Perovskite Solar Cell on Stainless Steel Substrate over 10% Efficiency for Building-Integrated Photovoltaics

Abstract

This study investigated the integration of perovskite solar cells (PSCs) on stainless steel (SS) substrates for application in building-integrated photovoltaics (BIPV). Using advanced atomic force microscopy measurements, we confirmed that enhanced substrate roughness increased the reflectance along an interface. Consequently, a remarkable final efficiency of 11.9% was achieved. Notably, PSCs, known for their exceptional efficiency of 26.1%, can overcome the inherent efficiency limitations of SS-based thin-film solar cells. In this study, a PSC with an efficiency of 14% was fabricated on a flexible SS substrate. This study is a significant step towards advancing sustainable energy solutions for BIPV applications. The global shift towards renewable energy sources has catalyzed intensive research and development efforts, rendering the exploration of alternative materials and manufacturing processes a priority. The success of PSCs on SS substrates underscores their promise to achieve a balance between efficiency and versatility in BIPV solutions. Moreover, our findings reveal that controlling the substrate surface characteristics can significantly enhance the performance of PSCs, offering a pathway toward greater energy efficiency and sustainability in the construction industry.

1. Introduction

The ever-increasing global demand for sustainable energy sources has propelled the relentless search for highly efficient solar cells, particularly those adaptable to diverse applications [1,2,3,4]. Two primary areas where solar energy seamlessly converges with architectural and transportation solutions are building-integrated photovoltaics (BIPV) and vehicle-integrated photovoltaics (VIPV) [5,6,7,8,9,10,11,12]. Thin-film solar cells, which can be manufactured on various substrates rather than on rigid crystalline silicon, have been extensively used in BIPV and VIPV. Among these, perovskites, capable of high efficiency, are currently being researched [13,14,15].

With the evolution of photovoltaic technologies, various solar-cell technologies have been meticulously tailored to function on flexible substrates. This endeavor spans a spectrum of materials, including amorphous silicon (a-Si:H), copper zinc tin sulfur selenide (CZTSSe), copper indium gallium selenide (CIGS), cadmium telluride (CdTe), dye-sensitized solar cells (DSSC), and organic photovoltaics (OPV) [16,17,18,19]. The development of solar cells on flexible substrates is a turning point in this field, facilitating innovative applications.

The quest for sustainable energy sources has sparked dynamic exploration of alternative substrates for photovoltaic applications. Among the several types of materials, stainless steel (SS) substrates have emerged as a compelling choice, boasting a constellation of unique advantages. These substrates are characterized by exceptional flexibility, proven durability against environmental stressors, and robust resistance to corrosion. With these advantages, SS substrates are applied in many fields. In the field of construction and architecture, SS panels play a crucial role in shaping structural elements, external facades, and roofing structures. The enduring stability they provide contributes significantly to the longevity of buildings. Also, in the automotive industry, stainless steel is integrated into vehicles as components and exterior materials. Its resilience against corrosion becomes particularly valuable in maintaining the overall lifespan and appearance of automobiles. Finally, in the field of electronics, stainless steel becomes an ally in providing durability and stability. It is employed in the outer casings and components of various electronic products. Therefore, applying solar cells to SS substrates makes it possible to utilize them in the above fields.

Consequently, SS substrates are considered the optimal choice for solar installations that require versatility and resilience, thereby fostering the prospect of innovative solar energy solutions.

The best research-cell efficiencies and power conversion efficiencies (PCE) of SS substrates for thin-film solar cells, including a-Si:H, CZTSSe, CIGS, CdTe, DSSC, and OPV, are graphically shown in Figure 1 [20,21,22,23,24,25,26].

Although SS substrates hold great promise, the historical challenge has been achieving high-efficiency solar cells that exceed the elusive 10% threshold for this unconventional material. However, this formidable endeavor has gained momentum with the advent of perovskite solar cells (PSCs). Notably, perovskite technology currently has the highest known efficiency in the solar cell realm (26.1%) [26]. PSCs have been effectively applied to steel substrates with a reported efficiency of 16.5% [27,28,29,30]. While previous studies have focused on the fabrication of perovskite solar cells (PSCs) on rigid SS substrates to achieve high efficiency, this paper distinguishes itself by successfully extending the fabrication of these cells to flexible substrates. Unlike the existing literature that predominantly addresses PSCs on rigid SS platforms, our work emphasizes the novel aspect of producing PSCs on flexible substrates, marking a significant departure from the conventional paradigm. This shift not only expands the scope of perovskite solar cell applications but also addresses the growing demand for flexible and lightweight photovoltaic technologies. In our study, while achieving a lower efficiency than this benchmark, we demonstrated a substantial efficiency of 11.9% by substituting the substrate with a steel substrate instead of a glass substrate. Furthermore, by extending our investigation to flexible substrates, the efficiency was enhanced to 14%. Additionally, we explored the effects of increased reflectance on current density when using SS substrates in future applications.

While PSCs demonstrate high efficiency on SS substrates, their practical application in BIPV, VIPV, etc., poses significant challenges. The potential leakage of lead (Pb) from PSCs raises environmental and human health concerns. In response to this issue, recent research has focused on developing robust strategies for immobilizing and capturing Pb, aiming to mitigate the associated risks [31,32]. These advancements in Pb immobilization and capture technologies are pivotal steps towards overcoming a critical barrier, thereby enhancing the viability of integrating PSCs into BIPV and VIPV systems in the near future.

2. Materials and Methods

2.1. Preparation of Materials

SS and polished SS substrates were obtained from POSCO. The polishing was carried out by rotating a buff with polished paper in the range of 240 to 320 mesh. Tin(4) oxide (SnO₂) 15% in H₂O colloidal dispersion was purchased from Alfa Aesar. N,N-Dimethylformamide (anhydrous, 99.8%) (DMF), dimethyl sulfoxide (anhydrous, 99.9%) (DMSO), Diethyl ether, chlorobenzene (anhydrous, 99.9%), acetonitrile (anhydrous, 99.8%), 4-tere-butylpyridine (96%), and bis (trifluoromethane) sulfonamide lithium salt (Li-TFSI) were purchased from Sigma-Aldrich. Methylammonium iodide (MAI) was purchased from Dyesol. Lead (2) iodide (PbI₂) was purchased from TCI. 2,2′,7,7′-Tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) was purchased from Lumtec.

2.2. Perovskite Solar Cell Fabrication

The SS substrates were rinsed with acetone, ethanol, and isopropanol for 15 min each under sonication, followed by an ultraviolet (UV)-ozone treatment for 30 min. For the diffusion barrier, 1 μm SiO₂ was deposited by PECVD on the SS substrate.

After the RF sputtering of Indium Tin Oxide (ITO) of approximately 150 nm, the substrate was annealed at 400 °C for 30 min. To deposit an electron transport layer (ETL), SnO₂ 15% in H₂O colloidal dispersion was diluted with deionized water (1:7). It was then spin-coated onto ITO at 4000 rpm (dynamic spin coating). To prepare the MAPbI₃ perovskite precursor solution, MAI (159 mg) and PbI2 (481 mg) were dissolved in DMF (636.6 μL)/DMSO (71 μL). Diethyl ether was dropped during spin coating. The substrate was dried on a hot plate at 60 °C for 1 min and then at 100 °C for 5 min. A spiro-OMeTAD solution was used as a hole transport layer (HTL). Spiro-OMeTAD (72.3 mg) powder was dissolved in 4-tertbutylpyridine (28.8 μL), Li-TFSI solution (520 mg in 1 mL of acetonitrile), and chlorobenzene (1 mL). The solution was then spin-coated onto a substrate. For the substrate configuration, a 20 nm MoOx sputter buffer layer and an ITO transparent electrode were deposited successively. Finally, a 100 nm Au electrode was thermally evaporated from the sample.

2.3. Measurement

The depth profile of the SS substrate was determined using dynamic secondary ion mass spectrometry (D-SIMS) (IMS-4fE7). The roughness of each substrate surface was measured using atomic force microscopy (AFM) (XE-100). The solar cell parameters of the devices were measured using a Xe lamp solar simulator (WACOM WXS-155S-10 class AAA) at 25 °C, with an airmass (AM) of 1.5, and 100 mW/cm² irradiation under a standard test condition (STC). Calibration was performed prior to the measurement using a modulated reference silicon solar cell. We measured the photocurrent as a function of the voltage using a source meter (Keithley 2400). All light I–V measurements were performed using a 0.075 cm² mask. The scan voltage setting time was 200 ms. X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer (Rigaku) with a Cu Kα source (1.54 nm). Scanning electron microscopy (SEM) was performed using a Quanta 250 FEG microscope (FEI). To evaluate the electrical properties of the device, external quantum efficiency (EQE) measurements were performed using a 150 W Xe lamp (QEX7, PV measurement, Point Roberts, WA, USA) and a monochromator to provide monochromatic light in the wavelength range from 300 to 850 nm. The absorbance and reflectance were measured using UV–vis spectroscopy (JASCO V-670).

3. Results and Discussion

3.1. Confirmation of SiO₂ Film for Diffusion Barrier

In this study, we constructed a PSC structure, illustrated in Figure 2, on an SS substrate using a substrate configuration. Notably, during the fabrication process, a 1 µm SiO₂ layer was meticulously deposited between the rear transparent electrode and SS substrate. The SiO₂ layer augmented the conductivity of the rear transparent electrode, ITO, via sputter deposition and subsequent annealing at 400 °C for 30 min. Notably, this annealing process, marked by a temperature of 400 °C, was the highest point in the solar cell fabrication sequence, primarily undertaken to thwart any potential impurity diffusion from the SS substrate.

This specific technique parallels practices in the fabrication of CZTSSe or CIGS solar cells, where processing temperatures typically fall in the range of 450–550 °C [20,33]. To substantiate the effectiveness of our impurity diffusion barrier, we conducted ITO deposition and annealing with and without SiO₂, followed by D-SIMS measurements. As shown in Figure 3, the outcomes confirmed that in the absence of SiO₂, impurities including Fe and Cr infiltrated the ITO layer. In contrast, in the presence of SiO₂, it acted as a robust diffusion barrier, successfully preventing impurities from infiltrating the ITO layer. This highlights the pivotal role of SiO₂ as an effective inhibitor of impurity diffusion in solar cells.

3.2. Perovskite Solar Cells on SS Substrates

Following the successful validation of our impurity diffusion barrier, we fabricated solar cells on an SS substrate by employing the same approach as that applied to glass substrates, that is, utilizing a substrate configuration. To streamline the fabrication process, a rigid substrate of equivalent thickness to glass, denoted as 1.1 t, was used. In this case, two types of SS substrates were prepared, differing in whether they were polished or not, and cells were subsequently manufactured. The outcomes of these cells are visually presented in a numerical distribution graph for each parameter, as depicted in Figure 4. Notably, the PSCs on w/o polished substrates demonstrated the highest efficiency, as low as 2.3% (

Table 1. Best parameters of PSCs on w/o polished and polished SS substrates.

	Unit	w/o Polished SS	Polished SS
PCE	%	2.30	11.9
JSC	mA/cm2	18.1	19.6
VOC	mV	457	988
FF	%	27.8	61.7

). This low efficiency is attributed to the roughness of the SS substrate. If the substrate is rough, the series resistance increases, resulting in low open-circuit voltage (V_OC) and fill factor (FF).

To address this challenge, solar cells were fabricated on polished substrates, leading to a substantial increase in efficiency of up to 11.9% (Figure 4). The most notable enhancements were observed in V_OC and FF, primarily attributed to the improved smoothness of the SS substrate, as confirmed by AFM measurements shown in Figure 5. Through AFM measurements, the roughness and morphology of the sample surface can be confirmed. Upon examining Figure 5a,b, a discernible vertical pattern is evident. This particular configuration aligns with findings in the literature [34], leading to the conclusion that the polishing process for this sample was exclusively conducted in the vertical direction. In particular, a detailed roughness analysis considering the peak roughness (R_p = peak height–valley depth), roughness variation (RMS), and average roughness (R_a) revealed that the polished substrate exhibited a roughness level more than double that of the original substrate (

Table 2. Roughness of w/o polished and polished SS substrates.

	Unit	w/o Polished SS	Polished SS
Rp	nm	962	447
RMS	nm	57.4	25.2
Ra	nm	32.2	16.8

).

We conducted absorbance, XRD, and SEM measurements to investigate the properties of the perovskite thin film (Figure 6). The results indicate that the perovskite thin film is well-formed, as evidenced by the absorbance and XRD analyses. However, SEM images reveal the presence of pinholes in the perovskite thin film. These pinholes are observed to act as defects, exerting an influence on both V_OC and FF. This suggests that the defects introduced by the pinholes have a discernible impact on the electrical characteristics of the perovskite solar cells, affecting both V_OC and FF. Consequently, SS substrate roughness is a critical fact, highlighting its significance in shaping the overall performance of perovskite solar cells.

This smoother surface facilitated improved thin-film coating, minimized defects, and enhanced junction formation, resulting in an anticipated increase in Voc and FF. Notably, the short-circuit current density (J_SC) increased even when the substrate configuration was considered. In Figure 7, the reflectance measurements demonstrate that the polished substrate exhibited over 10% higher reflectance across the entire wavelength range than the unpolished substrate. Figure 8 shows that the EQE results yielded higher EQE values for the polished substrate in the 400–800 nm wavelength range than for the substrate without polishing. The difference in J_SC between the two cells in this range amounted to 1.37 mA/cm². When the J_SC values obtained through light current-voltage (LIV) measurements were compared, the difference was found to be 1.5 mA/cm².

Thus, employing an SS substrate with a roughness level similar to that of a glass substrate can produce PSCs with V_OC and FF values on par with those of substrate configuration cells manufactured using high-efficiency glass substrates or other transparent flexible substrates, such as polyethylene terephthalate (PET) or polyimide. This indicates the feasibility of achieving high-efficiency cells with a significantly higher current density using SS substrates.

Additionally, this study extended the investigation to include flexible SS substrates with a thickness of 0.05 t. Solar cells were fabricated on these flexible substrates, yielding the following efficiency results. As shown in Figure 9 for reference, solar cells on glass substrates, which served as the benchmark, achieved an efficiency of approximately 15%. In contrast, solar cells on flexible SS substrates exhibited efficiencies with a range of 10–14%. The improved efficiency of the Perovskite Solar Cells (PSC) on flexible substrates compared to rigid substrates can be attributed to several factors.

First, flexible stainless steel (SS) substrates undergo a planarization process during manufacturing through a roll-to-roll method. This process leads to a notable improvement in surface roughness. Specifically, it has been reported that the roughness is reduced to R_p = 8.9 nm [35]. Additionally, studies on the fabrication of Copper Indium Gallium Selenide (CIGS) solar cells on flexible SS substrates indicate RMS of approximately 13.6 nm [36]. An examination of the actual reflectance of the flexible SS substrate utilized in our experiments revealed consistently high reflectance across the entire wavelength spectrum, as shown in Figure 10. The high efficiency of flexible SS substrates is thought to be due to the following reasons. This diversification in substrate options underscores the adaptability of perovskite solar cell technology and its potential for tailoring solutions for various applications.

4. Conclusions

Aiming at efficient solar cells adaptable to various applications, we investigated the potential of SS substrates. These substrates offer unique advantages such as flexibility, durability, and corrosion resistance, rendering them promising for photovoltaics, particularly BIPV and VIPV. Historically, SS substrates have encountered challenges in achieving efficiencies exceeding 10%. However, we harnessed the power of perovskite solar cells, which are known for their remarkable efficiency of 26.1%. By substituting glass with SS substrates, we achieved a substantial efficiency of 11.9%, which was further improved to 14% with flexible SS substrates.

Our study revealed that a smoother substrate is essential for enhancing parameters such as V_OC and FF. The polished SS substrate facilitated a better thin-film coating, reduced defects, and promoted efficient junction formation. This resulted in an increased J_SC and higher reflectance. Furthermore, our exploration of flexible SS substrates demonstrated their adaptability, with efficiencies in the range of 10–14%. This diversity in substrates underscores the versatility of the PSC technology.

In summary, our research revealed the potential of SS substrates for perovskite solar cells, offering high efficiency and high current density. This represents a critical step toward enriching the sustainable energy landscape, where SS substrates play a pivotal role in broader solar power integration.