Potassium iodide reduces the stability of triple-cation perovskite solar cells

Abstract

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices. Previous work on the use of potassium iodide (KI) in active layers to passivate defects in triple-cation mixed-halide perovskites has been shown to enhance their luminescence efficiency and reduce current-voltage hysteresis. However, the operational stability of KI passivated perovskite solar cells under ambient conditions remains largely unexplored. By investigating perovskite solar cell performance with SnO₂ or TiO₂ electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite-ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and - at concentrations >5 mol% in the precursor solution - modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.

Fig. 1. (a) Current density–voltage curves for champion devices incorporating the addition of 0, 5, 10 and 20% KI. Forward (J_sc to V_oc) and reverse (V_oc to J_sc) sweep directions are indicated by solid and dashed lines respectively. (b) Statistical box plots for PSCs performance (reverse scan V_oc to J_sc) determined from all devices.

Fig. 2. (a) SEM of triple-cation perovskite films containing various fractions of KI (X = 0, 5, 10 and 20% KI). Regions marked in yellow denote the presence of a secondary phase in addition to surrounding triple-cation perovskite grains. (b) Cross-section SEM images of TC perovskite devices; X = 0, 5, 10 and 20% KI.

Fig. 3. (a) XRD patterns of triple-cation perovskite films with different concentration of KI on ITO/SnO₂ (ETL) substrates. (b) Magnified view of XRD patterns of TC (X = 0–20%) films to show decrease in PbI₂ peak intensity around 2θ = 12.7° and (c) variation in (012) peak around 2θ = 31.8° with KI addition.

Fig. 4. Time-resolved photoluminescence of triple-cation perovskite films with KI added at 0% and 10% cast on (a) ITO/SnO₂ and (b) FTO/TiO₂.

Fig. 5. (a) J–V curves of champion triple-cation perovskite devices using c-TiO₂/mp-TiO₂ as the ETL. (b) A histogram of PCE of all TiO₂ ETL devices from the reverse-scan.

Fig. 6. Monitoring the reduction of perovskite crystallinity under accelerated aging conditions using integrated scatter from the (001) reflection for 0% KI (red) and 10% KI (yellow) samples. Films were placed in a sample chamber with a hotplate temperature of either 120 °C (solid lines) or 150 °C (dashed). The measured air temperature (T_air) and relative humidity (RH) for each experiment was T_air = 59 ± 3 °C, 62 ± 1 °C, 70 ± 2 °C, 69 ± 4 °C and RH = 33 ± 4%, 26 ± 1%, 19 ± 1%, 19 ± 3% for 0% KI, 10% KI at 120 °C and 0% KI, 10% KI at 150 °C, respectively.

Fig. 7. Long term stability of 0% KI and 10% KI films under accelerated aging conditions. Scattering intensity from the (001) reflection was monitored with films kept on a 43 °C hotplate with T_air and RH monitored. Note, that we recorded small differences in chamber conditions between each measurement run, with average T_air ≈ 41 °C and RH ≈ 41% for 0% KI and T_air ≈ 37 °C and RH ≈ 61% for 10% KI as shown in the lower panels, corresponding to absolute humidities of 22 gm⁻³ and 27 gm⁻³, respectively.

Fig. 8. Lifetime stability measurements of triple-cation perovskite devices created from precursor solutions with various levels of KI addition cast on (a) SnO₂ with data presented from an average value of a minimum of 6 devices, and (b) TiO₂ having an initial KI concentration of 0 and 10% from an average value of 6 cells.

Fig. 9. GIWAXS diffraction patterns of aged triple cation perovskite films with (a) 0% KI and (b) 10% KI. (c) Azimuthally integrated profiles from GIWAXS patterns recorded from aged devices (see text for details), highlighting degradation products formed in the region 0.65 ≤ Q ≤ 0.95 Å⁻¹. (d) XRD patterns from degraded films confirming KBr is still present for 10% and 20% KI addition.