This thesis covers research on the efficiency of green light emitting diodes (LEDs). The work presented focusses on the characterisation of conventional wurtzite (wz)-GaN based LEDs and novel zincblende (zb)-GaN based LEDs. The investigated wz-GaN based multiple quantum well structures have been found in other studies to have higher internal quantum efficiency (IQE) when the quantum wells (QWs) were grown at relatively high temperatures in MOVPE. In this thesis, more QW fluctuations were found as a result of using higher QW growth temperatures for green emitting material. QW fluctuations tended to localise the carriers away from defects, suppressing non-radiative recombination rate and this is hence a possible mechanism for improvement of the IQE of LED devices. Conventional wz-GaN based LEDs are limited in IQE by a strong in-build electric field when a large amount of indium is used to achieve green emission. Despite being a metastable phase, zb-GaN does not suffer from the same problem and exhibits good potential in green emission. A full MOVPE-grown zb-GaN LED structure was characterised in this thesis. The density of stacking faults (SFs) in the MOVPE-grown material is shown to be comparable to the state-of-art MBE-grown samples. A SF-induced alloy segregation effect was discovered in correlated structural and compositional characterisation and was further confirmed by the measurements in atom probe tomography (APT). Indium was found segregating next to the SFs whereas aluminium was found segregating to the SFs. Protruding surface features were produced by the growth of QWs and were suggested to correlate with SF bunches. Cathodoluminescence of a zb-GaN LED structure was characterised in both plan-view and in cross sections. Free exciton transition, donor-acceptor pair (DAP) emission and QW emission of 2.58 – 2.82 eV were observed in a Si doped layer, a Mg doped layer and QW layers, respectively. Low-energy QW emission peaks were also found in large wurtzite inclusions, which have been identified by in-depth correlated STEM and CL characterisation. The understanding given by this thesis on the luminescence and the defects of wz-GaN and zb-GaN based LEDs can provide useful suggestions for future growth strategies to achieve high efficiency green LEDs.