The modern era of photovoltaics started in 1954 when researchers at Bell
Labs in the USA accidentally discovered that pn junction diodes generated a voltage
when the room lights were on. Within a year, they had produced a 6% efficient Si pn
junction solar cell [23]. In the same year, the group at Wright Patterson Air Force Base
in the US published results of a thin-film heterojunction solar cell based on Cu2S/CdS
also having 6% efficiency [24].