Hybrid Gold-Based Perovskite Derivatives: Synthesis, Properties, and Prospects in Photovoltaics

: Hybrid gold-based perovskite derivatives typically exhibit low optical bandgaps and high optical absorption coe ﬃ cients, rendering them promising for photovoltaic applications. In this study, we successfully synthesized six new hybrid gold-based perovskite derivatives, namely [(C 6 H 8 N 2 )(AuI 4 )(AuI 2 )]


Introduction
Over the past twenty years, scientists have conducted intensive research to find economically friendly clean energy materials.In this context, perovskite materials have become a research hotspot for the next generation of energy-conversion semiconductor materials due to their superior economic properties, small exciton binding energy, long carrier diffusion length and lifetime, ease of solution processing, and tunable bandgap [1].Based on these advantages, perovskite materials can be applied to various optoelectronic devices, such as solar cells [2][3][4], photodetectors [5], light-emitting diodes (LEDs) [6], etc.However, the toxicity and instability of traditional lead-based perovskite materials have become the main factors limiting their further development [7][8][9].Therefore, researchers have been committed to finding new B-site elements to replace lead and develop environmentally friendly halide perovskite and perovskite-derivative materials [10].
Gold-based perovskite, as an environmentally friendly material, boasts good stability and a high optical absorption coefficient, among other advantages [11].Additionally, gold elements in the perovskite structure can exist in multiple oxidation states, maintaining the stability of the B-site gold element.Various gold-based perovskite derivatives can be synthesized by substituting A-site cations and X-site halide anions.Therefore, goldbased perovskites exhibit rich diversity, giving researchers a broad scope.Gold-based perovskites were first reported in the early 20th century, when Gupta and colleagues first discovered Cs2Au I Au III I6 [12].Subsequently, after more than a century of development, various new types of pure gold-based perovskites have been synthesized and extensively studied, such as K2Au I Au III I6, Rb2Au I Au III Br6 [13], LiAuI4, and RbAuCl4 [14].Recently, Kojima et al. found that the perovskite Cs2AuIAuIIII6 transitions from semiconductor to metallic properties under high-temperature and high-pressure conditions [15].Theoretical calculations by Debbichi et al. suggested that Cs2Au I Au III I6 possesses an appropriate bandgap and high optical absorption coefficient, potentially achieving over 20% photoconversion efficiency (PCE) [16].Additionally, Li et al. predicted a series of novel goldbased perovskites as promising solar cell absorbers, Cs2Au I SbCl6, Cs2Au I InCl6, and Cs2Au I BiCl6 exhibiting a spectroscopic limited maximum efficiency (SLME) of approximately 30% in a 0.5 mm thick film [17].
Recently, there has been a growing number of reports on organic-inorganic hybrid gold-based perovskites and their derivatives.In 2021, Fan et al. synthesized a series of narrow-bandgap hybrid organic-inorganic gold-based perovskites and their derivatives via a simple and efficient hydrothermal method, revealing their potential applications in the optoelectronic field [18].In 2024, Walusiak et al. reported a series of organic-inorganic hybrid gold-based perovskite derivatives and investigated their crystal structures and chemical properties [19].According to previous studies, these gold-based perovskite derivatives tend to form low-dimensional structures, typically containing [Au I I2] and [Au III I4] units composed of gold cations and iodide anions in their structures [20].These structural units exhibit strong intermolecular interactions, typically resulting in hybrid gold-based perovskite derivatives being dark-colored narrow-bandgap semiconductors.This makes them potential candidates for light-absorbing layer materials in solar cells, with related device studies demonstrating their potential application as solar cell absorbers [11,21].
However, despite the potential application values of organic-inorganic hybrid perovskites and their derivatives, reports on such materials are still limited, and research on them is relatively scattered.Therefore, it is essential to expand the variety of hybrid organic-inorganic perovskites and their derivatives further and explore their application potential.In this paper, we report six new organic-inorganic gold-based hybrid perovskite derivatives and have named these compounds based on the English abbreviations of their A-site cations: , and [(C10H26N2O3)(AuI4)(I3)](TOTA).Among these six compounds, 3AMP contains [Au III I6] octahedra, forming a zero-dimensional perovskite-like structure.In contrast, the remaining five compounds exhibit zero-dimensional non-perovskite structures.We obtained millimeter-sized single crystals of these gold-based perovskite derivatives by optimizing the hydrothermal reaction parameters.Then, we combined experimental characterization with theoretical calculations to study these materials' crystal structures, electrical properties, thermal stability, and optical properties.The results indicated that these compounds are narrow-bandgap materials with good thermal stability.First-principles calculations showed that these materials exhibited strong, light absorption coefficients in the visible light region, with SLME values reaching around 30% for 3AMPY, 2PEAI, DDA, and TOTA, suggesting significant potential for applications in the field of solar cells.We believe our work can further advance the research and application of organic-inorganic hybrid gold-based perovskite derivatives.

The Synthesis Process of Single Crystals
Using a straightforward and efficient hydrothermal reaction, we successfully prepared millimeter-scale single crystals of six organic-inorganic gold-based hybrid perovskite derivatives.The single-crystal (SC) synthesis process is shown in Figure 1a, taking the preparation of [(C4H14N2O) (AuI4)2] (OBA) as an example.An amount of 1 mmol of HAuCl4•4H2O (0.412 g) was dissolved in an appropriate amount of deionized water, stirred for 5 min to completely dissolve HAuCl4•4H2O, and then 1 mmol (0.64 µL) of 2,2′oxydiethylamine (OBA) was added, causing a noticeable reaction (generation of micro-SCs.Then, add 0.5 mL of hydroiodic acid (hydroiodic acid must be in excess to ensure the complete reaction of the reactants).After thoroughly stirring the reaction solution, transfer it to a reaction vessel lined with polytetrafluoroethylene and conduct a hydrothermal reaction in a temperature-controlled oven at 90 °C for 24 h.Then, slowly cool the reaction mixture to room temperature at 2 °C/hour to obtain the SCs.As shown in Figure 1b, all SCs were black and had polyhedral shapes, with crystal sizes in the millimeter range.
Subsequently, the single crystals were washed with deionized water and placed in an 80 °C oven to dry to remove any surface-adhered elemental I2.Single crystals of six compounds were obtained by replacing the organic component in the reaction system.The organic components used in this work are shown in Figure 1c.The control-of-variables experiment determined the above reaction parameters.According to the experience results, a low hydrothermal reaction temperature (below 60 °C) will result in insufficient energy in the reaction system, leading to the inability to generate single crystals.
Conversely, a high temperature (above 150 °C) can cause the decomposition of reaction products and the generation of elemental Au byproducts, as shown in Figure S1.Therefore, around 100 °C is an appropriate temperature for synthesizing single crystals in this series.The synthesis of other crystals was also based on this temperature.Furthermore, we found that properly reducing the cooling rate was beneficial for increasing the yield of single crystals.To balance the yield of single crystals with the reaction duration, we finally set the reaction conditions: reaction temperature of 90 °C, incubation for 24 h, and slow cooling to room temperature at 2 °C/hour.In our attempts to prepare thin films by redissolving those hybrid gold-based perovskite derivatives and via spin coating, we encountered solubility issues in most polar solvents and poor film formations, which is one of the challenges faced by many gold-iodide compounds.We are currently exploring the relevant film preparation technology to prepare solar cell devices in the lab.

The Analysis of Single-Crystal Structures
We conducted an X-ray single-crystal diffraction analysis on the six compounds, and their crystal structures are illustrated in Figure 2.Among them, 3AMP, 2PEAI, OBA, and TOTA belong to the monoclinic crystal system, exhibiting relatively good symmetry, while 3AMPY and DDA belong to the triclinic crystal system, displaying lower symmetry.The specific crystal structure parameters of six kinds of gold-based perovskite derivates are summarized in Table 1.The more specific crystallographic data are presented in Tables S1 and S2.

𝑃1
(2) Analyzing the composition of each crystal structure, we found three types of structural units, apart from the organic cations, including the [Au I I2] − , [Au III I4] − , and [I3] − units, composed of Au + and Au 3+ , and I − , as well as I − and I2, respectively.We investigated the formation mechanisms of different structural units in gold-based perovskite derivatives based on the possible redox reactions between Au cations and I anions.Equations ( 1)-( 4), depict the chemical reaction processes as follows: As depicted in Equations ( 1) and ( 2), in the reaction system, initially, Au 3+ undergoes oxidation by I − to form Au + , followed by the reaction of one unit of Au + with two units of I − to generate the [Au I I2] − unit [16].Subsequently, I − reacts with oxygen in the aqueous solution to form I2, and in the presence of excess I − , the surplus I − combines with I2 to form the [I3 − ] unit, as shown in Equation ( 3) [22].Finally, as indicated in Equation ( 4), at this stage, the [Au I I2] − unit in the reaction system reacts with the [I3] − unit, yielding the [Au III I4] − unit [19].
Then, we further investigated the crystal structures of these compounds.The six compounds were classified and analyzed based on the different structural units composing their crystal structures: (1) 3AMP and 3AMPY; (2) DDA and TOTA; and (3) 2PEAI and OBA.The detailed analysis is as follows:  In summary, the differences in crystal structures of these compounds arise from variations in the types of constituent units and their spatial positioning relationship.Raman spectroscopy is one of the most effective methods for studying the gold valence transitions of various organic-inorganic perovskite derivatives.To further demonstrate the accuracy of the valence state of Au elements in our crystal structure, we obtained the Raman spectra of these compounds through testing, as shown in Figure 5. Analyzing the Raman spectra, we found peaks corresponding to Au-I bonds near 130 cm −1 for all compounds, consistent with the literature reports [23].Further analysis, as shown in Figure 2a,b, revealed that 3AMP and 3AMPY contain [Au III I4] − and [Au I I2] − units, where the Au-I bond length in the [Au III I4] − units is longer than that in the [Au I I2] − units (see Figure S2).By comparing the longer Au-I bond in the [Au III I4] − units (around 2.62 Å) with that in the [Au I I2] − units (around 2.55 Å), we attributed the lower-lying modes, A1 (127.9 cm −1 ) and A2 (131.2 cm −1 ), to the [Au III I4] − units.Furthermore, the higher modes, B1 (130.4 cm −1 ) and B2 (133.8 cm −1 ) are indicative of [Au I I2] − units [24].Additionally, in 2PEAI, OBA, DDA, and TOTA, the Au ions and I ions form single [Au III I4] − units (Figure 2c-f), corresponding to individual Au-I modes (A3, A4, A5, and A6).The results of Raman spectroscopy testing corresponded well with our crystal structure, demonstrating the accuracy of the valence state of Au elements in the crystal structure.

The Characterization of Optical Properties and Thermal Stability
To ascertain the purity of the synthesized samples of the six compounds, we ground the single crystals into powder for X-ray diffraction experiments.The powder XRD peaks obtained from the experiment were compared with the theoretical simulated peaks of the single-crystal structures, as shown in Figure 6.The correspondence between the simulated diffraction peaks of the crystal structures and the experimentally obtained diffraction peaks confirmed the high purity of the synthesized materials.This assured the reliability of the subsequent optical, electrical, and thermal stability tests.Thermal stability is crucial for viable applications in optoelectronics, and a TGA/DTG (Thermogravimetric Analysis/Derivative Thermogravimetry) analysis was conducted on hybrid gold-based perovskite-derivative powders to understand their thermal decomposition pathways.Table 2 records their decomposition temperatures, and the TGA curves are shown in Figure S3.The thermal decomposition processes of the six compounds can be described using two modes: (1) For 3AMP, 2PEAI, 3AMPY, DDA, and TOTA, the thermal decomposition process can be divided into two steps: a.The first decomposition occurs around 100 °C, resulting in a weight loss of approximately 35%, corresponding to the organic cations.b.The second stage of decomposition begins at around 230 °C, corresponding to the loss of I2.The final products are AuI and Au.
(2) For OBA, the thermal decomposition process can be divided into three steps: a.At around 100 °C, approximately 20% of the weight is lost, corresponding to the weight percentage of organic cations.b.At around 180 °C, the second stage of decomposition begins, with the loss of mass corresponding to I2.At this point, the products are AuI3 and Au.c. Around 240 °C, the final decomposition reaction occurs, with AuI3 decomposing into Au and AuI.After this, the thermogravimetric curve stabilizes, indicating that the final decomposition products are AuI and Au.
Overall, due to the poor thermal stability of organic ions, all hybrid gold-based perovskite derivatives begin to decompose at around 100 °C.Their thermal stability is comparable to that of the widely used CH3NH3PbI3 [25].Considering that the operating temperatures of most optoelectronic devices are below 50 °C, it is evident that the thermal stability of these compounds is sufficient to support the device applications.
To investigate the optical properties of these compounds, we confirmed their light absorption ranges through ultraviolet-visible spectroscopy, as shown in Figure 7a.It can be observed that all compounds exhibited broad light absorption ranges, consistent with their black crystal morphology characteristics, indicating their excellent light absorption performance.Among these compounds, 3AMPY, 3AMP, OBA, and TOTA exhibited the broadest absorption peaks, reaching 1160 nm, 1150 nm, 1150 nm, and 1130 nm, respectively, while the absorption peaks of 2PEAI and DDA were relatively narrower, at 1090 nm and 1000 nm, respectively.According to the formula Eg = 1240/λg (where λg is the absorption threshold), the wider the absorption wavelength range, the smaller the bandgap.Therefore, all six abovementioned compounds exhibited relatively narrow bandgaps, around 1.10 eV.These materials featured broad absorption peaks ranging from visible light to near-infrared, demonstrating their potential applications in photodetection.

First-Principles Calculations
For elucidating the electronic structures and semiconductor properties of hybrid gold-based perovskite derivatives, we employed the Vienna Ab initio Simulation Package (VASP) with the Perdew-Burke-Ernzerhof (PBE) function to compute the band structures and density of the states (DOS) [26,27].As shown in Figure 8, the bandgap values obtained from the theoretical calculations for 3AMP, 3AMPY, 2PEAI, OBA, DDA, and TOTA were 1.04 eV, 1.03 eV, 1.03 eV, 0.92 eV, 1.17 eV, and 1.03 eV, respectively.These values were very close to the experimental values, demonstrating the accuracy of our computational results.For 3AMP, 3AMPY, 2PEAI, and DDA, the valence band maximum and conduction band minimum were not located in the same Brillouin zone, displaying typical characteristics of indirect bandgap semiconductors.Notably, OBA and TOTA exhibited valence-bandmaximum and conduction-band-minimum positions in Brillouin zones that were very close, indicating slight indirect bandgap properties.Through analysis, it was observed that the valence band maximum and conduction band minimum of all six materials were primarily composed of I-2p orbitals (shown in purple) and a small amount of Au-5d orbitals (shown in yellow).This indicates that the narrow bandgap characteristics of these materials originate from contributions of I-2p orbitals and partial Au-5d orbitals, which is consistent with previous reports.This suggests that the strong intermolecular interactions between the structural units composed of gold and iodine ions lead to the narrow bandgap of gold-based perovskite derivatives [21].
Then, the PBE method was utilized to calculate the optical absorption coefficients of these hybrid gold-based perovskite derivatives and evaluate their potential as absorber layers in solar cells.As shown in Figure 9a, b, all compounds exhibited excellent light absorption performance, with absorption coefficients in the visible region reaching the order of 10 5 cm −1 .Subsequently, SLME (Shockley-Queisser Limit Maximum Efficiency) calculations were performed to explore their potential as absorber layers in solar cells.The results, as shown in Figure 9c, d, indicated that when the film thickness was 500 nm, the SLME of DDA, 2PEAI, 3AMPY, and TOTA reached 32.06%, 31.89%,30.56%, and 30.00%, respectively.These values exceeded the theoretical maximum efficiency of mainstream hybrid perovskite materials [28], demonstrating the tremendous potential of these compounds in photovoltaic applications.The great SLME values were mainly attributed to their appropriate bandgaps (around 1.10 eV) and high optical absorption coefficients.However, OBA and 3AMP exhibited relatively lower SLME values, at 18.52% and 15.26%, respectively.This was due to their lower optical absorption coefficients.Recently, experimental values for methylammonium-formamidinium-based perovskite cells exceeded 20%, with the highest recorded value reaching 26.00%, approaching its theoretical maximum limit efficiency [29,30].Although SLME does not consider factors such as carrier mobility and intrinsic defects in actual thin films, our work still reveals the application potential of these hybrid gold-based perovskite derivatives, laying the foundation for future research on their practical applications.

Synthesis of Single Crystals
Millimeter-scale single crystals of gold-based perovskite derivatives can be obtained by the hydrothermal method under the appropriate parameters, the detailed steps of which are discussed in Section 2.

Characterizations
X-ray diffraction (XRD).XRD patterns of the films were obtained from an X-ray diffractometer (Miniflex 600, Rigaku, Japan) with Cu Kα radiation.Analysis of Single Crystal Structures.The SCs' structures were determined using a Bruker SMART APEX-II diffractometer equipped with a CCD detector (graphite-monochromatized Mo-Kα radiation, λ = 0.70 Å) at 200K.Data integration and cell refinement were performed using APEX3 software 1.5.The structure was analyzed by direct methods and refined using the SHELXTL 97 software 2.0 package.All nonhydrogen atoms of the structure were refined with anisotropic thermal parameters, and the refinements converged for Fo 2 >2σIJFo 2 .All the calculations were performed using the SHELXTL crystallographic software 2.0 package.Symmetry analysis on the model using PLATON revealed that no noticeable space group change was needed.In the refinement, the commands EDAP and EXYZ were used to restrain some of the related bond lengths and bond angles.Raman spectroscopy.Raman spectroscopy measurements were performed on a laser Raman microscopy system (RA-MAN 11, Nanophoton, Japan) at 300 K.The excitation laser wavelength was 532 nm, the energy was 2.67mW, and the detection range was 100 µm × 100 µm (the delay was 2 µm).UV−vis−NIR Absorption and Reflection Spectra Measurements.The UV−vis−NIR absorption and reflection spectra of powders were measured using a spectrophotometer (Cary 5000, Agilent, Malaysia) over the spectral range of 200−1400 nm.Thermogravimetric Analysis (TGA) Measurements.Thermogravimetric analysis (TGA) was conducted employing a TGA/DSC1/1600HT apparatus (Mettler Toledo, Switzerland).The specimens were positioned within platinum crucibles and subjected to incremental heating from an ambient temperature to 600 °C at a rate of 5 °C min −1 under a continuous nitrogen flow.Computational Methods.The first-principles density functional theory (DFT) simulations were performed with the Vienna ab initio simulation package (VASP) to study the geometric and electronic structures of the Au-based perovskites series.We adopted the Perdew-Burke-Ernzerhof (PBE) function in the generalized gradient approximation (GGA) to optimize their geometrical structure.For geometric optimization, the Monkhorst-Pack k-point grid was configured as 2*9*3, while for self-consistent calculations, it was adjusted to 4*6*7.Our investigations adopted the PBE function to calculate the band structures.Projector augmented wave pseudopotentials with a cut-off energy of 520 eV were employed.

Conclusions
In response to the limited research on the synthesis and application potential of goldbased perovskite derivatives, we efficiently synthesized single crystals of six such derivatives through parameter optimization.Through chemical reaction studies and crystal structure analyses, we elucidated the formation of various structural units in these derivatives, deepening our understanding of their compositions.Thermogravimetric testing demonstrated their good thermal stability, with decomposition temperatures around 100 °C, akin to that of CH3NH3PbI3.Integration of experimental characterization with theoretical calculations revealed optical bandgaps of 1.10 eV and high absorption coefficients across all materials.Notably, SLME calculations indicated that DDA, 2PEAI, 3AMPY, and TOTA achieved SLME values of around 30%, underscoring their potential in photovoltaic applications.This study establishes a robust framework for employing hybrid gold-based perovskite derivatives in photodetectors and photovoltaics.

Figure 2 .
Figure 2. (a-f) The schematic diagrams of the single-crystal structures of the hybrid gold-based perovskite derivates.

( 1 )
In the crystal structures of 3AMP and 3AMPY, the anions consist of [Au I I2] − and [Au III I4] − units.Specifically, 3AMP comprises [Au I I2] − and [Au III I4] − anion units, along with one divalent organic cation to achieve charge balance.It is noteworthy that in 3AMP, there exists an [AuI6] octahedron, forming a zero-dimensional perovskite-like structure.These octahedra are connected by Au•••I − -Au bonds formed by sharing I − ions in the vertical direction, with one [Au III I4] − unit and two [Au I I2] − units.Otherwise, 3AMPY comprises three [Au I I2] − and one [Au III I4] − anion units, balanced by two divalent organic cations.The [Au I I2] − and [Au III I4] − anion units exist independently, forming a zero-dimensional non-perovskite structure.This significant structural difference arises from the distinct spatial distribution of [Au I I2] − and [Au III I4] − ions between the two crystals.As shown in Figure 3a, in the crystal structure of 3AMP, the [Au I I2] − is positioned directly above the center of the [Au III I4] − unit.The Au•••I bond lengths between adjacent [Au I I2] − and [Au III I4] − are 3.336 Å and 3.364 Å, respectively.Therefore, considering the shorter distance between the structural units and appropriate spatial distribution, [Au I I2] − and [Au III I4] − can form [AuI6] octahedra.In the crystal structure of 3AMPY, the Au•••I bond lengths between [Au III I4] − and the [Au I I2] − unit positioned directly above it are 3.922 Å and 3.693 Å, respectively.The longer distances render the Au•••I bonds unstable, preventing the formation of [AuI6] octahedra.As a result, [Au I I2] − and [Au III I4] − units exist independently without forming an octahedral structure.(2) In the DDA and TOTA crystal structures, the anions consist of [Au III I4] − and [I3] − units.Specifically, DDA is balanced by two [Au III I4] − and one [I3] − anion units along with two divalent organic cations, while TOTA is balanced by one [Au III I4] − and one [I3] − anion units along with one divalent organic cation.As shown in Figure 4a,b, the length of the [I3] − structural unit is 5.876 Å.In both structures, the distance between the adjacent [Au III I4] − units is much greater than the length of [I3] − , allowing for the incorporation of [I3] − units.Consistent with previous studies, the presence of [I3] − units in hybrid gold-based perovskite derivatives facilitates charge transport.[23] Moreover, the closer the distance between [I3] − and [Au III I4] − units, the faster the carrier migration along [I3] − , resulting in better conductivity.This suggests that DDA and TOTA are expected to exhibit superior conductivity due to their favorable arrangement for charge transport.(3) In the crystal structures of 2PEAI and OBA, the anions consist of [Au III I4] − units.Specifically, 2PEAI is balanced by one [Au III I4] − anion unit and one monovalent organic cation, while OBA is balanced by two [Au III I4] − anion units and one divalent organic cation.Due to the absence of [Au I I2] − units, the [Au III I4] − units in these two crystals cannot form octahedra, resulting in zero-dimensional non-perovskite structures.Additionally, as shown in Figure 4c,d, because of the excessively short distances between the adjacent [Au III I4] − units, measured at 5.304 Å and 3.496 Å, respectively, [I3] − units cannot be accommodated in the structure.As a result, the anions consist solely of [Au III I4] − units.

Figure 5 .
Figure 5. Raman spectra for hybrid gold-based perovskite derivatives at 300 K.

Figure 6 .
Figure 6.(a-f) Powder XRD patterns of hybrid gold-based perovskite derivatives with the calculated XRD patterns of the single crystals.

Figure 7 .
Figure 7. (a) Ultraviolet-visible absorption spectra of hybrid gold-based perovskite derivatives and (b-g) the bandgap values calculated with the Tauc plot formula.

Figure 8 .
Figure 8. (a-f) The band structures and PDOS plots of hybrid gold-based perovskite derivatives.

Figure 9 .
Figure 9.The optical properties of hybrid gold-based perovskite derivatives.(a,b) Calculated absorption spectra of average optical directions.(c,d) SLME as a function of film thicknesses.

Table 1 .
Crystallographic parameters for hybrid gold-based perovskite derivates.

Table 2 .
The decomposition temperatures of hybrid gold-based perovskite derivatives.