The photoresponse of the devices was measured with a lock-in amplifier and a 5 Hz modulated signal (further details in the Methods Section). Figure shows the photoresponse of Device A1. At room temperature, two local maxima can be observed on the left and right sides of the charge neutrality point of graphene (CNP), while the phase of the signal jumps by 180° from the hole to the electron charge carrier regime of graphene (Figure ). The CNP varies only slightly when the device is cooled to 80 K, resulting in a nearly vertical line of no photoresponse as well as a phase jump in the responsivity and phase false-color maps (Figure , b; highlighted by a black dashed line), respectively. Surprisingly, in Figure the photoresponse vanishes at a diagonal line that is accompanied by a phase jump of 180° as observed in the corresponding values in Figure and Figure (highlighted by a dotted white line). The additional phase jump shows a change in polarity of the major charge carriers that are transferred to graphene and their exact sign was investigated by probing the transfer of charge carriers under illumination with time traces (Figure ) at the local maxima (P1A1 to P4A1). The rising part of the current trace under illumination cannot be fitted with a single exponential function, but consists of at least one fast and one slow component. While the slow component is in the range of tens of seconds at room temperature, it is even slower at lower temperatures (Figure ). In this respect, the slow time constant is likely related to trap states within the QD thin film and charging/discharging of those, which at low temperatures is significantly slower than the measurement time at light modulation of 5 Hz. The faster component, which is ascribed to the band alignment and charge carrier transfer to graphene, is comparable or faster than 100 milliseconds (Figure ). The results of both measurements, responsivity and time traces, allow identifying the two different regimes of charge carrier transfer shown in Figure depicting the sign of the majority photo-generated charge carriers transferred to the graphene channel. Under illumination, holes are transferred to the graphene channel at high temperatures (300 K) while electrons are accumulated in the QD thin film (yellow area in Figure ). At low temperatures (80 K), an inverse behavior is observed, namely, under illumination electrons are transferred to the graphene and holes are accumulated within the QD thin film (pink area in Figure ). The appearance of the responsivity and phase measurements when comparing Device A1 to Devices A2-A3 (Figure ) is highly similar, with only variation in the magnitude of photoresponse (with lower responsivity for thinner QD films), which proves the reproducibility of the obtained behavior in general. The transfer of holes to graphene at room temperature is in agreement with literature employing the same QD material-ligand-graphene combination, and a band alignment between graphene and the QD layer has been proposed as a reason for photo-induced charge transfer and photodetector operation mechanism. However, low-temperature measurements on PbS QD/graphene FETs have not been reported to date. Hence, to explain the observed behavior, we extend the room temperature band alignment model to low temperatures. For simplicity, we describe the QD thin-film energy levels and alignment as continuous bands. The Fermi level of the QD layer at 300 K is closer to the vacuum level with respect to the graphene (Figure ), and upon contacting the two materials, electrons from the QD layer are transferred to the graphene to equilibrate the Fermi levels, resulting in a Schottky-like potential barrier as shown in Figure . Upon illumination of the detector with energies larger than the bandgap of the QD, electron-hole pairs are generated within the QD thin film. The internal electric field, created by the potential barrier, will spatially separate charge carriers having electrons accumulated in the QD thin film and holes extracted to graphene resulting in the measured photogenerated current variations in the graphene channel. The Fermi level and the conduction band (CB) and valence band (VB) energies of QDs with respect to the vacuum energy depend on interface dipoles between the QD surface and ligands as well as on the intrinsic ligand dipole. They can be altered by QD size, stoichiometry, ligands, remote doping, and trap states. In Device A the ligands are exchanged with 1,2-ethandithiol and, additionally, the QD surface possesses PbSO3 and PbSO4 moieties from surface oxidation resulting in trap states with a deep energy distribution and highest state density located at the range of 0.1-0.3 eV. While at room temperature some of these trap states are thermally accessible and thus partially occupied, at lower temperatures the trap states will freeze out and the majority of these trap states may be depopulated. While for QDs no temperature-dependent CB and VB measurements have been reported, the temperature dependence of trap states in TiO2 nanoparticles was measured with electron paramagnetic resonance measurements, observing that holes were permanently trapped at surface oxides below 150 K and under illumination. The surface charges likely lead to changes in the surface dipole and, therefore, should influence the CB and VB energies positions dramatically. For the PbS QD/graphene FETs, a shift of the energy levels for the QDs, as shown in the lowtemperature scenario in Figure , can explain the observed temperature behavior for Devices A. Aligning the Fermi levels with graphene would result in electron transfer from graphene to the QD layer. The resulting potential barrier would lead to photogenerated holes being accumulated in the QD thin film and electrons transferred to graphene (Figure ). Additionally, applied gate voltages shift the Fermi level of graphene (-4.6 eV) by about 100 meV in either direction depending on the sign and magnitude of the applied gate voltage. Therefore both, temperature and gate voltage, can alter the band alignment between graphene and the QD thin film, and even lead to changes in the type of transferred photo-induced charge carriers.