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Enhanced Photovoltaic Performance of the Inverted Planar Perovskite Solar Cells by Using Mixed Phase Crystalline Perovskite Film with Trace Amounts of PbI as an Absorption Layer 2

Denghao Ma, Weijia Zhang, Zhaoyi Jiang, Dengyuan Song, Lei Zhang, and Wei Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04338 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Enhanced Photovoltaic Performance of the Inverted Planar Perovskite Solar cells by Using Mixed Phase Crystalline Perovskite Film with Trace Amounts of PbI2 as an Absorption layer Deng-Hao Ma†, Wei-Jia Zhang†* , Zhao-Yi Jiang†, Deng-Yuan Song‡, Lei Zhang‡, Wei Yu§*

†Center of Condensed Matter and Material Physics, School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China

‡Yingli Solar, 3399 Chaoyang North Street, Baoding 071002, China §College of Physics Science and Technology, Hebei University, Baoding 071002, China

ABSTRACT It is widely known that solvent engineering has a significant effect on the growth of high quality perovskite film. However, the crystal evolution process and crystal compositions of perovskite film when applying the two-step method are multifaceted, and the effects on the photoelectrical properties for the perovskite layer still require further study. In this study, the above two issues are systematically studied. Combining the XRD and photoluminescence spectra results, it was observed that the residual PbI2 decreased with the increase of DMSO in DMSO/DMF solvents, and that there were two types of local phase crystallites (disordered and ordered) in the perovskite films.The phase transitions of the crystals were investigated with the ratio of DMSO increasing in the mixed solvents. In addition, the interfacial defect states in the PbI2/perovskite inerface which can act as charge carrier trapping centers were significantly affected the photo(dark)-conductivity of the perovskite film. It was also observed that the photo-conductivity of the perovskite film with the DMSO 30%, which contains mixed phase (ordered/disordered) crystals and trace amounts of PbI2, was higher than the film with DMSO 60%. Finally, a high power conversion efficiency of 16.5% with trace amounts of PbI2 was achieved when the volume ratio of the DMSO was 30%. INTRODUCTION Organic-inorganic trihalide perovskite solar cells are considered to be a revolutionary new generation of photovoltaic devices. Beginning in 2012, the power conversion efficiency record has quickly increased1-8. At present, according to the National Renewable Energy Laboratory (NREL) report, the power conversion efficiency (PCE) of perovskite solar cells fabricated by Ecole Polytechnique Federale de Lausanne (EPFL) has reached 21%9, and this still can be improved greatly in the future. At the same time, these thin film solar cells, which are based on methylammonium triiodideplumbate (CH3NH3PbI3) halide perovskites, have many advantages, such as rich raw materials, advanced photoelectric properties, low temperature preparation, and so on3,10-13. Therefore, they have attracted much attention worldwide. As is well known, the performances of perovskite solar cells rely mainly on their structure, which is affected by the material properties and preparation technology. The common perovskite solar cell structures include formal (n-i-p) mesoporous structure, formal (n-i-p) planar structure and inverted (p-i-n) planar structure4,14,15. Despite the fact that at present the formal mesoporous perovskite solar cells have the highest energy conversion efficiency, they and the formal planar structure solar cells also exhibit have serious hysteresis and instability16,17. On the contrary, it has been reported that the inverted planar structure perovskite solar cells exhibit almost no hysteresis, and at present the stable conversion efficiency reaches up to 18%18,19.

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Previous studies have revealed that the most important reason for low PCE is a poor quality perovskite layer for the inverted planar perovskite solar cells. In order to improve its performance, it is necessary to optimize the microstructure of the perovskite film during the deposition process. Tomas et al. reported that two crystalline constitutions existed in the MAPbI3, which may be distinguished as disordered and ordered crystallites20. At present, combined with the crystallization kinetics principle of perovskite, the solvent engineering method is commonly used to control the perovskite crystallization, thus resulting in a high quality perovskite film. Seok et al. have presented a new species, a intermediate phase MAI-PbI2 -DMSO, and successfully introduced it into the perovskite crystal growth process by mixing the dimethyl sulfoxide (DMSO) into the butyrolactone precursor solution21. They discovered that the intermediate species greatly improved the perovskite quality. The following research showed that manifested that the crystallization of perovskite and PbI2 was suppressed by the DMSO due to its higher volatilization temperature and strong coordination with the Pb element, which benefited the growth of large crystals, and hence improved the perovskite layer quality. In addition, Han et al. found that the PbI2 film fabricated with DMF solvent showed a crystalline nature, while the PbI2 film based on DMSO solvent exhibited an amorphous nature, after which the microstructure properties of the fabricated perovskite films as precursor layers were clearly different22. It was found that the surface of the perovskite film became uniform and smooth after introducing DMSO into the mixed solvents, and the local phase crystallites may transited from disordered to ordered. Fan et al. also found that the obtained PbI2(DMSO)x complexes may tend to be closely packed by means of intermolecular self-assembly45. More recently, Zheng et al. confirmed the precise structure of the important adduct MA2Pb3I8(DMSO)2, which was identified as the intermediate phase by calculation using the DMF/DMSO mixed solvent23. Furthermore, Yang et al. suggested that the component of intermediate film was considerably affected by the composition of precursor solution24, and a pure MA2Pb3I8(DMSO)2 intermediate phase tended to grow along [110] to form aligned ordered phase perovskite crystals25. Although previous studies have emphasized the importance of solvent engineering in perovskite crystallization and the effects of the intermediate phases on the growth of perovskite crystal are thoroughly understood, the influences of solvent composition on the component of perovskite film and the details of the crystal growth process under annealing remain need further study at present, especially those of crystal evolution process and various crystalline phase crystallites (ordered or disordered). Moreover, their effects on the photoelectrical properties of the perovskite film still requires deep exploration. In this study, the microstructural evolution of the fabricated perovskite film using solvent engineering is systematically studied. By introducing the DMSO into DMF solvents, the mixed solvents have considerable effects on the MAPbI3 crystal growth and components of the perovskite films. The residual PbI2 or excess DMSO solvent may suppress the aligned phase perovskite crystal from forming, then resulting in the ordered phase crystallites content becoming lower. At the same time, the residual PbI2 in the perovskite film was decreased with the increasing volume ratio of the DMSO in the mixed solvents. Interestingly, the photoconductivity of the mixed phase perovskite crystal film with trace amounts of PbI2 was superior to that of the nearly pure ordered phase perovskite crystal film, and a high power conversion efficiency of 16.5% with trace amounts of PbI2 was achieved when the volume ratio of the DMSO was 30%. EXPERIMENTAL SECTION Treatment of Substrate and Synthesis of Precursors. Etched ITO glass used for substrate


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with the surface resistivity of 10 Ω/sq underwent ultrasonic cleaning for at least 15 min indeionized water, acetone and anhydrous ethanol successively, and then irradiation for more than 20 min in an ultraviolet ozone cleaner (UV/O3 cleaner) was required to remove the organic pollutants and increase the surface hydrophilicity. A series of PbI2 precursor solutions (1 mol/L) were prepared by pure DMF solvent, pure DMSO solvent and mixed solvents at different volume ratios (VDMSO/VDMF =30%, 60%, 80%, respectively). After stirring and heating at 70℃ overnight, the precursor solutions were filtered with a pin type filter (0.45 µm) in order to obtain a clear PbI2 solution. The concentration of MAI solution was 40 mg/mL, and isopropanol (IPA) was used for its solvent. The schematic diagram of the two-step process of perovskite films with various solvents is shown in Figure 1. Fabrication of Devices. PEDOT: PSS solution was spin-coated on the etched ITO glass, then the samples were kept at 130℃ for 12 min in a constant temperature drum wind drying oven to obtain the hole transport layer. The PbI2 and MAI precursor solutions were spin-coated one after another on the hole transport layer at a rotation speed of 3000 r/min, then the samples were kept at 100℃ for 1 h in a glove box filled with N2. The MAPbI3 absorption layer was developed at the interface due to the interdiffusion and interaction of PbI2 and MAI precursors. A chlorobenzene solution containing PC61BM was spin-coated on the absorption layer, and the electronic transport layer was obtained after annealing at 100℃ for 30 min in the glove box. Finally, the BCP blocking layer (7 nm) and Ag electrode (100 nm) were successively fabricated by evaporation. Figure 2 shows the SEM cross-sectional images of the completed device. Performance Measurement. In this paper, the top view of perovskite thin films fabricated on glass substrate and a cross-sectional image of the device were obtained by a NOVA NANOSEM 450 scanning electron microscope produced by FEI (USA). The XRD spectra of the films were obtained by a Bruker D8 type X-ray diffractometer at room temperature, the X-ray source of which was CuKα (40 kV, 40 mA, 0.15406 nm). The transmissivity (T) and reflectivity (R) of the film were obtained using a Japanese Hitachi U-4100 type spectrophotometer (wavelength range from 350 nm to 850 nm), and the optical absorption spectrum of the thin film was calculated by the T/(1-R) interference elimination method. The steady and time-resolved photoluminescence (PL) of the thin films were obtained using an FLS 920 steady/transient/microscopic fluorescence spectrometer produced by Edinburgh Instruments, and the excitation light source was a xenon lamp (450 W). The J-V characteristic curves were measured using an AM1.5 G (100 mW/cm2) simulating sun light source and Keithley 2400 meter. RESULTS AND DISCUSSION The surface morphological features and structural variation of the CH3NH3PbI3 (MAPbI3) perovskite films with different solvents were analyzed by means of the scanning electron microscopy (SEM) and XRD patterns, as shown in Figure 3. From these images, it is clearly observed that the solvents have significant effects on the microstruture and morphology of the perovskite films which exhibit a dramatic evolution. In addition, the differences between the films which were fabricated by spin-coating the MAI on the PbI2 precursor layers without annealing and with those annealing were very apparent. Figure 3(a) shows the XRD patterns of the prepared films without annealing, which are recorded to determine the composition of different precursors. It can be seen that the sample with pure DMF solvent shows the XRD spectral characteristics of the MAPbI3 perovskite film, which mainly includes five diffraction (XRD) patterns (each



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remarked with an *), and three intense peaks at around 14.06°, 28.40° and 31.8°. They are ( respectively represented as the (110), (220) and (310) planes of MAPbI3.26,27 Meanwhile, the impurity peaks at around 12.6°and 25.4°are also observed, which can be attributed to the PbI2 crystals in the sample, and indicates that the PbI2 is incompletely reacted with the MAI.7 When the ratio of DMSO in the mixed solvents was 30%, an additional weak peak at approximately 9.25o was observed in the sample, which could be indexed to the intermediate phase MA2Pb3I8(DMSO)2,23,28 which is the preferential alignment along the (022). In addition, theintensity of the MAPbI3 peak at 14.06o became weaker. As more DMSO was added in the mixed solvents, such as 60%, 80% or 100%, the intensity of the intermediate phase MA2Pb3I8(DMSO)2 gradually became stronger. Meanwhile, the MAPbI3 peak vanished when the ratio was 80%. Figure 3(b) shows the XRD of the MAPbI3 films with annealing. It can be observed that all of the prepared films show the XRD spectral characteristics of the MAPbI3 perovskite film. Meanwhile, the impurity peaks at around 12.6 ° and 38 ° are also observed, which can be attributed to the PbI2 crystals in the samples with DMF and DMSO 30% solvents, and indicates that the PbI2 is incompletely reacted with the MAI,7 but we cannot determine whether the trace amount of PbI2 distributed homogeneously within the film, and it is not controllable. In addition, the PbI2 diffraction peaks vanished when the volume ratios of DMSO were 60%, 80% and 100%, as shown in Figure 3(b). As has been previously reported, an appropriate amount of PbI2-(DMSO)x complexes emerged in the film with incorporation of DMSO, which can promote the PbI2 so that it completely converts to MAPbI3.29 For the prepared films, the intensities of several peaks at 14.06°, 28.40° and 31.8° first became stronger and then weakened as the volume ratio of the DMSO increased, and a maximal peak intensity at 14.06 was obtained when the ratio was 60%, which revealed that the incorporation of DMSO was effective for controlling the crystallization process and led to a high quality perovskite film being generated. However, with the ratio of DMSO further increasing in the range at 60 -100%, the characteristic peak intensities at 14.08°, 28.41° and 31.85° decreased, which indicated that the quality of the MAPbI3 crystal reduced. The SEM images of MAPbI3 films without annealing are given in Figure 3(c1)-(c5) to evaluate the morphologies. The perovskite film with pure DMF solvent shows clear perovskite crystal-like grains and boundaries with obvious pinholes on the surface. In a clear comparison, the c2 film exhibits a different crystal feature from that of the c1 film, especially due to the emergence of great numbers of pinholes on the surface. By carefully inspecting the morphology of the c2 film, it can be observed that the film mainly contains nanorod-shape crystals which are similar to those reported in the references for the MA2Pb3I8(DMSO)2 intermediate phase and granular grains which are the MAPbI3 crystals. At the same time, significant differences could be easily found for the c3 film, in which it appears that the density and average size of the pinholes became lower than those of the c2 film, and the granular grains vanished. When the ratio of DMSO in the mixed solvents increased, the average pinhole size gradually decreased and the nanorod crystals became larger. The intermediate films were then converted to MAPbI3 by annealing at 100oC for 1 h, the surface morphology are shown as Figures 3(d1)-(d5). Figure 3(d1) displays the morphology of MAPbI3 perovskite film with pure DMF solvent, which shows relatively dense and small in-plane grain size. With the increase in the ratio of DMSO in the mixed solvents, the crystal grain size gradually increased. When the ratio of the DMSO increases to 30~60% in mixed solvents, as


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shown in Figures 3(d2) and (d3), the MAPbI3 films became more dense and continuous. Compared to the film prepared by pure DMF solvent, the average grain size was significantly larger , as shown in Figures 3(e1) and (e2), and the films were more uniform and compact. As the proportion of the DMSO further increases, as shown in Figures 3(d4) and (d5), there is no obvious change for the MAPbI3 grain size, and the smoothness of the films worsens and more crystal planes are present in the film. Therefore, these results indicate that excess DMSO solvent can destroy the structure of the MAPbI3 crystals and reduce the smoothness of the prepared perovskite film. Due to its relatively low evaporation rate and high viscosity, pure DMSO was not very appropriate for forming a thin film by spin-coating. It should be noted that the diffraction peaks (110) of the films with DMSO30% and DMSO60% were significantly higher than those with pure DMF, DMSO80% and pure DMSO, as shown in Figure 4(a). The much stronger XRD intensity led to higher crystallization of perovskite with DMSO30% and DMSO60%. In addition, the intensity ratio of (110)/(310) in perovskite structure could well stand for the preferable crystal orientation.49 By plotting the relationship of the intensity ratio of (110)/(310) with the volume ratio of DMSO in the mixed solvents, as shown in Figure 4(b), it is clearly observed that when the ratio was below 60%, the intensity ratio of (110)/(310) increased with the ratio increasing. However, the intensity ratio of (110)/(310) decreased with the ratio further increasing (> 60%). Therefore, the composition and structure of intermediate film affects not only the perovskite crystal quality, but also the crystal orientation. In other words, the films with proper introduction of DMSO in CPS afford a preferable crystal orientation ([110] direction) during the annealing treatment. At the same time, with the ratio of DMSO further increasing in the range at 60 -100%, the characteristic peak intensities at 14.08°, 28.41° and 31.85° decreased, which indicated that the quality of the MAPbI3 crystal reduced. The above results could be explained as follows. As we all know that PbI2 film is firstly fabricated as the precursor layer, and its morphology and crystal structure have decisive impact on the properties of MAPbI3 layer. It was found that the coordination ratio between Pb and the solvent is 1 : 1 and 1 : 2 for DMF and DMSO, with the Pb– O bond length of 2.431˚ A and 2.386˚ A, respectively, indicating that DMSO has a stronger coordination ability with PbI2 than that of DMF. The strong interaction between DMSO and Pb2+ can retard the crystallization of PbI2, resulting in a uniform PbI2 film showing an amorphous character.22 Hence, the PbI2 film based on DMSO solvent showed amorphous nature while the PbI2 film based on DMF solvent exhibited crystalline nature. In addition, the amorphous feature of PbI2 precursor films based on DMSO retarded the crystallization of MAPbI3 resultant.22 Therefore, the higher DMSO content leads to lower MAPbI3 crystallinity. Figure 4(c) shows the normalized steady-state absorption and room temperature PL emission spectra. First, for the prepared sample with pure DMF solvent, the maximum PL spectral intensity was approximately centered at 1.61 eV and the absorption edge was positioned at around 1.60 eV, which are consistent with those shown in previous reports.6 Meanwhile, through careful observation, it can be found that the absorption tail at the energy was below the maximum intensity of the photon emission. This is likely attributed to the optical transition from the valence band to the band tail states which was below the conduction band minimum and was caused by the defects and traps in the perovskite film, which may be due to the grain boundaries or the disordered perovskite crystals.30 In addition, as the volume ratio of the DMSO in the mixed solvent was increased, the central PL emission spectra first displayed a red-shift tendency,



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followed by blue shift, and the PL intensity first increased then decreased. One interesting phenomenon that can be seen is that the absorption tail energy gradually became consistent with the maximum PL spectral intensity as the volume ratio of the DMSO increased in the range of 0 60%. However, it fell below the excitonic peak position again when the ratio of the DMSO was 80%. This possibly demonstrates that the localized state density in the band tail was the minimum for the film with DMSO 60% solvent in our five samples, which suggests that the film with DMSO 60% has a high ordered phase crystallite. Compared with the XRD results, this indicates that the films with a proper intermediate phase afford a preferable crystal orientation ([110] direction) during the annealing treatment. Furthermore, it may also be seen that the maximum PL intensity was observed when the DMSO volume ratio was 30%. This reveals that the PbI2 may have a passivation effect on the prepared perovskite film and leads to the enhancement of PL intensity, which will be explained in in the following sections. Figure 4(d) summarizes the evolution of the perovskite films with the increasing amount of DMSO. It shows that the prepared MAPbI3 film compositions are different as increasing the ratio, which should be related to the strong coordination effect of DMSO to PbI2. Figure 5(a1)-(a5) show the PbI2 films with various solvents before annealing, and the corresponding simulation diagrams are shown in Figure 5(b1)-(b4). Figure 5(a1) shows the mesoporous structural PbI2 film with pure DMF solvent. However, with the addition of DMSO in the DMF solvent, the morphology of the PbI2 film became significant different. It can be seen that the PbI2 film became smooth and there are several bulk crystals on the surface. Meanwhile, the size and density of the bulk crystals became larger with the ratio increasing, which is consistent with the previously report.45 As is well known that there is a strong intermolecular coordination between PbI2 and DMSO,31 and the DMSO has a high boiling point. Therefore, the bulk crystals may be the PbI2(DMSO)x complex. Previous studies have demonstrated that the morphology of the PbI2 precursor layer has an obvious influence on the morphology of perovskite film which was prepared by two-step spin-coating. In addition, the DMF solvent is advantageous to the crystallization of the PbI2 precursor, due to its low boiling point and easy removal. However, it results in difficulty for the MAI to penetrate the PbI2 precursor layer, and sufficiently react with the PbI2,32 at the same time, the reaction direction was not directional between MAI and PbI2, as shown in Figure 5(d1). Therefore, the pure DMF solvent is not conducive to the crystallization of MAPbI3, then results in the disordered pervoskite crystals generation with the residual PbI2 before annealing, as shown in Figure 5(e1). Then, it will converts to the disordered MAPbI3 crystals, as shown in Figure 5(f1). For the samples of Figure 5(d2)-(d4), with the addition of DMSO the reaction area between the PbI2 and MAI is increased, which is mainly attributed to that the DMSO solvent can effectively inhibit the crystallization of PbI2 as the precursor layer. Therefore, it promotes the formation of the PbI2-DMSO-MAI interphase, as shown in Figure 5(e2)-(e4), which is advantageous to the PbI2 sufficiently reacting with the MAI through the subsequent annealing process and completely converting to ordered phase MAPbI3 crystals, as shown in Figure 5(f2)-(f3). However, with the ratio of DMSO increasing in CPS, the coordinated DMSO in the complex would favor assembly of the PbI2 somehow. Therefore, the density of the as-formed PbI2(DMSO)x complexes was increased with the ratio of DMSO in CPS increasing. As previous report that the DMSO will be exchanged by MAI because of its smaller affinity toward PbI2 compared to MAI, which enables the complexes to deform their shape. The deformation occurs because the capillary and osmotic pressures are able to overcome the internal stresses in the


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system which may be related to the size and density of PbI2(DMSO)x complex.45 Therefore, excess of DMSO may give rise to an inhomogeneous film with pinholes, as shown in Figure 5(f4). From these images of Figure 5(g1)-(g4), which shows the cross-sectional SEM images of the MAPbI3 film with various solvents, it is evident that the film quality with DMSO60% solvent is the best. Figure 6 shows the steady PL spectra for the samples with different solvents. As is well known, the addition of DMSO in the mixed solvents causes a significant change of the microstructure for perovskite films. Specifically, it contains various phase crystalline MAPbI3 and residual PbI2. Edvinssion T et al have reported that the crystalline constitutions of MAPbI3 can be analyzed by fitting the photoluminescence spectra with Gaussian functions and the two fitting PL spectra indicate two kinds of local phases, which may be respectively distinguished as disorder and order crystallites.20 In addition, many studies have shown that there are various kinds of photoluminescence mechanisms for the perovskites films, such as radiative recombination and nonradiative recombination (which mainly focus on the auger recombination), and so on,33,34 as shown in Figure 7(c). Therefore, we use the same method to process our samples, and the results are shown in Figure 6(a)-(d). It can be seen that the PL spectra of films prepared with pure DMF and DMSO 30% contain three fitting peaks. However, the PL spectra of samples with DMSO 60% and DMSO 80% include only two fitting peaks. We take the sample prepared by using pure DMF solvent for example, in which the red fitting PL curve can correspond to a disordered phase crystallite, and the blue PL curve can be assigned to an ordered phase crystallite, according to the definition of Edvinssion T and Boschloo G.20 The results show that the charge quenching yield ratio R of the disordered phase versus the ordered phase crystallites first gradually increases then decreases with the ratio of DMSO increasing in the mixed solvent, and this represents a maximum value when the ratio is 60%,which is consistent with the XRD result, as shown in Figures 7(a) and (b), From the above results, it may be inferred that the PL emission of the disordered phase crystallites may be mainly attributed to the radiative recombination, and the auger recombination mechanism plays a major role in the PL emission of ordered phase crystallites, as shown in Figure 7(c). Meanwhile, they should all belong to band transition and may cause the PL peak position to shift. Beyond that, there is another interesting finding, namely that fitting PL peaks (purple curves) have been observed, located at around 1.55 eV, and the peak intensity became weaker with the content of residual PbI2 decreased, for the samples with pure DMF and MDSO 30% solvents. This may be attributed to the radiative trap states which has a close relation with the interfacial defect in the PbI2/perovskite crystals interface, which is associated with the appearance of PbI2 phases in the CH3NH3PbI3 film. Compared with the XRD results, they all contained the residual PbI2. In addition, it can also be found that the intensities of the fitting PL peaks decline as the content of residual the PbI2 decreased, and vanished when the ratio of the DMSO was above 60%. Therefore, the fitting PL peaks may be caused by the interfacial defect luminescence. The optical absorption spectra of the perovskite films prepared by mixing the solvents with different volume ratios of DMSO are presented in Figure 8. For all of the fabricated films, an absorption onset and sharp rise can be seen at around 800 nm and 780 nm, respectively, as shown in Figure 8(a), which is consistent with a previous report35. When the ratio of the DMSO was in the range of 0-60%, the absorption intensity was enhanced as the ratio increased. However, the intensity was decreased as the ratio further increased (above 60%). This, combined with the SEM images and XRD results, reveals that the morphology and PbI2 may together affect the optical absorption. Figure 8(b) shows the Urbach energy (EU) of the prepared perovskite films, where EU



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denotes the energy of the band tail, sometimes known as Urbach energy, which is weakly dependent upon temperature. It is often interpreted as the width of the band tail, due to the localized states in the normal band gap associated with disorder or low crystalline materials. The Urbach energy was found to vary substantially with the ratio of the DMSO, and is derived by the following formula:36,37

   0 exp( E / EU )

(1)

Where α0 is constant, and E represents the incident photon energy. It can be seen that as the ratio of the DMSO increased to 30%, the value of Eu decreased, and the content of PbI2 in the film also reduced with reference to the corresponding XRD spectrum, which suggests that the localized states in the band tail (possibly due to the defect state or trap state in the films) was decreased with the reduction of the PbI2. However, when the ratio was increased to 60%, the Eu was larger, and what is not consistent the results of PL spectra. Therefore, this may be attributed to the trace amount PbI2 has a certain passivation effect on the perovskite film.46,47 When the ratio was further increased, the Eu became larger, which may have been due to the introduction of excess DMSO, which then led to the enhancement of defect density in the band tail.

Table 1

Carrier lifetime extracted from TR-PL decay curves

Sample

τ1/ns

τ2/ns

DMF

2.1

81

30 % 60 % 80 % DMSO

2.7 3.2 1.8 1.7

93 90 77 67

In order to further analyze the relaxation and recombination process of carriers, the time-resolved photoluminescence (TRPL) of the perovskite thin films prepared by different volume ratios of DMSO have been measured, and the fitting curves by double exponential decay function are presented in Figure 9. The formula is as follows.38

I (t )  A0  A1 exp( 

t

1

)  A2 exp( 

t

2

)

(2)

A1 and A2 are the fitting parameters, and τ1 and τ2 are the lifetimes of the carriers. Based on previous reports, there are two main luminescence processes in perovskite samples. The first one, marked as τ1, is contributed to the bimolecular recombination. The other, marked as τ2, is contributed to recombination of free carriers in the radiative channel. A series of solid lines with the same color as the original data in Figure 9 represent the fitting results, and lifetimes τ1 and τ2 are listed in Table 1. As can be seen from the table, the τ1 changes faintly with the increase of the DMSO, while the τ2 shows a trend of first increasing then decreasing. When the content of the DMSO is 30%, the carrier lifetime is the longest. At first, many residual PbI2 precursors exist in the films and the defect density of the films is large, thus the lifetime τ2 is small. As the DMSO increases, the content of PbI2 in the thin film decreases, and the PL lifetime τ2 increases, which indicates the reduced carrier recombination in the film, and it may be associated with the passivation effect of trace amount PbI2 phases in the perovskite film. This will lead to the reduction of localized states density of the films which may causes the non-radiative


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recombination and the carrier lifetime decline,47,48 thus τ2 increases gradually. As the DMSO further increases, the τ1 and τ2 are both lower, which is due to excessive DMSO (> 60%), thus leading to the disordered phase crystallites content enhancing, in turn increasing the chance of the radiative recombination of carriers, which is consistent with the steady-state PL quenching. Figure 10(c) shows the change features of dark conductivity, photo-conductivity and photosensitivity of various samples with different solvents. It can be observed that the dark conductivity (σD) first decreased then increased with the addition of DMSO enhancing. In theory, as the proportion of DMSO in the mixed solvents increased, the content of residual PbI2 was reduced and the ration of ordered phase crystallites in the films increased, which should then lead to the dark conductivity of the sample with DMSO 30% becoming higher than the sample with pure DMF solvent. However, the experiment results show that the former is lower than the latter. Combined with the XRD and PL results, the reason for the experiment results may be associated with the trap states which have a close relation with the interfacial defects in the PbI2/perovskite interfaces. K.A.Nasyrov et al. suggested that the electrons may direct tunnels between traps,39 and the electrons are captured by a trap emission from the trap into the conduction band, followed by subsequent capture at another trap. For our own samples, as the PbI2 reduced, the density of the trap states decreased, thus the electron transport mechanism transformed from the direct tunneling between traps to the trapped electron. Therefore, the conductivity reduced, and the simulation transmission process of electrons in the film was shown in Figure 11. As the DMSO continued to increase, the dark conductivity first became larger then smaller, which shows the same rule as the disordered phase crystallites in the films. In addition, the photoconductivity (σph) is first increased and then decreased, which was the maximum when the ratio of DMSO was 30%. This is possibly associated with the trapped electron and the absorption ability. Specifically, the electrons which were captured by trap states may have been emitted from the trap state to the conduction band under the light conditions, as shown in Figure 12. Finally, as well known that the photosensitivity S of the prepared films can be calculated using the following formula:

S

 ph D

(3)

where σph is the photoconductivity and σD is the dark conductivity. Meanwhile, it can be seen that the dark conductivity was minimum and the photoconductivy was the maximum when the ratio of DMSO was 30%. Therefore, the photosensitivity of the perovskite film was significantly improved with the trace amount of PbI2 in the film. Figure 13 shows the main photovoltaic parameters for the prepared devices with different solvents, and the x axis displays the distinct solvents. From these images, it can be found that the results of the short circuit current density (Jsc), open circuit voltage (Voc) and power conversion efficiency (Eff) all display the same rule as the photoconductivity changing for these devices as the ratio of DMSO increases. At the same time, the results of fill factor (FF) show the same rule as the proportion of ordered phase crystallites and the evaluations of Urbach energy. In general, the short current density (Jsc) of the solar cell may mainly be affected by the photo-generated carrier density and recombination in the interfaces between different layers. In addition, it was previously reported that the trace residual PbI2 could change the grain to grain boundary bending from



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downward to upward, then reduce the carrier recombination at the interface between the perovskite layer and PEDOT layer.40,41 Therefore, combining this with the results of σph, it is easy to determine the reason for the Jsc increasing as the PbI2 reduced. When the DMSO continued to increase, the Jsc began to decrease, which can be explained by the reduction of absorption and enhancement of carrier recombination at the grain interface. As is well known, the Voc is mainly associated with the ability of carrier transporting and collection. Therefore, we used the VIM technology to analyze the carrier mobility and lifetime for these devices. By means of this technique, the short-circuit resistance (Rsc) can split into the contribution of the leak current, due to the physical shunt or dark shunt, and the contribution of the recombination current, due to the photo shunt.42,43 Figure 14(a) shows the current-voltage (J-V) curves which were measured under various illumination intensities for the device with pure DMF. Previous reports have indicated that the reciprocal of the short-circuit resistance (Rsc) could be obtained from the slope of the J-V curve at 0 V. In addition, Figure 14(c) shows the Rsc versus the reciprocal of the Jsc curve, of which the slope represents the collection voltage (Vcoll). Therefore, the (µτ)eff, which represents the effective carrier mobility and lifetime product, indicates the carrier collection ability for the perovskite film as the i-layer. The value of the (µτ)eff can be derived from the Vcoll using the following formula: 1 sc

Rsc  J  Vcoll  J

1 sc

 eff Vbi2 d i2

(4)

Where, Vbi is the built-in potential, and di is the perovskites film thickness. From Figure 14(b), the values of Vcoll can be acquired by the Rsc-Jsc-1 curves, then the (µτ)eff can be calculated with the di of approximatly 400 nm and the Vbi which can be estimated from the capacitance-voltage under dark conditions (as shown in Figure 14(b)). The value of the (µτ)eff increased from 1.574*10-8 to 5.28*10-8, then decreased to 1.954*10-8 cm2V-1 with the decrease of PbI2. This result clearly indicated that the proper residual PbI2 could effectively enhance the perovskite film quality and may have a certain passivation effect on the grain interface then reduce the carrier recombination. In addition, Yang Y et al. have suggested that the presence of PbI2 may help to inhibit the recombination between the electrons from perovskites and holes from the PEDOT,40 which is advantageous to the carrier transmission, after which the presence of PbI2 may then help to reduce the leak current and increase the Vbi. Thereby, the proper residual PbI2 can increase the Voc and improve the photoelectrical properties. In addition, as is well known, the fill factor FF is a ration of the maximum power Pmax and Voc × Jsc, where the Pmax is at a certain voltage position. We may use a dynamic perspective, which is the current reduction ratio (CRR) curve of the prepared cells, to investigate the main reasons for the changes in the FF parameters. Specifically, the CRR can be calculated using the following formula44:

1-

J out (V ) J SC

(5)

This is the reduction degree of the current density, which is experimentally measured by Jout under a certain voltage with respect to Jsc, as shown in Figure 15. From Figure 15, it can be observed that the CRR first decreased then increased as the DMSO increased, which is consistent


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with the change of the FF. Therefore, we can clearly see that the additional DMSO had a significant effect on the Jout in the region between the maximum current point and short-current point, in which the field may have been relatively strong. It was previously reported that a high collection of generated holes in the i/n interface may increase the CRR and enhance the built-in field, which is consistent with VIM results, in turn leading to a lower power loss.Thus, proper DMSO incorporation may enhance the fill factor for the planar structure perovskites solar cells. CONCLUSION In conclusion, in this study we demonstrated that not only the uniformity, compactness and phase composition, but also the trap states which may be attributed to the interfacial defect states in the PbI2/perovskite interface, play key roles in the performances of perovskite films and solar cells. We have explored the exact influence of solvent on the crystal evolution and crystal compositions of perovskite film, using the two step spin-coating method. Additionally, the relation of photoelectric performance of perovskite film with the microstructure properties was built. Our results illustrate that properly introducing a DMSO solvent into CPS can afford a preferable crystal orientation and a high crystal quality during the annealing treatment. The excess of DMSO may gave rise to an inhomogeneous film with pinholes, which is mainly attributed to the highly dense of the PbI2(DMSO)x and fully covered film with respect to the exchange of DMSO and MAI. With increasing the amount of DMSO, the content of residual PbI2 in the film decreases. Meanwhile, it was found that the photoelectric performance of the perovskite film with a trace amount of residual PbI2 is dramatically improved. We speculate that the above result may have a close relationship with the interfacial defect states which may act as the shallow traps and affect the carrier transport in the perovskite film. In addition, due to the outstanding properties of the mixed phase crystalline perovskite film with trace amounts of PbI2, a high power conversion efficiency of 16.5% was achieved. Together, our work has high-lighted the importance for controlling the phase composition and density of the interfacial defect states in the PbI2/perovskite interface to grow aligned and high-quality perovskite layers for high performance PSCs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. [email protected] Present Address

†BeiHang University XueYuan Road No.37, HaiDian District, BeiJing 100191, China §College of Physics Science and Technology, Hebei University, Baoding 071002, China Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by national natural science foundation project (No. 51572008). REFERENCES (1) Green, M. A.; Baillie, A. H.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics. 2014, 8, 506-514. (2) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional



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(34) Song, D. D.; Cui, P.; Wang, T. Wei, D.; Li, M. C.; Cao, F. H.; Yue, X. P.; Fu, P. F.; Li, Y. Y.; et al. Managing Carrier Lifetime and Doping Property of Lead Halide Perovskite by Postannealing Processes for Highly Efficient Perovskite Solar Cells. J. Phys. Chem. C. 2015, 119, 22812-22819. (35) Zhang, L. C.; Luo, D. Y.; Wu, J.; Hu, Q.; Zhang, W.; Chen, K.; Liu, T. H.; Liu, Y.; Zhang, Y.; et al. High-Performance Inverted Planar Heterojunction Perovskite Solar Cells Based on Lead Acetate Precursor with Efficiency Exceeding 18%. Adv. Funct. Mater. 2016, 26, 3508-2513. (36) Wassner, T. A.; Laumer, B.; Maier, S.; Laufer, A.; Meyer, B. K.; Stutzmann, M.; Eickhoff, M. Optical Properties and Structural Characteristics of ZnMgO Grown by Plasma Assisted Molecular Beam Epitaxy. J. Appl. Phys. 2009, 105, 023505-023512. (37) Ikhmayies, S. J.; Ahmad-Bitar, R. N. An Investigation of the Bandgap and Urbach Tail of Vacuum-Evaporated SnO2 Thin Films. Physica Scripta. 2011, 84, 143-148. (38) Pellet, N.; Gao, P.; Gregori, G.; Yang, T. Y.; Nazeeruddin, M. K.; Maier, J.; Grätzel, M. Mixed-Organic-Cation Perovskite Photovoltaics for Enhanced Solar-Light Harvesting. Angew. Chem. Int. Edit. 2014, 53, 3151-3155. (39) Nasyrov, K. A.; Gritsenko, V. A.; Novikov, Y. N. Two-Bands Charge Transport in Silicon Nitride Due to Phonon-Assisted Trap Ionization. J. Appl. Phys. 2004, 94, 4293-4299. (40) Chen, Q.; Zhou, H.; Song, T. B.; Luo, S.; Hong, Z.; Duan, H. S.; Dou, L.; Liu, Y.; Yang, Y. Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells. Nano Lett. 2014, 14, 4158-4162. (41) Li, J. B.; Chawla, V.; Clemens, B. M. Investigating the Role of Grain Boundaries in CZTS and CZTSSe Thin Film Solar Cells with Scanning Probe Microscopy. Adv. Mater. 2012, 24, 720-723. (42) Shah, A. V.; Meilland, F. S.; Berenyi, Z. J.; Ghahfarokhi, O. M.; Kumar, R. Diagnostics of Thin-Film Silicon Solar Cells and Solar Panels/Modules with Variable Intensity Measurements (VIM). Sol. Energy Mater. Sol. Cells. 2011, 95, 398-403. (43) Moon, T.; Jun, J. H.; Lee, H.; Yoon, W.; Kim, S.; Lee, B. K.; Lee, H. C.; Kim, W. Y. Additional Coating Effects on Textured ZnO:Al Thin Films as Transparent Conducting Oxides for Thin‐Film Si Solar Cells. Res. Appl. 2012, 20, 294-297. (44) Liu, B. F.; Bai, L. H.; Zhang, X. D.; Wei, C. C.; Huang, Q. Fill Factor Improvement in PIN Type Hydrogenated Amorphous Silicon Germanium Thin Film Solar Cells: Omnipotent N Type µc-SiOx:H Layer. Sol. Energy Mater. Sol. Cells. 2015, 140, 450-456. (45) Li, W. Z.; Fan, J. D.; Li, J. W.; Mai, Y. H.; Wang, L. D. Controllable Grain Morphology of Perovskite Absorber Film by Molecular Self-Assembly toward Efficient Solar Cell Exceeding 17%. J. Am. Chem. Soc. 2015, 137, 10399-10405. (46) Wang, L. L.; McCleese, C.; Kovalsky, A.; Zhao, Y. X.; Burda, C. Femtosecond Time-Resolved Transient Absorption Spectroscopy of CH3NH3PbI3 Perovskite Films: Evidence for Passivation Effect of PbI2. J. Am. Chem. Soc. 2014, 136, 12205-12208. (47) Zhang, T. Y.; Guo, N. J.; Li, G.; Qian, X. F.; Zhao, Y. X. A Controllable Fabrication of Grain Boundary PbI2 Nanoplates Passivated Lead Halide Perovskites for High Performance Solar Cells. Nano Energy. 2016, 26, 50-56. (48) Young, C. K.; Nam, J. J.; Noh, J. H.; Yang, W. S.; Seo, J.; Yun, J. S.; Huang, S. J.; Green, M. A. Beneficial Effects of PbI2 Incorporated in Organo-Lead halide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1502104-1502109.


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(49) Bai, Y.; Xiao, S.; Hu, C.; Zhang, T.; Meng, X. Y.; Li, Q.; Yang, Y. H. A Pure and Stable Intermediate Phase is Key to Growing Aligned and Vertically Monolithic Perovskite Crystals for Efficient PIN Planar Perovskite Solar Cells with High Processibility and Stability. Nano Energy, 2017, 34, 58-68.



B

A

ITO

C

MAI

PEDOT

MAPbI3

PbI2

PCBM

D

F Annealing

E

Figure 1. Schematic diagram of the two-step process of perovskite films with various solvents.

Ag PCBM/BCP

GLASS

-5.3eV -5.4eV -5.9eV -7.0eV

Figure 2. Cross-sectional SEM images of the fabricated perovskite device.


BCP

ITO

-4.8eV

PCBM

PEDOT:PSS

ITO

Perovskite

CH3NH3PbI3

-3.9eV

PEDOT

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 16 of 28

Ag

(a) 

PbI2  MAPbI3  MAI-DMSO-PbI2

 

Intensity (a.u.)



(110)



(220)

DMSO



DMSO80% DMSO60%



8

Annealing

DMSO30%



(112)







20 24

28 32

36 40

DMSO

DMSO60%



12

44 48

DMSO30%

16

20

24

28

32

36

DMF

40

44

48

2 (degree)

MAI+DMSO+PbI2 + PbI2 + CH3NH3PbI3 （110）

PbI2 + CH3NH3PbI3 （110）

 



(c2)

(a) (c1)

(330)

  

DMF

12 16

(b)

PbI2  MAPbI3

DMSO80%

2 (degree)

(c3) MAI+DMSO+PbI2 + CH3NH3PbI3 （110）

(c4) MAI+DMSO+PbI2

(c5) MAI+DMSO+PbI2

Unannealing

DMSO30%

DMF

DMSO60%

DMSO80%

DMSO

Annealing

(d1)

(d4)

(d3)

(d2)

(d5)

Pinholes

(e1)

50

25

DMF

(e2)

20

15 10

10

5

0

0

200

400

Size (nm)

600

DMSO

30

Percentage (%)

Percentage (%)

30

(e3)

35

DMSO60%

20

40

Percentage (%)

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


Intensity (a.u.)

Page 17 of 28

25 20 15 10 5

200

400 Size (nm)

600

0

200

Size (nm)

400

Figure 3. XRD patterns of the prepared films bansed on various solvents (a) without annealing, (b) with annealing. Surface SEM images for fabricated films (c1-c5) without annealing, (d1-d5) with annealing. The grain distribution of MAPbI3 (e1,e2 and e3) from SEM images was calculated by using images d1, d3 and d5, respectively.


600


(110)

(c)

(310)

DMSO

PL Intensity (a.u.)

(a)

DMSO60%

PL Intensity (a.u.)

32

PL Intensity (a.u.)

30

2 (degree)

PL Intensity (a.u.)

3

2

4 3

20000

(d)

DMSO30% DMSO60% DMSO80%

DMF

DMSO60%

2 1

0

0

40000

4 3

20000

DMSO30%

2 1

0

0

20000

3

10000

0

650

DMF

0

2

DMF

1

700

750

800

0

Absorbance (a.u.)

Internal ratio of (110)/(310)

1

Absorbance (a.u.)

(b) 4

16

2

0

DMF 14

3

DMSO80% 40000

DMSO30%

12

4

20000

Absorbance (a.u.)

Intensity (a.u.)

DMSO80%

5

Absorbance (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

Page 18 of 28

Wavelength (nm)

DMSO

DMSO 80%

DMSO 60%

DMSO 30% MAPbI3

+DMSO

+DMSO

+DMSO

PbI2

Figure 4. (a) XRD patterns of the perovskite films with various solvents. (b) Intensity ratio change of XRD patterns for (110)/(310). (c) Absorption (blue) and PL (black) spectra of fabricated perovskite films with various solvents. (d) Schematic illustrating the relation between the amount of DMSO and the component of perovskite film.


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DMSO+PbI2

1 2 3 4 5 DMF (a5) (a3) DMSO80% (a1) DMSO30% (a2) DMSO60% (a4) DMSO 6 7 8 9 10 11 12 13 DMSO30% DMSO60% DMSO80% DMF 14 15 16 Ⅰ 17 18 PbI2 PbI2 PbI2 PbI2 19 (b3) (b4) (b1) (b2) 20 21 22 23 MAI 24 25 26 27Ⅱ (c1) (c2) (c3) (c4) 28 29 (d3) (d4) 30 (d2) (d1) 31 32 33 the capillary and osmotic MAI+DMSO+PbI2 CH3NH3PbI2 34 pressures are different 35 Ⅲ 36 (e2) (e4) (e3) (e1) 37 Annealing Annealing 38 Annealing Annealing PbI2 39 40 41 42 (f3) (f4) (f1) (f2) 43 Disordered crystallites Ordered+disordered crystallites Ordered crystallites Disordered crystallites 44 45 Ⅳ 46 47 48 49 50 51 52 GLASS GLASS GLASS GLASS 53 DMF(g1) DMSO30% (g2) DMSO60% (g3) DMSO80% (g4) 54 55 Figure 5. Surface SEM images for PbI2 films based on solvents of (a1) DMF, (a2) DMSO 30%, (a3) DMSO 60%, 56 (a4) DMSO 80% and (a5) DMSO without annealing. Ⅰ-Ⅳ show the schematic of experimental procedures for 57 58 annealing routes. Cross-sectional SEM images of samples based on solvents of (b1) DMF, (b2) DMSO 30%, (b3) 59 DMSO 60% and (b4) DMSO 80%. 60



Final fit disorder order Trap-states

order-phase

(b)

interficial defect emission PbI2/perovskite disorder-phase

1.5

1.6

1.7

1.8

1.9

Final fit disorder order Trap-states

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

(a)

order-phase interficial defect emission PbI2/perovskite disorder-phase

1.5

2.0

1.6

1.7

Measured Final fit disorder order Trap-states

1.5

1.6

1.7

1.8

1.9

2.0

1.5

1.6

Energy (eV)

1.7

Intensity (a.u.)

Intensity (a.u.)

order-phase disorder-phase

1.7

1.8

2.0

1.8

1.9

Final fit disorder order

2.0

order-phase

disorder-phase

1.5

Measured Final fit disorder order

1.6

Measured Final fit disorder order Trap-states

1.9

(c)

Energy (eV)

1.5

1.8

2.0

1.9

2.0

1.6

1.7 1.8 1.9 Energy (eV)

2.0

Measured Final fit disorder order

1.5

1.6

Energy (eV)

1.7

1.8

1.9

2.0

Energy (eV)

Figure 6. Fitting of the PL spectra by three peaks (disordered phase, ordered phase and interfacial defect emission) for prepared perovskite films with various solvents (a) pure DMF and (b) DMSO 30%, and two peaks (disordered phase and ordered phase) for samples with different solvents (c) DMSO 60% and (e) DMSO 80%.


Intensity (a.u.)

Intensity (a.u.)

Final fit disorder order

1.6

1.7

1.9

Energy (eV)

(d)

1.5

1.8

Energy (eV)

Energy (eV)

Intensity (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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(b)

(a)

(110)

DMSO

6

DMSO80%

Intensity (a.u.)

R(Ordered/Disordered)

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


4

DMSO60% DMSO30%

2

0

DMF DMF


DMSO

12.0

12.5

13.0

13.5

14.0

14.5

15.0

15.5

16.0

2 (degree)

(c)

A

B

C

Disordered phase crystallites Ordered phase crystallites A Radiative recombination

hv

photon

B Auger recombination

Trap-state phonon

Conduction band

photon

C Radiative

Auger

Radiative

recombination

recombination

recombination

Interfacial defect state

Trap state

Residual PbI2/perovskite

Valence band

Figure 7. (a) The change trend of the ratio of ordered to disordered phases with the increase of DMSO. (b) XRD patterns for the perovskite films bansed on various solvents from 12° to 16 °. (c) Schematic diagrams of different carrier recombination.



60

(a)

DMF DMSO30% DMSO60% DMSO80% DMSO

5

1.5

1

(b)

50

EU(eV)

2

Absorbance  10 (a.u.)

40

30

0.5 0

20

400

500

600

700

800

DMF


Wavelength (nm)

Figure 8. Optical absorption spectra (a) and Urbach energy EU (b) of fabricated perovskite films with various solvents.

10

3


Intensity (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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10

2

50

100

150

200

Time (ns) Figure 9. TRPL of fabricated perovskite films with different solvents.


DMSO

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(a)

(c)

D

ph/D

-1

ph and D (Scm )

DMSO30%

(b)

Keithley 2400

1000

ph

1E-4

1E-5

100

ph/D

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


1E-6

V

10

1E-7 DMF


DMSO

Perovskite Glass substrate Figure 10. (a) DMSO30% sample with coplanar electrode. (b) Schematic conductivity measurement setup. (c) Dark conductivity, photo-conductivity and photosensitiveness of prepared films with different solvents.

× Direct tunneling between traps

DMF

(1)

electron

×

×

Electron emission and capture

DMSO30%

(2)

Figure 11. (1) Electron direct tunneling between traps without emission into the conduction band; (2) the electron, which is captured by a trap, subsequent emission from the trap into conduction band, and followed with subsequent capture at another trap.


electron


-4

2.0x10

Trapped electron emission

-1

Photoconductivity (s.cm )

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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-4

1.5x10

③

④

① ②

Trapped electron

Trapped electron emission

Electron emission -4

1.0x10

-5

5.0x10

0.0 DMF

DMSO 30%


Figure 12. Schematic diagrams of the trapped electron emission to the conduction band under the light condition.


DMSO

Page 25 of 28

1.10 17

2

JSC (mA/cm )

1.05 16 1.00

15

0.95

14

VOC (V)

0.90

13

0.85

12 DMF


DMSO

DMF


DMSO 16

70

Fill Factor (%)

14

65

12 10

60

Efficiency (%)

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


8

55

6 DMF

FMSO30% DMSO60% DMSO80%

DMSO

DMF


DMSO

Figure 13. Photovoltaic parameters of perovskite solar cells based on different solvents of pure DMF, DMSO 30%, DMSO 60%, DMSO 80% and pure DMSO.



1/RSC

Decreasing Intensity 4

10

12 -2

DMF device 0 -0.2

0.0

0.2

0.4

0.6

0.8

1.2 0.0

1.0

0.6

0.8

1.0

1.2

0 1.4

(c) DMF DMSO30% DMSO60% DMSO80% DMSO

1400 1200 2

0.4

Voltage (V)

Voltage (V)

1600

0.2

2

4


5

RSC ( cm )

2

6

-2

J (mA/cm )

(b)

(a)

15

C (10 F cm )

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 26 of 28

1000 800 600 400 200 60

80

100

-1

120

2

1/JSC (A cm )

140

Figure 14. (a) J–V curves of the perovskite device with pure DMF solvent. The slope of the J–V curves at 0V represents the reciprocal of the short‐circuit resistance (Rsc). (b) Mott−Schottky plots of the solar cells based on perovskite films with different solvents. (c) Short‐circuit resistance (Rsc) as a function of the reciprocal of the short‐circuit‐current density (Jsc). The slope of the Rsc versus Jsc−1 curve represents the collection voltage (Vcoll ), which indicates the carrier collection ability or the quality of the i layer.


Page 27 of 28

1.0

Current Reduction Ratio

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

DMSO 60% DMSO 80%

DMF DMSO 30%

Ag

0.2 0.0 0.0

Ag

BCP PCBM Perovskite

0.6 0.4

DMSO

PEDOT/PSS Glass/ITO

0.2

0.4

0.6

0.8

1.0

Voltage (V) Figure 15. Current reduction ratio curves of perovskite devices with different solvents.



1 2 3 4 5 6 7 8 DMF 9 10 11 12MAI 13 Annealing 14 15 16 17 18 19 Disordered phase crystals 20 21 22 10 23 24 25 10 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

Table of contents (TOC) graphic

DMSO60%

DMSO30% PbI2

PbI2

MAI Annealing

DMSO+PbI2

Mixed phase crystals (Disordered+Ordered)

ph/D

3

15

2

JSC(mA/cm )

2

Page 28 of 28

10

5

DMF DMSO30% DMSO60%

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

VOC(V)


MAI

PbI2 Annealing

DMSO+PbI2

Ordered phase crystals

Enhanced Photovoltaic Performance of the ... - ACS Publications

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