Anjana Wijesekara, Marc Walker, Yisong Han, David Walker, Steven, Huband, Ross A. Hatton

Advanced Energy Materials, 12 November 2021 | https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.202102766

Abstract: Unencapsulated organo-tin halide perovskite photovoltaic (PV) devices exhibiting record stability (for organo-tin perovskite PV devices) when tested under continuous one sun solar illumination in ambient air and under electrical load are reported. This exceptional stability is made possible by the use of a bathocuproine | copper cathode in an inverted device architecture. A series of experiments designed to elucidate the underlying reasons for the high stability show that, compared to conventional silver electrodes, compact copper electrodes are far more resistant to corrosion by I2 gas (evolved when organo-tin halide decomposes) and toward adverse morphological evolution and ingress of oxygen and water molecules through the top electrode into the device. The findings of these experiments show that copper should be the metal of choice for the reflective cathode in inverted tin perovskite PVs when the material interfacing the metal interacts strongly with it, enabling compact film formation and a stable interface toward copper diffusion into the adjacent charge transport layer.

Microcontact-Printed Nickel-Passivated Copper Grid Electrode for Perovskite Photovoltaics

Anjana Wijesekara, Marc Walker, Yisong Han, Ross A. Hatton

Applied Energy Materials, 16 August 2021 | https://pubs.acs.org/doi/pdf/10.1021/acsaem.1c01230

Abstract: We report a novel transparent copper-based grid electrode fabricated by microcontact printing that offers high stability toward oxidation in air. Passivation is achieved using a 2.5 nm thick layer of nickel buried just below the surface of the copper film from which the grid is etched. This approach to electrode passivation retains the advantages associated with using microcontact printing lithography for grid fabrication of very fast resist deposition and compatibility with the low cost, low toxicity etchant ammonium persulfate. The processes of patterned resist deposition and metal etching is complete within less than 1 min. Using this approach, a grid electrode with a far-field transparency of 82% across the wavelength range 300–900 nm and sheet resistance of 6.8 Ω sq−1 is demonstrated, which is shown to be a promising alternative to indium-tin oxide as the transparent electrode in perovskite photovoltaic devices.

High-Performance Transparent Copper Grid Electrodes Fabricated by Microcontact Lithography for Organic Photovoltaics

Phillip Bellchambers, Silvia Varagnolo, Corinne Maltby, Ross A. Hatton

Applied Energy Materials, 5 April 2021 | https://pubs.acs.org/doi/pdf/10.1021/acsaem.1c00469

Abstract: We report high-performance transparent copper grid electrodes on glass and plastic substrates that offer a higher Haacke figure-of-merit than conventional indium tin oxide electrodes and are well-matched to the requirements for organic photovoltaics (OPVs). The electrode is fabricated using microcontact lithography with a combination of molecular resist and low toxicity etchant, namely, hexadecanethiol and aqueous ammonium persulfate. This approach to electrode fabrication is much faster than conventional lithography because it takes <2 s to print the molecular resist layer and tens of seconds to etch the copper film, with both processes performed in ambient air. The grid line width achieved is >20 times narrower than is possible using conventional metal printing methods and so a grid pitch <30 μm is easily achieved without increasing metal coverage. The very small grid-line spacing relaxes the requirement to use highly conductive films to span the gaps between grid lines, reducing parasitic absorption losses. This is demonstrated using an extremely thin (10 nm) poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer. Additionally, we present evidence that it is not always necessary to embed the metal grid into the substrate or to planarize with a charge-transport layer, to avoid leakage current across the OPV device.

Transparent Fused Nanowire Electrodes by Condensation Coefficient Modulation

Jaemin Lee, Silvia Varagnolo, Marc Walker, Ross A. Hatton,

Advanced Functional Materials, 18 September 2020 | https://doi.org/10.1002/adfm.202005959

Abstract: Silver nanowire networks can offer exceptionally high performance as transparent electrodes for stretchable sensors, flexible optoelectronics, and energy harvesting devices. However, this type of electrode suffers from the triple drawbacks of the complexity of fabrication, instability of the nanowire junctions, and high surface roughness, which limits electrode performance and utility. Here, a new concept in the fabrication of silver nanowire electrodes is reported that simultaneously addresses all three of these drawbacks, based on an electrospun nanofiber network and supporting substrate having silver vapour condensation coefficients of one and near‐zero, respectively. Consequently, when the whole substrate is exposed to silver vapour by simple thermal evaporation, metal selectively deposits onto the nanofiber network. The advantage of this approach is the simplicity since there is no mask, chemical or dry metal etching step, or mesh transfer step. Additionally, the contact resistance between nanowires is zero and the surface roughness is sufficiently low for integration into organic photovoltaic devices. This new concept opens the door to the continuous roll‐to‐roll fabrication of high‐performance fused silver nanowire electrodes for myriad potential applications.

Embedded-grid silver transparent electrodes fabricated by selective metal condensation

Silvia Varagnolo, Keun-Woo Park, Jin-Kyun Lee, Ross A. Hatton

Journal of Materials Chemistry C, 16 September 2020 | https://doi.org/10.1039/D0TC02803GLink opens in a new window

Abstract: We report a new materials system for the fabrication of embedded silver grid electrodes with micron-sized linewidth >10 times narrower than can be achieved using the conventional printing techniques of the screen, inject and flexographic printing. Using micro-contact printed thin films of the highly fluorinated polymer poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl methacrylate) together with low-temperature heating of the substrate during metal deposition by thermal evaporation, we have fabricated embedded silver transparent grid electrodes on flexible plastic substrates without the need for a metal etching step or a separate grid-embedding step. This simplified approach to grid electrode fabrication is made possible by the very low condensation coefficient of Ag on areas of the substrate covered with the printed organofluorine layer, removing the need for harmful chemical etchants and slow chemical-intensive electrochemical deposition steps.

High Figure-of-Merit Transparent Copper–Zinc Oxide Window Electrodes for Organic Photovoltaics

H. Jessica Pereira and Ross A. Hatton

Front. Mater., 19 September 2019 | https://doi.org/10.3389/fmats.2019.00228

Abstract: We report a copper-zinc oxide bilayer electrode supported on flexible polyethene terephthalate (PET) with a sheet resistance of 11. 3 Ω sq⁻¹ and average transparency of 84.6% in the wavelength range of 400–800 nm. The copper film is perforated with a dense array of sub-micron diameter apertures fabricated using polymer-blend lithography, which imparts broad band anti-reflectivity. We demonstrate proof-of-principle that it is possible to fabricate the polymer mask by dip-coating which is a scalable deposition method compatible with roll-to-roll processing. During storage of the electrode at ambient temperature, the ZnO layer is spontaneously doped with copper from the underlying copper film and so the thin ZnO layer serves both as an anti-reflecting layer and an excellent electron transport layer. When compared with commercially available indium tin oxide coated (ITO) plastic substrates this electrode exhibits superior stability towards bending deformation, with no change in sheet resistance after bending through a 4 mm radius of curvature 100 times. Model inverted organic photovoltaic (OPV) devices using this electrode exhibit a champion power conversion efficiency of ~8.7%, which is the highest reported efficiency to date for an OPV device using a copper-based transparent electrode, outperforming identical devices using ITO coated plastic as the transparent electrode.

An Electrode Design Rule for Organic Photovoltaics Elucidated Using a Low Surface Area Electrode

[Press release]Link opens in a new window

G. Dinesha M. R. Dabera, Jaemin Lee, Ross A. Hatton

Advanced Functional Materials, 2019, doi.org/10.1002/adfm.201904749

Abstract: It is widely considered that charge carrier extraction in bulk-heterojunction organic photovoltaics (BHJ OPVs) is most efficient when the area of contact between the semiconductor layers and the electrodes is maximized and the electrodes are electrically homogeneous. Herein, we show that 90% of the electrode surface can in fact be insulating without degrading the efficiency of charge carrier extraction, provided the spacing of the conducting areas is less than or equal to twice the optimal thickness of the BHJ layer. This striking result is demonstrated for BHJ OPVs with both conventional and inverted device architectures using two different types of BHJ OPVs: namely PCDTBT:PC70BM and the ternary blend PBDB-T:ITIC-m:PC70BM. This finding opens the door to the use of a large pallet of materials for optical spacers and charge transport layers, based on a low density of conducting particles embedded in a wide band gap insulating matrix.

Selective deposition of silver and copper films by condensation coefficient modulation (Free access)

[Press release]Link opens in a new window

Silvia Varagnolo, Jaemin Lee, Houari Amari and Ross A. Hatton

Materials Horizons, 2019, DOI: 10.1039/c9mh00842j

Abstract: Whilst copper and silver are the conductors of choice for myriad current and emerging applications, patterning these metals is a slow and costly process. We report the remarkable finding that an extremely thin (∼10 nm) printed layer of specific organofluorine compounds enables selective deposition of copper and silver vapour, with metal condensing only where the organofluorine layer is not. This unconventional approach is fast, inexpensive, avoids metal waste and the use of harmful chemical etchants, and leaves the metal surface uncontaminated. We have used this approach to fabricate thin films of these metals with 6 million apertures cm⁻² and grids of ∼1 μm lines, through to 10 cm diameter apertures. We have also fabricated semi-transparent organic solar cells in which the top silver electrode is patterned with a dense array of 2 μm diameter apertures, which cannot be achieved by any other scalable means directly on an organic electronic device.

Stabilizing Silver Window Electrodes for Organic Photovoltaics Using a Mercaptosilane Monolayer 

Jaemin Lee, Marc Walker, Silvia Varagnolo, Steven Huband, Ross A. Hatton*

ACS Appl. Energy Mater. 2019, 2, 7, 5198-5205 DOI: https://doi.org/10.1021/acsaem.9b00878

Abstract: A single layer of the bifunctional molecule 3-mercaptopropyltrimethoxysilane is shown to be remarkably effective at improving the stability of optically thin silver film electrodes toward spontaneous morphological change and oxidation by airborne sulfur. Inclusion of this layer in the novel transparent electrode WO3(30 nm)/silver (13 nm)/sol–gel ZnO (27 nm) at the silver/ZnO interface improves the efficiency of organic photovoltaic devices using this electrode by 20%, such that the power conversion efficiency is very close to that achievable using a conventional indium–tin oxide glass electrode (9.6 ± 0.2% vs 10.0 ± 0.3%), with the advantage that the silver electrode has a sheet resistance one-third that of the ITO glass (∼4 ohm sq^–1). The mercaptosilane monolayer is also shown to retard silver diffusion into the ZnO layer while imparting a favorable ∼400 meV reduction in electrode work function. In addition to its utility inside the device, this molecular layer is shown to be useful for improving the stability of the silver film electrodes in top-illuminated semitransparent photovoltaics, since it can be deposited directly onto a completed device from the vapor phase.

Enhanced Oxidation Stability of Transparent Copper Films Using a Hybrid Organic-Inorganic Nucleation Layer

Philip Bellchambers, Dr. Marc Walker, Dr. Steven Huband, Aivaras Dirvanauskas, Dr. Ross A. Hatton

Abstract: A novel seed layer for the formation of slab‐like transparent copper films on glass and plastic substrates is reported, based on a mixed molecular monolayer and an ultra‐thin (0.8 nm) aluminium layer both deposited from the vapour phase, which substantially outperforms the best nucleation layer for optically thin copper films reported to date. Using this hybrid layer, the metal percolation threshold is reduced to <4 nm nominal thickness and the long‐term stability of sub‐10 nm films towards oxidation in the air is comparable to that of silver films of the same thickness fabricated using the best reported seed layer for optically thin silver films to date. The underlying reason for the remarkable effectiveness of this hybrid nucleation is elucidated using a combination of photoelectron spectroscopy, small angle X‐ray studies, atomic force microscopy and transmission electron microscopy.

Copper Substrate Electrode for Efficient Top-illuminated Organic Photovoltaics

H. Jessica Pereira & Ross A. Hatton

ACS Appl. Mater. Interfaces, 2019, 11 (1), pp 43–48 | DOI: 10.1021/acsami.8b18453Link opens in a new window

Abstract: It is now recognized that for solution processed organic photovoltaics (OPVs) to be manufactured on a large scale the thickness of the photoactive layer must be substantially increased beyond the currently used ≤150 nm. We show that copper can replace silver as the reflective substrate electrode in high performance top-illuminated OPVs without compromising device power conversion efficiency when the photoactive layer is thick enough to absorb the majority of incident photons on the first pass through the photoactive layer. Copper is one hundredth of the cost of Ag, enabling a significant reduction in the bill of materials for OPV manufacture.

Elucidating the Exceptional Passivation Effect of 0.8 nm Evaporated Aluminium on Transparent Copper Films (Free access)

Philip Bellchambers, Jaemin Lee, Silvia Varagnolo, Houari Amari, Marc Walker and Ross A. Hatton

Front. Mater., 2018, 5:71 | https://doi.org/10.3389/fmats.2018.00071Link opens in a new window

Abstract: Slab-like copper films with a thickness of 9 nm (~70 atoms) and sheet resistance of ≤ 9 Ω sq⁻¹ are shown to exhibit remarkable long-term stability toward air-oxidation when passivated with an 0. 8 nm aluminium layer deposited by simple thermal evaporation. The sheet resistance of 9 nm Cu films passivated in this way, and lithographically patterned with a dense array of ~6 million apertures per cm², increases by < 3.5% after 7,000 h exposure to ambient air. Using a combination of annular-dark field scanning transmission electron microscopy, nanoscale spatially resolved elemental analysis and atomic force microscopy, we show that this surprising effectiveness of this layer results from spontaneous segregation of the aluminium to grain boundaries in the copper film where it forms a ternary oxide plug at those sites in the metal film most vulnerable to oxidation. Crucially, the heterogeneous distribution of this passivating oxide layer combined with its very low thickness ensures that the underlying metal is not electrically isolated, and so this simple passivation step renders Cu films stable enough to compete with Ag as the base metal for transparent electrode applications in emerging optoelectronic devices.

Assessing the suitability of copper thiocyanate as a hole-transport layer in inverted CsSnI₃ perovskite photovoltaics

Anjana Wijesekara, Silvia Varagnolo , G. Dinesha M. R. Dabera , Kenneth P. Marshall,
H. Jessica Pereira & Ross A. Hatton

Scientific Reports, 2018, 8:15722, DOI:10.1038/s41598-018-33987-7Link opens in a new window

Abstract: We report the findings of a study into the suitability of copper (I) thiocyanate (CuSCN) as a hole transport
layer in inverted photovoltaic (PV) devices based on the black gamma phase (B-γ) of CsSnI₃
perovskite. Remarkably, when B-γ-CsSnI₃ perovskite is deposited from a dimethylformamide solution
onto a 180–190 nm thick CuSCN film supported on an indium-tin-oxide (ITO) electrode, the CuSCN layer
is completely displaced leaving a perovskite layer with high uniformity and coverage of the underlying
ITO electrode. This finding is confirmed by a detailed analysis of the thickness and composition of the film
that remains after perovskite deposition, together with photovoltaic device studies. The results of this
study shows that whilst CuSCN has proved to be an excellent hole-extraction layer for high performance
lead-perovskite and organic photovoltaics, it is unsuitable as a hole-transport layer in inverted B-γ-
CsSnI₃ perovskite photovoltaics processed from solution.

Fabrication of Copper Window Electrodes with ≈10⁸ Apertures cm⁻² for Organic Photovoltaics

H. Jessica Pereira, Joseph Reed, Jaemin Lee, Silvia Varagnolo, G. Dinesha M. R. Dabera and Ross A. Hatton

Adv. Funct. Mater. 2018, 1802893, DOI: 10.1002/adfm.201802893 Link opens in a new window

Abstract: We report a powerful approach to increasing the far-field transparency of copper film window electrodes which simultaneously reduces intra-band absorption losses for wavelengths < 550 nm and suppresses reflective losses for wavelengths > 550 nm. The approach is based on the incorporation of a random array of ~100 million circular apertures per square cm into an optically thin copper film, with a mean aperture diameter of ~500 nm. A method for the fabrication of these electrodes is described that exploits a binary polymer blend mask that self-organises at room temperature from a single solution, and so is simple to implement. Additionally, all of the materials used in electrode fabrication are low cost, low toxicity and widely available. We show that these nano-structured copper electrodes offer average far-field transparency of ≥ 80% and sheet resistance of ≤ 10 W sq^-1 when used in conjunction with a conventional solution-processed ZnO electron transport layer and demonstrate their utility in inverted organic photovoltaic devices.

A Comprehensive Guide to Solar Energy Systems
Chapter 12: Organic Photovoltaics by Ross Hatton

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The most advanced and research focused text on all aspects of solar energy engineering is a must have edition on the present state of solar technology, integration and worldwide distribution. In addition, the book provides a high-level assessment of the growth trends in photovoltaics and how investment, planning and economic infrastructure can support those innovations. Each chapter includes a research overview with a detailed analysis and new case studies that look at how recent research developments can be applied.

Cs_1−xRb_xSnI₃ light-harvesting semiconductors for perovskite photovoltaics

Kenneth P. Marshall, Shuxia Tao, Marc Walker, Daniel S. Cook, James Lloyd-Hughes, Silvia Varagnolo, Anjana Wijesekara, David Walker, Richard I. Walton and Ross A. Hatton

Mater. Chem. Front., 2018, Advance Article DOI:10.1039/C8QM00159FLink opens in a new window

Abstract: We show that films of the 3-dimensional perovskite Cs_1−xRb_xSnI₃ can be prepared from room temperature N,N-dimethylformamide solutions of RbI, CsI and SnCl₂ for x ≤ 0.5, and that for x ≤ 0.2 film stability is sufficient for utility as the light-harvesting layer in inverted photovoltaic (PV) devices. Electronic absorption and photoluminescence spectroscopy measurements supported by computational simulation, show that increasing x increases the band gap, due to distortion of the lattice of SnI₆ octahedra that occurs when Cs is substituted with Rb, although it also reduces the stability towards decomposition. When Cs_0.8Rb_0.2SnI₃ perovskite is incorporated into the model inverted PV device structure; ITO|perovskite|C60|bathocuproine|Al, an ∼120 mV increase in open-circuit is achieved which is shown to correlate with an increase in perovskite ionisation potential. However, for this low Rb loading the increase in band gap is very small (∼30 meV) and so a significant increase in open circuit-voltage is achieved without reducing the range of wavelengths over which the perovskite can harvest light. The experimental findings presented are shown to agree well with the predictions of density functional theory (DFT) simulations of the stability and electronic structure, also performed as part of this study.

Retarding oxidation of copper nanoparticles without electrical isolation and the size dependence of work function (Free access)

G Dinesha M R Dabera, Marc Walker, Ana M Sanchez, H. Jessica Pereira, Richard Beanland and Ross A Hatton

NATURE COMMUNICATIONS, 2017 DOI: 10.1038/s41467-017-01735-6Link opens in a new window

Abstract: Copper nanoparticles (CuNPs) are attractive as a low-cost alternative to their gold and silver analogues for numerous applications, although their potential has hardly been explored due to their higher susceptibility to oxidation in air. Here we show the unexpected findings of an investigation into the correlation between the air-stability of CuNPs and the structure of the thiolate capping ligand: Of the 8 different ligands screened, those with the shortest alkyl chain, -(CH2)2-, and a hydrophilic carboxylic acid end group are found to be the most effective at retarding oxidation in air. We also show that CuNPs are not etched by thiol solutions as previously reported, and address the important fundamental question of how the work function of small supported metal particles scales with particle size. Together these findings set the stage for the greater utility of CuNPs for emerging electronic applications.

Elucidating the role of the hole-extracting electrode on the stability and efficiency of inverted CsSnI3/C60 perovskite photovoltaics (Free access)

K. P. Marshall, M. Walker, R. I. Walton and R. A. Hatton

J. Mater. Chem. A, 2017 DOI: 10.1039/C7TA05967ALink opens in a new window

Abstract: The correlation between the stability of thin films of black (B)-γ CsSnI₃ perovskite in ambient air and the choice of supporting substrate is examined for the substrates: (i) soda-lime glass; (ii) indium tin oxide (ITO) glass; (iii) copper iodide (solution-processed)/ITO glass; (iv) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ITO glass; (v) and an optically thin (8 nm) gold film electrode. The performance of (ii)–(v) as the hole-extracting electrode in inverted photovoltaic (PV) devices with a simple bilayer architecture is compared for a test condition of 1 sun continuous solar illumination in air. CsSnI₃ film stability is shown to depend strongly on the density of pinholes and grain boundaries, although not on the preferred CsSnI₃ crystallite orientation. Solution-processed CuI is shown to be unsuitable as a hole-transport layer (HTL) for inverted CsSnI₃ PV devices because it is almost completely displaced by the CsSnI3 precursor solution during the spin coating process, and its large ionisation potential is poorly matched to the valence band edge of CsSnI3. Devices using an ITO (or Au) hole-extracting electrode with no HTL are found to be more stable than those using the archetypal HTL; PEDOT:PSS. Spectroscopic analysis of the CsSnI₃ layer recovered from PV devices after 24 hours testing in ambient air (with no device encapsulation) shows that ≤11% of the CsSnI₃ film thickness is oxidised to Cs₂SnI₆ due to air ingress, which shows that the deterioration in device efficiency under continuous illumination does not primarily result from a reduction in the light absorption capability of the perovskite film due to CsSnI₃ oxidation. Additionally, it is shown that SnCl₂ added during CsSnI₃ film preparation reduces the extent of p-type self-doping of the perovskite film and serves as an n-type dopant for the adjacent evaporated C₆₀ electron transport layer.

Copper light-catching electrodes for organic photovoltaics (Free access)

H. Jessica Pereira, Oliver S. Hutter, G. Dinesha M. R. Dabera, Luke Rochford & Ross. A. Hatton

Sustainable Energy & Fuels (2017) DOI:10.1039/c7se00077dLink opens in a new window

Abstract: 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.

Enhanced stability and efficiency in hole-transport-layer-free CsSnI₃ perovskite photovoltaics

K. P. Marshall, M. Walker, R. I. Walton & R. A. Hatton

Nature Energy 1, Article number: 16178 (2016) DOI:10.1038/nenergy.2016.178Link opens in a new window

Abstract: Photovoltaics based on tin halide perovskites have not yet benefited from the same intensive research effort that has propelled lead perovskite photovoltaics to >20% power conversion efficiency, due to the susceptibility of tin perovskites to oxidation, the low energy of defect formation and the difficultly in forming pinhole-free films. Here we report CsSnI3 perovskite photovoltaic devices without a hole-selective interfacial layer that exhibit a stability ∼10 times greater than devices with the same architecture using methylammonium lead iodide perovskite, and the highest efficiency to date for a CsSnI3 photovoltaic: 3.56%. The latter largely results from a high device fill factor, achieved using a strategy that removes the need for an electron blocking layer or an additional processing step to minimize the pinhole density in the perovskite film, based on co-depositing the perovskite precursors with SnCl2. These two findings raise the prospect that this class of lead-free perovskite photovoltaic may yet prove viable for applications.

High-performance silver window electrodes for top-illuminated organic photovoltaics using an organo-molybdenum oxide bronze inter-layer

Martin S. Tyler, Marc Walker, Ross A. Hatton

ACS Applied Materials & Interfaces (2016) DOI: 10.1021/acsami.6b02647 (Open Access)

Abstract: We report an organo-molybdenum oxide bronze that enables the fabrication of high performance silver window electrodes for top-illuminated solution processed organic photovoltaics without complicating the process of device fabrication. This hybrid material combines the function of wide band gap interlayer for efficient hole extraction with the role of metal electrode seed layer, enabling the fabrication of highly transparent, low sheet resistance silver window electrodes. Additionally, it is also processed from ethanol, which ensures orthogonality with a large range of solution processed organic semiconductors. The key organic component is the low cost small molecule 3-mercaptopropionic acid which: (i) promotes metal film formation and imparts robustness at low metal thickness.; (ii) reduces the contact resistance at the Ag / molybdenumn oxide bronze interface.; (iii) and greatly improves the film forming properties. Silver electrodes with a thickness of 8 nm deposited by simple vacuum evaporation onto this hybrid interlayer have a sheet resistance as low as 9.7 Ohms per square and mean transparency  80% over the wavelength range 400-900 nm without the aid of an anti-reflecting layer, which makes them well-matched to the needs of organic photovoltaics and applicable to perovskite photovoltaics. The application of this hybrid material is demonstrated in two types of top-illuminated organic photovoltaic devices.

An electrode design rule for high performance top-illuminated organic photovoltaics

Martin S. Tyler, Immad M. Nadeem and Ross A. Hatton

Materials Horizons (2016) DOI: 10.1039/C6MH00124F (Open access)

An electrode design rule for high performance top-illuminated bulk-heterojunction organic photovoltaics is proposed, that enables the device architecture to be simplified by removing the need for the electron selective layer at the interface with the low work function reflective electrode. This new guideline for electrode design is underpinned by device studies in conjunction with a study of the energetics at the interface between five widely used solution processed organic semiconductors of both electron donor and acceptor type, and a stable low work function reflective substrate electrode. The magnitude and distribution of space charge resulting from ground-state electron transfer from the electrode into each organic semiconductor upon contact formation is derived from direct measurements of the interfacial energetics using the Kelvin probe technique, which enables the variation in potential across the entire film thickness used in the devices to be probed.

Tin perovskite/ fullerene planar layer photovoltaics: improving the efficiency and stability of lead-free devices

Kenneth P. Marshall, Richard I. Walton, Ross A. Hatton*

J. Mater. Chem. A (2015), 3, 11631–11640. (Open access)

We report the first demonstration of orthorhombic CsSnI₃ films prepared from solution at room temperature that have defect densities low enough for use as the light harvesting semiconductor in photovoltaic devices even without using excess Sn in the preparative method, and demonstrate their utility in a model p–i–n photovoltaic device based on a CuI | CsSnI₃ | fullerene planar layer architecture. We also report an effective strategy for simultaneously improving both the efficiency and stability of these devices towards air exposure based on the use of excess of SnI₂ during CsSnI₃ synthesis from CsI and SnI₂. A combination of photoelectron spectroscopy, contact potential measurements and device based studies are used to elucidate the basis for this improvement and role of the excess SnI₂. The open-circuit voltage in these lead-free photovoltaic devices is shown to be strongly dependent on the degree of alignment between the perovskite conduction band edge and the lowest occupied molecular orbital (LUMO) in the fullerene electron transport layer. Furthermore, the energetics at the perovskite– fullerene interface are shown to be a function both of the LUMO energy of the fullerene and the nature of the interaction at the heterojunction which can give rise to a large abrupt vacuum level shift across
the interface. A champion open-circuit voltage of ~0.55 V is achieved using indene-C₆₀ bis-adduct as the electron extraction layer, which is twice that previously reported for a CsSnI₃ based PPV.

A Silver-Free, Reflective Substrate Electrode for Electron Extraction in Top-Illuminated Organic Photovoltaics

Special Issue on Organic Electronics

Martin S. Tyler, Oliver S. Hutter, Marc Walker, Ross A. Hatton^*

ChemPhysChem (2015), 16, 1203-1209 (Special Issue).

The choice of metals suitable as the reflective substrate electrode for top-illuminated organic photovoltaics (OPVs) is extremely limited. Herein we report a novel substrate electrode for this class of OPV architecture based on an Al | Cu | AlO_x triple layer structure, which offers a reflectivity comparable to that of Al over the wavelength range 400-900 nm, a work function suitable for efficient electron extraction in OPVs and high stability towards oxidation. In addition to demonstrating the advantage of this composite electrode over Al in model top-illuminated OPVs, we also present the results of a photoelectron spectroscopy study which show that an oxidised 0.8 nm Al layer deposited by thermal evaporation onto an Al | Cu reflective substrate electrode is sufficient to block oxidation of the underlying Cu by air or during deposition of a ZnO_1-x electron-transport layer, which is remarkable given that the self-limiting oxide thickness on Al metal is greater than 2 nm.

A Hybrid Copper:Tungsten Suboxide Window Electrode for Organic Photovoltaics

O. S. Hutter, R. A. Hatton*
Advanced Materials (2014), 27, 326-331.

We report a new type of window electrode for OPVs based on an ultra-thin bilayer of Cu and tungsten sub-oxide (WO_3-x) which derives its exceptional optical and electrical properties from spontaneous solid-state diffusion of Cu into the adjacent WO_3-x layer at room temperature. We show that this unpatterned Cu electrode can perform at least as well as ITO glass in efficient inverted OPVs by trapping light in a resonant optical cavity. In addition to the cost advantage associated with the use of Cu instead of Ag, this electrode also offers the advantages of: (i) a sheet resistance nearly half that of any unpatterned Cu film window electrode reported to date, at 6-7 Ω Sq^-1.; (ii) a simplified architecture as compared to triple layer Ag electrodes.; (iii) and, unlike Ag window electrodes, a transparency maxima well-matched to the peak of the solar photon flux, and thus to the emerging generation of narrow band gap polymers and non-fullerene electron acceptors.

An indium-free low work function window electrode for organic photovoltaics which improves with insitu oxidation

O. S. Hutter, H. M. Stec, R. A. Hatton*,

Advanced Materials (2013) 25, 284-288.

A low cost window electrode for organic photovoltaics which simultaneously removes the requirement for conducting oxide and conventional low work function electrodes and functions as a sink for oxygen/water in the heart of the device. Remarkably the functionality of this electrode, which is based on a 7.8 nm nano-structured Cu:Al film, improves upon insitu oxidation as demonstrated in bulk heterojunction organic photovoltaics.

Plasmon-active nano-aperture window electrodes for organic photovoltaics

H. M. Stec, R. A. Hatton*,

Advanced Energy Materials (2013) 3, 193-199.

A lithography-free method for the fabrication of optically-thin plasmon-active metal window electrodes with a dense array of nano-sized apertures on glass and plastic substrates is reported. These remarkably robust, low sheet resistance electrodes simultaneously concentrate light and extract charge carriers in solution processed and vacuum deposited organic photovoltaics and outperform indium-tin oxide electrodes on flexible substrates.

Ultra-high Voltage MultijunctionOrganic Solar Cells for Low-Power Electronic Applications

P. Sullivan, S. Schumann, R. Da Campo, T. Howells, A. Duraud, M. Shipman, R. A. Hatton*, T. S. Jones*

Advanced Energy Materials (2013) 3, 239-244.

Multijunction organic small molecule photovoltaic cells are demonstrated with open-circuit voltage values up to 7V. Current balancing is achieved in such complex structures through careful optical modelling leading to no loss in power conversion efficiency with as many as 5 stacked heterojunctions. Direct integration of single devices to batteries is shown, with charging observed under low light intensity and even monochromatic illumination

Nanoscale Geometric Electric Field Enhancement in Organic Photovoltaics

L-J. Pegg, R.A. Hatton*,

ACS Nano, 6 (2012) 4722-4730.

A novel nano-structured window electrode is utilized in conjunction with three-dimensional electrostatic modeling to elucidate the importance of geometric electric field enhancement (GEFE) effects at the electrode interfaces in organic photovoltaics. The results of this study show that nano-scale protrusions at the electrode surfaces dramatically improve the efficiency of photo-generated charge carrier extraction to the external circuit and that the origin of this improvement is the local amplification of the electric field in the vicinity of said protrusions.

Widely applicable coinage metal window electrodes on flexible polyester substrates applied to organic photovoltaics

H.M. Stec, R. A. Hatton*,

ACS Applied Materials and Interfaces (2012) 4(11) 6013-6020.

The fabrication, exceptional properties and application of 8 nm thick Cu, Ag, Au and Cu/Ag bi-layer electrodes on flexible polyethylene terephthalate and polyethylene naphthalate substrates is reported. The power conversion efficiency of 1 cm² organic photovoltaics (OPVs) employing 8 nm Ag and Au films as the hole-extracting window electrode exhibit performance comparable to those on indium-tin oxide, with the advantage that they are resistant to repeated bending through a small radius of curvature and are chemically well-defined. OPVs employing Cu and bilayer Cu:Ag electrodes exhibit inferior performance due to a lower open-circuit voltage and fill factor. Measurements of the interfacial energetics made using the Kelvin probe technique provide insight into the physical reason for this difference.

An Electrode Design Rule for Organic Photovoltaics Elucidated using Molecular Nanolayers

R. M. Cook, L-J. Pegg, S. L. Kinnear, O. S. Hutter, R. J. H. Morris, R. A. Hatton*,

Advanced Energy Materials 1(3) (2011) 440-447.

The extent to which spontaneous ground-state charge transfer from the donor layer to the hole-extracting electrode in bi-layer organic photovoltaics impacts device efficiency is investigated using a pair of model high work function electrodes based on silane nanolayer derivatized indium-tin oxide glass. Silane nanolayers at this interface are also shown to be a remarkably effective means of improving the interfacial stability under constant solar illumination.

Enhancing the Open-Circuit Voltage of Molecular Photovoltaics Using Oxidized Au Nanocrystals

L-J. Pegg, S. Schumann, R. A. Hatton*

ACS Nano 4 (2010) 5671–5678

Solution-processed surface-oxidized Au nanocrystals (o-AuNC) are shown to dramatically increase the open-circuit voltage (Voc) of OPV cells based on boron-subphthalocyanine chloride (SubPc)/C60 and chloro-aluminum phthalocyanine (ClAlPc)/C60 heterojunctions when incorporated at the interface between the hole-extracting electrode and the phthalocyanine donor layer. In addition, the cell-to-cell variation in Voc is reduced by up to 10-fold combined with a large reduction in the light intensity dependence of Voc, both of which are important advantages for practical application. The largest increase in Voc is achieved for SubPc/C60-based cells which exhibit a 45% increase to 1.09 ± 0.01 V—an exceptionally high value for a single junction small molecule OPV.