Design of all-inorganic hole-transport-material-free CsPbI₃/CsSnI₃ heterojunction solar cells by device simulation

Perovskite solar cells (PSCs) have been a focus in photovoltaic community owing to low fabrication cost and high efficiency. The certified power conversion efficiency (PCE) of the single-junction PSC has increased from 3.81% to 25.5% [1, 2]. The outstanding performance benefits from the excellent optoelectronic properties of halide perovskites, including suitable values of bandgap, strong absorption coefficient, high carrier mobility and long carrier diffusion length.

Most high-performance PSCs are based on hybrid organic-inorganic perovskites. Despite high efficiency, due to volatile and gyroscopic organic cation, hybrid organic-inorganic PSCs suffer from stability issues when exposed to moisture, illumination and raised temperature [3–5]. One method to improve stability of PSCs is to replace the organic cations with inorganic ones. Inorganic perovskite cesium lead halides CsPbX₃ (X = Cl, Br, I, or mixed halogens) have demonstrated improved stability against moisture and high temperatures of up to 400 °C [6–8]. Due to a suitable bandgap and good tolerance to humidity, oxygen and thermal stress, CsPbI₃ shows great prospect for stable PSCs. The CsPbI₃ solar cell has been a rapid growing research field and impressive progresses have been made [9–12]. Many strategies have been developed to fabricate thermodynamically stabilized CsPbI₃ solar cell and to improve cell efficiency. CsPbI₃ solar cells with PCE over 19% have been reported by several groups [10–12].

The stability of PSCs is limited by not only the perovskite material but also the carrier transport material and the electrode. Most high-performance PSCs employ the expensive Spiro-OMeTAD as the hole transport material (HTM). This is unfavorable for mass production. Many reports indicate that Spiro-OMeTAD also has stability issue [13, 14]. Thanks to the bipolar transport characteristic of perovskite materials [15], PSCs can work without HTM. Removing of HTM can simplify the architecture of PSCs and is a possible way to solve the instability caused by HTM. The pioneer work of HTM-free PSCs was conducted by Etgar et al [16]. Up to now, the highest efficiency of single-junction HTM-free CsPbI₃ solar cell is ∼14.6% [17], which is still much inferior to that of conventional ones. It is important to reveal the factors that limit the efficiency of HTM-free CsPbI₃ solar cells and to find ways to improve their performance.

In this work, a HTM-free solar cell with the CsPbI₃/CsSnI₃ heterojunction as light absorbing layer is proposed and systematically studied via numerically simulation. The effects of absorber thickness, defect density of absorber, work function of electrode and doping level in absorber are investigated. It is found that incorporation of a CsSnI₃ layer can greatly improve the efficiency of HTM-free CsPbI₃ solar cells. Based on simulation, suggestions to enhance the performance of the HTM-free CsPbI₃ solar cells are put forward.

The device simulation was carried out by using SCAPS-1D software [18], a one-dimensional solar cell simulation program developed by the Department of Electronics and Information of Ghent University in Belgium. It is one of the well-known applications for simulation of solar cells and has been adopted in many studies for the modeling and simulation of perovskite solar cells [19–21]. SCAPS-1D simulates the internal carriers transport of solar cell by calculating coupled Poisson equation, continuity equations and constitutive equations. The complete set of equations is the following:

$$\begin{eqnarray*}&&\left\{\begin{array}{c}\displaystyle \frac{\partial }{\partial x}\left[{\varepsilon }_{0}\varepsilon \displaystyle \frac{\partial {\rm{\Psi }}}{\partial x}\right]=-q\left[p-n+{N}_{D}^{+}-{N}_{A}^{-}+{p}_{t}-{n}_{t}\right]\\ -\displaystyle \frac{\partial {J}_{n}}{\partial x}-{U}_{n}+G=\displaystyle \frac{\partial n}{\partial t}\\ -\displaystyle \frac{\partial {J}_{p}}{\partial x}-{U}_{p}+G=\displaystyle \frac{\partial p}{\partial t}\\ {J}_{n}=-\displaystyle \frac{{\mu }_{n}n}{q}\displaystyle \frac{\partial {E}_{Fn}}{\partial x}\\ {J}_{p}=\displaystyle \frac{{\mu }_{p}p}{q}\displaystyle \frac{\partial {E}_{Fp}}{\partial x}\end{array}\right.\end{eqnarray*}$$

Where ${\varepsilon }_{0}$ is the vacuum permittivity, $\varepsilon$ is the relative permittivity, ${\rm{\Psi }}$ is the electrostatic potential, $q$ is the electron charge, $n$ is the density of free electron and $p$ is the density of free hole. ${N}_{D}^{+},\,{N}_{A}^{-},\,{p}_{t}$ and ${n}_{t}$ denote the ionized donor-like doping density, the ionized acceptor-like doping density, the trapped hole density and the trapped electron density, respectively. G is the optical generation rate, ${U}_{n}$ is the electron recombination rate, ${U}_{p}$ is the hole recombination rate. ${J}_{n}$ and ${J}_{p}$ are the electron current density and hole current density, respectively. ${\mu }_{n}$ and ${\mu }_{p}$ are used to account for the mobility of electron and hole, respectively. ${E}_{Fn}$ and ${E}_{Fp}$ represent the quasi Fermi energy level of electron and hole, respectively.

The parameters employed in the simulations are shown in table 1 [22–27]. Taking into account the high recombination rate at interfaces, the TiO₂/CsPbI₃ and CsPbI₃/C(Au) interfaces were inserted by an absorber layer (IDL1 and IDL2) with a thickness of 5 nm. The parameters of IDL1 and IDL2 are the same as the absorber except with a high-level defect density N_t of 10¹⁸ cm⁻³. The defect density of CsPbI₃ absorber is set as 2 × 10¹⁴ cm⁻³ except for studying the influences of the defect density, where it varies from 10¹³ cm⁻³ to 10¹⁸ cm⁻³. The work function of the front contact is 4.1 eV (FTO). The following parameters are set to be identical for all layers. The defect energy level is at the center of bandgap and the defect type is neutral. Its energetic distribution is Gaussian with a characteristic energy of 0.1 eV. The absorption coefficient α is calculated using α = A_α (hν−E_g )^1/2, where the absorption constant A_α is set as 10⁵. The thermal velocity of electron and hole is 10⁷ cm/s. Capture cross section of electron and hole is 2 × 10¹⁴ cm².

Table 1. Material parameters used in simulation.

Material properties	FTO [23]	TiO2 [24]	IDL1/ IDL2	CsPbI3 [25, 26]	Spiro-OMeTAD [27]	CsSnI3 [22]
Thickness(nm)	100	50	5	100–1000	170	0–1000
NA(cm−3)	0	4 × 1015	4 × 1015	1015–1020	2 × 1018	1020
ND(cm−3)	2 × 1019	1019	0	0	0	0
${\varepsilon }_{r}$	9	9	6	6	3	9.93
χ (eV)	4	4	3.95	3.95	2.45	3.6
Eg (eV)	3.5	3.2	1.73	1.73	3	1.3
${\mu }_{n}$(cm2/V s)−1	20	20	16	16	2 × 10–4	585
${\mu }_{p}$(cm2/V s)−1	10	10	16	16	2 × 10–4	585
Nt (cm−3)	1015	1015	1018	1013–1018	1015	1017,1018
NC (cm−3)	2.2 × 1018	1021	1.1 × 1020	1.1 × 1020	2.2 × 1018	1019
NV (cm−3)	1.8 × 1019	2 × 1020	8 × 1019	8 × 1019	1.8 × 1019	1018

Figure 1 depicts the schematic device configuration of FTO/TiO₂/CsPbI₃/C PSCs and figures 2(a)–(d) show the simulated performance as a function of CsPbI₃ thicknesses. For comparison, the results of corresponding cells with a Spiro-OMeTAD layer are also displayed. For both cells, the short circuit current J_SC increases with the increase of absorber thickness. This is because a thicker perovskite layer absorbs more sunlight and produces more photocarriers. As CsPbI₃ has a large absorption coefficient, a thickness of ∼1000 nm can absorb most incident sunlight and consequently J_SC would saturate with such a thickness. As shown in figure 2(e), in the HTM-free solar cell, the absence of HTM causes a downward bend of energy band of CsPbI₃ near the back-front (BF) electrode and thus results in a poorer collection of photocarriers and a smaller J_SC . Compared with the HTM-containing counterpart, the HTM-free cell has a much smaller open circuit voltage V_OC . This is due to the significant lowering of built-in barrier height in the HTM-free cell, as shown in figures 2(e) and (f). The built-in barrier height in the Spiro-OMeTAD-containing cell is mainly determined by difference in Fermi level between TiO₂ and Spiro-OMeTAD. As a result, for the cell with a Spiro-OMeTAD layer, V_OC depends weakly on CsPbI₃ thicknesses. For the HTM-free cell, the built-in barrier height depends on the thickness of CsPbI₃ and V_OC increases with the increases of absorber thickness, first dramatically and then slowly. Removing the Spiro-OMeTAD layer increases the fill factor FF slightly. The smaller FF in the cell with a Spiro-OMeTAD layer is due to the low mobility of Spiro-OMeTAD. This is verified through controlled simulation (not shown here). In the cell with a Spiro-OMeTAD layer, with the increases of absorber thickness, FF decreases slightly. In the HTM-free cell, when the absorber thickness increases, FF first decreases and then increases. In general, the fill factor depends on V_OC and various parameters that change the shape of current-voltage curves. For the present HTM-free cell, the changes of FF seems to correlate with that of V_OC . FF decreases when V_OC increases dramatically and increases when V_OC increases slowly. Overall, removing the Spiro-OMeTAD layer increases FF slightly but lowers J_SC , V_OC and PCE remarkably. The increase of barrier height is important to promote the cell efficiency in the HTM-free cell.

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Defects in solar cells act as the recombination center and have significant influences of cell performance. Figure 3 shows the effects of defect density within CsPbI₃ layers N_t (CsPbI₃) on PCE, V_OC , J_SC , and FF of cells with Au or C as the BF electrode. For cells with these two electrodes, the variation trend is similar. When N_t (CsPbI₃) is increased from 10¹³ to 10¹⁵ cm⁻³, PCE, Voc and Jsc decrease slowly. However, when N_t (CsPbI₃) is larger than 10¹⁵ cm⁻³, PCE, Jsc and FF exhibit a significant decline. To achieve a high performance, it is essential to keep N_t (CsPbI₃) lower than 10¹⁵ cm⁻³. In the following studies, N_t within CsPbI₃ absorber is kept to be 2 × 10¹⁴ cm⁻³ .

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In the HTM-free cell, the BF electrode contacts the absorber layer directly, causing a depletion region near the interface. Consequently, its work function influences the total barrier height and the cell performance. Figure 4(a) shows the J-V curves of the devices using BF electrodes with various work functions. When the work function is increased from 4.7 to 5.6 eV, both Jsc and Voc increase monotonically. It is notable that Voc increases from ∼0.43 V to ∼0.93 V as work function changes from 4.7 to 5.2 eV. In this range, with other parameters identical, the difference in Voc is exactly that in the work function of the BF electrode. As the work function approaches the top of valence band of CsPbI₃, the increase in Voc becomes slower. Such an increase of Voc with increasing work function of BF electrode is related to the rise of barrier height, as shown in figure 4(b). A larger work function of BF electrode means a larger difference in Fermi energy between BF electrode and the absorber layer and results in a larger built-in potential. Tuning the work function of BF electrode could effectively improve the cell performance of HTM-free cell. As Au and C with a work function of 5.1 eV and 5.0 eV are commonly used as the BF electrodes for perovskite solar cells, comparative studies between C and Au electrodes are performed in this work.

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The doping level of CsPbI₃ affects its Fermi level and the final built-in potential in the cell. Figure 5(a) shows J-V curves of FTO/TiO₂/CsPbI₃/C cells with different hole doping levels of CsPbI₃ N_A (CsPbI₃). With the increase of N_A (CsPbI₃), J_SC and V_OC exhibit an oppsite trend. In short, J_SC decreases and V_OC increases, first quickly and then slowly. As a result, for cells with both electrodes, PCE first increases and then decreases, as shown in figure 5(b). A moderate hole doping in CsPbI₃ can improve the performace of HTM-free CsPbI₃ solar cell. The increase of N_A (CsPbI₃) shifts the Fermi level downwards and increases the difference in Fermi level between TiO₂ and CsPbI₃. As a result, the barrier height formed between TiO₂ and CsPbI₃ increases, causing an increase of V_OC . Due to the increase of N_A (CsPbI₃) and the downward shift of Fermi level in CsPbI₃, the depletion region in CsPbI₃ becomes narrower and there is a downward bending of CsPbI₃ energy band at CsPbI₃/electrode interface (see figures 5(c) and (d)), which are harmful to the tranport and collection of photo-generated carriers and are responsible for the reduction of J_SC .

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The best values of PCE for FTO/TiO₂/CsPbI₃/C and FTO/TiO₂/CsPbI₃/Au cells with different N_A (CsPbI₃) after absorber thickness optimization are listed in table 2. All FTO/TiO₂/CsPbI₃/Au cells have a better cell performance than corresponding FTO/TiO₂/CsPbI₃/C cells. The best PCE is ∼15.60%, achieved in a cell with an Au electrode and a N_A (CsPbI₃) of 10¹⁶ cm⁻³. Due to the insufficient utilization of solar spectrum, this value is much inferior to that of hybrid organic-inorganic counterparts. With a relatively large bandgap (∼1.72 eV), sun light with a wavelength larger than 720 nm cannot be absorbed in CsPbI₃. Such a loss may be reduced by combing CsPbI₃ with another semiconductor with a smaller bandgap, such as CsSnI₃ (bandgap ∼1.3 eV). Inorganic lead-free CsSnI₃ has a high mobility and a high absorption coefficient [22, 28]. A critical issue of CsSnI₃ is the oxidization of Sn²⁺ through self-doping reaction. Various efforts have been paid to solve this issue. Through additive engineering, it has been possible to achieve ambient-air stability for several days [29]. As CsSnI₃ and CsPbI₃ have the similar perovskite structure, it is possible to construct a CsSnI₃/CsPbI₃ heterojunction as the absorption layer in a HTM-free solar cell. Figures 6(a) and (b) show the schematic device structure of the HTM-free CsSnI₃/CsPbI₃ heterojunction solar cell and its energy band diagram. As shown, the incorporation of CsSnI₃ could also facilitate the separation and transport of photo-generated carriers.

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Figure 7 displays the comparison of the J-V curves and quantum efficiency curves of two HTM-free solar cells using different light absorbing layers, CsPbI₃(350 nm) and CsPbI₃(350 nm)/CsSnI₃(200 nm). The incorporation of a CsSnI₃ layer improves J_SC and V_OC simultaneously. J_SC is increased from 17.7 mA cm⁻² to 22.2 mA cm⁻² and V_OC is increased from 0.73 V to 0.91 V. Correspondingly, PCE is increased from ∼8.9% to ∼15.5%. It is noted that J_SC , V_OC and PCE of FTO/TiO₂/CsPbI₃(1000 nm)/C cell are 19.6 mA cm⁻², 0.88 V and 13.6%, respectively. These values are all smaller than the corresponding performance parameters of the FTO/TiO₂/CsPbI₃(350 nm)/CsSnI₃(200 nm)/C cell. The increase of J_SC in FTO/TiO₂/CsPbI₃/CsSnI₃/C cell is partly due to the increase of absorber thickness and partly due to the extension of the absorption spectrum to the near infrared region. As shown in figure 7(b), the quantum efficiency of FTO/TiO₂/CsPbI₃/C reaches a maximum value in the wavelength range of 360–440 nm and drops near 720 nm, above which no light is absorbed. The incorporation of CsSnI₃ not only increases the quantum efficiency below 720 nm but also extends the absorption range to the infrared region. The increase of V_OC in FTO/TiO₂/CsPbI₃/CsSnI₃/C cell is due to combined effects. Voc could be expressed as V_OC = nk_B T/qln(J_L /J₀  + 1), where n is the ideality factor, k_B is the Boltzmann constant, T is the temperature, q is the electric charge, J_L is the photo-generated current density and J₀ is the dark saturation current density. With other parameters fixed, an increase of J_L from 17.7 mA cm⁻² to 22.2 mA cm⁻² lead a small increase of V_OC . By fitting the dark J-V curve with J = J₀ exp(qV/nk_B T), one can obtain the dark saturation current density J₀ , which is 2.03 × 10^–8 mA cm⁻² for the CsPbI₃ cell and 9.67 × 10^–15 mA cm⁻² for the CsPbI₃/CsSnI₃ cell. The calculated values of V_{OC_calc} [=nk_B T/q ln(J_L /J₀  + 1)] are 0.68V for the CsPbI₃ cell and 0.90 V for the CsPbI₃/CsSnI₃ cell, which are in good accordance with the V_OC (0.73 V and 0.91 V) obtained from the simulated J-V curves under illumination. Thus, the increase of V_OC in FTO/TiO₂/CsPbI₃/CsSnI₃/C cell is due to the combined effects of the increase of photocurrent and the decrease of dark saturation current density.

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Figure 8 shows the current density distribution and quantum efficiency for the FTO/TiO₂/CsPbI₃/CsSnI₃/C cells with different values of N_A (CsPbI₃) and N_t (CsSnI₃). For a fixed N_t (CsSnI₃), the increase of N_A (CsPbI₃) causes a decrease of quantum efficiency in the whole spectral range, especially for wavelength larger than 720 nm. From the distribution of current density, it is found that with an increase of N_A (CsPbI₃) the carrier transport between CsSnI₃ and CsPbI₃ is suppressed. As discussed in a previous section, with a higher hole doping concentration in CsPbI₃, the energy band of CsPbI₃ moves upward and there would be a downward bending of CsPbI₃ energy band at the CsPbI₃/CsSnI₃ interface (similar to that shown in figure 5(d)), which would suppress the carrier transport between CsSnI₃ and CsPbI₃. Thus, a low hole doping concentration in CsPbI₃ is beneficial for carrier transport in CsPbI₃/CsSnI₃ cells. With a fixed N_A (CsPbI₃), the increase of N_t (CsSnI₃) decreases the quantum efficiency and the photo-current from CsSnI₃. This is understandable as a higher defect concentration causes more recombination and less carrier collection.

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Figure 9 shows the absorber thickness optimization for the FTO/TiO₂/CsPbI₃/CsSnI₃/Au cell with a N_A (CsPbI₃) of 10¹⁵ cm⁻³ and a N_t (CsSnI₃) of 10¹⁷ cm⁻³. It can be found that with a fixed thickness of CsPbI₃, with the increase of CsSnI₃ thickness, J_SC and V_OC first increase dramatically and then show a saturation behavior. The maximum PCE is ∼19.99%, which occurs in a cell with the CsPbI₃(200 nm)/CsSnI₃(300 nm) as the absorbing layer. This efficiency is much larger than the maximum PCE obtained in the FTO/TiO₂/CsPbI₃/Au cell (14.43%) with an identical hole doping in CsPbI₃. Moreover, a cell efficiency above 19% can be achieved with various thickness combinations.

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Table 2 also summarizes the best cell performance for various FTO/TiO₂/CsPbI₃/CsSnI₃/C and FTO/TiO₂/CsPbI₃/CsSnI₃/Au cells obtained through absorber thickness optimization. The optimization process is similar as that displayed in figure 9. Three hole-doping levels in CsPbI₃ (10¹⁵, 10¹⁶ and 10¹⁷ cm⁻³) are studied. As the defect concentration in the CsSnI₃ films is usually quite high, the defect concentration in CsSnI₃ is set to be 10¹⁸ cm⁻³ and 10¹⁷ cm⁻³.

In all CsPbI₃/CsSnI₃ cells, the best PCE depends on the hole concentration in CsPbI₃. This is similar to that in cells with a single CsPbI₃ absorbing layer, but the dependence is different. In cells with a single CsPbI₃ absorbing layer, with the increase of N_A (CsPbI₃), the maximum PCE first increases and then decreases. The peak values of maximum PCE appears in the cell with a N_A (CsPbI₃) of 10¹⁶ cm⁻³. For FTO/TiO₂/CsPbI₃/C cell and FTO/TiO₂/CsPbI₃/Au cell, the peak value of maximum PCE is ∼13.87% and ∼15.60%, respectively. The cells with a CsPbI₃/CsSnI₃ absorbing layer show best performance at a N_A (CsPbI₃) of 10¹⁵ cm⁻³. An increase of N_A (CsPbI₃) to 10¹⁶ cm⁻³ cannot improve the cell performance. Instead, it causes a slight decrease of maximum PCE. A further increase of N_A (CsPbI₃) to 10¹⁷ cm⁻³ leads a much larger decrease of maximum PCE. As discussed above, a higher hole doping concentration in CsPbI₃ suppresses the carrier transport between CsSnI₃ and CsPbI₃ and decreases J_SC .

If N_A in CsPbI₃ is low, e.g. 10¹⁵ cm⁻³, even incorporation of a CsSnI₃ absorbing layer with a N_t of 10¹⁸ cm⁻³ can improve the cell performance considerably. For the cell with a BF Au electrode, the maximum PCE is enhanced from 14.43% to 15.67%. This improvement is comparable to that due to the increase of N_A (CsPbI₃) for the cell with a single CsPbI₃ absorbing layer. For the cell with a BF C electrode, the maximum PCE is enhanced more significantly, from 12.62% to 15.62%. This larger improvement originates from simultaneous enhancement of J_SC and V_OC . If N_A (CsPbI₃) is higher, the incorporation of a CsSnI₃ absorbing layer with a N_t of 10¹⁸ cm⁻³ would lead a much smaller improvement and even a deterioration of cell performance.

If N_t in the incorporated CsSnI₃ is reduced to 10¹⁷ cm⁻³, the incorporation of a CsSnI₃ layer can significantly improve cell performance for all three values of N_A (CsPbI₃) and both BF electrodes. The FTO/TiO₂/CsPbI₃/CsSnI₃/Au cell with a N_t (CsSnI₃) of 10¹⁷ cm⁻³ shows a best PCE of 19.99%, 19.85% and 18.28% for a N_A (CsPbI₃) of 10¹⁵, 10¹⁶ and 10¹⁷ cm⁻³, respectively. For FTO/TiO₂/CsPbI₃/CsSnI₃/C cell, the best PCE is 19.59%, 19.29% and 17.68% for a N_A (CsPbI₃) of 10¹⁵, 10¹⁶ and 10¹⁷ cm⁻³, respectively. These values of PCE are much larger than those in the counterpart cells with a single CsPbI₃ absorbing layer. For all three N_A (CsPbI₃) levels and both BF electrodes, the incorporation of a CsSnI₃ layer increases J_SC significantly due to a better utilization of solar spectrum. For the cell with a low N_A (CsPbI₃), J_SC and V_OC could be enhanced simultaneously. As a result, the improvement of cell performance is most significant.

It is noted that for CsPbI₃/CsSnI₃ cells, the efficiency of the cell with a BF C electrode is comparable to that with a BF Au electrode. This is different from cells with a single CsPbI₃ absorbing layer, where the efficiency of the cell with a BF Au electrode is considerably larger. It is also noted that the C electrodes are beneficial for improving the stability of PSCs, especially the HTM-free ones [30]. Thus, for the HTM-free CsPbI₃/CsSnI₃ heterojunction cells, C is a good choice for BF electrode as it can achieve desirable cell performance with improved stability and lowered fabrication cost.

In order to improve the cell performance of CsPbI₃ based HTM-free PSCs, CsPbI₃/CsSnI₃ heterojunction was proposed as the absorber and HTM-free CsPbI₃/CsSnI₃ PSCs were investigated systematically through numerical simulation by using SCAPS-1D. In the HTM-free cell with a single CsPbI₃ absorbing layer, though the cell performance can be improved through optimization of various parameters, e.g. absorber thickness, defect density of absorber, work function of electrode and doping level in absorber, its performance is greatly limited by the relatively large bandgap of CsPbI₃. Incorporation a narrow-bandgap perovskite CsSnI₃ in the cell extends the absorption spectrum and facilitate the separation and transport of photo-generated carriers. For the CsPbI₃/CsSnI₃ cell, it is beneficial to keep a low hole doing in CsPbI₃. With a hole concentration of 10¹⁵ cm⁻³ in CsPbI₃, even incorporation of a CsSnI₃ layer with a defect concentration of 10¹⁸ cm⁻³ can improve the cell efficiency from 14.43% to 15.67% and from 12.62% to 15.62% for the cell with a back-front Au electrode and a back-front C electrode, respectively. If the defect concentration in the incorporated CsSnI₃ is reduced to 10¹⁷ cm⁻³, the incorporation of a CsSnI₃ layer can increase the cell efficiency to 19.99% and 19.59% for the cell with a back-front Au electrode and a back-front C electrode, respectively. For the HTM-free CsPbI₃/CsSnI₃ heterojunction cells, C is a good choice for back-front electrode as it can achieve desirable cell performance with improved stability and lowered fabrication cost. The stability of CsSnI₃ has great impacts on the results. If the stability of CsSnI₃ can be further enhanced, these results would provide a theoretical basis for fabrication of CsPbI₃ based HTM-free PSCs.

This work was supported in part by the Zhejiang Provincial Natural Science Foundation of China under Grant LY19A040006, Grant LY21F010010 and Grant LY22A040002; in part by the National Natural Science Foundation of China under Grant 11504356 and Grant 51771175.

All data that support the findings of this study are included within the article (and any supplementary files).

There are no conflicts of interest to declare.