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]