Carlito S. Ponseca

Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination

Organometal halide perovskite-based solar cells have recently been reported to be highly efficient, giving an overall power conversion efficiency of up to 15%. However, much of the fundamental photophysical properties underlying this performance has remained unknown. Here, we apply photoluminescence, transient absorption, time-resolved terahertz and microwave conductivity measurements to determine the time scales of generation and recombination of charge carriers as well as their transport properties in solution-processed CH3NH3PbI3 perovskite materials. We found that electron-hole pairs are generated almost instantaneously after photoexcitation and dissociate in 2 ps forming highly mobile charges (25 cm(2) V(-1) s(-1)) in the neat perovskite and in perovskite/alumina blends; almost balanced electron and hole mobilities remain very high up to the microsecond time scale. When the perovskite is introduced into a TiO2 mesoporous structure, electron injection from perovskite to the metal oxide is efficient in less than a picosecond, but the lower intrinsic electron mobility of TiO2 leads to unbalanced charge transport. Microwave conductivity measurements showed that the decay of mobile charges is very slow in CH3NH3PbI3, lasting up to tens of microseconds. These results unravel the remarkable intrinsic properties of CH3NH3PbI3 perovskite material if used as light absorber and charge transport layer. Moreover, finding a metal oxide with higher electron mobility may further increase the performance of this class of solar cells.

Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite

Solar cells based on organometal halide perovskites have seen rapidly increasing efficiencies, now exceeding 15%. Despite this progress, there is still limited knowledge on the fundamental photophysics. Here we use microwave photoconductance and photoluminescence measurements to investigate the temperature dependence of the carrier generation, mobility, and recombination in (CH3NH3)PbI3. At temperatures maintaining the tetragonal crystal phase of the perovskite, we find an exciton binding energy of about 32 meV, leading to a temperature-dependent yield of highly mobile (6.2 cm(2)/(V s) at 300 K) charge carriers. At higher laser intensities, second-order recombination with a rate constant of γ = 13 × 10(-10) cm(3) s(-1) becomes apparent. Reducing the temperature results in increasing charge carrier mobilities following a T(-1.6) dependence, which we attribute to a reduction in phonon scattering (Σμ = 16 cm(2)/(V s) at 165 K). Despite the fact that Σμ increases, γ diminishes with a factor six, implying that charge recombination in (CH3NH3)PbI3 is temperature activated. The results underline the importance of the perovskite crystal structure, the exciton binding energy, and the activation energy for recombination as key factors in optimizing new perovskite materials.

Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study

Photoinduced electron injection dynamics from CdSe quantum dots to ZnO nanowires is studied by transient absorption and time-resolved terahertz spectroscopy measurements. Ultrafast electron transfer from the CdSe quantum dots to ZnO is proven to be efficient already on a picoseconds time scale (τ = 3-12 ps). The measured kinetics was found to have a two-component character, whose origin is discussed in detail. The obtained results suggest that electrons are injected into ZnO via an intermediate charge transfer state.

Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence.

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size ∼200 × 30 × 30 nm(3). The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light.

Iron sensitizer converts light to electrons with 92% yield

Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the (3)MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.

Ultrafast Terahertz Photoconductivity of Bulk Heterojunction Materials Reveals High Carrier Mobility up to Nanosecond Time Scale.

The few-picosecond (ps) decay of terahertz (THz) photoconductivity typically observed for conjugated polymer:fullerene blends (at excitation fluencies ~10(15) photons/cm(2) per pulse) is shown to be a result of charge pair annihilation for two polymer:PCBM blends. At a factor of 100 lower excitation density, the THz decay is in the hundreds of ps time scale, implying that very high carrier mobility (~0.1 cm(2) V(-1) s(-1)) prevails for long time after charge formation, of importance for free charge formation in organic solar cells.

Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, PCBM, and Spiro-OMeTAD: Role of Dark Carriers.

Despite the unprecedented interest in organic-inorganic metal halide perovskite solar cells, quantitative information on the charge transfer dynamics into selective electrodes is still lacking. In this paper, we report the time scales and mechanisms of electron and hole injection and recombination dynamics at organic PCBM and Spiro-OMeTAD electrode interfaces. On the one hand, hole transfer is complete on the subpicosecond time scale in MAPbI3/Spiro-OMeTAD, and its recombination rate is similar to that in neat MAPbI3. This was found to be due to a high concentration of dark charges, i.e., holes brought about by unintentional p-type doping of MAPbI3. Hence, the total concentration of holes in the perovskite is hardly affected by optical excitation, which manifested as similar decay kinetics. On the other hand, the decay of the photoinduced conductivity in MAPbI3/PCBM is on the time scale of hundreds of picoseconds to several nanoseconds, due to electron injection into PCBM and electron-hole recombination at the interface occurring at similar rates. These results highlight the importance of understanding the role of dark carriers in deconvoluting the complex photophysical processes in these materials. Moreover, optimizing the preparation processes wherein undesired doping is minimized could prompt the use of organic molecules as a more viable electrode substitute for perovskite solar cell devices.

Intrinsic femtosecond charge generation dynamics in single crystal CH3NH3PbI3

Hybrid metal–organic perovskite solar cells have astounded the solar cell community with their rapid rise in efficiency while maintaining low-cost fabrication. The intrinsic material photophysics related to the generation of free charges, their dynamics and efficiency, however, remains to be understood. As fabrication techniques improve, larger crystal grain sizes have been shown to be a critical factor for improving both the optical and transport properties of the hybrid metal halide perovskites. In this work, we use pulses of multi-THz frequency light in the ultra-broadband 1–30 THz (4–125 meV) range to observe the ac conductivity in large single crystal CH3NH3PbI3. Our spectra reveal the ultrafast dynamics and efficiencies of free charge creation and extremely high charge carrier mobility as high as 500–800 cm2 V−1 s−1. While quasi-equilibrium analysis of efficiencies through the Saha equation suggests a binding energy on the order of 49 meV, an observed reflectance feature appearing at high pump fluence occurs at 12 meV and is consistent with an orbital transition of the exciton, indicating a much lower Rydberg energy of 17 meV at room temperature. The signature of the exciton is found to vanish on a 1 ps time scale commensurate with the appearance of mobile carriers, consistent with thermal dissociation of the exciton to the continuum in the room temperature tetragonal phase.

Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film

We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film using time-resolved terahertz (THz) spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration, and recombination. The TRTS results show that upon femtosecond excitation above the absorption edge, the initial high photoconductivity (∼75 cm(2) V(-1) s(-1)) remains constant at least up to 8 ns, which corresponds to a diffusion length of 25 μm. Pumping below the absorption edge results in a mobility of 40 cm(2) V(-1) s(-1) suggesting lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-bandgap states. Furthermore, analysis of the THz kinetics reveals rising components of <1 and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.

Ultrafast Dynamics of Hole Injection and Recombination in Organometal Halide Perovskite Using Nickel Oxide as p-Type Contact Electrode

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.