TECHNICAL FIELD
[0001] The present disclosure relates to the field of photovoltaic devices. In particular, the present disclosure provides a two-terminal monolithic tandem solar cell.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed disclosure, or that any publication specifically or implicitly referenced is prior art.
[0003] In the recent era, thin-film silicon solar cell technology has become one of the most promising approach which attained great attention in the photovoltaic (PV) industry. To date, the highest power conversion efficiency (PCE) (more than 10%) is recorded by the state of the art single-junction PV devices in which thin-film silicon material is utilized. However, unabsorbed solar spectra and thermalization losses are the major hindrances of the single junction PV devices. Approximately 58% energy losses are contributed by the above-mentioned phenomena, which is in addition to further losses such as collection and reflection, resulting in the fundamental limit, i.e., Shockley-Queisser limit of ~33% for single-junction PV devices.
[0004] In order to surpass this limit, multi-junction or tandem solar cell (TSC) are employed in the PV industry to mitigate transmission and thermalization losses. It is worth noting that tandem devices designed using thin-film Si have shown the potential of higher conversion efficiency and overcome the downsides mentioned above for the single junction PV devices such as transmission and thermalization losses. However, the conventional TSC cannot function efficiently if materials of subcells of the TSC are not having certain relative energy levels with respect to each other, selection of materials, with random energy levels, for designing of the subcells may even lead to short-circuiting of the TSC, and the circuit associated with the TSC.
[0005] There is, therefore, a need in the art to provide a well-fabricated and efficient two-terminal monolithic tandem solar cell that can obviate the above mentioned limitations.

OBJECTS OF THE PRESENT DISCLOSURE
[0006] Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.
[0007] It is an object of the present disclosure to provide a tandem solar device including stacks of different bandgap absorber materials.
[0008] It is an object of the present disclosure to provide a tandem solar device having minimal thermalization/ transmission losses.
[0009] It is an object of the present disclosure to provide a tandem solar device having minimal non-absorption losses.
[0010] It is an object of the present disclosure to provide a flexible, tunable bandgap, cost-efficient two-terminal monolithic tandem solar device.

SUMMARY
[0011] Aspects of the present disclosure relates to the field of photovoltaic devices. In particular, the present disclosure provides a two-terminal monolithic tandem solar cell.
[0012] According to an aspect, the present disclosure discloses a two-terminal tandem solar device comprising: a top subcell configured to absorb first set of photons; a bottom subcell configured to absorb second set of photons; and an interlayer configured between the top subcell and the bottom subcell in order to achieve current-matching between the top subcell and the bottom subcell of the tandem solar device.
[0013] In an aspect, the solar device may comprise hydrogenated amorphous silicon (a-Si:H) based top subcell including Zinc Oxide (ZnO)| p-doped a-Si:H (p-a-Si:H)| intrinsic a-Si:H (i-a-Si:H)| n-doped a-Si:H (n-a-Si:H) architecture.
[0014] In an aspect, the solar device may comprise lead sulfide (PbS) carbon quantum dot (CQD) based bottom subcell including PbS-EDT(1, 2-ethanedithiol)| PbS-TBAI(tetrabutylammonium iodide)| MZO (Magnesium-doped Zinc Oxide)| ZnO| Silver (Ag)| Aluminum (Al) architecture.
[0015] In an aspect, the interlayer may be designed using Indium tin oxide (ITO).
[0016] In an aspect, the first set of photons, absorbed by the top subcell, are having a first pre-defined energy level, and the second set of photons, absorbed by the bottom subcell , are having a second pre-defined energy level; wherein, the first pre-defined energy level is higher than the second pre-defined energy level.
[0017] In an aspect, the solar device may comprise an inverted bottom subcell, in order to obtain unidirectional motion of electrons between the top subcell and the bottom subcell.
[0018] In an aspect, the top subcell may be configured to reduce thermalization losses, and the bottom subcell may be configured to reduce non-absorption losses of the solar device.
[0019] In an aspect, the solar device may comprise back reflectors for reducing unwanted heat in the solar device.
[0020] In an aspect, the back reflectors may comprise ZnO and Ag conductors.
[0021] In an aspect, the solar device comprises ZnO as an anode configured with the top subcell and aluminum (Al) as a cathode configured with the bottom subcell.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0022] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0023] The diagrams described herein are for illustration only, which thus are not limitations of the present disclosure, and wherein:
[0024] FIG. 1 illustrates a diagram representing structure of the proposed tandem solar device, to illustrate its overall working, in accordance with an embodiment of the present disclosure.
[0025] FIG. 2 illustrates graphs depicting impact of thickness of top subcell on EQE and J-V curve of the solar device, respectively, in accordance with an embodiment of the present disclosure.
[0026] FIG. 3 illustrates graphs depicting relation between various PV parameters and thickness of the top subcell of the solar device, in accordance with an embodiment of the present disclosure.
[0027] FIG. 4 illustrates graphs depicting impact of thickness of bottom subcell on EQE and J-V curve of the solar device, respectively, in accordance with an embodiment of the present disclosure.
[0028] FIG. 5 illustrates graphs depicting relation between various PV parameters and thickness of the bottom subcell of the solar device, in accordance with an embodiment of the present disclosure.
[0029] FIG. 6 illustrates J-V curve of standalone top/inverted bottom cell and tandem cell, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0030] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.
[0031] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[0032] As used herein the description and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0033] While embodiments of the present invention have been illustrated and described in the accompanying drawings, the embodiments are offered only in as much detail as to clearly communicate the disclosure and are not intended to limit the numerous equivalents, changes, variations, substitutions and modifications falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0034] Embodiments of the present disclosure relates to the field of photovoltaic devices. In particular, the present disclosure provides a two-terminal monolithic tandem solar cell.
[0035] According to an embodiment, the present disclosure discloses a two-terminal tandem solar device including: a top subcell configured to absorb first set of photons; a bottom subcell configured to absorb second set of photons; and an interlayer configured between the top subcell and the bottom subcell in order to achieve current-matching between the top subcell and the bottom subcell of the tandem solar device.
[0036] In an embodiment, the solar device can include hydrogenated amorphous silicon (a-Si:H) based top subcell including Zinc Oxide (ZnO)| p-doped a-Si:H (p-a-Si:H)| intrinsic a-Si:H (i-a-Si:H)| n-doped a-Si:H (n-a-Si:H) architecture.
[0037] In an embodiment, the solar device can include lead sulfide (PbS) carbon quantum dot (CQD) based bottom subcell including PbS-EDT(1, 2-ethanedithiol)| PbS-TBAI(tetrabutylammonium iodide)| MZO (Magnesium-doped Zinc Oxide)| ZnO| Silver (Ag)| Aluminum (Al) architecture.
[0038] In an embodiment, the interlayer can be designed using Indium tin oxide (ITO).
[0039] In an embodiment, the first set of photons, absorbed by the top subcell , are having a first pre-defined energy level, and the second set of photons, absorbed by the bottom subcell , are having a second pre-defined energy level; wherein, the first pre-defined energy level is higher than the second pre-defined energy level.
[0040] In an embodiment, the solar device can include an inverted bottom subcell, in order to obtain unidirectional motion of electrons between the top subcell and the bottom subcell.
[0041] In an embodiment, the top subcell can be configured to reduce thermalization losses, and the bottom subcell may be configured to reduce non-absorption losses of the solar device.
[0042] In an embodiment, the solar device can include back reflectors for reducing unwanted heat in the solar device.
[0043] In an embodiment, the back reflectors can include ZnO and Ag conductors.
[0044] In an embodiment, the solar device can include ZnO as an anode configured with the top subcell and aluminum (Al) as a cathode configured with the bottom subcell.
[0045] FIG. 1 illustrates a diagram representing structure of the proposed tandem solar device 100, to illustrate its overall working, in accordance with an embodiment of the present disclosure.
[0046] Primarily, TSC is the combination of different single-junction devices that responds to higher efficiencies with minimum thermalization/ transmission losses by stacking different wide/narrow bandgap absorber materials. Conventionally in tandem devices, higher and lower energy photons are absorbed by top and bottom subcells, respectively, which results in higher efficiency. In this regard, to resolve the above-mentioned complications associated with single junction devices, a new thin-film a-Si:H (Eg=1.7eV)/ PbS CQD (Eg=1.2eV) based 2-terminal monolithic tandem solar device is proposed in the present disclosure. Moreover, the focus of the present work is to project a flexible, tunable bandgap, cost-efficient 2-terminal monolithic tandem solar cell device by using two different appropriate bandgap absorber materials, i.e., a-Si:H/PbS CQD.
[0047] According to an embodiment, the proposed tandem solar device 100 (also referred to as solar device 100, or tandem solar cell 100, or solar cell 100, herein) can include a top subcell 102 and a bottom subcell 104, where the top subcell 102 and the bottom subcell 104 can be electronically coupled to each other through an interlayer 106. In one embodiment, the TRJ layer 106 can be configured between the top subcell 102 and the bottom subcell 104, such that it can facilitate current-matching between the top subcell 102 and the bottom subcell 104 of the tandem solar device 100. Further, to increase the efficiency of the tandem solar device 100, different absorber materials have been used for the effective utilization of both lower and higher wavelength photons.
[0048] In an embodiment, the top subcell 102 can be amorphous silicon (a-Si: H) based and configured to absorb a first set of photons having a first pre-defined energy level. In an embodiment, the solar device 100 can include hydrogenated amorphous silicon (a-Si:H) based top subcell including Zinc Oxide (ZnO)| p-doped a-Si:H (p-a-Si:H)| intrinsic a-Si:H (i-a-Si:H)| n-doped a-Si:H (n-a-Si:H) architecture. Further, the top subcell 102 can be configured to reduce thermalization losses of the solar device 100.
[0049] In an embodiment, the bottom subcell 104 can be lead sulfide carbon quantum dot PbS (CQD) based and configured to absorb a second set of photons having a second pre-defined energy level, where the first pre-defined energy level is higher than the second pre-defined energy level. For example, the first pre-defined energy level can be 1.7 electron-volt (eV) and the second pre-defined energy level can be 1.2 eV. In another embodiment, the solar device 100 can include lead sulfide (PbS) carbon quantum dot (CQD) based bottom subcell including PbS-EDT(1, 2-ethanedithiol)| PbS-TBAI(tetrabutylammonium iodide)| MZO (Magnesium-doped Zinc Oxide)| ZnO| Silver (Ag)| Aluminum (Al) architecture. Further, the bottom subcell 104 can be configured to reduce non-absorption losses of the solar device 100.
[0050] In an exemplary embodiment, the solar device 100 can accommodate the inverted bottom subcell 104, which can enable unidirectional motion of electrons between the top subcell 102 and the bottom subcell 104.
[0051] In an embodiment, the interlayer 106 can be designed using Indium tin oxide (ITO), which can act as a common electrode between the top subcell 102 and the bottom subcell 104, and thereby can enable current matching between both the subcells. In an exemplary embodiment, a 10 nanometre (nm) thick ITO layer can be used as the interlayer 106 in the proposed tandem solar device 100. In another exemplary embodiment, an appropriate lumped resistance (say, of 1016 ohms (?)) can be added to the common electrode to prevent the flow of current in the common electrode and to force the current to flow from anode to cathode. Physically, it can justified by the fact that the common electrode is acting like a resistor which allows the flow of current between anode and cathode without significant limitations and hence can be correlated with tunnel recombination junction (TRJ).
[0052] In another embodiment, the solar device 100 can include back reflectors for reducing unwanted heat in the solar device 100, where the back reflectors can be configured from ZnO and Ag conductors.
[0053] In yet another embodiment, the solar device 100 can include ZnO as an anode configured with the top subcell and aluminum (Al) as a cathode configured with the bottom subcell.
[0054] In a first embodiment, performance of the tandem solar device 100 can be analyzed by varying top subcell 102 absorber layer thickness (TOPALT) from 50 nm to 350 nm at constant bottom subcell 104 absorber layer thickness (BOTALT) of 200 nm (optimized thickness of standalone bottom subcell section). As illustrated in FIG. 2, graph (a) shows external quantum efficiency (EQE) curve for both, top and bottom subcells in tandem configuration. Further, graph (b), of the FIG. 2, depicts J-V curve of the overall tandem solar cell for the varying TOPALT. Moreover, FIG. 3 (a-d) summarizes PV parameters, including short-circuit current density (JSC), open-circuit voltage (VOC), form factor (FF) and PCE of the tandem solar device 100. In an embodiment, higher efficiency is obtained at optimized top subcell absorber layer thickness (TOPALT) of 200 nm.
[0055] In a second embodiment, analysis and optimization of PbS CQD inverted bottom subcell based on the impact of BOTALT can be performed. To analyze the solar device 100 performance, BOTALT can be varied from 50 nm to 350 nm at constant TOPALT of 200 nm. As illustrated in FIG. 4, graph (a) shows external quantum efficiency (EQE) curve for both, top and bottom subcells in tandem configuration. Further, graph (b), of the FIG. 4, depicts J-V curve of the overall tandem solar cell for the varying BOTALT. Moreover, FIG. 5 (a-d) summarizes PV parameters, including JSC, VOC, FF, and PCE of the tandem solar device 100. In an embodiment, higher efficiency is obtained at optimized bottom subcell absorber layer thickness (BOTALT) of 150 nm.
[0056] Hence, it can be concluded that 12% efficient a-Si:H|PbS-CQD based 2-terminal monolithic tandem solar device 100 with TOPALT and BOTALT value of 200 nm and 150 nm, respectively, can be the optimum device with JSC (10.5 mA.cm-2), VOC (1.59 V), FF (71.64%), and PCE (12%).
[0057] Fig. 6 illustrates J-V curve of standalone top/inverted bottom cell and tandem cell, in accordance with an embodiment of the present disclosure.
[0058] In an embodiment, from the individually obtained J-V curve of standalone cells and tandem cell, it can be clearly observed that the top subcell with minimum current density acts as a current limiting cell in comparison with bottom subcell. Therefore, JSC value of tandem solar device 100 is approximately equal to the JSC of the top subcell. Whereas, tandem VOC (1.59V) is equivalent to the sum of individual VOC of top and bottom subcells.
[0059] In an embodiment, the advantage of the proposed solar device 100 is that the material employed for top and bottom subcells such as a-Si:H has higher absorption coefficient (>105 cm-1), adjustable wide bandgap (1.6-1.9 eV), low deposition temperature (<3000C), which provides ease of manufacturing with low cost and flexibility. Besides, PbS CQDs are employed owing to flexibility, high stability and, an ultra-thin film with size-dependent tunable bandgap.
[0060] In addition, PbS CQD also offers the utilization of photons in the near-infrared region (NIR), which contributes 20% of the overall solar spectrum energy. Thus, the usage of PbS CQD with other wide bandgap materials in tandem solar cells could be a feasible approach to absorb the low energy photons of the solar spectra. Studies carried out over here in this work for a-Si:H/PbS CQD based 2-terminal monolithic tandem solar cell structure may pave a way for exploring this field even more deeply. It could also help in possible fabrication of solar cell structures with even higher efficiencies along with a major benefit of relative ease of fabrication than the other counterparts in the technology currently available.
[0061] The above results and values can also be verified with results and values obtained using simulation. In an exemplary embodiment, a simulation tool, for example, Silvaco ATLAS device simulator tool, can be used to design the a-Si:H/PbS CQD tandem solar cell, where two different materials with bandgap 1.7eV (a-Si:H) and 1.2eV (PbS CQD) are intended for tandem design of top and bottom subcells. The subcells can be configured to absorb the higher and lower energy photons of AM1.5 solar spectrum, respectively. Moreover, an ITO Interlayer can be configured in between the top and bottom subcells to provide the current matching between both the subcells of the tandem solar device. ZnO (100 nm) and Ag (100 nm) conductors can also be configured as a back reflector to reduce the unwanted heat in the tandem solar cell.
[0062] As used herein, and unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms "coupled to" and "coupled with" are used synonymously. Within the context of this document terms "coupled to" and "coupled with" are also used euphemistically to mean “communicatively coupled with” over a network, where two or more devices are able to exchange data with each other over the network, possibly via one or more intermediary device.
[0063] The terms, descriptions and figures used herein are set forth by way of illustration only. Many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated.
[0064] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE INVENTION
[0065] The present disclosure provides a tandem solar device including stacks of different bandgap absorber materials.
[0066] The present disclosure provides a tandem solar device having minimal thermalization/ transmission losses.
[0067] The present disclosure provides a tandem solar device having minimal non-absorption losses.
[0068] The present disclosure provides a flexible, tunable bandgap, cost-efficient two-terminal monolithic tandem solar device.

We Claims:

1. A two-terminal tandem solar device comprising:
a top subcell configured to absorb first set of photons;
a bottom subcell configured to absorb second set of photons; and
an interlayer configured between the top subcell and the bottom subcell in order to achieve current-matching between the top subcell and the bottom subcell of the tandem solar device.
2. The solar device as claimed in claim 1, wherein the solar device comprises hydrogenated amorphous silicon (a-Si:H) based top subcell including Zinc Oxide (ZnO)| p-doped a-Si:H (p-a-Si:H)| intrinsic a-Si:H (i-a-Si:H)| n-doped a-Si:H (n-a-Si:H) architecture.
3. The solar device as claimed in claim 1, wherein the solar device comprises lead sulfide (PbS) carbon quantum dot (CQD) based bottom subcell including PbS-EDT(1, 2-ethanedithiol)| PbS-TBAI(tetrabutylammonium iodide)| MZO (Magnesium-doped Zinc Oxide)| ZnO| Silver (Ag)| Aluminum (Al) architecture.
4. The solar device as claimed in claim 1, wherein the interlayer is designed using Indium tin oxide (ITO).
5. The solar device as claimed in claim 1, wherein the first set of photons, absorbed by the top subcell , are having a first pre-defined energy level, and the second set of photons, absorbed by the bottom subcell , are having a second pre-defined energy level;
wherein, the first pre-defined energy level is higher than the second pre-defined energy level.
6. The solar device as claimed in claim 1, wherein the solar device comprises an inverted bottom subcell, in order to obtain unidirectional motion of electrons between the top subcell and the bottom subcell.
7. The solar device as claimed in claim 1, wherein the top subcell is configured to reduce thermalization losses, and the bottom subcell is configured to reduce non-absorption losses of the solar device.
8. The solar device as claimed in claim 1, wherein the solar device comprises back reflectors for reducing unwanted heat in the solar device.
9. The solar device as claimed in claim 8, wherein the back reflectors comprise ZnO and Ag conductors.
10. The solar device as claimed in claim 1, wherein the solar device comprises ZnO as an anode configured with the top subcell and aluminum (Al) as a cathode configured with the bottom subcell.