Luminescent Cu(I) NHetPHOS complexes are very efficient emitters, which have successfully been deployed in organic light-emitting diodes (OLEDs). In this work, various important questions concerning Cu(I) complexes were addressed. By means of soft x-ray spectroscopy at the N K edge, ultraviolet photoelectron spectroscopy, and inverse photoemission spectroscopy, the electronic structure with particular focus on the occupied and unoccupied molecular orbitals, formed upon binding of the Cu- and N-atoms, and the HOMO and LUMO energies of these materials were investigated. Furthermore, new red-emitting Cu(I) complexes were developed. The principle structure of a not crystallizable NHetPHOS complex with a bridging bisphosphine ligand, yielding a new quantum efficiency record for both solution- and vacuum-processed organic light-emitting diodes with Cu(I) complexes as emitters, was determined by means of x-ray spectroscopy at the Cu K edge. Consequently, a breakthrough for the cost- and energy-efficient processing of organic light-emitting diodes from solution was enabled.