Highly Efficient Dye-Sensitized Solar Cells Achieved by Matching Energy Levels Between Pseudohalogen Redox Couples and Organic Donor−π−acceptor Cyanoacrylic Acid Dyes

A good match of energy levels between dyes and redox couples is an important factor to reduce energy loss and further improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). Pseudohalogens were introduced as an improved alternative to iodide/triiodide (I- /I3- ) couples to achieve this goal. Herein, we used SCN-/(SCN)2 and SeCN-/(SeCN)2 as redox couples combined with organic donor-pi-acceptor cyanoacrylic acid dyes, 3-{5-[N,N-Bis(9,9-dimethylfluoren-2 -yl)phenyl]thieno[3,2-b]thiophene-2-yl}-2-cyano-acrylic acid (C201) and 3-{5-[N,N-bis(4-diphenylamino)pheny]thieno[3,2-b]thiophen-2-yl}-2-cyano-acrylic acid (C207), respectively. Appropriate combination of SeCN-/(SeCN)2 couple (0.54 V vs. NHE) and C207 dye (0.75 V vs. NHE) yield an overall efficiency of 6.96 +/- 0.54 %, which is comparable to the widely used I-/I3- couples. Moreover, at the optimized SCN-/(SCN)2 concentration ratio (25mM/100 mM), the C201-based DSSCs achieved an efficiency up to 8.62 +/- 0.44 %, and the performance was 16 % higher than that of I-/I3- couples. The origin of this improvement was found to be the efficient regeneration of C201 dye with a reduced energy loss at a driving force of 0.19 eV using the SCN-/(SCN)2 couple. The electrochemical transient photovoltage decay spectroscopy studies further reveal that the pseudohalogen redox couples increases the redox potential to benefit the photovoltage of the cell and increase the electron lifetime. These results show the potential of exploiting redox couples with suitable heterogeneous electron-transfer rates and energy levels matched to selected dyes.