Molecular junctions and similar devices described by an energy dependenttransmission coefficient can have a high linear response thermoelectric figureof merit. Since such devices are inherently non-linear, the full thermodynamicefficiency valid for any temperature and chemical potential difference acrossthe leads is calculated. The general features in the energy dependence of thetranmission function that lead to high efficiency and also high power outputare determined. It is shown that the device with the highest efficiency doesnot necessarily lead to large power output. To illustrate this, we use a modelcalled the t-stub model representing tunneling through an energy levelconnected to another energy level. Within this model both high efficiency andhigh power output are achievable. Futhermore, by connecting many nanodevices itis shown to be possible to scale up the power output without compromisingefficiency in an (exactly solvable) n-channel model even with tunneling betweenthe devices.
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