David McMeekin

Abstract:

Controlling the crystallization process of perovskite thin films to obtain a high-quality material is one of the most challenging aspects for upscaling perovskite solar cell (PSC) technology. The use of non-halide lead sources, such as lead acetate, is a potential solution to this issue due to the fast perovskite crystallization process triggered by the facile removal of acetate during post-annealing. However, to date, lead acetate has been used exclusively as a precursor for the synthesis of methylammonium (MA) or caesium (Cs) based perovskites, which are unstable and less efficient. Here, we expand the lead acetate precursor route to form mixed A-cation perovskites, namely, formamidinium-caesium lead perovskite. High-quality large-area formamidinium-caesium mixed-cation perovskite films were produced by blade-coating a lead acetate-based precursor formulation in an ambient laboratory environment, with the use of NH4+ as a volatile cation to drive off acetate during annealing, leading to formation of PSCs with a power conversion efficiency (PCE) of up to 21.0%. Blade coated mini-modules with an aperture area of 10 cm2 displayed PCEs of up to 18.8%. The encapsulated PSCs showed excellent thermal stability, with no evidence of efficiency loss after 3300 hours at 65 °C.

Abstract:

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.

Abstract:

Back-contact electrodes for hybrid organic-inorganic perovskite solar cells (PSCs) eliminate the parasitic absorption losses caused by the transparent conductive electrodes that are inherent to conventional sandwich-architecture devices. However, the fabrication methods for these unconventional architectures rely heavily on expensive photolithography, which limits scalability. Herein, we present an alternative cost-effective microfabrication technique in which the conventional photolithography process is replaced by microsphere lithography in which a close-packed polystyrene microsphere monolayer acts as the patterning mask for the honeycomb-shaped electrodes. A comprehensive comparison between photolithography and microsphere lithography fabrication techniques was conducted. Using microsphere lithography, we achieve highly efficient devices having a stabilized power conversion efficiency (PCE) of 8.6%, twice the reported value using photolithography. Microsphere lithography also enabled the fabrication of the largest back-contact PSC to date, having an active area of 0.75 cm2 and a stabilized PCE of 2.44%.