Organic semiconductors (OSCs) are sparking great interest for their potential
to substitute inorganic-based technologies (e.g., silicon) in optoelectronic device applications.
In recent years, both organic photovoltaics (OPVs) and organic photodetectors
(OPDs) have reported dramatic and steady progress in performances, with
additional attractive characteristics of lightweight, colour-tunability/transparency,
mechanical exibility, and low-cost fabrication. However, for organic optoelectronics
to be ready for the market as a reliable alternative to silicon counterparts, further
performance and lifetime increase is required. This involves, for example, improving
photodetection speed and reducing dark currents in OPDs, and suppressing
OPV photodegradation mechanisms. In this thesis, both design of OSC molecular
structure and engineering of device active layer are utilised as powerful tools to improve
e ciency of photon-to-electron conversion processes in OPVs and OPDs and
to extend operational lifetimes, in both solution-processed and vacuum-evaporated
devices. Firstly, the e ect of di erent modi cations in OSC chemical structures
is analysed, including heteroatom substitution and end-group engineering in nonfullerene
acceptors, and geometry control in small-molecule donors. By detailed
investigation of molecular physics combining energetic (by air photoemission spectroscopy)
and structural (by Raman spectroscopy) characterisation, the role of these
modi cations on device performance is pinpointed. In particular, backbone rigidity
and ne-tuning of charge delocalisation along the molecule are found essential
to suppress chemical and morphological degradation during OPV operations, while the adoption of di erent donor conformations (twisted vs. planar) determines trapstate
distribution and photodetection speed in OPDs. Secondly, OPD active layer
characteristics are engineered by fabrication parameter tuning, taking into account
factors like donor:acceptor ratio and thickness. Here, it is found that high content
of donor polymer is crucial to suppress dark current and ensure fast photodetection
by morphology and energetic optimisation, while thickness increase in evaporated
blends can control molecular orientation and packing order, which is particularly
promising in fullerene-free OPDs.