P-type photoelectrodes for perovskite solar cell

Inverted p-i-n perovskite devices incorporating inorganic hole-transporters exhibit negligible small hysteresis and enhanced stability. In particular, NiO tends to be a very promising candidate as inorganic hole-transporter due to its high transmittance, suitable work function and low-lying valence band. NiO films fabricated through solution processing often have defects (e.g. pinholes) and uncontrolled impurities, which degrade their optical transmittance and carrier transport properties. These problems can be overcome by employing a more advanced deposition method such as electrostatic spray deposition (ESD). This technique not only allows growing NiO films with fewer defects but also facilitates the introduction of appropriate dopants. Furthermore, it enables the precise control of the film thickness, which is significant for light transmittance In this work, solution-processed and ESD grown NiO:Cu films have been compared as hole-transporter layers in perovskite solar cells. Copper has been added as a dopant to enhance the charge extraction efficiency and/or promote perovskite crystallization. In all cases higher power conversion efficiencies, arising from higher open circuit voltages and fill factors were obtained for cells using ESD NiO:Cu films as compared to solution-processed NiO:Cu film. Furthermore, the cells using Cu-doped NiO showed better performances than those fabricated with undoped NiO and there exists an optimum value of copper dopant for NiO film which is at 5at%. Conductivity and XRD measurements showed that the main influence of the Cu dopant is related to a modification of the perovskite crystallization. Apart from that, solution-processed NiO has been explored through two approaches, from substituting polar solvent in NiO precursor solution to non-polar solvent and through interface modification by incorporating cysteine to NiO film with the aim to form chemical bonds between NiO layer and perovskite. Solution-processed NiO(n-hexane) has less defects and shown similar hydrophilicity with ESD NiO:Cu, suggesting it to be a potential alternative for solution-processed NiO. On the other hand, Continuous efforts have been done to obtain perovskite film with high uniformity and integrity. It is observed that the slightest change in deposition parameters like spin-coating speed, spin-coating acceleration and even annealing time would affect the resulting perovskite film in a great extent. In short, the improved performance of perovskite solar cells with ESD-deposited NiO films as compared to solution-processed NiO films demonstrates that the NiO layer has a substantial influence on the overall operation of the solar cell. Electrostatic-spray deposited NiO films are denser and have less pinholes the solution-processed ones, thus reducing recombination phenomena and enhancing the power conversion efficiency. We have also demonstrated that the addition of an optimum amount of dopants, such as copper, into the NiO films also significantly improves the fill factor and Jsc. Our results indicate that Cu-doping results in an improved quality of the deposited perovskite films, while no enhancement of the conductivity was observed