Measurement of Relaxation Time of Excess Carriers in Si and CIGS Solar Cells by Modulated Electroluminescence Technique

Abstract

Excess carrier lifetime plays a crucial role in determining the efficiency of solar cells. In this paper, we use the frequency dependence of inphase and quadrature components of modulated electroluminescence (MEL) to measure the relaxation time (decay) of excess carriers. The advantage of the MEL technique is that the relaxation time is obtained directly from the angular frequency at which the quadrature component peaks. It does not need knowledge of the material parameters like mobility, etc., and can be used for any finished solar cells which have detectable light emission. The experiment is easy to perform with standard electrical equipment. For silicon solar cells, the relaxation time is dominated by recombination and hence, the relaxation time is indeed the excess carrier lifetime. In contrast, for the CIGS solar cells investigated here, the relaxation time is dominated by trapping and emission from shallow minority carrier traps.

Correlation between Charge Transport Length Scales and Dielectric Relaxation Time Constant in Hybrid Halide Perovskite Semiconductors

“…Khatavtar et al used a frequencydependent modulated electroluminescence technique to determine the relaxation time (τ d ) of Si and CIGS-based solar cells. 48 Here, the frequency response of I ph is used to determine the τ d of the photogenerated charge carrier in pristine and passivated MAPI-based PSCs. The τ d basically represents the response time of the system, and it is normally presented as Debye relaxation expression, if the relaxation of photogenerated charge carrier can be characterized by a single τ d .…”

“…The τ d basically represents the response time of the system, and it is normally presented as Debye relaxation expression, if the relaxation of photogenerated charge carrier can be characterized by a single τ d . 46,48 However, in our case, we used the Havriliak−Negami (H−N) relaxation expression to fit the experimental data because of asymmetry and broadness in frequency-dependent I ph response. 49 The H−N relaxation is a modification in the Debye relaxation expression and is given below…”

“…Because the difference in the I ph ( f ) maximum between the two devices is relatively small, we carried out this experiment on several devices in order to confirm that the two devices have distinct maximum value of I ph ( f ). Khatavtar et al used a frequency-dependent modulated electroluminescence technique to determine the relaxation time (τ d ) of Si and CIGS-based solar cells . Here, the frequency response of I ph is used to determine the τ d of the photogenerated charge carrier in pristine and passivated MAPI-based PSCs.…”

Defects characterization in thin films photovoltaics materials by correlated high-frequency modulated and time resolved photoluminescence: An application to Cu(In,Ga)Se2

“…While ultra-short laser pulses are used in TRPL experiments to record fast mechanisms (transport and recombination), the use of a slower illumination variation allows probing carrier dynamics in a way closer to the real excitation conditions and also to focus on slow mechanisms such as carrier detrapping. A trial has already been performed successfully by using modulated electroluminescence (MEL) on CIGS finished cells for trap detection [10], but on an opposite way, the maximum modulation frequency was limited at 200 kHz which might not be sufficient to probe fast carriers' dynamics as it happens in thin films materials with direct bandgap. Based on previous experiences in modulated techniques [3,11] we developed a HF-MPL setup extending the frequency range up to 10 MHz.…”

“…Furthermore, the mathematical background for data treatment of modulated luminescence [10] usually comes from the field of silicon crystalline wafers using the notion of effective lifetime. This is unclear in case of trapping events.…”