Brad Conrad

Effect of Acene Length on Electronic Properties in 5‐, 6‐, and 7‐Ringed Heteroacenes

Interest in organic semiconductors is motivated by their promise to offer a viable route to fabricating low-cost electronic devices on arbitrary substrates, and by the versatility of their chemical structures and physical properties, accomplished by means of molecular engineering. [ 1–3 ] Molecular modifi cations can yield soluble semiconductors that allow reduced complexity device fabrication using methods such as spin-coating, ink-jet printing, roll-to-roll processing and spray-deposition. [ 4–7 ]

Intrinsic Charge Trapping Observed as Surface Potential Variations in diF-TES-ADT Films

Spatial variations in surface potential are measured with Kelvin probe force microscopy for thin films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophenes (diF-TES-ADT) grown on SiO2 and silane-treated SiO2 substrates by organic molecular beam deposition. The variations are observed both between and within grains of the polycrystalline organic film and are quantitatively different than electrostatic variations on the substrate surfaces. The skewness of surface potential distributions is larger on SiO2 than on HMDS-treated substrates. This observation is attributed to the impact of substrate functionalization on minimizing intrinsic crystallographic defects in the organic film that can trap charge.

Spatial first-passage statistics of Al/Si(111)-(square root 3 x square root 3) step fluctuations.

Spatial step edge fluctuations on a multicomponent surface of Al/Si(111)-(square root 3 x square root 3) were measured via scanning tunneling microscopy over a temperature range of 720-1070 K, for step lengths of L=65-160 nm. Even though the time scale of fluctuations of steps on this surface varies by orders of magnitude over the indicated temperature range, measured first-passage spatial persistence and survival probabilities are temperature independent. The power law functional form for spatial persistence probabilities is confirmed and the symmetric spatial persistence exponent is measured to be theta=0.498+/-0.062 in agreement with the theoretical prediction theta=1/2. The survival probability is found to scale directly with y/L, where y is the distance along the step edge. The form of the survival probabilities agrees quantitatively with the theoretical prediction, which yields exponential decay in the limit of small y/L. The decay constant is found experimentally to be y(s)/L=0.076+/-0.033 for y/L<or=0.2.

Coverage dependent molecular assembly of anthraquinone on Au(111)

A scanning tunneling microscopy study of anthraquinone (AQ) on the Au(111) surface shows that the molecules self-assemble into several structures depending on the local surface coverage. At high coverages, a close-packed saturated monolayer is observed, while at low coverages, mobile surface molecules coexist with stable chiral hexamer clusters. At intermediate coverages, a disordered 2D porous network interlinking close-packed islands is observed in contrast to the giant honeycomb networks observed for the same molecule on Cu(111). This difference verifies the predicted extreme sensitivity [J. Wyrick et al., Nano Lett. 11, 2944 (2011)] of the pore network to small changes in the surface electronic structure. Quantitative analysis of the 2D pore network reveals that the areas of the vacancy islands are distributed log-normally. Log-normal distributions are typically associated with the product of random variables (multiplicative noise), and we propose that the distribution of pore sizes for AQ on Au(111) originates from random linear rate constants for molecules to either desorb from the surface or detach from the region of a nucleated pore.

Explaining the molecular structure of the bulk heterojunction with simple electronic measurements and marcus-hush theory for squaraine: PCBM organic photovoltaic devices

A unique donor material for small molecule organic solar cells is shown to be opto-electrically tunable with thermal annealing. The system is characterized and shown to make working devices (PCE=1.1 +/- 0.3%). Examination of the spectral response of the devices before and after thermal annealing show an interesting decrease in spectral response that is explained by examining both the material properties and through application of Marcus-Hush theory. The proposed explanation could lay the groundwork for a predictive model to analyze potential device candidates.

A study of squaraine small molecule donor materials for application in organic photovoltaics

A unique donor material for small molecule organic solar cells is shown to be opto-electrically tunable with thermal annealing. The system is characterized and shown to make working devices (PCE=1.1 +/- 0.3%). Examination of the spectral response of the devices before and after thermal annealing show an interesting decrease in spectral response that is explained by examining both the material properties and through application of Marcus-Hush theory. The proposed explanation could lay the groundwork for a predictive model to analyze potential device candidates.

Spatial First-passage Statistics of Al/Si(111)-(root3 x root3) Step Fluctuations

Spatial step edge fluctuations on a multicomponent surface of Al/Si(111)-(square root 3 x square root 3) were measured via scanning tunneling microscopy over a temperature range of 720-1070 K, for step lengths of L=65-160 nm. Even though the time scale of fluctuations of steps on this surface varies by orders of magnitude over the indicated temperature range, measured first-passage spatial persistence and survival probabilities are temperature independent. The power law functional form for spatial persistence probabilities is confirmed and the symmetric spatial persistence exponent is measured to be theta=0.498+/-0.062 in agreement with the theoretical prediction theta=1/2. The survival probability is found to scale directly with y/L, where y is the distance along the step edge. The form of the survival probabilities agrees quantitatively with the theoretical prediction, which yields exponential decay in the limit of small y/L. The decay constant is found experimentally to be y(s)/L=0.076+/-0.033 for y/L<or=0.2.

Spatial first-passage statistics ofAl∕Si(111)−(3×3)step fluctuations

Spatial step edge fluctuations on a multicomponent surface of Al/Si(111)-(square root 3 x square root 3) were measured via scanning tunneling microscopy over a temperature range of 720-1070 K, for step lengths of L=65-160 nm. Even though the time scale of fluctuations of steps on this surface varies by orders of magnitude over the indicated temperature range, measured first-passage spatial persistence and survival probabilities are temperature independent. The power law functional form for spatial persistence probabilities is confirmed and the symmetric spatial persistence exponent is measured to be theta=0.498+/-0.062 in agreement with the theoretical prediction theta=1/2. The survival probability is found to scale directly with y/L, where y is the distance along the step edge. The form of the survival probabilities agrees quantitatively with the theoretical prediction, which yields exponential decay in the limit of small y/L. The decay constant is found experimentally to be y(s)/L=0.076+/-0.033 for y/L<or=0.2.