Grey Christoforo

Abstract:

Multijunction photovoltaics (PVs) are gaining prominence owing to their superior capability of achieving power conversion efficiencies (PCEs) beyond the radiative limit of single-junction cells^1-8, where improving narrow bandgap tin-lead perovskites is critical for thin-film devices⁹. With a focus on understanding the chemistry of tin-lead perovskite precursor solutions, we herein find that Sn(II) species dominate interactions with precursors and additives and uncover the exclusive role of carboxylic acid in regulating solution colloidal properties and film crystallisation, and ammonium in improving film optoelectronic properties. Materials that combine these two function groups, amino acid salts, considerably improve the semiconducting quality and homogeneity of perovskite films, surpassing the effect of the individual functional groups when introduced as part of separate molecules. Our enhanced tin-lead perovskite layer allows us to fabricate solar cells with PCEs of 23.9, 29.7 (certified 29.26%), and 28.7% for single-, double-, and triple-junction devices, respectively. Our 1-cm² triple-junction devices show PCEs of 28.4% (certified 27.28%). Encapsulated triple-junction cells maintain 80% of their initial efficiencies after 860 h maximum power point tracking in ambient. We further fabricate quadruple-junction devices and obtain PCEs of 27.9% with the highest open-circuit voltage of 4.94 V. This work establishes a new benchmark for multijunction PVs.

Abstract:

Perovskite solar cells (PSCs) offer an efficient, inexpensive alternative to current photovoltaic technologies, with the potential for manufacture via high-throughput coating methods. However, challenges for commercial-scale solution-processing of metal-halide perovskites include the use of harmful solvents, the expense of maintaining controlled atmospheric conditions, and the inherent instabilities of PSCs under operation. Here, we address these challenges by introducing a high volatility, low toxicity, biorenewable solvent system to fabricate a range of 2D perovskites, which we use as highly effective precursor phases for subsequent transformation to α-formamidinium lead triiodide (α-FAPbI3), fully processed under ambient conditions. PSCs utilising our α-FAPbI3 reproducibly show remarkable stability under illumination and elevated temperature (ISOS-L-2) and “damp heat” (ISOS-D-3) stressing, surpassing other state-of-the-art perovskite compositions. We determine that this enhancement is a consequence of the 2D precursor phase crystallisation route, which simultaneously avoids retention of residual low-volatility solvents (such as DMF and DMSO) and reduces the rate of degradation of FA+ in the material. Our findings highlight both the critical role of the initial crystallisation process in determining the operational stability of perovskite materials, and that neat FA+-based perovskites can be competitively stable despite the inherent metastability of the α-phase.

Abstract:

As perovskite photovoltaics stride towards commercialization, reverse bias degradation in shaded cells that must current match illuminated cells is a serious challenge. Previous research has emphasized the role of iodide and silver oxidation, and the role of hole tunnelling from the electron-transport layer into the perovskite to enable the flow of current under reverse bias in causing degradation. Here we show that device architecture engineering has a significant impact on the reverse bias behaviour of perovskite solar cells. By implementing both a ~35-nm-thick conjugated polymer hole transport layer and a more electrochemically stable back electrode, we demonstrate average breakdown voltages exceeding −15 V, comparable to those of silicon cells. Our strategy for increasing the breakdown voltage reduces the number of bypass diodes needed to protect a solar module that is partially shaded, which has been proven to be an effective strategy for silicon solar panels.