Enhancing the performance of inverted perovskite solar cells by optimizing PEDOT:PSS with lysine additive

Over the past few years, inverted perovskite solar cells (PSCs), whose architecture is anode/hole transport layer (HTL)/perovskite/electron transport layer/cathode, have exhibited a high potential for industrial application due to their low preparation temperature and cost-effective solution processing. To obtain high-performance inverted PSCs, it is crucial to choose a suitable HTL material because it can affect the quality of the subsequently deposited perovskite absorbing layer. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is regarded as one of the most attractive HTL materials for inverted PSCs due to its inherent merits of a simple preparation process, good conductivity, uniform film formation, and low cost. However, the acidic property of the PEDOT:PSS solution and the strong hygroscopicity of the PEDOT:PSS film, which are caused by the sulfonic acid groups in the PSS chain, are detrimental to improving the photovoltaic performance and long-term stability of inverted PSCs. Therefore, it is urgent to adjust the acidity of the PEDOT:PSS solution and optimize the quality of the PEDOT:PSS film to enhance the photovoltaic performance of inverted PSCs based on PEDOT:PSS HTL. In this work, a simple and effective strategy, doping lysine additive into the PEDOT:PSS original solution, has been proposed to adjust the acidity of the PEDOT:PSS solution and improve the film quality of the PEDOT:PSS film. By doping an appropriate amount of lysine additive into the PEDOT:PSS original solution, the pH of the PEDOT:PSS solution was adjusted from 3.5 (original PEDOT:PSS solution) to 7 (20 wt% lysine-doped PEDOT:PSS solution) due to the neutralization reaction between the amino groups in the lysine additive and the sulfonic acid groups in the PSS chain. In addition, the lysine additive could restructure the distribution of sulfonic acid groups and sulfonate groups within the PEDOT:PSS film, which is beneficial for reducing the roughness of the PEDOT:PSS film and thereby improving the quality of the subsequently deposited perovskite film with a relatively larger grain size. Furthermore, a better energy-level matching between the lysine-doped PEDOT:PSS layer and the perovskite layer is achieved, which is helpful for enhancing hole extraction from the perovskite layer to the lysine-doped PEDOT:PSS HTL. As a result, the open-circuit voltage (V-OC) of the inverted PSCs based on lysine-doped PEDOT:PSS HTL increased from 0.94 V (reference device with undoped PEDOT:PSS film as HTL) to 1.04 V, the short-circuit current (J(SC)) improved from 20.81 mA/cm(2) (reference device) to 21.35 mA/cm(2), and the power conversion efficiency (PCE) rose from 15.71% (reference device) to 17.65%. In addition, the lysine-doped PSCs exhibit relatively high long-term stability due to the high quality of the perovskite film coated on the lysine-doped PEDOT:PSS HTL. The unencapsulated lysine-doped PSCs maintained 86.54% of its initial PCE value after being stored in a nitrogen atmosphere (25 degrees C) for 2160 hours, and kept over 85.88% of its initial PCE after being stored in an air environment (25 degrees C, 15% RH) for 360 hours, respectively. Taking all above into account, introducing lysine as an additive into PEDOT:PSS solution can not only moderately neutralize its acidity but also improve the crystallization performance and film quality of the perovskite film, providing a new strategy for achieving efficient and stable inverted PSCs.