## 1 Introduction  
Photodetectors are critical elements of emerging technologies, such as autonomous vehicles, environmental monitoring systems, security systems, mobile phone cameras and telecommunications systems [1, 2]. The most common types of photodetectors are photodiodes and phototransistors. A photodiode works by developing a p-n junction or Schottky junction, which separates the photoinduced electron-hole pairs, causing the generated electrons and holes to move in opposite directions, culminating in a photocurrent or photovoltage. In a phototransistor, electron-hole pairs are primarily separated by external bias and further regulated by a gate, resulting in a photocurrent. Photodetectors based on two-dimensional (2D) material heterostructures have remarkable responsivity, which has been confirmed through theoretical simulation models [3, 4]. Most importantly, out-of-plane 2D van der Waals heterostructure phototransistors can combine the advantages of photodiodes and phototransistors. The operation schematics of a photodiode and phototransistor are shown in Figure S1a,b in the Supporting Information, respectively. In the heterostructure phototransistor configuration, the photodiode (p-n junction) part separates electron-hole pairs under light illumination, in which electrons are injected to MoS\({}_{2}\), resulting in maximizing its electrical response to light in terms of photogain, as shown in Figure S1c. The phototransistor also minimizes the noise in its electrical output by operating the device in the depletion regime using appropriate gate voltage modulation. Recently, phototransistors based on WSe\({}_{2}\) and MoS\({}_{2}\) have shown promising avenues for optoelectronic devices, with promising carrier transport properties, high on/off ratio, high light absorption coefficient, scalable thickness down to the monolayer and, most significantly, easy to introduce doping [2; 5; 6; 7; 8; 9; 10; 11]. However, these devices have shown limited photresponsivity and detectivity due to the weak built-in electric field formed between WSe\({}_{2}\) and MoS\({}_{2}\)[6; 9; 10]. Therefore, further investigations regarding improvements in the strength of a built-in field for boosting the performance of the phototransistor based on WSe\({}_{2}\)/MoS\({}_{2}\) are still needed. Among 2D transition metal dichalcogenides (TMDs), p-doping techniques have been widely reported as an effective tool to manipulate the properties of heterostructure phototransistors, including chemical doping with XeF\({}_{2}\) vapors, PPh\({}_{3}\)/AuCl\({}_{3}\), low-energy phosphorus implantation, Te and H\({}_{2}\) gas, electrostatic split-gate configuration, ozone treatment and UV/ozone treatments [11; 12; 13; 14; 15; 16; 17]. Additionally, n-doping techniques have also been reported, including electron beam lithography, self-assembled monolayer, triphenylphosphine, lithium fluoride, nitrogen, zinc oxide and alkali metal compound-promoted chemical vapor deposition (CVD) [18; 19; 20; 21; 22; 23; 24; 25]. However, none of these methods can simultaneously achieve controllable, scalable and stable performance over an extended period without introducing chemical impurities. This severely limits the application of optoelectronic devices based on TMDs. Nevertheless, laser treatment has been used for defining p-n heterojunctions for fabricating high-performance photodetectors. It is a simple, efficient and selective p-doping approach for 2D TMDs [5; 26; 27]. In this study, the laser treatment technique is used for the achievement of p-doping WSe\({}_{2}\), forming a strong built-in electric field between p-doped WSe\({}_{2}\) and untreated MoS\({}_{2}\) in the p-doped WSe\({}_{2}\)/MoS\({}_{2}\) van der Waals heterojunction.