Architectures and criteria for the design of high efficiency organic photovoltaic cells
An organic photovoltaic cell includes an anode and a cathode, and a plurality of organic semiconductor layers between the anode and the cathode. At least one of the anode and the cathode is transparent. Each two adjacent layers of the plurality of organic semiconductor layers are in direct contact. The plurality of organic semiconductor layers includes an intermediate layer consisting essentially of a photoconductive material, and two sets of at least three layers. A first set of at least three layers is between the intermediate layer and the anode. Each layer of the first set consists essentially of a different organic semiconductor material having a higher LUMO and a higher HOMO, relative to the material of an adjacent layer of the plurality of organic semiconductor layers closer to the cathode. A second set of at least three layers is between the intermediate layer and the cathode. Each layer of the second set consists essentially of a different organic semiconductor material having a lower LUMO and a lower HOMO, relative to the material of an adjacent layer of the plurality of organic semiconductor layers closer to the anode.
1. An organic photovoltaic cell comprising:
an anode and a cathode, at least one of the anode and the cathode being transparent; and
a plurality of organic semiconductor layers between the anode and the cathode, each two adjacent layers of the plurality of organic semiconductor layers being in direct contact, the plurality of organic semiconductor layers comprising:
an intermediate layer consisting essentially of a photoconductive material having a lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO);
a first set of at least three layers between the intermediate layer and the anode, each layer of the first set consisting essentially of a different organic semiconductor material having a higher LUMO and a higher HOMO, relative to an adjacent layer of the plurality of organic semiconductor layers closer to the cathode; and
a second set of at least three layers between the intermediate layer and the cathode, each layer of the second set consisting essentially of a different organic semiconductor material having a lower LUMO and a lower HOMO, relative to an adjacent layer of the plurality of organic semiconductor layers closer to the anode.
2. The organic photovoltaic cell of claim 1 , wherein an energy difference between a HOMO of a layer closest to the anode of the first set and a LUMO of a layer to the second set closest to the cathode is between 0.5 eV and 3.0 eV.
3. The organic photovoltaic cell of claim 2 , wherein the energy difference between the HOMO of a layer closest to the anode of the first set and the LUMO of a layer to the second set closest to the cathode is at least 1 eV.
4. The organic photovoltaic cell of claim 2 , wherein an organic semiconductor material of each layer of the first set of at least three layers has a HOMO and a LUMO that is at least 0.026 eV higher relative to the material of an adjacent layer of the plurality of organic semiconductor layers closer to the cathode, and
wherein an organic semiconductor material of each layer of the second set of at least three layers has a HOMO and a LUMO that is at least 0.026 eV lower relative to the material of an adjacent layer of the plurality of organic semiconductor layers closer to the anode.
5. The organic photovoltaic cell of claim 1 , wherein a combined sum of the HOMO differences between adjacent layers consisting of the first set of at least three layers and the intermediate layer, and the LUMO differences between adjacent layers consisting of the second set of at least three layers and the intermediate layer, is between 0.15 eV and 1.0 eV.
6. The organic photovoltaic cell of claim 1 , wherein a combined sum of the HOMO differences between adjacent layers consisting of the first set of at least three layers and the intermediate layer, and the LUMO differences between adjacent layers consisting of the second set of at least three layers and the intermediate layer, divided by a sum of the number of layers in the first set and the second set, is at least 0.026 eV.
7. The organic photovoltaic cell of claim 1 , wherein a sum of the HOMO differences between adjacent layers consisting of the first set of at least three layers and the intermediate layer divided by the number of layers in the first set is at least 0.026 eV, and a sum of the LUMO differences between adjacent layers consisting of the second set of at least three layers and the intermediate layer divided by the number of layers in the second set, is at least 0.026 eV.
8. The organic photovoltaic cell of claim 1 , wherein the organic semiconductor materials of the first set and the second set are photoconductive materials.
9. The organic photovoltaic cell of claim 1 , wherein a layer of the first set of three layers adjacent to said intermediate layer and a layer of the second set of three layers adjacent to said intermediate layer are each no more than three monolayers thick of the respective organic semiconductor material.
10. The organic photovoltaic cell of claim 9 , wherein each layer of the first set and the second set is no more than three monolayers thick of the respective organic semiconductor material.
11. The organic photovoltaic cell of claim 9 , wherein layer thicknesses of the first set of at least three layers progressively increase toward the anode, and the layer thicknesses of the second set of at least three layers progressively increase toward the cathode.
12. The organic photovoltaic cell of claim 11 , wherein a layer of the first set of at least three layers closest to the anode and a layer of the second set of at least three layer closest to the cathode are no more than ten monolayers thick.
13. The organic photovoltaic cell of claim 1 , wherein material bandgaps of the first set of at least three layers progressively increase toward the anode.
14. The organic photovoltaic cell of claim 13 , wherein the anode is reflective.
15. The organic photovoltaic cell of claim 14 , wherein the largest material bandgap of the second set of at least three layers is smaller than smallest material bandgap of the first set of materials.
16. The organic photovoltaic cell of claim 1 , wherein material bandgaps of the second set of at least three layers progressively increase toward the cathode.
17. The organic photovoltaic cell of claim 16 , wherein the cathode is reflective.
18. The organic photovoltaic cell of claim 17 , wherein the largest material bandgap of the first set of at least three layers is smaller than smallest material bandgap of the second set of materials.
19. The organic photovoltaic cell of claim 1 , wherein the photoconductive material of the intermediate layer consists essentially of molecules that form an excimer in the excited state, the HOMO and the LUMO of the photoconductive material being that of the excimer,
wherein a LUMO and a HOMO of a layer of the first set of at least three layers adjacent to the intermediate layer are higher relative to the excimer, and a LUMO and a HOMO of a layer of the second set of at least three layers adjacent to the intermediate layer are lower relative to the excimer.
20. The organic photovoltaic cell of claim 1 , wherein the photoconductive material of the intermediate layer consists essentially of two different types of molecules that form an exciplex in the excited state, the HOMO and the LUMO of the photoconductive material being that of the exciplex,
wherein a LUMO and a HOMO of a layer of the first set of at least three layers adjacent to the intermediate layer are higher relative to the exciplex, and a LUMO and a HOMO of a layer of the second set of at least three layers adjacent to the intermediate layer are lower relative to the exciplex.