KEYWORDS: Solar cells, Solar energy, Organic photovoltaics, Concentrated solar cells, Photovoltaics, Solar concentrators, Energy efficiency, Ultraviolet radiation, Silicon, Fourier transforms
The most widespread expectations for the future role of organic solar cells are probably as an extremely low-cost, easily-replaceable, power-producing medium for a wide variety of portable applications. This picture has come about owing to the present-day relatively low efficiency and stability of organic solar cells compared to their far more mature inorganic counterparts. However, even with the highest-efficiency and most stable inorganic solar cells there are still serious questions as to whether such technology could ever be cost-competitive with fossil-fuelled power generation, except for special niche situations. We have recently proposed that very large parabolic dishes, if used to illuminate inorganic solar cells at solar intensities several hundred times larger than normal, could lead to fossil-competitive solar power generation. The paper will review the technical details and economic projections of such systems and will discuss the conditions under which it might be possible for them to employ organic solar cells.
We propose to modify the solar collector PETAL (Photon Energy Transformation & Astrophysics Laboratory) for astronomy. The mirror is a segmented parabolic dish collector, which has a relatively poor imaging quality. The conversion can be done by either of two principal methods: (1) phasing the surface of the collector itself or significant sections thereof; (2) transforming the structure into an optical interferometer by mounting small telescopes around its rim, and using fiber optics to combine the light at a common focus.
KEYWORDS: Solar cells, Amorphous silicon, Temperature metrology, Photovoltaics, Manufacturing, Organic photovoltaics, Silicon, Thin films, Polymers, Solar energy
The paper reviews first, the recent history of commercial thin film photovoltaic modules in terms of measurements we have performed on their true AM1.5 efficiency (under natural sunlight), and their observed long-term stability properties. Attention is focused on the first generation of single-junction amorphous silicon (a-Si) modules that became available during the 1980s, and we compare their performance with those of late-1990s models fabricated with multi-junction a-Si, CdTe and CuInSe2 thin films. The efficiency and stability of these modules are compared with corresponding measurements we have performed on the high-efficiency organic solar cells that were recently produced at the Johannes Keppler University of Linz. Finally, we review the economics of grid-connected PV systems in order to provide cost benchmarks for future organic competitors.
Bulk donor-acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing monochromatic efficiencies above 1%. The present paper reports the temperature and irradiance dependencies of full-spectrum photovoltaic parameters for such devices. The measurements were performed under real sun conditions and under a solar simulator. The sun provided a light source stable in intensity to within +/- 1% and closely approximating a true AM1.5 spectrum, whereas the simulator enabled the light intensity to be varied in the range 80 - 600 W m-2. The most interesting feature that was observed for these devices is that above a cell temperature of 20 degree(s)C the positive temperature coefficient observed for the short- circuit current exceeds in magnitude the negative temperature coefficient that was found for the open-circuit voltage. This means that, unlike the situation for conventional PV devices, these cells actually exhibit an increase in efficient with increasing temperature (reaching a value of 0.63% at 40 degree(s)C). We suggest that the observed behavior originates from the temperature dependence of the conductivity of the conjugated polymers-fullerene composite. This hypothesis is confirmed by the irradiance-resolved measurements performed at different cell temperatures. We observed a linear increase in the short-circuit current with light intensity over the whole ranges of irradiances and temperatures but maximum temperature influence is observed at highest light intensity.
The paper outlines several unique properties of C60 (environmental, electronic, and structural), compared to standard semiconductors such as Si and GaAs, that render it a most promising material for the fabrication of high-efficiency photovoltaic cells. Details are presented of an experimental Schottky barrier solar cell we have constructed using a single crystal of C60. Preliminary results of our experimental studies of the optical properties of crystalline and amorphous C60 films are discussed.
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