Defects in conventional semiconductors substantially lower the photoluminescence (PL)quantum yield (QY), a key metric of optoelectronic performance that directly dictates the maximum device efficiency. Two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as monolayer MoS2, often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. We show that the PL QY of as-processed MoS2 and WS2 monolayers reaches near-unity when they are made intrinsic by electrostatic doping, without any chemical passivation. Surprisingly, neutral exciton recombination is entirely radiative even in the presence of a high native defect density. This finding enables TMDC monolayers for optoelectronic device applications as the stringent requirement of low defect density is eased.Multiparticle Coulomb interactions are particularly pronounced in transition metal dichalcogenide (TMDC) monolayers, leading to a multitude of recombination pathways, each associated with the different quasiparticles produced by these interactions (1). The recombination rate of excitons formed by photogenerated carriers (2, 3), depends nonlinearly on the concentration. Because excitons interact with background charge to form trions (4-8), the Fermi level also controls the dominant recombination pathway. Thus, both the background carrier concentration and the generation rate must be tuned to investigate the complete effect of multiparticle interactions on TMDC photoluminescence (PL) quantum yield (QY).In this work, we simultaneously altered the photocarrier generation rate (G) by varying the incident pump power, and the total charge concentration (electron and hole population densities; N and P) by varying the back-gate voltage (Vg) in a capacitor structure (Fig. 1A). Surprisingly, we found that all neutral excitons recombine radiatively in as-processed monolayers of MoS2, resulting in near-unity QY at low generation rates. This high QY occurred despite a reported high

Room-temperature optoelectronic devices that operate at shortwave and midwave infrared wavelengths (1-8 μm) can be used for numerous applications 1-5 . To achieve the operating wavelength range needed for a given application, a combination of materials with different bandgaps (e.g. superlattice/heterostructure) 6,7 or the variation of semiconductor alloy composition during growth 8,9 is used; however, these approaches involve fabrication complexity and the operating range is fixed post-fabrication. Although wide-range, active, and reversible tunability of the operating wavelengths in optoelectronic devices after fabrication is a highly desirable feature, no such platform has been yet developed. Here, we demonstrate high-performance room-temperature infrared optoelectronics with actively variable spectra by presenting black phosphorus (bP) as an ideal candidate. Enabled by the * � � 𝐸𝐸 𝑔𝑔 𝑘𝑘 𝐵𝐵 𝑇𝑇 �� (2)where 𝑚𝑚 𝑒𝑒 * and 𝑚𝑚 ℎ * are the effective masses of electrons and holes, respectively, 𝑘𝑘 𝐵𝐵 is Boltzmann's constant, and 𝑇𝑇 is temperature 36,37 . Since 𝑚𝑚 𝑒𝑒 * and 𝑚𝑚 ℎ * in bP have similar values, the effective mass ratio (𝑚𝑚 𝑒𝑒 * /𝑚𝑚 ℎ * ) is much higher than that of other small bandgap semiconductors.According to equation (2), this results in suppressed Auger recombination (longer Auger lifetime), which leads to bP's theoretical QY limit being much higher than that of other small bandgap semiconductors in the high injection regime.