  P-n junctions based on vertically stacked single or few layer transition
metal dichalcogenides (TMDCs) have attracted substantial scientific interest.
Due to the propensity of TMDCs to show exclusively one type of conductivity, n-
or p-type, heterojunctions of different materials are typically fabricated to
produce diode-like current rectification and photovoltaic response. Recently,
artificial, stable and substitutional doping of MoS2 into n- and p-type has
been demonstrated. MoS2 is an interesting material to use for optoelectronic
applications due to the potential of low-cost production in large quantities,
strong light-matter interactions and chemical stability. Here we report the
characterization of the optoelectronic properties of vertical homojunctions
made by stacking few-layer flakes of MoS2:Fe (n-type) and MoS2:Nb (p-type). The
junctions exhibit a peak external quantum efficiency of 4.7 %, a maximum open
circuit voltage of 0.51 V, they are stable in air and their rectification
characteristics and photovoltaic response are in excellent agreement to the
Shockley diode model. The gate-tunability of the maximum output power, the
ideality factor and the shunt resistance indicate that the dark current is
dominated by trap-assisted recombination and that the photocurrent collection
depends strongly on the spatial extent of the space charge region. We
demonstrate a response time faster than 80 ms and highlight the potential to
integrate such devices into quasi-transparent and flexible optoelectronics.
