The mechanism behind the beneficial effect of light soaking on injection efficiency and photocurrent in dye sensitized solar cells

Abstract

Electrical and luminescence characterization was performed on 16 dye sensitized solar cells with different formulations, from different industrial and academic sources. Most of the cells were fabricated in pre-industrial pilot lines. The cells were put through a light soaking period up to 150 hours and then re-characterized. The results show the commonly observed increase in J sc with light soaking is due to a decrease in the conduction band energy (with respect to the electrolyte) and an increase in the injection rate and efficiency. The strong correlation between the luminescence decay lifetime (<200 ps to 5 ns) and the photocurrent (7 to 13 mA cm À2 ) shows that the luminescence decay is a useful monitor of injection rates in these cells. The very slow injection shown by some cells implies substantial losses at the injection step. The data point to a need to understand and improve the TiO 2 processing and dyeing conditions in the industrial setting as well as the need to focus injection studies on the full range of dynamics present in the cells.

The origin of an efficiency improving “light soaking” effect in SnO2 based solid-state dye-sensitized solar cells

“…1c. A light soaking effect has been previously observed for TiO 2 based liquid electrolyte DSCs, 14,15,18,19 with the enhancement occurring over time periods ranging from hours to days to even months. However such phenomena are not usually observed in solid-state cells, and certainly not to the extent observed here, where the photocurrent more than doubles over the measurement time.…”

“…This finding is interpreted to imply that enhanced electron transfer in that system is the mechanism behind the increase in photocurrent. 18 This correlation is surprising, not because of the inferred mechanism, but since the time-correlated single photon counting technique employed has a temporal resolution on the 100 ps timescale, which is much slower than the typical observed electron transfer rate of fs to ps.…”

“…This is in contrast to the longer term light soaking study performed by Listorti et al on TiO 2 based electrolyte cells who found a notable reduction in the recombination rate as a function of light soaking. 18 An estimate of the charge collection efficiency (h coll ) can be obtained from these measurements by simply balancing the rates for charge collection and recombination, as below:…”

Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis

“…These shifts occur when the acid/base characteristics of the electrolyte are changed intentionally, or change with time or light exposure. 41,42 . In DSSCs the shift in the capacitance curves has been assigned to adsorption/desorption of ions or dipoles that change the electric field between the TiO 2 and the electrolyte.…”

Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells

“…28 Recently, a detailed study of this process using time-resolved luminescence on DSSCs prepared with ruthenium complex dyes and various electrolytes determined that the improved J sc observed after light exposure can be attributed to an increased injection yield arising from a change in the kinetics of injection caused by a positive conduction band shift. 29 This paper also reported a reduction in the charge recombination after light exposure to mitigate voltage losses from the conduction band shift. For DSSCs prepared from a zinc porphyrin dye we previously reported that a short light exposure treatment produced simultaneous improvements in the J sc , V oc and fill factor for a zinc porphyrin DSSC.…”