Perovskite Solar Cells: The Role of Halogen


Student thesis: Doctoral Thesis

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  • Shiqiang LUO

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Awarding Institution
Award date3 Oct 2016


While energy shortage is always an issue, the impending exhaustion of fossil fuel resources makes it an ever increasingly pressing one. Photovoltaic technology brings hope in the struggle to alleviate this problem, but no solar cell has yet fulfilled the requirements of the viability of large scale production together with high efficiency and low cost. Fortunately, recently reported organic–inorganic halide perovskites, possessing the desirable properties of high absorption coefficient, long charge diffusion length, appropriate band gap, and solution processability, show great potential for photovoltaic applications. Within a few years, the power conversion efficiency of perovskite solar cells has increased from 3.8% to 21.1%. However, the role of chloride on the performance improvement of the chloride doped perovskite remains unclear and the perovskite solar cells suffered from the hysteresis effect (section 2.4). Our research started with hole transport material (HTM) free perovskite solar cells with a power conversion efficiency (PCE) of 1.7% and improved PCE to 10.4% using spiro-OMeTAD as HTM and further to 13.35% by applying spray method. The role of chloride was investigated in CH3NH3PbI3-xClx perovskite solar cells and a Br- ion migration process was revealed in CH3NH3PbI3-xBrx perovskite solar cells.

In Chapter 1, the aims and objectives of the thesis are given.

In Chapter 2, the history of the development of the perovskite solar cells is discussed in details and the deposition of the perovskite is classified in three methods. Then, modification of the perovskite solar cells by tuning the components of the perovskite is introduced. (Publication: J Mater Chem A, 2015, 3, 8992-9010)

In Chapter 3, the roles of chloride on the electronic performance and the crystallization of the perovskite are summarized. The crystal structure formation of CH3NH3PbI3-xClx is discussed based on the deposition methods and a new explanation about the (110)-oriented growth of CH3NH3PbI3 and CH3NH3PbI3-xClx is provided. (Publication: Materials, 2016, 9, 123.)

In Chapter 4, a mixture of PbI2 and PbCl2 was applied as lead salts for the perovskite. By optimizing the ratio of PbCl2 and the loading time of CH3NH3I solution on the lead salt layer, a PCE of 10.4% was achieved. A fast conversion from the mixture lead salts to the perovskite was found and this was explained by forming a new PbICl phase. (Publication: Mater Lett, 2016, 169, 236-240.)

In Chapter 5, a spray method was applied to solve the ripple shape problem by the two-step spin-coating method. A PCE of 13.35% was achieved. However, the spray method was conducted in ambient atmosphere and perovskite CH3NH3PbI3 was sensitive to moisture. To improve the stability, we added MABr in the PbI2 solution. A Br- ion migration process was revealed.

Finally, Chapter 6 summarizes the work demonstrated in this dissertation and suggests future work.