Influence of an Organic Salt-Based Stabilizing Additive on Charge Carrier Dynamics in Triple Cation Perovskite Solar Cells

Abstract

Besides further improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSC), their long-term stability must also be ensured. Additives such as organic cations with halide counter anions are considered promising candidates to address this challenge, conferring both higher performance and increased stability to perovskite-based devices. Here, a stabilizing additive (N,N-dimethylmethyleneiminium chloride, [Dmmim]Cl) is identified, and its effect on charge carrier mobility and lifetime under thermal stress in triple cation perovskite (Cs_0.05 MA_0.05 FA_0.90 PbI₃ ) thin films is investigated. To explore the fundamental mechanisms limiting charge carrier mobility, temperature-dependent microwave conductivity measurements are performed. Different mobility behaviors across two temperature regions are revealed, following the power law T^m , indicating two different dominant scattering mechanisms. The low-temperature region is assigned to charge carrier scattering with polar optical phonons, while a strong decrease in mobility at high temperatures is due to dynamic disorder. The results obtained rationalize the improved stability of the [Dmmim]Cl-doped films and devices compared to the undoped reference samples, by limiting temperature-activated mobile ions and retarding degradation of the perovskite film.

Keywords: microwave conductivity; mobile ions; mobility; perovskite solar cell; stability.

Figure 1

Band gap and mobility characterization of the reference and [Dmmim]Cl‐doped perovskite and device performance. a) Tauc‐plots of the reference and [Dmmim]Cl‐doped perovskite used to determine the band gap of both thin films. The reference and the doped perovskites exhibit a band gap of 1.545 eV. b) Quantum yield φ times the sum of electron and hole mobility Σµ as a function of laser fluence and injected charge carrier density. The quantum yield φ decreases with increasing laser intensity due to the fast recombination of charge carriers. Mobility is extracted at the plateau obtained at low laser fluences, showing that the [Dmmim]Cl‐doped perovskite exhibits a slight increase in charge carrier mobility compared to the reference perovskite. c) J–V characteristics of the solar cells with and without the dopant, in forward and reverse scan direction. The average solar cell parameters are collected in a table in the inset. The concentration of the [Dmmim]Cl in the precursor is 0.5 mol %.

Figure 2

Temperature‐dependent charge carrier mobility and transients. a) Charge carrier mobility at different temperatures is deduced from TRMC measurements. At low temperatures the slope of m = −0.5 indicates the scattering of charge carriers on optical phonons, whereas the slope of m = −2.0 at higher temperatures can be assigned to dynamic disorder. Since both curves coincide, the [Dmmim]Cl dopant does not seem to effect mobility. b) Corresponding transients to the mobility measurement in (a) for the reference perovskite, excited with a laser intensity of 3.6 × 10¹¹ photons per cm² which corresponds to a charge carrier density of 6.5 × 10¹⁵ cm⁻³.

Figure 3

Investigations of long‐term and thermal stability, including mobility, PLQY and OCVD measurements and PCE tracking. Same color coding for all Figures. a) Change in mobility over 72 days, revealing a slight increase in mobility for the [Dmmim]Cl doped perovskite compared to the reference. The inset shows the corresponding transients, which exhibit reduced recombination over time for both samples. b) PLQY for both samples after thermal annealing at 360 K. The PLQY of the reference thin film decreases quickly, whereas the doped perovskite resists the thermal stress. c) Normalized PCE of the reference and doped device stored at 60 then 85 °C. For higher temperatures, [Dmmim]Cl reduces degradation due to thermal stress. d) Mobile ion density in the perovskite device was measured with OCVD. The graph displays the mobile ion density at room temperature after different annealing steps with increasing temperature. The reference exhibits a gradual increase of mobile ion density at room temperature after the solar cell has been annealed at 330 K.