George R. Bird

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.

A new laser dye with potential for high stability and a broad band of lasing action: Perylene-3,4,9,10-tetracarboxylic acid-bis-N,N′(2′,6′ xylidyl)diimide

Abstract We have obtained high gain lasing action from a dye in the calss of perylene-tetracarboxylic acid diimides. These dyes have been used in the past as microcrystalline pigments and more recently as molecularly dissolved fluors having both an orange absorption color and orange fluorescence. In dimethylformamide this dye absorbs with a main maximum at 527 nm, well coupled to the exciting radiation from a frequency-doubled YAG exciting laser. The dye lases primarily in the 0′→ 1″ satellite of the fluorescence band from 566–585 nm, with intermittent output pulses to 605 nm. While this dye overlaps the lasing bands of the rhodamine dyes, we believe that it may be considerably more stable than even Rhodamine 6G, the most stable of that class.

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.

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.

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.

The rotational resonance Raman spectrum of nitrogen dioxide and the determination of electronic state symmetry from resonance Raman selection rules

Abstract The pure rotational Raman spectrum of nitrogen dioxide has been observed and shown to be consistent with existing determinations of molecular parameters. Upon observation at 600 Torr pressure and 0.4 cm −1 resolution a well-defined rotational spectrum is obtained. This spectrum is overlaid with a number of fluorescence lines. The fluorescence lines are separated from the Raman spectrum by a comparison of Stokes and anti-Stokes branches of the rotational spectrum. Out of seven strong fluorescence lines seen with 5145 A excitation, five probably are identifiable with vibration-rotation fluorescence progressions observed by Abe. The most striking feature of these observations is the potential use of the resonance Raman effect for the analysis of complicated electronic spectra. When this rotational spectrum is observed with excitation by 5309 A or 5145 A excitation, the Raman spectrum follows a -axis selection rules and the Q -branches are in the noise level or barely out of it. However, at 4880 A the Δ K = 2 Q -branches become a major feature of a spectrum, indicating that an appreciable part of the absorption at this wavelength is occurring through the operation of b - or c -axis selection rules. These findings are consistent with present notions of a 2 B 2 excited state dominating absorption at longer wavelengths, while at shorter wavelengths a 2 B 1 excited state becomes important. Given a tunable laser, one could map the relative importance of these two possible selection rules for NO 2 without any theoretical analysis more sophisticated than that presented in this paper. A simplified statement of the selection rules for resonance rotational Raman spectra of asymmetric tops has been developed in the course of this investigation. No attempt has been made to refine the rotational parameters of NO 2 since all of the lines seen areunresolved multiplets. Our data should be regarded as a search spectrum preliminary to investigation on a high resolution instrument.

Hydration of chloroaluminum phthalocyanine

Chloroaluminum phthalocyanine is shown to form a hydrate with two molecules of water, and this hydrate is shown to be identical with what was previously believed to be the result of a simple phase transformation.

A new family of lasing dyes from an old family of fluors

Abstract We report the development of a new series of laser dyes and describe the performance of these materials. This work was spurred by the realization that few laser dyes have an optimally shaped fluorescence emission for lasing action. Most of the dipole strength of the transition is concentrated in the 0.0 band, where lasing cannot occur, and very little is concentrated in the vibrational satellites 0–1 and 0–2 where lasing is possible. The new dyes are unsymmetrical materials which may be considered as a hybrid of a rhodamine molecule and a fluorescein molecule (fig. 1), and we consider them to be cyclized merocyanines. One dye in particular, 6-dimethylamino-9-o-carboxyphenylxanthen-3H-one (labelled SNH-8), has given power output and time stability better than the preferred lasing dye Rhodamine 6G. The effects of dye structure and of acid-base transitions on lasing activity will be discussed briefly. The theory of band-shaping and the detailed optical parameters of the new dyes will be presented separately.

Performance of some rhodamone dyes designed for high efficiency broadband lasing output

The performance of a family of rhodamone dyes designed for broadband lasing has been investigated. Conversion efficiencies of 60% and broadband lasing have been attained in this novel class of laser dyes. Lasing occurs in the 570 - 590 nm region with bandwidths twice that of high gain laser dyes such as rhodamine 6G and pyrromethene 580. The lasing characteristics of these stable dyes portends real benefits for the design of wavelength-agile systems.

Modeling, synthesis, and testing of materials and devices 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 devices 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 concentration density and suggests different hierarchies for utilization of Foerster 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 purification 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 solvents in which chloroaluminum phthalocyanine is not soluble. Testing of p-n and Schottky barrier cells is also discussed. The different capacitance vs. 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.