High-Performance Simple-Structured Planar Heterojunction

Sep 29, 2017 - (d) The transistor diode performance based on perovskite films with high- and low-quality precursors. ..... Jeon , N. J.; Noh , J. H.; ...
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High-Performance Simple-Structured Planar Heterojunction Perovskite Solar Cells Achieved by Precursor Optimization Kaixuan Wang, Zhenhua Lin,* Jing Ma, Ziye Liu, Long Zhou, Jianhui Du, Dazheng Chen, Chunfu Zhang, Jingjing Chang,* and Yue Hao State Key Laboratory of Wide Band Gap Semiconductor Technology, Shaanxi Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, 2 South Taibai Road, Xi’an 710071, China S Supporting Information *

ABSTRACT: Planar perovskite solar cells (PSCs) with perovskite and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) heterojunction have attracted much interest because they can be fabricated by a low-temperature process and exhibit high power conversion efficiency (PCE). Various compositional engineering and interface engineering approaches have been applied to produce high-performance PSC devices. In this study, we found that high-quality lead halide crystal precursor has a significant effect on the perovskite film quality and it could enhance perovskite film light absorption, reduce crystal defects, and suppress charge recombination, resulting in enhanced device performance (from 8.9 to 15.0% for CH 3 NH 3 PbI 3 and from 14.2 to 18.0% for MA0.7FA0.3PbI3−xClx). Moreover, electron and hole interlayer free devices were achieved by using high-quality PbI2 crystals and the devices exhibited a high PCE of 13.6 and 10.4% for glass and flexible substrates, respectively. This is important for simplifying the perovskite solar cell fabrication process without complex interface engineering involved, especially for printed PSCs.

1. INTRODUCTION

Besides the interface engineering approach, compositional engineering is thought to be another important method to enhance the perovskite solar cell efficiency and stability. Previously, the compositions, precursor aging condition, and precursor purity have been investigated and the results showed that the perovskite thin-film crystallinity, morphology, and device performance could be significantly enhanced by optimizing the precursor solutions.7,9,10,22 Moreover, much lower trap-state density and longer carrier diffusion length have been observed in perovskite single crystals compared to that in polycrystalline thin films.1 Hence, it is meaningful to improve the perovskite thin-film quality for enhancing the device performance. Meanwhile, investigation and analysis of film quality in perovskite devices are important to get more insight into electrical transport properties and enhance the device performance. In this study, we investigated the effects of both lead halide quality and interface structure on perovskite solar cell performance and found that the device using high-quality lead halide crystals achieved a remarkable fill factor (FF) of 0.80 even without any electron interlayer treatment. The device processed from high-quality PbI2 crystal without electron interlayer treatment exhibited a short-circuit current density (Jsc) of 19.8 ± 0.6 mA/cm2, an open-circuit voltage (Voc) of

Perovskite solar cells (PSCs) have received much attention in recent years due to their unique properties, such as high absorption coefficient, long charge-carrier lifetime, high efficiency, low cost, etc.1−4 To further enhance the power conversion efficiency (PCE) of PSCs, many strategies have been investigated, such as solvent engineering,5,6 compositional engineering,3,7−11 interface engineering,12−14 and so on. Among these methods, interface engineering has been widely used in PSCs and it was found that the interface modification could lower the contact resistance, improve the energy alignment, reduce the charge recombination, and enhance the charge extraction. Several interlayers have been used to enhance electron extraction and collection so as to improve the device efficiency.14,15 Among them, metal oxides, such as titanium dioxide (TiO2), zinc oxide (ZnO), tin dioxide (SnO2), and polyelectrolytes, such as fullerene surfactant (bis-C60), rhodamine 101, bathocuproine (BCP), etc. have been found to be potential candidates to act as the cathode interlayer because they can help to form good ohmic contacts between the cathode and the perovskite film, which could substantially improve the device performance and stability.11,12,16−19 However, for printed perovskite solar cells, the device performance is sensitive to the interlayer treatment because it is much affected by the interlayer thickness which is hard to control by using printing techniques.20 Hence, the interlayer free structure is desired.21 © 2017 American Chemical Society

Received: July 31, 2017 Accepted: September 18, 2017 Published: September 29, 2017 6250

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Figure 1. (a) Device architecture (ITO/PEDOT:PSS/perovskite/PCBM/Ag) of PSCs. (b) J−V curves of PSCs with high- and low-quality PbI2. (c, d) J−V curves of PSCs with or without the BCP interlayer. (e) J−V curves of PSCs by using the MA0.7FA0.3PbI3−xClx perovskite layer based on different qualities of mixed lead halide precursors. (f) J−V curves of best PSCs by using the MA0.7FA0.3PbI3−xClx perovskite layer based on highquality PbI2 and PbCl2 crystals.

improve the film crystallinity, enhance light absorption, increase the built-in field, and reduce the charge recombination. Most importantly, the simple-structured electron and hole interlayer free devices were achieved by using high-quality PbI2 crystals and the devices exhibited a high PCE of 13.6 and 10.4% for glass and flexible substrates, respectively. These results indicate that the efficient printed perovskite solar cells could be realized by using high-quality lead halide crystals without complex interlayer modification involved.

0.96 ± 0.02 V, and a FF of 0.79 ± 0.01, leading to a corresponding average PCE of 15.0%, and the PCE could be further promoted to 18.0% by using mixed lead halide crystalbased precursor. It was found that all of the photovoltaic parameters, such as Voc, Jsc, FF, and PCE were enhanced significantly by using the high-quality lead halide crystals. Moreover, the lead halide quality has a significant effect on the precursor solubility and high-quality lead halide crystals could promote the solubility of the precursor solution. Meanwhile, it was found that the high-quality lead halide crystals could 6251

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Table 1. Photovoltaic Performance of Perovskite Solar Cells with Different Process Conditionsa interlayer treatment low quality low quality high quality high quality a

none BCP none BCP

Voc (V) 0.91 0.92 0.96 0.97

± ± ± ±

0.01 0.02 0.02 0.02

Jsc (mA/cm2) 14.7 15.0 19.8 19.5

± ± ± ±

0.7 0.6 0.6 0.5

FF 0.63 0.76 0.79 0.80

± ± ± ±

0.02 0.01 0.01 0.01

PCE (%)

Rs (Ω cm2)

Rsh (kΩ cm2)

± ± ± ±

15.1 5.5 3.7 3.1

1.6 3.1 17.7 22.3

8.9 10.5 15.0 15.1

0.4 0.3 0.2 0.2

The device parameters were averaged over 12 devices.

Figure 2. Steady output characteristics of devices with high-quality PbI2 (a) and low-quality PbI2 (b) without the BCP interlayer. J−V curves of devices with high-quality PbI2 (c) and low-quality PbI2 (d) measured along the forward (from −0.2 to 1.1 V) and reverse (from 1.1 to −0.2 V) scans. The voltage step is 0.01 V, and the delay time is 100 ms.

2. RESULTS AND DISCUSSION The high-quality precursor is processed with ultrahighly pure PbI2 crystals, whereas low-quality precursor is processed with commercial PbI2 powders. It is unexpected that the high-quality crystals could enhance the precursor solubility, hence they are better for high-concentration solutions. PSC devices with the structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ CH3NH3PbI3/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/Ag have been fabricated. Figure 1 shows typical current density versus voltage (J−V) characteristics of devices with different PbI2 qualities and different electron interlayers. From Figure 1b and Table 1, it can be seen that the PSCs with low-quality PbI2 powder (denoted as control PSCs) delivered a PCE of 8.9%, with Jsc of 14.7 mA/cm2, Voc of 0.91 V, and FF of 0.63, whereas the PSCs using high-quality PbI2 crystal showed a PCE of 15.0%, which was much higher than that of control PSCs by 50−60%. The average Jsc, Voc, and FF parameters are substantially enhanced to 19.8 mA/cm2, 0.96 V, and 0.79, respectively. For control PSCs (Figure 1c), the BCP interlayer treatment significantly enhances the FF from 0.63 to 0.76,

whereas devices with high-quality PbI2 (Figure 1d) exhibited similar J−V characteristics without or with the electron interlayer such as BCP. These results indicate that the charge accumulation is minimal between perovskite/PCBM and Ag electrode for high-quality PbI2-based devices, which is desirable for simple device fabrication, such as that of printed PSCs. In this case, the PbI2 precursor quality plays an important role in determining the final device performance regardless of whether the electron interlayer is involved or not. Because during the device fabrication, the only difference between these two kinds of devices is the PbI2 quality, and all of the other conditions, such as the solvent used, materials used, and fabrication conditions, are the same, we can conclude that the performance difference is only caused by the PbI2 precursor quality. Meanwhile, PbCl2 crystals and PbBr2 crystals exhibited similar trends with higher performance achievement compared to that of low-quality PbCl2 powder and PbBr2 powder counterpart. It should be mentioned that the device efficiency could be further improved to 18.0% by using MA0.7FA0.3PbI3−xClx perovskite layer based on high-quality PbI2 and PbCl2 crystals. The Jsc, Voc, and FF are 21.9 mA/cm2, 1.04 V, and 0.79, respectively. 6252

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Figure 3. (a) IPCE spectra of the perovskite devices with different precursors. (b) Voc vs light intensity on a logarithmic scale for PSCs without the BCP interlayer. (b) C−V measurements of devices with different precursors at room temperature. The C−V measurement is performed using 30 mV, 100 kHz alternating current (ac) excitation with direct current bias from 0 to 1 V. The linear fittings are used for energy barrier analysis. (d) The transistor diode performance based on perovskite films with high- and low-quality precursors.

Moreover, the device also exhibited negligible hysteresis for different scan directions (Figure S1). Correspondingly, the device based on low-quality PbI2 and PbCl2 powder presented a low PCE of only 14.2% (Figure 1e). The results of steady-state current density and PCE at the maximum power point as a function of time are consistent with the J−V scan results, manifesting that our devices are stable and the final result is credible (see Figure 2a,b). Furthermore, it clearly shows that the high-quality PbI2 can enhance the Jsc and PCE of devices compared to those of the lower one. The J−V hysteresis behaviors of devices based on different precursors were also measured along the forward (from −0.2 to 1.1 V) and reverse (from 1.1 to −0.2 V) scans with a voltage step of 0.01 V and a delay time of 100 ms, as shown in Figure 2c,d. The J−V curves of the forward and reverse scans are well coincident for both devices with negligible hysteresis, which indicates that the devices are very stable due to the fullerene passivation23,24 and the PbI2 quality is not related to the device hysteresis. The corresponding device parameters are summarized in Table S1. The incident photon-to-electron conversion efficiency (IPCE) curves of both devices are shown in Figure 3a. The high-quality PbI2-based device shows higher IPCE value than that of the low-quality one, ranging from 400 to 800 nm. The calculated Jsc values by integrating the product of IPCE and solar irradiance are 15.5 and 19.3 mA/cm2 for the devices with low-quality and high-quality PbI2, respectively. These values were consistent with measured Jsc values derived from J−V curves. The enhancement in the IPCE suggests that high-quality PbI2

could enhance light absorption and charge transport and suppress charge recombination.25,26 Dark current analysis further reveals differences of devices with high- and low-quality PbI2 precursors. We observed that PSCs with high-quality PbI2 have a lower leakage current at reverse bias and higher on-current at forward bias. A high rectification ratio (defined as the ratio of forward-to-reverse bias current density) of 1.3 × 103 has been achieved from dark J−V characteristics of the devices with high-quality PbI2, whereas it is only 2.9 × 102 for devices with low-quality PbI2 (Figure S2 in the Supporting Information). The Voc is given by the equation:27,28 Voc = (nkT/q) ln(Jsc/Js + 1), where k is the Boltzmann constant, T is the temperature in kelvin, q is the elementary charge, n is the ideality factor, and Js is the (reverse bias) saturation current density. The Js values of devices with low and high-quality PbI2 are 1.9 × 10−2 and 8.6 × 10−3 mA/ cm2, respectively. Hence, smaller Js indicates larger Voc, whereas decreased Js implies reduced recombination loss in the PSC device. Furthermore, in contrast to the devices with low-quality PbI2 precursor, it is calculated that the devices with high-quality PbI2 have a higher shunt resistance (Rsh) and lower series resistance (Rs), which is consistent with the inferior leakage current and higher Jsc. Thus, the fill factor is enhanced substantially. To further figure out the PbI2 quality effect on the Voc and related recombination properties, the function between Voc and the logarithm of light intensity was depicted. Generally, the Voc is related to the energy differences between the quasi-Fermi 6253

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Figure 4. (a) SEM images of low-quality and high-quality PbI2 precursor-based perovskite thin films. (b) Cross-sectional SEM image for the highquality precursor-based device. (c) XRD patterns of MAPbI3 films fabricated with high-quality and low-quality PbI2. (d) UV−vis absorption spectra of MAPbI3 thin films fabricated with high-quality and low-quality PbI2.

different bias voltages, with ac excitation amplitude of 30 mV, at frequency of 100 kHz. For a Schottky diode, the junction capacitance showed a bias dependence according to the Mott− Schottky relation,37,38 C−2 = 2(Vbi − V)/(A2qεε0N), where Vbi is the built-in potential, V is the applied voltage, A is the device’s active surface, q corresponds to the elementary charge, ε accounts for the relative dielectric constant of the perovskite (assumed to be 6.5 as determined by the diffusion reflection),39 ε0 is the permittivity of vacuum, and N is assumed as the doping density of perovskite. The Vbi which accounts for the difference of the equilibrium Fermi levels at both sides of the contact could be estimated by the voltage corresponding to the maximal of capacitance that equals the flat-band condition. As shown in Figure 3c, the differences of Vbi and N in the devices were compared. We attained 0.82 V of Vbi and 4.20 × 1016 cm−3 of N from the devices with the low-quality PbI2 precursor. The Vbi and N were respectively enhanced to 0.93 V and 2.16 × 1016 cm−3 in the devices with the high-quality PbI2 precursor. Because there are many trapped states and ion migration in the perovskite thin films or at the interface and electrons and ions are easily trapped at the interface, Vbi will decrease due to the screening effect if electrons are trapped at interface states and more ions are piled.40 The larger N value indicates an increase of the surface doping density, which may be caused by charged impurities or defects of structure. A higher doping concentration results in enhanced recombination between the doped carrier and the photocarrier, leading to a large proportion of

levels of electrons and holes. In this case, the quasi-Fermi level position is determined by the free-carrier concentrations, which are affected by the balance between the photogeneration and recombination rates.29 As shown in Figure 3b, Voc increases linearly with logarithmic light intensity, and for a trap-free device, the slope of δVoc versus kT/q should equal to 1. In our case, the linear fitting suggests that the slope n = 1.1 and 1.38 for the PSC devices with high-quality and low-quality PbI2 precursors, respectively, implying that the Shockley−Read− Hall recombination (monomolecular recombination) is more dominant in the devices processed with low-quality PbI2 precursor,30,31 which indicates that the impurities or defects in the low-quality precursor can increase trap sites and thus promote trap-assisted recombination.32 Hence, the charge recombination of high-quality devices is suppressed due to lower trap density and the device performance is enhanced. We also extracted the FF as a function of light intensity to study the PbI2 quality effect on carrier extraction, as shown in Figure S3. It was found that the effective FF shows less dependence on different light intensities for a high-quality PbI2 precursor-based device, indicating that the recombination is minimal in this condition because the FF is related to charge recombination and carrier lifetime.33,34 Capacitance−voltage (C−V) measurements were taken in the dark at room temperature to investigate the electronic properties of MAPbI3 in actual devices with respect to the doping properties.35,36 The capacitance was measured against 6254

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Figure 5. (a) Steady-state PL emission spectra and (b) time-resolved PL spectra of perovskite thin films deposited on glass substrates.

crystals mean less grain boundaries existing in the thin films, indicating that the grain boundaries and impurities-related defects are reduced. Because it was speculated that the impurities could affect light absorption of perovskite thin films due to the lower crystallite size in the perovskite layer, the light absorption of the perovskite thin films with different precursors were investigated through UV−vis spectroscopy (Figure 4d). Obviously, the high-quality film exhibited increased light absorption in the range of 300−800 nm. This can be attributed to the higher crystallinity of the high-quality perovskite film. To figure out the photoconversion processes associated with the quality of the PbI2 precursor in the perovskite films, photoluminescence (PL) was carried out on both thin films, as shown in Figure 5a. The high-quality thin film exhibited a larger intensity of PL emission compared to that of the low-quality film. Therefore, the perovskite MAPbI3 with high quality could help devices possess a lower defect density and a lower charge recombination rate compared with the low-quality one. Consequently, the performance is enhanced substantially. The transiently photoexcited carriers were investigated by timeresolved PL measurements on the basis of perovskite films deposited on glass substrates. Perovskite thin films are known to be subject to two-carrier recombination under high fluorescence, which results in a fast PL decay.43,44 The PL decay of these two kinds of thin films is shown in Figure 5b, and the fitted parameters are summarized in Table S2. The average PL lifetimes for perovskite thin films prepared from high-quality and low-quality precursor were calculated to be 80.26 and 10.72 ns, respectively. Combining the steady-state and time-resolved PL study, the high-quality thin films displayed higher PL intensity and a slower PL decay rate, which indicates that the perovskite MAPbI3 with high quality has a high electron transport rate and a low carrier recombination rate compared to those of the low-quality counterpart. To further simplify the device fabrication step, holetransport-layer-free (HTL-free) MAPbI3/PCBM planar heterojunction solar cells were fabricated with the configuration of ITO/MAPbI3/PCBM/Ag. The hole-transport-layer-free devices have been reported in this structure, and it has been found that PEDOT:PSS has less effect on the charge transfer from MAPbI3 to the ITO electrode determined from PL spectra;

trap-assisted recombinations; hence, the Jsc and FF are reduced. The Vbi for the devices is in agreement with the Voc extracted from J−V characteristics. The smaller Vbi of devices based on the low-quality PbI2 precursor means lower driving force for the photogenerated carriers to be separated in PSC,41 which indicates that the devices with a low-quality PbI2 precursor have higher energy barrier. Hence, the reduced Vbi resulted in an increased probability of recombination and a decreased probability of charge-carrier extraction. This is consistent with the recombination mechanism.42 The charge transport properties were also studied by using a thin-film transistor diode. The perovskite thin film based on the high-quality PbI2 precursor exhibited higher current and conductivity, which is beneficial for efficient charge extraction and collection (Figure 3d). These results also correlate well with the J−V data and suggest that the high-quality PbI2 precursor could result in better device performance. To verify our views about film crystallinity, crystal size, and light-harvesting ability, the measurements of scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV−vis absorption were performed. As shown in Figure 4a, both perovskite thin films exhibited uniform and compact morphology, and the average crystal size of the high-quality perovskite film is larger than that of the low-quality one. It was thought that the impurities introduce more nucleation sites and induce a heterogeneous nucleation and crystal growth thus leading to smaller crystallites. Larger crystal size also means lower crystal defect density and trap sites existing in the thin films, which is beneficial for the efficient charge transport and reduced charge recombination. This is also confirmed by charge transport results. Thin-film crystallinity could be further confirmed by XRD measurements. As shown in Figure 4c, the two MAPbI3 thin films exhibit strong diffraction patterns at 14.2, 28.5, and 31.9°, which can be assigned to (110), (220), and (310) crystal planes of the tetragonal perovskite phase, respectively.8 However, the diffraction intensity of the high-quality film is much higher than that of the low-quality one, demonstrating that the former has better crystallinity and fewer defects, which is consistent with SEM results. According to the full width at half-maximum (FWHM) values (Figure S4), the crystallite size of the perovskite films could be estimated in terms of the Scherrer equation to be around 453 and 628 nm for low-quality and high-quality PbI2-based thin films, respectively. Larger 6255

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Figure 6. (a) J−V curves of perovskite solar cells fabricated on glass substrates with/without the PEDOT:PSS hole transport layer. (b) J−V curves of perovskite solar cells fabricated on poly(ethylene terephthalate) (PET) substrates without the PEDOT:PSS hole transport layer. Inset is the photograph of flexible devices.

perovskite solar cell fabrication process and could be the potential choice for the printed PSCs.

hence, the PEDOT:PSS interlayer could be avoided in this structure.45−47 Without using the hole-transporting layer, the devices are rapidly fabricated in comparison with devices containing hole-transporting layers. The simple structure and simplified fabrication process provide a potential for the commercialization of printing electronic devices. As shown in Figure 6a, the hole-layer-free device exhibited a maintained Jsc of 20.1 mA/cm2, slightly enhanced Voc of 1.00 V, and decreased FF of 0.67. Hence, the final device showed only a slightly decreased PCE of 13.5% compared to that of the control device (14.9%) with the PEDOT:PSS interlayer. To investigate the universality of this structure and highquality precursor, we achieved flexible HTL-free perovskite solar cells on PET substrates. The device fabrication method is similar to that of the glass substrates. The J−V curve of the flexible perovskite solar cell is shown in Figure 6b. The flexible solar cell devices achieved a best PCE of 10.4%, with a Jsc of 17.0 mA/cm2, a Voc of 0.97 V, and a FF of 0.63. The Jsc and FF slightly decreased compared to that of the glass substrates, which may be due to the increased surface resistance of the ITO electrode. The device hysteresis behavior was also checked, and only a slight hysteresis behavior was observed, which may be due to the trap states at the interface between ITO and perovskite thin films (Figure S5).23,47

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01099. Materials, fabrication, and characterization of perovskite solar cells; material characterization; the dark J−V characteristics; FF as a function of light intensity; FWHM peak for the perovskite thin films; fitted decay times of perovskite films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.L.). *E-mail: [email protected] (J.C.). ORCID

Jingjing Chang: 0000-0003-3773-182X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (61604119 and 61704131), Young Talent fund of China Association for Science and Technology, and the Fundamental Research Funds for the Central Universities.

3. CONCLUSIONS In conclusion, we have studied the lead halide crystal quality and interlayer structure effect on the performance of planar heterojunction perovskite solar cells. The high-quality lead halide crystals could promote the growth of perovskite crystals, which enhances the perovskite film light absorption, reduces crystal defects, and suppresses charge recombination. Moreover, a larger built-in field could be obtained for a high-quality lead halide-based device, which could enhance the charge collection and the charge transport. These advantages help the devices with high-quality lead halide to improve the fill factor to 0.80 and enhance the device performance substantially (from 8.9 to 15.0% for CH3NH3PbI3 and from 14.2 to 18.0% for MA0.7FA0.3PbI3−xClx). Meanwhile, HTL-free devices were successfully fabricated by using the high-quality PbI2 precursor and exhibited a high PCE of 13.6 and 10.4% for glass and PET substrates, respectively. This is important for simplifying the

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REFERENCES

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DOI: 10.1021/acsomega.7b01099 ACS Omega 2017, 2, 6250−6258

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DOI: 10.1021/acsomega.7b01099 ACS Omega 2017, 2, 6250−6258