO-based OSCs arises from the tailored absorption of AZO electrode in wavelength < 380 nm, serving as a UV filter to inhibit an inevitable degradation in ITO-based OSCs caused by the UV exposure.

Organic-inorganic perovskites are unique ferroelectrically semiconducting multifunctional materials. These perovskite materials have shown very promising prospective in sensing, renewable-energy, and lasing applications. This presentation reports the experimental studies on the fundamental mechanisms in controlling magneto-optic, photovoltaic, and thermoelectric functions. In magneto-optic properties we discovered that the electron-hole pairs are formed with spin-antiparallel and spin-parallel states within band structures under photoexcitation. In particular, the electron-hole pairs inevitably experience spin-mixing and spin-conserving behaviors between spin-antiparallel and spin-parallel states, leading to spin-dependent excited states. Therefore, using electron-hole pairs presents a unique mechanism to generate magneto-optic effects in excited states in organic-inorganic perovskites. In photovoltaic actions, we found by using magneto-photocurrent measurements that the spin-antiparallel and spin-parallel electron-hole pairs give high and low dissociation rates in the generation of photocurrent. This provides a new mechanism to enhance the photovoltaic efficiencies by controlling spins. Furthermore, we showed that the mutual interaction between electron-hole pairs can largely change the electron-hole binding energies during the charge dissociation. In addition, we found that the bulk polarization and electrode-interface polarization are mutually coupled through charge-drifting field, co-operatively controlling photovoltaic actions in perovskite solar cells. In thermoelectric properties we found that the surface dipoles can function as a new driving force to generate huge Seebeck effects in organic-inorganic perovskites. More importantly, our Seebeck studies have shown that the surface dipoles can change bulk semiconducting properties between n-type and p-type. This provides a new approach to tune the bulk semiconducting properties between N-type and P-type by using surface dipoles. In summary, this presentation intends to present the recent experimental progress on controlling the magneto-optic, photovoltaic, and thermoelectric properties in ferroelectrically semiconducting organic-inorganic perovskites.

Hybrid perovskites represent a new, disruptive, technology in the field of optoelectronics. They have the potential to overcome the performance limits of current technologies and achieving low cost and high integrability. Hybrid halide perovskite, e.g. CH₃NH₃PbX₃ [X = Cl, Br, or I], are usually deposited as polycrystalline thin-films with variable mesoscale morphology depending on the growth conditions. The obtained grain size ranges from tens to thousands of nm. Over the last two years the impressive improvement of photovoltaic performance has been driven by radical empirical evolution of the device architecture and processing methodologies. However, there is a considerable lack of understanding of material properties, both as pristine films and their embodiment in a device.
First, the seminar will focus on the understanding of the relationship between structure and optoelectronic properties of such complex materials, where the presence of various types of interactions and structural disorder play an important role. We will show that the electron-hole interaction is sensitive to the microstructure of the material. We find that by control of the material processing during fabrication, and of the local electric filed during thin film polarization, both free carrier and Wannier excitonic regimes are accessible, with strong implications for optoelectronic devices^1,2 In addition, we find that it is also possible to design the emissive properties for a single material composition by designing the processing routs³. Then, the role of interface engineering⁴, the effect of ion migration and interface doping^5,6 on charge extraction at metal-oxide and fully organic charge extractin layers will be elucidated to provide a guideline for the design of hysteresis free perovskites based hybrid solar cells.

[1] G. Grancini et al, The Journal of Physical Chemistry Letters, 5, 3836, 2014
[2] G. Grancini et al, Nature photonics 9 (10), 695-701, 2015
[3] V. D’Innocenzo et al, Journal of the American Chemical Society, 136, 17730, 2014
[4] Chen Tao et al, Energy & Environmental Science 8 (8), 2365-2370, 2015
[5] T Leijtens et al, Advanced Energy Materials, 2015, DOI: 10.1002/aenm.201500962
[6] M. De Bastiani et al, Advanced Energy Materials, 2015, in press

Nickel oxide (NiO) is a p-type semiconductor of high work function of 5.4 eV, which is close to the valence band edge level of CH₃NH₃PbI₃ perovskite (5.4 eV). The alignment of energy levels between NiO electrode interlayer with CH₃NH₃PbI₃ perovskite minimizes the interfacial energy losses for the hole transfer and optimizes the photovoltage output of device. Our previous work had reported the application of NiO_x as the ideal p-type electrode interlayer to fabricate the decent perovskite-based photovoltaics.^1-3 The experimental data showed that due to the improved wetting and better energy level alignment between light absorber (CH₃NH₃PbI₃ perovskite) and NiO_x, the photovoltage is significantly enhanced together with the overall photovoltaic parameters.^1-3
In this study, we applied the different approaches to prepare the p-type electrode interlayer, such as the solution-based,^1-2 sputtering,³ and nickel-oxidized⁴ process to simplify the fabrication and optimize the device performance. The efficient hole transfer at perovskite/NiO heterojunction was verified by photo-induced absorption spectroscopy, showing a broad spectral feature above 800 nm, the long-lived charge-separation state of NiO⁺/P^-.² The application of p-type metal oxide material has the advantages of providing robust device and the development of fully inorganic perovskite-based thin film solar cells. We believe the rapid significant advances in the development of perovskite-based solar cells in recent years will strengthen confidence in this promising technology.⁵
References
1. J. –Y. Jeng, K. –C. Chen, T. –Y. Chiang, P. –Y. Lin, T. –D. Tsai, Y. –C. Chang, T. –F. Guo, and P. Chen, Adv. Mater. 26, 4107 (2014).
2. K. –C. Wang, J. –Y. Jeng, P. –S. Shen, Y. –C. Chang, E. W. –G. Diau, C. –H. Tsai, T. Y. Chao, H. –C. Hsu, P. –Y. Lin, P. Chen, T. –F. Guo, and T. –C. Wen, Sci. Rep. 4, 4765 (2014).
3. K. –C. Wang, P. –S. Shen, M. –H. Li, S. Chen, M. –W. Lin, P. Chen, and T. –F. Guo, ACS Appl. Mater. Inter. 6, 11851 (2014).
4. W. –C. Lai, K. –W. Lin, T. –F. Guo, ad J. Lee, IEEE Trans. Electron Devices 62, 1590 (2015).
5. M. –H. Li, P. –S. Shen, K. –C. Wang, T. –F. Guo, and P. Chen, J. Mater. Chem. A 3, 9011 (2015).

Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 20%. We examine interfacial energetics of these materials with a set of candidates for alternative transport layer configurations including oxide as well as oxide-modified with organics and carbon nanotube systems by performing photoemission spectroscopy. Data resulting from studies of these systems permit identification of charge transfer mechanisms across the interface with chemical specificity. Our findings from this method are combined with transient absorption, time resolved microwave conductivity, cross-sectional scanning kelvin probe microscopy and user-facility based photoemission experiments to provide improved clarity regarding carrier dynamics at these interfaces of technological relevance. The implication of these results to the efficiency and stability of perovskite-based solar cells will be discussed along with device level data. Relationships between these carrier dynamics to both structural and compositional changes in the bulk materials perovskite material as elucidated by x-ray scattering and high-resolution TEM studies will also be presented.

Homoepitaxial growth of highly ordered and pure layers of rutile on rutile crystal substrates and anatase on anatase crystal substrates using atomic layer deposition (ALD) will be presented. The epilayers grow in a layer-by-layer fashion at low deposition temperatures but are still not well ordered on rutile. Subsequent annealing at higher temperatures produces highly ordered, terraced rutile surfaces that in many cases have fewer electrically active defects than the substrate crystal. The anatase epitaxial layers, grown at 250°C, have much fewer electrically active defects than the rather impure bulk natural crystals. Annealing the epilayers at higher temperatures increased band gap photocurrents in both anatase and rutile. ALD was also used to grow epitaxial layers of anatase (001) TiO₂ on the surface of SrTiO₃ (100) crystals with a 3% lattice mismatch. The epilayers grow as anatase (001) as confirmed by X-ray diffraction (XRD). Atomic force microscope images of deposited films showed epitaxial layer-by-layer growth up to about 10 nm whereas thicker films, of up to 32 nm, revealed the formation of 2-5 nm anatase nanocrystallites oriented in the (001) direction. The anatase epilayers were used as substrates for dye sensitization. The as received strontium titanate crystal were not sensitized with a ruthenium-based dye (N3) or a thiacyanine dye (G15), however photocurrent from excited state electron injection from these dyes was observed when adsorbed on the anatase epilayers. These results show that highly ordered anatase surfaces can be grown on an easily obtained substrate crystal.

Exposure to volatile organic compounds can lead to asphyxiation, pneumonia like conditions, comas, seizures and irreversible lung, kidney and central nervous system damage. Volatile organics are additionally extremely flammable and explosive, making their early detection in the immediate environment increasingly important. Metal oxide semiconductor (MOS) gas sensors present a potential technology to detect such gases.

Metal oxide semiconducting (MOS) gas sensors represent a cheap, robust and sensitive technology for detecting volatile organic compounds. MOS sensors have consistently been shown to lack sensitivity to a broad range on analytes, leading to false positive errors. In this study an array of five vanadium pentoxide thick-film sensors were produced. These were modified by incorporating a range of zeolites, namely β, Y, mordenite and ZSM5, into the bulk sensor material. Sensors were exposed to common reducing gases, namely acetone, ammonia, ethanol and toluene, at conecebtrations of 5-100 ppm at operating temperatures between 250°C and 350°C. A machine learning technique was applied to differentiate between the different gases. All sensors produced strong resistive responses (increases in resistance), with zeolite modified sensors producing stronger responses than the unmodified V₂O₅. This enhancement is a result of changes to the surface area, specific catalytic activity and effects on charge carriers in the materials. A support vector machine (SVM) was able to classify the data to a high degree of accuracy.

This talk will describe the use of cadmium sulfide quantum dots (CdS QDs) as visible-light photocatalysts for the reduction of nitrobenzene to aniline through six sequential photoinduced, proton-coupled electron transfers. At pH = 3.6-4.3 the internal quantum yield of photons-to-reducing electrons is 37.1% over 54 hours of illumination, with no apparent decrease in catalyst activity. Monitoring of the QD exciton by transient absorption reveals that, for each step in the catalytic cycle, the sacrificial reductant, 3-mercaptopropionic acid, scavenges the excitonic hole in ~5 ps to form QD^●–; electron transfer to nitrobenzene or the intermediates nitrosobenzene and phenylhydroxylamine then occurs on the nanosecond timescale. The rate constants for the single-electron transfer reactions are correlated with the driving forces for the corresponding proton-coupled electron transfers. This result suggests that electron transfer, not proton transfer is rate-limiting for these reactions. Nuclear magnetic resonance analysis of the QD-molecule systems shows that the photoproduct aniline, left unprotonated, serves as a poison for the QD catalyst by adsorbing to its surface. Performing the reaction at an acidic pH not only encourages aniline to desorb, but also increases the probability of protonated intermediates; the latter effect probably ensures that recruitment of protons is not rate-limiting.

Metal-oxide thin-films have attracted a lot of attention in the past years due to their ability to function as interfacial layers, performing efficient charge exchange with a variety of materials in novel electronics and energy applications. In the work presented here, dramatic effects in the optoelectronic properties of as-deposited molybdenum oxide thin-films by means of applying different oxygen partial pressures (1.00x10^-3, 1.37x10^-3, 1.98x10^-3, 2.28x10^-3 and 2.70 x10^-3 mbar) and different sputtering powers using DC reactive sputtering are reported.
Thin-films deposited under low oxygen partial pressure (1.00x10^-3 mbar) at high sputtering powers (250W) present the highest conductivity of around 3.22 S^.cm^-1, indicating that although the [O]/[Mo] ratio is low (~2.57 as extracted from Rutherford Backscattering Spectrometry), a semiconductor characteristic is still present in the films. As the oxygen partial pressure increases by only ~4.00 x10^-4 mbar, the conductivity of the resulting films drops by around 5 orders of magnitude (to 1.60x10^-5 S.cm^-1) having an [O]/[Mo] ratio of around 3.00. As the oxygen partial pressure is further increased up to 2.70 x10^-3 mbar, the conductivity drops further and reaches values of insulating materials of around 4.00x10^-10 S.cm^-1 with an [O]/[Mo] ratio greater than 3.00. Optical absorption measurements show drastic changes mostly within 0.60 eV - 2.50 eV region as the oxygen concentration in the films is steeply increased. Modification of the d-band occupancy is discussed and the observed effects are attributed to changes in the electron density at the defect band, i.e. as the oxygen partial pressure increases, electrons are released and empty out the defect band providing substantial changes to the optoelectronic properties of the films. High-resolution transmission electron microscopy (HRTEM) confirms an amorphous structure independently of the oxygen partial pressure within the investigated range [1].
Photoconductivity experiments within the 0.6 eV - 2.50 eV region of the MoO_x films confirm changes in the responsivity as a function of the oxygen partial pressure, namely the photo-responsivity peak matches the optical absorption of the defect band region of the films as well as it enhances over a broader spectral range in the case of the sub-stoichiometric films (MoO_2.57). In this work, additional information about the composition of the MoOx films at the nanoscale near the interface is obtained via energy electron loss spectroscopy (EELS), in order to get further insight into the interface properties of the films for specific devices. This work thus demonstrates an interesting and viable method for tuning the optoelectronic properties of MoO_x thin-films, which can be applied in combination with a wide range of materials e.g. in novel flexible electronics, photovoltaics, photo-detectors and more.
[1] A.L. Fernandes Cauduro, et al., Appl. Phys. Lett. 106, 202101 (2015).

The control of carrier extraction/injection from/to active layers is key to the optimization of all opto-electronic devices. These phenomena are dependent on bulk properties of both the active and contacting materials, but additionally depend on a third related parameter: properties of the formed interface. For the particular case of solar cells, optimum performances require that carrier extraction occurs at maximum energetic quasi-Fermi level splitting; this implies that the photogenerated electrons/holes are separated in the absorber and find a way out each into distinctive contacts, without recombining with each other. The contacts must therefore allow passage of one carrier type only—have a high conductivity and appropriate band alignment to the absorber— and prevent recombination at the interface—low interfacial mid-gap defect density.
In this contribution, we investigate the interface between crystalline silicon and oxygen-deficient molybdenum oxide (MoO_x, x