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Abstract:

Interfaces in thin-film photovoltaics play a pivotal role in determining device efficiency and longevity. Herein, we study the top surface treatment of mixed tin-lead (∼1.26 eV) halide perovskite films for p-i-n solar cells. We are able to promote charge extraction by treating the perovskite surface with piperazine. This compound reacts with the organic cations at the perovskite surface, modifying the surface structure and tuning the interfacial energy level alignment. In addition, the combined treatment with C₆₀ pyrrolidine tris-acid (CPTA) reduces hysteresis and leads to efficiencies up to 22.7%, with open-circuit voltage values reaching 0.90 V, ∼92% of the radiative limit for the band gap of this material. The modified cells also show superior stability, with unencapsulated cells retaining 96% of their initial efficiency after >2000 hours of storage in N₂ and encapsulated cells retaining 90% efficiency after >450 hours of storage in air. Intriguingly, CPTA preferentially binds to Sn²⁺ sites at film surface over Pb²⁺ due to the energetically favoured exposure of the former, according to first-principles calculations. This work provides new insights into the surface chemistry of perovskite films in terms of their structural, electronic, and defect characteristics and we use this knowledge to fabricate state-of-the-art solar cells. This article is protected by copyright. All rights reserved.

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:

Inverted-structure metal halide perovskite solar cells (PSCs) have attractive advantages like low-temperature processibility and outstanding device stability. The two-step sequential deposition method shows the benefits of easy fabrication and decent performance repeatability. Nevertheless, it is still challenging to achieve high-performance inverted PSCs with similar or equal power conversion efficiencies (PCEs) compared to the regular-structure counterparts via this deposition method. Here, we demonstrate an improved two-step sequential deposition technique via treating the bottom organic hole-selective layer with the binary modulation system composed of a polyelectrolyte and an ammonium salt. Such improved sequential deposition method leads to the spontaneous refinement of up and buried interfaces for the perovskite films, contributing to high film quality with significantly reduced defect density and better charge transportation. As a result, the optimized PSCs show a large enhancement in the open-circuit voltage by 100 mV and a dramatic lift in the PCE from 18.1% to 23.4%, delivering the current state-of-the-art performances for inverted PSCs. Moreover, good operational and thermal stability has been achieved upon the improved inverted PSCs. This innovative strategy helps gain a deeper insight into the perovskite crystal growth and defect modulation in the inverted PSCs based on the two-step sequential deposition method. This article is protected by copyright. All rights reserved.