Journal Article all-polymer solar cells polymer solar cells 3D structures Research Support, U.S. Gov't, Non-P.H.S. benzodithiophene-thieno[3,4-b]thiophene copolymer blend morphology blend self-organization diimide dimers n-Type polymer n-type organic semiconductors nanophase separation naphthalene diimide copolymers nonfullerene acceptors nonfullerene electron acceptors optical band gap organic photovoltaics organic solar cells polymer/polymer blend polymer/polymer blends tetraazabenzodifluoranthene diimide