Morphology control of organic halide perovskites by adding BiFeO3 nanostructures for efficient solar cell

The morphology of perovskite light-absorption layer plays an important role in the performance of perovskite solar cells (PSCs). In this work, BiFeO3 (BFO) nanostructures were used as additive for CH3NH3PbI3 (MAPbI3) via anti-solvent method. The addition of BFO nanostructures greatly enhanced the crystallinity, grain size and film uniformity of MAPbI3. As a result, the charge carrier mobility and electron diffusion length increased, leading to the increase of the short circuit current (JSC) of PSCs. This work provides a very simple but effective approach to improve the morphology of perovskite layer for efficient PSCs.

Enhancing electron diffusion length in narrow-bandgap perovskites for efficient monolithic perovskite tandem solar cells

Developing multijunction perovskite solar cells (PSCs) is an attractive route to boost PSC efficiencies to above the single-junction Shockley-Queisser limit. However, commonly used tin-based narrow-bandgap perovskites have shorter carrier diffusion lengths and lower absorption coefficient than lead-based perovskites, limiting the efficiency of perovskite-perovskite tandem solar cells. In this work, we discover that the charge collection efficiency in tin-based PSCs is limited by a short diffusion length of electrons. Adding 0.03 molar percent of cadmium ions into tin-perovskite precursors reduce the background free hole concentration and electron trap density, yielding a long electron diffusion length of 2.72 ± 0.15 µm. It increases the optimized thickness of narrow-bandgap perovskite films to 1000 nm, yielding exceptional stabilized efficiencies of 20.2 and 22.7% for single junction narrow-bandgap PSCs and monolithic perovskite-perovskite tandem cells, respectively. This work provides a promising method to enhance the optoelectronic properties of narrow-bandgap perovskites and unleash the potential of perovskite-perovskite tandem solar cells.

Electron Diffusion Length and Charge Separation Efficiency in Nanostructured Ternary Metal Vanadate Photoelectrodes

Ternary metal vanadates have recently emerged as promising photoelectrode materials for sunlight-driven water splitting. Here, we show that highly active nanostructured BiVO4films can be deposited onto fluorine-doped tin oxide (FTO) substrates by a facile sequential dipping method known as successive ionic layer adsorption and reaction (SILAR). After annealing and deposition of a cobalt phosphate (Co-Pi) co-catalyst, the photoelectrodes produce anodic photocurrents (under 100 mW cm-2broadband illumination, 1.23 Vvs. RHE) in pH 7 phosphate buffer that are on par with the highest reported in the literature for similar materials. To gain insight into the reason for the good performance of the deposited films, and to identify factors limiting their performance, incident photon-to-electron conversion efficiency spectra have been analyzed using a simple diffusion–reaction model to quantify the electron diffusion length (Ln; the average distance travelled before recombination) and charge separation efficiency (ηsep) in the films. The results indicate thatηsepapproaches unity at sufficiently positive applied potential but the photocurrent is limited by significant charge collection losses due to a shortLnrelative to the film thickness. The Co-Pi catalyst is found to improveηsepat low potentials as well as increaseLnat all potentials studied. These findings help to clarify the role of the Co-Pi co-catalyst and show that there could be room for improvement of BiVO4photoanodes deposited by SILAR ifLncan be increased.

Surface Engineering of Metal Oxide Photoelectrodes for Improved Band Alignment in Solar Water Splitting Cells

Several earth-abundant transition-metal oxides (e.g. Fe2O3, CoO, and Cu2O) possessing suitable band gaps for solar water splitting exist, but energy level alignment is often sub-optimal, i.e. the conduction and valence bands do not straddle the water oxidation and reduction potentials. Here, using a nanocrystalline-TiO2-based photoelectrochemical cell as a model system, we investigate the effect of tuning the semiconductor energy levels by adding Li+ ions to the electrolyte. The effect of LiClO4 addition on band edges, interfacial recombination resistance, electron diffusion length, and charge-separation efficiency were quantified by impedance spectroscopy and analysis of incident photon-to-current efficiency spectra. We find that the TiO2 band edges are shifted toward positive potentials by the addition of Li+, and that this increases the apparent electron diffusion length without affecting the charge-separation efficiency, most likely due to a change in the driving force for O2 reduction. These results should prove useful in the modeling and optimization of solar water splitting cells employing metal oxide photoelectrodes.