Additive Engineering For Perovskite Solar Cells

Solar energy is one of the most attractive renewable energies in the world due to its inexhaustibility.The generation of electricity by solar cells is more environmentally friendly though the cost of electricity from solar cells is higher than that from coal-fired stations nowadays.Silicon-based solar cells are the most widely used products in the field of photovoltaics,but the sophisticated production process make a huge difference to the cost of device fabrication.In contrast,solution-processed solar cells like perovskite solar cells and organic solar cells present a promising future because these devices are compatible with roll to roll industry.Unbelievably,the power conversion efficiency?PCE?of perovskite solar cell has reached 23.7%in only ten years,which is higher than that of multicrystalline silicon-based solar cell whose highest PCE is 22.3%and is close to the PCE of single crystal silicon-based solar cell whose champion value is 26.6%.Perovskite solar cell developed fast in the past ten years,but it will still take some time to meet mass production,one of the most important issues is to control the quality of perovskite films.Film quality control can be difficult because the kinetics of crystal growth is easily influenced by many factors,e.g.molar ratio of reactants,reactant purity,processing solvent,drying condition,annealing temperature,etc.To solve such a problem,considerable efforts have been paid to develop processing techniques for better film quality,including additive engineering,interface engineering and post-solvent annealing.Among them,additive engineering is the simplest and easiest technique.In this work,we introduce phenol,a common and low-cost material in chemical industry,as an additive in perovskite precursor solution.It is demonstrated that incorporation of phenol in perovskite solution is a facile and effective method to facilitate crystal growth of perovskite.The addition of phenol leads to larger average grain size,less grain boundaries,and decreased concentration of trap states.This additive eventually contributes to 20%enhancement in perovskite solar cells performance.Meanwhile,the additive can be removed completely from the perovskite films after vacuum and thermal annealing,thus resulting in high purity of perovskite.Noticeably,the additive is effective in a wide concentration range.Therefore,the method is ideal for industrial processing that places less stringent requirement on concentrations.To understand the underlying mechanism at the molecular level,we conducted a series of characteristic measurements.The results from SEM and XRD illustrate that the doped film has bigger crystalline grain size than the control sample.The steady-state and transient PL results together suggest that the defect-induced non-radiative recombination is suppressed and charge carrier lifetime is enhanced due to phenol-induced reduction of defect states.Besides,LC-MS results imply that upon quench and thermal annealing treatments,phenol with a slightly lower boiling point than that of the processing solvents can be removed completely from the perovskite film,facilitating higher purity of the perovskite crystals.Furthermore,the results of ¹H NMR,UV-vis and another XRD propose that the coordination between O atom in phenol and Pb in PbI₂ exists when phenol is introduced in perovskite precursor solution.Together the results mentioned above suggest such a molecular interaction contributes to the crystal growing process,resulting in increased size of perovskite grains.