Operating temperature and temperature gradient effects on the photovoltaic properties of dye sensitized solar cells assembled with thermoelectric–photoelectric coaxial nanofibers

Abstract

In this work, the coaxial electrospinning technique was used to prepare coaxial nanofibers (CNFs) consisting of N-type thermoelectric Nb doped SrTiO3 (Nb/STO) coated with a thin layer of TiO2, which was referred to simply as TiO2–Nb/STO. Then a certain amount of TiO2–Nb/STO CNFs was added into the TiO2 nanocrystalline layer to obtain the composite photoanode of dye-sensitized solar cells (DSSCs). Owing to the high electrical conductivity and straightforward electron transport channels for TiO2-Nb/STO CNFs, the device assembled with 10 wt% TiO2-Nb/STO CNFs demonstrated the highest power conversion efficiency (PCE) of 7.25%, which was significantly higher than the reference one (6.45%). In order to investigate the temperature gradient (ΔT) on the photovoltaic performances of DSSCs assembled with TiO2-Nb/STO CNFs, a cold source (0 °C) and a heat source (75 °C) was placed on the Pt counter electrode to create a negative ΔT (−7 K) and a positive ΔT (+7 K), respectively. The mechanism model of thermoelectric effect on the charge transfer dynamic process of DSSCs under different ΔT is established and explained. Data showed that in response to temperature fluctuations, the performance of both reference device and 10% device could be affected. The cold source enable the reference device to obtain the highest PCE of 6.75% due to an increase in open-circuit voltage (Voc). This behavior is mainly ascribed to the reduced internal carrier recombination rates, caused by decreased carrier concentrations at low operating temperature. Herein, the operating temperature refers to an average value between the temperatures on both sides of each device. On the contrary, 10% device applying hot source results in an increase of short-circuit current density (Jsc) and fill factor (FF), yielding the highest PCE of 7.61% despite the negative effects of high operating temperature on Voc. It is found that the hot source benefits the fast electron transfer for 10% device under the synergistic effect of thermoelectromotive force and fast charge carrier diffusion. These results further confirm that the improved photovoltaic performance of 10% device under the condition of hot source was primarily attributed to the thermoelectric Seebeck effect of TiO2-Nb/STO CNFs.