Various ceria and colloidal silica polishing slurries were used to polish fused silica glass workpieces on a polyurethane pad. Characterization of the slurries' particle size distribution (PSD) (using both ensemble light scattering and single particle counting techniques) and of the polished workpiece surface (using atomic force microscopy) was performed. The results show the final workpiece surface roughness is quantitatively correlated with the logarithmic slope of the distribution function for the largest particles at the exponential tail end of the PSD. Using the measured PSD, fraction of pad area making contact, and mechanical properties of the workpiece, slurry, and pad as input parameters, an Ensemble Hertzian Gap (EHG) polishing model was formulated to estimate each particle's penetration, load, and contact zone. The model is based on multiple Hertzian contact of slurry particles at the workpiece-pad interface in which the effective interface gap is determined through an elastic load balance. Separately, ceria particle static contact and single pass sliding experiments were performed showing~1-nm depth removal per pass (i.e., a plastic type removal). Also, nanoindentation measurements on fused silica were made to estimate the critical load at which plastic type removal starts to occur (P crit~5 3 10 À5 N). Next the EHG model was extended to create simulated polished surfaces using the Monte Carlo method where each particle (with the calculated characteristics described above) slides and removes material from the silica surface in random directions. The polishing simulation utilized a constant depth removal mechanism (i.e., not scaling with particle size) of the elastic deformation zone cross section between the particle and silica surface, which was either 0.04 nm (for chemical removal) at low loads (

P crit ). The simulated surfaces quantitatively compare well with the measured rms roughness, power spectra, surface texture, absolute thickness material removal rate, and load dependence of removal rate.

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated.Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ~487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ~2000 times less than predicted by Langevin theory. This in turn leads to the build-up of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films, due to the different scaling of electro-absorption and photo-induced absorption signals in transient absorption spectroscopy.

Solution-processable small molecule photovoltaics based on the novel molecular donor, benzodithiophene terthiophene rhodanine (BTR), recently have shown maximum power conversion efficiencies above 8 % for active layer thicknesses up to 400 nm, using post process solvent vapor annealing (SVA) with tetrahydrofuran (THF). Here we report an in-situ study on the morphology evolution during SVA using the moderate solvent THF and the good solvent chloroform (CF). The combination of real-time grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) allows us to draw a complete picture of the evolution of crystallinity and phase purity during post process annealing. We find that the relative crystallinity compared to the as-cast films is only modestly affected by SVA and solvent choice. However, both the phase purity and the characteristic domain sizes within the film vary significantly and are controlled by the solvent quality as well as exposure time. Using THF, films with high phase purity and desirable characteristic length scales of about 30 nm can be achieved, while the use of CF rapidly leads to excessive film coarsening and less preferable domain sizes on the order of 60 nm, too large for optimized charge separation.

Achieving efficient bulk-heterojunction (BHJ) solar cells from blends of solution-processable small-molecule (SM) donors and acceptors has proven particularly challenging due to the complexity in obtaining a favorable donor-acceptor morphology. In this report, we examine the BHJ device performance pattern of a set of analogous, well-defined SM donors -DR3TBDTT (DR3), SMPV1, and BTR -used in conjunction with the SM acceptor IDTTBM. Our examinations show that the nonfullerene "All-SM" BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ca. 4.5% (avg. 4.0%) when the solution-processing additive 1,8diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of "as-cast" BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum (EELS) analyses in scanning transmission electron microscopy (STEM) mode, carrier transport measurements via "metalinsulator-semiconductor carrier extraction" (MIS-CELIV) methods, and systematic recombination This article is protected by copyright. All rights reserved. 3 examinations by light-dependence and transient photocurrent (TPC) analyses, we show that DIO plays a determining role -establishing a favorable lengthscale for the phase-separated SM donoracceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall.

Nanoscale morphology and spin can have a significant impact on charge generation and short time scale recombination in organic photovoltaic materials. We reveal multiple efficient charge separation pathways and the suppression of triplet loss channels in a high-performing nematic liquid crystalline electron donor, benzodithiophene terthiophene rhodanine (BTR). BTR:PC 71 BM bulk heterojunction photovoltaic devices have been shown to exhibit charge generation quantum yields of ∼90% and power conversion efficiencies >9.5%, even in thick devices. Solvent vapor annealing increases device efficiency, delivering performance almost twice as high as that of untreated blend films, despite reduced exciton quenching. Broadband femtosecond transient absorption spectroscopy reveals both efficient hole and electron transfer on different time scales in the bulk heterojunction blends. BTR triplet excitons are formed due to subnanosecond bimolecular recombination in untreated blend films, though their formation is significantly suppressed after solvent vapor annealing. This treatment results in more crystalline BTR domains with three-dimensional percolation pathways that have an important impact on these terminal triplet loss channels formed through fast recombination of free charges. We propose that spin and nanoscale morphology have significant and interconnected roles in the prevention of loss channels that with careful control can lead to superior device performance in promising new photovoltaic materials.