Impact of HTM on lead-free perovskite solar cell with high efficiency

The Scaps-1d simulator was used to simulate a lead-free perovskite CH3NH3SnI3 based solar cell devices fabricated from different hole transport materials (HTM). This research looks at two organic and two inorganic HTM layers. The cell structure used in this study is FTO/TiO2/CH3NH3SnI3/HTM (variable)/Au(variable). Spiro-OMeTAD, PEDOT:PSS, CuO and Cu2O are the HTM materials used. The results show that utilizing CuO as an HTM produces better outcomes than other HTMs, with an efficiency of 28.45%. The thickness, acceptor concentration (NA), and defect density (Nt) of the perovskite layer on optoelectronic properties of the solar cell are focus of simulation studies. According to this study, an perovskite layer thickness of 1000 nm is suitable for a decent photovoltaic cell. Furthermore, by adjusting the HTM thickness and the defect density of HTM and absorber layer, promising findings of Jsc of 34.38 mAcm−2, Voc of 1.011 V, FF of 80.85% and PCE of 28.10% were obtained for Spiro-OMeTAD based PSC. Finally, in order to improve the device's performance, an anode material with high work function is required. Our findings reveal that using a thin absorber layer results in low photo generated charge carriers due to less absorption, but high carrier extraction. Although more carriers are created in the cell due to increased absorption, decreased collection efficiency is related to recombination, which decreases Voc for thick perovskite layers. Device efficiency is improved by increasing the doping density up to 1018 cm−3 in the perovskite layer due to built-in electric field across the solar cell. Again a very thin or thick HTL is not ideal for high PCE. For low recombination and a high fill factor, an HTM (Spiro-OMeTAD) of 1–100 nm is necessary. The great power conversion efficiency of organic HTM based lead-free PSC brings up the new possibilities for obtaining renewable energy.


Introduction
The need for energy will skyrocket in the future. The majority of today's energy usage is derived from fossil fuels, which will be depleted in the near future and it pollutes our environment. Because of this the renewable energy sources are in high demand as a result of industrialization and population growth. Solar energy is a promising source for this because it has no negative impact on our environment (Markose et al. 2020;Nocera 2017;Lipomi and Zhenan 2011;Huang et al. 2019;Yang et al. 2017;Shih et al. 2017). Photovoltaic devices are required as a part of a long-term global energy strategy. The energy research community has been paying close attention to perovskite solar cells as their has improved PCE day by day. Efficiency of this cell was relatively low at the initial time. But recently PSC's state of art certified PCE has a success rate of more than 25% (Shih et al. 2017;Kojima et al. 2009aKojima et al. , 2009bGao et al. 2018). Again, these perovskite materials have unique properties such as a high absorption coefficient, good charge carrier mobility, low exciton binding energy, and long charge carrier diffusion length (Kojima et al. 2009b;Lyu et al. 2017;Stranks et al. 2013;Su et al. 2014). As a result, it is indispensable to work on it. The photovoltaic effect can be used to transform this energy into electricity (Weiss et al. 2016). It is, however, a cheap and clean source of energy, and as a result, it has dominated the photovoltaic market for Si-based solar cells (Noel et al. 2014). Because of their excellent solar cell PCE, researchers investigated pure and modified methyl ammonium lead halide as a perovskite material (Burschka et al. 2013;Lee et al. 2012). Lead-based electronic equipment, on the other hand, have been strictly regulated by the European Union and other countries. The power conversion efficiency of these materials was extremely high. Lead, on the other hand, is not environmentally friendly and is damaging to both individuals and environment Caputo et al. 2019;Los et al. 2020;Guo et al. 2017;Li et al. 2018;Noh et al. 2013;You et al. 2014;Etgar et al. 2012).
To address this issue, several scientists and researchers have looked into the leadfree perovskite CH 3 NH 3 SnI 3 as a photovoltaic material due to the excellent optoelectronic capabilities. This material has a direct band gap of 1.3 eV, lying in the range of the absorber. Sevaral group of researchers have succeeded in fabricating organic-inorganic perovskite solar cells based on CH 3 NH 3 SnI 3 . In this work we develop organicorganic echo friendly perovskite solar cells using two organic HTMs (Spiro-OMeTAD, PEDOT:PSS). The electron transport material (HTM) is an important part of the PSC. The proper energy level for electron injection, strong electron mobility, chemical stability, low synthesis cost, and environmental friendliness make TiO 2 an attractive candidate material (Guo et al. 2019;Yang et al. 2019;Yu et al. 2016a;Bera et al. 2014). The band gap of TiO 2 material is appropriate for reducing the transmission of holes (Zhou et al. 2019). In PSC, the hole transport material is crucial for achieving high PCE. For minimum recombination of charge carriers at the interface, HTM requires higher value hole carrier mobility with fewer defects at the HTM/absorber layer. Cu 2 O as an HTM has recently been reported by Yu et al. to have strong hole mobility and appropriate energy level configuration with CH 3 NH 3 SnI 3 and photo-generated charges with higher life time (Yu et al. 2016a). It lowers the metal contact barrier height and lowers minority recombination loss at the anode. Because of this Cu 2 O is employed as an HTM. P Swaicka-Chudy et al. reported that as a HTM layer CuO provided better efficiency in solar cell than Cu 2 O (Sawicka-Chudy et al. 2019). Because of this we have chosen CuO as a HTM.
In this solar cell structure absorber layer is sandwiched between ETM and HTM where FTO and Au used as front and back electrode. A TiO 2 layer is utilized as an ETL towards the front electrode. Organic HTM is a Spiro-OMeTAD layer above the absorber that blocks electrons while allowing holes to flow towards the back electrode. Among all the organic HTM materials Spiro-OMeTAD is the most frequently used and benchmark material for high PCE solar cells. This is owing to CH 3 NH 3 SnI 3 perfect energy band alignment and great transparency of visible light (Jeyakumar et al. 2019). HTM not only aids hole conveyance, but it also prevents the contact between the back electrode and the absorber. The likely hood of collecting holes is determined by the thickness of HTM. As a result for high PCE solar cell HTM thickness optimization is required.
The conductivity of spiro-OMeTAD is about 3.5 × 10 -7 S/cm (Vivo et al. 2017) which is quite low. However, the conductivity of p-type spiro-ometad is higher than that of intrinsic Spiro-ometad. When spiro-OMeTAD is utilised, high J sc and V oc are obtained, indicating that Spiro-OMeTAD has a high charge collection efficiency. The quasi-Fermi level difference between ETM (TiO 2 ) and HOMO level of HTM (Spiro-OMeTAD) is responsible for the higher value of V oc (Reddy et al. 2016). V oc can be increased by lowering the HOMO energy level. Using Spiro-OMeTAD we have achieved a high FF with low series resistance (R s ) and high shunt resistance (R sh ).
Fill factor (FF), R s , and PCE are all determined by the thickness of the absorber, ETL, and HTL. Around 120 nm of compact TiO 2 is adequate for ETL. As a result of our prior findings, we may conclude that when thickness of the TiO 2 layer grows, then cell's R s grows. As a result, light transmission through TiO 2 diminishes. The latter diminishes J sc and PCE by affecting photogeneration in the perovskite layer. Fill factor falls as series resistance rises, and PCE suffers as a result. In addition, our research indicated that a doping density of 10 18 /cm 3 was appropriate for high fill factor TiO 2 . Various thicknesses are reported for perovskite (absorber) and HTM layers, and an optimal value for each layer is necessary. A thick(> 1000 nm) perovskite layer results in reduced hole collection efficiency due to an increase in R s and recombination inside the perovskite layer. A very thin (< 100 nm) perovskite layer causes low photocurrent because of less photo absorption. The probability of recombination is reduced since holes travel a shorter distance to reach the back electrode, device performance is improved when a thinner HTM (0.1-10 nm for Spiro-OMeTAD) is utilised. Despite this, an extremely thin HTM reduces the device's repeatability and stability because the counter electrode can penetrate through the HTM and touch the perovskite layer, shunting the device. But if we use a HTM of higher thickness which uniformly cover the rough perovskite layer, but it increases R s and lowers μ h . Furthermore, because CH 3 NH 3 SnI 3 contains crystallites, perovskite crystals can protrude through the HTM and contact the back electrode if a very thin HTL is utilised. To achieve a flat surface, HTL must be of an appropriate thickness which can cover the crystallites without altering the cell's R s (Bag et al. 2020). Moreover, a good HTM thickness improves the chances of collecting holes. If the layers are deposited using a spin-coating approach, the thickness of the layers can be adjusted by changing the spin-coating speed. To tune fill factor, energy band alignment, carrier concentration, and PCE, suitable doping in the CH 3 NH 3 SnI 3 absorber and Spiro-OMeTAD is necessary. In the absorber layer, dopants such as Mg, Na, Sn etc. were utilised Chang et al. 2019;Tang et al. 2017). The Mg dopant in CH 3 NH 3 SnI 3 lowered the conduction band and valance band levels to those of TiO 2 conduction band and Spiro-OMeTAD valance band, respectively. Magnesium doping enhanced the grain size, resulting in a pinhole-free film (Bag et al. 2020). The grain size of CH 3 NH 3 SnI 3 was similarly enhanced by sodium doping, and pinhole-free films were formed. Doping with tin is less difficult than doping with bismuth or titanium. The CH 3 NH 3 SnI 3 band gap can be changed by changing the Sn doping concentration (Bag et al. 2020).
Spiro-OMeTAD is commonly utilised for hole transfer. The impact of Spiro-OMeTAD and CH 3 NH 3 SnI 3 layer thickness and doping density on solar cell characteristics is investigated in this paper. With the cell design of FTO/ETM/perovskite absorber/HTM/Au, its impact on solar cell characteristics is examined. High PCE solar cells are created via device modelling, which involves selecting an appropriate HTM and fine-tuning variables such as HTM thickness and doping density, as well as the active layer thickness and doping density For this purpose we have used four different HTMs (Spiro-OMeTAD, PEDOT:PSS, CuO and Cu 2 O). In results and discussion section, we present our findings.
2 Device structure and simulation details SCAPS 1d software was used to perform a simulation of a Sn-based lead-free perovskite solar cell in this study. This simulation was carried out with a 1000 W/m 2 light source. We have investigated this study at 300 K and an air mass of AM 1.5G. In the configuration of the reported solar cell HTM is p-type CuO (variable), the absorber layer is CH 3 NH 3 SnI 3 , and the n-type TiO 2 as an ETM, it's used. In addition, FTO was chosen as the contact material, while anodes made of Ag, Cu, Au, and Pt were chosen.

Results and discussion
The energy band alignment and J-V graph of the reported PSC were displayed in Fig. 1a-b, respectively. For this, the used parameters shown in Tables 1 and 2. Inside the absorber layer, electron-hole pairs are formed after illumination. Electrons and holes travel towards ETL and HTL, respectively, due to the junction field. The cathode and anode collect the electrons and holes, respectively, and form a voltage. Using the initial electrical parameters the obtained values of J sc Voc, FF and PCE are 27.23 mA/cm 2 , 0.854 V, 71.08% and 16.54% respectively. We have determined the device's J sc by the active material's absorption coefficient, thickness, and mobility. The photo current will be higher if the absorption coefficient is higher (Kojima et al. 2009b;Stranks et al. 2013;Du et al. 2016;Lin et al. 2019a;Hao et al. 2014). The thickness of the absorber is the second crucial factor. It must be thick enough to absorb the incident solar radiation's greatest cut off wavelength (Lazemi et al. 2018;Lin et al. 2019a). Apart from that, movement is quite important for achieving a high Jsc. After illumination, the Jsc should be equivalent to solar cell current. For     , on the other hand, used substantially lesser and identical values of μ e (1.6 cm 2 /Vs) and μ h (0.16 cm 2 /Vs) and reported good J sc ( 30 mA/ cm 2 ) results. Again we found in some previous experimental literature that diffusion length is directly proportional to the mobility's square root (Minemoto and Murata 2015). As a result, for high charge carrier mobility, diffusion length increases, and recombination of charge carriers reduces. There could be different causes for the greater J sc when compared to recently published data (Devi and Mehra 2019;Khattak et al. 2019). The performance of the reported PSC is promising and reliable with the Sn-based PSC Bera et al. 2014). It is feasible to improve photovoltaic performance even more. PSC's energy band diagram is shown on Fig. 1a. At the absorber/ETL contact, we have observed a positive value of CBO which is roughly 0.35 eV. This positive spike blocks the flow of Photo-generated electrons towards the electrode (Minemoto and Murata 2015;Klenk 2001;Gloeckler and Sites 2005).

Absorber thickness
The thickness of the absorber, as indicated in Fig. 2, is a limiting parameter for cell performance. The absorber layer thickness can be adjusted to improve cell efficiency. Photon absorption and carrier collection have a reciprocal relationship in planar solar cells.
The thickness of the absorber in this reported PSC ranged from 100 to 4000 nm. Photo Absorber thickness(micrometer) Fig. 2 Impact of absorber thickness a on J sc , b on V oc , c on FF and d on PCE generated current is minimal for 100 nm thin absorber layer. Charge extraction, on the other hand, is high, as demonstrated by the high V oc , which indicates less recombination. Figure 2a, shows the increment of J sc with absorber thickness due to due to better light absorption. It immediately climbs from 26.2 mA/cm 2 to 34.56 mA/cm 2 for an increment of thickness from 100 to 1000 nm, then decreases until all light is absorbed, at which point it saturates at 35 mA/cm 2 . J sc is quite low around 100-200 nm due to decreased absorption, and utilising thicker (> 1000 nm) sheets has no benefit. The maximum value of J sc (~ 35 mA/cm 2 ) observed for a thickness of 1000 nm is mostly owing to the perovskite's high value of absorption coefficient (Lin et al. 2019a). As a result, a 800-1000 nm absorber thickness is appropriate. Since R s of the PSC increases, the fill factor falls with thickness. But a thick absorber surpasses the diffusion length and charge carriers are recombine.
Our findings demonstrate that the increase in J sc of the PSC is just 6.25% when comparing thicknesses of 400 nm and 1000 nm. This signifies that, while more charge carriers are generated as a result of increased absorption, a thicker cell's collection efficiency is reduced because of high recombination, as a result there is a decrease in V oc . This decrease in V oc with shown in Fig. 2b is linked to a decrease in the effective band gap and an increase in recombination (Lazemi et al. 2018). A decrease in V oc as thickness grows could be owing to increased recombination. Increasing absorber thickness, a modest decrease in FF (Fig. 2c) indicates a raise in R s in the PSC (Lin et al. 2019a). Cell efficiency (Fig. 2d) grows gradually until it reaches 1000 nm absorber thickness due to J sc alone, while efficiency saturation is due to both absorption and Jsc. Due to small absorption of light and reduced photogeneration, efficiency around 100 nm is low. Because of the highest absorption of light beyond 1000 nm, PCE remains steady at 28.45%, as indicated by constant J sc . From a non-optimized efficiency of 22.47 percent, an optimised thickness of 1000 nm increased efficiency to 28.45 percent. Adjusting the absorber thickness to 700-1000 nm results in excellent efficiency, according to our simulation.

Acceptor carrier concentration (NA) of CH 3 NH3SnI 3
Apart from the CH 3 NH 3 SnI 3 layer thickness, the acceptor density of holes in the perovskite layer has a considerable impact on the device performance of photovoltaic cells. Sn 2+ is transformed to Sn 4+ when the device is exposed to air in CH 3 NH 3 SnI 3 oxides. But, this procedure degrades the device's PCE and converts it to a p-type semiconductor (Ma et al. 2016). The formation of Sn 2+ to Sn 4+ is inhibited by the addition of SnO2 (Noel et al. 2014;Hao et al. 2014;Zhao et al. 2017). Feng et al. used Hall-effect measurement to calculate the dark carrier density of 10 14 cm −3 to 10 17 cm −3 (Hao et al. 2015). The hole concentration in this perovskite layer can be adjusted up to 10 19 cm −3 , according to Takashi et al. (Takahashi et al. 2013). As a result, the acceptor density of our reported perovskite layer was changed from 10 14 cm −3 to 10 19 cm −3 to see how acceptor doping concentration influences photovoltaic characteristics.
Cell characteristics for varied acceptor density in the CH 3 NH3SnI 3 layer are shown in Fig. 3a and b. For a very high acceptor density (> 10 19 /cm 3 ), Auger recombination can be highly relevant. The device's V bi can increase as the doping level rises above 10 14 /cm 3 . Strong Vbi enhances photogenerated carrier separation and cell efficiency. Nonetheless, a high level of doping promotes Auger recombination and lowers V oc . As a result, the maximum acceptor density in the perovskite layer in our study was set to 10 18 /cm 3 . In addition, due to a high recombination rate, very high p-type doping levels in the perovskite layer can restrict hole transportation from the absorber to the HTL. In our situation, the doping density isn't particularly great, hence no change in J sc or Voc was detected. Simultaneously, increasing the acceptor density from 10 14 cm −3 to 10 19 cm −3 can develop the better device's V bi . This high V bi forces the charge carriers towards the electrodes and boosts the charge collection efficiency and FF. As a result, cell performance improves with doping density. Our findings demonstrate that for N A = 10 14 /cm 3 to 10 18 /cm 3 there is a continuous increase in FF and PCE, and then for N A = 10 19 /cm 3 FF and PCE decline rapidly from 81.26 to 62.17 and 28.30 to 22.81, respectively. So the optimized value of N A of absorber layer is 10 18 /cm 3 .

Defect density (N t ) at absorber
We also examined the influence of absorber defect density was. In the absorber layer, defects are unavoidable. They can be found both in the bulk and on the surface. Defects in the perovskite absorber layer that appear as point defects include lattice vacancies, interstitial, Schottky, and Frenkel defects. The impurity defect in the absorber layer is caused by the self-doping process (Noel et al. 2014;Hao et al. 2014Hao et al. , 2015Ma et al. 2016). Deep or shallow levels in the energy band gap are introduced by these defects (Lee et al. 2018). Charge carriers can trap and enhance nonradiative electron-hole recombination as a result of these imperfections (Du et al. 2016;Lazemi et al. 2018). The diffusion length of charge carriers in Sn-based perovskite absorber layers can be extended up to ~ 3 μm by employing a tin-reduced precursor solution (Lin et al. 2019b). Because charge carrier diffusion length is proportional to defect density(Devi and Mehra 2019). To investigate the influence of electron diffusion length on photovoltaic responses, the defect density was changed from 10 18 to 10 14 cm −3 (Minemoto and Murata 2014) and the electron diffusion length was adjusted from 0.046 to 4.6 m. Lazemi et al. and some other researchers (Du et al. 2016;Lazemi et al. 2018;Hao et al. 2014) have all used a similar change in defect density. The defect density was altered from 10 14 to 10 16 cm −3 based on these research, and its influence on photovoltaic characteristics of the reported perovskite solar cell was displayed in Fig. 4.
It was discovered that as the defect density was reduced, the device's performance improved. The initial N t of the perovskite layer was adjusted at 1 × 10 16 cm −3 . Because the diffusion length of electrons and holes for this N t value is almost identical to empirically reported values (Yu et al. 2016a Fig. 3 Influence of absorber doping density on a PCE and b J sc defect density is 10 14 per cm −3 , with the values of J sc , V oc , FF and PCE are 34.17 mA/cm 2 , 0.999 V, 80.68% and 27.54% respectively. Now, Nt has decreased even further, from 10 14 to 0.5 × 10 14 cm −3 , with little variations in Jsc and FF, but significant changes in Voc and PCE.

Metal work function
A work function research was conducted using various anode materials to investigate the ohmic or rectifying behaviour at the metal/HTM junction. For PSC simulations, different metals such as Ag (4.74 eV), Cu (5.0 eV), Au(5.1 eV), and Pt (5.7 eV) were used as anodes (Kevin et al. 2011;Ming et al. 2018). Figure 5a,b shows the energy band alignment for different work function's metals as anode. As can be seen, the hole barrier layer grows as the work function of the contact materials decreases. The influence of the anode material on FF and PCE of the reported Perovskite solar cell PSC is shown in Fig. 5c,d. We can see that as the work function of the anode is reduced, PCE decreases. When the work function of the anode is less than CuO's work function, a rectifying Schottky barrier contact forms at the anode/CuO interface. As seen in Fig. 5a, this barrier obstructs the conduction of hole towards anode and lowering the fill factor with PCE. When the anode's work function is greater than the CuO's work function, an ohmic contact is produced at the anode/CuO junction. The hole transport at that interface is enabled by this ohmic contact. As a result, Jsc and PCE are showing signs of improvement (Fig. 6).

Effect of HTM thickness
We have employed four distinct HTMs (CuO, Cu 2 O, spiro-OMeTAD and PEDOT:PSS) and their thickness varying from 1 to 800 nm to investigate the effect of HTM thickness on optoelectronic capabilities. Except for Spiro-OMeTAD, we found that HTM thickness has no effect on optoelectronic properties of the PSC. Figure 7 depicts the role of HTM (spiro-OMeTAD) thickness. HTM thickness affects cell performance in the same way that absorber thickness does. For high efficiency, a thickness of around 1-100 nm is optimal. With a thickness of 10 nm, the highest value of Voc, Jsc, FF, and PCE were 1.011 V, 34.38 Page 13 of 20 455 mAcm −2 , 80.85%, and 28.10%, respectively. This suggests that the device has good charge transfer, less recombination, a high FF, and low R s . HTM may not completely cover up the projecting perovskite crystals which are present in the perovskite layer if the thickness is less than the optimum thickness. Our simulated results reveal that no HTM, as well as less than 10 nm or more than 100 nm, is ideal for high PCE. V oc reaches saturation and is independent of thickness when the HTM thickness is less than 10 nm. The device's V oc and recombination are linked. V oc can be influenced by factors that influence recombination, and this is true whether the HTM is thin (less than 10 nm) or not. A change in series resistance is indicated by a change in fill factor with HTM (spiro-OMeTAD) thickness. When the HTM thickness is increased from 0.1 nm to 10 nm, the fill factor rises to 80.85% and cell efficiency rises to 28.10%. When the thickness is increased to 800 nm, the FF and PCE are reduced to 73.19% and 25.46%, respectively. Because the path length of photo generated carriers to reach the bottom electrode is increased, raising R s . A thin HTM (< 10 nm) does not completely cover the perovskite layer, increasing the likelihood of recombination. When the HTM thickness increases from 0.1 to 10 nm, the fill factor improves due to increased coverage. As a result of the rise in R s with the thickness of HTL, the FF decreases.

Effect of secondary doping
By adding fresh dopants to the HTM, we may change the hole concentration, which has a secondary doping role in modifying the conductive polymer conduction. As a result, we investigated the effects of hole concentrations in the range of 10 14 -10 19 cm −3 on the photovoltaic parameters of the PSC, and the findings are shown in Fig. 7. In general, polymer doping is done to enhance charge carrier density and μ h . However, a large quantity of doping reduces μ h . As a result, the secondary doping density must be tuned such that the cell's performance is not harmed. We can only raise the efficiency of the Spiro-OMETAD HTM based PSC by increasing the hole density. Acceptor concentration was tuned from 10 14 to 10 19 /cm 3 to explore the effect of HTL doping density, and the results are presented in Fig. 7. Because of improvements in conductivity of HTL, the fill factor improves to 80.85. Fill factor reaches to this maximum value when the doping density is 1 × 10 18 cm −3 . When the acceptor doping level was equivalent to 10 18 cm −3 , we were able to attain a high PCE of 28.10% with V oc , J sc , and FF of 1.011 V, 34.38 mAcm −2 , and 80.85%. Finally, Fig. 8 shows the optimized J vs V characteristics of a spiro-OMeTAD HTM based PSC.

Effect of temperature
Another key aspect that affects the performance of PSC is temperature. The temperature of the PSC rises with absorption of light, thus temperatures are frequently greater than 300 K. In this work, we varied a temperature from 300 to 360 K and plotted the photovoltaic parameters against it (Fig. 9). Where increasing the temperature lowers the V oc and PCE. But increasing temperature increases FF and Jsc. The efficiency of all samples decreased as the temperature increased, which could be explained by the fact that at higher temperature, the electron absorbs more energy and goes to an unstable state, which increases the rate of recombination and lowering the PCE of the PSC (Mehmood et al. 2017;Jamalullail et al. 2018).

External Quantum efficiency
The ratio of the number of captured carriers to the number of incident photons is one of the solar cell's figures of merits, and it is characterized as external quantum efficiency (EQE). Figure 10 shows the quantum efficiencies of various HTMs. This diagram is drawn in the 300-900 nm wavelength range. The EQE increases at wavelengths larger than 300 nm, peaks about 400 nm, and then steadily drops after 700 nm. Because the EQE diagrams for all of the HTMs are so similar, we can conclude that the HTM has no bearing on the EQE of these perovskite solar cells.

Comparative study
We picked two organic and two inorganic HTMs to construct four distinct PSCs for a comparative investigation. The PSC structure is unchanged in this investigation; only the HTM layer is altered. We picked a thickness of 300 nm for all HTM layers in this work.  J sc is determined by the active material's absorption coefficient, thickness, and mobility. The photo current will be higher if the absorption coefficient is high. The thickness of the absorber is the second crucial factor. It must be thick enough to absorb the incident solar radiation's greatest cutoff wavelength. Apart from that, movement is quite important for achieving a high J sc . The high value of J sc was reached because current density is linearly related to charge carrier mobility. Another feature is that diffusion length is related to mobility's square root (Minemoto and Murata 2015). As a result, for high charge carrier mobility, diffusion length increases, and recombination of charge carriers reduces. There could be other explanations for J sc 's substantially higher worth. The Fermi energy level of the hole lowers as the acceptor doping concentration increases, and hence V oc rises. Another point to consider is that when the acceptor doping concentration rises, so does the built-in potential. As a result, charge separation improves, and V oc rises. Sn-based perovskite has oxidation problem. Sn 2+ is transformed to Sn 4+ (self-doping process) when the device is exposed to air in CH 3 NH 3 SnI 3 oxides. Unfortunately, this procedure degrades the device's performance. The formation of Sn 2+ to Sn 4+ is inhibited by the addition of SnO 2 (Noel et al. 2014;Devi and Mehra 2019;Lee et al. 2018). If a modest amount of the tetra-n-butylammonium (TBA) cation is doped in Sn-based perovskite, the film defects are successfully passivated. In mixed perovskite films, TBA cation doping improves surface coverage, suppresses Sn 2+ to Sn 4+ oxidation, and lowers background charge carrier density (Li et al. 2020). The high value of PCE produced in this study is achieved by optimising acceptor carrier concentration, defect density, and metal work function.

Conclusion
The Scaps-1d simulator was used to simulate a planar type lead free perovskite solar cell in this study. Several criteria, including perovskite layer thickness, N A of absorber, Nt at absorber, and work function of anode, were used to optimize photovoltaic characteristics. The absorber and HTM thicknesses and doping densities were changed to see how they affected V oc , J sc , fill factor, and efficiency. Cell efficiency rose from 22.47% to 28.45% depending on perovskite layer thickness between 100 and 1000 nm, owing primarily to an increase in J sc from 26.26 mAcm −2 to 34.56 mAcm −2 . J sc and PCE increase more slowly beyond 700 nm and saturate at 1000 nm due to saturated light absorption. Because photo generated charge carriers generated at the layer's centre will recombine when the thickness surpasses the diffusion length, long wavelength photon absorption due to band gap decrease in thicker films will be useless. The rise in J sc between 400 and 850 nm is only 6.2%, yet the increase in absorption is 18%. This demonstrates that not all produced carriers reach their intended electrodes. According to our findings, a perovskite layer thickness of 800 to 1000 nm is adequate for device application, and there is no benefit to using a thicker layer. Cell characteristics are also influenced by HTL thickness and secondary doping.
We investigated the performance of lead-free PSCs made from two organic and two inorganic HTMs in this work. Our findings demonstrate that among polymer HTMs, PEDOT:PSS with PCE of 28.43% outperforms Spiro-OMeTAD with a 28.10% efficiency. Furthermore, CuO, with a photovoltaic efficiency of 28.45%, outperforms Cu 2 O, which has a PCE of 28.34% among inorganic HTMs. The capacity to apply secondary doping and build flexible solar cells is a significant benefit of organic HTMs. In terms of secondary Page 17 of 20 455 doping in conductive polymers, we can conclude that secondary doping in Spiro-OMeTAD can effectively boost efficiency.