Vacuum sublimation of Dopant‐Free Crystalline Spiro‐OMeTAD films to enhance the Stability of Perovskite Solar Cells
Angel Barranco a, Carmen Lopez-Santos a b, Jesus Idigoras c, Francisco J. Aparicio a, Jose Obrero a, Javier Castillo-Seoane a b, Victor Lopez-Flores a, Lidia Contreras-Bernal c, Victor Rico a, Javier Ferrer d, Juan P. Espinos a, Ana Borras a, Juan A. Anta c, Juan R. Sanchez-Valencia a b
a Instituto de Ciencia de Materiales de Sevilla (ICMS), Consejo Superior de Investigaciones Científicas (CSIC), Universidad de Sevilla, C/ Américo Vespucio 49, Sevilla, Spain
b Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Spain, Avenida de la Reina Mercedes, Sevilla, Spain
c Pablo de Olavide University, Sevilla, Spain, Carretera de Utrera, km. 1, Montequinto, Spain
d Centro Nacional de Aceleradores, Universidad de Sevilla, CSIC, Spain, Av. Thomas A. Edison 7, Sevilla, Spain
nanoGe Perovskite Conferences
Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
Sevilla, Spain, 2020 February 23rd - 25th
Organizer: Hernán Míguez
Oral, Juan R. Sanchez-Valencia, presentation 034
Publication date: 25th November 2019

Organometal halide perovskites have emerged as one of the most promising photoabsorbent materials for efficient and cheap solar cells and have recently reached certified Power Conversion Efficiencies (PCE) of 25.2%.[1–4] The research in Perovskite Solar Cells (PSC) underwent an inflexion point when the group of Miyasaka first[5] and Park later[6] replaced the liquid electrolyte by solid‐state hole conductors (SSHCs), which increased significantly the PCE and stability of the devices. From this moment on, most of the researches on PSCs implement SSHC in the devices. The first and still most used SSHC is the organic molecule Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene].

Whereas the use of Spiro‐OMeTAD is widely spread, the undoped form of this material is reported to have very poor intrinsic hole mobility and conductivity, leading to high series resistance in the devices.[7,8] The poor charge transport properties of Spiro‐OMeTAD are solved in the literature by means of dopants. The most common additive for Spiro‐OMeTAD is lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), which dopes the system with Li+ cations, improving significantly the hole mobility.[9]

It is clear from the literature that a common strategy to enhance the charge transport properties of some SSHC is the addition of dopants in different relative concentrations. Unfortunately, the main handicap still hindering the eventual exploitation of PSCs is their poor stability under prolonged illumination, ambient conditions, and increased temperatures, which is partially hindered by the use of dopants or additives that can contribute to the degradation of the perovskite film.[7,9] For instance, Li+ cations are highly hygroscopic and induce moisture absorption.[10]

However and regardless of the significant number of examples that can be found in literature about Spiro‐OMeTAD layers as SSHC, negligible attention has been paid to the effect of its crystalline structure. This can be understood due to the highly disordered nature of solution‐processed films (the most widely spread) and to its amorphous‐growing tendency related to the spiro-center.[11,12] Despite the low intrinsic hole mobility reported for dopant‐free Spiro‐OMeTAD layers fabricated by wet approaches, the group of Bakr has recently reported significantly improved hole‐transporting properties of pristine Spiro‐OMeTAD single crystals.[12] Their results are supported by theoretical calculations performed by Brédas and Houk,[13,14] who reported a very high intrinsic hole conduction of crystalline Spiro‐OMeTAD layers along the π–π stacking direction of the crystal. Their findings demonstrated that the crystalline order of this SSHC is crucial to promote new charge transport pathways. Even though these results show the potential of crystalline Spiro‐OMeTAD layers to enhance the photovoltaic properties of PSCs, their implementation is not straightforward due to the antisolvent experimental strategy used to grow this organic molecule in its crystalline form.[12]

In this work,[15] we report the unprecedented sublimation under vacuum conditions of the most widely used SSHC in perovskite solar cells, the Spiro‐OMeTAD, in the form of dopant‐free crystalline layers. In addition, we demonstrate the enhanced stability of these layers acting as SSHC in PSCs in comparison with the solution‐processed counterpart. Our results reveal on one hand that the substrate temperature is a critical parameter controlling the microstructure and crystallinity of the layers. On the other hand, the implementation of these vacuum sublimated Spiro‐OMeTAD layers on PSCs have demonstrated two key aspects: i) a considerably increased PCE in comparison to the dopant‐free Spiro‐OMeTAD layers fabricated by a standard wet approach and ii) a significant enhancement of the stability of the cells, which have been tested under continuous illumination during 40 h and after annealing in air up to 200 °C.[15]

The authors thank the AEI, “Consejería de Economía y Conocimiento de la Junta de Andalucía” (US‐1263142), MINECO (MAT2016‐79866‐R, MAT2013‐42900‐P, FPA2016‐77689‐C2‐1‐R, and MAT2016‐76892‐C3‐2‐R) and the EU through cohesion fund and FEDER 2014‐2020 programs for financial support. J.R.S‐V. and A.B. acknowledge the project PlasmaPerovSol and funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska‐Curie grant agreement ID 661480. J.R.S‐V‐ and M.C.L‐S. thank the University of Seville through the VI PPIT‐US. Beam times at DESY (Hamburg, Germany) and at Elettra ring (Trieste, Italy) are acknowledged. The authors acknowledge DESY, a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA-III, at the P03/MinaXS beamline and the authors would like to thank Dr. Björn Beyersdorff for assistance. This research has received funding from the EU‐H2020 research and innovation programme under Grant Agreement No.654360 having benefitted from the access provided by Technische Universität Graz at Elettra—TUG in Trieste (IT) within the framework on the NFFA Europe Transnational Access Activity. F.J.A. and J.R.S‐V. acknowledge the “Juan de la Cierva” and “Ramon y Cajal” national programs, respectively.

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