Induced Oriented Attachment of ZnO Electron Transport Layer Enables Over 20% Efficiency in Solution-Processed Conventional Organic Solar Cells

The development of efficient and stable organic solar cells (OSCs) employing solution-processed zinc oxide (ZnO) electron transport layers (ETLs) is impeded primarily by structural disorder across multiple length scales and a high density of oxygen vacancy defects, particularly in conventional device architectures. Here, simultaneous modulation of macro- and microstructural ordering and surface defect passivation is achieved by meticulously designing dual-functional solid additives within the ZnO system to fine-tune nanoparticle stacking. Systematic analysis reveals that the solid additive (DIB) molecules strongly adsorb onto the surfaces of ZnO nanoparticles during film formation due to robust intermolecular interactions, providing steric hindrance that suppresses aggregation and promotes uniform film coverage. Upon subsequent mild thermal annealing, the moderately volatile DIB gradually sublimes, generating interparticle free volume that facilitates oriented attachment of ZnO nanoparticles guided by dipolar interactions, while concurrently enhancing interparticle Zn-O bonding to effectively passivate oxygen vacancies. This combination of structural regulation and defect passivation leads to ZnO films with improved electron mobility, reduced recombination losses, and favorable energy-level alignment. The resulting OSCs achieve a record power conversion efficiency of 20.1% (certified at 19.8%) among devices using ZnO-based ETLs, along with excellent thickness tolerance and operational stability. Notably, this strategy also demonstrates exceptional compatibility with flexible devices, delivering a record efficiency of 19.1%, and exhibits broad applicability across a range of DIB analogs.