Mechanically stacked photovoltaic tandem cells with optical intermediate filters

Description

[0001] This application claims the benefit of U.S. Provisional Application No. 62 / 126,326, filed February 27, 2015.

[0002] The present disclosure is directed to multi-junction photovoltaic devices and methods of fabricating, and more particularly multi-junction photovoltaic devices for solar cells.

[0003] Multi-junction photovoltaic cells provide superior performance over single junction solar cells by increasing the wavelengths of the solar spectrum that can be absorbed by the photovoltaic cell, and also minimize thermalization losses associated with absorption of photons with greater energy than the band gap of a photoactive material. A solar cell should cover the near infrared solar spectrum in order to maximize the performance of conventional multi-junction organic solar cell. Yet many photoactive materials, such as organic compounds, do not efficiently absorb and generate electrical energy in the near infrared solar spectrum. Quantum dot solar cells effectively cover the near infrared spectrum out to and beyond the wavelength of 1µm with reasonably high power conversion efficiency (>8%), as demonstrated by Chuang et al. in "Improved performance and stability in quantum dot solar cells through band alignment engineering." (Chia-Hao M. Chuang, Patrick R. Brown, Vladimir Bulović, and Moungi G. Bawendi, Improved performance and stability in quantum dot solar cells through band alignment engineering. 13 Nat. Mater., 796 (2014)). However, quantum dot solar cells also strongly absorb in the visible spectrum. The broad absorption spectrum of quantum dot solar cells therefore limits the performance of multi-junction organic solar cell by absorbing shorter wavelength photons and thereby preventing those photons from either transmitting or reflecting into other sub-cells that are tuned to absorb the short wavelength photons region.

[0004] Document US 2012 / 000518 A1 discloses a photoelectric conversion device in which a first photoelectric conversion module having a plurality of first photoelectric conversion elements formed on one surface of a first translucent insulated substrate and a second photoelectric conversion module having a plurality of photoelectric conversion elements formed on one surface of a second translucent insulated substrate are bonded together with the first photoelectric conversion elements and the second photoelectric conversion elements placed on an inner side. A wavelength selective reflective layer having waveform selectivity for reflecting only light having wavelength in a specific range is arranged between the first and second photoelectric conversion modules.

[0005] The present invention is directed to a multi-junction photosensitive device. The device includes at least two photoactive layers and at least one optical filter layer. In some embodiments, the multi-junction device has an increased absorption spectrum relative to a single junction device.

[0006] In one aspect of the invention, the optical filter can be configured to selectively tune the wavelengths of at least one of reflection, transmission, and absorption of incident light. In some embodiments, the optical filter is configured to transmit long wavelength light. In some embodiments, the optical filter is configured to reflect short wavelength light. In some embodiments, the optical filter is configured to transmit short wavelengths. In some embodiments the optical filter may reflect long wavelength light. In some embodiments the optical filter may be a photonic crystal. In some embodiments, the photonic crystal may comprise multiple layers of alternating materials with at least two different refractive indices. In some embodiments the photonic crystal may comprise a periodic grating of a waveguide material. In some embodiments, the optical filter may be a Distributed Bragg Reflector (DBR). In some embodiments, the optical filter may be a dichroic filter.

[0007] One aspect of the invention is directed to a multi-junction solar cell. In some embodiments the absorption of the solar cell covers at least some wavelengths of the solar spectrum. One aspect of the invention is directed to a multi-junction detector. In some embodiments the absorption of the multi-junction detector may comprise at least one wavelength outside the solar spectrum.

[0008] The present invention is also directed to a method of fabricating a multi-junction photosensitive device. This method according to the present invention is set out in independent claim 9. Fig. 1 shows a schematic illustration of an exemplary fabrication flow for an exemplary organic / quantum dot multi-junction solar cell from individual sub-cells. Fig. 2 shows a schematic illustration of the separation of the solar spectrum via an intermediate mirror in an exemplary organic / quantum dot multi-junction solar cell. Fig. 3A shows a schematic of a single junction solar cell with a DBR as an optical filter layer. Fig. 3B shows a graph of the measured photocurrent versus voltage for a single junction solar cell fabricated according to the layers in Fig. 3A. Fig. 4. shows a table summarizing the performance of a single junction solar cell with and without a DBR optical filter. Fig. 5 shows a schematic of an exemplary DBR as an optical filter layer. Fig. 6 shows a graph of the transmittance of light versus wavelength of light for the DBR of Fig. 5.

[0009] Fig. 1 shows a schematic illustration of an exemplary method of fabricating an exemplary multi-junction solar cell comprising sub-cells of organic and quantum dot photoactive layers.

[0010] As shown in Fig. 1.1, the organic 104 and quantum dot 112 photoactive layers are separately fabricated on different substrates, 108 and 116, respectively. The organic photoactive layer 104 is the top sub-cell and is fabricated on a transparent substrate, such as glass 108, and employs transparent contacts 102, 106. The quantum dot 112 photoactive layer is the bottom layer and is fabricated on a nontransparent substrate 116, and employs at least one transparent contact 110, but may also include at least one metal contact 114. In some embodiments the transparent contacts may be ITO, IZO, AZO or PEDOT:PSS. In some embodiments, sub-cells may be fabricated on an individual substrate via vacuum thermal evaporation or solution process.

[0011] After individual sub-cell fabrication, an optical filter layer 202, 302 is patterned on the top sub-cell, as shown in Fig. 1.2. In some embodiments the optical filter layer is patterned using a shadow mask, though other patterning methods may be used. Then, a grid 320 is patterned on both the top and bottom sub-cells, as shown in Fig. 1.3. In the present invention, the grid 320 employs a noble metal or metal coated with noble metal, such as Au or A1 / Au, and thereby avoids oxidation and other contamination.

[0012] Following grid deposition, both sub-cells are bonded via cold-welding by applying pressure along with or without a slight increase in temperature to encourage bonding. For reference, cold-weld bonding is described by Lee et al. in "Multiple growths of epitaxial lift-off solar cells from a single InP substrate" (Kyusang Lee, Kuen-Ting Shiu, Jeramy D. Zimmerman, Christopher K. Renshaw and Stephen R. Forrest, Multiple growths of epitaxial lift-off solar cells from a single InP substrate, 97 Appl. Phys. Lett., 101107 (2010)). For reference, cold-weld bonding is further described by Kim et al. in "Micropatterning of organic electronic devices by cold-welding" >Changsoon Kim, Paul E. Burrows, Stephen R. Forrest, Micropatterning of organic electronic devices by cold-welding. 288, Science, 831 (2000)). For reference, cold-weld bonding is further described by Ferguson et al. in "Contact adhesion of thin gold films on elastomeric supports: cold welding under ambient conditions" (Gregory S. Ferguson, Manoj K. Chaudhury, George. B. Sigal George M. Whitesides, Contact adhesion of thin gold films on elastomeric supports: cold welding under ambient conditions, 253 Science, 776 (1991)). For a cold-welding process, the surfaces of the epi-layer and the host substrate are generally pre-coated with layers of a similar noble metal (Au, Ni etc.), then appropriate pressure is applied between two metal interfaces. In some embodiments, metal contacts can be inserted in the regions that are cold-weld bonded to separately contact individual stacked elements. In various embodiments, the sub-cells do not require tunnel or interconnecting layers because sub-cells are connected by metal contacts in series, thereby considerably simplifying device fabrication.

[0013] Following cold weld bonding, the two sub-cells are joined to form a multi-junction solar cell, as shown in Fig 1.4. The incident light can pass through the glass layer 416 and select wavelengths of light are absorbed by the organic photovoltaic layer 412. Select wavelengths of light that are not absorbed by the organic photoactive layer 412 will reach the DBR, and said wavelengths will be transmitted or reflected depending on the configuration of the DBR. As shown in Fig 1.4, the photoactive layer beyond the DBR is a quantum dot photoactive layer 406. In one embodiment, one photoactive layer is organic photoactive layer absorbs light in the visible and ultraviolet spectrums and the other photoactive layer is a quantum dot photoactive layer that absorbs light in the near-infrared spectrum. In such an embodiment, the DBR may be configured to reflect light in the visible spectrum and transmit light in the near infrared spectrum. Light that passes through the organic photoactive layer, the DBR and the quantum dot photoactive layer may be reflected or absorbed by the metal contact layer 404. In some embodiments the metal contact layer may serve as both an electrical contact and as a mirror on the back of the multi-junction solar cell.

[0014] In a multi-junction series-connected solar cell, the currents delivered by each sub-cell should be equal to maximize the solar cell performance. Alternatively, in a parallel cell (achieved using intermediate contacts) the voltage should be equalized between cells to maximize performance. Some embodiments may be comprised of a dual near infrared cell stack connected in parallel with a single shorter wavelength device, when parallel connection is desired in the design. In some embodiments wherein the device is a photodetector that is sensitive to multiple wavelength signal channels, there is no requirement for either balance of voltage or current.

[0015] The exemplary embodiment in Fig. 1 represents a double stacked device. Other embodiments may include additional photoactive elements, such as doped silicon or titanium dioxide, that may absorb different wavelengths of the solar spectrum or other wavelengths of interest. Optical filters, such as 202 and 302, may or may not be placed between each additional photoactive layer in the stack. Further, air gaps between layers may or may not be present. In some embodiments this device may be a photovoltaic device. In other embodiments, this device may be configured to be a photodetector that may de-multiplex narrow spectral signals emitted from a signal source.

[0016] The present invention relates to a multi-junction photosensitive device. In some embodiments, the at least one optical filter layer in said device is a DBR. In some embodiments, the multi-junction device may comprise at least three photoactive layers, and at least two optical filter layers, wherein at least one optical filter layer is located between each photoactive layer.

[0017] In some embodiments, at least one photoactive layer may absorb wavelengths of light different than at least one other photoactive layer. In some embodiments, a photoactive layer below at least one other layer absorbs light in the near infrared. The near infrared spectrum includes wavelengths in the range of 0.75 to 1.4 microns. In some embodiments where the device is configured to be a photodetector, the wavelengths may include wavelengths up to 30 microns. In some embodiments, a photoactive layer, above at least one other layer, absorbs light in the ultraviolet spectrum. The ultraviolet spectrum includes wavelengths shorter than 400 nanometers. In some embodiments at least one photoactive layer absorbs a range of wavelengths of light spanning 10 nm different than at least one other photoactive layer.

[0018] In some embodiments, wavelength selective optical filter layers are placed between at least two photoactive layers of the multi-junction photosensitive device. In some embodiments an air gap between the photoactive layers may or may not be present.

[0019] In some embodiments, at least one photoactive layer is individually connected to a metal contact in regions that are cold weld or van der Waals bonded. In some embodiments, individual cells are connected in series. In some embodiments, individual cells are connected in parallel.

[0020] In some embodiments, at least one group of individual cells is connected in series and at least one group is connected in parallel. In some embodiments, at least one photoactive layer comprises materials chosen from organic photovoltaic, quantum dots, silicon, germanium, III-V semiconductors, kesterite, and perovskite. In the present invention, one photoactive layer comprises PbS quantum dots, and one photoactive layer comprises an organic photovoltaic layer.

[0021] In an embodiment, the device of the present invention is a photodetector configured to de-multiplex spectral signals emitted from a signal source.

[0022] The present invention is also directed to a method of fabricating a multi-junction photosensitive device. The method comprises the steps of fabricating a first subcell on a transparent substrate, wherein the first subcell comprises a first photoactive layer; fabricating a second subcell on a reflective substrate, wherein the second subcell comprises a second photoactive layer; patterning at least one optical filter layer on the first subcell; patterning a grid employing a noble metal or metal coated with a noble metal on each of the first and second subcells, located between and interconnecting the first and second subcells; bonding the first subcell to the second subcell using cold weld bonding; and, wherein the second photoactive layer comprises PbS quantum dots and the first photoactive layer comprises an organic photovoltaic layer. In some embodiments, at least one optical filter layer is a DBR. In some embodiments, the method further comprises fabricating at least one additional photoactive layer on a transparent substrate, and the transparent substrate may include a photoactive layer.

[0023] Fig. 2 shows a schematic illustration of the photon absorption process in an exemplary organic / quantum dot multi-junction solar cell. The exemplary mulitjunction solar cell is comprises a visible photovoltaic (VIS PV) which may be an organic photovoltaic layer (OPV), a DBR between two transparent contacts (ITO), a near infrared photovoltaic (NIR PV) which may be a quantum dot layer (QD cells), and a mirror on the back of the solar cell to reflect NIR light that is transmitted by the DBR and not initially absorbed by the NIR PV. The organic front sub-cell confines visible photons to the top sub-cell via a DBR, whereas only the near infrared light is transmitted into the bottom cell. The quantum dot bottom sub-cell has a metal contact reflector for near infrared light reflection. Using the intermediate optical filter, in this case a DBR, the solar spectrum can be selectively separated into sub-cells. A DBR is generally composed of alternating materials with at least two different refractive indices. Alternatively, a DBR may comprise a single material with a periodic variation of a characteristic (e.g. height) that results in variations of the effective refractive index. Reflection and transmission characteristics of the intermediate optical filter can be controlled by properly designing the DBR and / or the air gap thickness. Control over the reflection of the balance of photocurrent can be achieved between sub-cells to maximize the multi-junction solar cell performance. Also, due to the fact that the DBR will reflect select wavelengths of light back up through the organic layer, various embodiments may employ a thinner photoactive layer than necessary otherwise, because the optical path length of the organic layer will be lengthened as the light passes through the photoactive layer again after reflection.

[0024] A solar cell fabricated according to the layers shown in Fig. 3A demonstrates one envisioned benefit of an optical filter layer. The solar cell includes organic photoactive layers comprising DTDCPB:C 70 layers and an interconnecting layer between the organic photoactive layers. The photoactive layers are situated between transparent contact layers (ITO), and the DBR is situated on the rear, relative to incident light, of the solar cell. The DBR of Fig 3A comprises eleven alternating layers of Zinc Selenide and Magnesium Fluoride, configured such that the DBR has an onset of transmission of for light with a wavelength greater than 700 nm. The DBR of Fig 3B is abbreviated as DBR @ 700nm, ZnSe / MgF 2 and is described in more detail in Fig. 5. Fig 3B shows a graph of the measured photocurrent versus voltage for a solar cell fabricated according to the layers in Fig 3A, compared to a solar cell fabricated according to the same layers except for the DBR layer, as in Fig 3A. The results of the solar cell performance are summarized in Fig. 4 illustrate the increased power conversion efficiency of a solar cell when utilizing a DBR. Fig. 5 shows a schematic of the exemplary DBR shown in Fig 3A, and comprises a total of eleven layers of ZnSe and MgF 2 . The transmission of the solar cell of Fig 3A and the DBR alone is shown in the graph of Fig. 6, and illustrates an onset of transmission of light for wavelengths longer than 700 nm, while the wavelengths of light shorter than 700 nm are at least partially reflected.

[0025] Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the present disclosure herein. It is intended that the specification and examples be considered as exemplary only, with the scope of the present disclosure being indicated by the following claims.

Claims

1. A multi-junction photosensitive device comprising: a first subcell disposed on a first substrate (108, 416) and comprising a first photoactive layer (104, 412), wherein said first substrate is a transparent substrate; a second subcell disposed on a second substrate (116, 402) and comprising a second photoactive layer (112, 406), layer, wherein said second substrate is a reflective substrate; wherein the first and second subcells are located between the first substrate and the second substrate in a stacked configuration; a grid (320, 418) employing a noble metal or metal coated with a noble metal located between and interconnecting the first and second subcells, with the first and second subcells being bonded via cold weld bonding; at least one optical filter layer (302) located on the first subcell, between the first and second subcells; and wherein the second photoactive layer comprises PbS quantum dots and the first photoactive layer comprises an organic photovoltaic layer.

2. The device of claim 1, wherein the at least one optical filter layer is a Distributed Bragg Reflector.

3. The device of claims 1 or 2, wherein the first and second photoactive layers absorb different wavelengths of light.

4. The device of any of claims 1 to 3, wherein one of the photoactive layers absorbs a range of wavelengths of light spanning 10 nm that is not absorbed by the other photoactive layer.

5. The device of any of claim 1-4, wherein there is no air gap between the first and second subcells.

6. The device of any of claims 1-5, wherein the first and second subcells are connected in series.

7. The device of claims 1-5, wherein the first and second subcells are connected in parallel.

8. The device of claim 1, wherein the device is a photodetector configured to demultiplex spectral signals emitted from a signal source.

9. A method of fabricating a multi-junction photosensitive device, comprising the steps of: fabricating a first subcell on a transparent substrate (108, 416), wherein the first subcell comprises a first photoactive layer (104, 412); fabricating a second subcell on a reflective substrate (116, 402), wherein the second subcell comprises a second photoactive layer (112, 406); patterning at least one optical filter layer (202) on the first subcell; patterning a grid (320, 418) employing a noble metal or metal coated with a noble metal or each of the first and second subcells, located between and interconnecting the first and second subcells; and bonding the first subcell to the second subcell using cold weld bonding; wherein the second photoactive layer comprises PbS quantum dots and the first photoactive layer comprises an organic photovoltaic layer.

10. The method of claim 9, wherein the at least one optical filter layer is a Distributed Bragg Reflector.