Solar cell comprising an oxide-nanoparticle buffer layer and method of fabrication

Abstract

A buffer layer for protecting an organic layer during high-energy deposition of an electrically conductive layer is disclosed. Buffer layers in accordance with the present invention are particularly well suited for use in perovskite-based single-junction solar cells and double-junction solar cell structures that include at least one perovskite-based absorbing layer. In some embodiments, the buffer layer comprises a layer of oxide-based nanoparticles that is formed using solution-state processing, in which a solution comprising the nanoparticles and a volatile solvent is spin coated onto a structure that includes the organic layer. The solvent is subsequently removed in a low-temperature process that does not degrade the organic layer.

Claims

1. A method for forming an optoelectronic device, the method comprising: providing a first organic layer; forming a first buffer layer comprising a first plurality of oxide nanoparticles, wherein the first buffer layer is configured to protect the first organic layer during a sputter-deposition process; and forming a first transparent conductive electrode such that it is disposed on the first buffer layer, wherein the first transparent conductive electrode includes a first layer that is formed via sputter deposition.

2. The method of claim 1 wherein the first organic layer comprises a first perovskite.

3. The method of claim 1 wherein the first buffer layer is formed by operations including: providing a first solution comprising a solvent and the first plurality of nanoparticles, wherein the nanoparticles of the first plurality thereof comprise a wide bandgap oxide; distributing the first solution to form a nascent first buffer layer; and enabling the removal of the solvent from the first solution, wherein the solvent is removed while the temperature of the optoelectronic device remains less than or equal to 200° C.

4. The method of claim 1 wherein the first layer includes indium tin oxide (ITO).

5. The method of claim 1 further comprising forming the first organic layer on a first layer stack comprising a first absorption layer having a first energy bandgap (EG); wherein the first organic layer is formed such that is comprises a first perovskite having a second EG that is higher than the first EG; and wherein the first layer stack includes a second transparent conductive electrode that is between the first organic layer and the first absorption layer.

6. The method of claim 5 further comprising: forming a second organic layer on a second layer stack comprising the first organic layer and the first absorption layer, wherein the second organic layer comprises a second perovskite having a third EG that is higher than the second EG; forming a second buffer layer comprising a second plurality of oxide nanoparticles, the second organic layer being between the second layer stack and the second buffer layer; and forming a third transparent conducting electrode on the second buffer layer, the third transparent electrode being formed via a sputtering process.

7. The method of claim 6 wherein the first absorption layer comprises a third perovskite.

8. The method of claim 1 wherein the first plurality of oxide nanoparticles includes at least one nanoparticle comprising a material selected from the group consisting of zinc oxide (ZnO), tin oxide (SnO.sub.2), titanium oxide (TiO.sub.2), tungsten oxide (W.sub.2O.sub.3), aluminum oxide (Al.sub.2O.sub.3), silicon dioxide (SiO.sub.2), nickel oxide (NiO), and molybdenum oxide (MoO.sub.x).

9. The method of claim 8 wherein the oxide nanoparticles of the first plurality thereof are doped with a first dopant that is selected from the group consisting of aluminum, hydrogen, indium, and gallium.

10. The method of claim 1 wherein the optoelectronic device is formed such that it is a device selected from the group consisting of a solar cell, a light emitting diode, and an electrochromic.

11. A method for forming an optoelectronic device, the method comprising: providing a first organic layer disposed on a substrate, wherein the first organic layer comprises a first perovskite; forming a first buffer layer configured to protect the first organic layer during a sputter-deposition process, the first buffer layer comprising a first plurality of nanoparticles that includes at least one nanoparticle that comprises a wide-bandgap oxide; and sputter depositing a first layer such that it is disposed on the first buffer layer and first organic layer, wherein the first layer comprises a first material that is a transparent conductor, and wherein the first layer defines at least a portion of a first transparent conductive electrode.

12. The method of claim 11 wherein the first buffer layer is formed by operations including: providing a first solution comprising a solvent and the first plurality of nanoparticles, wherein the nanoparticles of the first plurality thereof comprise a wide bandgap oxide; distributing the first solution to form a nascent first buffer layer; and enabling the removal of the solvent from the first solution, wherein the solvent is removed while the temperature of the optoelectronic device remains less than or equal to 200° C.

13. The method of claim 11 wherein the first material includes indium tin oxide (ITO).

14. The method of claim 11 further comprising: forming the first organic layer on a first layer stack comprising a first absorption layer having a first energy bandgap (EG), wherein the first layer stack includes a second transparent conductive electrode that is between the first organic layer and the first absorption layer; wherein the first perovskite has a second EG that is higher than the first EG; and wherein the first absorption layer includes a second material that is selected from the group consisting of silicon, copper indium gallium selenide (CIGS), and a perovskite.

15. The method of claim 14 further comprising: forming a second organic layer such that it is disposed on a second layer stack comprising the first organic layer and the first layer stack, wherein the second organic layer comprises a second perovskite having a third EG that is higher than the second EG; forming a second buffer layer such that it is disposed on the second organic layer, the second buffer layer comprising a second plurality of nanoparticles that includes at least one nanoparticle comprising a wide-bandgap oxide; and sputter depositing a third layer such that the third layer is disposed on the second organic layer, wherein the third layer comprises a third material that is a transparent conductor, and wherein the third layer defines at least a portion of a third transparent conductive electrode.

16. The method of claim 14 wherein the second material comprises a third perovskite.

17. The method of claim 11 wherein the at least one nanoparticle includes a material selected from the group consisting of zinc oxide (ZnO), tin oxide (SnO.sub.2), titanium oxide (TiO.sub.2), tungsten oxide (W.sub.2O.sub.3), aluminum oxide (Al.sub.2O.sub.3), silicon dioxide (SiO.sub.2), nickel oxide (NiO), and molybdenum oxide (MoO.sub.x).

18. The method of claim 11 wherein the at least one nanoparticle is doped with a first dopant that is selected from the group consisting of aluminum, hydrogen, indium, and gallium.

19. The method of claim 11 wherein the optoelectronic device is formed such that it is a device selected from the group consisting of a solar cell, a light emitting diode, and an electrochromic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a single-junction, perovskite-based solar cell in accordance with an illustrative embodiment of the present invention.

(2) FIG. 2 depicts operations of a method suitable for forming solar cell 100.

(3) FIG. 3A depicts plots of current density as a function of applied voltage for two solar cells having structures analogous to that of solar cell 100.

(4) FIG. 3B depicts plots of efficiency over time for solar cells 302 and 304.

(5) FIG. 4 depicts a schematic drawing of a mechanically stacked tandem solar cell in accordance with the present invention.

(6) FIG. 5A depicts plots of the external quantum efficiency (EQE) for tandem solar cell 400.

(7) FIG. 5B depicts J-V curves for solar cell 100 and filtered and unfiltered solar cell 402.

(8) FIG. 6 depicts a schematic drawing of a cross-sectional view of the salient features of another tandem solar cell in accordance with the present invention.

(9) FIG. 7 depicts a schematic drawing of a cross-sectional view of the salient features of another tandem solar cell in accordance with the present invention.

(10) FIG. 8 depicts a schematic drawing of a cross-sectional view of an example of a three-junction solar cell device in accordance with the present invention.

DETAILED DESCRIPTION

(11) FIG. 1 depicts a single-junction, perovskite-based solar cell in accordance with an illustrative embodiment of the present invention. Solar cell 100 includes substrate 102, bottom contact 104, hole-selective layer 106, absorption layer 108, electron-selective layer 110 (also referred to as electron-acceptor layer 110), buffer layer 112, top contact 114, and antireflection coating 116, arranged as shown. Solar cell 100 generates output voltage, V1, when illuminated with light signal 118. Although the illustrative embodiment is a single-junction solar cell that incorporates a buffer layer in accordance with the present invention, it will be clear to one skilled in the art, after reading this Specification, that the present invention is more broadly applicable to other solar-cell architectures (tandem solar-cell structures, etc.), as well as for the fabrication of other structures, such as organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and electrochromics, among others optoelectronic devices.

(12) FIG. 2 depicts operations of a method suitable for forming solar cell 100. Method 200 begins with operation 201, wherein bottom contact 104 is formed such that it is disposed on conventional glass substrate 102. For the purposes of this Specification, the term “disposed on” (or “formed on”) is defined as “exists on” an underlying material or layer. This layer may comprise intermediate layers, such as transitional layers, necessary to ensure a suitable surface. For example, if a material is described to be “disposed (or grown) on a substrate,” this can mean that either (1) the material is in intimate contact with the substrate; or (2) the material is in contact with one or more transitional layers that reside on the substrate.

(13) Bottom contact 104 is a layer of ITO having a thickness suitable for providing low electrical sheet resistance. A typical value for the thickness of bottom contact 104 is approximately 170 nanometers (nm); however, one skilled in the art will recognize that any practical thickness can be used.

(14) At operation 202, hole-selective layer 106 is formed on bottom contact 104 to define a hole-selective contact for solar cell 100. Hole-selective layer 106 is a substantially smooth, hydrophilic layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which is formed by spin coating aqueous PEDOT:PSS onto bottom contact 104 and curing it. As will be appreciated by those skilled in the art, a variety of other materials can be used to form this layer, including, without limitation, small molecules such as spiro-OMeTAD, other polymers, such as poly triarylamine (PTAA), and inorganic materials, such as NiO.

(15) At operation 203, absorption layer 108 is formed on hole-selective layer 104. In the depicted example, absorption layer 108 comprises a CH.sub.3NH.sub.3PbI.sub.3 perovskite that is formed by dissolving lead(ii) acetate (PbAc) and methylammonium iodide (MAI) (1:3 molar ratio) in n,n-dimethylformamide (DMF) and spinning the resultant solution onto a substrate. The layer is initially dried at room temperature and then annealed at 100° C. for 5 minutes. One skilled in the art will recognize, after reading this Specification, that myriad perovskite layers can be used in absorption layer 108 without departing from the scope of the present invention. Other perovskites suitable for use in the present invention include, without limitation, Cs.sub.xFA.sub.1-xPb(Br.sub.yI.sub.1-y).sub.3, pure formamidinium based perovskite (FAPbI.sub.3), tin containing perovskites Cs.sub.xFA.sub.1-xPb.sub.zSn.sub.1-z(Br.sub.yI.sub.1-y).sub.3 etc. Further, one skilled in the art will recognize that many alternative methods can be used to form a layer of perovskite without departing from the scope of present invention.

(16) Absorption layer 108 and hole-selective layer 104 collectively define a p-type heterojunction within solar cell 100.

(17) At operation 204, electron-acceptor layer 110 is formed by spin-coating liquid-phase organic material onto the perovskite of absorption layer 108 and curing it via a low-temperature anneal. In the depicted example, electron-acceptor layer 110 is a layer of [60]PCBM having a thickness of approximately 20 nm. It is preferable that the layer of PCBM be thin to ensure good electron-extraction properties, while still achieving high optical transmission. Electron-acceptor layer 110 and absorption layer 108 collectively define an n-type heterojunction in solar cell 100. In some embodiments, a layer of lithium fluoride (LiF) is included as a passivation layer between electron-acceptor layer 110 and absorption layer 108. Electron-acceptor layer 110 is formed. In some embodiments, PCBM also prevents the development of an extraction barrier. In some other embodiments, the organic material can be an electrically-active material, such as [70]PCBM, spiro-OMeTAD or a polymer organic, such as PTAA.

(18) In some embodiments, electron-acceptor layer 110 includes a thin layer of bathocuproine (BCP), which is evaporated on the PCBM layer to improve hole blocking and electron extraction.

(19) For convenience, the layers of solar cell 100 between bottom contact 104 and buffer layer 112 (i.e., hole-selective layer 106, absorption layer 108, and electron-selective layer 110, as well as any additional associated layers) are referred to herein, collectively, as perovskite cell 122.

(20) At operation 205, buffer layer 112 is formed on electron acceptor layer 110. Buffer layer 112 comprises a layer of oxide nanoparticles 120, which is formed by dispersing a solution comprising the nanoparticles and isopropyl alcohol (IPA), spin coating the mixture onto layer 110 to form a nascent buffer layer, and enabling the removal of the solvent from the solution (e.g., via evaporation at room temperature or at a slightly elevated temperature in an oven or on a hotplate).

(21) Each of nanoparticles 120 comprises a wide-bandgap oxide, wherein “wide bandgap” is defined as an effective energy bandgap greater than 2.0 eV. In the depicted example, nanoparticles 120 comprise zinc oxide that is doped with approximately 2 mol % aluminum (i.e., AZO). Zinc oxide is a particularly attractive host material of nanoparticles 120 due to its deep valence level of approximately −7.6 eV. In addition, the use of aluminum-doped zinc oxide (AZO) nanoparticles reduces or eliminates the development of an extraction barrier that can arise from a misalignment of the work functions of ZnO and the ITO of top contact 114, thereby achieving a more ohmic top contact. However, one skilled in the art will recognize, after reading this Specification, that many alternative materials can be used in nanoparticles 120 without departing from the scope of the present invention. Alternative materials suitable for use in nanoparticles 120 include, without limitation, tin oxide (SnO.sub.2), titanium oxide (TiO.sub.2), tungsten oxide (W.sub.2O.sub.3), aluminum oxide (Al.sub.2O.sub.3), silicon dioxide (SiO.sub.2), nickel oxide (NiO), molybdenum oxide (MoO.sub.x), etc. Further, those skilled in the art will understand that the nanoparticles can be doped with many materials other than aluminum; including, without limitation, hydrogen, indium, gallium, and the like. A characteristic of a suitable dopant is that it can be alloyed into the host material (e.g., zinc oxide, etc.) and improve the conductivity of the host material. In some other embodiments, wide-bandgap oxide materials are undoped.

(22) It is an aspect of the present invention that forming buffer layer 112 using solution-based processing avoids some of the complications that arise in the prior art. For example, it is well known that bonds within a perovskite can be broken when material is deposited on the perovskite material using a high-energy process, such as sputtering. Since solution-based processing low-energy, its use enables buffer layer 112 to be formed on the perovskite material of absorption layer 110 with little or no damage.

(23) At operation 206, top contact 114 is formed on buffer layer 112 such that it is substantially transparent for light signal 118. For the purposes of this Specification, including the appended claims, “transparent” is defined as having a transmittance equal to or greater than 60%, where transmittance is typically measured at a wavelength of 550 nm, corresponding to the maximum of the human eye luminosity curve. To form top contact 114, a layer of ITO having a thickness within the range of approximately 60 nm to approximately 500 nm (preferably approximately 100 nm) is sputtered onto buffer layer 112 and then annealed at a temperature less than or equal to 200° C. It is another aspect of the present invention that the presence and robustness of the layers of buffer layer 112 enable the sputter deposition of the ITO as a transparent electrode with little or no damage to underlying electron-selective layer 110 or the perovskite material of absorption layer 108.

(24) In some embodiments, another material suitable for forming a substantially optically transparent, electrically conductive layer (e.g., AZO, IZO, HZO, etc.) is sputtered onto buffer layer 112.

(25) At optional operation 207, antireflection coating 116 is formed on top contact 114 via conventional evaporation. In the depicted example, antireflection coating 116 is a layer of magnesium fluoride (MgF.sub.2) having a thickness of approximately 150 nm; however, any suitable antireflection coating can be used in embodiments of the present invention. In some embodiments, an additional antireflection coating is formed on the bottom surface of substrate 102.

(26) Fully fabricated solar cells analogous to solar cell 100 were found to have an open circuit voltage (Voc) greater than 0.9 V, which evinces the efficacy of buffer layer 112 for protecting the integrity of the organic layers of the solar cell structure (i.e., absorption layer 108 and electron-selective layer 110).

(27) It should be noted that the materials and layer thicknesses provided herein are merely exemplary and that alternative materials and/or layer thicknesses can be used without departing from the scope of the present invention.

(28) FIG. 3A depicts plots of current density as a function of applied voltage for two solar cells having structures analogous to that of solar cell 100. Plot 300 shows measured current density vs. applied voltage for solar cells 302 and 304. Solar cell 302 includes a semi-transparent bottom contact 104 made of a substantially transparent layer of ITO. Solar cell 304 includes an opaque bottom contact 104 that includes layers of aluminum and silver. Measurement data was taken while illuminating solar cells 302 and 304 through aperture masks having apertures of 0.39 cm.sup.2 and 0.12 cm.sup.2, respectively.

(29) FIG. 3B depicts plots of efficiency over time for solar cells 302 and 304.

(30) The results shown in plots 300 and 306 demonstrate the efficacy of buffer layer 112 as a support layer for the formation of an operative top contact using conventional sputtering without damage to the underlying organic layers. First, the fill factor and voltage for solar cells 302 and 304 are substantially comparable, demonstrating that a top contact 114 disposed on an oxide-nanoparticle-based buffer layer (i.e., buffer layer 112) operates in normal fashion. Second, the fill factor and voltage for solar cells 302 and 304 are comparable. It should be noted that the lower current density measured for solar cell 302 can be attributed to the fact that its bottom contact does not function as a back reflector. Third, plot 306 shows that the inclusion of buffer layer 112 enables operation of the solar cells at room temperature with a stabilized power efficiency of 12.3% and 13.5% for solar cells 302 and 304, respectively.

(31) Table 1 below summarizes measurement data obtained for solar cells 302 and 304.

(32) TABLE-US-00001 TABLE 1 Photovoltaic parameters of single-junction perovskite-based solar cells having semi-transparent and opaque bottom contacts. J.sub.SC [mA/cm.sup.2] VOC [mV] FF Efficiency [%] Solar Cell 302 16.5 952 0.77 12.3 Solar Cell 304 18.8 938 0.77 13.5

(33) As discussed above, in some embodiments, the host material of nanoparticles 120 is doped to facilitate ohmic-contact operation of top contact 114. By employing doped oxide-based nanoparticles in buffer layer 112 (e.g., AZO), the work functions of the host material (e.g., ZnO) and the material of top contact 114 (e.g., ITO) can be more closely aligned. This mitigates the development of an extraction barrier that can impair operation of the solar cell.

(34) It is an aspect of the present invention that the ability to form a high-quality transparent contact on a structure comprising organic materials enables device structures that were difficult, if not impossible, to produce in the prior art. Examples of such structures include, without limitation, mechanically stacked tandem solar cells including a perovskite-based top cell, monolithically integrated tandem solar cells including a perovskite-based top cell, monolithically integrated tandem solar cells including perovskite-based top and bottom cells, and the like.

(35) FIG. 4 depicts a schematic drawing of a mechanically stacked tandem solar cell in accordance with the present invention. Solar cell 400 includes perovskite solar cell 100, silicon solar cell 402, and adhesive layer 404, which mechanically affixes the two solar cells in hybrid fashion.

(36) Silicon solar cell 402 is a conventional silicon-based solar cell comprising bottom contact 406, silicon cell 408, and top contact 410. One skilled in the art will recognize that, like solar cell 100, a conventional silicon-based solar cell normally includes additional layers, such as p-type and n-type amorphous silicon heterojunction layers, etc. In similar fashion to perovskite cell 122 described above, for convenience, the layers of silicon solar cell 402 between bottom contact 406 and top contact 410 (i.e., absorption layer, heterojunction and carrier-selective layers, etc.) are referred to herein, collectively, as silicon cell 408.

(37) Solar cell 602 is fabricated in substantially conventional fashion, beginning with the texturing of the top surface of a silicon substrate, which functions as the absorption layer of silicon cell 408.

(38) For example, silicon cell 408 begins as a silicon substrate that is an N-type, 280-μm-thick, double-side polished float-zone (FZ) wafer. The top surface of the substrate is textured by first depositing a 250-nm-thick, low-refractive-index silicon nitride layer by plasma-enhanced chemical-vapor deposition (PECVD) on its bottom surface and exposing the unprotected top surface to potassium hydroxide (KOH).

(39) After removal of the nitride layer from the bottom surface, intrinsic and n-type amorphous silicon (a-Si:H) layers (not shown) are deposited on the top surface of the substrate.

(40) In similar fashion, intrinsic and p-type a-Si:H films layers (not shown) are deposited on bottom top surface of the substrate to complete silicon cell 408.

(41) Bottom contact 406 is then formed by depositing a layer of silver (or other suitable electrically conductive material) having an appropriate thickness.

(42) Top contact 410 is formed by depositing, in conventional fashion, a layer of a transparent electrically conductive material. In the depicted example, top contact 410 comprises a layer of ITO having a thickness of approximately 500 nm.

(43) Once silicon solar cell 402 is complete, it is joined with solar cell 100 in conventional fashion. In the depicted example, the solar cells are affixed via adhesive layer 404, which is a layer of electrically insulating epoxy that is substantially transparent for light signal 118. In some embodiments, solar cells 100 and 402 are mechanically joined via another conventional method.

(44) FIG. 5A depicts plots of the external quantum efficiency (EQE) for tandem solar cell 400. Plot 500 shows measured EQE of individual solar cells 100 and 402 in response to light signal 118. Traces 502 and 504 show the EQE of the individual silicon and perovskite solar cells, respectively, under direct illumination by the light signal such that the light incident on the solar cells spans the full range of 300 nm to 1200 nm. Trace 506 shows EQE for individual solar cell 402 when illuminated by light signal 118 after it has passed through perovskite-based solar cell 100. Solar cell 402 is referred to as “unfiltered” when directly illuminated with light signal 118 and “filtered” when it is illuminated by the light signal after it has passed through solar cell 100.

(45) FIG. 5B depicts J-V curves for solar cell 100 and filtered and unfiltered solar cell 402.

(46) Table 2 below summarizes measurement data obtained for solar cells 100 and 402.

(47) TABLE-US-00002 TABLE 2 Photovoltaic parameters of individual solar cells 100 and 402, as well as stacked tandem configuration 400. J.sub.SC [mA/cm.sup.2] VOC [mV] FF Efficiency [%] Solar Cell 100 16.5 952 0.774 12.3 (individually) Solar Cell 402 38.3 587 0.754 17.0 (unfiltered) Solar Cell 402 13.3 562 0.762 5.7 (filtered) Tandem solar 18.0 cell 400

(48) The data shows that the efficiency of unfiltered solar cell 402 is 17.0%. It should be noted that the Voc of solar cell 402 is limited due to excess shaded area from the aperture mask used during its illumination, as well as the absorption of the light signal in top contact 114. In the mechanically stacked configuration, in which light signal 118 passes through solar cell 100 prior to impinging on solar cell 402, the combined efficiency of the two solar cells is 18.0% with a JSC of 13.3 mA/cm.sup.2 from the bottom cell (i.e., solar cell 402).

(49) It is an additional aspect of the present invention that a nanoparticle-based buffer layer, such as buffer layer 112, enables a sputtered ITO contact layer that acts as a barrier for moisture ingress, methylammonium egress, while the buffer layer itself protects the contact layer from halide-based corrosion.

(50) It is yet another aspect of the present invention that the inclusion of a buffer layer comprising oxide-based nanoparticles enables monolithically integrated tandem solar cells comprising perovskite-based top cells combined with bottom cells that can include any of a wide range of materials, such as silicon, CIGS, a lower EG perovskite, and the like.

(51) FIG. 6 depicts a schematic drawing of a cross-sectional view of the salient features of another tandem solar cell in accordance with the present invention. Solar cell 600 includes solar cell 100 and silicon solar cell 602, which collectively form a monolithically integrated tandem solar cell structure. For the purposes of this Specification, including the appended claims, the term “monolithically integrated” is defined as formed by depositing layers on a single substrate via one or more thin-film deposition processes and, optionally, patterning the deposited layers after deposition. The term monolithically integrated explicitly excludes structures wherein two or more fully fabricated devices are joined, after their fabrication on separate substrates, to form a unitary structure.

(52) Solar cell 602 is a silicon heterojunction solar cell that is analogous to solar cell 402 described above and with respect to FIG. 4; however, rather than conventional top contact 410, solar cell 602 includes center contact 604, which is dimensioned and arranged to support the monolithic integration of the layers of the solar cell 100.

(53) The fabrication of solar cell 602, up to the formation of center contact 604, is as described above and with respect to silicon solar cell 402.

(54) To form center contact 604, a thin (e.g., 20-nm thick) layer of ITO is deposited through a shadow mask to define multiple conductive regions (e.g., 1 cm by 1 cm). These ITO regions provide a recombination junction between the silicon and perovskite cells with minimal parasitic absorption. It should be noted that the ITO layer can be very thin because lateral conductivity is not of concern at this junction. It should also be noted that the shape of the ITO regions can be other than square.

(55) Once an appropriate center contact has been formed, the thin-film layer structure of solar cell 100 is formed on solar cell 602 in substantially the same manner as described above and with respect to FIGS. 1 and 2. It should be noted that, when formed in a monolithically integrated tandem configuration, substrate 102 is not included in solar cell 100.

(56) It should be noted, however, that the processes used to fabricate the constituent layers of solar cell 100 must be compatible with the solar structure on which they are formed. For example, in some embodiments, hole-selective layer 106 comprises a material other than PEDOT:PSS, such as NiO. An NiO layer, however, is typically annealed at 300° C. for several minutes after its deposition. Unfortunately, such an annealing step would lead to hydrogen loss in the doped amorphous silicon layers and/or crystallization of the amorphous silicon layers of silicon solar cell 602 and compromise their passivation properties. As a result, in accordance with the present invention, a hole-selective layer comprising NiO is annealed at a lower temperature for a longer period of time (e.g., at 200° C. for 10 hours) than would normally be used in the prior art.

(57) It is yet another aspect of the present invention that the formation of an effective buffer layer and top contact using low-temperature, solution-based processing enables monolithically integrated multi-junction solar cell structures (i.e., having two or more junctions) that are not possible in the prior art—specifically, multi-junction configurations that include perovskite absorption layers in every one of the stacked cells.

(58) FIG. 7 depicts a schematic drawing of a cross-sectional view of the salient features of another tandem solar cell in accordance with the present invention. Solar cell 700 includes solar cell 100-1 and solar cell 100-2, which collectively form a monolithically integrated, completely perovskite-based tandem solar cell structure.

(59) Each of solar cells 100-1 and 100-2 is analogous to solar cell 100, described above; however, perovskite cells 122-1 and 122-2 are formed such that the EG of perovskite cell 122-1 is lower than the EG of perovskite cell 122-2. In some embodiments, at least one of perovskite cells 122-1 and 122-2 has a regular (i.e., not inverted) architecture.

(60) In the depicted example, perovskite cell 122-1 comprises MAPbI.sub.3 perovskite having an EG of approximately 1.6 eV, while perovskite cell 122-2 comprises MAPbBr.sub.3 perovskite having an EG of approximately 2.3 eV.

(61) Center contact 702 is analogous to top contact 114; however, in some embodiments, center contact 702 includes a plurality of separate conductive regions as described above and with respect to center contact 604.

(62) Solar cell 700 is formed using sequential applications of method 200, described above. It should be noted that, since all operations of method 200 are performed at low temperature (i.e., 150° C.), there is no theoretical limit to the number of solar cell structures that can be included in a multi-junction solar cell. As a result, monolithically integrated stacked solar cell devices having three or more junctions can be made simply by employing method 200 the requisite number of times.

(63) Table 3 below summarizes a portion of the design space for a multi-junction solar cell, such as solar cell 700.

(64) TABLE-US-00003 TABLE 2 Theoretical performance for two-junction and three-junction solar cell structures. Si + 1.48 eV Si + 1.75 eV Si + 2.0 eV Perov. + Si, CIGS 1.48 eV Perov. 1.60 eV Perov. 2.0 eV Perov. Perov. Perov. 2.0 eV Perov. Dark Current (A) 4E−16 2.61E−21 4E−23 16.5 952 0.774 12.3 Photocurrent (A) 0.044 0.0298 0.0254 0.0147 0.0211 0.0147 0.0143 V.sub.OC (V) 0.83 1.12 1.23 1.569 2.18 2.69 3.52 SQ PCE (%) 32 30 28 27 41 36 45

(65) FIG. 8 depicts a schematic drawing of a cross-sectional view of an example of a three-junction solar cell device in accordance with the present invention. Solar cell 800 includes silicon solar cell 402 and solar cells 102-1 and 102-2, which are monolithically integrated on substrate 406.

(66) In order to fabricate solar cell 800, method 200 is performed twice. In the first performance of the method, a first perovskite absorption layer is formed such that it resides on layer stack 802 as part of perovskite cell 122-1. The first perovskite layer has an EG that is higher than the EG of silicon.

(67) Method 200 is then repeated such that a second perovskite layer is formed such that it resides on layer stack 804 as part of perovskite cell 122-2. The second perovskite layer has an EG that is higher than the EG of the first perovskite layer.

(68) As a result, each solar cell structure of solar cell 800 is operative for converting the energy of a different portion of the wavelength spectrum of light signal 118 into electrical energy.

(69) It is to be understood that the disclosure teaches only examples of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.