PacSurf2016 Session EH-TuM: Surfaces & Interfaces for Solar Cells and Solar Fuels

The performance of dye-sensitised solar cells (DSSCs) depends to a large degree on the material and surface structure of the photoelectrodes (most commonly only the anode is a photoelectrode). Electrospun photoelectrodes offer a very high surface area and inter-connectivity, which should be ideal for this particular application. We will present a study on electrospun titania and nickel oxide photoelectrodes and their application in DSSCs.

Titania (TiO₂) photoanodes are by far the most studied system for DSSCs. We have shown that the performace of electrospun titania (anatase) fibre-photoelectrodes can be significantly increased by forming a single walled carbon nanotube (SWCNT) composite material [1]. Figure 1 shows scanning electron micrographs (SEMs) of a typical photoanode of this type.

Photocathodes are much less efficient compared with anodes and are replaced by platinum group metal catalysts frequently. We have studied photocathodes that are based on nickel oxide (NiO). The base semiconductors have been ‘sensitised’ with quantum dots and organic fluorophores. The morphology and nanostructure of the materials have been varied and finally, electrocatalysts have been added. It was found that sensitising can improve the performance of some p-type photocathodes significantly, but even the best performing photocathodes did not compare with equivalent TiO₂ photoanodes.

References

[1] T. J. Macdonald, D. D. Tune, M. R. Dewi, C. T. Gibson, J. G. Shapter, T. Nann, ChemSusChem2015, 8, 3396–3400.

Energy band alignments at heterointerfaces play a crucial role in defining the functionality of semiconductor devices. The search for material systems with suitable electronic band alignments is one of the key challenges in developing novel photovoltaic technologies. Particularly the tuning of band offsets has enabled significant advances in efficiency for several state of the art thin film solar cell technologies. While the theoretical prediction of band alignments at semiconductor heterointerfaces is often very limited, interface experiments utilizing X-ray photoelectron spectroscopy (XPS) measurements can be employed to track the evolution of the electronic structure at the interface. In particular the V-IV binary chalcogenide Sb₂Se₃ has been identified as a prospective absorber material for the next generation of thin film solar cells [1], yet the understanding of interfacing this compound semiconductor to commonly employed buffer layers such as ZnS is still in its very infancy.

In recent years high throughput, combinatorial materials science methodology has gained tremendous interest [2]. In this work we demonstrate how combinatorial techniques can be utilized to increase the throughput of in-situ experiments to investigate the interface formation for respective semiconductor heterojunctions. Thin films are grown with intentional thickness gradients on the substrate, to minimize the required number of depositions and transfers. In addition, other deposition parameters such as the deposition temperature or composition can be systematically co-varied with thickness to cover several individual interface experiments on one substrate.

Using orthogonal thickness and temperature gradients a combinatorial in-situ interface experiment was set up, that provided a comprehensive dataset for the extensive investigation of the band alignment at the prototypical Sb₂Se₃/ZnS heterojunction. Our findings point to a large conduction band offset of ΔE_CB= 1.75 ±0.05 eV along with a potentially detrimental interfacial reaction at the junction. With this result, the study reveals why the choice of a ZnS buffer layer would limit device parameters and gives initial guidelines for alternative buffer layer materials with smaller electron affinities.

The method presented in this contribution can easily be applied for other material systems and will enable other researchers to quickly acquire comprehensive data for a more accurate picture of electronic processes across potential device interfaces.

[1] Y. Zhou et al. Nat. Photonics 2015, 9, 409.

[2] M. L. Green et al. J. Appl. Phys. 2013, 113, 231101.

The random networks of silver nanowires (AgNWs) have been noticed as next generation transparent electrode which satisfies the high conductivity, transmittance and flexibility at the same time. Thus, AgNWs have a great potential for replacement of indium-tin-oxide (ITO) transparent electrode which have been used most widely. Although the nature of the superb self-electrical, optical and mechanical properties, some disadvantages of the transparent electrode such as chemical, environmental stability and contact resistance are bottle-necks for practical applications.

A simple approach to improve the stability of AgNW electrode is to form low-reactive materials such as graphene, graphene oxide and alumina on the top of AgNWs as passivation layers. Chemical and environmental stability is increased and electrical resistance is decreased. However, there are some negative aspects which are losses of transmittance, increasing vertical electrical resistance and upper layer and unisotropic chemical etching.

In this study, plasma techniques are applied to augment the electrical and environmental properties of AgNWs transparent electrode. The electrical resistance is decreased and environmental stability is improved. Moreover, AgNWs with plasma treatment showed much improved thermal stability compared to untreated AgNWs. We applied the plasma treated AgNWs to flexible organic photovoltaic devices as transparent electrode, and its power conversion efficiency is similar to organic photovoltaics based on ITO/Glass.

References

[1] J. Photochem. Photobiol. C: Photochem. Rev. 11 (2011) 179.

[2] Bull. Chem. Soc. Jpn. 84 (2011) 1000.

[3] J. Phys. Chem. B 109 (2005) 16052.

[4] a) Chem. Commun. (2005) 3829; b) Chem. Mater. 21 (2009) 1543; b) J. Am. Chem. Soc. 132 (2010) 5858; c) ChemSusChem 4 (2011) 228.

[5] J. Am. Chem. Soc. 135 (2013) 16872.

[6] J. Am. Chem. Soc. 38 (2016) 2082.

[7] a) J. Am. Chem. Soc., 132 (2010) 11828; b) Energy Environ. Sci. 4 (2011) 4138; c) J. Am. Chem. Soc. 134 (2012) 6968; d) J. Am. Chem. Soc. 135 (2013) 10238.

High power conversion efficiency of perovskite-based solar cells offers promise for low-cost and scalable production of renewable energy. The need to harness solar energy has recently motivated the search to alternate ETL/HTL hybrid materials, specifically graphene/perovskite films. Hybrid organic-inorganic methylammonium lead halides, MAPbX₃ (X=I, Br, Cl)/mixed-halides (I_3-xCl_x, I_3-xBr_x) have been reported as light harvesting layers with their superior optoelectronic properties: tunable bandgap, long electron-hole diffusion lengths and high electron/hole mobility. Nevertheless, halide-based perovskites require in situ investigation for film growth, degradation and perovskite formation mechanisms to overcome detrimental effects of incomplete lead precursor conversion, inconsistent crystallite formation/film uniformity, and weak cation-anion-solvent coordination. Graphene-derived hybrids have recently emerged as an ETL/HTL replacement in these devices, however understanding the origin of interfacial chemical reactions between deposited perovskite films over graphene-derived materials is still lacking, especially at the ETL/perovskite/HTL interfaces. Moreover, effect of film thickness, stoichiometry control, overlayer/ underlayer morphology/composition, stability issues and cation-anion electrostatic interactions ought to be examined for better charge transport. To address scalability and stability issues, we examined degradation, nucleation and growth mechanisms in reduced graphene/graphite oxide (RGO) upon halide-based (I, Cl, Br) perovskite deposition. Chemical interactions were interpreted at perovskite/RGO interfaces for the grain size, orientation, boundaries, and surface/bulk effects using variable-temperature (≤600°C, Ar(g)) in situ spectroscopy (infrared absorption, micro-Raman, UV-vis-NIR, luminescence) and SEM, XRD, XPS, TEM, and AFM for morphology characterization. Controlled perovskite formation was achieved at room temperature for bromide/chloride-based perovskites resulting in improved chemical stability with heat (vs. iodide derivative). Overall, perovskite decomposition and oxygen-induced chemical reactions occurred at ≤150°C, eliminated hydroxyls/carboxyls in RGO, and maintained ethers/epoxides upon perovskite decomposition. Poor perovskite formation was observed on RGO due to varying electron affinity and reactivity of precursor halides, resulting in film degradation in air (O₂, H₂O). References: (1) M Acik, SB Darling. J. Mater. Chem. A (2016) 4, 6185-6235. (2) J Gong, SB Darling, F You, Energy Environ. Sci. (2015) 8, 1953-1968 (3) M Acik, G Lee, C Mattevi, M Chhowalla, K Cho, YJ Chabal. Nature Materials (2010) 9 (10), 840-845.

Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.

Photoelectrochemical water splitting is a promising route for efficient conversion of solar energy to chemical fuel. Among different photoelectrode materials, bismuth vanadate (BiVO₄) is one of the most actively investigated oxide semiconductors due to its moderate bandgap, favorable conduction band position, and relatively long photocarrier lifetimes.[1] However, under relevant operating conditions, pristine BiVO₄ thin films are subjected to degradation at the exposed surface facets. The degradation process in solution is accelerated by photoexcitation, which causes trapping of photogenerated holes at localized surface sites.[2] Therefore, developing approaches to stabilize these efficient semiconductor nanostructures requires a detailed understanding and control of charge separation, transport, and recombination mechanisms at their relevant length scales.

Here, we use photoconductive atomic force microscopy in combination with Kelvin probe force microscopy to correlate local surface morphology with generated photocurrent and contact potential difference maps at individual grain facets in polycrystalline BiVO₄ films. Furthermore, we employ scanning transmission X-ray microscopy to trace the changes in local chemical structure and composition in pristine and photodegraded BiVO₄.

The photocurrent and contact potential difference maps reveal the impact of different working conditions, such as bias voltage, excitation energy or excitation power, on the local charge carrier dynamics. Both for excitation above the bandgap (405 nm) and sub-bandgap illumination (532 nm), the photocurrent maps resolve the contributions from individual grains with nanometer spatial resolution. The photoelectrochemical performance of BiVO₄ can be significantly enhanced by varying the oxygen vacancy defects and hydrogen impurities, through hydrogen annealing. Therefore, we compare the photocurrent generation of pristine and hydrogen annealed BiVO₄. This careful analysis allows us to identify locally the charge transfer and loss mechanisms in these materials which ultimately contribute to desired photocurrent generation or undesired photocorrosion.[3]

[1] J. K. Cooper et al., The role of hydrogen in defining the n-type character of BiVO₄ photoanodes, Chem. Mater. 2016.

[2] F. M. Toma et al., Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes, Nat. Commun. 2016.

[3] J. Eichhorn et al., Nanoscale photocurrent imaging of monoclinic BiVO₄ films via atomic force microscopy, 2016 (in preparation).

Hybrid organic/inorganic perovskites define an emerging class of solar cell absorber materials which advances to the lead in maximum power conversion efficiencies in the area of thin-film solar cells.¹ In the photovoltaic device the electronic interaction between the perovskite absorber and adjacent charge extraction and transport layers is key to maximize cell functionality. We identified that device characteristics such as the open circuit voltage can be affected by the alignment of the electronic energy levels of an organic charge extraction layer with the electronic transport levels in the perovskite film.² Furthermore, we found that the doping characteristic of the underlying oxide substrate can be used to rigidly tailor the Fermi level position in a subsequently deposited perovskite film. For future applications such as the integration of a perovskite subcell into a tandem device, precise control over the electronic alignment processes is required.³

Here we present our recent findings in which we examine a selection of incrementally deposited oxide charge carrier transport layers on top of methylammonium and formamidinium lead iodide perovskite films. Functional n-type (e.g. TiO₂, MoO_x), p-type (e.g. NiO) and intrinsic oxides (e.g. Al₂O₃) are grown by pulsed laser and atomic layer deposition techniques on top of the perovskite absorber layer. We then use ultraviolet and X-ray photoemission spectroscopy (UPS/XPS) to determine the electronic energy level alignment at the oxide/perovskite interface while at the same time tracking changes in the interface chemistry. Using this approach we are able to explain band offset changes induced in the perovskite layer by chemical interactions with the oxide on top, changes in the electrostatic potential and the formation of defective surface layers. The results are not only used to give guidelines about how to embed oxide layers into perovskite photovoltaic devices but also inform to what extent the electronic structure of the perovskite is subject to extrinsic perturbations on a more universal scale.

[1] K. Emery, Best research‐cell efficiencies. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg, Ed. NREL: 2016

[2] P. Schulz, E. Edri, S. Kirmayer, G. Hodes, D. Cahen, A. Kahn, Energy Environ. Sci.2014, 7, 1377

[3] P. Schulz. L. L. Whittaker-Brooks, B. A. MacLeod, D. C. Olson, Y.-L. Loo, A. Kahn, Adv. Mater. Interfaces 2015, 2, 1400532