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Deepak
Professor,
 Materials Science and Engineering,
 Indian Institute of Technology-Kanpur ,
 Work Phone: (512) 2597353,
 Email: saboo[at]iitk.ac.in

Asha Sharma’s Thesis

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Thesis  Topic
 Ultraviolet organic light emitting diodes based on polysilanes: material and device degradation studies
Polysilanes are wide band gap organic semiconductors which could be potential candidates for ultraviolet or near ultraviolet light emitting diodes.  However, polysilanes are known to degrade rapidly upon exposure to ultraviolet light.  In this regard, a framework to understand the photodegradation in polysilanes is developed using experimental and theoretical methods.  The degradation in polysilanes is investigated by photoluminescence, absorption and spectroscopic ellipsometry in film and solution.  In addition to be being a strong function of chemical structure, photodegradation in polysilane is affected by environment, phase, excitation wavelength and intensity.  In the solution, the dominant mechanism is photoscission of Si-Si bonds and formation of silyl radicals that react with the environment.  In the film this mechanism exhibits a reduced rate due to cage effect.  In addition to this a second type of degradation is observed in the film, which is attributed to formation of defects or traps.  When the polysilane chemical structure is changed by adding longer alkyl chains or aryl groups, the dominant degradation mechanism seems to be defect or trap creation and degradation by scission of Si-Si bonds is not observed.
 Further, the electronic structure calculations are performed on model polymers to explain relative stability among polysilanes which differ in their side groups.  The semiempirical calculations are inadequate to explain the relative stability of the polysilanes in the excited state.  On the other hand, ab initio and density functional theory methods provide meaningful explanations, with striking correlation between the calculations and experimental observations.  The configuration interaction singles calculated potential energy curves demonstrate the relative stability of the polymers.  The presence of a local minimum in first excited triplet state increases the stability of aryl substituted polysilanes, such as poly(methylphenylsilane) and poly[bis(p-butylphenylsilane)].  In other words, Si-Si photoscission is reduced, this is in agreement with the experimental observation.
 Finally, organic light emitting diodes were fabricated using several new polysilanes.  Room temperature ultraviolet or near ultraviolet electroluminescence is realized in all cases, while prior to this work only poly[bis(p-butylphenylsilane)] was reported to show room temperature emission in near ultraviolet region.  From these devices, we also observe simultaneous emission in the visible region, which is not present in the photoluminescence spectra of these materials.  The visible emission is broad and has a Commission Internationale de l’Eclairage coordinate near to the equienergy point (0.36,0.35).  This suggests that polysilane based organic light emitting diodes can also be used as a white light source.  Furthermore, it can be modulated to a required white light coordinate by down converting the ultraviolet emission from the same device.  While the origin of ultraviolet emission in electroluminescence is ascribed to an excitonic emission from -* transition consistent with its presence in the photoluminescence spectrum, the origin of the visible spectrum is expected to be associated with the defects in the polysilane.
 A significant improvement in terms of device lifetime has been achieved by employing the changes in the device structure and encapsulation.  The life time increased from few seconds to an hour, though still not adequate for any commercial exploitation.  The device degradation mechanism is investigated through current-luminance-time measurements at constant voltage.  An electroluminescence intensity enhancement is observed in all devices due to thermochromic effect, and it is reversible.  However, the device degradation is non-recoverable type and is due to degradation in the emission efficiency of polysilane layer.