Design, synthesis and characterization of novel low band gap conjugated polymers for use in bulk heterojunction photovoltaic solar cells
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Abstract
This work details the synthesis of a functionalised all-donor polymer, poly(3-
hexylthioacetate thiophene) (P3HTT) [42] along with four new donor-acceptor (D-A) copolymers. Two of the four copolymers are based on fluorene (F) and carbazole (C) as electron donors copolymerised with DEBT to give, PFDEBT [62] and PCDEBT [63] respectively, while the other two are based on the same electron donors, but copolymerised with DES to give PFDES [64] and PCDES [65] respectively.
All the polymers exhibited good solubility in common organic solvents and good
thermal stability. The spectrophotometry analysis of polymer [42] in chloroform showed that it emits in the green region with 26% photoluminescence quantum yield (PL φ). The optical response of copolymers [62] and [63] in chloroform solution and thin films are similar; both have two broad absorption bands with peaks at short and long wavelengths which are within comparable range. In addition, both emit in the deep red region of the emission spectrum and have similar PL φ values, 28 % and 27 % respectively. In contrast, copolymers [64] and [65] have single broad absorption band and emit in the green region with PL φ values of 55% and 51% respectively.
The electrochemical responses of the polymers were measured using cyclic
voltammetry. The results obtained for polymer [42] were different from what was
expected; its band-gap was similar to that of P3HT and its energy levels were elevated. In contrast, the band-gaps of the copolymers were tuned in the range of 1.47 to 1.65 eV. The results showed that the acceptor units have exhibited the same effect of band-gap lowering in the two sets of the copolymers, for instance, the LUMO energy level of copolymers [62] and [63] is - 3.32 eV and those of copolymers [64] and [65] is - 3.57 eV. Conversely, different effect was observed in the values of the HOMO energy levels, for example, the values for [62] and [64] which contain the same donor (fluorene) are - 4.86 and - 5.07 eV
respectively; while those of copolymers [63] and [65] composed of carbazole are - 4.97 and - 5.04 eV respectively. This may suggest that the acceptor unit reduced the band-gap by lowering the LUMO energy level but at the same time elevated the HOMO energy level probably due to the presence of EDOT.
DSSCs devices based on polymer [42] were fabricated in combination with nc-TiO2 and ruthenium 535-bisTBA dye to give devices with the configuration,
glass/SnO2:F/nc-TiO2/dye/polymer/Au. The best device showed a PCE of 0.035% with Voc, Jsc and FF of 610 mV, 1.7 mA/cm2 and 34 % respectively. The copolymers were used in combination with PC6oBM to make BHJ solar cells having the structure, glass/ITO/PEDOT:PSS/copolymer:PC60BM/A1. The best performance was observed in the device based on copolymer [62] with 1: 1 polymer:PC60BM ratio which showed PCE of 1.45 % while the values of Voc, Jsc and FF are 647 mV, 5.57 mA/cm2 and 40% respectively.
hexylthioacetate thiophene) (P3HTT) [42] along with four new donor-acceptor (D-A) copolymers. Two of the four copolymers are based on fluorene (F) and carbazole (C) as electron donors copolymerised with DEBT to give, PFDEBT [62] and PCDEBT [63] respectively, while the other two are based on the same electron donors, but copolymerised with DES to give PFDES [64] and PCDES [65] respectively.
All the polymers exhibited good solubility in common organic solvents and good
thermal stability. The spectrophotometry analysis of polymer [42] in chloroform showed that it emits in the green region with 26% photoluminescence quantum yield (PL φ). The optical response of copolymers [62] and [63] in chloroform solution and thin films are similar; both have two broad absorption bands with peaks at short and long wavelengths which are within comparable range. In addition, both emit in the deep red region of the emission spectrum and have similar PL φ values, 28 % and 27 % respectively. In contrast, copolymers [64] and [65] have single broad absorption band and emit in the green region with PL φ values of 55% and 51% respectively.
The electrochemical responses of the polymers were measured using cyclic
voltammetry. The results obtained for polymer [42] were different from what was
expected; its band-gap was similar to that of P3HT and its energy levels were elevated. In contrast, the band-gaps of the copolymers were tuned in the range of 1.47 to 1.65 eV. The results showed that the acceptor units have exhibited the same effect of band-gap lowering in the two sets of the copolymers, for instance, the LUMO energy level of copolymers [62] and [63] is - 3.32 eV and those of copolymers [64] and [65] is - 3.57 eV. Conversely, different effect was observed in the values of the HOMO energy levels, for example, the values for [62] and [64] which contain the same donor (fluorene) are - 4.86 and - 5.07 eV
respectively; while those of copolymers [63] and [65] composed of carbazole are - 4.97 and - 5.04 eV respectively. This may suggest that the acceptor unit reduced the band-gap by lowering the LUMO energy level but at the same time elevated the HOMO energy level probably due to the presence of EDOT.
DSSCs devices based on polymer [42] were fabricated in combination with nc-TiO2 and ruthenium 535-bisTBA dye to give devices with the configuration,
glass/SnO2:F/nc-TiO2/dye/polymer/Au. The best device showed a PCE of 0.035% with Voc, Jsc and FF of 610 mV, 1.7 mA/cm2 and 34 % respectively. The copolymers were used in combination with PC6oBM to make BHJ solar cells having the structure, glass/ITO/PEDOT:PSS/copolymer:PC60BM/A1. The best performance was observed in the device based on copolymer [62] with 1: 1 polymer:PC60BM ratio which showed PCE of 1.45 % while the values of Voc, Jsc and FF are 647 mV, 5.57 mA/cm2 and 40% respectively.
Details
| Original language | English |
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| Award date | 2012 |