Jeffrey G. Tait

An electron beam evaporated TiO2 layer for high efficiency planar perovskite solar cells on flexible polyethylene terephthalate substrates

The TiO2 layer made by electron beam (e-beam) induced evaporation is demonstrated as an electron transport layer (ETL) in high efficiency planar junction perovskite solar cells. The temperature of the substrate and the thickness of the TiO2 layer can be easily controlled with this e-beam induced evaporation method, which enables the usage of different types of substrates. Here, perovskite solar cells based on CH3NH3PbI3−xClx achieve power conversion efficiencies of 14.6% on glass and 13.5% on flexible plastic substrates. The relationship between the TiO2 layer thickness and the perovskite morphology is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Our results indicate that the pinholes in the thin TiO2 layer lead to pinholes in the perovskite layer. By optimizing the TiO2 thickness, perovskite layers with substantially increased surface coverage and reduced pinhole areas are fabricated, increasing overall device performance.

Rapid composition screening for perovskite photovoltaics via concurrently pumped ultrasonic spray coating

Transitioning perovskite photovoltaics from the rapid progress in lab-scale devices to industrially viable large area modules is a key challenge for the economic breakthrough of the technology. In this work, we demonstrate ultrasonic spray coating as a scalable and versatile linear deposition technique for high efficiency perovskite photovoltaics. We show the versatility of concurrently pumped ultrasonic spray coating by rapidly and precisely optimizing precursor ratios based on PbCl2, Pb(CH3CO2)2·3H2O, PbBr2, CH3NH3Br, and CH3NH3I to achieve highly crystalline and pinhole-free layers. Initial power conversion efficiencies of 15.7% for small scale devices and 11.7% for 3.8 cm2 modules were achieved with current–voltage sweeps and tracked to 13.4% for devices and 10.4% for modules under continuous illumination and bias at the maximum power point. Process versatility is further demonstrated with the in situ bandgap control in CH3NH3PbIXBr3−X layers.

Ultrasonic Spray Coating of 6.5% Efficient Diketopyrrolopyrrole-Based Organic Photovoltaics

Ultrasonic spray coating was tailored for the deposition of diketopyrrolopyrrole-based polymer: fullerene films. Concurrently pumped spray coating was used to rapidly and precisely sweep photoactive layer thickness and donor:acceptor ratio. The highest reported efficiency for a spray-coated device of 6.5% was achieved and verified by external quantum efficiency. The short-circuit current density of the complete device stack was optically simulated as a function of thickness, agreeing with experimentally measured device performance. Charge carrier mobilities were studied as a function of fullerene loading, suggesting electron mobility limited performance. This paper shows that implementing the industrially relevant deposition technique of spray coating results in no performance loss relative to lab-based spin coating.

Interfacial Depletion Regions: Beyond the Space Charge Limit in Thick Bulk Heterojunctions

Space charge limited photocurrent is typically described as the limiting factor in carrier extraction efficiency for organic bulk heterojunctions with increasing thickness. It successfully characterizes the carrier extraction efficiency in these devices with thin to moderate thickness and dissimilar carrier mobilities. However, in this article we show that space charge limited photocurrent cannot solely explain the intensity dependent spectral response of extremely thick organic photovoltaics. In addition, interfacial depletion regions near the contacts contribute to the field distribution and carrier collection. Here, we describe charge collection efficiency with an optical p-i-n model, allowing for collection from band bending due to mobility-induced and interfacial-doping-induced space charge regions. We verify the model with up to 1400 nm thick spray-coated devices in both p-i-n (conventional) and n-i-p (inverted) architecture, including variations of thickness, illumination intensity, transport materials, and bifacial (semitransparent) devices.

Crystallisation dynamics in wide-bandgap perovskite films

Organic–inorganic metal halide perovskite materials have evolved as highly efficient photovoltaic materials with a controllable range of bandgaps. This trait offers exciting prospects for the application of perovskites as wide-bandgap thin-film top solar cells in tandem architectures with crystalline silicon bottom solar cells. In this work, we present a systematic material study on spin-coated methylammonium lead trihalide (CH3NH3Pb(I0.6Br0.4)3) that has a band gap of 1.77 eV, optimal for tandem architectures with crystalline silicon. Using a combination of X-ray diffraction, time-resolved photoluminescence, and scanning electron microscopy techniques, we determine the strong impact of annealing temperature and duration on perovskite film crystallinity, carrier lifetime, and average grain size. We further demonstrate a clear correlation between solar cell performance and crystallisation dynamics in the perovskite films. With optimised crystallisation of the perovskite films, our solar cells exhibit peak power conversion efficiency of 10.6% that stabilises at 9.0% after 10 minutes of maximum power point tracking. Finally, the activation energy for grain boundary mobility, and grain growth exponents are determined via quantitative analysis of grain growth kinetics, and hence, perovskite film quality.

Low-cost electrodes for stable perovskite solar cells

Cost-effective production of perovskite solar cells on an industrial scale requires the utilization of exclusively inexpensive materials. However, to date, highly efficient and stable perovskite solar cells rely on expensive gold electrodes since other metal electrodes are known to cause degradation of the devices. Finding a low-cost electrode that can replace gold and ensure both efficiency and long-term stability is essential for the success of the perovskite-based solar cell technology. In this work, we systematically compare three types of electrode materials: multi-walled carbon nanotubes (MWCNTs), alternative metals (silver, aluminum, and copper), and transparent oxides [indium tin oxide (ITO)] in terms of efficiency, stability, and cost. We show that multi-walled carbon nanotubes are the only electrode that is both more cost-effective and stable than gold. Devices with multi-walled carbon nanotube electrodes present remarkable shelf-life stability, with no decrease in the efficiency even after 180 ...

Light stability of ITO-free semi-transparent and opaque organic photovoltaic devices

Intrinsic light stability of transparent MoO3/Ag/TiO2 electrode is studied in four different polymer:fullerene solar cell configurations. We demonstrate that this stack can successfully replace ITO both in superstrate and substrate configurations required for non-transparent carriers. Although we observe that exposure of thin Ag layer directly to UV light reduces its conductivity, integration of a UV filter in the device package mitigates entirely this failure route. This enables us to show efficient and stable ITO-free cells both in superstrate and substrate configurations can reach identical stability as the ITO reference.

Blade coating of diketopyrrolopyrrole based organic photovoltaics from non-halogenated solvent systems

The need for scalable and environmentally friendly deposition of organic semiconductors is mounting, as lab-scale solution processed organic photovoltaic (OPV) devices have been certified with 10.8% efficiency. The replacement of toxic halogenated solvents is a priority in the commercialization of solution processed OPV. Here we introduce several solvent systems based on thiophene, tetraline, 1,2,4-trimethylbenzene, xylene, and anisole for the blade coating of diketopyrrolopyrrole-based polymer: fullerene OPV devices. These devices attain 6.1% efficiency, greater than the commonly used chloroform:ortho-dichlorobenzene solvent systems for the same polymers, and without involving the post-deposition annealing treatments nor additives typically implemented for non-halogenated inks. Furthermore, the solubility of the material was investigated with Hansen Solubility Parameters, and coatability on a variety of substrates was probed via contact angle measurements. This work enables a new non-halogenated solvent route for the scalable fabrication of organic electronics.