Impact of strength and size of donors on the optoelectronic properties of D--A sensitizers

A series of carbazole based sensitizers with either phenyl based donors (TBC, TMC, OMC, PC, TBR, TMR, OMR and PR) or aryl amine based donors (OMNC, CNC and HNC) as well as one without a donor group (CC) have been synthesized to understand the influence of the strength of the donor moiety on the optical, electrochemical and photovoltaic properties. Two different acceptor moieties such as cyano acrylic acid and rhodanine acetic acid were introduced and evaluated. Different substituents on the phenyl group have a significant impact on the light harvesting ability of the sensitizers. Among phenyl based donors, anisole based carbazole (OMC) shows the highest short circuit current (JSC) of 4.96 mA cm−2 with overall power conversion efficiency (PCE) of 2.69%. In the case of the sensitizers with aryl amine based donors, the increasing bulkiness of the donor group lead to increasing open circuit potential. Transient photocurrent and photovoltage measurements signify the importance of a bulky donor fragment in determining the open circuit potential of the dyes. Sensitizers with hexyloxy substituted phenyl amine as the donor group shows a JSC of 6.84 mA cm−2 with PCE of 3.33%. The overall investigation provides vital information about the influence of donor groups on the optoelectronic properties of the sensitizers for its photovoltaic applications.

Synthetic pathways for the preparation of intermediate compounds.

9-Octyl-9H-carbazole (CB)
Carbazole (1mol ratio) was added to a suspension of sodium hydroxide (1.2 mol ratio) in DMSO (13V) under nitrogen atmosphere.The mixture was stirred for 30 min and then 1-bromo octane (1.1 mol ratio) was added.After completion of the reaction, the mixture was extracted with CHCl 3 and dried over sodium sulphate and evaporated to afford CB.Product: brown liquid.

6-Iodo-9-octyl-9H-carbazole-3-carbaldehyde (CBAI)
To a two neck RB flask, CBA (1mol ratio) was dissolved in glacial acetic acid (16V) and then KI (0.67 mol ratio) and KIO 3 (0.4 mol ratio) were added.The mixture was stirred at 80C until iodine was fully consumed, then the mixture was cooled to room temperature, a pale brown colour solid appeared and filter the solid and then poured into 5% NaHSO 3 to remove KIO 3 .
General procedure for the preparation of amine donor aldehydes CBAI (0.0006 mol), amine donors (Bis-(4-methoxy-phenyl)-amine, Bis-(4-Hexyloxy-phenyl)amine, Bis(2,4-bis(hexyloxy)-[1,1-biphenyl]-4-yl)amine) (0.0006 mol) and sodium tertiary butoxide (0.0008 mol) were dissolved in toluene (6 ml).This solution was degassed under nitrogen for 20 min and then Pd[P(t-Bu) 3 ] 2 was added to the reaction mixture.The reaction was then brought to 80 °C and stirred for 12 hours.The reaction mixture was cooled to room temperature and plugged through a thin pad of NaSO 4 with DCM.The crude product was adsorbed in silica gel and purified by column chromatography to get the corresponding compounds.

Table S1 :
HOMO and LUMO values of carbazole based sensitizers obtained from DFT calculations