Space-charge-limited currents have been examined in a wide variety of n-i-n devices. If the devices were completely symmetric, the current-voltage characteristics would be identical for positive and negative bias, but in several devices differences between the two polarities were observed. In order to understand in which part of the device these differences originate, the influence of the contacts and interfaces on the JV characteristics were examined by using different metal top contacts, different n-layers and different i-layers. Ag and Al top contacts gave minor differences between the polarities, whereas with Cr contacts no differences were observed. Incorporation of a defect layer in the i-layer results in asymmetric JV curves. We have observed a small asymmetry in an experimental device, and a large asymmetry using AMPS modeling. N-i-n devices appear to be a sensitive probe for interface defects.