Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence

Careful interpretation of time-resolved photoluminescence (TRPL) measurements can substantially improve our understanding of the complex nature of charge-carrier processes in metal-halide perovskites, including, for instance, charge separation, trapping, and surface and bulk recombination. In this work, we demonstrate that TRPL measurements combined with powerful analytical models and additional supporting experiments can reveal insights into the charge-carrier dynamics that go beyond the determination of minority-charge-carrier lifetimes. While taking into account doping and photon recycling in the absorber layer, we investigate surface and bulk recombination (trap-assisted, radiative, and Auger) by means of the shape of photoluminescence transients. The observed long effective lifetime indicates high material purity and good passivation of perovskite surfaces with exceptionally low surface recombination velocities on the order of about $10\mathrm{cm}/\mathrm{s}$. Finally, we show how to predict the potential open-circuit voltage for a device with ideal contacts based on the transient and steady-state photoluminescence data from a perovskite absorber film and including the effect of photon recycling.

Figure 1

Optical pump terahertz probe spectroscopy measurements of a ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{3-x}{\mathrm{Cl}}_x$ perovskite film. (a) Measured normalized conductivity change $\mathrm{\Delta }\sigma$ of the sample over delay time $t_{\text{pump}}$ (blue line). The black dotted line represents the modeled conductivity change. (b) Real and imaginary parts of the derived complex mobility $\mu$ for different probe frequencies $f$ at 1 ns after photoexcitation. The real part accounts for the sum of electron and hole mobility.

Figure 2

Normalized photoluminescence decays from measurements (open symbols) of a ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{3-x}{\mathrm{Cl}}_x$ perovskite film for different excitation fluences: (a),(c) on a semilogarithmic scale, while (b),(d) on a double-logarithmic scale. Solid lines represent global fits including radiative and trap-assisted recombination for the bottom row (w/o Auger) and Auger recombination for the top row (w/ Auger).

Figure 3

(a) Normalized photoluminescence intensity as a function of the injected excess-charge-carrier concentration $\mathrm{\Delta }n$ derived from solving the continuity equation. Open symbols represent experimental data for the indicated excitation fluences. The solid line shows the best fit from simulation. Furthermore, dashed lines illustrate regions where low level injection (LLI) and high level injection (HLI) conditions hold. Here, the dotted black line displays the excess-charge-carrier concentration under illumination corresponding to an intensity of one sun. (b) Bulk lifetimes of the individual recombination processes: SRH, Auger, and radiative recombination without (solid line) and with corrections due to photon recycling (diamonds). The black curve shows the effective bulk lifetime ${\tau }_{\text{bulk}}$. (c) Apparent internal luminescence quantum efficiency $Q_i^{\text{lum}}$ (solid line) and corrected for photon recycling (diamonds).

Figure 4

Surface lifetime ${\tau }_s$ as a function of the layer thickness $d$ for a given mobility of $20{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and equal surface recombination velocities $S$ at the two surfaces of the sample. The blank area in the lower right corner cannot be occupied even by extreme high surface recombination velocities because charge-carrier diffusion towards the surfaces becomes the limiting process in this region and, thus, is defining the boundary. The dotted black line indicates the possible surface lifetimes for the observed layer thickness of $d=311\mathrm{nm}$. The marked origin reflects the lower limit in the case of no present defect-related recombination in the bulk, when ${\tau }_s$ is simply given by the obtained SRH lifetime ${\tau }_{\mathrm{SRH}}\approx 950\mathrm{ns}$ upon one-sun illumination.

Figure 5

Photoluminescence spectrum of a ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_{3-x}{\mathrm{Cl}}_x$ perovskite thin film (black open circles, left $y$ axis). The black line represents a fit to determine the temperature of the sample. Absorption coefficients (red lines and open squares, right $y$ axis) are obtained by photothermal deflection spectroscopy (${\alpha }_{\mathrm{PDS}}$) as well as extracted from the photoluminescence spectrum (${\alpha }_{\mathrm{PL}}$) by applying the generalized Planck radiation law.