Theoretical study of time-resolved luminescence in semiconductors. III. Trap states in the band gap

Abstract

In the third part of this series, we study the influence of trap states in the band gap of semiconductors on the time-resolved luminescence decay (TRL) after a pulsed excitation. The results based on simulations with Synopsys TCAD® and analytical approximations are given for p-doped Cu(In,Ga)Se2 as a working example. We show that a single trap can be mostly described by two parameters which are assigned to minority carrier capture and emission. We analyze their influence on the luminescence decay and study the difference between a single trap and an energetic Gaussian trap distribution. It is found that trap states artificially increase the TRL decay and obscure the recombination dynamics. Thus, there is a demand for experimental methods which can reveal the recombination of minority carriers in a TRL experiment without trapping effect. In this regard, a variation of the device temperature, the excitation frequency, the injection level, as well as a bias illumination may be promising approaches. We study these methods, discuss advantages and disadvantages, and show experimental TRL for prove of concept. At the end, we validate our approach of simulating only band-to-band radiative recombination although photoluminescence spectra often exhibit free-to-bound radiative recombination of charge carriers.

Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials

“…Intensity-dependent minority carrier lifetimes consistent with SRH recombination have been reported for CIGSe. [45] Alternatively, an exponential decay of Δn(t) can be the result of minority carrier detrapping, [27] detailed in Section 2.5.…”

“…Detailed theoretical and experimental discussion regarding the role of various absorber properties and measurement conditions on the PL decay signal are reported by Maiberg et al, [2,26,27] Ahrenkiel, [1,28] and Kuciauskas et al [4] First, carrier redistribution processes can dominate the early decay signal, as illustrated in Figure 1a. As generation follows a Beer-Lambert absorption profile, carrier diffusion will occur during initial times following the excitation pulse; this process may include energetic relaxation of carriers due to gradients in the absorber or due to potential (i.e., electrostatic or band gap) fluctuations, as illustrated.…”

“…Following carrier redistribution, radiative and nonradiative recombination in addition to properties such as surface recombination, carrier drift in an electric field, absorber inhomogeneity, material degradation, and minority carrier trapping can have a significant impact in the measured signal. [1,2,27,[29][30][31][32] An illustration of various process which influence the TRPL decay is shown in Figure 1b. Further details regarding the role of processes shown in Figure 1 in relation to the PL decay are discussed throughout the text.…”

Discriminating between surface and bulk recombination in organic solar cells by studying the thickness dependence of the open-circuit voltage

“…In many cases, it may be possible to identify the dominant recombination mechanism, i.e. Shockley-Read-Hall, radiative or Auger, from an analysis of luminescence yield 12 , luminescence transients 13,14 , ideality factor [15][16][17][18][19] , doping level 17,20,21 , and numerical simulations 22,23 . However, distinguishing between recombination at the interface between absorber and electrodes (surface recombination) and recombination in the bulk of the absorber (bulk recombination) can be extremely challenging 24,25 .…”

Chalcopyrite compound semiconductors for thin film solar cells

“…Only recently it was pointed out that the decay time of the photoluminescence signal gives information on the photocarrier lifetime only in special cases. In particular the measured decay time can be much longer than the lifetime due to trapping and detrapping effects [93]. The influence of detrapping effects can be detected by the temperature dependence of the decay behaviour [94].…”