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 \u3c 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\u27 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.\u2

Direct evidence for interface state annealing in the negative bias temperature instability response Direct evidence for interface state annealing in the negative bias temperature instability response

Using a rapid data acquisition methodology, the authors examine the time dependent recovery of the "permanent" component of charge build-up due to the negative bias temperature instability in Si based p-channel field effect transistors in inversion and n-channel devices in accumulation. The authors find clear evidence for recovery of the charge associated with interface states for elevated temperatures (!150 C) and for extended times (t recover $ 20 000 s). Recovery appears to begin at shorter times for p-channel devices than for n-channel. An explanation is advanced both for the mechanism of interface state annealing and for the difference observed between p and n channel devices