Defect-Passivation and Layered-Phase Formation in an Antimony

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Defect-Passivation and Layered-Phase Formation in an Antimony-Based Hybrid Iodide Perovskite-Derivative through Chlorine-Incorporation Goutam Paul, and Amlan J. Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02958 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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The Journal of Physical Chemistry

Defect-Passivation and Layered-Phase Formation in an Antimony-Based Hybrid Iodide Perovskite-Derivative through Chlorine-Incorporation Goutam Paul and Amlan J. Pal* School of Physical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

Corresponding Author *Tel.: +91-33-24734971. Fax: +91-33-24732805. E-mail: [email protected] ORCID Amlan J. Pal: 0000-0002-7651-9779

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ABSTRACT

Inclusion of chlorine in (CH3NH3)3Sb2I9 is an efficient route for a phase transformation of the antimonybased perovskite-derivative from its 0D dimer phase to a 2D layered phase. As such, a 2D phase is preferred as an active material in solar cells for facile carrier transport. We revisit chlorine incorporation in the 0D antimony-based hybrid iodide perovskite-like material in terms of defect-passivation vis-à-vis a phase-transformation to a layered phase. Scanning tunneling spectroscopy (STS) studies and thereby density of states (DOS) spectra of the materials provided progression of band-edges during the phase transformation. Surface potential and local conductivity of the perovskite-derivative films, measured through Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), respectively, inferred that chlorine-incorporation not only prompted a layered phase formation but also passivated the intrinsic defects present during the synthesis of the materials. Since passivation of defects is crucial in any solar cells, we have optimized planer heterojunction solar cells through energy levels of the layered material, namely chlorine-incorporated antimony-based hybrid iodide perovskite-derivatives including the carrier-transporting components.


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INTRODUCTION Beyond methylammonium lead triiodide (MAPI) and materials having multications,1 partial metalsubstitution,2,3 and mixed-halides4-6 in a AMX3 structure separately or in conjunction, elpasolite )7,8 and trivalent metal-based perovskite-derivatives (

(

9-11

are being considered

recently in solar cell applications. Here A and M represent a monovalent and a bivalent cation, respectively, and X is a halide in AMX3 perovskite structure. In the elpasolite, monovalent and a trivalent metal, respectively, whereas in

and

are a

structures, Bi3+ and Sb3+ are

specially considered at the B-site due to their isoelectronic configuration (s2p0) as in lead ions (Pb2+).12,13 The elpasolites have mostly indirect and/or too large band gap and have a complex processing route in achieving device-quality thin-films. The trivalent metal based perovskite-derivatives, on the other hand, have (M2I9)3− bioctahedra rather than corner-sharing MI6-octahedra, and thus form a lower-dimensional (0D) structure.9 The face-sharing bioctahedra have a size of around 1 nm and are separated by organic groups.14 Any form of 0D structures including

may not be suitable for solar cell applications due to

quantum confinement effect and thereby widening of gap and more importantly a hopping nature of conduction process leading to a poor carrier-extraction capability. They however offer a better stability in devices due to suppression of ion migration.15 In a recent work, Jiang et al. provided an efficient route for a phase transformation of MA3Sb2I9 (MA: methylammonium) from the 0D dimer phase to a 2D layered phase and also formation of high-quality films for solar cell applications.16 Chlorineincorporation has been the route for a chemical-composition engineering in achieving the layered material, which also yielded a higher solar cell efficiency than that with their 0D phase. The transformation to a 2D layered phase has occurred due to a lower decomposition enthalpy of cholorineincorporated MA3Sb2I9 in its 2D layered-phase than that of the 0D dimer phase; for the pristine MA3Sb2I9, the dimer phase has a lower decomposition enthalpy making the 0D phase stable.16 Efficient conduction process, which in turn enhances carrier-extraction capability, is also largely affected by defects which inevitably form in a material during their growth.17,18 The defects having ACS Paragon Plus Environment

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energies outside the gap may have a lesser adverse effect on the conduction process; the deep-level defects forming within the band gap, on the other hand, will have a much crucial impact on their optoelectronic properties.17,19 To be specific, in solar cell applications, the defect-sites forming within the gap act as recombination or trapping centers and thereby hinder carrier transports in a large manner.20 In this work, we revisit MA3Sb2I9 system upon chlorine-incorporation in terms of defect-passivation vis-à-vis transformation from the 0D dimer phase to a 2D layered phase. We have combined atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (C-AFM), and scanning tunneling spectroscopy (STS) to deliberate on the grain size and crystallinity, defect passivation, conductivity, energy-dependent density of states (DOS), and thereby band-edges of the antimony-based ternary iodide perovskite-derivative system upon controlled chlorine-incorporation, that is under the influence and passivation of defects.

EXPERIMENTAL SECTION Materials. Methylammonium chloride, CH3NH3Cl, antimony(III) iodide (SbI3), dimethylsulfoxide (DMSO), γ-butyrolactone (GBL), and chlorobenzene were purchased from Sigma-Aldrich Chemical Company. Methylammonium iodide (CH3NH3I) was procured from Dyesol Limited, Australia. The materials were stored in a nitrogen-filled glovebox and used without further purification. Fabrication of Perovskite Thin-Films. To form thin-films of the pristine antimony-based hybrid iodide perovskites, (CH3NH3)3Sb2I9 or MA3Sb2I9, we have adopted a one-step solution-process followed by an antisolvent treatment. Briefly, stoichiometrically-balanced amounts of CH3NH3I and SbI3 were dissolved in a mixed solvent of DMSO and GBL (3:7 v/v) and stirred overnight in the glovebox. The solution was spun on pre-heated substrates at 1000 rpm for 10 s followed by spinning at 4000 rpm for 20 s; ~0.3 mL of chlorobenzene was dripped on the spinning substrate at the twentieth second for a fast deposition crystallization (FDC) of the perovskite-like film followed by annealing at 105 °C for 30 min. For chlorine-incorporation in the perovskite-derivative to form MA3Sb2ClxI9-x, a stoichiometric amount ACS Paragon Plus Environment

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of CH3NH3Cl was added to the CH3NH3I precursor while following the above-mentioned filmformation protocol. Presence of chlorine in the perovskite-derivative was established from X-ray photoelectron spectroscopy (XPS) (Figure S5 in the Supporting Information). Characterization of Perovskite Thin-Films. Thin-films of the perovskite-derivatives were characterized by optical absorption spectroscopy and X-ray diffraction (XRD) patterns; measurements were carried out by using Cary 5000 UV−vis−NIR spectrophotometer and a Rigaku Smart Lab X-Ray Diffractometer (wavelength of Cu Kα radiation = 1.5406 Å), respectively. The film morphology, surface potential (surface work function) imaging, and conductivity mapping were measured with a Nanosurf Easyscan2 atomic force microscope (AFM) by using Al-coated, Pt/Cr-coated, and Pt/Ir-coated tips, respectively. While the conductivity mapping was carried out in a contact-mode, the former ones were done in a non-contact mode. In addition, the pristine and chlorine-incorporated perovskite-like materials were characterized with a Nanosurf Easyscan2 scanning tunneling microscope (STM) in ambient condition. From tunneling current versus voltage characteristics, we derived differential tunnel conductance spectroscopy, which has a correspondence to the materials’ density of states (DOS) spectrum, to locate their band-edges. For measurements with a STM, ultrathin films were deposited on arsenic-doped silicon(111) substrates having a resistivity of 3–10 mΩ cm; a bias was applied to a Pt/Ir (80%/20%) tip with respect to the substrate electrode. Set current of tip-approach was 1.0 nA at 2.0 V. Device Fabrication. Devices were fabricated on patterned indium tin oxide (ITO) coated glass substrates, which were cleaned through a usual protocol followed by UV−ozone (UVO) treatment for 15 min. As a hole-transport layer (HTL), a 40 nm thick thin-film of copper-doped NiO (5 at%) was first formed on the substrates following a well-documented method.21 The HTL-coated substrates were subjected to a short UVO treatment (3 min) and pre-heated to 70 °C for 10 min to improve its filmmorphology and also to facilitate adhesion with the subsequent perovskite-derivative layer. A perovskite layer was then formed on the HTL; content of chlorine with respect to iodine in the perovskite was varied from 0 to 50% in several steps. As an electron-transport layer (ETL), a solution of surfacemodified ZnO nanoparticles (20 mg/mL in chloroform), which were synthesized by following a typical ACS Paragon Plus Environment

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hydrolysis route,22 were spun on the active layer at 3000 rpm for 30 s, followed by a 20 min annealing process at 100 °C. Finally, aluminum strips as a top electrode (100 nm) was thermally evaporated orthogonal to the ITO ones. The active area of each cell was 10 mm2. Device Characterization. Current-voltage (I–V) characteristics of the planer heterojunction solar cells were recorded under a dark and 1 sun illumination conditions by a Keithley 2636 Electrometer using LabTracer software. Contacts with the electrodes were made by using a three-axes micropositioners having pressure-loaded spring-based probes. As a source for illumination, we used a 300 W solar simulator (Newport-Stratfort model 76500) attached with an AM1.5 ﬁlter placed below the glovebox. Intensity of the simulated solar light on the devices was 100 mW/cm2. Areas outside the cell were covered during recording of I–V characteristics under the illumination to avoid any contribution from neighboring regions or cells.

RESULTS AND DISCUSSION Optical Absorption Spectroscopy. The effect of chlorine-incorporation at the iodine-site of MA3Sb2I9 has first been investigated by UV−vis−NIR absorption spectroscopy. In Figure 1a, we have represented optical absorption spectra and corresponding Tauc plots of pristine and chlorineincorporated MA3Sb2I9 thin-films at different chlorine-to-iodine ratios (Cl/I). It can be seen that the gap shrunk from 2.3 to 2.0 eV at a particular content of chlorine-incorporation; the material retained the lower gap as the chlorine-content in the material was increased further (Figure 1b). Such a nature represents withdrawal of quantum-confinement effect in 0D MA3Sb2I9 while forming a layeredstructure in MA3Sb2ClxI9-x at a Cl/I ratio of 0.3 and beyond. It may be stated that a chlorine-to-iodine ratio of 0.3 amounts to x ~ 2.1 in MA3Sb2ClxI9-x. The results are in agreement with the previous report and band structure calculations, which in addition inferred that the indirect-nature of gap turned to a quasi-direct band gap upon chlorine-incorporation.16 It may also be stated that such a quasi-direct nature of band gap, as in MAPI, is known to slow down the radiative recombination process of photoinduced carriers benefitting photovoltaic activities. ACS Paragon Plus Environment

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X-ray Diffraction Patterns. We then proceeded to characterize the perovskite-like materials through their X-ray diffraction patterns in investigating their phase-purity and crystal-structures. In Figure 2, we have presented the patterns of pristine and chlorine-incorporated antimony-based ternary iodide perovskite-derivatives. Diffraction patterns of pristine MA3Sb2I9 show strong peaks at 16.6˚ and 24.8˚ representing (002) and (003) planes, respectively. The patterns match well with the reported results of the material indicating formation of their 0D crystal structure.23 In addition, several peaks appeared in the 25-30° region. Upon chlorine incorporation, the peaks at 24.8° and 29.4° remained static, while other peaks in the region disappeared. The results indicate a phase transformation from the 0D dimer phase to the 2D layered phase, as evidenced from simulated XRD patterns of the dimer and the layered phases.16 Interestingly, the observed transformation in the XRD patterns and optical absorption spectrum occurred at the same chlorine content, that is at a chlorine-to-iodine ratio of 0.3.

(a)

(b)

with Cl/I = 0.0 0.1 0.2 0.3 0.4 0.5

1.0 1.6

1.8

2.2

2.4

2.6

2.8

Energy (eV)

0.5

0.0

2.0

Optical Gap (eV)

2

1.5

3.0

MA3Sb2I9-xClx

( h ) (a.u.)

2.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5

2.0

1.5 400

600

800

1000

1200

0.0

Wavelength (nm)

0.1

0.2

0.3

0.4

0.5

Chlorine/Iodine

Figure 1. (a) UV−vis−NIR absorption spectra and Tauc plots (inset) of chlorine-incorporated MA3Sb2I9 with a range of chlorine-to-iodine (Cl/I) ratios. (b) Optical gap of the materials obtained from Tauc plots as a function of the Cl/I ratio.


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Cl/I= 0.0

0.1

0.2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

0.4

0.5

10

20

30

2

40

50

60

(Degree)

Figure 2. XRD patterns of MA3Sb2ClxI9-x with a range of chlorine-to-iodine (Cl/I) ratios (as specified in legends).

Scanning Tunneling Spectroscopy. To know the influence of chlorine-incorporation on the band edges of MA3Sb2ClxI9-x, we have characterized them through scanning tunneling spectroscopy (STS). Differential tunnel conductance (dI/dV) spectrum of a semiconductor has a correspondence to its density of states (DOS), from which one may locate the conduction band (CB) and valance band (VB) edges with respect to Fermi energy of a semiconductor.24,25 We have therefore recorded tunneling current as a function of tip voltage for all the perovskite-like materials in their ultrathin-form. Since the bias was applied to the tip, the peak closest to 0 V (which is aligned to the Fermi energy) in the positive voltage region of dI/dV spectrum implied withdrawal of electrons and hence the VB energy. Similarly the position of CB edge could be obtained from the location of the first peak in the negative voltage region of dI/dV verses voltage plots. Figure 3 represents typical dI/dV spectra of pristine and chlorineincorporated MA3Sb2ClxI9-x perovskite-derivatives. Since STS is an extremely localized mode of


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measurement, it was necessary to record spectra at many points on the surface of each ultrathin-film. From each measurement, we obtained a set of VB- and CB-edges to finally draw histogram of their energies (Figure S1 in the Supporting Information). From the DOS spectrum and also from the histogram of band-energies, we infer that the pristine MA3Sb2I9 exhibited a weak p-type conductivity, as its Fermi energy is closer to VB edge as compared to the CB one. The transport gap as obtained from the difference between VB and CB edges appears a little wider as compared to the optical gap; such a difference is explicable since exciton binding energy has to be provided to an exciton (optical gap) to form a free electron and a hole in the CB and VB separated by a transport gap. Upon chlorineincorporation, VB-edge could be found to move away from the Fermi energy leading to an increase in the transport gap. Band structure calculations of MA3Sb2I9 inferred that while the VB is composed of I(5p) and Sb(5s) orbitals, the Sb(5p) orbitals solely formed the CB.26 The position of VB-edge should hence be sensitive to the choice of halide in the perovskite-derivative; with incorporation of chlorine in place of iodine, the valence atomic orbitals changes from 5p to 3p leading to lesser orbital-overlap and thereby a shift of the band-edge away from the Fermi energy and hence a widening of the transport gap. We may add that a shift of VB-edge away from the Fermi energy can also be due to passivation of defect states above the band. When we analyze STS results of MA3Sb2ClxI9-x having higher chlorine contents, a decrease in the gap is observed at Cl/I = 0.3. The value of chlorine-content at which the change occurred matches well with the optical spectroscopy results exhibiting a decrease in the band gap due to a phase transformation from 0D to 2D-layered structure at the same amount of chlorine-incorporation. In the dI/dV spectrum of the materials with Cl/I = 0.3 (Figure 3), the CB-edge could be seen to be shifted towards the Fermi energy inferring a week n-type conductivity. Upon further increase in the chlorine-content, none of the edges exhibited a significant shift implying that the halogen orbitals did not contribute to band formation of MA3Sb2ClxI9-x in their 2D-layered structure.


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(a) CB

Cl/I = 0.0

VB

-2

(b)

CB

Energy (eV)

0.1

0.2

dI/dV (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

-1

0

EF

1

0.4

VB 2

0.5

EF -2

-1

0

0.0

0.1

0.2

0.3

0.4

0.5

Chlorine/Iodine 1

2

Tip Voltage (V)

Figure 3. (a) dI/dV spectra (b) and band-edges of MA3Sb2ClxI9-x with a range of chlorine-to-iodine (Cl/I) ratios. In (a), the arrows in the forward and reverse bias direction point to peaks indicating location of CB and VB-edges, respectively. Fermi energy (EF) is aligned to 0 V in both the figures. Morphology, Surface Potential, and Conductivity Mapping of MA3Sb2ClxI9-x Thin-Films. We then proceeded to investigate the role of chlorine-incorporation on the morphology and surface potential of the perovskite thin-films through AFM and KPFM, respectively. Earlier, KPFM studies have been employed to investigate the charge carrier generation, transport, and extraction mechanism in MAPIbased solar cell structures.27 Figure 4 represents AFM topographies of pristine and chlorineincorporated materials in their thin-film form. We observe that with an increase in chlorine content, the grain size increased continuously with respect to the pristine material; however, surface roughness could be seen to have appeared. Chlorine-incorporation prompting an improvement in grain size at a cost of surface roughness can have a complex effect on photovoltaic performances based on the thin-film. While a larger grain size may facilitate carrier transport,28 a rough surface would however result in pinholes thus affecting the device performance adversely. Solar cell characteristics, as presented in a


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later section, evidenced that a large degree of surface roughness at a Cl/I content of above 0.3 had a negative effect on device performance.

Figure 4. AFM images of MA3Sb2ClxI9-x with a range of chlorine-to-iodine (Cl/I) ratios. As such, surface work function of semiconducting thin-films plays a crucial role in optoelectronic devices. Fermi energy can be derived from the surface work-function which has a major role in electronic transport process across metal-semiconductor interfaces.29,30 To study the role of chlorineincorporation on the electronic properties of MA3Sb2I9 thin-films, we have mapped the surface potential of different films. Figures 5a and 5b show the surface potential images of the thin-films and the distribution of contact potential difference (VCPD) throughout the surface of the films, respectively. Topography of the regions, where the KPFM measurements were carried out, has been presented in Figure S2 in the Supporting Information along with the surface potential image. Through KPFM, we have measured the VCPD as:31 eVCPD = ФTip – ФSample where ФTip and ФSample are work-functions of the tip and the sample, respectively. The former was estimated with reference to a highly oriented pyrolytic graphite (HOPG) having a known work-function


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of 4.60 eV.32 The measured VCPD between tip and HOPG was 0.08±0.01 eV, from which we can conclude that Фtip was 4.68±0.01 eV. The surface potential images of the thin-films (Figure 5a) bring out the extent of non-uniformity in the surface potential. The distribution of potential, which is clearly depicted in Figure 5b, shows that the average surface work-function depends on the chlorine-content (Figure 5c). The energy changes abruptly at a chlorine-to-iodine ratio of 0.3. This supports an occurrence of a structural phase transition in the perovskite-derivatives at a Cl/I ratio of 0.2-0.3. This result has matched well with the STM analysis where we have observed a change of band position occurring at that particular cholorinecontent implying a phase transition from a 0D to a 2D phase. At higher chlorine-to-iodine ratios, the 2D phase continued to exist with its perovskite-like form. Apart from its average value, the distribution of contact potential (VCPD) throughout the surface of the films could be seen to depend clearly on the chlorine-content (Figure 5b). The distribution is very wide in the pristine material; presence of point defects on the surface leads to such a (broad) distribution limiting the photovoltaic performance based on the material. The distribution narrows down upon chlorine incorporation (Figure 5b). A narrowing down of the surface-potential distribution implies that the defects were being passivated through chlorine-incorporation. At a chlorine-to-iodine ratio of 0.3 and above, while the average surface potential increased, excess chlorine in the 2D-phase may have introduced further defects, as evidenced from the degree of widening in the surface-potential distribution. The distribution in the 2D phase of the perovskite-derivatives is narrower than that of the pristine one having a 0D-phase.


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(a)

0.2V

Cl/I=0.4

-0.6 V

(b)

0.1

0.2 0.3

0.4 0.5 -0.5

-0.4

-0.3

-0.2

-0.1

Surface Work Function (eV)

Cl/I = 0.0

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.1

(c)

5.0

4.9

4.8

4.7

0.0

0.1

0.2

0.3

0.4

0.5

Chlorine/Iodine

0.0

Contact Potential Diffence (V)

Figure 5. (a) Contact potential images (b) distribution of contact potential difference, and (c) average surface work-function of MA3Sb2ClxI9-x with a range of chlorine-to-iodine (Cl/I) ratios. To acquire further evidence of defects passivation in the perovskite-derivatives and to study its effect on charge transport processes in thin-films, we have characterized the perovskite-derivatives through CAFM measurements. Apart from surface topography, C-AFM measurements also provide mapping of local conductivity on the surface (Figure 6a). With a bias voltage of 1.0 V being applied between the tip and the substrate electrode, current is measured and mapped throughout the surface. Figure 6a represents the conductivity map of pristine and chlorine-incorporated perovskite-like thin-films. The


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corresponding topography of the regions where the C-AFM measurements were carried out is presented in Figure S3 in the Supporting Information. When we analyze current on the surface by drawing histograms and compare them, we find that the average value of current increased with chlorineincorporation in the 0D systems (Figure 6b). The increase in conductivity again suggests defectpassivation leading to fewer trap-sites on the surface of the materials. When the system transforms into a 2D phase upon further chlorine-incorporation, the current increases further due to a combined effect of defect-passivation and structural phase-transition towards a higher dimension which as such provides a better platform for carrier conduction due to lesser number of quantum confinement directions. A large amount of chlorine-incorporation however affected the film quality in an adverse manner resulting in a decrease in conductivity. In Figure 6c, we have compared the RMS roughness and average current evidencing their correlation in support of our inference. In regular three-dimensional polycrystalline perovskites thin-films, ion migration, which occurs through grains via defects and grain boundaries, has an impact on its stability and conductivity.33 Here, the increased conductivity in lower-dimensional MA3Sb2I9 upon chlorine incorporation is hence due to an increase in grain size and defect passivation along with an improved film-morphology. The results hence show that at an optimum value of chlorine content in the perovskite-derivative, we may obtain a better photovoltaic performance.


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Cl/I=0.0

4 µm

Cl/I=0.1

4 µm

0 nA

Cl/I=0.3

4 µm

20 nA

20 nA

Cl/I=0.4

0 nA

4 µm

20 nA

Cl/I=0.2

4 µm

0 nA

20 nA

Cl/I=0.5

4 µm

0 nA

20 nA

0 nA

20 nA

0 nA

Cl/I= 0.0

(b)

0.1

0.2

0.3

RMS Roughness (nm)

80

60 10 40 5 20

0

0.4

15

(c)

0.0

0.1

0.2

0.3

0.4

0.5

Average Current (nA)

(a)

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Chlorine/Iodine

0.5

0

5

10

15

20

Current (nA)

Figure 6. (a) Conductivity mapping (b) distribution of current at 1.0 V, and (c) a comparison between RMS roughness (from AFM topographies) and average current of MA3Sb2ClxI9-x with a range of chlorine-to-iodine (Cl/I) ratios. Solar Cell Characteristics. We then fabricated p-i-n heterojunction solar cells to study the photovoltaic performance of pristine and chlorine-incorporated MA3Sb2ClxI9-x thin-films. We have used thin-films of Cu@NiO and ZnO nanoparticles as hole- and electron-transporting layers, respectively. ACS Paragon Plus Environment

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From STS studies, it could be inferred that the pristine and chlorine-incorporated MA3Sb2ClxI9-x formed a type-II band-alignment at the Cu@NiO/perovskite and perovskite/ZnO interfaces (Figure S4 in the Supporting Information). In Figure 7a, we have presented current-voltage (I-V) characteristics of a device based on pristine MA3Sb2I9 under dark and illuminated conditions. While the dark characteristics were rectifying in nature, the I-V under light exhibited photovoltaic characteristics. While the shortcircuit current density (JSC) was low (1.4 mA/cm2), open-circuit voltage (VOC) and fill-factor (FF) were moderate under 1 sun illumination condition (VOC = 0.73 V and FF = 40%). We then characterized the Cu@NiO/MA3Sb2ClxI9-x/ZnO heterojunction devices under dark and illuminated conditions. Under dark, all the heterojunctions returned rectifying I-V characteristics. Such a nature implies that all the sandwich structures would act as solar cells under illumination. The I-Vs of devices under illumination exhibited photovoltaic characteristics (Figure 7b). The solar cell parameters, which have been enlisted in Table 1, depended on the chlorine-content in MA3Sb2ClxI9-x. As can be seen from the Figure 7b and Table 1, the VOC of all the devices remained within 0.73-0.78 V with marginal variation. The JSC on the other hand improved significantly upon chlorine incorporation. The increase in JSC upon chlorine-incorporation originated due to the passivation of defects in the perovskitederivatives. In materials with defects, they create trap-states thereby affecting the photovoltaic performance of the device in an adverse manner. Passivation of such defects increasing the conductivity of MA3Sb2ClxI9-x hence improves the Cu@NiO/perovskite and perovskite/ZnO interfaces for efficient carrier transport. Moreover, the phase transformation of MA3Sb2ClxI9-x upon chlorine incorporation (for Cl/I ≥ 0.3) from 0D to 2D increased the optical absorption spectrum in the higher wavelength region and thereby augmented the charge transport process through the active layer. As a result, the magnitude of JSC increased in 2D material based devices. A larger content of chlorine in antimony halide perovskite-derivatives however deteriorated the film morphology which in turn prompted a decrease in its photovoltaic performance.


16

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0

-1

-2

Dark Illumination -3 -0.2

0.0

0.2

0.4

0.6

(b) (CH3NH3)3Sb2ClxI9-x

2

Current Density (mA/cm )

(a) (CH3NH3)3Sb2I9

2


1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8

1.0

0

Cl/I =

-2

0.0 0.1 0.2 0.3 0.4 0.5 -4 -0.2

0.0

Voltage(V)

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 7. (a) I–V characteristics of the Cu@NiO|MA3Sb2I9|ZnO heterojunction under dark and 1 sun illumination conditions. (b) I–V characteristics of heterojunctions based on MA3Sb2ClxI9-x with a range of chlorine-to-iodine (Cl/I) ratios. While the characteristics under dark were rectifying in nature, the IVs under 1 sun illumination are shown in the figure.

Table 1. Photovoltaic parameters of Cu@NiO|MA3Sb2ClxI9-x|ZnO heterojunctions with a range of chlorine-to-iodine (Cl/I) ratios. Cl/I Ratio in MA3Sb2ClxI9-x

JSC (mA/cm2) VOC (V)

Fill-Factor (%)

PCE (%)

0.0

1.40

0.73

48

0.50

0.1

1.79

0.77

55

0.77

0.2

1.94

0.78

54

0.87

0.3

3.72

0.78

54

1.55

0.4

3.56

0.77

52

1.44

0.5

3.07

0.75

49

1.19

As presented in Table 1, the power conversion efficiency (PCE) optimized to 1.55% in devices based on MA3Sb2ClxI9-x with a chlorine-to-iodine ratio of 0.3. It may be stated here that to study reproducibility of the solar cells, at least ten devices based on each material were fabricated in batches and characterized. Though the PCE value remained a little low as compared to the reported PCE value,16


17


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further improvement in this direction can be envisaged through a band-engineering and optimization of device structures.

CONCLUSIONS In summary, we have shown that the phase-transformation of MA3Sb2I9 from a 0D dimer phase to a 2D layered one upon chlorine-incorporation is associated with passivation of defects in the antimonybased perovskite-derivative. STS studies inferred a significant change in band-energies due to the phasetransformation processes. The defect-passivation upon chlorine-incorporation has also been evidenced through a narrowing in the distribution of local surface potential measurements. Investigation on the local conductivity showed that the conductivity of antimony-halide perovskite-derivatives increased upon defect-passivation prompted by chlorine-incorporation. The 2D phase of the material allowing facile carrier transport along with passsivation of defects has been found to improve photovoltaic performance in the band-optimized solar cell devices. We have observed that the efficiency of devices increased from a paltry 0.5 % in the 0D system to a moderate 1.5% in a defect-passivated 2D system having a chlorine-to-iodine ratio of 0.3.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.*******. Histogram of CB and VB-edges, KPFM topographies, and C-AFM topographies of (CH3NH3)3Sb2I(Cl)9 perovskite-derivatives, band-diagram of p-i-n heterojunctions based on the perovskites, XPS of MA3Sb2ClxI9-x with Cl/I = 0.3 (Figures S1 – S5) (PDF)


18

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS A.J.P. acknowledges the JC Bose National Fellowship (SB/S2/JCB-001/2016) of SERB. G.P. acknowledges the CSIR Junior Research Fellowship Number 09/080(1042)/2017-EMR-I (Roll no. 523509).

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23


-0.5

Cl/I = 0.0

Cl/I = 0.3

-0.4 -0.3 -0.2 -0.1 0.0 Contact Potential Difference (V)

(CH3NH3)3Sb2ClxI9-x

2

(CH3NH3)3Sb2ClxI9-x


ToC Graphic

Count

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

0

Cl/I =

-2

-4 -0.2

0.0 0.3

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)


24

Defect-Passivation and Layered-Phase Formation in an Antimony

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