Resonant free-carrier absorption in 2D hybrid organic-inorganic perovskites: The Rashba effect or small polarons?

The Rashba effect, which shifts the conduction band extremum from wavevector k = 0 to vertical bar k vertical bar = k(0) not equal 0 with an energy gain of E-R, has been frequently invoked to explain outstanding optoelectronic properties in hybrid organic-inorganic perovskites (HOIPs). For two-dimensional (2D) layered HOIPs, the only direct experimental evidence of the Rashba effect to date is resonant free-carrier absorption (FCA), which has been attributed to direct optical transition between the Rashba subbands at vertical bar k vertical bar = k(0) not equal 0 with photon energy h omega = 4E(R). Here, we show that in layered perovskites, small polarons induced by a strong short-range coupling between electrons and optical phonons can also give rise to a resonant FCA at h omega = 4E(b), with E-b being the polaron binding energy. The Rashba effect- and small-polaron-induced resonant FCA responses can be distinguished via an applied electric field, which moves the resonance peak to a higher or lower photon energy in the former but splits the peak into two in the latter, suggesting that electric-field-modulated FCA can help prove and quantify the Rashba effect and/or small polarons in layered perovskites. Published under license by AIP Publishing.