Paul Panayotatos

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.

Achievements and limitations in optimized GaAs films grown on Si by molecular‐beam epitaxy

A systematic study of the growth of high‐quality films of GaAs on Si substrates has been performed for applications in devices, particularly in optoelectronic devices for cointegration in optical interconnects. The effort for optimized active layers was approached through the separate optimization of substrate preparation, growth time parameters, and postgrowth treatment. In particular, the study of growth involved the investigation of the effect of silicon substrate orientation, post‐growth treatment, as well as multilayer and, especially, silicon buffer layers. For quantification of film quality, a number of characterization methods were used both in situ: reflected high‐energy electron diffraction (RHEED); and ex situ: optical, electrical [current versus voltage (I‐V), capacitance versus voltage (C‐V), deep‐level transient spectroscopy (DLTS), Hall], transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron channeling patterns, x‐ray double‐crystal diffractometry (DDX). Schot...

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.

Microwave performance of GaAs-on-Si MESFETs with Si buffer layers

The incorporation of silicon-buffer layers is shown to be critical in attaining optimized microwave performance for GaAs on silicon MESFETs. A current gain cutoff frequency (f/sub t/) of 18 GHz and maximum power cutoff frequency (f/sub max/) of 30 GHz is reported for relaxed geometry devices. The low parasitic capacitance and excellent device isolation make this structure suitable for monolithic integration. >

Modelling the effects of field-dependent quantum efficiency in organic semiconductor Schottky barrier solar cells

Abstract The external characteristics of a β-metal-free phthalocyanine Schottky barrier solar cell are modelled. The model utilizes representative circuit parameters and calculates the photogeneration current based on depletion region absorption and the field dependence of photogeneration. Projected power efficiencies are calculated for the case where the effects of series resistance and absorption in the front contact are overcome. The field dependence of photogeneration of this particular material severely limits the solar cell efficiency. However, the model should be the applicable to any material with a similar field dependence of photogeneration. In addition, a method is suggested utilizing Forster radiationless transfer to improve the generation efficiency in organic photovoltaic devices.

Dependence of structural and compositional characteristics of chromium metal films as a function of deposition rate during the fabrication of metal/insulator/semiconductor solar cells

Abstract Metal/insulator/semiconductor solar cells on single-crystal and polycrystalline silicon substrates have been fabricated under various electrode metal (chromium) deposition rates. The effects of deposition rate on the electrical, structural and compositional characteristics of the resulting films have been studied. Auger electron spectroscopy and transmission electron microscopy results correlate well with the electrical characterization of the resulting devices. It is found that the oxygen content, which is inversely proportional to the rate of metal deposition, determines the structure of the resulting metal films which can be amorphous or polycrystalline. The oxygen content also has a beneficial effect on the performance of the solar cells by inducing a reduction in the electrode metal (chromium) work function.

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.

Parameter extraction for a figure-of-merit approach for the comparison of optical interconnects

A methodology is described for parameter extraction from modeling for the determination of the appropriate figures of merit (FOMs) to be used for comparisons between different architectures of optical interconnects and their electrical counterparts. The key FOM selected for performance is that of data throughput, defined as the product of bitrate and interconnect length. Parameter extraction proceeds through modeling of a prototype optical transceiver introduced by GTE laboratories. Optical power budget modeling is performed as it applies to a simple basic optical interconnect, which can then be extended to the modeling of arrays, by incorporating channel crosstalk limitations. Through this modeling, the specific limitations to the data throughput are explored and several trade-offs such as those between gain and speed, power and delay-times, and power and bit error rate (BER) are quantified

Molecular beam epitaxy GaAs on Si: material and devices for optical interconnects

(100)CdZnTe epilayers are grown by hot wall beam epitaxy (HWBE) on (100) GaAs. The CdZnTe epilayers are used as substrates for the growth of Hg1_CdTe (x = 0.27 - 0.32) layers by closed space vapor phase epitaxy. The Hg1CdTe layers have an x-ray rocking curve width of 59 10 arc sec measured across a 1 inch wafer. The layers are p-type with a hole concentration of 2 - 4.1016crri3 and a mobility of 250 - 350cm2/Vs at 77 K. Photoconductivity decay measurements give the lifetime of excess carriers which is governed by recombination via Shockley-Read centres at T < 250 K. Linear arrays of planar, photovoltaic detectors are fabricated by implantation of B ions. The elements size is 50 50,arn2, the space between the elements is 50prn. The R0A product, the responsivity and the cut-off wavelength of the p-n junction are measured at 192 K. The variation of the responsivity across a 16 element array is less than 4 %, the cut-off wavelength varies between 4.77,um and 4.83μm.

Optical interconnect reliability

In order to make appropriate technology choices for the system implementation of optical interconnects, the issues of reliability and manufacturability must be included in any figure of merit (FOM) calculation.