Importance of interfaces in hybrid perovskite solar cells

Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 22%. In this talk we discuss the role of the interface in optimizing device performance as measured by both power conversion efficiency and stability. We present an examination of different perovskite active layers and interfacial electronic structure of these remarkable materials with functional oxide and organic contact layers. Interface formation of the active layer with different carrier transport materials has direct implications for performance of the resulting devices. We present interface studies, which permit identification of charge transfer mechanisms across the interface with chemical specificity and insight into the requirements for realizing high performance devices. Our findings from surface science approaches are combined with time resolved spectroscopy, structural studies and device level studies to validate impacts on carrier dynamics and demonstrate their technological relevance of interfacial insights

Magnetic Dipolar Interactions in Solid Gold Nanosphere Dimers

We report the first observation of a magnetic dipolar contribution to the nonlinear optical (NLO) response of colloidal metal nanostructures. Second-order NLO responses from several individual solid gold nanosphere (SGN) dimers, which we prepared by a bottom-up approach, were examined using polarization-resolved second harmonic generation (SHG) spectroscopy at the single-particle level. Unambiguous circular dichroism in the SH signal was observed for most of the dimeric colloids, indicating that the plasmon field located within the interparticle gap was chiral. Detailed analysis of the polarization line shapes of the SH intensities obtained by continuous polarization variation suggested that the effect resulted from strong magnetic-dipole contributions to the nanostructure’s optical properties

Three-Dimensional Interfacial Structure Determination of Hollow Gold Nanosphere Aggregates

The boundary regions between hollow gold nanospheres (HGNs) comprising an extended aggregate were examined using 3-D electron tomography. The images obtained from these experiments allowed for precise determination of the 3-D arrangement of the HGNs within the aggregate and revealed structural heterogeneities that were not resolvable with traditional two-dimensional techniques. These features included particle necking, point contacts, lattice pinholes, and HGN cavities that were joined by pores. The theoretical influence of these interfacial substructures on nanoscale plasmon properties was assessed using finite difference time domain (FDTD) numerical simulations. These results demonstrated the prospective impact of 3-D imaging techniques on the development of complete-structure descriptions of nanoscale optical properties

Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C₆₀

The time scales for interfacial charge separation and recombination play crucial roles in determining efficiencies of excitonic photovoltaics. Near-infrared photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (<i>e</i>.<i>g</i>., C₆₀). However, little is known about crucial photochemical events that occur on femtosecond to nanosecond time scales at such heterojunctions. Here, we present transient absorbance measurements that utilize a distinct spectroscopic signature of charges within SWCNTs, the absorbance of a trion quasiparticle, to measure both the ultrafast photoinduced electron transfer time (τ_pet) and yield (ϕ_pet) in photoexcited SWCNT–C₆₀ bilayer films. The rise time of the trion-induced absorbance enables the determination of the photoinduced electron transfer (PET) time of τ_pet ≤ 120 fs, while an experimentally determined trion absorbance cross section reveals the yield of charge transfer (ϕ_pet ≈ 38 ± 3%). The extremely fast electron transfer times observed here are on par with some of the best donor:acceptor pairs in excitonic photovoltaics and underscore the potential for efficient energy harvesting in SWCNT-based devices

Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films

In spite of the rapid rise of metal organic halide perovskites for next-generation solar cells, little quantitative information on the electronic structure of interfaces of these materials is available. The present study characterizes the electronic structure of interfaces between semiconducting single walled carbon nanotube (SWCNT) contacts and a prototypical methylammonium lead iodide (MAPbI₃) absorber layer. Using photoemission spectroscopy we provide quantitative values for the energy levels at the interface and observe the formation of an interfacial dipole between SWCNTs and perovskite. This process can be ascribed to electron donation from the MAPbI₃ to the adjacent SWCNT making the nanotube film <i>n</i>-type at the interface and inducing band bending throughout the SWCNT layer. We then use transient absorbance spectroscopy to correlate this electronic alignment with rapid and efficient photoexcited charge transfer. The results indicate that SWCNT transport and contact layers facilitate rapid charge extraction and suggest avenues for enhancing device performance

Trap-limited carrier recombination in single-walled carbon nanotube heterojunctions with fullerene acceptor layers

Single-walled carbon nanotube (SWCNT)-fullerene (C₆₀) bilayers represent an attractive “donor-acceptor” binary system for solar photoconversion, where the kinetics of photoinduced processes depend critically on the properties of the interface between the two materials. Using photoconductivity measurements we identify the kinetic scheme that describes the free carrier kinetics in such bilayers where the dominant SWCNT species is the (7,5) semiconducting nanotube. Following charge separation, the carrier kinetics, covering up to four orders of magnitude in volumetric hole density, are described by a recombination process that is limited by capture and emission at traps or states at the SWCNT-C₆₀ interface. The high-frequency mobility of holes in the (7,5) SWCNT phase is lower than in multichiral films, potentially due to differences in SWCNT defect density for nanotubes that have been purified more aggressively. The results obtained here provide fundamental insights into the transport and recombination of both charges and excitons within SWCNT thin films and bilayers, and point to several potential ways to improve SWCNT-C₆₀ photovoltaic devices.8 page(s

Photoluminescence Imaging of Polyfluorene Surface Structures on Semiconducting Carbon Nanotubes: Implications for Thin Film Exciton Transport

Single-walled carbon nanotubes (SWCNTs) have potential to act as light-harvesting elements in thin film photovoltaic devices, but performance is in part limited by the efficiency of exciton diffusion processes within the films. Factors contributing to exciton transport can include film morphology encompassing nanotube orientation, connectivity, and interaction geometry. Such factors are often defined by nanotube surface structures that are not yet well understood. Here, we present the results of a combined pump–probe and photoluminescence imaging study of polyfluorene (PFO)-wrapped (6,5) and (7,5) SWCNTs that provide additional insight into the role played by polymer structures in defining exciton transport. Pump–probe measurements suggest exciton transport occurs over larger length scales in films composed of PFO-wrapped (7,5) SWCNTs, compared to those prepared from PFO-bpy-wrapped (6,5) SWCNTs. To explore the role the difference in polymer structure may play as a possible origin of differing transport behaviors, we performed a photoluminescence imaging study of individual polymer-wrapped (6,5) and (7,5) SWCNTs. The PFO-bpy-wrapped (6,5) SWCNTs showed more uniform intensity distributions along their lengths, in contrast to the PFO-wrapped (7,5) SWCNTs, which showed irregular, discontinuous intensity distributions. These differences likely originate from differences in surface coverage and suggest the PFO wrapping on (7,5) nanotubes produces a more open surface structure than is available with the PFO-bpy wrapping of (6,5) nanotubes. The open structure likely leads to improved <i>intertube</i> coupling that enhances exciton transport within the (7,5) films, consistent with the results of our pump–probe measurements