Chang-Hua Liu, Nanditha M. Dissanayake, Seunghyun Lee, Kyunghoon Lee, Zhaohui Zhong
Direct extraction of above bandgap photoexcited hot carriers prior to relaxation can lead to 66% power conversion efficiency. However, hot carriers undergo rapid thermalization to the energy band edge in bulk semiconductor, thereby setting the Shockley-Queisser limit (31%) for single-junction photovoltaic. Here we report evidence of direct nonequilibrium hot carriers extraction from graphene by careful gate-dependent photocurrent study. Scanning photocurrent excited by femtosecond pulse laser shows unusual gate dependence compared with continuous wave (CW) laser excitation. Power dependence studies further confirm that the photocarriers extracted at the metal/graphene contact are nonequilibrium hot carriers. Hot carrier extraction is found to be most efficient near the Dirac point where carrier lifetime reaches maximum. These observations not only provide direct evidence of hot carrier extraction from graphene, but also open the door for graphene based hot carrier optoelectronics.
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http://arxiv.org/abs/1203.5681
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