Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency

Abstract

Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.

Fig. 1. XRD and optical characterisation of Cs_xM compounds. (a) XRD spectra of perovskite upon addition of Cs investigating the series Cs_x(MA_0.17FA_0.83)_(1–x)Pb(I_0.83Br_0.17)₃, abbreviated as Cs_xM where M stands for “mixed perovskite”. Cs_xM with x = 0, 5, 10, 15%. (b) The corresponding (dashed lines) and photoluminescence (PL) spectra (solid lines).

Fig. 2. Thermal stability and film formation dependence on processing conditions. UV-visible absorption of Cs₀M (a) and Cs₁₀M (b) films annealed at 130 °C for 3 hours in dry air with the corresponding images. (c) Absorption spectra of as-fabricated films (at room temperature) without the subsequent annealing step, the Cs₀M film is red (red line, image of red film). Upon addition of cesium, the Cs₁₀M film turns black (black line, image of black film). The inset XRD data show that Cs₁₀M has the characteristic perovskite pattern whereas Cs₀M does not. (d) When the spin coating processing temperature, T _{Glove box}, is kept at 18 °C, Cs₀M does not form a perovskite phase even after annealing at 100 °C for 1 hour (solid red line). The perovskite phase forms, however, when using a processing temperature of 25 °C (dashed black line). Cs₁₀M forms readily the perovskite phase at 18 °C (solid black line). The inset images show representative Cs₀M films at 18 °C (red), 25 °C (black); or for Cs₁₀M at 18 °C (black).

Fig. 3. Cross-sectional scanning electron microscopy (SEM) images of (a) Cs₀M (b) Cs₅M and (c) low magnification Cs₅M devices.

Fig. 4. Statistics of 40 controls (Cs₀M) and 98 Cs-based (Cs₅M) devices as collected over 18 different batches. We note that all device parameters and the standard deviation (S.D.), a metric for the reproducibility, improved: the V _oc improved from 1121 ± 25 (n = 40) to 1132 ± 25 mV (n = 98), the J _sc improved from 21.06 ± 1.53 to 22.69 ± 0.75 mA cm^–2, the FF improved from 0.693 ± 0.028 to 0.748 ± 0.018, and the PCE improved from 16.37 ± 1.49 to 19.20 ± 0.91%. 20 independent devices show efficiencies >20%.

Fig. 5. JV and stability characteristics (a) current–voltage scans for the best performing Cs₅M device showing PCEs exceeding 21%. The full hysteresis loop is reported in Table S4 (ESI†). The inset shows the power output under maximum power point tracking for 60 s, starting from forward bias and resulting in a stabilized power output of 21.1% (at 960 mV). The voltage scan rate for all scans was 10 mV s^–1 and no device preconditioning, such as light soaking or forward voltage bias applied for a long time, was applied before starting the measurement. (b) Aging for 250 h of a high performance Cs₅M and Cs₀M devices in a nitrogen atmosphere held at room temperature under constant illumination and maximum power point tracking. The maximum power point was updated every 60 s by measuring the current response to a small perturbation in potential. A JV scan was taken periodically to extract the device parameters. This aging test resembles sealed devices under realistic operational conditions (as opposed to “shelf stability” of devices kept in a dry atmosphere in the dark and measured periodically). The device efficiency of Cs₅M drops from 20% to ∼18% (red curve, circles) where it stays relatively stable for at least 250 h. This is not the case for Cs₀M (black curve, squares).