Sequential crystallization driven by regioisomeric volatile additives in high-efficiency organic solar cells

Abstract

Exciton dissociation and charge transport in bulk-heterojunction organic solar cells (BHJ-OSCs) highly depend on the active layer morphology; however, their complex internal structure makes morphology modulation difficult. We report, an efficient additive strategy that enables sequential crystallization of donor and acceptor in BHJOSCs, significantly enhancing vertical component distribution and phase separation morphology, thereby leading to improved device performance. In this context, the structure-performance relationships of the regioisomeric additives 1,2-diiodobenzene (oDIB), 1,3-diiodobenzene (mDIB), and 1,4-diiodobenzene (pDIB) were systematically explored to elucidate their distinct roles in morphology control and efficiency enhancement. An enhanced vertical composition gradient was observed, characterized by donor enrichment near the bottom and acceptor accumulation toward the top of the active layer, thereby enabling more efficient carrier transport and suppressing charge recombination. Additionally, the accelerated film formation promoted favorable phase separation and high crystallinity, as verified by grazing-incidence wide-angle X-ray scattering, which collectively contributed to improved exciton dissociation and charge extraction. Transient absorption spectroscopy revealed that devices treated with additives exhibited faster hole transport dynamics, especially with mDIB. The mDIBtreated PM6:L8-BO device achieved a power conversion efficiency (PCE) of 18.61%, and the D18:L8-BO reached a remarkable PCE of 19.42%.