Nanostructured copper electrodes for organic photovoltaics

This thesis describes a body of original research focused on the development of a viable alternative to the indium-tin oxide (ITO) glass window electrode used in organic photovoltaic (OPV) devices, based on the use of ultra-thin Cu films. The first results chapter describes a low cost, robust Cu | Al bilayer window electrode that simultaneously functions as the low work function electron-extracting electrode and as a sink for oxygen/water molecules in OPVs. When the electrode is exposed to air, an ultra-thin oxide layer forms at its surface without any increase in surface roughness, and the sheet resistance of the electrode actually decreases. However, this electrode has the disadvantage of a lower far-field transparency than ITO glass. The second results chapter describes how the transparency of ultra-thin Cu films can be increased to a level comparable to that of ITO glass across most of the spectrum over which OPVs harvest light using an overlayer of tungsten sub-oxide (WO3-x) which is spontaneously doped with Cu, increasing both its refractive index and electrical conductivity. Unfortunately these electrodes are not air stable. The third results chapter describes how the developments described in the previous two chapters might be integrated to realise an electrode that is both air-stable and highly transparent. The final results chapter describes a very different approach to coupling light into an OPV based on a Cu electrode with a dense array of sub-optical wavelength apertures. These electrodes absorb light strongly, concentrating it as surface plasmon excitations. It is shown that this trapped light can be absorbed by the light harvesting organic semiconductor in organic photovoltaics so that electrodes with very low far-field transparency can perform as well as more transparent electrodes

Heat shock factor 1 regulates lifespan as distinct from disease onset in prion disease

Prion diseases are fatal, transmissible, neurodegenerative diseases caused by the misfolding of the prion protein (PrP). At present, the molecular pathways underlying prion-mediated neurotoxicity are largely unknown. We hypothesized that the transcriptional regulator of the stress response, heat shock factor 1 (HSF1), would play an important role in prion disease. Uninoculated HSF1 knockout (KO) mice used in our study do not show signs of neurodegeneration as assessed by survival, motor performance, or histopathology. When inoculated with Rocky Mountain Laboratory (RML) prions HSF1 KO mice had a dramatically shortened lifespan, succumbing to disease ≈20% faster than controls. Surprisingly, both the onset of home-cage behavioral symptoms and pathological alterations occurred at a similar time in HSF1 KO and control mice. The accumulation of proteinase K (PK)-resistant PrP also occurred with similar kinetics and prion infectivity accrued at an equal or slower rate. Thus, HSF1 provides an important protective function that is specifically manifest after the onset of behavioral symptoms of prion disease

Growth and Characterization of Sb2Se3 Single Crystals for Fundamental Studies

Three methods of growing bulk crystalline samples of Sb2 Se3 to provide material for basic studies have been investigated and preliminary racterization is reported. These growth methods were: A) melt-growth, similar to vertical Bridgman, b) dynamic vapor transport over a temperature gradient (Piper-Polich method) and c) a static vapor method in which the source material is transported in nearly iso-thermal conditions. The melt-growth method produced the largest single crystals (up to 4 mm diameter), while the vapor methods both yielded polycrystalline boules with mm-sized grains. Powder XRD confirmed the boules to comprise orthorhombic Sb2 Se3, having lattice parameters a = 11.7808 Å b = 3.9767 Å and c = 11.6311 Å. Cleavage facets were parallel to (100). Raman peaks at 191 (A g and 211 cm -1 were excited anisotropically. FTIR reflectance features showed some sensitivity to s- and ppolarization

CSS Antimony Selenide Film Morphology and High Efficiency PV Devices

Knowledge of close-space sublimation (CSS) Sb 2 Se 3 growth conditions is vital for proper understanding of PV performance, and optimization of Sb 2 Se 3 devices. In this work, various growth parameters have been studied and the resulting Sb 2 Se 3 films have been characterized using SEM, XRD and optical transmission measurements, thus illustrating the desired properties for high device performance. PV devices were fabricated using TiO 2 as a window layer combined with P3HT or PTB7 as the hole transport material, resulting in V oc =0.42 V, J sc =33.4 mAcm -2 , FF = 43.2% and PCE = 6.06% for P3HT

Copper light-catching electrodes for organic photovoltaics

Optically thin copper films with a random array of sub-optical wavelength apertures couple strongly with light in the wavelength range 600-800 nm due to excitation of surface plasmonic resonances. Herein we show that this trapped light can be used to excite electronic transitions in a nearby strongly absorbing organic semiconductor before the plasmonic excitations dissipate their energy as heat into the metal. This energy transfer process is demonstrated using model small molecule and polymer photovoltaic devices (based on chloro-aluminium phthalocyanine : C60 and PCE-10 : PC70BM heterojunctions respectively) in conjunction with a nano-hole copper electrode formed by thermal annealing an optically thin Cu film supported on polyethylene terephthalate. The efficiency of this process is shown to be highest for wavelengths in the range 650-750 nm, which is part of the solar spectrum that is weakly absorbed by today’s high performance organic photovoltaic devices, and so these findings demonstrate that this type of electrode could prove useful as a low cost light catching element in high performance organic photovoltaics

Vegard Relation and Raman Band Reference Data Generated from Bulk Crystals of Kesterite-Phase Composition Series Cu2ZnSnS4xSe4–4x (CZTSSe, 0 ≤ x ≤ 1)

Solid solutions in the series Cu2ZnSnS4xSe4–4x (CZTSSe) are of interest for PV applications. The purpose of this work was to grow bulk crystalline samples over the entire composition range to allow the Vegard relation (lattice parameter variation with composition) and the systematic behavior of Raman bands to be defined to generate reference data. Samples with 0 ≤ x ≤ 1 were synthesized from the elements and grown into crystalline form from solution in either KCl/NaCl eutectic or elemental Sn. Details of the crystal growth outcomes, including the use of a quartz seed plate to make thick film samples, are described. Ordered kesterite-type material was formed upon crystallization, and X-ray diffraction demonstrated linear Vegard relationships, with the lattice parameters varying with composition as a (Å) = −0.268(3)x + 5.6949(17) and c (Å) = −0.516(6)x + 11.345(3). Raman spectroscopy yielded two dominant peaks, these being kesterite A modes associated with the Se and S modes in CZTSe and CZTS. These varied in wavenumber linearly as ωCZTSe (cm–1) = (44.6 ± 1.6)x + (194.6 ± 0.8) and ωCZTS (cm–1) = (7.1 ± 1.3)x + (329.0 ± 0.8). Crystallization was also shown to promote ordering. The variation of lattice parameters with composition exhibited significant differences from those observed in previous studies. Also, while the Raman S mode behavior differed from previous reports, the Se modes were similar. These differences are discussed

Chemical etching of Sb2Se3 solar cells: surface chemistry and back contact behaviour

The effect of (NH4)2S and CS2 chemical etches on surface chemistry and contacting in Sb2Se3 solar cells was investigated via a combination of x-ray photoemission spectroscopy (XPS) and photovoltaic device analysis. Thin film solar cells were produced in superstrate configuration with an absorber layer deposited by close space sublimation. Devices of up to 5.7% efficiency were compared via current–voltage measurements (J–V) and temperature-dependent current–voltage (J–V–T) analysis. XPS analysis demonstrated that both etching processes were successful in removing Sb2O3 contamination, while there was no decrease in free elemental selenium content by either etch, in contrast to prior work. Using J–V–T analysis the removal of Sb2O3 at the back surface in etched samples was found to improve contacting by reducing the potential barrier at the back contact from 0.43 eV to 0.26 eV and lowering the series resistance. However, J–V data showed that due to the decrease in shunt resistance and short-circuit current as a result of etching, the devices show a lower efficiency following both etches, despite a lowering of the series resistance. Further optimisation of the etching process yielded an improved efficiency of 6.6%. This work elucidates the role of surface treatments in Sb2Se3 devices and resolves inconsistencies in previously published works

Direct Silicon Heterostructures With Methylammonium Lead Iodide Perovskite for Photovoltaic Applications

We investigated the formation of photovoltaic (PV) devices using direct n-Si/MAPI (methylammonium lead tri-iodide) two-sided heterojunctions for the first time (as a possible alternative to two-terminal tandem devices) in which charge might be generated and collected from both the Si and MAPI. Test structures were used to establish that the n-Si/MAPI junction was photoactive and that spiro-OMeTAD acted as a “pinhole blocking” layer in n-Si/MAPI devices. Two-terminal “substrate” geometry devices comprising Al/n-Si/MAPI/spiro-OMeTAD/Au were fabricated and the effects of changing the thickness of the semitransparent gold electrode and the silicon resistivity were investigated. External quantum efficiency and capacitance–voltage measurements determined that the junction was one-sided in the silicon—and that the majority of the photocurrent was generated in the silicon, with there being a sharp cutoff in photoresponse above the MAPI bandgap. Construction of band diagrams indicated the presence of an upward valence band spike of up to 0.5 eV at the n-Si/MAPI interface that could impede carrier flow. Evidence for hole accumulation at this feature was seen in both Kelvin-probe transients and from unusual features in both current–voltage and capacitance–voltage measurements. The devices achieved a hysteresis-free best power conversion efficiency of 2.08%, V OC 0.46 V, J SC 11.77 mA/cm2, and FF 38.4%, demonstrating for the first time that it is possible to create a heterojunction PV device directly between the MAPI and n-Si. Further prospects for two-sided n-Si/MAPI heterojunctions are also discussed

Natural Band Alignments and Band Offsets of Sb2Se3 Solar Cells

Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems