Chemical Reduction of Iodine Impurities and Defects with Potassium Formate for Efficient and Stable Perovskite Solar Cells

Abstract

The performance of perovskite solar cells (PSCs) is negatively affected by iodine (I2) impurities generated from the oxidation of iodide ions in the perovskite precursor powder, solution, and perovskite films. In this study, the use of potassium formate (HCOOK) as a reductant to minimize the presence of detrimental I2 impurities is presented. It is demonstrated that HCOOK can effectively reduce I2 back to I− in the precursor solution as well as in the devices under external conditions. Furthermore, the introduced formate anion (HCOO−) and alkali metal cation (K+) can reduce the defect density within the perovskite film by modulating perovskite growth and passivating electronic defects, significantly prolonging the carrier lifetime and reducing the J–V hysteresis. Consequently, the maximum efficiency of the HCOOK‐doped planar n–i–p PSCs reaches 23.8%. After 1000 h of operation at maximum power point tracking under continuous 1 sun illumination, the corresponding encapsulated devices retain 94% of their initial efficiency.

A comparison of molecular iodine evolution on the chemistry of lead and tin perovskites

“…Very recently, Sun et al reported the use of potassium formate (HCOO − K + ) as an effective and oxygen-stable iodine reductant. 95 The study demonstrates that iodine formation within the precursor solution can be mitigated via the addition of formate anions yielding CO 2 and both I − and H + ions. These products are benign to the function of the PSCs enabling a high performance of 23.8% and good accelerated stability in ambient conditions.…”

“…These products are benign to the function of the PSCs enabling a high performance of 23.8% and good accelerated stability in ambient conditions. 95…”

“…55 This effect can be visually identified as a yellowing of the FAI powder where I 2 quickly reacts with I − , forming I 3 − and is often observed when FAI is stored improperly with exposure to oxygen. 95 This oxidation of I − occurs via the FAI equivalence with FA and HI; the latter of which undergoes favourable oxidation with oxygen as discussed. This oxidation occurs on short timescales in FAI, possibly owing to the formation of sym -triazine as a product.…”

“…96 To this end, the incorporation of poor-quality FAI, containing I 2 /I 3 − , dramatically reduces the stability and performance of PSCs. 55,95 Alkylammonium bromides and chlorides offer greater stability to oxidation under ambient owing to better stabilisation of the negative charge on the bromide and chloride ions, yielding higher redox potentials ( E °) (Table 1). Consequently, in mixed cation-halide systems, it may prove beneficial to use bromide and chloride formamidinium salts where possible, incorporating iodine via more stable methylammonium, alkali metal or lead salts.…”

Design and optimization of Cs2SnI6 based inorganic perovskite solar cell model: numerical simulation

“…By using the Shockley-Read-Hall model, it can be inferred (assuming uniform carrier distribution) that an enhancement in defect density will lead to an enhancement in the concentration of a series of deep level impurities. These deep level impurities not only become the recombination centers of electrons and holes, reducing the lifetime of charge carriers, but also become charged centers after ionization, causing scattering effects on charge carriers, hindering their migration, reducing the diffusion length of charge carriers, and reducing the conductivity of the device [65,66]. In addition, after introducing shallow level impurities into the device, the electrons or holes generated by ionization of shallow level impurities can transition to the corresponding CB or VB, which increases the number of carriers in the pn junction, enhances the strength of the built-in electric field and current density.…”