Phosphorene Nanoribbon-Augmented Optoelectronics for Enhanced Hole Extraction

Phosphorene nanoribbons (PNRs) have been widely predicted to exhibit a range of superlative functional properties; however, because they have only recently been isolated, these properties are yet to be shown to translate to improved performance in any application. PNRs show particular promise for optoelectronics, given their predicted high exciton binding energies, tunable bandgaps, and ultrahigh hole mobilities. Here, we verify the theorized enhanced hole mobility in both solar cells and space-charge-limited-current devices, demonstrating the potential for PNRs improving hole extraction in universal optoelectronic applications. Specifi- cally, PNRs are demonstrated to act as an effective charge-selective interlayer by enhancing hole extraction from polycrystalline methylammonium lead iodide (MAPbI(3)) perovskite to the poly(triarylamine) semiconductor. Introducing PNRs at the hole-transport/MAPbI(3) interface achieves fill factors above 0.83 and efficiencies exceeding 21% for planar p-i-n (inverted) perovskite solar cells (PSCs). Such efficiencies are typically only reported for single-crystalline MAPbI(3)-based inverted PSCs. Methylammonium-free PSCs also benefit from a PNR interlayer, verifying applicability to architectures incorporating mixed perovskite absorber layers. Device photoluminescence and transient absorption spectroscopy are used to demonstrate that the presence of the PNRs drives more effective carrier extraction. Isolation of the PNRs in space-charge-limited-current hole-only devices improves both hole mobility and conductivity, demonstrating applicability beyond PSCs. This work provides primary experimental evidence that the predicted superlative functional properties of PNRs indeed translate to improved optoelectronic performance.

Charge carrier and exciton dynamics within hybrid lead halide perovskites of mixed composition and dimensionality

Marine Eva Fedora Bouduban

Research is nowadays usually directed by its potentiality to birth technological applications of relevance in the present environmento-socio-economical context, which can be seen as fair. This can however appear frustrating to us scientists, as it sometimes embodies an obstacle to our curiosity and creative freedom. As a consequence, research topics exhibiting both high impact potential and strong scientific interest, in terms of novel ideas and self-questioning driving force, can be seen as some kind of Holy Grail. Hybrid lead halide perovskites (HOIPs) seem to fit this profile: demonstrating an extraordinary potential for optoelectronic applications, their high versatility in terms of composition and dimensionality allow the exploration of a large number of fascinating solid-state physics phenomena. This thesis faithfully reflects this double nature: each of the projects described is dedicated to a different type of perovskites, at the compositional of dimensional level, or to questions pertaining their embodiment in application-intended devices. Chapters 1 provides the scientific background necessary to the understanding of this work, from the fundamental concepts underlying light-matter interaction and semiconductor photophysics to the specificities of HOIPs and their optoelectronic devices. The second chapter presents the main concepts related to the experimental methods of use in this work. In particular, we discuss the interpretation of femtosecond time-resolved transient absorbance measurements and their analysis, as well as the nature of the electroabsorption signal and the information it holds about of the systems under study. In chapters 3 and 4, we discuss 3D HOIPs of mixed composition (MAyFA1âyPbI3âxBrx.), and address both the photophysics in their bulk and at their interfaces with electron-transporting and hole-transporting materials used in photovoltaic devices (SnO2 and spiro-OMeTAD). We show the formation of bulk charge transfer (CT) excitons between domains of different iodide/bromide proportion, and suggest this to be at the origin of a sustained charge separation, yielding better photovoltaic performance. Similarly, using a novel experimental strategy, we propose that charge injection at the SnO2/perovskite interface is mediated by interfacial CT excitons, and thus, slowed down. Furthermore, our experimental strategy also confirms the importance of additives in photovoltaic-intended spiro-OMeTAD layers, by demonstrating that injected charges accumulate in undoped spiro-OMeTAD. Chapter 5 constitute the transition towards perovskite systems of lower dimensionality: it presents a fundamental study of MAPbBr3 (MA = CH3NH3) nanoparticle aggregates, in solutions involving a distribution of 3D nanoparticles and quasi-2D nanoplatelets of different thicknesses. We highlight inter-structure interactions in the form of a cascade of energy and charge transfer, the latter being mediated by the formation of interparticle CT excitons. In chapter 6, we continue our exploration of the multi-dimensionality of HOIPs by focusing on 3D/2D perovskite bilayers, where the 2D layer involves the cations phenethylammonium (PEA) and 4-fluorophenethylammonium (FPEA), in the context of their photovoltaic performance. In particular, we demonstrate a strong relationship between the photophysics at 3D/2D interfaces and the crystal growth and orientation of the 2D layer, directly determined by the structure of its organic cation. We go one step

EPFL2019

Zr-doped indium oxide electrodes: Annealing and thickness effects on microstructure and carrier transport

Christophe Ballif, Mathieu Gérard Boccard, Aïcha Hessler-Wyser, Quentin Thomas Jeangros, Federica Landucci, Monica Morales Masis, Oscar Esteban Rucavado Leandro

Zr-doped indium oxide (In2O3:Zr) has been shown to satisfy the requirements of low resistance, wide band gap, and high infrared transmittance for application as a front contact in broadband solar cells. However, the reduction of indium usage in front of transparent electrodes is still an unsatisfied requirement. With the goal of reducing the amount of indium while leveraging its properties, in this work, In2O3 :Zr films with reduced thickness compared to those standardly used in solar cells are studied. 100 to 15-nm-thick films were sputtered at room temperature and annealed in distinct atmospheres to study the links between thickness, microstructure, and optoelectronic properties. As-deposited films exhibit an amorphous microstructure embedding bixbyite In2O3 nanocrystals. Annealing in neutral (N-2) or reducing atmosphere (H-2) allows a slight growth of these crystallites but the layers remain mostly amorphous. Whereas annealing in air results in polycrystalline films with an average grain lateral size ranging from 350 to 500 nm. The large crystalline grains formed during air annealing lead to increased electron mobility for all thickness: up to 100 cm(2)V(-1)s(-1) for 100-nm-thick films and up to 50 cm(2)V(-1)s(-1) for 15-nm-thick films, which is remarkable for such thin polycrystalline films. Conversely, H-2 annealing ensures high free-carrier densities (>1 x 10(20) cm(-3)) but not high mobilities, still achieving conductivities between 1000 and 2000 S cm(-1), with the films less than 50-nm-thick keeping high broadband transmittance. The possibility of thinning down In2O3:Zr to a few tens of nanometers while keeping both high lateral conductivity and good transparency makes this material a promising candidate to reduce the amount of indium in optoelectronic applications, such as flexible touch screens and solar cells.

AMER PHYSICAL SOC2019

Polymeric room-temperature molten salt as a multifunctional additive toward highly efficient and stable inverted planar perovskite solar cells

Ulf Anders Hagfeldt, Ziwei He, Jiajia Suo, Chao Tang, Jiabao Yang, Bowen Yang

The inferior power conversion efficiency (PCE) compared to their regular counterparts (n-i-p) and undesirable stability issues of inverted (p-i-n) perovskite solar cells (PSCs) are the foremost issues hindering their commercialization. Here, for the first time, we demonstrate a polymeric room-temperature molten salt (poly-RTMS), namely poly(1-vinyl-3-ethyl-acetate) imidazole tetrafluoroborate (PEa), as a novel type of additive to modulate the perovskite crystallization and its electronic properties. The PEa poly-RTMS containing multiple chemical anchoring sites along with strong bonding stability can firmly bond to Pb ion defects at grain boundaries and the interface of the perovskite film via coordination bond, which effectively passivates the electronic defects and enhances the photo-, thermal-, and moisture-stability of perovskite films. As a result, the PEa-modified inverted PSCs show striking performance improvements over the control with the PCE exceeding 21.4% and excellent long-term operational stability, maintaining over 92% of the initial efficiency for 1200 hours under continuous full sun illumination at 70-75 degrees C. This strategy opens a new avenue to modulate the properties of perovskites for optoelectronic applications.