Suppressing charge recombination in a methylammonium-free wide-bandgap perovskite film for high-performance and stable perovskite solar cells

Qiufeng Ye ac, Wenzheng Hu a, Junchi Zhu a, Ziyu Cai a, Hengkang Zhang a, Tao Dong a, Boyang Yu a, Feiyang Chen a, Xieli Wei a, Bo Yao a, Weidong Dou a, Zebo Fang *a, Feng Ye *b, Zhun Liu *a and Tie Li *c
aDepartment of Physics and Zhejiang Engineering Research Center of MEMS, Shaoxing University, Shaoxing 312000, P. R. China. E-mail: yeqiufeng21@usx.edu.cn; csfzb@usx.edu.cn; liu6zhun@163.com
bShangyu Coll, Shaoxing University, Shaoxing 312300, P. R. China. E-mail: yefeng@usx.edu.cn
cScience and Technology on Microsystem Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China. E-mail: tli@mail.sim.ac.cn

Received 10th February 2024 , Accepted 29th April 2024

First published on 3rd May 2024


Abstract

Wide-bandgap (WBG) formamidinium–cesium (FA–Cs) hybrid lead iodide–bromide mixed perovskites (∼1.7 eV) have gained great attention with the potential of enabling highly efficient tandem photovoltaics when integrated with crystalline silicon and other low-bandgap solar cells. However, their power conversion efficiencies (PCEs) are still insufficient compared to their methylammonium (MA) counterparts, mainly owing to the high open-circuit voltage (VOC) deficits (>0.43 V). Here, by incorporating rubidium iodide (RbI) in the FA0.8Cs0.2Pb(I0.75Br0.25)3 perovskite precursor, the film crystallinity and bulk defects are significantly optimized. In addition, we propose an all-around interface engineering strategy sequentially constructing a surface heterojunction and using trioctylphosphine oxide (TOPO), which can significantly passivate grain boundaries and undercoordinated defects, as well as optimize the energy band. As a result, the target MA-free WBG n–i–p solar cells at 1.685 eV have achieved a record efficiency of 23.35% and a high VOC of 1.30[thin space (1/6-em)]V (with a record voltage deficit of 0.385 V). Most importantly, the unencapsulated solar cells also display impressive air storage stability, operating stability and thermal stability. Moreover, a PCE of 19.54% on a 1 cm2 WBG solar cell and a PCE of 21.31% on a 0.04 cm2 p–i–n inverted WBG solar cell are also demonstrated.



Broader context

MA-free wide-bandgap perovskite materials (e.g., FAxCs1−xPbIyBr3−y) have attracted increasing attention because of their unique advantages (e.g., wide band-gap (∼1.7 eV), and ideal thermal stability) for perovskite-based tandems (e.g., perovskite/Si). However, realizing high-efficiency MA-free WBG devices is a challenge. VOC deficits in MA-free WBG perovskite systems are still too high compared with those of their MA counterpart mainly caused by charge recombination in the perovskite bulk or interface. Focusing on this point, here we propose an all-around passivation strategy by incorporating RbI, constructing a surface heterojunction, and using TOPO in MA-free FA0.8Cs0.2Pb(I0.75Br0.25)3 PSCs, which can optimize the film morphology, enhance the light absorption and passivate the undercoordinated defects. As a result, the charge recombination can be significantly reduced. A device performance as high as 23.35% and record voltage deficit of 0.385 V are achieved. The unencapsulated devices also display impressive long-term air stability, light-soaking operating stability and thermal stability.

Introduction

Owning to their impressive optoelectronic properties and the relative ease of the manufacturing processes, single-junction metal halide perovskite solar cells (PSCs) have attracted great attention in recent decades as their certified PCE has skyrocketed to 26.1%, approaching the record efficiency of crystalline silicon.1–6 Despite the rapid growth of performance, single-junction PSCs are fundamentally subjected to the Shockley–Queisser (S–Q) theoretical limits based on the single absorbing layer.7–10 The tandem design of combining a wide-bandgap (∼1.7 eV) perovskite top cell with Si or low-bandgap perovskite bottom cells (∼1.1 eV) is an effective approach to further improve the PCE potential of PSCs, attributed to addressing the energy losses triggered by the sub-bandgap transmission and thermalization.11,12 Therefore, it is essential to fabricate high-efficiency and highly stable wide bandgap (WBG) PSCs.

WBG perovskites are typically prepared by regulating the ratio of I to Br at the X site of the crystal lattice, ABX3.13 Among them, WBG PSCs with an optimal bandgap generally approaching 1.7 eV show promise, and by controlling the growth of the perovskite crystals,14–16 inhibition of ion migration,17,18 and also passivation engineering in the bulk or at the interface,19–21 there have been significant improvements. For instance, Zhu et al. introduced mixed-ammonium (F5PEA+–PEA+) 2D-PPA into (FA0.64MA0.20Cs0.15)Pb0.99(I0.79Br0.2)3 perovskite (∼1.68 eV) precursor, which contributes to prolonged carrier lifetimes and reduced trap density. PSCs with a PCE of 21.1% and improved stability were achieved.22 Yang et al. used ionic coupled potassium sorbate (ICPS) to inhibit the perovskite defects at Cs0.05(FA0.84MA0.16)0.95Pb(I0.75Br0.25)3 WBG (∼1.67 eV) perovskite grain boundaries, along with an efficiency of 22.00% and a VOC of 1.272 V.23 Recently, by applying recrystallized perovskite arrays on the surface of the WBG perovskite for enhancing the coordinated light management ability and surface passivation, Shen et al. fabricated (CsMAFA)Pb(I, Br)3 (∼1.63 eV) PSCs with the PCE reaching 23.23%, which is the best performance of the reported WBG PSCs.24 However, under illumination, the perovskite materials with MA on the A site are easy to segregate under relatively low photon doses;25 in contrast, the formamidinium–cesium hybrid lead iodide–bromide mixed perovskites (FA(1−y)CsyPb(BrxI(1−x))3) are much more stable and less likely to segregate ions in perovskite structures, with much higher photon doses required to induce halide separation.26–28

There are therefore more efforts made on the development of FA(1−y)CsyPb(BrxI(1−x))3 perovskites (∼1.7 eV) with the potential of long-term phase stability. To improve the optoelectronic properties of the perovskite, passivate the carrier traps, and increase ion activation energy, Fang et al. used 2-thiopheneethylammonium chloride (TEACl) in 2021, and the target MA-free device with 1.68 eV delivered a champion PCE of 20.31% exhibiting superior stability.29 Dimethylamine (DMA) also replaces MA to improve stability. Zhu and colleagues recently suggested a technique that involves merging the quick crystallization of Br with a mild gas-quenching approach to produce 1.75 eV Cs0.3FA0.6DMA0.1Pb(I0.7Br0.3)3 perovskite films with enhanced texture and lower defect density, and the PCE of 20.1% with a high VOC of 1.33 V was realized.30 However, the VOC deficits in the MA-free WBG perovskite system are still too high (>0.43 V) compared with that of their MA counterpart (0.39 V) mainly caused by charge recombination in the perovskite bulk or interface.

Here, we report that the addition of rubidium iodide (RbI) to FA0.8Cs0.2Pb(I0.75Br0.25)3 perovskite precursors can simultaneously improve the crystallinity and efficiency of MA-free WBG PSCs. Besides, we also propose an all-around interfacial engineering strategy sequentially constructing a surface heterojunction and using trioctylphosphine oxide (TOPO), which can significantly passivate grain boundaries and undercoordinated defects, as well as optimize the energy band. Combined with the above methods, the charge recombination in the perovskite bulk and at the interface is significantly reduced. Finally, the target MA-free WBG n–i–p solar cells at 1.685 eV achieve the PCE of 23.35% and a champion VOC of 1.30[thin space (1/6-em)]V, which is the highest performance and the lowest voltage deficit (0.385 V) of the reported FA–Cs based WBG PSCs. The unencapsulated solar cells also display impressive long-term air storage stability, light-soaking operating stability and thermal stability, retaining 94% of the initial PCE after 3400 hours in ultra dry air (∼10% humidity) and 85% of the original PCE after 300 hours of MPP tracking operation.

Results and discussion

Previous research has shown that the rubidium iodide additive can effectively improve the film morphology and thus, the device performance for MAPbI3,31 FA0.83Cs0.17−xRbxSn0.5Pb0.5I3,32 and FAPbI333 perovskite solar cells. Here, we firstly verified the effect of RbI additive in the WBG FA0.8Cs0.2Pb(I0.75Br0.25)3 thin films. We prepared the perovskite films by a one-step spin-coating process using chlorobenzene (CB) as the antisolvent. We used scanning electron microscopy (SEM) to observe the top-view and cross-sectional film morphology with different excess RbI concentrations, as illustrated in Fig. 1(a)–(d) and Fig. S1 (ESI). A turtle-backed surface appears on the primary film along with relatively small grains and the average grain size is 426.3 nm (Fig. 1(e)). According to the histogram in Fig. 1(f)–(h), the average grain size of the RbI-containing perovskite sample will rise sharply to 1063.2 nm, with a distribution ranging up to 1200 nm, and among these films, it has the most uniform and smooth morphology when containing 4 mg per ml RbI. The cross-sectional SEM of the RbI-introduced device also shows the larger perovskite grain size and optimized smoothness, which contribute to the lower non-radiative recombination.34
image file: d4ee00666f-f1.tif
Fig. 1 Top-view SEM images of the WBG perovskite films doped with different concentrations of RbI: (a) without RbI, (b) 2 mg per ml RbI, (c) 4 mg per ml RbI and (d) 6 mg per ml RbI. The grain size statistics of the WBG perovskite films doped with different concentrations of RbI: (e) without RbI, (f) 2 mg per ml RbI, (g) 4 mg per ml RbI and (h) 6 mg per ml RbI. AFM images of the WBG perovskite films doped with different concentrations of RbI: (i) without RbI, (j) 2 mg per ml RbI, (k) 4 mg per ml RbI and (l) 6 mg per ml RbI.

We also tested the corresponding WBG perovskite films by atomic force microscopy (AFM) (Fig. 1(i)–(l)). Perovskites and carrier transport layers normally come into closer contact with each other when the surface roughness of the film is smaller.35 The averaged roughness (Ra) of the perovskite film without RbI is 22.2 nm, and then it reduces to 17.5, 14.9 and 19.5 nm for the samples with the RbI concentration of 2, 4 and 6 mg ml−1, respectively, indicating that the target film is more advantageous for carrier transmission across the interface contact.

Fig. S2 (ESI) shows the energy dispersive X-ray spectroscopy (EDS) results to investigate the presence and distribution of Rb element. Fig. S2a (ESI) confirms that Rb was uniformly incorporated in the WBG FA0.8Cs0.2Pb(I0.75Br0.25)3 perovskite. To study the change in bonding nature by doping 4 mg per ml RbI, we used X-ray photoelectron spectra (XPS) as illustrated in Fig. S3 (ESI), the peaks corresponding to the Rb 3d core level were detected in the FA0.8Cs0.2Pb(I0.75Br0.25)3 film. The Cs 3d peaks shifted to stronger binding energy by 0.15 eV and the I 3d peaks by 0.10 eV mainly due to a stronger interaction in the perovskite lattice resulted by the incorporation of RbI, which is beneficial for the reduction of vacancy defects.

Using X-ray diffraction (XRD), we can investigate the different phases of perovskite films grown on ITO substrates with varying concentrations of excess RbI doping (Fig. 2(a)). On the one hand, the strength of the peak is enhanced with the increase of the concentrations of RbI doping corresponding to the improved crystallinity and larger grain size of the perovskite films, as also corroborated in the above SEM and AFM. What's more, as seen in Fig. 2(b) and the Tauc plot in Fig. S4 (ESI), with the addition of RbI as a dopant, the peak shifted towards a lower angle, which might be attributed by the expansion of the lattice parameter and a red shift of the bandgap was observed. The bandgap of the control film will shift from 1.693 eV to 1.685 eV of the target film with 4 mg per ml RbI, as well as the highest absorption intensity. Therefore, we believe that the excess RbI concentration of 4 mg ml−1 is the most promising condition for the preparation of FA0.8Cs0.2Pb(I0.75Br0.25)3 film.


image file: d4ee00666f-f2.tif
Fig. 2 (a) XRD patterns of the WBG perovskite films doped with different concentrations of RbI. (b) UV-vis of the WBG perovskite films doped with different concentrations of RbI. (c) The PL spectra of the WBG perovskite films doped with different concentrations of RbI. (d) The TRPL spectra of the WBG perovskite films doped with different concentrations of RbI. (e) Illuminated JV curves of the WBG solar cell devices doped with different concentrations of RbI. (f) The statistical distributions of WBG solar cell devices doped with different concentrations of RbI.

Then, steady state photoluminescence (PL) and time-resolved photoluminescence (TRPL) decays are also carried out to confirm the above conclusions. The red-shift of the PL peak supported the statement of the reduction of the WBG perovskite bandgap when RbI was introduced (Fig. 2(c)). Besides, steady PL showed the most strong intensity when the RbI concentration is 4 mg ml−1. Meanwhile, the TRPL of the perovskite films (Fig. 2(d)) showed that the lifetime of the FA0.8Cs0.2Pb(I0.75Br0.25)3 perovskite had been elongated to 148.1 ns (Table S1, ESI). These results indicate that the RbI additive can significantly reduce the charge recombination in the perovskite films.

We prepared the solar cells with varying concentrations of excess RbI additive to confirm the passivation effect (Fig. 2(e)–(f)). As shown in Fig. S5 (ESI), the EQE intensity enhanced with the increase of the RbI concentration, which is attributed to the improvement of the film quality. What's more, the absorption cutoff edge also red-shifted in EQE with the increase of the RbI concentration, which was mainly caused by the decrease of the bandgap. Thus, the integrated current density is eventually increased. The champion target device exhibited all-around promotion for the device performance, showing a PCE of 20.11% with a VOC of 1.233[thin space (1/6-em)]V, JSC of 21.46 mA cm−2 and FF of 76.01%, while the control PSC shows a sufficient PCE of 15.29%, along with VOC, JSC, and FF of 1.164[thin space (1/6-em)]V, 18.98 mA cm−2, 69.22%, respectively (Table S2, ESI). Moreover, we also fabricated the p–i–n inverted PSCs using the optimized RbI doped FA0.8Cs0.2Pb(I0.75Br0.25)3 films with the structure of ITO/NiOx/RbI-doped perovskite/PCBM/BCP/Ag. The best device delivered an excellent PCE of 21.31% with a VOC of 1.198 V and impressive FF of 84.51%, which is promising for preparing high-efficiency tandem solar cells (Fig. S6, ESI).

For further pushing the performance of WBG PSCs, reducing the recombination at the interface becomes necessary. Our previous work has shown that the surface heterojunction (SHJ) on the perovskite film can effectively enhance the light absorption and reduce the interfacial charge recombination.36 Hence, we also introduced FAI on the RbI-doped FA0.8Cs0.2Pb(I0.75Br0.25)3 film surface to in situ generate a relatively narrow bandgap perovskite, and it can be found that the bandgap red-shifts from 1.686 eV to 1.668 eV and the strength of absorption enhanced, indicating the formation of SHJ (Fig. S7, ESI). The SHJ-passivated solar cell exhibited an improved PCE, mainly benefitting from the increase of VOC and JSC (Fig. S8 and Table S3, ESI).

Furthermore, we firstly explored the possibility of using trioctylphosphine oxide (TOPO) to modify the surface defects for the WBG perovskite film, and the effect of TOPO on the surface of the perovskite is illustrated in Fig. 3(a). SEM and AFM were carried out to certify that TOPO could fill the grain boundary and optimize the interface contact (Fig. S9 and S10, ESI). XPS was used to observe the change in bonding nature after TOPO modification as shown in Fig. S11 (ESI). The peaks corresponding to the P 2p core level root in TOPO were detected, as illustrated in Fig. 3(b). In the primary film, Pb2+ peaks at 143.17 and 138.56 eV were observed, which then move toward lower binding energies at 142.68 and 137.82 eV with TOPO (Fig. 3(c)). Furthermore, Fig. 3(d) illustrates how the characteristic peaks in the I 3d spectra shift from 630.56 eV to lower binding energies, 629.98 eV and 618.54 eV. As a result of the strong coordination effect between the TOPO molecule and Pb2+, the TOPO molecule and perovskite octahedron exhibit strong interaction.


image file: d4ee00666f-f3.tif
Fig. 3 (a) Schematic diagram of sequential passivation strategies of the perovskite film surface. The XPS results of (b) P 2p, (c) Pb 4f, and (d) Cs 3d. (e) The PL spectra of the WBG perovskite films with and without TOPO. (f) The TRPL spectra of the WBG perovskite films with and without TOPO. (g) The energy alignment of the perovskite surface with and without TOPO.

We then characterized the optical properties for the films with and without TOPO. As illustrated in Fig. S12 (ESI), we observe that TOPO will not change the perovskite bandgap but enhance the absorption intensity, which might contribute to the higher current response. PL and TRPL measurement was also carried out to study the interface charge recombination capability (Fig. 3(e) and (f)). Indeed, the PL intensity was enhanced significantly and the peak was blue-shift after TOPO post-deposition treatment, suggesting the reduced nonradiative recombination. We also carried out the fluence intensity dependence of the PL spectra for the WBG perovskite films. Fig. S13 (ESI) shows that after the TOPO treatment, the slope of integrated photoluminescence intensity versus excitation fluence decreased from about 1.81 to 1.49, indicating reduced interface recombination.37 The results of TRPL analysis also found a significant increase in the lifetime of the WBG perovskites with TOPO when compared to the samples without, indicating that TOPO actually passivates the surface defects effectively (Table S4, ESI).

In solar cells, it is effective to improve the device performance by optimizing the energy alignment. Thus, we performed ultraviolet photoelectron spectroscopy (UPS) to assess the surface energy alignment as plotted in Fig. S14 (ESI). The TOPO caused a beneficial p-type doping level, leading to a decreased ECBM between the perovskite and spiro-OMeTAD, which in turn reduced the hole extraction gap and facilitated smoother hole transfer at the interface (Fig. 3(g)).38 The charge transport properties were further studied by PL and TRPL measurement between the perovskite layer and spiro-OMeTAD. Fig. S15a (ESI) shows a significant PL quenching with TOPO treatment. The TRPL results showed that the lifetime of the perovskite layer with TOPO can be significantly reduced (Fig. S15b, ESI), indicating that efficient hole transfer occurred from the perovskite to spiro-OMeTAD.39 These results proved that TOPO can assuredly optimize hole transport at the interface.

Then, we carried out a conductive atomic force microscopy (c-AFM) test. Fig. 4(a)–(b) show c-AFM figures of the perovskite films with and without TOPO, which show heterogeneous surface tunneling current distributions over the perovskite grains and grain boundaries (GBs). A high microscopic current signal at the GBs (JGB), definitely representing high trap density, makes the GBs contribute to the leakage current in perovskites.40 According to the c-AFM line profiles in Fig. 4(c), in comparison with the film without TOPO, the current flow through the perovskite grains with TOPO was reduced. In addition, we also observed that the spatial distribution of microscopic conductivity becomes more uniform. These results indicate that TOPO properly passivated the defective GBs in the WBG perovskite film.


image file: d4ee00666f-f4.tif
Fig. 4 c-AFM images of (a) without TOPO and (b) with TOPO WBG perovskite films. (c) Line profiles of local current flow for the black and red lines in the c-AFM images. (d) Nyquist plots of electrochemical impedance spectroscopy (EIS) under dark conditions with and without TOPO. (e) Light-intensity-dependent JV measurements for the corresponding devices. (f) The JV characteristic curves of the devices without/with TOPO in the dark.

Multidimensional tests on PSCs were conducted to investigate the charge transfer and recombination mechanism behind the enhanced photovoltaic performance of TOPO-treated WBG PSCs. Initially, we examined the electrochemical impedance spectrum measurement (EIS) to explore the charge recombination dynamics of the devices (Fig. 4(d)). The TOPO device demonstrated a significant improvement in recombination resistance (Rrec) over the control device, suggesting that TOPO had an effective effect on defect suppression.41 We also tested the devices response under different light conditions. Firstly, we found that both of the devices with and without TOPO showed a linear JSCversus light intensity relationship (insets in Fig. 4(e)), indicating the absence of an interfacial barrier or carrier imbalance with TOPO introduced. Defect-assisted recombination within devices may result in a deviation of the ideal factor from 1.42 Afterwards, the VOC of the corresponding solar cells as a function of the light intensity is illustrated in Fig. 4(e) and Fig. S16 (ESI), the obtained ideal factor n values of the control and TOPO-introduced device are 1.65 and 1.39, respectively. Furthermore, the target device also demonstrated a smaller dark current than the control sample (Fig. 4(f)). Overall, these results prove that the optimized interface created by using TOPO effectively suppresses recombination in PSCs.

We utilized the TOPO molecule to enhance carrier lifetime and regulate surface-band alignment, resulting in the configuration of PSCs with the following structure: ITO/SnO2/RbI-doped FA0.8Cs0.2Pb(I0.75Br0.25)3/FAI/TOPO/spiro-OMeTAD/Au, as shown in Fig. 5(a). The best performances of the control and TOPO-treated solar cells are displayed in Fig. 5(b). The champion TOPO-treated device delivers a PCE of 23.32% with an impressively high VOC of 1.282[thin space (1/6-em)]V and FF of 81.2%, while the control device shows a relatively low PCE of 21.87%, along with VOC and FF of 1.241 V and 78.7%, respectively. Fig. 5(c) shows that the control and TOPO-treated solar cells all deliver high external quantum efficiency (EQE) of over 90%, and the integrated current densities closely matched with the measured JSC. The EQE of the TOPO-treated device shows a slightly stronger response than the control counterpart, which is mainly attributed to the increased absorption intensity.


image file: d4ee00666f-f5.tif
Fig. 5 (a) The scheme of device architecture of the TOPO passivated WBG perovskite solar cells. (b) Illuminated JV curves of the WBG solar cell devices without/with TOPO passivation. (c) The device EQE spectra of the WBG solar cell devices without/with TOPO passivation. (d) Stabilized power output (SPO) under maximum power-point conditions of the WBG solar cell devices without/with TOPO passivation. (e) Efficiency histogram of the WBG solar cell devices without/with TOPO passivation. (f) Plots of PCE against VOC deficits for FA–Cs based WBG PSCs reported in this work and in the literatures from Table S5 (ESI).

The stabilized power outputs (SPOs) of the TOPO-treated and control cells are 22.58% and 20.46%, respectively, which well matches with the PCEs measured by the JV curves, as shown in Fig. 5(d). The performance distributions of 40 devices for each type are shown in Fig. 5(e) to verify the reproducibility. We also found that the hysteresis index can be reduced from 9.8% to 2.9% after TOPO modification (Fig. S17, ESI), indicating that the defects and ion migrations have been significantly suppressed. The highest VOC of 1.30[thin space (1/6-em)]V was achieved for some individual TOPO-treated devices, corresponding to a VOC deficit lower than 0.385 V (Fig. S18, ESI). This is the highest performance and the lowest voltage deficit (0.385 V) of the reported FA–Cs based WBG PSCs (Fig. 5(f) and Table S5, ESI). In addition, we found that TOPO could be extended to improve the VOC of the relatively narrow bandgap (1.55 eV) hybrid solar cells. We prepared the PSCs using the regular structure of ITO/SnO2/FAxMA1−xPbI3/TOPO/spiro-OMeTAD/Au. The control device exhibits a PCE of 21.57% with VOC of 1.133[thin space (1/6-em)]V, JSC of 23.98 mA cm−2 and FF of 78.76%, while the TOPO-treated device shows a higher PCE of 23.50%, with VOC of 1.162 V, JSC of 24.38 mA cm−2 and FF of 83.09% (Fig. S19 and Table S6, ESI), which proves the general applicability of this strategy.

To further understand the device physics and the possible mechanism for the enhanced efficiency in the TOPO-passivated device, we used the 1-D drift and diffusion model as implemented in the solar cell capacitance simulator program 1, 2. Firstly, the validity of the device modeling needs to be confirmed by comparison with the experimental results. As shown in Fig. S20 (ESI), the device structure of the perovskite solar cell consists of Au/spiro-OMeTAD/defective interface layer/perovskite/SnO2/ITO. The physical parameters for the layers are listed in Table S7 (ESI). We note that the defective interface layer represents the effective interface region between the hole transport layer and the perovskite. Thus, the most immediate factor for the TOPO passivation is to reduce the defect concentration in the defective interface layer, which is usually assumed to reduce interfacial recombination. In comparison with the TOPO passivated device (Fig. S20b, ESI), the electronic property (band edges) of the defective interface layer has also been modified significantly from our experimental characterization. Thus, the influence of the modified band energy level needs further identification.

Meanwhile, the exact thickness of the defective interface layer is unknown, and thus we can compare the typical perovskite device parameters with our experimental measurements to fit the approximate thickness. As shown in Fig. S21 (ESI), we have compared the simulated main performance parameters (VOC, PCE, JSC, and fill factor) with the experimental measurements by varying the defective interface layer thickness. This indicates that the performance parameters from the defective interface layer with 7 nm thickness show a good agreement with our experiment measurements.

As shown in Fig. 6(a) the simulated JV curve almost replicates the experimentally measured values. Then, we simulated the JV curve for the TOPO-passivated device with up-shifted band edges in the defective interface layer from the measured values (Fig. S22, ESI). The results indicate that the up-shifted band edges only have improvement for the fill factor, which still can’t explain the enhanced VOC. Therefore, we further consider the device performance influenced by the interfacial defect concentration. As shown in Fig. S23 (ESI), the VOC and fill factor of the device significantly decreased when the critical defect concentration was above 1014 cm−3, which suggests that the main contribution of increased efficiency comes from the interfacial defect passivation. Thus, the optimized performance in the TOPO passivated device can be understood by the simulation from the device with the low defect concentration and the up-shifted band edges in the defective interface layer (Fig. 6(b)). On the other hand, we also further estimated the p-type doping effect from the TOPO passivation due to the up-shifted band edges. As shown in Fig. 6(c) and (d), this extra doping effect has a synergistic improvement on the fill factor, therefore providing a higher efficiency under the maximum power point conditions.


image file: d4ee00666f-f6.tif
Fig. 6 JV curves for the simulated vs. experimental measured typical perovskite solar cell (a) and the TOPO passivated solar cell. (b) Effect of doping on the photovoltaic performances of TOPO passivated devices, (c) VOC and PCE, and (d) JSC and fill factor.

Next, we found that the TOPO-treated WBG PSCs exhibited much improved operational stability. Fig. 7(a) shows that the TOPO-modified unencapsulated PSC retained 94% of its initial PCE after being stored for 3400 hours in ultra dry air (maintained at 25 °C in ∼10% humidity), and the details of the evolution of device performance are shown in Table S8 (ESI). The control solar cell maintained only 54.8% of its original PCE after 3400 h. To verify the tolerance under light soaking, the maximum power point (MPP) operation stability of the encapsulation device was tested in a nitrogen environment (Fig. 7(b)), which shows that the TOPO-treated device maintained 85% of its initial efficiency after 300[thin space (1/6-em)]hours of operation. To measure the thermal stability, we baked the unencapsulated solar cells on a hotplate at 65 °C in an N2 glove box for 280 h. Our device that had been subjected to TOPO treatment maintained 66.1% of its initial efficiency, whereas the control device retained only 31.6% (Fig. 7(c)). Meanwhile, an excellent humidity stability with a less than 12% efficiency drop is demonstrated for the WBG perovskite solar cells under ambient air (in 30–40% humidity) over 350 h (Fig. 7(d)).


image file: d4ee00666f-f7.tif
Fig. 7 (a) The storage stability of the corresponding PSCs exposed in ultra dry air (25 ± 3 °C, relative humidity (RH) of ≈10%) and in dark. (b) Maximum power point (MPP) tracking of unencapsulated without/with TOPO PSCs under 100 mW cm−2 white light-emitting diode illumination around 40 °C in a N2 environment. (c) Thermal stability of the unencapsulated without/with TOPO PSCs heated at 65 °C in a N2 environment. (d) The humidity stability of the corresponding PSCs exposed in ambient air (25 ± 3 °C, relative humidity (RH) of 30–40%).

Overall, the improved crystal quality and suppressed defect density of the WBG perovskite determined the stability of the devices.

Moreover, we also prepared large-scale (1 cm2) WBG PSCs combining the above passivation strategies in the film bulk and at the interface, which is of great significance for the preparation of tandem solar cells (Fig. S24, ESI).43 We obtained an exhilarating PCE of 19.54% from a reverse scan with VOC of 1.16[thin space (1/6-em)]V, JSC of 22.54 mA cm−2 and FF of 74.88% in WBG perovskite solar cells.

Conclusions

In conclusion, we report an all-around passivation strategy by incorporating RbI, constructing a surface heterojunction and using trioctylphosphine oxide (TOPO) in MA-free WBG FA0.8Cs0.2Pb(I0.75Br0.25)3 PSCs, which can significantly optimize the film morphology, passivate grain boundaries and undercoordinated defects, and improve the surface-band alignment. Thus, the charge recombination at the perovskite bulk and interface is significantly suppressed. Indeed, the target MA-free WBG n–i–p solar cells at 1.685 eV achieve the PCE of 23.35% and a VOC of 1.30[thin space (1/6-em)]V, which is the highest performance and the lowest voltage deficit (0.385 V) of the reported FA–Cs based WBG PSCs. The unencapsulated devices also displayed impressive long-term air stability, thermal stability, and light stability. Moreover, an excellent PCE of 19.54% on 1 cm2 solar cells and a record PCE of 21.31% on a 0.04 cm2 p–i–n MA-free WBG solar cell are also demonstrated.

Author contributions

Ye Q. wrote the paper, provided guidance and revised the manuscript. Hu W. and Zhu J. participated in the experiments. Hu W, Cai Z., Zhang H., Dong T., Yu B., Chen F., Yao B. and Dou W. contributed to experimental characterizations. Li T. and Fang Z analyzed the results and revised the manuscript. Liu Z. and Wei X. carried out the device simulation. Ye F. helped with sample preparation and characterizations. Ye Q. directed and supervised the project. All authors contributed to the general discussion.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Pro. Qi Jiang and Pro. Jiangsheng Xie for their help with the experiments. This work was financially supported by the China Postdoctoral Science Foundation (Grant No. 2022M723281), Natural Science Foundation of Zhejiang Province, China (Grant No. LQ22F040001), Shaoxing Public Welfare project (Grant No. 2023A11002), and also by the Science and Technology Planning Project of Shaoxing City (Grant No. 2022B41001).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee00666f
These authors contributed equally to this work.

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