Isovalent Alkali Metal Doping as Shallow Complex Acceptors in Monovalent Copper (I) Based Semiconductors

The cation-deficient Cu(I)-based semiconductors show the greatest potential in photovoltaic devices that require p-type doping technology for tuning the key device parameters. Such doping can enhance the solar cell performance by reducing nonohmic contacts and series resistance and increasing the open-circuit voltages related to the built-in potential of the heterojunction devices. Notable examples include p-type Cu(In,Ga)Se₂ and Cu₂O photoabsorbers, which are known to improve solar cell efficiency by the substantial increase in hole concentration with alkali metal dopants (e.g. Na) [1, 2], though the mechanism behind the improvement remains unclear. Similarly, p-type doping is pivotal for the high-performance hole transport materials (HTM) for perovskite/organic solar cells, enhancing charge extraction to the external electrodes, ensuring high-workfunction ohmic contacts, and adjusting the Fermi level position to the absorbers. Wide-gap p-type Cu(I)-based semiconductors, such as γ-CuI are investigated for their potential for long-term stability as replacements for conventional organic HTMs. However, to date, device performance improvements have been limited, making extrinsic carrier doping an important step for the development of Cu(I)-based HTMs.
This paper elucidates the p-type doping mechanism in Cu₂O with isovalent Na impurity, opening up a potential route for p-type doping in other Cu(I)-based semiconductors. First-principles calculations suggest Na interacts with the abundant native V_Cu defect in Cu₂O to form the dopant–vacancy (Na_i–2V_Cu) complex, which acts as a shallow acceptor with a lower formation energy than V_Cu [3]. Coulomb repulsion between the impurity and neighboring Cu cations reduces the formation energy of the acceptor-type V_Cu, increasing the net hole concentration. Based on this doping approach, we demonstrate γ-CuI with the Cs impurity distinctly improves the carrier concentration controllability for single crystals and thin films in the range 10¹³–10¹⁹ cm^–3. First-principles calculations indicate that dopant–vacancy complexes (i.e., Cs_i–3V_Cu–V_I and Cs_i–4V_Cu–V_I) could be the origin of increased p-type conductivity, broadening the applicability of isovalent impurity doping in Cu(I)-based semiconductors [3].
Building on this scheme, we found that the variation in the impurity sizes of alkali metals enables both p- and n-type doping when the host material is within the doping limits. Such ambipolar control was attained in Cu₃N polycrystalline thin films. The largest Cs enables p-type conductivity by forming (Cs_N–6V_Cu) acceptor complex, while the smallest Li facilitates n-type conductivity originating from a simple interstitial impurity (Li_i) donor [4].
In summary, we propose a novel route for p-type doping in binary Cu(I)-based semiconductors, such as Cu₂O, CuI and Cu₃N, via the formation of impurity–defect complexes through the interaction between V_Cu and alkali metal impurities. This study presents an effective strategy for overcoming doping bottlenecks in Cu(I)-based semiconductors for use in optoelectronic devices.
[1] F. Pianezzi et al., Phys. Chem. Chem. Phys. 16, 8843 (2014).
[2] T. Minami et al., Appl. Phys. Lett. 105, 212104 (2014).
[3] K. Matsuzaki et al., J. Am. Chem. Soc. 144, 16572 (2022).
[4] K. Matsuzaki et al., J. Am. Chem. Soc. 146, 24630 (2024).