A study on light sensitization behavior in (Mph)2CuCl4 low-dimensional hybrid material with tetrahedral units and extended absorption up to the NIR region

To develop toxic-free hybrids and hybrid perovskites with transition metals, copper is a potential candidate moreover, Cu settles to a layered structure with higher stability. Depending on the size of the organic cation, the organic inorganic copper halide hybrid settles into a lower dimensional perovskite structure or as a lower dimension hybrid material which lack the octahedral factor and form a regular array of tetrahedral units with organic cations affixing to them. Due to their unique optical properties, these hybrid structures have been comprehensively investigated for light emitting applications. According to the structural study, the synthesized morpholinum copper chloride ((Mph)2CuCl4) hybrid has inorganic tetrahedral units organized in layers, with organic bilayers between them. The band gap of the material was found to be 1.44 eV with good absorbance. This work examined the light sensitization properties of the organic inorganic hybrid semiconductor (Mph)2CuCl4. For that, a device of structure fluorinated tin oxide glass substrate/TiO2 compact layer/TiO2 mesoporous layer/(Mph)2CuCl4/Spiro-OMeTAD/Au was fabricated and encountered solar cell parameters and impedance.


Introduction
The rapid growth of research on hybrid organic inorganic metal halide perovskites (HOIP) began in 2009 when Miyasaka et al disclosed the application of methylammonium lead iodide and methylammonium lead bromide as light sensitizers over titanium dioxide (TiO 2 ) electron transport material [1]. Their appealing photophysical characteristics are a high absorption coefficient, variable band gap, and effective c harge transport properties [2][3][4]. HOIPs have a general stoichiometry of AMX 3 , where A is the organic cation, M is a metallic cation, and X is a halide variation [5][6][7]. The network of corner-sharing octahedra (MX 6 ) 4− crystallizes with organic cation A at the 12-fold coordination sites to create the perovskite structure [8]. Big concerns of these HOIPs include lead (Pb)-toxicity and poor stability [9,10]. Pb toxicity can be reduced by substituting tin (Sn), bismuth (Bi), or copper (Cu) for Pb [11][12][13]. Sn and Bi-based perovskites are more discussed in the case of lead-free perovskites in which the stability of Sn-based perovskites is inferior, and the performance of Bi-based perovskites is poor [14,15]. However, the case of Cu-based perovskites is different. Cubased HOIPs do not require an inert atmosphere to process and possess appealing optical characteristics [16]. The smaller ionic radius of Cu (73 picometer) leads to a two-dimensional (2D) settlement, which is more stable than Pb, Sn, and Bi-based perovskites [17]. Switching to large-sized organic cations can also increase stability by boosting the settlement to a 2D structure [18,19]. In 2D perovskites, the (MX 6 ) 4− octahedra are arranged as layers, and organic cations are placed between the layers [20]. An important outcome of moving to larger organic cations for Cu-substituted Pb-based HOIPs is the development of hybrid organic-inorganic metal halide (HOIH) with (MX 4 ) 2− tetrahedral repeating units surrounded by organic cations resembling layered perovskite [21,22].
HOIH semiconductors attract much attention from the scientific community because of their outstanding stability and optical properties [23,24]. Some of the important works are listed here. A zero-dimensional (0D) x-ray sensitive HOIH emitter (C 7 H 11 N 2 ) 2 CuBr 4 with an isolated organic cation bilayer and tetrahedral units was described by Timothy M. McWhorter et al in 2021 [25]. In 2021, Lu et al presented two chiral HOIHs, (R/Smethyl benzyl ammonium) 2 CuCl 4 and (R/S-methyl benzyl ammonium) 2 CuBr 4 , for spin filtering in spintronic applications [26]. In 2023, Ioannou, A et al reported on the structural and optical properties of two Cu-based HOIHs, (CH 3 SC(NH2) 2 ) 2 CuBr 4 and (CH 3 SC(NH2) 2 ) 2 CuCl 4 . All these materials have repeating layers of tetrahedral units and stacking organic bilayers between them, similar to 2D perovskites, to achieve A 2 MX 4 stoichiometry [27]. These reported HOIH have one thing in common: long-term endurance with no signs of deterioration. Upon considering the organic cations suitable for HOIH, morpholinum (C 4 H 10 NO + ) is a wellknown contender.
Owczarek et al reported that morpholinum organic cation forms a HOIH structure with the Bi-halide in the inorganic part, which settles into tetrahedral repeating units [28]. Szklarz et al reported a similar HOIH with tetrahedral units in morpholinum tetrafluoroborate [29]. These works helped to finalize our organic cation selection.
In this work, we report morpholinum copper chloride (Mph) 2 CuCl 4 HOIH, and examine its light sensitization behavior. The HOIH material was synthesized by reacting morpholine hydrochloride ((C 4 H 9 NO.HCl))(Mph. HCl) and copper (2) chloride (CuCl 2 ). The structural investigation revealed their stoichiometry and formation of tetrahedral inorganic building blocks. The optical features of the material admired us for investigating its light sensitization behavior and extracted band gap from the Kubelka-Munk plot. The light sensitization analysis was accomplished by employing the well-known mesoporous perovskite solar cell design with (Mph) 2 CuCl 4 as the active layer. The solar cell characteristics and the impedance spectrum of the device were examined.
Preparation of (Mph) 2 CuCl 4 : Mph. HCl to CuCl 2 had a molar (M) ratio 1:1. We dissolved 1.23 gram (g) of Mph. HCl in 20 milliliters (mL) of ethanol via sonication. After that, 1.34g of CuCl 2 is slowly added to the Mph. HCl solution and continued sonication for 30 minutes (min). The mixture is stirred for 3-4 hours (hrs) at 50°C and filtered to create the precursor solution of (Mph) 2 CuCl 4 HOIH. A part of the solution was kept in the oven at 65°C overnight to increase the molarity and then transferred, the solution into a beaker and maintained at 4°C. We kept it undisturbed for two weeks to grow crystals depicted in the inset image in figure 1(b) also, the molarity of Mph. HCl is maintained at 0.5M to avoid crystallization in (Mph) 2 CuCl 4 HOIH precursor solution, which is used for thin film preparation and device fabrication [30].
Device fabrication: We sprayed 8 volume (V)% of TTIP in IPA over the FTO glass substrate, followed by 30 min of sintering at 450°C, producing a 40 nanometer(nm) thick compact TiO 2 layer. After that, we dispersed TiO 2 paste in ethanol at a weight ratio 1:6 via overnight stirring. This dispersion was deposited over the compact layer of TiO 2 by spinning it at 3000 rotations per minute (rpm) for 30 seconds(s) and then sintering it at 450°C for 30 min, a mesoporous TiO 2 electron transport layer (ETL) was created. Spin-coating the precursor solution for 30 s at 2000 rpm twice resulted in the deposition of a 150 nm thick (Mph) 2 CuCl 4 HOIH layer onto the mesoporous TiO 2 layer. The HOIH film was then annealed at 65°C for 48 hrs to achieve moisture stability. To create the hole transporting layer (HTL), spiro-OMeTAD, Li-TFSI, and 4-TBP were blended in chlorobenzene following a general process. It was spin-coated at 4500 rpm for 30 s to form the HTL layer. On top of it, a 70 nm thick gold (Au) metal was thermally evaporated to obtain the following device structure: FTO glass substrate/TiO 2 compact/TiO 2 mesoporous/(Mph) 2 CuCl 4 / spiro-OMeTAD/Au [31,32].
A Cu-Kα source with a wavelength of 1.54 angstrom (Å) was used in IISER Bhopal, India, to obtain (Mph) 2 CuCl 4 single crystal x-ray diffraction (SXRD) data in D8 Venture. An ultravioletvisible-near infrared

Structural analysis
The inset picture in figure 1(b) shows the deep-green colored crystals formed by gradually cooling the (Mph) 2 CuCl 4 precursor solution to 4°C and holding that temperature. SXRD examination verified that we received (MX 4 ) 2− tetrahedral repeating units, as shown in figure 1(a) [33]. According to Li et al., the size of the organic cation determines the feasibility of forming a perovskite or nonperovskite structure [34]. Since the morpholinum cation is large enough, it does not favor the formation of (CuCl 6 ) 4− octahedral units; instead, it settles between (CuCl 4 ) 2− tetrahedral units. Along the bc-plane, the neighboring isolated tetrahedra arrange themselves into single layers. It is clear from figure 1(a) that each layer has variations in the intertetrahedral spacing. The stack of organic bilayers created by Coulombic and weak Wander Vaal interactions between these tetrahedral layers is evidenced by figure 1(a) [34,35]. The lengths of the Cu-Cl bonds in a tetrahedral unit are 2.28 Å , 2.28 Å , 2.25 Å , and 2.29 Å shown in the inset image in figure 1 (b). Similarly, the angles between Cl-Cu-Cl in the tetrahedral unit are 168.91°and 163.13°. Figure 1(a) shows that in (Mph) 2 CuCl 4 tetrahedra are not arranged uniformly. The closest and farthest distance between neighboring tetrahedral units is 2.97 Å and 4.61 Å, respectively. Figure 1(a) shows that although the substance lacks a perovskite structure, stoichiometry has an A 2 MX 4 form. Using the VESTA 3.5.8 software program, we retrieved the crystallographic structure. Moreover, figure 1(b) displays the XRD pattern of the crystal produced by the same tool. The most prominent reflection in the XRD pattern is the reflection from the (200) plane at 8.5°. SXRD indicates that (Mph) 2 CuCl 4 belongs to the noncentrosymmetric space group of the orthorhombic lattice system. The lattice parameters, unit cell volume, and space group details are listed in table 1. Figures 2(a) & (b) depicts FESEM images of thin films of (Mph) 2 CuCl 4 with spans of 10 micrometer (μm) and 4 μm, respectively. Figure 2(a) shows the regular arrangement of rod-like microstructures in both the horizontal and vertical axes. The picture also shows that microstructure creation is more frequent in the horizontal than in the vertical direction. The (Mph) 2 CuCl 4 thin film FESEM image in figure 2(b), which has a shorter span, supports the homogeneity in creating these microstructures. Figure 2(c), the surface profile of the (Mph) 2 CuCl 4 thin film indicates fast crystallization that supports the creation of the microstructure in the FESEM images. The thickness of the film was determined to be 150 nm by measuring the smooth area, which represents the wipedoff portion. Figure 2

Optical analysis
The nonuniformity of (Mph) 2 CuCl 4 thin film prompted to take DRS. The Kubelka-Munk (KM) function, which is derived from the DRS of (Mph) 2 CuCl 4 , is plotted against the incident energy in eV in figure 3(a), depicting the Kubelka-Munk plot of (Mph) 2 CuCl 4 for direct band gap materials [38]. A line drawn from the point exhibits a sharp decline in the KM function intersected at 1.44 eV, designated band gap(E g ). It is evident from the absorption spectrum in figure 3(b) that the substance has two prominent broad absorption peaks in the UVVis-NIR range. The first peak spans the wavelength range from 240 to 563 nm, and the second peak spans from 563 to 870 nm. In most reported cases, Cu-based HOIPs and HOIHs have narrow multiple absorption peaks spanning the UV-Vis-NIR wavelength range [27,[39][40][41]. We obtained two broad peaks across the region with enough absorbance for light sensitization here, which prompted us to examine the use of (Mph) 2 CuCl 4 in solar cells.

Solar cell analysis
Light emission applications are the primary focus of research on Cu-based hybrids. Nevertheless, the acquired band gap and high absorbance inspire us to study the solar cell application of the (Mph) 2 CuCl 4 hybrid. A simple mesoporous perovskite solar cell configuration is considered with the (Mph) 2 CuCl 4 hybrid semiconductor as the active material. The fabrication process is shown in figure 4. FTO glass substrate/Compact  Impedance analysis is also performed to investigate why the solar cell performs poorly. Using a source input voltage of 10 mV and frequencies ranging from 0.1 hertz (Hz) to 10 6 Hz, we performed a dark mode analysis on the device to determine the charge transfer and recombination resistance. Figure 5(b) displays the Newton algorithm-generated fitted and simulated Nyquist plot produced by the device employing software eissa 1. An inset image in figure 5(b) shows the equivalent circuit suitable for the (Mph) 2 CuCl 4 solar cell. The nonideal capacitive impedance is represented by the constant phase element (CPE) with a phase factor (n) of 0.93. The estimated capacitance (C) is 1.01 microfarad (μF), connected to the dielectric response of the (Mph) 2 CuCl 4 HOIH layer. The resistance R 1 , faced by the charge carriers associated with the top and bottom contact to underlying HTL or ETL interface resistances and ohmic resistance of the device, equals 120 ohm (Ω). Since we employed the standard perovskite solar cell architecture, which was designed for high output current density, we may eliminate the ohmic resistance of the connections [44,45]. If the area normalized series resistance is greater than or equal to 10 Ωcm 2 , it is classified as very high. The value of our solar cell for the same is 10.3 Ωcm 2 (120 * 0.09 = 10.3) [46]. This very high area-normalized R 1 is responsible for insufficient charge extraction and voltage drop within the device and eventually causes the low PCE in the (Mph) 2 CuCl 4 solar cell [43,47,48]. The charge recombination resistance, denoted by R 2 , is 128 kΩ coupled with CPE in the equivalent circuit diagram. More than 1000 Ωcm 2 is the favorable area normalized shunt resistance value. That is, 11520 Ωcm 2 (128000 * 0.09 = 11520) in the case of our solar cell [45]. Our solar cell showed a better FF (45.64%) due to this area having normalized high R 2 values, which point to low trap density, less charge recombination, and less leakage current [49][50][51]. This high R 2 is responsible for producing a J sc = 0.044 mAcm −2 even in the presence of high series resistance [48]. Hence, the high value of R 1 emerged as the primary cause of the performance drop in (Mph) 2 CuCl 4 solar cells.

Conclusion
The synthesis and structural analysis of an (Mph) 2 CuCl 4 hybrid semiconductor were demonstrated in the current study. The inorganic segment of this material settles into CuCl 4 tetrahedral units arranged in layers with an organic bilayer in between them. The material displayed UVVis-NIR absorption with a promising quantity absorption coefficient and band gap at 1.44 eV. Our investigation into the behavior of (Mph) 2 CuCl 4 under illumination in this study was prompted by its notable optical properties. To do this, we developed a mesoporous perovskite solar cell architecture employing (Mph) 2 CuCl 4 as the active layer instead of the perovskite material. The obtained solar cell parameters are J sc = 0.044 mAcm −2 , V oc = 0.52 V, FF = 45%, and PCE=0.01%. Impedance analysis and calculated solar cell resistance parameters from the J-V plot of the (Mph) 2 CuCl 4 solar cell confirmed that the high resistance faced by the charge carriers during transport hampers its performance.