The degradation mechanism is compared in organic light-emitting devices (OLEDs) fabricated by solution-coating to that in vacuum-deposited OLEDs. Devices comprising various host materials made by vacuum-deposition or solution-coating are investigated. Changes in devices electroluminescence (EL) spectra during prolonged electrical driving are compared and analyzed. Hole-only devices are also utilized, and employed to study the effects of charges and excitons, separately and combined. The results reveal that the faster degradation of solution-processed devices relative to their vacuum-deposited counterparts under electrical stress is due to a faster aggregation of the host materials. Interactions between excitons and polarons in the emitting layers of the devices induce this aggregation phenomenon. Although this phenomenon affects both vacuum-deposited and solution-coated emitting layers, it is found to occur much faster in the later. The findings shed light on the root causes of the limited stability of solution-processed OLEDs.