Defects in the active layers of MOCVD-grown InGaN/GaN blue-emitting multiple quantum well LED heterostructures on (111) silicon substrates are examined by conventional and high resolution transmission electron microscopy (HRTEM). The quantum wells contain small gaps, interfacial steps, sphalerite stacking and well thickness variations. No characteristic phase separation of the InGaN into In-rich clusters is observed within the wells. Despite the defective active layers and the absence of In clusters, the internal quantum efficiency of the device quantum wells is 23%, hence mechanisms by which carriers are localised in the quantum wells via the defects are considered. The roles of well gaps, thickness variations, stacking faults and sphalerite grains in exciton confinement are discussed.