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Microcavity organic solar cells (OSCs) consisting of different photoactive layers were systematically investigated to demonstrate the potential of using this strategy for efficient light-harvesting. The device structure was carefully optimized to achieve maximal photocurrent (J max) through optical simulation based on transfer matrix method. The results showed that by replacing ITO with ZnO/Ag/ZnO multilayer structure as transparent electrode on the glass substrate, the microcavity OSCs exhibited lower J max compared to the ITO-based devices for the active layers with absorption spectra beyond 800 nm. Optical properties within the device indicated that the decrease of the J max was ascribed to the lower transmittance of ultrathin metal film, which offset the improved absorption of low energy photons near the band edge of the photoactive layers by the microcavity light-trapping effects. Based on these results, top-illuminated microcavity configuration was designed for the ultra-narrow bandgap bulk heterojunction (BHJ) systems, which allowed a significant increase of the J max , reaching the values as high as those of devices built on ITO-coated substrates. Our results can provide a guideline for the further optimization of microcavity effect, especially for the highly efficient nonfullerene BHJs with strong near-infrared (NIR) absorptions to facilitate the fabrication of high-performance large-area flexible OSC devices. • Microcavity can enhance light absorption near the band edge of photoactive layers. • Bottom-illuminated structure is not suitable for ultra-narrow bandgap active layers. • Low transmittance of ZnO/Ag/ZnO is limiting factor in bottom-illuminated devices. • Top-illuminated microcavity exhibit higher photocurrent for near-infrared absorbers. [ABSTRACT FROM AUTHOR]