This chapter builds on the understanding of pn diodes developed in Chaps. 12 and 13 to analyze technologically important optoelectronic devices that rely on pn junctions. Examples of light-absorbing devices include photodetectors, fast photodiodes used in telecommunications, and single-photon detectors. An expansive section on solar cells introduces the reader to the spectral irradiance from the sun, basic concepts such as the current-voltage characteristic of photovoltaic cells under illumination, and different loss mechanisms that limit the power conversion efficiency of solar cells. This leads naturally to the so-called Shockley-Queisser limit, i.e., the ultimate (thermodynamic) limit to the power conversion efficiency of single-junction solar cells, along with examples of approaches for overcoming the Shockley-Queisser limit. The progress in efficiency of different photovoltaic technologies, ranging from monocrystalline silicon cells to hybrid inorganic-organic perovskites, is analyzed in relation to the Shockley-Queisser limit. The chapter concludes with an introduction to semiconductor light emitters, including light-emitting diodes (LEDs), devices for solid-state lighting, and diode lasers.