Critique of charge collection efficiencies calculated through small perturbation measurements of dye sensitized solar cells

Through experiment and simulation, we critically examine the conventional method of extracting the charge collection efficiency, η_c, of dye-sensitized solar cells (DSSCs). We demonstrate that the collection efficiency extracted by measuring transient decay rates at short circuit deviates considerably from the true value as η_c decreases. This discrepancy arises from two sources of error: first, transient voltage decay rates are especially sensitive to perturbation size near short circuit, and second, the decay rates are influenced by transient equilibration between trapped and free charge during the measurement. Only if the collection efficiency is high (>90%) do the true and measured values coincide well. This finding finally elucidates the disparity between steady-state and transient measurements of the collection efficiency; the former is more accurate for devices with a low η_c, and the latter for devices with a high η_c. For the first time, we present a fully nondimensional model of the DSSC, allowing us to fit device parameters without extensive material knowledge. The resulting simulations and fitting of solid-state dye sensitized solar cells additionally enable us to quantify the inaccuracy of small perturbation measurements of the collection efficiency when η_c is below 90%.