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TiO 2 nanorods were synthesized by the hydrothermal methods with ethanol-HCl and water-HCl solutions, respectively, and CH 3 NH 3 PbI 3-x Cl x perovskite solar cells based on them were fabricated. The power conversion efficiency (PCE) of the best solar cells based on TiO 2 nanorods with water-HCl solution is higher than that with ethanol-HCl solution. The dimension, morphology, optical property, and photogenerated charge behavior of the two kinds of samples were investigated. The results indicate that the better performance of solar cells based TiO 2 nanorods with water-HCl solution than that with ethanol-HCl solution could be attributed to its special orientation, high conductivity, improved morphology, good optical property, fast charge transfer and reduced charge recombination. The PCE of 11.8% was achieved using the TiO 2 nanorods with water-HCl solution, which is the highest in the reported TiO 2 nanorods based cells.

Extending photoelectric response to the near-infrared (NIR) region using upconversion luminescent (UCL) materials is one promising approach to obtain high-efficiency perovskite solar cells (PSCs). However, challenges remain due to the shortage of highly efficient UCL materials and device structure. NaCsWO3 nanocrystals exhibit near-infrared absorption arising from the local surface plasmon resonance (LSPR) effect, which can be used to boost the UCL of rare-earth-doped upconversion nanoparticles (UCNPs). In this study, using NaCsWO3 as the LSPR center, NaCsWO3@NaYF4@NaYF4:Yb,Er nanoparticles were synthesized and the UCL intensity could be enhanced by more than 124 times when the amount of NaCsWO3 was 2.8 mmol %. Then, such efficient UCNPs were not only doped into the hole transport layer but also used to modify the perovskite film in PSCs, resulting in the highest power conversion efficiency (PCE) reaching 18.89% (that of the control device was 16.01% and the PCE improvement was 17.99%). Possible factors for the improvement of PSCs were studied and analyzed. It is found that UCNPs can broaden the response range of PSCs to the NIR region due to the LSPR-enhanced UCL and increase the visible light reabsorption of PSCs due to the scattering and reflection effect, which generate more photocurrent in PSCs. In addition, UCNPs modify the perovskite film by effectively filling the holes and gaps at the grain boundary and eliminating the perovskite surface defects, which lead to less carrier recombination and then effectively improve the performance of PSC devices.