Design of multifunctional near-infrared organic heterojunction and double hole transport layer to improve efficiency and stability of perovskite solar cells

• Single-walled carbon nanotubes acts as a double hole transport layer. • Multifunctional bulk organic heterojunction provides additional light harvesting. • Heterojunction and buffer layer enhance the stability of perovskite solar cells. Currently, the high-efficiency lead organic–inorganic hybrid perovskite solar cells (PSCs) can only utilize a relatively small fraction of sunlight (300–800 nm) due to the limited tunability of optical bandgap, which is the key factor limiting the further improvement of the photoelectric conversion efficiency (PCE) of PSCs. Here, we report a method to blend the narrow band gap non-fullerene acceptor molecule Y18 and the wide band gap donor molecule PM6 to form a multifunctional bulk organic heterojunction active layer. To avoid the loss of voltage and fill factor caused by the compromised hole transport layer (HTL), we introduce the single-walled carbon nanotubes as a buffer layer to fabricate a double HTL. The champion device displays a PCE improvement from 20.05% to 22.82% with a relative increase of 13.81%. After a deep examination of the mechanism, organic heterojunction provides the multifunctional roles as follows: (1) providing additional harvesting to the near-infrared light ranging of 800–950 nm; (2) passivating the perovskite surface and grain boundary to reduce the defect density; (3) promoting hole extraction from perovskite to HTLs by forming cascade energy levels. Moreover, the stability of PSCs is considerably enhanced because the introduction of organic heterojunction and buffer layer design isolated the natural degradation, which enhances the tolerance towards water and oxygen. Finally, a module group is fabricated by connecting two series modules in parallel, which is served as a power source to light the diode screen. This work demonstrates a buffer layer design for fabricating organic heterojunction-perovskite integrated solar cells to expand the near-infrared response, which provides a novel approach for designing integrated solar cells in the future.