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Hybrid perovskites are popular materials for realizing low cost, third generation solar cells. Certified power conversion efficiencies of laboratory scale devices now exceed 20%. Despite engineering advances, which have led to these advances, hybrid perovskites are still plagued by instabilities. The best known relate to moisture instabilities. Less, however, is understood about intrinsic limitations of these materials, stemming from both anion and cation migration under illumination or under bias. Understanding the microscopic origin of these instabilities as well as suppressing them represents one of the last major open scientific questions, preventing their commercialization. In this talk, I therefore summarize what is known about anion and cation migration and discuss results from microscopic, spatially-resolved optical measurements we have conducted on both thin films and solar cells.

Hybrid perovskite (MAPbI3, MA=CH3NH3) based solar cells have attracted great interest recently due their fast growing power conversion efficiencies. Over the last 5 years solar conversion efficiencies have risen from 3.8 to 22 %. To further increase these power conversion efficiencies, a better understanding of underlying charge recombination mechanisms in hybrid perovskite thin films is needed. The solution-based fabrication of hybrid perovskite-based solar cells additionally means that there can be significant spatial heterogeneities in the dominant charge recombination mechanism across films. This can translate into spatially-varying photovoltaic performance. To address these issues, we have therefore conducted spatially-resolved measurements of carrier recombination in MAPbI3 thin films. This has led to the observation of large variations in local carrier trap densities. These variations, in turn, directly impact local photoluminescence quantum yields as well as photovoltaic performance. We have also conducted work to better understand the optical response of mixed halide perovskite thin films, which undergo phase separation upon illumination.

Semiconductor nanostructures possess a number of applications in solar energy conversion. This includes using colloidal quantum dots in solar cells and, more recently, using analogous nanostructures such as nanowires (NWs) and nanosheets (NSs) in photocatalytic applications. Here we describe recent work to understand the photocatalytic response of solution-synthesized cadmium chalcogenide NWs and NSs within the context of hydrogen generation. Various CdX (X = S, Se) systems such as core/shell NWs and hybrid metal nanoparticle decorated NWs and NSs have been studied. In all cases, femtosecond transient differential absorption spectroscopy has been used to reveal relevant carrier relaxation processes in these materials as well as the flow of charges across the different heterointerfaces that are present. By correlating these transient absorption results to results from accompanying hydrogen generation efficiency measurements, we have, in turn, rationalized the response of these materials, clarifying the role that different heterojunctions play in establishing both charge separation and hydrogen generation efficiencies.

Semiconductor nanostructures possess a number of applications in solar energy conversion. This includes using colloidal quantum dots in solar cells and, more recently, using analogous nanostructures such as nanowires (NWs) in photocatalytic applications. Here we describe recent work to understand the photocatalytic response of solution-synthesized CdSe NWs within the context of hydrogen generation. Various CdSe NW-based systems such as core/shell structures and hybrid metal nanoparticle/semiconductor NW hybrid systems have been studied. In all cases, femtosecond transient differential absorption spectroscopy has been used to reveal relevant carrier relaxation processes in these materials as well as the flow of charges across the different heterointerfaces that are present. By correlating these transient absorption results to results from accompanying hydrogen generation efficiency measurements, we have, in turn, rationalized the response of these materials, clarifying the role that different heterojunctions play in establishing both charge separation and hydrogen generation efficiencies.

Molecules and nanoparticles exhibit unique properties such as emission intermittency (i.e. blinking) and spectral wandering which are only observable at the single molecule level. Until recently, single molecule detection has mainly been restricted to fluorescent entities. Now techniques such as photothermal imaging and spatial modulation microscopy allow single molecules and nanoparticles to be detected, regardless of their ability to emit light. This opens up the possibility of discovering new absorptive properties and even photophysics that have previously been masked within ensemble measurements. In this talk, I describe the use of these direct absorption techniques for two case studies. The first is on CdSe nanowires, a material that has potential uses in renewable energy applications. Through direct absorption measurements, we obtain the first absorption size series of any nanowire system and reveal the complex interplay between competing effects such as confinement and dielectric contrast, which determine the optical response of the wires. In a second case study, we focus on graphene oxide, an important precursor in the production of chemically-derived graphene, another important material for renewable energy applications. We show, for the first time, the direct observation of laser-induced, single layer GO reduction through correlated changes to its absorption and emission. Acquired absorption/emission movies illustrate the initial stages of single layer GO reduction, its transition to reduced-GO (rGO) as well as its subsequent decomposition upon prolonged laser illumination. These studies reveal GO’s photoreduction life cycle and through it native GO/rGO absorption coefficients as well as their intrasheet distributions and spatial heterogeneities. Beyond these two case studies, the demonstrated single nanostructure absorption technique offers exciting possibilities for directly interrogating other systems that are not necessarily fluorescent, revealing insights beyond simple ensemble averages.

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