Molecular Engineering of Tetraphenylbenzidine-Based Hole Transport

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Molecular Engineering of TetraphenylBenzidine Based HTM for Perovskite Solar Cell Maebienne Anjelica Baroro Gapol, Mannix Padayhag Balanay, and Dong Hee Kim J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12651 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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The Journal of Physical Chemistry

Molecular Engineering of Tetraphenyl-Benzidine Based HTM for Perovskite Solar Cell

Maebienne Anjelica B. Gapol,a Mannix P. Balanay,b and Dong Hee Kim*a a

Department of Chemistry, Kunsan National University, Kunsan, 573-701, Republic of Korea

b

Department of Chemistry, School of Science and Technology, Nazarbayev University,

Astana Kazakhstan

*Corresponding Author:

Tel:

+82-63-469-4576

Fax:

+82-63-469-4571

Email: [email protected]

ACS Paragon Plus Environment


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Abstract Experimental and theoretical HOMO energy correlation of tetraphenyl-benzidine (TPB) based hole transport materials (HTMs) was successfully achieved through adiabatic ground state oxidation potential calculation using LC-ωPBE. Similarly, the trends in the computed excitation energies and hole reorganization energies of the HTMs are in agreement with the experimental band gaps and hole mobilities, respectively. Using these established correlations, the calculated properties of novel TPB based HTMs were analyzed and among the derivatives, TPB with attached fluorene (Fl) has lesser absorption in the visible region, lower hole reorganization energy, and deeper HOMO level compared to the reference. These properties signify that Fl could be a promising HTM in perovskite solar cells since this material will not compete with the perovskite absorption, will be efficient for hole transport due to its better hole mobility and will eventually enhance the open-circuit voltage of the device. All of these factors could improve the efficiency of perovskite solar cell.


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I.

Introduction Various studies have been dedicated to the exploration and development of

alternative types of solar cells with lower cost, better efficiency and stability compared to silicon photovoltaics. Nowadays, many research are focused on the study of perovskite solar cells (PeSC). These emerging photovoltaics employ a perovskite absorber, CH3NH3PbI3, which has advantageous properties such as high absorption coefficient, long charge diffusion length between 100-1000 nm, weak exciton binding energy and direct band gap of 1.55 eV corresponding to an absorption onset at 800 nm 1. The improvement of photovoltaic conversion efficiency (PCE) in PeSCs undergoes an exponential increase throughout the years, from a mere 3.8 % 2 in 2009, it now has a PCE of 22.1% 3. There is still a lot of room for improvements in PeSC, since the theoretical efficiency limit is at 31%4, this can be done by optimizing the absorber and hole transport material (HTM). Discovery of alternative hole transport material is one of the major areas of research in PeSC dedicated to the enhancement of device efficiency and stability. Spiro-OMeTAD (Fig. 1), which is the commonly used HTM in PeSC, possesses several disadvantages such as complicated synthetic route, high cost, low conductivity and hole mobility. The latter two drawbacks can be resolved through the addition of dopants

5,6

. However, some of the

additives used have been observed to hasten device degradation due to their hydroscopic properties7-9,

wherein, the presence of water causes voids formation and decomposition of

the perovskite layer10, thus decreasing the solar cell efficiencies. On the other hand, studies have shown that the usage of dopant-free HTM in PeSC led to improve stability. This is attributed to better shielding of the perovskite layer from water by the hydrophobic HTM in the absence of deliquescent additives7,8. Since perovskite degradation is the principal limitation in the potential market distribution of PeSC, the search for novel dopant-free HTM which could protect the perovskite layer will help in the



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commercialization of PeSC 11. Tetraphenyl-benzidine (TPB) (Fig. 1) and its analogues have been widely used as HTM in organic light- emitting diode due to their high hole mobilities

12,13

. TPB has been

used as dopant-free HTM in PeSC; however, its HOMO level (-5.52 eV) is lower than the perovskite absorber which is unsuitable for hole transfer leading to lower efficiency in PeSC 14

. Further modification in its structure, such as the introduction of ethylenedioxythiophene

(TPBS) or N-ethylcarbazole (TPBC), has increased the HOMO level leading to better device efficiency that is almost comparable to that of the doped Spiro-OMeTAD

15

. In this paper,

different tetraphenyl-benzidine derivatives are considered in the hope of creating a dopantfree HTM for more efficient and stable PeSC. OCH3

OCH3

N

O O S

H3CO

N

N

OCH 3

H3CO

N

N

OCH 3 N

N

N

N

OCH3

OCH3

Spiro-OMeTAD

N

TPBC

N

TPB

N

TPBS

N

TPD

N

N

NPB

Fig. 1 Molecular structures of hole transport materials. In PeSC, proper alignment of the HTM’s highest occupied molecular orbital (HOMO) in relation to the perovskite absorber and gold electrode is crucial for the charge transport of holes and device performance (Fig. 2). Thus, the calculation of the energy levels of a novel molecular structure is the most vital step in identifying if it can act as HTM in PeSC. However, many theoretical studies on HTMs for PeSCs are plague by imprecise calculations of the energy levels 16-18. Therefore, new methodology for the calculation of the energy levels


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is needed in order to properly screen the materials computationally. Calculation of oxidation potentials has provided better correlation to experimental energy levels in organic dyes used in DSSC 19. Since the dyes in DSSC and HTMs in PeSC have some structural similarity, the use of oxidation potentials to calculate energy levels of HTMs is promising. On the other hand, HTMs with high hole mobility are required for better hole extraction and collection from the perovskite layer without the use of additives. Employing theoretical methods in the screening of novel molecules with promising properties has successfully led to the creation of high hole mobility organic material20. Thus, it is anticipated that the use of theoretical methods will lead to the creation of a more strategic approach in the molecular design of highly efficient dopant-free HTMs for PeSC.

Fig. 2 The working principle of perovskite solar cell. To tackle these problems, we investigated the use of different theoretical methods in the calculation of the energy level of reference molecules and designed new structures for use as HTM in PeSC. This study is divided into three parts. First is the evaluation of methods for the energy level calculation of known molecules. Secondly, the investigation of the calculated properties of the published HTMs in relation to hole mobility and PeSC device performance. And lastly, molecular engineering of novel HTMs using electron-donating and electronaccepting substituents.



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II.

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Theoretical Methodology In hybrid density functional theory (DFT) level calculations, calculated ground state

oxidation potential (GSOP) is closer to the experimental values compared to the calculated HOMO energy level19,21-23. Thus, calculation of the GSOP may provide better values corresponding to experimental HOMO levels of HTMs. GSOP is equal to the free energy difference between the neutral and oxidized state. This involves the calculation of the Gibbs free energy of a molecule in gas and the free energy of solvation

19,24

. However, this

technique is computationally expensive, especially when dealing with large molecules. An approximate method in acquiring the GSOP is to calculate the vertical energy difference (GSOPv) in solution between neutral and oxidized species, both at the geometry of the ground-state structure25. On the other hand, it was found that the adiabatic energy difference (GSOPa) between the neutral and cationic geometry of the molecule yielded results closer to the experimental value compared to the vertical approach22,23. In this study, the calculated HOMO, GSOPv, and GSOPa values were considered and compared to the experimental value. Schematic representation of the vertical and adiabatic GSOP are shown in Fig. 3a.


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(a)

(b)

(c)

Fig. 3

Representation of the potential energy curves used in determining the (a) vertical and adiabatic ground state oxidation potentials (GSOPv and GSOPa, respectively), (b) hole reorganization energy, and (c) electron reorganization energy.

As shown in Fig. 3a, the GSOPv and GSOPa is calculated using the equations,

GSOP = EM − E M


(1)


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GSOP = EM − E M

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(2)

where E(M) is the ground state energy of the neutral molecule, E+(M) is the cationic energy of the neutral structure and E+(M+) is the ground state energy of the cationic structure. On the other hand, the LUMO which is equal to the excited state oxidation potential (ESOP) can be calculated by adding the first excitation energy to the GSOP. Another important property to address with regards to HTM is the hole transport process. For less ordered systems, charge transport occurs predominantly by the hopping model 26-28, wherein, the hole transfer process is represented as follows, ∗ ∗ M

+ M

→ M

+ M

(3)

where M+(a/b) is the molecule in cationic state and M*(a/b) is a neighboring molecule in neutral state. The charge hopping rate, K, between two neighboring molecules can be expressed by using Marcus theory 29,

K =

ћ

exp − ! "

(4)

where V denotes the electronic coupling between two adjacent molecules, λ is the reorganization energy, T is the absolute temperature and kB is the Boltzmann constant. As presented in eqn. 4, V and λ are the contributing factors that affect the charge transport. The λ is defined as the geometric relaxation of the molecules in a crystal and its surroundings due to the movement of charge carriers. On the other hand, V relies on the relative order of the molecules in the crystal20. For analogous molecules, there is a negligible difference in V 30,31. Thus, only the reorganization energy was considered in this study. The reorganization energies were calculated by

λ /% = λ± + λ' = (E ± M − E± M ± ) + (EM ± − EM )


(5)

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where E(M) and E±(M) are the energies of the neutral and cationic or anionic states with the optimized geometry of the neutral molecule, respectively. On the other hand, E(M±) and E±(M±) are the energies of the neutral and cationic or anionic states with the optimized cationic or anionic geometry, respectively. The hole reorganization energy (λ+) was calculated using the cationic states while the electron reorganization energy (λ-) was calculated using the anionic states. A schematic representation for the calculation of the reorganization energies is shown in Fig. 3b-c. Here, the hole and electron reorganization energies were calculated to give insight on the hole and electron mobility of the molecule.

III.

Computational Details All calculations in this paper were done using Gaussian 0932. Five hybrid functionals

were used to benchmark the HOMO level. The functionals employed for benchmarking the HOMO level were two global hybrids (B3LYP and B3PW91), one hybrid-meta-GGA (M062X), and two range-separated hybrid (LC-ωPBE and ωB97XD) using 6-31G(d,p) basis set. The ground state geometry optimization was done with solvation effects in dichloromethane (DCM) using the conductor-like polarized continuum model (C-PCM) framework since DCM was used as the solvent in the measurement of the HOMO level of the published HTMs (Spiro-OMeTAD, TPBC, TPBS and TPB) in PeSC. Using the ground-state geometry optimized from the best functional in the previous calculation, time dependent density functional theory (TD-DFT) calculations were used to benchmark the HOMO-LUMO band gap through calculation of the first excitation energy (E1) employing B3LYP, M06-2X, and LC-ωPBE at 6-31G(d,p) in DCM. The neutral ground-state optimized geometries in solution of all the molecules were subjected to frequency calculation at the same level of theory in the optimization step. This is to confirm that no imaginary frequency has been reported which ensures energetic minima in the respective potential energy surfaces.



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IV.

Results and Discussion

A.

Assessment of Theoretical Methodology The calculated HOMO, GSOPv and GSOPa of TPBC were obtained using the

different theoretical methods. The calculated values are listed in Table S1 and the corresponding absolute errors are shown in Fig. 4. For global hybrid functionals, B3LYP and B3PW91, the calculated HOMO and GSOPs yield the same absolute errors (~0.6-0.7 eV). On the other hand, M06-2X, LC-ωPBE and ωB97XD calculated GSOPs closer to the experimental values with an average absolute error of 0.2 eV. Conversely, the range-separated functionals have pronounced underestimated calculated HOMO. Considering all the functionals, the mean absolute errors (MAE) of each type of calculation are -0.45, 0.32 and 0.43 eV for HOMO, GSOPv, and GSOPa. This shows that the calculation of GSOPv using different functionals will yield the least erroneous results. Furthermore, the calculation of the GSOPs using LC-ωPBE provided the smallest absolute errors. Thus, LC-ωPBE was used for the calculation of the GSOP levels.

Fig. 4

The absolute errors of the calculated HOMO, vertical and adiabatic ground state oxidation potentials (GSOPv and GSOPa, respectively) of TPBC using different functional in DCM using C-PCM framework. The mean absolute errors (MAE) of each calculation are shown inside the parenthesis.


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Using LC-ωPBE, GSOP calculations were done for Spiro-OMeTAD and TPB based molecules to confirm the validity of the theoretical method. The bar graphs in Fig. 5 show the order of the experimental HOMO (HOMOexp) and calculated values of the reference molecules. The order of the HOMOexp is Spiro-OMeTAD > TPBS > TPBC > TPD > NPB > TPB. The calculated GSOPv of these references does not follow the order of the HOMOexp with NPB having the lowest value. On the other hand, excluding the GSOPa of SpiroOMeTAD, the calculated GSOPa of the reference TPB based molecules follow the HOMOexp order. The calculated GSOPa of the differently structured Spiro-OMeTAD has the lowest value in contrary to the experimental ordering of energy levels. This finding proved two major points. First, GSOPa calculation of structurally dissimilar molecules using one functional will give erroneous results. Second, the calculation of GSOPa using LC-ωPBE will give us correct relative HOMO levels for TPB based molecules. The corresponding graphical correlation between the experimental and calculated values are shown in Fig. S1 and the numerical values are listed in Table S2. As can be seen, the calculated GSOPa showed better correlation compared to the calculated GSOPv. The following equation produced by the correlation of the HOMOexp and GSOPa will be used in the computation of the GSOP of the derivatives which is equal to the predicted HOMOexp.

GSOP = 0.9227 GSOP + 0.5195

(6)

The commonly employed HOMO energy level calculation using B3LYP was also analyzed using the reference molecules. The calculated HOMO values of Spiro-OMeTAD, TPBC and TPBS were similar to the computed values of Chi et al.

33

using the same functional. The

HOMO of the reference molecules calculated using B3LYP does not follow the experimental trend (Fig. 5). This shows that using B3LYP for these types of molecules will give inaccurate results. Also, this demonstrates that the correlation of experimental and calculated values is functional dependent. Thus, careful examination of the theoretical method to be used is



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necessary for accurate calculation of the HOMO levels of the HTM for PeSC.

Fig. 5

Experimental HOMO (HOMOexp) versus the calculated vertical and adiabatic ground state oxidation potential (GSOPv and GSOPa, respectively) using LC-ωPBE and calculated HOMO using B3LYP in DCM employing the C-PCM framework.

In experiments, the LUMO is acquired by adding the optical gap (first excitation energy, E1) to the HOMO energy level. Here, the E1 of TPBC calculated using TD-DFT were 3.04, 3.97 and 3.59 eV for B3LYP, LC-ωPBE and M06-2X, respectively. Compared to the experimental band gap of 2.94 eV15, TD-B3LYP//LC-ωPBE calculated the closest E1. Also, the simulated absorption spectrum using this method (Fig. 6a) is similar with the measured UV-Vis spectra. Calculation of the E1 of the TPB based reference molecules using the same theoretical method and correlating it to the experimental band gap gave an almost linear relationship (Fig. 6b). The corresponding values for the experimental band gaps and calculated first excitation energy are listed in Table S2. The calculated E1 was then corrected using the following equation of the correlation to get the predicted experimental band gap.


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E12344 = 0.7531 E1 + 0.6403

(7)

Furthermore, excited state oxidation potential (ESOP) which is equated to the LUMO level was calculated using

ESOP = 89:; + ?? .

(a)

(8)

(b)

Fig. 6 (a) Simulated absorption spectra of TPBC calculated using different functional in DCM using C-PCM framework. (b) Experimental band gap of the reference molecules versus the calculated first excitation energy using TD-B3LYP//LC-ωPBE in DCM using C-PCM framework.

B.

Investigation of the properties of reference molecules To provide some baseline to our theoretical methodology, the hole reorganization

energies (Table S3) of the reference molecules were analyzed. TPD and NPB have relatively higher λ+ compared to the other reference molecules even though their experimental hole mobilities are much larger. This deviation from Marcus theory29, wherein hole reorganization energy decreases as hole mobility increase, could be due to the different experimental conditions such as varied temperature and electric field which are factors that influence the hole mobility26; thus, these two reference molecules were not included in the correlation of the experimental hole mobility and hole reorganization energy. In the same experimental



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conditions with the reference molecules excluding TPD and NPB, Spiro-OMeTAD which has the lowest experimental hole mobility, produced the lowest λ+. This inconsistency to the theory could be due to the different structure of the Spiro-OMeTAD compared to the TPB molecules. This further proves that property calculations of dissimilar molecules will not reproduce relative results compared to the experimental values. On the other hand, a linear correlation following Marcus theory was found between hole reorganization energy and the experimental hole mobility of TPBC, TPBS and TPB (Fig. 7a). This finding could mean that the transfer integral of these molecules are almost similar as observed in analogue molecules 30,31

or its contribution is smaller compared to the hole reorganization energy due to the

exponential dependence of the hopping rate on hole reorganization energy (eqn 4). Thus, we can predict the hole mobility of the TPB based novel designs by examining the hole reorganization energy solely. Also, all of the molecules exhibited lower hole reorganization energy compared to electron reorganization energy. This implies that the HTMs have larger hole mobility than electron mobility 16. (a)

(b)

Fig. 7 (a) Hole reorganization energies versus the experimental hole-drift mobilities of TPBC, TPBS and TPB calculated using LC-ωPBE in DCM employing C-PCM framework. (b)The simulated absorption spectra of TPBC, TPBS and TPB using TDB3LYP//LC-ωPBE in DCM using C-PCM framework.


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For the following discussions, only the published TPB based HTMs used in PeSC, namely TPBC, TPBS, and TPB, were examined. As can be seen in Fig. 7b, TPBS has the most red-shifted spectra while TPBC has the highest extinction coefficient. The red-shifted spectra of TPBS is in agreement with the experimental results

15

. Only TPBC and TPBS

absorbs in the visible region while TPB has an onset wavelength at around 420 nm. Thus, the lower Jsc of TPBS compared to TPBC can be attributed to the more red-shifted absorption range of TPBS. This is due to the filtering of light by the HTMs before it reaches the perovskite absorber that leads to the lower absorption of the perovskite

7,34-36

. Therefore,

when designing candidate HTMs, molecules with less absorption in the visible region is more suitable. On the other hand, the exciton binding energy is the energy needed to separate the exciton into free charge carriers. It is the difference between the transport gap and the optical gap37. The smaller exciton binding energy is advantageous for larger Jsc 17. TPBC and TPBS have a very small exciton binding energy difference of 0.07 eV (Table S4). Even though TPBS has a lower exciton binding energy, the lower hole reorganization energy corresponding to higher hole mobility, lower HOMO level and blue-shifted absorption of TPBC contributes to its better Jsc and device performance. Lastly, electron density gives a qualitative information on the transfer integral of the molecules. While LUMO electron density is associated with electron transfer, the HOMO electron density is associated with hole transfer. More inter-orbital overlaps in HOMO of neighboring molecules corresponds to larger hole transport integral 17. Thus, the delocalized HOMO electron density of the TPB based HTMs over the whole molecule (Fig. 8) is favorable for hole transport 16,17.



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TPBC

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TPBS

TPB

LUMO

HOMO Fig. 8 Frontier molecular orbitals of TPBC, TPBS and TPB.

C.

Effect of electron donor and acceptor substituents Different moieties (Fig. 9) commonly used in organic solar cells as electron donors

and acceptors were used to modify the TPBC reference. Electron Donors S S

R

S

Fl

Th N

N

R=

S

S

CPT

BDT

Electron Acceptors

S N

S

O

N

O

N

S N

N

S S

N

N S

TT

TPD

BT

NT

Fig. 9 Electron-donating and electron-accepting substituents used to modify TPBC.

As shown in Fig. 10, all derivatives except TPD, met the required energy levels as HTM for CH3NH3PbI3 based PeSC. TPD has a lower HOMO level compared to that of CH3NH3PbI3 which is not favorable for hole transport. Relating the energy level to device performance, the lower HOMO level, which corresponds to GSOP in this study, is ideal in


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increasing VOC since VOC is dependent on the difference between the Fermi level of the conduction band of the TiO2 and the HOMO level of the HTM. Higher VOC has been measured in HTMs in PeSC with lower HOMO energy levels compared to reference HTM 7,35,38-41

. This was also shown in the higher VOC of TPBC and TPBS based PeSC compared to

Spiro-OMeTAD based devices due to their lower HOMO15. Since all of the new designs have a lower GSOP compared to the reference molecules except CPT and TT, higher VOC relative to TPBC will be expected when using these novel molecules as HTM in perovskite solar cells. Also, all the designs have a higher ESOP level compared to the conduction band of CH3NH3PbI3. This property is advantageous for blocking the electrons created in the perovskite layer leading to minimized charge recombination 42,43.

Fig. 10 Ground and excited state oxidation potentials (GSOP and ESOP, respectively) of TPBC and derivatives calculated using LC-ωPBE and TD-B3LYP//LC-ωPBE/, respectively, in DCM using C-PCM framework.

Examining the reorganization energies, except for CPT and TT, the derivatives have a lower λ+ compared to TPBC (Fig. 11a). Contrasting the electron donor and acceptor based



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derivatives, most of the electron-acceptor substituted derivatives have a smaller λ+ compared to the electron-donor substituted derivatives excluding TT. Using the linear relationship between hole mobility and λ+, the predicted hole mobility of Th, Fl, BDT, TPD, BT and NT will be greater than that of TPBC. The anticipated greater hole mobility of these molecules may aid in better charge extraction of holes from the perovskite layer. This will be advantageous since efficient charge extraction will lead to minimize hysteresis which is one of the serious problems in PeSC

44

.

Also, since these molecules have lower GSOP levels

and better hole mobility, the avoidance of dopants, which is used for further lowering of the GSOP and enhancing the hole mobility, could be attained. Furthermore, all the compounds have a lower hole reorganization energy compared to the electron reorganization energy except CPT which means that hole mobility predominates over electron mobility. Fig. 11 shows the absorption spectra of the derivatives with respect to TPBC. For the electron-donor substituted derivatives, the maximum wavelength (λmax) is red shifted compared to TPBC (Fig. 11b). The trend for the λmax is TPBC < Fl < Th < BDT

Molecular Engineering of Tetraphenylbenzidine-Based Hole Transport

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