This concise volume provides an introduction to the working principles, design, and construction of air-stable inverted organic light-emitting diodes (OLEDs), which lead to the realization of practical flexible electronics. The first part of the book reviews the history of the three generations of inverted OLEDs: hybrid organic inorganic light-emitting diodes (HOILEDs), metal oxides and organic electron injection layer, describing the materials, fabrication techniques, device structure, applications, and technological challenges involved in each case.  The second part of the book focuses on the carrier injection mechanism in OLEDs. The book will be of interest to students and researchers working on organic optoelectronics.