The active layer morphology with a domain size of 10–20 nm and large interfacial area plays a significant role in governing the device power conversion efficiency (PCE) of semitransparent polymer solar cells (ST-PSCs). However, the unbalanced aggregation rate of a polymer donor and a near infrared non-fullerene acceptor led to a large phase separation domain. In order to inhibit the intense aggregation of a non-fullerene acceptor, a third component which increases the nucleation and growth barrier of the non-fullerene acceptor can be added to balance aggregation leading to a suitable domain size of donors and acceptors. In order to verify our idea, PTB7-Th and non-fullerene acceptor IEICO-4F were selected as a model system. IEICO-4F tends to form strong aggregation due to its planar conformation. In order to suppress the intense aggregation, an amorphous molecule NCBDT-4Cl which has good miscibility with IEICO-4F is added to increase the nucleation barrier of IEICO-4F because NCBDT-4Cl can be dispersed evenly in the IEICO-4F phase. In situ UV-vis absorption spectra were recorded to monitor the nucleation and growth process of IEICO-4F during the drying process. The results show that the growth time of IEICO-4F is reduced to 45 s from 122 s by incorporating 20 wt% NCBDT-4Cl, resulting in reduced aggregation of IEICO-4F. The small IEICO-4F aggregates disperse in the PTB7-Th matrix and form refined bicontinuous pathways. The optimized ternary ST-PSCs achieve a PCE of 10.31% with an average visible transmittance (AVT) of 20.6%.