Non-Thermal Annealing Fabrication of Efficient Planar Perovskite

Feb 19, 2015 - The Journal of Physical Chemistry Letters 2016 7 (14), 2735-2741 ..... Vapor-Assisted Solution Deposition and Application as Solar Cell...
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Non-Thermal Annealing Fabrication of Efficient Planar Perovskite Solar Cells with Inclusion of NH4Cl Yani Chen,† Yixin Zhao,‡ and Ziqi Liang*,† †

Department of Material Science, Fudan University, Shanghai 200433, China College of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

‡

S Supporting Information *

resultant film.15 The lead halide perovskite morphology in the one-step methods is usually controlled by nucleation and crystal growth during film formation. Additives have proved to be an effective method of facilitating homogeneous nucleation or modulating the crystallization kinetics. For instance, by adding CH3NH3Cl, 1,8-diiodooctane (DIO), or HI to their corresponding standard precursor solutions, the uniform CH3NH3PbI3 or CH3 NH 3PbI3−xCl x or HC(NH2 ) 2PbI3 films were made, respectively.16−19 All these methods, however, require thermal annealing to remove the additives and crystallize perovskite thin films. Inappropriate thermal annealing such as excessive annealing time or over high temperature would result in undesirable pinholes, intraholes, or cracks through the films due to the fast evaporation of solvents or decomposition of perovskite.20,21 Additionally, improper heat treatments could account for the uncontrolled reproducibility of device performance. More importantly, polyethylene terephthalate (PET), the typical substrate for making a flexible solar cell, cannot suffer thermal annealing over 100 °C. As a result, a nonthermal method of fabricating perovskite solar cells is highly required, which enables high-throughput and high-reproducibility of manufacturing flexible electronics. Herein we for the first time report a facile one-step solution method of fabricating high-quality perovskite films without the need of thermal annealing. The method is adding a small amount of NH4Cl to the standard precursor solutions of CH3NH3I and PbI2, followed by spin-coating to produce CH3NH3PbI3 thin films with full surface coverage. To demonstrate how such delicate NH4Cla very cheap compound and widely used as fertilizermodulates the annealing-free process, we investigate the classical perovskiteCH3NH3PbI3and conventional planar heterojunction architecture using representative holeand electron-transporting layers of PEDOT:PSS and PC61BM, respectively. The best PCE close to 10% and an impressive fill factor of 78% were obtained on these unannealed CH3NH3PbI3 film based bilayer solar cells with an active area of 11 mm2. This efficiency is comparable to that of a more recent literature report based on identical materials and device structure.22 Importantly, our unannealed devices exhibited excellent reproducibility of performance and no hysteresis behavior was observed. All perovskite films in the following discussion were prepared via one-step solution method on PEDOT:PSS substrate, which

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rganic−inorganic hybrid perovskite based solar cells have attracted considerable research attention and emerged as the forerunner in the next generation photovoltaic technology in the past two years.1−4 The power conversion efficiency (PCE) of perovskite solar cells has most recently skyrocketed to a certified 20.1%5 from 3.8%6 back in 2009. These organic−inorganic hybrid perovskite compounds adopt ABX3 structure, where the A cations are organic (typically CH3NH3+, C2H5NH3+, HC(NH2)2+) and the B cations are metal ions (Pb2+, Sn2+, Cu2+), while the X anions are halides (Cl−, Br−, I−).7 In a typical perovskite structure, the CH3NH3+ cation resides at the eight corners of the cubic unit, while the Pb2+ cation is located at the body centers of an octahedral [PbX6]4− cluster.8 Owing to specific structural properties, such organic−inorganic hybrid perovskites are known to exhibit a plethora of appealing features such as high absorption coefficient, tunable bandgaps, decent ambipolar charge mobility, remarkably low exciton binding energy, substantially long electron and hole diffusion lengths, and long-term charge life.9−11 Currently, the most commonly studied organic−inorganic hybrid perovskite are triiodide perovskite CH3NH3PbI3and mixed halide perovskiteCH3NH3PbI3−xClx and CH3NH3PbI3−xBrx. Along with advantages of costeffective solution-processability, perovskite solar cells’ efficiency is now comparable to state-of-the-art copper indium gallium diselenide (CIGS) solar cells or commercialized silicon solar cells, fulfilling the requirements for the large-scale deployment of solar energy, although there are stability and environmental problems for perovskite solar cells to overcome before commercialization.12 At present, one of the main issues encountered in perovskite solar cell fabrication lies in a fine-control of the film morphology such as surface uniformity and surface coverage, which is of paramount importance to achieve high-performance solar cells.13 Poor perovskite morphology causes electrical shunt or induces traps, which deleteriously impacts charge dissociation and transport while largely increasing the probability of charge recombination. Currently, the most frequently applied deposition methods for the perovskite layer include one-step precursor solution deposition, two-step sequential deposition, dual-source vapor deposition, and vapor assisted solution process.8,14 Among these fabrication techniques, one-step deposition of an equimolar mixture of CH3NH3I and PbI2 precursor solution is the simplest way. More importantly, this method is one of the most possible ways for realizing the large-area full-printing manufacturing. However, it often involves the formation of needle-shaped solvation intermediates (CH3NH3PbI3·DMF and CH3NH3PbI3·H2O), which leads to incomplete coverage of the © XXXX American Chemical Society

Received: January 6, 2015 Revised: February 17, 2015

A

DOI: 10.1021/acs.chemmater.5b00041 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

occurrs within the first 10 min of annealing, and the formed crystal structure is unchanged even after longer annealing. After 45 min of thermal annealing, a small amount of PbI2 phase shows up, due to the decomposition of perovskite after long-time annealing. For simplicity, the following disucssion uses the 95 °C and 10 min for annealed films since the crystal structures of perovskite are established under such thermal annealing condition and maintained afterward. In addition, grazingincident wide-angle X-ray scattering (GIWAXS) patterns of unannealed and annealed films were measured and are shown in Figure 2b,c, respectively, indicating that the unannealed perovskite film shows finer ordering structures and better orientation than the annealed film. These findings are further supported by scanning electron microscopy (SEM) images. Morphological variations of CH3NH3PbI3 films made from binary and ternary mixture solution are shown in Figure 3. Figure 3a shows that the

resembles the device structure for reliable comparison. The perovskite film without thermal annealing was directly formed via spin-coating from a ternary mixture solution in dimethylformamide (DMF) consisting of CH3NH3I, PbI2, and NH4Cl additive with a molar ratio of 1:1:0.5. This unannealed film became dark brownish immediately after spin-coating, as indicated in Figure 1a which displays a highly smooth and

Figure 1. (a) Photograph and (b) optical absorption spectrum of perovskite film fabricated without thermal annealing from ternary mixture solution of CH3NH3I, PbI2 and NH4Cl.

reflective film with full surface coverage. To further understand the evolution of this unannealed perovskite film, the optical absorption spectrum was measured as shown in Figure 1b, characteristic of CH3NH3PbI3.23 This indicates that the formation of perovskite crystals from ternary mixture solution was completed during the spin-coating step and therefore no thermal treatment is further required. Such formation of perovskite crystals under no thermal annealing is further confirmed by cyclic voltammetry (CV) measurements (Figure S1 in the Supporting Information), which shows the conduction band (CB) and valence band (VB) of −3.9 and −5.4 eV vs vacuum, respectively, typical of CH3NH3PbI3.24,25 X-ray diffraction (XRD) measurement was used to further determine the formation of perovskite phases and crystallization using ternary precursor compositions. Figure 2a presents the

Figure 3. Top-view SEM images of CH3NH3PbI3 thin films prepared on glass/PEDOT:PSS substrates: (a) from standard binary solution of CH3NH3I and PbI2 and (b) from the ternary mixture solution of PbI2, CH3NH3I, and NH4Cl without thermal annealing and (c) after thermal annealing at 95 °C for 10 min. (d) Possible mechanisms in the formation of CH3NH3PbI3 film that is spin-coated from the ternary mixture solution without thermal annealing.

CH3NH3PbI3 film fabricated from a standard binary solution is discontinuous and of low surface coverage, which consists of isolated needle-like crystals having submicrometer gain size and distinct grain boundaries. This is consistent with previous reports.16 Upon addition of NH4Cl to the standard binary precursors, film morphology with small-sized compactly packed crystallites yet no apparent crystal gain boundaries is seen in Figure 3b. By inclusion of NH4Cl, the film becomes uniform with noticeably improved surface coverage. However, under thermal annealing, the crystalline textures of the perovskite film become rough while grain boundaries are formed, as shown in Figure 3c. Crystalline domains are also found to increase with thermal annealing,27 which is in accord with XRD results in Figure 2. Moreover, such annealing yields the film fractures (Figure 3c), which will cause electrical shunt28 or induce traps and therefore are detrimental to device performance. These morphology differences between Figure 3b and 3c can be explained by the variation of intensity ratio of XRD patterns (Figure 2a), which suggests the perovskite regrows under thermal annealing. These results suggest that NH4Cl strongly affects the crystallization process of CH3NH3PbI3 and in some way acts as a glue to interconnect separated parts of CH3NH3PbI3, avoiding the formation of elongated large crystal plates and incomplete coverage. Yet the exact role of chlorine in regulating the morphology of perovskite film still remains unclear.29−31 However, it was reported that ammonia can reversibly intercalate into or form a new coordination complex with the perovskite crystal lattice or network.32 Thus, it seems reasonable that adding NH4Cl to the standard precursor solution of CH3NH3I and PbI2 could lead to the rapid formation of an intermediate crystal structure like PbI2·CH3NH3I···x·NH4Cl. Then during the film-

Figure 2. (a) XRD profiles of CH3NH3PbI3 films deposited on glass/ PEDOT:PSS substrates showing the effect of thermal annealing time at 95 °C and GIWAXS patterns of CH3NH3PbI3 film prepared from ternary mixture solution of PbI2, CH3NH3I, and NH4Cl: (b) without and (c) under thermal annealing at 95 °C for 10 min.

XRD patterns of the perovskite films made from the ternary mixture solution with different thermal annealing conditions. The unannealed perovskite film shows two intense peaks at 14.12° and 28.42°, which are assigned to (110) and (220) crystal planes of the CH3NH3PbI3 perovskite structure, respectively.26 No impurity peak is found in the XRD spectrum of unannealed film, indicating that the formation of a perovskite phase was completed with no aid of thermal annealing. It is also seen that the intensity ratio of (110) to (220) diffraction peaks is increased after 10 min annealing at 95 °C and remains almost the same with prolonged annealing time, suggesting that the recrystallization B

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Chemistry of Materials forming via spin-coating under room temperature the intermediate crystal structure will realease NH4Cl. Removal of NH4Cl in the spin-coating step is confirmed by the energy dispersive X-ray (EDX) spectrum (Figure S2 in the Supporting Information), which displays no measurable chloride remaining in the bulk film. The above mechanisms in the perovskite formation are described in Figure 3d. This issue of chloride loss in perovskite film prepared from PbCl2 + 3CH3NH3I or PbI2 + CH3NH3I + xCH3NH3Cl precursors has recently caught more attention.16,30,33 The loss pathways are generally believed to orginate from the sublimation of CH3NH3Cl30 or degradation of CH3NH3Cl into the volatile hydrochloric acid (HCl) and methylamine (CH3NH2) species facilitated by residual water.34 The fertilizer NH4Cl is well-known to slowly decompose into NH3 and HCl under ambient conditions. As a result, in the ternary mixture solution of PbI2, CH3NH3I, and NH4Cl, excess chlorides could disappear from intermediate PbI2·CH3NH3I···x·NH4Cl via the release of highly volatile NH4Cl or via the decomposition into NH3 and HCl. This process can readily proceed during perovskite crystallization due to the low vapor point of NH4Cl and spin-induced low pressure in the step of film-forming.35 In addition, the small size of NH4Cl may account for its facile intrusion−escape process in layered perovskites, similar to that of as-mentioned NH3.32 The release of chloride further catalyzes the transformation of CH3NH3PbI3 as presented in Figure 3d, which is likely to induce structural rearrangements, to some degree. As a consequence, no thermal annealing is necessary for the formation of flat and uniform CH3NH3PbI3 film by inclusion and fast release of NH4Cl. Finally, the influence of film morphology on the performance of the photovoltaic cell was examined based on CH3NH3PbI3 film made from ternary mixture solution. The perovskite solar cells were constructed with a typical configuration of ITO/ PEDOT:PSS/perovskite/PC61BM/Ca/Al device with an active area of 0.11 cm2. The device architecture is schematically shown in Figure 4a, and the corresponding cross-sectional image is presented in Figure 4b. Figure 4c,d displays the current density versus voltage (J−V) curves of NH4Cl treated CH3NH3PbI3 planar photovoltaic cells without and under thermal annealing, respectively. In the dark, all the devices show good diode behavior. Under AM 1.5G simulated light with an intensity of 100 mW cm−2, a PCE of 9.32% was obtained for the unannealed device under reverse scan with short-circuit current density (JSC) of 13.11 mA cm−2, open-circuit voltage (VOC) of 0.91 V, and fill factor (FF) of 78.11%. Large FF results mainly from greatly reduced recombination in our device. Note that obtained JSC and thus efficiency are somehow underestimated due to the notable reflection loss using our nitrogen-filled sample holder for measurements. By contrast, the annealed device gives a PCE of 6.26% under reverse scan. The deterioration of device performance is mainly orginated from the siginificant drop of VOC (to 0.73 V) and FF (to 60.38%), which is presumably related to the film fractures during thermal annealing (Figure 2c).17 The measured cell performance parameters are summarized in Table 1. To quantify the photon-to-electron conversion, the external quantum efficiency (EQE) curves were measured and shown in Figure 4e. The maxmum EQE of the devices reaches ∼70% at 520 nm, in which the annealed cell exhibits a slightly higher EQE value than the unannealed cell, in agreement with their moderate difference in JSC. Lastly, we fabricated a flexible unannealed perovskite solar cell on PET substrate, exhibiting the JSC of 14.98 mA cm−2, VOC of 0.92 V, FF of 49.20%, and hence a PCE of 6.80% under reverse scan, as shown in Figure 4f. Importantly, the

Figure 4. (a) Schematic depiction of ITO/PEDOT:PSS/ CH3NH3PbI3/PC61BM/Ca/Al device architecture. (b) Cross-sectional SEM image of the device structure. J−V characteristics of regular ITO/ PEDOT:PSS/CH3NH3PbI3/PCBM/Ca/Al solar cells measured in the dark and under light with an intensity of 100 mW/cm2 at forward and reverse bias, respectively, (c) without thermal annealing and (d) under thermal annealing at 95 °C for 10 min. (e) EQE spectra of the unannealed and annnealed devices. (f) J−V characteristics of flexbile devices on PET substrates, a photograph of which is displayed in the inset. (g) Efficiency histograms of unannealed, annnealed, and flexible and unannealed CH3NH3PbI3 planar solar cells. Note that Gaussian distributions were fit to obtain the average and standard deviations.

Table 1. Device Performance Parameters under Both Forward and Reverse Scans devices unannealed annealed flexible and unannealed

sweep direction

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

forward reverse forward reverse forward reverse

12.81 13.11 13.72 14.21 14.85 14.98

0.91 0.91 0.73 0.73 0.92 0.92

77.39 78.11 60.38 60.34 48.80 49.20

9.02 9.32 6.05 6.26 6.70 6.80

solar cell performance of unannealed devices including that on glass and PET substrates exhibited much better reproducibility than that of annealed devices, as statistically shown in Figure 4g. This indicates that our nonthermal fabrication method holds great promise of low-temperature, roll-to-roll processing of flexible thin film solar cells. Hysteresis in the J−V curve is now a well-known issue for perovskite solar cells, and it has been speculated to correlate with the process of trapping/detrapping of charge carriers,36,37 ferroelectricity,36,38 ion migration,39 and charge accumulation.40 We measured the J−V curves using both forward (short-circuit to open-circuit) and reverse (open-circuit to short-circuit) scans with a slow scanning rate of 10 mV/s. As shown in Figure 4c, negligible hysteresis is found in the unannealed devices, which is presumably attributable to largely reduced traps in the unannealed perovskite absorbers prepared from NH4Cl-added C

DOI: 10.1021/acs.chemmater.5b00041 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials precursors. Owing to the flat, uniform perovskite films with full surface coverage, defect density within or around the interface is greatly reduced, leading to reduced charge recombination and improved charge separation and transport. By comparison, the annealed devices exhibit a slightly noticeable hysteresis (Figure 4d), which is also related to thermal annealing induced cracks of peroviskite film. In summary, we have demonstrated a facile one-step solution method for nonthermal fabrication of the high-quality CH3NH3PbI3 perovskite film for solar cells. The key is addition of NH4Cl to the standard binary precursor solution, which strongly affects the crystallization process and results in the smooth CH3NH3PbI3 film with full surface coverage without the need of thermal annealling. Power conversion efficiency of 9.32% and impressive fill factor of 78% were obtained from nonthermal annealing processed planar CH3NH3PbI3/PCBM bilayer heterojunction solar cells. These room-temperature fabricated perovskite solar cells exhibited good reproducibility, and no hysteresis behavior was observed. Our nonannealing fabrication method holds enormous potential for high-throughput solutionprinting of flexible perovskite thin film solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details including materials, sample preparation, characterization, and device fabrication and measurements; Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Recruitment Program of Global Experts in China, the start-up funds from Fudan University and National Natural Science Foundation of China (NSFC) Grant No. 51473036 (Z.L.) and 51372151 (Y.Z.).

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b00041 Chem. Mater. XXXX, XXX, XXX−XXX