We first developed a stable wide band gap perovskite solar cell using a novel crystallization technique. Achieving the long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to achieving this goal. The commonly employed dimethyl-formamide/dimethyl-sulfoxide solvent preparation method results in a poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature dimethyl-sulfoxide-free processing method that utilizes dimethylammonium chloride as an additive to control the perovskite intermediate precursor phases. By controlling the crystallization sequence, we tune the grain size, texturing, orientation (corner-up versus face-up) and crystallinity of the formamidinium (FA)/caesium (FA)yCs1–yPb(IxBr1–x)3 perovskite system. A population of encapsulated devices showed improved operational stability, with a median T80 lifetime (the time over which the device power conversion efficiency decreases to 80% of its initial value) for the steady-state power conversion efficiency of 1,190 hours, and a champion device showed a T80 of 1,410 hours, under simulated sunlight at 65 °C in air, under open-circuit conditions. This work highlights the importance of material quality in achieving the long-term operational stability of perovskite optoelectronic devices.
We later developed a narrow band gap material that is composed of a Pb/Sn mixture to create a tandem architecture. This work involved the incorporation of the formamidinium (FA+ ) and caesium as replacement cation for methylammonium MA+ . As we have shown in our previous work, the (FA,Cs)Pb(Br,I)3 perovskite is significantly more thermally stable than its MA-based counterpart. However, we identify a significant challenge, where these cations have a significantly lower solubility in the ACN/MA solvent system. Hence, we explored the alkylamine group in hopes of identifying a methylamine replacement that can solubilize these more thermally stable cations in an aprotic solvent that will not damage the underlying perovskite layer. Wo observed that many of the amines could actually dissolve the precursor formamidinium-based tin/lead precursor salts. For example, we were able to fabricate a (FA(1-x)MAx)Pb(Br(1-y)Iy)3 perovskite film using an acetonitrile solvent mixed with n-butylamine CH₃(CH2)₃NH2, diethylamine (CH₃CH2)2NH, Isopropylamine (C3H9N) and dipropylamine (C6H15N). Although thin films were able to form a perovskite film, for most compositions, residual 2D material from the chosen amine would reside in the film. We conducted thorough crystallographic and optical band gap studies on these films to fully characterize them.
