European Chemistry Congress

Biography

Biography: Dieter M. Gruen

Abstract

The cost of solar electricity must be reduced by a factor of two in order to compete in the marketplace with fossil or nuclear fuels. This can be done by hybrid or co-generation of solar power accomplished by simultaneous conversion of both of PV and thermal energy in the same facility. The configuration that has been elaborated is called HYCSOS (Hybrid Conversion Solar System). To reach the stated goal of this effort requires solar cells that maintain conversion efficiency at temperature of 400 degrees Centigrade and above. The design of such cells is the subject of this presentation. Operating solar cells at high temperatures requires semiconductors with band-gaps of 3 ev or higher to minimize the generation of thermal carriers that counteract and severely diminish the photocurrent induced by the absorption of light. However wide band-gap semiconductors are only minimally effective in absorbing the solar spectrum. The new cell design described here circumvents this problem by surrounding nanowire wide band-gap semiconductors with highly light absorbing graphene. It has recently been established theoretically and by careful experimental measurements that nanowires function as light pipes with transmission modes that are highly concentrated at the nanowire surfaces thus providing an effective mechanism for the interaction of solar radiation with the graphene coating. It has been shown that nanowires of micron lengths surrounded by graphene absorb all of the solar radiation incident on them in part because the absorptivity of graphene is wavelength independent. Another unique property of graphene is its very high hole mobility which makes it possible for this substance to serve as its own hole conductor. This exceedingly important characteristic enables a simple cells design in which a metal foil that supports the nanowires is the electron conducting electrode and an optically transparent graphene top coating is the hole conducting electrode. This solar cell is composed entirely of inorganic, non-resource limited, environmentally benign materials that are highly temperature stable. It has also been shown theoretically and experimentally that graphene lends itself to multiple carrier generation which is strongly enhanced in the concentrating solar power (CSP) applications envisioned here. It is therefore conceivable that the conversion efficiency of the cells described here could exceed the Shockley/Queisser limit. Some time ago, this author together with colleagues, showed that a graphene/wide band-gap diode rectifies currents at temperatures approaching 900 degrees Centigrades. This pathbreaking result encourages the belief that a high temperature photo-diodes can be realized using a similar approach.