Development of Nb-TiO2/Nb2O5 functional photoanode and flexible metal-mesh cathode for dye-sensitized solar cells and perovskite solar cells

Abstract

Inexpensive alternatives for converting solar energy into electricity such as dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) have received great attention. To further improve the power conversion efficiency (PCE) of DSSCs, we developed a scalable synthesis of Nb-doped TiO2 nanoparticles (NPs) used as the DSSC photoanode, and a facile synthesis of ultrasmall Nb2O5 NPs are used as an effective blocking layer between TiO2 photoanode and transparent conductive oxide (TCO) substrate. First, the Nb-doped TiO2 photoanode was made by simply mixing TiO2 paste with functionalized Nb2O5 via sol-gel. By doping 2 mol% Nb into TiO2, the positive shift of conduction band minimum enhances the electron injection and the improved electron conductivity facilitates the electron transport, leading to 18.9% improvement of PCE as compared with the standard DSSCs. Second, a new synthesis route of ultrasmall Nb2O5 NPs (2-5 nm) was developed by adding NbCl5 into ethanol with acetic acid as stabilizer via UV ozone treatment. The Nb2O5 NPs-coated thin film functions as an effective blocking layer between a mesoporous TiO2 and TCO. It thereby reduced the recombination at the TiO2/FTO/electrolyte interface. Furthermore, it serves as an ultrathin and compact hole blocking layer in PSCs, resulting in a moderate hysteresis effect and an improved photovoltaic efficiency from high-transmittance, low capacitance and low series resistance. A high transparent, flexible plastic counter electrode (CE) with pulse-electrodeposited platinum NPs was developed for light-weight bifacial flexible DSSCs. This CE consists of the size less than 30 nm PtNPs decorated on the transparent ITO-PEN to provide superior electrical conductivity and excellent catalytic ability. Utilizing this PtNPs CE and a low-temperature processed photoanode, full-plastic bifacial DSSCs are fabricated and tested with a remarkable PCE of 5.00% under rear-side illumination, which approaches 90% of the 5.72% PCE under front-side illumination. Further, a new type of embedded metal-mesh transparent electrode (EMTE) with in-situ electrodeposited catalytic PtNPs was developed as a high-performance CE. The thick but narrow nickel micro-mesh fully embedded in a plastic film provides superior electrical conductivity, optical transmittance, and mechanical stability to the novel electrode. PtNPs decorated selectively on the nickel micro-mesh surface provide the catalytic function with minimum material cost and without interfering with optical transparency. Using this PtNP-decorated nickel EMTE as the CE and titanium foil as the photoanode, unifacial flexible DSSCs are fabricated with a PCE of 6.91%. By replacing the titanium foil with a transparent ITO-PEN photoanode, full-plastic bifacial DSSCs are fabricated and tested, demonstrating a remarkable PCE of 4.87% under rear-side and 5.67% PCE under front-side illumination, among the highest ratio in published results. These promising results reveal the enormous potential of this hybrid transparent CE in scalable production and commercialization of low-cost and efficient flexible DSSCs.