Alessandro Sepe

Atmospheric Influence upon Crystallization and Electronic Disorder and Its Impact on the Photophysical Properties of Organic–Inorganic Perovskite Solar Cells

Recently, solution-processable organic-inorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance, one of the critical factors is the processing conditions of the perovskite film, which directly influences the photophysical properties and hence the device performance. Here we study the effect of annealing parameters on the crystal structure of the perovskite films and correlate these changes with its photophysical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces an improved crystallinity and large grain domains as compared to nitrogen. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. We note that the limiting nonradiative pathways (i.e., maximizing PLQE) is important for obtaining the highest device efficiency. This indicates a critical impact of the atmosphere upon crystallization and the ultimate device performance.

Electronic Structure of Low‐Temperature Solution‐Processed Amorphous Metal Oxide Semiconductors for Thin‐Film Transistor Applications

The electronic structure of low temperature, solution-processed indium–zinc oxide thin-film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm2 V−1 s−1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution-processed oxide thin films as a function of annealing temperature and environment using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop-off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub-bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution-processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy-induced donor levels.

2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

Polymer Crystallization as a Tool To Pattern Hybrid Nanostructures: Growth of 12 nm ZnO Arrays in Poly(3-hexylthiophene)

Well-ordered hybrid materials with a 10 nm length scale are highly desired. We make use of the natural length scale (typically 10-15 nm) of the alternating crystalline and amorphous layers that are generally found in semicrystalline polymers to direct the growth of a semiconducting metal oxide. This approach is exemplified with the growth of ZnO within a carboxylic acid end-functionalized poly(3-hexylthiophene) (P3HT-COOH). The metal-oxide precursor vapors diffuse into the amorphous parts of the semicrystalline polymer so that sheets of ZnO up to 0.5 μm in size can be grown. This P3HT-ZnO nanostructure further functions as a donor-acceptor photovoltaic system, with length scales appropriate for charge photogeneration.

Structure formation in P3HT/F8TBT blends

The structure evolution of all-polymer solar cells based on the blends of poly(3-hexylthiophene) (P3HT) and poly[(9,9-dioctyluorene)-2,7-diyl-alt-(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2′′-diyl] (F8TBT) was investigated. The P3HT/F8TBT system exhibits crystallization-driven structure formation similar to the P3HT/phenyl-C61-butyricacidmethylester (PCBM) blend despite the existence of a miscibility gap, which was determined for a blend containing regio-random P3HT. The lamellar crystallization of regio-regular P3HT was not perturbed by the addition of F8TBT. X-ray scattering studies indicate that F8TBT is segregated to the interlamellar amorphous phase, establishing a bulk heterojunction framework with the crystalline lamellae of P3HT. The excess F8TBT is accommodated at the film–substrate interface and at amorphous grain boundaries. The structural studies were correlated with the photovoltaic device performance of blend films that consisted of large P3HT spherulites. These device results emphasize the importance of a mesoscopic F8TBT network that separates the P3HT crystal domains. Our results suggest that the nanostructure formation in P3HT/F8TBT blends is determined by P3HT crystallization, resulting both in a 10 nm crystalline morphology and a F8TBT mesoscopic segregation network, both of which are beneficial for exciton dissociation.

Compatibilization of All-Conjugated Polymer Blends for Organic Photovoltaics

Compatibilization of an immiscible binary blend comprising a conjugated electron donor and a conjugated electron acceptor polymer with suitable electronic properties upon addition of a block copolymer (BCP) composed of the same building blocks is demonstrated. Efficient compatibilization during melt-annealing is feasible when the two polymers are immiscible in the melt, i.e. above the melting point of ∼250 °C of the semicrystalline donor polymer P3HT. To generate immiscibility at these high temperatures, the acceptor polymer PCDTBT is equipped with fluorinated side chains leading to an increased Flory-Huggins interaction parameter. Compatibilization in bulk and thin films is demonstrated, showing that the photovoltaic performance of pristine microphase separated and nanostructured BCPs can also be obtained for compatibilized blend films containing low contents of 10-20 wt % BCP. Thermodynamically stable domain sizes range between several tens of microns for pure blends and ∼10 nm for pure block copolymers. In addition to controlling domain size, the amount of block copolymer added dictates the ratio of edge-on and face-on P3HT crystals, with compatibilized films showing an increasing amount of face-on P3HT crystals with increasing amount of compatibilizer. This study demonstrates the prerequisites and benefits of compatibilizing all-conjugated semicrystalline polymer blends for organic photovoltaics.

Highly Planarized Naphthalene Diimide–Bifuran Copolymers with Unexpected Charge Transport Performance

The synthesis, characterization, and charge transport performance of novel copolymers PNDIFu2 made from alternating naphthalene diimide (NDI) and bifuran (Fu2) units are reported. Usage of potentially biomass-derived Fu2 as alternating repeat unit enables flattened polymer backbones due to reduced steric interactions between the imide oxygens and Fu2 units, as seen by density functional theory (DFT) calculations and UV–vis spectroscopy. Aggregation of PNDIFu2 in solution is enhanced if compared to the analogous NDI–bithiophene (T2) copolymers PNDIT2, occurring in all solvents and temperatures probed. PNDIFu2 features a smaller π–π stacking distance of 0.35 nm compared to 0.39 nm seen for PNDIT2. Alignment of aggregates in films is achieved by using off-center spin coating, whereby PNDIFu2 exhibits a stronger dichroic ratio and transport anisotropy in field-effect transistors (FET) compared to PNDIT2, with an overall good electron mobility of 0.21 cm2/(V s). Despite an enhanced backbone planarity, the smaller π–π stacking and the enhanced charge transport anisotropy, the electron mobility of PNDIFu2 is about three times lower compared to PNDIT2. Density functional theory calculations suggest that charge transport in PNDIFu2 is limited by enhanced polaron localization compared to PNDIT2.

The effect of heat treatment on the internal structure of nanostructured block copolymer films

We report on the temperature dependence of the nanostructure of thin block copolymer films, as studied using in situ grazing-incidence small-angle x-ray scattering (GISAXS). We focus on spin-coated poly(styrene-b-butadiene) diblock copolymer thin films featuring lamellae perpendicular to the substrate. In situ GISAXS measurements elucidate the structural changes during heat treatment at temperatures between 60 and 130 °C. Thermal treatment below 100 °C does not destroy the perpendicular lamellar order. In contrast, treatment between 105 and 120 °C leads to a broad distribution of lamellar orientations which only partially recovers upon subsequent cooling. Treatment at 130 °C leads to severe changes of the film structure. We attribute the change of behavior at 100 °C to the onset of the glass transition of the polystyrene block and the related increase of long-range mobility. Our results indicate that the perpendicular lamellar orientation for high molar mass samples is not stable under all conditions.