Investigation on Low-temperature Annealing Process of Solution-processed TiO₂ Electron Transport Layer for Flexible Perovskite Solar Cell

Abstract

Flexible perovskite solar cells (PSCs) have received increasing attention in wearable and portable devices over the past ten years. The low-temperature process of electron transport layer plays a key role in fabricating flexible PSCs. In this paper, we improve the performance of flexible PSCs by controlling the thermodynamic procedure in the low-temperature annealing process of solution-processed TiO₂ layers and modulating the precursor concentration of (6,6)-phenyl c₆₁ butyric acid methyl ester (PC₆₁BM) deposited on fluorine-doped tin oxide (FTO)/TiO₂ substrate. The results show that slowing down evaporation rate of residual solvent and adopting PC₆₁BM of appropriate precursor concentration are confirmed to be effective methods to improve the performance of flexible PSCs. We also demonstrate carbon electrode-based flexible PSCs. Our work expands the feasibility of low temperature process for the development of flexible perovskite photodetectors and light-emitting diodes, as well as flexible PSCs.

1. Introduction

Organometal halide perovskite solar cells (PSCs) have aroused great interest in academia and industries [1,2,3,4,5]. The environmental issues concerning the footprint of PSC manufacturing processes are also of great importance [6]. Flexible PSCs expand the feasibility for the application in wearable and portable devices [7,8,9,10]. Recently, the power conversion efficiency (PCE) of flexible PSCs has rocketed to 18.4% [10].

There are many papers on low fabrication oxide. For example, Wang et al. adopted solution-processed amorphous WO_X as electron selective layer for PSCs [1], and Eze et al. Fabricated efficient planar PSCs using solution-processed amorphous WO_X/fullerene C₆₀ as electron extraction layer [2]. In this paper, we use TiO₂ as electron transport layer (ETL). However, the traditional TiO₂ ETL-based PSCs require high temperature sintering (>400 °C) [11,12,13,14,15], which exceeds the thermal budget of flexible substrates such as FTO-coated polyethylene terephthalate (PET) (≤150 °C). Therefore, low temperature process of TiO₂ ETL has been intensively studied [16,17,18,19,20]. Yang et al. and Feleki et al. demonstrated magnetron sputtering and electron beam to prepare highly dense TiO₂ layer, respectively [16,17]. A solution process was also reported to fabricate compact TiO₂ ETL [18].

In 2019, based on solution process, Chu et al. studied the influence of treatment time of UV/O₃ process following the annealing process of TiO₂ film on a hotplate on the properties of TiO₂ ETL and discovered that 80 min-UV/O₃-treating endowed the flexible PSCs with a best PCE of 7.33% owing to UV/O₃-induced removal of residual organic solvent as well as enhanced hydrophilicity and conductivity of TiO₂ films [19]. In the same year, You et al. investigated the effect of different solvent used to re-disperse TiO₂ nano-sol on the quality of solution-processed TiO₂ ETL prior to the annealing process of TiO₂ film on a hotplate and demonstrated flexible PSCs of better PCE of 15.8% based on N, N-dimethylformamide (DMF) solvent, which has high zeta potential and low surface tension [20].

In this paper, we prepare the nano TiO₂ films with solution process and perform the annealing process on a hotplate at low temperature (≤150 °C). We control the morphology of TiO₂ ETL by adjusting evaporating rate of the organic solvent in the annealing process on a hotplate, which leads to the change of thermodynamic process in the growth and formation of TiO₂ films. Furthermore, we optimize the ETL with PC₆₁BM of different precursor solution to reduce the defects on the surface of TiO₂ layer [21]. It is worth noting that all the exploring experiments are carried out on the rigid substrates at first, and then we prepare the carbon electrode-based flexible PSCs based on the optimized preparing process.

2. Experiments

2.1. Materials

FTO (7–8 Ω/square, Hunan Yiyang Xiangchen Tech Co., Ltd., Yiyang, China), low-temperature TiO₂ nanoparticle (LT-TiO₂ NP) (Shanghai Mater Win New Materials Co., Ltd., Shanghai, China), PC₆₁BM (Xi’an Polyme Light Technology Corp., Xi’an, China), DMF (99.8%, Shanghai Mater Win New Materials Co., Ltd., Shanghai, China), dimethyl sulfoxide (DMSO, 99.8%, Shanghai Mater Win New Materials Co., Ltd., Shanghai, China), methyl ammoniur iodide (CH₃NH₃I, >99.5%, white powder in appearance, Shanghai Mater Win New Materials Co., Ltd., Shanghai, China), lead(II) iodide (PbI₂) (99.99%, yellow crystalline powder in appearance, Xi’an Polyme Light Technology Corp., Xi’an, China), 2, 2′, 7, 7′-tetrakis-(N, N-dip-methoxyphenylamine)-9, 9′-spirobifluorene solution (Spiro-OMeTAD) (≥99.5%, yellow solution in appearance, Xi’an Polyme Light Technology Corp., Xi’an, China).

2.2. Device Fabrication

2.2.1. Cleaning of FTO Substrate

First, the FTO conductive glass was cleaned with deionized water (Beijing Information Science and Technology University, Beijing, China) and detergent with ultrasonic vibration cleaners (KQ-100E, Skymen Cleaning Technology Shenzhen Co., Ltd., Shenzhen, China) for 20 min. Next, FTO was cleaned with ethanol (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) for 20 min. Then, FTO was cleaned with a mixed solution of isopropyl alcohol (Sinopharm Chemical Reagent Co., Ltd., Beijing, China), acetone (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) and deionized water in a volume ratio of 1:1:1 for 20 min. Finally, the cleaned conductive glass was treated with UV light cleaner (BZS250GF-TC, Shenzhen Huiwo Technology Co., Ltd., Shenzhen, China) for 15 min.

2.2.2. Formation of Electron Transport Layer

LT-TiO₂ NP solution was deposited on the FTO substrate by spin-coating at 2000 rpm for 30 s and then dried at 150 °C for 20 min on a hot plate to form a nano TiO₂ layer.

(1) Giving the heating mode, we determine three kinds of annealing procedures (Process 1): (a) The annealing temperature was directly raised to 150 °C (Direct method); (b) The annealing temperature was raised from room temperature to 150 °C at rate of 8 °C/min (Proportional method); (c) The as-grown nano TiO₂ layer had been delayed for one hour before the annealing temperature was raised from room temperature to 150 °C at rate of 8 °C/min (Delayed method);

(2) Based on Process 1(c), we demonstrated an interface modifying procedure (Process 2): Depositing PC₆₁BM of different precursor concentration (5, 10, 15, 20, 25 mg/mL) on nano TiO₂ layer by spin-coating at 1500 rpm for 30 s. After the deposition, PC₆₁BM was left in the ambient condition for forty minutes to evaporate slowly with no annealing process. It is worth noting that the PC₆₁BM was dissolved in chlorobenzene (Shanghai Mater Win New Materials Co., Ltd., Shanghai, China).

2.2.3. Formation of Perovskite Light-absorption Layer

First, the PbI₂ was fully dissolved in a mixed solvent of DMF and DMSO in the volume ratio of 0.95:0.05 by vigorous stirring to form 600 mg/mL precursor solution. The methyl ammonium was fully dissolved in anhydrous isopropanol to form 70 mg/mL precursor solution. Then, the PbI₂ precursor solution was deposited on the PC₆₁BM layer by spin-coating at 1500 rpm for 30 s. Immediately, the CH₃NH₃I precursor solution was drop-cast on the as-grown PbI₂ film at 1500 rpm for 30 s and then dried at 150 °C for 20 min.

2.2.4. Formation of Hole Transport Layer

The hole transport layer was formed by spin-coating Spiro-OMeTAD on perovskite layer at 3000 rpm for 30 s. After the spin-coating, Spiro-OMeTAD was left in the ambient condition for 40 minutes to evaporate slowly with no annealing process.

2.2.5. Formation of Counter Electrode

The soot from the burning candle was collected with an FTO glass substrate to form a sponge-like carbon electrode, which was then pressed against the prepared hole transport layer [22].

2.3. Characterization

Field emission transmission electron microscope (FE-TEM) (Tecnai G2 F20, FEI, Hillsboro, OR, USA), field emission scanning electron microscope (FE-SEM) (SU8020, Hitachi, Tokyo, Japan) images were obtained for structural and morphological characterization of TiO₂ films, PC₆₁BM layers and perovskite films. The current-voltage (J–V) curves were obtained under standard simulated air-mass (AM) 1.5 sunlight generating from a solar simulator (Sol 3A, Oriel, Newport, RI, USA). UV-vis absorption spectra were characterized by ultroviolet visible absorption spectrometer (Avantes, Apeldoom, The Newtherlands). Photoluminescence (PL) spectrum was measured with PL testing system (LabRAW HR800, HORIBA Jobin Yvon, Paris, France). All the characterization of devices was performed in ambient atmosphere at room temperature.

3. Results and Discussion

In this paper, we investigate the effect of low temperature process of conventional TiO₂-based ETL on the performance of PSCs. Hence, we have characterized the structure of TiO₂ nanoparticles with TEM, as demonstrated in Figure 1, showing the average size of about 5 nm.

Firstly, we investigate the influence of thermodynamic process on the growth and formation of nano TiO₂ films. The detailed information about experiments is shown in the second paragraph of experimental Section 2.2.2. The SEM images are shown in Figure 2. The J–V curves of PSCs and the corresponding data extracted from J–V curves are shown in Figure 3 and

Table 1. Data extracted from reverse scanning J–V curves of perovskite solar cells (PSCs) based on Process 1. The letter Di, Pr, De represent films prepared with Direct method, Proportional method, and Delayed method, respectively. The device active area is 0.25 cm² (0.5 cm × 0.5 cm). The meaning of Process 1, Direct method, Proportional method and Delayed method is described in the second paragraph of experimental Section 2.2.2.

Samples	PCE a (%)	Voc b (V)	Jsc c (mA/cm2)	FF d
Di	0.26	0.55	1.94	0.27
Pr	0.11	0.47	0.93	0.25
De	0.96	0.66	4.82	0.30

Notes: a. PCE: power conversion efficiency; b. V_OC: open-circuit voltage; c. J_SC: short-circuit current; d. FF: fill factor.

, respectively.

It is clearly observed from Figure 2a that there are large holes remained in nano TiO₂ film prepared with Direct method. Figure 2b shows that these holes locate at valleys of FTO where the surface curvature changes greatly. Some voids can still be discovered from Figure 2c and d obtained from nano TiO₂ film prepared under Proportional method, however, they are smaller compared with Figure 2a,b. Figure 2f shows the FTO is effectively covered by nano TiO₂ film with few defects fabricated under Delayed method. We can also observe that the nano TiO₂ layer and FTO surface fit well with each other, indicating the conformal growth of nano TiO₂ on FTO substrate.

We can conclude that the morphology of nano TiO₂ film deposited on FTO substrate can be controlled by optimizing annealing process and nano TiO₂ film exhibits high quality when Delayed method was introduced, which is ascribed to the slower evaporating rate of the residual solvent, resulting in conformal covering of TiO₂ over FTO substrate. Generally, films with fewer cracks and defects endow the PSCs with better performance, which is also confirmed by reverse scanning J–V curves of the devices in Figure 3 and data extracted from reverse scanning J–V curves in Table 1.

Secondly, as shown in Figure 2, the TiO₂ nanoparticles are not connected tightly with each other, which may lead to leakage of PSCs. Therefore, we studied the effect of PC₆₁BM on the interface between nano TiO₂ layer and perovskite layer. The detailed information about experiments is shown in the third paragraph of experimental Section 2.2.2. The SEM images are shown in Figure 4.

As shown in Figure 4d,f,h,j,l, it is clearly observed that there are some light islands distributed on the dark substrate. We can determine that the light islands are peaks of nano TiO₂ layer and the dark substrate is PC₆₁BM comparing with Figure 4b. From the magnified SEM images, we can conclude that the islands disappear gradually with the concentration of PC₆₁BM increasing from 5 to 25 mg/mL. When the concentration of PC₆₁BM is 5 mg/mL, the PC₆₁BM just filled the valleys of nano TiO₂ layers. With the concentration of PC₆₁BM increased to 15 mg/mL, the nano TiO₂ film was well covered by PC₆₁BM and we can hardly find the light islands, leaving few cracks. While the concentration further rises to 25 mg/mL, the nano TiO₂ layer was fully covered by PC₆₁BM, giving smooth and uniform film.

As demonstrated in previous report [23], it is too hard for thick ETL to transfer the electron from the absorption layer to FTO substrate and instead relatively thin ETL leads to a serious recombination of electrons and holes. Hence, we can infer that the device optimized with PC₆₁BM of 15 mg/mL exhibits the best performance, in which PC₆₁BM filled the most valleys of nano TiO₂ layer, leaving few cracks, preventing the contact between FTO substrate and perovskite layer and improving the electron transporting efficiency.

Finally, to investigate the influence of PC₆₁BM substrate on the formation and properties of perovskite films, we studied the SEM images and absorbance spectra of perovskite films deposited on PC₆₁BM of different concentration (5, 10, 15, 20, 25 mg/mL), absorbance spectra of PC₆₁BM of different concentration and PL spectra of perovskite films deposited on PC₆₁BM of different concentration, as shown in Figure 5, Figure 6 and Figure 7, respectively.

As shown in Figure 5, when the PC₆₁BM precursor solution concentration is lower than 15 mg/mL, a good grained and uniform perovskite films were obtained. In contrast, obviously different perovskite surface morphologies are observed on 20 and 25 mg/mL PC₆₁BM substrate, where the crystallinity of the perovskite films is distinctly deteriorated with serious grain boundary distortion and large holes.

From Figure 6a, we can observe that the absorption spectra signify the similar light-harvesting capabilities over 400 to 900 nm regardless of the different PC₆₁BM concentration adopted. All five films exhibit approximately the same absorption edge at about 790 nm with no red-shift or blue-shift, corresponding to optical bandgap of perovskite. Particularly, perovskite films deposited on 25 mg/mL PC₆₁BM manifest inferior light-harvesting capabilities, which is caused by the internal defects resulted from crystal distortion, and it is consistent with Figure 5j. From Figure 6b, we can observe that the PC₆₁BM exhibits higher light-harvesting capabilities with the concentration increasing. However, perovskite films deposited on 15, 20 and 25 mg/mL PC₆₁BM manifest similar absorbance, which indicates that light-harvesting capabilities of perovskite films deposited on PC₆₁BM layers decrease with the precursor concentration (≥15 mg/mL) of PC₆₁BM.

Figure 7 shows PL spectra of perovskite films deposited on PC₆₁BM of different concentration. It is clearly observed that the peak of photoluminescence is fairly weak when nano TiO₂ layer has not been optimized with PC₆₁BM, being explained by the existence of via holes in nano TiO₂ layer, which leads to leakage. Furthermore, from Figure 7, we can observe that the PL intensity decrease as the concentration of PC₆₁BM increasing from 5 to 15 mg/mL. However, when the concentration of PC₆₁BM is further increased to 25 mg/mL, the PL intensity raises up again and reaches the maximum value.

As demonstrated in a previous report, the PL intensity of the films is closely related to quenching of excitons resulted from the following two reasons, radiative relaxation of excited electrons back to the ground state of perovskite and electron injection from light-absorption layer into electron transport layer [24]. Hence, we can infer that the PC₆₁BM of 15 mg/mL deposited on nano TiO₂ layer guaranteed the high-quality interface between ETL and perovskite layer, resulting in more efficient injection of electrons from perovskite layer into nano TiO₂ layer, leading to low PL intensity, which is consistent with Figure 5f.

We also have performed J–V characteristics of PSCs optimized with PC₆₁BM, as shown in Figure 8 and

Table 2. Data extracted from reverse scanning J–V curves of perovskite solar cells optimized with different concentration of PC₆₁BM. -rev represents reverse scanning. The number 5, 10, 15, 20, 25 represent different concentration of PC₆₁BM. The device active area is 0.25 cm² (0.5 cm × 0.5 cm).

Samples	PCE a (%)	Voc b (V)	Jsc c (mA/cm2)	FF d
5-rev	0.81	0.94	2.81	0.31
10-rev	2.52	0.90	8.76	0.32
15-rev	5.48	1.01	14.74	0.37
20-rev	3.14	1.02	9.94	0.31
25-rev	0.26	0.35	2.52	0.29

Notes: a. PCE: power conversion efficiency; b. V_OC: open-circuit voltage; c. J_SC: short-circuit current; d. FF: fill factor.

. From Figure 8 and Table 2, we can discover that following the increasing of the concentration, the PCE extracted from reverse scanning J–V curves shows up a different trend about approximately 0.81%, 2.52%, 5.48%, 3.14% and 0.26% from the perovskite solar cells with PC₆₁BM concentration of 5, 10, 15, 20 and 25 mg/mL respectively. It is clearly observed that the PCE reaches the maximum value when the concentration is increased up to 15 mg/mL, and then decreases with the further increasing of the concentration, which is consistent with the above discussion.

Finally, we demonstrate the carbon electrode-based flexible solar cell of PCE of 3.24% and 2.60% for reverse and forward scanning, respectively [25]. The carbon electrode is fabricated with spongy film composed of carbon nanoparticles, which leads to unstable contact in characterization, resulting in poor device performance. The golden electrode-based device is under investigation. Moreover, we are investigating the optimization process for perovskite layer and hole transport layer and the optimization process for the interface between layers to improve the device performance.

4. Conclusions

We have discovered a method to improve the performance of PSCs by optimizing low-temperature annealing process of solution-processed TiO₂ ETL. In this work, we have received better nano TiO₂ films using Process 1c (the as-grown nano TiO₂ film had been delayed for one hour before the annealing temperature was raised from room temperature to 150 °C at rate of 8 °C/min). Moreover, the surface of nano TiO₂ film optimized with 15 mg/mL PC₆₁BM endows the device with better performance. Finally, we have demonstrated the carbon electrode-based flexible perovskite solar cell, and the golden electrode-based device is under investigation. Our results can serve as a platform to fabricate high-efficiency flexible PSCs and have a potential in flexible and wearable devices.