Rethinking Dye Sensitized Solar Cell Technology For “Real-World” Conditions

Candidate: Tamara Al Faouri
Supervisor: Dr. Bryan Koivisto

ABSTRACT

A family of bichromic-bipodal dyes with R2Dd-π-Dp-A motif; containing two triphenylamine (TPA) redox active units as donor (D) with R-substituents (-H, -OMe, -SMe, -OHex) on the distal TPA (Dd), thiophene π-spacers bridging the D units, and two cyanoacetic acid acceptor (A) units, were studied in dye-sensitized solar cells (DSSCs) under direct sunlight and diffuse light. The best performing dye under diffuse light was further tested with three types of electrolytes (I-/I3-, [Co(bpy)3]2+/3+ (TFSI)2/3, [Cu(dmp)2]+/2+ (TFSI)1/2) at direct and diffuse light. The structure-performance relationship of the dyes was evaluated based on the number of π-spacers and the type of R-substituents. It was found that 0-1 π-spacers had a beneficial electrochemical influence on the proximal TPA (Dp) donor abilities, and the R-substituent affected the donating ability and electrolyte interactions of the Dd. This resulted in the independent tuning of the two TPA units that ultimately improved regeneration and injection rates in this family of dyes. The best performing DSSCs were ones with -SMe under direct sunlight (3.18%), due to the S-I soft-soft interaction and the large molar extinction coefficient of -SMe dyes. A new methodology was developed to test this family of dyes under diffuse light conditions. An inverse relationship of DSSC performance with incident light was observed for all dyes, and the number of π-spacers increased the efficiency for dyes with -OMe and -SMe substituents but decreased it for dyes without any substituents (-H). iv

Different thermodynamic and kinetic approaches explained the effect of π-spacer number and R-substituent type on device’s performance. Overall, the dyes without any substituents were the best performing ones, with highest performance efficiency of (0.79% at 1 mW/cm2). Cu-based electrolyte, at all concentrations, was most suitable for DSSCs under diffuse light conditions, compared to I-/I3- that was most suitable for direct incident light, attributed to its large open circuit potentials and low diffusion limitations at low light conditions. Co-based electrolytes provided high performance DSSCs and open-circuit potentials only in DSSCs with -SMe dye under diffuse light, and in DSSCs with -H dye under direct sunlight conditions. The overall low performance of Co-electrolyte was correlated with its mass transport limitations, and the enhanced performance was associated with the relative dye’s fast electron transfer rates