Photovoltaic sandwich cells

The solar to electric power conversion efficiency of dye-sensitized solar cells of laboratory scale (0.158 cm2), validated by an accredited photovoltaic calibration laboratory, has reached 11.1% under standard reporting conditions, i.e., air mass 1.5 global sunlight at 1000 Wm-2 intensity and 298 K temperature, rendering it a credible alternative to conventional p-n junction photovoltaic devices [68]. Photovoltaic performance data obtained with a sandwich cell under illumination by simulated AM 1.5 solar light using complex 26 are shown in Fig. 16. At 1 sun the 26-sensitized solar cell exhibited 17.73 =b 0.5 mA current, 846 mV potential, and a fill factor of 0.75 yielding an overall conversion efficiency of 11.18%. 

Quite pronounced photovoltaic effects have been observed in Mx 1150 nm -chlorophyll a M2 sandwich cells, where Mx = A1 or Cr and M2 = Hg or Au.7e>77 These are ascribed to a Schottky barrier at the junction with metal Mx which has a lower work function than M2. If the Mi junction is the front (illuminated) electrode then the photovoltaic action spectrum is identical with the absorption spectrum of the chlorophyll. If it is at the rear, the action spectrum shows an inner filter effect, because only light absorbed in the region of the barrier is effective. Figure 11 shows the performance of a typical cell, which has a power conversion efficiency of ca. 10-3% at 745 nm. The best efficiency, 5 x 10-a%, was achieved by a Cr chlorophyll a Hg cell. Photovoltaic properties have also been reported in the cell A11 Mg phthalocyanine Ag, which has a Schottky barrier of height ca. 0.6 eV at the A1 junction.78 At 690 nm, the power conversion efficiency was ca. 10-2%. It has been shown that oxidized A1 contacts to Cu phthalocyanine are blocking.79... 

The plasma polymerization of TCl JQ and TCNE was carried out at 13.56 MHz from the gas phase and semiconductive polymeric films were obtained [88, 89]. The electrical conductivities of the films obtained ranged fix m 10 " to 10 S cm and the Al/polymer/ITO (indium tin oxide) sandwich cells made from the films showed rectifying behavior and photovoltaic response. Photoconductivity was also observed in the films. Infrared, ultraviolet, and X-ray photoelectron spectroscopy were utilized to characterize the structure, and these results as well as those from electrical measurements confirmed that a conjugated structure with delocalized 7c-electrons has been formed in the films. 

Chi Jb was purchased from Sigma (St. Louis, MO) and was used without further purification. The photovoltaic cells were prepared by interposing the Langmuir-Blodgett films of Chi b between two dissimilar metal electrodes, i.e. aluminium and silver, with work functions ((()), such that A1 pigment Ag Both metal electrodes were evaporated under vacuum according to the method previously described. The active area of the cell is 0. 5 cm. Before the deposition of Chi b monolayer, the top aluminium electrode was covered with one monolayer of cadmium arachidate transferred at a surface pressure of 30 mN m l. The sandwich cells contained monolayers of Chi transferred on top of the arachidate layer at a surface pressure of 20 mN m l. 

We have next studied the effect of temperature on the photovoltaic properties of the sandwich cells of Chi in the temperature range of 248-318 K. Fig. 4 shows the typical temperature dependence of the short-circuit photocurrent of a Chi cell illuminated at 656 nm with a light... 

A. Desormeaux, J.-J. Max, and R. M. Leblanc, Photovoltaic properties of Al/Langmuir-Blodgett films/Ag sandwich cells incorporating either chlorophyll a, chlorophyll b or zinc porphyrin derivative, J. Phys. Chem. submitted (1989). 

J. A. Bardwell, and J. R. Bolton, Monolayer studies of 5 (4-carboxy-phenyl)-10,15 20-tritolyl-porphyrin. II. Photovoltaic study of multilayer sandwich cells, Photochem. Photobiol. 40 319(1984). 

The detector in a spectrometer must produce a signal related to the intensity of the radiation falling on it. For instruments operating in the visible region a photovoltaic or barrier-layer cell is the simplest and cheapest available. Current produced when radiation falls on a layer of a semiconductor material, e.g. selenium, sandwiched between two metallic electrodes, is proportional to the power of the incident radiation and can be monitored by a galvanometer. Barrier layer cells are robust and are often used in portable instruments but they are not very sensitive and tend to be unstable during extended use. 

More recently, emission of light by a metal-CP-metal sandwich has been observed [235]. This is again a thin-film device, analogous to conventional MIM devices [230]. To some extent, such a light-emitting diode (LED) can be considered as the reciprocal of a photovoltaic cell. In the latter, absorption of a photon creates an electron-hole pair that is collected in the external circuit, whereas in the former, recombination of an electron and a hole that have been injected from the electrodes generates an emitted photon. LEDs using CPs are discussed in Section V.C. 

Aside from dye-based cells, a good deal of study has been devoted to photovoltaic effects in polymer-based devices. Single-layer sandwich-type devices composed of aluminum—polyphenjienevinjiene—ITO have been fabricated with solution-cast polymer layers of These devices display... 

Devices with luminescent polymer films sandwiched between high and low work function electrodes were originally fabricated to be LEDs. These same devices, however, can be operated as photodetectors or photovoltaic cells. With no externally applied bias, the polymer layer has a built-in electric field, because of the difference in work function of the two electrodes, which tilts the energy bands (Fig. VII-3). When light is absorbed by the polymer, some of the electron-hole pairs that are created are separated by the electric field. The holes are then pushed by the field to one electrode and the electrons are pushed to the other anode. The carriers that reach the electrodes provide a voltage that can either be used as a measure of the light intensity or as a source of energy. 

In evaporation-intercalation devices solar energy conversion would, at least in the more efficient case of a thermal system, not be converted by exciting electrons and rapidly separating them from holes, but by transferring atoms or molecules across a phase boundary by evaporation which is usually a very efficient process. It is, consequently, neither necessary to use materials which are well crystallized like those developed for photovoltaic cells nor is it necessary to prepare sophisticated junctions. A compacted polycrystalline sheet of a two-dimensional material which is on one side placed in contact with an electrolyte, sandwiched between the layer-type electrode and a porous counter electrode, as it is used in fuel cells, would constitute the central energy conversion unit. Some care would have to be taken to choose an electrolyte which is suitable for intercalation reactions and which is not easily evaporated through leaks in the electrodes. Thin layers of polymeric or solid electrolytes would seem to be promising. 

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