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A photodetector is described along with corresponding materials, systems, and methods. The photodetector comprises an integrated circuit and at least two optically sensitive layers. A first optically sensitive layer is over at least a portion of the integrated circuit, and a second optically sensiti

A photodetector is described along with corresponding materials, systems, and methods. The photodetector comprises an integrated circuit and at least two optically sensitive layers. A first optically sensitive layer is over at least a portion of the integrated circuit, and a second optically sensitive layer is over the first optically sensitive layer. Each optically sensitive layer is interposed between two electrodes. The two electrodes include a respective first electrode and a respective second electrode. The integrated circuit selectively applies a bias to the electrodes and reads signals from the optically sensitive layers. The signal is related to the number of photons received by the respective optically sensitive layer.

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1. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer;

1. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer, a thickness of the third optically sensitive layer is less than a thickness of at least one of the first optically sensitive layer and the second optically sensitive layer, a third dark current of the third optically sensitive layer is less than a second dark current of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode. 2. The photodetector of claim 1, wherein the integrated circuit selectively applies a bias to the electrodes and reads signals from the optically sensitive layers, wherein the signal is related to the number of photons received by the respective optically sensitive layers. 3. The photodetector of claim 1, wherein a thickness of the third optically sensitive layer is greater than a thickness of at least one of the first optically sensitive layer and the second optically sensitive layer. 4. The photodetector of claim 1, wherein a thickness of the second optically sensitive layer is less than a thickness of the first optically sensitive layer. 5. The photodetector of claim 1, wherein the third optically sensitive layer is relatively completely absorbent of light in a first wavelength interval and relatively completely transmissive of light outside the first wavelength interval. 6. The photodetector of claim 5, wherein the second optically sensitive layer is relatively completely absorbent of light in a second wavelength interval and relatively completely transmissive of light outside the second wavelength interval, wherein the second wavelength interval includes and is larger than the first wavelength interval. 7. The photodetector of claim 6, wherein the first optically sensitive layer is relatively completely absorbent of light in a third wavelength interval, wherein the third wavelength interval includes and is larger than the second wavelength interval. 8. The photodetector of claim 1, wherein at least one of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap, and wherein the nanocrystals are quantum confined to have an effective bandgap more than twice the bulk bandgap. 9. The photodetector of claim 1, wherein at least one of the optically sensitive layers includes nanocrystals comprising nanoparticles, wherein a nanoparticle diameter of the nanoparticles is less than a Bohr exciton radius of bound electron-hole pairs within the nanoparticle. 10. The photodetector of claim 1, wherein a first diameter of nanocrystals of the first optically sensitive layer is greater than a second diameter of nanocrystals of the second optically sensitive layer. 11. The photodetector of claim 1, wherein a first diameter of nanocrystals of the first optically sensitive layer is less than a second diameter of nanocrystals of the second optically sensitive layer. 12. The photodetector of claim 1, wherein at least one of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 electronvolts (eV), and wherein the nanocrystals are quantum confined to have a bandgap more than 1.0 eV. 13. The photodetector of claim 1, wherein each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than 0.5 eV, and wherein the nanocrystals in the first optically sensitive layer are quantum confined to have a bandgap of approximately 1.8 eV, the nanocrystals in the second optically sensitive layer are quantum confined to have a bandgap of approximately 2.2 eV, and the nanocrystals in the third optically sensitive layer are quantum confined to have a bandgap of more than approximately 2.5 eV. 14. The photodetector of claim 1, wherein each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV. 15. The photodetector of claim 14, wherein the nanocrystals of the third optically sensitive layer are quantum confined to a bandgap corresponding to 490 nm wavelength. 16. The photodetector of claim 14, wherein the nanocrystals of the third optically sensitive layer are quantum confined to a bandgap of approximately 2.5 eV. 17. The photodetector of claim 14, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap corresponding to 560 nm wavelength. 18. The photodetector of claim 14, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap of approximately 2.2 eV. 19. The photodetector of claim 14, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 1.8 eV. 20. The photodetector of claim 14, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 1.2 eV. 21. The photodetector of claim 14, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 0.9 eV. 22. The photodetector of claim 14, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 0.7 eV. 23. The photodetector of claim 14, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 650 nm wavelength. 24. The photodetector of claim 14, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 700 nm wavelength. 25. The photodetector of claim 1, further comprising a fourth optically sensitive layer, wherein the fourth optically sensitive layer is over at least a portion of the third optically sensitive layer. 26. The photodetector of claim 25, wherein each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV. 27. The photodetector of claim 26, wherein the nanocrystals of the fourth optically sensitive layer are quantum confined to a bandgap corresponding to 490 nm wavelength. 28. The photodetector of claim 26, wherein the nanocrystals of the fourth optically sensitive layer are quantum confined to a bandgap of approximately 2.5 eV. 29. The photodetector of claim 26, wherein the nanocrystals of the third optically sensitive layer are quantum confined to a bandgap corresponding to 560 nm wavelength. 30. The photodetector of claim 26, wherein the nanocrystals of the third optically sensitive layer are quantum confined to a bandgap of approximately 2.2 eV. 31. The photodetector of claim 26, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap corresponding to 630 nm wavelength. 32. The photodetector of claim 26, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap corresponding to 650 nm wavelength. 33. The photodetector of claim 26, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap corresponding to 670 nm wavelength. 34. The photodetector of claim 26, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap corresponding to 700 nm wavelength. 35. The photodetector of claim 26, wherein the nanocrystals of the second optically sensitive layer are quantum confined to a bandgap of approximately 1.8 eV. 36. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 800 nm wavelength. 37. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 900 nm wavelength. 38. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 1000 nm wavelength. 39. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 1300 nm wavelength. 40. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 1650 nm wavelength. 41. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 3 um wavelength. 42. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap corresponding to 5 um wavelength. 43. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 1.2 eV. 44. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 0.9 eV. 45. The photodetector of claim 26, wherein the nanocrystals of the first optically sensitive layer are quantum confined to a bandgap of approximately 0.7 eV. 46. The photodetector of claim 1, further comprising a fourth optically sensitive layer over at least a portion of the third optically sensitive layer. 47. The photodetector of claim 46, wherein the third optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light and visible green light and transmissive to visible red light and infrared light. 48. The photodetector of claim 46, wherein the second optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light, visible green light, and visible red light. 49. The photodetector of claim 46, wherein the first optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light, visible green light, visible red light and infrared light. 50. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer, a thickness of the third optically sensitive layer being less than a thickness of at least one of the first optically sensitive layer and the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, the at least two optically sensitive layers including a fourth optically sensitive layer, wherein the fourth optically sensitive layer is over at least a portion of the third optically sensitive layer, wherein a thickness of the fourth optically sensitive layer is less than a thickness of one of the first optically sensitive layer, the second optically sensitive layer, and the third optically sensitive layer. 51. The photodetector of claim 50, wherein a thickness of the fourth optically sensitive layer is less than a thickness of the third optically sensitive layer. 52. The photodetector of claim 51, wherein a thickness of the third optically sensitive layer is less than a thickness of the second optically sensitive layer. 53. The photodetector of claim 52, wherein a thickness of the second optically sensitive layer is less than a thickness of the first optically sensitive layer. 54. The photodetector of claim 50, wherein the fourth optically sensitive layer is relatively completely absorbent of light in a first wavelength interval and relatively completely transmissive of light outside the first wavelength interval. 55. The photodetector of claim 54, wherein the third optically sensitive layer is relatively completely absorbent of light in a third wavelength interval and relatively completely transmissive of light outside the third wavelength interval, wherein the third wavelength interval includes and is larger than the fourth wavelength interval. 56. The photodetector of claim 55, wherein the second optically sensitive layer is relatively completely absorbent of light in a second wavelength interval, wherein the second wavelength interval includes and is larger than the third wavelength interval. 57. The photodetector of claim 56, wherein the first optically sensitive layer is relatively completely absorbent of light in a first wavelength interval, wherein the first wavelength interval includes and is larger than the second wavelength interval. 58. The photodetector of claim 50, wherein a fourth dark current of the fourth optically sensitive layer is less than at least one of a third dark current of the third optically sensitive layer, a second dark current of the second optically sensitive layer, and a first dark current of the first optically sensitive layer. 59. The photodetector of claim 50, wherein a third dark current of the third optically sensitive layer is less than at least one of a fourth dark current of the fourth optically sensitive layer, a second dark current of the second optically sensitive layer, and a first dark current of the first optically sensitive layer. 60. The photodetector of claim 50, wherein a second dark current of the second optically sensitive layer is less than at least one of a fourth dark current of the fourth optically sensitive layer, a third dark current of the third optically sensitive layer, and a first dark current of the first optically sensitive layer. 61. The photodetector of claim 50, wherein a first dark current of the first optically sensitive layer is less than at least one of a fourth dark current of the fourth optically sensitive layer, a third dark current of the third optically sensitive layer, and a second dark current of the second optically sensitive layer. 62. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer, at least one of the optically sensitive layers comprises a nanocrystal material having photoconductive gain and a responsivity of at least approximately 0.4 amps/volt (A/V);wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode. 63. The photodetector of claim 62, wherein the responsivity is achieved when a bias is applied between the respective first electrode and the respective second electrode, wherein the bias is approximately in a range of 1 volt to 5 volts. 64. The photodetector of claim 63, wherein the bias is approximately 0.5 volts. 65. The photodetector of claim 63, wherein the bias is approximately 1 volt. 66. The photodetector of claim 63, wherein the bias is approximately 1.2 volts. 67. The photodetector of claim 63, wherein the bias is approximately 1.5 volts. 68. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, the third optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light and transmissive to visible green light, visible red light, and infrared light, the second optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light and visible green light and transmissive to visible red light and infrared light, and the first optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light, visible green light, and visible red light. 69. The photodetector of claim 68, wherein the first optically sensitive layer is absorptive to infrared light. 70. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer;wherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer; andwherein a fourth optically sensitive layer is over at least a portion of the third optically sensitive layer, the fourth optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 490 nm wavelength;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode. 71. The photodetector of claim 70, wherein the third optically sensitive layer comprises a nanocrystal material having an absorption onset of less than approximately 560 nm wavelength. 72. The photodetector of claim 71, wherein the second optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 630 nm wavelength. 73. The photodetector of claim 71, wherein the second optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 650 nm wavelength. 74. The photodetector of claim 71, wherein the second optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 670 nm wavelength. 75. The photodetector of claim 71, wherein the second optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 700 nm wavelength. 76. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 800 nm wavelength. 77. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 900 nm wavelength. 78. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 1000 nm wavelength. 79. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 1300 nm wavelength. 80. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 1650 nm wavelength. 81. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 3 um wavelength. 82. The photodetector of claim 71, wherein the first optically sensitive layer comprises a nanocrystal material having an absorption onset at approximately 5 um wavelength. 83. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer, a thickness of the third optically sensitive layer being less than a thickness of at least one of the first optically sensitive layer and the second optically sensitive layer, a third dark current of the third optically sensitive layer being less than a first dark current of the first optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode. 84. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer, a thickness of the third optically sensitive layer being less than a thickness of at least one of the first optically sensitive layer and the second optically sensitive layer, a second dark current of the second optically sensitive layer being less than a first dark current of the first optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode. 85. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to an 800 nm wavelength. 86. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to a 900 nm wavelength. 87. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to a 1000 nm wavelength. 88. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to a 1300 nm wavelength. 89. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to a 1650 nm wavelength. 90. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to a 3 μm wavelength. 91. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than approximately 0.5 eV, the nanocrystals of the first optically sensitive layer being quantum confined to a bandgap corresponding to a 5 μm wavelength. 92. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer; andwherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer;wherein a fourth optically sensitive layer is over at least a portion of the third optically sensitive layer;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode, each of the optically sensitive layers comprises nanocrystals of a material having a bulk bandgap of less than 0.5 eV, the nanocrystals in the first optically sensitive layer being quantum confined to have a bandgap corresponding to approximately 800 nm wavelength, the nanocrystals in the second optically sensitive layer being quantum confined to have a bandgap corresponding to approximately 630 nm wavelength, the nanocrystals in the third optically sensitive layer being quantum confined to have a bandgap corresponding to approximately 560 nm wavelength, and the nanocrystals in the fourth optically sensitive layer being quantum confined to have a bandgap corresponding to approximately 490 nm wavelength. 93. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer;wherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer; andwherein a fourth optically sensitive layer is over at least a portion of the third optically sensitive layer;wherein a local absorption maximum is absent from an absorption spectrum of at least one of the first optically sensitive layer, the second optically sensitive layer, the third optically sensitive layer, and the fourth optically sensitive layer; andwherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode. 94. A photodetector comprising: an integrated circuit;a plurality of optically sensitive layers;wherein a first optically sensitive layer is over at least a portion of the integrated circuit;wherein a second optically sensitive layer is over at least a portion of the first optically sensitive layer;wherein a third optically sensitive layer is over at least a portion of the second optically sensitive layer; andwherein a fourth optically sensitive layer is over at least a portion of the third optically sensitive layer, the fourth optically sensitive layer comprises a nanocrystal material absorptive to at least visible blue light and transmissive to visible green light, visible red light, and infrared light;wherein the optically sensitive layers are interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode.