Patent ID: 12191415

DETAILED DESCRIPTION

The following detailed description, which references to and incorporates the drawings, describes and illustrates one or more examples of a two-terminal tandem photovoltaic cell and example methods of fabricating the same. These examples, offered not to limit but only to exemplify and teach embodiments of inventive multi-junction photovoltaic cells, are shown and described in sufficient detail to enable those skilled in the art to practice what is claimed. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. The disclosures herein are examples that should not be read to unduly limit the scope of any patent claims that may eventually be granted based on this application.

The word “exemplary” is used throughout this application to mean “serving as an example, instance, or illustration.” Any system, method, device, technique, feature or the like described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other features.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise.

Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention(s), specific examples of appropriate materials and methods are described herein.

Also, the use of “or” means “and/or” unless stated otherwise. Similarly, “comprise,” “comprises,” “comprising” “include,” “includes,” and “including” are interchangeable and not intended to be limiting.

It is to be further understood that where descriptions of various embodiments use the term “comprising,” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language “consisting essentially of” or “consisting of.”

By “about” is meant a quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length that may vary by as much as 30, 25, 20, 25, 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1% to a reference quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length. With respect to ranges of values, the claims may, some instances, encompass each intervening value between the upper and lower limits of the range to at least a tenth of the lower limit's unit, unless the context clearly indicates otherwise. Further, the claims may, in some instances, encompass any other intervening values.

FIG.1is a schematic illustration of an exemplary multi-junction photovoltaic cell10. The cell10is a two terminal (2-T) construction, with electrical terminals on a top subcell12and a bottom subcell for carrying current to a load16.

The top subcell12may include a light absorption layer having a relatively large bandgap, for example, a layer of that includes, consists of, or consists essentially of perovskite material. Other suitable materials may be used for the top light absorption layer, for example, other organic materials, such as those used in solar cell applications, polymer dots, a polyacetylene polymer, hydrogenated amorphous silicon, carbon balls, any combination of the foregoing, or the like. The relatively large bandgap of the top light absorption layer may be in the range of about 1.5 eV-3 eV.

The bottom subcell14may include a light absorption layer having a relatively small bandgap, for example, a crystalline silicon (c-Si) material, or Gallium arsenide, cadmium telluride, bismuth vanadate, or other semiconducting light absorber with a suitable band gap to act as a long wavelength light absorber in a tandem structure, which may be used as a substrate for the cell10. Other suitable materials may be used for the bottom subcell14. The relatively small bandgap of the bottom light absorption layer may be in the range of about 0.5 eV-1.5 eV.

A wide bandgap oxide conductor layer, such as an oxide layer13that includes, consists of, or consists essentially of oxygen and one or more elements, e.g., titanium (e.g., a titania (TiO2) layer) is included in the top subcell12. The oxide conductor layer13may also include other large bandgap oxides that facilitate hole tunneling by virtue of defect band conduction, including, for example, SnO2, SrTiO3, any suitable combination these metal oxides (including TiO2), and the like. The wide or large bandgap of the oxide conductor may be in the range of about 3 eV-5 eV. The properties of the oxide may include that it has an absorption of about 20% or less of light having energies between 0.5 eV and 3.0 eV, and it may have a resistance between the absorption layers of the tandem cell of less than 1000 ohms (or conductance of the oxide film greater than 0.001 mhos), e.g., producing a voltage loss of less than 1 V in certain embodiments of the cell, and less than 0.1 V in some embodiments. For example, in some embodiments of the cell, the oxide conductor film between the absorption layers may have a conductivity of greater than about 0.01 mho/cm2and in some instances, greater than about 0.1 mho/cm2.

Included in the bottom subcell14is an emitter region15of the Si substrate that contacts the titania layer13of the top subcell12. The conductive oxide layer13, e.g., a TiO2layer, and emitter region15may be configured to provide a highly ohmic conduction path between the subcells12,14. In certain embodiments, this may be accomplished by fabricating the cell10such that a relatively conductive TiO2/p+-Si emitter interface is achieved between the titania layer13and the emitter region15. Thus, with this structure, the top and bottom subcells12,14are in direct contact with each other and lack a conventional interconnect layer, as found in known multi-junction cells. The multi-junction cell10may include any of the materials, structures, and additional layers disclosed herein and may be manufactured using any of the fabrication methods disclosed herein.

FIG.2is a schematic cross-sectional perspective view of an exemplary, efficient, monolithic, two-terminal (2-T) perovskite/Si tandem solar cell100that was prepared.

The cell100does not utilize a conventional interconnection layer and instead places a perovskite top cell102in direct contact with the Si homojunction bottom cell104. A highly ohmic contact may be formed between a TiO2layer106deposited by atomic-layer deposition (ALD) and a p+-Si region122. Despite the absence of an intentional recombination layer in the cell100, under certain deposition conditions the conductive contact may be produced spontaneously between the two subcells102,104in the monolithic tandem device100. The conductive contact is consistent with the formation of an atomic-scale recombination layer due to a defective interphase region at the TiO2/p+-Si interface.

In the examples prepared, the contact resistance between the top and bottom cells102,104was dependent on the band alignment at the TiO2/p+-Si interface and on the relative doping densities of the TiO2and Si, which were sensitive to the TiO2preparation method.

The perovskite top cell102may include any suitable perovskite material and conductive layers. In the example shown, the perovskite top cell102of the tandem device100includes an n-i-p structure with a stack comprising a cp-TiO2(compact TiO2) layer (ALD-TiO2)106, a ms-TiO2(mesoporous-TiO2) layer107, a perovskite layer108, a conductive polymer layer110, for example, a layer of Spiro-OMeTAD (2,2′,7,7′-Tetraakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene), or other known conductive polymer layers, a protective layer112, such as a layer of metal oxide that includes, consists of, or consists essentially of oxygen and one or more elements, e.g., molybdenum (MoOxlayer112), a transparent conductor layer114, such as a layer of indium zinc oxide (IZO), and a conductor layer116, for example, a metal grid such as an Au grid or any other suitable metal such as Cu, Al, Ag or the like. An anti-reflection layer118, such as an anti-reflection foil, may be applied over the top of the subcell102, contacting the transparent conductor and conductor layers114,116, or other known techniques for light management of reflection losses in solar cells.

The ALD-TiO2layer106may be deposited so that it is uniform and conformal with a low surface roughness of about 0.77 nm. Whilst largely amorphous in its initially-deposited state, the compact TiO2layer106may be crystallized by annealing at 400° C. A compact TiO2layer106thickness of about 50 nm may be used, covered by an ms-TiO2layer107of thickness of about 70-80 nm, and an ultra-thin PCBM (Phenyl-C61-butyric acid methyl ester)/PMMA (Poly(methyl methacrylate)) passivation layer (not shown) to improve the cell voltage and reduce hysteresis in the current density vs. voltage (J-V) characteristics.

The perovskite layer108may include any suitable perovskite material. The TiO2hole tunnel layer may also be used with other top cell absorber materials included in the layer108, as well. For example, multiple cation perovskites, which have consistently outperformed their single-cation originators, may be used in the cell100. The perovskite layer108may fabricated using an anti-solvent one-step method. A composition of Cs0.05Rb0.05FA0.765MA0.135PbI2.55Br0.45may be used for the layer108, as it may yield stable films with an appropriate bandgap (Eg=1.63 eV).

Current matching between the two subcells102,104may be obtained by deposition of a relatively thin (about 310 nm) perovskite layer108. To reduce the parasitic absorption of the Spiro-OMeTAD hole blocking layer110while reducing pinholes, the perovskite layer108thickness may be reduced to about 120 nm.

Before sputtering the IZO layer114having a thickness of about 40 nm for the top contact, the protective MoOxlayer112having a thickness of about 40 nm may be deposited to protect the Spiro-OMeTAD layer110from sputtering damage.

Optical losses on the top surface of the cell100may be reduced by the use of narrow metallic grids (e.g., Au grids) for the conductor layer116, where the grids have a width of about 30 μm and a spacing of about 1 mm. This may result in grid loss of about 3%.

The bottom cell104includes a crystalline silicon (c-Si) substrate120, which may be an n-type Si wafer with a passivated bottom surface126and bottom side texturing124. The p+-Si emitter region122may be formed in the C—Si substrate by doping, as described below. Conductor layers128,130and132may be formed on the bottom of the c-SI substrate102forming the bottom terminal.

The bottom cell104may be a Si homojunction cell. An advantage of using a Si homojunction cell is a greater tolerance to high temperature processing, which may be used to achieve high-conductivity crystalline titania via annealing.

FIG.3is a cross-sectional scanning-electron microscope (SEM) image200of an example of the tandem photovoltaic cell100illustrated byFIG.2, from the top conductor surface116to the p+-Si layer122. The image200shows a Si emitter region202, an ALD-TiO2layer204, an ms-TiO2layer206, a perovskite layer208, a Spiro-OMeTAD layer210, an MoOxlayer, an IZO layer212, and an Au conductor layer214. The anti-reflection layer is not included in the image200due to its relatively large thickness of about 1 mm.

FIG.4is a flowchart diagram50illustrating an exemplary method of making the tandem photovoltaic cell100illustrated inFIG.2.

In step52, the Si subcell is fabricated by forming the p-type emitter region on the c-Si substrate. This may be accomplished with the following steps. A 200 μm-thick n-type float zone (FZ) Si wafer with a resistivity of about 1 Ωcm may be used as the substrate. Both sides of the wafer may be chemically polished and coated with a thick SiO2layer via a 1050° C. dry oxidation process. The wafer may then immersed in 1% (volume percentage) hydrofluoric acid (HF) to remove the SiOxon the back side while the front side was protected with a coating of photoresist. A texturing process is then performed on the back side, which is then further covered with SiOxusing a wet-oxidization process. The oxide on the front side of the wafer may then be removed and photolithography may be used to define the active region for boron-doping the p+ emitter region. Boron-diffusion may be performed by transferring the wafer into a Tempress furnace at 930° C. with deposition for 25 minutes and oxidation for 20 minutes. Next, phosphorus-diffusion doping (POCl3) is performed on the rear-side with a back-to-back approach at a diffusion temperature of 780° C. for 30 minutes. The borosilicate glass on the front side and the phosphosilicate glass on the rear side are then removed using an HF etch. The textured rear side may then be passivated with SiNx(refractive index of about 1.9, 70 nm thick) deposited by plasma-enhanced chemical-vapor deposition (PECVD). The back side can utilize a point-contact scheme. Photolithography may be used to define the contact opening for the rear side of the wafer, followed by a reactive-ion etch (RIE). An HF etch may be then used to remove the passivation layers in the contact regions. A Cr/Pd/Ag metal stack may be thermally evaporated on the rear side of the wafer, and can then be lifted off with acetone in an ultrasonic bath. The Cr/Pd/Ag rear-contact stack is then covered with an Ag (100 nm)/Al (500 nm) contact capping layer using thermal-evaporation. On front side, the non-active area may be covered with about 20 nm Al2O3deposited by ALD and about 100 nm SiNxdeposited by PECVD.

In step54, the compact TiO2or ALD-TiO2layer is prepared. The compact TiO2layer may be derived from one of three different precursors: tetrakisdimethylamidotitanium (TDMAT), titanium tetraisopropoxide (TTIP), or titanium tetrachloride (TiCl4). The selected precursor may be deposited directly on top of p+-Si emitter region of the Si wafer after removal of the Si native oxide from the wafer with an HF etch. Immediately after the HF etch, the Si solar cells are transferred to an ALD chamber for TiO2deposition.

For deposition using the TDMAT precursor, an Ultratech Fiji 200 Plasma ALD System may be used for the deposition, with the reaction temperature fixed at 150° C. The TDMAT precursor may be maintained at 75° C. Prior to ALD, a 0.10 s pulse of H2O may be applied to the partially completed cell. Each ALD cycle may consist of a 0.015 s pulse of H2O followed by a 0.10 s pulse of TDMAT. Between each precursor pulse, a 15 s purge under a constant 0.02 L min−1flow of research-grade N2(gas) may be used. While idle, the ALD system was maintained under a continuous N2(gas) purge and at a pressure of about 0.5 Torr.

A thermal ALD system may be employed for depositing TiO2films using the TTIP and TiCl4precursors, with N2(gas) as the purge gas. For the TiCl4precursor, the reactor temperature may be 75° C. and H2O may be used as the oxidant. The chamber N2gas flow may be 200 ccm. Each ALD cycle can consist of a 0.75 s pulse of TiCl4followed by a 0.050 s pulse of H2O. Between each precursor pulse, a 0.75 s purge under a constant 300 sccm flow of research-grade N2(gas) can be used. The deposition rate may be about 0.76 Å/cycle by spectroscopic ellipsometry.

For the TTIP precursor, the source may be heated to a temperature of 40° C., and the reactor set to a temperature of 230° C. H2O may also be used as the oxidant. Each ALD cycle may consist of a 1 s pulse of TTIP followed by a 0.5 s pulse of H2O. Between each precursor pulse, a 2 s purge under a constant 300 sccm flow of research-grade N2(gas) may be used. The deposition rate was determined to be about 0.023 Å/cycle.

To complete the tandem photovoltaic cell, an about 70-80 nm mesoporous TiO2layer may be deposited on the ALD-TiO2layer by spin-coating diluted TiO2paste (TiO2:ethanol=1:12, weight %) at a speed of 5000 rpm (step56).

The as-prepared mesoporous TiO2and ALD-TiO2layers undergo a high temperature annealing treatment at 400° C. for 20 min, which may be performed out in air (step58). Immediately after cooling to room temperature from the annealing step, the cell substrate may be further passivated with an ultra-thin layer by spin-coating a poly(methyl methacrylate)/Phenyl-C61-butyric acid methyl ester (PMMA/PCBM) mixture (1:3, weight %) in chlorobenzene at 5000 rpm/s with a ramp of 5000 rpm/s for 30 s. The partially completed cell may then be subsequently baked on a hotplate at 100° C. for 10 min.

Next, the perovskite layer is deposited on the compact TiO2layer (step60). A multiple cation perovskite precursor solution (Cs0.05Rb0.05FA0.765MA0.135PbI2.55Br0.45) may be used that contains 1.2 M PbI2, 1.1 M FAI, 0.20 M PbBr2, 0.20 M MABr, 0.091 M CsI, and 0.039 M RbI in 1 mL of anhydrous DMF:DMSO (8:2, volume ratio). Different spin-coating speeds and precursor solution concentrations may be used to select the desired perovskite film thickness.

A one-step process may be used to apply the perovskite layer. Alternatively, a two-step process may be used to apply the perovskite layer. For example, the multiple cation perovskite precursor solution may be deposited by spin-coating at 2000 rpm with a ramp rate of 200 rpm s−1for 10 s, and then again at 4000 rpm with a ramp of 1000 rpm s−1for 25 s. During the second step, about 100 μl chlorobenzene may be poured on the spinning substrates 5 s prior to the end of the spin cycle.

To reduce the perovskite thickness and ensure current matching, a 1 mL of the multiple cation perovskite precursor solution may be diluted with 0.2 mL anhydrous DMF:DMSO (8:2, v/v) to obtain a 1.0 M concentration of PbI2. In addition, the chlorobenzene dropping may be conducted earlier, about 8 s prior the end of the second spinning stage.

To further reduce the perovskite film thickness for the tandem device, the second stage of the spin-coating process may be modified additionally by increasing the ramp speed from 1000 rpm s−1to 4000 rpm s−1, and changing the chlorobenzene dropping to about 10 s prior to the end of the program.

In all cases, the spin-coated perovskite films may be then annealed for 30 minutes at 100° C.

In step62, a spiro-OMeTAD film is applied over the perovskite layer. To do this, a spiro-OMeTAD precursor solution may be prepared by dissolving 72.5 mg spiro-OMeTAD, 28.5 μL 4-tert-butylpyridine and 17.5 μL of lithium bis(trifluoromethanesulfonyl)imide solution (520 mg/mL in acetonitrile) in 1 mL of chlorobenzene. The spiro-OMeTAD thin film may then be deposited by spin-coating the precursor solution at 3500 rpm with a ramp of 3500 rpm s−1for 30 s. After spin-coating the Spiro-OMeTAD solution, the partially-completed cell substrate may be placed in a humidity-controlled box for 12 hours to allow the oxidation of the Spiro-OMeTAD film prior to electrode/contact layer deposition.

In step64, an about 10 nm layer of MoOxis deposited on the cell over the Spiro-OMeTAD film by thermal evaporation at a rate of 0.05 nm/s under a high vacuum of 8×10−7Torr.

The transparent conductor is then fabricated by sputtering 40 nm of IZO on the MoOx(step66). The sputtering may be performed for 60 min with 30 W of RF power under an Ar plasma, with a chamber pressure of 1.5 mTorr.

To complete the tandem solar cell, Au fingers with a period of 1 mm and width of 30 μm (3% shading) may be deposited on the cell using e-beam evaporation though a shadow mask (step68). A textured anti-reflection foil may be applied on the top surface of the completed tandem device (step70).

FIGS.5A-Bare transmission-electron microscopy (TEM) and high-resolution TEM (HRTEM) images, respectively, of the TiO2/p+-Si hetrojunction interface of an exemplary multi-junction photovoltaic cell.

FIGS.5A-Bare transmission-electron microscopy (TEM) and high-resolution TEM (HRTEM) images250,300, respectively, of the TiO2/p+-Si hetrojunction interface of an exemplary multi-junction photovoltaic cell. The TEM image250ofFIG.5Ashows the TiO2layer258of the tandem cell, the SixTixOxlayer254of TiO2/p+-Si hetrojunction interface, and the Si substrate252. The HRTEM image300is a higher resolution image of the interface.

FIG.6shows a graph350of the photovoltaic J-V performance exhibited by tandem photovoltaic cells fabricated according to the methods described herein, with the corresponding photovoltaic metrics summarized in Table S1 ofFIG.13. The inset shows the efficiency evolution under continuous illumination for >1 h at the bias that yielded the maximum power point for the first J-V scan.

The J-V data were obtained at relatively slow scan rates (0.1V/s and 0.01V/s) in both the reverse and forward directions, and exhibited negligible hysteresis. The highest efficiency of 23.2% under 100 mW/cm2of simulated Air Mass 1.5G illumination was obtained with an open-circuit voltage (Voc) of 1.703 V, a short-circuit current density (Jsc) of 17.2 mA cm−2, and a fill factor (FF) of 0.792. The perovskite subcell alone exhibited a standalone efficiency of ˜18.9%, and Si solar subcells fabricated including ALD-TiO2on a p+-Si emitter, all other components being the same as those in our tandem design exhibited η=14.8%. An unencapsulated tandem cell example retained ˜97% of its original efficiency (˜22.5%) after 4000 s of continuous illumination in N2(gas) (FIG.6, inset).

Due to the large number of material interfaces, optical losses must be reduced in an efficient multi-junction cell, such as those disclosed herein. The example tandem devices prepared as described herein had a low average reflection over the 400-1200 nm spectral region, partly due to the attachment of a textured foil to the front surface (see graph400ofFIG.7). The graph400shows absorbance (1−R, where R is the reflectance) of an example tandem photovoltaic device (dashed-dot line), external quantum efficiency (EQE) of the perovskite top subcell (left-side line), and EQE of the c-Si bottom subcell (right-side line).

Spectral response analysis revealed that the fabricated tandem cells yielded excellent current matching between subcells, with only a slightly larger integrated current density of ˜17.5 mA/cm2for the perovskite top subcell as compared with a current density of ˜17.2 mA/cm2for the Si bottom subcell. Better light management, via tuning of the perovskite composition and removal of optical absorption and reflection from the Spiro-OMeTAD contact, may allow for higher short-circuit current densities.

Minority-carrier lifetime measurements of example cells indicated that the TiO2only provided a weak passivation effect on the p+-Si emitter, hence increased passivation of the p+-Si surface may further improve the open-circuit voltage of the tandem cell. Strategies including enhancement of the p+-doping density to reduce the emitter thickness and hence absorption, or optimizing the p+-Si and TiO2interface with respect to passivation, may also be beneficial.

Efficient operation of the tandem photovoltaic cells disclosed herein requires efficient charge transfer between the p+-Si emitter and TiO2layer. Specifically, photogenerated electrons collected in the TiO2layer should be able to recombine, while incurring little voltage loss, with corresponding holes from the Si emitter region. The n-type character of TiO2layer would be expected to produce a rectifying p-n heterojunction with p-type Si emitter region. However, the J-V characteristics of the disclosed tandem photovoltaic devices did not exhibit S-shaped curves, nor fill-factor losses, that would be expected if a rectifying contact were present between the two subcells. The existence of facile electrical contact between TiO2layer and p+-Si emitter was experimentally confirmed in prepared tandem cells. The contact resistivity (pc) of both films with respect to p+-Si was determined via the method devised by Cox and Strack. The contact resistivity so derived includes not only the desired metal oxide/p+-Si contact resistivity, but also includes contributions from the bulk oxide as well as the oxide/Al contact. These experimental measurements confirmed that the contact between TiO2and p+-Si was highly ohmic, characterized by a resistivity less than 30 mΩcm2, surpassing that of an ITO/p+-Si combination (˜230 mΩcm2).

Annealing the TiO2layers played helped to achieve the desired low resistance contact between subcells. Indeed, superior performance for the TiO2/p+-Si interface was obtained after annealing the structure at 400° C. in ambient air, which produced more than a ten-fold reduction in the derived contact resistance (compare Tables S2 and S3 ofFIG.13). The interconnect-free tandem cells described herein thus have dual advantages of higher performance as well as more facile fabrication.

The TiO2layer106may be prepared using different ALD precursors, which each exhibited mutually different J-V characteristics (see graph450ofFIG.8). Ohmic, highly conductive behavior was observed after annealing the TiO2using tetrakisdimethylamidotitanium (TDMAT) as the ALD precursor (FIG.8, green solid line); however, very low conductivity (>10 Ωcm2) in the low-bias region was obtained when titanium tetrachloride (TiCl4) was used as the ALD precursor instead of TDMAT, despite nominally identical processing conditions (FIG.8, blue solid line). The use of titanium tetraisopropoxide (TTIP) as the ALD precursor resulted in intermediate performance, displaying conductive but distinctly non-linear J-V behavior (FIG.8, yellow solid line). The conductivity of the TiO2/p+-Si test structures shown inFIG.8correlated well with the behavior of the example prepared tandem devices, with η=21% for a tandem cell fabricated with TTIP as the precursor and η=3.6% for a tandem cell fabricated with TiCl4as the precursor.

The interfacial band alignment facilitates carrier transport between TiO2and p+-Si. Based on X-ray photoelectron spectroscopic (XPS) measurements of the electron affinity (χTiO2) for our TiO2samples (4.35-4.7 eV) and the ionization energy of Si (I.E.Si), taken as 5.15 eV, neglecting any surface dipole contribution the band alignment at an idealized TiO2/p+-Si junction should result in an energy gap of Δ=EcTiO2−EvSi=χTiO2−I.E.Si≈0.45-0.8 eV, between the top of the Si valence band and the bottom of the TiO2conduction band (FIG.9).FIG.9is an example simulated band diagram500of the TiO2/p+-Si interface of an exemplary tandem cell at equilibrium.

Experimental determinations of Δ that include the surface dipole require combining data from several techniques, and have only been reported rarely for TiO2/p-Si interfaces. Values of Δ between 0.45 eV and 0.8 eV have been obtained depending on the 1-2 nm interlayer composition, supporting the observation of a sensitivity to processing. A non-vanishing gap at the TiO2/p+-Si interface prohibits at 0 V band-to-band tunneling between the TiO2conduction band and the Si valence band, due to a lack of overlap in the bulk density of states at equilibrium (seeFIG.9), as would occur in a tunnel diode. Sub-gap states in TiO2due to a band-tail or defect band may alter this situation, but the mechanism is essentially the same as a defect-mediated pathway. The necessary band overlap between the TiO2conduction band and the Si valence band occurs at a threshold reverse voltage, but carriers must nevertheless tunnel through the sum of the depletion and interlayer widths, estimated to be 10's of nanometers for TiO2doping in the range of 1017-1019 cm−3. This distance is at the upper limit of what is physically reasonable, and indicates that band-to-band tunneling at reverse bias is only likely to occur when both depletion regions are very small, corresponding to high doping. At forward bias, the band overlap is decreased and band-to-band tunneling becomes prohibited. In this case, current could be carried via the thermionic emission of conduction-band electrons from TiO2over the barrier due to the conduction-band offset, EgSi−Δ, to Si, but this mechanism would predict a strong tradeoff between the forward and reverse current, contrary to the observed ohmic behavior (i.e., large gaps Δ would provide a small barrier for the forward current while enlarging the threshold voltage for reverse current, and vice versa).

These considerations indicate that the presence of a pristine interfacial energy gap A is not readily compatible with the observed highly conductive contact between the TiO2and p+-Si layers. A more likely alternative is the presence of a substantial density of localized mid-gap states at the interface between Si and TiO2. Such interfacial states can facilitate band-to-band tunneling at reverse bias and act as generation-recombination centers at all bias voltages. In such a scenario, electrons can move into and out of the defects states via local capture/emission as well as tunneling (FIG.9). As generation-recombination centers, the interface states would have a substantial influence on charge transport by facilitating recombination of carriers at forward bias without requiring carriers to cross over the interfacial barrier. At reverse-bias, every recombination center could become a source of generation, and high conductivity may be obtained by thermally generated carriers. Conceptually, this situation is similar to having a recombination-layer of atomic dimensions between the TiO2and p+-Si created in situ and intrinsically via the native material contact, without the introduction of substantial optical losses.

Numerical drift-diffusion models based on SCAPS simulation software were used to investigate the impact of interfacial generation-recombination centers on the junction current. These models were designed to compute the current across a TiO2/p+-Si heterojunction assuming ohmic metal contacts on both sides, and therefore mainly addressed the junction current, with only minor contributions from bulk conduction through the small layers thicknesses (50 nm and 100 nm for TiO2and Si, respectively). Shockley-Read-Hall (SRH) recombination centers were added at the simulated TiO2/p+-Si interface to physically correspond to localized states that are expected to form in the interfacial energy gap. Such defects are likely to occur at a high density, given the relatively low degree of lattice matching between TiO2and c-Si, the possibility of precursor remnants, Si dangling bonds, and the presence of a 1-2 nm amorphous alloy interlayer254observed in our samples between the two bulk crystals (FIGS.5A-B). Interlayers are known to have a profound effect on the interface dipole or band-alignment of semiconductor-semiconductor contacts, as well as on the mechanisms of charge transfer, and may therefore play a key role in the experimental tandem cells. For simplicity in the modeling, the contribution of the interlayer capacitance was neglected, while the defect density of the interlayer was captured in the interfacial SRH parameters. Tunneling due to defects was not accurately modeled due to a lack of detailed knowledge of the interface parameters, but calculations with tunneling processes included are presented inFIG.10to illustrate qualitatively the behavior that results from this effect.

FIG.10is a graph550showing example simulated J-V characteristics of TiO2/p+-Si heterojunctions with varying gaps Δ in the range of 0.4-0.9 eV, and with a high density of neutral mid-gap defects (recombination velocities Sn=Sp=105cm/s), and all other parameters being equal. The various TiO2/p+-Si hetrojunctions are usable in examples of the tandem cell. A single neutral mid-gap SRH defect was included in the simulation with Sn=Sp=105cm/s. The dashed curves are computed with tunneling to defects included in the modeling.

These characteristics were computed using the SCAPS application software. The J-V characteristics of graph550bear a striking resemblance to the experimental behavior of the TiO2/p+-Si heterojunctions the disclosed tandem cells (shown inFIG.8) in that they both exhibit the full range of qualitative characteristics seen experimentally, namely highly conductive ohmic behavior (e.g. Δ=0.4), asymmetric exponential-type curves (Δ=0.5, 0.6 eV) and strong rectification (Δ=0.7-0.9 eV). The detrimental effect of a large band offset can only be compensated by higher recombination velocities up to the physical limit of Sn,p=vth≈107 cm/s, likely ruling out high defect-mediated conductivity for band offsets greater than ˜0.7 eV. A somewhat less trivial prediction of the SRH model concerns the balance of carrier densities at the interface. The interfacial carrier densities should be balanced to achieve maximal conductivity (in particular vnn0≈vpp0where n0,p0are the equilibrium carrier densities at the interface and vn,ptheir quasi-recombination velocities).

The interfacial conductivity of the prepared TiO2/p+-Si heterojunctions benefitted from a high substrate doping (graph600ofFIG.11, square dots), which is consistent with donor type defects at the interface that act to deplete the hole density. Donor defects are frequently present at both TiO2and unpassivated-Si surfaces in the form of oxygen vacancies and dangling bonds (Pbcenters), respectively. The SRH theory thus consistently accounts for the ohmic conductivity between TiO2/p+-Si. The graph600shows an example simulated small voltage resistivity of a TiO2/p+-Si hetrojunction at various p-Si acceptor doping levels. Simulated small voltage resistivity

(ρ=dVdI⁢❘V=0)
with the TiO2donor density fixed at 1018cm−3and variable p-Si acceptor doping. Measurements are shown as the square data points and range. Calculations for neutral (solid lines), acceptor-type (dotted lines) and donor-type (dot-dashed lines) are shown to demonstrate the important effect of defect charge on the interfacial carrier balance.

According to the SRH theory outlined above, the diverse interface behavior seen in the graph450ofFIG.8with respect to preparation conditions likely results from variations in the band offsets between the TiO2conduction-band edge and the Si valence band (Δ), the TiO2doping density, and the interfacial defect properties. The experimental data suggest that a small Δ, in addition to a conductive TiO2layer, may be helpful to obtain a high-conductivity contact between p+-Si and TDMAT-ALD TiO2. The built-in voltage of a perovskite cell is determined in part by the work function of the n-type TiO2selective contacts (corrected for surface dipole contributions), and thus has an effect on the maximum open-circuit voltage. Larger n-type selective contact work functions reduce the built-in voltage and therefore the achievable quasi-Fermi level splitting, in contrast to the experimental observations that the perovskite cells on TDMAT TiO2function without substantial losses. The observed cell performance is thus indicative of an additional role for the mesoporous TiO2layer that is inserted in the cell architecture between the compact TiO2layer and the perovskite—to improve film quality and electron extraction. Experimental measurements indicated that the conduction band in compact TDMAT-ALD TiO2is energetically lower-lying than in other TiO2preparations. Due to its smaller work function, inclusion of the solution-processed mesoporous TiO2layer therefore appears to maintain the built-in voltage of the cell. An example simulated band diagram650(FIG.12) of the complete tandem photovoltaic cell structure ofFIG.2summarizes the key findings that contact between the high-work function cp-TiO2layer and p+-Si is facilitated by interface defects, and that high cell voltages for the complete tandem cell100rely on the inclusion of the lower work function mesoporous TiO2to maintain the built-in voltage of the perovskite subcell.FIG.12shows the example simulated band diagram650of the cell100at illuminated open-circuit. The inset depicts the two critical energetic offsets Δ and δ, respectively, defined as the valence-to-conduction band offset at the TiO2—Si interface and the difference in work functions between the solution-processed mesoporous TiO2layer and that of the ALD compact TiO2layer.

FIG.13shows tables S1, S2, S3 summarizing experimental photovoltaic metrics of an example tandem photovoltaic cell and Hall-effect measurements regarding various states of the TiO2layer of a tandem photovoltaic cell.

Disclosed herein are examples of two-terminal perovskite-Si tandem photovoltaic devices that function without a conventional interlayer between their sub-cells. An enabling feature of the devices is the formation of an atomic-scale recombination layer between a compact TDMAT-ALD TiO2layer and the pt-Si emitter region, which produces a highly conductive, relatively ohmic contact between the two materials. The contact resistance, interfacial band offsets, defect densities, and doping densities are dependent on processing, but therefore present several handles for tunability. The twin advantages of the disclosed cells in reducing optical losses and in reducing processing steps brings the perovskite-Si pairing tandem structure closer to delivering its full potential.

Under experimental conditions, the prepared tandem cells yielded efficiencies as high as about 23.2% under 100 mW/cm2of Air Mass 1.5G simulated sunlight, with an open-circuit voltage of 1.703 V, a short-circuit current density of about 17.2 mA/cm2, and a fill factor of 0.792.

Although the foregoing description discloses exemplary tandem cells, it should be understood that the cells and structures disclosed herein may be used in multi-junction cells having more than two subcells, for example, three, four or more subcells fabricated into a single photovoltaic cell.

The disclosed cells may also be electrically connected together, for example, in series or parallel, to form a module comprising a plurality of cells, such as a solar module or panel.

The foregoing description is illustrative and not restrictive. Although certain exemplary embodiments have been described, other embodiments, combinations and modifications involving the invention will occur readily to those of ordinary skill in the art in view of the foregoing teachings. Therefore, this invention is to be limited only by the following claims, which cover at least some of the disclosed embodiments, as well as all other such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.