Regulating the Crystallization of Mixed-Cation Perovskite for Inverted Planar Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Awarding Institution
Award date9 Aug 2019


Since the pioneering work reported in 2013, the inverted planar perovskite solar cells (PSCs), with the features of simple fabrication, negligible hysteresis and mechanical flexibility, have drawn intensive attention worldwide. Recently, the record power conversion efficiency (PCE) of the inverted planar PSC has been pushed up to 21.5%, which is on par with that of the commercially available technologies (e.g., crystalline silicon, cadmium telluride). However, the poor long-term stability of the PSCs is still a major hurdle towards the commercialization of PSCs.

The thesis was aimed to simultaneously achieve “superior stability” and “high efficiency” for inverted planar PSCs. In order to solve the instability from the root whilst preserving high PCE, the thesis was mainly focused on the growth of high-quality perovskites with intrinsically improved stability. By exploring various approaches, such as modification of hole transport materials, the rational design of intermediates, morphology engineering of PbI2 scaffold and diammonium cation incorporation, the crystallization of perovskite was finely regulated. As a result, PSCs with PCE exceeding 18% achieved superior long-term stability even exposed to humidity, light soaking and heat.

Specifically, the thesis was started by surface modification of a non-wetting hole transport material (HTM), Poly-TPD, to fabricate high-performance MAPbI3 based inverted planar PSCs. With a facile ultraviolet-ozone modification method, the surface wettability of poly-TPD layer was tuned without affecting its bulk properties. Hence, crystallization of perovskite with large grain size and full film coverage was achieved. Benefiting from the high-quality perovskite film, well-matched energy level alignment and hydrophobic property of poly-TPD, the resulting PSCs showed a high PCE of 18.19% with significantly enhanced stability as compared to the PEDOT: PSS counterparts. This work highlighted the critical roles of the energy level and hydrophobicity of the HTM in enhancing the efficiency and stability of the inverted planar PSCs.

With the Poly-TPD based device architecture, we sought to replace the instable MAPbI3 with the Cs0.15FA0.85PbI3 perovskite possessing superior intrinsic stability. By coupling a PbI2-(CsI)0.15-(FAI)x intermediate complex with preheating in a two-step method, crystallization of Cs0.15FA0.85PbI3 to form uniform and compact films were achieved in air with a relative humidity (RH) of 70±10%. It was because that the intermediate complex reduced the FAI concentration in the second step and the preheating suppressed the detrimental impact of moisture. Consequently, the air-processed PSCs delivered a PCE of 15.56%. Although the PCE is not satisfied as a result of insufficiently high quality of the perovskite film, the superior stability of the Cs0.15FA0.85PbI3 upon thermal- and photo-stresses revealed its great potential for practical application.

To further improve the film quality of Cs0.15FA0.85PbI3 derived from a two-step method, a novel approach that enabled effective morphology engineering of the precursor layer (typically, PbI2) was developed. Through integrating CsI incorporation with toluene dripping in air, a porous and intercrossed PbI2-(CsI)0.15 nanorods scaffold was constructed. This porous scaffold simultaneously featured with high porosity and dense deposition of the precursors, thus guaranteeing the depth-independent crystallization of high-quality Cs0.15FA0.85PbI3, as evidenced by the photoluminescent (PL) spectra. Consequently, a higher PCE of 16.85% was obtained, and the device retained 85% of its initial PCE after exposed to continuous light soaking at 60 °C in air (RH: 60±10%) for 300 h.

Moreover, the porous PbI2-(CsI)0.15 scaffold was also extended to fabricating Cs-FA PSCs with bulky propane-1,3-diammonium (PDA) cations incorporation. It was found that by increasing the PDA content, the PCE of the Cs0.15FA0.85-xPDAxPbI3 PSC first increased and then drastically decreased. The highest PCE of 18.10% obtained by Cs0.15FA0.83PDA0.02PbI3 was superior to that of the Cs0.15FA0.85PbI3 (16.82%). Through systematic investigations, it was revealed that the PDA incorporation profoundly affected the crystallization of perovskite, and the PDA content-dependent PCE was attributed to a competition between the enhanced defect passivation and the emerged excitonic effect with increased PDA content. Moreover, the Cs0.15FA0.83PDA0.02PbI3 PSC exhibited almost 1.5 times increased stability than the Cs0.15FA0.85PbI3 PSC, with 83% of its initial PCE retained after 500 h exposure under continuous light soaking at 60 °C with a RH of 60±10%. This work not only provided a practical strategy to enhance the device stability without sacrificing the PCE, but also deepened our understanding on the effects of diammonium cation incorporated in 3D perovskite.

    Research areas

  • Perovskite solar cell, Crystallization, Photovoltaic