The long term stability of non-encapsulated amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon single and tandem cell structures was tested by means of light soaking (AM 1.5,T = 50°C), damp heat testing (T = 85°C, humidity = 85%) and high temperature treatment (T = 150°C) up to 2000 h to simulate a variety of harsh environmental conditions. In order to study the influence of the TCO front contact and backside contact on the long term stability, cells deposited on different substrates and prepared with different backside configurations were examined. As prepared (non-encapsulated) a-Si:H and μc-Si:H diodes show very similar effects after light soaking, damp heat testing and temperature treatment. Both solar cell types show no significant variation of the solar cell parameters even after 2000 h of damp heat testing. After light soaking a-Si:H diodes exhibit the well known distinct degradation of the fill factor while the bulk properties of the investigated μc-Si:H diodes remain nearly unchanged. © 2006 IEEE.