Eric F. Manley

Room Temperature Phase Transition in Methylammonium Lead Iodide Perovskite Thin Films Induced by Hydrohalic Acid Additives

Although reactive additives have been employed in perovskite solar cells to enhance film morphology and significantly increase device performance, little is known about the effect of these additives on perovskite structural and optical properties. Here we report a systematic study of how the properties of methylammonium lead iodide perovskite (CH3 NH3 PbI3 ) are influenced by hydrohalic acid additives (HX; X=I, Br, Cl) in the precursor solution. Detailed structural and optical spectroscopic analysis reveals that all three acids affect the optical properties and alter the unit cell lattice parameters. Depending on the identity and concentration of HX, optical bandgaps widen or compress: addition of HBr yields a wider bandgap, whereas HI compresses the gap at high concentrations; HCl, on the other hand, has no significant effect on the bandgap. These changes can be understood by correlating them with the types of defects present in polycrystalline perovskite thin films in combination with the structural strain induced in very small crystallites. The presence of extra halides from HX in the precursor solution enables filling of the lattice vacancies in the perovskite, thereby altering metal-halogen-metal bond connectivity and consequently cell volumes and optical bandgaps. Remarkably, a room temperature tetragonal→cubic phase transition is observed for CH3 NH3 PbI3 films treated with high HX concentrations. Further insights into this anomalous phase transformation are obtained from in situ variable-temperature X-ray diffraction in the 25-55 °C (298-328 K) range, revealing a monotonic fall in transition temperature with increasing precursor solution HX concentration.

Hole-Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials

Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC → donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.

Aggregation control in natural brush-printed conjugated polymer films and implications for enhancing charge transport

Significance Shear-printing of electroactive polymers using natural brushes is a promising film deposition technique for printed electronics capable of microstructure control and electrical properties enhancement over large areas. Nevertheless, the interplay between film printing parameters, microstructure development, and charge transport is not well-understood. We report that natural brush-printing greatly enhances charge transport by as much as 5.7× through control of polymer nanofibril aggregate growth and backbone alignment, attributable to the oriented squamae of the natural hair. However, while brush shear-induced aggregation enhances charge transport, we show that backbone alignment alone does not guarantee charge transport anisotropy. These results provide additional understanding of shear-induced enhanced charge transport and set processing guidelines for high-performance printed organic circuitry. Shear-printing is a promising processing technique in organic electronics for microstructure/charge transport modification and large-area film fabrication. Nevertheless, the mechanism by which shear-printing can enhance charge transport is not well-understood. In this study, a printing method using natural brushes is adopted as an informative tool to realize direct aggregation control of conjugated polymers and to investigate the interplay between printing parameters, macromolecule backbone alignment and aggregation, and charge transport anisotropy in a conjugated polymer series differing in architecture and electronic structure. This series includes (i) semicrystalline hole-transporting P3HT, (ii) semicrystalline electron-transporting N2200, (iii) low-crystallinity hole-transporting PBDTT-FTTE, and (iv) low-crystallinity conducting PEDOT:PSS. The (semi-)conducting films are characterized by a battery of morphology and microstructure analysis techniques and by charge transport measurements. We report that remarkably enhanced mobilities/conductivities, as high as 5.7×/3.9×, are achieved by controlled growth of nanofibril aggregates and by backbone alignment, with the adjusted R2 (R2adj) correlation between aggregation and charge transport as high as 95%. However, while shear-induced aggregation is important for enhancing charge transport, backbone alignment alone does not guarantee charge transport anisotropy. The correlations between efficient charge transport and aggregation are clearly shown, while mobility and degree of orientation are not always well-correlated. These observations provide insights into macroscopic charge transport mechanisms in conjugated polymers and suggest guidelines for optimization.

Naphthalene Bis(4,8-diamino-1,5-dicarboxyl)amide Building Block for Semiconducting Polymers

We report a new naphthalene bis(4,8-diamino-1,5-dicarboxyl)amide (NBA) building block for polymeric semiconductors. Computational modeling suggests that regio-connectivity at the 2,6- or 3,7-NBA positions strongly modulates polymer backbone torsion and, therefore, intramolecular π-conjugation and aggregation. Optical, electrochemical, and X-ray diffraction characterization of 3,7- and 2,6-dithienyl-substituted NBA molecules and corresponding isomeric NBA-bithiophene copolymers P1 and P2, respectively, reveals the key role of regio-connectivity. Charge transport measurements demonstrate that while the twisted 3,7-NDA-based P1 is a poor semiconductor, the planar 2,6-functionalized NBA polymers (P2-P4) exhibit ambipolarity, with μe and μh of up to 0.39 and 0.32 cm2/(V·s), respectively.

Indolo-naphthyridine-6,13-dione thiophene building block for conjugated polymer electronics: Molecular origin of ultrahigh n-type mobility (Conference Presentation)

We present the synthesis and characterization of four conjugated polymers containing a novel chromophore for organic electronics based on an indigoid structure. These polymers exhibit extremely small band gaps of ∼1.2 eV, impressive crystallinity, and extremely high n-type mobility exceeding 3 cm2 V s–1. The n-type charge carrier mobility can be correlated with the remarkably high crystallinity along the polymer backbone having a correlation length in excess of 20 nm. Theoretical analysis reveals that the novel polymers have highly rigid nonplanar geometries demonstrating that backbone planarity is not a prerequisite for either narrow band gap materials or ultrahigh mobilities. Furthermore, the variation in backbone crystallinity is dependent on the choice of comonomer. We find that electron mobility can be correlated to the degree of order along the conjugated polymer backbone. Finally, we use this novel system to begin to understand the complicated effect of alkyl chain variation on the solid state packing in all 3 dimensions.

In situ GIWAXS analysis during spin-coating of solvent and additive effects on organic electronic thin-film microstructure evolution

Electronic Thin Film Microstructure Evolution The influence of solvent and processing additive for spin-coating organic electronic polymer thin films is investigated by in situ grazing incident wide angle x-ray scattering (GIWAXS) techniques to assess how device optimization choices impact pathways and kinetics of crystallization, as well as the resulting morphology of organic electronic thin films. Films of PTB7, P3HT and pDTS(FBTTh2)2 are spun from solutions in chloroform (CF), chlorobenzene (CB), and 1,2dichlorobenzene (DCB) and solutions with 1% and 3% v/v of the processing additives 1chloronapthalene (CN), diphenyl ether (DPE), and 1,8-diiodooctane (DIO). Through this in situ characterization multiple crystallization pathways are identified, with i) the single-solvent systems exhibiting rapid (i.e., <3s) crystallization after a solvent boiling point dependent film thinning transition, and ii) the solvent + additive mixture systems exhibiting different crystallization pathways with distinct intermediate morphological forms correlated with additive molecular structures and demonstrating crystallization formation times ranging from minutes (CN, DPE) up to hours (PTB7:DIO). It is also clear that the polymer/small molecule solution properties play a key role in how the additive interacts, demonstrated by solution self-aggregating PTB7, solution free-chain P3HT and small molecule p-DTS(FBTTh2)2 having divergent interactions with select additives. These results shed light on how spin casting parameters impact morphological formation and provide an important knowledge base that can be used by device engineers to better direct performance optimization studies. This research was supported by the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award Number DE-SC0001059. Use of Beamline 8-ID-E of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.