Tuning the energy level of organic sensitizers for high-performance dye-sensitized solar cells

Cost-effective organic sensitizers will play a pivotal role in the future large-scale production and application of dye-sensitized solar cells. Here we report two new organic D-p-A dyes featuring electron-rich 3,4- ethylenedioxythiophene- and 2,2'-bis(3,4-ethylenedioxythiophene)-conjugated linkers, showing a remarkable red-shifting of photocurrent action spectra compared with their thiophene and bithiophene counterparts. On the basis of the 3-{5'-[N,N-bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2'-bis(3,4- ethylenedioxythiophene)-5-yl}- 2-cyanoacrylic acid dye, we have set a new efficiency record of 7̇6% for solvent-free dye-sensitized solar cells based on metal-free organic sensitizers. Importantly, the cell exhibits an excellent stability, keeping over 92% of its initial efficiency after 1000 h accelerated tests under full sunlight soaking at 60 °C. This achievement will considerably encourage further design and exploration of metal-free organic dyes for higher performance dye-sensitized solar cells. We have also scrutinized the physical origins of the relatively low photocurrent and photovoltage obtained with an ionic liquid electrolyte compared to a volatile acetonitrilebased electrolyte through transient and modulated photoelectrical measurements.