Re-evaluating the role of phosphinic acid (DINHOP) adsorption at photoanode surface on the performance of dye-sensitized solar cells

Dineohexyl phosphinic acid (DINHOP) is a popular amphiphilc molecular insulator considered to be the most efficient co-adsorbent (co-grafter) for the improvement of photovoltaic performance of TiO2 based hybrid solar cells. Although the effect of its incorporation on the improvement of cell performance has been well demonstrated, the mechanisms through which it affects the photovoltaic and electrodynamic parameters of the cell are not yet clear. Here we re-examine the mechanism through which the co-adsorbent DINHOP affects the photovoltaic and electrodynamic parameters of dye-sensitized solar cells. Although DINHOP is widely belived to inhibit (passivate) recombination across the TiO2/electrolite interface, we demonstrate that this is true only for a very high concentration (e.g. 750 µM) of DINHOP, co-sensitized with dye. For the mostly used DINHOP concentrations (e.g. 75 µM and 375 µM), observed increases on the diffusion coeficient and the recombination rate could be diretly associated to a decrease of total-intra gap states in TiO2. Influence of DINHOP concentration on the dynamic parameters (transport and recombination) of charge carriers and band-edge movement of semiconducting TiO2 photoanode has been estimated and interpreted using small perturbation techniques (Stepped Light-Induced Transient Measurements, SLITM) and a continuity equation model, respectively. We demonstrate that for different concentrations, incorporated DINHOP molecules affect the surface of nanostructured TiO2 photoanode in different ways. For a DINHOP concentration as low as 75 µM, the conduction band edge of TiO2 moves upward due to the combined effects of charge accumulation and a decrease in total number of intra-gap states. The decrease of total intra-gap states enhances both the transport and recombination rates of the charge carriers by the same fraction due to a transport-limited recombination process. On the other hand, adsorption of DINHOP molecules at higher concentrations such as 375 µM and 750 µM modifies the distribution of intra-gap states, affecting the nonlinear recombination parameter of charge carriers at the anode-electrolyte interface. In all the cases, incorporation of DINHOP results in an overall improvement of the solar cells efficiency, with maximum (⁓ 14% compared with the reference one) for a concentration of 375 µM, were no inhibition of recombination was observed.