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

Abstract

We observe a strong ''light-soaking'' effect in SnO 2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the device's short-circuit photocurrent and efficiency increase significantly over 20-30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing opticalpump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of freeelectrons in the SnO 2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer occurs. To verify this hypothesis, we perform capacitance and charge extraction measurements which indicate a shift in the surface potential of SnO 2 of up to 70 mV with light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate and ensuing photocurrent. The cause for the shift in surface potential is not clear, but we postulate that it is due to the photoinduced charging of the SnO 2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. Understanding temporally evolving processes in DSCs is of critical importance for enabling this technology to operate optimally over a prolonged period of time. This work specifically highlights important changes that can occur at the dye-sensitized heterojunction, even without direct light absorption in the metal oxide.

Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells

“…In particular, our frequency-dependent capacitance provides the fi rst direct evidence that light soaking can decrease bulk-electrical polarization within organo-metal halide perovskites. [22][23][24][25][26][27][28] However, it still demands further in-depth investigations to understand how bulk and interface parameters are internally coupled in light soaking and hysteresis phenomena to control the useful and nonuseful photovoltaic processes in perovskite solar cells. However, neutralizing the defects at electrode interfaces and bulk perovskite fi lm can enhance the transport of the dissociated charge carriers to respective electrodes, increasing the FF during light illumination.…”

Architecture designed ZnO hollow microspheres with wide-range visible-light photoresponses

“…This phenomenon can be explained by the "light soaking" effect, which has been observed in the photoanodes made from wide band-gap metal oxides, such as SnO 2 , TiO 2 , etc. [35][36][37] The light soaking effect of UV can be observed not only in charge generation but also in the charge decay aer light switch-off. Both the response time and recovery time of charge transport under visible-light irradiation were over 30 s (Fig.…”

A Ga-doped SnO2 mesoporous contact for UV stable highly efficient perovskite solar cells

“…By determining the maximum power point and keeping the system at the corresponding voltage for extended periods of time, a PCE value can be determined that resembles the PCE of a device under real-world working conditions. 38 For completeness forward and backward scans were measured, and they can be found in ESI SI 3 † (note that hysteresis is quite severe; this may be due to the reversal of the "light soaking" effect described by Tiwana et al 39 under short circuit conditions). External quantum efficiency (EQE) spectra are shown in ESI SI 4; † the integrated J sc is in good agreement with the value extracted from J-V curves.…”

“…Note that the stabilized PCE is in some cases even higher than the PCE extracted from the J-V curve; this can be attributed to light soaking which has been reported to improve the properties of SnO 2 (ref. 39) and spiroOMeTAD. 40 The PCE of Ga-doped m-SnO 2 based PSCs rivals that of m-TiO 2 based PSCs constructed in our lab (ESI SI 5 †).…”