Highly efficient dye-sensitized solar cells with NiSe2 counter electrodes: Effect of morphology on the electrocatalytic activity and the photovoltaic performance

As the third generation solar cell, dye-sensitized solar cells (DSSCs) have attracted tremendous amount of interest in scientific research and industrial applications during the past 25 years because of their simple production procedures, low-cost, environmental friendliness of raw materials, and relatively high power conversion efficiency (PCE). As an important component, counter electrode (CE) not only transfers electrons flowing from the external circuit to the electrolyte, but also catalyzes the reduction of oxidized electrolyte at the CE/electrolyte interface. So far, Pt as a noble metal has been the most commonly used CE for laboratory research due to its excellent electrocatalytic activity, high conductivity and good chemical stability. Unfortunately, the high cost and low abundance ratio of Pt create a big obstacle for large-scale applications of DSSCs. Therefore, exploiting Pt-free CE materials with high electrocatalytic activity and low cost is vital for the practical applications. Recently, a lot of Pt-free materials, such as graphene, carbon, conductive polymers and metal compounds (including carbide, oxide, sulfide, nitride and selenide), have been proposed for use as the CE of DSSCs. Among those materials, metal selenides have drawn great interest due to their distinctive electronic structures, interesting chemical behaviors and distinctive physical properties. The electrocatalytic activity of a substance not only depends on the chemical composition but also relies on the morphology of materials. While the investigation on developing new electrocatalysts has been extensively conducted, the morphology effect on the electrocatalytic activity has rarely been studied. In this paper, a simple low temperature hydrothermal method was used to synthesize NiSe2 with nanorod (r-NiSe2) and nanoparticle (p-NiSe2) morphologies. A drop-cast method was used to fabricate CE films without any post-treatment. Since the only difference for those two CEs used in DSSC is their morphology, it allows us to study the morphology effect on the electrocatalytic activity and solar cell performances. By changing the drop-cast times, the loading amount from 5.0 to 80.0 μg/cm2 could be controlled. The PCE of DSSCs increases with increasing the loading amount up to 10 μg/cm2 and then decreases with further increasing the loading amount of NiSe2. Thus, the best loading amount has been determined to be 10 μg/cm2. All the measurements in this paper are based on the best loading amount. The DSSC with r-NiSe2 CE achieves PCE of 8.52%, which is higher than that for the p-NiSe2 based DSSC (8.13%) and even better than the Pt based DSSC (8.20%) at comparable conditions. This indicates that NiSe2 nanorod is better than NiSe2 nanoparticle as the CE of DSSCs towards higher photovoltaic performance. To elucidate the catalytic activity of r-NiSe2 and p-NiSe2, electrochemical impedance spectroscopy, Tafel polarization curve and cyclic voltammetry were conducted. The experimental results indicate that the r-NiSe2 CE exhibits higher electrocatalytic activity than p-NiSe2 CE, which explains the reason that the DSSC based on r-NiSe2 CE has the higher PCE than the p-NiSe2 CE. The result implies that by varying the morphologies of CE materials could also fulfill the improvement of solar cell's performance. © 2017, Science Press. All right reserved.