Structural, Optical, and Chemical Characteristics of High‐Quality PbI₂ Thin Films via Chemical Solution Deposition with Thermal Annealing

Abstract

Lead iodide (PbI 2 ) thin films are interesting semiconductor materials arranged in quasi 2D layers, [1] that have an optical bandgap near the ultraviolet (E g = 2.4 eV) spectrum and are composed of atoms with a high atomic mass. [2,3] These characteristics of PbI 2 allow its use in diverse applications, [4][5][6][7][8] with a recent interest in large-area visible, X-ray, and γ-ray radiation detectors. [2,9] Additionally, PbI 2 thin films are excellent precursors in the chemical synthesis of lead halide perovskite CH 3 NH 3 PbI 3 (MAPI) thin films, [10][11][12] which are photovoltaic materials of great importance. However, whether PbI 2 can be used as a high-performance optoelectronic material or efficient chemical precursor, strongly depends on achieving favorable structural properties. [3,[12][13][14] Moreover, the understanding of how the different deposition conditions affect the multiple stacking conformations that the compact PbI 2 layers take [15] is challenging. Different structural configurations known as polytypes [16] can be achieved but are difficult to reproduce, [3] and can lead to diverse microstructures, [17][18][19] that in turn produce varying optical and electrical properties.Different polytypes can be obtained according to the annealing conditions [20] and the implemented synthesis methodology. [21] Diverse previous studies have reported the synthesis of PbI 2 employing thermal evaporation, physical vapor deposition, laser ablation, atomic layer deposition, spray pyrolysis, and spin-coating with favorable crystalline properties. [12][13][14]22,23] The literature shows that the most common polytype is the 2H, albeit the 4H and 12R polytypes have been reported [24,25] at annealing treatments of 150 and 170 °C, respectively. Achieving the 4H and 12R polytypes may improve the optoelectronic properties in large-area radiation sensors, as well as favoring the quality and stability of perovskite thin films. Thus, the use of PbI 2 thin films in such purposes requires the examination of the structural properties at specified synthesis parameters.Therefore, in this work, we explore the synthesis of PbI 2 thin films via chemical bath deposition (CBD), a technique that has not been extensively studied for the deposition of PbI 2 thin films, only few reports exist in the literature. [3,26] CBD, when done correctly, enables the deposition of uniform, homogeneous, and large-area thin films that are strongly adhered to almost any kind of substrate. This technique does not require complex infrastructure, ergo it is low cost and easy to implement, and has been proven successful for the synthesis of numerous metal iodide thin films. [27][28][29][30] However, achieving good-quality films is complicated, because it requires fine tuning of the chemical formulation (reaction solution), as shown by the work of Kariper [3] where the structure and optical properties of the PbI 2 thin films are significantly influenced by the pH of the reaction solution.

Photocurrent Enhancement by Copper Incorporation in Chemical-Solution-Synthesized Inorganic Lead Perovskite Thin Films

“…For the CsPbI 1.5 Br 1.5 perovskite, the Cs 3d 5/2 and I 3d 5/2 signals are energy located at 724.80 and 619.29 eV binding energy, respectively, which are in correspondence to other reports in the literature . Also, we can observe the presence of the Pb 4f 7/2 located at 138.79 eV, associated with a Pb 2+ state and a low-intensity signal related to metallic Pb . The binding energy values of the Br 3d 5/2 and Cs 4d 5/2 are found to be 68.81 and 76.05 eV, which are similar results to what various authors have reported elsewhere …”

“…10 Also, we can observe the presence of the Pb 4f 7/2 located at 138.79 eV, associated with a Pb 2+ state 43 and a low-intensity signal related to metallic Pb. 38 The binding energy values of the Br 3d 5/2 and Cs 4d 5/2 are found to be 68.81 and 76.05 eV, which are similar results to what various authors have reported elsewhere. 44 By analyzing the photoelectron spectra for the (Cs 0.95 :Cu 0.01 )PbI 1.5 Br 1.3 Cl 0.1 perovskite thin film, there exists evidence supporting that Cu atoms induced a change in the local chemical environment of the perovskite, which consists in an overall chemical shift to lower binding energies of around 0.57 ± 0.08 eV for all photoelectron signals.…”

“…In correspondence, the I 3d spectrum also shows two new peaks, on both the higher and the lower binding energy side of the main photoemission peak, suggesting that there may be iodides within the perovskite unit cell that have less electronic density in the case of the higher binding energy side peak and other iodides with more electronic density in the case of the lower binding energy side peak, meaning that local chemical environment of the iodides are more clearly affected by the presence of Cu and Cl in this perovskite. The Pb 4f also shows the presence of two additional signals in the case of the (Cs 0.95 :Cu 0.01 )­PbI 1.5 Br 1.3 Cl 0.1 perovskite thin film, although in this case, the peaks have been reported before as coming from the formation of a minoritarian lead oxide , at the surface of the film and the remanent presence of metallic lead …”

“…The Pb 4f also shows the presence of two additional signals in the case of the (Cs 0.95 :Cu 0.01 )-PbI 1.5 Br 1.3 Cl 0.1 perovskite thin film, although in this case, the peaks have been reported before as coming from the formation of a minoritarian lead oxide 25,38 at the surface of the film and the remanent presence of metallic lead. 38 Further corroboration for copper incorporation into a perovskite structure is done by analyzing the Cu 2p spectra (the Cl 2p spectra are not shown due to the relatively low photoionization cross-section 45 and concentration of this atom, not allowing the measurement of quality experimental data), where for the CsPbI 1.5 Br 1.5 film, no Cu 2p photoemission peaks are detected. In contrast, for the (Cs 0.95 :Cu 0.01 )PbI 1.5 Br 1.3 Cl 0.1 perovskite thin film, there is a clear presence of a main photoelectron peak located at 931.99 eV binding energy and a strong satellite feature at 942.19 eV binding energy; these strongly suggest that the chemical state of copper in the perovskite is a Cu 2+ state.…”

“…The Raman spectra for the perovskite thin films are found in the right section of Figure , with the PbI 2 spectrum included as a reference, displaying the signature peaks of the 2H–PbI 2 type structure. , In the case of the CsPbI 1.5 Br 1.5 thin film, the spectrum shows a sharp Raman band at 124 cm –1 and two broad bands at 74 and 299 cm –1 , which are related to the transverse-optical (TO) and longitudinal-optical (LO) modes of the cubic perovskites. − The peak at 74 cm –1 is related to the TO 2 mode, the narrow band at 124 cm –1 corresponds to the TO 3 mode, and the broad band at 299 cm –1 corresponds to the second-order phonon LO 2 mode. Although the LO 1 mode is not observed as a distinct peak due to the strong overlap with the TO modes, a broad shoulder around 150 cm –1 can be attributed to this phonon mode.…”