Bulk-heterojunction photovoltaic cells with donor-acceptor ratio varied across the film prepared by evaporative spray deposition using ultradilute solution method

We fabricated three types of bulk-heterojunction (BHJ) photovoltaic cells with the electron donor and acceptor ratio varied across the film. One has a stacked layer structure of a donor-rich layer (2 : 1) the near cathode and an acceptor-rich layer (1 : 2) near the anode (DA-BHJ), another has a stacked structure with the inverse order (AD-BHJ), and the other has a constant ratio (1 : 1) as a reference. These were fabricated by evaporative spray deposition using ultradilute solution (ESDUS) method. The DA-BHJ cell showed significantly higher energy conversion efficiency than the AD-BHJ cell and the reference BHJ cell where the three exhibited almost identical light absorption.