Journal Article 2D materials 2D Coulomb interactions anisotropic emission atomically thin organic-inorganic heterostructures binding energy exciton pumping graphene open-circuit voltage organic semiconductors organic-inorganic photoluminescence phototransistors photovoltaic cells positive and negative trions quasi-Fermi level splitting transition metal dichalcogenides type II interlayer trions type-I heterostructures ultrathin