Interface Design for Metal Halide Perovskite Solar Cells

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Interface Design for Metal Halide Perovskite Solar Cells Philip Schulz ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00404 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Energy Letters

Interface Design for Metal Halide Perovskite Solar Cells

Philip Schulz

CNRS, Institut Photovoltaique d'Ile de France (IPVF), UMR 9006, 30 route départementale 128, 91120, Palaiseau, France National Renewable Energy Laboratory, Golden CO, 80401, USA

ABSTRACT: With the rise of thin-film photovoltaics, the research community brought the interfaces within the device into the spotlight for performance enhancements. This perspective lays out why this notion is especially applicable for solar cells based on Metal Halide Perovskite (MHP) absorber materials, for which we reach a significant improvement of device functionality and stability through the means of engineering the energetics and electronic properties at the interface. After examining recent key developments in chemical and electronic structure characterization of MHPs that led to this avenue, we will iterate the next steps towards interlayer tailoring and analysis. The future goal is to derive a comprehensive model for interface energetics in perovskite solar cells and to set up respective design rules for stable high efficiency cells. This approach also pertains to the broader scope of MHP-based optoelectronic devices for which charge carrier transport across the interfaces and surface recombination play a major role.

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MANUSCRIPT TEXT: As our future energy supply will be dependent on our ability to access renewable energy sources, bridging the gap between fundamental materials research and technologic application appears to be more imperative than ever before. In order for photovoltaics to reach multi-terawatt levels as is required for the massive decarbonization of the energy economy, we need novel materials that go one step beyond the current silicon-based technology. Thus, the research community has dedicated substantial effort in seeking energy materials that can be manufactured at high volume, at low cost and that are highly efficient, reliable and stable.1 Absorber materials based on the emerging class of metal halide perovskites (MHP) are on a route to fulfill these needs and have risen to newfound prominence in recent years due to their disruptive success in the research field of solar cell applications.2 At the heart of every photovoltaics related technology is the understanding and control of the principles of semiconductor physics, which relies on the perfection and periodicity of crystalline lattices. This condition is clearly disrupted at junctions and interfaces between absorber and selective contact materials required for extracting charge carriers on opposite ends of the device. Interestingly, in most relevant photovoltaic technologies, the classical p-n homojunction between differently doped parts of the same material has become largely irrelevant. Instead, heterojunctions between different materials are preferred because they offer a higher degree of freedom in the design of devices and ultimately higher performance. However, heterojunctions also impose more stringent requirements on the characterization and understanding of the properties of these interfaces. This argument becomes even more relevant when we consider the fact that the trend in photovoltaics is toward higher efficiency devices (such as tandem devices) that have an even higher degree of complexity and a larger number of interfaces. The main loss process in photovoltaics (and in most other optoelectronic devices) is non-radiative recombination of electrons and holes via defect states. To suppress this recombination both the defect density and the energetic alignment at the interface between two materials need to be characterized, understood, and optimized (see figure 1). We can thus not only consider interfaces and surfaces as one of the largest ‘defect’ for the solid-state system, but also regard the interface as one of the primary contributors to the mode of operation and functionality of a device.3 With the latest developments in electronic and energy applications this shift in paradigms becomes


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ACS Energy Letters

even more prevalent and the emerging challenges in the field of photovoltaics demonstrate this trend very distinctively. The quality of light absorber materials has reached such a high level that once an incident photon has been absorbed, the carrier lifetime (monocrystalline Si: ms) vastly exceeds the average diffusion time (µs) to the interfaces.4 Therefore, recombination in the bulk becomes negligible whereas recombination at the interfaces featuring longer carrier residence times dominates the observed loss mechanisms.5 Note that this assumption for negligible recombination in the bulk generally requires careful evaluation for novel absorber materials. The role of grain boundaries or internal interfaces can lead to significant non-radiative losses in the bulk, particularly for nanocrystalline and quantum dot absorber films. We can express the main physical loss mechanisms of the surface recombination (loss) current JS of electrons at a hole contact by: = ∆ ⁄ with q the unit charge, S the surface recombination velocity, n0 the electron density at the hole contact in thermal equilibrium, ∆EF the quasi Fermi level splitting and the thermal energy kBT. Hence, for a given ∆EF, JS is large for high surface recombination velocities and high charge carrier densities. S is mainly determined by the density of defects which can act as recombination centers, whereas n0 is determined by the potential drop at the electrodes. The higher the built-in voltage, the lower n0 at the hole contact and also the lower the equilibrium hole concentration p0 at the electron contact. This example underlines that the defect density and the energy level alignment at the interfaces between photovoltaic absorber materials and their respective contacts are the main contributions to recombination losses. While these considerations are universally true for conventional photovoltaic systems, we need to put specific emphasis on this issue as we move beyond crystalline silicon based solar cells. MHPs as a class of new absorber materials with tunable wide optical band gap in perovskite solar cells (PSC) offer the opportunity to implement advanced concepts like tandem solar cell architectures to increase power conversion efficiency in a scalable and cost-effective manner.6 However, these light absorbers face very particular challenges in regard of respective efforts in interface engineering as we lack a comprehensive model on the detailed nature, density and origin of defect states. Impedance spectroscopy measurements of MHP based solar cell stacks suggest that surface recombination has a pronounced impact on the reduction of the attainable photovoltage in PSCs.7 The



result is mirrored by recently developed numerical simulations of transient photoluminescence data of MHP/charge transport layer stacks, which elucidates the significant role of interface recombination in MHP optoelectronic devices.8 Furthermore, in a recent study we underline the importance of interfaces for the stability of PSCs. In a remarkable result we used a combination of a tailored oxide bottom electrode and a newly synthesized organic hole transport layer which enabled us to fabricate high efficiency PSCs with over a thousand hours of device lifetime under constant operation in ambient and without any means of encapsulation.9 This study demonstrates that even physical properties such as compositional stability, which were inherently attributed to the perovskite bulk, are effectively controlled via the interface layout (figure 2a). We attribute this remarkable improvement in stability to a more chemically inherent interface by replacing TiO2 with SnO2 and less permeability to extrinsic ions through the top contact and hole transport material.

Figure 1 - Schematic of interface related loss mechanisms in a simplified photovoltaic device. After photoexcitation (1) and loss-less transport of charge carriers to the contact interfaces, carrier extraction can be impaired by (2) interfacial energy barriers due to imperfect band alignment, (3) defect driven interface recombination S and (4) back recombination. ECBM, EVBM, EF are the conduction band minimum, valence band maximum and Fermi level, respectively.


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ACS Energy Letters

In another example we developed a surface engineering process to deploy the next generation of perovskitebased quantum dot solar cells.10 In this specific case the treatment of a CsPbI3 quantum dot film with a formamidinium iodide salt solution led to a better interconnection between the dots within the array and hence a higher charge carrier mobility ultimately resulting in a record power conversion efficiency of over 13.4% (figure 2b). Thus, key question to be addressed in future research endeavors will be to better understand and resolve persisting entanglements of interface and bulk effects. In the following we will discuss the underpinnings that led to these recent results and depict the groundwork for further improvement. Starting with the definition, MHPs are named after the preferential crystallization in cubic, tetragonal or orthorhombic ABX3 structures with A mostly being an organic cation (e.g. methylammonium CH3NH3+ (MA) or formamidinium CH5N2+ (FA), B a divalent metal cation (e.g. Pb2+) and X a halide anion (e.g. I-), see figure 3a). The material class now constitutes a new type of emerging solar cell platform that operates at performance levels competing with the most efficient thin-film photovoltaic devices.2,11 MHP compounds demonstrate remarkable semiconductor properties, many of which suggest a potential for their use beyond traditional photovoltaics (e.g. long-lived charge carriers, room temperature optical Stark effect).12-16 In particular, the electronic properties of the compounds remain unperturbed even throughout marked structural and chemical stress.17 This defect tolerance of the halide perovskites, which are typically highly disordered, is in marked contrast to the traditional single crystal III-V semiconductors, although the carrier-cooling rates and photovoltaic performance are comparable across these material systems. The high values achieved for power conversion efficiencies of PSCs are attributed to the strong optical absorption with comparably low recombination rates in the bulk. Despite the high degree of disorder and large density of defects in the bulk and internal interfaces, this leads to large diffusion length for both types of charge carriers.18 Yet, a precise characterization of these properties in thin-films needs to account for perturbations at grain boundaries in the material as well as electronic coupling at interfaces to the charge transport layers. These interdependencies between bulk and interface properties are particularly hard to decouple in MHPs. For example, changing the interface between perovskite absorber and charge transport layer impacts multiple properties simultaneously. First and foremost, recombination of charge carriers at the interface is affected. At the same time a change in the chemical potential can drive ion migration in the



perovskite absorber and thus alter the bulk optoelectronic properties. These limitations in understanding and differentiation have been identified as the major obstacle to advance this emerging PSC technology.19-21 To solve this issue a clear assessment of the electronic structure of MHPs within the complex device architecture is required.22 In particular, the interplay of the perovskite film with typical transport layer

Figure 2 - a) Interface layout and evolution of power conversion efficiency of perovskite solar cell with specifically tailored hole- and electron transport layer as well as MoOx/Al electrode for enhanced stability. Reprint from ref 9. b) Scheme and device cross section for FAI surface treated CsPbI3 quantum dot solar cell with record power conversion efficiency Reprint from ref 10.


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ACS Energy Letters

materials such as conductive oxides, organic semiconductors, metal contacts or nanocomposites such as carbon nanotubes (CNT) is of high relevance for the fundamental understanding and application. In the device, the interfaces to these transport layers determine the rates of charge extraction and at the same time perturb or even template the electronic structure in the absorber film at the nanoscale.23-28 A versatile approach to describe the effects imposed on the electronic structure of the functional layers is to determine the energy level positions and construct the energy diagram for the layer stack. Subsequently, we can identify physical mechanisms such as interface barrier heights and the degree of band-bending that are either beneficial or detrimental for the device operation. With respect to this methodology, photoemission spectroscopy has proven to be a powerful tool for the investigation of energy level alignment at interfaces in hybrid materials systems.29 Using direct and inverse photoemission spectroscopy, we can determine the single particle band gap and the onsets of the electronic bands from the Fermi level, a set of parameters which is key for describing the mode of operation and governing efficient charge harvesting at the cell terminals.23,30 However, given the metastable nature of the compounds and the photon flux induced changes of the probed specimen, a dedicated analysis routine is required as even exposure to vacuum conditions and white light bias could alter the observed results.31 A compilation of the opportunities and shortcomings of these methods can be found in the study of Hoye et al.32 while we give a direct example for the deliberate testing of metastability and accompanying defect tolerance of MHP absorber materials in Steirer et al.17 With the awareness of these methodological peculiarities the evaluation of interfaces between MHP absorber film and transport layer materials can be targeted. Initial studies were focused on a subset of MHP / organic transport layer interfaces.23,24 These systems showed an absence of changes in the energy level positions of the perovskite layer. A first guideline inspired from these studies has been that the choice of those organic transport layers should be such, that the positions of its electronic energy levels (ionization energy and electron affinity) should match the respective levels in the MHP to not limit the attainable device performance.33 In contrast to this interfacial energy level alignment, photoemission spectroscopy experiments of MHP / oxide film interfaces reveal that a pronounced change in the Fermi level position as well as band bending in the



MHP layer can be observed.24,34 On the one hand adoption of the doping type in the perovskite film from the oxide substrate layer along with the fairly unperturbed energetic alignment to adjacent organic transport layers hints to why perovskite films can be implemented in a large variety of conventional and inverted device architectures. On the other hand, band bending in the perovskite film induced by an adjacent oxide transport layer can result in adverse effects on device functionality. Figure 3b shows an example for the analysis of a high work function oxide interlayer integration between absorber film and organic transport layer which eventually led to diminished device performance. The exact mechanism for this effect has yet to be resolved; for instance Hard X-ray photoemission spectroscopy measurements and complementary characterization techniques such as UV-Vis spectroelectrochemistry can yield additional information of the buried interface.3537

A further study indicates that interface state densities related to the MHP film could determine the energy

level alignment.38 More generally, we still need to resolve the exact cause of these effects and its full implication on device functionality. In particular, it remains unknown how the observed changes directly impact charge carrier transfer across the interface. We need to highlight, that the chemical reactivity of MHP’s with oxide layers can be pronounced (as in the case of the MHP/MoO3 interface)34 and hence changes of the electronic properties at the interface as described above can also originate from chemical reactions at said interface. For example, various groups reported that reversible chemistry occurs at the solid-state TiO2/CH3NH3PbI3 interface, which is readily encountered in the

Figure 3 - a) Orthorhombic ABX3 perovskite crystal structure. b) Example for layer stack with a complex interlayer between perovskite absorber and organic hole transport materials (HTM). c) The energy diagram derived from photoemission spectroscopy studies reveals that the high work function molybdenum oxide in the interlayer causes band bending in the perovskite film detrimentally affecting charge carrier extraction and hence cell performance. Reprint from ref 34.


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ACS Energy Letters

most common PSC architectures.39 A targeted modification of this interface, for instance by producing excess PbI2, is reported to improve the electronic coupling between the MHP absorber and the oxide charge transport layer.40 Analogously, the need for chemical inertness can be expanded onto the many interfaces present in MHP-based optoelectronic devices.41,42 However, it had been the replacement of the bottom oxide layer that added the most significant enhancement of device stability in our most recently reported device architecture.9 These considerations play a significant role in the current development of alternative charge transport layer configuration for PSCs. A promising approach has been the use of CNT and related nanomaterials for charge carrier extraction from the absorber.43 Beside their convincing performance in the device, CNT interlayers depict a unique study systems for interface engineering in perovskite electronics. Most recent work assesses the use of semiconducting single walled carbon nanotubes (s-SWCNT) as alternative charge carrier extraction layers in PSCs by combining photoemission and transient optical spectroscopy.44 Initial results on the respective s-SWCNT / hybrid perovskite interfaces have shown that a preferential charge transfer and energetic alignment in the ground state would in fact be beneficial for rapid hole extraction. Eventually, the narrow material-specific optical spectroscopic signature of the s-SWCNT has been used to unambiguously track the kinetics of carrier diffusion, interfacial charge transfer and recombination as laid out in Figure 4.44,45 Specific knowledge of these interfacial design principles allowed us to formulate a more robust integration of MHP absorber films into the device geometry.45 In summary, the research field of MHP-based solar cells saw a significant surge in the past five years. The

Figure 4 - a) Ground state electron transfer from MHP to s-SWCNT leads to n-doping of s-SWCNT at the interface with preferential band bending for hole extraction. Reprint from ref 44. b) This unique energetic alignment results in ultrafast excited state charge transfer with suppressed recombination rates. Reprint from ref 45.



community was able to considerably extend its understanding of basic physical properties of MHPs and identify candidates for adjacent transport layer configurations. Nonetheless, we still lack the description of the exact function of each component in the hybrid material and their impact on the macroscopic physical properties such as electronic transport characteristics. With respect to interfacing the absorber film to charge transport layers, the current understanding comprises the effect of the electronic energy level alignment on the device functionality only for the most common layer combinations. Yet, the full implications of the interface formation on recombination and barrier formation at the interface convoluted with changes of the bulk properties and the related device physics remain a major obstacle. This challenge to describe the desired energy level alignment between these electronically dissimilar compounds deters us from formulating clear guidelines for prospective material combinations. In order to overcome these limitations, our body of most recent work illustrates the avenue to generate a universal picture of the fundamental electronic processes across interfaces in perovskite solar cells with the following three steps:

•

First, a reliable methodology needs to be chosen and further developed to capture the most relevant physical and chemical properties of the interface system. Most likely this can be achieved by combining advanced optical and electron spectroscopies, to correlate charge carrier dynamics with the interface energetics. Focus will be on a precise measurement of energy barriers, interface defects, and associated carrier transfer and recombination rates, while carefully refined measurement protocols need to take transient changes and sample damage into account.

•

Second, using the tools developed in step one a more comprehensive model for the electronic processes at the interfaces and a clear correlation to the device physics needs to be established. This step involves a broad screening of potential transport layer materials beyond the classical organic semiconductor and transparent conductive oxides while evaluating their interaction with the perovskite absorber. Concomitantly performed electrical measurements of the full device then enable a verification of the determined charge transfer barriers and recombination velocities.

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Third, the correlation between interface and bulk properties of (hybrid) MHP compounds needs to be investigated to fine-tune the resultant electronic heterostructures needed for novel devices. This effort will

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ACS Energy Letters

require advanced characterization and analysis routines with variable depth resolution (such as synchrotron based XPS methods) as well as synthesis routes for dedicated layer-by-layer grown study systems, e.g. through CVD processes with improved precision and reproducibility.

Following through with these steps could endow the research community with a more comprehensive picture encompassing not only the full implications of the energetic alignment on the electrical properties of the layer stack but also the atomistic origin of the electronic interactions between the material systems. On an immediate time scale a primary goal remains to gain insight and better guidelines for the right material choices as well as film processing to boost efficiency, stability and reliability of perovskite photovoltaics. In the long run, we want to leverage a profound understanding of the interfaces in hybrid organic inorganic compounds for novel electronic applications, e.g. in the form of MHP heterostructures. Particularly the insight gained regarding defects and carrier transport in such heterostructures could enable novel MHP-based technologies like hot-carrier solar cells, novel tunable quantum-well lasers, and room-temperature qubits.

BIOGRAPHY: Philip Schulz holds a position as Research Director for Physical Chemistry and New Concepts for Photovoltaics at CNRS. In this capacity he leads the “Interfaces and Hybrid Materials for Photovoltaics” group at IPVF via the “Make Our Planet Great Again” program, which was initiated by the French President Emmanuel Macron. Before that, he has been a postdoctoral researcher at NREL from 2014 to 2017, and in the Department of Electrical Engineering of Princeton University from 2012 to 2014. Philip Schulz received his Ph.D. in physics from RWTH Aachen University in Germany in 2012.

ACKNOWLEDGEMENTS: This work has been supported by the hybrid perovskite solar cell program of the National Center for Photovoltaics funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Office of Solar Energy Technology under Award Number DE-AC3608GO28308DOE with the National Renewable Energy Laboratory (NREL). I thank the French Agence Nationale de la Recherche for funding under the contract number ANR-17-MPGA-0012. I thank Prof. T.



Kirchartz, Dr. J. J. Berry, Dr. G. Teeter and Dr. N. M. Haegel for intense discussions and fruitful exchanges which served as a base for this perspective manuscript.

QUOTES: “This example underlines that the defect density and the energy level alignment at the interfaces between photovoltaic absorber materials and their respective contacts are the main contributions to recombination losses.” “This study demonstrates that even physical properties such as compositional stability, which were inherently attributed to the perovskite bulk, are effectively controlled via the interface layout.” “Focus will be on a precise measurement of energy barriers, interface defects, and associated carrier transfer and recombination rates, while carefully refined measurement protocols need to take transient changes and sample damage into account.”

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ACS Energy Letters

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Interface Design for Metal Halide Perovskite Solar Cells

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