Performance Improvement Of Inverted Planar Heterojunction Perovskite Solar Cell Via Morphology Control And Interface Engineering

Organic-inorganic hybrid perovskite solar cells(PSCs) are considered as one of the most promising next-generation solar cells due to their advantages of low-cost precursors, high power conversion efficiency (PCE) and ease of processing. In the past few years, the PCEs have jumped from 3.81% to over 20% for PSCs. Recent developments demonstrate that perovskite exhibits bipolar semiconducting characteristic, which allows for the construction of planar heterojunction(PHJ) perovskite solar cells. PHJ PSCs can avoid the use of high-temperature sintered mesoporous metal oxides, which enable simple processing and the fabrication of flexible and tandem perovskite solar cells. In planar heterojunction, hole/electron transport layers are introduced between perovskite film and anode/cathode. The most practical methods for efficiency improvement of PSCs are morphology control of the perovskite films and interface engineering. As the light absorber, perovskite films play a significant role in PSCs performance. Solution engineering, various of deposition process, annealing optimization are introduced to control perovskite films morphology, such as large perovskite grain size, high aspect ratio and pinhole free. The hole and electron transporting layers are expected for enhancing exciton separation, charge transportation and collection. Further, the supporting layer for perovskite film not only plays an important role in energy-level alignment, but also affects perovskite film morphology, which both determine the device performance. In addition, interfacial layers also affect device stability.In this thesis, we focused on performance improvement of inverted PHJ PSCs devices via perovskite morphology control and interface engineering, and carried out the following two works:(1)The precursor of solution-processed perovskite thin films is one of the most central components for high efficiency PSCs. By charging the mole ratio of CH3NH3I, PbI2, PbCl2 and the concentration of precursor solution, high quality of perovskite films was realized. We also find that partial chlorine substitution can accelerate the crystalline nucleation process of perovskite, which make the perovskite gain long charge carrier diffusion distances and high charge carrier mobilities. Moreover, thermally and solvent annealing were introduced to increase the crystallinity and grain size of perovskite film, which realizes high quality of perovskite films with large grain size, high aspect ratio and pinhole free.(2) Firstly, by charging the thicknesses of PEDOT:PSS hole transport layer(HTL) and PCBM electron transport layer(ETL), which could make the HTL and ETL cover the entire underlayer, as well as decrease recombination and series resistance when the dissociated carriers travel the HTL of ETL to anode or cathode. Furthermore, polymer EDOT-DPP was introduced between PEDOT:PSS (HTL) and perovskite layer as interface layer. EDOT-DPP was selected for hole transport interface layer due to its high occupied molecular orbital(HOMO) falls somewhere in between PEDOT:PSS’s and perovskite’s, which could enhance exciton separation, charge transportation and collection. As the lowest unoccupied molecular orbital (LUMO) of EDOT-DPP is higher than perovskite’s, EDOT-DPP efficiently blocks the flow of electrons. Finally, PFN-Br was selected for cathode interface improving as its property of formation of appropriate dipoles for better energy level alignment at the electrode/ETL interface and improvement of electron selectivity at the cathode by efficiently blocking holes; and also the improvement of interfacial charge transport properties.