Sequentially Deposited Antimony-Doped CH

preheated to 60 °C. The PbI2-SbI3 solution was then spun at 2500 rpm for 30 s that .... spectroscopy (STS) has recently been demonstrated as a unique...
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Sequentially Deposited Antimony-Doped CH3NH3PbI3 Films in Inverted Planar Heterojunction Solar Cells with a High Open-Circuit Voltage Soumyo Chatterjee, Uttiya Dasgupta, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *

ABSTRACT: We introduce antimony-doped hybrid perovskite compounds in planar inverted solar cells. Here, we report in-depth and systematic studies on the formation of the perovskite layer through a modified two-step spin-coat method. In this method, the “loading time” of CH3NH3I on a “wet” PbI2 layer was varied in achieving a complete conversion to the perovskite material. The “loading time” that in turn also controlled morphology of the perovskite layer along with the antimony content in perovskite compounds was varied to optimize the solar cell performances. The effect of dopant content has affected the band diagram, which was drawn from density of states of the components as derived from their scanning tunneling spectroscopy. The solar cell parameters were then correlated with the experimental band diagram of the heterojunctions. In Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM p−i−n heterojunctions, we have achieved a high open-circuit voltage of 1.13 V with an energy conversion efficiency of 12.8%. The solar cell parameters have been correlated with junction properties, which have been studied through in-depth analysis of diode characteristics.

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INTRODUCTION Since the introduction of hybrid organic−inorganic perovskites, solar cells based on such materials have earned considerable attention due to their low cost, solution processability, and promising energy conversion efficiency (η).1 With perovskites being the prime light-harvesting material, surface morphology of thin films in controlling the interface of heterostructures,1−3 stoichiometry,4,5 and optical properties6 of the perovskites have always been topics of extensive research. Apart from morphology and stoichiometry, compositional alteration in the perovskite compound affected photovoltaic parameters due to an alteration in their energy levels.7 So far, in the traditional CH3NH3PbI3 material, substitutions at all three ionic sites have been studied. Chloride doping at the anionic site lengthened the carrier diffusion length, resulting in an increase in η.8 Bromide in the form of CH3NH3PbI(Br)3, on the other hand, due to its wide optical band gap and a strong photoluminescence (PL) emission in the visible region, adversely affected solar cell parameters.9 Replacement of methylammonium, CH3NH3+ (MA+), with formamidinium, CH(NH2)2+ (FA+), resulted in a better photovoltaic performance due to the extended light-harvesting capability (reduced band gap energy), longer charge diffusion length, and superior photostability of the latter.10 When the organic cations are substituted by inorganic cesium (Cs+) ions, the corresponding compound showed significantly enhanced photostability and moisture tolerance due to a better ionic interaction.11 Substitution at the metal site tuned the band gap and thus the window of solar cell operation.12,13 In this direction, homovalent tin (Sn2+), strontium (Sr2+), calcium (Ca2+), etc., © 2017 American Chemical Society

have been incorporated in the conventional perovskite structure.14−16 These studies inferred that a greater ionic nature of interaction between the metal dopant and iodide lowers the band gap of the compound.16 Among heterovalent dopants, bismuth (Bi3+), gold (Au3+), indium (In3+), and antimony (Sb3+) have been considered.17,18 The trivalent dopants as such tuned the band gap of the material in a large manner. The band gap in turn affected energy alignment at both of the interfaces (n−i and i−p) and thereby the opencircuit voltage (VOC) of perovskite solar cells. In a TiO2| CH3NH3Pb(Sb)I3|spiro-OMeTAD direct n−i−p structure with ITO and gold as electrodes, VOC could be found to depend strongly on the antimony content.18,19 Here, spiro-OMeTAD is the commonly used abbreviated form of 2,2′,7,7′-tetrakis(N,Ndi-4-methoxyphenylamino)-9,9′-spirobifluorene, and ITO represented the semitransparent indium tin oxide electrode. In both of these reports, the structures were not the best in extracting holes and suffered from low electron mobility in TiO2 leading to carrier recombination.20 These issues hence restricted one from achieving a high VOC despite a wider band gap of the perovskite upon Sb-doping. Moreover, a single-step deposition technique was opted that did not provide any control over crucial factors like morphology, crystallization, and remnant PbI2 amount of the perovskite film.20 It is therefore essential to fabricate inverted solar cells through a low-cost and simplified film-deposition technique with an improved Received: July 15, 2017 Revised: August 26, 2017 Published: August 29, 2017 20177

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As an electron-transport layer (ETL), PCBM was spun from a 20 mg/mL solution in chlorobenzene at 2500 rpm followed by another round of annealing at 100 °C for 20 min on a hot plate. To complete device fabrication, aluminum as a top electrode was thermally evaporated in a vacuum chamber, which was fitted to the glovebox. The active area of the cells was 10 mm2. Characterization of the Materials. The materials were characterized by optical absorption and photoluminescence (PL) spectroscopy, X-ray diffraction (XRD) patterns, and energy dispersive X-ray (EDX) analysis. The optical measurements were carried out with a Shimadzu UV-2550 spectrophotometer and Spex Fluoromax 4P emission spectrophotometer, respectively. A Bruker D8 Advanced X-ray powder diffractometer (Cu Kα radiation, λ = 1.54 Å) was used to record the XRD patterns, and a Jeol JEM-2100F TEM was used to obtain the EDX spectra. Surface morphology of the films was recorded with a Nanosurf Easyscan2 atomic force microscope (AFM). The materials were further probed with a Nanosurf Easyscan2 scanning electron microscope (STM) in an ambient condition. The differential conductance (dI/dV) spectra that have correspondence to the DOS of the materials enabled us to locate the conduction and valence band-edges (CB and VB, respectively) of the inorganic and highest occupied molecular orbitals and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of the organic semiconductors. Characterization of the Devices. Current−voltage (I−V) characteristics of the devices under dark and 1 sun illumination conditions were recorded with Keithley 2636A electrometer using LabTracer software. Electrodes of the devices, which were kept in the glovebox, were connected to the electrometer via 3axis micropositioners having pressure-loaded spring probe contacts. A 300 W Solar Simulator (Newport-Stratfort model 76500) attached with an AM1.5 filter placed outside the glovebox acted as a source for illumination through the base of the glovebox. The intensity of the simulated solar light on the device was 100 mW/cm2. While I−V characteristics were recorded under an illumination condition, regions outside the cell area were covered to avoid any contribution from neighboring areas or cells. To record the external quantum efficiency (EQE) spectrum of the solar cells, a 1/8 m monochromator (Oriel Cornerstone 130) was used to disperse the simulated solar illumination; the corresponding photocurrent was measured with a Hewlett-Packard 34401A digital multimeter. Impedance spectroscopy on the devices was recorded with a Solartron impedance analyzer model 1260A driven by SMaRT software. A test voltage of 10 mV rms was swept in the 1 Hz to 100 kHz range (10 points/decade).

controllability, so that the true essence of heterovalent doping by antimony can be envisaged. In this work, we targeted to form p−i−n inverted heterojunctions with antimony-doped organic−inorganic hybrid perovskites. We, in addition, used a two-step spin-coating technique to form doped-perovskite thin films for in-depth control of its morphology and thereby the photovoltaic activities. Use of Cu@NiO and PCBM as carrier-selective materials has significantly reduced the cost and complexity of the device fabrication method along with more balanced carrier extraction. The band diagram of Cu@NiO|CH3NH3Pb(Sb)I3| PCBM heterojunctions drawn from scanning tunneling spectroscopy (STS) and the corresponding density of states (DOS) of the individual materials inferred the possibility of achieving a high VOC in the proposed dopant-content-optimized planar structures.

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EXPERIMENTAL SECTION Materials. Methylammonium iodide was purchased from Dyenamo AB. Lead(II) iodide (99%), antimony(III) iodide (98%), and N,N-dimethylformamide (anhydrous 99.8%) (DMF) were purchased from Sigma-Aldrich Chemical Co. Chlorobenzene (99.5%) and isopropyl alcohol (HPLC grade) were purchased from Merck. Phenyl-C61-butyric acid methyl ester (PCBM) (99%) was purchased from M/s SES Research, Houston, TX. The materials, which were stored in a nitrogenfilled glovebox to prevent adsorption of humidity and oxygen, were used without further purification or treatment. Formation of Perovskite Thin Films and Fabrication of Devices. In practice, the ITO-coated substrates were first cleaned through a usual protocol followed by UV−ozone (UVO) treatment for 20 min. To form a hole-transport layer (HTL) of 40 nm thickness, 5 at. % copper-doped NiO (Cu@ NiO) was spun from their hydroxide solutions at 3000 rpm for 30 s; the film was then annealed at 425 °C for 15 min.21 The HTL-coated substrates were subjected to a short UVO treatment followed by immediate transfer to a glovebox for perovskite layer formation, for which a two-step or sequential spin-coat method was adopted.2 The PbI2−SbI3 precursor (1 M PbI2 in anhydrous DMF with x% SbI3; x% = 0, 3, 5, 8, and 10) was prepared by stirring the mixed-solution overnight at 70 °C. Prior to the deposition, the substrates were preheated to 60 °C. The PbI2−SbI3 solution was then spun at 2500 rpm for 30 s that resulted in a transparent and reddish-yellow colored Pb(Sb) film. Immediately thereafter, 0.3 mL of CH3NH3I solution in isopropanol (10 mg/mL, kept at 70 °C) was loaded onto the “wet” Pb(Sb)I2 layer with a variety of loading times (τload) ranging from 0 to 60 s before spinning off the excess CH3NH3I solution to form thin films. The films were then solvent-annealed at 100 °C for 20 min to complete formation of the perovskite film. During the solvent-annealing process, the film along with a small glass vial were placed on a hot plate and covered with a Petri dish. The smaller glass vial filled with isopropanol:DMF (100:1 v/v) mixed solvent was kept close to the perovskite film so that the solvent vapor could promote morphology and crystallinity of the perovskite layer. The final perovskite film turned out to be transparent and brownish in color. We, in addition, optimized a range of other parameters, such as substrate preheat temperature, concentration of the CH3NH3I solution, and so forth for a higher device efficiency.

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RESULTS AND DISCUSSION Importance of Two-Step Spin-Coat Method. Several approaches have so far been reported to form thin films of CH3NH3PbI3, such as solution-processed one-step and twostep or sequential deposition methods, vacuum deposition, and so forth.22 Among these, the two-step method provides a better control on morphology of perovskite films along with an easier processability. Such controls over morphology and thereby also on the interface with carrier-transport layers are extremely important in achieving efficient photovoltaic performances.23 In a two-step approach, the morphology of the final perovskite film depends strongly on the condition of PbI2 film deposited in the first step.24 Because PbI2 tends to form a flat and layered structure, a compact and uniform PbI2 film is 20178

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compared to those in a device based on the undoped perovskite. The efficiencies were found to be on the lower side mainly due to a presence of PbI2 residue in perovskite films. Interestingly, VOC of the devices increased monotonically with an increase in the antimony content (up to 8%) inferring widening of band gap of the perovskite. As the doping concentration increased beyond 8%, the device efficiency decreased with a significant drop in fill factor; a lower value of VOC was evidenced. This could be attributed to an energy level mismatch between the perovskite and the transporting layers due to a higher content of antimony incorporation. Scanning Tunneling Spectroscopy, Density of States, and Band Diagram. Scanning tunneling spectroscopy (STS) has recently been demonstrated as a unique tool to determine the density of states (DOS) of semiconductors and thereby band-mapping in junctions.29 We therefore aimed to correlate the results with the energy-level diagram of the heterojunctions. To do so, we recorded tunneling current as a function of voltage through ultrathin films of the individual components, that is, Cu@NiO, CH3NH3Pb(Sb)I3, and also PCBM. From the DOS spectra, we located their HOMO (or VB) and LUMO (or CB) energies. Because STS is a localized mode of measurement, we measured tunneling current at many points on all of the ultrathin films. Typical DOS spectra of Cu@NiO, a couple of CH3NH3Pb(Sb)I3, and PCBM are shown in Figure 2. Spectra of other CH3NH3Pb(Sb)I3 have been placed in Figure S1 for clarity. The first peak in the positive tip-voltage from 0 V, implying withdrawal of electrons from the semiconductor, inferred the location of HOMO (or VB) level. Similarly, the first peak in the negative voltage resulted in LUMO (or CB) energies. The range of measurements on each of the materials has been summed as histogram of HOMO (or VB) and LUMO (or CB) energies; such histograms have been placed in the inset of respective DOS spectra (Figure 2 and Figure S1). We may add here that the band-edges of the materials did not depend on the thickness of the individual layer. As such, with an increase in the antimony content, the band gap of the perovskite widened (Figure 3); there has been a little shift in the relative position of Fermi energy toward the CBedge that implied introduction of electrons in the system upon antimony-doping. The widening of the band gap has also been observed in optical absorption spectroscopy and corresponding d(αhν)2/d(hν) versus hν plots (see Figure S2), which show that the optical gap increased from 1.58 to 1.71 eV due to antimony incorporation. The difference between the band gap obtained from DOS spectra and the optical gap from absorption spectroscopy is generally explained by taking exciton binding energy into account. In the present system, the first peak in the DOS spectrum may also arise due to trap-states, thus lowering the value of measured band gap. Here α, h, and ν represent the absorption coefficient, Planck’s constant, and frequency, respectively. From the HOMO (or VB) and LUMO (or CB) energies of the components, we formed an energy level diagram of the p− i−n heterojunctions (Figure 3). The diagrams, when viewed for each antimony content in the perovskite, infer that there may be an optimum antimony content at which the energy levels of the p−i−n heterojunction would yield the highest VOC while retaining a type-II band alignment at both p−i and i−n interfaces. The energy diagrams are hence concurrent with the solar cell characteristics (Table 1), which inferred an optimized VOC of 0.97 V and corresponding η of 7.68% in the device based on the perovskite with 8% antimony content.

usually formed. This opens some challenges for the two-step deposition process while using the conventional dip-coating method in the second step. Usually a long exposure to the CH3NH3I solution (in isopropanol) is opted to achieve a complete conversion to perovskite. This often leads to poorer surface morphology (due to dissolution with the polar solvent) of the film, creates lead deficiency and thus affecting device performance.25,26 In contrast to such dip coating, spinning of a CH3NH3I solution in the second step not only reduces unnecessary waste of materials but also offers more controllability of the film deposition process. Moreover, coverage of the perovskite film and its morphology are reported to be advantageous for device application while using the twostep spin-coat technique.2,24,27,28 Antimony Incorporation in CH3NH3PbI3 and Solar Cell Characteristics. In this work, we first varied the antimony content in CH3NH3PbI3 perovskite to achieve an energy-leveloptimized p−i−n heterojunction. We then proceeded to control morphology of the optimized perovskite layer and monitored its role on photovoltaic activities. To study the effect of antimony dopant on device performance, we have fabricated devices in a Cu@NiO| CH3NH3Pb(Sb)I3|PCBM configuration. I−V characteristics of the heterojunction devices under illumination are shown in Figure 1; solar cell parameters of the devices have been listed in

Figure 1. Current−voltage characteristics of Cu@NiO|CH3NH3Pb(Sb)I3|PCBM heterojunctions under 1 sun illumination with different Sb3+-content as stated in the legends.

Table 1. The results inferred a noticeable effect of antimony addition in CH3NH3PbI3 on the device parameters. The device with pristine CH3NH3PbI3 exhibited an η of 6.50%, which improved upon antimony incorporation. The composition with 8% antimony led to an optimized efficiency of 7.68%. This increase in device efficiency is largely due to ∼9% enhancements in short-circuit current (JSC) and ∼7% increase in VOC as Table 1. Photovoltaic Parameters of Cu@NiO| CH3NH3Pb(Sb)I3|PCBM Heterojunctions with Different Sb3+-Content percentage of antimony in CH3NH3Pb(Sb)I3

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

0% 3% 5% 8% 10%

14.17 15.05 15.20 15.53 11.16

0.90 0.92 0.94 0.97 0.89

51 50 51 51 32

6.50 6.92 7.29 7.68 3.18 20179

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Figure 2. Typical DOS spectra of Cu@NiO, a couple of CH3NH3Pb(Sb)I3 with antimony content of 0 and 8%, and PCBM. Solid and broken vertical lines point to the peaks closest to 0 V locating the position of VB (HOMO) and CB (LUMO) edges, respectively, with respect to the Fermi energy, which was set to 0 V. The inset of each plot contains histograms of VB (HOMO) and CB (LUMO) edges of the materials.

We may add here that we have used UVO-treated Cu@NiO films as a hole-transport layer. UVO-treatment has been an effective route to modulate the work-function of the HTL without affecting its pristine band gap and thereby a better η in perovskite solar cells.30 The process also modifies the properties of the oxide through a removal of surface adsorbents, thereby improving the wettability and hence a better interface formation possibility.31 A prolonged UVO treatment, on the other hand, degrades the device performance due to excess oxygen content in Cu@NiO films. The improved p−i interface along with a shift in Fermi energy of the oxide led to an efficient photoinduced charge-transfer process from the perovskite to Cu@NiO. Such a charge-transfer has been inferred by observing an efficient PL-quenching of the perovskite when it was formed on UVO-treated Cu@NiO layer as compared to

Figure 3. Band diagram of Cu@NiO|CH3NH3Pb(Sb)I3|PCBM heterojunctions without and with antimony dopant as obtained from the DOS spectra and histograms thereof.

Figure 4. (a) Optical absorbance spectra and (b) corresponding d(αhν)2/d(hν) versus hν plots of CH3NH3Pb0.92Sb0.08I3 thin films formed with different loading times of CH3NH3I solution onto “wet” Pb(Sb)I2, as stated in the legends. 20180

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Figure 5. (a) XRD patterns of CH3NH3Pb0.92Sb0.08I3 thin films with τload = 40 s. The peaks at 12.7° and 38.7°, which are marked with asterisks, represent (001) and (003) planes, respectively, as signatures of PbI2 (JCPDS file 07-0235). In the inset, the section containing peaks representing (001) of PbI2 and (110) of CH3NH3PbI3 is shown. (b) τload dependence of the XRD patterns showing only the 10−15° region. τload is shown in the legends.

shown in Figure 4b. The plots showed a distinct peak at 1.69 eV, which was independent of τload. Because the gap is known to depend on antimony content, the invariance of band gap with τload infers that the dopant concentration was not affected through this approach, which however controlled the morphology of the thin film (as discussed in a later section). To ensure antimony incorporation in the perovskite structure, we carried out EDX measurements at several locations to balance the localized nature of the measurement. Figure S4 presents the EDX spectrum corresponding to the optimized antimony content (8%), whereas the targeted and obtained atomic percentages of antimony for all of the doping concentrations are tabulated in Table S1. The results evidence a presence of antimony in the compounds as a replacement of lead with an atomic percentage close to the desired value. We then studied the time evolution of Pb(Sb)I2 conversion into CH3NH3Pb(Sb)I3 perovskite through XRD measurements. The XRD patterns showed the conventional diffraction peaks of pure CH3NH3PbI3 (Figure 5) that mostly matched the reported results.28 Apart from signatures of unreacted PbI2 at 12.7° and 38.7°, no additional peaks appeared in the spectra, ruling out the formation of separate compounds with the dopant and hence inferring the possible substitutional nature of antimony dopant. Interestingly, the intensity of the peaks depended on τload. With increasing τload, the intensity of diffraction patterns of the perovskite increased, whereas the peak due to PbI2 shed its intensity, evidencing its conversion to the desired perovskite. The amount of PbI2 residue in the film was found to be negligible when τload was more than 50 s. It may be stated here that a small amount of residual PbI2 has been found to be beneficial in perovskite solar cells.35−37 Such a PbI2 layer provides a platform for a better crystallization of the perovskite layer, probably due to a reduced lattice strain with the HTL. Moreover, at the perovskite/HTL interface, PbI2 reduces the surface potential of the perovskite grain boundaries, thereby facilitating hole-transport and electron-blocking capabilities and thereby a decrease in carrier recombination possibilities. On the other hand, a serious amount of carrier recombination and a decline in charge transport were evidenced in devices having a thicker and also a smaller or

that when the material was deposited on an untreated oxide layer (see Figure S3). In the figure, the PL of CH3NH3Pb(Sb)I3 could moreover be seen to be a little blue-shifted as compared to that of CH3NH3PbI3 due to a widening of the band gap upon antimony-doping. The results are hence in concurrence with STS studies, which evidenced widening of the band gap in antimony-doped perovskites. Formation and Morphology of the Perovskite: Loading Time Dependence. We then proceeded to vary the loading time (τload) of the CH3NH3I solution onto a “wet” (as deposited) Pb(Sb)I2 layer. In general, in a two-step solution approach, the spin-coated PbI2 film is annealed at 70 °C as the first step.32,33 This leads to rapid nucleation and growth of PbI2 crystals from the amorphous precursor. Here, by omitting the annealing treatment, we aimed to use the amorphous domains of the “wet” PbI2 precursor film for facile penetration of CH3NH3I solution and reaction. As such, CH3NH3I has a stronger binding capacity with Pb2+ than DMF molecules; the lead ions can therefore substitute the solvent molecules to selfassemble into an ordered perovskite crystal.34 Also, the energy requirement for Pb−I ionic bond formation is higher than that for the PbI2/CH3NH3I dipolar interaction, so that the desired reaction (intercalation) to form CH3NH3PbI3 occurs preferentially to complete perovskite conversion. To infer a correlation between loading time and optical density of the films, we have recorded the respective UV− visible absorption spectra (Figure 4a). All of the perovskite films absorbed a broad range of the spectra covering until the 750 nm region and resembled that of the perovskite. As evidenced from the figure, the absorbance initially grew monotonically with τload evidencing gradual conversion to the perovskite; at a τload above 40 s, the absorbance started to decrease, implying a damage of the perovskite film due to an overexposure to isopropanol. Apparently, from these absorbance spectra, the presence of any unreacted PbI2 in the perovskite film could not be investigated due to a high absorption coefficient of the perovskite that would suppress the signatures of residual PbI2, if any, at around 493 nm. The optical gap of the perovskite could be estimated from the onset of d(αhν)2/d(hν) versus hν plots for direct band gap semiconductors; such spectra of the perovskite for each τload are 20181

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Figure 6. AFM topographies of CH3NH3Pb0.92Sb0.08I3 thin films with different τload values.

no amount of PbI2. It is hence crucial to optimize the amount of PbI2 residue in the final perovskite film. To study the role of loading time on the crystallinity of the perovskite films further, we have calculated the crystallite size using the Williamson−Hall (WH) method. Such results are shown in Table S2. The method allowed us to estimate the increasing tendency in the crystallite size with τload. Such a behavior is due to the fact that a longer loading time favors the intercalation of CH3NH3I into the PbI2 film, facilitating formation of larger perovskite crystals. It is worthy to add that an introduction of Sb3+ in the perovskite lattice leads to the formation of a perovskite, which is more compatible to an ideal cubic perovskite structure. Such an increment in the stability can be evidenced by calculating the Goldsmith's tolerance factor (t) for both pristine and antimony-doped CH3NH3PbI3. This factor for the doped one has a calculated value closer to unity (0.93) as compared to the undoped perovskite (0.8138), implying a higher structural stability after doping. When we recorded AFM images of the films at different τload (Figure 6), it appeared that the loading time controlled the morphology also. With an increase in τload, the crystallite size enlarged; a higher level of surface roughness however appeared. The residual PbI2 together with crystallite size and surface roughness may have a complex effect on photovoltaic performance. While a little amount of PbI2 may be beneficial, a larger crystallite size may facilitate carrier transport. A rough surface may however result in pin-holes thus adversely affect the device performance. We therefore have plotted the PbI2 content (in the form of the intensity ratio between peaks at 12.7° and 14.1° in the XRD spectra) and the surface roughness (RMS) from AFM images. The comparison, as presented in Figure 7, infers that the loading time effectively reduces the amount of PbI2 residue at the expense of surface roughness. To achieve an optimized device performance, it is therefore expected that a loading time of 40 s may probably provide the best solar cell with a perovskite film having neither a large amount of PbI2 residue nor a high surface roughness. In the figure, we have added the solar cell efficiencies of devices based on perovskite layers formed using different loading times. The results concur with the thesis that an optimum morphology of the perovskite should have an optimized amount of PbI2 residue and also an appropriate surface roughness to yield an efficient solar cell. The detailed solar cell characteristics are being discussed in the next section. Morphology of CH3NH3Pb(Sb)I3 Layer and Solar Cell Characteristics. We have made an in-depth analysis of the impact of loading time on photovoltaic performance of the

Figure 7. Optimization of τload on the basis of PbI2 conversion and film roughness. The blue data points denote the values for optimized conversion and roughness. Solar cell efficiencies corresponding to each loading time are also included in the plot.

devices under dark and standard white light illumination conditions (1 sun). While the characteristics for the morphology-optimized structure (τload = 40 s) under dark and 1 sun have been presented in Figure 8a, part (b) of the plot contains τload variation of the characteristics under 1 sun. The diode and solar cell parameters have been summed in Table 2 for comparison. The dark characteristics evidenced a higher rectification ratio with an increase in τload; the ratio reached above 100 at a τload of 40 s. Beyond 40 s, an increased roughness lowered the ratio resulting in inferior diode characteristics. Under illumination, I−V characteristics of the devices evidenced similar tendency. While all of the diodes acted as solar cells, photovoltaic parameters could be found to increase monotonically with an increase in τload in the 0−40 s range. This can be attributed to a gradual improvement in the conversion of Pb(Sb)I2:CH3NH3I onto CH3NH3Pb(Sb)I3 during the loading process. Such a conversion resulted in a decrease in the thickness of the insulating PbI2 layer between HTL and perovskite layers and thereby facilitates hole extraction. The optimum device performance was obtained at a loading time of 40 s with an efficiency of 10.5% and a significantly high VOC of 1.13 V. With a higher loading time, the devices yielded a poorer performance that may be attributed to an inferior film quality. We also aimed to check the consistency in the device performance by fabricating devices through a number of 20182

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Figure 8. (a) I−V characteristics of the Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM heterojunction under dark and 1 sun illumination conditions. In both of the characteristics, the loading time was 40 s. (b) I−V characteristics of heterojunctions based on perovskite layers formed through different loading times. Characteristics only under 1 sun illumination are shown.

obtained in that particular batch. The observed efficiency is still somewhat lower than the reported value of 16.4% in an indirect heterojunction (p−i−n) with oxides as a hole-transport material,39 even though the certified η of conventional perovskite solar cells with direct heterostructure (n−i−p) reached over 22%.40 Hysteresis in I−V Characteristics. Photocurrent hysteresis has been a major hindrance in organometal trihalide perovskite solar cells especially for devices with planar geometry.41 To study the hysteretic behavior of our devices, we have recorded I−V characteristics under a voltage loop. Typically, a forward (−V to +V) and a reverse scan (+V to −V) tend to underestimate and overestimate devices’ efficiency, respectively. In the present study, such a tendency, although present, was not too significant (Figure 9). The measured variation in JSC was about 4.3%, and η of the device changed by as little as 1.2%, that is, from 10.5% to 11.7%, due to a small change in VOC and fill factor in the two sweep directions. While the presence of excess ions (PbI2 residue) and trap states at the surface and grain boundaries in perovskite films can be attributed to cause hysteresis in I−V characteristics, an improved quality of the perovskite film, a more balanced electron (Je) and hole (Jh) flux due to the inverted planar structure, and effective passivation of the trap states by the top PCBM layer are assumed to suppress the hysteresis.41−43 The results presented here, that is, suppression of hysteresis, have hence brought out the further advantage of the two-step

Table 2. Diode Characteristics and Photovoltaic Parameters of Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM Heterojunctions with Different Loading Times of CH3NH3I loading time (s)

rectification ratio, I+2V/I−2V

JSC (mA/cm2)

VOC (V)

fill factor

efficiency (%)

0 10 20 30 35 40 45 50 60

35 40 47 81 98 137 110 82 38

15.53 15.76 15.94 16.12 16.21 16.29 16.15 16.08 15.23

0.97 0.98 1.02 1.07 1.12 1.13 1.10 1.08 1.05

0.51 0.51 0.52 0.54 0.56 0.57 0.55 0.54 0.55

7.68 7.87 8.45 9.31 10.17 10.49 9.77 9.38 8.80

batches following an identical procedure. As presented in Figure S5, the histogram of cell efficiencies showed a decent reproducibility over batch-to-batch production. Inconsistency in the performance of perovskite solar cells is generally ascribed to the difficulties in controlling the active layer formation procedure.28 Precise control over each step of sequential film formation improved the consistency of device output. It may be worth mentioning that in a particular batch of device fabrication with a loading time of 40 s, efficiency of the cells was reached until 12.8% (see Figure S6). In the figure, we have presented typical I−V characteristics under dark and illumination conditions along with a histogram of efficiencies

Figure 9. (a) Current−voltage characteristics of the optimized perovskite solar cell under different voltage sweep directions (scan speed 0.1 V/s) as stated in the legends and (b) variation of difference factor with respect to τload. 20183

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loss, which is however comparable to similar reports with usual VOC’s.46,47 Here q and EG denote electronic charge and band gap of the active material, respectively. A recipe for lowering recombination loss in conventional devices would hence be still applicable to these perovskite solar cells for further performance enhancement. To get further insights, we analyzed the I−V characteristics under dark condition to compare the diode turn-on voltages. The turn-on voltage, which is an indicator of the built-in field within the device, increased due to antimony-doping (see Figure S6). Moreover, the presence of Sb3+ in place of Pb2+ in the perovskite attracted iodide ions toward the I-sites, decelerating PbI2 elimination from the perovskite.19 This would improve the interfaces and hence Voc. We may add here that a thin layer of PbI2 in the final perovskite device also plays a beneficial role in achieving a high VOC.35−37 Because PbI2 has a higher band gap (Eg = 2.3 eV) than the perovskite, its thin layer can bend the VB edge of the perovskite down in energy, creating a gradient or cascaded-form in the band structure.5 This resulted in an upward band-bending at the perovskite|HTL interface that could lower the electron backflow possibility and hence an improved charge collection and in-turn a higher VOC. Improvement of the JSC, on the other hand, has resulted from the additional carriers introduced through dopants. Also, high crystallinity of the perovskite layer on the treated HTL-coated substrates favored light-harvest and carrier-transport phenomena. However, the values of both JSC and fill factor were unfortunately below the reported results, restricting us from achieving a still higher-efficient device. To obtain further insights on charge-transfer processes in the devices, we have recorded impedance spectra of the solar cells under an illumination condition. The Nyquist plots for devices formed with perovskite layers having different τload values are presented in Figure S7. The plots are dominated by a large distinct semicircle in the low frequency region with a small arc in the high frequency section. Such a double-arc impedance spectrum is quite common in mesoscopic sandwiched structures; the spectrum can be fitted to an electrical analogue containing two parallel combinations of capacitor-resistor networks (CP−RP) connected in series. In general, the semicircle in the high-frequency region is dictated by interfacial charge transfer between the perovskite layer and adjacent charge-transport layers; the features in the low-frequency region are governed by charge accumulation and recombination within the active layer.48 When Nyquist plots for different τload values are analyzed, the recombination resistance (RRec) maximized at τload = 40 s. Such a reduction in carrier recombination led to efficient charge separation in the optimized device. The presence of a significant amount of PbI2 residue in the perovskite layer (when τload < 40 s) facilitated charge accumulation in the layer and thereby increased recombination pathways.33 At τload > 45 s, pinholes present in the perovskite layer raised the number of shunts and hence aided recombination. While the series resistance (RS) remained mostly unaltered, the τload-dependence of RRec/RS matched well with the variation of fill factor and VOC of the solar cells because both of these parameters are known to be affected adversely by carrier-recombination phenomenon. To seek a deeper insight, we plotted the ratio between the resistances (RRec/RS) as a function of τload and correlated the plot with the variation of fill factor and VOC of the corresponding devices (Figure 11). Such a graphical compar-

synthesis process with a control over conversion of PbI2:CH3NH3I to CH3NH3PbI3 in general and the antimonydoped perovskite in particular. The amount of hysteresis in I−V characteristics can be quantified by a “difference factor” as44 η − ηforward difference factor = reverse ηreverse where ηreverse and ηforward are the efficiencies in the reverse and forward voltage scans, respectively. We proceeded to measure the “difference factor” as a function of loading time (τload); the corresponding plot of the factor versus loading time, as presented in Figure 9b, shows that the I−V’s became less hysteretic with an increase in τload. In the same figure, we have added the loading-time dependence of PbI2 content in the perovskite as obtained from XRD analysis (from Figure 7). The trends of “difference factor” and PbI2 content with the loading time matched quite well, implying that excess ions from PbI2 residue must have caused the hysteresis in I−V characteristics. We may emphasize here that the scan speed was kept moderate to achieve a widest hysteresis, preventing the capacitive effect arising out of a fast scan rate and also photoinduced alterations due to a slow scan rate.45 When we studied the external quantum efficiency (EQE) spectrum of the solar cells, we observed its similarity with the optical absorption spectrum of the heterojunction (Figure 10).

Figure 10. EQE spectrum of a Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM heterojunction device and optical absorption spectrum of the heterojunction.

In the champion device containing an almost completely converted perovskite material, charge carriers in the perovskite layer were efficiently extracted through the electrodes. As a result, the EQE value of the device reached ∼62% across a broad range of wavelengths, ensuring an excellent light-harvest and conversion in the devices. Junction Properties. Among reports on perovskite solar cells with antimony-doped CH3NH3PbI3 as the absorber material,18,19 the obtained VOC of 1.13 V in our devices can be considered to be on the higher side. From the point of view of the band structure, Sb3+-doping in conventional perovskites engineers the bands and widens the optical gap (Figure 3). In addition, the work-function deepening of Cu@NiO through UVO treatment leading to a larger built-in field between the HTL and the ETL has been instrumental in yielding a larger VOC.43 However, the qVOC/EG ratio was only ∼66% (and EG − qVOC ≈ 0.57 eV) in the device, implying the margin of potential 20184

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od for each τload (Table S2), device histogram of Cu@ NiO|CH3NH3Pb0.92Sb0.08I3|PCBM heterojunctions (Figure S5), I−V characteristics of loading time optimized Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM heterojunction device and corresponding efficiency histogram (Figure S6), dark I−V characteristics of 8% Sb doped and undoped perovskite devices (Figure S7), and Nyquist plots of Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM heterojunction devices for different τload (Figure S8) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-33-24734971. Fax: +91-33-24732805. E-mail: [email protected].

Figure 11. Ratio of recombination to series resistances (RRec/RS), fill factor, and VOC versus τload. The blue data points denote the values for optimized VOC and fill factor values.

ORCID

Amlan J. Pal: 0000-0002-7651-9779 Notes

ison inferred that the performance of the perovskite solar cells was optimized at τload = 40 s due to a large recombinationresistance and a small series-resistance.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.J.P. acknowledges the JC Bose Fellowship (SB/S2/JCB-001/ 2016) of SERB. S.C. and U.D. acknowledge the DST INSPIRE Fellowship [IF 140158] and CSIR Junior Research Fellowship no. 09/080(0843)/2012-EMR-I (roll no. 519699), respectively. We acknowledge financial assistance from a SERIIUS project bearing grant no. IUSSTF/JCERDC-SERIIUS/2012.

CONCLUSION In summary, we report in-depth and systematic studies on the formation and improvement of the planar perovskite solar cells in which antimony dopant was introduced and optimized in the perovskite. In this work, we have established the benefits of a modified two-step spin-coat method for formation of the perovskite layer. We have shown that a “wet” PbI2 layer in combination with “loading time” of CH3NH3I can lead to a complete conversion of PbI2 into the perovskite material. We further have observed that an optimized residual PbI2 content and surface roughness of the perovskite can maximize the efficiency of the solar cells. From STS-assisted DOS measurements, we have deliberated on the effect of antimony inclusion in the perovskite compound and thereby on the band diagram of p−i−n heterojunctions. With a control over band alignment and morphology of the perovskite layer, we have achieved a high VOC of 1.13 V with an efficiency of 12.8% in a device with CH3NH3Pb0.92Sb0.08I3. A thorough analysis of diode characteristics and impedance spectroscopy inferred that the performance of the morphology-optimized Sb-doped perovskite solar cells maximized when the device has a large recombinationresistance and a small series-resistance. The work thus presents a detailed approach to routinely control morphology in fabricating planar and highly efficient doped-perovskite solar cells.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06963. Typical DOS spectra of CH3NH3Pb(Sb)I3 with Sb content of 3%, 5%, and 10% along with their respective histograms (Figure S1), optical absorption spectra and corresponding d(αhν)2 /d(hν) versus hν plots of CH3NH3Pb(Sb)I3 with different Sb content (Figure S2), PL spectra of CH3NH3Pb0.92Sb0.08I3, d(αhν)2/d(hν) versus hν plots of CH3NH3Pb0.92Sb0.08I3 for each τload (Figure S3), EDX spectrum of CH3NH3Pb0.92Sb0.08I3, targeted and obtained percentage of elements (Figure S4 and Table S1), measured crystallite size of CH3NH3Pb0.92Sb0.08I3 by Williamson−Hall (WH) meth20185

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