Intermediate-band photosensitive device with quantum dots embedded in energy fence barrier

Abstract

A plurality of layers of a first semiconductor material and a plurality of dots-in-a-fence barriers disposed in a stack between a first electrode and a second electrode. Each dots-in-a-fence barrier consists essentially of a plurality of quantum dots of a second semiconductor material embedded between and in direct contact with two layers of a third semiconductor material. Wave functions of the quantum dots overlap as at least one intermediate band. The layers of the third semiconductor material are arranged as tunneling barriers to require a first electron and/or a first hole in a layer of the first material to perform quantum mechanical tunneling to reach the second material within a respective quantum dot, and to require a second electron and/or a second hole in a layer of the first semiconductor material to perform quantum mechanical tunneling to reach another layer of the first semiconductor material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an intermediate band solar cell.

FIGS. 2A and 2B are energy-band diagrams for a cross-section of an inorganic quantum dot in an inorganic matrix material, with the lowest quantum state in the conduction band providing the intermediate band.

FIGS. 3A and 3B are energy-band diagrams for a cross-section of an inorganic quantum dot in an inorganic matrix material, with the highest quantum state in the valence band providing the intermediate band.

FIG. 4 is an energy band diagram for the intermediate band solar cell of FIG. 1, with inorganic quantum dots in an inorganic matrix material, and with the lowest quantum state in the conduction band providing the intermediate band.

FIG. 5 illustrates a cross-section of the array of quantum dots in the device in FIG. 1, as generally idealized and as formed in colloidal solutions.

FIG. 6 illustrates a cross-section of the array of quantum dots in the device in FIG. 1, if produced using the Stranski-Krastanow method.

FIG. 7 is an energy band diagram for a cross-section of an inorganic quantum dot in an inorganic matrix material, illustrating de-excitation and trapping of a passing electron.

FIG. 8 illustrates a cross-section of an array of quantum dots like that shown in FIG. 5, modified to include a tunneling barrier.

FIGS. 9A and 9B are energy-band diagrams for a cross-section of a quantum dot including tunneling barriers with a lowest quantum state above the band gap providing the intermediate band.

FIGS. 10 is an energy band diagram for a solar cell based on the design in FIG. 1, with quantum dots modified to include the tunneling barrier, and with the lowest quantum state above the band gap providing the intermediate band.

FIGS. 11A and 11B are energy-band diagrams for a cross-section of a quantum dot including tunneling barriers with a highest quantum state below the band gap providing the intermediate band.

FIG. 12 is an energy band diagram for a solar cell based on the design in FIG. 1, with quantum modified to include the tunneling barrier, and with the highest quantum state below the band gap providing the intermediate band.

FIG. 13 illustrates a cross-section of the array of quantum dots modified to include the tunneling barrier, if produced using the Stranski-Krastanow method.

FIGS. 14 and 15 demonstrate tunneling through a rectangular barrier.

FIG. 16 demonstrates a triangular tunneling barrier.

FIG. 17 demonstrates a parabolic tunneling barrier.

FIG. 18 illustrates a structure of GaAs/InAs intermediate band fence barrier (DFENCE) solar cell. Path A shows transport along on-dot sites through the GaAs buffer, Al_xGa_1-xAs fences, InAs wetting layers, and InAs quantum dots. Path B shows charge transport along off-dot sites through the GaAs buffer, InAs wetting layers and Al_xGa_1-xAs fences.

FIGS. 19A and 19B are energy-band diagrams for cross-sections of a DFENCE structure from FIG. 18. FIG. 19A illustrates an on-dot band diagram (along line “A” in FIG. 18) and FIG. 19B illustrates an off-dot band diagram (along line “B” in FIG. 18). As the thin InAs wetting layer 1832 has negligible impact on tunneling, it is not represented in FIG. 19B.

FIG. 20 is a plot of ground state transition energy versus quantum dot radius of (R) for the structure in FIG. 18, with the thickness of fence barrier fixed to t=0.1 R for aluminum fractions of x=0, 0.1, 0.2, and 0.3. Here, l is the dot length and l=R, d is the thickness of the surrounding GaAs layer and d=5 nm, and L is the distance between quantum dots in the plane of the substrate surface and L=1 nm+2 R. The trace for x=0 corresponds to a structure having tunneling barriers.

FIG. 21 is a graph of the carrier escape rate versus quantum dot radius for the same structures as in FIG. 20.

FIG. 22 is a graph of current density versus voltage for GaAs DFENCE heterostructures as a function of the number of stacked quantum dot layers N (x=0.2).

FIG. 23 is a graph of power conversion efficiency versus number of quantum dot layers (N) for quantum dots with a radius of 8 nm when x increases from 0 to 0.2. The DFENCE structure is otherwise as described in FIG. 20 (t=0.1 R=0.8 nm; d=5 nm; L=1 nm+2 R=17 nm).

FIG. 24 is a graph of power conversion efficiency versus intermediate band energy level calculated for: (a) the ideal conditions proposed in the paper A. Luque and A. Marti, Phys. Rev. Lett. 78, 5014 (1997) (“Luque model”), (b) the Luque model for GaAs with the band gap of 1.426 eV, and (c), (d) and (e) a respective upper limit of the GaAs/InAs DFENCE model with x=0.2, 0.1 and 0. The labeled data in curve (a) is the bulk band gap assumed that corresponds with the intermediate band level on the abscissa to achieve maximum efficiency.

FIG. 25 illustrates a structure of an InP/InAs intermediate band fence barrier (DFENCE) solar cell. Path A shows transport along on-dot sites through the InP buffer, Al_0.48In_0.52As fences, InAs wetting layers, and InAs quantum dots. Path B shows charge transport along off-dot sites through the InP buffer, InAs wetting layers and Al_0.48In_0.52As fences.

FIGS. 26A and 26B are energy-band diagrams for cross-sections of a DFENCE structure from FIG. 25. FIG. 26A illustrates an on-dot band diagram (along line “A” in FIG. 25) and FIG. 26B illustrates an off-dot band diagram (along line “B” in FIG. 25). As the thin InAs wetting layer 2532 has negligible impact on tunneling, it is not represented in FIG. 26B.

FIG. 27 is a plot of ground state transition energy versus quantum dot radius of (R) for the structure in FIG. 25, with the thickness of fence barrier fixed to t=0.1 R. Here, t is the dot length and l=R, d is the thickness of the surrounding GaAs layer and d=5 nm, and L is the distance between quantum dots in the plane of the substrate surface and L=1 nm+2 R. The data is also included for the same structure with no tunneling barriers.

FIG. 28 is a graph of the carrier escape rate versus quantum dot radius for the structure as in FIG. 25, and an equivalent structure having no tunneling barriers.

FIG. 29 is a graph of the carrier escape rate versus quantum dot radius for the structure as in FIG. 25. In view of the escape rate in FIG. 28 appearing to be zero, the y-axis scale in FIG. 29 is adjusted to more clearly show the escape rate for the DFENCE structure.

FIG. 30 is a graph of power conversion efficiency versus number of quantum dot layers (N) for quantum dots with a radius of 8 nm. The DFENCE structure is otherwise as described in FIG. 27 (t=0.1 R=0.8 nm; d=5 nm; L=1 nm+2 R=17 nm).

FIG. 31 is a graph of power conversion efficiency versus intermediate band energy level calculated for: the ideal conditions proposed in the Luque model, the Luque model for InP with the band gap of 1.34 eV, an upper limit of the InP/InAs DFENCE model. The labeled data on the ideal Luque model curve is the bulk band gap assumed that corresponds with the intermediate band level on the abscissa to achieve maximum efficiency.

FIG. 32 illustrates the relationship between lattice constant, peak absorption wavelength, and energy gap for a variety of common compound semiconductors. Ternary and quaternary combinations of these semiconductors (in between the points shown) provide lattice matched materials having different energy gaps.

Claims

1. A device comprising: a first electrode and a second electrode;a plurality of layers of a first semiconductor material disposed in a stack between the first electrode and the second electrode; anda plurality of dots-in-a-fence barriers, each dots-in-a-fence barrier consisting essentially of a plurality of quantum dots of a second semiconductor material embedded between and in direct contact with two layers of a third semiconductor material, wherein each dots-in-a-fence barrier is disposed in the stack between and in direct contact with a respective two of the layers of the first semiconductor material,wherein each quantum dot provides at least one quantum state at an energy between a conduction band edge and a valence band edge of the adjacent layers of the first semiconductor material, wave functions of said at least one quantum state of the plurality of quantum dots to overlap as at least one intermediate band, andthe layers of the third semiconductor material are arranged as tunneling barriers to require a first electron and/or a first hole in a layer of the first material to perform quantum mechanical tunneling to reach the second material within a respective quantum dot, and to require a second electron and/or a second hole in a layer of the first semiconductor material to perform quantum mechanical tunneling to reach another layer of the first semiconductor material.

2. The device of claim 1, wherein the third semiconductor material is lattice matched to the first semiconductor material.

3. The device of claim 1, wherein the first semiconductor material is GaAs, the second semiconductor material is InAs, and the third semiconductor material is AlxGa1-xAs with x>0.

4. The device of claim 3, wherein: each InAs quantum dot has an average lateral cross-section of 2 R and a height of l, with 2 nm≦R≦10 nm;each AlxGa1-xAs layer has a thickness t, with 0.1 R≦t≦0.3 R; andeach GaAs layer disposed between two dots-in-a-fence barriers has a thickness d, with 2 nm≦d≦10 nm.

5. The device of claim 4, wherein: a period of a quantum dot unit cell within a respective dots-in-a-fence barrier is L, with 2 R≦L≦2 R+2 nm; anda period of a quantum dot unit cell between adjacent dots-in-a-fence barriers is Lz, with Lz=+d+t.

6. The device of claim 4, wherein 6 nm ≦R≦8 nm.

7. The device of claim 6, wherein there are 1010 to 1012 quantum dots per square centimeter.

8. The device of claim 3, wherein a first layer of the plurality of GaAs layers nearest to the first electrode is n-doped, a second layer of the plurality of GaAs layers nearest to the second electrode is p-doped, and other layers of the plurality of GaAs layers are intrinsic.

9. The device of claim 3, wherein said at least one quantum state in each quantum dot includes a quantum state above a band gap of InAs providing an intermediate band.

10. The device of claim 3, wherein said at least one quantum state in each quantum dot includes a quantum state below a band gap of InAs providing an intermediate band.

11. The device of claim 1, wherein the first semiconductor material is InP, the second semiconductor material is InAs, and the third semiconductor material is Al0.48In0.52As.

12. The device of claim 11, wherein: each InAs quantum dot has an average lateral cross-section of 2 R and a height of t, with 2 nm≦R≦12 nm;each Al0.48In0.52As layer has a thickness t, with 0.1 R ≦t≦0.3 R; andeach InP layer disposed between two dots-in-a-fence barriers has a thickness d, with 2 nm≦d≦12 nm.

13. The device of claim 12, wherein: a period of a quantum dot unit cell within a respective dots-in-a-fence barrier is L, with 2 R≦L≦2 R+2 nm; anda period of a quantum dot unit cell between adjacent dots-in-a-fence barriers is Lz, with Lz=l+d+t.

14. The device of claim 11, wherein a first layer of the plurality of InP layers nearest to the first electrode is n-doped, a second layer of the plurality of InP layers nearest to the second electrode is p-doped, and other layers of the plurality of InP layers are intrinsic.

15. The device of claim 11, wherein said at least one quantum state in each quantum dot includes a quantum state above a band gap of InAs providing an intermediate band.

16. The device of claim 11, wherein said at least one quantum state in each quantum dot includes a quantum state below a band gap of InAs providing an intermediate band.

17. The device of claim 1, wherein a first layer of the plurality of layers of the first material nearest to the first electrode is n-doped, a second layer of the plurality of layers of the first material nearest to the second electrode is p-doped, and other layers of the plurality of layers of the first material are intrinsic.

18. The device of claim 1, wherein said at least one quantum state in each quantum dot includes a quantum state above a band gap of the second semiconductor material providing an intermediate band.

19. The device of claim 1, wherein said at least one quantum state in each quantum dot includes a quantum state below a band gap of the second semiconductor material providing an intermediate band.