Exciton Formation Dynamics and Band‐Like Free Charge‐Carrier Transport in 2D Metal Halide Perovskite Semiconductors

Abstract

Metal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high‐efficiency photovoltaic applications. Low‐dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge‐carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature‐dependent measurements of free charge‐carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in‐plane mobilities. Enhanced charge‐phonon coupling is shown to reduce charge‐carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge‐carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge‐carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature‐dependent interplay of exciton and free‐carrier populations in 2D MHPs. Furthermore, such sustained free charge‐carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.

Untangling free carrier and exciton dynamics in layered hybrid perovskites using ultrafast optical and terahertz spectroscopy

“…For excitation at 410 nm, it can be assumed that j ≈ 1 at early times since the excitation energy is well above the excitonic absorption feature and an equilibrium between excitions and free carriers has not yet been established [20]. For (PEA) 2 PbI 4 excited at 410 nm, we find the effective charge-carrier mobility to be 5.8 ± 0.9 cm 2 V −1 s −1 , consistent with other reports in literature [10,20,23]. Similarly, photoexciation at 450 nm yeilds a charge-carrier mobility value of 5.4 ± 0.2 cm 2 V −1 s −1 An effective charge-carrier mobility of 1.9 ± 0.5 cm 2 V −1 s −1 is estimated from the decay traces following resonant exciation to the excitonic states at 514 nm.…”

“…To confirm that we are only observing free-charge carriers and not excitons, photoconductivity spectra between 0.5-3.5 THz were obtained at excitation wavelengths of 410 and 514 nm (see SI, figure S6). In both cases, the real and imaginary photoconductivity components have a free charge-carrier (Drude-like) response that has been observed in the THz photoconductivity spectra of perovskites including MAPbI 3 and LPKs [23,55,68]. There is also no evidence of a resonance feature, confirming that we are likely probing only a freecarrier response.…”

“…This value is somewhat suprising considering that the freecarrier concentration should be much lower here and closer to its equilibrium value, so we would expect the charge-carrier mobility to be significantly lower. This discrepency between the expected mobility and the what is predicted by the Saha equation was also mentioned in the work by Motti et al and was attributed to a sustained, long-lived free charge-carrier population being present [23].…”

“…For context, solar cells under AM 1.5 conditions have lower carrier densities at 10 15 -10 16 cm −3 [21] but higher carrier densities of 10 15 cm −3 or greater are required for applications in LEDs and lasing [22]. Furthermore, the accuracy of the Saha equation is questioned for layered perovskites in a recent work Motti et al which found discrepencies between the free charge-carrier density predicted by the Saha equation and the experimentally obtained photoconductivity results [23].…”

Repairing humidity-induced interfacial degradation in quasi-2D perovskite solar cells printed in ambient air

“…Combined with the narrowed PL band tails, it could be confirmed that the suppressed interface defect favored the mitigation of energetic disorder near the band edge and shielded the carriers from nonradiative relaxation. 26,27…”