Jorge Ávila

Trap-Assisted Non-Radiative Recombination in Organic-Inorganic Perovskite Solar Cells

Zernike Institute for Advanced Materials University of Groningen Nijenborgh 4, 9747 AG , Groningen , The Netherlands E-mail: wetzelaer@mpip-mainz.mpg.de M. Scheepers, A. Miquel Sempere, C. Momblona, J. Ávila, Dr. H. J. Bolink Instituto de Ciencia Molecular Universidad de Valencia C/J. Beltran 2, 46980 Paterna , Spain E-mail: henk.bolink@uv.es [+] Present address: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz Germany

Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions

Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p–i–n and n–i–p CH3NH3PbI3 solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current–voltage (J–V) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the J–V hysteresis.

Quantification of spatial inhomogeneity in perovskite solar cells by hyperspectral luminescence imaging

Vacuum evaporated perovskite solar cells with a power conversion efficiency of 15% have been characterized using hyperspectral luminescence imaging. Hyperspectral luminescence imaging is a novel technique that offers spectrally resolved photoluminescence and electroluminescence maps (spatial resolution is 2 micrometer) on an absolute scale. This allows, using the generalized Planck’s law, the construction of absolute maps of the depth-averaged quasi-Fermi level splitting (Δμ), which determines the maximum achievable open circuit voltage (Voc) of the solar cells. In a similar way, using the generalized reciprocity relations the charge transfer efficiency of the cells can be obtained from the hyperspectral images. Very strong inhomogeneity, both in quasi-Fermi level splitting (Δμ) and in charge transfer efficiency, are found in these vacuum deposited perovskite solar cells. This implies that these efficient solar cells are still far from perfect as many areas in the device do not or only partially participate in the photon to electron conversion processes.

Anionic Cyclometalated Iridium(III) Complexes with a Bis-Tetrazolate Ancillary Ligand for Light-Emitting Electrochemical Cells

A series of monoanionic Ir(III) complexes (2-4) of general formula [Ir(C^N)2(b-trz)](TBA) are presented, where C^N indicates three different cyclometallating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline), b-trz is a bis-tetrazolate anionic N^N chelator (H2b-trz = di(1H-tetrazol-5-yl)methane), and TBA = tetrabutylammonium. 2-4 are prepared in good yields by means of the reaction of the suitable b-trz bidentate ligand with the desired iridium(III) precursor. The chelating nature of the ancillary ligand, thanks to an optimized structure and geometry, improves the stability of the complexes, which have been fully characterized by NMR spectroscopy and high-resolution MS, while X-ray structure determination confirmed the binding mode of the b-trz ligand. Density functional theory calculations show that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are mainly localized on the metal center and the cyclometalating ligands, while the bis-tetrazolate unit does not contribute to the frontier orbitals. By comparison with selected classes of previously published cationic and anionic complexes with high ligand field and even identical cyclometallating moieties, it is shown that the HOMO-LUMO gap is similar, but the absolute energy of the frontier orbitals is remarkably higher for anionic vs cationic compounds, due to electrostatic effects. 2-4 exhibit reversible oxidation and reduction processes, which make them interesting candidates as active materials for light emitting electrochemical cells, along with red, green, and blue emission, thanks to the design of the C^N ligands. Photoluminescence quantum yields range from 28% (4, C^N = pqu, red emitter) to 83% (3, C^N = dfppy, blue emitter) in acetonitrile, with the latter compound reaching 95% in poly(methyl methacrylate) (PMMA) matrix. In thin films, the photoluminescence quantum yield decreases substantially probably due to the small intersite distance between the complexes and the presence of quenching sites. In spite of this, surprisingly stable electroluminescence was observed for devices employing complex 2, demonstrating the robustness of the anionic compounds.

Efficient wide band gap double cation – double halide perovskite solar cells

In this work we study the band gap variation and properties of the perovskite compound Cs0.15FA0.85Pb(BrxI1−x)3 as a function of the halide composition, with the aim of developing an efficient complementary absorber for MAPbI3 in all-perovskite tandem devices. We have found the perovskite stoichiometry Cs0.15FA0.85Pb(Br0.7I0.3)3 to be a promising candidate, thanks to its band gap of approximately 2 eV. Single junction devices using this perovskite absorber lead to a maximum PCE of 11.5%, among the highest reported for solar cells using perovskites with a band gap wider than 1.8 eV.

Influence of mobile ions on the electroluminescence characteristics of methylammonium lead iodide perovskite diodes

In this work, we study the effect of voltage bias on the optoelectronic behavior of methylammonium lead iodide planar diodes. Upon biasing the diodes with a positive voltage, the turn-on voltage of the electroluminescence diminishes and its intensity substantially increases. This behavior is reminiscent of that observed in light-emitting electrochemical cells (LECs), single-layer electroluminescent devices in which the charge injection is assisted by the accumulation of ions at the electrode interface. Because of this mechanism, performances are largely independent from the work function of the electrodes. The similarities observed between planar perovskite diodes and LECs suggest that mobile ions in the perovskite do play an important role in device operation. Besides enhanced electroluminescence, biasing these devices can also result in improved photovoltaic performance.

Impact of the use of sterically congested Ir(III) complexes on the performance of light-emitting electrochemical cells

The synthesis, structural and optoelectronic characterization of a family of sterically congested cyclometalated cationic Ir(III) complexes of the form [Ir(C^N)2(dtBubpy)]PF6 (with dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine and C^N = a cyclometalating ligand decorated at the 4-position of the pyridine ring and/or the 3-position of the phenyl ring with a range of sterically bulky substituents) are reported. This family of complexes is compared to the unsubstituted analogue complex R1 bearing 2-phenylpyridinato as cyclometalating ligand. The impact of sterically bulky substituents on the C^N ligands on both the solid state photophysics and light-emitting electrochemical cell (LEEC) device performance is investigated. X-ray diffraction analysis of complexes 1a, R2, 2a, and 1b show an increasing internuclear distance in the solid state, within these four complexes. Emission studies in solution and neat film show that the chosen substituents essentially do not impact the emission energy. The photoluminescence quantum yields (ΦPL) are in the same range (ΦPL ∼ 25–31%), except for 1b, which shows a lower ΦPL of 12%. All complexes exhibit similar monoexponential emission lifetimes in the submicrosecond regime. LEECs based on R1, 1a, 1b and R2 were fabricated, showing yellow luminescence and moderate efficiencies and lifetimes. The arguably best performing LEEC device, showing the highest luminance (737 cd m−2), current efficiency (7.4 cd A−1) and EQE (2.6%), employed emitter 1a.

Influence of doped charge transport layers on efficient perovskite solar cells

Planar vacuum deposited p–i–n methyl ammonium lead tri-iodide perovskite solar cells are prepared with different electron and hole transporting layers, either doped or undoped. The effect of these layers on the solar cells performance (efficiency and stability) is studied. The main benefit of using doped layers lies in the formation of barrier free charge extraction contacts to the electrodes. However, this comes at the cost of increased residual absorption (reducing the current density and efficiency of the cells) and a decreased stability. A generic solar cell structure using undoped charge extraction layers is presented, containing a thin layer of a strong electron acceptor in between the transparent electrode and the hole transport layer, that leads to efficiencies of 18% and a significant (>5 times) improvement of the stability.

Can we use time-resolved measurements to get steady-state transport data for halide perovskites?

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. Howev-er, as yet no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state pa-rameters. To answer the latter question, we revisited pulsed excitation, and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We per-formed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed TR-MC and TR-PL measurements as function of excitation intensity on the same samples of dif-ferent MAPbI3 thin films, and find good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of photo-transport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.

Strontium insertion in methlyammonium lead iodide: long charge carrier lifetime and high fill factor solar cells(Conference Presentation)

Organic-inorganic (hybrid) lead halide perovskites are taking the lead among the emerging photovoltaics technologies, thanks to the demonstration of power conversion efficiencies exceeding 20 %. Hybrid perovskites have a wide spectrum of desirable properties; they are direct bandgap semiconductors with very high absorption coefficients, high and balanced hole and electron mobility, and large diffusion length. A unique feature of these materials is their versatility in terms of bandgap energy, which can be tuned by simple exchange of their components. In this paper we present vacuum and hybrid deposition routes for the preparation of different organic-inorganic lead perovskite thin films, and their incorporation into efficient solar cells. The influence of the type of organic semiconductors used as hole/electron transport layer in p-i-n solar cells will be presented. We also discuss their electroluminescence properties, either for applications in light-emitting diodes or as a diagnostic tool of the optical and electronic quality of perovskite thin films. Finally, the effect of additives and dopants in the perovskite absorber as well as in the charge selective layers will be described.