Conformal Loading Effects of P3CT-Na Polymers on the Performance of Inverted Perovskite Solar Cells

Abstract

The conformal loading effects of P3CT-Na polymers on ITO/glass samples were investigated using different concentrations of P3TC-Na/water solution, which significantly influenced the device efficiency of the resultant inverted perovskite solar cells. The obtained water-droplet contact angle images, surface morphological images, photoluminescence spectra and X-ray diffraction patterns show that the hydrophilic moiety of the P3CT-Na polymers plays an important role in the conformal loading effects, thereby resulting in a smoother perovskite crystalline film due to the formation of merged grains. It is noted that the average power conversion efficiency increases from 14.83% to 17.27% with a decrease in the concentration of the P3CT-Na/water solution from 60 wt% to 48 wt%.

1. Introduction

Over the last ten years, the power conversion efficiency (PCE) of perovskite thin-film solar cells has rapidly increased to more than 20%, mainly due to the superior material characteristics of perovskite crystalline thin films and the relative ease of device fabrication [1,2,3,4,5,6,7,8]. Perovskite thin-film solar cells can be mainly classified into two types: regular perovskite solar cells and inverted perovskite solar cells. In regular perovskite thin-film solar cells, TiO₂ materials were widely used as the electron transport layer (ETL), which can result in a high PCE of 20% when a doped Spiro-OMeTAD organic thin film was used as the hole transport layer (HTL) [9,10,11,12]. In order to reduce the fabrication temperature from 450 °C to 200 °C, SnO₂ crystalline films were used to replace the anatase TiO₂ films as the ETLs for regular perovskite solar cells [13,14,15,16]. However, regular perovskite solar cells are difficult to integrate with silicon solar cells to form two-terminal tandem solar cells because of the relatively low conductivity of p-type transparent metal oxides [17,18,19,20]. Fortunately, the PCE of inverted perovskite thin-film solar cells can also be larger than 20%. On the other hand, inverted perovskite thin-film solar cells have been successfully integrated with silicon solar cells to form two-terminal tandem solar cells, resulting in a high PCE of 29.8% [21]. In inverted perovskite solar cells, the most widely used HTLs are conductive PEDOT:PSS polymer films [22,23,24,25], NiO_x inorganic films [26,27,28,29], CuO_x inorganic films [30,31,32], WO_x inorganic films [33,34,35], poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) polymer films [36,37,38,39], poly(3-(4-carboxybutyl)thiophene-2,5-diyl)-X (P3CT-X) polymer films (X: Na, K, Rb and Cs) [40,41,42,43] and p-type self-assembled monolayers (SAMs) [44,45,46,47]. Among these HTLs, P3CT-X and p-type SAM ultra-thin layers are also used as a hole modification layer (HML) to modify the surface work function of transparent anode electrodes (TCEs) [48,49]. The surface wettability of the modified TCEs also influences the formation of perovskite crystalline films. Our previous report showed that the higher regioregularity of P3CT-Na polymers can simultaneously result in the higher open-circuit voltage (V_OC), short-circuit current density (J_SC) and fill factor (FF) values of the resultant inverted perovskite solar cells, mainly due to the formation of larger perovskite grains [50]. Additionally, the wetting time of a perovskite precursor solution on P3CT-Na/ITO/glass substrates can largely influence the interfacial quality between the MAPbI₃ crystal film and the P3CT-Na polymer, which dominates the FF values of the resultant perovskite solar cells [51]. In other words, the loading effects of P3CT-Na polymers on TCEs (ITO or FTO) highly influence the device efficiency of inverted perovskite solar cells.

To explore the loading effects of P3CT-Na polymers on ITO/glass samples, the concentration of a P3CT-Na precursor solution was varied from 60 wt% to 24 wt%. Our experimental results show that the better conformal loading of P3CT-Na polymers on ITO/glass substrates results in a relatively smooth CH₃NH₃PbI₃ (MAPbI₃) crystalline film, thereby increasing the V_OC, J_SC and FF values simultaneously.

2. Material Preparation, Device Architecture and Characterization

The inverted perovskite solar cell contains a transparent ITO film, a P3CT-Na ultra-thin film, a MAPbI₃ film, a phenyl-C₆₁-butyric acid methyl ester (PCBM) film and an Ag film. After the fabrication of the PCBM thin films, the samples were treated with a BCP/IPA solution [52]. Each sample has four cells. The active area of each cell is 10 mm². The fabrication process of the inverted MAPbI₃ thin-film solar cells and the material preparation and characterization are described in the Supplementary Materials. Figure 1 displays the device architecture. To investigate the conformal loading effects of P3CT-Na polymers, the concentration of a P3CT-Na precursor solution was varied from 60 wt% to 24 wt%. Surface morphological images, water-droplet contact angle images, photoluminescence (PL) spectra, X-ray diffraction (XRD) patterns and transmittance spectra were measured in order to investigate the conformal loading effects of P3CT-Na polymers on the material properties of MAPbI₃ crystalline films.

3. Results

3.1. Photovoltaic Performance

Figure 2 displays the current density–voltage (J–V) curves of the MAPbI₃ solar cells. The device performance (V_OC, J_SC and FF values) is presented in Table 1. The series resistance and shunt resistance values were obtained by calculating the slopes of the fitting curves at the V_OC and J_SC points, respectively. When the concentration of the P3CT-Na precursor solution is decreased from 60 wt% to 48 wt%, the V_OC, J_SC and FF values simultaneously are increased, thereby improving the average PCE from 14.83% to 17.27%. Additionally, the V_OC, J_SC and FF values decreases from 1.001 V, 22.18 mA/cm² and 77.8% to 0.900 V, 19.95 mA/cm² and 68.4%, respectively, with a decrease in the concentration of the P3CT-Na precursor solution from 48% to 24%. In other words, the 48 wt% P3CT-Na precursor solution can be used to form the conformal loading of P3CT-Na polymers on ITO/glass samples, which effectively collects photo-generated holes from the MAPbI₃ crystalline film. It is noted that the trend of the J_SC values is inversely proportional to the trend of the series resistance (R_S) values, which can be used to evaluate the conformal loading efficiency of the P3CT-Na polymers. The higher J_SC values and lower R_S values correspond to the better conformal loading of the P3CT-Na polymers on the ITO/glass samples. Additionally, the better conformal loading efficiency of the P3CT-Na polymers also results in larger shunt resistance (R_SH) values, which is partially related to the carrier recombination at the MAPbI₃–P3CT-Na interface. Figure S1 displays the J–V curves of the MAPbI₃ solar cells in different scanning directions. The V_OC hysteresis (J_SC hysteresis) increases from 0.010 V (0.23 mA/cm²) to 0.016 V (0.48 mA/cm²) with a decrease in the concentration of the P3CT-Na precursor solution from 48 wt% to 24 wt%, which means that the conformal loading efficiency of the P3CT-Na polymers on the ITO/glass samples influences the formation of defects in the MAPbI₃ crystalline thin films, thereby dominating the device efficiency of the solar cells.

3.2. Characterization and Discussion

The atomic force microscopic (AFM) images of the ITO/glass samples and the P3CT-Na-coated ITO/glass samples are plotted in Figure 3. The surface roughness (Rq) values of the ITO/glass samples, 24 wt% P3CT-Na-coated ITO/glass samples, 36 wt% P3CT-Na-coated ITO/glass samples and 48 wt% P3CT-Na-coated ITO/glass samples are 5.49 nm, 5.67 nm, 5.72 nm and 5.67 nm, respectively, which indicates that the thickness values of the P3CT-Na layers are far smaller than the R_q values of the ITO thin films. Additionally, the transmittance spectra of the four P3CT-Na/ITO/glass samples overlap, which can be used to confirm the ultra-thin P3CT-Na layers (see Figure S2 in the Supplementary Materials). When the ITO/glass samples were coated with the 60 wt% P3CT-Na precursor solution, the Rq values significantly decreased from 5.49 nm to 4.91 nm. Additionally, the island-like surface feature (see Figure 3a) becomes a blurred feature in the AFM image (see Figure 3e), which means that the rough ITO surface can be filled by the multi-layered P3CT-Na polymers. The water-droplet contact angle images of the P3CT-Na-coated ITO/glass samples are plotted in Figure 4. It is noted that the small differences between the contact angles are reasonable because the contact surfaces are P3CT-Na polymers. The trend of the contact angle values is inversely proportional to the trend of the PCE values (see Table 1). This trend can be used to understand that the better conformal loading of the P3CT-Na polymers results in smaller contact angles (i.e., better wettability), thereby increasing the interfacial quality between the MAPbI₃ crystal and the P3CT-Na polymer, which can be used to explain the lower R_S values and the higher R_SH values.

In order to explore the conformal loading effects of the P3CT-Na polymers on the formation of MAPbI₃ crystalline films, the AFM images, XRD patterns and PL spectra were measured (see Figure 5 and Figure 6). When the concentration of the P3CT-Na precursor solution is higher than 36 wt%, the trend of the Rq values of the MAPbI₃ crystalline films is proportional to the trend of contact angle values of the P3CT-Na/ITO/glass samples (see Figure 4). It is noted that the smaller Rq value correspond to a burled image, as shown in Figure 5b. In other words, the better conformal loading of the P3CT-Na polymers results in better interfacial contact between the MAPbI₃ crystal and the P3CT-Na polymer, which forms shallower depths between the MAPbI₃ grains. The better contact quality at the MAPbI₃–P3CT-Na interface can be used to explain the higher collection efficiency of the photo-generated holes, thereby resulting in the largest J_SC value (see Table 1). Additionally, the mean grain sizes of the MAPbI₃ crystalline films increases from 330 nm to 422 nm with a decrease in the concentration of the P3CT-Na precursor solution from 60 wt% to 24 wt%, which means that the lower density of the P3CT-Na polymers on the ITO/glass samples results in fewer nucleation sites during the formation of the MAPbI₃ crystalline films, thereby forming the larger grains (see Figure S3 in the Supplementary Materials). The trend of the PL intensities (see Figure 6a) is inversely proportional to the trend of the mean MAPbI₃ grain sizes, which indicates that lower PL intensities are due to stronger PL quenching from defects at the grain boundaries in the top region of the MAPbI₃ crystalline films. The emission peaks of the PL spectra are fixed at a wavelength of 770 nm, which means that the light emissions are mainly from the inner region of the MAPbI₃ grains. Figure 6b displays the main XRD patterns of the MAPbI₃/P3CT-Na/ITO/glass samples. When the concentration of the P3CT-Na precursor solution decreases from 60 wt% (48 wt%) to 48 wt% (24 wt%), the main XRD peak increases (decreases) from 14.038° (14.052°) to 14.052° (14.031°). The larger XRD peak corresponds to a shorter d spacing, which means that there is compressive stress in the MAPbI₃ crystalline film when the concentration of the P3CT-Na precursor solution is 48 wt%. In other words, the crystal growth in the MAPbI₃ crystalline films is most influenced by the P3CT-Na-coated ITO/glass samples when the concentration of the P3CT-Na precursor solution is 48 wt%.

4. Conclusions

Our experimental results show that the concentration of the P3CT-Na precursor solution largely influences the device performance of P3CT-Na HTL-based MAPbI₃ solar cells. When the concentration of the P3CT-Na/water solution is 48 wt%, the V_OC, J_SC and FF of the solar cells simultaneously increase to the highest values. The AFM images, water-droplet contact angle images, PL spectra and XRD patterns show that the conformal loading efficiency of the P3CT-Na polymers on the ITO/glass samples strongly influences the surface, structural and excitonic properties of the MAPbI₃ crystalline films, which explains the trends of the V_OC, J_SC and FF values.

Author Contributions

Conceptualization, S.H.C.; methodology, Q.B.K. and J.-R.W.; validation, Q.B.K. and J.-R.W.; formal analysis, Q.B.K. and S.H.C.; data curation, A.C., S.-E.C., C.-C.L. and S.-Y.L.; writing—original draft preparation, S.H.C. and J.-R.W.; writing—review and editing, S.H.C.; supervision, S.H.C.; funding acquisition, S.H.C. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. A top view and cross-sectional view of the device architecture.

Figure 2. The current density–voltage curves of the P3CT-Na HTL-based MAPbI3 solar cells under one sun illumination (100 mW/cm2, AM 1.5G). The concentration of the P3CT-Na precursor solution was varied from 60 wt% to 24 wt%.

Figure 3. The atomic force microscopic images: (a) ITO/glass; (b) 24 wt% P3CT-Na/ITO/glass; (c) 36 wt% P3CT-Na/ITO/glass; (d) 48 wt% P3CT-Na/ITO/glass; (e) 60 wt% P3CT-Na/ITO/glass; (f) schematic diagram.

Figure 4. The water-droplet contact angle images: (a) 60 wt% P3CT-Na/ITO/glass; (b) 48 wt% P3CT-Na/ITO/glass; (c) 36 wt% P3CT-Na/ITO/glass; (d) 24 wt% P3CT-Na/ITO/glass.

Figure 5. The atomic force microscopic images of the MAPbI3/P3CT-Na/ITO/glass samples: (a) 60 wt% P3CT-Na; (b) 48 wt% P3CT-Na; (c) 36 wt% P3CT-Na/; (d) 24 wt% P3CT-Na.

Figure 6. The excitonic and crystal structures of the MAPbI3/P3CT-Na/ITO/glass samples: (a) photoluminescence spectra; (b) main X-ray diffraction patterns.

Table 1

The device performance statistics of the P3CT-Na HTL-based MAPbI₃ solar cells under one sun illumination (100 mW/cm², AM 1.5G). The data were calculated using eight cells for each statistic.

P3CT-Na/Water Concentration	VOC (V)	JSC (mA/cm2)	FF (%)	PCE (%)	RS (Ω)	RSh (kΩ)
60 wt%	0.945 ± 0.002	21.06 ± 0.11	74.5 ± 2.3	14.83 ± 0.56	60 ± 10	17.2 ± 2.0
48 wt%	1.001 ± 0.001	22.18 ± 0.10	77.8 ± 2.2	17.27 ± 0.58	54 ± 9	19.3 ± 1.4
36 wt%	0.990 ± 0.002	20.95 ± 0.09	74.6 ± 2.2	15.47 ± 0.59	55 ± 12	15.2 ± 2.4
24 wt%	0.900 ± 0.004	19.95 ± 0.16	68.4 ± 2.4	12.28 ± 0.59	57 ± 13	7.5 ± 2.3

Supplementary Materials

The following can be downloaded at https://www.mdpi.com/article/10.3390/pr10081444/s1, Figure S1: The light J–V curves of the P3CT-Na HTL-based MAPbI₃ solar cells in the forward and backward scanning directions, Figure S2: The transmittance spectra of the P3CT-Na/ITO/glass samples, Figure S3: The calculation of the mean sizes from the atomic force microscopic images of the MAPbI₃/P3CT-Na/ITO/glass samples.

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