Nanoscale structure, dynamics and power conversion efficiency correlations in small molecule and oligomer-based photovoltaic devices

Photovoltaic functions in organic materials are intimately connected to interfacial morphologies of molecular packing in films on the nanometer scale and molecular levels. This review will focus on current studies on correlations of nanoscale morphologies in organic photovoltaic (OPV) materials with fundamental processes relevant to photovoltaic functions, such as light harvesting, exciton splitting, exciton diffusion, and charge separation (CS) and diffusion. Small molecule photovoltaic materials will be discussed here. The donor and acceptor materials in small molecule OPV devices can be fabricated in vacuum-deposited, multilayer, crystalline thin films, or spin-coated together to form blended bulk heterojunction (BHJ) films. These two methods result in very different morphologies of the solar cell active layers. There is still a formidable debate regarding which morphology is favored for OPV optimization. The morphology of the conducting films has been systematically altered; using variations of the techniques above, the whole spectrum of film qualities can be fabricated. It is possible to form a highly crystalline material, one which is completely amorphous, or an intermediate morphology. In this review, we will summarize the past key findings that have driven organic solar cell research and the current state-of-the-art of small molecule and conducting oligomer materials. We will also discuss the merits and drawbacks of these devices. Finally, we will highlight some works that directly compare the spectra and morphology of systematically elongated oligothiophene derivatives and compare these oligomers to their polymer counterparts. We hope this review will shed some new light on the morphology differences of these two systems

FOURIER TRANSFORM MICROWAVE SPECTROSCOPY OF ARGON CHLOROCYCLOBUTANE

$^{a}$ H. Kim and W.D. Gwinn J. Chem. Phys. 44, 865, (1966)Author Institution: Department of Chemistry, Wesleyan UniversityThe microwave spectrum of chlorocyclobutane was reported in $1966^{a}$. We have measured the microwave spectrum of argon chlorocyclobutane as part of our on-going research on bonding and dynamics of four-membered ring containing van der Waals complexes. Preliminary results show that the four-membered ring remains puckered in the complex. An analysis based partially on the nuclear quadrupole coupling projections of the $^{35}Cl$ nucleus will be presented

Zinc oxide nanorod growth on gold islands prepared by microsphere lithography on silicon and quartz.

International audienceGold islands, vapor deposited on silicon and quartz by microsphere lithography patterning, are used to nucleate arrays of ZnO nanorods. ZnO is grown on approximately 0.32 microm2 Au islands by carbothermal reduction in a tube furnace. Scanning electron microscopy (SEM) and energy dispersive atomic X-ray spectroscopy (EDS) confirm that the gold effectively controls the sites of nucleation of ZnO. Atomic force microscopy (AFM) shows that approximately 30 nm diameter nanorods grow horizontally, along the surface. Alloy droplets that are characteristic of the vapor-liquid-solid (VLS) mechanism are observed at the tips of the nanorods. The spatial growth direction of VLS catalyzed ZnO nanorods is along the substrate when they nucleate from gold islands on silicon and quartz. The energy of adhesion of the VLS droplet to the surface can account for the horizontal growth

Optical injection probing of single ZnO tetrapod lasers

The properties of zinc oxide (ZnO) nanotetrapod lasers are characterized by a novel ultrafast two-color pump/stimulated emission probe technique. Single legs of tetrapod species are isolated by a microscope objective, pumped by 267 nm pulses, and subjected to a time-delayed 400 nm optical injection pulse, which permits investigation of the ultrafast carrier dynamics in the nanosize materials. With the optical injection pulse included, a large increase in the stimulated emission at 400 nm occurs, which partially depletes the carriers at this wavelength and competes with the normal 390 nm lasing. At the 390 nm lasing wavelengths, the optical injection causes a decrease in the stimulated emission due to the energetic redistribution of the excited carrier depletion, which occurs considerably within the time scale of the subpicosecond duration of the injection pulse. The effects of the optical injection on the spectral gain are employed to probe the lasing dynamics, which shows that the full width at half maximum of the lasing time is 3 ps

Ultrafast Intramolecular Exciton Splitting Dynamics in Isolated Low-Band-Gap Polymers and Their Implications in Photovoltaic Materials Design

Record-setting organic photovoltaic cells with <b>PTB</b> polymers have recently achieved ∼8% power conversion efficiencies (PCE). A subset of these polymers, the <b>PTBF</b> series, has a common conjugated backbone with alternating thieno­[3,4-<i>b</i>]­thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2–8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular (“pseudo”) charge-transfer state in isolated <b>PTBF</b> polymers in solution, in the absence of the acceptor phenyl-C₆₁-butyric acid methyl ester (<b>PCBM</b>) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole–electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed

Correction to “Effects of Additives on the Morphology of Solution Phase Aggregates formed by Active Layer Components of High-Efficiency Organic Solar Cells”

Correction to “Effects of Additives on the Morphology of Solution Phase Aggregates formed by Active Layer Components of High-Efficiency Organic Solar Cells