Abstract
1. Introduction
Perovskite solar cells (PSCs) have attracted extensive attentions as the most promising candidate for the new generation photovoltaic technologies. The power conversion efficiency (PCE) has developed rapidly from 3.8% to a certified 25.5% within about ten years due to the superior optoelectronic properties of perovskites, such as high light absorption coefficient, long charge diffusion length, desirable/tunable bandgap and high defect tolerance[
Since the first report of FPSC with an efficiency of 2.62% in 2013 by Kumar et al. intensive efforts on investigating flexible substrates, transparent electrodes, charge transport materials, perovskite films and interfacial layers have been made to achieve the current record efficiency over 21% with small device area (<1 cm2)[
In this mini review, we will focus on the recent progress of FPSCs including efficiency improvement of small-scale FPSCs, R2R processed large-area devices and emerging flexible photovoltaic technologies. We will summarize the criteria/requirements for different component of a FPSC, such as flexible substrate, transparent electrode, charge transport layer and perovskite film. The innovative techniques on coating methods and perovskite precursors for R2R process are discussed. The emerging photovoltaics based on deformable and stretchable FPSCs are demonstrated. Finally, we will conclude and address the remaining challenges and potential future directions of this research field.
2. Device components of high-performance FPSCs.
A FPSC contains several essential components, including a substrate, two electrodes with at least one being transparent, a perovskite layer and two charge transport layers (CTLs) for electrons or holes respectively. The major thickness of the whole device comes from the substrate since the perovskite layer and electrodes are always very thin (< 1μm) and thus the mechanical flexibility of the device is mainly influenced by the substrate. On the other hand, flexible substrate plays a key role on the photovoltaic performance of the flexible device because the following layers deposited on it could be influenced by its chemical and mechanical properties. Therefore, the selection of flexible substrate is important for the whole fabrication process. Unlike traditional glass substrates, flexible substrates usually have high roughness and low-temperature processing requirement. Hence, it is necessary to precisely optimize the deposition conditions of electrodes, CTLs and perovskite layers to ensure both high efficiency and mechanical robustness of resultant flexible devices.
2.1. Flexible substrates
A desirable flexible substrate for FPSCs should have high optical transmittance, high thermal tolerance, low roughness, high resistance to chemical solvents, robust mechanical flexibility and good oxygen and water barrier properties. However, it is hard to integrate all these properties in one kind of substrate. Currently, there are mainly three types of flexible substrates applied in FPSCs, such as plastic polymer substrates, metal foils, and glass substrates.
Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films are the most used flexible polymer substrates for FPSCs due to their high optical transmittance, mechanical robustness, good chemical resistance and R2R processability. The current FPSCs with record efficiency are prepared on these polymer substrates[
Metal foils have better thermal resistance, lower water and oxygen transmission rate and higher mechanical durability. They can be used as substrates, electrodes and even CTLs in one cell simultaneously, which significantly simplifies the fabrication process. Ti and Cu foils are the most used materials for flexible substrate[
A glass substrate with thickness lower than several hundred micrometers could become mechanically flexible[
2.2. Transparent electrode
ITO is the most used transparent electrode due to high optical transmittance and low resistance. While the high production cost is a drawback for the mass production due to the presence of noble metal. Therefore, other transparent conductive oxides electrodes have been explored as alternatives to prepare FPSCs, such as aluminum-doped zinc oxide (AZO)[
Solution-processable metal nanowire/mesh are promising materials for transparent electrodes due to high conductivity, transparency and flexibility[
Figure 1.(Color online) (a) Schematic illustration of the PET/Ni-mesh substrate, Ni-mesh:PH1000 hybrid electrode, and perovskite device; the top right image shows the SEM image of Ni-mesh. The right is the optical image of PET/Ni-mesh substrate. Reprinted with permission from Ref. [
Carbon-based materials, such as graphene and carbon nanotubes, are investigated as transparent electrodes for FPSCs due to their high transmittance and conductivity[
PEDOT:PSS is a widely used conductive polymer material in flexible and stretchable electronics[
2.3. Charge transport layers
In a working PSC, a charge transport layer would extract charge carriers from perovskite absorber and transport to corresponding electrode. Therefore, the band structure, film morphology and mobility are key factors for the selection and preparation of CTLs for high performance and stable PSCs[
In FPSCs with n–i–p structure, low temperature processed ZnO, SnO2, TiO2 are usually used inorganic electron transport layers (ETLs)[
Figure 2.(Color online) (a) TEM image of planar layer and porous planar layer coated on ITO/glass substrate and EDS mapping. (b)
2.4. High-quality perovskite layer
The processing approaches and compositions of perovskite films on flexible substrates are different from counterparts on rigid ones due to the different properties of substrates. The preparation methods at low temperature and compatible with R2R mass production are highly desirable for practical applications.
Laser annealing is an efficient method crystalizing perovskite films at room temperature at a fast rate. Jeon et al. first used a laser with wavelength of 1064 nm to anneal the perovskite film via photo-thermal heating induced by the light absorption of ITO and PEDOT:PSS layers[
Figure 3.(Color online) (a) Schematic description of perovskite film formation process by laser annealing method. Reprinted with permission from Ref. [
Composition tailoring for perovskite precursor is necessary since the direct transfer of perovskite deposition from rigid to flexible substrates usually leads to poor morphology and low performance[
3. R2R fabrication
Low cost and high efficiency are two advantages promising the commercialization of PSCs in the near future. R2R fabrication might be a breakthrough to realize commercialization due to the cost-effective high throughout mass production. In order to realize R2R production of FPSCs, uniform and large-area deposition of sequential layers should be achieved. The R2R deposition of CTLs have already been established in other photovoltaic technologies, such as organic solar cells[
Galagan et al. used dimethyl sulfoxide (DMSO) and 2-butoxyethanol as solvents for perovskite precursor to prepare FPSCs with a best PCE of 13.5% by R2R slot-die coating method[
Dai et al. reported that the addition of ammonium chloride (NH4Cl) into the perovskite precursor solution could form high quality perovskite film with good contact with substrates by retarding the crystallization process[
Kim et al. employed tert-butyl alcohol (tBuOH) as an eco-friendly anti-solvent to prepare highly crystalline and uniform FA based perovskite film by gravure printing method on flexible substrates[
Figure 4.(Color online) (a) Diagram showing R2R processing for the fabrication of FPSCs. (b) Photograph of fully R2R processed PSCs. (c) The
Although significant improvements have been obtained on photovoltaic performance of R2R processed FPSCs, the current efficiency could not reach the commercialization level. More efforts are needed to study the deposition of high quality perovskite film by R2R compatible approaches. Higher efficiency (~20%) can been expected.
4. Emerging FPSCs
Due to the easy fabrication of perovskite films, different kinds of FPSCs have been successfully prepared on various flexible substrates extending the applicability and facilitating opening an emerging photovoltaic market. The deformable and lightweight FPSCs could be integrated with different functional devices to construct self-powdered smart systems.
The plastic substrates such as PET and PEN would cause environmental pollution due to the long decomposition periods[
Figure 5.(Color online) (a) Photographs of silk derived electrodes using natural silkworm cocoons as raw materials, which show high transmittance and various deformation including wave, spiral, bowknot, flower and paper crane. Reprinted with permission from Ref. [
The foldable and stretchable FPSCs are highly desirable for wearable electronics and vehicle- and building integrated photovoltaics. Kaltenbruner et al. fabricated FPSCs on PET substrates with a thickness of 1.4 μm[
The application of flexible power supply by FPSCs has also been demonstrated. Zhao et al. reported a safe, flexible and self-powdered wristband system by integrating high performance zinc-ion batteries with FPSCs[
5. Conclusion and outlook
In summary, FPSCs are supposed to be a promising breakthrough for the next generation photovoltaic technology with a high possibility for commercialization. In addition, the successful fabrication of lightweight and deformable FPSCs applied in various self-powdered systems could open an emerging and pioneering market. Therefore, the development of FPSCs will play a critical role in the commercialization pathway of PSCs. Interestingly, the fabrication techniques of FPSCs have been developed very fast and the efficiency over 21% has been achieved. The integration with R2R process has been reported by several groups recently with promising performance achieved in large-area devices. In this review, we summarize the recent development of FPSCs, focusing on the key issues of flexible devices, R2R process and emerging applications of FPSCs, which provides a guideline for the future development of this field.
To further improve the device performance, we should carefully check and optimize each component of the devices. In the efficiency evolution of FPSCs, many efforts have been made in optimizing flexible substrates, transparent electrodes, CTLs and perovskite absorbers for high performance FPSCs. For the three types of flexible substrates, including plastic polymer substrates, metal foils and flexible glasses, the polymer substrate-based devices demonstrate high efficiency and easy fabrication. Moreover, it is convenient to functionalize the polymer substrates (e.g. coating hydrophobic or antireflection layers), which will definitely enhance the performance of the FPSCs.
For transparent electrodes, ITO-based devices exhibit the highest efficiency while relatively poor mechanical flexibility. PEDOT:PSS could be a better electrode for stretchable and deformable devices. However, perovskites may degrade on the acidic PEDOT:PSS surfaces, leading to poor stability of the devices, so surface modification on PEDOT:PSS is needed, such as coating a suitable CTL on the surface.
The low-temperature processed materials with pre-synthesized nanoparticles/nanocrystals structure should be a useful strategy to prepare efficient CTLs for FPSCs. In addition, the CTLs with porous structure could successfully overcome the drawback of high roughness of flexible substrates. More efforts are needed to suppress the non-radiative recombination and facilitate the charge carrier transfer at the CTL/perovskite interfaces. Similarly, low-temperature crystallization technologies of perovskite films are needed. In view of mass production, laser annealing may be a promising method to crystalize the perovskite films at room temperature with fast rate. The fabrication of large-scale FPSCs by R2R procession along with laser annealing is expected to be a feasible strategy.
The development of R2R fabrication of FPSCs is still at early stage and the record efficiency for fully R2R processed devices is only 13.8%. There is a considerable room for further improvements. The key is to deposit uniform and pinhole-free wet perovskite precursor films by R2R compatible methods. Novel additives and precursor compositions need to be further probed. The deformable and stretchable FPSCs have been demonstrated as power supplies by integrating with sensors, batteries, and aircrafts. Potential future applications could be power sources for industrial monitoring and tactical security applications.
In the future studies on FPSCs, our focus should be on but not be limited to the following aspects: (1) Enhancement of long-term stability. The plastic polymer substrates have limited water and oxygen barrier abilities. Additional encapsulations on top and bottom of devices are necessary. (2) Large-area R2R fabrication. Although the record efficiency for FPSCs is over 21%, the corresponding device area is relatively small. Besides, the spin-coating method is not compatible with mass production. More efforts should be focused on the deposition of perovskite film with large scale by R2R compatible coating methods. (3) Extending the applicability of FPSCs. Due to high efficiency and mechanical flexibility, FPSCs have great potential to be integrated with different systems as power supply. The successful application in the field of wearable/flexible electronics maybe a preferential commercialization option. Overall, the rapid advancements along with the practical challenges in FPSCs suggest a bright future for this active field.
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