Critical review of recent progress of flexible perovskite solar cells

Abstract

Perovskite solar cells (PSCs) have emerged as a ‘rising star’ in recent years due to their high-power conversion efficiency (PCE), extremely low cost and facile fabrication techniques. To date, PSCs have achieved a certified PCE of 25.2% on rigid conductive substrates, and 19.5% on flexible substrates. The significant advancement of PSCs has been realized through various routes, including perovskite composition engineering, interface modification, surface passivation, fabrication process optimization, and exploitation of new charge transport materials. However, compared with rigid counterparts, the efficiency record of flexible perovskite solar cells (FPSCs) is advancing slowly, and therefore it is of great significance to scrutinize recent work and expedite the innovation in this field. In this article, we comprehensively review the recent progress of FPSCs. After a brief introduction, the major features of FPSCs are compared with other types of flexible solar cells in a broad context including silicon, CdTe, dye-sensitized, organic, quantum dot and hybrid solar cells. In particular, we highlight the major breakthroughs of FPSCs made in 2019/2020 for both laboratory and large-scale devices. The constituents of making a FPSC including flexible substrates, perovskite absorbers, charge transport materials, as well as device fabrication and encapsulation methods have been critically assessed. The existing challenges of making high performance and long-term stable FPSCs are discussed. Finally, we offer our perspectives on the future opportunities of FPSCs in the field of photovoltaics.

Introduction

Whilst human civilization continues to march forward, accompanied by an impending energy shortage, the demand for clean and renewable energy is an important issue still to be addressed. Among the rising tide of renewables, solar energy is abundant and less dependent on geographical locations compared with other forms of clean energy including wind, geothermal, and hydroelectric power. The metal halide perovskites with a formula of ABX₃ (A = Cs⁺, CH₃NH₃⁺, or CH(NH₂)₂⁺; B = Sn²⁺ or Pb²⁺; X = Cl⁻, Br⁻, or I⁻), have gained considerable attention because of their potential for low-cost and simple manufacture among the many types of solar cells, making it a rising star amongst the next-generation technology. The best power conversion efficiency (PCE) of perovskite solar cells (PSCs) is now recorded as 25.2% [1], achieving the highest value among thin-film solar cells. A significant advantage of perovskite solar cells over other types of thin-film solar cells is the abundance of precursor materials, making them suitable for mass production. Other types of third-generation solar cells, including organic photovoltaics (OPVs), dye-sensitized solar cells (DSSCs), quantum dot solar cells (QDSCs) and so on, with each suffering from either complicated fabrication processes or relatively low PCEs, preventing them from commercial exploitation. Thus, PSCs could become an encouraging candidate for the 4th generation solar cell technologies in terms of manufacture cost and device efficiency [2].

The efficiency of flexible PSCs (FPSCs) has reached up to 19.5% within 6 years (see Fig. 1). Advantages of FPSCs such as high power-to-weight ratio, compatibility with irregular surfaces, high portability, etc. have surpassed their rigid counterparts. Comparing performances with other types of flexible solar cells, i.e. OPVs, DSSCs, cadmium telluride (CdTe) and thin-film silicon solar cells, etc., (see Table 1), FPSCs are more likely to be commercialized in the near future due to their better PCEs and simpler manufacture process.

With the rapid advancing of the fifth generation of mobile networks (5G) and 4 K resolution screens, larger and faster cell phones are in high demand. For portable electronic devices, ultra-light and ultra-flexible are principal priorities. From the outset of 2019, flexible and foldable cell phones have been introduced by Huawei and Samsung, marking 2019 as the first year with wide-spread commercial flexible electronics, showcasing their potential as a future trend. Batteries, supplying power to most portable electronics, should ideally be foldable or rollable within the electronic device configuration. Furthermore, PSCs are deemed to manifest advantageous capabilities in near-space applications [19], and the weight of the spacecraft is extremely important. Thus, cheap and ultra-light FPSCs could be combined with these devices to supply power at any time and anywhere.

Another important reason why many researchers continue working towards efficient and stable perovskite solar cells is their economic advantage. Chang et al. performed a complete assessment, estimating the manufacturing costs and levelized cost of energy (LCOE) to be in the range of 87–140 dollars per square meter, and 3.5–4.9 cents per kilowatt per hour, respectively [20]. Whilst the FPSCs can be manufactured via a roll-to-roll process, the LCOE can be certainly decreased. Standard perovskite PVs can be cost-competitive with c-Si (∼$0.02/kWh [21]) and thin-film CdTe modules (∼$0.04/kWh [22]), and the effort is still underestimated their economic potential due to the choice of module production, with the use of costly materials (e.g., Ag or Au), and slow throughput deposition techniques (e.g., thermal evaporation). Song et al. demonstrated a more promising PSC structure and pragmatic manufacturing process to further decrease the manufacturing cost to as low as 6.3 cents per kilowatt per hour [23].

In this article, we start with the introduction of FPSCs, followed by the critical assessment of their constituent materials, including flexible substrates, charge transport materials, perovskite absorbers, and metal electrodes. We then discuss the fabrication and encapsulation methods for FPSCs. Following this, current challenges will be discussed, primarily focusing on device stability and fabrication of large-area devices. Finally, we offer our perspectives on the future research directions of FPSCs.