J. Cent. South Univ. (2020) 27: 1104-1133

DOI: https://doi.org/10.1007/s11771-020-4353-7

Solution-processed perovskite solar cells

CHANG Jian-hui(常建辉), LIU Kun(刘坤), LIN Si-yuan(林思远), YUAN Yong-bo(袁永波),

ZHOU Cong-hua(周聪华), YANG Jun-liang(阳军亮)

Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Perovskite solar cells (PSCs) have emerged as one of the most promising candidates for photovoltaic applications. Low-cost, low-temperature solution processes including coating and printing techniques makes PSCs promising for the greatly potential commercialization due to the scalability and compatibility with large-scale, roll-to-roll manufacturing processes. In this review, we focus on the solution deposition of charge transport layers and perovskite absorption layer in both mesoporous and planar structural PSC devices. Furthermore, the most recent design strategies via solution deposition are presented as well, which have been explored to enlarge the active area, enhance the crystallization and passivate the defects, leading to the performance improvement of PSC devices.

Key words: perovskite solar cells; mesoporous structure; planar structure; solution process; large-scale deposition techniques

Cite this article as: CHANG Jian-hui, LIU Kun, LIN Si-yuan, YUAN Yong-bo, ZHOU Cong-hua, YANG Jun-liang. Solution-processed perovskite solar cells [J]. Journal of Central South University, 2020, 27(4): 1104-1133. DOI: https://doi.org/10.1007/s11771-020-4353-7.

1 Introduction

Organic-inorganic hybrid halide perovskite materials have recently become one of the most intensively investigated optoelectronic materials due to their merits of high absorption coefficient [1], high carrier mobility [2, 3], long electron-hole diffusion length [4, 5], tunable direct bandgap [6, 7], and low exciton binding energy [8, 9]. The power conversion efficiency (PCE) of perovskite solar cells has been significantly jumped from 3.8% to 25.2% in less than 10 years [10-17]. Not only is this PCE nearing that of market leading single crystal silicon cells, but also the solution processability and compatibility with printing and roll-to-roll (R2R) manufacturing process offer the potential for low-cost photovoltaic technology.

In general, organic-inorganic hybrid halide perovskite materials possess an ABX₃ structure, where A-site typically utilizes methylammonium (MA⁺),formamidine (FA⁺), or Cs⁺, B-site can be Pb²⁺ or Sn²⁺ and X-site is halide ion (Cl^-, Br^-, I^-). There are several methods for depositing the perovskite light absorbing layer in PSC devices, such as solution process, co-evaporation, chemical vapor deposition (CVD), close space sublimation [18-21]. As compared with other types of deposition methods, the solution process exhibits the advantages of low cost and the compatibility with large-scale, high-throughput R2R manufacturing processes.

PSCs are typically classified into two types of device architectures, i.e., conventional structure (n-i-p) and inverted structure (p-i-n)(Figures 1(a)-(c)), and conventional structural PSCs include mesoporous structure and planar structure, of which organic small molecular or polymer materials are normally used as the hole transport layer (HTL). For the mesoporous structural PSCs (meso-PSCs), the electron transport layer (ETL) includes both the compact layer and the mesoporous layer. However, the fabrication of mesoporous metal oxides typically requires high sintering temperature over 400 °C to create the conductive phase, limiting the development of flexible meso-PSCs. While the planar structural PSCs do not require the induction of mesoporous frameworks and high-temperature sintering, which makes the fabrication of PSCs be more diverse at the temperature lower than 150 °C. The state-of- the-art PCEs of planar n-i-p PSCs are more than 23%, of which SnO₂ is used as the dense ETL [12]. Due to the advantages of low-temperature solution processing, low hysteresis effects and compatibility with flexible substrates, the inverted planar PSCs have drawn tremendous attention and show high efficiency close to that of the meso-PSCs [22-24]. Many efforts in device fabrication, composition engineering, post-treatment engineering and material optimization have been devoted to optimizing the efficiency and properties of the inverted planar PSCs [25].

This review first provides an overview on solution-processed meso-PSCs (Section 2), including PSCs using mesoporous TiO₂ (mp-TiO₂) or doped mp-TiO₂ as electron-transporting materials, as well as PSCs using mesoporous Al₂O₃ (mp-Al₂O₃) as the scaffold. Sections 3 describes solution- processed perovskite layer for planar structural PSCs, including spin coating deposition and scalable deposition such as doctor blading, slot-die coating, spray coating and other deposition techniques. Section 4 summarizes the ETL and HTL for solution-processed conventional planar PSCs, while Section 5 summarizes the HTL and ETL for solution-processed inverted planar PSCs. Section 6 summarizes the solution-processed top electrodes. Section 7 provides concluding remarks and summarizes the outlook.

2 Solution-processed meso-PSCs

2.1 PSCs using mp-TiO₂ as electron-transporting material

Following the pioneered works in dye- sensitized solar cells, PSCs were successfully fabricated based on mp-TiO₂ scaffold. Besides the first report by MIYASAKA in 2009 using perovskite nanocrystallites as “sensitizers” and also the liquid-state electrolyte as hole-transporting material [10], GRATZEL et al [26] deposited perovskite film directly on mp-TiO₂ by sequential processes in 2013. The PbI₂ solution was first deposited onto mp-TiO₂, and subsequently transformed into the MAPbI₃ perovskite by soaking it in MAI solution. As shown in Figure 2(a), perovskite film of one-step process has a large grain size, but the distribution is uneven, and the coverage is relatively low. However, perovskite film prepared from sequential two-step deposition has uniformly-distributed grain size, with higher coverage (Figure 2(b)). The device achieved the PCE of 15.0% with short-circuit current (J_sc) of 20.0 mA/cm², open-circuit voltage (V_oc) of 0.993 V, and fill factor (FF) of 0.73 (Figure 2(c)).

Figure 1 Schematic illustration of PSCs with conventional mesoporous structure (a), conventional planar structure (b), and inverted planar structure (c)

Figure 2 SEM images of perovskite films fabricated via one-step (a) and two-step (b) sequential deposition on FTO substrate, (c) J–V curves for a best-performing cell measured at a simulated AM 1.5G solar irradiation of 96.4 mW/cm² (solid line) and in the dark (dashed line) [26] (Copyright 2013 Nature)

In 2016, GRATZEL et al [27] embedded small and oxidation-stable rubidium (Rb⁺) cation into “A site” and created a cationic blended perovskite. The results indicated that Rb⁺ could modify the crystal lattice of perovskite and favor the growth of perovskite black-phase, leading to the PCE up to 21.8% with J_sc of 22.8 mA/cm², V_oc of 1.18 V and FF of 0.81. In addition, excellent photo-stability was achieved, and 95% of its initial efficiency was retained though the RbCsMAFA device had been thermal treated at 85 °C for 500 h.

The PCE of mp-TiO₂ based PSCs was further improved via the modification in HTL material. In 2018, SEO et al [15] synthesized a fluorene- terminated hole-transporting material (N²,N²,N⁷, N<sup>7`</sup>-tetrakis(9,9-dimethyl-9H-fluoren-2-yl)-N<sup>2</sup>,N<sup>2′</sup>, N<sup>7</sup>,N<sup>7′</sup>-tetrakis (4-methoxyphenyl)-9,9`-spirobi [fluorene]-2,2`,7,7`-tetraamine, which exhibited a better fine-tuned energy level and higher glass transition temperature, as compared with the typical HTL material 2,2’,7,7’-Tetrakis-(N,N-di-4- methoxyphenylamino)-9,9-spirobifluorene (Spiro- OMeTAD). Thus, the PCE up to 23.2% could be achieved with J_sc of 24.91 mA/cm², V_oc of 1.144 V and FF of 0.883. The devices also exhibited better thermal stability.

Although meso-PSCs with metal electrode have achieved optimistic PCEs, these devices suffer from the stability issues due to the corrosion behavior of perovskite materials. Thus, carbon- electrode was developed to be used as the electrode instead of metal ones. In 2014, HAN et al [28] filled the mesoscopic skeleton of “mp-TiO₂/mp- ZrO₂/C” with precursor containing MAI, HOOC(CH₂)₄NH₃I (5-AVAI) and PbI₂ in the solvent of γ-butyrolactone (Figure 3(a)). The 5-AVAI was found to play a key role in carbon-electrode PSCs, in which the group of “—COOH” in 5-AVAI could bind to the inner surface of the mesoporous skeleton and the terminal group of “—NH₃⁺” could serve as “nucleation site” for the subsequent growth of perovskite crystallites. As a result, “anchoring effect” appears, and it can enhance the loading behavior of perovskite crystallites on the skeleton. The TEM images (Figures 3(c) and (d)) show that uniform and compact loading could be achieved after the addition of 5-AVAI, leading to the upgraded J_sc and PCE, as shown in Figure 3(b). Such strategy has been verified as well when using TiO₂ nanoparticle-binding carbon-film as the top electrode in the hole-conductor-free meso-PSCs [29-32].

2.2 PSCs using doped mp-TiO₂ as electron- transporting material

Hysteresis is one of the key issues in PSCs, especially for those based on mp-TiO₂. The doping process is developed to treat mp-TiO₂ for hopefully decreased hysteresis. GRATZEL et al [33] doped mp-TiO₂ using Li and hysteresis behavior was greatly decreased (Figures 4(a) and (b)). X-ray photoelectron spectrometric (XPS) results showed that Li-treatment could induce the conversion of partial Ti⁴⁺ to Ti³⁺, as shown in Figures 4(c)-(f). Besides, the concentration of sub-bandgap states was reduced as well (Figure 4(g)), which improved the electron transport within mp-TiO₂ (Figure 4(h)). Besides Li, other elements have also been used as the doped materials, such as Nb [34], Y [35], Co [36].

Figure 3 Schematic of fully printable carbon-electrode meso-PSCs (a) (The mesoporous layers of TiO₂ and ZrO₂ have a thickness of ~1 and 2 mm, respectively, and are deposited on a FTO-covered glass sheet. They are infiltrated with perovskite by drop-casting from solution); (b) J-V curves under simulated AM 1.5 solar irradiation at an intensity of 100 mW/cm² measured at room temperature; (c) and (d) TEM images of MAPbI₃ and (5-AVA)_x(MA)_1-xPbI₃ [28] (Copyright 2016 Science)

Though TiO₂ has been widely used in PSCs as a famous ETL, the active photocatalysis effect is thought as a shortage in application of optoelectronic devices, especially for solar cells. Such feature is mainly caused by the UV sensitivity of TiO₂. In 2017, LIU et al [29] observed a kind of irreversible light-soaking behavior in mp-TiO₂ based PSCs. With scanning round goes on, the PCEs increases, and the value returns to its starting point even though the device was recovered in dark for 1 to 4 days. Such behavior is different from the previously reported “reversible behavior” [37], it was caused by UV light-induced oxygen vacancies in TiO₂. The oxygen vacancies lead to “N-type” doping effect and increase the film conductivity and improve interfacial charge transfer.

2.3 PSCs using mp-Al₂O₃ as scaffold

Besides mp-TiO₂, other mesoporous materials have been used as the scaffold in PSCs as well. In 2012, SNAITH et al [38] reported a “meso- superstructured solar cell” using mp-Al₂O₃ instead of mp-TiO₂ (Figure 5(a)). As the Al₂O₃ was applied in the device, the V_oc was over 200 mV higher than the mp-TiO₂ based PSCs (Figure 5(b)). For TiO₂-based PSCs, the electrons transport to the TiO₂ and the holes transfer to the Spiro-OMeTAD after light absorption in perovskite. For Al₂O₃-based PSCs, the electrons remain in perovskite phase until they are collected by compact TiO₂-coated FTO electrode, and they must transport throughout the perovskite thickness. Therefore, the Al₂O₃ scaffold did not act as an n-type layer in PSCs. The charge generation in both kinds of PSCs was performed by photoinduced absorption (PIA), as shown in Figure 5(c). The PIA spectrum of TiO₂-based perovskite revealed feature in near-IR due to the free electrons in Ti, but the Al₂O₃-based perovskite exhibited no PIA signal, confirming the role of “scaffold”. Furthermore, it was proved that the charge collection in Al₂O₃-based PSCs was faster by a factor of over 10 than that of TiO₂-based PSCs, indicating that the electron diffusion was faster through the perovskite than the n-type TiO₂ (Figure 5(d)).

Figure 4 Cross-sectional SEM image of PSCs (a); J-V curves of PSCs with (red) and without (black) Li doping (b); doped mp-TiO₂ layer for the O 1s peaks (c) and the undoped control (d); Ti 3s and Li 1s peaks slightly visible at 55 eV for Li-doped (e) and the signal for the undoped TiO₂, which reveals the absence of the peaks related to Ti 3s (dashed line at 61.5 eV) and Li 1s, dashed line at 54.9 eV (f); Charge transport lifetime as function of the J_sc prepared with (red) and without (black) Li-doped TiO₂(g); Charge extracted at open circuit as function of the voltage for no treated samples (black) and Li-doped samples (red) [33] (h) (Copyright 2016 Nature)

Figure 5 Schematic illustrating charge transfer and charge transport in a perovskite-sensitized TiO₂ solar cell (left) and a noninjecting Al₂O₃-based solar cell (right) (a); J-V characteristics for Al₂O₃-based cells (red solid trace crosses for high efficiency of Al₂O₃-based cells, red dashed line with crosses for high V_oc of Al₂O₃-based cells, black trace with circles for a perovskite-sensitized TiO₂ solar cell, purple trace with squares for a planar-junction diode with structure FTO/compact TiO₂/MAPbI₂Cl/Spiro-OMeTAD/Ag) (b); PIA spectra of mp-TiO₂ films (black circles) and Al₂O₃ films (red crosses) coated with perovskite with (solid lines) and without (dashed lines) Spiro-OMeTAD hole transporter (c);Charge transport lifetime determined by small-perturbation transient photocurrent decay measurement of perovskite-sensitized TiO₂ cells (black circles) and Al₂O₃ cells (red crosses), both with lines to aid the eye (Inset shows normalized photocurrent transient for Al₂O₃ cells (red trace with crosses) and TiO₂ cells (black trace with circles)) (d) [38] (Copyright 2012 Science)

3 Solution-processed perovskite layer for planar PSCs

Perovskite is a kind of materials with chemical formula ABX₃ (A=MA⁺, FA⁺, Cs⁺; B=Pb²⁺, Sn²⁺; X=I^-, Br^-, Cl^-), has aroused worldwide attention as excellent photoactive materials due to the impressive intrinsic optoelectronic properties including high absorption coefficient, long charge recombination lifetime, facile deposition, large carrier diffusion length and tunable bandgap, etc [13, 39-41]. So far, remarkable progress has been made in enhancing the efficiencies and stabilities of the PSCs. The enhancement is largely attributed to the improved crystallization, morphology, surface coverage of perovskite films owing to innovations in deposition techniques or modified strategies of perovskite films [13, 40, 42-45]. Therefore, enormous deposition techniques and optimization strategies including modifications and post treatments have been widely studied.

3.1 Spin coating deposition

3.1.1 One-step deposition

One-step solution deposition is widely used to fabricate perovskite film, in which a γ-butyrolactone (GBL), dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) solvent solution containing mixed MAI and PbI₂ precursors is directly deposited onto substrates [25]. JENG et al [46] reported the first inverted PSCs based on the perovskite films prepared by spin-coating a DMF precursor solution of equimolar MAI and PbI₂, achieving a 3.9% PCE. Through composition engineering, the perovskite MAPbI_3-xCl_x prepared by spin-coating of MAI and PbCl₂ (3:1 molar ratio) in a DMF solution was utilized as the photoactive layer of inverted PSCs, delivering the PCE up to 10% by SNAITH’s group [25] and up to 11.5% by YANG’s group [22], respectively.

Furthermore, additives engineering is proved to be another efficient strategy to improve film quality. For example, HEO et al [47] demonstrated a hysteresis-less inverted PSCs with a PCE of 18.1% by using hydroiodic acid (HI) as the additive, leading to pinhole-free dense MAPbI₃ films. CORREA-BAENA et al [48] reported a PSCs- based Rb-containing mixed cationic perovskite, of which the hysteresis was suppressed and the carrier lifetime and V_oc were increased. The addition of as little as 1% RbI in precursor solution could suppress the excess PbI₂ in the film. A champion stabilized efficiency of 20.3% for SnO₂ based planar PSCs was achieved.

Poor device performance based on one-step spin-coating deposition mainly arises from non- continuous rough perovskite films, which may be due to the fast-uncontrollable film crystallization. Leakage current derived from poor morphology of perovskite films results in large device efficiency loss and poor device reproducibility [49, 50].

3.1.2 Two-step deposition

To control grain size and crystallization of perovskite films more effectively, various two-step solution deposition techniques have been intensively studied. BURSCHKA et al [26] reported a sequential deposition process of perovskite films in meso-PSCs, in which PbI₂ dissolved in a DMF solution is firstly introduced into the mesoporous metal oxide films and subsequently transformed into the perovskite after tens of seconds by dipping it into a solution of MAI in iso-propyl alcohol (IPA). YANG’s group [51] reported another two-step solution deposition, named vapor-assisted solution deposition, for fabricating highly efficient planar PSCs. The resulting perovskite films with full surface coverage, small surface roughness and large grain size were obtained via in-situ reaction of the as-deposited film of PbI₂ with MAI vapor. The corresponding conventional planar PSCs with the structure configuration of FTO/TiO₂/perovskite/ HTL/Ag achieved a PCE of 12.1%.

BEIN et al [52] prepared MAPbI_3-xCl_x film by immersing the PbI₂ film in a heated solution of a mixture of MAI and MACl in IPA. They highlighted the criticality of chloride in MA lead halide perovskite via a controlled MACl into the MAI immersion solution. The photoluminescence lifetime of the photoexcited species in the device reaches values exceeding 300 ns influenced by the presence of chloride. The resulting devices based on TiO₂ ETL exhibited the PCE approaching 15%. However, the perovskite films fabricated by this immersion process consisted of discontinuous perovskite particles, leading to a low coverage film and deteriorate device performance.

Apart from fabricating the meso-PSCs and conventional planar PSCs, a modified two-step solution deposition, named interdiffusion method, was developed in the inverted planar PSCs (Figure 6). The deposition and thermal annealing process at a low temperature of less than 150 °C resulted in full interdiffusion of spun PbI₂/MAI stacking bilayers [53]. Based on the interdiffusion, homogeneous, uniform and pin-hole free MAPbI₃ films formed and an efficient inverted device with a PCE up to 15.4% was obtained. Later, BI et al [54] studied the influence of wettability of various HTL materials such as PEDOT:PSS, polyvinyl alcohol (PVA), crosslinked compact-OTPD on the perovskite film morphology of inverted devices.

Figure 6 Schematic of interdiffusion deposition (a); SEM images of PbI₂ film (b) and annealed MAPbI₃ film deposited by an interdiffusion deposition (c) and one-step deposition (d) [53] (Copyright 2014 RSC)

They found that non-wetting HTLs benefited the formation of perovskite films with larger grain sizes, which significantly reduced the trap density of perovskite films and boosted the PCE to 18.3%. In another study, by inserting a tunneling layer, such as insulating polystyrene (PS), Teflon, or polyvinylidene-trifluoroethylene copolymer (PVDF- TrFE), between perovskite and ETL, the interface carrier recombination was suppressed due to the hole blocking effect of the interfacial insulating polymer. By using this nonlattice-matching structure, the PSCs fabricated by two step spin-coating achieved a high PCE of 20.3% and exhibited water-resistant [55].

3.2 Scalable deposition

The past few years have witnessed the successful application of spin-coating deposition in the fabrication of highly efficient PSCs. However, spin coating is not suitable for scalable production due to non-uniform large-area films resulted from centrifugal force. Therefore, various scalable deposition techniques have been proposed to satisfy the potential mass production of PSCs without the sacrifices of device performance. Some representative scalable techniques including doctor- blade coating, slot-die coating, spray coating will be briefly discussed below [19, 56-58].

3.2.1 Doctor-blade coating

Doctor-blade coating is a simple, low-cost, high-throughput deposition process for the fabrication of planar PSCs. In order to deposit a uniform and pin-hole free thin film, a precursor solution of PbI₂ and MAI dissolved in a specific solvent is dropped onto substrate, and subsequently swiped by a blade at a certain speed [57](Figure 7(a)). In this deposition, the film thickness can be controlled by the concentration of precursor solution, the gap between blade and substrate, the sweeping speed of blade across substrate, surface tension force, annealing temperature, etc [59-63].

JEN et al [56] firstly reported the doctor- bladed PSCs based on the uniform MAPbI_3-xCl_x perovskite films. The doctor-bladed perovskite films were deposited in ambient condition, achieving a self-assembled large crystalline domains and improved device stability. Later, the impact of doctor-blading speed on the perovskite film morphologies was studied. For example, HUANG’s group [64] chose a large speed of over 50 mm/s in the Landau–Levich region and incorporated surfactants into the precursors solution, enabling doctor-blading of smooth perovskite films with small surface roughness. They found that a very small amount of surfactants dramatically altered the fluid drying dynamics, increased the adhesion of the perovskite ink to the underlying non-wetting substrates and passivated the charge traps. The resulted PSCs achieved PCEs over 20% in small area and module efficiencies of 15.3% measured at aperture area of 33.0 cm². Similar to PSCs based on spin-coated perovskite films, PSCs based on doctor-bladed perovskite films exhibit improved performance in efficiency and stability through composition engineering [64-68]. For example, a bilateral alkylamine (BAA) additive was incorporated into the perovskite doctor-blading process and could passivate the grain surface effectively. This defect-passivation effect resulted in the enhancement of device performance and stability, achieving a PCE of 20% (aperture,1.1 cm²) and keeping 90% of the initial efficiency after 500 h under light.

3.2.2 Slot-die coating

Slot-die coating is a R2R compatible process for processing large-area PSCs due to its continuous ink supply. It involves the transportation of perovskite solution from the ink reservoir of slot-die head to the substrate and the formation of perovskite film under elevated temperature (Figure 7(b)) [69].

Based on the modification of a 3D printer, slot-die coating was successfully used in the scalable fabrication of PSCs after the point nozzle was substituted by a mini slot-die head with metal shims [58]. Later, a PC-controlled system with temperature-control enabled the fully slot-die coated PSCs with exception of evaporated metal electrode. Furthermore, combined with gas- quenched treatment, this process achieved more dense and uniform PbI₂ films [70]. In addition, ZHU et al [71] demonstrated a slot-die coating process for the fabrication of PSCs at room temperature and in ambient environment, achieving a reverse J-V sweep PCE of 18%. In particularly, a robust perovskite precursor ink with long wet-film processing wind used in this slot-die coating process could be also utilized in other deposition methods such as doctor-blade coating, spray- coating and spin coating.

3.2.3 Spray coating

As another scalable deposition approach, spray coating exhibits great potential in the fabrication of thin films on varied substrates. Spray coating process involves the solution atomization into droplets under ultrasound, droplet spray under gas flow and material crystallization with solvent evaporation (Figure 8(a)) [62, 72].

Figure 7 Schematic illustration of doctor-blade coating (a) and slot-die coating (b) [18] (Copyright 2019 Wiley)

Figure 8 Schematics of spray coating (a) [51] and micro-gravure printing (b) [74] (Copyright 2019 RSC)

BARROWS et al [53] introduced one-step spray coating of the blend precursor solution containing MAI and PbCl₂ into the MAPbI_3-xCl_x film deposition. Followed by thermal annealing, the PSCs achieved a PCE of 11%. Furthermore, by adjusting the composition of DMF and GBL mixed solvents, the largest MAPbI_3-xCl_x crystal grains at a 8:2 volume ratio of DMF:GBL were obtained via spray coating due to the flux balance, resulting in a high PCE up to 18.3% in inverted planar PSCs [72]. However, spray coating may be impractical for the potential use due to material loss and low-resolution patterning.

3.2.4 Other deposition techniques

On the pathway toward mass production of large-area PSCs and modules, gravure printing combined with scalable R2R fabrication might be a possibility (Figure 8(b)). The advance of well- defined edges, controllable thickness and high material utilization make the gravure-printing are the most compatible techniques with R2R process in large-scale production. Gravure printing is frequently used for the deposition of patterned structures in polymer solar cell modules [73, 74], and the potential of gravure printing as a scalable printing to fabricate perovskite devices has been demonstrated. HU et al [61] reported high oriented, large-area, ultra-long MAPbI₃ perovskite nanowires fabricated by a large-scale R2R micro-gravure printing process on flexible polyethylene terephthalate (PET) substrate in ambient environment. Thereafter, TONG et al [75] reported high-quality Cs-doped triple cation perovskite film fabricated by R2R micro-gravure printing onto PET substrate. Furthermore, GONG et al [76] fabricated a SnO₂ ETL on a PEN-based ITO substrate used R2R micro-gravure printing process and deposited triple-cation (MA/FA/Cs) perovskite film used a R2R sequential deposition process, i.e., first Cs-doped PbI₂ was deposited by R2R micro- gravure printing and then triple-cation halide films was deposited by R2R slot-die coating. The PCEs of triple-cation flexible PSCs fabricated using R2R sequential printing was up to 10.56%.

In addition, various other attempts, such as inkjet printing [18, 77, 78], screen printing [79, 80], hot-casting [81], soft-cover deposition [82, 83], have also been used to fabricate PSCs.

3.3 Post-treatment

The perovskite film morphology involving surface coverage, grain size, crystallinity and uniformity, etc., is crucial to the device performance. Post-treatment, as an efficient strategy of enhancing film quality, have been intensively studied [19]. The schematics of some representative post-treatments including solvent annealing, Ostwald ripening and methylamine gas treatment are illustrated in Figure 9.

The solvent-annealing treatment is an effective strategy for enhancing the crystallinity and quality of MAPbI₃ films [84, 85]. SEOK’s group designed a novel solvent-engineering strategy, through which the use of a mixed solvent of GBL and DMSO followed by a toluene drip, extremely homogeneous perovskite layer via a MAI-PbI₂-DMSO intermediate phase could be formed. The PSCs using this solvent-engineering technology reached a certified PCE of 16.2% [86]. HUANG’s group utilized another solvent annealing treatment of dissolving the PbI₂ and MAI at the interfaces by DMF vapor to increase the MAPbI₃ grain size and crystallinity, leading to performance enhancement with the PCE over 14% and the diffusion length over 1 μm resulting from deeper diffusion of precursor ions and molecules than in all-solid state [84]. A cryogenic process of cooling as-cast precursor films down and subsequently blow-drying with nitrogen gas could control the perovskite film deposition, achieving anti-solvent-free highly efficient PSCs [87].

Figure 9 Schematics illustrating representative post-growth treatments for perovskite films, i.e., Ostwald ripening, solvent annealing and methylamine gas treatment [19] (Copyright 2018 Nature)

Interestingly, the MAPbI₃ shows a unique soft-matter nature. A surface tuning strategy of modifying the interface between perovskite and HTL by surface agents employed in the fabrication of perovskite films without inducing the formation of recombination or degradation centers [88]. A methylamine (CH₃NH₂) induced defect-healing (MIDH) behavior, which resulted from the reversible reaction of CH₃NH₃PbI₃ and CH₃NH₂ and the formation of an intermediate CH₃NH₃PbI₃·xCH₃NH₂ liquid phase, was observed. The performance of PSCs treated by MIDH exhibited a dramatically enhanced efficiency due to film morphology improvement [89]. On the other hand, ZHAO et al [90] reported an Ostwald ripening process for the formation of compact, large-grain and pinhole-free MAPbI_3-xI_x films. The MABr treatment via spin coating with proper concentration was proved to be an effective approach to construct high-quality perovskite films with larger grain size.

As mentioned above, uniform and pin-hole free perovskite films with large grain size are crucial to the formation of high-performance PSCs for the reduced defects caused by grain boundaries and density of trap states. Therefore, defect passivation can effectively improve the film quality, thereby obtaining the highly efficient PSCs. The (6,6)-phenyl C61-butyric acid methyl ester (PCBM) layer spun onto the light absorbing layer can passivate the charge trap states on the surfaces and boundaries of perovskite materials, yielding a hysteresis-free device with minimized charge recombination and enhanced efficiency [91,92]. As compared to PCBM passivation strategy, HUANG et al [93] reported one more effective ionic defect passivation strategy by utilizing quaternary ammonium halides (QAHs) in the fabrication of perovskite film, leading to remarkably increased V_oc and improved efficiency due to reduced surface trap density and long carrier lifetime. The PCEs of PSC devices containing MAPbI₃ and FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃ were 20% and 21%, respectively.

In addition, an exciting surface defect passivation strategy was reported by YOU’s group, and the certificated PCE of planar PSCs increased up to 23.32% with a high V_oc of 1.18 V. They developed an organic halide salt, phenethylammonium iodide (PEAI), by which the defects of FA/MA mixed perovskite films were effectively passivated and non-radiative recombination caused by defects became much less [12]. The above post-treatment is a popular strategy to increase V_oc. LIN et al [94] reported a n-butylamine (BA) treatment to construct a 2D/3D stacking structures for the fabrication of highly stable PSCs. It is different to n-butylammonium iodide (BAI) treatment, (BA)₂PbI₄ produced by reaction of BA with MAPbI₃ has better protection due to its more organic ligands and the mixture of 2D perovskites, leading to the PCE to 19.56%.

4 ETL and HTL for solution-processed conventional planar PSCs

4.1 ETL for solution-processed conventional planar PSCs

The ETL materials usually play an essential role in the performance of PSCs. Qualified ETL materials must meet the following conditions: (1) good energy-level alignment with perovskite for efficient charge transfer and hole blocking; (2) high electron mobility to guarantee fast electron transport within the ETL; (3) high transmittance to ensure full use of sunlight; (4) stability; (5) low cost and easy processing [39, 95-97].

4.1.1 TiO₂ planar ETL

Generally, TiO₂ was used as ETL in planar PSCs. In 2014, EPERON et al [40] first reported efficiencies above 10% in fully solution-processed MAPbI_3-xCl_x PSCs with planar heterojunction architecture and then reported the PCEs up to 14.2% in FA lead trihalide PSCs [41]. Initially, the low PCE of planar n-i-p PSCs was due to the poor morphology of perovskite film because of the lack of a mesoporous layer and the high defect density at the perovskite/TiO₂ interface, which lead to the short diffusion length and carrier lifetime, as well as large non-radiative recombination loss at the interface.

Therefore, a lot of work was focused on the modification of perovskite morphology and passivated perovskite/TiO₂ interface defects. Additive engineering is a common strategy to improve the morphology of perovskite films [42, 43]. ZHOU et al [44] presented a possible microscopic mechanism concerning the perovskite nucleation and grain growth in the presence of weak coordination additives through introducing the weak coordination molecule acetonitrile (ACN) as the additive into the PbI₂/DMF solution. The perovskite film displayed gradually enlarged grain size by increasing its concentration, with the grain size in the range of a few hundreds of nanometers to over 1 μm (Figure 10(a)). Finally, the planner configuration PSCs of ITO/TiO₂/Perovskite/Spiro- OMeTAD/Au with ACN-aided perovskite films achieved an average PCE of 17.43% and the best PCE of 19.7%.

CHEN et al [45] proposed a novel approach to modulate the nucleation and growth of perovskite crystals in planar PSCs by intermixing precursor- capped inorganic nanoparticles of PbS (Figure 10(b)). Through this intermixing-seeding growth technique, substantial morphological improvements were realized in perovskite films, such as increased crystal domains, enhanced coverage, and uniformity. ZHOU et al [98] reported a MA/EtOH (CH₃NH₂/ethanol) additive-based solution process to fabricate high-quality perovskite films. These led to substantial reduction in defect density and extended charge carrier diffusion lengths in the resulted film (Figure 10(c)). Subsequently, the planar PSCs were fabricated by thick perovskite absorber (~650 nm) achieved a PCE of 19.01%.

However, it was found that the TiO₂/perovskite owns the charge barrier. Thus, it would lead to charge accumulation and inefficient charge transfer at the interface [39]. In addition, the TiO₂ film normally contains many oxygen vacancies at the interface that could reduce the long-term stability of solar cell devices. In order to overcome the limitations of TiO₂, a few strategies have been implemented. TAN et al [99] reported a contact-passivation strategy using chlorine-capped TiO₂ (TiO₂-Cl) colloidal nanocrystal film that mitigates the interfacial recombination and improves interface binding in low-temperature planar PSCs. The smooth, pinhole-free mixed cation-halide perovskite FA_0.85MA_0.15Pb_2.55Br_0.45 films with uniform and large grains were formed on TiO₂-Cl (Figure 11(a)), and the PSCs reached a PCE of 21.4%. SUN et al [100] developed a sandwiched TiO_x-Au-TiO_x composites structure to act as the electron conductor to improve the efficiency (Figure 11(b)). Au-NPs can enhance the conductivity and decreased surface potential of TiO_x film. KONG et al [101] reported a planar structure PSCs based double layer structural ETL, i.e., TiO₂ compact layer and SnO₂ amorphous layer, achieved a maximum PCE of 21.4% (Figure 11(c)). After modification with SnO₂, the PSCs based TiO₂/ SnO₂ ETL exhibited a good stability under the UV light because the SnO₂ layer can shield the charge transfer passes.

4.1.2 SnO₂ planar ETL

In order to further improve the PCEs of PSCs, SnO₂ is used to replace TiO₂ as ETL due to its deep conduction band and good energy level, high bulk electron mobility (up to 10^-3 cm² V^-1 s^-1), low- temperature process and excellent chemical stability [39, 102-104]. In recent years, most planar heterojunction PSCs used SnO₂ as the ETL have achieved highly efficient PCEs [105-109]. In 2016, YOU et al [110] developed a regular n-i-p planar structural PSCs with solution-processed SnO₂ nanoparticles as the ETL on ITO substrate, which was baked on a hot plate in ambient air at 150 ^oC for 30 min. The PSCs with a planar structure of ITO/SnO₂/(FAPbI₃)_0.97(MAPbBr₃)_0.03/Spiro-OMeTAD/Au achieved a PCE close to 20%. Now, PSCs based SnO₂ ETL have achieved the PCE as high as 23.3% [12].

Modifying SnO₂ by adding dopants or additives can effectively increase the electron mobility of SnO₂ and engineer energy levels that would better match with those of perovskites. YAN et al [111] developed a SnO₂/PCBM ETL in planar PSCs. In the process of spin-coating perovskite precursor solution, the PCBM could be re-dissolved by DMF and DMSO, which would allow only ultra-thin PCBM to be retained at the interface and some dissolved PCBM infiltrate into perovskite grain boundaries. The PCBM in PSCs plays a role of passivate both the SnO₂/perovskite interface and perovskite grain boundaries and promote electron transfer. The PSCs with an unpassivated SnO₂ ETL achieved a PCE of 16.3% and with a SnO₂/PCBM ETL achieved a PCE of 18.7% (Figure 12(a)). Fullerene C60 was also introduced onto SnO₂ by CAROUS et al to partially fill the surface trap states in SnO₂ and passivated both of the C60/perovskite and SnO₂/C60 interfaces [112]. Meanwhile, the better energy alignment could effective suppression of charge recombination and thus enhanced V_oc.

Figure 10 Top-view SEM images of perovskite films obtained by two-step spin coating using different ratios of ACN in DMF (a) [44] (Copyright 2016 Wiley); Schematic of the ligand exchange process and the proposed nucleation and growth routes of perovskite crystal films (b) (The assisted formation of lead-halide octahedrons in the vicinity of an MAI-capped PbS nanoparticle [45], Copyright 2016 RSC); Current density-voltage characteristics of devices with ITO/perovskite/Au configuration utilized for estimating the defect density in perovskite films (c) [98] (Copyright 2017 Wiley)

Figure 11 Top view of perovskite film on TiO₂-Cl (a) [97] (Copyright 2017 Science); Schematic illustration of device structure ITO/TiO_x-Au-NPs/perovskite/HTL/Ag (b) (Au-NPs are sandwiched between two TiO_x layers [100], Copyright 2015 Wiley); Cross-sectional SEM image of c-TiO₂/a-SnO₂ ETL (c) [101] (Copyright 2018 Wiley)

LIU et al [113] realized an EDTA-complexed SnO₂ (E-SnO₂. EDTA: Ethylene diamine tetraacetic acid) ETL by complexing EDTA with SnO₂ in planar PSCs (Figure 12(b)). As compared to normal SnO₂, the electron mobility of E-SnO₂ ETL increases by about three times. The PSCs with E-SnO₂ ETL achieved PCE as high as 21.60%. HU et al [114] applied the commercially accessible SnO₂ and home-made TiO₂ NPs as a combined electron transport bilayer to achieve planar PSCs that acquired a high PCE of 20.50%.

Recently, new progress has been made in the modification of SnO₂. HUI et al [115] used carboxylic-acid- and hydroxyl-rich red-carbon quantum dots (RCQs) to dope low-temperature solution-processed SnO₂, which increased its electron mobility by ~ 20 times from 9.32×10^-4 to 1.73×10^-2 cm²/(Vs), and a champion PCE for n-i-p heterojunction PSCs up to 22.77% has been achieved. In addition, KCl [116] and graphitic carbon nitride (g-C₃N₄) quantum dots [117] could effectively passivate the defect at the ETL/ perovskite interface and eliminate the oxygen- vacancy-reduced trap centers. As the result, the PCEs of planar PSCs can exceed 22%.

4.1.3. ZnO planar ETL

ZnO is a promising ETL material due to its charge carrier mobility, carrier lifetime and electron diffusion coefficient that is orders of magnitude higher than that of TiO₂ [118]. In 2016, HEO et al [119] fabricated low-temperature processable ZnO ETL for MAPbI₃ PSCs with an average PCE of 17.6%. However, the chemical instability at the interface is a major challenge. The extended heat-treatment made the perovskite change from dark brown to right yellow due to ZnO induced proton-transfer reactions at the ZnO/perovskite interface, leading to eventual decomposition of the perovskite film into PbI₂ [120]. Therefore, a range of approaches including optimizing deposition techniques, heteroatom doping and interface modification have been successfully established to enhance the stability of the ZnO/perovskite interface and the photovoltaic performance. AZMI et al [121] incorporated the alkali cations K⁺ into ZnO, which could improve the electron mobility, raise the Fermi energy level and passivate the defects of ZnO ETLs (Figure 13(a)). Thus, PSCs fabricated with K:ZnO ETL achieved the highest PCE of 19.9% and better stability. TAVAKOLI et al [122] demonstrated improved photovoltaic performance and stability by introducing monolayer graphene (MLG) at the interface between the ZnO ETL and perovskite absorber, resulting in stable PSCs with the PCE up to 19.81% (Figure 13(b)).

4.1.4 Other planar ETLs

There are some other binary ETL materials in addition to TiO₂ and SnO₂, including metal binary oxide, metal sulfide and selenide and metal ternary oxide. These materials exhibit some drawbacks regarding their application as the ETLs, but intriguing properties have been shown when integrated in PSCs.

Figure 12 J-V curves of PSCs using SnO₂ and SnO₂/PCBM ETLs under reverse and forward voltage scanning (a) [111] (Copyright 2016 RSC); J-V curves of PSCs using EDTA, SnO₂ and E-SnO₂ ETLs with inset showing device configuration (b) [113] (Copyright 2018 Nature)

Figure 13 Schematic illustration of energy levels of ZnO-ETLs and perovskite film (a) [121] (Copyright 2018 ACS); Schematics of perovskite films annealed on the top of ZnO ETL and MLG/ZnO ETL (b) [122] (Copyright 2019 RSC)

In₂O₃ can be used as an ETL material because it is an n-type semiconductor that possesses a wide band gap (~3.75 eV), a high electron mobility (~20 cm² V^-1 s^-1) and good thermal stability. In 2016, QIN et al [123] fabricated In₂O₃/PCBM- based planar PSCs with a PCE of 14.8% via a sol-gel route to deposit In₂O₃ and modify the surface with the PCBM. By further optimizing the morphology of In₂O₃ ETL and offset the negative effect from pinholes, PSCs based In₂O₃ ETL have achieved the PCE higher than 16% [124, 125]. Nb₂O₅ has comparable electronic properties and chemical stability with TiO₂. LIN et al [126] first reported Nb₂O₅ as the ETL in planar PSCs and achieved a PCE of 17.2%, exhibiting the great potential in PSCs. Furthermore, WANG et al [127] reported a low-temperature solution-processed Nb₂O₅ nanoparticle-formed film as the ETL in PSCs, and the highest PCE up to 20.22% was achieved with a very high V_oc of 1.19 V. In addition, metal ternary oxide, such as Zn₂SnO₄ [127, 128], BaSnO₃ [129, 130], SrSnO₃ [131] were also employed as the ETLs in planar PSCs.

In addition to oxides, many metallic sulfides and selenides including CdS [132, 133], In₂S₃ [134], TiS₂ [135, 136] and SnS₂ [137] were also employed as ETLs in planar PSCs. DONG et al [132] used CdS thin films in planar PSCs and achieved a PCE of 16.5%. XU et al [134] successfully prepared In₂S₃ ETL via a solvent-thermal method and reached an impressive PCE of 18.83%. In 2018, 2D-TiS₂ nanosheets were prepared by a simple solution exfoliation by YAN et al [135]. Using TiS₂ thin films prepared from solution with an optimized concentration, PSCs show a high PCE of 17.37%. 2D-SnS₂ is also a promising ETL in planar PSCs. Therefore, ZHAO et al [137] fabricated 2D SnS₂-based PSCs and realized a PCE of 20.12%.

4.2 HTL for solution-processed conventional planar PSCs

The HTL is used to extract and transport the holes produced by perovskite and block the electron transport from perovskite to anode, as well as avoid carrier recombination. The HTL plays a critical role in boosting the PCEs of PSCs, and the commonly used HTLs in conventional PSCs, both mesoporous and planar structures, are Spiro-OMeTAD and poly[bis(4-phenyl)(2,5,6-trimethylphenyl)amine(PTAA). Spiro-OMeTAD is very commonly applied as the HTL in high-efficiency conventional structure PSC devices because of their suitable energy level, high solubility, amorphous character and high glass transition temperature. It has also been found to be the most suitable HTL for the fabrication of PSCs due to its facile nature and high device performance. In 2012, all-solid-state DSSCs based on MAPbI₃ demonstrated a PCE of 9.7%, in which Spiro-OMeTAD was used as the HTM for the first time [138]. However, pure Spiro-OMeTAD exhibits lower conductivity and instability, several approaches are reported to overcome these issues. Bis(trifluoromethane)sul-fonimide lithium salt (LiTFSI) and tert-butyl pyridine (TBP) are widely doped in Spiro-OMeTAD. JIANG et al [12] reported the high PCE of 23.32% in PSCs with Spiro-OMeTAD HTL. Up to now, Spiro-OMeTAD is the most efficient HTL for PSCs, the only concern is the stability [106, 110, 139].

PTAA was reported to be a very effective HTL for high-performance PSCs. As a replacement of Spiro-OMeTAD, PTAA was first employed as the HTL in the conventional structural PSCs by SEOL et al and delivered a PCE of 12.0% [140]. SNAITH et al [141] achieved a best PCE of 23.6% under 14 suns illumination and retain its PCE more than 90% after 150 h under 10 suns with PTAA as the HTL. This has been proved to be an ideal candidate to tackle the instability of device, especially under the illumination and the elevated the temperature. However, in most cases, PTAA is used as the HTL in inverted structure PSC, which will be discussed later.

5 HTL and ETL for solution-processed inverted planar PSCs

The performance of perovskite films largely depends on the surface properties of HTL and ETL. Especially, whether the contact of the perovskite layer and HTL or ETL is good determines the extraction efficiency of the photogenerated charges, thereby greatly affecting the PCEs of planar PSCs. On the other hand, the matched energy levels between the perovskite layer and HTL or ETL are crucial to minimize the energy loss during charge extraction.

5.1 HTL for solution-processed inverted planar PSCs

5.1.1 Organic HTLs

As a highly conductive organic polymer blend with high work function (WF) and high optical transmittance [142-146], poly(3,4- ethylenedioxythio phene): poly(styrenesulfonate) (PEDOT: PSS) is the primary candidate that used as the HTL in planar PSCs due to its orthogonal solubility with perovskite precursor solutions [46, 147-154]. Although a reported high PCE above 18% for inverted PSCs with PEDOT:PSS as the HTL and MAPbI₃ as the photoactive layer [47], in most of the cases, the PCEs of inverted PSCs are in a range of 14%-16% because of the energy loss at the PEDOT:PSS/MAPbI₃ interface [53, 155-160]. This energy loss results from the mismatch between the Fermi-level of PEDOT:PSS (-5.2 eV to -4.9 eV) and the valence band (VB) of MAPbI₃ (-5.4 eV) [161]. On the other hand, the absence of high lying lowest unoccupied molecular orbital (LUMO) level in PEDOT:PSS makes it fail to block electrons falling into the HTL. This feature of PEDOT:PSS results in obvious interface recombination and reduction of quasi-Fermi energy level splitting, and then produces a low V_oc, which is another pathway for the energy loss [161]. Moreover, the acidic and hygroscopic PEDOT:PSS may induce instability caused by reactions with adjacent components and moisture filtration.

Considerable efforts have been devoted to the modification of PEDOT:PSS, such as film doping. For example, a polymer electrolyte PSS-Na or dopamine was doped into PEDOT:PSS to tune the WF of PEDOT:PSS, giving rise to highly efficient PSCs with a PCE above 15% [147, 162]. An inverted PSC with 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) doped PEDOT:PSS as the HTL achieved an improved PCE from 13% to 17%, which resulted from the reduced HOMO level and enhanced electrical conductivity [163]. In addition, an interfacial engineering strategy by doping PEDOT:PSS with CsI was developed to enhance hole extraction from the perovskite layer to the HTL and suppress charge recombination at the perovskite/CsI-PEDOT:PSS interface, realizing inverted PSCs with excellent PCE of 20.22% [164]. Except doping strategy, the post-treatment of PEDOT:PSS with solution washing is another effective strategy to improve its conductivity and morphology by removing insulating PSS from PEDOT:PSS films. For example, OUYANG et al [165] reported that the PEDOT:PSS film treated by the MAI solution exhibited significantly enhanced conductivity.

In order to minimize the energy loss during the hole extracting from the perovskite layer to the HTL, the HTL materials with deep lying HOMO levels are desired to replace PEDOT:PSS as the HTL. Poly (N,N′-bis(4-butylphenyl)-N,N′- bis(phenyl)benzidine) (poly-TPD) with HOMO level of -5.2 eV was then selected as HTL material to extract holes because it can block electrons efficiently and can suffer from the lateral spin coating process of perovskite layer. The average efficiency of solution-processed inverted PSCs based on a perovskite layer sandwiched between a poly-TPD layer and a PCBM layer reached 13.8% [144]. The issue is that the HOMO of poly-TPD still does not match well with the VB of MAPbI₃ [166], which leads to relatively low V_oc value of 1.04 V [167].

As another popular hole transport materials in organic electronics [168, 169], PTAA has a deeper HOMO level of -5.25 eV than that of poly-TPD, making it more promising in inverted planar PSCs (Figure 14) [54, 170]. However, the poor hole mobility of PTAA (10^-3-10^-2 cm²/V·s) make it inefficient in hole extracting. The doping PTAA with F4-TCNQ was then investigated to reduce resistivity of HTL. Based on F4-TCNQ doped PTAA, inverted PSCs with thick PTAA exhibited enhanced V_oc and FF without sacrifice of J_sc, boosting the PCE up to 17% [54, 171].

Figure 14 Device structure of inverted planar heterojunction PSCs: ITO/PTAA/MAPbI₃/PCBM/C60/ BCP/Al [172] (Copyright 2016 Nature)

5.1.2 Inorganic HTLs

As alternatives to organic materials such as PEDOT:PSS and PTAA, inorganic HTL materials have drawn broad attention due to their better stability. P-type metal oxides, such as NiO_x, Cu-doped NiO_x, cuprous oxide (Cu₂O) and copper oxide (CuO), etc., are frequently employed as the HTL materials since they have good electron blocking capability [173, 174].

As compared to PSCs based on PEDOT:PSS, inverted PSCs with NiO_x as the HTL exhibited a remarkable enhanced photovoltaic parameters including V_oc of 0.92 V, J_sc of 12.43 mA/cm², FF of 0.68 and PCE of 7.8%, superior to that of PEDOT:PSS based PSCs with V_oc of 0.62 V, J_sc of 9.39 mA/cm², FF of 0.66 and PCE of 3.9% [175]. Similar to PTAA, NiO_x also faced the issue of high resistivity due to its insufficient hole mobility. Adjusting its WF or doping NiO_x by metal elements may be an effective approach to improve the PSCs performance. For example, a co-doping strategy of employing Ni_xMg_yNi_1-x-yO as HTL material to extract the charge carriers was introduced, achieving a high PCE of 18.4% [176]. A PCE exceeding 20% could be achieved in inverted planar PSCs based on the high-quality HTL material of copper doped NiO (Cu:NiO). The PSCs with doping NiO_x with 2,2′-(perfluoronaphthalene- 2,6-diylidene) dimalononitrile (F6TCNNQ) molecules as the HTL could achieved the PCE as high as 20% [177].

Due to the low-lying VBs of Cu₂O and CuO that match well with the VB of the perovskite layer, a relative low energy loss existed in inverted PSCs with metal oxide based HTLs. DING’s group [174] reported the fabrication procedure of Cu_xO based inverted planar PSCs. Cu₂O and CuO films were firstly prepared by a facile low-temperature deposition and used as HTLs, exhibiting remarkably enhanced V_oc, J_sc and PCE. Apart from metal oxide, some materials such as graphene oxide (GO) [178], copper thiocyanate (CuSCN) [179], poly[(9,9-dioctyl-fluorenyl-2,7-diyl)-co-(4,4’-(N-(4-secbutyl-phenyl) diphenylamine)] (TFB) [180], etc., are also employed as the HTL materials in inverted PSCs. For instance, an ultrathin TFB film was employed as the HTL, achieving a PCE as high as 20% in inverted PSCs based on one-step coated perovskite layer.

5.2 ETL for solution-processed inverted planar PSCs

5.2.1 Fullerene and fullerene derivatives as ETL

In inverted PSCs, fullerene and its derivatives, such as ICBA, ICTA, and PC₇₁BM, etc. [49, 85, 181], are the most popular ETL materials due to their excellent electron mobility and hole-blocking capability. Among them, PCBM is one representative electron acceptor [25, 166, 182, 183]. Firstly, fullerene-based materials were used in inverted PSCs to form a donor-acceptor (D-A) structure based on MAPbI₃/C₆₀ (or PCBM, etc.) planar heterojunction, the corresponding device achieved a maximum PCE of 3.9% for effective charge separation at the D-A interface [46]. With the rapid development of PSCs in terms of materials, structures and modification technologies, the PCEs of fullerene-based inverted devices have been continuously improved [53, 54, 84, 181].

The use of doped fullerene or fullerene derivatives as the ETL materials is an effective strategy to improve electron transport capability or improve film coverage. For example, oleamide [184], graphdiyne (GD) [185] and PS [55] was added into PCBM to realize electron transport enhancement. A charge-carrier-balance strategy that used the PCBM modified by poly(methylmethacrylate) (PMMA) additive and solvent-treated PEDOT:PSS as the ETL material and HTL material, respectively, was developed in inverted PSCs, achieving an enhanced PCE of 18.72% [186].

5.2.2 Non-fullerene ETLs

Although fullerene or fullerene derivatives are the dominant ETL materials in inverted PSCs, the poor film morphological stability and energy level adjustment restrict their broader applications. Therefore, an urgent need to develop new non-fullerene materials as the ETLs has promoted the emergency of new techniques or strategies.

Han’s group developed n-doped (n+) TiO_x to extract selectively electrons, achieving highly efficient devices based on MAPbI₃-PCBM architecture [176]. Perylenetetracarboxylic dianhydride (PTCDA), a typical perylene derivative, was employed as the ETL in inverted PSCs, achieving a maximum PCE of 16.5%. By inserting pyrene-substituted silicon phthalocyanine (SiPc- Py-2) into the perovskite/PTCDA interface, the PSCs boosted the PCE up to 19.2%, which resulted from the passivation effect [187]. A donor-acceptor (D-A) structured non-fullerene ETL materials, in which triphenylamine (TPA) and (3-cyano-4,5,5- trimethyl-2(5H)-furanylidene)malononitrile (3CN) were employed as the donor group and acceptor group, respectively, was utilized in the fabrication of inverted PSCs, exhibiting a comparable PCE of 19.2% with that of PCBM based PSCs [188].

6 Solution-processed top electrode

In PSCs, the top electrode can be prepared by a solution process. More importantly, PSCs with solution-processed top electrode can be semi- transparent, making them applicable tandem solar cells [189]. In addition, it also shows the feasibility for the low-cost, large-scale and high-throughput roll-to-roll production of solid-state devices. Up to now, various solution-processed top electrodes have been reported in PSCs, mainly metal-nanowire [190] and carbon materials [191].

It is strongly believed that the integration of Ag nanowires (AgNWs) network is the most promising solution processable electrodes for using in PSCs. 2016, DAI et al [192] fabricated PSCs using solution-processed AgNWs as transparent top electrode with markedly enhanced device performance, as well as stability, by evaporating an ultrathin transparent Au layer beneath the spin- coated AgNWs forming a composite transparent metallic electrode. However, when the AgNWs network is prepared by spin coating, the solvent in the AgNWs dispersion will corrode the perovskite layer, resulting in a decrease in the PCE and poor long-term stability. In addition, spin coating is not suitable for large-scale preparation. In order to reduce solvent damage to the perovskite films, spray coating was utilized onto the PSC devices. LEE et al [193] deposited AgNWs electrodes on the top of the Spiro-OMeTAD HTL by spray coating at moderate temperatures and achieved a PCE of 7.45%. ZHANG et al [194] reported a planar heterojunction PSC employing AgNWs as the top electrode and ZnO nanoparticles as the ETL. The PSC exhibited the highest PCE of 9.21%. It is worth nothing that the spray coated AgNWs are found to be weakly connected, which leads to the poor conductivity as well as low stability. To solve this problem, HAN et al [195] spray coated an additional layer of ZnO nanoparticle to form an AgNWs/ZnO composite electrode. The conductivity of the AgNWs/ZnO composite electrode was enhanced due to the improvement of the interconnection between AgNWs by filling the voids between AgNWs with ZnO nanoparticles. Finally, the high PCE of 13.27% with an averaged transmittance of 16.3% was achieved. In addition, inkjet printing can also be used to prepare AgNWs electrode. XIE et al [196] reported the preparation of transparent AgNWs top electrode for PSCs using inkjet printing process. By inserting a thin layer of polyethylenimine (PEI), the charge injection barrier between PC₆₁BM and AgNWs electrode was minimized.

Carbon based materials have received much attention for use as conducting electrode in PSCs, due to their low cost and high stability. Importantly, it can be prepared by the solution processed. In addition, the carbon electrode is compatible with processes such as blade coating, slot–die coating, and screen printing. Thus, fully printing can be realized to prepare large-area PSCs [197]. MEI et al [28] printed carbon black/graphite composite material as the top electrode and achieved highly efficient and long-term stable conventional mesoporous structure PSCs [28]. Today, the PCE of such PSCs has reached 16.26% [198]. However, mesoporous carbon electrodes prepared by printing require high temperature annealing and are not compatible with flexible devices. Embedment carbon electrode is produced by a considerably simplified fabrication process which avoids high temperature, as a promising alternative to mesoporous carbon electrode. WEI et al [199] reported a new type of inkjet-printed carbon electrode. By designing the carbon with CH₃NH₃I ink to transform PbI₂ in situ to CH₃NH₃PbI₃, an interpenetrating seamless interface between the CH₃NH₃PbI₃ active layer and the carbon hole- extraction electrode was instantly constructed, with a markedly reduce charge recombination as compared to that with the carbon ink alone. As a result, a considerably high PCE up to 11.60% was achieved. In addition, the process of covering the carbon electrode directly on the perovskite layer has attracted more and more attention, because this device structure and fabrication strategy is the simplest [200-202]. JIN et al [203] prepared a graphite/carbon black composite electrode by spin coating and fabricated PSCs with a structure of ITO/ZnO/CH₃NH₃PbI₃/graphite/cabon black and the best device yielded 10.2% PCE.

7 Conclusions and outlook

In conclusion, the performance of PSCs fabricated by solution process has improved dramatically. By optimizing the composition of perovskite materials, selecting the appropriate charge transport materials and reasonably modifying the interface, the PCE of small-area PSCs have increased to the level of single crystal silicon solar cells. At the same time, large-area PSC modules have also developed rapidly. During the past few years, a lot of attempts have been conducted on large-scale manufacturing processes and perovskite precursor ink engineering, resulting in rapid increase in the efficiency of PSC modules, e.g., mini-module with a PCE of 16.4% and an aperture area 63.7 cm² has been demonstrated [204]. However, there are still many issues for accelerating the commercialization of solution- processed PSCs, such as stability, toxicity and large-scale fabrication.

The stability issue of PSCs is one of the important factors hindering the commercialization. Although the stability of PSCs has been improved from several minutes to over one year, its improvement is not good enough for practical applications, which is still far away as compared with silicon solar cells. In order to enhance the stability of PSCs, systematic engineering including perovskite materials, charge transport materials, electrode material, structure design and encapsulation techniques should be considered. For example, the addition of two-dimensional perovskite has significantly improved the stability of PSCs. In particular, encapsulation plays a vital role in improving the stability and would help to accelerate the commercialization of PSCs.

As far as the toxicity of lead-based perovskite is concerned, researchers are trying to search for lead-free materials with similar optoelectronic properties. With theoretical calculations and experimental characterizations, a series of ions, like Sn²⁺，Ge²⁺, Cu²⁺, Bi³⁺, Sb³⁺, etc., in ABX₃ structure can replace Pb²⁺ to form lead-free perovskites. Excitingly, organic-inorganic tin-based PSCs have now achieved PCE more than 10%, and there is still much room for improvement. The stability of Sn-based PSCs is far away from the requirement of commercialization. The origin of the instability is that facile oxidation of Sn²⁺ to Sn⁴⁺ and can decompose after exposure to water in air like the lead-based perovskite. So, the other huge challenge is how to restrain the oxidation of Sn²⁺ and improve the air stability of Sn-based perovskite materials.

PSCs fabricated by scalable printing and coating technologies have been impressive progress in the last few years. The control on nucleation and crystal growth of perovskite film are the key developments for the rapid progress in coated and printed large-area PSCs. The ink engineering allows a slow drying and processing time window for subsequent non-solvent treatment appears to be a promising route for reproduce the high-quality perovskite films in a variety large-scale printing or coating techniques, especially for R2R process. The in-situ and post-treatments, such as gas quenching, vacuum induced crystallization, elevated substrate temperatures, etc., can further enhance the quality of large-area perovskite films and accordingly highly efficient large-area PSCs. It is believed that the PCEs of scalable large-area PSCs or modules fabricated via solution-processed coating or printing techniques will be much closed to the ones fabricated via spin coating deposition. It is also the key point of future development of PSCs.

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(Edited by HE Yun-bin)

中文导读

溶液法制备钙钛矿太阳电池

摘要：钙钛矿太阳电池已经成为光伏应用中最有前途的候选者之一。它具有低成本、低温溶液涂布或印刷工艺制备及兼容大规模卷对卷制造工艺等优点，展现出巨大的商业化前景。在这篇文章中，我们重点综述了介孔和平面结构钙钛矿太阳电池器件中溶液法沉积电荷传输层、钙钛矿吸收层及顶电极层。此外，还重点概括了可大规模溶液法制备钙钛矿薄膜的工艺方法、钙钛矿薄膜结晶性增强及表面缺陷钝化等，进而提高钙钛矿太阳电池性能。

关键词：钙钛矿太阳电池；介孔结构；平面结构；溶液法制备；大规模制备

CHANG Jian-hui, LIU Kun and LIN Si-yuan contributed equally to this work.

Foundation item: Projects(51673214, 51673218, 61774170) supported by the National Natural Science Foundation of China; Project(2017YFA0206600) supported by the National Key Research and Development Program of China

Received date: 2020-01-14; Accepted date: 2020-02-28

Corresponding author: YANG Jun-liang, PhD, Professor; Tel: +86-731-88660256; E-mail: junliang.yang@csu.edu.cn; ORCID: 0000- 0002-5553-0186; YUAN Yong-bo, PhD, Professor; E-mail: yuanyb@csu.edu.cn; ORCID: 0000-0002-4606- 4611; ZHOU Cong-hua, PhD, Professor; E-mail: chzhou@csu.edu.cn; ORCID: 0000-0002-7565-9016