Both organic photovoltaics (OPVs) and perovskite solar cells (PSCs) are gaining attention due to their fast-growing power conversion efficiency (PCE) and potential for indoor or aerospace applications. However, this success is based on semi-empirical choice of specific molecules and interfacial optimization, where performance origin is still partially unknown. Especially, systematic understanding of molecular-structure-dependent energetics and energy-level alignment in organic/hybrid interfaces is highly needed. In this thesis, fine-tuned packing in different organic molecules and interfacial energetic alignment in perovskite/hole transport layer (HTL) interface is extensively studied, combining ambient photoemission spectroscopy, surface photovoltage, and computational simulations.

Firstly, in OPV materials, various molecular structures and packing motifs are investigated, analyzing the correlation with their energetics and trap distribution. For high-performance non-fullerene acceptors (NFAs), molecular rigidity originated from side-chain design is shown to determine energetic disorder and trap formation. Besides, due to strong electric quadrupole moments, NFA energetics is tuned by packing-orientation control in thin films. The impact of packing-dependent energetic shift is further shown to govern recombination loss and device open-circuit voltage (VOC). This energetic analysis is also extended to bulk heterojunctions, as demonstrated in polymer:fullerene blends, where blending-induced energetic shifts are identified for different polymer structures. These findings provide important structure-property-performance relationships and guidance for organic molecular designs for high-efficiency OPVs.

Secondly, in PSC HTLs, interfacial energetic alignment is studied by sequential PEDOT:PSS dedoping. Beneficial band-bending is found for dedoped PEDOT:PSS, reducing non-radiative recombination and leading to higher VOC and PCE.

Finally, the impact of two-dimensional (2D) perovskite interlayer is investigated by tuning cation concentrations. Here, an extraction barrier is observed in phenethylammonium-based 2D layers, but surface-defect suppression reduces recombination loss and increases VOC. Moreover, significant HTL energetic shift is found, correlated to HTL-to-perovskite dopant diffusion. The study reveals the potential of 2D perovskites to improve PSC performance by reducing this phenomenon.