Potential of Perovskite Doping Unlocked: Shaping The Future of Energy and Electronics

Doping of metal halide perovskites (MPHs) is the next, essential step towards the implementation of perovskite semiconductor technology for electronic devices. Nonetheless, only a limited number of successful doping instances have been documented, and it is frequently mistaken for additives, grain passivation, or surface functionalization, none of which affect the semiconductor's charge carrier density. Due to the ionic character of the perovskite crystal, the introduction of heterovalent ions is accompanied by the counter ions that balance out the potential doping effect. Here we present a use of homovalent yet metastable ions, in particular Sm²⁺, to substitute Pb²⁺ in MAPbI₃ (MA = methylammonium). Sm²⁺ ions spontaneously oxidize to Sm³⁺, once incorporated into the crystal lattice and each of them releases one electron to the conduction band. The oxidation of samarium ions is confirmed by the analysis of the Sm 3d core level in the X-ray photoelectron spectrum (XPS). Residual content of the oxidized form of Sm³⁺ present in the doping solution allows to observe a slight shift of the Sm 3d peak towards higher binding energy, suggesting the environment change of the Sm³⁺ ion, once introduced inside the perovskite film. Together with the shift of the XRD pattern upon doping, this supports the hypothesis that the dopant ion is incorporated and stabilized in the crystal lattice. At the same time, the crystal structure of the doped perovskite layer is conserved and no phase separation is observed from the XRD patterns. Ultraviolet photoelectron spectroscopy (UPS) shows a shift of the Fermi level (<i>E_F</i>) by around 0.5 eV towards the conduction band, proving the doping to be n-type. The increase of the free electron density in the conduction band is the direct reason for the conductivity increase for the doped films by 3 orders of magnitude. Using the Mott-Schottky method, the ionized charge carrier density is estimated to be 10¹⁷ cm^-3 for the sample showing the highest conductivity increase, which suggests the ionized dopant concentration in the doped perovskite film to be around 0.01% (Pb density in MAPbI₃ ≈ 20²¹ cm^-3). The discrepancy between this result and the doping concentration stemming from the XPS measurement, which is calculated to be around 20%, leads to two possible explanations. Either the majority of the Sm²⁺ introduced to the perovskite polycrystalline film resides at grain boundaries and therefore does not act as a dopant or the dopant is only partially oxidized in normal conditions, exhibiting the freeze-out effect. The dopant activation energy (the energy necessary to oxidize all the Sm²⁺ ions to Sm³⁺) of around 350 meV seems to support the latter hypothesis and is in agreement with the energy between the <i>E_F</i> and conduction energy (<i>E_C</i>). The presented method allows to reach the highest to-date conductivity increase for the n-type and may become a protocol for the efficient n-type doping of MHPs. To illustrate one of its applications, we fabricated perovskite solar cells (PSC) with poly(triaryl amine) (PTAA) as a hole transporting layer (HTL) (p-side) and Sm-doped MAPbI₃(n-side), without the electron transporting layer (ETL). Additionally, we used gold as an electron collecting electrode (<i>WF_Au</i> = 5.22 eV). The strain-induced ohmic contact between highly doped perovskite and Au allows to minimize the series resistance at this interface and an efficient electron collection. This shows the potential of flexible electrode selection for the doped PSC. Therefore, the ETL-free n-doped PSC retains the same PCE as the reference sample (with PCBM as an ETL), in spite of the simplified architecture that decreases the fabrication cost and the number of interfaces.