Two dimensional numerical simulations of carrier dynamics during time-resolved photoluminescence decays in two-photon microscopy measurements in semiconductors

Abstract

We use two-dimensional numerical simulations to analyze high spatial resolution time-resolved spectroscopy data. This analysis is applied to two-photon excitation time-resolved photoluminescence (2PE-TRPL) but is broadly applicable to all microscopic time-resolved techniques. By solving time-dependent drift-diffusion equations, we gain insight into carrier dynamics and transport characteristics. Accurate understanding of measurement results establishes the limits and potential of the measurement and enhances its value as a characterization method. Diffusion of carriers outside of the collection volume can have a significant impact on the measured decay but can also provide an estimate of carrier mobility as well as lifetime. In addition to material parameters, the experimental conditions, such as spot size and injection level, can impact the measurement results. Although small spot size provides better resolution, it also increases the impact of diffusion on the decay; if the spot size is much smaller than the diffusion length, it impacts the entire decay. By reproducing experimental 2PE-TRPL decays, the simulations determine the bulk carrier lifetime from the data. The analysis is applied to single-crystal and heteroepitaxial CdTe, material important for solar cells, but it is also applicable to other semiconductors where carrier diffusion from the excitation volume could affect experimental measurements.

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

“…The first component of the decay τ 1,TRPL < 1 ns (in reported studies and in samples studied here) is attributed to drift in the space charge field of the junction, interface recombination, and bulk recombination. [ 26–28 ] In high‐efficiency solar cells drift rate is higher than recombination rate, which is evident from τ 1,TRPL increase to 2.2 ns at higher injection (Figure 3a) due to screening of the pn junction field by photogenerated carriers and resulting lower drift rate. From τ 1,TRPL ≈ 0.5 ns, we estimate charge carrier mobility μ = l τ 1,TRPL −1 E SCF −1 , where space charge field strength E SCF ≈ 6000 Vcm −1 and depletion width l ≈ 1.5 μm are from the equilibrium band diagram in Figure 1b.…”

Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

“…Second, because of the large L d , passivation at both contacts (front and back) becomes increasingly important, and the largest current limitation for CdTe solar cells appears to be recombination at the back contact (e.g., recombination velocity >10 5 cm s −1 for CdTe/ZnTe and CdTe/Te interfaces). Third, methods are needed to identify recombination loss locations in devices when L d exceeds absorber thickness, and such advanced characterization will probably include electro‐optical and materials modeling.…”

Charge Carrier Lifetime Fluctuations and Performance Evaluation of Cu(In,Ga)Se2 Absorbers via Time‐Resolved‐Photoluminescence Microscopy

“…Hence, GB recombination cannot be assessed from the second slope of the decay, being mainly representative of bulk recombination, in agreement with previous literature. [ 20,22 ] On the contrary, initial times are more influenced by GB recombination.…”