Highly and homogeneously conductive conjugated polyelectrolyte hole transport layers for efficient organic solar cells

High mobility, stable and thick hole-transporting layer (HTL) materials are highly desirable for organic solar cells (OSCs). In this work, two pH neutral conjugated polyelectrolytes (CPEs), namely PCPDT-T and PCPDT-2T, based on 3,4-dithia-7H-cyclopenta[a] pentalene and thienyl units with a self-doping effect were designed and synthesized. Between the two CPEs, the PCPDT-T exhibits higher, more homogeneous mobility and appropriate work function, which makes it an ideal HTL material for OSCs. The single-junction OSCs modified with the PCPDT-T HTL showed an outstanding power conversion efficiency of 9.3%, which is much higher than that of the PEDOT:PSS-modified devices (8.0%). Notably, the PCPDT-T HTL exhibits excellent thickness insensitivity in fabricating OSC devices, i.e. OSC devices with very thick PCPDT-T interlayers over 50 nm still demonstrated high power conversion efficiencies over 7.1%, which is very compatible to meet the requirement for future roll-to-roll printing.