We report on PbSe thin films serving as an absorber in solar cell structures, CdS͑100 nm͒/Sb 2 S 3 ͑250 nm͒/PbSe͑100-250 nm͒. The cells are prepared by sequential chemical deposition of the films on a commercial SnO 2 :F coated sheet glass. These cells show V oc of 690 mV and J sc of 3.5 mA/cm 2 and a conversion efficiency of 0.69% under sunlight. Two distinct routes are taken to deposit PbSe thin films of 100-250 nm thickness: N,N-dimethylselenourea ͑SU͒ or sodium selenosulfate ͑SS͒ as the source of selenide ions in the bath. These films are p-type with an electrical conductivity of 0.5 ͑⍀ cm͒ −1 and optical bandgaps of 0.68 eV ͑SU͒ and 0.85 eV ͑SS͒ at a film thickness of 150 nm. Without the PbSe absorber, a CdS/Sb 2 S 3 cell has V oc of 595 mV, but its J sc is low, 0.003 mA/cm 2 . In CdS/Sb 2 S 3 /PbSe ͑SU or SS͒ cells, J sc is higher by a factor of thousand, and it is consistently higher for a larger PbSe film thickness. Thus, PbSe performs the role of a p + optical absorber in the cell, contributing to the J sc of the cell.PbSe crystallizes in a rocksalt structure ͓face-centered cubic ͑fcc͒, a = 0.6147 nm, PDF 06-0354͔. The energy band analysis suggests that the extrema of the valence band and conduction band occur at the L-point of the fcc Brillouin zone. 1 The bandgap ͑L 6v + − L 6c ͒ is 0.145 eV at 4 K and 0.278 eV ͑E g direct͒ at 300 K. In the 100-400 K temperature range relevant to ͓photovoltaic ͑PV͔͒ device operation, bulk-PbSe exhibits a positive temperature coefficient for E g of 0.53 meV/K. The room-temperature bandgap of 0.28 eV of PbSe implies an optical absorption onset ͓ g ͑m͒ = 1.240/E g ͑eV͔͒ for electromagnetic radiation of wavelength 4.43 m. Hence, Wien's displacement law governing the wavelength of maximum spectral irradiance ͓ m ͑m͒ = 2890/T ͑K͔͒ from a black/gray body emitter suggests PbSe as an effective detector of radiation from heat sources with surface temperature 652 K ͑379°C͒. Thus, the application of PbSe as an IR detector and specifically for heat-seeking missile guidance in the post-1947 era was the initial motivation toward the development of thin-film techniques for this material. 2 Nanocrystalline PbSe and Solar CellsPrince and Loferski's, 3 and later on, Shockley and Quiesser's 4 efficiency analyses for the PV conversion of solar energy by semiconductor absorbers as a function of their optical bandgap ͑E g ͒ kept PbSe irrelevant with sub-1% conversion efficiency for solar radiation under the detailed balance scheme. This situation for PbSe as a solar cell absorber changed in the late 1990s. The concept of quantum dot ͑QD͒ solar cells, in which the electron-hole pairs generated by photons in a semiconductor nanocrystal could be transported into an external circuit by electron/hole transporters, 5 brought forth PbSe as a solar cell material. Until recently, it was also held that multiexciton generation ͑MEG͒, from a single energetic photon with energy greater than 2E g in a QD semiconductor could enhance the lightgenerated current density in solar cells toward the blue side of ...