We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC61BM, or PFBT2Se2Th:PC71BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC61BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT2Se2Th:PC71BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems.