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Vertically Oriented BiI3 Template Featured BiI3/Polymer Heterojunction for High Photocurrent and Long-Term Stable Solar Cells Shuang Ma, Yi Yang, Cheng Liu, Molang Cai, Yong Ding, Zhan'ao Tan, Pengju Shi, Songyuan Dai, Ahmed Alsaedi, and Tasawar Hayat ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10266 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Vertically Oriented BiI3 Template Featured BiI3/Polymer

Heterojunction

for

High

Photocurrent and Long-Term Stable Solar Cells Shuang Maa,b, Yi Yanga,b, Cheng Liua,b, Molang Caia,b,*, Yong Dinga,b,*, Zhan’ao Tana,c, Pengju Shia,b, Songyuan Daia,b,d,*, Ahmed Alsaedid, Tasawar Hayatd,e a. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China. b. Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, China. c. Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. d. NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia. e. Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan.

ABSTRACT: Nontoxic and stable materials are one of the necessaries for commercialization of solar devices. However, most lead-free absorber has limited light absorption range as well as poor morphology. In this work, the vertically oriented BiI3 template induced by Li-bis(trifluoromethanesulfonyl)imide (Li-TFSI) dopant is intimately integrated with the light-absorbing polymer to form organic-

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inorganic BiI3/polymer heterojunction absorber in solar cells. Compared with the dopant-free BiI3/polymer, the broaden light absorption of the doped BiI3/polymer enhances the external quantum efficiency (EQE) of device beyond 500 nm, as well as extending EQE edge from 650 nm to 750 nm, which significantly increase the shortcircuit current (Jsc) of the device from 1.3 to 3.7 mA cm-2. The polymer top layer is further optimized to improve the charge extraction, which achieved the highest Jsc record (7.8 mA cm-2) of BiI3-based solar cells and efficiency of 1.03 %. Moreover, the encapsulated device shows no degradation after storage in ambient condition for nearly 2 years. KEYWORDS:

photovoltaics,

oriented

BiI3/polymer

heterojunction,

spectral

broadening, controllable orientation, high photocurrent, stability

1. INTRODUCTION In recent years, new thin film solar cells including organic photovoltaics (OPVs), dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs) have developed rapidly and achieved increased efficiency.1-4 The toxicity of material is one of the issues for commercialization of thin film solar devices, such as the lead content in PSCs. Ge-, Sn-, Sb- and Bi-based materials are used as the alternative substitutes for lead-containing absorbers in solar cells, such as AGeX3, ASnX3, A3Sb2X9, A3Bi2X9, ABi3X10 (A = MA (methylammonium), FA (formamidinium) or Cs; X = Cl, Br or I), et al.5-12 Ge2+ and Sn2+ are easily oxidized to a higher valence state due to the highenergy-lying 5s orbitals, leading to the instability issues of Ge- and Sn-based perovskite materials in ambient condition.13-14 Sb- and Bi-based perovskite-like hybrid compounds have inherent low-dimensional structure, resulting in less impressive performance.7, 15 In recent years, double perovskite materials have drawn


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attention due to their potential as a stable and green alternatives for optoelectronic applications. However, their transport ability and band structure are still barely satisfactory, and only few double perovskite materials have been synthesized and applied in solar cells.13 Exploration of novel nontoxic and stable materials is crucial for commercialization and has attracted great attention. Inorganic metal halide, bismuth iodide (BiI3), has been found to be an alternative lead-free absorber with favorable photovoltaic properties and solution processability.16-18 BiI3 has a rhombohedral crystal structure (space group R3) with dimensional parameters marked in Figure 1.19 This semiconductor possesses a layered crystal structure with strong ionic bond of Bi and I within single I-Bi-I layers, and weak van der Waals forces between adjacent I-Bi-I layers.20-21 Consequently, BiI3 crystal has anisotropic optical and electrical properties, and control of vertical orientation of BiI3 films is important for better carrier transport for optoelectronic application. The electronic mobility of BiI3 single crystal was reported to be ~600 cm2 V-1 s-1, and it can be improved to ~1000 cm2 V-1 s-1 by Sb-doping.22 The absorption coefficient of BiI3 in the visible region of the solar spectrum (>105 cm-2)18 is comparable to Si23, GaAs24 and perovskite materials.25 Moreover, BiI3 films can be prepared through a simple solution process due to its high solubility in solvent like tetrahydrofuran (THF) and N, N-dimethylformamide (DMF) (in excess of 400 mg mL-1),16, 18 which is beneficial to the commercialized development. These properties indicate that BiI3 has good potential as a nontoxic absorber for solar devices.



Figure 1. (a) The crystal structure of BiI3, (b) the atom arrangement within single IBi-I layer, and the molecular structure of Li-TFSI. Nevertheless, the literature-reported bandgap value of BiI3 that is derived from experimental measurement or theoretical calculation ranges from 1.6 to 2.0 eV.16, 20-21, 26

The short-circuit current (Jsc) of these BiI3-based solar cells is limited (< 6 mA cm-2)

by its relatively narrow absorption edge of ~650 nm, leading to the insufficient lightharvesting at longer wavelength range. The planar heterojunction BiI3-based solar cells with the structure of FTO/TiO2/BiI3/HTL (hole transport layer)/Au was prepared by Anna J. Lehner et al.27 Although employing HTL with deeper valence band maxima (VBM) can improve the open-circuit voltage (Voc) from 0.22 to 0.42 eV, the Jsc decreased from 3.85 to 1.70 mA cm-2. Umar H. Hamdeh et al. used solvent-vaporannealing (SVA) process to increase grain size and reduce porosity of solutionprocessed BiI3 film. They found that a layer of BiOI formed at the surface when processing BiI3 in air, which can facilitate hole extraction and prevent film dewetting during SVA. By this process, a large improvement in Jsc was achieved from 1.23 ± 0.2 to 5.0 ± 0.9 mA cm-2.16 Phase-pure rhombohedral BiI3 thin-films can be obtained by a versatile gas phase iodination of Bi2S3 reported by Devendra Tiwari and coworkers.17 Based on these films, devices (FTO/TiO2/BiI3/F8/Au, F8 is Poly(9,9-di-n-


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octylfluorenyl-2,7-diyl)) achieved the best Voc of 600 mA but the Jsc of 5.28 mA cm-2 is still low. The Jsc would be enhanced by complementing the light absorption of BiI3 with another absorber that has long wavelength absorption. Homogeneous distribution of absorbers is required for the formation of bulk-heterojunction (BHJ) as found in OPV devices. The polymer crystal in the bulk is generally less than 100 nm to enable the charge transport in the film before quenching.28 In this work, we report controllable vertical orientation of the BiI3 template induced by lithium salt (Li-TFSI). The gap between oriented BiI3 grains was optimized below 100 nm and filled by polymer with longer

light

absorption

wavelength,

poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-

yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4b]thiophene-)-2-carboxylate-2-6-diyl)]

(PTB7-Th).

The

oriented

BiI3/polymer

heterojunction is formed with pinhole-free polymer top layer. Compared with the dopant-free BiI3-based structure, the doped BiI3-based heterojunction can convert the light at longer-wavelength region at 500 to 750 nm, which dramatically improves the photon utilization rate and boosts the photocurrent. Moreover, suppressed charge recombination was observed for the doped BiI3-based device compared to the dopantfree analogue. The best efficiency of 1.03% was obtained with a remarkably improved Jsc of 7.8 mA cm-2. Notably, the device exhibited superior stability and the efficiency showed almost no degradation after exposure in air for nearly 2 years when simply encapsulated by a UV-curing coating. 2. RESULTS AND DISCUSSION The growth orientation of BiI3 crystals related to dopant in precursor was studied by Umar H. Hamdeh and co-workers.29 They found that doping solvents with high



Gutmann donor number (DN) such as dimethylsulfoxide (DMSO) or N-methyl-2pyrrolidone (NMP) results in preferred grain orientation within thin films. What’s more, it was noted that the grain size doesn’t show evident numerical trends for different solvent dopant. However, these results are not universally suitable for LiTFSI dopant in this work. Li-TFSI is a Lewis base containing S=O electron-donor groups, which can form coordination complex with the Lewis acid BiI3.16 As can be observed in the scanning electron microscope (SEM) images, especially the crosssection images (Figure 2), when the doping concentration increased, the film shows vertical orientation with increasing grain size and meanwhile enlarged cracks. The increase in grain size agrees with the decreased full-width half-maximum (FWHM) as indicated in Figure S1. To further determine this out-of-plane growth direction over substrate, X-ray diffraction (XRD) spectra were measured as shown in Figure 2(g). The peaks marked with asterisk of 12.8°, 25.7°, 41.6° and 43.7° correspond to the (003), (006), (300) and (303) crystal planes of BiI3.18 It is obvious that the peak intensity of (003) plane decreases in the doped film, accompanied by the increased peak intensity of (300) plane, which implies more out-of-plane orientation than inplane. Furthermore, to determine the dopant distribution on the surface and inside the film, X-ray photoelectron spectroscopy (XPS) depth profiling of the doped film was conducted, as presented in Figure 2(h) and Figure S2. C-1s peak at 284.8 eV was used as the standard reference.30 Considering the spectra may shift after etching by Xray beam, I-3d peak was also normalized to calibrate binding energy scale since the C-1s reference peak is very weak after etching. It was found that F and N elements, the characteristic elements of Li-TFSI with the characteristic peaks at 688.7 eV and 399.7 eV, only existed on the surface of doped BiI3 film.31-32 We infer from this that


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the BiI3/Li-TFSI intermediate phase is not stable, and Li-TFSI remains in the solvent to the last stage of crystallization because of its high solubility in DMF. However, further research is required to determine the specific physical mechanisms of this growth orientation.

Figure 2. (a-c) SEM top-view and (d-f) cross-section of BiI3 films with different concentration of Li-TFSI dopant (named as BiI3-x, x is doping concentration), (g) XRD patterns of FTO, FTO/BiI3 and FTO/BiI3-4, (h) F-1s depth profiling of BiI3-4 film accompanied by XPS spectrum of Li-TFSI. The electron mobility of BiI3 films with gradient doping ratio are compared in Figure 3, which was obtained from the electron-only devices with the structure of



FTO/TiO2/BiI3/BCP/Au. The detailed calculation process is shown in Supporting Information. It can be found that BiI3-4 film has the best electrical property, which is most reasonably benefitted from the vertical orientation. The carrier transport within I-Bi-I planes is naturally easier than that through the planes that are combined by van der Waals interactions. The resistivity value in the direction along I-Bi-I planes was reported two orders of magnitude lower than that normal to planes.33 This anisotropy effect in conductivity was also found for other absorbers such as Sb2Se3 and BiSI that possess layered crystal structure.34-35 Doping even at the 2 mg mL-1 level demonstrated some out-of-plane growth orientation thus better charge transfer compared with dopant-free film. However, when the doping ratio of Li-TFSI is 6 mg mL-1, the charge transport decreases because of the increased film thickness and shunting paths through enlarged pinhole dimension. We expected to select some materials as the HTL for the above vertically oriented BiI3 to fabricate FTO/TiO2/BiI3/HTL/Au device. The important factors that should be taken into consideration include well-matched energy levels, favorable absorption and transport properties, solution processability. According to these requirements, four different organics were employed as the HTL, including poly[N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine] (Poly-TPD), PTB7-Th, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene4,8-dione)] (PBDB-T), and 2,2’,7,7’-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’spirobifluorene (spiro-OMeTAD). The energy level diagram of these materials in BiI3-based solar cell is shown in Figure S3 according to previous references.14, 36-38 The performance parameters related to different HTLs are listed in Table S1. For some of these structures, an MoO3 layer (~10 nm) was added between BiI3 and HTL


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to buffer the large energy level gap. Although the devices with spiro-OMeTAD as HTL have high Jsc because of its high conductivity, the 4-tert butylpyridine (TBP) dopant in spiro-OMeTAD precursor dissolves BiI3 film during spin-coating process, which means poor repeatability. Moreover, the expensive cost and multiple dopant process

also

limit

its

application.

The

device

with

the

structure

of

FTO/TiO2/BiI3/PTB7-Th/Au achieves the optimal Voc and FF. As a result, we chose this structure for further optimization.

Figure 3. (a) J1/2 versus Vapp-Vbi (linear coordinates) and (b) J versus Vapp-Vbi (double logarithmic

coordinates)

of

electron-only

devices

with

the

structure

of

FTO/TiO2/BiI3/BCP/Au (the labelled value in figure is the doping concentration). The current-voltage (J-V) characteristic curves, external quantum efficiency (EQE) spectra and the parameters of devices with different doping levels are shown in Figure 4(b, c) and Table S2. Compared with the dopant-free devices, the doped BiI3based devices show improvement for every photovoltaic parameter, especially the Jsc. The Voc and FF of the best-performance device also increase, which may be related to the restrained recombination loss in the optimal structure. The recombination dynamics in the doped BiI3-based device will be further discussed thereinafter in the transient photovoltage decay test and ideal factor analysis. The most predominantly



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increased parameter is the Jsc, which is in good agreement with the integrated current derived from EQE. Obviously, the EQE of the doped BiI3-based devices are enhanced beyond 500 nm wavelength, and the edge is broadened from 650 nm to 750 nm. Combining with the evidence of absorption spectrum (Figure 4(a)), it can be concluded that in the doped BiI3-based device, the polymer converts photons at the longer wavelength range of 500-750 nm.39 It is known that the excitons generated in polymer absorber must diffuse to heterojunction for dissociation and collection.40 For the device based on oriented BiI3 template, PTB7-Th permeates into the gap between BiI3 crystals. As a result, due to the sufficient interface area in this heterojunction, most of the excitons generated in PTB7-Th can be separated at the interface of BiI3 and PTB7-Th. By contrast, for the dopant-free BiI3 without vertical orientation, BiI3 and PTB7-Th form planer heterojunction. Only few excitons generated in PTB7-Th can be separated due to the reduced interface area and the large exciton binding energy of PTB7-Th.41 Thus, the EQE of corresponding device shows no obvious photo-response beyond 650 nm. Another reason for the increased Jsc that needs to be taken into consideration is the light scattering inside the device. Because of the different morphology of BiI3 controlled by doping Li-TFSI, the light utilization rate could be changed which also contributes to the device Jsc variation. To investigate this, light harvesting efficiency (LHE) and absorbed photon-to-current conversion efficiency (APCE) are estimated using the following equations:42-43 LHE = (1 ― R)(1 ― 10 ―A) APCE = EQE/LHE


(1) (2)

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where R is reflectance and A is absorption of the FTO/TiO2/BiI3/PTB7-Th film. LHE and APCE spectra are plotted in Figure S7 and Figure 4(d). Even though LHE spectra show some difference when tuning doping level, APCE spectra follow the same trend with EQE, which indicates that the light scattering only has slight effect on the device Jsc.

Figure 4. (a) Ultraviolet-visible (UV-Vis) absorption spectra of BiI3, PTB7-Th and BiI3/PTB7-Th films on FTO substrates, (b) J-V, (c) EQE and (d) APCE curves of dopant-free BiI3-based device and doped BiI3-based devices with different doping levels. The concentration of PTB7-Th precursor was also optimized (Figure 5(a, b) and Table S3). When the concentration is low, the poor efficiency of corresponding devices is due to the incomplete coverage of polymer top layer; when the



concentration reaches 10 mg mL-1, the top layer becomes thicker, and photogenerated carriers cannot be effectively transferred through it to the counter electrode before quenching. Ultimately, the best efficiency of 1.03% and Jsc of 7.8 mA cm-2 were achieved when the concentration is 6 mg mL-1. As shown in the cross-section and top-view SEM images of BiI3-4/PTB7-Th-6 heterojunction in Figure 5(c, d), PTB7-Th fills the crevices and completely covers the oriented BiI3 layer below to form heterojunction structure. The best J-V curves with forward and reverse scans are shown in Figure S8.

Figure 5. (a) J-V curves and (b) EQE spectra of solar cells prepared by 4, 6, 8 or 10 mg mL-1 PTB7-Th precursor, (c) SEM cross-section and (d) top-view image of BiI34/PTB7-Th-6 heterojunction structure. To further investigate the charge recombination kinetics, transient photovoltage decay test and ideal factor analysis of the dopant-free and doped BiI3-based devices


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(BiI3-4) were performed (Figure 6). Transient photovoltage decay is commonly used to reflect the photoinduced carrier recombination dynamics of the entire heterojunction across the solar cells.44 The doped BiI3-based device shows a much slower photovoltage decay than the dopant-free device in Figure 6(a). The carrier lifetime (τ) under different photovoltage can be further calculated from the transient photovoltage decay result using the following equation (3):45 𝜏 = ― 𝐾𝐵𝑇𝑒 ―1(𝑑𝑉𝑜𝑐/𝑑𝑡) ―1

(3)

where e is the elementary charge, T is temperature, and KB is the Boltzmann constant. The relationship between the carrier lifetime τ and photovoltage Voc is shown in Figure 6(b). The improved carrier lifetime indicates the suppressed carrier recombination across the solar device, which results from complex reasons including the interfacial recombination at c-TiO2/BiI3 and BiI3/PTB7-Th, and the recombination within BiI3 film.



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Figure 6. (a) Transient photovoltage decay curves, (b) extracted lifetime of the injected carriers versus photovoltage of dopant-free device and doped BiI3-based device, the plot of the relationship of (c) -dV/dJ and (Jsc-J)-1, (d) ln(Jsc-J) and V+RsJ. The ideal factor of J-V characteristics was also analyzed to further explain the reduced recombination in doped BiI3 structure using the following equation:46

[ (

𝐽 = 𝐽𝐿 ― 𝐽0 exp

𝑒(𝑉 + 𝐽𝑅𝑠) 𝐴𝐾𝐵𝑇

) ― 1] ―

𝑉 + 𝐽𝑅𝑠 𝑅𝑠ℎ

(4)

where J is the current density, JL is the light-induced constant current density (equal to Jsc), J0 is the dark reverse saturated current density, V is the bias voltage, Rs is the series resistance, KB is the Boltzmann constant, A is the ideal factor (1 < A < 2), T is the absolute temperature, and Rsh is the shunt resistance.47 For a real device, equation (4) can be deduced as:


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ln (𝐽𝑠𝑐 ― 𝐽) = (𝑉 + 𝑅𝑠 × 𝐽)𝑒𝐴 ―1𝐾𝐵 ―1𝑇 ―1 + ln 𝐽0 ― d𝑉 d𝐽 = 𝐴𝐾𝐵𝑇(𝐽𝑠𝑐 ― 𝐽) ―1𝑒 ―1 + 𝑅𝑠

(5) (6)

When the J is zero, the Voc can be expressed as: 𝑉𝑜𝑐 ≈ 𝐴𝐾𝐵𝑇𝑒 ―1ln (𝐽𝑠𝑐𝐽0 ―1)

(7)

The fitting curves of -dV/dJ versus (Jsc-J)-1 and ln(Jsc-J) versus V+RsJ are plotted in Figure 6(c, d), from which we can calculate the Rs, A and J0 as 3.086 Ω cm2, 6.098 and 0.330 mA cm-2 for doped BiI3-based device (5.216 Ω cm2, 7.141 and 0.194 mA cm-2 for dopant-free device), respectively. Therefore, the doped BiI3-based device suffers from less recombination loss than undoped device. According to equation (7), the calculated Voc values of dopant-free device and doped BiI3-based device are 0.352 and 0.385 V. These results are close to the Voc values obtained from the J-V measurement. The stability of films and solar cells was also studied, as demonstrated in Figure 7. The absorption spectra of BiI3 and BiI3/PTB7-Th films stored in N2 atmosphere show no difference after 210 days (Figure S9). The brown BiI3 films stored in air were found gradually oxidized into BiOI and finally form yellow Bi2O3 according to the evidence of absorption edge shift in Figure 7(a).48 Depth-profiling XPS was conducted to further analyze the chemical state of elements in the oxidized films, as shown in Figure S10. The oxidation level on and beneath the surface of BiI3 and BiI34 films is demonstrated by comparing the integrated intensity of lattice O-1s peaks. It is observed that the doped film has higher oxidation level, which agrees with the macroscopic film appearance in the inset of Figure 7(a). In the case of device, the stability of the whole structure is often more important other than certain layers. Owing to the coated polymer layer, BiI3 is prevented from contact with air thus the



BiI3/PTB7-Th film shows remarkable stability. The film remained brown and its absorbance only slightly declined after 20 days exposure in air. The simply encapsulated device was stored in ambient condition and the performance parameters were measured at certain time intervals. After nearly 2 years, the efficiency maintains about 90% of the peak value. This result shows the potentiality of BiI3 as the nontoxic and stable optoelectronic material.

Figure 7. (a) UV-vis spectra of BiI3 and BiI3/PTB7-Th films stored in ambient condition (RH = 45~85%, T = 25~30 ºC). The inset is the photograph of unencapsulated films stored under ambient condition. (b) Stability of encapsulated solar cells stored under ambient condition for 660 days. 3. CONCLUSION In summary, the vertical orientation of BiI3 crystal was controlled by doping LiTFSI into the precursor, and this oriented BiI3 template was intimately combined with a light-absorbing polymer to form heterojunction structure. In this structure, the polymer can complementarily convert the light in the longer wavelength range, resulting in the significantly improved Jsc compared with the analogue dopant-free BiI3-based devices. The reduced charge recombination of doped BiI3-based device is


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also estimated by transient photovoltage decay and ideal factor analysis. The top efficiency of 1.03% was obtained with a remarkably improved Jsc of 7.8 mA cm-2, which is the highest photocurrent value for BiI3-based solar cells. Moreover, the simply encapsulated device showed good stability in that its efficiency remained about 90% of the highest value after storage under ambient condition for 2 years. Our work proposes a design of organic-inorganic heterojunction structure for BiI3-based solar cells by orientation engineering, and suggests that the HTL with good light absorption capability can contribute to the device photocurrent by architectural design. This also demonstrates the potential of BiI3 material as a nontoxic and stable potential choice for optoelectronic application in the future. We believe that the performance of BiI3-based solar cells will be further improved through crystallization control, dopant studies, interfacial engineering and structure design. 4. EXPERIMENTAL METHODS 4.1. Materials and precursor preparation. F-doped SnO2 (FTO) glass with a sheet resistance of 15 Ω sq-1 was purchased from Wuhan Geao (China). Hydrochloric acid (37 wt% in water), acetone (99.8%, extra dry), isopropanol (99.8%, extra dry) and chlorobenzene (99.8%, extra dry) were purchased from Acros Organics. Titanium diisopropoxide, bis(acetylacetonate), lithium bis(trifluoromethylsulphonyl)imide (LiTFSI), N, N-dimethylformamide (DMF) and bismuth (III) iodide (BiI3, 99.999%) were purchased from Alfa Aesar. PTB7-Th was purchased from 1-Material. All these commercially available materials were used as received without further purification. 200 mg mL-1 BiI3 was completely dissolved in DMF. 200 mg mL-1 Li-TFSI dissolved in DMF was used as the dopant, and different volume of it was added into BiI3 precursor for certain doping concentration. The BiI3 precursor was filtered through a



0.45 μm-diameter filter before spin coating. PTB7-Th with gradient concentration was dispersed sufficiently in chlorobenzene to prepare HTL. 4.2. Device fabrication. FTO glass was patterned to 2 mm width by zinc powder (95.0%) and hydrochloric acid (37 wt% in water). Then, the 1.5×1.5 cm2 substrate was ultrasonically cleaned twice by detergent, water, ultrapure water, acetone and isopropanol for 15 min, followed by 150 °C drying. 0.6 mL titanium diisopropoxide and 0.4 mL bis(acetylacetonate) were mixed with 7 mL anhydrous isopropanol and sprayed on clean FTO substrate at 450 °C in air. The flow rate of oxygen carrier gas is 30 L min-1. FTO coated with compact-TiO2 (c-TiO2) was heated at 450 °C for 30 min and gently cooled down to room temperature. Filtered BiI3 precursor was then spin-coated on c-TiO2 at 3000 rpm in N2 glove box (oxygen ≤ 0.1 ppm, water ≤ 0.1 ppm), followed by thermal treatment at 60 °C for 10 min. Then, PTB7-Th solution was spin-coated on BiI3 film at 3000 rpm, and gold electrode of 30 nm was thermally evaporated on the top of device under 3×10-4 Pa. The active area of devices was 0.04 cm2. 4.3. Characterization. The device in this paper was illuminated by a solar simulator (SAN-EI, AAA grade) under standard AM1.5G (100 mW cm-2) calibrated with a standard silicon cell. J-V test was carried out using Keithley 2400 Source Measure Unit in N2 atmosphere. The scan delay is 0 s, and the scan speed is 93 mV s-1. Systems model QE-DLI (Enli Technology Co., Ltd.) was used to measure external quantum efficiency (EQE) in air after calibrated with a standard silicon cell. Absorption spectra were tested by Shimadzu UV-2450 UV-visible spectrophotometer. SEM images were obtained by HITACHI SU8010 (Japan). Crystallization property was determined by X-ray diffract meter (Rigaku miniflex 600) at a scanning rate of 5° min-1. Transient photovoltage decay were measured using the Zahner electrochemical


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workstation combined with a module for fast intensity transients. The intensity of light source is 1000 W m-2. XPS measurements were conducted using ESCALAB 250Xi XPS system (Thermo Fisher Scientific) with Al Kα X-ray (1486.6 eV) as the radiation source. Charging problems or beam damage were excluded by comparing the single XPS measurements sequentially. ASSOCIATED CONTENT Supporting information Additional data including FWHM of (003) and (300) peak in XRD, the depthprofiling XPS for BiI3 and BiI3-4 films, the energy level diagram of BiI3-based solar cells with different HTLs, the photovoltaic parameters and statistics of devices with different HTLs, the photovoltaic parameters and statistics of devices with different LiTFSI doping concentrations, the photovoltaic parameters and statistics of devices with different PTB7-Th precursor concentrations, transmittance, reflection and LHE of FTO/TiO2/BiI3/PTB7-Th films, the optimal J-V curves with forward and reverse scans, the absorption spectra of BiI3 and BiI3/PTB7-Th films stored in N2 atmosphere for 210 days, and depth-profiling XPS for the oxidized BiI3 and BiI3-4 films. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected]. *Email: [email protected]. Notes



The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the National Key Research and Development Program of China (2016YFA0202400), the 111 Project (B16016), the National Natural Science Foundation of China (51572080, 51702096, U1705256), and the Fundamental Research Funds for the Central Universities (2017XS080, 2017XS081, 2018QN063). REFERENCES (1) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jacobsson, T. J.; Grätzel, M.; Hagfeldt, A. The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10 (3), 710-727. (2) Ostroverkhova, O. Organic Optoelectronic Materials: Mechanisms and Applications. Chem. Rev. 2016, 116, 13279-13412. (3) Graetzel, M.; Janssen, R. A.; Mitzi, D. B.; Sargent, E. H. Materials Interface Engineering for Solution-Processed Photovoltaics. Nature 2012, 488, 304-312. (4) https://www.nrel.gov/pv/cell-efficiency.html (5) Kopacic, I.; Friesenbichler, B.; Hoefler, S. F.; Kunert, B.; Plank, H.; Rath, T.; Trimmel, G. Enhanced Performance of Germanium Halide Perovskite Solar Cells through Compositional Engineering. ACS Appl. Energy Mater. 2018, 1, 343-347. (6) Singh, T.; Kulkarni, A.; Ikegami, M.; Miyasaka, T. Effect of Electron Transporting Layer on Bismuth-Based Lead-Free Perovskite (CH3NH3)3Bi2I9 for Photovoltaic Applications. ACS Appl. Mater. Interfaces 2016, 8, 14542-14547. (7) Zhang, Z.; Li, X.; Xia, X.; Wang, Z.; Huang, Z.; Lei, B.; Gao, Y. High-Quality (CH3NH3)3Bi2I9 Film-Based Solar Cells: Pushing Efficiency up to 1.64%. J. Phys. Chem. Lett. 2017, 8, 4300-4307.


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