Novel triple-doping strategy promises performance boosts
Over the past decade, one family of materials has dominated discussions on solar cell technology: perovskites. Their distinctive crystal configuration and three-part structure allow for many possible permutations and combinations of elements. This, alongside continuous improvements in fabrication techniques, has seen the power conversion efficiency of perovskite solar cells (PSCs) increase from 3.8 % in 2009 to 25.7 % in 2022. PSCs also produce more power than commercial silicon cells in low-light and indoor conditions. But they still face one major challenge – their intrinsic, long-term stability lags far behind equivalent solar cells.
In a new paper published in Nano Energy [DOI: /10.1016/j.nanoen.2023.108268], a group of Italian researchers report on the development of a strategy that they say improves both the stability and efficiency of inverted (p-i-n) perovskite solar cells. Previous studies had shown that the insertion of particular additives into perovskite precursor inks can chemically passivate the resulting device, optimising the charge transfer and improving its overall efficiency. This group of researchers expanded on those observations, combining three of the most promising additives in the literature to create “a universal multi-additive strategy” for optimising perovskite formulations.
Each of the three doping additives play a different role in the passivation of solar devices. The first, ionic liquid BMIM-BF4 (1-Butyl-3-methylimidazolium tetrafluoroborate), operates at the grain boundaries, improving short circuit current density (Jsc) and fill factor (FF). The second, alkylamine ligand Oleylamine (OAm), controls crystal orientation, and enhances the open circuit voltage (Voc). The third additive, BHC (benzylhydrazine hydrochloride) is a reducing agent that prevents the formation of defect sites by reducing the oxidation of I- to I2.
To test the combined impact of the additives, the authors chose two perovskite formulations – one with a narrow bandgap (NBg) and the other with a wide bandgap (WBg). Each of these formulations were triply-doped and then assembled into inverted structure solar devices. Pristine versions of the perovskites were used to make identical versions of the devices to act as controls. The additives were shown to improve the photovoltaic parameters of both perovskites; Jsc, Voc, and FF were all enhanced. That resulted in improved power conversion efficiencies too, with an increase from 16.35 % to 20.31 % for the NBg perovskite, and from 14.91 % to 20.20 % for the WBg. In addition, they tested an “upgraded architecture” in which a different hole transport layer material was used with the narrow bandgap perovskite. This resulted in the highest efficiency they measured for single device, reaching a maximum value of 21.3 %. The wide bandgap perovskite was also shown to operate well in low light conditions, and to combine favourably with a commercially available silicon heterojunction, boosting its efficiency.
In addition, the team investigated the impact of their triple-additives on device stability as it operated in harsh conditions – under continuous light soaking (> 1000 hours illumination) and thermal stress (reaching a temperature of 85°C), both in air. For both perovskites, T80, a metric used to quantify a device’s lifetime, was greatly extended by the presence of additives. Its value increased from 27 to 135 h for the NBg, and from 56 to 305 h for the WBg.
To examine whether their approach could apply more generally, the authors used the same additives to dope a different perovskite formulation (CsFA). Its stability improved by ~ 300 % compared to the pristine CsFA device. The most stable of these devices retained 100 % of their starting efficiency after 1500 h of thermal stress, without the need for a diffusion barrier.
The authors conclude that their study “… has demonstrated a universal strategy to achieve high efficiency and stability in any perovskite precursor, with the goal of paving the road for the commercialization of perovskite solar technology.”
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Luigi Angelo Castriotta, Emanuele Calabro, Francesco Di Giacomo, Sathy Harshavardhan Reddy, Daimiota Takhellambam, Barbara Paci, Amanda Generosi, Luca Serenelli, Francesca Menchini, Luca Martini, Mario Tucci, Aldo Di Carlo. “A universal multi-additive strategy to enhance efficiency and stability in inverted perovskite solar cells,” Nano Energy 109 (2023) 108268. DOI: /10.1016/j.nanoen.2023.108268