Organic photovoltaic cells are partiuclarly sensitive to exciton harvesting and are thus, a useful platform for the characterization of exciton diffusion. While device photocurrent spectroscopy can be used to extract the exciton diffusion length, this method is frequently limited by unknown interfacial recombination losses. We resolve this limitation and demonstrate a general, device-based photocurrent-ratio measurement to extract the intrinsic diffusion length. Since interfacial losses are not active layer specific, a ratio of the donor- and acceptor-material internal quantum efficiencies cancels this quantity. We further show that this measurement permits extraction of additional device-relevant information regarding exciton relaxation and charge separation processes. The generality of this method is demonstrated by measuring exciton transport for both luminescent and dark materials, as well as for small molecule and polymer active materials and semiconductor quantum dots. Thus, we demonstrate a broadly applicable device-based methodology to probe the intrinsic active material exciton diffusion length.

Understanding how structural and synthetic factors influence the complex refractive index of quantum dot (QD) solids is crucial to tailoring the light−matter interactions of QD-containing photonic and optoelectronic devices. However, neat QD films are challenging to accurately model as they are a mixture of inorganic core/shell materials and surrounding organic ligands. Furthermore, both the QD film morphology and the complex refractive index vary due to particle size, ligand chain length, and the deposition process. Here, we study the complex refractive index of neat CdSe/CdS core/shell QD films by using variable-angle spectroscopic ellipsometry to derive the effective complex refractive index using Kramers−Kronig consistent dispersion models. We use this information in conjunction with intrinsic refractive index data of CdSe and CdSe/CdS QDs extracted from solution-state absorption data and effective medium approximations (EMA) to describe neat QD films. We find that EMAs can successfully be used as a tool to approximate the complex refractive index of QD films. This information allows us to better understand packing variations between QD films and more accurately predict the absorption in QD thin films, including those made with core/shell heterostructures.

Conjugated block copolymers have the potential to improve solution processed optoelectronic devices such as organic photovoltaics (OPVs), but significant synthetic challenges exist and systematic studies investigating structure− property relationships are lacking. We demonstrate a new route to conjugated block copolymers via copper-catalyzed click coupling and apply this method to synthesize a series of poly(3-hexylthiophene)-block-poly(9,9-dioctylfluorene) (P3HT-b-PF) conjugated block copolymers with varying block weight fractions. The resulting block copolymers are comprised of two conjugated polymers joined by a flexible, nonconjugated linker. The series of conjugated block copolymers prepared enables an investigation into the role of polymer block lengths and composition on crystallization and self-assembly behavior. Grazing incidence wide-angle X-ray scattering measurements indicate the formation of highly oriented P3HT and/or PF crystallites in thermally annealed block copolymer films. Crystallization of either P3HT or PF blocks is predominant in all block copolymers studied, but at intermediate ratios crystallization of both blocks is observed.

The small size of colloidal nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to nanophotonics, including bright, tunable photoluminescence (PL). However, exploring the properties of solid QD assemblies at the nanoscale has proven challenging because of the limitations in the nanoscale QD patterning methods. Generally, the precise placement of QD solids is difficult to achieve, especially for tall structures with multiple QD layers, and when it is achieved the patterns often cannot withstand the further processing steps required for final device construction. Direct electron beam lithography of QDs has emerged as a straightforward patterning process that does not require ligand exchange and results in structures that retain bright PL. Here, we demonstrate that direct patterning QD films on substrates treated with a self-assembled monolayer of octadecyltrichlorosilane allows us to create feature sizes as thin as 30 nm with heights of multiple layers and characterize the pattern resolution, robustness, and placement accuracy. These structures withstand sonication in a variety of solvents, and the structures are placed within 20 nm of their intended location nearly 100% of the time. We further show how this patterning method can be applied to nanophotonics by measuring the complex refractive index of the QD materials to model the absorption and scattering cross sections of QD structures of various sizes and shapes. These simulations reveal that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators.

Organic photovoltaic cells are partiuclarly sensitive to exciton harvesting and are thus, a useful platform for the characterization of exciton diffusion. While device photocurrent spectroscopy can be used to extract the exciton diffusion length, this method is frequently limited by unknown interfacial recombination losses. We resolve this limitation and demonstrate a general, device-based photocurrent-ratio measurement to extract the intrinsic diffusion length. Since interfacial losses are not active layer specific, a ratio of the donor-and acceptor-material internal quantum efficiencies cancels this quantity. We further show that this measurement permits extraction of additional device-relevant information regarding exciton relaxation and charge separation processes. The generality of this method is demonstrated by measuring exciton transport for both luminescent and dark materials, as well as for small molecule and polymer active materials and semiconductor quantum dots. Thus, we demonstrate a broadly applicable device-based methodology to probe the intrinsic active material exciton diffusion length.