Researchers have developed atomically thin p-n junctions using van der Waals (vdW) heterostructures, achieving the ultimate quantum limit for electronic and optoelectronic components. These junctions are composed of transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂), each just one unit cell thick. The junctions exhibit gate-tunable diode-like current rectification and photovoltaic response, with tunnelling-assisted interlayer recombination of majority carriers playing a key role in their electronic and optoelectronic properties. Sandwiching the junction between graphene layers enhances the collection of photoexcited carriers. The absence of a depletion region in atomically thin junctions leads to a different physical mechanism for rectification compared to conventional p-n junctions. Band profiles show that most of the voltage drop occurs across the vertical p-n junction, leaving no appreciable potential barriers in the lateral transport direction. Under reverse bias, substantial potential barriers result from band bending in the lateral direction. The current is governed by tunnelling-mediated interlayer recombination between majority carriers at the bottom (top) of the conduction (valence) band of MoS₂ (WSe₂). Two possible physical mechanisms or a combination of both explain the rectification: Shockley-Read-Hall (SRH) recombination mediated by inelastic tunnelling of majority carriers into trap states in the gap, and Langevin recombination by Coulomb interaction. The rectifying I-V characteristics are understood by increasing interlayer recombination rate at higher forward biases. The current under forward bias can be tuned by varying carrier densities through electrostatic gating. The junctions also exhibit a gate-tunable photovoltaic response, with the short-circuit current density reaching a maximum at Vg = 0 V. The maximum photoresponsivity is ~2 mA/W, measured using a focused 532-nm laser. The photovoltaic response is attributed to spontaneous charge separation at the junction, compatible with photoluminescence (PL) characteristics of the MoS₂/WSe₂ junction. PL spectra show strong quenching of emission only at the junction area, indicating rapid charge carrier separation. Spontaneous dissociation of a photogenerated exciton into free carriers is driven by large band offsets across the atomically sharp interface. Charge separation may also be understood in terms of highly asymmetric charge transfer rates in a heterostructure with type II band alignment. These charge transfer and separation processes are analogous to those in excitonic organic solar cells. In atomically thin p-n junctions, charge transfer processes are expected to be fast and efficient since exciton (or minority carrier) diffusion is not required. The observation of significant PL quenching and photocurrent generation in the p-n junction indicates that exciton dissociation at the junction is significantly faster than otherResearchers have developed atomically thin p-n junctions using van der Waals (vdW) heterostructures, achieving the ultimate quantum limit for electronic and optoelectronic components. These junctions are composed of transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂), each just one unit cell thick. The junctions exhibit gate-tunable diode-like current rectification and photovoltaic response, with tunnelling-assisted interlayer recombination of majority carriers playing a key role in their electronic and optoelectronic properties. Sandwiching the junction between graphene layers enhances the collection of photoexcited carriers. The absence of a depletion region in atomically thin junctions leads to a different physical mechanism for rectification compared to conventional p-n junctions. Band profiles show that most of the voltage drop occurs across the vertical p-n junction, leaving no appreciable potential barriers in the lateral transport direction. Under reverse bias, substantial potential barriers result from band bending in the lateral direction. The current is governed by tunnelling-mediated interlayer recombination between majority carriers at the bottom (top) of the conduction (valence) band of MoS₂ (WSe₂). Two possible physical mechanisms or a combination of both explain the rectification: Shockley-Read-Hall (SRH) recombination mediated by inelastic tunnelling of majority carriers into trap states in the gap, and Langevin recombination by Coulomb interaction. The rectifying I-V characteristics are understood by increasing interlayer recombination rate at higher forward biases. The current under forward bias can be tuned by varying carrier densities through electrostatic gating. The junctions also exhibit a gate-tunable photovoltaic response, with the short-circuit current density reaching a maximum at Vg = 0 V. The maximum photoresponsivity is ~2 mA/W, measured using a focused 532-nm laser. The photovoltaic response is attributed to spontaneous charge separation at the junction, compatible with photoluminescence (PL) characteristics of the MoS₂/WSe₂ junction. PL spectra show strong quenching of emission only at the junction area, indicating rapid charge carrier separation. Spontaneous dissociation of a photogenerated exciton into free carriers is driven by large band offsets across the atomically sharp interface. Charge separation may also be understood in terms of highly asymmetric charge transfer rates in a heterostructure with type II band alignment. These charge transfer and separation processes are analogous to those in excitonic organic solar cells. In atomically thin p-n junctions, charge transfer processes are expected to be fast and efficient since exciton (or minority carrier) diffusion is not required. The observation of significant PL quenching and photocurrent generation in the p-n junction indicates that exciton dissociation at the junction is significantly faster than other