Efficient Top-Illuminated Organic-Quantum Dots Hybrid Tandem Solar

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Efficient Top-Illuminated Organic-Quantum Dots Hybrid Tandem Solar Cells with Complementary Absorption Jinhui Tong, Xiaokun Yang, Yang Xu, Weiwei Li, Jiang Tang, Haisheng Song, and Yinhua Zhou ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00045 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 12, 2017

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ACS Photonics

Efficient Top-Illuminated Organic-Quantum Dots Hybrid Tandem Solar Cells with Complementary Absorption

Jinhui Tong,†,# Xiaokun Yang,†, # Yang Xu,† Weiwei Li,‡ Jiang Tang,† Haisheng Song, *,† and Yinhua Zhou *,†,║

†

Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic

Information, Huazhong University of Science and Technology, Wuhan 430074, China ‡

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ║

Research Institute of Huazhong University of Science and Technology in Shenzhen,

Shenzhen 518057, China

Corresponding authors: *E-mail address: [email protected] (Y.H.Z.), [email protected] (H.S.S.)

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Abstract Organic and quantum dots (QDs) semiconductors are promising to build low-cost hybrid tandem solar cells since they are both fully solution-processable, and have tunable bandgaps and absorption spectra. The challenges for high-performance organic-QDs tandem solar cells are to balance the photocurrent in sub-cells and construct an efficient charge-recombination layer (CRL) to maximize the efficiency of the whole tandem cell. In this work, we report a top illuminated organic-QDs hybrid tandem solar cell that employs an organic-based front sub-cell and a PbS QDs-based back sub-cell where the organic absorber complements the absorption deficiency of QDs film in the range of 650-900 nm. The hybrid tandem solar cell is

monolithically

integrated

and

electrically

connected

with

a

Spiro-MeOTAD/MoO3/Ag/PEIE CRL. A conversion efficiency of 7.4% is achieved for the hybrid tandem cells. The tandem solar cells exhibit an open-circuit voltage of 1.12 V, which is nearly the sum of the VOC of individual sub-cells, and a fill factor up to 56%, confirming the effectiveness of CRL for building organic-QD hybrid tandem cells.

Keywords: hybrid tandem solar cells; complementary absorption; top-illuminated structure; charge recombination layer

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Solar cell technology has been considered as one of the most effective solution to produce clean energy. Among them, solution-processed solar cells such as quantum-dots (QDs) and organic have drawn intense attention due to their low-temperature processing, mechanical flexibility and compatibility with roll-to-roll manufacturing.1-6 Pursuing high efficiency is important for a photovoltaic device. However, solar cells with a single light-absorbing layer are only capable to utilize a fraction of the solar spectrum, limiting the device efficiency.7-9 To improve the efficiency of solution-processed solar cells, an effective method is to build tandem structure.10-15 Tandem devices comprising materials with complementary absorption spectra can extend the light absorption spectra and therefore obtain high power conversion efficiency (PCE).12, 16-19 For series-connected tandem solar cells, they can deliver a higher open-circuit voltage (VOC), nearly sum of the sub-cells to enrich their application fields. PbS QDs have a size-dependent and tunable bandgap, and display a wide absorption range from the visible to the near infrared.20, 21 However, due to the discrete energy level of QDs, there exists an absorption valley in the QDs-based films that loses the light harvesting efficiency. For example, the QDs we used here have a size and bandgap of about ~3 nm and 1.38 eV respectively. However, there is an absorption valley exists in the range of 700-850 nm as shown in Figure 1a. It is possible to achieve higher solar cell performance when smaller bandgap QDs are used in the tandem cells. The reason why we use the PbS QDs with a size of 3 nm is because the 3-nm QDs deliver high power conversion efficiency up to 10.6%,1 but still with an EQE valley. The work here is try to compensate the EQE valley with organic active layers. In Figure 1a, the PbS QDs showed the first excitonic peak centered at 3


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900 nm, which corresponded to a mean diameter of 2.9 nm based on the empirical equation: E0=0.41+(0.0252d2+0.283d)-1 where E0 is the band gap and d is the diameter. The absorption valley, centered at approximately 800 nm, is due to the discrete energy levels in our PbS QDs ensemble with tight size distribution and strong quantum confinement.22 Beside extending the absorption range by using materials with different bandgaps, introducing another absorber that has strong absorption to complement the deficiency of PbS QDs and building tandem devices is another strategy to improve the light harvesting. In recent decades, organic semiconductors with different bandgaps have been intensively synthesized and developed for photovoltaic applications.23-25 Among them, diketopyrrolopyrrole (DPP)-based polymers typically display low bandgap.26-28 One of the DPP-based polymers, PMDPP3T (Figure 1a) absorbs up to 950 nm and holds a strong absorption band in the range of 700-900 nm, which complements the absorption of PbS QDs (about 3 nm diameter).27 Thus it could be a promising candidate to integrate with PbS QDs to produce hybrid tandem solar cells.10, 29 It should be noted that blending the two materials in a single-junction cell to achieve high efficiency is not feasible from the view of fabrication. The QDs and PMDPP3T:PCBM films are fabricated from different solvents. The QDs films fabrication also include a layer-by-layer spin coating process with ligand exchange. Charge transport will possibly be another issue if the two materials are simply blended for photovoltaic applications. Recently, Kim reported the organic-PbS QDs hybrid tandem solar cells exhibited a maximum PCE of 5.33%, consisting of a polymer front sub-cell and a QD back sub-cell.30 Two primary challenges existed in their organic-QD tandem solar cells: (1) Absorption between the QD and the polymer: fullerene sub-cells were not spectrally complementary; (2) 4


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Device structure of the two sub-cells: the smaller bandgap QDs with broad absorption was used in front sub-cell while the organic one worked in the back cell. The incident light was first absorbed by the QD front cell that leads to a small portion of light absorbed by the organic-absorber based back sub-cell. That would result in unbalanced photocurrent in the two sub-cells and photocurrent loss for the whole tandem cells. In order to achieve synergistic effect of two sub-cells, a feasible approach is to switch the layer sequence of the QD layer and organic sub-cells in the vertically-stacked tandem cells. However, switching the sequence of sub-cells will need to address the deposition compatibility of solvent formulations. The solvents to process the QD sub-cell should not deteriorated the underlying charge recombination layer (CRL) and the organic sub-cell. It would be a tough work to develop such solvents that can prevent the damage to the underlying CRL and organic active layers. In this work, the above complementary absorption of the QDs and the organic absorbers, and light illumination sequence on sub-cells for photocurrent balancing are addressed. A DPP-based organic absorber with spectrally complementary absorption to PbS QD (3 nm size) was utilized as the front sub-cell absorber. A new device architecture is designed to enable light to illuminate from the top electrode and reach the organic sub-cell first to balance the photocurrent.

Low

band-gap

polymer

poly[[2,5-bis(2-hexyldecyl-2,3,5,6-tetrahydro-

3,6-dioxopyrrolo 3,4-c]pyrrole-1,4-diyl]-alt-[3′,3″-dimethyl-2,2’:5’,2”-terthiophene]-5,5”-diyl] (PMDPP3T, Figure S1) (Eg = 1.3 eV) blends with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the front sub-cell that has a strong absorption band in the range of 700-900 nm, which well complements the absorption of PbS QDs (~ 3 nm diameter)22. As for the top electrode, we employ transparent conductive film for photocurrent collection.31 The light can 5


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illuminate from the top transparent electrode and therefore the organic sub-cell could absorb the light first to achieve high balanced photocurrent. The top transparent electrode is a conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The organic-QDs hybrid tandem cells exhibit an open-circuit voltage (VOC) of 1.12 V, short-circuit current of 11.8 mA cm-2, and fill factor of 0.56, yielding a power conversion efficiency (PCE) of 7.4%, higher than both sub-cells with solution-processed top electrode. Our device performance is also among the highest PCE reported so far for organic-QDs hybrid tandem solar cells.30, 32

Results and Discussion Figure 1b shows the device structure of an organic-QD hybrid tandem solar cell. PbS CQD films were fabricated by layer-by-layer spin coating according to the published reports33. Experimental details are included in the supporting information. The full stack consisted of an ITO substrate, a ZnO/PbS back sub-cell, a Spiro-MeOTAD/MoO3/Ag/PEIE CRL and a PMDPP3T:PC61BM front cell. Previously, Wang et al. reported the efficient CRL for MoO3/ITO/AZO/TiO2

for

QDs

tandem

solar

cells.34

Here

we

applied

Spiro-MeOTAD/MoO3/Ag/PEIE as the CRL with easier processing. Transparent conducting polymer PEDOT:PSS is employed as the top electrode to allow the light to illuminate first the organic sub-cell. The PEDOT:PSS electrode is prepared by transfer-printing technique (hereafter referred to as PEDOT:PSS-T). The details of the film-transfer lamination process can be found in our previous reports.19, 31, 35 Prior to transfer of the PEDOT: PSS film, the surface of PMDPP3T: PCBM is treated by 6


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a short-time of plasma (about 5 s) to turn the surface to hydrophilic which will improve the adhesion between the PEDOT:PSS film and the PMDPP3T:PCBM surface and assist the separation of the PEDOT:PSS from the PDMS substrate. To investigate the plasma treatment effect on the organic solar cell performance, we fabricated the devices with structure of ITO/PEIE/PMDPP3T:PC61BM (180 nm)/MoO3/Ag where the PMDPP3T:PC61BM was treated by air plasma for 5 s. As shown in Figure 2a, the cells with and without plasma treatment exhibited almost identical J-V characteristics that indicates the short-time plasma treatment doesn’t not deteriorate the solar cell performance. The single-junction PbS QD solar cells have the sequence stacking structure: ITO/ZnO/PbS/Spiro-MeOTAD/MoO3/Ag. Figure 2b shows the J-V characteristics of the QD cells with or without Spiro-MeOTAD as the hole-transporting layer. Without the Spiro-MeOTAD layer, the current-voltage characteristics exhibited an S-shaped kink. This is due to the unfavorable hole extraction between PbS and MoO3 as reported previously.2 After adding a 150 nm LiTFSI-doped Spiro-MeOTAD layer, the S-shape kink disappeared (Figure 2b). Spiro-MeOTAD is commonly used as a hole-transporting layer in perovskite solar cell.36 The role of Spiro-MeOTAD film here also helps to improve the hole exaction from the PbS QD films. Furthermore, the Spiro-MeOTAD would also protect PbS layer when depositing MoO3 layer on top of it. Before the integration of hybrid tandem structure, thicknesses of the PbS QD layer and organic layer in single-junction solar cells were optimized respectively to match the photocurrents. Figure 3a shows the performance of the PbS single-junction cells (ITO/ZnO/PbS/Spiro-MeOTAD/MoO3/Ag) as a function of thickness of the PbS QDs films. 7


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The maximum efficiency of 4.8% was achieved at the absorber thickness of 170 nm. Further increasing the film thickness resulted in a reduction of PCE. The PCE decreased to 3.8% and 3.2% when the thickness of PbS QD film was 240 nm and 280 nm respectively. So, the 170 nm was used as the thickness of the QDs film in the back QDs sub-cell. For the organic active layer, its thickness affects not only the performance of the organic sub-cell, but also the efficiency of the PbS QDs sub-cell, because it is the front sub-cell and acts as a filter for the PbS QDs sub-cell. To estimate current match between the front and back sub-cells, PbS QDs single-junction solar cells (ITO/ZnO/PbS/Spiro-MeOTAD/PEDOT:PSS-T, light illuminated from the PEDOT:PSS-T side) were also tested with the organic active layer as the filter (quartz/PEIE/PMDPP3T:PC61BM). The thickness of the PMDPP3T:PC61BM were varied (80, 110, 180 nm). As shown in Figure 3b, the JSC of the PbS QDs cells decreases from 19.3 to 13.1 mA cm-2 with the increase of the organic active layer thickness. When the PMDPP3T:PC61BM filter thickness increased to 180 nm, the JSC of PbS is about 13.1 mA cm-2 which is close to a 180 nm single-junction PMDPP3T:PC61BM cell. Therefore, the thickness of the organic front sub-cell is selected as 180 nm. Figure 4a shows the current density-voltage (J-V) characteristics of the hybrid tandem solar cells and the corresponding single-junction solar cells. Their device structures are glass/ITO/ZnO/PbS/Spiro-MeOTAD/MoO3/Ag

(device

A),

glass/ITO/PEI/PMDPP3T:

PC61BM/PEDOT:PSS-T (device B) and glass/ITO/ZnO/PbS/Spiro-MeOTAD(O2)/MoO3(10 nm)/Ag (0.5 nm)/PEIE/ PMDPP3T:PC61BM/PEDOT:PSS-T (device C). Their performance is summarized in Table 1. The single-junction device A yields a VOC = 0.56 V, a JSC = 21.2 mA cm-2, and a FF = 41%, resulting in a PCE = 4.8%. The device B shows a VOC = 0.60, a JSC = 8


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12 mA cm-2, and a FF = 50%, leading to a PCE = 3.6%. The hybrid tandem cell C exhibits a VOC = 1.12 V, a JSC = 11.8 mA cm-2, and a FF = 56%, obtaining a PCE = 7.4%, which is higher than the control single-junction solar cells. When illuminated from the bottom (ITO) side, the cell delivers a PCE of about 6.6% (Fig. 4a). The lower efficiency is mainly due to the lower JSC resulting from the unbalanced photocurrent in the sub-cells. The external quantum efficiencies (EQE) of the PbS and organic sub-cells are shown in Figure 4b. The EQE spectra of the organic and the PbS cells can well compensate each other. Calculated JSC by integrating EQE curves for the front and back cells are 11.0 mA cm-2 and 11.2 mA cm-2, respectively, which demonstrates the good current match of the front and back sub-cells. The hybrid tandem solar cell yields a VOC of 1.12 V, which is close to the sum of the VOC values of the two component cells (0.60 V for the organic and 0.56 V for the QD sub-cells).

Conclusions We have achieved efficient organic-QD hybrid tandem solar cells via selecting optical absorption-complementary organic and QD sub-cells, and designing a top-illuminated device structure. The transparent conducting polymer top electrode allows incident light enter the tandem cells through the organic sub-cells side to achieve balanced photocurrent. With the development of the effective charge recombination layer of Spiro-MeOTAD/MoO3/Ag/PEIE and the thickness optimization of sub-cells, a power conversion efficiency of 7.4% has been achieved for the hybrid organic-QDs tandem solar cells. The hybrid device show a VOC of up to 1.12 V that correspond to the sum of the VOC of the individual sub-cells, and a higher FF 9


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comparing with that of the individual sub-cells. The organic-QD hybrid combination can expand the spectral response of QD absorbers and compensate for the low VOC of QDs sub-cell. This shows that the hybrid tandem structure is an effective strategy to integrate some material systems in one device and compensating their respective inherent difficulties. To further enhance the power conversion of the hybrid tandem solar cells, development and selection of photoactive layers with proper bandgaps are highly desirable. Multijunction (with 3-4 junctions) could be the way to record-high efficiency based on the optimization experience of III-V multijunction solar cells.

ASSOCIATED CONTENT

Supporting Information The supporting Information is available free of charge on the ACS Publication website. Energy diagram of the hybrid tandem cell (Figure S1), and Experimental details including the information of materials, device fabrication and characterization.

Acknowledgements The work is supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 21474035, 51403071), the Fundamental Research Funds for the Central Universities, HUST (Grant No. 2016JCTD111), and Science and Technology Program of Shenzhen (JCYJ20160429182443609).

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Author contributions #

J.H.T. and X.K.Y. contributed equally to this work. J.H.T. and Y.H.Z. conceived the

project. J.H.T., X.K.Y. and Y. X. carried out the characterization and fabrication of the solar cells. W.W.L prepared the PMDPP3T materials. Y.H.Z., H.S.S. and J.T. directed the work. J.H.T. wrote the initial draft of the manuscript. All the authors revised and approved the final version the manuscript.

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[28] Li, W.; Hendriks, K. H.; Furlan, A.; Roelofs, W. S.; Wienk, M. M.; Janssen, R. A. Universal correlation between fibril width and quantum efficiency in diketopyrrolopyrrole-based polymer solar cells. J. Am. Chem. Soc. 2013, 135, 18942-8. [29] Liu, Y.; Chen, C. C.; Hong, Z.; Gao, J.; Yang, Y. M.; Zhou, H.; Dou, L.; Li, G.; Yang, Y. Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency. Sci. Rep. 2013, 3, 3356. [30] Kim, T.; Gao, Y.; Hu, H.; Yan, B.; Ning, Z.; Jagadamma, L. K.; Zhao, K.; Kirmani, A. R.; Eid, J.; Adachi, M. M.; Sargent, E. H.; Beaujuge, P. M.; Amassian, A. Hybrid tandem solar cells with depleted-heterojunction quantum dot and polymer bulk heterojunction subcells. Nano Energy 2015, 17, 196. [31] Zhou, Y.; Khan, T. M.; Shim, J. W.; Dindar, A.; Fuentes-Hernandez, C.; Kippelen, B. All-plastic solar cells with a high photovoltaic dynamic range. J. Mater. Chem. A 2014, 2, 3492. [32] Speirs, M. J.; Groeneveld, B. G.; Protesescu, L.; Piliego, C.; Kovalenko, M. V.; Loi, M. A. Hybrid inorganic-organic tandem solar cells for broad absorption of the solar spectrum. Phys. Chem. Chem. Phys. 2014, 16, 7672-6. [33] Hu, L.; Li, D.-B.; Gao, L.; Tan, H.; Chen, C.; Li, K.; Li, M.; Han, J.-B.; Song, H.; Liu, H.; Tang, J. Graphene Doping Improved Device Performance of ZnMgO/PbS Colloidal Quantum Dot Photovoltaics. Adv. Funct. Mater. 2016, 26, 1899-1907. [34] Wang, X.; Koleilat, G. I.; Tang, J.; Liu, H.; Kramer, I. J.; Debnath, R.; Brzozowski, L.; Barkhouse, D. A. R.; Levina, L.; Hoogland, S. Tandem colloidal quantum dot solar cells employing a graded recombination layer. Nat. Photonics 2011, 5, 480-484. [35] Jiang, F.; Liu, T.; Zeng, S.; Zhao, Q.; Min, X.; Li, Z.; Tong, J.; Meng, W.; Xiong, S.; Zhou, Y. Metal electrode–free perovskite solar cells with transfer-laminated conducting polymer electrode. Optics Express 2015, 23, A83-A91. [36] Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897-903.

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ACS Photonics

Figure 1 (a) Absorbance spectra of the PbS QDs and PMDPP3T films. Chemical structure of the PMDPP3T polymer; (b) device structure of the top-illuminated organic-PbS QDs hybrid tandem solar cells

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ACS Photonics

(b)

ITO/PEIE/PMDPP3T:PC61BM/MoO3/Ag

5

-2

10

Current density / mA cm

-2

(a) Current density / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

With plasma treatment Without plasma treatment

0 -5 -10 -15

5

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ITO/ZnO/PbS/Spiro-OMeTAD/MoO3/Ag

0 Without spiro-OMeTAD layer With spiro-OMeTAD layer

-5 -10 -15 -20

-20

-0.2

0.0

0.2

0.4

0.6

0.8

-0.2

Voltage / V

0.0

0.2

0.4

0.6

Voltage / V

Figure 2 (a) J-V curves of PMDPP3T:PC61BM-based single-junction solar cells with or without plasma treatment; (b) J-V curves of single-junction PbS QDs solar cells with or without Spiro-MeOTAD as the hole-extraction layer.

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(a)

ITO/ZnO/PbS/Spiro-OMeTAD/MoO3/Ag

(b) 10 -2

4.8


4.4 4.0

PCE / %

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ACS Photonics

3.6 3.2 2.8 2.4 2.0

60

120

170

240

280

0

With 80 nm PMDPP3T:PCBM filter With 110 nm PMDPP3T:PCBM filter with 180 nm PMDPP3T:PCBM filter Without filter

-10

-20 -0.2

0.0

0.2

0.4

0.6

Voltage / V

PbS thickness / nm

Figure 3 (a) PCE versus film thickness of PbS QDs single-junction cells. The line is fitted using

B-Spline model based on the experimental data (filled circle); (b) J–V characteristics of PbS QDs

solar

cells

(ITO/ZnO/PbS/Spiro-MeOTAD/PEDOT:PSS-T,

illuminated

from

the

PEDOT:PSS-T side) under illumination of 100 mW cm-2 using different thicknesses of PMDPP3T: PC61BM films as the filter.

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(b) 4

70

0

60

-4

50

-8

40

EQE / %

-2

(a)


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-12 -16

-24 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

30 20

Organics sub-cell PbS QD sub-cell PbS QD sub-cell with filter Tandem (bottomilluminated) Tandem (top illuminated)

-20

PbS QDs sub-cell PMDPP3T:PC61BM sub-cell

10 0 400

1.2

500

600

700

800

900

1000 1100

Wavelength / nm

Voltage / V

Figure 4 (a) J-V characteristics of PbS QDs sub-cell, organic sub-cell, and tandem cell (illumination from both sides). (b) EQE of the sub-cells in the hybrid tandem solar cell. PbS QDs sub-cell:

glass/ITO/ZnO/PbS/Spiro-MeOTAD/MoO3/Ag;

glass/ITO/PEI/PMDPP3T:

PC61BM/PEDOT:PSS-T;

glass/ITO/ZnO/PbS/Spiro-MeOTAD/10

Organic and

nm

Ag/PEIE/PMDPP3T:PC61BM/PEDOT:PSS-T.

18


tandem MoO3/0.5

sub-cell: cell: nm

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ACS Photonics

Table 1 Photovoltaic performance of single-junction ITO/ZnO/PbS/Spiro-MeOTAD/MoO3/Ag;

B:

and

tandem cells.

A: glass/

glass/ITO/PEIE/PMDPP3T:PC61BM/

PEDOT:PSS-T; and C: glass/ITO/ZnO/PbS/Spiro-MeOTAD/MoO3 (10 nm)/Ag (0.5 nm)/PEIE/ PMDPP3T:PC61BM/PEDOT:PSS-T (top illuminated).

Devices

VOC (V)

JSC (mA cm-2)

FF (%)

PCE (%)

A (QD sub-cell)

0.56

21.2

41

4.8

B (organic sub-cell)

0.60

12

50

3.6

C (hybrid-tandem)

1.12

11.8

56

7.4

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ACS Photonics

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Table of Contents Graphic

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Efficient Top-Illuminated Organic-Quantum Dots Hybrid Tandem Solar

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