Hole-transport layer molecular weight and doping effects on

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC

Surfaces, Interfaces, and Applications

Hole-transport layer molecular weight and doping effects on perovskite solar cell efficiency and mechanical behavior Inhwa Lee, Nicholas Rolston, Pierre-Louis Brunner, and Reinhold H. Dauskardt ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05567 • Publication Date (Web): 11 Jun 2019 Downloaded from http://pubs.acs.org on June 11, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 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

ACS Applied Materials & Interfaces

Hole-transport layer molecular weight and doping effects on perovskite solar cell efficiency and mechanical behavior Inhwa Lee1‡, Nicholas Rolston2‡, Pierre-Louis Brunner3, and Reinhold H. Dauskardt1* 1

Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA

2

Department of Applied Physics, Stanford University, Stanford, CA, USA

3 Solaris

Chem Inc., St-Lazare, Quebec, Canada

KEYWORDS: PTAA, molecular weight, perovskite solar cell, mechanical properties, stability

ABSTRACT: The effect of tuning molecular weight (Mn) in PTAA to increase both mechanical properties of the film and electrical properties of perovskite solar cells is reported. Perovskite solar cell devices are fabricated to investigate the effect of Mn on power conversion efficiency (PCE). Moisture stability for various Mn is also studied in PTAA films exposed to mechanical loads in humid environments. Furthermore, cohesion and tensile tests are employed to determine the mechanical properties of PTAA, where higher Mn leads to more robust films. In order to elucidate the effect of Mn on the debonding kinetics, a viscoelastic fracture kinetic model is proposed as a

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 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 2 of 24

function of Mn, and the debonding mechanism is found to be dependent on Mn. Finally, the effect of small-molecule based dopants on the mechanical stability of PTAA is investigated.

INTRODUCTION Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) has attracted significant attention as a hole transport layer (HTL) for perovskite solar cells due to its high hole mobility1 (10−2–10−3 cm2V−1s−1), appropriate work function2, excellent electron blocking capacity3 and good solubility4. PTAA has emerged as a representative HTL candidate in perovskite solar cells since achieving high power conversion efficiencies (PCE) exceeding 20%.5 In addition to device performance, PTAA has superior mechanical robustness in terms of cohesion compared to other HTL candidates such as spiro-OMeTAD and P3HT.6 Previous work has demonstrated that undoped PTAA has a cohesion energy (Gc) over 9 J/m2, the highest reported Gc value among HTL materials.7 The moisture resistance of adjacent materials to the perovskite layer is also important because the perovskite layer is highly susceptible to degradation in the presence of moisture.8 Recent work has showed that common Li- additives used as dopants for HTL materials are hygroscopic, which results in faster decomposition of the perovskite layer.6,9 However, the moisture resistance of PTAA has not been reported, even though the HTL also functions as a barrier layer to prevent water molecules from directly contacting the perovskite layer. Previous work has shown that using HTL materials with hydrophobic and moisture-resistant properties significantly improves the stability of perovskite solar cells10, 11. Therefore, careful understanding on the moisture resistance of PTAA is required to engineer more reliable perovskite solar cells. Recent commercialization of various molecular weights (Mn) in PTAA also suggests a pathway to increase both mechanical and electrical properties of PTAA. It is well known that the Mn can


2

Page 3 of 24 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


significantly change the viscosity, physical, thermomechanical and electrical properties of polymers12, 13. Despite the recent commercial accessibility to a range of PTAA Mn, the effect of Mn of PTAA has not been reported in any study to date. Previous work showed that increasing Mn of P3HT can significantly improve Gc—although this led to a trade-off in device performance— with these more robust films exhibiting decreased PCE from increased trap states.14 This improvement in reliability at the expense of efficiency has been observed previously in perovskite solar cell devices15, and solving this trade-off remains a critical challenge for enabling the successful commercialization of perovskites, where reliability has too long been an afterthought. In this work, the effects of Mn of PTAA on the electrical and thermomechanical properties are investigated. Eight PTAA samples—each with different Mn and polydispersity index (PDI)—were prepared and fabricated as HTLs for perovskite solar cells in order to examine the device characteristics. The samples were also tested by a double cantilever beam (DCB) test to investigate the cohesion depending on the Mn. Then, the debond surfaces were examined by atomic force microscopy (AFM) to explore plastic deformation during the DCB test. Furthermore, the influence of Mn on decohesion rate is shown and a viscoelastic kinetic model is used to explain the decohesion mechanism of PTAA. Finally, correlations between the PCE, cohesion, and moisture resistance are discussed to highlight optimal PTAA properties for reliable and efficient perovskite solar cells.

RESULTS AND DISCUSSION Device performance. Perovskite solar cells were fabricated in the conventional, n-i-p architecture—where PTAA (Figure 1a) was spin-coated on top of a MAPbI3 perovskite—to


3


Page 4 of 24

determine the effect of PTAA Mn on device performance. The perovskite and charge transport layers used were identical to a previous study in order to obtain high performing devices with measured current values closely matched to EQE integration.16 The J-V curve for the bestperforming cell at each Mn is shown in Figure 1b and the PCE plotted as a function of Mn in Figure 1c and the tested Mn information of PTAA is listed in Table 1. Devices fabricated from all Mn yielded PCE values exceeding 16%, except for the lowest Mn = 6.0 kDa. This reduction in performance suggests that for low Mn, the perovskite film was not readily planarized, resulting in a lower fill factor (Table 1). The polydispersity index (PDI), which measures the relative distribution in Mn within a polymer, was also varied for the PTAA and shown to have an effect on PCE. In particular, PTAA with a lower PDI approaching 1—a value which indicates ideal uniformity and no variation in Mn—markedly improved PCE from 15.6 % for PDI = 5.0 to 17.1 % for PDI = 1.4 (Figure 1d). While the PCE of 17.1% obtained in this study is respectable but not as high as record devices, the values achieved were a result of several factors that include recombination in the amorphous, nanoparticle-based TiO2 layer used as the electron transport layer that reduced Voc and the lack of a mesoporous TiO2 layer to enable thicker perovskite films that reduced Jsc.


4

Page 5 of 24 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


Figure 1. (a) Chemical structure of PTAA (b) Device architecture (inset) and J-V curves of PTAA based perovskite solar cells, power conversion efficiency (PCE) as function of (c) molecular weight (Mn) and (d) polydispersity index (PDI), respectively. The minimal variation in PCE observed in the devices is an indication that the electronic properties of PTAA are largely unchanged across the entire range of Mn studied. A related study performed on P3HT across a similar range of Mn for organic solar cells showed a large decrease in PCE with increasing Mn due to thicker films reducing photon harvesting. 14 A similar increase in thickness was observed for PTAA with higher Mn (Figure S1); however, the primary reason that the PCE was not compromised for PTAA-based devices is a result of the perovskite absorber layer—which has longer carrier lifetimes than organic solar cell absorber layers incorporating P3HT 17—and is thus less sensitive to layer thickness of charge transport layers. 18 Additionally, an oxidized, Li-based dopant (LiTFSI) was used to enhance the conductivity of the PTAA 19, which


5


Page 6 of 24

further reduces any potential thickness dependence on PCE for the range of 40-100 nm observed in the PTAA films (Figure S1).

Table 1. Weight average molecular weight (Mw), number average molecular weight (Mn), and polydispersity index (PDI) for the PTAA samples used in this study. PDI is defined by the ratio of Mw to Mn. In addition, figures of merit are listed for the best performing perovskite solar cell fabricated with each of the PTAA samples as the hole transport layer in the n-i-p architecture. Mw [kDa]

Mn [kDa]

PDI

Voc

Jsc

FF

PCE [%]

10

7.1

1.4

1.03 ± 0.01

21.0 ± 0.2

0.79 ± 0.01

17.1 ± 0.3

20

12.5

1.6

1.02 ± 0.00

20.6 ± 0.1

0.80 ± 0.01

17.0 ± 0.2

30

21.5

1.4

1.02 ± 0.01

20.6 ± 0.2

0.78 ± 0.01

16.5 ± 0.3

30

6

5.0

1.02 ± 0.01

21.0 ± 0.4

0.73 ± 0.02

15.6 ± 0.7

50

22.7

2.2

1.00 ± 0.01

21.2 ± 0.1

0.77 ± 0.01

16.5 ± 0.4

115

34.8

3.3

1.01 ±0.01

21.0 ± 0.1

0.78 ± 0.01

16.6 ± 0.2

175

38

4.6

1.04 ± 0.00

20.4 ± 0.3

0.78 ± 0.02

16.5 ± 0.6

350

76.1

4.6

1.04 ± 0.00

20.7 ± 0.1

0.77 ± 0.02

16.6 ± 0.5

Tensile testing. To investigate the effect of Mn on mechanical properties of PTAA such as the tensile modulus and failure strain, a tensile test was employed. Figure 2a shows stress and strain curves of free-standing PTAA films with varying Mn. The tensile modulus is saturated above 35 kDa (Figure 2b) When Mn is low, entanglement between chains is not dominant, enabling the chains to slide easily past each other and resulting in less resistance to mechanical loading. However, the entanglement density increases with Mn, and thus higher Mn of PTAA increased the tensile modulus according to the entanglement density. Since entanglement physically links multiple chains together, higher entanglement density suppresses material deformation. However, no significant increases in the tensile modulus occur as polymer chains reach the entanglement


6

Page 7 of 24 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


chain length. 20, 21 Figure 2c shows that PTAA has a higher crack onset strain with increasing Mn. At low Mn, chain slippage results in correspondingly low values of crack onset. In contrast, fracture occurs at a higher strain as Mn increases because more deformation is caused by entanglement between polymer chains. Here, the crack onset strain is defined as the strain when noticeable load drop is observed during the tensile testing.

Figure 2. (a) Stress-strain curves of PTAA with different Mn (6.0, 12.5, 21.5, 22.7, 34.8, 38.0, 76.1 kDa) (b) Tensile moduli and (c) crack onset strain of the PTAA films with varying molecular weight. The dotted lines in (b) and (c) are guides to the eye.

Cohesion in PTAA depending on the Mn. Gc measurements were performed using double cantilever beam (DCB) tests on each of the PTAA samples deposited on glass functionalized with an aminosilane to improve bonding with the substrate.22 The cross-section of the architecture used for DCB testing is shown in Figure 3a, where an identical glass substrate was bonded to the metal layer to complete the specimen. A uniaxial load was then applied perpendicular to the PTAA layer at a controlled displacement rate, and Gc was calculated from the resulting load-displacement curve as a crack was driven through the PTAA. The measured Gc is plotted as a function of Mn in Figure 3b, where a clear increase is observed—from 1.54 ± 0.24 J/m2 for Mn = 7.1 kDa to 17.31 ± 5.57 J/m2 for Mn = 76.1 kDa and does not follow the same trend of the tensile modulus, which is


7


Page 8 of 24

plateaued above 35 kDa (Figure 2b). Furthermore, a roughly linear increase is observed in Gc with increasing Mn, showing a direct correlation.

Figure 3. (a) Schematic of the sandwiched DCB specimen for measuring the debond energy (b) Measured critical debond energy (Gc) as a function of molecular weight (c) XPS spectra on the cohesively debonded PTAA surfaces and (d) roughness data measured by AFM with varying Mn.

In all cases, the PTAA elemental constituents of carbon, oxygen, and nitrogen were observed on the debonded surfaces as determined by X-ray photoelectron spectroscopy data (Figure 3c), which indicates that the cohesion of PTAA layer was measured as opposed to adhesion with an adjacent layer. The mechanism for the significant increase in Gc is likely due to increased plastic deformation for the higher Mn samples, as a thicker plastic zone at the crack tip allows for the dissipation of more energy. Figure 3d supports this hypothesis by showing a noticeable increase in the average roughness, Rq, of the fractured samples with higher Mn. A rougher fracture interface


8

Page 9 of 24 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


indicates that more plastic deformation occurred during the DCB test and suggests that the crack was deflected as it meandered through the thicker PTAA layer with higher Mn.

AFM Analysis on the debond surfaces of PTAA. Figure 4a-h shows AFM images on debond surfaces after DCB testing. In figure 4a and b, the surface morphology is quite flat and no discernible features are observed in the polymer. However, when the Mn increases over 10 kDa, the appearance of grain-like features is seen on the fractured surface, an effect that likely results from polymer entanglement. Also, the growth in size of the entanglement features is clearly noticeable with higher Mn. From Figure 4a of 6.0 kDa and Figure 4e of 22.7 kDa, the increase in Rq is due to the agglomeration growth by entanglement, rather than from plastic deformation, which is not a dominant effect during the DCB test. Also, the very smooth fracture surfaces indicate that crack deflection did not occur. However, for Mn over 34.8 kDa (Figure 4f), the fracture surface exhibited increased roughness with evidence of local plastic deformation and associated higher Gc. The rougher fracture surfaces also induce crack deflection providing an additional contribute to the linear increase in Gc with increasing Mn. In Figure 2, the tensile modulus also plateaued above 35 kDa and then plastic deformation occurred at higher Mn values. When the grain feature size grows—an effect which is assumed to be the result of entanglement—the tensile modulus increases. However, because tensile modulus is the initial elastic response to mechanical loading, the tensile modulus is saturated at the same time that grain feature sizes reach a plateau.


9


Page 10 of 24

Figure 4. (a) AFM images of cohesively debonded PTAA surfaces with varying Mn of (a) 6.0 (b) 7.1 (c) 12.5 (d) 21.5 (e) 22.7 (f) 34.8 (g) 38.0 and (h) 76.1 kDa, respectively.

Crack growth mechanism. Figure 5 shows a visualization at the crack tip based on the experimental results in DCB testing. When Mn is lower than 20 kDa, the polymer chain length is too short to create chain entanglement. Therefore, the crack could easily propagate due to chain slippage (Figure 5a). However, as Mn increases to 35 kDa, the entanglement is more dominant as the chain length increases. As a result, in addition to the chain slippage, chain scission is added during the crack propagation (Figure 5b). Also, the grain feature size grows with increasing Mn by increased entanglement in this Mn range. However, when Mn is higher than 35 kDa, even though grain feature size is plateaued, the chain connections between the grains induce plastic deformation and dissipate more strain energy. Therefore, crack deflection occurs and results in the highest Gc of 17.3 J/m2. We expect these mechanisms are also connected to the viscoelastic zone formed at the crack tip. In the next section, we model decohesion kinetics to explicitly examine viscoelastic behavior and its relation to Mn.


10

Page 11 of 24 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


Figure 5. Schematic illustration for the debonding mechanism at the crack tip depending on Mn, ranging from (a) low (Mn < 20 kDa) to (b) med (20 35kDa)

Decohesion kinetics of PTAA under humid environments. Humidity is an important factor that limits the long-term stability of perovskite solar cells.23, 24 In order to investigate the resistance of PTAA to humid environments, decohesion kinetics were studied by placing the micromechanical test system in an environmental chamber of 50 % RH at 25 °C. Figure 6a plots the decohesion growth rate, da/dt, curves in the PTAA layer measured over several orders of magnitude from ~10-4 m/s to below 10-11 m/s as a function of the applied driving force, G. Depending on the Mn, the decohesion behavior is significantly varied. Note that the two highest Mn samples experienced a change in the crack path from cohesive to adhesive failure during exposure (Figure S2), an effect which was likely related to their high cohesion values exceeding the interfacial adhesion of the PTAA with the substrate and were therefore not included in Figure 6.


11


Page 12 of 24

Figure 6. (a) Decohesion growth rate in PTAA layer as a function of applied driving force with varying Mn. (b) Transition temperature, T, (c) crack opening displacement, δc and (d) rate sensitivity, n, calculated from a kinetic model fitted with varying Mn. All samples had a sigmoidal decohesion curve—a characteristic commonly observed for polymer materials regardless of the Mn—while the curve for low Mn samples was approximately linear due to small plastic deformation. For sigmoidal behavior in polymers, the curves can be categorized by three regions.25-27 When the decohesion growth rate is higher than 10-6 (da/dt > 10-6 m s-1), molecular relaxation time is limited, and the driving force is approximately equal to Gc. However, when the decohesion growth rate becomes slower (10-8 – 10-6 m s-1), viscoelastic behavior dominates. Therefore, the behavior in this intermediate region has high dependence on humidity, temperature and time and occurs at a value below Gc, known as subcritical crack growth. When the decohesion growth rate is lower than 10-9 m s-1, a threshold debonding driving force,


12

Page 13 of 24 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


Gth, is observed. The value of Gth is meaningful for device design because if the applied driving forces are below Gth, debonding growth does not occur at all in the same environmental condition. The gap between Gc and Gth is highly affected by the molecular relaxation, and thus the gap expands with increasing Mn (Figure S3). For the molecular relaxation, a viscoelastic debonding model at the crack tip was used based on previous work in PV backsheets and organic solar cells as follows28-30:

𝑑𝑎

𝜋

𝑑𝑡 = 8

𝐶𝑎(𝑇 ― 𝑇𝑔)

1

𝛿𝑐

𝐶𝑏 + (𝑇 ― 𝑇𝑔)

1

𝐺𝑛 ∙ 10

(1)

𝜀𝑦(𝛿𝑐𝜀𝑦𝐸1)𝑛 where a is the crack length, t is the time, δc is the crack opening displacement, εy is the yield strain of the material, E1 is the modulus at unit time, n is the rate sensitivity of the material, Ca = 17.1 and Cb = 51.6 K-1 from Williams-Landel-Ferry (WLF) time-temperature superposition shift factor31 and Tg is the glass transition temperature. In the model, the core assumption is that the decohesion rate, da/dt, can be approximated by the rate of plastic zone formation at the crack tip. To determine Tg, δc and n, equation (1) was fitted to the experimental data of Figure 6a. From the fitting, it was found that Tg saturated with increasing Mn as shown in Figure 6b due to the change in free volume. With increasing Mn, the free volume decreases because high Mn with long chain length has fewer chain ends per total units.32 Therefore, higher energy is required to cause molecular motion, resulting in higher Tg. In our study, calculated Tg values from the fitting are plateaued at ~ 45 ⁰C, which corresponds to one of the transition temperatures of PTAA.33 Previous work demonstrated that there are two phase changes at 46 and 98 ⁰C, which shows that this fitting model is in good agreement with the experimental results. Unlike Tg, the values of δc and n are linearly related to the Mn as shown in Figure 6c and 6d. When Mn is low, cracks easily propagate because the short polymer chains experience minimal


13


Page 14 of 24

plastic deformation. Therefore, δc of a few nm is enough to induce crack propagation. However, when Mn is high, entangled chains disturb crack propagation and induce plastic deformation, and thus higher δc is required for crack propagation. Unlike the increasing trend of δc, n—characterized as the rate sensitivity of the material with a value of zero indicating a perfectly elastic material— decreases with increasing Mn. Therefore, from the decreasing trend of n with increasing Mn, we can assume that the modulus of PTAA rises with increasing Mnas shown in Figure 2. For the high PDI sample, the fitting results do not align with the trend and thus indicate that this model prediction is not applicable to high PDI values.

The effect of CoTFSI additive. In order to improve charge transport, high performing perovskite devices incorporate small-molecule based dopants into the PTAA 34, which increases charge carrier density and PTAA film conductivity. However, the primary concern of this dopant strategy is the reduction in mechanical and operational stability 6 observed when using a traditional, Li-based dopant—LiTFSI—a volatile material that does not interact strongly with the PTAA polymer chains. In contrast, a Co-based dopant (CoTFSI), tris(2-(1H-pyrazol-1-yl)-4-tertbutylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209), has been shown as a successful alternative 35 that improves stability in devices with SpiroOMeTAD and P3HT as hole transport materials compared to LiTFSI.

36, 37

Also, CoTFSI directly oxidizes PTAA and causes

morphology changes by reducing the distance between polymer chains as shown in Figure 7a. Devices fabricated with CoTFSI only instead of LiTFSI did not perform as efficiently with higher series resistance (Figure S5), which could be attributed to not improving PTAA conductivity as effectively or modifying the PTAA work function and creating a hole extraction barrier. Here, we report on the effect of morphology changes in CoTFSI on the cohesion and mechanical properties


14

Page 15 of 24 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


of PTAA. Figure 7b shows the increasing trend of Gc values as a function of CoTFSI concentration. While the rougher surfaces with higher concentration on debond surfaces after DCB testing support this Gc results (Figure 7c and S4), the underlying mechanism is not clear from this result alone.

Figure 7. (a) Schematic diagrams illustrating mechanisms for interaction between PTAA chains (b) Measured critical debond energy (Gc), (c) roughness data, (d) Stress-strain curves, (e) tensile modulus, and (f) toughness as a function of CoTFSI concentration

Therefore, we conducted tensile tests to investigate the effect of CoTFSI on the mechanical properties of PTAA. Figure 7d shows the stress/strain curves of PTAA films with varying dopant concentration. The tensile moduli and toughness increase with increasing CoTFSI concentration (Figure 7e and 7f), and these results indicate that the PTAA thin films become more robust and ductile with the addition of CoTFSI. Since tensile modulus and toughness are mutually exclusive properties, the increase in both quantities with doping indicates that CoTFSI-doped PTAA films are more mechanically robust compared to pristine PTAA. These trends are also observed in P3HT,


15


Page 16 of 24

where polymer networks via pi-pi stacking prevent sliding out of chains, and the gradual disruption of pi-pi stacking as more tensile stress is applied dissipates more energy.38 Similarly, the interacted region by CoTFSI prevents the sliding out of chains—which increases the modulus—and the other amorphous domains contribute to the plastic deformation that results in both increased modulus and toughness.

CONCLUSIONS This work studied the effect of molecular weight on the electrical and mechanical properties of PTAA to determine the optimal range of parameters for compatibility with perovskite solar cells. Increasing molecular weight had little effect on device performance, while a large increase in mechanical characteristics in terms of cohesion, tensile modulus and yield strain was observed. Debond kinetics were used to characterize the effect of moisture on the mechanical properties of PTAA, and it was found that although higher Mn leads to more robust films, these PTAA films were also more susceptible to molecular relaxation and subcritical crack growth. Future work investigating the stability of perovskite solar cells based on molecular weight will need to replace the volatile Li-based additives in the PTAA with alternative doping strategies such as CoTFSI and replace the MA cation in the perovskite with cesium and/or formamidinium, which have been shown to be more resistant to environmental degradation.

EXPERIMENTAL METHODS Materials: ITO-glass (Xin-Yan Technology) with a sheet resistance of 10 Ω □–1 and dimensions of 2 cm x 2 cm were used as substrates Perovskite materials. Methylammonium iodide (MAI, Dyesol) and lead acetate trihydrate (PbAc2, Sigma-Aldrich CAS No. 6080-56-4) were


16

Page 17 of 24 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


prepared in a 40 wt % solution with a 3:1 molar ratio in N,N-dimethylformamide (DMF, Acros) with hypophosphorous acid (HPA, Sigma-Aldrich CAS No. 6303-21-5) added to the solution. Specifically, 60 µL of 1:100 HPA:DMF solution was added to 940 µL of DMF to form 1 mL of perovskite solution. Charge transport materials. Compact titania (TiO2) was prepared by mixing two vials, one of which contained 360 µL titanium isopropoxide (Sigma Aldrich, CAS No. 54668-9) added to 2.5 mL of ethanol (Acros) and the other of which contained 35 µL of 2M HCl in 2.5 mL of ethanol (Acros). C60 (M.E.R. Corporation) was used as received. Poly(triarylamine) (PTAA, Solaris Chem.) with varying Mn was prepared in toluene (Acros) at a concentration of 15 mg mL–1 with the addition of 10 µL Li-bis(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile (170 mg mL–1) and 6 µL distilled tert-butylpyrridine (tBP) as dopants to enhance conductivity and film formation. Solaris Chem has characterized the Mn information of PTAA using gel permeation chromatography (GPC). Perovskite Solar Cell Device Fabrication: The substrates were cleaned by sequential sonication in a 1:10 Extran:DI water solution, DI water, acetone, and IPA for 15 min periods. All solutions were filtered through a 0.2 µm PTFE filter immediately prior to deposition. The TiO2 precursor solution was spin-coated in ambient at 2000 rpm for 30 s, followed by annealing at 150 °C for 1 h. C60 was thermally evaporated to a thickness of 15 nm to reduce hysteresis and cover any pinholes/voids in the nanoparticle TiO2 layer. The perovskite precursor solution was spin-coated in dry air ( 1,000 Hour Operational Stability. Nature Energy 2018, 3 (1), 68-74. 10. Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J., Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells. Nano Lett. 2014, 14 (10), 5561-5568. 11. Leijtens, T.; Giovenzana, T.; Habisreutinger, S. N.; Tinkham, J. S.; Noel, N. K.; Kamino, B. A.; Sadoughi, G.; Sellinger, A.; Snaith, H. J., Hydrophobic Organic Hole Transporters for Improved Moisture Resistance in Metal Halide Perovskite Solar Cells. ACS applied materials & interfaces 2016, 8 (9), 5981-5989. 12. Fetters, L.; Lohse, D.; Richter, D.; Witten, T.; Zirkel, A., Connection between Polymer Molecular Weight, Density, Chain Dimensions, and Melt Viscoelastic Properties. Macromolecules 1994, 27 (17), 4639-4647. 13. Ishikawa, H.; Xu, X.; Kobayashi, A.; Satoh, M.; Suzuki, M.; Hasegawa, E., Effect of Molecular Mass of Poly (3-Alkylthiophene) on Electrical Properties. J. Phys. D: Appl. Phys. 1992, 25 (5), 897. 14. Bruner, C.; Dauskardt, R., Role of Molecular Weight on the Mechanical Device Properties of Organic Polymer Solar Cells. Macromolecules 2014, 47 (3), 1117-1121. 15. Rolston, N.; Printz, A. D.; Tracy, J. M.; Weerasinghe, H. C.; Vak, D.; Haur, L. J.; Priyadarshi, A.; Mathews, N.; Slotcavage, D. J.; McGehee, M. D., Effect of Cation Composition on the Mechanical Stability of Perovskite Solar Cells. Advanced Energy Materials 2018, 8 (9), 1702116. 16. Watson, B. L.; Rolston, N.; Printz, A. D.; Dauskardt, R. H., Scaffold-Reinforced Perovskite Compound Solar Cells. Energy Environ. Sci. 2017, 10 (12), 2500-2508. 17. Garcia-Belmonte, G.; Boix, P. P.; Bisquert, J.; Sessolo, M.; Bolink, H. J., Simultaneous Determination of Carrier Lifetime and Electron Density-of-States in P3ht: Pcbm Organic Solar Cells under Illumination by Impedance Spectroscopy. Sol. Energy Mater. Sol. Cells 2010, 94 (2), 366-375. 18. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342 (6156), 341344. 19. Hawash, Z.; Ono, L. K.; Qi, Y., Moisture and Oxygen Enhance Conductivity of Litfsi‐ Doped Spiro‐Meotad Hole Transport Layer in Perovskite Solar Cells. Advanced Materials Interfaces 2016, 3 (13), 1600117.


21


Page 22 of 24

20. Tummala, N. R.; Risko, C.; Bruner, C.; Dauskardt, R. H.; Brédas, J. L., Entanglements in P3ht and Their Influence on Thin‐Film Mechanical Properties: Insights from Molecular Dynamics Simulations. J. Polym. Sci., Part B: Polym. Phys. 2015, 53 (13), 934-942. 21. Rodriquez, D.; Kim, J.-H.; Root, S. E.; Fei, Z.; Boufflet, P.; Heeney, M.; Kim, T.-S.; Lipomi, D. J., Comparison of Methods for Determining the Mechanical Properties of Semiconducting Polymer Films for Stretchable Electronics. ACS applied materials & interfaces 2017, 9 (10), 8855-8862. 22. Zhang, F.; Srinivasan, M., Self-Assembled Molecular Films of Aminosilanes and Their Immobilization Capacities. Langmuir 2004, 20 (6), 2309-2314. 23. Smith, I. C.; Hoke, E. T.; Solis‐Ibarra, D.; McGehee, M. D.; Karunadasa, H. I., A Layered Hybrid Perovskite Solar‐Cell Absorber with Enhanced Moisture Stability. Angew. Chem. 2014, 126 (42), 11414-11417. 24. Park, N.-G., Perovskite Solar Cells: An Emerging Photovoltaic Technology. Mater. Today 2015, 18 (2), 65-72. 25. Kook, S.-Y.; Dauskardt, R. H., Moisture-Assisted Subcritical Debonding of a Polymer/Metal Interface. J. Appl. Phys. 2002, 91 (3), 1293-1303. 26. Lawn, B., Diffusion-Controlled Subcritical Crack Growth in the Presence of a Dilute Gas Environment. Materials Science and Engineering 1974, 13 (3), 277-283. 27. Wiederhorn, S. M.; Freiman, S. W.; Fuller, E. R.; Simmons, C., Effects of Water and Other Dielectrics on Crack Growth. Journal of Materials Science 1982, 17 (12), 3460-3478. 28. Novoa, F. D.; Miller, D. C.; Dauskardt, R. H., Environmental Mechanisms of Debonding in Photovoltaic Backsheets. Sol. Energy Mater. Sol. Cells 2014, 120, 87-93. 29. Novoa, F. D.; Miller, D. C.; Dauskardt, R. H., Adhesion and Debonding Kinetics of Photovoltaic Encapsulation in Moist Environments. Progress in Photovoltaics: Research and Applications 2016, 24 (2), 183-194. 30. Bruner, C.; Novoa, F.; Dupont, S.; Dauskardt, R., Decohesion Kinetics in Polymer Organic Solar Cells. ACS applied materials & interfaces 2014, 6 (23), 21474-21483. 31. Williams, M. L.; Landel, R. F.; Ferry, J. D., The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers and Other Glass-Forming Liquids. J. Am. Chem. Soc. 1955, 77 (14), 3701-3707. 32. Fox Jr, T. G.; Flory, P. J., Second‐Order Transition Temperatures and Related Properties of Polystyrene. I. Influence of Molecular Weight. J. Appl. Phys. 1950, 21 (6), 581-591. 33. Barard, S.; Heeney, M.; Chen, L.; Cölle, M.; Shkunov, M.; McCulloch, I.; Stingelin, N.; Philips, M.; Kreouzis, T., Separate Charge Transport Pathways Determined by the Time of Flight Method in Bimodal Polytriarylamine. J. Appl. Phys. 2009, 105 (1), 013701. 34. Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I., Iodide Management in Formamidinium-LeadHalide–Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356 (6345), 1376-1379. 35. Song, Z.; Liu, J.; Wang, G.; Zuo, W.; Liao, C.; Mei, J., Understanding the Photovoltaic Performance of Perovskite–Spirobifluorene Solar Cells. Chemphyschem 2017, 18 (21), 30303038. 36. Ma, Y.; Fan, J.; Zhang, C.; Li, H.; Li, W.; Mai, Y., Enhanced Charge Collection and Stability in Planar Perovskite Solar Cells Based on a Cobalt (Iii)-Complex Additive. RSC Advances 2017, 7 (60), 37654-37658. 37. Jung, J. W.; Park, J.-S.; Han, I. K.; Lee, Y.; Park, C.; Kwon, W.; Park, M., Flexible and Highly Efficient Perovskite Solar Cells with a Large Active Area Incorporating Cobalt-


22

Page 23 of 24 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


Doped Poly (3-Hexylthiophene) for Enhanced Open-Circuit Voltage. Journal of Materials Chemistry A 2017, 5 (24), 12158-12167. 38. Son, S. Y.; Kim, J.-H.; Song, E.; Choi, K.; Lee, J.; Cho, K.; Kim, T.-S.; Park, T., Exploiting Π–Π Stacking for Stretchable Semiconducting Polymers. Macromolecules 2018, 51 (7), 2572-2579. 39. Kanninen, M., An Augmented Double Cantilever Beam Model for Studying Crack Propagation and Arrest. International Journal of fracture 1973, 9 (1), 83-92. 40. Dupont, S. R.; Novoa, F.; Voroshazi, E.; Dauskardt, R. H., Decohesion Kinetics of Pedot: Pss Conducting Polymer Films. Adv. Funct. Mater. 2014, 24 (9), 1325-1332.


23


Page 24 of 24

Table of Contents


24

Hole-transport layer molecular weight and doping effects on

Recommend Documents