On the impact of selective donor:acceptor structural ordering in PBDB-T:ITIC organic solar cells

Abstract

The bulk heterojunction (BHJ) is the standard configuration of the photoactive layer in single-junction organic solar cells. Therein, electron-rich (a donor polymer) and electron deficient (a small molecule acceptor) organic semiconductors are intimately blended to form a complex 3D network of crystallites and vitrified regions that altogether determine the final device performance. Studying the relationship between said performance and the structural order achieved in the photoactive materials individually is desirable to discern the underlying structure–function relationship in organic solar cells, hence isolating, with no ambiguities, the role played by the structural order achieved in the donor and acceptor domains themselves on the device performance. This work precisely tackles the archetypal PBDB-T:ITIC blend to demonstrate how the structural order of the donor and acceptor fractions in the BHJ can be selectively tailored through an adequate selection of co-solvents during the film formation process. When using chloroform as unique solvent, both components exhibit a low degree of order in the BHJ. Conversely, the use of ortho-xylene yields BHJs in which PBDB-T shows enhanced structural order. Furthermore, the addition of 1,8-diiodooctane as co-solvent is found to spark the crystallization of ITIC without effect on the degree of order of PBDB-T, resulting in downgraded photovoltaic performance. Overall, BHJs in which ITIC remains in its vitrified state are beneficial, in which case the structural order achieved by PBDB-T has little to no effect. Notwithstanding, BHJs that contain ordered materials (donor and/or acceptor) show virtually no degradation after more than 3.5 years of shelf storage. This study, thus, pinpoints to (i) the microstructure attained by the acceptor domains and (ii) the presence of crystalline domains of either type (donor and/or acceptor) as the main determinants of the maximum achievable performance and the shelf stability of organic solar cells, respectively.