Thermal Evaporation: an Alternative Deposition Method of Self-Assembled Monolayers for Lossless Interfaces in p-i-n Perovskite Solar Cells

Ahmed Farag^a^,^b, Thomas Feeney^a, Ihteaz M. Hossain^b, Fabian Schackmar^a^,^b, Paul Fassl^a^,^b, Kathrin Küster^c, Rainer Bäuerle^d, Marco A. Ruiz-Preciado^a^,^b, Mario Hentschel^e, David B. Ritzer^a^,^b, Alexander Diercks^a, Yang Li^a^,^b, Bahram Abdollahi Nejand^a^,^b, Felix Laufer^a, Roja Singh^a, Ulrich Starke^c, Ulrich W. Paetzold^a^,^b

^aKarlsruhe Institute of Technology, Light Technology Institute, Engesserstrasse 13, 76131 Karlsruhe, Germany

^bKarlsruhe Institute of Technology, Institute of Microstructure Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

^cMax Planck Institute for Solid State Research, Stuttgart, Germany, Germany

^dInnovationLab GmbH, Heidelberg-Germany, Speyerer Straße, 4, Heidelberg, Germany

^e4th Physics Institute and Research Center SCoPE, Pfaffenwaldring 57, 70569 Stuttgart, Germany

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)

London, United Kingdom, 2023 June 12th - 14th

Organizers: Tracey Clarke, James Durrant and Trystan Watson

Poster, Ahmed Farag, 049
Publication date: 30th March 2023

Given the key role that hole transport layers (HTLs) play in the photovoltaic performance of perovskite solar cells (PSCs), identifying a proper HTL is pivotal to advance the technology. Among the various HTLs employed in the inverted p-i-n PSC architecture, self-assembled monolayers (SAM-HTLs) have presented themselves as a prime candidate.^1,2 Their close to perfect energetic alignment, hole selectivity, and self-assembling behavior, result in almost lossless contacts and minimize interfacial recombination. The importance of SAM-HTLs further manifests in the fact that many high efficiency single-junction PSCs and perovskite-based monolithic tandems use the PACz derivatives (i.e. 2PACz, MeO-2PACz, and Me-4PACz). Until now, these materials have been exclusively deposited via solution-based methods, which limits their versatility and restricts process flexibility. To overcome this challenge, we present the first vacuum-based evaporation of common SAM-HTLs.³

At first sight, it would be reasonable to assume that the quality of the SAMs itself and, thus, of the SAM-HTLs/perovskite interface, degrades if they are deposited via thermal evaporation. However, X-ray photoelectron spectroscopy (XPS) measurements provide no evidence towards the emergence of new signals in evaporated SAMs compared to their solution-processed counterparts. Furthermore, XPS characteristic peak positions and relative area of the evaporated films exhibit no shift or remarkable change compared to solution-processed and original powder samples. In addition, reflection-absorption infrared spectroscopy measurements emphasize that the evaporated SAM-HTLs maintain the self-assembling behavior expected in solution-processed counterparts, independent of the evaporated film thickness, which is essential for fabricating efficient PSCs.

Morphological, compositional, and microstructural analysis indicate no remarkable difference in the crystallization dynamics of perovskite thin films deposited onto SAM-HTLs with either deposition method. Furthermore, a dramatic enhancement in surface wettability is achieved for evaporated SAM-HTLs, tackling the previously reported poor surface wettability issue of solution-processed Me-4PACz.

Photoluminescence quantum yield, together with time resolved photoluminescence spectroscopy data demonstrate the high optoelectronic quality of the HTL/perovskite interface is preserved or slightly enhanced when employing evaporated SAM-HTLs. In addition, the comparable minority charge carrier lifetime in half-stacks employing either solution-processed or evaporated SAM-HTLs indicates no remarkable difference in the charge carrier dynamics. Furthermore, evaporating SAM-HTLs over micrometer-sized textured surfaces results in conformal coverage and consequently an almost lossless HTL/perovskite interface. Thus, in addition to the process flexibility that evaporating SAMs offer, PSCs employing evaporated SAM-HTLs outperform their solution-processed counterparts both on planar and textured substrates.

References:

1. Al-Ashouri, A. et al. Conformal monolayer contacts with lossless interfaces for perovskite single junction and monolithic tandem solar cells. Energy Environ. Sci. 12, 3356–3369 (2019).

2. Al-Ashouri, A. et al. Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction. Science (80-. ). 370, 1300–1309 (2020).

3. Farag, A. et al. Evaporated Self-Assembled Monolayer Hole Transport Layers: Lossless Interfaces in p-i-n Perovskite Solar Cells. Adv. Energy Mater. 2203982 (2023) doi:10.1002/AENM.202203982.

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