Thin and lightweight solar cells of high specific power that are flexible and elastic under tensile strain are appealing for emerging applications in aerospace, unmanned aerial vehicles and integration in automobiles, buildings, textiles and moving parts of machinery1. Key elements that will enable the flexible solar cell technology to further evolve are the development of low-cost and abundant materials for the device substrate, electrodes and functional layers combined with the capability to be printed using high-yield low-cost methods. Promising candidates are the emerging photovoltaic technologies based-on solution-processed semiconductors, such as organics2, colloidal quantum dots3 and organic-inorganic halide perovskites4. Particularly, perovskite solar cells (PSCs) have already reached commercially viable power conversion efficiencies (PCEs) up to 25.2%5-8 for rigid devices fabricated on glass substrates and up to 20.7% for flexible ones fabricated on plastics9-13. This progress was realized by tailoring the composition and dimensionality of the perovskite absorber, advancements related to low‐temperature deposition methods to improve the perovskite layer’s properties, and interface materials to improve the energy level alignment14-20.
However, the majority of the existing solar cells, including perovskite ones, are fabricated either on glass or on plastic substrates coated with indium tin oxide (ITO) or fluorinated tin oxide. These substrates increase the device thickness and weight (especially glass) and are unsuitable when considering fully solution printing capability of solar cells21-23. More importantly, the conductive ITO layer (a commonly utilized electrode material for a vast variety of optoelectronic devices), is both expensive and mechanically fragile. Low-cost, Earth-abundant, and easily recyclable materials for substrates and electrodes of solar cells are desired to overcome this bottleneck. Efforts to replace ITO with alternative electrode materials include graphene14, single wall carbon nanotubes24, doped-metal oxides25, silver nanowire (Ag NW) networks26,27 and conducting poly(3,4-ethylenedioxythiophene) polystyrene sulfonate23,28. To avoid the use of glass and plastics, sustainable paper substrates derived from cellulosic pulps have been recently explored due to their abundance, environmental friendly properties, renewability and sufficient mechanical tolerance29. However, these papers are commonly made of microcrystals/microfibers, which scatter significant portion of the incident light and, hence reduce their transparency30. To increase transparency, nanocellulose papers have been recently explored, but still require the additional deposition of conductive electrode materials – thus increasing the complexity of device fabrication31-36. Development of sustainable materials combining both high transparency and adequate conductivity to serve as FSBEs for solar cell application and beyond will offer advancements in many fields of renewable energy harvesting devices.
In this study, we report that super-reduction of polyoxometalate (POM) clusters grafted onto a cellulose nanocrystaline (CNC) matrix can create a highly conductive and transparent nanopaper, which serves as a free-standing device bottom electrode for the fabrication of substrate-free PSCs printed in the air via a facile solution shearing method. The devices exhibited a PCE of 19.89% combined with a total thickness of only 0.8 μm, a weight of 5.1 g m-2 and a power-to-weight ratio of 38,470 W Kg-1, which render it the thinnest, lightest, and yet high PCE solar cell made to date1,28. The composite nanopaper acted as a conformal coating that enabled practically unchanged device efficiency upon heating at 95oC under continuous illumination in inert environment. It also protected the device layer from decomposition when immersed in a mild citric acid water solution for 100 days and enabled superior mechanical robustness upon multiple bending and folding cycles and upon the application of strain.
Development of FSBE
Cellulose paper substrates are commonly made from fibers having large diameters up to 20 μm, which backscatter a significant portion of the incident light and are either optically opaque or semitransparent. Furthermore, the large size and the high packing density of these fibers hinders their use as a reversibly deformable paper substrate. To overcome the inherent limitations of microcrystalline cellulose papers, nanopapers made from cellulose nanofibers or nanocrystals (NCs) were recently developed37. Nonetheless, despite being sufficiently transparent, these nanopapers are highly-insulating, thus requiring additional electrode materials prior to the device fabrication. Instead of depositing additional electrode, which makes the device fabrication more complex, our work has focused on endowing the nanopaper with high conductivity without compromising its transparency. To this end, we explored the suitability of a well-known family of solution-processed molecular oxides namely polyoxometalate (POM) clusters. We have previously demonstrated that nearly metallic conductivities can be achieved by highly reduced POM clusters of either Keggin or Wells-Dawson type38. Here, we designed and developed a highly transparent and conducting nanopaper consisted of super-reduced POM clusters grafted onto cellulosic NCs of small size. The cellulose NCs were synthesized via a multistep procedure initiated by acid hydrolysis of the cellulosic pulp as explained in Methods. In brief, a concentrated sulfuric acid solution was first added in the pulp causing the dissolution of the amorphous regions of the cellulose molecular chains whereas the highly crystalline regions remained intact. The molecular diagram in Fig. 1a depicts two of the crystalline cellulose chains comprising the NCs, which formed through multiple hydrogen bond interactions between these chains. The procedure requires repeatable purification steps (Supplementary Fig. 1); it results in the formation of a white powder consisted of the needle-like NCs with diameters of 30-50 nm and lengths of 150-350 nm (Fig. 1b). Next, these NCs were mixed with urea (NH2CONH2), a well-known thermal stabilizer for cellulose and reducing agent for POMs, and ammonium metatungstate hydrate ((NH4)6(H2W12O40).xH2O) POM nanoparticles with an average size of 1 nm (Fig. 1c) and dissolved in a mixture of deionized (DI) water, sodium hydroxide (NaOH) and hydrochloric acid (HCl). The specific POM was selected among a large variety of POM clusters tested, because it was capable of undergone a very high degree of reduction which was retained for more than a year under storage. It was also very stable to heating up to 150 oC and 1.5 AM simulated light exposure for many weeks. After extensive optimization of the concentration of the constituents, the resultant ink was printed on a solid support via a solution shearing method; a simple, fast and a low-cost procedure that is suitable for up-scaling. In the printed nanopaper the cellulosic NCs exhibited a moderate degree of orientation with only few POM aggregates between them (Supplementary Fig. 2). Transmission electron microscopy (TEM, Fig. 1d) and X-ray diffraction (XRD, Supplementary Fig. 3) indicated that POM clusters were grafted onto the cellulose NCs. Remarkably, the printed nanopaper was very conductive for a wide range of printing conditions and resultant thicknesses (Fig. 1e and Supplementary Table 1). The conductivity originated from the high degree of reduction, termed as super-reduction, of POM clusters in the cellulose:urea:POM. To unravel the origin of super reduction of POMs, we performed an extended study of cellulose:POM and urea:POM composites (Supplementary Note 1 includes the detailed discussion about the urea-induced super-reduction of POM). X-ray photoelectron (XPS) and UV-Vis absorption (Supplementary Fig. 4), Fourier transform infrared (FTIR) (Supplementary Fig. 5), ultra-violet photoelectron (UPS, Supplementary Fig. 6) spectroscopy measurements taken in urea:POM composites indicated electron transfer from urea molecules to POM38,39, as illustrated in Fig. 1f . Notably, there was no evidence for POM reduction in cellulose when urea molecules were not present. Urea bears two lone pairs per molecule that lying above and below the molecular plane and are hence prone to transfer to a low lying LUMO molecule like POM40. XPS results revealed that about 12 excess electrons were up taken by each cluster, which resulted in a large negative charge of -18 per cluster. This negative charge is partially compensated by the positive charge of amines in urea molecules that were coordinated and anchored in the outer shell of POMs (Supplementary Fig. 5). The sr-POM clusters were subsequently uniformly anchored onto the cellulose NCs through the formation of extended hydrogen bond interactions (Fig. 1g and Supplementary Fig. 7). This prohibited the formation of large aggregates and increased the visible transparency (Supplementary Fig. 4d). Regarding the electronic structure of srPOMs, UPS measurements revealed that their valence band maximum (VBM) is extended up to the Fermi level (Supplementary Note 2 and Supplementary Fig. 6). This VBM has mainly O2p character, while the conduction band minimum (CBM) is composed of W5d atomic orbitals with the 5dxy to have the lower energy40-42. According to the electronic structure proposed by Nishimoto et al. for super-reduced α-Keggin POMs40, we consider our srPOM clusters as the total of four identical substructures consisted of metal triangle features originating from the formation of tungsten-tungsten bonds upon the uptake of excess electrons. The three additional electrons per metal triangle substructure are placed in 5dxy orbitals with e and a1 in symmetry as shown in frontier molecular orbital (MO) diagram of the substructure (Fig. 1h, middle). With a total of 12 excess electrons per cluster, half of the metals are in the 4+ state. In order to preserve their skeleton, the super-reduced structure squeezes out oxygen bridging two neighboring W atoms, which form covalent metal bonds. The decreased metal-metal distance in the metal triangular configuration enhances the overlap of these orbitals and lowers their relative energies. Once the substructures are connected into srPOM, the four a1 and eight e frontier MOs containing the excess electrons split into groups of one a1 and three t2 (for a1) and two e, three t1 and three t2 (for e) MOs, respectively (Fig. 1h, right). The 12 excess electrons fill the MOs with lower energies (t2, a1 and e in symmetry), which are located near the Fermi level. As a result, the srPOMs exhibit semi-metallic behavior and endows the resultant nanopaper with conductivity values up to 18 S cm-1. The transparency of the nanopaper is also very high for a wide range of thicknesses (Fig. ij and Supplementary Table 2) reaching 96.4% transmittance (at 546 nm). This is the synergistic result of the high transparency within the visible spectrum of srPOM clusters when they are uniformly grafted onto cellulose NCs (Supplementary Fig. 3). However, the high transparency mainly stems from the reduced light backscattering efficiency caused by the small NCs of 30-50 nm in diameter43; the light backscattering efficiency is inversely proportional to the size of the NC. As a result, the light propagation in the forward direction is significantly strengthened. Notably, a highly transparent (94.5 % at 550 nm) cellulose nanopaper has been recently reported through in situ photo-polymerization of the conductive polymerizable deep eutectic solvent (PDES) monomer44, albeit with a very low conductivity of 0.0013 S cm-1.
Solar cell fabrication and performance
Among the low-cost thin film printing techniques, solution shearing has the advantage of yielding minimum waste of precursor solution (<10% of the amount required for spin-coating) and allowing for direct thin film crystallization near the edge of the meniscus45. We applied the solution shearing printing method in the fabrication of all device layers including the nanopaper which served as a free-standing device bottom electrode (Fig. 2a). Moreover, we combined this method with a vacuum flash-assisted (VASP) processing for the deposition of the perovskite film46. The combination of solution shearing with VASP enabled the formation of high-quality perovskite layers, having large grains on the order of micrometer (Supplementary Fig. 8), and superior optoelectronic properties (Supplementary Fig. 9). The PSC architecture was: CNC (160 nm)/tin oxide (SnO2, 40 nm)/phenyl-C61-butyric acid methyl ester (PCBM, 10 nm)/perovskite (330 nm)/Spiro-OMeTAD (70 nm)/Ag NWS (160 nm) (Fig. 2b) having a total thickness of 770 nm (substrate included). The SnO2/PCBM bilayer and Spiro-MeOTAD were used as the electron and hole transport layers (ETLs/HTLs), respectively. The light absorber was a quadruple cation RbCsMAFA (Rb:CsMAFA 10:90) perovskite having the triple cation perovskite chemical structure of Cs0.5(MA0.17FA0.83)0.5Pb(Br0.17I0.83)3, where Rb is rubidium, Cs is caesium, MA and FA are methylammonium and formamidinium, respectively47. Fig. 2c depicts a representative cross-section scanning electron microscopy (SEM) image of the complete PSC device. High-angle annular dark-field (HAADF) image and energy dispersive X-ray spectroscopy (EDS) mapping in scanning tunneling electron microscope (STEM) mode confirmed the homogeneous distribution of Rb and Cs elements in the perovskite layer obtained through printing (Fig. 2d). Notably, for the top cathode we adopted a non-aqueous processing procedure for the deposition of an orthogonal array of Ag NW ink (see Supplementary Note 3 for more details). Ag NWs were embedded into a cellulose matrix instead of the commonly used, hydroscopic PEDOT:PSS. Our process enabled the fabrication of high-elastic Ag NW orthogonal arrays (Supplementary Fig. 10), without affecting the quality of the perovskite layer underneath.
Figure 3a presents the current density-voltage (J-V) characteristics of the 0.25 cm2 champion device fabricated on nanopaper after an extensive optimization study (Supplementary Figs. 11 and 12). The device exhibited high open circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) that gave rise to a maximum PCE of 19.72% in the forward scan (FS) and 19.89% in the reverse scan (RS) (Table 1). Remarkably, this nanopaper-based PSC exhibited negligible hysteresis, high external quantum efficiency (EQE) above 90% in the visible, a stable VOC over time, and a mean PCE value of 19.32% (Supplementary Fig. 13). Due to the high optical transparency of both the free-standing bottom electrode and the top electrode, it also exhibited a small semi-transparency (i.e. an average transmittance (AVT) of 11.86%), which could further improved, albeit on the expense of the device efficiency (Supplementary Fig. 14). Interestingly, a larger area (1×1 cm2) device delivered a PCE of 17.35% and a stabilized PCE of 17.16% (Fig. 3b and Supplementary Table 3). This also originated from the successful combination of solution shearing and VASP methods for the printing of perovskite absorber as indicated by the high efficiencies obtained in various PSCs based on either ITO/glass or ITO/PET substrates (Supplementary Fig. 15 and Supplementary Tables 4-6). Notably, a PCE of 22.08% achieved in the rigid device (ITO/glass) represents a record for PSCs processed via solution shearing and is among the highest efficiencies reported to date for printed PSCs45. The nanopaper-based device represents the best performing flexible perovskite solar cell, including devices fabricated on plastic/ITO substrates (Supplementary Tables 7 and 8). Besides the high efficiency, the device is extremely thin (i.e., 0.8 μm thick), and lightweight (5 g m-2), leading to an unprecedented power-to-weight ratio of 38,470 W Kg-1 for the champion cell (38,060 W Kg-1 at MPP). Clearly, the nanopaper-based PSCs offer a great advantage for lightweight power generation.
Stability and recyclability
During operation, solar cells are exposed to sunlight. As a result, they are constantly heated due to the absorption of thermal radiation. It has been reported that solar modules operating in hot climate may reach temperatures of over 65 °C48. Moreover, device fabrication may require high temperature post perovskite deposition processes, such as annealing of charge transport overlayer and device encapsulation. To track the effect of continuous heating, we measured the variation of PCE of the encapsulated cells that were heated at 95°C for 1,000 hours under continuous 1 sun illumination at maximum power point (MPP) tracking in a nitrogen atmosphere (Fig. 3c). Remarkably, during light exposure at 95°C, the device retained 99% of its initial performance. The high thermal stability of our PSCs is attributed to the superior properties of the perovskite absorber and the nanopaper. With our solution shearing printing combined with VASP, we were able to deposit highly-oriented perovskite layers having large (on the order of micrometer in size) grains, thus reducing the deleterious effect of grain boundary defects. This was verified by the high stability obtained in several types of rigid and flexible PSCs tested upon continuous illumination in nitrogen (Supplementary Fig. 15) or in ambient air (Supplementary Fig. 16). However, the stability of the nanopaper-based device was superior compared to others, which indicates that CNC layer has also a contribution. To explain such remarkable stability of CNC-based devices we can consider the close matching of the thermal expansion coefficients of perovskite (1.5x10-4 K-1) and the cellulosic substrate (1.3x10-4 K-1). This results in minimum thermally-induced device stress upon heating. Moreover, the addition of urea in cellulose matrix has been previously shown to highly expand the thermal stability of the resultant paper49. This is because carboxylic acids acting as hydrolysis-induced degradation sites in cellulose are partially neutralized by forming a stable salt with the respective amines. Moreover, the addition of urea and POM in the cellulosic matrix resulted in a more hydrophobic nanopaper in comparison to pristine cellulose sample (Supplementary Fig. 17), indicating that a reduced water content within the composite nanopaper. We note that a similar concept of amine molecules addition, termed as the “alkaline reserve in cellulose conservation science”, is applied by librarians and archivists to preserve historic acidified papers and books in the best possible manner.
An additional advantage of renewable energy devices is the ability to be fully-recyclable using low-cost methods in order to have minimal effect on environmental pollution. Glass and metal electrodes, however, present significant constrains for the recyclability of solar cells even when they are fabricated from solution-processed absorbers and interlayers. Perovskite layers have been recently shown to be fully recyclable using appropriate solvent combination50. However, the use of glass and metals adds significant drawbacks to make the existing PSCs environmentally-friendly. Our nanopaper-based PSCs are glass and ITO-free and undergo full degradation when continuously exposed to a mild citric acid (5% wt) water solution heated at >155oC (see also Supplementary Note 4 for details). Notably, the device exhibited a remarkable stability to decomposition when continuously immersed for 100 days (Fig. 3d). The device recyclability was only initiated by heating at 155 oC and above, where the hydrolysis of cellulosic material -probably in both electrodes- is initiated according to a scheme presented to Supplementary Fig. 18. The degradation reaction results in the formation of a furan-based bio-oil. After the cellulosic material has been transformed and removed, the citric acid etches and dissolves the SnO2/PCBM layer, whereas water decomposes the perovskite absorber. Notably, this experiment not only demonstrates the full recyclability or our device in a mild acidic solution but it also proves its high resistance to moisture-induced degradation for 100 days before the application of heat. This could be a consequence of suppressed carboxylic acid initiated hydrolysis of cellulose upon urea amino groups’ coordination as explained above.
Flexibility and stretchability studies
For practical applications, the mechanical durability of PSCs under bending, folding and application of strain (Supplementary Fig. 19) is of significant importance. To evaluate the flexibility of nanopaper-based PSCs, we subjected the devices to bending (Fig. 4a) and folding (Fig. 4b) tests. In the first case, tensile strain is applied to the perovskite absorber whereas in the second case the absorber is subjected to compressive strain. Our device displays excellent mechanical bending/folding stability even after a 2,000-cycle bending/folding test at curvature radii of 2.0, 1.0 and 0.5 mm. Negligible changes are observed in all device performance characteristics (JSC, VOC and FF) upon bending and release for 2000 cycles. Similar results were obtained for the folding stability tests where the perovskite absorber is subjected to compressive instead of tensile strain. The small decline in efficiency after 2000 folding cycles is mainly due to a loss in VOC. On the contrary, a small increase in the device FF after the repeated folding tests could be due to an increase in mobility in the perovskite layer due to the decrease in the inter-lattice distance during folding51. This extraordinary bending and folding stability of our flexible PSCs built on nanopaper (Fig. 4c), can be explained on the basis that cellulose nanocrystals with such small size can be reversibly transform during each cycle of mechanical deformation. This is a great advantage of our nanopaper relative to the regular paper commonly used (which consists of large cellulose fibers of the order of 20 μm) as well as the plastic substrates. The small size of the building blocks of our nanopaper results in the formation of a nanoporous network in each layer that adsorbs strain more easily compared to rigid substrates. Besides being flexible, our device was also subjected to the application of tensile strain and found quite durable for strains up to 20% (Fig. 4d and Supplementary Fig. 20). A significant failure caused by crack formation and propagation was initiated first in the perovskite layer only when the strains were above 20% (Supplementary Fig. 21). Cracks in the nanopaper substrate appeared for strains over 60%, which indicates that our FSBE is capable of very high mechanical strength. However, upon keeping the applying strain below 20% the device performance was quite stable for multiple cycles of strain and release (Figs 4 e,f), indicating that fabrication of a solar cell on a conformal nanopaper composed of small nanocrystals is very promising for emerging applications such as wearable electronics.
In this study, we present a choice of materials and device fabrication methods that allowed the development of highly stable all-printable, fully recyclable perovskite solar cells with a record figure of merits including being lightweight and ultra-thin, reaching very high efficiency and record power-to-weight, and demonstrating enhanced thermal and mechanical durability. These cells were fabricated on highly transparent and conducting nanopapers composed of cellulose nanocrystals grafted with super-reduced semi-metallic polyoxometallate clusters. The nanopaper served as the device free-standing bottom electrode. The application of a non-aqueous solution processed cellulose:Ag NW composite top electrode, synergistically contributed to the fabrication of all-printable devices using a solution shearing printing method. The printed solar cells were highly resistant to water-induced decomposition but they could be easily recycled upon immersion in a citric acid solution and application of heating. Our concept of using abundant natural materials for the fabrication of critical device components is expected to be readily applicable in a large variety of optoelectronic devices with the aim to decrease their fabrication cost and eliminate environmental pollution caused when these devices reach the end of their lifetime.