Journal Article Review polymer solar cells Se···O conformational locks anode interlayers applications cathode interlayer conjugated polymers efficiency heterostructures high conductivities low bandgap molecular design molybdenum oxide n-doping nanostructured designs non-fullerene acceptors optical properties organic photovoltaic cells organic photovoltaics organic solar cells organic thin-film transistors plasmonic materials polaritons poly(vinylpyrrolidone) printable self-organization method solar energy tandem devices thermoplasmonics