n-Type Doping and Energy States Tuning in ... - ACS Publications

n‑Type Doping and Energy States Tuning in CH3NH3Pb1−xSb2x/3I3 Perovskite Solar Cells

Jing Zhang,*,† Ming-hui Shang,*,‡ Peng Wang,† Xiaokun Huang,† Jie Xu,† Ziyang Hu,† Yuejin Zhu,*,† and Liyuan Han*,§ †

Department of Microelectronic Science and Engineering, Ningbo University, Zhejiang 315211, China School of Materials Science and Engineering, Ningbo University of Technology, Zhejiang 315016, China § Photovoltaics Materials Unit, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan ‡

S Supporting Information *

ABSTRACT: Built-in field and energy band alignment decide the charge separation and transportation in perovskite solar cells. Composition change in perovskites to tune the energy states is thus valuable to try. In contrast to the equivalent substitution of Pb, here trivalent Sb is for the first time incorporated into CH3NH3PbI3, with a tuned optical band gap from 1.55 to 2.06 eV. Density function theory (DFT) calculations unveil the enlarged energy band gap and n-type doping property by Sb with more valence electrons than Pb. n-Type doping by Sb elevates the quasi-Fermi energy level of the perovskite/TiO2 and promotes electron transport in the working solar cell. Thus, the doped perovskite solar cell gains a lot in photovoltage while maintaining a high photocurrent, resulting in enhanced performance of 15.6% (0.956 sun, AM1.5). The results highlight the method of n/p-type doping of perovskites by heterovalent elements and its tunability to the energy states. stability.13−15 However, quite limited research pointed at the nor p-doping of the perovskite by element change.16 Quite recently, heterovalent substitutions of Pb in perovskites were also reported. Experiment and DFT calculation of a Cs3Sb2I917 perovskite show the enlarged band gap above 2.0 eV. Though the energy state of the materials may not be suitable for a TiO2/perovskite/HTM n−i−p type solar cell structure, the calculated direct band gap and small effective mass of Cs3Sb2I9 are attractive and illuminative to us; trivalent Sb can partially substitute or dope the Pb cation in the MAPbI3 perovskite, tuning the energy levels in the TiO2/perovskite/ HTM solar cell to achieve better performance. Here for the first time, Sb3+ is incorporated in the Pb2+ site (noted as Sb-X%, X% is the Sb percentage) to effectively tailor the optoelectrical property of the perovskite. The perovskite energy band is widely tuned from 1.55 to 2.06 eV. A small amount Sb doping is proved to be n-type doping by both theory calculations and experimental results. It effectively changes the built-in field of the n−i−p solar cell junction and promotes the electron transport. A significant improvement of the photovoltage and performance are achieved in Sb-doped perovskite solar cells.

R

ecent rapid development of perovskite solar cells is revolutionizing the third-generation photovoltaic research field, with the latest certified power conversion efficiency reaching over 20%.1−3 Efficient solar cells stem from efficient carrier generation in the perovskite layer, separation at the interfaces, and collection in the hole/electron transport layers (HTMs/ETMs). Electron and hole Fermi level splitting, which drives carrier separation and transportation, determines the photovoltage of the solar cell. Therefore, large Fermi level splitting and designed energy band alignment are key factors for enhancing device performance.4 Element doping of the materials can effectively modulate the energy states and carrier concentration, which is successfully utilized in perovskite and quantum dot solar cells. Through ndoping of TiO25,6 and p-doping of NiO6 in p−i−n type perovskite solar cells, improved charge transportation and interface energy alignment were successfully achieved, resulting in high photovoltaic performances. n-Type-doped PbS quantum dots effectively enlarged the built-in field in a p−n type quantum dot solar cell and boosted its photovoltage.7 As the core material in perovskite solar cells, organic−inorganic hybrid CH3NH3PbI3 is reported to be a good p-type, intrinsic, or good n-type semiconductor.8 Chemical management of the organic cation and anion9 exhibits versatile tunability of the energy band structure,10,11 carrier mobility,12 and moisture © XXXX American Chemical Society

Received: July 1, 2016 Accepted: August 14, 2016

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DOI: 10.1021/acsenergylett.6b00241 ACS Energy Lett. 2016, 1, 535−541

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ACS Energy Letters

Figure 1. XRD patterns of Sb-X% films: (a) 0−8%, (b) 10−100% (PbI2 peak at 12.6°; the star and triangle represent perovskite and CH3NH3Sb2/3V1/3I3 phases, respectively), (c) UV−visible absorption spectra, and (d) the estimated energy band of Sb-X%. The error bar is 5%.

The preparation details of Sb3+ incorporated perovskite and other experimental details are depicted in the Supporting Information. The pure CH3NH3PbI3 exhibits conventional perovskite diffraction peaks with Miller indices noted18 in Figure 1a. Increasing the Sb doping content to 8%, the X-ray diffraction (XRD) pattern remains similar (Figure 1a), which implies that the Sb is doped in the perovskite structure without an impurity phase. The Sb-1% doped samples are well crystallized with long-range orientation of the lattice and no impurity phase of the doped sample with a high-resolution transmission electron microscope (HRTEM, Figure S1). The lattice parameters derived from the refined XRD are a = b = 8.852 Å and c = 12.632 Å for pure CH3NH3PbI3. When X% increases to 10%, c reduces consistently from 12.632 to 12.507 Å (Table S1). The volume of the lattice (Table S1) shrinks by obviously shortening the parameter c, owing to the smaller ionic radius of Sb3+ (0.76 Å) compared to that of Pb2+ (1.33 Å).19 The enlarged peaks at 28.11 and 28.36° in Figure 1a are indexed to (004) and (220) planes, respectively. The peak at 28.11° gradually grows higher by increasing the Sb content. Such behavior is different from that of CH 3 NH 3 Pb(I1−xBrx)320,21 and CH3NH3Pb1−aSnaI3−xClx22 alloys, where the Br− and Sn2+ increase the lattice symmetry to allow the (004) diffraction peak disappear. It indicates that replacing Pb with Sb distorts the crystallization because of the different ionic radii of Pb2+ and Sb3+ and differing valence states. When Sb (trivalent group VA element) replaces the Pb atom in a perovskite lattice, metal site vacancies are required to keep the system chemically neutral.23 Therefore, the formula for Sbdoped material is written as CH3NH3Pb1−xSb2x/3Vx/3I3, with V

marking the vacancy. By entirely substituting Pb2+ with Sb3+, the Sb-100% (Figure 1b) exhibits a set of new diffraction peaks, which is similar to recently reported XRD of layered structure Cs3Sb2I9.17 The layered structure CH3NH3Sb2/3V1/3I3 with tolerance factor α = 1.06 is believed to be established in Sb100% and is consistent with the conclusion that α > 1 leads to a layered structure.24 Due to the existence of peaks similar to Sb0, Sb-100%, and a deviated peak at 24.6°, the Sb-10−75% samples might be ascribed to the mixture phases of CH 3 NH 3 Sb 2 / 3 V 1 / 3 I 3 , CH 3 NH 3 Pb 1 − x Sb 2 x / 3 V x / 3 I 3 , and CH3NH3PbI3. The crystalline size and surface morphologies of Sb-X% are similar for Sb doping (Sb-0−10%). Crystal domains become smaller with a further increase of the Sb content (X% = 25− 75%). By all Sb substitution, the film displays a new morphology (Figure S2 and Table S1). As shown in Figure 1c, the films with Sb incorporation show strong absorption as the pure perovskite film. The onset absorption of the composites is monotonically a blue shift of the wavelength from 790 to 595 nm with enhanced Sb content. The inset of Figure 1c shows the corresponding colors of the films. With the Sb composition increase, the color varies from dark brown for CH3NH3PbI3 to brown for X% = 50−75% Sb content film and further turns to reddish orange with X% = 90−100%. The band gap Eg of Sb-X% is determined by plotting (αhν)2 versus the excitation energy hν. The optical Eg is gradually enlarged from 1.55 eV for pure CH3NH3PbI3 to 1.57 eV at X% = 1% and ultimately to 2.06 eV for whole Sb substitution. Sb-0−25% samples show sharp onset of the absorption edge, while larger Sb content (Sb-25−75%) shows a gradual onset edge. It might stem from the transition from a 536


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Figure 2. (a) I−V curves (0.956 sun, AM1.5), scan rate 0.1 V/s; (b) EQE spectra of the Sb-X% solar cells.

Voc and Jsc suffer great lose with higher Sb ≥ 25% content. The non-Pb-containing device Sb-100% shows low photovoltaic performance. External quantum efficiency (EQE) spectra are presented in Figure 2b, showing the same tendency change as the Jsc of the solar cells. The EQE remained over 80% throughout the visible spectra for the Sb-1% device, higher than the contrast device. Due to the similar optical absorption of Sb-1% with the pure perovskite film, the charge transport and collection property of the Sb-1% device might be especially good for achieving a high Jsc. Further rapid reduction of Jsc with increasing Sb (>10%) on one hand is due to reduced light absorption, and on the other hand, the poor carrier transport ability may damage the Jsc. The photovoltage is determined by the “built-in voltage”: the difference between the quasi-Fermi energy of the perovskite/ TiO2 and the HOMO (the highest occupied molecule obit) energy of the HTM, where the quasi-Fermi energy is owed to the buildup of charge.27,28 Compared to the undoped perovskite solar cell, the Voc is increased a lot by Sb doping (Sb% < 10%), larger than the conduction band movement. It implies that the quasi-Fermi energy level under illumination is greatly uplifted by Sb doping. The dark I−V curves (Figure S5) of the solar cells illustrate greater diode turn-on voltages for the Sb-1% and Sb-10%, indicating that Sb doping modulates the built-in field to be larger than that of the undoped device. The Fermi level and the donor density of the doped perovskite are further checked by Mott−Schottky plots of the spiro-OMeTAD/perovskite p−n junction (Figure 3a). The intercept of the linear fit, which represents the Fermi level of the perovskite under dark conditions, moves negatively after Sb doping. It verifies that Sb doping pushes the Fermi level toward the conduction band, leading to the enlarged built-in field between the HTM and perovskite. The slope of the Mott− Schottky plots indicates Sb-1% and Sb-10% are n-type doped, where the calculated doping density of Sb-1% is enhanced to 5.2 × 1017 cm−3, compared to 4.0 × 1017 cm−3 of the pure perovskite film. The donor density does not show further increase at higher Sb content Sb-10% (4.3 × 1017 cm−3), which might due to the increased trap states in the higher Sb content films. Designed Hall effect measurements29 further support the enhanced carrier density by Sb doping (Figure 3b). The electron concentration in Sb-1% is higher than that in the undoped film and varied similar to the Mott−Schottky method. On the basis of Mott−Schottky and Hall effect measurements, substantial increment of Voc in Sb-1% is understandable; well-

direct band gap to an indirect band gap semiconductor or the impurity states near the band edge.25 X-ray photoelectron spectroscopy (XPS) valence band spectra in Figure S3 are used to check the valence band edge of Sb-X%. The valence band maximum (VBM) remains unchanged until the Sb content reaches 25%. For higher Sb content, the VBM moves far from the zero energy level. On the basis of the measured Eg and the XPS valence band edge potentials, the energy band structures of the Sb hybrid photoanodes are estimated in Figure 1d. The optical Eg can only be considered as an estimation because there may be exciton absorption or impurity absorption at the band edge.26 By substituting Pb with Sb ≤ 25%, the conduction band minimum (CBM) gradually up-shifts to vacuum level with the VBM changed a little. By a further increase in the Sb content (50−100%), there is a simultaneous upshift of the conduction band and a downshift of the valence band, which results in a larger band gap. The performances of Sb-X% mesoporous heterojunction solar cells are tested and shown in Figure 2a with photovoltaic parameters in Table 1. The photovoltaic values here represent Table 1. Photovoltaic Parameters of Sb-X% Solar Cells (0.956 sun, AM1.5) X [%]

Jsc [mA cm−2]

Voc [V]

FF

eff [%]

0 0.5 1 4 10 25 50 75 100

21.53 22.0 21.82 20.5 17.37 1.53 1.68 0.72 0.465

0.89 0.945 0.985 0.968 0.937 0.700 0.648 0.587 0.513

0.652 0.62 0.692 0.60 0.557 0.636 0.718 0.332 0.454

13.1 13.5 15.6 12.4 9.48 0.712 0.818 0.147 0.113

the statistical average of several batches of solar cells (Figure S4). Encouragingly, the photovoltage (Voc) of the devices increases with the Sb content enhanced to 10%. The Sb-1% device shows remarkable increment of Voc to 0.985 V when the photocurrent (Jsc) is raised a little. Thus, the performance of the Sb-1% device remarkably enhances to 15.6%, much better than 13.1% of the original device. Even with 10% Pb substitution, the preformance of the Sb-10% device still remains at 9.48% with a high Voc of 0.937 V. However, both 537


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Figure 3. (a) Mott−Schottky plots of a spiro-OMeTAD/perovskite p−n junction, (b) electron density of the Sb-doped perovskite film by Hall effect measurements.

accumulated electrons under illumination further up-shift the quasi-Fermi energy level, and therefore, the larger built-in field is constructed in a TiO2/doped perovskite/spiro-OMeTAD junction. DFT calculations are performed to gain a detailed understanding of the energy states of Sb-doped materials. CH3NH3PbI3 lattice parameters are calculated to be a = b = 8.91 Å and c = 13.11 Å, which is in line with reported theoretical30 and our experimental results. The lattice parameters of CH3NH3Pb0.75Sb0.25I3 were calculated to be a = b = 8.83 Å and c = 12.87 Å. The shrinkage of the lattice is consistent with our XRD experimental results. The vacancy is not considered for its density is rather low in the doping situation. It mainly acts as trap states in the band gap. In the geometric structure (Figure 4a), the Sb−I bond length (3.17 Å) is shorter than the bond length of Pb−I (3.24 Å) in CH3NH3Pb0.75Sb0.25I3, and the Pb−I is lengthened obviously compared with 3.16 Å in pure perovskite. These results are ascribed to the following: (i) the size of the Sb atom is smaller than that of Pb, which directly results in a much shorter Sb−I length; (ii) moreover, the Sb atom can provide more valence electrons than Pb. Thus, the chemical interaction between Sb−I is enhanced, while the bonding in Pb−I is weakened, resulting in longer Pb−I length. Figure 4b displays the calculated partial density of states (PDOS) of pure and CH3NH3Pb0.75Sb0.25I3 perovskite to reveal the energy band structure. The Fermi energy level is clearly moved up to the bottom of the conduction band, unveiling the n-type doping31 property of Sb because more valence electrons of Sb 5p3 results in an electron excess in the system. In pure CH3NH3PbI3, Pb 6p and I 5p contribute to the majority of the CBM and VBM, respectively, yielding a direct band gap (Figure S6). CH3NH3Pb0.75Sb0.25I3 still processes the direct band gap. The position of the VBM shifts down by 0.1 eV, and the CBM upshifts 0.15 eV, resulting in a larger energy gap (ΔE) compared to that of the pure perovskite. The composition of the VBM (I 5p) is uninfluenced by Sb. Hybridized Sb 5p and I 5p states are induced in the conduction band edge, and at the same time, the Pb 6p states are pushed up to higher energy with respect to I 5p states at the VBM. The enlarged gap ΔE is responsible for the blue shift of UV−visible absorption spectra. Here, Sb 5p is identified as external impurity states in the band gap. In fact, the PL spectra (Figure S7) of the Sb-X% film exhibit decreased PL intensity in Sbincorporated samples, which also indicates increased trap states

Figure 4. (a) Geometric structure of pure and CH3NH3Pb0.75Sb0.25I3 perovskite, (b) calculated PDOS for both samples, and (c,d) XPS core-level spectra [(c) Pb 4f; (d) I 3d].

in Sb-incorporated perovskite films. DFT calculation successfully explains the experimental results of the n-type doping, and the CBM and VBM change tendency also verifies that the trap states are induced by Sb doping. Chemical bonding is also checked by XPS core-level spectra. Both binding energies of Pb 4f and I 3d (Figure 4c,d) move to lower energy with a Sb content increase; meanwhile, the binding energy difference between the Pb 4f7/2 and I 3d5/2 is reduced by 0.12 eV in Sb-1% and 0.2 eV in Sb-75% compared with that of Sb-0. The phenomena is caused by the new 538


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Figure 5. (a) Dn and τn plots and (b) Ln plot. The error bars are 5%.

chemical environment of Sb incorporation; the Sb 5p3 devotea more valence electrons to the system than Pb 6p2, which weakens the Columbic interaction between core and extranuclear electrons in both Pb2+ and I− (reduced binding energy). Sb possesses weaker electronegativity compared to Pb; thus, Sb−I strongly bonds and consequently reduces the Pb−I binding energy. The XPS results greatly match with the reduced Pb−I length in DFT calculation. When 1 mol of Sb is doped in CH3NH3Pb1−xSb2x/3Vx/3I3, it devotes 3 mol of valence electrons to the system, resulting in 1 mol of electron excess and thus n-doping; it also brings 0.5 mol Pb vacancies and crystalline lattice distortion, both of which can trap electrons. Therefore, it is a two-sided effect of Sb doping. The electron transport in the doped solar cell under illumination is checked by intensity modulated spectroscopies (IMPS and IMVS).32 The effective diffusion coefficient Dn (Figure 5a) is derived from IMPS measurements (Figure S8) to elucidate the electron diffusion property in the devices. There is a significantly enlarged Dn for 0.5 and 1% Sb-doped perovskite solar cells that is gradually reduced with further increasing the Sb content. The large Dn of Sb (X% = 0.5−10%) doped samples promotes electron transfer in solar cells. The nearly 20 times increment of Dn from 2.9 × 10−7 cm2 S1− for the pure device to 5.0 × 10−6 cm2 S1− for Sb-1% is striking. In fact, Dn is electron density/ voltage-dependent33

10%), which is due to the defects induced by Sb augment charge recombination. The varied electron lifetime is also proved by electrochemical impedance measurement in Figure S9. The diffusion length Ln = (τnDn)1/2 defines the average distance that an electron travels before it recombines with either the absorber or the hole conductor. Due to the great improvement of Dn in the Sb-1% solar cell, the Ln (Figure 5b) is improved to 1.5 μm, more than twice the 0.65 μm in the pure perovskite solar cell. Therefore, the Jsc of Sb-1% is enhanced because the long charge diffusion length guarantees efficient charge transport and collection. Further reduction of Ln in higher-Sb%-doped devices is ascribed to the increased trap states, which deteriorates the Jsc. In summary, we used trivalent Sb incorporated in CH3NH3PbI3 material in contrast with the previously homovalent alloys. Sb doping modulates the energy states in TiO2/perovskite/HTM solar cells. The reduced Pb−I bonding by stronger Sb−I interaction leads to an enlarged band gap. Meanwhile, Sb also induces vacancy in crystalline and impurity states in the band gap. Subtle Sb content leads to n-doping of the material, making the electron density in conduction band increase and the quasi-Fermi energy level to be elevated. Therefore, the built-in potential in Sb-1% is enlarged, which results in substantial increment of Voc and the improvement of electron transportation. The Sb-1% doped solar cell shows improved performance. Other photoelectric propertys of pure Sb-100% material can be further explored. Our work might be promisingly helpful for discovering new properties beyond the traditional CH3NH3PbI3 perovskite by judiciously choosing the element to dope or replace Pb.

Dn = D0e α(Efn − Ef0)/ kBT where kB is the Boltzmann constant and T the temperature, Efn and Ef0 are the quasi-Fermi levels of electrons under illumination and in the dark, respectively, D0 represents the diffusion coefficient in the dark, and α is a constant. n-Type doping of Sb induces more electrons in the conduction band and upshifts the quasi Fermi energy level E fn under illumination. Thus, the energy difference between the n and p sides is enlarged to promote electron−hole separation/ transportation, with Dn enlarged so much. The plots of Dn with respect to photovoltage (Figure S8b) are also strong evidence that electron transport is enlarged rapidly with Voc/built-in energy. However, the electron mobility decreased rapidly at Sb4−10% concentration due to electron scattering by the defects (Pb vacancies and distorted lattice). The electron lifetime τn is obtained from IMVS to characterize recombination in perovskite solar cells (Figure 5a). It is clear that τn drops with a Sb content increase (0.5−

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00241. Experimental details, HRTEM of Sb-1%, SEM of film morphologies, XPS valence band spectra, device performance distribution, dark I−V curves of the small Sb content, calculated energy band structures, PL spectra of perovskite films, IMPS plots of Sb incorporated solar cells, EIS of Sb-doped solar cells, detailed composition, and additional parameters (PDF) 539


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (M.-h.S.). [email protected] (J.Z.). [email protected] (Y.Z.). [email protected] (L.H.).

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Guojia Fang, Suhuai Wei, Xiangmei Duan, and Shihao Wei for productive discussions. This work was supported by the National Natural Science Foundation of China (Contract Grant Numbers 51302137, 11374168, and 11547033), the Natural Science Foundation of Ningbo (No. 2015A610032), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, and the K.C. Wong Magna Fund in Ningbo University, China.

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REFERENCES

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Letter

ACS Energy Letters (32) Zhao, Y.; Nardes, A. M.; Zhu, K. Solid-State Mesostructured Perovskite CH3NH3PbI3 Solar Cells: Charge Transport, Recombination, and Diffusion Length. J. Phys. Chem. Lett. 2014, 5, 490−494. (33) Guillen, E.; Ramos, F. J.; Anta, J. A.; Ahmad, S. Elucidating Transport-Recombination Mechanisms in Perovskite Solar Cells by Small-Perturbation Techniques. J. Phys. Chem. C 2014, 118, 22913− 22922.

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