Full paperImpact of interlayer application on band bending for improved electron extraction for efficient flexible perovskite mini-modules
Introduction
In the past few years, perovskite solar cells (PSCs) have achieved impressive efficiencies (above 22%) [1] thanks to their high absorption coefficient, long charge-carrier diffusion length and defect tolerance [2], [3]. In addition, PSCs can be developed on flexible substrates paving the way to high-throughput roll-to-roll manufacturing, lightweight mobile applications and advanced building-integration. Low-temperature processing is necessary to realize PSCs on flexible plastic substrate, which forbids the use of mesoporous TiO2 scaffold (> 450 °C sintering step) as highly efficient electron transport layer (ETL) [1], [4]. For these reasons, many efforts have been focused on developing efficient ETLs via low-temperature deposition [5], [6].
Commonly, flexible PSCs are developed on indium tin oxide (ITO)-coated plastic substrates [7]. However, there is still a lack of understanding and investigation of charge extraction at the interface between low-temperature ETLs and alternative transparent conductive oxides (TCOs). Al-doped ZnO (AZO) is a promising alternative to ITO for its low cost, composition based on earth-abundant elements and for its high near-infrared transmittance which opens the way to new flexible thin-film tandem concepts [8]. So far, only moderate device efficiency (~ 7%) has been obtained for flexible PSCs based on AZO [9]. Improvements in performances (13.2%) were achieved using undoped ZnO as ETL [8], which, however, poses a source of instability for CH3NH3PbI3 (MAPI) under moderate annealing temperatures (~ 100 °C) due to deprotonation of the CH3NH3+ cation [10]. Therefore, low-temperature deposited ETLs and ETL/AZO interfaces need to be developed and engineered to have reduced energy barriers for electrons extraction and low interface recombination losses.
In order to get one step closer to industrial implementation, upscaling from individual solar cells to module sizes is required.
A thin-film solar module is fabricated by serial interconnection of solar cells, which is mainly based on three patterning steps, two scribes (P1 and P3) are necessary for electrical separation of the front and rear contacts between cells, while one scribe (P2) is needed to create an electrical interconnection between the anode and the cathode of adjacent cells [11]. The area between P1 and P3 does not contribute to the photocurrent (dead area); hence, it is necessary to keep the scribing lines and their separations as narrow as possible.
Only few works on flexible perovskite mini-modules have been reported up to now. Di Giacomo et al. developed a flexible perovskite mini-module on ITO/PET substrate, with an efficiency of 3.1% onto an active area of 7.9 cm2 with scribing procedures based on masking, laser definition and self-patterning [12]. Yeo et al. demonstrated a flexible perovskite mini-module with 8.1% efficiency onto an active area of 10 cm2 (P1 obtained by wet-etching of ITO substrate, P2 by mechanical scribing and P3 by use of shadow masks) [13]. These scribing methods for serial interconnections are not suitable for high speed and large-area industrial manufacturing. Therefore, more reliable, high speed and scalable patterning methods should be considered in order to minimize dead area losses improving mini-module performances. Recently, Dagar et al. reported fully laser-scribed flexible perovskite mini-module onto an active area of 12 cm2 with an efficiency of 5.7% and 8.8% when measured in forward and backward direction, respectively [14]. Limited charge transport and detrimental pile-up at ETL interface could result in hysteresis and poor efficiencies [15].
These mini-module performances (not steady-state values and with significant hysteresis) still lag far behind the performances of small flexible PSCs (< 0.2 cm2 with efficiencies > 14%) [14].
Here we report flexible PSCs with sputtered AZO as TCO and thermally evaporated C60 as ETL. Limited charge transport due to an unfavorable band bending at ETL/TCO interface is observed, which constrains device performances. We show that interfacial modification by spin coated poly(ethylenimine) ethoxilated (PEIE) or thermally evaporated lithium fluoride (LiF) gives rise to favorable band bending and enhanced charge extraction at the interface with TCO. Particularly, LiF interfacial modification induces a stronger modulation of electrostatic potential with a more severe downward band bending at ETL/TCO interface compared to PEIE one.
Flexible PSCs with steady-state efficiency of 14.8% are demonstrated, using LiF as interlayer between AZO and C60 in a traditional n-i-p structure.
In addition, the use of vacuum deposited interlayer and ETL opens the opportunity to mini-module up-scaling. We demonstrate monolithically-integrated flexible perovskite mini-module with a steady-state efficiency of 10.5% onto an aperture area of 10.2 cm2, relying only on laser scribing as patterning method for P1, P2 and P3 interconnections. A geometric fill factor (GFF) of ~ 94% is achieved.
Section snippets
Results and discussion
We have developed PSCs on a flexible substrate which is commonly used as front encapsulation foil for flexible Cu(In,Ga)Se2 (CIGS) solar modules. This substrate exhibits a higher transmittance and orders of magnitude lower water-vapor transmission rate (WVTR) values with respect to commonly used uncoated PET and PEN flexible substrates. As we already reported [8], highly near infrared (NIR)-transparent AZO is used as TCO.
Thermally evaporated C60 is used as ETL to suppress J-V hysteresis and to
Conclusions
We observed that the interface between AZO and C60 represents a bottleneck for flexible PSC performances. As observed by UPS, an unfavorable band bending arises at the ETL/TCO interface. We successfully improved the power conversion efficiency of n-i-p PSCs by application of specific interlayers. With both solution-processed PEIE and vacuum deposited LiF interlayers we are able to achieve an absolute gain in efficiency of 2% with respect to the device without interlayers (from 12.8% to 14.8%
Device fabrication
PSCs are grown on flexible foil which is used as moisture barrier front sheet for encapsulation in flexible CIGS modules. 5 cm × 5 cm size flexible substrates are washed by hand followed by ultrasonic soap and water baths. The substrates are dried in vacuum for one week and cut into 4 quarters. Prior to further processing, 600 nm of compact ZnO:Al layer is deposited at room temperature by RF sputtering from a ceramic ZnO target (containing 2 at% Al2O3). The sheet resistance of as-deposited film
Acknowledgements
Financial funding from Swiss National Science Foundation (SNF)-NRP70, PV2050 (project NO.: 407040_153976 and 407040_153916), SNF-NanoTera and Swiss Federal Office of Energy (SYNERGY: 20NA21_150950), NanoTera (project Synergy Gateway) and FP7 APPOLO project (609355). A special thank goes to Roger Ziltener for his contribution in mini-modules realization.
Stefano Pisoni received his M.S. degree in Materials Engineering and Nanotechnology at Polytechnic of Milan in 2015 with master thesis project in Prof. Henry Snaith’s group at University of Oxford. He is currently a Ph.D. student under the supervision of Prof. Ayodhya N. Tiwari at Empa-Laboratory for Thin Films and Photovoltaics, Switzerland. His current research is focused on development and characterization of flexible perovskite solar cells and mini-modules for thin tandem applications.
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Cited by (0)
Stefano Pisoni received his M.S. degree in Materials Engineering and Nanotechnology at Polytechnic of Milan in 2015 with master thesis project in Prof. Henry Snaith’s group at University of Oxford. He is currently a Ph.D. student under the supervision of Prof. Ayodhya N. Tiwari at Empa-Laboratory for Thin Films and Photovoltaics, Switzerland. His current research is focused on development and characterization of flexible perovskite solar cells and mini-modules for thin tandem applications.
Fan Fu joined Laboratory of Thin Films and Photovoltaics headed by Prof. Ayodhya N. Tiwari at Empa in 2014, and received his Ph.D. degree from Department of Electrical Engineering and Information Technology, ETH Zuerich, in 2017. Now he is working as a postdoctoral scientist in Prof. Christophe Ballif’s group at EPFL. His research interest involves developing efficient and stable perovskite solar cells for multi-junction applications.
Roland Widmer received his Ph.D. in Chemistry in 2002 from the University of Bern (Switzerland) and is currently deputy group leader in the Empa’s nanotech@surfaces Laboratory. He has a strong background in experimental surface physics and chemistry, and follows an experimental approach building on state-of-the-art scanning probe methods (UHV temperature-controlled STM/STS, electrochemical STM) combined with structural and spectroscopic methods based on photoelectron emission techniques (XPS, UPS, XPD, ARPES). His focus is on the fabrication and understanding of complex surfaces in terms of atomic and electronic structure as well as the understanding of their reactivity on the molecular and atomic level.
Romain Carron received his M.S. degree in physics from the Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland, and the Ph.D. degree in physics from EPFL in the field of quantum nanostructures for infrared light emission. After a stay in the industry, he is since 2015 part of the Laboratory for Thin Films and Photovoltaics of Prof. Ayodhya N. Tiwari at Empa, Switzerland. His research interests center on thin film photovoltaics and notably the CIGS technology, with a focus on absorbers and device fabrication, as well on optical, electrical and material characterization.
Thierry Moser received his B.S and M.S degree in Materials Science at ETH Zuerich. He is currently a Ph.D. student under the supervision of Prof. Ayodhya N. Tiwari at Empa-Laboratory for Thin Films and Photovoltaics, Switzerland. His current research is focused on development and characterization of NIR-transparent perovskite solar cells for thin tandem applications and implementation of large-area scalable deposition processes.
Oliver Groening graduated in experimental physics from the University of Fribourg (Switzerland) in 1994. In 1999 he received his PhD for the investigation of the field emission properties of carbon nanostructures in the group of Prof. Louis Schlapbach also in Fribourg. In 2001 Dr. Groening joined Empa, where he helped building up of the nanotech@surfaces laboratory for which he is the deputy laboratory head since 2011. His research interest spans from the investigation of chemical reactions on intermetallic surfaces to the synthesis of novel graphene derived nanostructures for electronic applications.
Ayodhya Nath Tiwari is the head of the Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Material Science and Technology, and a Professor at ETH (Swiss Federal Institute of technology) Zürich, Switzerland. He is the Chairman and founder of Flisom Company producing flexible CIGS thin film solar modules with roll-to-roll manufacturing processes. He received his PhD degree from Indian Institute of Technology, Delhi, India in 1986. His research interests include science and technology of thin films for energy conversion and conservation, compound semiconductors, thin film solar cells based on chalcogenides, kesterites and perovskites.
Dr. Stephan Buecheler received his PhD degree from the Swiss Federal Institute of Technology in Zurich (ETH Zürich) in 2010. He is currently group leader in the Laboratory for Thin Films and Photovoltaics at the Swiss Federal Laboratories for Materials Science and Technology (Empa) with main research focus on the materials science and device physics of different thin film photovoltaic and solid state battery technologies.
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Present address: Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Ecole Polytechnique Fèdèrale de Lausanne (EPFL), Rue de la Maladière 71b, 2002 Neuchâtel, Switzerland.