Improved photovoltaic performance of Pb-free AgBi2I7 based photovoltaics

Hybrid perovskites based on bismuth are good candidates for developing lead-free and air-stable photovoltaics, but they have historically been constrained by poor surface morphologies and large band-gap energies. Monovalent silver cations are incorporated into iodobismuthates as part of a novel materials processing method to fabricate improved bismuth-based thin-film photovoltaic absorbers. However, a number of fundamental characteristics prevented them from achieving better efficiency. We examine bismuth iodide perovskite made of silver with improvements in surface morphology and a narrow band gap, and we achieve high power conversion efficiency. AgBi2I7 perovskite was used in the fabrication of PSCs as a material for light absorption, and its optoelectronic proficiencies were also studied. We reduced the band gap to 1.89 eV and achieved a maximum power conversion efficiency of 0.96% using the solvent engineering approach. Additionally, simulation studies verified an efficiency of 13.26% by using AgBi2I7 as a light absorber perovskite material.

a lot of interest in developing halide perovskite solar cells that are non or low-toxic and air stable. As a result, efforts have been made to develop perovskite solar cells and seek alternatives to lead. Many perovskites based on tin (Sn) 9 and germanium (Ge) 10 have been investigated to address the issue of toxicity. Song et al. 11 designed and manufactured Sn-based perovskite materials containing caesium (Cs) as a cation, with CsSnI 3 and CsSnBr 3 perovskites achieving efficiencies of 3.04% and 1.83%, respectively. In addition, Ke et al. 12 used a unique hollow 3-D perovskite [enFASnI 3 ] as a light absorber material in perovskite solar cells (PSCs), with a 7.1% efficiency. Mhaisalkar and colleagues used a Ge-based AGeI 3 perovskite-like material. 13 These Sn and Ge based perovskite-like materials have a high efficiency, but they must be handled with caution since they are air sensitive and need an inert environment to be stable. The perovskite structure is distorted by the quick shi in the oxidation states of Sn and Ge by +2. Moreover, when compared to Pb, these Sn and Ge based perovskites are unable to reach high efficiency. However, instability and poor performance of Sn and Ge based devices under ambient conditions owing to disproportionation are disappointing.
There have been several reports on the use of copper (Cu) as a metal ion in PSCs, utilising a lead-free approach. Mathews et al., 14 Ahmad et al., 15 and Li et al. 16  We have studied the solution engineering approach to improve the photovoltaic performance of silver-based bismuth iodide perovskites as light absorbers. To our knowledge, this is the rst report on the use of SBI perovskites as light absorbers in the presence of DMF and DMF : MeOH. The impact of the solvent engineering strategy on the fabrication of SBI perovskite solar cells may be immediately seen in their efficiency and photovoltaic properties. In the present work, we employed DMF and MeOH, two different solvents in an appropriate ratio, and with molar ratio 1 : 2 of AgI and BiI 3 respectively. AgBi 2 I 7 DMF (SBI-D) and AgBi 2 I 7 DMF : MeOH (SBI-DM) were spin coated onto the conductive glass electrode (FTO) at 1500 rpm for 30 seconds (Scheme 1). Other fabrication data are provided in the ESI. † A convincing demonstration of the equimolar ratio of both the solvent (DMF : MeOH) gives the highest PCE as compared to only 0.96% with DMF under 1 sun illumination conditions and 30-40% humidity.
PXRD was used to characterise the phase purity and formation of the SBI perovskite material synthesized with both DMF and DMF : MeOH solvents, with the ndings shown in Fig. 1A. The formation and crystalline nature of SBI-D (DMF : MeOH = 1 : 0) and SBI-DM (DMF : MeOH = 0.5 : 0.5) perovskite materials were revealed by PXRD peak patterns. The growth of SBI-D and SBI-DM is supported by the appearance of a prominent diffraction peak in the (333) plane.
The stability of perovskite may be assessed using the Goldschmidt tolerance factor (t); when the value of "t" is between 0.8 and 1, it implies that the perovskite structure is stable. 36 Another important component in the cubic crystal is the ionic radius of A, which should not be either large (then, t > 1) or too small (then, t < 0.8) in comparison to the ionic radius of B. If the ionic radius of A is substantially bigger than that of B, it will not t inside the BX 8 octahedron. This could originate from a distinct perovskite structure. The tolerance factor and octahedron ratio are described by eqn (a) and (b): where r A , r B , and r X stand for ionic radii of A, B, and X present in the perovskite (ABX 3 ) structure. At t = 1, the predicted perfect cubic structure was seen. In order to produce a stable octahedron for a cubic cell, the octahedron factor (m) should be between 0.44 and 0.72. The effective ionic radii of silver (Ag) and bismuth (Bi) are 1.26 Å and 1.03 Å, respectively, which are appropriate for the creation of a stable cubic perovskite structure. We may infer and remark on the stability of our perovskite based on this nding, and therefore, it can also be employed as a light absorber material in photovoltaic applications.
UV/vis spectroscopy was used to determine the optical characteristics of SBI-D and SBI-DM thin lms (Fig. 1B). The absorption spectra of these perovskite materials were nearly identical. The optical band gaps for SBI-D and SBI-DM may be calculated using UV absorption spectra and linear extrapolation of Tauc plots (Fig. S1 †). 37 The optical band gap was calculated to be 1.89 eV using the Tauc plot ((ahn) n against hn), where a, h and n are the absorption coefficient, Planck's constant, and excitation frequency respectively. The computed optical band gap reveals that the produced perovskite has high absorbance and has the potential to be used as a light absorber in solar cells.
Surface morphology investigation was done using FE-SEM to evaluate the impact of engagement and solvent content on the perovskite SBI material. The surface morphology of SBI-D and SBI-DM is shown in Fig. 2A-D, and it was obvious that only the DMF solvent produced a rod-like shape, whereas the DMF : MeOH combination produced a distorted rod-like morphology that was converted to a uniform surface morphology. This might be due to perovskite suppressing and regulating the quick crystallisation process. It has been discovered that a pinhole-free, smooth layer can improve the photovoltaic efficiency of perovskite solar cells.
When a photon strikes a perovskite material, it generates an electron and hole pair. The produced electron was excited and moved towards the LUMO (lowest unoccupied molecular orbital) level, from where it was transferred to conductive glass (FTO) through mesoporous-TiO 2 and blocking-TiO 2 LUMO levels. The produced electrons are contained in FTO glass, while the remaining hole in the perovskite material is carried via the HTM (hole transport material) to complete the circuit. Scheme 2 explains the entire electron transfer mechanism in simple terms. All of the energy levels of the conductive glass, ETL, and HTM, such as bl-TiO 2 , mp-TiO 2 , Spiro-MeOTAD, and Au, have been extracted from previously published literature. In addition, using UV-visible and cyclic voltammetry (CV) techniques, the HOMO-LUMO energy levels of our perovskite materials SBI were estimated similarly to previous reports.
The optical band gap of SBI was determined using a Tauc plot, whereas the onset reduction potential (E red ) was determined using a CV graph (Fig. S2 †). We determine the energy levels of the SBI perovskite material using eqn (i) and (ii). 38 All of the CV measurement details are included in the ESI. † Here, E g and E red stand for the optical band gap and onset reduction potential, and E CB and E VB are conduction and valence band energy levels. Henceforth, we compared our computed SBI perovskite energy levels (E CB and E VB ) to the ETL and HTM for smooth charge transfer. The results indicate an excellent correlation between the HOMO and LUMO levels, implying that SBI perovskite has potential as a light absorber material.
Moreover, a PSC device was designed using AgBi 2 I 7 as a perovskite light absorber; comprehensive device fabrication details are included in the ESI †. The device's photovoltaic performance was measured aer it was fabricated under ambient conditions (30-40% humidity). The short circuit photocurrent density-voltage curve was used to assess the device's photovoltaic performance. Fig. 3 shows the photovoltaic performance of SBI-D and SBI-DM under one sun conditions (1.5 AM; 100 mW cm −2 ). Table 1 contains all of the photovoltaic parameters recorded by the devices. The maximum PCE of 0.96% was reached using SBI-DM PSCs, which is higher   than the PCE of PSCs manufactured with SBI-D as a light absorber. Additionally, Fig. S5, ESI † provides the box charts of the J sc , FF, V oc , and PCE for SBI-D and SBI-DM. Moreover, SBI-DM perovskite as a light absorber attained a higher open circuit voltage than SBI-D PSCs. However, AgBi 2 I 7 synthesized from DMF and MeOH solutions revealed homogeneous grains and a thin uniform layer, providing an easy interaction with surrounding charge transfer layers. It is observed from the XRD and SEM results that the solvent in the precursor solution signicantly affects the crystallization and morphology of AgBi 2 I 7 . This solvent engineering approach showed the improvement in morphology without affecting the perovskite structure, which resulted in an increase in efficiency.
We summarized all data in Table 1 for comparison with other reported photovoltaic performances of Pb free PSCs. Recently, there has been a tremendous increase in the production of Pb-free PSCs. The research and development of non-toxic perovskite materials for photovoltaic applications has garnered considerable interest from researchers. 0-D (CH 3 -NH 3 ) 3 Sb 2 I 9 perovskite was introduced by Hebig et al. 21 as a potential contender for lead-free perovskite solar cells. Additionally, a PCE of 0.49% was obtained by using solvent engineering techniques that included a toluene drop during the spin-coating procedure. Li et al. 16 created a novel form of photovoltaic material with a band gap of 1.64 eV, although they only managed to obtain a 0.46% efficiency. For PSCs, Qiu et al. 39 used a Cs 2 SnI 6 light absorber, but the manufactured device had a subpar PCE of 0.86%. A thin le of AgBi 2 I 7 perovskite with a direct band gap of 1.93 eV was designed by Shao et al., 40 although the efficiency was only 0.83%. 2018 had seen the utilization of the (CH 3 NH 3 ) 3 Sb 2 I 9 perovskite material in solar cells by Chatterjee et al., 41 and the manufactured PSC device had the best PCE of 0.57% without any dopant. In earlier research, Zhang et al. 42 designed a novel perovskite structure; the light absorber (Cs 2 NaBiI 6 ) had good optoelectronic properties, however its PCE was less than 1%. Very stable Pb free PSCs have also been developed using all inorganic perovskite structures, although only 0.62% efficiency was attained. 43 Zuo et al. 44 had used variation in halide ions using iodide and bromide ions, with (NH 4 ) 3 Sb 2 I 9 having the maximum PCE of 0.5%. However, Yokoyama et al. 45 and Ahmad et al. 46 respectively designed Sn-based perovskites (CH 3 NH 3 SnBr 3 ) and Sn-incorporated materials (MA 3 (Bi 1−x Sn x ) 2 I 9 ), where the PCE was higher with the doped light absorber (MA 3 (Bi 1−x Sn x ) 2 I 9 ). When compared to previous reported lead-free perovskite devices, the photovoltaic performance of our SBI-D and SBI-DM based PSC devices was superior.
Research is still underway to develop a high-performance, stable device that can meet all energy demands. In this viewpoint, various attempts such as encapsulation, insertion of metal ions, doping of metal ions, and multistep fabrication were done. Different Pb-free PSCs have been developed and manufactured. We also looked into the electrical and optical properties of perovskite light absorbers and charge transport layers, which are important for improving the efficiency of perovskite solar cells. Computational studies were conducted to determine the inuence of ETL, HTM, and perovskite material thickness during solar cell fabrication. Understanding the variable performance characteristics of PSCs (FF, V oc , J sc , PCE) in terms of thickness variation was important. The SCAPS-1D programme was used to simulate the AgBi 2 I 7 lead-free perovskite. 47 The J-V curve and performance metrics for the FTO (500 nm)/TiO 2 (100 nm)/AgBi 2 I 7 (varying)/Spiro-MeOTAD (100 nm)/ Au device architecture are shown in Fig. 4a and b. According to the numerical simulation, the AgBi 2 I 7 lead-free perovskite's greatest PCE, with a perovskite layer thickness of 500 nm, was 13.26%. When the perovskite material thickness is extended from 100 nm to 500 nm, the J sc value rises while the V oc value somewhat declines but not enough to have an impact on performance. The outstanding AgBi 2 I 7 perovskite PCE of 13.26% at optimum 500 nm thickness was employed for simulation purposes (Fig. 4).
ETL and HTM effects on performance had been examined to note the impact of perovskite material thickness. The J-V curve of a simulation with increasing TiO 2 thickness is shown in Fig. S3a. † A little drop in J sc was seen when the TiO 2 thickness increased from 100 nm to 500 nm. Additionally, this drop in J sc impacts the device PCE as the TiO 2 thickness increases, with only a minimal impact on V oc and FF (Fig. S3b †). As a consequence, simulation ndings show that TiO 2 at a thickness of 100 nm is far more effective than that of the ETL. Fig. S4a † shows the J-V curves of AgBi 2 I 7 perovskite with various thicknesses of HTMs (Spiro-MeOTAD), which exhibited almost little change in J sc and V oc . The simulated results of the perovskite parameter demonstrate that, with the exception of V oc , all other parameters showed a declining tendency as the thickness of the HTM increased (Fig. S4b †). As a result of its superior performance, modelling of the AgBi 2 I 7 perovskite was done at a 100 nm TiO 2 and HTM thickness. Recently, computational studies on lead-free perovskite solar cells were also carried out by Mobin et al. 46 and relative efficiencies of approximately 12% for the perovskite materials MA 3 (Bi 1−x Sn x ) 2 I 9 were reported. To summarize our conclusions, we used a solvent engineering technique to fabricate a silver based bismuth perovskite (AgBi 2 I 7 ) material as a light absorber for PSCs. The photovoltaic efficiency was improved by combining DMF and MeOH in an optimum ratio. The obtained device performance results demonstrate the efficacy of the solvent engineering method. Likewise, using AgBi 2 I 7 as a light absorber in the device construction of PSCs resulted in a good PCE (0.96%) and V oc (650 mV). In the future perspective, improving system performance may be achieved by comprehending the crystallization process as well as by investigating appropriate charge transport layers and solvents. To improve the optical characteristics of AgBi 2 I 7 , less noxious metals might be inserted or doped into the structure. Additionally, AgBi 2 I 7 can be applied in different energy-harvesting scenarios.

Conflicts of interest
There are no conicts to declare.