Radiation Study of the Organic Photovoltaic Cell P3HT:PCBM
The need for inexpensive, dependable, radiation hard solar cells for use in space applications has led to attention being focused on organic semiconductor based solar cells. Such cells are lightweight, flexible and are potentially useful in conformal coverage applications. While these solar cells are less efficient (presently < 12%) than traditional silicon or III-V semiconductor based solar cells, the reduced efficiency is compensated for by their lower weight. This leads to a higher specific power and hence a lower load for launch. Furthermore, their flexibility is a particularly positive attribute since this renders them less vulnerable to vibration damage during the launch process. It must also be added that since one envisages solution processing deposition of the organic cells on very large area sheets (roll by roll technology) one can then also imagine a scenario in which a chosen panel area can be simply tailored from a large roll, thereby speeding up the process of solar panel production. Before this somewhat futuristic approach to low power solar panel production can become a reality for space applications, a full evaluation/understanding of their behavior in a radiation environment is necessary. In this work, a detailed study has been performed on the archetypal organic photovoltaic P3HT:PCBM. The interest of the applicability of organic photo-cells for use in space based solar panels is derived from the recognition that unusual' conditions exist which are not generally addressed by the organic photo-cell community. The defense presentation will cover the findings of pre-irradiation, irradiation, and post-irradiation characteristics; determination of the physical mechanisms resulting in the dominant photo-carrier loss mechanism, and a detailed investigation of the radiation effects. Transient photo-voltage (TPV) measurements were utilized to evaluate carrier relaxation times in P3HT:PCBM based photo-cells over a wide range of open circuit voltages. Satisfactory agreement is found with data obtained by low frequency impedance measurements. This data set offers valuable insight into the loss mechanism to help material scientists develop new material that has better power conversion efficiency. Furthermore, the results are promising for the development OPV technology for space based applications. We find that the experimental data is inconsistent with the theoretical behavior expected based on the generally accepted Langevin recombination model. In particular, the Langevin coefficient is three orders of magnitude smaller than the theoretical one and appears to be dependent on the carrier density. For the low light levels, the relaxation time variation is determined by the RC time constant behavior of the photodiode.'
OPV, Radiation Effects, Organic Photovoltaic, Solar Power
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