High efficiency small molecule tandem photovoltaic devices

Abstract

A high efficiency small molecule tandem solar cell is disclosed. The tandem cell may include a first subcell comprising a first photoactive region and a second subcell comprising a second photoactive region. The first and second photoactive regions are designed to minimize spectral overlap and maximize photocurrent. The device may further include an interconnecting layer, disposed between the first subcell and the second subcell, that is at least substantially transparent.

Claims

1. A tandem photovoltaic device comprising: a first subcell comprising a first photoactive region; a second subcell comprising a second photoactive region; and a separating layer between the first subcell and the second subcell, wherein the separating layer comprises an interconnecting layer comprising bathophenanthroline (BPhen):C.sub.60; wherein: the first photoactive region comprises a first donor material and a first acceptor material forming a first donor-acceptor heterojunction; the second photoactive region comprises a second donor material and a second acceptor material forming a second donor-acceptor heterojunction; the first donor material comprises 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c] [1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) or a derivative thereof; the first acceptor material comprises C.sub.60; the second donor material comprises dibenzo([f,f′]-4,4′,7,7′-tetraphenyl)diindeno[1,2,3-cd:1′,2′,3′-lm]perylene (DBP) or a derivative thereof; the second acceptor material comprises C.sub.70; and the first subcell is the front subcell and the second subcell is the back subcell, such that incident light enters the device through the first subcell and light that is not absorbed by the first subcell is passed to the second subcell.

2. The tandem photovoltaic device of claim 1, wherein the first donor-acceptor heterojunction is chosen from a mixed heterojunction and a hybrid planar-mixed heterojunction.

3. The tandem photovoltaic device of claim 1, wherein the second donor-acceptor heterojunction is chosen from a mixed heterojunction and a hybrid planar-mixed heterojunction.

4. The tandem photovoltaic device of claim 1, wherein the first and second donor-acceptor heterojunctions are chosen from mixed heterojunctions and hybrid planar-mixed heterojunctions.

5. The tandem photovoltaic device of claim 1, wherein the second donor material comprises DBP.

6. The tandem photovoltaic device of claim 5, wherein the first donor material comprises DTDCTB.

7. The tandem photovoltaic device of claim 1, wherein the separating layer further comprises a charge recombination layer.

8. The tandem photovoltaic device of claim 7, wherein the charge recombination layer comprises a material chosen from Al, Ag, Li, LiF, Sn, and Ti.

9. The tandem photovoltaic device of claim 7 further comprising a charge collecting layer between the separating layer and one of the first and second photoactive regions, wherein the charge collecting layer comprises a metal oxide.

10. The tandem photovoltaic device of claim 9, wherein the metal oxide is chosen from MoO.sub.3, V.sub.2O.sub.5, ZnO, and TiO.sub.2.

11. The tandem photovoltaic device of claim 1, wherein the first donor material comprises DTDCTB.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.

(2) FIG. 1 shows a block diagram of an exemplary tandem photovoltaic device according to the present disclosure.

(3) FIG. 2a shows an exemplary optimized front subcell. FIG. 2b shows an exemplary optimized back subcell. FIG. 2c shows an exemplary tandem solar cell device according to the present disclosure.

(4) FIG. 3A shows the extinction coefficients (k) of DTDCTB:C.sub.60 films measured as functions of volume ratio. The inset shows a plot of k at λ=450 nm and λ=700 nm as a function of the C.sub.60 percentage. FIG. 3B shows the EQE of an optimized DTDCTB:C.sub.60 cell. The inset shows the structure of the optimized cell and the chemical structure of DTDCTB.

(5) FIG. 4A shows an exemplary tandem solar cell device according to Example 1. FIG. 4B shows the extinction coefficient of 1:1 DTDCTB:C.sub.60 and 1:10 DBP:C.sub.70 blends along with the AM 1.5G solar spectrum. FIG. 4C shows the EQE spectrum of an exemplary cell with PTCBI used as the buffer layer interconnecting the front and back subcells.

(6) FIG. 5A shows the simulated optical field distribution within the exemplary tandem cell, comparing a 5 nm thick conventional PTCBI with a similarly thick BPhen:C.sub.60 mixed buffer. FIG. 5B shows the fourth quadrant J-V characteristics of an exemplary tandem and front cell utilizing PTCBI and BPhen:C.sub.60 as the interconnecting layer.

(7) FIG. 6 shows the spectrally corrected J-V characteristics of an exemplary front cell, back cell, and tandem device.

(8) FIG. 7A shows both simulated and measured optimized tandem cell EQEs of the exemplary tandem device of Example 1. FIG. 7B shows the measured and calculated 4th quadrant J-V characteristics of the exemplary tandem device of Example 1.

(9) FIG. 8A shows performance parameters under different light intensities for the exemplary tandem with PTCBI as the interconnecting layer. FIG. 8B shows performance parameters under different light intensities for the exemplary tandem with BPhen:C.sub.60 as the interconnecting layer.

(10) FIG. 9 shows the results of measuring the responsivity and rip of the exemplary tandem solar cells of Example 2, with BPhen or BPhen:C.sub.60/BPhen as the cathode buffer layers with intensities ranging from 0.2 to 4 suns.

(11) FIG. 10 shows the chemical structure of DTDCPB.

(12) FIG. 11 shows an exemplary tandem device according to Example 3.

(13) FIG. 12 shows the extinction coefficient of 1:1 DTDCTB:C.sub.60 and the 1:1 DTDCPB:C.sub.70 blends along with the AM 1.5G solar spectrum.

(14) FIG. 13 shows the simulated EQE spectrum of the exemplary tandem device of Example 3.

(15) FIG. 14 shows the simulated optical field distribution within the exemplary tandem cell of Example 3 with a 5 nm thick BPhen:C.sub.60 mixed buffer.

(16) FIG. 15 shows the fourth quadrant J-V characteristics of the front subcell, back subcell, and tandem of the exemplary device of Example 3.

(17) The terms “electrode” and “contact” are used herein to refer to a layer that provides a medium for delivering photo-generated current to an external circuit or providing a bias current or voltage to the device. That is, an electrode, or contact, provides the interface between the active regions of an organic photosensitive optoelectronic device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. Anodes and cathodes are examples. U.S. Pat. No. 6,352,777, incorporated herein by reference for its disclosure of electrodes, provides examples of electrodes, or contacts, which may be used in a photosensitive optoelectronic device.

(18) In a photosensitive optoelectronic device, it may be desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoconductive active interior region. That is, the electromagnetic radiation must reach a photoconductive layer(s), where it can be converted to electricity by photoconductive absorption. This often dictates that at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. In some cases, such a contact should be transparent or at least semi-transparent. An electrode is said to be “transparent” when it permits at least 50% of the incident electromagnetic radiation in relevant wavelengths to be transmitted through it. An electrode is said to be “semi-transparent” when it permits some, but less than 50% transmission of ambient electromagnetic radiation in relevant wavelengths. The opposing electrode may be a reflective material so that light which has passed through the cell without being absorbed is reflected back through the cell.

(19) As used and depicted herein, a “layer” refers to a member or component of a photosensitive device whose primary dimension is X-Y, i.e., along its length and width. It should be understood that the term layer is not necessarily limited to single layers or sheets of materials. In addition, it should be understood that the surfaces of certain layers, including the interface(s) of such layers with other material(s) or layers(s), may be imperfect, wherein said surfaces represent an interpenetrating, entangled or convoluted network with other material(s) or layer(s). Similarly, it should also be understood that a layer may be discontinuous, such that the continuity of said layer along the X-Y dimension may be disturbed or otherwise interrupted by other layer(s) or material(s).

(20) As used herein, a “photoactive region” refers to a region of the device that absorbs electromagnetic radiation to generate excitons. Similarly, a layer is “photoactive” if it absorbs electromagnetic radiation to generate excitons. The excitons may dissociate into an electron and a hole in order to generate an electrical current.

(21) In the context of the organic materials of the present disclosure, the terms “donor” and “acceptor” refer to the relative positions of the HOMO and LUMO energy levels of two contacting but different organic materials. If the LUMO energy level of one material in contact with another is lower, then that material is an acceptor. Otherwise it is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material, and for holes to move into the donor material.

(22) As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.

(23) Disclosed herein is a tandem photovoltaic device comprising: a first subcell comprising a first photoactive region; and a second subcell comprising a second photoactive region; wherein: the first photoactive region comprises a first donor material and a first acceptor material forming a first donor-acceptor heterojunction; the second photoactive region comprises a second donor material and a second acceptor material forming a second donor-acceptor heterojunction; the first donor material comprises DTDCTB or a derivative thereof; and the second donor material comprises a material chosen from DBP and DTDCPB, and a derivative thereof. An example device schematic according to the present disclosure is shown in FIG. 1.

(24) Accordingly, a subcell, as used herein, means a component of the device which comprises at least one donor-acceptor heterojunction. The donor-acceptor heterojunction may be chosen from those known in the art, such as a planar heterojunction, a bulk heterojunction, a mixed heterojunction, and a hybrid planar-mixed heterojunction. In certain embodiments, the first donor-acceptor heterojunction is a mixed heterojunction or a hybrid planar-mixed heterojunction. In certain embodiments, the second donor-acceptor heterojunction is a mixed heterojunction or a hybrid planar-mixed heterojunction. In certain embodiments, the first and second donor-acceptor heterojunctions are chosen from mixed heterojunctions and hybrid planar-mixed heterojunctions.

(25) When a subcell is used individually as a photosensitive optoelectronic device, it typically includes a complete set of electrodes. In a tandem photovoltaic device, such as those of the present disclosure and as shown in FIG. 1, each subcell may include an electrode, and the subcells may be divided by a separating layer. As known in the art for tandem devices, the separating layer may comprise at least one charge transfer layer, at least one electrode, or at least one charge recombination layer. In some tandem configurations, it is possible for the subcells to utilize a common, i.e., shared, electrode, charge transfer layer or charge recombination layer. In other cases, the subcells do not share common electrodes or charge transfer layers. The subcells may be electrically connected in parallel or in series.

(26) In some embodiments, the charge transfer layer or charge recombination layer may be chosen from Al, Ag, Au, MoO.sub.3, Li, LiF, Sn, Ti, WO3, indium tin oxide (ITO), tin oxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZO), or zinc indium tin oxide (ZITO). In some embodiments, the charge transfer layer or charge recombination layer may comprise metal nanoclusters, nanoparticles, or nanorods. In some embodiments, the charge recombination layer comprises a thin metal layer. In certain embodiments, the charge recombination layer is less than or equal to about 20 Å thick, such as, for example, less than or equal to about 15 Å, 10 Å, or 5 Å thick. The small thickness can allow light to reach the back subcell.

(27) One of the electrodes of the present disclosure may be an anode, and the other electrode a cathode. It should be understood that the electrodes should be optimized to receive and transport the desired carrier (holes or electrons). The term “cathode” is used herein such that in a non-stacked PV device or a single unit of a stacked PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a PV device, electrons move to the cathode from the photo-conducting material. Similarly, the term “anode” is used herein such that in a PV device under illumination, holes move to the anode from the photoconducting material, which is equivalent to electrons moving in the opposite manner.

(28) As described above, the first donor material in the first subcell comprises DTDCTB, the molecular structure of which is shown in FIG. 3B, or a derivative thereof. DTDCTB primarily absorbs in the orange-to-NIR spectral region. The second donor material in the second subcell is chosen from DBP, DTDCPB and a derivative thereof. The structure of DTDCPB is shown in FIG. 10. DBP and DTDCPB absorb primarily in the green spectral region. Thus, DTDCTB in the first subcell, and DBP or DTDCPB in the second subcell show considerable separation between their absorption maxima, thereby minimizing spectral overlap and maximizing photocurrent.

(29) The first and second acceptor materials may be chosen from suitable materials known in the art. Examples of suitable acceptor materials include but are not limited to perylenes, naphthalenes, fullerenes, and fullerene derivatives (e.g., PCBMs, ICBA, ICMA, etc.). Non-limiting mention is made to those chosen from C.sub.60, C.sub.70, C.sub.76, C.sub.82, C.sub.84, or derivatives thereof such as Phenyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]PCBM), Phenyl-C.sub.71-Butyric-Acid-Methyl Ester ([70]PCBM), or Thienyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]ThCBM), and other acceptors such as 3,4,9,10-perylenetetracarboxylic-bisbenzimidazole (PTCBI), hexadecafluorophthalocyanine (F.sub.16CuPc), and derivatives thereof. Suitable acceptor materials may be chosen to be consistent with the desire of the present disclosure to minimize spectral overlap and maximize photocurrent.

(30) In some embodiments, the first acceptor material and the second acceptor material each comprise a material independently chosen from fullerenes and derivatives thereof. In some embodiments, the fullerenes are chosen from C.sub.60 and C.sub.70. In certain embodiments, the first acceptor material comprises C.sub.60. In certain embodiments, the second acceptor material comprises C.sub.70. In certain embodiments, the first acceptor material comprises C.sub.60 and the second acceptor material comprises C.sub.70. In certain embodiments, the second donor material comprises DBP and the second acceptor material comprises C.sub.70.

(31) In some embodiments of the present tandem devices, either the first subcell or the second subcell may act as the front subcell or the back subcell. The front subcell and the back subcell are used herein in the following manner: light enters the device through the front subcell and light that is not absorbed by the front subcell is passed to the back subcell. In certain embodiments, the front subcell may be provided with an electrode (anode or cathode as the case may be) that is substantially transparent in order to allow light to pass into the tandem cell. In some embodiments, the back subcell may be provided with a reflective electrode in order to allow incident light to be reflected back through the tandem device.

(32) In some embodiments, the first subcell is the front subcell. In certain of these embodiments, the first acceptor material is C.sub.60. In certain of these embodiments, the first acceptor material is C.sub.60 and the second acceptor material is C.sub.70. In some embodiments, the first subcell comprises an anode, and the second subcell comprises a cathode.

(33) The subcells of the present disclosure may further comprise additional layers known in the art for photovoltaic devices. For example, the subcells may further comprise buffers layers, such as one or more charge collection/transporting layers and/or one or more blocking layers, such as exciton blocking layers (EBLs).

(34) With regard to materials that may be used as an exciton blocking layer, non-limiting mention is made to those chosen from bathocuproine (BCP), bathophenanthroline (BPhen), 1,4,5,8-Naphthalene-tetracarboxylic-dianhydride (NTCDA), 3,4,9,10-perylenetetracarboxylicbis-benzimidazole (PTCBI), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), tris(acetylacetonato) ruthenium(III) (Ru(acac)3), and aluminum(III)phenolate (Alq2 OPH), N,N′-diphenyl-N,N′-bis-alpha-naphthylbenzidine (NPD), aluminum tris(8-hydroxyquinoline) (Alq3), and carbazole biphenyl (CBP). Examples of blocking layers are described in U.S. Patent Publication Nos. 2012/0235125 and 2011/0012091 and in U.S. Pat. Nos. 7,230,269 and 6,451,415, which are incorporated herein by reference for their disclosure of blocking layers.

(35) In some embodiments, one or more blocking layers are located between an electrode and one of the photoactive regions. In some embodiments, one or more blocking layers are located between each of the photoactive regions and the corresponding electrodes.

(36) In some embodiments, one or more charge collecting/transporting layers are located between an electrode and one of the photoactive regions. In some embodiments, one or more charge collecting/transporting layers are located between each of the photoactive regions and the corresponding electrodes. In some embodiments, one or more charge collecting/transporting layers are located between the separating layer and one or both of the photoactive regions. In certain embodiments, the charge collecting/transporting layers comprise a material chosen from metal oxides. In certain embodiments, the metal oxides are chosen from MoO.sub.3, V.sub.2O.sub.5, ZnO, and TiO.sub.2

(37) In addition, the tandem devices may further comprise at least one smoothing layer.

(38) As discussed above, and as shown in FIG. 1, the first and second subcells may be divided by a separating layer. The separating layer may comprise at least one charge transfer layer, at least one electrode, or at least one charge recombination layer.

(39) In some embodiments, the separating layer further comprises an interconnecting layer. The interconnecting layer may comprise a mixture of a wide energy gap material and either an electron conducting material or a hole conducting material. The wide energy gap material will block excitons while the electron conducting material or hole conducting material will transport electrons or holes, respectively. For example, in an exemplary optimized tandem device according to the present disclosure, shown in FIG. 11, the separating layer comprises a thin Ag layer (0.1 nm) (i.e., the charge recombination layer) and an interconnecting layer. In this particular embodiment, the interconnecting layer comprises a mixture of BPhen and C.sub.60. BPhen constitutes the wide energy gap material and blocks excitons, and C.sub.60 is an electron conducting material and transports electrons to the charge recombination layer.

(40) Thus, the wide energy gap material should have a HOMO-LUMO energy gap larger than the HOMO-LUMO gap of the nearest photoactive material, which in the particular embodiment of FIG. 11 is the acceptor C.sub.60. The electron conducting material should have a LUMO energy level equal to or lower than the LUMO energy level of the nearest photoactive material, which in the particular embodiment of FIG. 11 is the acceptor C.sub.60. In some embodiments, the electron conducting material comprises the same material as the nearest acceptor material. Thus, electron conducting materials may be chosen from the acceptor materials described herein and as known in the art. In a device configuration where a hole conducting material is appropriate, the hole conducting material should have a HOMO energy level higher than the HOMO energy level of the nearest photoactive material. In some embodiments, the hole conducting material comprises the same material as the nearest donor material.

(41) Suitable wide energy gap materials include, but are not limited to, bathocuproine (BCP), bathophenanthroline (BPhen), p-Bis(triphenylsilyl)benzene (UGH-2), (4,4′-N,N′-dicarbazole)biphenyl (CBP), N,N′-dicarbazolyl-3,5-benzene (mCP), poly(vinylcarbazole) (PVK), phenanthrene and alkyl and/or aryl substituted phenanthrenes, alkyl and/or aryl substituted derivatives of benzene, triphenylene and alkyl and/or aryl substituted triphenylenes, aza-substituted triphenylenes, oxidiazoles, triazoles, aryl-benzimidazoles, adamantane and alkyl and/or aryl substituted adamantanes, tetraarylmethane and its derivatives, 9,9-dialkyl-fluorene and its oligomers, 9,9-diaryl-fluorene and its oligomers, spiro-biphenyl and substituted derivatives, corannulene and its alkyl and/or aryl substituted derivatives, and derivatives thereof.

(42) It is also advantageous for the interconnecting layer to be substantially transparent, or ideally entirely transparent, across the spectral region absorbed by the subcells, so as to permit as much relevant light as possible to pass from the front subcell to the back subcell. By mixing a transparent wide energy gap material with either an electron conducting material or a hole conducting material, the electron or hole conducting material may be sufficiently diluted, rendering the interconnecting layer transparent or substantially transparent at relevant wavelengths resulting in improved device performance.

(43) Layers and materials may be deposited using techniques known in the art. For example, the layers and materials described herein can be deposited or co-deposited from a solution, vapor, or a combination of both. In some embodiments, organic materials or organic layers are deposited or co-deposited using vacuum evaporation, such as vacuum thermal evaporation, organic vapor phase deposition, or organic vapor-jet printing.

(44) It should be understood that embodiments described herein may be used in connection with a wide variety of structures. Functional organic photovoltaic devices may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Additional layers not specifically described may also be included. Materials other than those specifically described may be used. The names given to the various layers herein are not intended to be strictly limiting.

(45) Other than in the examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, analytical measurements and so forth, used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.

(46) Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, unless otherwise indicated the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

(47) The devices and methods described herein will be further described by the following non-limiting examples, which are intended to be purely exemplary.

Example 1: DTDCTB:C.SUB.60 .and DBP:C.SUB.70 .Subcells

(48) Materials and device fabrication. Devices were grown on indium tin oxide (ITO) coated glass substrates. Prior to use, DTDCTB, DBP, C.sub.60, and C.sub.70 were purified once using temperature-gradient sublimation. The ITO surface was cleaned in a series of detergents and solvents and treated with ultraviolet (UV)-ozone for 10 min before thin film deposition. Neat films were deposited using vacuum thermal evaporation in a chamber with a base pressure of ˜10.sup.−7 torr at a rate of 0.1 nm/2, except for the Ag nanoparticle charge recombination layer, which was deposited at 0.005 nm/s. The components of the DTDCTB:C.sub.60 and BPhen:C.sub.60 layers were co-deposited at 0.1 nm/s with the rate for each material adjusted to achieve the desired volume ratio. The components of DBP:C.sub.70 were co-deposited at 0.2 nm/s. The growth rates and thicknesses were monitored using quartz crystal monitors and calibrated by ex situ variable-angle spectroscopic ellipsometry. The 100 nm thick Ag cathodes were deposited through a shadow mask with an array of circular, 1 mm diameter openings that defined the device areas.

(49) Device characterization. Following cathode deposition, samples were transferred into a glove box filled with ultrapure (<0.1 ppm) N.sub.2 for testing. The current density-voltage (J-V) characteristics were measured at various incident light intensities using AM 1.5G solar illumination from a filtered Xe lamp, with intensities adjusted using neutral density filters. The intensity was measured using a National Renewable Energy Laboratory (NREL) traceable Si reference cell. The J.sub.SC and η.sub.P were corrected for spectral mismatch. The EQE was measured using monochromated light from a 200 Hz chopped Xe-lamp and calibrated with a NIST-traceable Si detector. Errors quoted correspond to device-to-device variations on the same substrate. The data for J.sub.SC and η.sub.P also include a systematic error of 5%.

(50) Optical simulations. Structure optimization and device performance simulations were carried out based on the transfer-matrix approach with the calculation of exciton diffusion and carrier collection lengths as parameters. The real and imaginary indices of refraction, n and k, respectively, for 30 nm thin films deposited on silicon, were measured by ellipsometry (at wavelengths of 300-1600 nm) using the Cauchy model with Gaussian oscillators. To determine the optimal structure for current matching and efficiency for the tandems, the light intensity dependent J-V data from the constituent subcells was used. The mismatch factors M for the subcells were calculated from the measured EQE. For the tandem cell, this is obtained from the simulated power conversion efficiency η.sub.P with the simulator spectrum divided by the reference AM 1.5G 1 sun solar spectrum.

(51) As embodied herein, a small molecule tandem solar cell was formed with a front subcell and a back subcell. The front subcell adjacent to the transparent anode comprised the primarily orange-to-NIR absorbing donor, 2-({7-(5-{dip-tolylamino)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) blended with C.sub.60 (together, DTDCTB:C.sub.60). The front subcell was paired with a UV-to-yellow absorbing dibenzo{[f,f′]-4,4′,7,7′-tetraphenyl}diindeno[1,2,3-cd:1′,2′,3′-lm]perylene (DBP) mixed with C.sub.70 (together, DBP:C.sub.70) back subcell.

(52) In the front subcell, the C.sub.60 intermolecular charge transfer (CT) absorption feature in the green was greatly reduced when diluted in DTDCTB, thus providing a spectrally complimentary system with the back sub-cell. Both sub-cells had single junction efficiencies of ≥6.0% and were current matched in the stack. As a result, the tandem solar cell broadly covered the solar spectrum from λ=350 nm to 900 nm, achieving an efficiency of 10.0±0.5% under standard illumination. The high efficiency utilized the principle of non-overlapping spectral sensitivity between subcells that is unique to excitonic cells, along with a nearly optically lossless BPhen:C.sub.60 electron filtering layer connecting the subcells.

(53) To determine the characteristics of the front subcell, a single junction DTDCTB:C.sub.60 cell was formed. The low band gap DTDCTB absorbs at wavelengths as long as λ=900 nm. To separate the front and back subcell absorption spectra, the blue-green absorbing C.sub.60 was used as the acceptor, whereas the broadly absorbing C.sub.70 was employed solely in the back subcell. The extinction coefficients (k) of the DTDCTB:C.sub.60 films measured as functions of volume ratio are shown in FIG. 3A. The absorption of a neat C.sub.70 film is also shown for comparison. The C.sub.60 showed two peaks at λ=360 nm and λ=450 nm, corresponding to Frenkel-type and intermolecular CT excitations. The CT feature results from electrons excited from the HOMO of one molecule to the LUMO of a nearby C.sub.60 molecule and is hence sensitive to C.sub.60 concentration. In contrast, the intramolecular Frenkel transition absorption strength is proportional to the total number of C.sub.60 molecules. The neat DTDCTB film exhibited a maximum absorption centered at λ=700 nm.

(54) To analyze the Frenkel and CT absorption in the DTDCTB:C.sub.60 mixed film, k at λ=450 nm and λ=700 nm as a function of the C.sub.60 percentage was plotted in the inset of FIG. 3A. The DTDCTB Frenkel absorption peak at λ=700 nm linearly decreased with C.sub.60 concentration, as expected. On the other hand, the C.sub.60 CT peak at λ=450 nm was significantly reduced even at modest dilutions, eliminating losses caused by subcell absorption overlap from the green-absorbing feature as the fraction of C.sub.60 was reduced.

(55) An optimized DTDCTB:C.sub.60 cell, shown in FIG. 3B, had the following structure: ITO/MoO.sub.3 (10 nm)/DTDCTB:C.sub.60 (60 nm, 1:1 ratio by vol.)/C.sub.60 (20 nm)/BPhen (8 nm)/Ag (100 nm). The MoO.sub.3 served as the anode buffer layer due to its large work function, high transmittance, and low resistance. The BPhen was used as the exciton blocking buffer layer adjacent to the cathode. The device exhibited V.sub.OC=0.82±0.01 V, a short circuit current density of J.sub.SC=13.7±0.7 mA/cm.sup.2 (spectral mismatch factor of M=0.93±0.01), and FF=0.55±0.01. This corresponds to a power conversion efficiency of 6.2±0.3% at standard illumination. At this blend ratio, the CT absorption by C.sub.60 was suppressed to only 40% of its value in the neat film. Accordingly, the DTDCTB:C.sub.60 cell showed an external quantum efficiency of EQE>55% at λ=700 nm, falling off to <25% at λ<500 nm as shown in FIG. 3B. As further shown, this NIR absorbing front subcell has minimal spectral overlap with the principally green-absorbing DBP:C.sub.70 back subcell.

(56) The tandem device is shown in FIG. 4A. To optimize the subcells, a single junction front-only cell approximating the DTDCTB:C.sub.60 subcell was fabricated by inserting a 0.1 nm thick Ag layer, used for charge recombination and plasmonic field enhancement, followed by a 40 nm thick MoO.sub.3 spacer located beneath the Ag contact. This is shown in FIG. 2a. Compared with the optimized single junction cell, the thickness of the neat C.sub.60 layer was reduced from 20 nm to 5 nm to move the front cell closer to the cathode to increase the absorption by the active DTDCTB:C.sub.60 layer and reduce the CT absorption of the C.sub.60 layer. The optimized structure of the back subcell, shown in FIG. 2b, was: ITO/MoO.sub.3 (5 nm)/DBP:C.sub.70 (30 nm, 1:10 ratio by vol.)/C.sub.70 (7 nm)/BPhen (7 nm)/Ag (100 nm), which resulted in J.sub.SC=11.3±0.6 mA/cm.sub.2 (M=1.00±0.01), V.sub.OC=0.90±0.01 V, FF=0.61±0.01 and η.sub.P=6.2±0.3%. The thickness of the DBP:C.sub.70 mixed layer was increased in comparison to other iterations from 25 nm to 30 nm. This increased the back subcell absorption to match J.sub.SC with the front DTDCTB:C.sub.60 subcell.

(57) For the absorption ability of each subcell: the extinction coefficient of the 1:1 DTDCTB:C.sub.60 and the 1:10 DBP:C.sub.70 blends, along with the AM 1.5G solar spectrum, are shown in FIG. 4B. The DBP:C.sub.70 film exhibited a broad spectral response at λ<700 nm (blue-yellow range), while the DTDCTB:C.sub.60 layer primarily absorbs from λ=500 nm to 900 nm (red and NIR). Stacking these two subcells allowed absorption to span the wavelengths from λ=350 nm to 900 nm. This allowed coverage of a large portion of the solar spectrum with only minimal overlap between the constituent subcells. The tandem thus harvested light efficiently with good current match between subcells.

(58) FIG. 4C shows the EQE spectrum of the tandem OPV with the commonly used 3,4,9,10-perylenetetracarboxylic-bisbenzimidazole (PTCBI) used as the buffer layer interconnecting the front and back subcells. Both the measured EQEs of the subcells agreed with calculated DBP:C.sub.70 subcell EQE>60% at λ<600 nm and calculated DTDCTB:C.sub.60 subcell peak EQE=50% at λ=700 nm. Compared with previous small molecule tandems, the DTDCTB:C.sub.60 front cell had a higher response in the orange-to-NIR spectral region and was nearly transparent at λ<500 nm. Thus the front cell with the NIR material leaves a “hole” space for blue-yellow light that allows the back subcell to absorb efficiently.

(59) The black line in FIG. 4C shows the calculated EQE of the tandem cell. The sum of the measured EQE of the subcells is shown by a dashed line. The measured tandem cell EQE was higher than that measured for both individual component subcells and was nearly identical to their sum, with the exception of a <10% loss between λ=550 nm and 700 nm where the subcells exhibited a small absorption overlap. The tandem device performance parameters were: J.sub.SC=9.2±0.4 mA/cm.sup.2 (M=0.96±0.01), V.sub.OC=1.72±0.01 V, FF=0.58±0.01. This resulted in an initial η.sub.P=9.2±0.4%.

(60) To improve device performance, the transparent exciton blocking and electron conducting BPhen:C.sub.60 electron filter was used as the interconnect layer. FIG. 5A shows the simulated optical field distribution within the tandem cell, comparing a 5 nm thick conventional PTCBI with a similarly thick BPhen:C.sub.60 mixed buffer. The lower portion is the front cell and the top portion is the back cell. The dashed rectangular region indicates the position of the interconnecting layer. As FIG. 5A shows, the BPhen:C.sub.60 interconnecting layer is transparent, in striking contrast with the PTCBI buffer. The simulation also showed that both subcells fit within the first interference maximum of the optical field.

(61) FIG. 5B shows the fourth quadrant J-V characteristics of the tandem and front cell utilizing PTCBI and BPhen:C.sub.60 as the interconnecting layer. As expected, when PTCBI was replaced with BPhen:C.sub.60, J.sub.SC increased from 11.6±0.6 mA/cm.sup.2 to 12.3±0.6 mA/cm.sup.2 in the front subcell, with no significant change in FF. Hence, the efficiency of the front subcell increased from 5.7±0.3% to 6.0±0.3%. Additionally, as shown in FIG. 5A, the optical field in the DBP:C.sub.70 subcell was enhanced when using BPhen:C.sub.60 leading to a corresponding increase in current. Consequently, J.sub.SC of the tandem cell was increased to 9.9±0.5 mA/cm.sup.2, whereas the V.sub.OC and FF remained unchanged.

(62) FIG. 6 shows the fourth quadrant J-V characteristics results for another tandem cell with the DTDCTB:C.sub.60 layer reduced by 5 mm (as in FIG. 2c). The tandem control cell with PTCBI had J.sub.SC=9.3±0.4 mA/cm.sup.2, V.sub.OC=1.73±0.01 V, and FF=0.58±0.01%. The overall efficiency was 9.3±0.5% with a mismatch factor of 0.96. Compared with the control cell, the tandem cell with a BPhen:C.sub.60 mixed buffer as the interconnecting layer has a higher J.sub.SC of 10.2±0.4 mA/cm.sup.2, and V.sub.OC=1.73±0.01 V (the same). Though the FF dropped to 0.56±0.01 compared with the former cell, the overall efficiency was increased to 9.9±0.5% owing to the increase in J.sub.SC being more than enough to compensate for the lower FF. The detailed data of the spectrally corrected parameters of different front, back and tandem cells is presented in Table 1.

(63) TABLE-US-00001 TABLE 1 Spectrally corrected device performance of the front DTDCTB:C.sub.60 PM-HJ cell, , the back DBP:C.sub.70 PM-HJ subcell and Tandem cell with different interconnecting layers. V.sub.OC J.sub.SC PCE (%) Devices (V) (mA/cm.sup.2) FF AM 1.5 G M Front 0.83 ± 0.01 11.9 ± 0.6 0.59 ± 0.01 5.8 ± 0.2 0.93 (PTCBI) Front 0.83 ± 0.01 12.8 ± 0.6 0.57 ± 0.01 6.1 ± 0.3 0.92 (BPhen:C.sub.60) Back 0.90 ± 0.01 11.3 ± 0.5 0.60 ± 0.01 6.1 ± 0.3 1.00 Tandem 1.73 ± 0.01  9.3 ± 0.4 0.58 ± 0.01 9.3 ± 0.5 0.96 (PTCBI) Tandem 1.73 ± 0.01 10.2 ± 0.4 0.56 ± 0.01 9.9 ± 0.5 0.95 (BPhen:C.sub.60)

(64) A thin layer of silver was also employed in between the two cells to act as a recombination center of electrons and holes in order to make the current flow. Using BPhen:C.sub.60 as an interconnecting layer along with Ag nanoparticles as a charge recombination layer, optical modelling suggested that the optimized tandem cell should employ a front sub-cell mixed layer thickness of 55 nm to 60 nm, and a back subcell thickness between 30 nm and 35 nm. The resulting experimental cell, whose structure and optimized layer thicknesses are shown in FIG. 4A matched the modeled performance summarized in Table 1.

(65) The optimized tandem cell EQEs, both simulated and measured, are shown in FIG. 7A. The optimized tandem cell EQE using a BPhen:C.sub.60 interconnecting layer is similar to that of the tandem employing a conventional PTCBI buffer, although the measured front subcell peak EQE increased from 50% to 53% (Compare FIG. 4C). The solid black line, showing the modeled EQE of the tandem cell, shows a fairly evenly distributed conversion efficiency over the visible and NIR spectrum.

(66) FIG. 7B shows the measured and calculated 4th quadrant J-V characteristics. The experimental characteristics of the individual subcells were measured at 100 mW/cm.sup.2 where both have an efficiency of ˜6.0%. The calculated tandem J-V characteristics agreed with the measurements, suggesting that the models of the optical field distribution and the charge collection are predictive of performance, thereby simplifying future device layer thickness design. The optimized tandem OPV cell achieved J.sub.SC=9.9±0.5 mA/cm.sup.2 (M=0.95±0.01), V.sub.OC=1.72±0.01 V, FF=0.59±0.01, with η.sub.P=10.0±0.5%. This represented an approximately 60% improvement over the discrete cell efficiencies comprising the stack. Furthermore, the tandem V.sub.OC was equal to the sum of the constituent sub-cells, suggesting that the interconnecting charge recombination layer used was lossless.

(67) FIG. 8 shows the device performance parameters under different light intensities. FIG. 8A depicts the performance of the tandem with PTCBI and 8B shows the that of the tandem with BPhen:C.sub.60 as the interconnecting layer. In both cases, V.sub.OC increased linearly with increasing light intensity. FF also had an upward trend. The photo responsivity decrease with increasing light intensity, however, caused an overall decreasing trend of power conversion efficiency. It is also worth noting that the efficiency of the tandem device with PTCBI decreases more slowly than that of the BPhen:C.sub.60.

Example 2: High Intensity with DTDCTB:C.SUB.60 .and DBP:C.SUB.70 .Subcells

(68) It has been shown that optimized tandem structures employ subcells whose currents are approximately matched at the maximum power point (MPP) of operation. The power conversion efficiency penalty that is related to any mismatch in constituent subcell efficiencies is defined as:
Δη=1−(J.sub.MTV.sub.MT)/(J.sub.M1V.sub.M1+J.sub.M2V.sub.M2)

(69) The optimal design corresponds to Δη=0. Here, J.sub.M and V.sub.M refer to the current density and voltage at the MPP. The subscript T refers to the tandem, and 1,2 refer to the two subcells. According to the simulated J-V characteristics of the subcells in FIG. 7B, the optimized tandem device in the previous example embodiment has Δη=0.6%. This is close to the ideal scenario where J.sub.MT=J.sub.M1=J.sub.M2 and M.sub.MT=V.sub.M1=V.sub.M2.

(70) In one further tandem cell structure, the BPhen cathode buffer (7 nm, below the AG contact) was replaced with a high electron conductivity, exciton blocking compound BPhen:C.sub.60 (5 nm)/BPhen (2 nm) electron filter that reduces bimolecular recombination at the buffer/acceptor interface. The electron filter increased η.sub.P of the back cell, but also increased the FF and J.sub.SC differences between the sub-cells, which ultimately caused a larger mismatch. The tandem efficiency was 9.6±0.5%, since J.sub.SC decreased to 9.0±0.4 mA/cm.sup.2. The current mismatch resulted in Δη=4.8%; a slight reduction in efficiency from the tandem device in Example 1. The reduced tendency for bimolecular recombination and exciton-polaron quenching suggested, however, that the use of this compound buffer could result in improved response at high intensity.

(71) Thus, the responsivity and η.sub.P of the tandem solar cells was measured, with BPhen or BPhen:C.sub.60/BPhen as the cathode buffer layers with intensities ranging from 0.2 to 4 suns. The results are shown in FIG. 9. The responsivity of the tandem cell with BPhen decreases from 0.112±0.005 NW to 0.090±0.004 A/W, while the tandem with the mixed buffer shows only a minor (5%) change from 0.092±0.004 NW to 0.087±0.004 A/W. The tandem cell with the mixed cathode buffer shows no efficiency change up to 1 sun intensity, after which it decreases due to series resistance. At 4 suns, both solar cells exhibit a power conversion efficiency of 9.0%. Accordingly, use of the mixed cathode buffer in the form of BPhen:C.sub.60/BPhen may be preferred in applications involving high intensities.

Example 3: DTDCTB:C.SUB.60 .and DTDCPB:C.SUB.70 .Subcells

(72) As further embodied herein, a small molecule tandem solar cell may be formed with a front subcell and a back subcell. The front subcell adjacent to the transparent anode may comprise the primarily orange-to-NIR absorbing donor, (DTDCTB) blended with C.sub.60 (DTDCTB:C.sub.60). The front subcell may be paired with a UV-to-yellow absorbing (2-[(7-{4-[N,N-Bis(4-methylphenyl)amino]phenyl}-2,1,3-benzothiadiazol-4-yl)methylene]propanedinitrile (DTDCPB) mixed with C.sub.70 (DTDCPB:C.sub.70) back subcell. The chemical structure of DTDCPB is shown in FIG. 10.

(73) In the front subcell, the C.sub.60 intermolecular charge transfer (CT) absorption feature in the green is greatly reduced when diluted in DTDCTB, thus providing a spectrally complimentary system with the back sub-cell. The resulting tandem solar cell broadly covers the solar spectrum from λ=350 nm to 900 nm, achieving a simulated efficiency of 9.8% under standard illumination. The high efficiency again utilizes the principle of non-overlapping spectral sensitivity between subcells that is unique to excitonic cells, along with a nearly optically lossless BPhen:C.sub.60 electron filtering layer connecting the subcells.

(74) The tandem device is shown in FIG. 11. For the absorption ability of each subcell: the extinction coefficient of the 1:1 DTDCTB:C.sub.60 and the 1:1 DTDCPB:C.sub.70 blends, along with the AM 1.5G solar spectrum, are shown in FIG. 12. The DTDCPB:C.sub.70 film exhibits a broad spectral response at λ<700 nm (blue-yellow range), while the DTDCTB:C.sub.60 layer primarily absorbs from λ=500 nm to 900 nm (red and NIR). Stacking these two subcells allowed absorption to span the wavelengths from λ=350 nm to 900 nm. This allowed coverage of a large portion of the solar spectrum with only minimal overlap between the constituent subcells. The tandem thus harvested light efficiently with good current match between subcells.

(75) FIG. 13 shows the simulated EQE spectrum of the tandem OPV. As before, the DTDCTB:C.sub.60 front cell has a higher response in the orange-to-NIR spectral region and is nearly transparent at λ<500 nm. Thus the front cell with the NIR material laves a “hole” space for blue-yellow light that allows the back subcell to absorb efficiently. The black line in FIG. 13 shows the calculated EQE of the tandem cell. The tandem device's calculated performance parameters are: J.sub.SC=9.1 mA/cm.sup.2 V.sub.OC=1.72, FF=0.62. This results in an η.sub.P=9.8%.

(76) To improve device performance, the transparent exciton blocking and electron conducting BPhen:C.sub.60 electron filter is again used as the interconnect layer. FIG. 14 shows the simulated optical field distribution within the tandem cell with a 5 nm thick BPhen:C.sub.60 mixed buffer. As shown, the BPhen:C.sub.60 interconnecting layer is transparent, allowing the light from the front cell to pass unabsorbed to the back cell.

(77) FIG. 15 shows the fourth quadrant J-V characteristics of the tandem, front, and back cell of this example. The detailed data of the spectrally corrected parameters of different front, back and tandem cells is presented in Table 2. The optimized tandem cell efficiency of 9.8% represents a significant improvement over the individual cells comprising the stack, as shown in Table 2. Furthermore, the tandem V.sub.OC is equal to the sum of the constituent sub-cells, suggesting that the interconnecting charge recombination layer used was lossless.

(78) TABLE-US-00002 TABLE 2 Cells J.sub.SC (mA/cm.sup.2) V.sub.OC (V) FF PCE (%) Front 9.1 0.82 0.57 4.3 Back 9.1 0.90 0.68 5.6 Tandem 9.1 1.72 0.62 9.8