Rain on Methylammonium Lead Iodide Based Perovskites: Possible Environmental Effects of Perovskite Solar Cells.

Synthesis of CH<sub>3</sub>NH<sub>3</sub>Sr<sub><em>x</em></sub>Pb<sub>(1-<em>x</em>)</sub>I<sub>3</sub> with Less Pb Content and Its Application in All-Solid Thin Film Solar Cells

Hybrid organic-inorganic lead halide perovskite APbX3 pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid-state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite-based photovoltaics is to extend their optical-absorption onset further into the red to enhance solar-light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN=CHNH3 (+), FA) and methylammonium (CH3 NH3 (+), MA) cations in the A position of the APbI3 perovskite structure. This combination leads to an enhanced short-circuit current and thus superior devices to those based on only CH3 NH3 (+). This concept has not been applied previously in perovskite-based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials.

Behaving like atomically precise two-dimensional quantum wells with non-negligible dielectric contrast, the layered hybrid organic-inorganic lead halide perovskites (HOIPs) have strong electronic interactions leading to tightly bound excitons with binding energies on the order of 500meV. These strong interactions suggest the possibility of larger excitonic complexes like trions and biexcitons, which are hard to study numerically due to the complexity of the layered HOIPs. Here, we propose and parametrize a model Hamiltonian for excitonic complexes in layered HOIPs and we study the correlated eigenfunctions of trions and biexcitons using a combination of diffusion MonteCarlo and very large variational calculations with explicitly correlated Gaussian basis functions. Binding energies and spatial structures of these complexes are presented as a function of the layer thickness. The trion and biexciton of the thinnest layered HOIP have binding energies of 35 and 44meV, respectively, whereas a single exfoliated layer is predicted to have trions and biexcitons with equal binding energies of 48meV. We compare our findings to available experimental data and to that of other quasi-two-dimensional materials.

To enhance the stability in humidity is very crucial to hybrid organic-inorganic lead halide perovskites in a broad range of applications. This report describes a coating stratergy of perovskite nanocrystals via polymethylmethacrylate-introduced ligand-assisted reprecipitation, using the interactions between the Pb cations on the surface of perovskite nanocrystals and the functional ester carbonyl groups in polymethylmethacrylate framework. The hydrophobic framework shields the open metal sites of hybrid organic-inorganic lead halide perovskites from being attacked by water, effectively retarding the diffusion of water into the perovskite nanocrystals. The as-prepared films demonstrate high resistance to heat and moisture. Additionally, the introduction of polymethylmethacrylate into ligand-assisted reprecipitation can effectively control the bulk precipitation and promote the stability of the perovskite solution.

In hybrid organic-inorganic lead halide perovskite solar cells, the energy loss is strongly associated with nonradiative recombination in the perovskite layer and at the cell interfaces. Here, a simple but effective strategy is developed to improve the cell performance of perovskite solar cells via the combination of internal doping by a ferroelectric polymer and external control by an electric field. A group of polarized ferroelectric (PFE) polymers are doped into the methylammonium lead iodide (MAPbI3 ) layer and/or inserted between the perovskite and the hole-transporting layers to enhance the build-in field (BIF), improve the crystallization of MAPbI3 , and regulate the nonradiative recombination in perovskite solar cells. The PFE polymer-doped MAPbI3 shows an orderly arrangement of MA+ cations, resulting in a preferred growth orientation of polycrystalline perovskite films with reduced trap states. In addition, the BIF is enhanced by the widened depletion region in the device. As an interfacial dipole layer, the PFE polymer plays a critical role in increasing the BIF. This combined effect leads to a substantial reduction in voltage loss of 0.14 V due to the efficient suppression of nonradiative recombination. Consequently, the resulting perovskite solar cells present a power conversion efficiency of 21.38% with a high open-circuit voltage of 1.14 V.

Low temperature (< 150C) solution-processed organic-inorganic hybrid lead halide perovskites are an emergent class of material which has attracted the full attention of the academic community. Apart from its striking optoelectronic properties and economical production cost, the chemical versatility for facile properties-tuning is its primary appeal. Recent works have demonstrated the potential of this material system for light-harvesting (i.e. solar cell) and lightemitting (e.g. LEDs, lasers, etc.) devices with ground-breaking efficiencies.

Diversifying Progress in Solar

Hybrid organic-inorganic lead halide perovskite microwires are potential building blocks for realizing on-chip integrated optoelectronic devices. However, the length-controllable synthesis of one-dimensional hybrid perovskite microwires has been rarely reported, especially the ones with lengths in the millimeter scale. Herein, methylammonium lead bromide (MAPbBr3) and formamidinium lead bromide (FAPbBr3) micro and milliwires are demonstrated using single-crystal PbBr2 microwires synthesized via template-free solution-phase growth as the lattice framework. Following the PbBr2 template, the as-converted perovskite microwires possess controllable lengths ranging from tens to thousands of micrometers. In addition, Fabry-Perot (FP) lasing was realized in both MAPbBr3 and FAPbBr3 microwires, attesting to their excellent crystal quality and the efficient optical confinement of the natural cavity. These unique properties result in the first demonstration of FP perovskite microwire lasers with submillimeter lengths. More interestingly, the microwire lasers show excellent photostability under repetitive pulsed laser excitation for over 8 × 106 cycles. Such findings demonstrate that the solution-converted hybrid lead bromide microwires have excellent optoelectronic performances promising for practical applications, and the size controllability indicates that this novel fabrication process may be feasible for large-scale industrial production.

The photoluminescence (PL) variations of organic-inorganic hybrid lead halide perovskites in different atmospheres are well documented, while the fundamental mechanism still lacks comprehensive understandings. This study reports the reversible optical and electrical properties of methylammonium lead bromide (MAPbBr3 or CH3NH3PbBr3) single crystals caused by air infiltration. With the change in the surrounding atmosphere from air to vacuum, the PL intensity of perovskite single crystals decreases, while the conductivity increases. By means of first-principles computational studies, the shallow trap states are considered as key elements in PL and conductivity changes. These results have important implications for the characterization and application of organic-inorganic hybrid lead halide perovskites in vacuum.

The organic-inorganic hybrid lead halide perovskite fever stems from a rapid rise in power conversion efficiency of perovskite solar cells (PSC) to more than 25 % within 10 years, which makes them comparable to commercial silicon solar cells. Apart from these conventional 3D perovskites, the emergence of Ruddlesden-Popper (RP) hybrid perovskites was inspired from its remarkable moisture stability by the incorporation of hydrophobic cations as well as its “soft” multidimensional layered structure through chemical engineering. The unique “soft” structure provides an opportunity to explore their ferroelectric properties, which could potentially have a strong impact on carrier dynamics due to the presence of spontaneous polarizations in nanodomains and thus suppress recombination loss in solar cells. However, the structure-function properties and the factors determining the tuning of ferroelectric properties in RP perovskites are still open questions. Besides, in the application of RP perovskite-based LEDs, mechanisms for the detrimental bottleneck “efficiency roll-off” are still unclear and under debate. Therefore, the main focus of this thesis is to investigate the structure-function relations affecting the ferroelectric properties and to uncover the mechanisms behind “efficiency roll-off” from the view of electric field dependent carrier dynamics in RP perovskites. Piezoresponse force microscopy and time-resolved transient reflection spectroscopy are the main tools for these studies. Importantly, our findings disclose composition engineering as an approach to tune the ferroelectric properties and exciton coupling as the key to electrically control the quenching phenomenon, which provides fresh opportunities to unlock new functionalities for perovskite optoelectronic devices.

Hybrid organic-inorganic lead halide perovskites (HOIPs) have received significant attention because of their impressive performances in the fields of solar cells and photoelectric detection. In the past five years, great efforts have been made to improve the crystallinity, reduce grain boundaries, and enhance the stabilities of perovskite films. Compared with films, HOIP single crystals possess fewer grain boundaries and stronger optoelectronic properties and can be applied in optoelectronic devices. As the most popular HOIP member, single crystals of MAPbX3 (X = Br, Cl) are deemed as important candidates for ultraviolet-visible photodetectors, in which the crystal structure anisotropy largely affects the detection performance. In this study, high-quality cubic single crystals of MAPbBr3 and MAPbCl3 were successfully grown from solutions. Taking advantages of their smooth (100) facets, planar metal-semiconductor-metal photodetectors were fabricated using Au interdigitated electrodes. The optoelectronic performances under nonpolarized and linearly polarized lights were explored. The optoelectronic performances were dependent on linearly polarized lights. Interestingly, both responsivity and external quantum efficiency were greatly enhanced under the excitation with linearly polarized lights. Moreover, the polarization-related optical absorptions and the electron densities within the (100) plane could be used to interpret different optoelectronic performances of single crystals of MAPbX3 (X = Br, Cl) under various linearly polarized lights.

Investigation of thickness dependent efficiency of CsPbX3 (X = I, Br) absorber layer for perovskite solar cells

Hybrid organic-inorganic lead halide perovskites (HOIPs) have appealed to researchers on account of excellent optoelectronic properties. Compared with films which possess grain boundaries, HOIPs single crystals with fewer defects behave excellent transport and recombination performances. In the family of HOIPs, single crystals of MAPbX3 (MA = CH3NH3 +, X = Cl, Br or I) are recognized as the most competitive candidates for optoelectronic applications. However, the photodetectors based on MAPbX3 have difficulties in detecting weak signals for lacking of gains without structure optimizations and extra energy transfer channels. In this study, taking advantage of MAPbBr3 single crystal (100) facets, planar metal-semiconductor-metal (MSM) photodetectors were fabricated with Au zigzag electrodes and modified Au nanoparticles (NPs) to realize localized Au surface plasmons (SPs). Compared to device without Au NPs, 2 times enhancement of photocurrent and responsivity have been achieved under 630 nm photon irradiation and 5 V bias. Furthermore, the surface metal structures can inhibit ionic migration to a certain extent. Potential mechanisms of the enhancements and suppressions are discussed in details to reveal the applications of this technique.

Organic-inorganic hybrid lead halide perovskites already reach very high power conversion efficiencies above 22% on architectures employing mesoporous TiO2, but the carrier injection processes across the different interfaces are still not fully understood. Here we use ultrafast broadband transient absorption spectroscopy to determine time constants and yields for hole and electron injection. We show that hole transfer from the perovskite valence band (VB) to the hole-transport material (HTM) H101 at the perovskite/HTM interface occurs in less than 500 fs, but is limited by imperfections of the contact layer and poor infiltration of the HTM into the mesoporous structure. Electron injection from the perovskite conduction band (CB) into the CB of mesoporous TiO2 is only a small channel (25%). Electron transport inside mesoporous MAPI/TiO2 architectures therefore mainly occurs via the perovskite. We also show that electron injection from H101 into the perovskite is feasible for excitation at 400 nm resulting in light-harvesting of high-energy photons by the HTM. Accurate absolute NIR absorption coefficients for CB electrons in mesoporous TiO2 are provided.

Hybrid organic-inorganic lead halide perovskites have emerged as a promising material for high-efficiency solar cells, yet challenges related to crystallization and defects limit their performance and stability. This study investigates the use of perovskite quantum dots (QDs) as crystallization seeds to enhance the quality of FAPbI3 perovskite films and improve the performance of perovskite solar cells (PSCs). We demonstrate that CsPbI3 and CsPbBr3 QDs effectively guide the crystallization process, leading to the formation of larger crystals with preferential orientations, particularly the (001) and (002) planes, which are associated with reduced defect densities. This seed-mediated growth strategy resulted in PSCs with power conversion efficiencies (PCEs) of 24.75% and 24.11%, respectively, compared to the baseline efficiency of 22.05% for control devices. Furthermore, devices incorporating QD-treated perovskite films exhibited remarkable stability, maintaining over 80% of their initial PCE after 1000h of simulated sunlight exposure, a significant improvement over the control. Detailed optoelectronic characterization revealed reduced non-radiative recombination and enhanced charge transport in QD-treated devices. These findings highlight the potential of QDs as a powerful tool to improve perovskite crystallization, facet orientation, and overall device performance, offering a promising route to enhance both efficiency and stability in PSCs.