Heat-Recovery Solar Cell

Heat-recovery (HERC) solar cell—a concept of solar cells utilizing the heat for improving the power conversion efficiency—is presented. HERC solar cell, characterized by an absorber hotter than electrodes, recovers heat as electricity to have high conversion efficiency exceeding the detailed balance limit when carrier-energy filtering is employed. Being different from hot-carrier solar cells, HERC solar cell does not require fast carrier extraction within the thermalization time, which largely improves its feasibility (feasible even with Si). An increment in the conversion efficiency originates from thermoelectricity produced by the temperature difference. Requirements for materials of the filtering layers are also given based on a nonideal device simulation with the thermoelectric properties.

Figure 1

Schematic diagrams for HERC solar cell: (a) the basic structure and (b) a function of the energy-filtering layers.

Figure 2

$I\text{−}V$ curves for different values of the barrier height $E_b(=0,0.2,0.4,0.6,0.7\mathrm{eV})$, obtained by the local equilibrium approximation (solid curves) and the full solution of the rate equations [Eqs. (1) and (2)] in the nonequilibrium theory (filled circles), for ${\tau }_{\mathrm{out}}={10}^{-9}\mathrm{s}$. Simulations are performed for $100$-$\mu \mathrm{m}$ $\mathrm{Si}$ under irradiation of 6000-K blackbody spectrum (AM0) at 1-sun condition with $T_c=300\mathrm{K}$.

Figure 3

(a) Two-dimensional contour plot of the power conversion efficiency in ($E_b$, $V$) plane are shown for (${\tau }_{\mathrm{out}}$, $T_{\mathrm{ph}})=({10}^{-9}\mathrm{s}$, 450 K). Unstable parameter regions where the heat supply is depleted, $P_{Q_{\mathrm{in}}}+P_T<0$ and $P_{Q_{\mathrm{in}}}<0$, are superposed in the figure. (b) Max power conversion efficiency ${\eta }_{max}$ is plotted as a function of ${\tau }_{\mathrm{out}}$ for $T_{\mathrm{ph}}=450\mathrm{K}$ with and without an IR absorber (red squares and blue triangles, respectively), and for $T_{\mathrm{ph}}=300\mathrm{K}$ (black circles). Simulations were performed for $100\mu \mathrm{m}$ $\mathrm{Si}$ under irradiation of 6000 K blackbody spectrum (AM0) at 1 sun condition with $T_c=300\mathrm{K}$.

Figure 4

Theoretical limit on the conversion efficiency ${\eta }_{max}$ in HERC solar cells (solid curves) with (squares) and without (triangles) an IR absorber, plotted for $300\mathrm{K}\le T_{\mathrm{ph}}\le 450\mathrm{K}$ and $T_c=300\mathrm{K}$. For comparison, we also show ${\eta }_{max}$ for HERC solar cells without energy filtering ($E_b=0$, dashed) at $T_{\mathrm{ph}}>T_c(=300\mathrm{K}$) with (open squares) and without (open triangles) an IR absorber, and ${\eta }_{max}$ for a normal solar cell with $T_c=T_{\mathrm{ph}}$ (dotted).

Figure 5

Theoretical limiting efficiency of HERC solar cell (with an IR absorber) with various absorber materials: $\mathrm{Ge}$, ${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_2$, $\mathrm{Si}$, $\mathrm{Ga}\mathrm{As}$, and $\mathrm{Cd}\mathrm{Te}$, respectively, with the thicknesses, 100, 2, 100, 2, $2\mu \mathrm{m}$, the band gaps, 0.67, 1.04, 1.12, 1.42, 1.56 eV, the effective mass parameters (DOS value), $m_e^{\ast }/m_0=0.88,0.09,1.08,0.067,0.09$ for electrons, and $m_h^{\ast }/m_0=0.29,0.73,0.55,0.47,0.64$ for holes [18, 23, 24, 25]. Simulations are performed for $T_{\mathrm{ph}}=350,400,450\mathrm{K}$ and $T_c=300\mathrm{K}$ under 1 sun, AM0 condition with 6000-K blackbody spectrum. The SQ limit for normal solar cell [2] (solid curve), and max efficiencies of hot-carrier solar cells [7] with the same carrier temperatures, $T_{\mathrm{carrier}}=350,400,450\mathrm{K}$ (dashed curves), are also shown for comparison. Calculation for a hot-carrier solar cell is performed based on the method presented in the original paper [7] with the same excitation condition.

Figure 6

Nonideal device simulation: (a) a schematic drawing of the device (with an IR absorber, electrodes are omitted in the figure for simplicity). (b) Maximum efficiency of HERC solar cell (with an IR absorber) plotted as a function of $T_{\mathrm{ph}}$ for different values for thermoelectric parameters of the energy filtering layers, $\kappa /\sigma =0$, ${10}^{-4.5}$, ${10}^{-4.0}$, ${10}^{-3.5}$, ${10}^{-3.0}$, ${10}^{-2.7}$, and ${10}^{-2.5}{\mathrm{V}}^2/\mathrm{K}$. Simulations are performed based on the nonideal device model with a $100$-$\mu \mathrm{m}$ $\mathrm{Si}$ absorber, illumination of 1 sun of 6000-K blackbody spectrum (AM0), and $T_c=300\mathrm{K}$.

Figure 7

Numerical data for (a) ${\eta }_{max}$ in a wide range of $T_{\mathrm{ph}}$ ($300\mathrm{K}\le T_{\mathrm{ph}}\le 2000\mathrm{K}$) and the optimal parameters at $\eta ={\eta }_{max}$ for HERC solar cell with and without an IR absorber (red square and blue triangles, respectively): (b) an optimal extraction time ${\tau }_{\mathrm{out}}$, (c) an optimal barrier height $E_b$, and (d) an optimal bias voltage $V$ and internal voltage $V^{\mathrm{cell}}$. Simulations are performed for $100$-$\mu \mathrm{m}$ Si under irradiation of the 6000-K blackbody spectrum (AM0) at 1-sun condition with $T_c=300\mathrm{K}$.