The PERL cell, to date, is one of the closest approximations to an ultimate Shockley diode, single-junction cell design for low injection conditions, with fine light trapping features, extensive oxide passivations, reduced ohmic contact regions, and an Al reflecting back contact. The only major features it is missing are heterojunction barriers to its ohmic contact regions and possibly back surface texturing. Chapter 2 of the First Edition  discussed this device in detail with its picture even featured on this book ’s cover fly sheet. A champion version has demonstrated the highest crystalline silicon efficiency so far, even approaching the 25.2% efficiency of the even more expensive GaAs single junction, one sun solar cell also of standard, low injection, Shockley diode design. However, the latter also had the additional Nobel Prize-winning double heterostructure barriers to it ohmic contact regions. Neither the Si point contact nor the PERL cell design (requiring photolithography) is cost competitive for one sun applications. The lower open – circuit voltages of the IBC and HIT cell compared to the PERL support the former being high injection devices described by Figure 4.5b where the PERL is a low injection device described by Figure 4.5a.