Herein, we report a novel peel-and-stick process to fabricate efficient TFSCs onto virtually any substrates regardless of materials, roughness and rigidness without changing the material deposition conditions and performance of TFSCs. The peel-and-stick process includes two steps: 1) peeling-off fully fabricated TFSCs in water from the nickel (Ni) coated Si wafer used for fabrication and 2) attaching the peeled-off TFSCs to the surface of any substrate. The peeling process relies on the phenomenon of water-assisted subcritical debonding at interface between Ni and silicon dioxide (SiO 2 ), which separates the metallic layer together with TFSCs from the original Si wafer14,15. Since the peel-and-stick process does not require any fabrication on the final target substrate, it circumvents all the fabrication challenges associated with these nonconventional substrates discussed above. Importantly, the efficiency of the transferred TFSCs on any target substrate remains the same as the as-fabricated TFSCs on Si wafers. The procedures of the peel-and-stick process are illustrated in Figure 1. First, a Si/SiO 2 wafer is coated with a Ni film (300 nm) by electron-beam (e-beam) evaporation and subsequently TFSCs are deposited on top of the metallic layer using regular TFSC fabrication procedures (Figure 1a). Second, a thermal release tape (NittoDenko®) is attached to the top of the TFSCs serving as a temporary transfer holder. A transparent protection layer (ProTek®) is spin-casted in between the TFSCs and the thermal release tape to prevent the TFSCs from the tape polymer contamination and direct contact with water. Third, the entire structure is soaked in a water bath at room temperature. Inside the water bath, an edge of the thermal release tape is slightly peeled back to promote water penetration into the Ni and SiO 2 interface. The Ni and SiO 2 interface is separated due to the water-assisted subcritical debonding14,15, leading to the peeling-off the TFSCs from the original Si/SiO 2 wafer (Figure 1b). Finally, the thermal release tape holding the peeled-off TFSCs is heated at 90°C for a few seconds to weaken its adhesion to the TFSCs. The TFSCs are then attached to various surfaces using common adhesive agents, such as double sided tapes or Polydimethylsiloxane (PDMS) (Figure 1c). After removing the thermal release tape, only the TFSCs are left on the target substrate, such as cell phone, paper, metal foils, plastics and textile (Figure 1d).

Figure 1 Procedures of the peel-and-stick process. (a) As-fabricated TFSCs on the original Si/SiO 2 wafer. (b) The TFSCs are peeled off from the Si/SiO 2 wafer in a water bath at room temperature. (c) The peeled off TFSCs are attached to a target substrate with adhesive agents. (d) The temporary transfer holder is removed and only the TFSCs are left on the target substrate. Full size image

To demonstrate the peel-and-stick process, we use the a-Si:H TFSCs as our model system. The fabrication conditions for the a-Si:H TFSCs are identical to those that would have been usually used for fabricating TFSCs on Si wafers (See the method section for the TFSC fabrication details). Figure 2a (left image) shows a representative optical image of the as-fabricated a-Si:H TFSCs on the Ni coated Si/SiO 2 wafer before the peel-and-stick process, as also described in figure 1a. The big and small round circles correspond to solar cells with an area of 0.28 cm2 or 0.05 cm2, respectively. After peeling-off the TFSCs in a water bath (Figure 1b), the Si wafer is clean and reusable (Figure 2a, middle image) and the TFSCs are held temporarily by the thermal release tape (Figure 2a, right image). Notably, the TFSCs after the peel-and-stick process show no visible damages. Next, the peeled-off TFSCs are attached to virtually any objects, including cell phone, business card and building window (Figure 2b) and these objects are previously inaccessible due to the incompatibility issues with the existing TFSC fabrication facilities. The peel-and-stick process provides a simple way for integrating TFSCs into buildings, clothes and many other nonconventional substrates.

Figure 2 TFSCs at different stages of the peel-and-stick process. (a) As-fabricated TFSCs on the original Ni coated Si/SiO 2 wafer (left). The donor Si/SiO 2 wafer is clean and reusable after the peeling-off step (middle). The TFSCs are held by a temporary transfer holder (right). (b) TFSCs on cell phone (left), business card (middle) and building window (right). Full size image

Importantly, the a-Si:H TFSCs show nearly identical efficiency before and after the peel-and-stick process. Figure 3 shows the current-voltage (I–V) characteristics of representative TFSCs before and after the peel-and-stick process to a sheet of stainless steel (left) or a soda-lime glass slide (right) and the I–V characteristics are indistinguishable, implying that no damages are induced in the TFSCs during the peel-and-stick process. Table 1 summarizes the average performance metrics over 20 solar cells with area of 0.05 cm2 and 0.28 cm2 respectively, showing η = 7.4 ± 0.5% and 5.2 ± 0.1% before the peel-and-stick process and η = 7.6 ± 0.5% and η = 5.3 ± 0.1% after the peel-and-stick process. The efficiency difference in different sizes of solar cells is caused by large series resistance in larger solar cells16. Nevertheless, more important thing is that both solar cells have nearly identical efficiencies before and after the peel-and-stick process with only 5% variation that is within measurement errors. These results illustrate several key advantages of the present peel-and-stick process: versatility in substrate choices, high fidelity to original TFSC performance, simplicity and scalability of the procedures and additional cost-saving features with reusable original Si/SiO 2 wafers.

Table 1 Statistic summary of the average performance metrics over 20 a-Si:H TFSCs before and after the peel-and-stick process with only 5% variation that is within the measurement errors Full size table

Figure 3 Comparisons of the TFSC performances before and after the peel-and-stick process. The representative I–V characteristics (below average performance) of the as-fabricated TFSCs (green lines with stars) are the same as those after transferring the TFSCs (red lines with dots) to stainless steel (left) and soda-lime glass (right). Full size image

The applications of TFSCs may require intentional bending or non-planar shaping17,18. The peel-and-stick process also enables TFSCs to be integrated with flexible or curved surfaces (e.g., wavy building roof, helmets and portable electronics). To demonstrate this, a-Si:H TFSCs are transferred on a flexible sheet of stainless foil (~0.2 mm thick) and manually bended as shown in figure 4a (inset). As a result, I-V characteristics of the TFSCs remain the same after bending the flexible sheet with a range of bending radius from ∞ down to 7 mm (Figure 4a). In addition, the solar cell performances are unchanged over 3000 cycles of bending with bending radius about 10 mm (Figure 4b), demonstrating the mechanical flexibility and robustness of the transferred TFSCs. It should be noted that the mechanical properties of the final solar cells are not determined by the peel-and-stick process, but rather by the intrinsic material properties and dimensions of the TFSCs (e.g., a-Si:H as an active material, indium tin oxide (ITO) as an electrode).