Organic solar cells (OSCs) are an alternative photovoltaic technology with a promising potential for a cost-effective, clean and sustainable source of energy. The performance of the solution-processable fullerene-based OSCs has been extensively investigated. However, the weak absorption in the long wavelength range, limited electronic property tuning capability and expensive purification processes are some of the limiting fac-Remarkable progress has been made in the development of high-efficiency solution-processable nonfullerene organic solar cells (OSCs). However, the effect of the vertical stratification of bulk heterojunction (BHJ) on the efficiency and stability of nonfullerene OSCs is not fully understood yet. In this work, we report our effort to understand the stability of nonfullerene OSCs, made with the binary blend poly[(2,6-(4, 8-bis(5-(2-ethylhexyl) thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′] dithiophene-4,8-dione)] (PBDB-T):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2b:5,6-b′] dithiophene (ITIC) system. It shows that a continuous vertical phase separation process occurs, forming a PBDB-T-rich top surface and an ITIC-rich bottom surface in PBDB-T:ITIC BHJ during the aging period. A gradual decrease in the built-in potential (V 0 ) in the regular configuration PBDB-T:ITIC OSCs, due to the interfacial reaction between the poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer and ITIC acceptor, is one of the reasons responsible for the performance deterioration. The reduction in V 0 , caused by an inevitable reaction at the ITIC/PEDOT:PSS interface in the OSCs, can be suppressed by introducing a MoO 3 interfacial passivation layer. Retaining a stable and high V 0 across the BHJ through interfacial modification and device engineering, e.g., as seen in the inverted PBDB-T:ITIC OSCs, is a prerequisite for efficient and stable operation of nonfullerene OSCs.