Indoor light harvesting perovskite solar cells on TCO-free ultrathin deformable substrates

Perovskite solar cells (PSCs) are receiving renewed interest since they have reached high power conversion efficiency and show potential for application not only on rigid and flexible substrates but also on mechanically deformable substrates for integration on non-planar curvilinear surfaces. Here we demonstrate PSCs fabricated on transparent conducting oxide (TCO) free ultra-thin polyethylene terethapthalate (PET) substrates capable of efficiently harvesting indoor light even under compressive strain. Interface engineering with PTAA improved the shunt resistance and band alignment at perovskite-hole transport layer interface which resulted in enhanced charge extraction, leading to 114% improvement in PCE from 5.57 % to 11.91% under 500 lux indoor LED (4000K) illumination. The champion device exhibited a PCE of 18.37% under 250 lux cool white LED (4000K) light. The maximum power output (Pmax) of the devices varied from 13.78 to 25.38 µW/cm2 by changing the indoor light illumination from 250 to 1000 lux, respectively. Moreover, the devices showed impressive performance even after mechanical deformation and retained 83% and 76 % for 1 sun and indoor light, respectively, under 30% compressive strain. Our approach pays way for fabrication of efficient indoor light harvesting PSCs on mechanically deformable substrates for integration on non-planar surfaces prone to compressive strain.