Synthesis And Photovoltaic Performance Of Phenothiazine-Based Dyes With Bilateral And Trilateral π-Conjugation Extensions

Dye-sensitized solar cell (DSSC) is an emerging photovoltaic technology with a great potential for large scale manufacturing at low cost. World energy requirement is going skyscraper due to enormous domestic and industrial usage. It is estimated that energy requirement from2030to2050will be far more as existing requirements. Fossil fuel resources are continously consumed, which are estimated to be finished in next50to60years. Furthermore, the consumption of these resources is also producing noxious gases, which are responsible for environmental issues. Exploration of new energy resources is one of the most important issues of the time. Currently, several renewable energy sources have been developed but they all have reservations being toxic or expensive. To convert sunlight into electrical energy is the cheapest way to overcome this problem. DSSC technology has already got repute since1991, now the need is to develop efficient, appropriate and cheap materials. Organic materials are easily available at considerable low cost which can be feasibly designed into active sensitizers for DSSCs.In the first chapter the introduction of dye-sensitized solar cell technology has been given and various components, operating principle, and characteristics of the cells have elaborated. Fundamental requirements and problems associated to the dye-sensitized solar cells are discussed in detail. In the second chapter, recent developments in metal free sensitizers regarding electron donors, π-spacers and anchoring units have reviewed and their role in photophysical and photovoltaics has described. In the third, four and fifth chapter the design of our work, synthesis of the dyes, testing and results has been described.Phenothiazine moiety was selected as donor because it has butterfly shaped non planar configuration. Its extra electron rich sulfur atom impart role to make it strong electron donor than other amines like triphenylamine, carbazole, tetrahydroquinoline etc. Its structure was modified by the introduction of phenyl ring at N-atom with Ulmann’s reaction and then various alkoxy chains like; methoxy, octyloxy and dodecyloxy were incorporated through nucleophilic substitution reaction. Five dyes Z0, Zl, Z2, Z3and Z4containing phenyl, anisole, octyloxyphenyl, dodecyloxyphenyl and bis-octyloxyphenyl moieties were designed and synthesized. These modifications not only increase its electron donating ability but also make it suitable for DSSCs. Because increased conjugation and hydrophobic alkyl chains have positive impact on the photovoltaic performance. It was also found that dye Z2with octyloxy chain gave better efficiency as compared to the dye Z3with dodecyloxy chain. Plausibly, due to formation of compact structure which reduces the recombination reactions. Furthermore, Z2was midified and π-conjugation extension at7-position was carried out to form dye Z4. A comparison of lateral and bilateral conjugation extended dyes was made for DSSCs. Results revealed that bilateral conjugation extension (Z4) has extended UV absorption range and reduced the HOMO-LUMO gap, which indicates that its electron donor-acceptor strength has increased. The bilateral conjugation extended dye Z4has delivered an efficiency of5.73%. This efficiency was due to better light harvesting capability of the sensitizer. In second part of study, phenol and octyloxypheyl groups were introduced on phenothiazine skeleton at7-position and dyes Z5and Z6were synthesized. The impact of hydroxy and octyloxy substituents on photovoltaic properties were studied. Absorption maximum of the dye Z6with octyloxy substitutition was increased and HOMO-LUMO gap was decreased as compared to the dye Z5with hydroxy substitution. The results showed that octyloxy group (Z6) was better than hydroxy unit in respect of suppression of recombination, which leads to an over all power conversion efficiency of6.32%. Phenothiazine moiety was further modified by incorporating π-spacers and anchoring units and dyes Z7, Z8, Z9and Z10were synthesized. The dyes Z7contain two anchoring group and Z9containing two π-spacers and two anchoring groups. While dyes Z8contain one π-spacer and one anchoring group and dye Z10contain two spacers and one anchoring group. It was found that dye Z9with two π-spacers and two anchoring units have increased the electron injection rate, which lead to an increase in the short-circuit current density. Dye Z9has higher short-circuit current density as compared to all other dyes Z0-Z8and Z10. Maximum overall power conversion efficiency of6.82%was obtained from the dye Z9. Though efficiency of the dye (Z9) with dianchoring units was increased but its open-circuit voltage was decreased as compared to single anchoring unit dye Z0-Z4, Z6and Z8. The electron lifetime of the dyes Zl, Z2, Z4and Z8was also higher than the dye Z9. In order to increase the open-circuit voltage and resistance of recombination, further modifications in the design of Z8dye were made by the introduction octyloxyphenyl moiety at7-position and π-spacer furan (Z11) or thiophene (Z12) at3-position and trilateral π-conjugation was extended. It was found that Z12dye with thiophene unit gave better UV absorption spectrum as compred to the dyes Z0-Z11. The dye Z12has highest short-circuit photocurrent density than all other dyes Z0-Z11. From this design the open-circuit voltage and resistance of recombination of the dye Z12were also increased considerably and an overall power conversion efficiency of7.33%was obtained. Finally, the results are summarized under the heading of summary.