Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity Efficiency

A heated semiconductor light-emitting diode at low forward bias voltage $V<k_{B}T/q$ is shown to use electrical work to pump heat from the lattice to the photon field. Here the rates of both radiative and nonradiative recombination have contributions at linear order in $V$. As a result the device’s wall-plug (i.e., power conversion) efficiency is inversely proportional to its output power and diverges as $V$ approaches zero. Experiments directly confirm for the first time that this behavior continues beyond the conventional limit of unity electrical-to-optical power conversion efficiency.

Figure 1

(a) LED output optical power versus input electrical power. The inset shows the experimental temperature dependence of the emission spectrum for a typical device. The lines denote the results of a numerical model. The discrete markers denote experimental data. (b) Band diagram at 26 mV forward bias. Peltier heat exchange occurs at various positions within the device. When a small forward bias is applied, most of the energy required to inject electrons and holes into the InGaAsSb active region is drawn from the lattice and the device operates as a heat pump. (CB, conduction band; VB, valence band.)

Figure 2

Light-current and current-voltage characteristics for the LED. Measurable current flows and measurable light is emitted even as $qV$ is reduced well below the average energy $\hslash \omega$ of the emitted photons.

Figure 3

(a) External quantum efficiency ${\eta }_{\mathrm{EQE}}$ versus current and (b) wall-plug efficiency $\eta$ versus output optical power. External quantum efficiency ${\eta }_{\mathrm{EQE}}$ asymptotically approaches a constant value in the low-bias regime (where $V<k_{B}T/q$). As a result, in this regime output power is linear in current while input power is quadratic, and $\eta$ is inversely proportional to output power. This behavior continues beyond unity efficiency indicating electroluminescent cooling.

Figure 4

(a) Cooling power versus current at high temperature and (b) low-bias external quantum efficiency and zero-bias (ZB) resistance versus temperature. The parabolic dependence of cooling power on current is indicative of a linear cooling process competing with a quadratic heating process. Increases in ${\eta }_{\mathrm{EQE}}$ and reductions in $R$ lead to improved heat pumping power as the temperature rises. Numerical results suggest that this trend should continue to higher temperature.