Modulated CH3NH3PbI3−xBrx film for efficient perovskite solar cells exceeding 18%

The organic-inorganic lead halide perovskite layer is a crucial factor for the high performance perovskite solar cell (PSC). We introduce CH3NH3Br in the precursor solution to prepare CH3NH3PbI3−xBrx hybrid perovskite, and an uniform perovskite layer with improved crystallinity and apparent grain contour is obtained, resulting in the significant improvement of photovoltaic performance of PSCs. The effects of CH3NH3Br on the perovskite morphology, crystallinity, absorption property, charge carrier dynamics and device characteristics are discussed, and the improvement of open circuit voltage of the device depended on Br doping is confirmed. Based on above, the device based on CH3NH3PbI2.86Br0.14 exhibits a champion power conversion efficiency (PCE) of 18.02%. This study represents an efficient method for high-performance perovskite solar cell by modulating CH3NH3PbI3−xBrx film.

Scientific RepoRts | 7:44603 | DOI: 10.1038/srep44603 energy band of perovskite is expanded and leads to a higher photovoltage (V OC ) of the device. Thirdly, the modulation of Br is beneficial to the interfacial energy match between TiO 2 /perovskite/spiro-OMeTAD, the injection and extraction of charge carriers are improved and thus the photocurrent and fill factor of the device are enhanced. Under optimal conditions, the device with CH 3 NH 3 PbI 2.86 Br 0.14 exhibits high reproducibility and a champion PCE of 18.02%.

Results
Crystallinity and morphology. CH 3 NH 3 PbI 3−x Br x perovskites were prepared by controlling different iodide/bromide molar ratio in perovskite precursor solutions. The resultant mixed methylammonium lead halides are termed as CH 3 NH 3 PbI 3 , CH 3 NH 3 PbI 2.91 Br 0.09 , CH 3 NH 3 PbI 2.89 Br 0.11 , CH 3 NH 3 PbI 2.86 Br 0.14 , CH 3 NH 3 PbI 2.8 Br 0.2 , and CH 3 NH 3 PbI 2 Br, respectively. Figure 1 shows the XRD patterns of the CH 3 NH 3 PbI 3−x Br x perovskite films. The two main peaks for all CH 3 NH 3 PbI 3−x Br x films approximately located at 14.2° and 28.4° can be indexed to the (110) and (220) planes 24,25 , confirming the presence of the tetragonal perovskite phase in all of the prepared films. In order to examine the influence of Br amount on the crystallinity of the perovskite films, XRD patterns of CH 3 NH 3 PbI 3−x Br x films at 13.5°~15.5° were measured and shown in Figure S1. The FWHM values of the strong diffraction peaks of the (110) face are calculated using JADE 6.0 26,27 . The FWHM values of the CH 3 NH 3 PbI 3−x Br x films are 0.17°, 0.18°, 0.16°, 0.14°, 0.13° and 0.12°, corresponding to the CH 3 NH 3 PbI 3−x Br x , x = 0, 0.09, 0.11, 0.14, 0.20, and 1, respectively. Small FWHM value means a sharper XRD pattern [28][29] . On the other hand, the characteristic (110) peak has shifted from CH 3 NH 3 PbI 3 at 14.30° to CH 3 NH 3 PbI 2 Br at 14.09°, corresponding to a reduction in the lattice parameter from a = 0.616 nm to a = 0.604 nm with the increase in the Br content due to the difference in the ionic radius of Br − (1.96 A) and I − (2.2 A), which is consistent with the literatures 22,30 .
Top-view SEM images of as-prepared CH 3 NH 3 PbI 3−x Br x hybrid perovskite films are shown in Fig. 2. It can be seen that the Br amount has a mild influence on the grain size of the hybrid perovskite. CH 3 NH 3 PbI 2.86 Br 0.14 and  Scientific RepoRts | 7:44603 | DOI: 10.1038/srep44603 CH 3 NH 3 PbI 2.8 Br 0.2 present compact, pinhole-free perovskite films and enlarged average grain size, which will minimize the grain boundary energy and be beneficial to the charge transportation. For CH 3 NH 3 PbI 2 Br, there are some pinholes appearing in the film, marked by white circles. The pinholes will produce direct contacts of spiro-OMeTAD and electron transporting layer, leading to a shunting path which probably decreases fill factor and open-circuit voltage for the device with CH 3 NH 3 PbI 2 Br 17 . Figure 3 shows the cross-view SEM images as-prepared CH 3 NH 3 PbI 3−x Br x perovskite films. Surprisingly, there are some stark contrasts of the perovskite film in vertical direction. The cementite appears as small granular grain in the capping perovskite layer for CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 2.91 Br 0.09 . With the increase of Br amount from CH 3 NH 3 PbI 3 to CH 3 NH 3 PbI 2 Br, the big grain contour becomes gradually obvious. The grain size is commensurate with its thickness among CH 3 NH 3 PbI 2.86 Br 0.14 , CH 3 NH 3 PbI 2.8 Br 0.2 and CH 3 NH 3 PbI 2 Br, most of the grain boundaries are perpendicular to the mesoporous layer to minimize the grain boundaries. As well known that the less grain boundaries will facilitate the transportation of charge 29,31 , thus it is expected that the fill factor of the corresponding device is improved by adjusting Br amount in CH 3 NH 3 PbI 3−x Br x hybrid perovskite.
However, for CH 3 NH 3 PbI 2 Br, there are many voids at the bottom of the perovskite layer which may be attributed to the dewetting during anti-solvent washing and subsequent growth 29,31,32 . In the specific experimental operation of CH 3 NH 3 PbI 2 Br, the viscosity of the precursor solution may vary owing to different compositions in the precursor solution and the methylammonium halide was composed entirely of MABr. As the nucleation grows the material dewets at the interface between solution-phase and solid-phase, meaning that it is pulled away from the regions surrounding the crystals, resulting in the formation of large voids in the film. So a distinguishing crystallization behavior was obtained without MAI in the precursor solution. Many voids in the perovskite layer will result in inferior photovoltaic performance of PSCs.
Band gap. Figure 4 shows the UV-visible absorption spectra of the CH 3 NH 3 PbI 3−x Br x perovskite films with different iodide/bromide ratio. It can be seen that with the increase of Br amount from CH 3 NH 3 PbI 3 to CH 3 NH 3 PbI 2 Br, the absorption edges of the hybrid perovskite films shift towards the shorter wavelength 33 . The bandgaps of crystallized perovskite films are 1.565, 1.579, 1.585, 1.591, 1.597, and 1.725 eV, corresponding to the CH 3 NH 3 PbI 3−x Br x , x = 0, 0.09, 0.11, 0.14, 0.20, and 1, respectively, from the Taus plots 34 shown in Figure S2. The corresponding absorption edge and band gap are listed in Table 1. The blue shift is found that: 782 nm for CH 3 NH 3 PbI 3 is shifted to 709 nm for CH 3 NH 3 PbI 2 Br, consistent with the variety of the energy band levels of perovskite materials in Figure S3. According to the literatures 34-36 and the energy level principle 2,3,12,37,38 , with the increase of Br amount in CH 3 NH 3 PbI 3−x Br x the conduction band of the perovskite ascends and the valence band descends, resulting in the absorption edge shifted towards to shorter wavelength, which indicates the formation of wider energy band and a higher photovoltage (V OC ) of the device.
Charge carrier lifetime. Time resolved photoluminescence (TRPL) intensity decay measurements of CH 3 NH 3 PbI 3−x Br x perovskite films offer a quantitative information on the dynamics of charge carrier recombination 39 , the samples were composed of FTO/CL-TiO 2 /mp-TiO 2 /perovskite, and corresponding results are presented in Fig. 5, of which the fitted time constants charge carrier lifetime (τ ) are 52, 39, 35, 34, 33, and 15 ns, corresponding to the CH 3 NH 3 PbI 3−x Br x , x = 0, 0.09, 0.11, 0.14, 0.20, and 1, respectively. The shorter τ value means that the charge more fast transfers from the perovskite to TiO 2 layer [40][41][42][43][44] . The significantly reduced τ value with the increase of Br amount in the hybrid perovskite infers an efficient charge extraction process occurred at the perovskite/TiO 2 interface. The CH 3 NH 3 PbI 2 Br yields the smallest τ value, meaning that the charge carriers extract very quickly, which may be attributed to the high conduction band of the perovskite. The high conduction band energy level is beneficial to the charge injection between the perovskite and the electron selective electrode, however, the device of CH 3 NH 3 PbI 2 Br presents the worst performance.
Charge transferring resistances. Nyquist plots of CH 3 NH 3 PbI 3−x Br x films are shown in Fig. 6, and the corresponding equivalent circuit is inset. The radius of semi-circles corresponds to the interfacial charge transfers resistance 45,46 . It can be seen that with the increase of X (Br amount), the radii first decrease, and then increase. When X = 0.14, the radius reaches the minimum, indicating the smallest interfacial charge transfers resistance among all CH 3 NH 3 PbI 3−x Br x films. Therefore, the device based on CH 3 NH 3 PbI 2.86 Br 0.14 film possesses superior charge injection characteristics and low internal resistance, resulting in better photovoltaic performance than others.
Photovoltaic performance. J-V curves (reverse scan) of the perovskite solar cells based on CH 3 NH 3 PbI 3−x Br x hybrid perovskite are shown in Fig. 7 and the corresponding photovoltaic parameters are listed in Table 1. For the device based on ancestral CH 3 NH 3 PbI 3 film, a PCE of 15.60% is obtained with short-circuit current  Furthermore, the performance of the device based on CH 3 NH 3 PbI 2 Br shows a dramatic downward. The reason for the increase and then decrease of power conversion efficiency (PCE) of the PSCs with the increase of Br amount lies in V OC , J SC and FF. From the aspect of PV energy devices 3,17,37,38 , with the increase of Br amount, the conduction band of the perovskite ascends and the valence band descends, the energy difference (Δ E 1 ) between TiO 2 Femi level and the valence band of perovskite expends, the energy difference (Δ E 2 ) between the conduction band of perovskite and TiO 2 Femi level also expends. The expanded Δ E 1 increases V OC , however appropriate Δ E 2 produces small energy loss and large J SC and FF; and excess Δ E 2 by excessive Br leads to an opposite result. From the aspect of morphology, owing to a large amount of voids in CH 3 NH 3 PbI 2 Br perovskite by dewetting, the charge extraction process is limited at the interface of perovskite/TiO 2 . Combined above factors, photovoltaic parameters of the PSCs based on CH 3 NH 3 PbI 3−x Br x increase and then decrease of with the increase of Br amount.
Noticeably, owing to different preparation conditions, materials and devices structure, the optimized bromide/iodide ratio may be different.  Figure S4 and Table S1 show the J-V curves and the photovoltaic parameters of the devices based on CH 3 NH 3 PbI 3−x Br x films (x = 0, 0.09, 0.14, and 1) under both reverse and forward bias scans. It can be seen that there are still noticeable hysteresis effects and instabilities for all samples. However, the device based on the champion film CH 3 NH 3 PbI 2.86 Br 0.14 shows the highest reverse PCE (18.02%) and forward PCE (14.62%).
Statistic results of the cell performance are provided in Fig. 8 as histogram charts. It can be found that the devices with CH 3 NH 3 PbI 2.86 Br 0.14 shows better performance and the average PCE is 17.37%. Meanwhile, the histogram chart demonstrates the high reproducibility of the devices (Each team is calculated from a batch of 50 cells). The corresponding incident-photon-to-current conversion efficiency (IPCE) spectra of PSC devices are shown in Figure S5.
J-V curves of the devices based on CH 3 NH 3 PbI 2.86 Br 0.14 with various dwell time from 30 ms to 800 ms are shown in Figure S6, it can be seen that there are minor fluctuations. Figure S7 shows the steady-state photocurrent

Discussions
In summary, we introduce CH 3 NH 3 Br in perovskite precursor solution to prepare CH 3 NH 3 PbI 3−x Br x , leading to the significant improvement of photovoltaic performance of perovskite solar cell. Firstly, uniform perovskite layer with improved crystallinity and apparent grain contour is obtained, and less grain boundaries will facilitate the transportation of charges. Secondly, owing to the introduction of Br, the energy band of perovskite is expanded, resulting in a higher photovoltage of the device. Thirdly, the appropriate regulation of energy level by Br is beneficial to interfacial energy match and the charge injection and extraction at between TiO 2 /perovskite/ spiro-OMeTAD and enhances photocurrent, which is certified by time-resolved photoluminescence intensity decay measurements. Under optimal conditions, the device based on CH 3 NH 3 PbI 2.86 Br 0.14 achieves a champion PCE of 18.02% with a stabilized output efficiency of 17.68% at the maximum power point.

Materials and Reagents.
All of the materials were purchased from the Sigma-Aldrich Corp., if not specified. The Spiro-OMeTAD was from Luminescence Technology Corp. CH 3 NH 3 I was synthesized according to literature 48 . Precursor solution preparation. 461 mg PbI 2 (99.9985%, Alpha Aesar) and 78 mg DMSO were mixed in 702 μ L DMF solution at room temperature. Then, mixed methylammonium halides (MAX, X = I, Br) were added to the above solution, the mole ratio of PbI 2 : MAX: DMSO was controlled at 1:1:1. The molar ratio of MAI: MABr in MAX was controlled at: 1:0, 10:1, 8:1, 6:1, 4:1, 0:1, respectively. Lastly, the solutions were stirred at room temperature for 1 h in order to form a completely dissolved perovskite precursor solution. The resultant perovskites were termed as CH 3  Device fabrication. Fluorine-doped tin oxide-coated (FTO) glass (Pilkington, TEC-8, 8Ω/sq) was cleaned by UV-ozone treatment for 15 min, followed by cleaning with detergent and ethanol consecutively. The compact TiO 2 blocking layers were deposited on FTO glass, which was prepared by spin-coating 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol, Aldrich) in 1-butanol (99.8%, Aldrich) solution, at 500 rpm for 5 s and 2000 rpm for 30 s and then dried at 125 °C for 5 min. A mesoporous TiO 2 film was deposited on compact TiO 2 layer by spin-coating TiO 2 paste diluted in ethanol. TiO 2 paste was prepared as described previously [49][50][51] . After drying at 100 °C for 5 min, the film was annealed at 450 °C for 30 min, providing a thickness of ca. 200 nm. The mesoporous TiO 2 film was immersed in 0.02 M aqueous TiCl 4 solution at 80 °C for 20 min. After washing with deionized water and alcohol, the film was heated at 500 °C for 30 min. 20 μ L precursor solution was dipped onto the mesoporous TiO 2 layer and then spun at 4000 rpm for 25 sec. During spinning, 0.5 mL of diethyl ether was slowly dripped on the rotating substrate within the first 7 s. The film was finally heated to 60 °C for about 2 min and followed by 100 °C for 10 min.
Characterization. The current density-voltage (J-V) curves were measured using a Keithley 2420 source-measure unit under AM1.5 G illumination at 100 mW·cm −2 provided by an Oriel Sol 3 A solar simulator in ambient environment. The light intensity was adjusted using a NREL-calibrated Si solar cell equipped with KG-2 filter. The devices had an active area of 0.125 cm 2 without metal mask. The devices were measured by reverse (2.0 to − 0.1 V) and forward (− 0.1 to 2.0 V) voltage scanning at a scan step of about 21.2 mV (100 data points in total). The pre-sweep delay time was 40 ms, the dwell time at each voltage step was 30 ms. Surface morphologies were characterized by field-emission scanning electron microscopy (SEM, Hitachi S-8000, Japan). The crystalline structures were examined using X-ray diffraction (XRD, Bruker AXS, D8 Advance). Impedance spectra (IS) for the solar cell were measured on a ZAHNER IM6e electrochemical workstation under 1 sun AM 1.5 illumination, by applying a 0 V DC bias and a 5 mV voltage perturbation in the frequency range from 0.1 to 10 6 Hz. The impedance spectra were analyzed with Zview software. IPCE curves were measured as a function of wavelength from 300 nm to 800 nm using the Newport IPCE system (Newport, USA). The time-resolved photoluminescence spectrum was acquired using the time-correlated single-photon counting technique (Pico harp 300), and the excitation light pulse was provided using a picosecond diode laser at a wavelength of 760 nm with a repetition frequency of 1 MHz (PDL 800B).