Optical Property and Stability Study of CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 Ruddlesden Popper 2D Perovskites for Photoabsorbers and Solar Cells and Comparison with 3D MAPbI 3

: Three dimensional (3D) perovskite solar cells (PSCs) are a promising candidate for third-generation photovoltaics (PV) technology, which aims to produce efﬁcient photon conversion devices to electricity using low-cost fabrication processes. Hybrid organic-inorganic perovskites for-lmed using low-cost solution processing are explored here, which have experienced a stu-pendous rise in power conversion efﬁciency (PCE) over the past decade and serve as a prime candidate for third-generation PV systems. While signiﬁcant progress has been made, the inherent hygroscopic nature and stability issue of the 3D perovskites (3DPs) are an impediment to its commercialization. In this work, we have studied two-dimensional (2D) organometallic halide (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n − 1 Pb n I 3n+1 ) layered perovskites in the Ruddlesden Popper structure, represented as BA 2 MA 3 Pb 4 I 13 for the n = 4 formulation, for both photoabsorbers in a two-terminal architecture and solar cells, given that these material are considered to be inherently more stable. In the two-terminal photo absorber devices, the photocurrent and responsivity were measured as a function of incoming laser wavelength, where the location of the peak current was correlated to the emission spectrum arising from the 2DP ﬁlm using photoluminescence (PL) spectroscopy. The 2D (BA)2(MA)3Pb4I13 ﬁlms were then integrated into an n-i-p solar cell architecture, and PV device ﬁgures of merit tabulated, while our 3D MAPbI 3 served as the reference absorber material. A comparative study of the 3DP and 2DP ﬁlm stability was also conducted, where freshly synthesized ﬁlms were inspected on FTO substrates and compared to those exposed to elevated humidity levels, and material stability was gauged using various material characterization probes, such as PL and UV-Vis optical absorption spectroscopy, scanning electron microscopy and X-ray diffraction. While the PCE of the 3D-PSCs was higher than the 2D-PSCs, our results conﬁrm the enhanced environmental stability of the 2DP absorber ﬁlms compared to the 3DP absorbers, suggesting their promise to address the stability issue broadly encountered in 3D PSCs toward third-generation PV technology. nm at all of the three environmental conditions examined, which are not present in the spectra of the 3DP ﬁlm. The optical resonances in the 2DP ﬁlms may be attributable to quantum conﬁnement and excitonic effects given the reduced physical dimension of the 2DP crystalline structure.


Introduction
Clean alternative energy sources are critical to addressing one of humanity's grand challenges, given our dependence on fossil fuels which produce harmful CO 2 emissions that exacerbates global warming. With the worldwide demand for energy expected to triple by the end of the century, and coupled with the limited planetary fossil fuel reserves, the need to develop high-efficiency, low-cost energy harvesting devices as a clean energy source is imperative and urgent. Renewable energy sources based on energy harvesting organic cation (such as an aliphatic or aromatic alkylammonium) that functions as a spacer between the inorganic sheets, and n is the number of inorganic layers held together. The layered structure reduces the Goldschmidt tolerance factor's limitations on cation size and unlike 3DPs, allows bulky A' cations to be accommodated via van der Waals interactions between the inorganic sheets.
In this study, we have examined the optoelectronic properties of 2DRPs absorber, specifically ((CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 ), and its integration into a solar cell architecture is also discussed, where a comparative analysis is conducted with 3DP absorbers, specifically MAPbI 3 . In the first part of this work, we discuss the synthesis approach used for the ((CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 ) 2DP, which may be considered to be part of the broader 2D layered materials family that are emerging as promising candidates for optoelectronics [49][50][51]. The 2DP was synthesized using previously reported approaches [52][53][54][55], and material characterization studies on the environmental stability of the absorber were conducted using techniques such as steady state photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The optoelectronic properties of the 2DP absorber were deduced from photoabsorber measurements through the determination of the responsivity which was correlated to the PL emission characteristics of the intrinsic absorber. Through our work it is clear the 2DPs offer promising photoabsorption characteristics while also being more environmentally stable under ambient conditions compared to MAPbI 3 , to address stability concerns intrinsic to these 3DP photoabsorbers.

Synthesis and Characterization of 2DRP
In the 2DRP phase, the inorganic layer of the octahedra comprise of Pb atoms that share the halide ions with its neighboring metal atom in a 2D plane. The organic groups self-assemble through van der Waals interactions to form butylammonium (BA) and methylammonium (MA) layers where the ammonium head of the organic groups, namely CH 3 (CH 2 ) 3 NH 3 and CH 3 NH 3 , binds to Pb 4 I 13 molecules through the van der Waals interaction. We synthesized our 2DP (BA) 2 (MA) 3 Pb 4 I 13 formulation using the process illustrated in the Figure 1a. The PbO powder (2232 mg) was dissolved in a mixture of 57% w/w aqueous HI solution (10.0 mL) and 50% aqueous H 3 PO 2 (1.7 mL) with heating at 130 • C and subsequent boiling under constant stirring for about 25 min. Into this solution, CH 3 NH 3 Cl (507 mg) was subsequently added, which initially caused the precipitation of a black powder that quickly re-dissolved under stirring to yield a clear bright-yellow solution. Additionally, n-CH 3 (CH 2 ) 3 NH 3 I (248 µL) and HI acid (5 mL) were mixed separately in a glass vial and this mixture was then added dropwise to the previous solution under vigorous stirring for 10 min. The solution was then left to cool down to room temperature which resulted in the formation of dark black, rectangular-shaped platelets. The crystals were isolated by suction filtration and thoroughly dried under reduced pressure [52,56,57], as shown by the powder collected in a vial in Figure 1b-left. The crystal structure for the n = 4 formulation is depicted in Figure 1b-right, where the metal-halide octahedral sheets of the AMX 3 complex are connected by the intercalated BA bilayer with an effective gap of~15.6Å. The repeat unit of the ordered Pb 4 I 13 stack is~25.11 Å, which influences the bandgap for the structure [56], for the inorganic moiety within the unit cells along the stacking direction [52,58,59]. The inherent stability of the 2DP powder was gauged us steady-state PL measurements, gathered over the course of 1.5 years, for crystals stored at ambi temperatures in a desiccator. A slight blue shift of 29 nm in the peak position with time was evide but the crystals were still photoactive with a well-defined emission peak, and a FWHM that w largely unchanged over the 1.5 years of observation time. (d) The 2DP crystals were dispersed solution and spin cast into thin films on an oxidized Si substrate with metal electrodes that w deposited using a shadow mask. Here the Ip was measured as a function of voltage and the top in shows the schematic of the device, with the fabricated version of the device seen in the bottom in As noted previously, one of the advantageous features of 2DPs is their higher stab ity, given that the slow loss of the MA + cation in MAPbI3 films gradually leads to its d solution to the yellow PbI2 phase after exposure to moisture-rich ambient environmen The inherent stability of the 2DP powder was gauged using steady-state PL measurements, gathered over the course of 1.5 years, for crystals stored at ambient temperatures in a desiccator. A slight blue shift of 29 nm in the peak position with time was evident, but the crystals were still photoactive with a well-defined emission peak, and a FWHM that was largely unchanged over the 1.5 years of observation time. (d) The 2DP crystals were dispersed in solution and spin cast into thin films on an oxidized Si substrate with metal electrodes that were deposited using a shadow mask. Here the I p was measured as a function of voltage and the top inset shows the schematic of the device, with the fabricated version of the device seen in the bottom inset.
As noted previously, one of the advantageous features of 2DPs is their higher stability, given that the slow loss of the MA + cation in MAPbI 3 films gradually leads to its dissolution to the yellow PbI 2 phase after exposure to moisture-rich ambient environments, even for short durations. The hydrophobic character of the R cation in the 2DP, as well as the highly orientated structure and tight packing lowers the density of grain boundaries, contributing to its increased stability within the lower dimensionality crystalline structure. In order to examine the storage stability of our synthesized 2DP crystals, we used steady state PL spectroscopy and observed the variation in optical properties over a period of~1.5 years, and the data are shown in Figure 1c. From this analysis, it is clear that an indisputable PL peak was present even after 1.5 years of storage at room temperature in an ordinary desiccator, though there was a slight blue shift amounting to~29 nm within the one year period. Incidentally, Xing et al. [60] conducted PL measurements as a function of time for 2DP, though the type of perovskite they used comprised of a PEA head group which is slightly different from the BA head group we used in our 2DP, and their stability study was conducted over a much shorter time scale. Specifically, in their findings, the PL peak positions remained unchanged for their 2DP over a time span of 60 min. In contrast, the PL peak of their 3DP film shifted from 460 nm to 515 nm within 10 min of illumination under the same conditions. The main conclusion drawn from their work was that the 2DP did not exhibit any significant variation in PL emission peak location with time, and hence was more stable, whereas their 3DP PL peak position red-shifted by 55 nm even over short durations of exposure, i.e. 10 min. It appears our study for measuring stability of the 2DP powder over a time-span of 1.5 years is unique as there seems to be no direct, equivalent study in the open literature for comparative purposes. Here we hypothesize that the slight blue shift of~29 nm (from 652.6 nm to 623.2 nm) we observe in the peak location may have origins from bond length variations, including due to the van der Waals interaction of the BA head group with the outer layers of the PbI octahedral planes, as seen from the structural schematic of Figure 1b. After the initial period, however, the PL peak position nonetheless stabilizes with time for the extended period we studied. Additionally, the fullwidth-half-maximum (FWHM) was largely unchanged, which validates the high-degree of environmental stability of our 2DP crystals synthesized using the approach described in Figure 1a.
The organic cations in 2DPs operate as insulating barriers that confine charge carriers in two dimensions, and also serve as dielectric moderators to control electrostatic pressure exerted on the photogenerated electron-hole pairs. A crystallographically organized 2D multiple-quantum-well electronic structure emerges spontaneously through a bottom-up self-assembly synthesis process described in Figure 1, owing to the particular arrangement of alternating organic and inorganic layers. For (BA) 2 (MA) 3 Pb 4 I 13 (n = 4) compounds, the exciton binding energy is closer to that of the MAPbI 3 (with n → ∞), and the excitons are expected to be almost ionized at room temperature, leading to charge-carrier transport dominated by free carriers. To characterize our synthesized 2DRP crystals, we prepared the perovskites films on top of SiO 2 /Si substrates. For making the perovskite solution, the 0.118M of Pb 2+ cations were dissolved in anhydrous DMF, while the SiO 2 /Si substrates were sequentially cleaned using ultra-sonication in distilled water (DI), acetone and isopropyl alcohol (IPA). Finally, the substrates were placed in an oxygen plasma for 20 min to further clean the substrates. The films were synthesized using the hot-casting approach, where the substrates were first pre-heated to 125 • C on a hot plate for 10-15 min, and~80 µL of precursor solution was drop cast onto the hot substrate and spin coated at a speed of 5000 rpm for 20 s. After spin-coating, the films were then immediately transferred to the hot plate for annealing. Afterwards,~100 nm of Ag was deposited using e-beam evaporation at a base pressure of~10 −5 Torr, with a shadow mask over the underlying spin coated 2DP film.
The subsequent optoelectronic characteristics of the fabricated two-terminal devices were measured using a Keysight B1500A semiconductor parameter analyzer interfaced to a vacuum probe stage, where transport measurements were made at room temperature at a pressure of~5 × 10 −7 Torr. A metal cap was placed on the optically transparent window of the vacuum chamber in order to conduct the dark current measurements, while measurements in the presence of broadband lights were performed after removing the cap and shinning broadband radiation on the device at 100% power. The compliance current was kept at 100 µA to prevent joule-heating induced damage to the 2DP film due to excessive power dissipation. Here the photocurrent I p was measured as a function of voltage, as shown by the data in Figure 1d, where I p = I Light − I Dark ; here I Light and I Dark are the current measured under illumination and in the dark, respectively. The top inset in Figure 1d shows the schematic of the device with two electrode terminals contacting the underlying 2DP film, while the fabricated version of the device is displayed in the bottom inset. The I p generally increased with increasing bias voltages as shown in Figure 1d, which is expected for semiconducting behavior in order to excite carriers across the bandgap of the 2DP.
In order to understand the photoresponse of our 2DP further, we conducted wavelength dependent measurements of the I p , as shown by the data in Figure 2a, where the incoming tunable laser wavelength λ ranged from 400 nm to 1100 nm. The maximum I p was obtained at~775 nm which is consistent with the band gap value of the spin coated 2DP film, yielding an E g of~1.6 eV. Additionally, it is also clear that there is diminished optical activity within the NIR region for our 2DP films, as expected. The photoresponsivity R was then calculated using R = J ph /P, where J ph is photocurrent density (in A/cm 2 ) and P is tunable laser power (in W/cm 2 ). From this we determined the peak R~1.28 × 10 −4 A/W at λ~775 nm, as shown by the data in Figure 2b, which is also consistent with the photocurrent response.
Solar 2022, 2, FOR PEER REVIEW 6 shinning broadband radiation on the device at 100% power. The compliance current was kept at 100 μA to prevent joule-heating induced damage to the 2DP film due to excessive power dissipation. Here the photocurrent Ip was measured as a function of voltage, as shown by the data in Figure 1d, where Ip = ILight − IDark; here ILight and IDark are the current measured under illumination and in the dark, respectively. The top inset in Figure 1d shows the schematic of the device with two electrode terminals contacting the underlying 2DP film, while the fabricated version of the device is displayed in the bottom inset. The Ip generally increased with increasing bias voltages as shown in Figure 1d, which is expected for semiconducting behavior in order to excite carriers across the bandgap of the 2DP.
In order to understand the photoresponse of our 2DP further, we conducted wavelength dependent measurements of the Ip, as shown by the data in Figure 2a, where the incoming tunable laser wavelength λ ranged from 400 nm to 1100 nm. The maximum Ip was obtained at ~775 nm which is consistent with the band gap value of the spin coated 2DP film, yielding an Eg of ~1.6 eV. Additionally, it is also clear that there is diminished optical activity within the NIR region for our 2DP films, as expected. The photoresponsivity R was then calculated using / , where Jph is photocurrent density (in A/cm 2 ) and P is tunable laser power (in W/cm 2 ). From this we determined the peak R ~1.28×10 -4 A/W at λ ~775 nm, as shown by the data in Figure 2b, which is also consistent with the photocurrent response. Figure 2. (a) The Ip was measured as a function of λ over the 400 to 1100 nm spectral range for the 2DP film photoabsorber where the peak in Ip occurring at ~775 nm was consistent with the peak position of the PL emission spectrum for our n = 4, 2DP formulation. (b) Responsivity R as a function of λ over the same spectral range, and again, the highest R coincided with the PL peak emission observed in our 2DP photoabsorber. The laser power used was ~200 μW and the laser spot size of the fiber optic cable irradiating our device defined an active area of ~5 μm 2 . (a)

Responsivity (A/W)
Wavelength (nm) Figure 2. (a) The I p was measured as a function of λ over the 400 to 1100 nm spectral range for the 2DP film photoabsorber where the peak in I p occurring at~775 nm was consistent with the peak position of the PL emission spectrum for our n = 4, 2DP formulation. (b) Responsivity R as a function of λ over the same spectral range, and again, the highest R coincided with the PL peak emission observed in our 2DP photoabsorber. The laser power used was~200 µW and the laser spot size of the fiber optic cable irradiating our device defined an active area of~5 µm 2 .

Comparative Characterization of 3D and 2D PSCs
After our electronic transport measurements on the synthesized 2DP films, we then proceeded toward their integration in the PSC fabrication process, and alongside we also prepared our 3DP (MAPBI 3 ) films for comparative analysis. Both films were formed on top of FTO substrates and their optical properties were studied using PL spectroscopy which was measured using the LabRAM HR Evolution spectrometer (HORIBA Scientific) equipped with a 532 nm excitation laser. Figure 3a shows the PL spectra comparatively for the spin-coated 3DP and 2DP films, where a blue shift in the emission peak for the 2DP film (~739.4 nm) is present, in comparison to the 3DP film (~773.4 nm) which corresponds to~1.67 eV and~1.60 eV, respectively. We also characterized the 3DP and 2DP films using UV-Vis optical absorption spectroscopy, measured using the Agilent CARY 5000 spectrophotometer, and the data are shown in Figure 3b,c, respectively. From the absorbance data, the optical bandgap E g was calculated using the Tauc plot, where (αhυ) 2 was plotted as a function of the energy, hυ here υ is the frequency of the incoming light source and h is the Planck's constant. Using the Tauc equation (αhv) n = A hv − E g , the x-intercept to this plot represents the optical E g . Since organic-inorganic metal halide perovskites are direct bandgap semiconductors, the n was assumed to be 2, and α the absorption coefficient, was calculated using the absorption data according to 2.303 × Absorption Thickness . The thickness of the 3DP was~400 nm and that of the 2DP film was~360 nm, based on spinning parameters used. From the linear extrapolation of the plot, the E g for the 3DP film was determined to be~1.59 eV, as shown in Figure 3b, while the E g for the 2DP film was determined to bẽ 1.67 eV, as shown in Figure 3c. These values are in close agreement with the PL emission peaks which were calculated to be~1.60 eV and~1.67 eV, respectively, from the data in Figure 3a.
For extracting the PV device parameters, we fabricated PSCs using the 3DP and 2DP absorbers in the n-i-p architecture, as shown by the schematic in Figure 4a. The working principle in this regular architecture proceeds according to an incoming photon being absorbed within the 2D or 3D absorber layer, and leading to the generation of electron-hole pairs or bound excitons in the active layer. These excitons are then separated and collected in the respective electron transport layer (ETL) and HTL, and subsequently generate a current at the terminals. The deposition and spin coating conditions for the MAPbI 3 and ((CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 ) are summarized in Figure 4b-top and Figure 4b-bottom, respectively. For the 3DP film deposition process illustrated in Figure 4b-top, the one-step solvent exchange process was used with the Lewis-base-acid adduct in 4:1 anhydrous DMF:DMSO with precursors MAI (1.5 M) and PbI 2 (1.5 M), while the solution was stirred overnight at RT. Figure 4b-top shows the spinning conditions with an initial 800 rpm for 5 s, followed by 4500 rpm at 20 s using 100 µL solution. For the antisolvent, 0.1 mL of toluene was slowly dripped on the rotating substrate in the last 5 s, which is important for the controlled reaction kinetics and nucleation of the film. The substrate was then transferred to a hotplate for annealing at 125 • C for 30 min. On the other hand, the 2DP ((CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 ) film was deposited using the hot-casting technique on a pre-heated FTO substrate at~125 • C, as shown by the film synthesis process flow in Figure 4b-bottom, where the 2DP precursor was deposited at 5000 rpm for 20 s and annealing was conducted at 125 • C for 3 min.
For the n-i-p fabrication process on FTO substrates, c-TiO 2 and Spiro-OMeTAD served as the ETL and HTL, respectively, and e-beam evaporated Au was used as the collector electrode. The details of the entire fabrication process and deposition conditions used for the 3D PSC and 2D PSC are shown in Figure 4c-top and Figure 4c-bottom, respectively. The FTO glass substrates (Zhuhai Kaivo/China) were sequentially cleaned by detergent, DI water, and acetone and sonicated with isopropyl alcohol in an ultrasonic bath for 30 min. This was then followed by UV-O 3 treatment in a UV-ozone oven for 20 min. After this, we then spin-coated 0.15 M solution of titanium diisopropoxide bis(acetylacetonate) in ethanol at 4000 rpm for 20 s to form a compact titanium dioxide (c-TiO 2 ) layer, which served as the ETL that was subsequently annealed at 510 • C for 30 min. Next, we transferred the TiO 2 coated substrates into the glove box and spin coated the MAPbI 3 (1.5 M) solution at 800 rpm for 5 s followed by 4500 rpm for 20 s and 0.1 mL of toluene was slowly dripped on the rotating substrate in the last 5 s (as also shown in Figure 4b-top). Then the films were annealed at 125 • C for 30 min. Once the substrates cooled down, spiro-OMeTAD (used as received 2,2 ,7,7 -tetrakis-(N,N-di-p-methoxyphenylamine)-9,9 -spirobifluorene from Lumtec) solution was spin coated at 4000 rpm/20 s on top of the 3DP layer as the HTL. Finally,~80 nm of Au was deposited using e-beam evaporation at a pressure of 10 −5 Torr. The PSC device fabrication process for the 2D PSC is similar and is shown in Figure 4c-bottom. Here, the preheated TiO 2 deposited substrate was immediately transferred to the spin coater using the hot-casting approach already discussed, where 0.118 M solution of the 2D precursor in DMF was used and annealed at 125 • C for 3 min. As noted, the rest of the device fabrication process for the 2DP PSC was similar to the 3D PSC fabrication process.
Solar 2022, 2, FOR PEER REVIEW 8 Figure 3. (a) The steady state PL spectra for our 2DP and 3DP films deposited directly on unetched FTO/glass substrates, which shows a peak emission occurring at 1.60 eV for the 3DP film and a peak position occurring at 1.67 eV for the 2DP photoabsorber. From the absorbance data measured using UV-Vis optical absorption spectroscopy, the optical bandgap Eg was determined using the Tauc approach for the (b) 3DP film, and the (c) 2DP film, where Eg was determined to be ~1.59 eV and 1.67 eV, respectively; these results were largely in alignment with the PL emission data.  Figure 3. (a) The steady state PL spectra for our 2DP and 3DP films deposited directly on unetched FTO/glass substrates, which shows a peak emission occurring at 1.60 eV for the 3DP film and a peak position occurring at 1.67 eV for the 2DP photoabsorber. From the absorbance data measured using UV-Vis optical absorption spectroscopy, the optical bandgap E g was determined using the Tauc approach for the (b) 3DP film, and the (c) 2DP film, where E g was determined to be~1.59 eV and 1.67 eV, respectively; these results were largely in alignment with the PL emission data.  Table 1.
For the n-i-p fabrication process on FTO substrates, c-TiO 2 and Spiro-OMeTAD served as the ETL and HTL, respectively, and e-beam evaporated Au was used as the collector electrode. The details of the entire fabrication process and deposition conditions used for the 3D PSC and 2D PSC are shown in Figure 4c-top and Figure 4c-bottom, respectively. The FTO glass substrates (Zhuhai Kaivo/China) were sequentially cleaned by detergent, DI water, and acetone and sonicated with isopropyl alcohol in an ultrasonic bath for 30 min. This was then followed by UV-O3 treatment in a UV-ozone oven for 20  Table 1.
The electrical characteristics of the fabricated PSCs were measured under one-sun optical illumination, i.e., 100 mW-cm −2 , using the Oriel LSH-7320 LED solar simulator connected to a source meter unit from Ossila (Model: X200). The light was calibrated with a standard mono-Si solar cell (PVM-396, PV Measurements Inc., Boulder, CO, USA) certified by the US National Renewable Energy Laboratory (NREL). The optimized J-V Characteristic of the 3D PSC is shown in Figure 4d, which resulted in a maximum PCE of~15.69%, with an open circuit voltage V oc = 1.06 V, short circuit current density J sc of 21.89 mA-cm −2 , and a fill factor of FF = 67.50%. The J-V Characteristic for the 2D PSC is shown in Figure 4e, and a PCE of~3.33% resulted, with a V oc = 1.13 V, J sc~5 .5 mA-cm −2 and FF = 53.51%. The PV figures of merit for both absorber devices is summarized in Table 1. Table 1. Summary of optimized photovoltaic parameters used for the n-i-p PSCs fabricated, as shown in Figure 4, based on the 3DP (MAPbI 3 ) and 2DP ((CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 ) solar cell absorbers.

Stability Comparison of 3DP and 2DP Absorbers
To gain insights into the degradation paths related to the 2D and 3D photoabsorbers, we spin coated the 3DP and 2DP absorbers on top of FTO substrates, as shown by the film deposition processes outlined already in Figure 4b, top and bottom, respectively. Their properties were examined as a function of environmental storage using the FEI Quanta environmental SEM, the Rigaku Ultima III X-ray diffractometer for the XRD analysis, and UV-vis spectroscopy. For the SEM analysis, three cases were considered, i.e., (i) freshly deposited films, (ii) films exposed to room temperature ambient (RT) for a duration of 8 h at~25 • C and~45 ± 5% RH (uncovered), and (iii) films exposed to~90% RH using an in-house simulated humidity chamber at ambient temperature of~25 • C for 30 min. SEM images corresponding to 3DP films are shown in Figure 5a-(i) for freshly deposited films, (ii) ambient (RT) exposed films, and (iii) RH~90% exposed films. The freshly deposited films in Figure 5a-(i) appear dense, and showed a uniform morphology with a small grain size, which is very well aligned with previous reports. The RT exposed films in Figure 5a-(ii) reveal signs of void formation, particularly at boundaries separating the grains. The increase in porosity and cracking suggests degradation pathways for the 3DP, contributing to its diminished stability. In Figure 5a-(iii)" the films have largely disintegrated, and the substrate was also discolored. Similarly, SEM images corresponding to the 2DP films are shown in Figure 5b-(i) for freshly deposited films, which shows the fresh 2DP film is very uniform, smooth and dense, though some emergent boundary porous regions are evident. In Figure 5b-(ii) the films are exposed to RT ambient for a duration of 8 h at 25 • C and 45 ± 5% RH (uncovered), and Figure 5b-(iii) represents films exposed to RH using an in-house simulated humidity chamber at ambient temperature of~25 • C for 30 min. In contrast to the 3DP, the 2DP showed an identical surface morphology for both fresh and RT exposed films (Figure 5b-(i,ii)). Thus, unlike the case for the 3DP film, the 2DP film still appears smooth and dense, as noted in Figure 5b-(ii), but the onset of degradation is evident when the 2DP films were exposed to 90% RH, as in Figure 5b-(iii). Regarding the appearance of the two films, the 2DP and 3DP films turned yellow in color upon exposure to a RH of~90%. However, from the comparative analysis, it appears the 2DP films are denser and smoother and can sustain exposure to moisture-rich ambient conditions, well beyond that possible with our 3DP films.
The XRD analysis for both 3DP and 2DP films was also conducted and the respective XRD spectra are shown in Figure 6a Figure 6a-(iii). In prior work by Jia et al [61] they examined the bare FTO (220) peak occurring at~26 • , although our (200) peak is present at~21 • and this shift in angular position may have to do with the bare FTO substrate itself interacting with the degraded perovskite. We also note that additional peaks were obtained at lower angles for the 2DP, for example at~6.7 • and~8.9 • , when the films were exposed to the 90% RH condition. These maybe attributable to the lower order crystal planes in the 2DP, i.e., (060) and (080) and their presence may have origins in solvated phases that do not retain their original phase purity and are dissociated into BA + , MA + , and (PbI x ) 2−x ions. Similarly, the XRD spectra of the 2DP film shown in 6b-(i-iii), reveals the distinct crystallinity of the 2DP films with the two dominant planes at~14.1 • ,~28.41 • and also at~43.1 • indexed to the 2D perovskite crystallographic (111), (202) planes and (313) planes, respectively, as shown in Figure 6b-(iii) for the freshly deposited films. As shown in Figure 6a-(iii), fresh films showed characteristic peaks of 3DP, while the 3DP peaks subsided in samples stored at RT (Figure 6a-(ii)). Interestingly for the 2DP films, fresh and lab ambient films (Figure 6b-(ii,iii)) preserved the characteristic peaks in both environmental ambient. Notably, upon exposing both absorbers in RH~90%, the characteristics peaks diminished for both absorbers which is observed in Figure 6a-(i),b-(i), respectively. Taking the (110) peak and (111) peak for 3DP and 2DP films, respectively, the grain size for the fresh and RT exposed films was calculated using the Scherrer relation, S = Kλ β cos θ , where S is the estimated crystalline grain size, λ = 1.5406 is the wavelength of the X-rays used in the diffractometer, β is the FWHM in radians, θ is the diffraction Bragg angle, and K is the Scherrer constant or shaping factor. The K value ranges from 0.94, 0.89 for the 3DP films and is~0.9 for the 2DP films assuming spherical crystallites [52][53][54]. The K for our 3DP and 2DP films was thus taken to be~0.9, assuming spherical crystals likely for our samples, which is within the 0.89 and 0.94 range. These values of S were computed and are summarized in Table 2 for both fresh and RT exposed 3DP and 2DP films, where S~15.2 nm for 3DP-fresh which increased to~30.1 nm when the films were exposed to RT. This is consistent with the SEM images in Figure 5a-(ii), where increased porosity of the films is clearly visible in the SEM images. On the contrary, for the 2DP films, S~29.4 nm for fresh films and when the samples were exposed to RT, the S~16.18 nm. The porosity of the film is not greatly altered in these samples, which is reflected in the SEM images shown in Figure 5b-(ii). Thus, the increased porosity at grain boundaries in the 3DP manifests itself as potential degradation pathways for the RT exposed films, supporting their diminished stability, in contrast to the 2DP, which is largely unchanged morphologically through the correlations we have made using SEM and XRD data. Solar 2022, 2, FOR PEER REVIEW 11 grains. The increase in porosity and cracking suggests degradation pathways for the 3DP, contributing to its diminished stability. In Figure 5a-(iii),, the films have largely disintegrated, and the substrate was also discolored. Similarly, SEM images corresponding to the 2DP films are shown in Figure 5b-(i) for freshly deposited films, which shows the fresh 2DP film is very uniform, smooth and dense, though some emergent boundary porous regions are evident. In Figure 5b-(ii) the films are exposed to RT ambient for a duration of 8 h at 25 °C and 45 ± 5% RH (uncovered), and Figure 5b-(iii) represents films exposed to RH using an in-house simulated humidity chamber at ambient temperature of ~25 °C for 30 min. In contrast to the 3DP, the 2DP showed an identical surface morphology for both fresh and RT exposed films (Figure 5b-(i,ii)). Thus, unlike the case for the 3DP film, the 2DP film still appears smooth and dense, as noted in Figure 5b-(ii), but the onset of degradation is evident when the 2DP films were exposed to 90 % RH, as in Figure 5b-(iii). Regarding the appearance of the two films, the 2DP and 3DP films turned yellow in color upon exposure to a RH of ~90%. However, from the comparative analysis, it appears the 2DP films are denser and smoother and can sustain exposure to moisture-rich ambient conditions, well beyond that possible with our 3DP films. Figure 5. Surface morphology characterization studies conducted on spin coated 3DP and 2DP on top of FTO substrates for three cases, (i) freshly deposited films at t = 0, (ii) exposed to room temperature ambient (RT) for a duration of 8 h at 25˚C and 45 ± 5 % RH (uncovered), and (iii) exposed to 90% RH in an inhouse simulated humidity chamber at ambient temperatures for 30 min. SEM images corresponding to 3DP films that are (a)-(i) freshly deposited, (ii) ambient RT exposed, and (iii) exposed to RH > 90%. While the freshly deposited films in (a)-(i) appear dense, the RT exposed films in (a)-(ii) reveal signs of void formation, particularly at boundaries separating the grains, while in (a)-(iii), the films have disintegrated, and the substrate is also discolored. SEM image corresponding to 2DP films that are (b)-(i) freshly deposited, (ii) ambient RT exposed, and (iii) exposed to RH > 90%. Unlike the case for the 3DP film, the 2DP film still appears smooth and dense in (ii) after it is Figure 5. Surface morphology characterization studies conducted on spin coated 3DP and 2DP on top of FTO substrates for three cases, (i) freshly deposited films at t = 0, (ii) exposed to room temperature ambient (RT) for a duration of 8 h at 25 • C and 45 ± 5% RH (uncovered), and (iii) exposed to 90% RH in an inhouse simulated humidity chamber at ambient temperatures for 30 min. SEM images corresponding to 3DP films that are (a)-(i) freshly deposited, (ii) ambient RT exposed, and (iii) exposed to RH > 90%. While the freshly deposited films in (a)-(i) appear dense, the RT exposed films in (a)-(ii) reveal signs of void formation, particularly at boundaries separating the grains, while in (a)-(iii), the films have disintegrated, and the substrate is also discolored. SEM image corresponding to 2DP films that are (b)-(i) freshly deposited, (ii) ambient RT exposed, and (iii) exposed to RH > 90%. Unlike the case for the 3DP film, the 2DP film still appears smooth and dense in (ii) after it is freshly formed in (i), but the onset of degradation is evident when the 2DP films are exposed to 90% RH (iii). However, from the comparative analysis, it appears the 2DP films are denser and smoother and can sustain exposure to ambient conditions, well beyond that possible with the 3DP films.  Figure 5. For the 3DP films, the amplitude of the (110) peak is diminished significantly even upon exposure to the RT conditions in (ii), while for the 2DP film the (111) peak is strongly active even after exposure to the conditions in (ii). From the (110) and the (111) peaks for the 3DP and 2DP films, respectively, the grain size S at t = 0 for freshly deposited films and RT exposed samples was computed using the Scherrer formula and the data are summarized in Table 2. Besides the morphology and structural characterization conducted above, we also performed UV-Vis optical absorption spectroscopy on both the 3DP and 2DP films on top of FTO substrates for three environmental ambients to gauge stability. The onset of the optical absorption edge for the 3DP freshly synthesized film occurs at ~775 nm, as shown by the data in Figure 7a. On the other hand, this absorption edge starts at a shorter λ, less than 700 nm for the 2DP film in Figure 7b. For the RT exposed 3DP samples in Figure 7a, there is already a reduction in the absorbance below 625 nm, while for the RT exposed 2DP samples in Figure 7b, an improvement in the absorbance is seen for all wavelengths in the measurement range below the optical absorption edge, i.e., <700 nm, suggesting i) are exposed to 90% RH for 30 min., (a)-(ii) and (b)-(ii) exposed to RT for 8 h, and (a)-(iii) and (b)-(iii) at t = 0 for freshly deposited films; further details on the specific environmental conditions are provided in the caption of Figure 5. For the 3DP films, the amplitude of the (110) peak is diminished significantly even upon exposure to the RT conditions in (ii), while for the 2DP film the (111) peak is strongly active even after exposure to the conditions in (ii). From the (110) and the (111) peaks for the 3DP and 2DP films, respectively, the grain size S at t = 0 for freshly deposited films and RT exposed samples was computed using the Scherrer formula and the data are summarized in Table 2. Table 2. The grain size S for the fresh and RT exposed samples was computed using the Scherrer formula from the (110) and the (111) peaks for the 3DP and 2DP films, respectively. Besides the morphology and structural characterization conducted above, we also performed UV-Vis optical absorption spectroscopy on both the 3DP and 2DP films on top of FTO substrates for three environmental ambients to gauge stability. The onset of the optical absorption edge for the 3DP freshly synthesized film occurs at~775 nm, as shown by the data in Figure 7a. On the other hand, this absorption edge starts at a shorter λ, less than 700 nm for the 2DP film in Figure 7b. For the RT exposed 3DP samples in Figure 7a, there is already a reduction in the absorbance below 625 nm, while for the RT exposed 2DP samples in Figure 7b, an improvement in the absorbance is seen for all wavelengths in the measurement range below the optical absorption edge, i.e., <700 nm, suggesting that the 2DP is performing optimally as a photoabsorber and is more stable at these environmental conditions. For the 3DP exposed to RH~90%, there is once again a general reduction in the optical absorption based on the intensity, and a sharp rising edge occurs at λ < 525 nm, which maybe attributable to the breakdown of the 3DP film into PbI 2 that is optically active at shorter λ, closer to the near-UV region. Equivalently, for the 2DP film, several optical resonances surface between 550-650 nm at all of the three environmental conditions examined, which are not evident in the spectra of the 3DP film. The optical resonances in the 2DP films may be attributable to quantum confinement and excitonic effects given the reduced physical dimension of the 2DP structure based on the crystalline lattice. From the summary of the optical absorption spectroscopy data, it is clear the 2DP film is more stable upon exposure to moisture-rich environments, compared to the 3DP film whose optical absorption spectra shows a steady decline in the environmental ambients tested, compared to the freshly deposited films.
Solar 2022, 2, FOR PEER REVIEW resonances in the 2DP films may be attributable to quantum confinement and excito effects given the reduced physical dimension of the 2DP structure based on the crystall lattice. From the summary of the optical absorption spectroscopy data, it is clear the 2 film is more stable upon exposure to moisture-rich environments, compared to the 3 film whose optical absorption spectra shows a steady decline in the environmental am ents tested, compared to the freshly deposited films.  Figure 5. The onset of the opt absorption edge for the 3DP freshly synthesized film at t = 0 occurs at ~775 nm in (a), while absorption edge starts at a shorter λ, less than 700 nm for the 2DP film in (b). For the RT expo 3DP samples in (a), there is already a reduction in the absorbance below 625 nm, while for the exposed 2DP samples in (b), an improvement in the absorbance is seen for all wavelengths in measurement range below the optical absorption edge, i.e., <700 nm. For the 3DP exposed to R there is once again a general reduction in the optical absorption based on the intensity, and a sh rising edge occurs at λ < 525 nm which maybe attributable to the breakdown of the 3DP film i PbI2 which is optically active at shorter λ, closer to the near-UV region. Equivalently, for the 2 film, several optical resonances surface between 550-650 nm at all of the three environmental c ditions examined, which are not present in the spectra of the 3DP film. The optical resonances in 2DP films may be attributable to quantum confinement and excitonic effects given the redu physical dimension of the 2DP crystalline structure.

Conclusions
In summary, we characterized the photoresponse of reduced dimensiona (BA)2(MA)3Pb4I13 2DP crystallites, where the photocurrent and responsivity were me ured as a function of wavelength at room temperature. The synthesized 2DP was co pared with the 3DP MAPbI3 absorber and the bandgap of both the MAPbI3 and 2DRP w determined to be ~1.6 eV and ~1.67 eV, respectively, based on the PL emission spectra a equivalent values for Eg were determined to be ~1.59 eV for MAPbI3 and 1.67 eV for 2 using the Tauc approach, respectively. The PV device figures of merit for the 2DP a 3DP absorbers in the n-i-p architecture on FTO substrates were also extracted and co pared. We then conducted a comparative material stability analysis in elevated moist ambients, where the fresh films, ambient exposed films (RT and RH > 90%) were examin using SEM, XRD, and UV-Vis and all of our material probes indicated that the 2DP fil were more robust upon exposure to elevated moisture levels, though degradation in  Figure 5. The onset of the optical absorption edge for the 3DP freshly synthesized film at t = 0 occurs at~775 nm in (a), while this absorption edge starts at a shorter λ, less than 700 nm for the 2DP film in (b). For the RT exposed 3DP samples in (a), there is already a reduction in the absorbance below 625 nm, while for the RT exposed 2DP samples in (b), an improvement in the absorbance is seen for all wavelengths in the measurement range below the optical absorption edge, i.e., <700 nm. For the 3DP exposed to RH, there is once again a general reduction in the optical absorption based on the intensity, and a sharp rising edge occurs at λ < 525 nm which maybe attributable to the breakdown of the 3DP film into PbI 2 which is optically active at shorter λ, closer to the near-UV region. Equivalently, for the 2DP film, several optical resonances surface between 550-650 nm at all of the three environmental conditions examined, which are not present in the spectra of the 3DP film. The optical resonances in the 2DP films may be attributable to quantum confinement and excitonic effects given the reduced physical dimension of the 2DP crystalline structure.

Conclusions
In summary, we characterized the photoresponse of reduced dimensionality (BA) 2 (MA) 3 Pb 4 I 13 2DP crystallites, where the photocurrent and responsivity were measured as a function of wavelength at room temperature. The synthesized 2DP was compared with the 3DP MAPbI 3 absorber and the bandgap of both the MAPbI 3 and 2DRP was determined to be~1.6 eV and~1.67 eV, respectively, based on the PL emission spectra and equivalent values for E g were determined to be~1.59 eV for MAPbI 3 and 1.67 eV for 2DP using the Tauc approach, respectively. The PV device figures of merit for the 2DP and 3DP absorbers in the n-i-p architecture on FTO substrates were also extracted and compared. We then conducted a comparative material stability analysis in elevated moisture ambients, where the fresh films, ambient exposed films (RT and RH > 90%) were examined using SEM, XRD, and UV-Vis and all of our material probes indicated that the 2DP films were more robust upon exposure to elevated moisture levels, though degradation in RH 90% ambients caused both films to degrade. Thus, we validated the increased stability of 2DP compared to the 3DP based on environmental storage conditions.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.