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Bournonite CuPbSbS3: An electronically-3D, defect-tolerant, and solution-processable semiconductor for efficient solar cells
Journal Pre-proof Bournonite CuPbSbS3: an electronically-3D, defect-tolerant, and solution-processable semiconductor for efficient solar cells Yuhao Liu, Bo Yang, Muyi Zhang, Bing Xia, Chao Chen, Xueping Liu, Jie Zhong, Zewen Xiao, Jiang Tang PII:
S2211-2855(20)30132-4
DOI:
https://doi.org/10.1016/j.nanoen.2020.104574
Reference:
NANOEN 104574
To appear in:
Nano Energy
Received Date: 16 December 2019 Accepted Date: 3 February 2020
Please cite this article as: Y. Liu, B. Yang, M. Zhang, B. Xia, C. Chen, X. Liu, J. Zhong, Z. Xiao, J. Tang, Bournonite CuPbSbS3: an electronically-3D, defect-tolerant, and solution-processable semiconductor for efficient solar cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104574. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Graphical abstract
Bournonite
CuPbSbS3:
an
electronically-3D, defect-tolerant, and
solution-processable semiconductor for efficient solar cells Yuhao Liu†a, Bo Yang†a, Muyi Zhanga, Bing Xiaa, Chao Chena, Xueping Liuc, Jie Zhongc, Zewen Xiaoa,*, Jiang Tanga,b,* a
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
b
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China c
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China *Corresponding Authors: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail addresses: [email protected] (Z. Xiao); [email protected] (J. Tang)
† These authors contributed equally to this work. Keywords: Bournonite, BDCA solution, thin film solar cell, electronic dimensionality, defect tolerance
Abstract The absorber layer is a key component of thin-film solar cells. Based-on the recently-proposed electronic dimensionality concept, a promising solar cell absorber material should be electronically high-dimensional, as is the case for all the mainstream absorbers, such as Si, GaAs, CIGS, CdTe, and CH3NH3PbI3. In this work, we propose an electronically three-dimensional semiconductor, bournonite CuPbSbS3, as a prospective efficient solar cell absorber material. Our density functional theory calculations indicate that CuPbSbS3 exhibits desired optoelectronic properties, such as
a nearly direct bandgap, high optical absorption coefficients, appropriate p-type doping, and defect tolerance. Experimentally, we developed a butyldithiocarbamate acid (BDCA) solution process for depositing high-quality CuPbSbS3 thin films and built the first CuPbSbS3 solar cells. Our CuPbSbS3-based thin film solar cells achieved a preliminary power conversion efficiency of 2.23% (open circuit voltage of 699 mV), highlighting the potential of this semiconductor for thin film photovoltaics.
1. Introduction In the past few decades, great progress has been made for thin-film solar cells, the second generation photovoltaic technique. Particularly, solar cells based on Cu(In,Ga)Se2 (CIGS), CdTe, and CH3NH3PbI3 absorbers have achieved high power conversion efficiency (PCE) of 22.6%, 22.1%, and 25.2%, respectively [1-3]. However, while lead halide perovskites has the long-term stability issue, CIGS and CdTe suffer from the scarcity of raw materials (In in CIGS and Te in CdTe), limiting their terawatt-scale commercial application. Therefore, low-cost earth-abundant and stable absorber materials for thin film solar cells need to be explored. While no good solutions have been found yet for stabilizing CH3NH3PbI3 for several years and for replacing the Te in CdTe, the trivalent In in CIGS can be mutated by two earth-abundant elements, Zn and Sn, to form kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) [4-6]. However, the record efficiency of kesterite solar cells has been stagnated at 12.6% after years of extensive study [7], significantly lower than the theoretical efficiency limit based solely on the bandgap consideration. The
efficiencies of kesterite solar cells are mainly limited by the large open-circuit voltage (VOC) deficit, defined as Eg/q − VOC, where Eg is the band gap of the absorber and q is the electron charge [8, 9]. Density functional theory studies reveal that the large VOC deficits for kesterite solar cells are attributed to the high density of cation-on-cation antisites which introduce absorption band-tail states and act as nonradiative recombination centers [10]. On the other hand, In can also be directly replaced by trivalent elements such as Bi and Sb, forming the layered ternary Cu(Sb,Bi)(S,Se)2 compounds, which, however, exhibit indirect bandgaps and undesired anisotropic carrier transport and optical properties [11-13].
Based on the recently-proposed concept of electronic dimensionality [14], a promising photovoltaic absorber should have high electronic dimensionality, which is the case for all efficient solar cells reported so far. It is noticeable that the three-dimensional crystal structure is a prerequisite for high electronic dimensionality. In fact, all the mainstream thin-film solar cell absorber materials, such as CIGS, CdTe, and CH3NH3PbI3, have 3D crystal structures [15-19]. Unfortunately, the above-mentioned Cu(Sb,Bi)(S,Se)2 compounds as well as many other earth-abundant post-transition metal chalcogenides (e.g., SnS, Sb2Se3, NaSbS2, etc.) crystallize into low-dimensional crystal structures, which is attributed to the structural distortion that necessarily occurs for stabilizing the filled lone-pair s2 states [20-22]. In fact, despite the remarkable progress, the record efficiencies for these low-dimensional chalcogenide absorber materials are far lower than the mainstream absorber materials.
Therefore, it is still of significance to research for efficient earth-abundant solar cell absorber materials, particularly from those which are both structurally and electronically 3D.
It is noted that among binary heavy post-transition metal chalcogenides, Pb-containing chalcogenides are the few ones that crystallize into 3D rocksalt-type crystal structure with a higher coordination number of 6, due to the less active lone pair states of Pb [23]. We expect that the incorporation of PbS into layered chalcogenides might reconstruct the crystal structures to form 3D multi-component chalcogenides. Using Cu(Sb,Bi)S2, which have been intensively studied as absorber materials, as prototypes, incorporating PbS leads to the formation of 3D CuPb(Sb,Bi)S3, as shown Fig. 1a. In this study, we focus on CuPbSbS3, a natural mineral named as bournonite, which has a bandgap of 1.3 eV suitable for single-junction solar application. Herein, we particularly emphasize, by density functional theory (DFT) calculations, that CuPbSbS3 is electronically 3D, with desired optoelectronic properties for solar cell application, such as a nearly direct bandgap, good carrier transport properties, and high optical absorption coefficients. Additionally, our defect calculations predict CuPbSbS3 is defect-tolerant and exhibits natural weak p-type conduction if synthesized under S-rich conditions. We developed a low-cost and facile solution process to obtain phase-pure, crack-free, and compact CuPbSbS3 thin films with grains in micrometer level. The CuPbSbS3-based thin film solar cells with the glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au architecture
achieved a high preliminary PCE of 2.23%, suggestive of a bright future for further material investigation and solar cell optimization.
2. Results and discussions We first show, by DFT calculations, that CuPbSbS3 is not only structurally 3D, but also electronically 3D, and has much better optoelectronic properties as compared with layered CuSbS2. As seen in Fig. 1b, for CuSbS2, the bandgap is indirect, with the HSE-calculated value of 1.56 eV, which is close to the reported experimental value [24]. Such indirect bandgap is typical for layered chalcogenide semiconductors with lone-pair cations [23], and is not ideal for thin-film solar cell application. For 3D CuPbSbS3, the HSE-calculated bandgap is direct, with both the VBM and CBM located at the Γ point (see Fig. S1 in the Supporting Information). The HSE-calculated bandgap value is 1.61 eV , larger than the reported experimental values of 1.20−1.31 eV [25, 26]. The bandgap overestimation should be caused by the neglect of the spin−orbit coupling (SOC) effect, which is profound for compounds containing heavy post-transition elements like Pb [27]. By including the SOC, the bandgap of CuPbSbS3 is reduced to 1.29 eV (see the red arrow in Fig. 1c), which is attributed to the Pb 6p state splitting-induced CBM lowering. Additionally, because of the lack of inversion symmetry in CuPbSbS3 (the space group is Pmn21), the SOC induces profound Rashba splitting, with the doubly spin-degenerated band splitting into two bands shifted with respect to each other. As a result, the CBM is slightly off the Γ point, leading to a weakly indirect bandgap. The direct bandgap at the Γ point is 1.37
eV, only 0.08 eV larger than the indirect bandgap. Nevertheless, CuPbSbS3 can be regared as a nearly direct bandgap semiconductor, similar to the case of CH3NH3PbI3 [28-30]. These results are consistent with those in an early theoritical study by Wallace et al [31]. However, for CuPbSbS3, the direct bandgap transition at the Γ point is forbidden (Fig. S2). The optical transition gradually becomes allowed when move off the Γ point . As a result, the optical absorption coefficient right above the bandgap is relatively low (Fig. S3). Interestingly, the Rashba effect in Pb halide perovskites has been proposed to contribute to the experimentally-observed long carrier lifetimes [32-34], which leads to the expectation that the Rashba effect in CuPbSbS3 may also help to prolong the carrier lifetime for high photovoltaic performance. As seen from the projected densities of states (DOSs) (Fig. 1d), the VBM consists of the antibonding states of S 3p and Cu 3d/Pb 6s/Sb 5s orbitals, while the CBM is composed of the antibonding states of Cu 4s/Pb 6p/Sb 6p and S 3p orbitals. That is, all atoms in CuPbSbS3 contribute to the VBM and CBM, indicating a 3D electronic dimensionality. Because of the high electronic dimensionality, the band edges of CuPbSbS3 are dispersive, indicating good carrier transport properties.
Fig. 1. (a) Schematic illustration of crystal structure revolution from layered CuSbS2 to 3D CuPbSbS3 by incorporating PbS; (b) Band structure of CuSbS2 calculated with HSE hybrid functional. (c) Band structure of CuPbSbS3 calculated with the HSE hybrid functional and the inclusion of spin−orbit coupling effect. Transition at Γ point is direct but forbidden. (d) HSE+SOC-calculated total and projected densities of states (DOSs) of CuPbSbS3. Note that the DOSs of Cu 4s and 4p orbitals are magnified by 10 times for a clearer view.
Defect properties of an absorber materials must be appropriate also for producing efficient solar cells. It is known that some quaternary compounds like kesterites Cu2ZnSn(S,Se)4 [35-37] and double perovskites Cs2(Ag,In,Tl)BiBr6 [38, 39] suffer from the easy formation of deep cation-on-cation antisites, because the Cu, Zn, and Sn in Cu2ZnSn(S,Se)4 have similar tetrahedrally-bonded environments while the Ag/In/Tl and Bi in Cs2(Ag,In,Tl)BiBr6 exhibit similar octahedrally-bonded environments. For kesterites, the cation disorder issue can be solved by replacing Zn
by a distant-atom Ba to form Cu2BaSn(S,Se)4 where the large Ba atom exhibits [Ba(S/Se)8] polyhedra entirely different from the [Sn(S,Se)4] tetrahedra and the exchange of Ba and Cu/Sn is no longer energetically favored [40]. Similarly, the three cations Cu+, Pb2+, and Sb3+ exhibit totally different coordination environments (see Fig. 1a), which leads to the expectation that the formation of cation-on-cation antisites is energetically unfavored. To confirm this, we performed DFT study on the defect properties of CuPbSbS3. Fig. 2a shows the calculated charge-state transition levels of intrinsic point defects in CuPbSbS3. Among these defects, 6 defects (VPb, VCu, CuPb, PbSb, Cui, and SbPb) have relatively low ∆Hf values and are the dominant defects, while the other defects including those with deep states (e.g., VS, Pbi, SPb, VSb etc.) have high ∆Hf values (> 1eV) such that their densities are low, e.g. 106 cm−3 for room temperature process and 1014 cm−3 for 600 K process (assuming all defects formed at 600 K are frozen to room temperature). Interestingly, all the dominant defects are shallow defects, without deep states in the bandgap. VPb, VCu, CuPb, and PbSb are shallow acceptors, while Cui and SbPb act as shallow donors. These results indicate CuPbSbS3 should be defect-tolerant, which is desired for high-efficiency solar cells.
The calculated defect formation enthalpies and thus the equilibrium Fermi level (EF) depends on the chemical condition for synthesis (Fig. S4−S8). Fig. 2b, c and d show the calculated chemical potentials (∆µCu, ∆µPb, ∆µSb) at three different ∆µS, representing S-rich, S-moderate, and S-poor conditions, respectively. Under the S-rich condition, the EF is located at p-type region. At the chemical potential point 4, the EF
is determined be at 0.38 eV above the VBM (Fig. 2e), indicating suitable weak p-type conduction for efficient solar cells. As the chemical potential moving to point 1 (Fig. 2f), the EF is shifted towards the mid-bandgap due to the enhanced formation of SbPb donors and the suppressed formation of CuPb, VCu, and PbSb. Under a S-moderate condition, e.g., at the point 45 (Fig. 2g), the EF is further shifted slightly beyond the mid-bandgap (by only 0.07 eV), however, the electron density for which is too low to be measured by Hall effect. Further changing the chemical potential to S-poor does not push the EF to measurable n-type region, but pull the EF back to 0.62 eV above the VBM (Fig. 2h), due to the suppressed formation of n-type SbPb. These results indicate the semiconducting behavior of CuPbSbS3 ranges from weak p-type under the S-rich condition to intrinsic under S-moderate and S-poor conditions. Obviously, for solar cell application, the S-rich condition is preferred for synthesizing CuPbSbS3 to generate suitable carrier density.
Fig. 2. (a) Calculated charge-state transition levels of intrinsic defects in CuPbSbS3; (b‒d) 3D maps of the calculated chemical potentials (∆µCu, ∆µPb, ∆µSb) for the quaternary Cu-Pb-Sb-S system at three given ∆µSb values that represent S-rich, S-moderate, and S-poor conditions, respectively; (e‒h) Calculated formation enthalpies of intrinsic defects in CuPbSbS3 as a function of the Femi level EF at chemical potential points 4, 1, 45, and 63, respectively.
While CuPbSbS3 powder has been synthesized by methods such as solid-states reactions [25, 41] and colloidal method [26], there is no report on thin film fabrication. For a prospective solar cell absorber candidate, it is of significance to develop a facile and low-cost approach for fabricating high-quality thin film. It is well known that butyldithiocarbamate acid (BDCA), a mercapto acid obtained by combination reaction from carbon disulfide (CS2) and n-butylamine (C4H11N), can easily dissolve a variety
of metal oxides and metal hydroxides, forming sulfur-based organometallic complex precursor solutions, which have been widely used for depositing metal chalcogenide films [5, 42-45]. Herein, we present a BDCA solution process for CuPbSbS3 thin film deposition. As schematically illustrated in Fig. 3, firstly, Cu2O, PbO, and Sb2O3 were dissolved in BDCA solvent, respectively, forming sulfur-based precursor solutions of Cu, Pb, and Sb. Then the three precursor solutions were mixed in an optimized ratio of Cu:Sb:Pb = 0.9:1:1.25. Finally, the mixture was spin-coated on ITO/CdS substrate, followed by a thermal annealing process carried out inside a N2 filled glovebox.
Fig. 3. Schematic illustration for the synthesis of CuPbSbS3 film. The obtained CuPbSbS3 films show an ash black color and uniform surface morphology. Fig. 4a shows the X-ray diffraction (XRD) pattern of thin film deposited on CdS/ITO substrate (i.e., CuPbSbS3/CdS/ITO). All the diffraction peaks are attributed to the orthorhombic CuPbSbS3 (space group Pmn21), and no diffraction from impurity phases is observed. The oxidation states of Cu, Pb, Sb, and S elements were determined to be +1, +2, +3, and −2, respectively, from the X-ray photoelectron spectroscopy (XPS) measurements (Fig. S9). These results indicate single-phase
CuPbSbS3 thin films were successfully obtained by the BDCA solution method. The as-obtained CuPbSbS3 film is polycrystalline and has a smooth surface free of cracks (Fig. S10), a prerequisite to prevent short-circuit in solar cells. As seen from the thermogravimetric analysis (TGA) curve (Fig. S11), CuPbSbS3 thin films are thermally stable below 480 °C, indicating good thermostability for long-term stable solar cells. Optical properties of CuPbSbS3 film were characterized by UV-Vis-NIR. Fig. 4b shows the absorption coefficients (α) of a 250 nm thick CuPbSbS3 film, which was derived from the measured transmittance and reflection spectrum shown in Fig. S12. The α initiates from around 1.3 eV and reaches 104 and 105 cm−1 at 1.5 and 2.0 eV, respectively. The inset of Fig. 4b shows the (αhυ)2/3–hυ Tauc plot for direct forbidden transition. The direct forbidden bandgap is determined to be 1.31 eV from the linear region. From the measured ultraviolet photoelectron spectroscopy (UPS) spectrum shown in Fig. 4d, the work function (Φ) was determined to be 4.86 eV. As seen from the valence band spectrum shown in the inset of Fig. 4c, EF level is 0.38 eV above the VBM, indicating weak p-type conduction, as predicted for synthesis under S-rich condition (note the BDCA solution always contain excess sulfur and the samples are annealed in a sulfur-rich environment). From the measured Φ, EF (with respect to VBM), and Eg, the VBM and CBM levels are determined to be −5.24 and −3.93 eV, respectively.
Fig. 4. (a) XRD patterns of the CuPbSbS3 thin film on CdS/ITO substrate after two-step annealing (100 ℃ for 10 min and then 320 ℃ for 2 min); (b) Bandgap dependent absorption coefficient of CuPbSbS3 film, inset is the Tauc plot (n=2/3); (c) UPS spectra of CuPbSbS3 film, the inserts showed the magnified spectra and linear fitting in the range of -2.0~1.5 eV.
From the above results, CuPbSbS3 semiconductor exhibits natural weak p-type conductivity. It is critical to select a suitable n-type semiconductor to construct efficient heterojunction solar cells by matching the electronic band edges [46]. Herein CdS was chosen as the n-type partner, as its CBM is slightly lower than CuPbSbS3, which could facilitate the electron ejection into the ITO electrode. To facilitate the photogenerated hole transport from CuPbSbS3 absorber to Au electrode, Spiro-OMeTAD, which has a slightly more shallow VBM than CuPbSbS3, was selected as the hole transport layer. Thus, our solar cell devices were built with the glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au architecture, with the schematic and SEM cross-sectional device structures and the corresponding band energy alignment shown in Fig. 5a, b, and c, respectively. The current−voltage characteristics of our best CuPbSbS3 solar cells under 100 mW cm−2 simulated AM1.5G irradiation are shown in Fig. 5d. The as-fabricated device showed a best PCE of 2.23% (a short-circuit current density (JSC) of 8.19 mA cm−2, an open circuit voltage (VOC) of 699 mV, and a fill factor (FF) of 39%), which is high for a novel absorber material explored on the elementary stage. The mean PCE was 2.03% by averaging the 20 CuPbSbS3 thin-film cells (Fig. S13). When exposed the as-fabricated device to air, the PCE was reduced to 1.80 % after one month (Fig. S14), which may be attributed to the poor air stablity of Spiro-OMeTAD, a common issure for Spiro-OMeTAD-based solar cells [47, 48]. External quantum efficiency (EQE) spectrum of the CuPbSbS3 solar cells was also measured to study the photoresponse characteristics (Fig. 5e, solid
black line). To further validate our JSC value, we derived the current density of 9.37 mA cm−2 by integrating the EQE spectra with standard AM 1.5G, which basically agreed with the experimental value of 8.19 mA cm-2. The EQE approaches approximately 60% in the wavelength range of 450 to 500 nm, which is high for a novel absorber material. Remarkably, the EQE drops at ~700 nm (~1.8 eV), which should be attributed to the insufficient light absorption right above the bandgap (see Fig. 4b). By comparing the calculated maximum achievable EQE of CuPbSbS3 (the dashed black line in Fig. 5e) calculated from the transmission spectrum of ITO/CdS and ITO/CdS/CuPbSbS3 (Fig. S15), it is found that the n-type CdS layer also contributes to the EQE in the waveleng range of 370−500 nm. We expect that the EQE of CuPbSbS3-based solar cells may be further improved by strategies such as bulk defect reduction, interface passivation and absorber thickness optimization.
Fig. 5. Photovoltaic device structure and electrical characteristics. (a) Schematic; (b) SEM cross-sectional; and (c) band structure of CdS/CuPbSbS3 device structure; (d) J−V curves of CuPbSbS3 solar cell performance in the dark (cyan line) and under AM 1.5G irradiation (red line); (e) EQE and integrated JSC of the as-fabricated CdS/CuPbSbS3 device (solid black line and blue line). For comparison, the calculated maximum achievable EQE of CuPbSbS3 film is shown by the dashed black line. 3. Conclusions In summary, we have demonstrated that CuPbSbS3 is prospective absorber material for low-cost and high-efficiency solar cells by a combination of DFT and experimental study. Our DFT calculations suggest that CuPbSbS3 exhibit 3D electronic dimensionality (a prerequisite for high-efficiency solar cells), with a suitable direct bandgap of 1.3 eV, high optical absorption efficients, appropriate p-type conductivity, and defect tolerance. We developed a low-cost and facile solution process to obtain phase-pure, crack-free, and compact CuPbSbS3 thin films with grains in micrometer level. The CuPbSbS3-based thin film solar cells with the glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au
architecture
achieved
a
high
preliminary PCE of 2.23% and a high open circuit voltage of 699 mV, indicative of a bright future for further efficiency improvement.
4. Methods 4.1 Materials and chemicals High-purity Cu2O (Aladdin, 99.999%), PbO (Aladdin, 99.999%), Sb2O3 (Sinopharm, 99.999%), CS2, (Sinopharm, 99.99%), C4H11N (Sinopharm, 99.99%), ethanol
(Sinopharm,
99.99%),
Spiro-OMeTAD
(Feiming,
99.999%),
4-tert-butylpyridine (tBP) (Sigma, 99.99%), lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI) (Sigma, 99.99%), acetonitrile (Sigma, 99.99%), and chlorobenzene (Sigma, 99.99%) were used as received without any further purification.
4.2 Preparation of CuPbSbS3 precursor solution 0.5723 g (4 mmol) of Cu2O was added into a mixture of 5 mL of CS2 and 10 mL of ethanol. Afterwards, 8 ml of C4H11N was dripped into the mixture with a speed of 2 drops/second under high-speed magnetic stirring, followed by a further stirring for 10 min. The Pb and Sb precursor solutions containing 0.6696 g (3 mmol) of PbO and 1.166 g (4 mmol) of Sb2O3, respectively, were prepared with similar procedure. Subsequently, measured Cu, Pb, and Sb precursor solutions were mixed with a mole ratio of approximately 0.9:1:1.25 and stirred for 30 min to produce the solution for subsequent spin-coating.
4.3 CuPbSbS3 film and device fabrication CuPbSbS3 films were deposited on prepared 2.5×2.5 cm2 glass/ITO/CdS substrates by spin-coating (800 rpm for 10 s, 2500 rpm for 60 s), in which the CdS layer was deposited by chemical bath deposition [49]. Then, the film was preheated on a hot plate at 100 ℃ for 10 min and subsequently annealed at 320 ℃ for 2 min. The as-fabricated glass/ITO/CdS/CuPbSbS3 substrates were used for the characterization of
CuPbSbS3.
Solar
cell
devices
were
fabricated
with
the
glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au architecture. Spiro-OMeTAD solution,
prepared by mixing 73 mg of Spiro-OMeTAD powder, 30 µL of tBP, 19 µL of 520 mg mL−1 prepared solution of Li-TFSI in acetonitrile, and 1 mL of chlorobenzene, was deposited on glass/ITO/CdS/CuPbSbS3 substrate by spin-coating at a speed of 2500 rpm for 30 s. The Au electrodes (≈100 nm) were thermally evaporated through a standard mask on the top of the devices. All the film and device fabrication processes except for electrode deposition were conducted in glovebox filled with dry pure nitrogen
(H2O,
O2
<
0.1
ppm).
Each
substrate
has
9
standard
Glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au devices with active area of 0.095 cm2 (Fig. S16).
4.4 Materials and device characterization The phase determination of the CuPbSbS3 films was carried out by XRD (Philips, X pert pro MRD, with Cu Kα radiation, λ = 1.54 Å). The oxidation states of Cu, Pb, Sb, and S elements were determined by the XPS using Al Kα excitation (EDAX Inc. Genesis, 300 W). The morphology of CuPbSbS3 film was observed by SEM (FEI Nova NanoSEM450, with Au coating). TGA (ProRyan Corporation, Mettler toledo, TG/DSC1, 10 ℃/min, N2 flow) was conducted to study the weight loss behavior of naturally dried CuPbSbS3 precursor solution upon increasing temperatures. Transmittance (T) spectrum of CuPbSbS3 thin films was measured by UV-vis-near IR spectrophotometer (Perkin-Elmer Instruments, Lambda 950 using integrating sphere). The absorption coefficient (α) was estimated by α = ln(1/T)/d, where d is the film thickness determined from the SEM cross-section image of CuPbSbS3 film. UPS
(Specs UVLS, He I excitation, 21.21 eV, referenced to the Femi edge of argon etched gold) was performed for measuring the work function and the valence band structure. The performance of CuPbSbS3-based solar cell devices was measured by Keithley 2420 source-meter under a solar simulator with an Xe light source (450 W, Oriel, model 9119) and an Air Mass 1.5G filter, offering simulated 1 standard Sun illumination. The external quantum efficiency (EQE) curve were measured using the light source generated by a 300 W xenon lamp of Newport (Oriel, 69911) and then split into specific wavelengths by a Newport Oriel Cornerstone 130 1/8 Monochromator (Oriel, model 74004).
4.5 DFT calculations DFT calculations were performed using the projector-augmented wave (PAW) method and either the Perdew−Burke−Ernzerhof gereralized gradient approximation (PBE-GGA) functional [50] or the Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional [51], as implemented in the VASP 5.4 code [52]. The PBE-GGA and a plane-wave cutoff energy of 500 eV were used for the geometry optimization of perfect crystals and defect models, while the HSE and a cutoff energy of 400 eV was used for the calculations of fundamental and defect properties. Particularly, for the fundamental and defect properties of CuPbSbS3, the SOC effect was included to correctly describe the Pb 6p states. Primitive unit cells and Γ-centered k-meshes with k-spacing of 0.2 Å−1 were used for perfect crystals, while a 192-atom supercell (i.e., with 2×2×2 primitive cells) and the Γ-only k-mesh were used for defects. For the
geometry optimization of defects, the atomic coordination was relaxed, while the lattice parameters were constrained at the PBE-relaxed values (a = 8.284 Å, b = 9.029 Å and c = 7.853 Å) for the perfect crystals to meet the dilute limit condition. For charged defects, the common posteriori corrections (including image-charge, potential alignment and band filling corrections) were applied to the calculated total energies to correct the finite-size effects using the scheme in literature [53, 54]. The calculated dielectric constants (εxx = 7.4602, εyy = 7.2527, and εzz = 7.5647) are used for the image charge correction. For a defect (D) in a charge state q, the formation enthalpy ∆HD,q (µ, EF) is calculated through the equation ∆HD,q (µ, EF) = (ED,q ‒ EH) ‒ Σnαµα + q(EF+EV),
(1)
where (ED,q ‒ EH) is the energy difference between the defect and host supercells. nα indicates the number of α atoms added (nα > 0) or removed (nα < 0) when a defect is formed, and µα is the chemical potential of the α atom, which can be expressed with respect to that of the elemental phase (µαel) by µα = µαel + ∆µα. EF is the Fermi level referred to the host VBM, EV. The µα value varies depending on the experimental synthesis conditions, and the allowed µα values are constrained within the polygons of CuPbSbS3 in the Cu‒Pb‒Sb‒S quaternary system drawn using the CHESTA code [55], where all competing phases that can be retrieved in the Inorganic Crystal Structure Database (ICSD) are considered. From the calculated ∆HD,q, the defect transition level ε(q/q’) between two charge states q and q’ is obtained as the EF where ∆HD,q(EF) = ∆HD,q’(EF).
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (61725401), the National Key R&D Program of China (2016YFA0204000), the China Postdoctoral Science Foundation (2018M642825), and the HUST Key Innovation Team for Interdisciplinary Promotion (2016JCTD111, 2017KFXKJC003). Z. Xiao acknowledges the supports from the Thousand Young Talents Program of China, the Startup Fund of Huazhong University of Science and Technology (HUST) and the Director Fund of Wuhan National Laboratory for Optoelectronics (WNLO). The authors thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices (CNCD), WNLO-HUST.
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Dr. Yuhao Liu obtained his Bachelor’s degree from Ludong University of Physics and Optical Engineering in 2012/06. From 2012/09 to 2017/06, he studied at the College of Physics at Huazhong University of Science and Technology, and he received his Ph.D. degree in 2017/6. Currently, he is a post-doctoral fellow in Wuhan National Laboratory for Optoelectronics (WNLO) at Huazhong University of Science and Technology. His research interest is Sb-based thin film solar cells fabricated by solution methods.
Dr. Bo Yang received his Ph.D. degree in Wuhan National Laboratory for Optoelectronics (WNLO) from Huazhong University of Science and Technology, Wuhan, China. He is currently a postdoc in the WNLO. His current research interests include novel thin film solar cells and X-ray detection devices.
Muyi Zhang received his B. E. degree at China University of Petroleum in 2017. He is pursuing his M. S. degrees of both Huazhong University of Science & Technology and MINES ParisTech PSL currently. He focuses on the research of photoelectric materials and devices as well as the X-Ray detector at Wuhan National Laboratory for Optoelectronics.
Bing Xia received his B. E. degree of Optoelectronics Information Science and Engineering from Huazhong University of Science and Technology in June 2019. He is pursuing his M. S. degrees of Wuhan National Laboratory for Optoelectronics (WNLO). His research interest focus on theoretical modeling and design of novel chalcogenide solar cell absorber materials.
Dr. Chao Chen received his B.S. degree in School of Physics at Huazhong University of Science and Technology in 2014/06. From 2014/09-2019/02, he studied in Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology as a doctoral candidate and received his Ph.D. degree in 2019/02. Currently, he is a post-doctor in Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology. His research interests are thin film solar cells and photodetectors.
Xueping Liu received her Master’s degree in Materials Science and Engineering from Wuhan university of technology. She is currently a PhD candidate in Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK. Her research interest is developing new materials and engineering for high performance perovskite devices.
Prof. Jie Zhong received his Ph.D. in December 2011 in Central South University. In 2015, he joined State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology as associate professor. His research interest is focus on developing new materials and techniques for solution-processed high efficiency flexible organic-inorganic hybrid perovskite solar cells.
Prof. Zewen Xiao received his Bachelor and Master degrees from Xi’an Jiaotong University in 2010 and 2012, respectively, and his Ph.D. degree in Materials Science from Tokyo Institute of Technology in 2015. He worked as a postdoctoral fellow at The University of Toledo from 2015 to 2016. Then, he worked as a specially appointed assistant professor at Tokyo Institute of Technology from 2016 to 2018. Since 2018, he holds a full professor position in Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology. His Materials Innovation Group focuses on designing novel optoelectronic semiconductors for applications such as solar cells and light emission by a combined theoretical and experimental approach. He was awarded “National 1000 Young Talents” project in 2019. He is an editorial board member of Frontiers of Optoelectronics.
Prof. Jiang Tang received his BSc (2003) from University of Science and Technology of China and Ph.D (2010) from University of Toronto under the supervision of Prof. Edward Sargent. After one year and half postdoctoral research at IBM T.J. Watson Research Center, he joined in Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology as a full professor. His research interests are exploration of new semiconductors for optoelectronic devices. Specifically, he pioneered in antimony selenide (Sb2Se3) thin film solar cells, constructed lead-free halide perovskites X-ray detectors with low detection limit, and developed stable and efficient white emissive halide perovskites. He has published more than 100 papers in well-known journals such as Nature, Nature Materials, Nature Photonics, Nature Energy, Nature Communications and so on, with a total citation of more than 9,000. He was awarded the NSFC fund for distinguished young scholars. He is the editor of Frontiers of Optoelectronics, Editorial Advisor Board member of Solar RRL and has delivered more than 50 invited or plenary talks at various international conference or prestigious universities.
Highlights
DFT calculations show that CuPbSbS3 exhibits a 3D electronic dimensionality, a suitable direct bandgap of 1.3 eV, and high optical absorption coefficients. Defect calculations indicate that CuPbSbS3 exhibits weak p-type conduction under S-rich condition, which is suitable for heterojunction solar cell. Phase pure, large grained CuPbSbS3 thin films were fabricated by the BDCA solution method. CuPbSbS3 solar cells were constructed for the first time, achieving 2.23% device efficiency.
Declaration of Interest Statement There are no conflicts to declare.
S2211-2855(20)30132-4
DOI:
https://doi.org/10.1016/j.nanoen.2020.104574
Reference:
NANOEN 104574
To appear in:
Nano Energy
Received Date: 16 December 2019 Accepted Date: 3 February 2020
Please cite this article as: Y. Liu, B. Yang, M. Zhang, B. Xia, C. Chen, X. Liu, J. Zhong, Z. Xiao, J. Tang, Bournonite CuPbSbS3: an electronically-3D, defect-tolerant, and solution-processable semiconductor for efficient solar cells, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104574. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Graphical abstract
Bournonite
CuPbSbS3:
an
electronically-3D, defect-tolerant, and
solution-processable semiconductor for efficient solar cells Yuhao Liu†a, Bo Yang†a, Muyi Zhanga, Bing Xiaa, Chao Chena, Xueping Liuc, Jie Zhongc, Zewen Xiaoa,*, Jiang Tanga,b,* a
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
b
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China c
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China *Corresponding Authors: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail addresses: [email protected] (Z. Xiao); [email protected] (J. Tang)
† These authors contributed equally to this work. Keywords: Bournonite, BDCA solution, thin film solar cell, electronic dimensionality, defect tolerance
Abstract The absorber layer is a key component of thin-film solar cells. Based-on the recently-proposed electronic dimensionality concept, a promising solar cell absorber material should be electronically high-dimensional, as is the case for all the mainstream absorbers, such as Si, GaAs, CIGS, CdTe, and CH3NH3PbI3. In this work, we propose an electronically three-dimensional semiconductor, bournonite CuPbSbS3, as a prospective efficient solar cell absorber material. Our density functional theory calculations indicate that CuPbSbS3 exhibits desired optoelectronic properties, such as
a nearly direct bandgap, high optical absorption coefficients, appropriate p-type doping, and defect tolerance. Experimentally, we developed a butyldithiocarbamate acid (BDCA) solution process for depositing high-quality CuPbSbS3 thin films and built the first CuPbSbS3 solar cells. Our CuPbSbS3-based thin film solar cells achieved a preliminary power conversion efficiency of 2.23% (open circuit voltage of 699 mV), highlighting the potential of this semiconductor for thin film photovoltaics.
1. Introduction In the past few decades, great progress has been made for thin-film solar cells, the second generation photovoltaic technique. Particularly, solar cells based on Cu(In,Ga)Se2 (CIGS), CdTe, and CH3NH3PbI3 absorbers have achieved high power conversion efficiency (PCE) of 22.6%, 22.1%, and 25.2%, respectively [1-3]. However, while lead halide perovskites has the long-term stability issue, CIGS and CdTe suffer from the scarcity of raw materials (In in CIGS and Te in CdTe), limiting their terawatt-scale commercial application. Therefore, low-cost earth-abundant and stable absorber materials for thin film solar cells need to be explored. While no good solutions have been found yet for stabilizing CH3NH3PbI3 for several years and for replacing the Te in CdTe, the trivalent In in CIGS can be mutated by two earth-abundant elements, Zn and Sn, to form kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) [4-6]. However, the record efficiency of kesterite solar cells has been stagnated at 12.6% after years of extensive study [7], significantly lower than the theoretical efficiency limit based solely on the bandgap consideration. The
efficiencies of kesterite solar cells are mainly limited by the large open-circuit voltage (VOC) deficit, defined as Eg/q − VOC, where Eg is the band gap of the absorber and q is the electron charge [8, 9]. Density functional theory studies reveal that the large VOC deficits for kesterite solar cells are attributed to the high density of cation-on-cation antisites which introduce absorption band-tail states and act as nonradiative recombination centers [10]. On the other hand, In can also be directly replaced by trivalent elements such as Bi and Sb, forming the layered ternary Cu(Sb,Bi)(S,Se)2 compounds, which, however, exhibit indirect bandgaps and undesired anisotropic carrier transport and optical properties [11-13].
Based on the recently-proposed concept of electronic dimensionality [14], a promising photovoltaic absorber should have high electronic dimensionality, which is the case for all efficient solar cells reported so far. It is noticeable that the three-dimensional crystal structure is a prerequisite for high electronic dimensionality. In fact, all the mainstream thin-film solar cell absorber materials, such as CIGS, CdTe, and CH3NH3PbI3, have 3D crystal structures [15-19]. Unfortunately, the above-mentioned Cu(Sb,Bi)(S,Se)2 compounds as well as many other earth-abundant post-transition metal chalcogenides (e.g., SnS, Sb2Se3, NaSbS2, etc.) crystallize into low-dimensional crystal structures, which is attributed to the structural distortion that necessarily occurs for stabilizing the filled lone-pair s2 states [20-22]. In fact, despite the remarkable progress, the record efficiencies for these low-dimensional chalcogenide absorber materials are far lower than the mainstream absorber materials.
Therefore, it is still of significance to research for efficient earth-abundant solar cell absorber materials, particularly from those which are both structurally and electronically 3D.
It is noted that among binary heavy post-transition metal chalcogenides, Pb-containing chalcogenides are the few ones that crystallize into 3D rocksalt-type crystal structure with a higher coordination number of 6, due to the less active lone pair states of Pb [23]. We expect that the incorporation of PbS into layered chalcogenides might reconstruct the crystal structures to form 3D multi-component chalcogenides. Using Cu(Sb,Bi)S2, which have been intensively studied as absorber materials, as prototypes, incorporating PbS leads to the formation of 3D CuPb(Sb,Bi)S3, as shown Fig. 1a. In this study, we focus on CuPbSbS3, a natural mineral named as bournonite, which has a bandgap of 1.3 eV suitable for single-junction solar application. Herein, we particularly emphasize, by density functional theory (DFT) calculations, that CuPbSbS3 is electronically 3D, with desired optoelectronic properties for solar cell application, such as a nearly direct bandgap, good carrier transport properties, and high optical absorption coefficients. Additionally, our defect calculations predict CuPbSbS3 is defect-tolerant and exhibits natural weak p-type conduction if synthesized under S-rich conditions. We developed a low-cost and facile solution process to obtain phase-pure, crack-free, and compact CuPbSbS3 thin films with grains in micrometer level. The CuPbSbS3-based thin film solar cells with the glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au architecture
achieved a high preliminary PCE of 2.23%, suggestive of a bright future for further material investigation and solar cell optimization.
2. Results and discussions We first show, by DFT calculations, that CuPbSbS3 is not only structurally 3D, but also electronically 3D, and has much better optoelectronic properties as compared with layered CuSbS2. As seen in Fig. 1b, for CuSbS2, the bandgap is indirect, with the HSE-calculated value of 1.56 eV, which is close to the reported experimental value [24]. Such indirect bandgap is typical for layered chalcogenide semiconductors with lone-pair cations [23], and is not ideal for thin-film solar cell application. For 3D CuPbSbS3, the HSE-calculated bandgap is direct, with both the VBM and CBM located at the Γ point (see Fig. S1 in the Supporting Information). The HSE-calculated bandgap value is 1.61 eV , larger than the reported experimental values of 1.20−1.31 eV [25, 26]. The bandgap overestimation should be caused by the neglect of the spin−orbit coupling (SOC) effect, which is profound for compounds containing heavy post-transition elements like Pb [27]. By including the SOC, the bandgap of CuPbSbS3 is reduced to 1.29 eV (see the red arrow in Fig. 1c), which is attributed to the Pb 6p state splitting-induced CBM lowering. Additionally, because of the lack of inversion symmetry in CuPbSbS3 (the space group is Pmn21), the SOC induces profound Rashba splitting, with the doubly spin-degenerated band splitting into two bands shifted with respect to each other. As a result, the CBM is slightly off the Γ point, leading to a weakly indirect bandgap. The direct bandgap at the Γ point is 1.37
eV, only 0.08 eV larger than the indirect bandgap. Nevertheless, CuPbSbS3 can be regared as a nearly direct bandgap semiconductor, similar to the case of CH3NH3PbI3 [28-30]. These results are consistent with those in an early theoritical study by Wallace et al [31]. However, for CuPbSbS3, the direct bandgap transition at the Γ point is forbidden (Fig. S2). The optical transition gradually becomes allowed when move off the Γ point . As a result, the optical absorption coefficient right above the bandgap is relatively low (Fig. S3). Interestingly, the Rashba effect in Pb halide perovskites has been proposed to contribute to the experimentally-observed long carrier lifetimes [32-34], which leads to the expectation that the Rashba effect in CuPbSbS3 may also help to prolong the carrier lifetime for high photovoltaic performance. As seen from the projected densities of states (DOSs) (Fig. 1d), the VBM consists of the antibonding states of S 3p and Cu 3d/Pb 6s/Sb 5s orbitals, while the CBM is composed of the antibonding states of Cu 4s/Pb 6p/Sb 6p and S 3p orbitals. That is, all atoms in CuPbSbS3 contribute to the VBM and CBM, indicating a 3D electronic dimensionality. Because of the high electronic dimensionality, the band edges of CuPbSbS3 are dispersive, indicating good carrier transport properties.
Fig. 1. (a) Schematic illustration of crystal structure revolution from layered CuSbS2 to 3D CuPbSbS3 by incorporating PbS; (b) Band structure of CuSbS2 calculated with HSE hybrid functional. (c) Band structure of CuPbSbS3 calculated with the HSE hybrid functional and the inclusion of spin−orbit coupling effect. Transition at Γ point is direct but forbidden. (d) HSE+SOC-calculated total and projected densities of states (DOSs) of CuPbSbS3. Note that the DOSs of Cu 4s and 4p orbitals are magnified by 10 times for a clearer view.
Defect properties of an absorber materials must be appropriate also for producing efficient solar cells. It is known that some quaternary compounds like kesterites Cu2ZnSn(S,Se)4 [35-37] and double perovskites Cs2(Ag,In,Tl)BiBr6 [38, 39] suffer from the easy formation of deep cation-on-cation antisites, because the Cu, Zn, and Sn in Cu2ZnSn(S,Se)4 have similar tetrahedrally-bonded environments while the Ag/In/Tl and Bi in Cs2(Ag,In,Tl)BiBr6 exhibit similar octahedrally-bonded environments. For kesterites, the cation disorder issue can be solved by replacing Zn
by a distant-atom Ba to form Cu2BaSn(S,Se)4 where the large Ba atom exhibits [Ba(S/Se)8] polyhedra entirely different from the [Sn(S,Se)4] tetrahedra and the exchange of Ba and Cu/Sn is no longer energetically favored [40]. Similarly, the three cations Cu+, Pb2+, and Sb3+ exhibit totally different coordination environments (see Fig. 1a), which leads to the expectation that the formation of cation-on-cation antisites is energetically unfavored. To confirm this, we performed DFT study on the defect properties of CuPbSbS3. Fig. 2a shows the calculated charge-state transition levels of intrinsic point defects in CuPbSbS3. Among these defects, 6 defects (VPb, VCu, CuPb, PbSb, Cui, and SbPb) have relatively low ∆Hf values and are the dominant defects, while the other defects including those with deep states (e.g., VS, Pbi, SPb, VSb etc.) have high ∆Hf values (> 1eV) such that their densities are low, e.g. 106 cm−3 for room temperature process and 1014 cm−3 for 600 K process (assuming all defects formed at 600 K are frozen to room temperature). Interestingly, all the dominant defects are shallow defects, without deep states in the bandgap. VPb, VCu, CuPb, and PbSb are shallow acceptors, while Cui and SbPb act as shallow donors. These results indicate CuPbSbS3 should be defect-tolerant, which is desired for high-efficiency solar cells.
The calculated defect formation enthalpies and thus the equilibrium Fermi level (EF) depends on the chemical condition for synthesis (Fig. S4−S8). Fig. 2b, c and d show the calculated chemical potentials (∆µCu, ∆µPb, ∆µSb) at three different ∆µS, representing S-rich, S-moderate, and S-poor conditions, respectively. Under the S-rich condition, the EF is located at p-type region. At the chemical potential point 4, the EF
is determined be at 0.38 eV above the VBM (Fig. 2e), indicating suitable weak p-type conduction for efficient solar cells. As the chemical potential moving to point 1 (Fig. 2f), the EF is shifted towards the mid-bandgap due to the enhanced formation of SbPb donors and the suppressed formation of CuPb, VCu, and PbSb. Under a S-moderate condition, e.g., at the point 45 (Fig. 2g), the EF is further shifted slightly beyond the mid-bandgap (by only 0.07 eV), however, the electron density for which is too low to be measured by Hall effect. Further changing the chemical potential to S-poor does not push the EF to measurable n-type region, but pull the EF back to 0.62 eV above the VBM (Fig. 2h), due to the suppressed formation of n-type SbPb. These results indicate the semiconducting behavior of CuPbSbS3 ranges from weak p-type under the S-rich condition to intrinsic under S-moderate and S-poor conditions. Obviously, for solar cell application, the S-rich condition is preferred for synthesizing CuPbSbS3 to generate suitable carrier density.
Fig. 2. (a) Calculated charge-state transition levels of intrinsic defects in CuPbSbS3; (b‒d) 3D maps of the calculated chemical potentials (∆µCu, ∆µPb, ∆µSb) for the quaternary Cu-Pb-Sb-S system at three given ∆µSb values that represent S-rich, S-moderate, and S-poor conditions, respectively; (e‒h) Calculated formation enthalpies of intrinsic defects in CuPbSbS3 as a function of the Femi level EF at chemical potential points 4, 1, 45, and 63, respectively.
While CuPbSbS3 powder has been synthesized by methods such as solid-states reactions [25, 41] and colloidal method [26], there is no report on thin film fabrication. For a prospective solar cell absorber candidate, it is of significance to develop a facile and low-cost approach for fabricating high-quality thin film. It is well known that butyldithiocarbamate acid (BDCA), a mercapto acid obtained by combination reaction from carbon disulfide (CS2) and n-butylamine (C4H11N), can easily dissolve a variety
of metal oxides and metal hydroxides, forming sulfur-based organometallic complex precursor solutions, which have been widely used for depositing metal chalcogenide films [5, 42-45]. Herein, we present a BDCA solution process for CuPbSbS3 thin film deposition. As schematically illustrated in Fig. 3, firstly, Cu2O, PbO, and Sb2O3 were dissolved in BDCA solvent, respectively, forming sulfur-based precursor solutions of Cu, Pb, and Sb. Then the three precursor solutions were mixed in an optimized ratio of Cu:Sb:Pb = 0.9:1:1.25. Finally, the mixture was spin-coated on ITO/CdS substrate, followed by a thermal annealing process carried out inside a N2 filled glovebox.
Fig. 3. Schematic illustration for the synthesis of CuPbSbS3 film. The obtained CuPbSbS3 films show an ash black color and uniform surface morphology. Fig. 4a shows the X-ray diffraction (XRD) pattern of thin film deposited on CdS/ITO substrate (i.e., CuPbSbS3/CdS/ITO). All the diffraction peaks are attributed to the orthorhombic CuPbSbS3 (space group Pmn21), and no diffraction from impurity phases is observed. The oxidation states of Cu, Pb, Sb, and S elements were determined to be +1, +2, +3, and −2, respectively, from the X-ray photoelectron spectroscopy (XPS) measurements (Fig. S9). These results indicate single-phase
CuPbSbS3 thin films were successfully obtained by the BDCA solution method. The as-obtained CuPbSbS3 film is polycrystalline and has a smooth surface free of cracks (Fig. S10), a prerequisite to prevent short-circuit in solar cells. As seen from the thermogravimetric analysis (TGA) curve (Fig. S11), CuPbSbS3 thin films are thermally stable below 480 °C, indicating good thermostability for long-term stable solar cells. Optical properties of CuPbSbS3 film were characterized by UV-Vis-NIR. Fig. 4b shows the absorption coefficients (α) of a 250 nm thick CuPbSbS3 film, which was derived from the measured transmittance and reflection spectrum shown in Fig. S12. The α initiates from around 1.3 eV and reaches 104 and 105 cm−1 at 1.5 and 2.0 eV, respectively. The inset of Fig. 4b shows the (αhυ)2/3–hυ Tauc plot for direct forbidden transition. The direct forbidden bandgap is determined to be 1.31 eV from the linear region. From the measured ultraviolet photoelectron spectroscopy (UPS) spectrum shown in Fig. 4d, the work function (Φ) was determined to be 4.86 eV. As seen from the valence band spectrum shown in the inset of Fig. 4c, EF level is 0.38 eV above the VBM, indicating weak p-type conduction, as predicted for synthesis under S-rich condition (note the BDCA solution always contain excess sulfur and the samples are annealed in a sulfur-rich environment). From the measured Φ, EF (with respect to VBM), and Eg, the VBM and CBM levels are determined to be −5.24 and −3.93 eV, respectively.
Fig. 4. (a) XRD patterns of the CuPbSbS3 thin film on CdS/ITO substrate after two-step annealing (100 ℃ for 10 min and then 320 ℃ for 2 min); (b) Bandgap dependent absorption coefficient of CuPbSbS3 film, inset is the Tauc plot (n=2/3); (c) UPS spectra of CuPbSbS3 film, the inserts showed the magnified spectra and linear fitting in the range of -2.0~1.5 eV.
From the above results, CuPbSbS3 semiconductor exhibits natural weak p-type conductivity. It is critical to select a suitable n-type semiconductor to construct efficient heterojunction solar cells by matching the electronic band edges [46]. Herein CdS was chosen as the n-type partner, as its CBM is slightly lower than CuPbSbS3, which could facilitate the electron ejection into the ITO electrode. To facilitate the photogenerated hole transport from CuPbSbS3 absorber to Au electrode, Spiro-OMeTAD, which has a slightly more shallow VBM than CuPbSbS3, was selected as the hole transport layer. Thus, our solar cell devices were built with the glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au architecture, with the schematic and SEM cross-sectional device structures and the corresponding band energy alignment shown in Fig. 5a, b, and c, respectively. The current−voltage characteristics of our best CuPbSbS3 solar cells under 100 mW cm−2 simulated AM1.5G irradiation are shown in Fig. 5d. The as-fabricated device showed a best PCE of 2.23% (a short-circuit current density (JSC) of 8.19 mA cm−2, an open circuit voltage (VOC) of 699 mV, and a fill factor (FF) of 39%), which is high for a novel absorber material explored on the elementary stage. The mean PCE was 2.03% by averaging the 20 CuPbSbS3 thin-film cells (Fig. S13). When exposed the as-fabricated device to air, the PCE was reduced to 1.80 % after one month (Fig. S14), which may be attributed to the poor air stablity of Spiro-OMeTAD, a common issure for Spiro-OMeTAD-based solar cells [47, 48]. External quantum efficiency (EQE) spectrum of the CuPbSbS3 solar cells was also measured to study the photoresponse characteristics (Fig. 5e, solid
black line). To further validate our JSC value, we derived the current density of 9.37 mA cm−2 by integrating the EQE spectra with standard AM 1.5G, which basically agreed with the experimental value of 8.19 mA cm-2. The EQE approaches approximately 60% in the wavelength range of 450 to 500 nm, which is high for a novel absorber material. Remarkably, the EQE drops at ~700 nm (~1.8 eV), which should be attributed to the insufficient light absorption right above the bandgap (see Fig. 4b). By comparing the calculated maximum achievable EQE of CuPbSbS3 (the dashed black line in Fig. 5e) calculated from the transmission spectrum of ITO/CdS and ITO/CdS/CuPbSbS3 (Fig. S15), it is found that the n-type CdS layer also contributes to the EQE in the waveleng range of 370−500 nm. We expect that the EQE of CuPbSbS3-based solar cells may be further improved by strategies such as bulk defect reduction, interface passivation and absorber thickness optimization.
Fig. 5. Photovoltaic device structure and electrical characteristics. (a) Schematic; (b) SEM cross-sectional; and (c) band structure of CdS/CuPbSbS3 device structure; (d) J−V curves of CuPbSbS3 solar cell performance in the dark (cyan line) and under AM 1.5G irradiation (red line); (e) EQE and integrated JSC of the as-fabricated CdS/CuPbSbS3 device (solid black line and blue line). For comparison, the calculated maximum achievable EQE of CuPbSbS3 film is shown by the dashed black line. 3. Conclusions In summary, we have demonstrated that CuPbSbS3 is prospective absorber material for low-cost and high-efficiency solar cells by a combination of DFT and experimental study. Our DFT calculations suggest that CuPbSbS3 exhibit 3D electronic dimensionality (a prerequisite for high-efficiency solar cells), with a suitable direct bandgap of 1.3 eV, high optical absorption efficients, appropriate p-type conductivity, and defect tolerance. We developed a low-cost and facile solution process to obtain phase-pure, crack-free, and compact CuPbSbS3 thin films with grains in micrometer level. The CuPbSbS3-based thin film solar cells with the glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au
architecture
achieved
a
high
preliminary PCE of 2.23% and a high open circuit voltage of 699 mV, indicative of a bright future for further efficiency improvement.
4. Methods 4.1 Materials and chemicals High-purity Cu2O (Aladdin, 99.999%), PbO (Aladdin, 99.999%), Sb2O3 (Sinopharm, 99.999%), CS2, (Sinopharm, 99.99%), C4H11N (Sinopharm, 99.99%), ethanol
(Sinopharm,
99.99%),
Spiro-OMeTAD
(Feiming,
99.999%),
4-tert-butylpyridine (tBP) (Sigma, 99.99%), lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI) (Sigma, 99.99%), acetonitrile (Sigma, 99.99%), and chlorobenzene (Sigma, 99.99%) were used as received without any further purification.
4.2 Preparation of CuPbSbS3 precursor solution 0.5723 g (4 mmol) of Cu2O was added into a mixture of 5 mL of CS2 and 10 mL of ethanol. Afterwards, 8 ml of C4H11N was dripped into the mixture with a speed of 2 drops/second under high-speed magnetic stirring, followed by a further stirring for 10 min. The Pb and Sb precursor solutions containing 0.6696 g (3 mmol) of PbO and 1.166 g (4 mmol) of Sb2O3, respectively, were prepared with similar procedure. Subsequently, measured Cu, Pb, and Sb precursor solutions were mixed with a mole ratio of approximately 0.9:1:1.25 and stirred for 30 min to produce the solution for subsequent spin-coating.
4.3 CuPbSbS3 film and device fabrication CuPbSbS3 films were deposited on prepared 2.5×2.5 cm2 glass/ITO/CdS substrates by spin-coating (800 rpm for 10 s, 2500 rpm for 60 s), in which the CdS layer was deposited by chemical bath deposition [49]. Then, the film was preheated on a hot plate at 100 ℃ for 10 min and subsequently annealed at 320 ℃ for 2 min. The as-fabricated glass/ITO/CdS/CuPbSbS3 substrates were used for the characterization of
CuPbSbS3.
Solar
cell
devices
were
fabricated
with
the
glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au architecture. Spiro-OMeTAD solution,
prepared by mixing 73 mg of Spiro-OMeTAD powder, 30 µL of tBP, 19 µL of 520 mg mL−1 prepared solution of Li-TFSI in acetonitrile, and 1 mL of chlorobenzene, was deposited on glass/ITO/CdS/CuPbSbS3 substrate by spin-coating at a speed of 2500 rpm for 30 s. The Au electrodes (≈100 nm) were thermally evaporated through a standard mask on the top of the devices. All the film and device fabrication processes except for electrode deposition were conducted in glovebox filled with dry pure nitrogen
(H2O,
O2
<
0.1
ppm).
Each
substrate
has
9
standard
Glass/ITO/CdS/CuPbSbS3/Spiro-OMeTAD/Au devices with active area of 0.095 cm2 (Fig. S16).
4.4 Materials and device characterization The phase determination of the CuPbSbS3 films was carried out by XRD (Philips, X pert pro MRD, with Cu Kα radiation, λ = 1.54 Å). The oxidation states of Cu, Pb, Sb, and S elements were determined by the XPS using Al Kα excitation (EDAX Inc. Genesis, 300 W). The morphology of CuPbSbS3 film was observed by SEM (FEI Nova NanoSEM450, with Au coating). TGA (ProRyan Corporation, Mettler toledo, TG/DSC1, 10 ℃/min, N2 flow) was conducted to study the weight loss behavior of naturally dried CuPbSbS3 precursor solution upon increasing temperatures. Transmittance (T) spectrum of CuPbSbS3 thin films was measured by UV-vis-near IR spectrophotometer (Perkin-Elmer Instruments, Lambda 950 using integrating sphere). The absorption coefficient (α) was estimated by α = ln(1/T)/d, where d is the film thickness determined from the SEM cross-section image of CuPbSbS3 film. UPS
(Specs UVLS, He I excitation, 21.21 eV, referenced to the Femi edge of argon etched gold) was performed for measuring the work function and the valence band structure. The performance of CuPbSbS3-based solar cell devices was measured by Keithley 2420 source-meter under a solar simulator with an Xe light source (450 W, Oriel, model 9119) and an Air Mass 1.5G filter, offering simulated 1 standard Sun illumination. The external quantum efficiency (EQE) curve were measured using the light source generated by a 300 W xenon lamp of Newport (Oriel, 69911) and then split into specific wavelengths by a Newport Oriel Cornerstone 130 1/8 Monochromator (Oriel, model 74004).
4.5 DFT calculations DFT calculations were performed using the projector-augmented wave (PAW) method and either the Perdew−Burke−Ernzerhof gereralized gradient approximation (PBE-GGA) functional [50] or the Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional [51], as implemented in the VASP 5.4 code [52]. The PBE-GGA and a plane-wave cutoff energy of 500 eV were used for the geometry optimization of perfect crystals and defect models, while the HSE and a cutoff energy of 400 eV was used for the calculations of fundamental and defect properties. Particularly, for the fundamental and defect properties of CuPbSbS3, the SOC effect was included to correctly describe the Pb 6p states. Primitive unit cells and Γ-centered k-meshes with k-spacing of 0.2 Å−1 were used for perfect crystals, while a 192-atom supercell (i.e., with 2×2×2 primitive cells) and the Γ-only k-mesh were used for defects. For the
geometry optimization of defects, the atomic coordination was relaxed, while the lattice parameters were constrained at the PBE-relaxed values (a = 8.284 Å, b = 9.029 Å and c = 7.853 Å) for the perfect crystals to meet the dilute limit condition. For charged defects, the common posteriori corrections (including image-charge, potential alignment and band filling corrections) were applied to the calculated total energies to correct the finite-size effects using the scheme in literature [53, 54]. The calculated dielectric constants (εxx = 7.4602, εyy = 7.2527, and εzz = 7.5647) are used for the image charge correction. For a defect (D) in a charge state q, the formation enthalpy ∆HD,q (µ, EF) is calculated through the equation ∆HD,q (µ, EF) = (ED,q ‒ EH) ‒ Σnαµα + q(EF+EV),
(1)
where (ED,q ‒ EH) is the energy difference between the defect and host supercells. nα indicates the number of α atoms added (nα > 0) or removed (nα < 0) when a defect is formed, and µα is the chemical potential of the α atom, which can be expressed with respect to that of the elemental phase (µαel) by µα = µαel + ∆µα. EF is the Fermi level referred to the host VBM, EV. The µα value varies depending on the experimental synthesis conditions, and the allowed µα values are constrained within the polygons of CuPbSbS3 in the Cu‒Pb‒Sb‒S quaternary system drawn using the CHESTA code [55], where all competing phases that can be retrieved in the Inorganic Crystal Structure Database (ICSD) are considered. From the calculated ∆HD,q, the defect transition level ε(q/q’) between two charge states q and q’ is obtained as the EF where ∆HD,q(EF) = ∆HD,q’(EF).
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (61725401), the National Key R&D Program of China (2016YFA0204000), the China Postdoctoral Science Foundation (2018M642825), and the HUST Key Innovation Team for Interdisciplinary Promotion (2016JCTD111, 2017KFXKJC003). Z. Xiao acknowledges the supports from the Thousand Young Talents Program of China, the Startup Fund of Huazhong University of Science and Technology (HUST) and the Director Fund of Wuhan National Laboratory for Optoelectronics (WNLO). The authors thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices (CNCD), WNLO-HUST.
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Dr. Yuhao Liu obtained his Bachelor’s degree from Ludong University of Physics and Optical Engineering in 2012/06. From 2012/09 to 2017/06, he studied at the College of Physics at Huazhong University of Science and Technology, and he received his Ph.D. degree in 2017/6. Currently, he is a post-doctoral fellow in Wuhan National Laboratory for Optoelectronics (WNLO) at Huazhong University of Science and Technology. His research interest is Sb-based thin film solar cells fabricated by solution methods.
Dr. Bo Yang received his Ph.D. degree in Wuhan National Laboratory for Optoelectronics (WNLO) from Huazhong University of Science and Technology, Wuhan, China. He is currently a postdoc in the WNLO. His current research interests include novel thin film solar cells and X-ray detection devices.
Muyi Zhang received his B. E. degree at China University of Petroleum in 2017. He is pursuing his M. S. degrees of both Huazhong University of Science & Technology and MINES ParisTech PSL currently. He focuses on the research of photoelectric materials and devices as well as the X-Ray detector at Wuhan National Laboratory for Optoelectronics.
Bing Xia received his B. E. degree of Optoelectronics Information Science and Engineering from Huazhong University of Science and Technology in June 2019. He is pursuing his M. S. degrees of Wuhan National Laboratory for Optoelectronics (WNLO). His research interest focus on theoretical modeling and design of novel chalcogenide solar cell absorber materials.
Dr. Chao Chen received his B.S. degree in School of Physics at Huazhong University of Science and Technology in 2014/06. From 2014/09-2019/02, he studied in Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology as a doctoral candidate and received his Ph.D. degree in 2019/02. Currently, he is a post-doctor in Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology. His research interests are thin film solar cells and photodetectors.
Xueping Liu received her Master’s degree in Materials Science and Engineering from Wuhan university of technology. She is currently a PhD candidate in Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK. Her research interest is developing new materials and engineering for high performance perovskite devices.
Prof. Jie Zhong received his Ph.D. in December 2011 in Central South University. In 2015, he joined State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology as associate professor. His research interest is focus on developing new materials and techniques for solution-processed high efficiency flexible organic-inorganic hybrid perovskite solar cells.
Prof. Zewen Xiao received his Bachelor and Master degrees from Xi’an Jiaotong University in 2010 and 2012, respectively, and his Ph.D. degree in Materials Science from Tokyo Institute of Technology in 2015. He worked as a postdoctoral fellow at The University of Toledo from 2015 to 2016. Then, he worked as a specially appointed assistant professor at Tokyo Institute of Technology from 2016 to 2018. Since 2018, he holds a full professor position in Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology. His Materials Innovation Group focuses on designing novel optoelectronic semiconductors for applications such as solar cells and light emission by a combined theoretical and experimental approach. He was awarded “National 1000 Young Talents” project in 2019. He is an editorial board member of Frontiers of Optoelectronics.
Prof. Jiang Tang received his BSc (2003) from University of Science and Technology of China and Ph.D (2010) from University of Toronto under the supervision of Prof. Edward Sargent. After one year and half postdoctoral research at IBM T.J. Watson Research Center, he joined in Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology as a full professor. His research interests are exploration of new semiconductors for optoelectronic devices. Specifically, he pioneered in antimony selenide (Sb2Se3) thin film solar cells, constructed lead-free halide perovskites X-ray detectors with low detection limit, and developed stable and efficient white emissive halide perovskites. He has published more than 100 papers in well-known journals such as Nature, Nature Materials, Nature Photonics, Nature Energy, Nature Communications and so on, with a total citation of more than 9,000. He was awarded the NSFC fund for distinguished young scholars. He is the editor of Frontiers of Optoelectronics, Editorial Advisor Board member of Solar RRL and has delivered more than 50 invited or plenary talks at various international conference or prestigious universities.
Highlights
DFT calculations show that CuPbSbS3 exhibits a 3D electronic dimensionality, a suitable direct bandgap of 1.3 eV, and high optical absorption coefficients. Defect calculations indicate that CuPbSbS3 exhibits weak p-type conduction under S-rich condition, which is suitable for heterojunction solar cell. Phase pure, large grained CuPbSbS3 thin films were fabricated by the BDCA solution method. CuPbSbS3 solar cells were constructed for the first time, achieving 2.23% device efficiency.
Declaration of Interest Statement There are no conflicts to declare.