Abstract
Perovskite solar cells (PSCs), which utilize perovskite-structured material as the active layer, have demonstrate remarkable power conversion efficiency. A solar cell must have a built-in electric field suitable for separating and extracting photo-generated electrons and holes in the active layer for their highly efficient operation. However, the band structures at the interface and carrier dynamics in PSCs remain insufficiently understood. Recently, we evaluated the surface state of perovskite solar cells using terahertz (THz) emission spectroscopy (TES) that measured the waveforms of THz emissions from the samples excited by femtosecond laser pulses [1]. In this study, the samples were prepared by sequentially spin coated MeO-2PACz (hole transport layer) and Cs0.05(FA0.76MA0.24)0.95Pb(I0.76/Br0.24)3 (perovskite layer) on ITO (transparent electrode), and introduced the IPA solution as a passivating agent at the interface of the perovskite layer for one sample, while the other was prepared without IPA solution. The effect of the IPA solution on the photocarriers was investigated using TES with these two kinds of samples. The photocarriers in the samples were photoexcited using a femtosecond laser as illustrated in Fig. 1(a). The photocarriers were accelerated by the built-in electric field at the interface and by internal diffusion, generating a transient current that produced THz waves. The THz waves were measured using an optical pump-probe technique to investigate the photoexcitation process in the picosecond time domain. As shown in Fig. 1(b), THz waves were observed from both samples, and were in phase with the signals from InAs. This suggests that the THz emission results from the photo-Dember effect, which is caused by the difference in diffusion velocities of photo-generated electrons and holes in the vicinity of the surface, leading to an ultrafast diffusion current. Additionally, as observed around 22 ps, the passivated sample exhibited faster relaxation of THz waves compared to the non-passivated sample, indicating that passivation may accelerate the relaxation of the current. Our results suggest that TES provides valuable insights into the role of interface passivation in enhancing the carrier dynamics and overall efficiency of PSCs.
References
[1] T. Mochizuki et al., Photonics 9, 316 (2022).
