Can two-dimensional graphdiyne-based materials be novel materials for perovskite solar cell applications?

Two-dimensional (2D) graphdiyne (GDY)-based materials have attracted attention in the solar cell research community owing to their unique physicochemical properties and hydrophobic nature which can serve as moisture resistance from the surrounding medium. Benefiting from these, the performance and stability ofperovskite solar cells (PSCs) can be greatly improved via the addition of 2D GDY-based materials. This mini-review summarizes the recent development of 2D GDY-based materials for PSC application. The roles of 2D GDY-based materials, such as hole transporting material, electron transporting material, dopant material in perovskite film and interfacial layer, are discussed in detail. Moreover, we provide future perspectives in this field, aiming to help further progress efficient and stable 2D GDY-based materials in PSCs.


Introduction
Organic-inorganic hybrid perovskite (OIHP), with the crystal structure of ABX 3 (figure 1(a)), has attracted great attention as a solar harvesting material, owing to outstanding optoelectronic properties, ease of processing and the fact that it can be fabricated at low cost [1,2]. Within a decade of research investigation, the certified power conversion efficiency (PCE) of the solid-state OIHP solar cell has now reached 25.70%, which is compatible with silicon-based solar cells [3]. In * Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. addition to solar cells, OIHP has also been applied in lightemitting diodes [4], lasers [5], photodetectors [6], and so on.
Since the first finding of two-dimensional (2D) graphene in 2004, atomically thin 2D layered materials have attracted interest in solar cell application [7][8][9]. It has been demonstrated that 2D materials not only enhance device performance, but also improve device stability, owing to their unique structural features and excellent physicochemical properties. As such, understanding the mechanism and the roles of how 2D materials contribute to improved device performance and stability is of great importance. Recently, experimental synthesis of graphdiyne (GDY) [10] has expanded rapidly in energy storage and conversion, catalysis, gas separation, water remediation, sensors, biomedical, nanophotonics and so on [11][12][13]. Similar to graphene with sp 2 hybridized carbon atoms that are tightly packed into a 2D honeycomb crystal lattice [14], GDY has been categorized Future perspectives Two-dimensional (2D) materials have been widely used in solar cell application due to their merits of improving moisture resistance, optimizing energy levels and facilitating charge transfer with low carrier recombination loss in the devices. By exploring the application of 2D graphdiyne-based materials in perovskite solar cells (PSCs), a new class of strategies can be developed to optimize the performance and stability of PSCs before they can be commercialized in the future.
In this mini-review, we pay attention to the successful application of 2D GDY-based materials in highly efficient and stable perovskite solar cell (PSC) application. We begin with a discussion of the basic working principle of PSCs. Then, we discussed the progress of GDY in PSCs. Finally, we provide the future perspective and conclusion on the application of GDY in PSCs. We believe that this mini-review can help readers to have a deeper understanding of 2D GDY-based material characteristics and applications in PSCs.

How do PSCs work?
The pioneering work of PSCs began in 2009 with dyesensitized solar cells, in which the perovskite was utilized as the dye, and a liquid electrolyte remained in the structure [16]. However, due to the dissolution issue of OIHP in the liquid electrolyte, the corresponding PSCs showed inferior stability. A few years later, the liquid electrolyte was replaced with the solid state hole transporting layer (HTL) of 2,20,7,70-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,90spirobifluorene (spiro-OMeTAD) [17]. Figures 1(c)-(f) show the most common device architectures of PSCs, whereby the perovskite layer is sandwiched between the electron transporting layer (ETL) and HTL. When the PSC is illuminated with light, the perovskite layer absorbs sunlight radiation, and the free electrons/free holes are formed inside the perovskite layer. Benefiting from the low binding energy of the excitons in the perovskite material (i.e. typically <50 meV), the free electrons/free holes can be immediately injected/extracted to the cathode/anode with very low charge recombination loss. The basic working mechanism of PSCs is depicted in figure 1(g).

2D GDY-based materials as hole transporting material (HTM)
The first PSCs derived from GDY were reported in 2015 by Xiao et al in regular mesoporous-based MAPbI 3 PSCs. In this work, the GDY was synthesized on the surface of copper via a cross-coupling reaction using hexaethynylbenzene and then introduced in poly(3-hexylthiophene) P3HT/chlorobenzene (CB) solution to serve as HTL. Figures 2(a) and (b) show the photographs of the GDY powder and corresponding CB solution. With GDY, a strong π-π stacking interaction between GDY with P3HT was formed, which not only partially transferred the electron from P3HT to GDY, but also served as p-type doping to facilitate the hole transportation within the P3HT matrix. As a result, a higher current density, J SC (21.70 vs. 18.30 mA cm -2 ) and fill factor (FF) (0.71 vs. 0.68) were attained for the modified device, with the champion PCE of 14.58% (vs. 11.53% for the control device) [18].
On the other hand, Li et al showed that the addition of GDY solution (i.e. dispersed in methanol by ultrasonication for 48 h, which was the most stable solution with no sediment after a few hours, figure 2(c)) in conjugated polyelectrolytes of poly[3-(4-carboxylbutyl)] thiophene-K (P3CT-K) boosted conductivity of the HTL and modulated the perovskite film morphology with higher crystallinity, reduced grain boundaries and improved HTL/perovskite interface contact. Consequently, the leakage current of the device and charge recombination at HTL/perovskite were significantly suppressed, which led to higher J SC (22.80 vs. 20.60 mA cm -2 ) and FF (0.81 vs. 0.78) for the champion MAPbI 3 device with inverted structure, with the best PCE of 19.50% (vs. 16.80% for the control device) [20].
However, P3CT-K HTL tends to form aggregation in the CB solution, which could further restrict the device performance. To solve the problem, GDY oxide (GDYO) was doped in P3CT-K film [21]. With the addition of GDYO, the aggregation of P3CT-K was reduced and the main chain of P3CT-K was unlocked. Furthermore, a homogeneous surface coverage film with reduced hydrophobicity film surface was formed, which was beneficial for facilitating the perovskite crystal growth with optimized crystallinity and enlarged the perovskite grain sizes. Consequently, the charge carrier transfer rate was optimized and the charge recombination was restrained for the modified device with GDYO dopant, yielding a champion PCE of 19.06% (vs. 17.02% for the control device) with higher J SC (22.65 vs. 21.15 mA cm -2 ) and FF (0.79 vs. 0.76) [21]. Similar observations (i.e. promoting the charge extraction, accelerating the charge transportation and suppressing the charge recombination) were obtained after GDYO was introduced in nickel oxide (NiO x )-based HTL for inverted planar MAPbI 3 PSCs. In contrast to GDYO-P3CT-K nanocomposite film [21], GDYO in NiO x formed a non-wetting surface and facilitated perovskite growth with fewer grain boundary defects. A champion PCE of 19.14% (vs. 16.93% for the control device) along with a higher FF of 0.81 (vs. 0.78) were obtained for the inverted MAPbI 3 device. Without encapsulation, the modified device maintained 90.41% of initial PCE after being stored in nitrogen (N 2 ) atmosphere for 13 d [22].

2D GDY-based materials as electron transporting material (ETM)
ETL composed of 2D GDY was first utilized in 2015 in inverted planar MAPbI 3−x Cl x PSCs. The addition of GDY in PCBM increased the electron mobility (i.e. from 2.98 to 5.32 × 10 -4 cm 2 V -1 s -1 ) and passivated the underneath  layer of perovskite film with fewer surface trap-states. Taking advantage of these, electron was extracted more readily to the cathode with reduced charge recombination at the perovskite/ETL interface. A champion PCE of 14.80% (vs. 13.60% for the control device) along with low photocurrent hysteresis was attained for the modified device [23]. Later, dual-doping GDY method in ETLs was proposed to further improve the device performance and its stability. Compared to single-layer doping, dual-doping GDY formed better interfacial contact and repaired the defects of the individual ETL (PCBM and ZnO) more effectively, with reduced surface roughness and good electrical properties. Consequently, the leakage current, the charge recombination and the electron accumulation at the cathode interface were significantly suppressed for the modified device. A champion PCE of 20.00% (vs. 16.59%) along with higher J SC (24.06 vs 21.34 mA cm -1 ) and FF (0.79 vs. 0.75) was achieved for the corresponding device. After being stored in N 2 atmosphere for 30 d, the unencapsulated champion device maintained 86% of initial PCE. By contrast, the control device degraded quickly to 21% of the initial PCE under similar storage conditions [24].
On the other hand, Chen et al demonstrated that chemical modification of GDY with alkyl azide (CH 3 (CH 2 ) 16 CH 2 N 3 ) increased its solubility in most of the organic solvents (figures 3(a) and (b)). After being introduced in PCBM film (denoted as GDY-Tz-CH 2 (CH 2 ) 16 CH 3 ), the electron mobility was increased, which ultimately facilitated the electron extraction and an 18.6% increase of PCE was obtained for the champion MAPbI 3 Cl 3−x device with the best PCE of 19.26%. By contrast, the control device showed a lower PCE value of 16.24% [25].
Zhang et al reported that the addition of GDY in SnO 2 film induced the formation of C-O σ bond between the bridge oxygen atom on SnO 2 and the carbon atom on the diacetylenic linkage of GDY, which contributed to (i) increasing electron mobility from 2.61 × 10 -4 to 1.09 × 10 -3 cm 2 V -1 s -1 , (ii) increasing the vertical current of SnO 2 film from 4.86 to 9.13 nA (also known as electrical conductivity) and (iii) optimizing the energy level between the conduction band of SnO 2 and the perovskite. Furthermore, it was found that the enhanced hydrophobicity of SnO 2 film after modification further inhibited the heterogeneous perovskite nucleation, which regulated the perovskite crystal growth with larger grain sizes, higher crystallinity and fewer defect states (i.e. surface and interfacial defects). With GDY in SnO 2 film, the grain size and the trap density were increased from 221 to 304 nm and reduced from 6.18 to 4.46 × 10 15 cm -3 , respectively. Due to these advantages, electron extraction was facilitated, and charge recombination was suppressed for the modified device, finally leading to a champion PCE of 21.11% (vs. 19.17% for the control device) with negligible photocurrent hysteresis. Meanwhile, the modified device without encapsulation also demonstrated higher ambient stability by maintaining 93% of initial PCE after being stored in ambient air for 1000 h [27]. Yao et al found that the chemical interaction between hydrophilic carboxy and hydroxyl groups of GDYO with the uncoordinated Sn of SnO 2 suppressed oxygen vacancy of SnO 2 . This interaction suppresses the surface defect of SnO 2 and improves the SnO 2 /perovskite interface contact with reduced trap-state density. With the addition of GDYO in SnO 2 , energy band alignment was optimized, which further promoted electron extraction/injection, suppressed charge recombination and accumulation at SnO 2 /perovskite layer interface. A higher electron mobility (8.15 × 10 -3 vs. 8.18 × 10 -4 cm 2 V -1 s -1 ) was obtained for the modified film. After modification, an enhancement of over 10% was achieved for the modified device, with the best PCE of 21.23%. By contrast, the control device delivered a lower PCE value of 19.19%. Meanwhile, the modified device also performed higher device stability under thermal stress at 80 • C and under one sun illumination at 25 • C in N 2 atmosphere when compared to the control device (figures 3(c) and (d)) [26].

2D GDY-based materials as dopant in perovskite film
Meng et al demonstrated that the addition of GDY in perovskite formed a bulk heterojunction film, which created an extra route for electron extraction to the cathode. According to the authors, electrons could be first extracted by the GDY located in the grain boundaries and the interface, then injected into the ETL. As a result, electron extraction/transportation was facilitated with the reduction of charge recombination, yielding in higher PCE of 20.55% for regular planar structure FA 0.85 MA 0.15 Pb(I 0.85 Br 0.15 ) 3 device (vs. 20.06% for the control device). More importantly, the hydrophobic GDY in grain boundaries and interface could suppress the formation of PbI 2 , yielding higher device ambient stability. Under dark ambient conditions (i.e. 30% RH at room temperature), the unencapsulated modified device maintained 95% of initial PCE after 140 d [28].
By contrast, Li et al showed that the GDY formed an intermediate adduct with PbI 2 , which induced the formation of high crystalline MAPbI 3 film with larger grain size and reduced grain boundaries and restrained the formation of iodide vacancies ( figure 4(a)). As a result, a champion PCE of 21.01% (vs. 16.69% for the control device) was achieved for the modified device with reduced photocurrent hysteresis. In N 2 atmosphere, the modified device without encapsulation maintained 85% of initial PCE after 33 d, attributed to the GDY inhibiting the formation of PbI 2 [29].
Cao et al demonstrated that the addition of anthraquinonemodified GDY quantum dots (GDY-AQ QDs) in perovskite film not only induced the formation of high crystalline perovskite film with larger grains size and fewer grain boundary defects, but could also interact with unsaturated Pb 2+ in perovskite to passivate the defects in perovskite films. Thanks to these advantages, charge accumulation/recombination was suppressed, the charge transfer was facilitated and the perovskite decomposition rate was inhibited. Finally, the modified device showed a champion PCE of 21.03% (vs. 19.43% for the control device) with enhanced device stability [31].
On the other hand, pyridinic nitrogen-doped GDY (N-GDY) retarded the perovskite growth and crystallization process ( figure 4(b)), inhibited the ion migration and suppressed the phase segregation of the perovskite film. Thanks to the pyridinic N atom in N-GDY, the deep-level trap states like Pb-I antisite defects and under-coordinated Pb atoms were passivated. As a result, a champion PCE of 22.38% (vs. 19.64% for the control device) was achieved for the modified device, attributed to the effective charge extraction with reduced charge recombination. Meanwhile, the modified device also exhibited higher ambient, thermal and light stability when compared to the control device (figures 4(c)-(e)) [30]. Huang et al demonstrated that fluorinated-doped GDY (F-GDY) and nitrogen-doped GDY (n-GDY) both could reduce surface defects and improve surface and bulk crystallinity than the pristine GDY. Compared to the control MAPbI 3 film and after being doped with GDY, MAPbI 3 with n-GDY and F-GDY exhibited excellent film morphology with pinholes-free, full surface coverage and high uniformity films. Among these dopants, the MAPbI 3 film with F-GDY exhibited the largest grain size with reduced grain boundaries, facilitating charge transport and enhancing the devices' FF. By contrast, the MAPbI 3 film with n-GDY exhibited superior electrical conductivity, attributed to the imine N in n-GDY and showed better ability for electron-donor. After modification, higher PCEs of 18.10%, 18.07%, and 16.03% were obtained for the devices with n-GDY, F-GDY and GDY, respectively. The control device showed a lower PCE of 13.61% [32].

2D GDY-based materials as interface layer
In addition to serving as HTM, ETM or dopant in perovskite film, GDY has also been used as the interfacial layer in PSCs. For example, Li et al showed that the addition of GDY in cross-linkable fullerene [6,6]-phenyl-C61-butyric styryl dendron ester (PCBSD) improved film orientation of the ETL, thereby inducing the formation of high crystalline perovskite film with larger grain size and reduced grain boundaries. Meanwhile, GDY-PCBSD nanocomposite film at TiO 2 /perovskite interface enhanced electron mobility, facilitated the charge extraction and tailored the energy-level alignment of the device. Consequently, a champion PCE of 20.19% (vs. 17.38% for the control device) was achieved for the modified device. More importantly, the GDY could provide an adhesive film network with sufficient solvent resistance for the modified film, preventing moisture invasion and suppressing the perovskite decomposition rate. Thus, a higher device lifetime (i.e. ambient and light stability) was obtained for the modified device [33].
Later, Li et al demonstrated that the addition of chlorinesubstituted GDY (Cl-GDY) in [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) could reduce the surface roughness of PCBM film surface by minimizing the aggregation of the PCBM nanoparticles. As a result, Cl-GDY induced a better electron extraction and faster electron transport channel to the cathode with reduced charge accumulation between the perovskite layer and the cathode. Finally, Cl-GDY-PCBM nanocomposite film at perovskite/ZnO interface in an inverted cell delivered a champion PCE of 20.34% (vs. 17.08% for the control device) along with higher stability in N 2 atmosphere. The champion device maintained 70% of the initial PCE after being stored for more than 450 h [34].
With the deposition of GDY/ anhydrous CB solution during the anti-solvent dripping process (average thickness of the GDY nanosheets of ∼2 nm), the internal defects state density of the perovskite films was passivated, which could be linked to the chemical interaction between the oxygencontaining group of GDY with the exposed lead atom in the defects of the perovskite layer. With GDY treatment, charge recombination in perovskite film was passivated and led to a longer free charge carrier lifetime (i.e. 86.9 vs. 51.8 ns). In addition to defect passivation, the GDY could also serve as a hole collector and transport channel, facilitating the separation and diffusion of the free electrons and the free holes in the devices. A champion PCE of 19.60% (vs. 18.30% for the control device) along with higher ambient, light and thermal stabilities were attained for the modified device. The higher stability of the modified device was attributed to the GDY interface layer enhancing moisture resistance and suppressing the degradation process of the perovskite film [35].

Limitations of 2D GDY-based materials
Even though 2D GDY-based materials show great application in PSCs, challenges remain for theory and practice. For example, synthesizing 2D GDY-based material at the laboratory scale is time-consuming and has low yield. In addition, the precursor of hexaethynylbenzene to synthesize GDY is expensive and unsuitable for industry application. Among the graphyne family, only GDY has been successfully synthesized for solar cell application. Thus, it is crucial to develop a common approach for mass production, as well as synthesize new types of grapyhnes with lower cost for practical applications. Another challenge of GDY is the difficulty in controlling the single layers of GDY sheets in solution, which could be attributed to the strong interaction between the GDY layers. On the other hand, multilayered GDY sheets contain a small portion of the acetylenic bonds, which can reduce the percentage of sphybridized carbon atoms and limit its application in solar cells application. As a result, the development of new methods to synthesize monocrystallized GDY, monolayer GDY, few-layer GDY and nanostructured GDY with designed size and morphologies still remains a huge challenge and needs further efforts [12,36,37].

Conclusion and future outlooks
We have systematically reviewed the roles of 2D GYD-based materials in PSC application. It should be noted that GDY has shown great potential in PSCs and has been utilized as HTM, ETM, dopant in perovskite film and interface layer (table 1). Based on the discussion, we found that GYD can (i) enhance the electrical properties of the free charge carriers (i.e. carrier mobility and carrier conductivity), (ii) optimize the energy level alignment of the device, (iii) improve film surface property, (iv) prevent moisture invasion of the perovskite film, (iv) create a new route for the charge transport, and so on (figure 5). All the reviewed works demonstrate that 2D GDY-based materials can be used as a novel material for PSC application. To further establish GDY as a promising 2D material for PSC application, it is suggested that 2D GDY-based materials should be further explored in other perovskite materials beyond MAPbI 3 , such as FAPbI 3 , CsPbI 3 or mixed OIHP materials. It is believed that the performance and stability of the devices can be further increased when 2D GDYbased materials are doped into these absorber layers, attributed to the fact that the MAPbI 3 is thermodynamically unstable, undergoes a structural transformation readily, and starts to decompose when the cell operating temperature has reached 85 • C [38]. Furthermore, the addition of 2D GDY-based materials in these perovskite materials can guide the researchers to have a deeper understanding of the light-matter interaction within the device system. In addition to PCBM ETL and P3CT-K HTL, 2D GDY-based materials can also be mixed together with other kinds of fullerene derivatives [39] and other kinds of commonly used p-type conjugated polymers in organic solar cells [40][41][42]. Meanwhile, careful control of the functional groups of GDY on 2D GDY-based materials should receive attention to facilitate the chemical interaction between 2D GDY-based materials with perovskite/ETL or HTL, to realize highly efficient charge transport, passivate the surface or bulk defects and decelerate the degradation process of the devices. Although 2D GDY-based materials are hydrophobic and can enhance the moisture resistance of the device, it is important to encapsulate the device to guarantee device stability and prevent lead leakage. Research collaboration across multidisciplinary fields is required to have a deeper understanding of the physical and chemical properties of 2D GDY-based materials.
In conclusion, we strongly believe that the development of 2D GDY-based materials will provide a promising new resource for incorporation into high-performance PSCs to realize lowcost solar cell technologies for commercialization.