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2020 “Welding” Flexible Transparent Electrode in 15.2% Flexible Organic Solar Cells SC Yaowen Li flexible OSCs over 15 and 6000 bending cycles

2020 “Welding” Flexible Transparent Electrode in 15.2% Flexible Organic Solar Cells.

Realizing Ultrahigh Mechanical Flexibility and >15% Efficiency of Flexible Organic Solar Cells via a “Welding” Flexible Transparent Electrode

First Author:Xiaobin Chen

Corresponding Authors: Hongwei Gu, Yaowen Li

DOI: 10.1002/adma.201908478

Contents

Highlights

The scientist reported that realizing the ultra-high-mechanical flexibility and >15% efficiency of flexible organic solar cells by “welding” flexible transparent electrodes.

  1. The power conversion efficiency (PCE) of flexible organic solar cells (OSC) still lags behind that of rigid devices. Mechanical stability cannot meet the needs of flexible electronic products currently due to the lack of high-performance flexible transparent electrodes (FTE).
  2. This paper proposes the so-called “welding” concept to design an FTE with a close combination of the upper electrode and the lower substrate.
  3. The upper electrode composed of solution-treated Al-doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by using the capillary force effect and secondary growth of AZO, which can reduce the junction resistance of AgNWs.
  4. The polyethylene terephthalate is modified by embedding AgNW, and then the AgNW is used to connect with the AgNW in the upper mixed electrode to enhance the adhesion between the electrode and the substrate.
  5. The single-junction flexible organic solar cell based on this welded FTE exhibits high performance, reaching a record power conversion efficiency of 15.21%. In addition, the power conversion efficiency of flexible organic solar cells is less affected by the area of the device and shows strong bending durability even under extreme test conditions.
  6. Through this welding strategy, the key bottlenecks related to FTE in terms of photoelectric and mechanical properties have been fully resolved.
Summary

  In February 2020, Advanced Materials magazine published an ultra-high mechanical flexibility and >15% efficiency of flexible organic solar cells by “welding” flexible transparent electrodes, which developed by Professor Yongfang Li and Yaowen Li from Chinese Academy of Sciences. In order to prove its feasibility as a flexible OSC electrode, this paper studies fullerene and non-fullerene active layer materials with various band gaps. All flexible OSCs showed PCE equivalent to the corresponding rigid devices and achieved a record high efficiency of 15.21%. It is important that the PCE of flexible organic solar cells is less affected by the device area, and even under extreme test conditions, the device shows strong bending durability.

Background

  Metal nanowires, especially silver nanowires (AgNWs), are considered the most promising FTE materials due to their excellent transmittance, high conductivity and flexibility. However, the solution-processed AgNW network usually has a low coverage (less than 40%) and high junction resistance will greatly reduce the conductivity and even the working stability of the device, because the AgNW junction locally concentrates heat by radiating under current flow. In addition, random stacking and low-adhesion of AgNW on plastic substrates may cause device short circuits and reduce mechanical peeling stability.

  Although this tangled problem can be partially alleviated by coating a conductive polymer (such as PH1000) on the AgNW network film, the high parasitic absorption in the long-wavelength region and the acidity of PH1000 will reduce light collection and deteriorate device stability.

  In view of the advantages of combining AgNW network with PH1000 in enhancing coverage, adhesion and conductivity, some researchers have tried to replace PH1000 with ZnO to obtain high transmittance, favorable interface energy and low-temperature solution processability. Under the synergy of capillary force and electric bridge, the ZnO solution can easily fill the AgNW network and weld the connection points to achieve full coverage of AgNWs: ZnO hybrid electrode with reduced junction resistance.

  In order to further improve the junction resistance and transmittance of this hybrid electrode, using a precise silver grid pattern to perform dry etching of the pre-deposited junction-free metal film (AgNN). The obtained 15 nm thick AgNN/ZnO hybrid electrode can achieve a low sheet resistance of 35.2 Ω sq-1 and a high light transmittance of 91.6% (not including poly(ethylene terephthalate) (PET) substrate).

  However, the complexity and high energy consumption of manufacturing processes such as vacuum thermal evaporation, electrospinning and ion beam etching technology will undoubtedly limit their practical applications. In order to simplify the process, Chen et al. By doping the polyelectrolyte into the AgNW-water solution, a method of ionic electrostatic charge repulsion is proposed to reduce the aggregation and junction resistance of AgNW. The resulting hybrid AgNWs:ZnO electrode contains a grid-like AgNW pattern, showing excellent photoelectric properties and a smooth surface.

  Therefore, it is desired to precisely control the composition and structure through the solution process to manufacture high-performance and low-cost hybrid electrodes, which is urgently needed to promote the development of flexible electronic products.

  This paper proposes an integrated FTE design welding strategy, including the upper electrode and the underlying substrate, to match the most advanced non-fullerene near-infrared absorption and robust flexible active layer materials. For the upper electrode, the AgNW-based solution is used to process the hybrid, and the composition is precisely controlled through the capillary force effect and the secondary growth of Al-doped ZnO (AZO) to weld unfavorable AgNW junctions.

  The insulating underlying PET substrate has also been modified by embedding AgNW in the UV curing resin, thereby realizing the connection between the AgNW in the upper hybrid electrode and the underlying substrate. In this way, improvements are not only achieved in terms of photoelectric properties (such as conductivity and transmittance), but also in terms of the adhesion of the upper electrode to the substrate and the previous poor morphology of the upper hybrid electrode.

  The resulting welded AgNW-based FTE exhibits a low sheet resistance (Rsh) of ≈18 Ω sq-1, the highest light transmittance at 550 nm (excluding PET substrate) is about 95%, and the surface is smooth. The mechanical stability is good in bending and peeling tests.

Key Results
Schematic diagram of electrode fabrication
Schematic diagram of electrode fabrication

Figure 1. a) Schematic diagram of electrode fabrication. b) Sheet resistance, c) Conductivity statistics, and d) Em-Ag/PH1000, Em-Ag/AgNWs: AZO-SG and AgNWs: AZO-SG electrode optical transmission spectrum and activity The normalized absorption spectrum of the layer. All transmittances of FTE include PET substrate. e) Transmittance (λ = 550 nm) is plotted as a function of the sheet resistance of the film. Illustration: Photo of Em-Ag/AgNWs: AZO-SG FTE.

  A schematic diagram of the FTE manufacturing process is shown in Figure 1a. Firstly, the PET substrate is modified by coating the AgNW film, and then an ultraviolet curing resin is applied to protect the coating from the air and promote the adhesion of AgNW to the PET substrate, thereby forming an embedded AgNW substrate (Em-Ag) . Interestingly, the resulting Em-Ag substrate still exhibits significantly improved conductivity (sheet resistance = 130 Ω sq-1, conductivity = 7.7 × 104 S m-1), which indicates that the incompletely embedded AgNW provides an additional charge transfer channel is provided.

  In order to prove the feasibility of using this welding FTE in a flexible OSC, a comprehensive evaluation of the conduction mechanism, morphology, electrical and optical properties, and mechanical stability was carried out. In this paper, the conductive polymer PH1000 is coated on the Em-Ag substrate (Em-Ag/PH1000), and the AgNWs:AZO-SG hybrid electrode is coated on the bare PET substrate (AgNWs:AZO-SG) for comparison. In the case of Em-Ag/AgNWs:AZOSG FTE, the Rsh calculated based on the statistical results is 18 Ω sq-1 (average value) and the standard deviation is 0.66, which is much lower than that of the traditional glass/ITO electrode. Ag/PH1000 (90 Ω sq-1) and AgNWs: AZO-SG (28 Ω sq-1) FTE (Figure 1b). This result is also consistent with their respective conductivity values, as shown in Figure 1c.

  As shown in Figure 1d, under the Rsh of 18 Ω sq-1, the welded FTE exhibits an average transmittance of up to 84% in the range of 500-1000 nm, without any degradation at long wavelengths. This characteristic can well match the absorption of the most advanced non-fullerene active layer materials extending into the near-infrared region. In contrast, when the traditional conductive polymer PH1000 is used as the upper electrode (Em-Ag/PH1000), the average light transmittance is only 75% (Rsh is 90 Ω sq-1), which can even be reduced to 70% 1000 Nano. Our team also noticed that the Em-Ag-free AgNWs: AZO-SG FTE showed slightly enhanced transmittance, although its Rsh was 1.5 times higher than that of soldered FTE, indicating that Em-Ag plays a key role in enhancing conductivity effect.

  The figure of merit (FoM) of a transparent electrode is defined by the ratio of direct current conductivity to optical conductivity (σDCOp), and is usually used to accurately evaluate the trade-off between Rsh and transmittance. As shown in Figure 1e, the FoM value of the welded FTE reaches the maximum value of 498, Rsh is 18 Ω sq-1, and the transmittance at 550 nm is about 95% (excluding the substrate), while the comparison AgNWs: AZO-SG and Em -Ag/PH1000 FTE is only 322 and 130 respectively. The observed high FoM value is close to 500, showing high transmittance (inset in Figure 1e), due to the careful selection of Em-Ag/AgNEs:AZO-SG FTE and the well-welded composition.

Cross-sectional SEM images

Figure2. a-c) Cross-sectional SEM image: a) Em-Ag/PH1000, b) Em-Ag /AgNWs:AZO-SG, and c) the AgNWs:AZO-SG FTEs. d) With inward And the increase in the number of bending cycles of the outward bending test, the sheet resistance of various FTEs changes. e) The adhesion value of various FTEs, illustration: schematic diagram of adhesion measurement.

  As shown in Figure 2d, the Rsh/R0 of AgNWs:AZO-SG grown on bare PET substrates slightly increased after 1200 times of inward bending, indicating that the brittleness of AZO has little effect on bending durability. In contrast, when the PET substrate is modified with Em-Ag, the Rsh/R0 change of Em-Ag/AgNWs:AZO-SG FTE is negligible, regardless of the bending direction, which is consistent with the behavior of conductive polymers- Base electrode (Em-Ag/PH1000). This result shows that the effect of brittle AZO on flexibility can be further reduced due to the synergistic effect of the underlying layer Em-Ag and AZO-SG auxiliary upper electrode.

  To clarify this point, FTE cross-sectional SEM imaging was performed. Figures 2a-c show that all components, including AgNW, PET, UV curable resin, and PH1000, can be clearly distinguished among various FTEs. In the case of Em-Ag/AgNWs:AZO-SG FTE, it is observed in this paper that the upper AgNWs are connected to the AgNWs exposed by the UV curable resin at the interface. As expected, this interface welding will enhance the bonding strength between the upper electrode and the substrate and reduce the stress on the substrate, thereby helping to enhance the bending durability. In addition, this interface welding can also enhance the adhesion between the electrode and the substrate.

  Through 90° peeling measurement, the adhesion force between the AgNWs:AZO-SG layer and the Em-Ag substrate reached 1.45 N mm-1. In contrast, as shown in Figure 2e, the adhesion of Em-Ag/PH1000 and AgNWs:AZO-SG FTE without interfacial welding is significantly reduced to 58% and that of Em-Ag/AgNWs:AZO-SG. 73.2% FTE, respectively. Due to overall improvements in optoelectronic and mechanical properties, welded FTE is a promising candidate for use as electrodes in flexible OSCs. Obviously, these advantages should be accompanied by improved device performance.

energy alignment of the devices

Figure3. a) Schematic diagram of device energy alignment b) Schematic diagram of flexible organic solar cell and donor PBDB-T-2F and acceptor Y6 in the active layer.

  From the device energy level diagram in Figure 3a, the Em-Ag/AgNWs:AZO-SG FTE shows a good alignment of energy levels with the non-fullerene active layer material. The small energy shift between the work function of the electrode and the LUMO energy level of the non-fullerene acceptor (such as Y6 (4.1 eV)) can effectively reduce the energy loss of the device.

  Therefore, the flexible OSCs of Em-Ag/AgNWs:AZO-SG/active layer/MoO3/Al (Figure 3b) with an inverted structure is manufactured using PBDB-T-2F:Y6, PBDB-T-2F:IT-4F, and PTB7-Th: PC71BM as the active layer material. It is worth noting that, due to the matching energy level and high electron extraction capability of AZO in FTE, there is no additional electron transport layer is added to the device, which can be used to simplify the device structure.

Photovoltaic performance of the PBDB-T-2FY6-based flexible OSCs

Figure 3 c–f) Photovoltaic performance of flexible OSC based on PBDB-T-2F:Y6: c) J-V curve under AM1.5G 100 mW cm-2 irradiation; d) EQE spectrum and integral Jsc, e) Jsc versus natural logarithm of the light intensity fitted by the linear relationship, and f) Voc versus natural logarithm of light intensities fitted by a linear relationship.

  As shown in Figure 3c, the J-V curve of the flexible OSCs using PBDB-T-2F:Y6 as the active layer were measured under AM1.5G 100 mW cm−2 irradiation. In order to verify the fluctuation of Jsc in various FTEs, the external quantum efficiency (EQE) spectra of the corresponding devices were checked to evaluate the photoresponse of the entire absorption region (Figure 3d).

  The result of EQE curve here is using Enlitech QE-R system to test. The QE-R software comes with a calculation function for the integrated current density Jsc (EQE) of the AM1.5G spectrum. As expected, the devices based on high light transmittance AgNWs:AZO-SG and Em-Ag/AgNWs:AZO-SG FTE exhibit significant photoresponse in the range of 500-900 nm, and the integrated Jsc value is consistent with the value obtained from the J-V measurement, and the deviation is less than 4%

  In order to further understand how the optical and electrical characteristics of FTE affect the performance of the device, the related photophysical process was studied. The carrier recombination process of each device is evaluated by measuring the dependence of Jsc on various light intensities, where the data conforms to the power law Jsc ∝ Plightα. As shown in the curves of log-Jsc and log Plight of flexible OSCs in Figure 3e, the similar slope (α value) shows that due to the smooth surface and good wettability, the quality of the active layer is less affected by the FTE of the underlying layer.

  The α values of Em-Ag/AgNWs:AZO-SG FTE devices are close to the same, indicating that the bimolecular recombination in the active layer is negligible. According to the relationship of Voc ∝ (nkT/q)ln(Plight), the carrier recombination behavior is evaluated through the dependence of Voc on light intensity measurement (Figure 3f). Em-Ag/AgNWs: AZO-SG FTE device with a significantly reduced slope of 1.12 kT/q (close to kT/q), which indicates that the trap state is low, which may be due to the carrier recombination reduction layer between the electrode and the active material . The enhanced exciton dissociation efficiency further confirms this behavior.

Photovoltaic performance parameters of the PBDB-T-2FY6-based OSCs

Table 1. Photovoltaic performance parameters of OSCs based on PBDB-T-2F:Y6 under AM1.5G 100 mW cm-2 illumination. The values of Jsc(EQE) [Jcal in the above table] and Jsc(IV) [Jsc in the above table], and the comparison error between the two is within 3%.

EQE is defined as the ratio of the number of output electrons to the number of incident photons. Jcal can be calculated from EQE curve and photon flux spectrum of the indoor light source.

The formula is as follows:

EQE formula

  The EQE test results can be used to integrate the spectrum of the indoor light source (measured by the spectrometer) to obtain Jcal, or called Jsc (EQE), to verify the comparison of the Jsc (IV) measured by the IV under the indoor light. The difference between them should be less than 5%. Therefore, the EQE curve of PV and the illuminance spectrum of incident light are necessary for accurate measurement in indoor PV testing.

  It is worth noting that the PCE of the flexible OSCs based on Em-Ag/AgNWs:AZO-SG FTE is 15.21%, the open circuit voltage (Voc) is 0.832 V, the Jsc is 25.05 mA cm-2, and the fill factor is 72.97 (FF). %. As far as we know, 15.21% of the PCE is the highest value reported by a single-junction flexible OSCs so far.

flexible OSC with a 4 mm bending radius

Figure 4 a) Photograph of a flexible OSC with a bending radius of 4 mm on a cyclic bending machine. b) Schematic diagram of inward and outward bending tests. c-e) The relative PCE attenuation of flexible OSCs is based on: c) Em-Ag/PH1000, Em-Ag/AgNWs: AZO-SG and AgNWs: AZO-SG FTE in relation to the bending cycle with a radius of 4 mm; d) Em-Ag /AgNWs: The relationship between AZO-SG FTE and the bending radius after 1200 times of inward bending, and e) Em-Ag/AgNWs: AZO-SG FTE and the bending cycle with inward bending radius of 4 mm.

  Bending durability is another key factor for high-performance flexible OSCs. Therefore, the obtained flexible OSCs were subjected to bending tests in two directions, 1200 consecutive bending with a radius of 4 mm to evaluate their stability to mechanical bending (Figure 4a, b). As we all know, the conductive polymer PH1000 has excellent mechanical flexibility due to its plastic properties. As shown in Figure c, the flexible OSCs based on Em-Ag/PH1000 FTE retained its initial efficiency of 90.8% and 89.5% of the inward bending cycle after 1200 times of bending in the outward direction.

  In order to further understand the mechanical stability of the flexible OSCs, bending tests were conducted under different bending radii. As shown in Figure 4d, the device is able to maintain a robust efficiency (PCE) after 1200 bending cycles based on Em-Ag/AgNWs:AZO-SG FTE at a bending radius of 0-8 mm. Even under extreme conditions where the device is fully folded inward (Rc = 0 mm), 81.7% of the initial PCE can be tolerated. In addition, the flexible OSCs with Em-Ag/AgNWs:AZO-SG FTE was surprisingly able to maintain 75% of its initial PCE value after 6000 bending cycles with an inward bending radius of 4 mm (Figure 4e).

Conclusion

  This paper successfully developed a welding FTE with Em-Ag/AgNWs:AZO-SG structure to solve the problem of mismatch between the transmission/absorption spectrum of FTE and the most advanced non-fullerene active layer. The fine-tuned AgNW network combines the capillary force effect and the secondary growth of AZO in the solution to effectively avoid the parasitic absorption of the electrode composition, and the deposited AZO welds the junction of the AgNW. The exposed AgNWs in the underlying Em-Ag are further combined with the AgNWs in the upper AgNWs:AZO-SG layer, thereby enhancing the mechanical properties of FTE. Welding FTE shows good photoelectric properties, smooth surface, and flexibility.

  Therefore, based on Em-Ag/AgNWs:AZO-SG FTE, flexible organic solar cells use various bandgap effective layers to achieve power conversion efficiency comparable to glass/ITO devices, and the power conversion efficiency is as high as 15.21% and a record high. 12.28%, which are single-junction flexible organic solar cells with small area and large area respectively. More importantly, these devices show strong mechanical properties in bending and peeling. The strategy in this article provides new functions for the emerging FTE, which is expected to promote the development of flexible electronic devices to high performance and large area.

Article Information

Realizing Ultrahigh Mechanical Flexibility and >15% Efficiency of Flexible Organic Solar Cells via a “Welding” Flexible Transparent Electrode

Xiaobin Chen, Guiying Xu, Guang Zeng, Hongwei Gu,* Haiyang Chen, Haitao Xu, Huifeng Yao, Yaowen Li,* Jianhui Hou, and Yongfang Li

DOI: 10.1002/adma.201908478

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