  We investigated the electronic and optoelectronic properties of vertical van
der Waals heterostructure photodetectors using layered p type GaSe and n type
InSe, with graphene as the transparent electrodes. Not only the photocurrent
peaks from the layered GaSe and InSe themselves were observed, also the
interlayer optical transition peak was observed, which is consistent with the
first-principles calculation. The built-in electric field between p-n
heterojunction and the advantage of the graphene electrodes can effectively
separate the photo-induced electron-hole pairs, and thus lead to the response
time down to 160 {\mu}s. Making use of the interlayer as well as intralayer
optical transitions of the vertical layered p-n heterostructure and graphene
electrodes, we could achieve high performance multi-color photodetectors based
on two-dimensional layered materials, which will be important for future
optoelectronics.
