The work presented in this thesis has been motivated by the potential of organic photovoltaics (OPVs) to meet the requirements of envisioned photovoltaic (PV) manufacturing methods and future PV applications. Electron transport layers (ETLs) have been instrumental in breaking the power conversion efficiency (PCE) boundaries of solution-processed PVs. New ETLs must enable higher PCEs to be attained and be compatible with large scale manufacturing that benefits from the use of low temperature processes. In this thesis with a view to increase sustainability and compatibility with flexible or foldable substrates I have developed novel ETLs for OPVs processed at an annealing temperature of 150⁰C, by modifying ZnO ETLs with MgO.
Firstly, I demonstrate OPVs incorporating a ‘‘bilayer’’ ZnO/MgO ETL. These ETLs have a more uniform top surface and a lower work function compared to single ZnO ETLs which is expected to be beneficial to electron extraction. Furthermore, I demonstrate that insertion of the thin (∼ 10 nm) MgO interlayer in devices leads to an increase in the shunt resistance (RSH) and to a reduction in recombination losses that boost the short circuit current density (JSC) and fill factor (FF), enhancing the PCE by ∼ 10%.
Secondly, I demonstrate an Mg-doped ZnO ETL formed from a single solution (via the direct addition of the Mg precursor in the ZnO precursor solution). Previous use of such an ETL for OPVs has been limited, and only investigated inconnection with annealing temperatures of ∼ 300⁰C. I demonstrate that Mg doping (∼1%) in the ZnO ETL reduces leakage currents and recombination losses, whilst leaving the work function of the ETL unaffected. A concomitant increase of the JSC, open circuit voltage (VOC) and FF leads to an ∼ 18% enhancement of the PCE.