Di-n-alkylphosphinic acids as coadsorbents for metal-free organic dye-sensitized solar cells

Three di-n-alkylphosphinic acids (DPAs) with different chain lengths (1, 8, 16) were adopted as coadsorbents in dye-sensitized solar cells (DSCs) with organic sensitizer D149. The adsorption behavior of these coadsorbents on nanoporous TiO2 surface through POTi bond was confirmed by FT-IR spectra. And the performance of all devices was detected on the basis of photocurrent–voltage (J–V) characteristics and electrochemical impedance spectroscopy (EIS). It was found that the amount of dye adsorption gradually decreased with increasing alkyl chain length of DPAs, which was contributed to the competitive adsorption between dye D149 and coadsorbents. In spite of this, di-n-hexadecylphosphinic acid (DHdPA) performed best both in the improvement of short-circuit current density (Jsc) and open-circuit voltage (Voc). The increase of open-circuit photovoltage was ascribed to the negative movement of the conduction band edge and the retardation of electron recombination. Although the dye adsorption amount reduced to a great degree, the break up of dye aggregation mainly contribute to the enhancement of short-circuit current density. The overall conversion efficiency was further improved from 5.53% to 6.09% with DHdPA as the coadsorbent for D149 based device.