Goal of research
Development of the model describing the charge carrier generation, recombination and transport processes in promising polymers for use in spacecraft and solar energy technologies.
state-of-art computer simulation methods of the charge carrier transport and translational diffusion in disordered solids;
advanced technique for measuring radiation-induced conductivity due to electrons with energy up to 50 keV combined with the radiation-induced variant of the time of flight method.
Empirical base of research
The modified, semi-empirical Rose-Fowler-Vaisberg model featuring a two- exponential trap distribution has been successfully parameterized for a number of polymers widely used in spacecraft and solar energy technologies.
A model describing charge carrier recombination in amorphous organic semiconductors accounting for spatial correlations has been developed which allows assessing the deviation of the recombination rate constant from its Langevin value.
Results of research
The modified Rose-Fowler-Vaisberg model featuring a two- exponential trap distribution has been developed to treat the radiation-induced conductivity in promising polymers for use in spacecraft and solar energy technologies. Comparative numerical calculations have been done for this model based on the conventional multiple trapping formalism or the two-exponential trap distribution both for pulsed and continuous irradiations in a small-signal regime. The last model resolved for the first time the fundamental problem of the data mismatch between pulsed and long-time irradiations observed in such wide-spread common polymers as polyethyleneterephthalate (Mylar) and polyimide (Kapton). It has been experimentally shown that unusual behavior of the radiation-induced conductivity in polyethyleneterephthalate in small-signal irradiations is due to the presence of deep traps allowing no thermal detrapping which greatly distorts the conventional dispersive transport of charge carriers.
A model describing charge carrier recombination in amorphous organic semiconductors accounting for spatial correlations has been developed which allows assessing the deviation of the recombination rate constant from its Langevin value but also can describe the effect of polar molecules with constant dipole or quadrupole moments on the decrease of a recombination rate.
Charge carrier transport through thin films of new polymers – polyaryleneketones copolymers has been investigated to detect a switching effect at pre-breakdown electric fields inducing an insulator-conductor transformation in a polymer. It has been established that there appears a strong non-linearity (super linearity) in co-PAEK in the range of field from 5 to 40 m which testifies in favor of the Onsager mechanism of the charge carrier generation. Increasing the relative number of phthalide groups in co-PAEK from 0.97/0.03 to 0.5/0.5 did not practically change the radiation-induced conductivity which reveals that the conduction process is due to isolated charge carriers (it is not a collective effect) and shows that an electric field is still too low to induce the switching effect.
On the basis of the completed research, a number of R&D applications for spacecraft and solar energy technologies have been developed:
a. A new criterion to assess the maximum electric field leading to an initiation of electrostatic discharges (ESD) from a polymer bulk into the semiconductor crystal has been developed for semiconductor chips in plastic cases. An engineering methodology of selecting the bulk conductivity of the case dielectric to exclude ESD between charged case polymer and the chip crystal.
b. Photo-generation and mobility of charge carriers in films of the molecularly doped polymer composite PCDTBT – PCBM designed for polymer solar cells have been studied as a function of a dopant (fullerene) concentration. It has been demonstrated that for the weight ratio of PCDTBT and PC70BM approaching 1:1, electron and hole mobilities are getting close making a carrier transport more balanced. Modification of the photoactive composite by addition of a sensitizer improves the film quality without degrading parameters of a solar cell.
Level of implementation, recommendations on implementation or outcomes of the implementation of the results
Recommendations have been given for using the developed engineering methodology of selecting the bulk conductivity of the case dielectric to exclude ESD between charged case polymer and the chip crystal.
Recommendations have been given for eliminating the photo degradation of polymer solar cells based on PCDTBT:PC70BM.