Stability at Scale: The Challenges of Durable Module Interconnects for

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Stability at Scale: The Challenges of Durable Module Interconnects for Perovskite Photovoltaics Jeffrey A. Christians, Fei Zhang, Rosemary C Bramante, Matthew O. Reese, Tracy H Schloemer, Alan Sellinger, Maikel F.A.M. van Hest, Kai Zhu, Joseph J. Berry, and Joseph M. Luther ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01498 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Energy Letters

Stability at Scale: The Challenges of Durable Module Interconnects for Perovskite Photovoltaics Jeffrey A. Christians,1,3 Fei Zhang,1 Rosemary C. Bramante,1 Matthew O. Reese,1 Tracy H. Schloemer,2 Alan Sellinger,1,2 Maikel F. A. M. van Hest,1 Kai Zhu,1 Joseph J. Berry,1 Joseph M. Luther1*

Affiliations: 1

National Renewable Energy Laboratory, Golden, CO 80401, USA

2

Department of Chemistry, Colorado School of Mines, Golden, CO 80401, USA

3

Department of Engineering, Hope College, Holland, MI 49423, USA

*Correspondence to: [email protected]

Abstract: Uniting efficiency, scalability, and stability is the next frontier for perovskite solar cells. Stability tests conducted on efficient perovskite solar cell mini-module architectures reveal promising stability yet also the stability challenges of scale up.

TOC Graphic:

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Photovoltaic modules must invariably have three characteristics: efficiency, scalability, and stability. Halide perovskite solar cells, while generating excitement throughout the research community, have yet to deliver on all three of these aspects simultaneously. Multiple groups world-wide have surpassed 20% power conversion efficiency (PCE). In scaling, Yang et al. recently reported a 10.36 cm2 mini-module with 17.9% active area PCE1 and Deng et al. reported a 180 m hr-1 coating procedure with modules of 14.6% PCE at 57.2 cm2.2 However we note that further progress in efficiency at scale is still required to achieved the figures met for panels of other thin film technologies like CdTe and CIGS.3 Recently, we identified the most critical interface-related degradation mechanisms in the standard perovskite solar cell architecture and mitigated them to dramatically improve device operational stability of small-area cells.4 While these efforts continue, research into device stability has intensified as it has become clearer that this aspect of the technology will likely dictate its eventual success or failure.5 While stability tests on individual cells have yielded dramatic device architecture and concomitant stability improvements,6 eventually the field must move toward the combination of efficiency, scalability, and stability. Impressive stability results have been reported by Grancini et al.7 for a sealed, ca. 10% efficient mini-module based on the TiO2/ZrO2/perovskite/carbon 2 ACS Paragon Plus Environment

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architecture which Mie et al. pioneered.8 Nevertheless, improving the efficiency of devices with this architecture such that they are competitive with existing commercial technologies remains a distinct challenge. To better understand the challenges associated with the goal of efficiency, stability, and scale we have fabricated well-performing perovskite mini-modules using our most stable previously reported cell architecture4 and subjected them to stability tests. Here, the mini-modules were spin-coated and consisted of a SnO2 electron transport layer9 deposited on indium tin oxide (ITO), an alloyed formamidinium lead iodide-based perovskite

active

layer

(the

precursor

solution

stoichiometry

was

(FA0.79MA0.16Cs0.05)0.97Pb(I0.84Br0.16)2.97,4,10 hereafter referred to as FAMACs), an EH44 (EH44 = 9H-(2-ethylhexyl)-N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-9H-carbazole-2,7-diamine)

hole-

transport layer,11 and lastly a MoOx/Al contact.12 The device design consisted of 4 monolithically interconnected cells with a combined active area of 7.25 cm2, a total area (including the area of the scribes) of 7.84 cm2, and a geometric fill factor of 92.5%. The best performing mini-module (Figure 1a) achieved an active-area PCE of 14.9% with a short-circuit current density (JSC) of 5.75 mA cm-2, an open-circuit voltage (VOC) of 4.19 V, and a fill factor (FF) of 0.62. Typical devices with this architecture had PCE exceeding 12% (Table S1) and showed negligible hysteresis under the scan conditions utilized (Figure S1). A photograph of a typical device with this architecture is shown in Figure 1b and a schematic representation of the interconnect design used is shown in Figure 1c.



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Figure 1. (a) Current density-voltage (J-V) curve of champion ITO/SnO2/FAMACs/EH44/MoOx/Al perovskite solar cell mini-module. The total area was 7.84 cm2 while the performance characteristics are based on the active area of 7.25 cm2 (geometric fill factor 92.5%). The mini-modules consisted of 4 interconnected cells, as shown in the photograph (b) where the monolithic interconnect design is shown in the schematic (c). (d) The stability of a mini-module was monitored under constant illumination at ~70 mW cm-2 intensity (lamp spectrum shown in Figure S2) under constant load in a nitrogen atmosphere.

Figure 1d depicts the performance of a well-performing mini-module of this architecture under constant operation in a nitrogen atmosphere. The device was constantly operated by holding under resistive load during the course of the experiment except when the performance of the device was monitored by conducting a J-V sweep every 30 min. During 100 hrs of constant operation in these conditions (ISOS-L-2 conditions in N2),13 the performance of the mini-module retained ~92% of its performance relative to the initial PCE. While this stability is promising, it trails behind the stability of similar single-cell devices under the same test conditions; comparable small-area single-cell devices conducted in the same inert environment show no


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ACS Energy Letters

degradation over 400 hrs (Figure S3) and our previous experiments on similar devices demonstrated only a ~2% relative decrease in performance over 1500 hrs of testing.4 In the case of modules, the interconnects themselves can serve as an avenue for degradation,3 particularly given the redox reactions observed between halide perovskites and a wide array of metals.14,15 Even in the case of the MoOx/Al contacts employed in this study, where the self-passivating nature of the oxide-metal interface imparts dramatic stability gains,12 reactions between the MoOx and perovskite at the interconnects will likely be of utmost importance,16 as will defects in the MoOx layer which can lead to direct contact with Al.4 We also find that interconnect design is not merely a function of the metal contact used, interconnect degradation can be dramatically different in devices with very similar architecture and the same metal contacts. At the single cell level, architectures featuring either EH44 or spiro-OMeTAD hole transport layers both show reasonable stability using MoOx/Al contacts.4,12 Based on this previous cell-level result, we would expect reasonable stability for both spiro-OMeTAD- and EH44-based mini-modules with EH44 likely somewhat outcompeting spiro-OMeTAD.4,11 In contrast to this expectation, when mini-modules with these architectures are stressed, the EH44based devices perform as expected while the spiro-OMeTAD devices experience extremely rapid and complete device failure with interconnect damage clearly visible to the naked eye (Figure S4). Further work elucidating the differences present in these systems which result in this divergent behavior will likely prove critical to the design of stable interconnects. Achieving all three of efficiency, scalability, and stability are necessary for any photovoltaic technology, perovskite solar cells not excepted. The perovskite solar cell community has thus far experienced continued progress in each of these areas individually and even in two of these aspects in tandem: efficiency and stability;4,10 efficiency and scale;1,2



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stability and scale.7 Through the stability assessment of an efficient and stable n-i-p perovskite solar cell architecture at mini-module scale, we highlight the challenges facing the community in achieving all three of these necessary goals. Importantly, we show that while much of the understanding of stability and degradation pathways gained at the cell level does indeed translate to larger area modules, there are key differences and aspects which must be treated separately, motivating a range of studies at the material, cell, and module level.17 Specifically, we reveal that the design of the interconnects and understanding associated with interconnect-specific degradation pathways, currently woefully understudied aspects of perovskite solar cell modules, will be critical to the development of commercially successful perovskite solar cell modules.


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ACS Energy Letters

Supporting Information The Supporting Information is available free of charge on the ACS website at DOI: XXXX Includes: materials and detailed fabrication and characterization methods, additional device characterization data, stability testing system lamp spectrum, single cell stability data, and photographs of interconnects before and after stressing.

Acknowledgements This work was authored by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy, Solar Energy Technologies Office of the Energy Efficiency and Renewable Energy under Contract No. DEAC36-08-GO28308 with the National Renewable Energy Laboratory. J.A.C. was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy Postdoctoral Research Award under contract number DE-SC00014664. We thank Andrée Paquin and Francis Bélanger of PCAS Canada for supplying 2,7-dibromocarbazole as a precursor for the synthesis of the EH44 hole transport material used in this study. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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Stability at Scale: The Challenges of Durable Module Interconnects for

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