Robbie Oliver (he/him)

Abstract:

High temperature post-deposition annealing of hybrid lead halide perovskite thin films—typically lasting at least 10 min—dramatically limits the maximum roll-to-roll coating speed, which determines solar module manufacturing costs. While several approaches for “annealing-free” perovskite solar cells (PSCs) have been demonstrated, many are of limited feasibility for scalable fabrication. Here, this work has solvent-engineered a high vapor pressure solvent mixture of 2-methoxy ethanol and tetrahydrofuran to deposit highly crystalline perovskite thin-films at room temperature using gas-quenching to remove the volatile solvents. Using this approach, this work demonstrates p-i-n devices with an annealing-free MAPbI₃ perovskite layer achieving stabilized power conversion efficiencies (PCEs) of up to 18.0%, compared to 18.4% for devices containing an annealed perovskite layer. This work then explores the deposition of self-assembled molecules as the hole-transporting layer without annealing. This work finally combines the methods to create fully annealing-free devices having stabilized PCEs of up to 17.1%. This represents the state-of-the-art for annealing-free fabrication of PSCs with a process fully compatible with roll-to-roll manufacture.

Abstract:

Vacuum deposition is a solvent-free method suitable for growing thin films of metal halide perovskite (MHP) semiconductors. However, most reports of high-efficiency solar cells based on such vacuum-deposited MHP films incorporate solution-processed hole transport layers (HTLs), thereby complicating prospects of industrial upscaling and potentially affecting the overall device stability. In this work, we investigate organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) as alternative, low-cost, and durable HTLs in all-vacuum-deposited solvent-free formamidinium-cesium lead triodide [CH(NH₂)₂]_0.83Cs_0.17PbI₃ (FACsPbI₃) perovskite solar cells. We elucidate that the CuPc HTL, when employed in an “inverted” p–i–n solar cell configuration, attains a solar-to-electrical power conversion efficiency of up to 13.9%. Importantly, unencapsulated devices as large as 1 cm² exhibited excellent long-term stability, demonstrating no observable degradation in efficiency after more than 5000 h in storage and 3700 h under 85 °C thermal stressing in N₂ atmosphere.

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:

Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but photoinduced halide segregation compromises photovoltaic device performance. This study explores the influence of a hole-transport layer, necessary for a full device, by monitoring halide segregation through in situ, concurrent X-ray diffraction and photoluminescence measurements to disentangle compositional and optoelectronic changes. This work demonstrates that top coating FA0.83Cs0.17Pb(Br0.4I0.6)3 perovskite films with a poly(triaryl)amine (PTAA) hole-extraction layer surprisingly leads to suppression of halide segregation because photogenerated charge carriers are rapidly trapped at interfacial defects that do not drive halide segregation. However, the generation of iodide-enriched regions near the perovskite/PTAA interface enhances hole back-transfer from the PTAA layer through improved energy level offsets, increasing radiative recombination losses. It is further found that while passivation with a piperidinium salt slows halide segregation in perovskite films, the addition of a PTAA top-coating accelerates such effects, elucidating the specific nature of trap types that are able to drive the halide segregation process. This work highlights the importance of selective passivation techniques for achieving efficient and stable wide-bandgap perovskite photovoltaic devices.