1. Introduction
Over the last few decades, solar cells have become a key technology due to their ability to convert sunlight into electrical energy. Silicon-based solar cells are already established in the marketplace, but their costly and tedious fabrication motivates the examination of alternative systems. Dye-sensitized solar cells (DSCs) are a promising solution. The sandwich-type DSC structure is easy to manufacture with a broad variety of sensitizers and offers lower-cost devices. The photoanode typically consists of a fluorine-doped tin oxide (FTO)-coated glass substrate with a layer of a sintered nanoparticulate semiconductor (usually TiO2 in n-type DSCs), onto which a sensitizer is adsorbed.
The high scientific interest in DSCs has resulted in the development of techniques for measuring their performance. The necessity for optimized and reproducible measurements is critical. The most broadly used characterization methods for DSCs are current density–voltage plots (
J–
V curves), external quantum efficiency (EQE) measurements, and electrochemical impedance spectroscopy (EIS). The solar-to-electrical energy conversion efficiency (
η) is a solar-irradiance-dependent unit, which can be written as Equation (1) [
1]:
where
η is presented as a function of the short-circuit current density (
JSC), the open-circuit voltage (
VOC), the fill factor (
ff), and total incident solar power of the cell (
PIN). Due to the relationship between
η and
PIN, the correct characterization of DSCs depends not only on the calibration of the irradiance to a standard 1 Sun at an air mass (AM) of 1.5, but also on the active area of the DSC, which needs to be smaller than the total surface of the area of TiO
2 [
2,
3]. This is achieved by masking the cell, such that only a small circular region of a functionalized semiconductor is irradiated. Use of unmasked cells without a set irradiance area results in an overestimation of
η.
Incident photon-to-current conversion efficiency (IPCE) or EQE is the spectral response of a solar cell to the light in terms of current [
4]. The IPCE can be calculated according to Equation (2) [
4]:
For accurate measurements, factors including the calibrated light at 1 Sun, cell masking, and temperature control for long measurements have to be considered.
Electrochemical impedance spectroscopy is a well-known technique used for studying electrical properties of different materials [
5,
6]. A DSC has a complicated structure involving multiple interfaces, at which many electronic processes take place simultaneously. In contrast to resistance, impedance is not limited to one circuit element and can therefore be used to describe a system as a more complex and general circuit. EIS is based on the application of a small alternating current (AC) perturbation (
Ṽ) at a fixed frequency (
f) over a sample in equilibrium at stationary bias [
7]. The impedance at a specific frequency is denoted as
Z(f) and is presented in Equation (3):
where
Ĩ is an AC modulation. The impedance can be seen as a frequency-dependent differential resistance of the
I–
V curve, because of the small amplitude of the AC voltage modulation (
Figure 1) [
8]. For the full impedance spectrum, a measurement is made in a frequency range from hertz (Hz) to kilohertz (kHz).
EIS results are typically presented in Nyquist and Bode plots, of which the fitting results in parameters including the series resistance (R
s), the resistance (R
Pt), and the capacitance (C
Pt) of a counter electrode, the recombination resistance (R
rec), the chemical capacitances (C
μ), and the diffusion resistances of charge carriers in an electrolyte (Ws). Impedance plots include the real (Z’) and imaginary (Z’’) parts. Classically, a Nyquist plot consists of three semicircles at an open circuit potential (
Figure 2a). The high-frequency region before the beginning of the curve depicts the series resistance. The first semicircle at high
f is associated with the counter electrode, the second semicircle is associated with the semiconductor–electrolyte interface, and the last semicircle at low
f is associated with the diffusion of the electrolyte. The resistance value can be estimated by the width of the arc along the abscissa [
9]. The Bode plot provides an important representation of resistances from the plateaus (
Figure 2b). The electron lifetime (τ) is a critical parameter, which is inversely proportional to the maximum frequency (
fmax), and can be extracted from the Bode plot [
10].
EIS spectra can be recorded with different irradiation intensities, circuit conditions, and frequency ranges. Under carefully chosen conditions, a significant number of essential processes can be distinguished according to the spectral shapes of an impedance response, including electron transport in TiO
2, electron recombination at a TiO
2–electrolyte interface, and charge transfer at a counter electrode [
11]. Data extracted from
J–
V curves can be explained in more detail with the help of EIS. For example, EIS parameters including the recombination resistance, chemical capacitance, transport resistance, and diffusion length contribute to the value of
JSC for a DSC. This allows for a more precise explanation about the limiting factors of the DSC performance. Many studies have been reported in order to investigate the correct interpretation of EIS results [
12,
13,
14,
15,
16,
17].
On the other hand, to the best of our knowledge, there is no discussion in the literature of how reproducible EIS results for DSCs are. Usually, published data refer only to one measured cell. It is a common, but by no means universal, practice for current density–voltage measurements to be presented in the literature for two or more cells. A reproducibility study of DSCs with the standard dye N719 showed only a small deviation in
η of 5.76 ± 0.14% [
18]. In order to broaden this investigation to gain insight into the reproducibility of EIS measurements for DSCs, we performed the impedance analysis of DSCs functionalized with two commercially available dyes, N719 and SQ2. The Nyquist profiles are considerably different for N719 and SQ2 (
Figure 3) [
19,
20,
21], and in this paper, we examine the impedance reproducibility for DSCs sensitized with a metal complex and an organic dye.