p‐Type Doping and Alloying of CuI Thin Films with Selenium

Abstract

The impact of the intentional selenium doping of CuI thin films is investigated concerning crucial crystalline, electrical and optical properties. For selenium contents in between 0.1 at.% and 1 at.%, the carrier density can be systematically adjusted by the selenium supply during growth between cm and cm while transparency and crystallinity remain unaffected. By temperature‐dependent Hall‐effect measurements, a carrier freeze out is observed and the binding energy of the selenium dopant is determined. The long‐term electrical stability in combination with cappings is significantly improved compared to undoped or oxygen doped CuI. However, for selenium contents exceeding 1 at.%, major crystalline changes are observed that are presumably correlated to a phase transformation. Transmission and electrical measurements suggest that the solubility limit of Se in CuI is about 1 at.% since a degradation of the transparency and decreasing free hole densities are observed for Se contents exceeding 1 at.%. Hence, the doping limit for Se in CuI corresponds to 1 at.%.

Suppression of Rotational Domains of CuI Employing Sodium Halide Buffer Layers

“…These values are also closest to the bulk properties with the smallest impact of interface conduction and mobilities that represent the joint effect of single-crystalline and large domain epitaxial CuI. This corresponds to a slight increase in hole mobilities compared to the literature of domain epitaxial PLD-CuI with μ = 20 cm 2 /(V s), , presumably due to a significant impact of the rotational domains on the electrical transport. After 22 h (purple line), the carrier density already increased to p = (7.2 ± 0.3) × 10 16 cm –3 (averaged over the 25 μm thick crystal), while the mobilities decreased to μ = (20.4 ± 0.9) cm 2 /(V s).…”

“…However, the cleavage symmetry of such samples is completely unaffected by the reappearance of rotational domains and still determined by the single-crystalline parts of the thin film. Hall and resistivity measurements were only obtained for samples that simultaneously possess single-crystalline and twinned CuI and revealed slightly enhanced hole mobilities for PLD CuI with μ = 24 cm 2 /(V s) compared to previous reports maximizing at μ = 20 cm 2 /(V s). , The crystals are also nondegenerate if measured directly after growth but become degenerate after at least 22 h of atmospheric exposure. The low critical thickness for single-crystalline CuI at ∼2 μm currently prevents the growth of volume single crystals with sufficient mechanical robustness as free-standing films and requires further optimizations of the PLD growth or an ex situ transfer to another growth technique.…”

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films

“…In comparison with most TCOs, CuI has a much larger VB dispersion, with a small hole effective mass ~0.3 m 0 . A record hole mobility μ of ~43.9 cm 2 V –1 s –1 in single-crystalline CuI has been reported 21 , which is two orders of magnitude higher than that of commonly used p-type NiO (μ < 0.1 cm 2 V –1 s –1 ) 22 – 24 Appropriate extrinsic acceptor doping (e.g., by substituting the iodine with chalcogen impurities like S or Se) can be employed to further increase the p-type conductivity of CuI 25 , 26 In the low excitation density (i.e., the density of photogenerated electron-hole pairs less than the corresponding Mott density) regime, the optical properties of CuI are dominated by excitonic transitions. For instance, the absorption spectrum exhibits two sharp peaks at ~3.06 eV (Z 1,2 excitonic absorption related to the doubly degenerated VBM at the Γ point) and ~3.7 eV (Z 3 transition due to the split-off band), respectively 27 , 28 while two prominent peaks at ~3.06 eV (corresponding to the free-exciton recombination) and ~2.95 eV (related to the bound exciton recombination or donor-acceptor pair recombination) respectively can be observed in photoluminescence (PL) measurements.…”

Hole-Doping to a Cu(I)-Based Semiconductor with an Isovalent Cation: Utilizing a Complex Defect as a Shallow Acceptor

“…17,18 Copper iodide (γ-CuI) crystallized in a zinc-blende-type cubic is one of the promising inorganic HTMs available by lowtemperature processing due to the wide direct band gap of 2.95 eV and high hole mobility of 12−44 cm 2 /(V s) for single crystals, although the intrinsic hole concentration remains at performance has been achieved yet for perovskite solar cells with γ-CuI HTMs; 17 therefore, extrinsic carrier doping would be an important step toward the development of Cu(I)-based HTMs. 23,24 This paper focuses on the elucidation of the mechanism of p-type doping in Cu 2 O with the isovalent Na impurity 25 to open an approach to p-type doping in γ-CuI. Our firstprinciples calculations suggest Na interacts with the abundant native V Cu defect in Cu 2 O to produce the dopant−vacancy (Na i −2V Cu ) complex, which acts as a single shallow acceptor with a lower formation energy than the V Cu .…”

“…The earth-abundant Cu­(I)-based semiconductors, mostly known as cation-deficient hole conductors, hold the greatest promise in photovoltaic devices that require p-type doping technology for tuning the key device parameters. p-Type doping in the photoabsorbers can enhance the photovoltaic cell performance by reducing nonohmic contacts and series resistance and increasing the open-circuit voltages related to the built-in potential of the junction. , The prominent examples are Cu­(In,Ga)­Se 2 (CIGS) and Cu 2 O photoabsorbers that improve solar cell efficiency by the substantial increase in hole concentration using alkali metal dopants, although the mechanism behind the improvement is still under debate. − Similarly, p-type doping is pivotal for the high-performance hole transport materials (HTM) for perovskite/organic solar cells to improve charge extraction efficiency from the absorbers to the external electrodes, ensuring high-workfunction ohmic contacts and adjusting the Fermi level position to the corresponding absorbers. − Wide-gap p-type Cu­(I)-based semiconductors are investigated for the great potential of the long-term stability to replace conventional organic HTMs. , Copper iodide (γ-CuI) crystallized in a zinc-blende-type cubic is one of the promising inorganic HTMs available by low-temperature processing due to the wide direct band gap of 2.95 eV and high hole mobility of 12–44 cm 2 /(V s) for single crystals, although the intrinsic hole concentration remains at (4–9) × 10 16 cm –3 . − So far, little enhancement of device performance has been achieved yet for perovskite solar cells with γ-CuI HTMs; therefore, extrinsic carrier doping would be an important step toward the development of Cu­(I)-based HTMs. , …”