Electrostatics govern both charge extraction and recombination in solar cells and are conventionally discussed in terms of doping densities, built-in fields, and band alignments. In perovskite solar cells, however, additional electrostatic mechanisms can dominate device operation.

Voltage-dependent PL measurements reveal that substantial charge-carrier densities can remain in the perovskite absorber even under external short-circuit conditions, causing recombination losses during extraction and reducing the extracted current.¹ Possible origins include low charge-transport-layer mobilities, extraction barriers, but also electric-field screening by redistributed mobile ionic species of the perovskite layer. Since conventional voltage-dependent PL measurements are typically performed under slow JV-scan conditions, mobile ions can approach a quasi-steady-state distribution and screen the internal field. Fast hysteresis measurements, in contrast, distinguish this ionic steady state from an ion-frozen regime, thereby revealing ion-induced current losses.² Here, I present scan-speed and voltage-dependent PL by recording the PL signal with a photodetector during the fast hysteresis scan. This approach separates electronic extraction losses from ionic current losses associated with field screening, indicating a high density of mobile ions.

Even though lead-halide perovskites exhibit very low equilibrium doping densities in the dark, electrostatic effects may also play an important role in recombination through the charge-neutrality condition. Several studies have suggested that illumination can induce an imbalance between free electron and hole densities, referred to as photodoping.^3–5 Here, I show that the excitation-intensity dependence of steady-state PL in perovskite films yields ideality factors between 1 and 2 across the investigated range. In the absence of contact-induced electrostatics, this behavior is consistent with photodoping mediated by a high density of trap states, leading to an effective ideality factor close to 1.5 over a wide range of excitation conditions.

The simultaneous requirement of high trap-state densities for photodoping and high mobile-ion densities for efficient field screening raises the question of whether these defect populations are physically connected. Do recombination and charge extraction in perovskite solar cells ultimately need to be described within a unified framework based on mobile defect centers?

1.           Akel, S. et al. Adv. Energy Mater. 14, 2401800 (2024).

2.           Thiesbrummel, J. et al. Nat. Energy 9, 664–676 (2024).

3.           Stranks, S. D. et al. Phys. Rev. Appl. 2, 1–8 (2014).

4.           Feldmann, S. et al. Nat. Photonics 14, 123–128 (2020).

5.           Yuan, Y. et al. Nat. Mater. 2024 233 23, 391–397 (2024).