Crystalline Silicon Interfaces with Adhesive Polymer Semiconducting Glue and Thiazolothiazole Redox Active Organic Small Molecules

1 online resource (112 pages) : PDF

2020

University of North Carolina at Charlotte

Crystalline silicon is an extremely versatile semiconductor material used for electronics and photovoltaic devices. An organic conductive glue based on a blend of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and d-sorbitol was examined for laminating conductors to crystalline silicon. PEDOT:PSS functions as a high-work-function solution processable conductor and exhibits an ohmic contact on p-type silicon. On n-type silicon it forms a rectifying contact, which behaves as a solar cell. D-sorbitol has adhesive properties and enhances the conductivity of the PEDOT:PSS polymer blend. D-sorbitol blended with PEDOT:PSS (s-PEDOT:PSS) was brought into contact with n-Si to form a rectifying contact that could be laminated to any sort of conductor. Conductive glue could prove especially useful for laminating to textured silicon or novel micro- or nanostructured silicon materials. However, when the d-sorbitol is added to the PEDOT:PSS and brought into contact with silicon, current–voltage characteristics suggest minority carrier trap states are formed, leading to charge recombination at the silicon/polymer interface. To avoid creating electron trap states where recombination occurred, the surface of the silicon was modified by covalently bonding organic molecules to the silicon. The silicon surface was covered completely with methyl groups in a two-step chemical reaction, which reduced trap state formation and charge recombination by protecting the silicon surface from oxidation. This created hybrid Si/polymer solar cells with no photogenerated current loss and enabled photovoltaic devices to be produced using s-PEDOT:PSS contacts to n-Si. Further extending the concept of bonding organic molecules to the silicon crystal, a redox active small molecule was attached to the silicon surface. This molecule was chosen and synthesized for a number of reasons. Its reactivity with the silicon surface was essential, so an alkene moiety was used. Its ability to accept and donate electrons was also key, so a viologen was chosen. Thiazolo[4,5-d]thiazole bridged extended-viologens have a lower redox potential relative to the silicon conduction band, and can be reduced more easily than traditional viologens. Diallylpyridinium-thiazolo[4,5-d]thiazole dibromide ((Allyl2TTz)2+(Br-)2) was therefore synthesized and was used for all experiments involving silicon surface-bound redox active molecules. This compound was studied in solution to determine its properties as an electron shuttle with a silicon electrode. Bare silicon is a poor catalyst, however it did reduce (Allyl2TTz)2+(Br-)2 in solution with a dramatic increase in current vs electrolyte solution in cyclic voltammetry experiments. Next (Allyl2TTz)2+(Br-)2 was covalently bonded to the silicon surface to see whether it would retain the properties of a solution containing (Allyl2TTz)2+(Br-)2 and whether it would be catalytic for related reactions. The chemically modified silicon surface was found to pass more current to an acidic solution than methylated silicon on the first sweep of CV, but after that did not outperform Me-Si surfaces. This suggests that the molecule was reduced on the surface but did not reduce protons in solution.

doctoral dissertations

Materials science
Chemistry
Chemistry, Organic

Ph.D.

PEDOT:PSS
Silicon
Sorbitol
Surface Modification
Viologen

Nanoscale Science

Walter, Michael

Poler, Jordan
Zhang, HaiTao
Bejger, Christopher
Schmedake, Tom

Thesis (Ph.D.)--University of North Carolina at Charlotte, 2020.

This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). For additional information, see http://rightsstatements.org/page/InC/1.0/.

Copyright is held by the author unless otherwise indicated.

Cohen_uncc_0694D_12643

http://hdl.handle.net/20.500.13093/etd:1924