Letter pubs.acs.org/NanoLett
Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells Nicholas De Marco,†,‡ Huanping Zhou,†,‡ Qi Chen,†,‡ Pengyu Sun,† Zonghao Liu,†,‡ Lei Meng,† En-Ping Yao,† Yongsheng Liu,†,‡ Andy Schiffer,‡ and Yang Yang*,†,‡ †
Department of Materials Science and Engineering, ‡California NanoSystems Institute, University of California, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: Hybrid perovskites have shown astonishing power conversion efficiencies owed to their remarkable absorber characteristics including long carrier lifetimes, and a relatively substantial defect tolerance for solution-processed polycrystalline films. However, nonradiative charge carrier recombination at grain boundaries limits open circuit voltages and consequent performance improvements of perovskite solar cells. Here we address such recombination pathways and demonstrate a passivation effect through guanidinium-based additives to achieve extraordinarily enhanced carrier lifetimes and higher obtainable open circuit voltages. Time-resolved photoluminescence measurements yield carrier lifetimes in guanidinium-based films an order of magnitude greater than pure-methylammonium counterparts, giving rise to higher device open circuit voltages and power conversion efficiencies exceeding 17%. A reduction in defect activation energy of over 30% calculated via admittance spectroscopy and confocal fluorescence intensity mapping indicates successful passivation of recombination/trap centers at grain boundaries. We speculate that guanidinium ions serve to suppress formation of iodide vacancies and passivate under-coordinated iodine species at grain boundaries and within the bulk through their hydrogen bonding capability. These results present a simple method for suppressing nonradiative carrier loss in hybrid perovskites to further improve performances toward highly efficient solar cells. KEYWORDS: Perovskite, solar cell, guanidinium, passivation, open circuit voltage, carrier lifetime
1.0. INTRODUCTION The hybrid organic−inorganic perovskite has emerged as a strong candidate for photovoltaic cells, achieving an unprecedented rise in performance from 3.8% PCE1 as a liquid-based solar cell to an initial 9.7% in the solid state,2 eventually soaring rapidly to reach over 20% in just 5 years.3 This alluring material possesses several key attributes of an ideal solar cell absorber, such as a favorable band gap, high absorption coefficient, long ambipolar carrier diffusion lengths, high carrier mobility, and a relatively high defect tolerance.1,4−9 Furthermore, its capability to be processed via low temperature solution techniques renders it substantially more cost-effective than the wellestablished silicon solar technology. As such, perovskite has become highly attractive as an affordable and scalable nextgeneration photovoltaic technology with the potential to match the continuously increasing global energy demands. The ability of an absorber material to effectively generate and extract charge carriers is of paramount importance to create a highly efficient solar device. Theoretical studies have shown defect energy levels to lie relatively shallow,10,11 and it has been commonly accepted that grain boundaries do not contribute largely to recombination losses in perovskite. However, such recombination centers in perovskite have recently been deemed less benign than previously believed.12,13 Hence, there still remains considerable room to improve the performance of © 2016 American Chemical Society
perovskite solar cells. In order to effectively mitigate nonradiative carrier loss and enhance efficiency, it is necessary to suppress recombination pathways within the perovskite film itself. As a result of the solution-processed nature of perovskite, under-coordinated ions may exist at grain boundaries and surfaces, as has been recently described, that can act as charge carrier trap/recombination centers.14,15 Passivation of such species has been demonstrated by Abate and co-workers, where an iodopentafluorobenzene (IPFB) post-treatment was used to successfully passivate under-coordinated iodine ions.14 In a similar manner, Noel et al. used the Lewis bases, thiophene and pyridine, to passivate under-coordinated Pb ions and achieve carrier lifetimes of perovskite films up to 2 μs.15 Internal passivation methods have also been reported. For instance, Chen and collaborators demonstrated a self-induced passivation effect due to residual PbI2 to serve as recombination barriers.16 In addition, Br and Cl have been recently suggested to preferentially locate at grain boundaries where they serve to assist in suppressing recombination and decoupling electron− hole pairs.17,18 Thus, one may speculate that there is a high appeal to utilize the compositional flexibility of perovskite and Received: October 5, 2015 Revised: December 14, 2015 Published: January 20, 2016 1009
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Figure 1. (a) Current−voltage characteristics of the champion GA and MA reference devices. The optimized GA device shows an improved Voc and preserved FF and Jsc yielding an overall efficiency improvement, as depicted in the inset. (b) Device averages demonstrating the consistency of enhanced performance characteristics for GA addition.
Figure 2. (a) Molar and weight ratios ranging from pure MA (ref) to pure GA (Film 7) used in this study; (b) corresponding film colors for preannealed (top) and postannealed (bottom) GA films; (c) absorption spectra for varying GA content with a log scale plot in the inset.
decay enabling an order of magnitude enhanced carrier lifetime over that of pure MAPbI3, open circuit voltages as high as 1.112 V, and improved device performances over 17% PCE. A reduced defect activation energy and enhanced carrier lifetime within a full device indicate passivation effects as a result of the GA inclusion. We believe that the hydrogen bonding capability of GA provides enhanced grain size and continuity and more significantly serves to effectively passivate under-coordinated iodine species between adjacent crystalline grains.
explore new species for suppression of nonradiative recombination pathways. In this light, the organic molecule CH6N3+, more commonly known as guanidinium (GA), was investigated as an additive in MAPbI3 to observe influences on structure, film quality, and performance. GA possesses an approximate zero dipole moment that has been hypothesized to have influence on bias-induced ionic motion and hysteric effects.19,20 However, GA is substantially larger in size (278 pm) than the commonly employed methylammonium (MA; CH3NH3+) cation (217 pm), and ought to not form a 3D perovskite structure as the sole A-cation.21,22 Interestingly, GA has been well investigated as an additive for dye-sensitized solar cells (DSSCs) to improve performance.23 In such devices, many claim that GA serves to passivate TiO2 surfaces. More recently its contributions have been shown to lie within the interaction of the dye/liquid electrolyte interface.23 In this manuscript we demonstrate a passivation effect through partial GA incorporation in perovskite films, producing significant mitigation of nonradiative
2.0. RESULTS AND DISCUSSION Mixed solutions of MA and GA precursors (MAI, GACl, GAI) with molar ratios ranging from 1:0 to 0:1 (MA:GA) were studied. Perovskite films were fabricated via a sequential twostep deposition technique using spin-coating similarly to previous reports24 (detailed in the Supporting Information). To directly observe the effects of GA on device performance, current−voltage (J−V) measurements were employed for varying GA content with a planar device architecture of ITO/ 1010
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Figure 3. (Top) Scanning electron microscopy (SEM) and (Bottom) atomic force microscopy (AFM) of (a,c) MA and (b,d) GA films, respectively. Scale bars are 1 μm for the SEM images, and the width of AFM images is 5.0 μm. An increase in film continuity and slightly improved surface roughness is observed via GA inclusion, showing an improved morphology.
champion device (due a large sacrifice in the Jsc and FF), this result shows that even higher open circuit voltages may be attainable in the future. A prominent reddish color change of the preannealed perovskite film, and a corresponding increase in transparency postannealing, was observed for incremental amounts of GA addition, as shown in Figure 2a,b. It was found that a molar ratio of 2:1 (MA:GA) was the upper limit of GA inclusion for successful 3D perovskite formation indicated by a distinct yellow resultant film color. UV−visible absorption spectroscopy was utilized to investigate the optical effects of GA inclusion. Figure 2c shows the absorption spectra for increasing GA content, from which it is readily apparent that an increase in GA content results in a decrease in light harvesting capability, as evidenced by the reduction in Jsc. The band gap (Eg) remains relatively unaffected with GA incorporation, varying between 1.53 and 1.55 eV for different GA amounts corresponding to absorption tails spanning 790−800 nm. The calculated band gap for MA and GA were 1.56 and 1.55 eV, respectively. We note that this small fluctuation within this range is commonly observed for reference samples prepared with identical conditions, and is based on morphological and film quality differences between samples. Tauc plots are provided in Figure S2 to show variation in optical band gap. Interestingly, the reduction in Jsc from GA incorporation does not appear to result from a shift in band gap, but rather, a reduction in absorption intensity across the spectrum for increasing GA content. This indicates that GA does not directly substitute for MA, as if this were the case the band gap ought to shift in accordance with the large size of the GA cation. This result is in contradiction with previous theoretical studies that suggest substitution of MA with the larger GA cation would increase
TiO2/perovskite/spiro-OMeTAD/Au. Intriguingly, we observed a prominent enhancement of Voc through GA incorporation. It was found that for only a small quantity of GA incorporation, ranging between a 30:1 and 6:1 molar ratio of MA:GA, the Voc could be improved while preserving Jsc. An optimal molar ratio of 6:1 (MA:GA), hereinafter denoted as GA when directly compared to the reference MA device, provided the best performance characteristics. Figure 1a shows the J−V curve of the champion device in comparison with the reference (MA) sample, demonstrating an enhancement of Voc from 1.025 V in the standard MA-based device to 1.071 V for GA. Furthermore, a slight increase in fill factor (FF) from 75.00 to 75.31 for MA and GA was observed, respectively. The Jsc remained relatively unaffected with GA inclusion, achieving 21.24 mA/cm2 compared to 21.27 mA/cm2 for the pure MA device. The much improved Voc and slightly increased FF led to an overall power conversion efficiency (PCE) gain of 17.13% compared to 16.35% for the MA reference device. The average device characteristics (Figure 1b) show the consistency of the observed performance enhancement for GA addition. We can see that GA provides an average increase in V oc of approximately 50 mV, while preserving the Jsc and improving the FF from 74.87% to 76.04%. These improvements lead to a device performance average of 16.27% over the 15.75% PCE of its MA-based counterpart. As the GA content was increased over a molar ratio of 6:1 (MA:GA), a steady decrease in Jsc was observed. For molar ratios above 6:1 up to 3:1 (MA:GA), a Voc between 1.07 and 1.09 V was consistently achievable, where a high of 1.112 V was achieved, as depicted in Figure S1. It can be observed that this increased Voc comes at the cost of Jsc, and did not lead to the highest performing device. Despite inferior overall efficiencies of the higher Voc devices to that of the 1011
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Figure 4. Confocal fluorescence intensity images for (a) MA and (b) GA (scale bar is 5 μm). Darker low-intensity regions are prominent in the case of MA, which we attribute to grain boundary regions. (c) Corresponding histogram for the intensity range of fluorescence images of MA and GA, respectively. A magnified view of (d) MA and (e) GA films with 4 μm linear intensity profiles shown in (f). The MA film shows a prominent drop across the darker region, whereas the GA remains relatively constant across.
does not affect the perovskite structure, while a larger amount will ultimately disrupt the crystallinity. To further explore the nature of GA incorporation within the perovskite crystal, we conducted XRD measurements for pure GA (no MA) content, as provided in Figure S4b. From the spectra we can see that the characteristic perovskite peaks diminish entirely as a result of the GA cation inclusion. While this confirms that GA does not incorporate into the lattice, this spectra cannot be well interpreted as there appear to be other factors as a result of the thin film fabrication process that affect the spectra. Therefore, we cannot ascribe this XRD spectra to be an accurate representation of a pure GAPbI3 phase. A thorough study on the structural characteristics of the pure GAPbI3 single crystal phase is thus necessary and is currently under investigation for future work. Nevertheless, these results demonstrate that GA cannot serve as a direct substitution for MA within the perovskite crystal lattice, suggesting that its role in enhanced performance characteristics lie elsewhere. One plausible explanation for the enhanced Voc is facilitation of improved film morphology resulting from GA that may reduce recombination at interfaces. It is believed that introducing organic ammonium cations could help facilitate crystal growth to improve film uniformity and compactness.26 Indeed, this effect is observed through scanning electron microscopy (SEM) images provided in Figure 3a,b that depict morphological impacts of the resulting films. A slight improvement in film continuity is observed for GA-based films as there appears to be higher grain continuity with fewer small grain protrusions and less prominent grain boundaries present. To investigate such features, atomic force microscopy (AFM) was used to measure the surface profiles of the MA and
the band gap due to a reduction in antibonding interactions between Pb and I.19 This would indeed be the case if GA were to successfully substitute for MA in the perovskite crystal structure. However, as there is no experimental evidence regarding the crystal structure for GAPbI3 to show how it incorporates into the perovskite crystal lattice, calculations based on the GA ion in perovskite must make such an assumption. We believe that an excess amount of the oversized GA within the perovskite film likely disrupts the crystallinity of the 3D structure that constitutes the lower absorption, consequent reduction in Jsc, and unsuccessful perovskite formation over the upper molar ratio limit. Evidence for this claim is provided in the following characterization results. Ultraviolet photoelectron spectroscopy (UPS) was further used to observe Fermi level and valence band edge positions of the MA and GA champion device (Figure S3). There is a negligible Fermi level (Ef) shift, with values located at approximately 4.59 and 4.54 eV for MA and GA, respectively, indicating an n-type nature of the perovskite film. A slight change in the valence band energy level (EVB) is observed. The pure-MA reference device yields an EV value of 5.6 eV in comparison to a slight shift to 5.72 eV for GA. Interestingly, this downward shift does not seem to affect the hole transfer between perovskite and Spiro-OMeTAD. In order to observe structural impacts of GA on the perovskite crystal, X-ray diffraction (XRD) analysis was conducted (Figure S4a). Both in the case of MA and GA, strong characteristic perovskite 110 and 220 peaks were observed, located at approximately 14.15° and 28.5° (2θ), respectively, in accordance with previous results.25 Along with the absorption spectra and observed film color change, these results show that a small amount of GA 1012
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Figure 5. (a) Comparison of relative PL intensities and (b) TRPL spectra of pure MA and GA PVSK films deposited on glass substrates. GA shows an increased PL intensity over 4 times MA and an enhanced carrier lifetime an order of magnitude over that of MA.
GA films (Figure 3c,d). GA shows a slight reduction in surface roughness of 12.7 nm in comparison to 13.2 nm of pure MA. To further observe any morphological trends, we fabricated films with higher GA content (Figure S5). An even further improved film continuity and larger grain regions are apparent in the case of the SEM image for increasing GA content, suggesting an improved morphology and reduced surface roughness. Interestingly, it would appear that the higher GA (Film 2) content provides a smoother and more continuous morphology than the optimized GA content (Film 1) according to the SEM images. However, AFM measurements yield a surface roughness of 16.3 nm for Film 2, which is larger than that of Film 1. This indicates that though there is a larger grain continuity for increased amounts of GA, the larger surface roughness may reduce interfacial contact that would affect charge extraction across the perovskite transport layer junctions. Consequentially, the very slight decrease in surface roughness for GA (Film 1) compared to the reference (MA) does not provide conclusive evidence that the observed performance improvements may be attributed to an improved film roughness and interfacial contact effects. Such a small change in surface roughness is commonly observed within a given sample due to local topographical variations across different regions of the surface, resulting from the imperfect nature of the spin-casting technique. It has recently been suggested that the microstructure can greatly affect local carrier lifetime in perovskite.12 In particular, it was shown through PL microscopy that between different grains the local carrier lifetime varies considerably, and that the dark low-intensity regions of the PL image, corresponding to grain boundaries, represent a region of reduced carrier lifetime. In this light, we conducted confocal fluorescence microscopy to correlate observed morphological features to the carrier dynamic effects of GA films, where a low intensity (darker) region corresponds to a reduced lifetime. The fluorescence intensity mapping of the MA and GA films are depicted in Figure 4. The samples were prepared on a glass substrate identically to the two-step procedure, as specified in the Supporting Information. Darker regions are clearly distinguishable in the case of the MA reference (Figure 4a). In the case of the GA film (Figure 4b), we observe a reduced quantity of the prominent dark regions. In correlation to our SEM and AFM images, we presume that the larger regions represent morphological differences observed between the film surfaces of MA and GA. Individual grains are not highly distinguishable
due to the limited resolution of this technique, however, we can observe what appears to be small grains with sizes between 500 nm to 1 μm, which is in accordance with grain sizes observed under SEM. The darker regions surrounding these grains are believed to be grain boundaries. Accordingly, we can observe less prominent darker grain boundaries surrounding individual grains in the case of GA, indicating that recombination within these grain boundary regions is successfully suppressed. Histogram plots (Figure 4c) of average intensity show a higher average intensity in the case of the GA sample. The mean intensity values, within a relative intensity range of 0−255, were 140.47 in the case of GA and 131.54 for MA. We can also observe over an order of magnitude increase in the number of counts for GA, indicating that it fluoresces more strongly. Furthermore, 4 μm linear intensity profiles were taken for both the MA and GA samples, as shown in Figure 4d,e. Plotting these linear intensity scans as a function of distance (Figure 4f) yields a consistent profile for GA, whereas there is a strong dip in the profile in the case of MA. These results indicate that the darker regions in MA indeed correspond to a reduced intensity, and corresponding lifetime, suggesting that GA serves to passivate grain boundaries. Photoluminescence spectroscopy is a useful tool to extract information regarding charge carrier dynamics in semiconductor materials. Specifically, the quality of the material can be analyzed by determining the degree of nonradiative recombination loss. The carrier dynamics of the GA perovskite films were carefully examined through photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements (Figure 5). The perovskite films for this measurement were prepared on glass substrates with an identical sequential twostep deposition technique as used for device fabrication. To our surprise, GA shows a large rise in PL intensity over four times that of the pure MA devices, as shown in Figure 5a. Even more intriguingly, the TRPL spectra shows a substantial enhancement in carrier lifetime (τ), superseding that of the reference by an order of magnitude. The curves display biexponential character and were fitted accordingly with both fast and long decay components, where the long decay component (τ) can be described as free carrier recombination.16 Figure 5b portrays an enhanced τGA of 800 ns for the GA film compared to τMA = 80 ns in the case of the MA reference. It is rather intriguing that such an exceptional improvement in carrier lifetime does not yield an as significantly improved performance. In principle, an improved carrier lifetime ought to result in a larger open circuit 1013
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Figure 6. (a) Normalized log scale transient photovoltage decay measurements for MA and GA samples. GA shows an improved carrier lifetime of 7.95 μs compared to 3.15 μs for MA. (b) Carrier lifetimes for varying light intensities of both MA and GA samples. GA consistently achieves approximately twice the lifetime of MA-based devices at different light intensities.
spectroscopy provides a useful route to extract the energy levels of defects residing within semiconductor materials. The capacitance is directly associated with the charging and discharging of trapped charge carriers within the material, as has been described in further detail in our previous report.28 Occupancy of these trap sites depends on the location of the Fermi level, where traps below the Fermi level are assumed filled. Accordingly, by varying the AC voltage frequency and the temperature of the system we can probe these trap levels per Fermi level shift and extract information regarding the defect activation energy of the system. A comparison of defect activation energies determined from admittance spectroscopy measurements is provided in Figure S7. The activation energy of the MA-based device was calculated as 21.11 meV, which is in accordance with our previous report.29 We can observe that the defect activation energy of the GA-based device is significantly lower (14.86 meV), indicating that recombination within the bulk of the film has been successfully suppressed. Among the native point defects within perovskite, iodine vacancies have shown to be most detrimental for charge trapping and nonradiative recombination due to their deep lying energy levels.30 It was suggested that chloride inclusion can alter the lattice constant of perovskite and prevent the formation of iodide vacancies. Because GA does not directly substitute for MA into the perovskite crystal lattice, its interactions between adjacent crystalline domains of the perovskite film may similarly affect the lattice constant, thereby suppressing the formation of deep iodide vacancies, as evidenced by the reduced defect activation energy. On a similar note, passivation of under-coordinated iodine species has recently been demonstrated via postfilm formation treatments using the IPFB Lewis acid.14 As a free cation, GA is a Lewis acid owed to the difference in covalent character between the central C atom and surrounding nitrogen. The C− N partial charge difference for GA within the solid state perovskite film has been shown to be greater than that for the free GA cation according to recent theoretical studies.19 In fact, the unique hydrogen bonding capability of GA has been previously demonstrated in metal−organic frameworks (MOFs).31 The symmetry of the amine groups allows the GA molecule to form six hydrogen bonds with neighboring octahedra of the MOF, which enhanced its mechanical stability. Hence, we expect an increased quantity of hydrogen bonding capability between the partial negative (δ−) iodine ions of exposed PbI64− of neighboring perovskite crystals and the
voltage as a reduction in minority carrier Shockley−Read−Hall type recombination (due to a reduced defect density) would yield larger Fermi level splitting. However, the complexity of a fully assembled device under operating conditions is much greater than that of a bare perovskite film during optical measurements. It is probable that there are other factors within the device, such as the selective contacts, that create additional bottlenecks that pose additional limitations on device performance regardless of the quality of the perovskite film itself. With this concept in mind, we may still correlate the enhanced carrier lifetime to the enhanced device performance characteristics, mainly, the open circuit voltage, for which a less prominent improvement is observed. Interestingly, it was observed that additional GA species reduces the PL intensity and carrier lifetime, indicating that exceeding the 6:1 molar ratio will have a reverse effect. The PL and TRPL spectra for additional GA content is provided in Figure S6. This result is in accordance with the observed morphological trends, however, it contradicts our observed performance trends, as a higher Voc can still be obtained through use of Film 2, but a reduced Jsc and FF led to reduced performances. Transient photovoltage decay was conducted to further explore carrier dynamics through investigation of carrier dynamics within a fully assembled device. In this technique, illuminated devices are probed with a light pulse to generate a photovoltage perturbation. The voltage decay rate thereafter, which represents the change in carrier density versus time, is observed to extract information regarding carrier lifetime within the device. Figure 6 shows the photovoltage decay spectra for the reference MA and GA samples. The carrier lifetimes were taken as the time for which the photovoltage decayed to 1/e of its peak intensity.27 The MA reference device shows a charge carrier lifetime of 3.15 μs, in agreement with previous reports (Figure 6a).25 The GA sample shows an improved charge carrier lifetime of 7.95 μs, over twice that of the MA device, verifying suppression of carrier recombination. Figure 6b depicts the extracted carrier lifetimes as a function of light intensity. GA consistently yields carrier lifetimes approximately twice that of MA-based devices, providing further validation of this result. We note here that use of both GAI and GACl yielded comparable lifetime enhancements, signifying that the observed effects are indeed a result of incorporation of the guanidinium ion. Carrier recombination dynamics in solar devices are largely influenced by defects within the absorber material. Admittance 1014
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Nano Letters partial positive (δ+) ammonium ions of the organic GA species. Accordingly, we propose that GA preferentially resides at grain boundaries and forms hydrogen bonds with under-coordinated iodine species to effectively suppress these charge trapping/ recombination regions. Furthermore, the hydrogen bonding capability between neighboring grains may aid in grain growth during film formation, facilitating the higher film continuity and larger grain regions observed through SEM images. It is worth noting that incorporation of the passivant GA molecule directly into the precursor solution, as opposed to postsurface treatments, may allow for the passivant species to address deeper internal defect regions that are otherwise inaccessible through postfilm formation surface treatments. This may be one advantage in using additive-based passivation methods in future works. However, the majority of the GA species ought to reside at grain boundaries and interfaces between the perovskite film and selective contacts. Despite the tremendous growth in performances of perovskite solar cells, there still remain two critical issues unresolved that prevent its commercialization in (1) the current−voltage hysteresis, and (2) the inherent instability of the perovskite film. Hysteresis effects are commonly observed for forward and reverse current−voltage scans of perovskite solar cells. GA has been hypothesized to potentially solve hysteresis in perovskite films owed to its zero dipole moment.19,20 To compare the hysteric effects between the GA and MA-based devices, we conducted forward and reverse current−voltage scans for the champion devices, provided in Figure S8. As can be observed, hysteresis is slightly more prominent for GA addition in comparison to MA alone. At first we may be surprised at this result since it is in contradiction with what we would expect according to the zero dipole moment of GA and the work from Giacomo et al.19 However, since this GA ion is not confined within the perovskite crystal, the ionic molecule would thus be responsive to an external electrical influence. Tress et al. have recently conducted thorough studies on hysteresis, for which they attributed such effects to migration of ionic species to interfaces that can effectively screen the applied electric field.32 Since the GA species do not directly substitute for MA into the crystal structure and are rather weakly hydrogen-bonded to the under-coordinated ionic species mainly at grain boundaries (due to its approximate zero dipole moment19), the GA ions ought to be more responsive to an external electrical influence. As previously mentioned, the amount of GA species located at grain boundaries will be significantly more than those residing internally at defect regions within the bulk. Consequentially, the more mobile GA species between interfaces and at grain boundaries would readily migrate to the junction under the influence of an applied bias, leading to further internal electric field cancellation and enhanced hysteric effects. Now turning our attention toward the stability of perovskites, it is inarguably the most pressing issue preventing perovskite from commercialization as perovskite films exposed to ambient environment will degrade within a matter of hours to days depending on the humidity level.25,33 The stability for the GAbased device is compared to the standard MA-based device (Figure S9). The devices were stored under a dry oxygen environment and only exposed to ambient environment during measurement. We can see that the both devices are relatively stable for approximately 7.5 days (180 h), where afterward both devices undergo a rapid decrease of the initial PCE. Interestingly, the rate of PCE drop for the pure MA-based device is much higher than that of the device with GA
inclusion, where the GA device maintains over 80% of its initial PCE compared to approximately 60% for the pure MA device. While there are several factors involved in device stability, and conclusive evidence regarding the role of GA in enhanced stability requires further investigation, we speculate that the following as a potential explanation of the observed results: As grain boundaries are highly susceptible to invasion by incident water molecules, the less prominent, and therefore vulnerable, grain boundaries as observed from the SEM images could mitigate the effects of attacking water molecules. Moreover, since water molecules will primarily attack the more susceptible grain boundary regions, the weakly bound GA ions located in these regions will be the first to interact with the incoming water molecules. If this were the case, it would preserve the crystalline perovskite domains for longer than films without GA present. Nevertheless, a thorough study on the role of GA in device stability is required and is currently under investigation.
3.0. CONCLUSION In summary we have demonstrated a simple route to significantly enhance carrier lifetimes and open circuit voltages in hybrid perovskite solar cells via GA-based additives. Timeresolved photoluminescence and photovoltage decay techniques were used to extract charge carrier dynamics, demonstrating an exceptional improvement in carrier lifetime for GA-based films 1 order of magnitude larger than the pure MA films that led to open circuit voltages exceeding 1.1 V. Fluorescence spectroscopy results showing a reduced PL intensity quenching at grain boundary regions in correlation with a reduced defect activation energy measured by admittance spectroscopy indicate successful passivation of nonradiative recombination/ trap centers within the perovskite film. We propose that the large hydrogen bonding capability of the GA molecule allows for effective passivation of under-coordinated iodine species located at grain boundaries both at the surface and internally within the bulk. SEM and AFM images show a slight improvement in film quality with a lower surface roughness, higher degree of film continuity, and less distinct grain boundaries that may correlate to the observed stability of the GA-based device. The introduction of the GA species as a perovskite precursor overcomes the limitations of surface treatments by allowing for infiltration of the GA passivant into internal regions within the perovskite bulk, where undercoordinated species may reside. This study demonstrates a simple and effective route to improve carrier lifetime and film quality through precursor additives, bypassing the need for additional processing steps and material usage to improve performances of perovskite solar cells.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04060.
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Details regarding device fabrication and characterization procedures along with supporting figures (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 1015
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(21) Kieslich, G.; Sun, S.; Cheetham, A. K. Chem. Sci. 2014, 5, 4712− 4715. (22) Szafranski, M.; Jarek, M. CrystEngComm 2013, 15, 4617−4623. (23) Jeanbourquin, X. A.; Li, X.; Law, C.; Barnes, P. R. F.; HumphryBaker, R.; Lund, P.; Asghar, M. I.; O’Regan, B. C. J. Am. Chem. Soc. 2014, 136, 7286−7294. (24) Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G. Nat. Nanotechnol. 2014, 9, 927−932. (25) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Science 2014, 345, 542−546. (26) Li, X.; Ibrahim Dar, M.; Yi, C.; Luo, J.; Tschumi, M.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Han, H.; Grätzel, M. Nat. Chem. 2015, 7, 703−711. (27) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Science 2013, 342, 341−344. (28) Duan, H.-S.; Zhou, H.; Chen, Q.; Sun, P.; Luo, S.; Song, T.-B.; Bob, B.; Yang, Y. Phys. Chem. Chem. Phys. 2015, 17, 112−116. (29) Chen, Q.; Zhou, H.; Fang, Y.; Stieg, A. Z.; Song, T.-B.; Wang, H.-H.; Xu, X.; Liu, Y.; Lu, S.; You, J.; Sun, P.; McKay, J.; Goorsky, M. S.; Yang, Y. Nat. Commun. 2015, 6, 7269. (30) Du, M. H. J. Mater. Chem. A 2014, 2, 9091−9098. (31) Li, W.; Thirumurugan, A.; Barton, P. T.; Lin, Z.; Henke, S.; Yeung, H. H. M.; Wharmby, M. T.; Bithell, E. G.; Howard, C. J.; Cheetham, A. K. J. Am. Chem. Soc. 2014, 136, 7801−7804. (32) Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M. Energy Environ. Sci. 2015, 8, 995− 1004. (33) Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L. ACS Nano 2015, 9, 1955−1963.
N.D., H.Z., and Y.Y. generated the idea. N.D., H.Z., and Q.C. discussed the direction and experimental details. N.D. conducted device fabrication and all characterization with the assistance of L.M. for UPS measurements, Z.L. for photovoltage decay, P.S. and E.Y. for admittance spectroscopy, and A.S. for device fabrication. N.D. wrote the manuscript with input from H.Z., Q.C., and Y.Y. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by UC Solar MRPI (Grant No. 442551-YY-69090) as well as grants from the National Science Foundation (Grant Numbers ECCS-1202231, Program Director: Dr Radhakisan S. Baheti; and ECCS-1509955, Program Director: Dr. Nadia El-Masry), Air Force Office of Scientific Research (Grant Number FA9550-12-1-0074, Program Manager Dr. Charles Lee) and UCLA Internal Funds. The authors graciously acknowledge the Advanced Light Microscopy/Spectroscopy Lab at California NanoSystems Institute at UCLA for their assistance with fluorescence imaging.
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DOI: 10.1021/acs.nanolett.5b04060 Nano Lett. 2016, 16, 1009−1016
