Researchers from Sungkyunkwan University (SKKU), Korea University, Seoul National University and Korea's Frontier Energy Solution have demonstrated the feasibility of using the van der Waals stacking (vdWS) strategy to overcome the inefficiencies of flexible perovskite solar cells (f-PSCs).
The team explained that using halide perovskites can significantly increase the power conversion efficiency (PCE) of f-PSCs due to the low-temperature processability, ultrathin dimensions, low weight, and excellent optoelectronic properties of such cells. Several efforts were also made to develop the charge transporting layer (CTL), interface engineering between CTLs and perovskite, and highly-crystalline perovskite films on a flexible substrate, which further increased the f-PSC PCE to 22.44%. However, the efficiency is only 87% of the conventional glass-based PSCs. The f-PSC efficiency is affected by the physical process limitations caused by the flexible substrates' flexibility. Polymer substrates, such as polyethylene naphthalate (PEN) or polyethylene terephthalate (PET), or tin-doped indium oxide (ITO) are typically used as flexible substrates for f-PSCs.
The deformability and softness of the flexible substrates lead to distortion or bending during suction on the vacuum basement of a bar-coater or the vacuum chuck of a spin-coater, resulting in a nonuniform deposition of perovskite and the deterioration of different post-treatment effects. Holding the flexible substrate on a rigid substrate, such as slide glass, using tape or other sticking techniques can help overcome these issues. However, an air gap often develops between the rigid and flexible substrates after holding the flexible substrate on the rigid substrate, leading to an inefficient heat-transfer behavior.
Defect passivation at the inner grain boundary and surface is necessary to improve the PSC photovoltaic performance. The deformity of a flexible substrate limits the application of different post-treatment processes due to inferior process uniformity. Additionally, the thermal coefficients of the flexible substrates vary depending on the substrate material. Although the perovskite annealing process critically influences the perovskite layer quality during the growth and nucleation of perovskite crystals, a limited number of studies were performed focusing on the perovskite film heat transfer on the flexible substrate.
In the recent study, the researchers used a vdWS-induced lift-off process to achieve a highly efficient and reproducible fabrication of f-PSCs and f-PSMs. A polydimethylsiloxane (PDMS) layer was used between the top ITO/PEN flexible substrate layer with a vdW bond and the bottom glass substrate with a covalent bond to address the physical process limitations of the flexible substrates.
PDMS, a solution-processable polymer, was selected due to its adequate adhesion between PEN and glass substrates. Moreover, the cured PDMS layer demonstrated superior stability against different solvents used during the PSC fabrication. Initially, the PDMS layer was spin-coated on the glass substrates. During the PDMS layer curing, a siloxane bonding occurred between the PDMS and glass layers.
Patterned flexible ITO/PEN substrates were then placed on the PDMS layer, and the as-prepared heterosubstrate was subjected to a vacuum for a specific duration to form the vdW bonds between the PEN and PDMS and eliminate the residual gap. Subsequently, solvent engineering, a typical glass-based fabrication technique, was used to manufacture the PSCs.
After the completion of the electrode deposition, the fabricated f-PSCs were lifted off gently. The PEN/ITO/ tin oxide (SnO2)/perovskite structure device was detached easily from the glass/PDMS substrate due to the low adhesion energy of the vdW bond between the PEN and PDMS.
A conventional tape-holding process was also used to fabricate a glass/air-gap/PEN/ITO/SnO2/perovskite structure. The tape-holding process was used as a control and compared with the vdWS to demonstrate the effect of vdWS on the heat transfer.
COMSOL Multiphysics was used to perform the heat-transfer simulation using a time-dependent heat-transfer physical model with glass/PDMS/PEN/ITO/SnO2/perovskite structure and glass/air-gap/PEN/ITO/SnO2/perovskite structure.
A new vacuum-assisted post-treatment process was used for defect passivation owing to its excellent compatibility with the vdWS substrate. Oleylamine (OLA) was spin-coated on a perovskite film (vdWS/OLA), which was then subjected to a low vacuum for three min (vdWS/OLA/vacuum), leading to the formation of chemical bonds between the uncoordinated halide and lead ions of the perovskite film and OLA.
A scanning electron microscope, photoluminescence spectrometers, a spherical aberration (Cs)-corrected scanning transmission electron microscope, and a potentiostat were used to characterize all perovskite films. Researchers also performed mechanical and environmental stability tests and assessed the PV performance of all f-PSCs, PSCs, and f-PSMs.
The vdWS-ITO/PEN/PDMS/glass substrate provided an air-gap-free interface for uniform heat transportation, rigidity for compatibility with different processes, and high bond strength between the PDMS and flexible substrate for a nondestructive lift process. Thus, highly crystalline and uniform perovskite films were obtained on the flexible substrates using the vdWS method during the perovskite annealing process.
The defects at the grain boundaries and top and bottom interfaces on the perovskite films were effectively minimized using the vacuum-assisted passivation post-treatment, improving the efficient charge transport of the f-PSCs. This vacuum-assisted process facilitated the penetration of the OLA solution as a passivator in the deep valley of the perovskite grain boundary, leading to effective defect passivation.
Thus, the f-PSC displayed an impressive performance similar to glass-based PSCs. Although a minor decrease in the current was observed in the f-PSCs owing to the parasitic absorption by the PEN substrates, the reduction did not adversely affect the perovskite quality during the lift-off process.
Moreover, the vdWS/OLA/vacuum devices demonstrated better mechanical and environmental stabilities as they maintained over 90% of their initial PCE after each heat, light, and humidity stability test for 300 h, and 81% of their initial PCE after 1000 bending cycles.
The best f-PSC with a 0.14 cm2 active area achieved PCEs of 22.54% and 41.23% under one sun and 1000 lx illuminations, respectively. The vdWS process also displayed a scalable uniformity, with 18.35% PCE achieved in an f-PSM with a 48.90 cm2 active area using an upscaled vdWS/OLA/vacuum process.
The findings of this study demonstrate that the vdWS strategy can considerably increase the efficiency of f-PSCs by effectively overcoming their limitations.