Decoding the Vertical Phase Separation and Its Impact on C8-BTBT/PS Transistor Properties

Disentangling the details of the vertical distribution of small semiconductor molecules blended with polystyrene (PS) and the contact properties are issues of fundamental value for designing strategies to optimize small-molecule:polymer blend organic transistors. These questions are addressed here for ultrathin blends of 2,7-dioctyl[1]­benzothieno­[3,2-<i>b</i>]­[1]­benzothiophene (C8-BTBT) and PS processed by a solution-shearing technique using three different blend composition ratios. We show that friction force microscopy (FFM) allows the determination of the lateral and vertical distribution of the two materials at the nanoscale. Our results demonstrate a three-layer stratification of the blend: a film of C8-BTBT of few molecular layers with crystalline order sandwiched between a PS-rich layer at the bottom (a few nm thick) acting as a passivating dielectric layer and a PS-rich skin layer on the top (∼1 nm) conferring stability to the devices. Kelvin probe force microscopy (KPFM) measurements performed in operating organic field-effect transistors (OFETs) reveal that the devices are strongly contact-limited and suggest contact doping as a route for device optimization. By excluding the effect of the contacts, field-effect mobility values in the channel as high as 10 cm² V^–1 s^–1 are obtained. Our findings, obtained via a combination of FFM and KPFM, provide a satisfactory explanation of the different electrical performances of the OFETs as a function of the blend composition ratio and by doping the contacts

Chiral Organization and Charge Redistribution in Chloroaluminum Phthalocyanine on Au(111) Beyond the Monolayer

The nontrivial effect of molecular dipoles on the surface work function of metals is explored for the unidirectional ordered arrays forming the first and second layers of chloroaluminum phthalocyanine on Au(111). This phthalocyanine is a nonplanar molecule with permanent electric dipole perpendicular to its molecular π-plane that can adopt two distinct configurations (Cl-up and Cl-down) when adsorbed on surfaces. The ordered array forming the first layer is known to consist of all Cl-up molecules, whereas the less-studied second layer is formed by molecules in the Cl-down configuration. The inverted orientation of the molecules in these two layers constitutes our benchmark system to investigate the influence of the dipole array orientation on the surface work function. The present study includes an experimental and theoretical approach that combines diverse imaging and spectroscopic scanning probe microscopies, in ultrahigh vacuum, with first-principles density functional theory-based atomistic simulations. Experiment and theory show a chiral organization transferred from the first layer to the growing film that is reflected in the electronic structure. We demonstrate that the obtained surface work function changes are smaller in magnitude than expected from a dipolar approximation because of charge rearrangement at and beyond the monolayer. We provide understanding of the crucial interplay between the interlayer and organic/metal interactions and quantify their effect on the electron density distribution and on the work function changes

Gaining Further Insight into the Solvent Additive-Driven Crystallization of Bulk-Heterojunction Solar Cells by <i>in Situ</i> X‑ray Scattering and Optical Reflectometry

The use of solvent additives has become a successful strategy to control the structural evolution upon film formation in bulk-heterojunction (BHJ) solar cells. Nonetheless, a complete understanding of the additive’s role in the phase separation mechanisms and organization of donor and acceptor materials in BHJs is still lacking. In this work we gain further insight about the specific role that a widely used additive, 1,8-octanedithiol (ODT), has in the crystallization of both PCPDTBT and PC₇₁BM, directly after wet film deposition using blade-coating. By <i>in situ</i> X-ray scattering and optical reflectometry, we correlate the additive-driven crystallization with the evolution of film composition from the earliest time of solvent evaporation. It is shown that ODT influences the evolution of both PCPDTBT and PC₇₁BM. ODT leads to prolonged crystallization time during the ODT-drying dominated period corresponding to an overall solvent content (<i>x</i>) of 75 wt % > <i>x</i> > 15 wt % and delays the onset of PC₇₁BM aggregation to <i>x</i> < 20 wt %, being pushed out of the crystalline polymer domains

Interplay between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance–voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency

Instability and Surface Potential Modulation of Self-Patterned (001)SrTiO₃ Surfaces

The (001)­SrTiO₃ crystal surface can be engineered to display a self-organized pattern of well-separated and nearly pure single-terminated SrO and TiO₂ regions by high temperature annealing in oxidizing atmosphere. By using surface sensitive techniques we have obtained evidence of such a surface chemical self-structuration in as-prepared crystals and unambiguously identified the local composition. The contact surface potential at regions initially consisting of majority single terminations (SrO and TiO₂) is determined to be Φ­(SrO) < Φ­(TiO₂), in agreement with theoretical predictions, although the measured difference ΔΦ ≤ 100 meV is definitely below calculations for ideally pure single-terminated SrO and TiO₂ surfaces. These relative values are maintained if samples are annealed in UHV up to 200 °C. Annealing in UHV at higher temperature (400 °C) preserves the surface morphology of self-assembled TiO₂ and SrO rich regions, although a non-negligible chemical intermixing is observed. The most dramatic consequence is that the surface potential contrast is reversed. It thus follows that electronic and chemical properties of (001)­SrTiO₃ surfaces, widely used in oxide thin film growth, can largely vary before growth starts in a manner strongly dependent on temperature and pressure conditions

Threshold-Voltage Shifts in Organic Transistors Due to Self-Assembled Monolayers at the Dielectric: Evidence for Electronic Coupling and Dipolar Effects

The mechanisms behind the threshold-voltage shift in organic transistors due to functionalizing of the gate dielectric with self-assembled monolayers (SAMs) are still under debate. We address the mechanisms by which SAMs determine the threshold voltage, by analyzing whether the threshold voltage depends on the gate-dielectric capacitance. We have investigated transistors based on five oxide thicknesses and two SAMs with rather diverse chemical properties, using the benchmark organic semiconductor dinaphtho­[2,3-b:2′,3′-<i>f</i>]­thieno­[3,2-<i>b</i>]­thiophene. Unlike several previous studies, we have found that the dependence of the threshold voltage on the gate-dielectric capacitance is completely different for the two SAMs. In transistors with an alkyl SAM, the threshold voltage does not depend on the gate-dielectric capacitance and is determined mainly by the dipolar character of the SAM, whereas in transistors with a fluoroalkyl SAM the threshold voltages exhibit a linear dependence on the inverse of the gate-dielectric capacitance. Kelvin probe force microscopy measurements indicate this behavior is attributed to an electronic coupling between the fluoroalkyl SAM and the organic semiconductor

Structure Formation in Low-Bandgap Polymer:Fullerene Solar Cell Blends in the Course of Solvent Evaporation

The drying process of the bulk heterojunction (BHJ) layer has a strong impact on the solar cell performance for the well-investigated material system P3HT:PC₆₁BM. For higher performing low-bandgap polymers and C₇₁ fullerene derivatives, no comprehensive studies of the BHJ structure evolution during film drying are available. In this work we investigate the structure formation of the low-bandgap polymer poly­{[4,40-bis­(2-ethylhexyl)­dithieno­(3,2-<i>b</i>;20,30-<i>d</i>)­silole]-2,6-diyl-<i>alt</i>-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) in the transition from wet to solid by in-situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry simultaneously. The nucleation and crystallization of PSBTBT differs from the interface-induced crystallization of P3HT and occurs partially in the solution. It is shown that PSBTBT:PC₇₁BM blend nanomorphology and optoelectronic device properties are rather insensitive to the drying process in the investigated temperature range of 40–85 °C. This is beneficial for fast drying at elevated temperatures which enables high throughput fabrication of efficient organic photovoltaics