Photodetector based on materials like SnSe shows the more photocurrent and slow response in air. In vacuum, photocurrent reduces but response becomes fast. While doped SnSe based Photodetector behave complety opposite. Does this depend on type of doping ( n- type or p-type).
Bear with me a little as I start off far afield then work in toward your question.
In metal oxide sensors (insulators at room T but semiconductors at operating temperatures), surface oxygen depletes a (temperature dependent) debye length of its electrons and thereby alters the conductivity of the surface layer. When organic gases pass over and react with the oxygen, removing it from the surface, an electron is donated back to the surface layer restoring it toward it's un-oxygenated conductivity level.
The sensor's operating temperature is elevated (for band-gap & faster reaction kinetics) and photodetectors are not, but since they are both semiconducting materials that have the ability to be gated by adjacent charge, a mechanism like the MOS- oxygen interaction may be happening in your photodetectors.
In the case of sensors, hydroxides grab electrons like oxygen does, but are not easily removable, so in sensor applications they poison the sensitivity of the sensor.
Consider reading about the basic mechanisms of MOS sensors to see if some of the mechanisms apply to your photodetectors.
You may think two different types of defect states here to understand better. Both can enhance photocurrent however one has relatively fast response than other. 1.
associate to bulk defects. e.g. point defects due to Se/-vacancies, which are there in bulk crystal during synthesis. 2. associate to surface defects and interface. e.g. surface adsorbed molecules (OH-,O2- etc.), oxide formation at surface, interface states between TMD layer and SiO2, which has slow time-response.
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