Ronald R. Sauers

Investigations of materials and device structures for organic semiconductor solar cells

In our work with the phthalocyanines and perylenes, we have formulated a hierarchy of placement of dyes in p-n heterojunction de- vices to optimize the short-circuit current density. Computer modeling of Schottky barrier cells, with parameters fit to experimental results and incorporating field-dependent carrier generation, were used to optimize the power efficiency. The model predicts an optimum carrier concentra- tion density and suggests different hierarchies for utilization of Forster radiationless energy transfer. Synthesis and purification of materials is also discussed. In terms of purity, most materials used in the literature are shown to have been quite below solar grade. A newly devised pu- rification technique is introduced. A hydration mechanism is shown to exist for chloroaluminum phthalocyanine, previously thought immune to hydration. The latter mechanism had been mistaken before for a simple phase transformation and can be induced by various different treatments with organic nonsolvents for chloroaluminum phthalocyanine. Testing of p-n and Schottky barrier cells is also discussed. The different capaci- tance versus voltage (C-V) spectroscopies are compared, and the case for the small-signal method is argued over the triangular voltage sweep. Several cautions on the interpretation of the C-V curves are noted.

Improved p-n heterojunction solar cells employing thin film organic semiconductors

Abstract Scanning of organic dye pairs suitable for the fabrication of p-n heterojunction solar cells has led to the fabrication of new structures with improved response. The choice of dyes as well as the proper hierarchy among the materials in the cells was suggested by the electronic and optical dye characteristics so that more of the excitons generated by the absorbed quanta were delivered at the dye-dye interface. The cells were fabricated by vacuum evaporation of thin (200–400 A) dye layers on tin oxide covered glass. Both old and newly synthesized organic semiconductors were used. The major drawback of the one-dye cells used in the past is the limitation on the portion of the solar spectrum covered by the dye absorption spectrum. In the p-n heterojunction cells studied here this limitation has been successfully overcome and full visible solar spectrum coverage has been achieved. Internal current generation quantum efficiencies of the order of 7% have been demonstrated. Overall solar cell efficiencies of these structures, which are not thickness optimized, are still in the area of 0.1%. This is twice as high as the values previously reported for some p-n junction solar cells with organic semiconductors that did not take dye hierarchy and loss of energy through internal molecular transitions into account.

Addition of ethyl diazoacetate to bicyclic olefins

Abstract Several new derivatives of the tricyclo(3.2.1.0 2,4 octane system have been synthesized. The key reaction involved addition of ethyl diazoacetate to norbornenes and norbornadiene.

An ab initio study of some heterocyclic 6π-carbenes

Ab initio calculations on a series of 6π-electron heterocyclic ring systems containing O, N, and S atoms provided evidence for aromatic stabilization ranging from 7.6 to 25.5 kcal/mol. The most highly stabilized system was the imidazolyl carbene which had an aromatic stabilization energy comparable to furan and pyrrole. Proton affinity data was consistent with literature pKa measurements of the conjugate acids.

Properties of the nucleic-acid bases in free and Watson-Crick hydrogen-bonded states: computational insights into the sequence-dependent features of double-helical DNA

The nucleic-acid bases carry structural and energetic signatures that contribute to the unique features of genetic sequences. Here, we review the connection between the chemical structure of the constituent nucleotides and the polymeric properties of DNA. The sequence-dependent accumulation of charge on the major- and minor-groove edges of the Watson–Crick base pairs, obtained from ab initio calculations, presents unique motifs for direct sequence recognition. The optimization of base interactions generates a propellering of base-pair planes of the same handedness as that found in high-resolution double-helical structures. The optimized base pairs also deform along conformational pathways, i.e., normal modes, of the same type induced by the binding of proteins. Empirical energy computations that incorporate the properties of the base pairs account satisfactorily for general features of the next level of double-helical structure, but miss key sequence-dependent differences in dimeric structure and deformability. The latter discrepancies appear to reflect factors other than intrinsic base-pair structure.

An approach to the optimal design of p-n heterojunction solar cells using thin film organic semiconductors☆

In recent work with p-n heterojunction solar cells constructed with pairs of photoconductive organic dyes, we have obtained short-circuit current yields of over 19%. The best results to date have been obtained with the pair chloroaluminum phthalocyanine (ClAlPc) and the bis-methylimide of perylenetetracarboxylic acid (DMP). The preferred configuration is SnO2/DMP/ClAlPc/Ag. This configuration has the advantage of placing the shorter wavelength absorber DMP against the transparent SnO2 input surface and facilitating Forster radiationless energy transfer from the DMP phase to the region of the active DMP/ClAlPc junction. It also makes best use of the electron-accepting and electron-donating properties of the two dyes. Through tests of single-dye Schottky cells with a variety of metal electrodes, we have learned that the Fermi levels of these two dyes lie too close together for optimum cell performance. Synthetic variants of the simple perylene dye and of ClAlPc have been prepared for stronger electron-accepting and electron-donating properties respectively.

Diffuse functions in natural bond orbital analysis

We show that diffuse function augmentation of Pople basis sets at the 6‐311G RHF and B3LYP levels strongly impact conclusions drawn from natural bond orbital (NBO) analysis. The large spatial extent of high quantum number Rydberg orbitals introduced by augmentation contribute importantly to the valence space of neighboring atoms due to the likely inadequacy of the 311 valence functions. In contrast, lesser anomalies are found for augmentation of double zeta type 6‐31G. The energetic anomalies found for bond and antibond NBO descriptions, made nonlocal by augmentation, are most serious for molecules with four or more heavy atoms. For these cases augmentation can lead to nonphysical results. NBO results using Dunning‐type correlation consistent orbitals exhibit much weaker basis set dependence than those using the Pople basis sets.© 2006 Wiley Periodicals, Inc. J Comput Chem 2007

Shaping the absorption and fluorescence bands of a class of efficient, photoactive chromophores: synthesis and properties of some new 3H-xanthen-3-

Abstract An approach to the systematic design of absorption and fluorescence band shapes is developed from considerations of the solvatochromism and electronic perturbation of unsymmetrical chromophores. The predicted spectral shifting and line broadening has been demonstrated within a series of newly synthesized substituted 6-amino-3H-xanlhen-3-ones 1 . A new method of presenting bandwidth data is developed which utilizes dipole strength-weighted moments of absorption bands.

A computational study of proton and electron affinities

Proton and electron affinities were calculated for a series of 28 carbanions and radicals using MP2/6-31 and B3LYP methods. Structural and electronic factors that control anion and radical stabilization were examined by natural bond orbital analyses. New examples of hyperconjugation were found for lone pairs and radical centers and adjacent C-H∗ and C-C∗ orbitals. Correlations between %-s-character of CH bonds and anions with proton affinity were poor.

Cyclopropanes: Calculation of NMR spectra by ab initio methodology

Abstract 1H and 13C nuclear magnetic resonance chemical shifts for a select group of cyclopropyl systems were calculated using ab initio/gauge-independent atomic orbital methodology. Long range nucleus independent chemical shifts attributable to cyclopropane ring currents were estimated. The results demonstrate the viability of this protocol to simulate unusual long range shielding/deshielding effects associated with cyclopropyl systems. Examination of the molecular orbitals of bicyclo[3.1.0]-hexyl systems revealed a highly delocalized HOMO for the anti-conformer (pseudo-chair form). Natural atomic population analyses were used to examine putative correlations between atomic charges and chemical shifts.