In situ grown bifacial graphene stabilizes composite electrode for efficient perovskite solar cells


 Instability of rear electrodes undermines the long-term operational durability of efficient perovskite solar cells (PSCs). Here, a composite electrode of copper-nickel (Cu-Ni) alloy stabilized by in situ grown bifacial graphene is designed. The alloying makes the work function of Cu suitable for regular PSCs and Cu-Ni is the ideal substrate for preparing high-quality graphene via chemical vapor deposition, which simultaneously protects the device from oxygen, water and internal components reaction. To rivet the composite electrode with semi-device, a thermoplastic copolymer is employed as an adhesive layer during hot pressing. The resultant device achieved power conversion efficiency of 24.34% with significantly improved stability; the devices without encapsulation retained 97% of their initial efficiency after the damp heat test at 85oC with relative humidity of 85% for 1440 hours and the encapsulated devices maintained 95% of their initial efficiencies after maximum power point tracking under continuous 1 sun illumination for 5000 hours.


Metal halide perovskite solar cells (PSCs) have attracted great attention in both academia
and industry owing to their excellent optoelectronic performance and low manufacturing costs [1][2][3][4][5][6] . However, for PSCs to realize commercialization, they must survive the long-term natural erosion imposed by oxygen, moisture, light and heat 7, 8 . Thanks to the optimization of the perovskite materials, charge transport materials and the interface layers [9][10][11][12] , the 3 operational stability of PSCs has made a great progress, but one of the key functional layers, rear electrode, is still prone to fail, which limits the overall durability of efficient PSCs [13][14] .
Silver (Ag) and aluminum (Al) are commonly used rear electrodes, whereas they tend to react with migrated halide anions from perovskite to form resistive compounds such as AgI and AlI3 [15][16][17] . In the case of gold (Au), although the formation enthalpy of Au-I is much higher, Au atoms can diffuse into the perovskite to form deep-level defects of AuPb, which act as efficient non-radiative recombination centers 18 . In addition, the use of noble metals comes at a high price and implies more stringent requirements for vacuum and temperature, which will significantly increase the manufacturing costs. To suppress the interaction between perovskite and metal atoms, introducing a thin buffer layer to separate them without hindering the charge transport has been reported as a successful strategy 19,20 .
However, materials from the perovskite or the electrode tend to penetrate that buffer layer on a long timescale because achieving a thin but uniform and compact coverage on a large area with buffer layers that are usually formed by solution-processable small molecules or polymers is hard 21 , calling for an increased intrinsic chemical stability of rear electrodes for PSCs.
Copper (Cu) is a potential candidate due to its relative inertness to migrated perovskite components and has been widely used in inverted PSCs 22 . However, its work function (WF) of 4.65 eV limits its application in efficient regular PSCs, which usually use 2,2′,7,7′-4 tetrakis(N,N-dip-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) or poly[bis(4-phenyl) (  This process also smoothens the surface of the substrate, thereby preparing it for in situ 6 growth of graphene, as demonstrated by the atomic force microscopy (AFM) (Extended Data Fig. 2, H).
A uniform but thin layer of graphene is required here to meet the demand of being an efficient barrier without introducing extra defects or hindering the carrier transport. 27 We focus on the characterization of one side since the quality of graphene on both sides should be identical in a CVD method. Using CH4 as the carbon source, with respective flow rates Therefore, CNG-10 is expected to be the most suitable composite electrode among the three samples.

Assembly and performance of CNG devices
Before the assembly of an integral device, the effect of graphene on the WFs of Cu-Ni alloy is measured by ultraviolet photoelectron spectroscopy (UPS). As shown in Extended  additives can be chosen to boost the conductivity of the copolymer. However, to guarantee the stability of the interface, stable materials with good dispersity will be the priority selection.

Stabilizing mechanism of CNG electrode
To reveal the stabilizing mechanism of CNG, we first studied their resistance to water and oxygen. As shown in Extended Data Fig. 8, A to E, the contact angle with water in air rises from 74.9° for Ag electrode to 102° for CNG-10, which should be beneficial for 11 repelling water. Even if water is dropped and stays at the surface of CNG-10 for 24 hours, the color of perovskite remains black, while the perovskite under Ag turns yellow (Extended Data Fig. 8, F and G). To exclude the effect of electrode thickness, the permeation rates of water and oxygen for CNG-10 electrodes are measured and drop to 3.32% and 5.38% of the initial values for Cu-Ni alloy after equipped with air-tight and hydrophobic graphene (Extended Data Fig. 9, A and B). The reduced permeability meets the requirements for encapsulation 33  in the aged Ag device have migrated over the whole depth of the device while I - (Fig. 3D) and Cu - (Fig. 3F) in the aged CNG-10 device are almost confined to their original layers.

Stabilizing effect of CNG electrode on devices
CNG-10 devices without encapsulation retained 97% of their initial efficiency after the aging test of heating at 85 o C with ca. 85% RH for 1440 hours, whereas the PCEs of the control devices dropped to 56.5% of their initial PCEs after 936 hours (Fig. 4A). For the operational stability test, another control sample using Cu-Ni alloy with bifacial sprayed graphene as the electrode (SG device) was tested. All the encapsulated devices were measured under MPPT with continuous 1 sun illumination; the CNG-10 devices retained 95% of their initial PCEs after 5000 hours with a small deviation across five individual cells, while the PCEs of the SG and Au devices dropped to 59% and 30% of their initial PCEs after 2500 and 1250 hours, respectively, with much larger deviations (Fig. 4B). The 13 inferior performance of SG device may be ascribed to numerous pores in overlapped graphene.

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