Surface Passivation Treatment to Improve Performance and Stability of Solution‐Processed Metal Oxide Transistors for Hybrid Complementary Circuits on Polymer Substrates

Funder: Engineering and Physical Sciences Research Council; Id: http://dx.doi.org/10.13039/501100000266Funder: (EPSRC)Abstract: Hybrid integration of n‐type oxide with p‐type polymer transistors is an attractive approach for realizing high performance complementary circuits on flexible substrates. However, the stability of solution‐processed oxide transistors is limiting the lifetime and reliability of such circuits. Oxygen vacancies are the main defect degrading metal oxide transistor performance when ambient oxygen adsorbs onto metal oxide films. Here, an effective surface passivation treatment based on negative oxygen ion exposure combined with UV light is demonstrated, that is able to significantly reduce surface oxygen vacancy concentration and improve the field effect mobility to values up to 41 cm2 V−1 s−1 with high on–off current ratio of 108. The treatment also reduces the threshold voltage shift after 2 days in air from 5 to 0.07 V. The improved stability of the oxide transistors also improves the lifetime of hybrid complementary circuits and stable operation of complementary, analog amplifiers is confirmed for 60 days in air. The suggested approach is facile and can be widely applicable for flexible electronics using low‐temperature solution‐processed metal oxide semiconductors

A facile low temperature route to deposit a TiO2_2 scattering layer for efficient dye-sensitized solar cells

Herein, we demonstrate a facile low temperature chemical bath deposition approach to deposit a light scattering layer on a nanostructured mesoporous TiO$_2$ bottom layer in a dye-sensitized solar cell architecture. Large TiO$_2$ nanoparticles were formed on the top surface of photoanode electrodes $\textit{via}$ hydrolysis of TiCl$_4$ at 70 $^{\circ}$C. We controlled the size and agglomeration of these TiO$_2$ nanoparticles by altering the concentration of TiCl$_4$ in the chemical bath during the hydrolysis process. Electron microscope images revealed that mono-dispersed scattering particles having uneven surfaces with diameters between 100 to 300 nm formed on the mesoporous titania layer. The scattering behavior of the formed titania overlayer was confirmed by the improved reflectance in the diffuse reflectance spectrum of the films. We also observed a significant improvement in the density of states near the band-edge of titania for the TiCl$_4$ treated electrodes as well as a considerable decline in the sub-band gap absorption states. Consequentially, enhancement in the photovoltaic parameters of TiCl$_4$ treated based solar cells is achieved which led to a power conversion efficiency of 8.54% for the cell having an optimum content of large titania particles on the top surface compared to 7.10% for the pristine titania based solar cell.Nava Technology Limited, Iran Nanotechnology Initiative Council, Nyak Technology Limited, Engineering and Physical Sciences Research CouncilThis is the author accepted manuscript. The final version is available from Royal Society of Chemistry via http://dx.doi.org/10.1039/C6RA13273

Reversible Removal of Intermixed Shallow States by Light Soaking in Multication Mixed Halide Perovskite Films.

The highest reported efficiencies of metal halide perovskite (MHP) solar cells are all based on mixed perovskites, such as (FA,MA,Cs)Pb(I1-x Br x )3. Despite demonstrated structural changes induced by light soaking, it is unclear how the charge carrier dynamics are affected across this entire material family. Here, various (FA,MA,Cs)Pb(I1-x Br x )3 perovskite films are light-soaked in nitrogen, and changes in optoelectronic properties are investigated through time-resolved microwave conductivity (TRMC) and optical and structural techniques. To fit the TRMC decay kinetics obtained for pristine (FA,MA,Cs)Pb(I1-x Br x )3 for various excitation densities, additional shallow states have to be included, which are not required for describing TRMC traces of single-cation MHPs. These shallow states can, independently of x, be removed by light soaking, which leads to a reduction in the imbalance between the diffusional motion of electrons and holes. We interpret the shallow states as a result of initially well-intermixed halide distributions, which upon light soaking segregate into domains with distinct band gaps.Z.A.-G. acknowledges funding from a Winton Studentship and ICON Studentship from the Lloyd’s Register Foundation. M.A.-J. thanks Cambridge Materials Limited and EPSRC (Grant Number EP/M005143/1) for their funding and technical support. S.D.S. acknowledges the Royal Society and Tata Group (UF150033) for funding

Manipulating Color Emission in 2D Hybrid Perovskites by Fine Tuning Halide Segregation: A Transparent Green Emitter.

Halide perovskite materials offer an ideal playground for easily tuning their color and, accordingly, the spectral range of their emitted light. In contrast to common procedures, this work demonstrates that halide substitution in Ruddlesden-Popper perovskites not only progressively modulates the bandgap, but it can also be a powerful tool to control the nanoscale phase segregation-by adjusting the halide ratio and therefore the spatial distribution of recombination centers. As a result, thin films of chloride-rich perovskite are engineered-which appear transparent to the human eye-with controlled tunable emission in the green. This is due to a rational halide substitution with iodide or bromide leading to a spatial distribution of phases where the minor component is responsible for the tunable emission, as identified by combined hyperspectral photoluminescence imaging and elemental mapping. This work paves the way for the next generation of highly tunable transparent emissive materials, which can be used as light-emitting pixels in advanced and low-cost optoelectronics

Static and Dynamic Disorder in Triple-Cation Hybrid Perovskites

A detailed understanding of the carrier dynamics and emission characteristics of organic-inorganic lead halide perovskites is critical for their optoelectronic and energy harvesting applications. In this work, we reveal the impact of the crystal lattice disorder on the photo-generated electron-hole pairs through low-temperature photoluminescence measurements. We provide strong evidence that the intrinsic disorder forms a sub-bandgap tail density of states, which determines the emission properties at low temperature. The PL spectra indicate that the disorder evolves with increasing temperature, changing its character from static to dynamic. This change is accompanied by a rapid drop of the PL efficiency, originating from the increased mobility of excitons/polarons, which enables them to reach deep non-radiative recombination centers more easily

Potassium- and Rubidium-Passivated Alloyed Perovskite Films: Optoelectronic Properties and Moisture Stability.

Halide perovskites passivated with potassium or rubidium show superior photovoltaic device performance compared to unpassivated samples. However, it is unclear which passivation route is more effective for film stability. Here, we directly compare the optoelectronic properties and stability of thin films when passivating triple-cation perovskite films with potassium or rubidium species. The optoelectronic and chemical studies reveal that the alloyed perovskites are tolerant toward higher loadings of potassium than rubidium. Whereas potassium complexes with bromide from the perovskite precursor solution to form thin surface passivation layers, rubidium additives favor the formation of phase-segregated micron-sized rubidium halide crystals. This tolerance to higher loadings of potassium allows us to achieve superior luminescent properties with potassium passivation. We also find that exposure to a humid atmosphere drives phase segregation and grain coalescence for all compositions, with the rubidium-passivated sample showing the highest sensitivity to nonperovskite phase formation. Our work highlights the benefits but also the limitations of these passivation approaches in maximizing both optoelectronic properties and the stability of perovskite films.Engineering and Physical Sciences Research Council (grant number: EP/M005143/1

The impact of oxygen on the electronic structure of mixed-cation halide perovskites

Alloyed triple A-cation perovskites containing a mixture of Cs, methylammonium (MA) and formamidinium (FA) cations are attracting intense attention because of their high photovoltaic performance and relative stability. However, there is limited fundamental understanding of their vacancy defect behaviour and influence of molecular oxygen on their electronic and stability properties. In this combined computational-experimental study, we investigate the (FA,MA,Cs)Pb(I,Br)3 model system with its simulated atomistic structure presented for the first time and supported by X-ray diffraction data. We examine how iodide vacancies and O2 molecules influence the local geometry and electronic structure. Our calculations, supported by Kelvin Probe contact potential difference and photoluminescence measurements, show that introduction of O2 leads to a p-doped triple-cation perovskite, and passivates iodide vacancies resulting in enhanced luminescence efficiency. These results have important implications for the performance and stability of mixed-cation perovskites in optoelectronic devices.This work was supported by the EPSRC Programme Grant ‘Energy Materials: Computational Solutions’ (EP/K016288/1) and the HPC Materials Chemistry Consortium for Archer computational time (EP/L000202/1). Z.A.-G. acknowledges funding from a Winton Studentship, and ICON Studentship from the Lloyd’s Register Foundation. S.D.S acknowledges the Royal Society and Tata Group (UF150033). K.G. acknowledges the Polish Ministry of Science and Higher Education within the Mobilnosc Plus program (Grant No. 1603/MOB/V/2017/0). M.A. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 841386

Charge extraction via graded doping of hole transport layers gives highly luminescent and stable metal halide perovskite devices.

One source of instability in perovskite solar cells (PSCs) is interfacial defects, particularly those that exist between the perovskite and the hole transport layer (HTL). We demonstrate that thermally evaporated dopant-free tetracene (120 nm) on top of the perovskite layer, capped with a lithium-doped Spiro-OMeTAD layer (200 nm) and top gold electrode, offers an excellent hole-extracting stack with minimal interfacial defect levels. For a perovskite layer interfaced between these graded HTLs and a mesoporous TiO2 electron-extracting layer, its photoluminescence yield reaches 15% compared to 5% for the perovskite layer interfaced between TiO2 and Spiro-OMeTAD alone. For PSCs with graded HTL structure, we demonstrate efficiency of up to 21.6% and an extended power output of over 550 hours of continuous illumination at AM1.5G, retaining more than 90% of the initial performance and thus validating our approach. Our findings represent a breakthrough in the construction of stable PSCs with minimized nonradiative losses.Cambridge Materials Limite

Generalised Framework for Controlling and Understanding Ion Dynamics with Passivated Lead Halide Perovskites

Metal halide perovskite solar cells have gained widespread attention due to their high efficiency and high defect tolerance. The absorbing perovskite layer is as a mixed electron-ion conductor that supports high rates of ion and charge transport at room temperature, but the migration of mobile defects can lead to degradation pathways. We combine experimental observations and drift-diffusion modelling to demonstrate a new framework to interpret surface photovoltage (SPV) measurements in perovskite systems and mixed electronic ionic conductors more generally. We conclude that the SPV in mixed electronic ionic conductors can be understood in terms of the change in electric potential at the surface associated with changes in the net charge within the semiconductor system. We show that by modifying the interfaces of perovskite bilayers, we may control defect migration behaviour throughout the perovskite bulk. Our new framework for SPV has broad implications for developing strategies to improve the stability of perovskite devices by controlling defect accumulation at interfaces. More generally, in mixed electronic conductors our framework provides new insights into the behaviour of mobile defects and their interaction with photoinduced charges, which are foundational to physical mechanisms in memristivity, logic, impedance, sensors and energy storage

Microsecond Carrier Lifetimes, Controlled p-Doping, and Enhanced Air Stability in Low-Bandgap Metal Halide Perovskites.

Mixed lead-tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite-perovskite tandem solar cells. Previous reports on lead-tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.EPSRC (EP/M005143/1 and DTP funding) Royal Society ER