2020ConductiveHole-SelectivePassivatingContactsforCrystallineSiliconSolarCells

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/339420326 Conductive Hole-Selective Passivating Contacts for Crystalline Silicon Solar Cells Article in Advanced Energy Materials · February 2020 DOI: 10.1002/aenm.201903851 CITATIONS8 READS292 11 authors, including: Some of the authors of this publication are also working on these related projects: NSF of Hebei province View project Si Solar Cell Research View project Feng Li Yingli 26 PUBLICATIONS 137 CITATIONS SEE PROFILE Jianhui Chen Hebei University 70 PUBLICATIONS 459 CITATIONS SEE PROFILE All content following this page was uploaded by Jianhui Chen on 28 February 2020. The user has requested enhancement of the downloaded file. www.advenergymat.de 1903851 (1 of 8) © 2020 WILEY-VCH Verlag GmbH in addition, it is linked to the use of an electric field provided by fixed charges in dielectric materials (field-effect passivation). [18] In these materials, generally, no free electrons exist. This is why the passivation materials are usually not conductive. Like- wise, the materials that possess many free electrons, including inorganic materials like metals, ITO, and organic conductive polymers like poly(3,4-ethylenedioxythiophene) (PEDOT), do not provide a passivation effect. [19–21] The passivation effect is typically evaluated by means of the effective minority carrier lifetime (τ eff ), and a high-quality passivation is always accom- panied by a high minority carrier lifetime. [22] If the conductivity Defect state passivation and conductivity of materials are always in opposi- tion; thus, it is unlikely for one material to possess both excellent carrier transport and defect state passivation simultaneously. As a result, the use of partial passivation and local contact strategies are required for silicon solar cells, which leads to fabrication processes with technical complexities. Thus, one material that possesses both a good passivation and conductivity is highly desirable in silicon photovoltaic (PV) cells. In this work, a passivation- conductivity phase-like diagram is presented and a conductive-passivating- carrier-selective contact is achieved using PEDOT:Nafion composite thin films. A power conversion efficiency of 18.8% is reported for an industrial multicrystalline silicon solar cell with a back PEDOT:Nafion contact, dem- onstrating a solution-processed organic passivating contact concept. This concept has the potential advantages of omitting the use of conventional dielectric passivation materials deposited by costly high-vacuum equipment, energy-intensive high-temperature processes, and complex laser opening steps. This work also contributes an effective back-surface field scheme and a new hole-selective contact for p-type and n-type silicon solar cells, respec- tively, both for research purposes and as a low-cost surface engineering strategy for future Si-based PV technologies. L. Wan, C. Zhang, K. Ge, Dr. X. Yang, Dr. W. Yan, Dr. L. Yang, Prof. Y. Xu, Dr. J. Chen Hebei Key Lab of Optic-Electronic Information and Materials College of Physics Science and Technology Hebei University Baoding 071002, China E-mail: chenjianhui@hbu.edu.cn Dr. F. Li, Z. Xu, Prof. D. Song State Key Laboratory of Photovoltaic Materials however, thus far, this does not happen to current metal or ITO and dielectric materials. A conductive polymer, PEDOT:PSS, should be able to form a conductive passivation material because the PSS species has an excellent passivation effect, being comparable to that of high-temperature annealed SiO 2 , [23] and PEDOT has a good conductivity, like that of ITO. [24] However, PEDOT:PSS has a core−shell structure, and after the use of a PSS dispersant to solubilize the material for assembly, the structure is composed of a conductive PEDOT core and an insulated PSS shell [25,26] ; thus, an additive, such as ethylene glycol (EG) [24,27] or dimethyl sulfoxide (DMSO), [28–30] must be used to destroy the PSS shell structure and to obtain a good conductivity. [30] However, it is noted that the additive also destroys the passivation. This is why a conductive passivation material has eluded researchers so far. Thus, although a host of previous work focus on the PEDOT:PSS/c-Si heterojunc- tion solar cells, such as organic−inorganic hybrid cells [24,31–34] and the BackPEDOT concept developed by Schmidt et al. [35–38] has already presented the emphasis on hole selectivity and not passivation. In this work, we discovered a co-existing region of good conductivity and passivation experimentally by incorporating a PEDOT and Nafion solution to form a composite film, referred to as PEDOT:Nafion herein. Our previous work found that a Nafion film can provide an excellent passivation effect, rivalling the best a-Si:H(i) in PV field. [39] We produced a pas- sivation−conductivity phase-like diagram and achieved optional conductivity and passivation by controlling the PEDOT/Nafion ratio. This led us to develop an organic passivating contact cell concept, in which passivation and hole selectivity occur at the PEDOT:Nafion/Si organic−inorganic hybrid back interfaces without the need for a high-temperature process and compli- cated laser opening steps. The device shows good performance with a simple solution-based process. 2. Results and Discussion The centre panel in Figure 1 shows the functionalized phase- like diagram of a conductive passivation material that is based on a plot of the conductivity versus minority carrier lifetime. This diagram portrays three functional regions: (i) a [C] region, where the materials have good conductivity but nearly no pas- sivation ability; (ii) a [C+P] region, where the materials have a good conductivity and provide passivation; and (iii) a [P] region, where the materials have good passivation ability but poor con- ductivity. The transition between the regions is determined by the Nafion/PEDOT ratio (x). For 0 400 S cm −1 but do not provide an acceptable lifetime, suggesting a poor passivation ability. In the [C+P] region (0.3 2.8), the conductivity dramatically degrades, resulting in a transition to a passivation function only. Clearly, traversing the region diagram from low to high Nafion con- centrations results in a transition from “conductive materials” to “conductive passivation materials” and then to “passivation materials”. Note that the single Nafion thin film has a conduc- tivity of 2.7 × 10 −6 S cm −1 and a good passivation effect with a 25 ms lifetime. The lifetime reported here is an effective carrier lifetime at an injection level of 10 15 cm −3 , and lifetime versus injection level curves can be seen in Figure S3 Supporting Information. Unlike other dopants, such as EG or DMSO, the appropriate Nafion addition does not destroy the conductivity of the PEDOT because the electrical connection of the PEDOT-rich grains is maintained after the Nafion addition (see TEM image in Figure 1a,b). Excess Nafion leads to weak phase separation, and PEDOT grains are limited in the Nafion matrix, which provides an electrical barrier between the conductive PEDOT grains (Figure 1c). We observe that the pristine film in the [C] region consists of disconnected films, whereas after the Nafion addi- tion, the density and smoothness of the films in the [C+P] and [P] regions increase and have slightly decreased RMS values, as shown in the AFM images in Figure 1d–f. On the other hand, the Nafion addition remarkably increases the wettability of the PEDOT:PSS solution on the Si surface, which is indi- cated by an improved contact angle that decreased from 44.6° to 11.1° (see the insets in Figure 1d–f). This is an advantage of the PEDOT:Nafion solution, which has demonstrated a very good coating ability without the need of other surfactants, such as Triton-100 and Zonyl, that have been reported in previous work. [34,40–42] We apply the abovementioned three materials in the [C], [C+P], and [P] regions to actual solar cell devices. Here, we use a conventional multicrystalline Si (mc-Si) solar cell as an example. Figure 2a shows the design of the solar cell, which mainly consists of a p-type mc-Si substrate (Figure S4, Supporting Information) followed by a familiar p−n + junction on the front side and a polymer−Si organic−inorganic hybrid junction on the rear side that is coated by a PEDOT:Nafion thin film. In addition, a SiN x antireflection layer on the black Si tex- tured surface (see the top of Figure 2) was followed by firing a Ag finger pattern and evaporating the whole back Ag electrode on the front and rear sides, respectively. The PEDOT:Nafion film provides three functions: 1) it selectively transports holes from the Si base, such as a back surface field (BSF), favoring the majority carrier transport and suppressing minority car- rier recombination; 2) it passivates back surface defects; and 3) it forms an electrical Ohmic contact. The general operation Adv. Energy Mater. 2020, 1903851 www.advenergymat.dewww.advancedsciencenews.com © 2020 WILEY-VCH Verlag GmbH b) High-temperature firing Al BSF in this work; c) Al BSF [45] ; d) Multi-PERC [45] with the Al 2 O 3 /SiN x stack locally ablated by a laser. www.advenergymat.dewww.advancedsciencenews.com © 2020 WILEY-VCH Verlag GmbH they were p-type, multicrystalline, and had a resistivity of ≈1.5 Ω·cm and thickness of 180 µm by diamond wire cutting. After the organic layer coating, a Ag metal film was thermally evaporated to form a back electrode (Figure S12, Supporting Information). Characterization: The lifetime was measured by using transient photoconductance decay techniques on the WCT-120 apparatus. The conductivity was measured on glass substrates with a Ag coplanar electrode configuration. [60] The microimages of the PEDOT:Nafion film produced with different ratios were illustrated by high-resolution transmission electron microscopy (HRTEM) by dropping the solution into a copper mesh. The surface topography and roughness of the samples were observed by atomic force microscopy (AFM). The contact angles were measured by fitting a mathematical expression to the shape of the drop and then calculating the slope of the tangent to the drop at the liquid−solid−vapor (LSV) interface line (Drop Shape Analyser, DSA100, Kruss). The work function of the PEDOT:Nafion thin film was measured on a Thermo Scientific ESCALab 250Xi (He I, 21.22 eV) by ultraviolet photoelectron spectroscopy (UPS). The reflectance spectra was characterized by UV–Vis–NIR spectrophotometer (Hitachi U4100). The PV performances of the solar cells were characterized by current density−voltage (J−V) measurements under standard test conditions (AM1.5, 100 mW cm −2 , and 25 °C) and the EQE (R3011, Enlitech). V oc as a function of light intensity was analyzed by Suns-V oc measurement by the Sinton tool. The spatial distribution of the EL intensity of solar cell devices were investigated using a high-resolution EL mapping measurement (LIS-R1, BT Imaging Pty Ltd.). Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements J.C. would like to thank Dr. Benjamin S. Flavel and Dr.Han Li at the Karlsruhe Institute of Technology for fruitful scientific discussions. The authors gratefully acknowledge support from the National Natural Science Foundation of China (Nos. 61804041 and 11604072), Outstanding Youth Science Foundation of Hebei province (No. F2019201367), Natural Science Foundation of Hebei Province (No. F2019204325), and Scientific and Technological Research Project of Hebei Province (No. QN2015008). Conflict of Interest The authors declare no conflict of interest. www.advenergymat.dewww.advancedsciencenews.com © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1903851 (8 of 8)Adv. 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