A Time-Resolved Photophysical Study of Hybrid Organic-Inorganic Perovskite Photovoltaic Materials

The search for new photovoltaic materials has been driven by the combined need to exploit sources of energy that are clean and sustainable, while simultaneously doing it in a cost effective manner. In this light, hybrid organic-inorganic perovskites have recently emerged as an extremely promising photonic material, and their application as a functional photovoltaic layer has resulted in device efficiencies that rival long established silicon based technologies. Rapid progress in device efficiencies have occurred over the last years (22.1% being the current record). However, the simultaneous growth in basic studies of the material have not resulted in a conclusive understanding of the fundamental process that occur subsequent to photoexcitation. Hence, there is a pressing need to identify the pathways that currently limit device performance and provide direction for future work in materials and device engineering. Towards this goal, we investigate two perovskite compositions (CH3NH3PbI3 and (FAPbI3)0.85(MAPbBr3)0.15) using time-resolved (THz and electroabsorption) spectroscopic techniques. Chapter 1 and 2 provide a general introduction into the investigated system and the experimental techniques that have been used. In chapter 3, we detail time-resolved THz measurements and report on experimental evidence for carrier recombination through an indirect transition, as well as a direct recombination pathway that is present at higher carrier densities. We calculate temperature dependent carrier mobilities (at THz frequencies) and bimolecular recombination constants. Through which we identify phonon scattering as the primary limiting mechanism for carrier transport, and temperature dependent bimolecular recombination that is mediated by the relative mobility of the charge carriers. Analysis of the complex photoconductivity spectra using the Drude-Smith model revealed a large difference in carrier scattering between the two perovskite films that could be attributed to the significantly different morphologies. In chapter 4 we apply time-resolved electroabsorption spectroscopy (TREAS) to insulated CH3NH3PbI3 layers and investigate the macroscopic carrier transport dynamics under applied electric fields. Transport within the 40nm perovskite grain was discovered to be diminished by a factor ¿ 2 relative to the high frequency mobility obtained through THz spectroscopy. The averaged carrier mobility across the 280nm film length was reduced by a factor ¿ 4, due to the presence of grain boundaries and defects. Preliminary investigations also identified spectral signatures associated with carrier accumulation at the perovskite interface and delayed extraction at the contacts. Chapter 5 deals with complete solar cells formed using (FAPbI3)0.85(MAPbBr3)0.15 as the active layer and we report the first application of the TREAS technique to complete perovskite devices. Our results reveal that the improved morphology of the film results in film averaged mobilities that are near the intrinsic values obtained using THz spectroscopy. Analysis of transient absorption spectra revealed an electroabsorption signature, its dynamics can be correlated with the disassociation of a transient excitonic species to form free charge carriers

Charge Separation Pathways in a Highly Efficient Polymer:Fullerene Solar Cell Material

PBDTTPD is one of the best conjugated polymers for solar cell applications (up to 8.5% efficiency). We have investigated the dynamics of charge generation in the blend with fullerene (PCBM) and addressed highly relevant topics such as the role of bulk heterojunction structure, fullerene excitation, and excess energy. We show that there are multiple charge separation pathways. These include electron transfer from photoexcited polymer, hole transfer from photoexcited PCBM, prompt (<100 fs) charge generation in intimately mixed polymer:fullerene regions (which can occur from hot states), as well as slower electron and hole transfer from excitons formed in pure PBDTTPD or PCBM domains (diffusion to an interface is necessary): Very interestingly, all the charge separation pathways are highly efficient. For example, the yield of long-lived carriers is not significantly affected by the excitation wavelength, although this changes the fraction of photons absorbed by PCBM and the amount of excess energy brought to the system. Overall, the favorable properties of the PBDTTPD:PCBM blend in terms of morphology and exciton delocalization allow excellent charge generation in all circumstances and strongly contribute to the high photovoltaic performance of the blend

Correlation of fluorescence microscopy, electron microscopy, and NanoSIMS stable isotope imaging on a single tissue section.

Correlative light and electron microscopy allows localization of specific molecules at the ultrastructural level in biological tissue but does not provide information about metabolic turnover or the distribution of labile molecules, such as micronutrients. We present a method to directly correlate (immuno)fluorescent microscopy, (immuno)TEM imaging and NanoSIMS isotopic mapping of the same tissue section, with nanometer-scale spatial precision. The process involves chemical fixation of the tissue, cryo sectioning, thawing, and air-drying under a thin film of polyvinyl alcohol. It permits to effectively retain labile compounds and strongly increases NanoSIMS sensitivity for 13C-enrichment. The method is illustrated here with correlated distribution maps of a carbonic anhydrase enzyme isotype, β-tubulin proteins, and 13C- and 15N-labeled labile micronutrients (and their anabolic derivates) within the tissue of a reef-building symbiotic coral. This broadly applicable workflow expands the wealth of information that can be obtained from multi-modal, sub-cellular observation of biological tissue

Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material

PBDTTPD is a conjugated polymer with high power conversion efficiency if used in organic solar cells together with fullerene derivatives. We have investigated forthe first time the excited state dynamics of pristine PBDTTPD thin film as well as the ultrafast evolution of charge carriers in PBDTTPD:PCBM bulk heterojunction blend using femtosecond transient absorption spectroscopy. In the latter, charges appear within the time resolution of the experiment (80% of charges survive after 1 ns; the rest recombines (most probably geminately) on the 200 ps time scale

Dynamics of Photocarrier Separation in MAPbI3 Perovskite Multigrain Films under a Quasistatic Electric Field

Applying time-resolved electroabsorption spectroscopy for the first time to methylammonium lead triiodide perovskite (MAPbI3) thin films under reverse bias, we monitored optically the ultrafast evolution of the local counter-electric field produced by the drift of photogenerated electrons and holes in opposite directions. Under an externally applied electric field of |E| < 10^5 V cm–1, the carriers were found to reach a separation of 40 nm within ∼1 ps. This distance corresponds to the average dimensions of crystalline grains in the active film, at the boundaries of which charges were trapped. An intragrain average carrier drift mobility of μ± = 23 cm^2 V–1 s–1 was inferred. Subsequent charge detrapping, migration through the entire film, and accumulation at its insulated surfaces caused a blue shift of the perovskite absorption edge that arose within tens of picoseconds, owing to a trap-limited electron drift mobility μn = 6 cm^2 V–1 s–1. Charge recombination was entirely suppressed between field-separated photocarriers generated at initial densities of n0 ≤ 2 × 10^16 cm–3. Accumulation of electrons at the interface between a mesoporous TiO2 electron-transport layer and a multigrain MAPbI3 film was also observed, which was indicative of delayed charge injection through a poor contact junction

Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions

Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site. The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization