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Defect Passivation via Graded Fullerene Heterojunction in Low Bandgap Pb-Sn Binary Perovskite Photovoltaics Adharsh Rajagopal, Po-Wei Liang, Chu-Chen Chueh, Zhibin Yang, and Alex K-Y. Jen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00847 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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Fabrication process and SEM image of MAPb0.5Sn0.5I3 thin films with (a) planar heterojunction (PHJ), (b) bulk heterojunction (BHJ) and (c) graded heterojunction (GHJ) structure. 152x101mm (300 x 300 DPI)

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(a) J-V curve of low bandgap Pb-Sn binary PVSC employing PHJ, BHJ and GHJ perovskite structures. (b) EQE and integrated current density of GHJ PVSC. 165x64mm (300 x 300 DPI)


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Figure 3. (a) Plot of ln(α) vs E(=hγ) used to extract Urbach energy of perovskite films with PHJ and GHJ structure. Light intensity dependence of (b) open-circuit voltage and (c) short-circuit current density for devices employing PHJ and GHJ perovskite structure. 165x50mm (300 x 300 DPI)


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Nyquist plot of PVSCs employing (a) PHJ and (b) GHJ perovskite structure. Comparison of extracted (c) Bulk recombination resistance, Rbuk and (d) Surface recombination resistance, Rsurface values at different applied bias. 152x124mm (300 x 300 DPI)


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(a) J-V curve along with photovoltaic performance metrics and (b) EQE along with integrated current density for low bandgap binary Pb-Sn GHJ PVSCs employing IC60BA as electron transporting layer. 165x64mm (300 x 300 DPI)


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Change in XRD intensity over time for low bandgap binary Pb-Sn perovskite thin film with (a) PHJ and (b) GHJ structures. (c) Comparison of contact angle for perovskite films with PHJ and GHJ structures. (d) Stability of corresponding photovoltaic devices in glovebox and ambient (RH = 60 ± 5 %) tracked by measurement of PCE at periodic intervals for 15 d. 152x124mm (300 x 300 DPI)


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ACS Energy Letters

Defect

Passivation

via

Graded

Fullerene

Heterojunction in Low Bandgap Pb-Sn Binary Perovskite Photovoltaics Adharsh Rajagopal,a,

‡

Po-Wei Liang,a,

‡

Chu-Chen Chueha, Zhibin Yang,a and Alex K.-Y.

Jena,b,c* a

Department of Materials Science and Engineering, University of Washington, Seattle, WA

98195, USA b

c

Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong

Department of Materials Science & Engineering, City University of Hong Kong, Kowloon,

Hong Kong ‡

Adharsh Rajagopal and Po-Wei Liang contributed equally to this work.

* Address correspondence to: [email protected]


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Abstract Development of low bandgap (~1.2 eV) Pb-Sn binary perovskites are exciting and have recently gained immense attention because of their high photovoltages, lowered Pb toxicity and its pivotal role in realizing perovskite tandem solar cells. Defect passivation in this class of perovskite alloys has immense potential to further reduce the photovoltage deficit, but are relatively unexplored. Here, we investigate and report the passivation of defect sites in low bandgap CH3NH3Pb0.5Sn0.5I3 perovskite through the incorporation of fluoroalkyl-substituted fullerene (DF-C60) via a graded heterojunction (GHJ) structure. Graded distribution of DF-C60 successfully reduced number of trap sites and the resultant films had characteristically lower Urbach energy, dominant bimolecular recombination, and higher surface / bulk recombination resistance. The improved optoelectronic quality of films with GHJ structure reflected in improved performance for corresponding photovoltaic devices, with best PCE up to 15.61% and a remarkably high Voc of 0.89 V. A Voc ~ 92% of the Shockley-Queisser (SQ) limit achieved here is comparable to that of state-of-the-art inorganic technologies and is the best among perovskite solar cells (PVSCs) till date. Additionally, through stability studies we find that though GHJ with DF-C60 can slow down degradation due to moisture penetration, the oxidative susceptibility of Sn in binary perovskites sharply constraints overall stability.

TOC GRAPHICS


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Organic-inorganic hybrid perovskite referring to a material class with a general crystal structure of AMX3 (A: organic cation, M: metal cation, and X: halogen) has received great attention in photovoltaic field over recent years because of its solution processability and excellent optoelectronic properties.1–5 Recently, remarkable power conversion efficiency (PCE) of 22.1% comparable to values of inorganic photovoltaic technologies has been achieved.6 However, there are several critical concerns that hinder the commercialization of perovskite solar cells (PVSCs).3,7 One of them is the toxicity of the lead (Pb), a key constituent in most of the employed hybrid perovskite compositions including the state-of-the-art high-performance PVSCs.3,8 To ensure PVSCs become a more environmental friendly and sustainable technology, it is critical to develop other metal alternatives to replace Pb fully or partially in the perovskite compositions. To date, many metal alternatives have been explored to replace Pb in hybrid perovskites.9– 15

Among them, Tin (Sn) is the most attractive candidate since it has similar outer shell

electronic configuration, coordination geometry, ionic radius, and complete solid solubility with Pb.12-13,16-18 Nevertheless, initial works on Sn-based PVSCs suffered from inferior device performance and stability due to the Sn oxidative susceptibility, which has a weakened inert-pair effect compared to Pb.19–22 The oxidation of Sn2+ to Sn4+ significantly deteriorates the semiconductor nature of hybrid perovskites and adversely affects the photovoltaic device operation. Lately, introduction of additives (like SnF2 and antioxidants) into the perovskite precursor solution and control of the processing method / atmosphere have somewhat alleviated impact of Sn oxidation and improved the processability of Sn-based perovskites.23–29 Though, pure Sn-based perovskites still suffer from low PCE (~8%),30-31 significant advancement has been made in the performance of low bandgap (~1.2 eV) Pb-Sn binary alloys which have high


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photovoltages, lowered Pb toxicity, and are pivotal for realizing all perovskite tandem solar cells.28,32–34 By optimizing through new additives and processing conditions, Yan’s group have recently reported a certified PCE of 17.01% using (FASnI3)0.6(MAPbI3)0.4 composition.32 To further improve the optoelectronic quality and reduce photovoltage deficit for this class of low bandgap perovskites, it is important to reduce the density of trap sites, which adversely impacts the open-circuit voltage (Voc), charge collection efficiency and device performance.35-36 Trap sites usually stem from facile point defect formation in the perovskite crystals and uncoordinated bonding at the film surface.1, 37-38 Particularly for Sn-based perovskites, Sn cation vacancies have been identified as an additional critical factor that could deteriorate perovskite semiconductor properties.39-40 Recent studies reporting perovskite-fullerene interactions/ heterojunctions in Pb-based perovskites provide some insights for overcoming these issues due to defects. First, the trap sites in perovskite thin films could be passivated by applying fullerene derivatives, like C60 or PCBM.41–43 As a result, the electronic properties of the derived device could be effectively enhanced and the hysteresis phenomena were significantly suppressed.42–44 Second, the fullerene derivatives could be applied as an efficient electron-transporting layer (ETL) for PVSCs since it has appropriate charge transport properties and lowers the resistance at perovskite / electron-transporting layer (ETL) interface.45 Fullerene incorporation within perovskite has recently been capitalized for developing highly efficient and stable PVSCs. The fluoroalkyl-substituted fullerene, DF-C60, has also been recently developed by our group not only to passivate the defects at perovskite grain boundaries but also increase the hydrophobicity of perovskite surface.46 It has also been demonstrated as an efficient ETL to enhance the device performance of PVSCs. It is worth to note that a graded fullerene distribution forms in perovskite thin-film surface during the evolution of fullerene /


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perovskite heterojunction due to the low surface energy of fluoroalkyl functional group.46 Recently, other research groups have also reported similar graded fullerene distribution structure and demonstrated it as an effective method to enhance the charge extraction efficiency.44,47-48 These results highlight the efficacy of forming graded heterojunction using fullerene derivatives to passivate trap sites and improve device performance. Translation of these important insights to low bandgap (Eg) binary Pb-Sn PVSCs is interesting and pivotal to further reduce photovoltage deficit. In this work, we incorporate fluoroalkyl-substituted fullerene, DF-C60, in low bandgap binary Pb-Sn perovskites to form a graded heterojunction (GHJ) structure. Graded distribution of DF-C60 in CH3NH3Pb0.5Sn0.5I3 (MAPb0.5Sn0.5I3) contributed to beneficial defect passivation and lower Urbach energy in resultant films, reduced number of trap sites, dominant bimolecular recombination, and higher resistance to surface / bulk recombination. The improved optoelectronic quality of films with GHJ structure has resulted in improved performance for corresponding photovoltaic devices. With additional interlayer and process optimization, devices with GHJ perovskite structure showed a high PCE of 15.61% and a remarkably high Voc of 0.89 V. A Voc ~ 92% of the Shockley-Queisser (SQ) limit is comparable to those from the state-ofthe-art inorganic counterparts and is the best among PVSCs to date.5 Additionally, we found that the application of DF-C60 in GHJ devices can slow down moisture penetration and the oxidative susceptibility of Sn in binary perovskites is that constraints overall stability. To have a comparative control, the additive formulation was first modulated for the MAPb0.5Sn0.5I3-based devices. PEDOT:PSS and C60 were used as hole- and electron-transporting layers (ETL and HTL), respectively. Further device fabrication details are provided in experimental section. Tin fluoride (SnF2) additives were added to suppress the oxidation of Sn2+


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and reduce the hole density in the resulting Sn-based perovskite films as reported in literature.17,25,49 In addition, a same mole ratio of excess lead iodide (PbI2) were added to the formulation to maintain Pb:Sn stoichiometry in the precursor solution and capitalize on the beneficial impacts of excess PbI2 in terms of defect passivation.50–53 As the overall additive (1:1 PbI2:SnF2) ratio was increased from 0-20 mol%, the Eg remained unchanged (~ 1.22 eV) and ensured that the film stoichiometry in relatively unperturbed by additive incorporation (Figure S1a). A cooperative effect of excess metal halides is expected to contribute to improvement in perovskite film quality. This reflected in the device performance, shown in Figure S1b, wherein the photovoltaic performance metrics increases correspondingly with increased amount of additive ratio. The optimized condition is found to be MAPb0.5Sn0.5I3 perovskite with 20% molar ratio additives (Table S1). The corresponding device has an open-circuit voltage (Voc) of 0.69V, a short-circuit current (Jsc) of 24.2 mA/cm2, a fill factor (FF) of 0.68, and a PCE of 11.39% (Figure 2b and S2a). When additive ratio was increased beyond 20 mol%, precipitation of secondary phases were evident and adversely impacted the morphology of thin films. The above optimized perovskite formulation is used as a control device hereafter and referred to as planar heterojunction, PHJ (Figure 1a). To ascertain the optimal way of introducing DF-C60 into the MAPb0.5Sn0.5I3 film and have its beneficial impact maximized, we compared two different processing approaches. Fabrication details of different fullerene/perovskite heterojunction structures (Figure 1) are provided in the experimental section. Essentially, we investigate incorporation of DF-C60 into MAPb0.5Sn0.5I3 film via addition into perovskite precursor solution and toluene anti-solvent. We rationally term these approaches correspondingly as bulk heterojunction (BHJ) and graded heterojunction (GHJ) structures.


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ACS Energy Letters

First, the DF-C60 solution was mixed with MAPb0.5Sn0.5I3 precursor before the toluene washing treatment to form a perovskite-fullerene bulk heterojunction (BHJ) as illustrated in the Figure 1c, which has been individually adopted by several groups to yield high-performance Pbbased PVSCs.46,48,54 From our previous result,46 the DF-C60 and MAPbI3 mixture slightly increased the perovskite grain size. However, as observed from the scanning electron microscopy (SEM) images shown in Figure 1b (BHJ), the morphology of the optimized DF-C60 and MAPb0.5Sn0.5I3 mixture does not change significantly and is similar to the morphology of pristine perovskite. From the device point of view, the performance decreased dramatically when using this perovskite-fullerene BHJ (Figure 2a and S2b). This result might arise from the difference in crystallization kinetics between MAPbI3 and MAPb0.5Sn0.5I3 perovskite which is evident from the literature. Experimentally, the Sn-based perovskite was observed to crystallize faster and the transformation time from precursor to perovskite phase is rapid.20,55 This would possibly constraint the diffusion of DF-C60 and its limited time in moving toward the surface / grain boundary which resulted in trapped DF-C60 within the films for such a BHJ structure. Poor device performance compared to the planar heterojunction (PHJ) structure (Figure 2a) is likely resulting from an uncontrollable distribution of fullerenes in the bulk / close to the HTL side hindering the charge collection efficiency and the trapped DF-C60 introducing potential recombination sites. An alternate method to introduce DF-C60 into the MAPb0.5Sn0.5I3 film is to in-situ treat the as-cast perovskite film with toluene solvent containing DF-C60 during the spin-coating, as shown in the Figure 1c. This fabrication process would result in a unique fullerene-perovskite graded heterojunction (GHJ) structure like the one reported by Han et al.47 The graded distribution of DF-C60 in the perovskite layer facilitated by this approach successfully overcomes the limitation


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of the BHJ approach discussed above. The SEM image of GHJ perovskite thin film (Figure 1b) showed the disappearance of grain texture compared to pristine perovskite (PHJ) similar to the observation by Han et al.47 implying successful formation of fullerene heterojunction by this fabrication process. More importantly, this reflected in an improved device performance, as depicted in Figure 2a. The average PCE of the fabricated PVSC improved to 12.88 ± 0.42% by utilizing this GHJ approach (Table 1). The best device showed a PCE of 13.46% with a Voc of 0.73 V, a Jsc of 26.3 mA/cm2, and a FF of 0.70 when measured under forward scan. Small hysteresis was observed in the J-V measurement with various scan rates, as shown in the Figure S3.34 Figure 2b shows the corresponding EQE spectrum for the GHJ PVSC. The integrated photocurrent density (26.2 mA/cm2) is in good agreement with the Jsc derived from the J-V measurement. Considering the unchanged thickness (300-350 nm as measured by thin film profilometry) and light absorption of the PHJ, BHJ and GHJ thin films (Figure S4), the enhanced photocurrent and Voc in GHJ should be attributed to an improved charge collection and reduced recombination / transport losses resulting from favorable perovskite-fullerene interaction. To characterize the improved optoelectronic quality of films with GHJ structure compared to PHJ structure, we characterized the absorption band edges. As shown in Figure 3a, the bandedge of perovskite thin film with GHJ structure is sharper than the film with PHJ structure. Urbach energy (Eu), which is the energetic disorder at the band edge, can be derived using the relation: = exp ( ⁄ ) , where α is the absorption coefficient, E(=ℎ ) is the photon energy, and Eu is the Urbach energy. The control PHJ sample has Eu ~ 40 meV, whereas the GHJ sample has an Eu of ~ 32 meV, which is ~20% lower (Figure 3a,). This indicates that the incorporated DF-C60 with a gradient distribution lowers density of trap sites and sharpens band


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edge in Pb-Sn binary perovskites. Films with GHJ structure will thus have a lower trap-assisted recombination, which explains the improvement in Voc for GHJ compared to PHJ. To further elucidate the improved optoelectronic quality and understand charge recombination process in Pb/Sn-binary perovskites with different structures, light-intensity dependent current-voltage characterization measurement was performed. Figure 3b-c show the light-intensity-dependence of Voc and Jsc, respectively. The Voc is related to the splitting of the quasi-Fermi levels of electrons and holes with the quasi-Fermi level positions set by the freecarrier concentration, which in turn are determined by the balance between the photogeneration and recombination rates.34 The relationship between Voc and light intensity (I) is: Voc = nkT/e ln(I/Io + 1), where n is the ideality factor, e is the elementary charge, k is the Boltzmamm constant, and T is the temperature.56 For a trap-free, complete bimolecular recombination, n should be close to unity. On the other hand, if trap-assisted Shockley-Read-Hall (SRH) recombination dominated, the ideality factor n approaches 2. In the case of PHJ, the ideality n ~ 1.86 at low incident light intensities (< 10 mW/cm2) implying dominant SRH type recombination in this region. However, at the higher irradiance regime from 10 to 100 mW/cm2, the ideality n ~ 1.30 approaches bimolecular limit as demonstrated in the previous reported papers.28,34,15,57-58 In the case of the GHJ devices, the n ~ 1.08 was observed across the entire range of light intensity (from 2 to 100 mW/cm2). Further, according to the power law dependence between Jsc and I,58 the light intensity dependence of Jsc (Figure 3c) demonstrate that the GHJ PVSC has a higher slope of 0.92 compared to 0.85 for PHJ PVSCs, indicating the reduced extent of non-radiative trap assisted recombination with GHJ.48 These results definitively establish that bimolecular recombination is dominating in the GHJ devices and trap states in the GHJ perovskite thin films are far lesser than the PHJ thin films. The incorporated DF-C60 thus contribute towards


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passivation of defect states in perovskite thin-films which effectively enhances the perovskite optoelectronic quality and improves device performance. Electrochemical impedance spectroscopy (EIS) has been widely used to measure the charge transport dynamics in solar cells, such as chemical capacitance, recombination lifetime, and transport properties.59–61 To confirm the charge dynamics in the fabricated devices, the EIS for both PHJ and GHJ PVSCs were measured. The Nyquist plots of the PHJ and GHJ devices at different applied voltage are shown in Figure 4a and 4b. The frequency used for all EIS measurements is ranged from 1 Hz to 1 MHz. All devices were applied with a bias ranging from 0 V to 0.7 V. To decouple the recombination process, we fitted the data to a commonly used equivalent circuit model as shown in the Figure S5, where Rsurf-Csurf represented the surface recombination, Rbulk-Cbulk represented the bulk recombination, and Rseries, Rinternal and Cgeo represented the series resistance, internal resistance, and geometric capacitance, respectively.41 Recombination processes in devices were compared using the surface and bulk charge recombination resistance (Rsurf and Rbulk) as reported in previous studies.41,59–61 Both Rsurf and Rbulk of GHJ PVSC are observed to be higher than the corresponding values of PHJ PVCS (Figure 4c and 4d). These results further confirm that the unique device structure allows the fullerene derivative to not only efficiently passivate the defects state near perovskite surface, but also in the bulk perovskite. After mechanistically understanding and validating the optoelectronic merits of DF-C60 incorporation via a GHJ in MAPb0.5Sn0.5I3, devices were further engineered to improve the PCE through modifying choice of ETL and increasing perovskite thickness. In our recent work, we have demonstrated that Indene-C60 bis-adduct, IC60BA, is a more suitable ETL material for MAPb0.5Sn0.5I3 PVSCs compared to C60 because of better energy level alignment and reduced


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nonradiative recombination losses.34 Adapting from this work, C60 was replaced in our device architecture with IC60BA to achieve optimized contact at MAPb0.5Sn0.5I3 / ETL interface. Further, the active layer thickness was increased from 300-350 nm to 500-550 nm by increasing the initial precursor solution concentration from 1 M to 1.4 M. The combined optimization of interlayer material and active layer thickness resulted in a much-improved performance for low Eg GHJ PVSCs (Figure 5a) and had negligible J-V hysteresis as reported in our previous work.34 All performance metrics were increased (Voc: 0.87 ± 0.02 V, Jsc: 26.23 ± 0.61 mA/cm2, FF: 0.69 ± 0.02, PCE: 15.18 ± 0.42 %) and the best device reached a PCE of 15.61% (Table 1). The Jsc improvement corresponds well with the increase in EQE around infrared (NIR) region as expected for thicker films (Figure 5b). More importantly, the Voc of devices reached an impressive value (considering low Eg ~1.22 V of perovskite absorber) of 0.89 eV, representing the highest value achieved so far for low Eg Sn-based PVSCs. Moreover, this corresponds to a remarkable 92% of the SQ limit (Voc,SQ for Eg = 1.22 eV is 0.97 eV), comparable to those of the state-of-the-art inorganic counterparts and is the best for any PVSC reported to date.5 Finally, taking into consideration the hydrophobic nature of DF-C60, the ambient stability of GHJ films was assessed in comparison to PHJ using XRD (Figure 6a-b). The contact angle for the perovskite thin films with GHJ (122.5°) was significantly higher than PHJ (52.5°) demonstrating the increased hydrophobicity in GHJ due to preferential gradient distribution of DF-C60 (Figure 6c). The perfuoroalkyl groups in DF-C60 near the film surface can effectively slow down the permeation of moisture and resulting degradation of perovskite thin films. This was reflected in much stable XRD peak intensity for GHJ films compared to PHJ films when exposed to an ambient environment with relative humidity (RH) = 60 ± 5% (Figure 6a-b). The stability of devices with PHJ and GHJ films were further evaluated (Figure 6d). Both PHJ and


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GHJ devices were relatively stable when kept inside glovebox indicating the applicability of these devices with proper encapsulation. However, when devices were stored in ambient (60 ± 5%), GHJ devices showed slightly better stability compared to PHJ devices but PCE dropped to 50% of its original value in 15 d. These results imply that though GHJ with DF-C60 can slow down degradation due to moisture penetration, oxidative instability of Sn in perovskites sharply limits the performance. Further efforts to improve intrinsic stability towards oxidation and lowering oxygen permeation are thus critical to make improvements in the stability front for low Eg binary Pb-Sn perovskites. In summary, an efficient low bandgap Pb-Sn binary PVSC with a GHJ structure was demonstrated. The graded distribution of fluoroalkyl-substituted fullerene, DF-C60, in perovskite thin films effectively passivated defects and improved absorber optoelectronic quality. Using GHJ, high-performance low bandgap PVSCs with PCE up to 15.61% were realized with additional interlayer optimization. Strikingly, the Voc of devices reached 0.89 eV which is the highest value achieved so far for low Eg Sn-based PVSCs. This corresponds to an impressive 92% of the SQ limit for Eg ~ 1.22 eV and represent best among any PVSC reported to date. We find that though the GHJ with DF-C60 can slow down moisture induced degradation, oxidative susceptibility of Sn still constraints the stability severely. With future efforts on stability front, the GHJ structure demonstrated in this work shows the promise for realizing remarkably high Voc and device performance using less toxic binary Pb-Sn low bandgap PVSCs. Experiment Section Materials: ITO glass (

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