Alastair Buckley

Efficient planar heterojunction mixed-halide perovskite solar cells deposited via spray-deposition

We report the use of ultra-sonic spray-coating under ambient conditions as a deposition technique for the fabrication of planar heterojunction CH3NH3PbI3−xClx perovskite solar cells. We make a first optimization of processing parameter space using this deposition technique, and explore the role of the temperature of the substrate during spray-casting, the volatility of the casting solvent and the post deposition anneal on determining the efficiency of the resultant solar cells. We find that maximum device efficiency is correlated with the creation of dense films having a surface coverage above 85%. When such films are incorporated into a solar cell device, power conversion efficiencies of up to 11% are realized. These results demonstrate that spray-coating can be used in the large-area, low-cost manufacture of high efficiency solution-processed perovskite solar cells.

Energy transfer dynamics in polyfluorene-based polymer blends

The Forster energy transfer mechanisms in polyfluorene-based polymer blends have been investigated by picosecond time-resolved photoluminescence spectroscopy. Blends of poly(9,9-dioctylfluorene), (F8), and poly(9,9-dioctylfluorene-co-benzothiadiazole), (BT), ranging from 50% to 0.01% BT have been investigated from room temperature down to 12 K. In high concentration BT/F8 films, nearest neighbour transfers from F8 to BT, characterised by an F8 fluorescence lifetime of 12 ps, totally dominate. In low concentration BT/F8 films energy transfer between F8 and BT still occurs at nearest neighbour sites, but exciton migration to these sites amongst F8 chain segments now also contributes to the measured dynamics.

Efficient energy transfer in organic thin films—implications for organic lasers

We show that efficient nonradiative (Forster) energy transfer between solid films of two highly luminescent perylene dyes blended in a solid film can be used to control the amplified spontaneous emission (ASE) emitted from the films under pulsed optical excitation. Perylene orange, which acts as the donor, and perylene red, which is the acceptor, are doped into a host matrix of poly(methylmethacrylate) (PMMA). We report the ASE behavior as a function of acceptor concentration, and observe a sudden change in the spectral position of the ASE at an acceptor:donor concentration of 1:9 by weight. Below this concentration, emission is at 590 nm, which is characteristic of ASE from undoped perylene orange:PMMA blends, whereas films with higher acceptor concentrations produced ASE spectra centered at 620 nm, which is characteristic of perylene red:PMMA blends. In order to understand this behavior, the rate constant for energy transfer between the dyes was measured and found to be 5.0±0.2×1011 s−1 (mol/dm3)−1. We used this to deduce an upper limit for the stimulated emission rate of 4.9±0.2×108 s−1.

Highly efficient visible-light driven photochromism: developments towards a solid-state molecular switch operating through a triplet-sensitised pathway.

We introduce a new highly efficient photochromic organometallic dithienylethene (DTE) complex, the first instance of a DTE core symmetrically modified by two Pt(II) chromophores [Pt(PEt(3))(2)(C≡C)(DTE)(C≡C)Pt(PEt(3))(2)Ph] (1), which undergoes ring-closure when activated by visible light in solvents of different polarity, in thin films and even in the solid state. Complex 1 has been synthesised and fully photophysically characterised by (resonance) Raman and transient absorption spectroscopy complemented by calculations. The ring-closing photoconversion in a single crystal of 1 has been followed by X-ray crystallography. This process occurs with the extremely high yield of 80%--considerably outperforming the other DTE derivatives. Remarkably, the photocyclisation of 1 occurs even under visible light (>400 nm), which is not absorbed by the non-metallated DTE core HC≡C(DTE)C≡CH (2) itself. This unusual behaviour and the high photocyclisation yields in solution are attributed to the presence of a heavy atom in 1 that enables a triplet-sensitised photocyclisation pathway, elucidated by transient absorption spectroscopy and DFT calculations. The results of resonance Raman investigation confirm the involvement of the alkynyl unit in the frontier orbitals of both closed and open forms of 1 in the photocyclisation process. The changes in the Raman spectra upon cyclisation have permitted the identification of Raman marker bands, which include the acetylide stretching vibration. Importantly, these bands occur in the spectral region unobstructed by other vibrations and can be used for non-destructive monitoring of photocyclisation/photoreversion processes and for optical readout in this type of efficiently photochromic thermally stable systems. This study indicates a strategy for generating efficient solid-state photoswitches in which modification of the Pt(II) units has the potential to tune absorption properties and hence operational wavelength across the visible range.

PCDTBT based solar cells: one year of operation under real-world conditions.

We present measurements of the outdoor stability of PCDTBT:PC71BM based bulk heterojunction organic solar cells for over the course of a year. We find that the devices undergo a burn-in process lasting 450 hours followed by a TS80 lifetime of up to 6200 hours. We conclude that in the most stable devices, the observed TS80 lifetime is limited by thermally-induced stress between the device layers, as well as materials degradation as a result of edge-ingress of water or moisture through the encapsulation.

Air processed organic photovoltaic devices incorporating a MoOx anode buffer layer

Molybdenum oxide (MoOx) has been shown to act as an efficient hole extraction layer in organic photovoltaic (OPV) devices. However, exposing MoOx films to air is problematic as it is hygroscopic; the uptake of moisture having a negative impact on its electronic properties. Here, we use spectroscopic ellipsometry to characterise the uptake of water, and fabricate PCDTBT:PC70BM based OPVs to determine its effects on device performance. We then show that thermally annealing MoOx reduces its hygroscopicity, permitting it to be processed in air. Using this process, we create air-processsed OPVs having PCEs (power conversion efficiencies) of up to 5.36%.

The role of the hole-extraction layer in determining the operational stability of a polycarbazole:fullerene bulk-heterojunction photovoltaic device

We have made a comparative study of the relative operational stability of bulk-heterojunction organic photovoltaic (OPV) devices utilising different hole transport layers (HTLs). OPV devices were fabricated based on a blend of the polymer PCDTBT with the fullerene PC70BM, and incorporated the different HTL materials PEDOT:PSS, MoOx and V2O5. Following 620 h of irradiation by light from a solar simulator, we find that devices using the PEDOT:PSS HTL retained the highest efficiency, having a projected T80 lifetime of 14 500 h.

Ray-Optics Modelling of Rectangular and Cylindrical 2-Layer Solar Concentrators

In the present work we quantify the light intensity reaching the side faces of an externally-coated, rectangular luminescent solar concentrator, and the facets of a cylindrical one. Ray-tracing is used: an analytical model has been constructed and discretized. The main novelties reside in the attribution of a finite thickness and attenuation coefficient to the external layer, and in the comparison between two geometries that have been measured against each other only in the homogeneous limit so far. In previous studies the external material is usually treated as infinitely thin. A physical thickness allows, instead, to calculate the ray-paths, to quantify the absorption losses and to evaluate the efficiency of the concentrator as function of the external layer depth. A set of numerical experiments has been performed, in order to evaluate the efficiency of the concentrator when the thickness and material properties of the outer layer are changed, and to compare the performance of the rectangular to the one of the cylindrical device under various conditions. Qualitatively we find the bilayer device to have greater optical efficiency than a comparable homogenous version. For the cylindrical geometry the factor of improvement over the homogenous device is more strongly dependent on both the thickness and the attenuation of the luminescent layer than for the rectangular geometry.

Dependence on material choice of degradation of organic solar cells following exposure to humid air.

ABSTRACT Electron microscopy has been used to study the degradation of organic solar cells when exposed to humid air. Devices with various different combinations of commonly used organic solar cell hole transport layers and cathode materials have been investigated. In this way the ingress of water and the effect it has on devices could be studied. It was found that calcium and aluminum in the cathode both react with water, causing voids and delamination within the device. The use of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was found to increase the degradation by easing water ingress into the device. Replacing these materials removed these degradation features.

Solution processed nickel oxide anodes for organic photovoltaic devices

Nickel oxide thin films have been prepared from a nickel acetylacetonate (Ni(acac)) precursor for use in bulk heterojunction organic photovoltaic devices. The conversion of Ni(acac) to NiOx has been investigated. Oxygen plasma treatment of the NiO layer after annealing at 400 °C affords solar cell efficiencies of 5.2%. Photoelectron spectroscopy shows that high temperature annealing converts the Ni(acac) to a reduced form of nickel oxide. Additional oxygen plasma treatment further oxidizes the surface layers and deepens the NiO work function from 4.7 eV for the annealed film, to 5.0 eV allowing for efficient hole extraction at the organic interface.