Photomultiplication type quasi-planar all-polymer photodetectors (PM-QAPDs) are fabricated with P3HT/PY-IT as active layers by employing a sequential spin-coating method. The part of PY-IT can penetrate into a P3HT layer to emerge isolated electron-traps formed with PY-IT surrounded by P3HT. The trapped electron distribution near an Al electrode will determine the spectral response range of PM-QAPDs. Broadband PM-QAPDs can be achieved with a 0.25 μm thick P3HT layer and a ultra-thin PY-IT layer prepared from 1 mg/ml solution, exhibiting a broad response from 320 to 870 nm. An external quantum efficiency (EQE) value of optimal PM-QAPDs approaches 16 000% at 360 nm under −12 V bias. When the thickness of the P3HT layer is increased to 2.4 μm, the PM-QAPDs exhibit a narrowband response from 630 to 870 nm, which can be well explained according to the Beer–Lambert law. The work may provide a smart strategy to adjust response range of PM-QAPDs by alerting the thickness of the donor layer.

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