Co-sensitization of benzoxadiazole based D–A–π–A featured sensitizers: compensating light-harvesting and retarding charge recombination

Abstract

Triphenylamine (TPA) dyes usually show relatively narrow spectral response range with respect to indoline and porphyrin based dyes. To optimize light-harvesting, WS62 and WS64 are molecularly engineered on the basis of D–A–π–A model. We employ TPA in the absence or presence of long alkoxy-chain as the electron donor, benzoxadiazole as the auxiliary acceptor, a 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) unit as the π-bridge, and cyanoacetic acid as the anchor group. The incorporated electron-withdrawing unit of benzoxadiazole enhances light harvesting by decreasing the molecular energy gap and red-shifting absorption spectra. Moreover, three D–π–A-featured dyes (S0, S1 and S2) with different length of the π-bridge are developed as co-sensitizers for WS62 and WS64. As demonstrated, the co-sensitization effect is critically dependent upon the π-conjunction length in the three co-adsorbent dyes. Dye S2 containing dithiophene unit as the π-bridge shows a promising co-sensitization result for enhancing photovoltaic efficiency. In contrast, S1 and S0 with fewer thiophene units negatively contribute to photovoltaic performances. The cocktail co-sensitization of WS62 and WS64 with S2 can compensate the peak valley of IPCE adsorbed by electrolyte near 400 nm and compact the surface of TiO₂ to retard charge recombination, essentially for the optimization of photovoltaic performances. Solar cells based on the co-sensitization of WS64 and S2 show a high efficiency of 7.9% (V_OC of 738 mV, J_SC of 14.9 mA cm⁻² and FF of 0.72), exhibiting a significant improvement by 41% compared to the WS64 alone sensitized devices under the same conditions. The charge transfer resistance (R_CT) of the co-sensitized DSSCs is larger than that of DSSC comprising only WS62 or WS64 by around 10-fold, indicating that the unfavourable charge transfer from TiO₂ to electrolyte is efficiently blocked by the cocktail co-sensitization of S2. These findings pave a way regarding how to choose the proper and matchable co-sensitizers for further increasing the photovoltaic performances of pure organic sensitizers.