Combining Modulated Techniques for the Analysis of Photosensitive Devices

Abstract

niques involve optical perturbations. [9,10] IS has been used to analyze transport and recombination processes together through the response of the device to an electrical perturbation. [11,12] IMPS and IMVS techniques have been widely applied to study dye-sensitized solar cells, [13,14] photoelectrodes for water oxidation, [15][16][17] and recently are gaining attention in the field of perovskite solar cells (PSCs). [18,19] The analysis of the data obtained with these techniques is generally made through the characteristic time constants. [20][21][22] While such an analysis is very useful in certain cases, this strategy limits the amount of information that can be obtained from these techniques. With regard to IS, experimental data are usually analyzed by modeling the internal electrochemical processes of the device using an equivalent circuit model (EC), [23] built from passive electrical elements such as resistances, capacitances, and inductances. [24][25][26] These parameters in turn can provide key information regarding the nature of the operation of the device and its limitations. However, the choice of the correct EC could be a difficult task, because several ECs can reproduce the same experimental spectra. Therefore, there exists a need to combine the analysis methods of these three techniques to overcome their individual limitations and extract the maximum knowledge from the experimental data.Before discussing how these techniques are related, we present a scheme of a photosensitive device showing the basic light to electricity conversion mechanism in Figure 1. In these systems, when a photon flux (f) reaches the device, part of these photons can be absorbed, generating electronhole pairs (represented by absorbed current, j a ). The pairs that do not recombine and are successfully separated, provide the free-photogenerated current (j ph ) in the light absorber material. Part of this current is lost as a recombination current (j rec ), and the remaining current ( j e = j ph − j rec ) is extracted at the external contacts of the device. j rec , and thus j e , are functions of the external voltage (V e ).In Figure 2, the basic operation of IS, IMPS, and IMVS is shown. Figure 2b-d shows the excitation and measurement signals involved in IS, IMPS, and IMVS, respectively. Note that all variables with a tilde (  ) indicate AC perturbations, while variables with overbars ( ) represent steady-state signals. Thus, the steady-state at which the measurements are performed is defined by φ j , , eV and e j . As shown in Figure 2b, for IS (in potentiostatic mode), an AC small-perturbation in voltage Small-perturbation techniques such as impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) are useful tools to characterize and model photovoltaic and photoelectrochemical devices. While the analysis of the impedance spectra is generally carried out using an equivalent circuit, the intensity-modulated spectroscopies are often analyzed th...

Recent Progress of Light Intensity‐Modulated Small Perturbation Techniques in Perovskite Solar Cells

“…. [18] Please note that either one of the quantities, that is, photocurrent or photovoltage, can be negative. Here, the photovoltage is taken negative.…”

“…QðωÞ . [18] More details of this relation and the electrical EC for the same are discussed in Section 2.2.…”

“…Therefore, the transfer function of IMVS can be written as . [ 18 ] Please note that either one of the quantities, that is, photocurrent or photovoltage, can be negative. Here, the photovoltage is taken negative.…”

“…The relation of EIS, IMPS, and IMVS can be expressed as . [ 18 ] More details of this relation and the electrical EC for the same are discussed in Section 2.2.…”

“…e) General EC and its simplification to EIS, IMPS, IMVS, and steady-state condition. Reproduced with permission [18]. Copyright 2021, Wiley VCH.…”