Contactless Determination of Optimal Chloride Concentration for Power Conversion Efficiency in CH₃NH₃Pb(Cl,I)₃ Using Photoluminescence Spectroscopy

Abstract

We applied room-temperature photoluminescence (PL) spectroscopy for the compositional engineering of a CH₃NH₃Pb(Cl,I)₃ light harvester in an alloy-based perovskite solar cell. This spectroscopic characterization determines the optimal Cl concentration where the power conversion efficiency shows its maximum in a contactless and non-destructive manner. The PL quenching ratio evaluated from the comparative PL studies between the films grown on glass/ZrO₂ and SnO₂:F/TiO₂ substrates exhibited its maximum at a Cl concentration of 10 mol%, which agrees with the Cl concentration determined from the current–voltage measurement-based device performance. We also discuss the possible reasons for the coincidence mentioned above regarding the charge extraction effect induced by Cl incorporation.

1. Introduction

Much attention has been recently paid to an inorganic–organic lead halide perovskite CH₃NH₃PbI₃ (MAPbI₃) as a new class of light harvester in a solar cell [1,2]. These perovskite materials demonstrate rapid progress in their performance as a solar cell, the central part of which is owing to their material properties such as their direct nature in the interband transition, high absorption coefficient, and high charge carrier mobility [3,4,5]. Furthermore, since their first discovery in 2009 [6], the power conversion efficiency (PCE) is now around 25%, exceeding that of other emerging photovoltaic technologies such as dye-sensitized solar cells and organic photovoltaics [7,8,9]. These properties and the possibility of flexible synthesis have made halide perovskites promising for next-generation photovoltaics [10,11,12,13].

To enhance the PCEs of the device, many groups have proposed the mixing of halide anions (X = Br, Cl) in the MAPbI₃ structure [1,14,15]. Chloride ions have positive roles in manufacturing high-quality perovskite films. For example, both diffusion lengths for electrons and holes of mixed halides are larger than those of iodide [16,17].

Despite the enormous profits implemented by chloride addition, the question related to an optimal ratio is still a topic that is under debate. Throughout this paper, one defines the term ‘optimal ratio’ as the Cl concentration showing the highest PCE in the perovskite-based MAPb(Cl,I)₃. Although some research groups proposed that the optimal ratio should be around 0.33 [18], a look through the literature has revealed that the optimal ratio depends on the device preparation methods and growth conditions [11,19]. In other words, the determination of the optimal ratio requires the preparation and characterization of many devices with different Cl concentrations. This could indeed be time-consuming because electrical contact processes are needed to evaluate the PCE.

Among well-established optical techniques, photoluminescence spectroscopy (PLS) is a non-contact characterization tool. This technique is suitable for materials intended for optoelectronic applications. So far, there have been several reports on the PL properties of MAPb(Cl,I)₃ films with charge extractors [17,19,20], attributing their observed PL quenching to the charge extraction from the light harvester. Because it is well known that charge extraction plays an essential role in PCE determination, we reached a conjecture that this technique can be used as a contactless and non-destructive determination tool for the optimal ratio. Furthermore, the PL measurement does not require electrical contact on the samples, which expectedly enables rapid characterization by applying, e.g., a composition-spread technique [21]. For the verification, we characterized and compared PL properties and current–voltage (J–V) characteristics by preparing several MAPb(Cl,I)₃ films whose Cl concentration ranges from 0 to 25 mol%.

After the introduction, the paper begins with a description of the methods for film preparation, spectroscopic characterizations, electrical characterizations, and theoretical calculations. It will then go on to the results and discussions related to the optical properties of our prepared films concerning device performance from the viewpoint of compositional engineering. Finally, we draw our conclusions.

2. Experimental and Calculation Procedures

We first deposited porous titanium oxide (mesoporous TiO₂) on the blocking layer (compact TiO₂). The preparation methods consisted of spin coating (dip-casting) and screen printing.

• Mesoporous oxide and absolute ethanol were mixed in a weight ratio of 2:7.
• We applied them using spin coating.
• We fixed the substrate in a spin coat.
• We drip-cast 0.2 mL of mesoporous oxide solution to spread throughout.

Then, we dried it at 125 °C. Furthermore, if the substrate does not have a porous layer, the perovskite can be formed only on an extremely thin film, so that the porous layer is necessary. Therefore, we focused on ZrO₂ as an insulative porous material. In the case of ZrO₂ as well, we drip-cast it.

We adopted the two-step solution strategy, whose detailed prescriptions were given elsewhere [12]. First, we created Pb(Cl,I)₂ layers. Then, we dissolved powders of lead iodide (PbI₂) 1.3 M and lead chloride (PbCl₂) in X mol% in a solvent composed of N,N-dimethylformamide, and dimethyl sulfoxide in a ratio of 9:1. If the addition of PbCl₂ was required, we added it when mixing the iodide solution. We drip-cast it on SnO₂:F (FTO) substrate, and then it was annealed at 100 °C and cooled to room temperature. Next, we dissolved methylammonium iodide in iso-propanol. We drip-cast it at 4000 rpm for 30 s for film formation, followed by annealing and cooling to room temperature. We measured the PL for MAPb(Cl,I)₃ films grown on glass/ZrO₂ and FTO/TiO₂. For J–V characteristics, we prepared a hole transport layer consisting of CuSCN as a top layer [22].

We used a monochromator and a charge-coupled device as a detecting unit and a 325 nm line from the HeCd laser as an excitation light source for the room-temperature PL measurements [23,24].

We performed an ab initio calculation using the Green function method-based package of Akai-KKR [25,26] to evaluate the electronic energy band structures and density of states distributions on CH₃NH₃PbI₃ and systems alloyed with CH₃NH₃PbCl₃ [27]. KKR indicates the initials of Korringa, Kohn, and Rostoker, who invented this method. The strength of the KKR method is the ability to take a coherent potential approximation often used for alloy systems into account [28,29]. We calculated them using a conventional unit cell whose lattice constants are a = b = c = 5.68 nm. In addition, we used the Perdew–Burke–Ernzerhof (PBE) functional, which belongs to generalized gradient approximation (GGA) functionals.

3. Results and Discussion

We evaluated the Cl concentration dependence of the PL spectra in MAPb(Cl,I)₃ films grown on (1) glass/ZrO₂ and on (2) FTO/TiO₂ [19,30,31]. The study of the first structures was intended to quantify the number of photocarriers absorbed in the light harvesting layer [32]. In this structure, ZrO₂, an insulator, was used instead of TiO₂ (a charge extractor) as a scaffold [33,34]. As will be explained later in detail, by evaluating the PL’s intensity ratio between the first and the second structures, hereafter, we aim to evaluate a quantity related to the charge extraction efficiency and hence the PCE.

Figure 1a,b show the spectra and integrated intensities of the PL in the film grown on glass/ZrO₂, respectively, as a function of the Cl concentrations. The central photon energy of the PL spectrum was about 1.61 to 1.59 eV for each Cl concentration [35]. The observed energy is in good agreement with the reported bandgap energy of MAPbI_3, ranging from 1.5 to 1.6 eV [36,37].

The central photon energy did not shift even with chloride addition [13]. Chloride addition tends to blueshift the bandgap energy because the bandgap energy of MAPbCl₃ is significantly larger than that of MAPbI₃ [37]. This observation is probably due to the cancellation of the bandgap energy shift with the carrier localization effect. It is well known that the bandgap fluctuation introduced by Cl incorporation induces the red-shifting carrier localization [38]. As can be understood from Figure 1b, the integrated PL intensity decreases as the Cl concentration increases, which is in good agreement with the ‘theoretical’ concentration dependence shown in Figure 2 [26]. Here, we plotted the density of states spectrally integrated between the gap energy and 3.5 eV, which is considered to be proportional to the number of photocarriers absorbed in the light harvesting layer.

Figure 3a,b show PL’s spectra and intensities, respectively, for the film grown on FTO/TiO₂ [39,40]. The PL is quenched, compared to the first samples’ cases shown in Figure 1b. This result is probably due to photocarriers’ charge extraction from the light harvesting layer into the electron transport layer [41]. Letting the integrated PL intensity of the first and second structures be I₁ and I₂, we define an ‘efficiency’ term hereafter as [(I₁ − I₂)/I₁] to evaluate the effect induced by replacing ZrO₂ with TiO₂. We plot this quantity as a function of the Cl concentration in Figure 3c. We notice an interesting concentration dependence. The maximum efficiency found itself at 10 mol% of chlorine, which is in good agreement with the PCE shown in

Table 1. Summary of device performance of CH₃NH₃Pb(Cl,I)₃ solar cells with different Cl concentrations obtained from photocurrent density–voltage (J–V) curves. The device parameters were from the data obtained in a forward scanning direction. It is noted that the hysteresis in the J–V curve is negligible.

Concentration	Jsc	Voc	FF	PCE
mol%	mA/cm2	V		%
0	6.8	0.81	0.56	7.7
2	8.3	0.87	0.60	9.7
10	20.7	0.92	0.62	12.0
25	9.6	0.90	0.60	10.6

[42,43]. We measured the current–voltage (J–V) curves in MAPb(Cl,I)₃ devices and summarized their parameters, such as PCE in Table 1. We can safely conclude that the ‘efficiency’ is intimately related to the charge extraction efficiency. Our results indicate that PL spectroscopy could determine the optimal ratio by evaluating the ‘efficiency’ because the charge extraction efficiency is known to be one of the principal components determining the PCE of solar cells. However, it is not good practice to draw a conclusion by using only one characterization tool. For further justification, we conducted a similar study with time-resolved differential absorption spectroscopy, which showed an identical concentration dependence to the case of PL spectroscopy [44,45].

Figure 4a,b are SEM images of CH₃NH₃PbI₃ and CH₃NH₃Pb(Cl,I)₃ with a Cl concentration of 25 mol% [14,46]. The particle size of CH₃NH₃PbI₃ of ca. 150 nm is significantly different from that of CH₃NH₃Pb(Cl,I)₃ (ca. 400 nm). A previous study insisted that photocarriers’ localization length scale is comparable to the length scale of the particles’ sizes. Because photocarriers’ localization length likely influences the PCE of the device, our observed change in the particles’ size may have some impact on the PCE. Further studies are necessary to clarify this issue.

4. Conclusions

The present research aimed to examine the usefulness of PL spectroscopy for the compositional engineering of a MAPb(Cl,I)₃ light harvester. The comparative investigation of the Cl concentration dependence of PL intensities for the films on glass/ZrO₂ and FTO/TiO₂ has shown its maximum at a Cl concentration of 10 mol%, which is in good agreement with that determined from J–V measurements. This spectroscopy can determine the optimal Cl ratio in a contactless manner, which will expectedly speed up the material development cycle.