α-DTC₇₀ Fullerene Performs Significantly Better than β-DTC₇₀ as Electron Transporting Material in Perovskite Solar Cells.

Abstract

In this work, two new C₇₀ isomers, α and β bis(2-(thiophen-2-yl)ethyl)-C₇₀-fullerene mono-adducts (DTC₇₀), were synthesized, characterized and used as electron transporting materials (ETMs) in perovskite solar cells (PSCs). Our results show that the α isomer improves both the J_sc and FF values of the devices, when compared to the results for the β-isomer and to those for phenyl-C₇₀-butyric acid methyl ester (PC₇₁BM), used as control. Devices based on α-DTC₇₀ achieved a power conversion efficiency (PCE) of 15.9%, which is higher than that observed with PC₇₁BM (15.1%).

Graphical Abstract

Introduction

In the past few years, the development of organic-inorganic hybrid perovskite solar cells (PSCs) has received special attention because of the fast improvement of their PCE, from 3.8% in 2009 to 25.2% in 2019,¹ easy fabrication procedures and low cost.^{2, 3} These materials are promising for the production of new devices with enhanced properties for commercial applications.^4–10

Among the different device configurations, inverted p-i-n planar PSCs have emerged as a new alternative due to their simple device architecture and low temperature manufacturing processes, while still keeping considerably high performances.¹¹ Another advantage of planar inverted PSCs is their versatility regarding both the hole transporting materials (HTMs) and the ETMs.^12–16 In this regard, fullerenes are among the best ETMs in inverted PSCs because of their deep lying lowest unoccupied molecular orbital energies (LUMO) that match with the conduction band of the perovskite¹⁷ and their ability to accept multiple electrons.^{2, 18} Additionally, due to their low reorganization energy, fullerenes can accelerate photoinduced charge separation and slow down charge recombination in the dark.¹⁹

One of the most used fullerene derivatives in PSCs fabrication is the phenyl-C₆₀-butyric acid methyl ester (PC₆₁BM), but many other fullerenes have been successfully incorporated in PSCs.^{2, 20} It is worth noting that the function of fullerenes in PSCs is not only limited to accepting and transporting electrons. A wide variety of fullerene derivatives bearing different functional groups have been used with several purposes: as ETM compact layers, as interfacial modification layers, as additives, as cathode buffer layers and as double layers.^{2, 3, 20}

Charge recombination and hysteretic behaviour are some of the drawbacks that affect the performance of PSCs. Recently, it has been shown that the introduction of heteroatom-containing motifs on fullerene derivatives can effectively passivate the trap sites of the surface of the perovskite layer by interaction with undercoordinated Pb²⁺ cations.²¹ As a result of this interaction, charge recombination and hysteresis can be supressed, thus improving the overall photovoltaic performance.²¹ It has been reported that fullerene derivatives passivate the trap sites that present both on the surface and in the grain boundaries of the perovskite layer.^{22, 23}

The functionalization of fullerenes has been widely studied to improve their solubility as well as to modify and control their physical and optical properties.^23–26 When functionalizing C_70, different isomers can be obtained, making the purification process difficult. Some of us have reported that the use of a α-C₇₀ fulleropyrrolidine as the ETM improves the performance exhibited by a regioisomeric mixture. Such difference was attributed to the energy disorder provided by the different α, β and γ isomers present in the mixture. Similar results were reported in other articles.^27–29 In general, the use of regioisomeric mixtures in photovoltaic devices often results in lower PCEs when compared to their corresponding pure isomers.^{21, 30–35} When comparing pure isomers to regioisomeric mixtures, most of the research so far has focused mainly on the α-isomer, which are more easily prepared than their β counterparts. Recently, we have reported a very straightforward methodology to selectively synthesize pure C₇₀ β-isomers.³⁶ Additionally, we also reported that the introduction of thiophene moieties in the fullerene cage increases the interaction of the ETM and the perovskite layer, thus raising the photovoltaic performance. Here we report the synthesis and characterization of the α and β bis(2-(thiophen-2-yl)ethyl)-C₇₀-fullerene (DTC₇₀) mono-adducts and their performances in PSCs when used as the ETMs.

Results and Discussion

The main motivation for this work was to probe the influence of the position of the functional groups in the two structurally different isomers and their impact on the photovoltaic performance as well as to gain a better understanding of their interaction with the perovskite layer. The α-DTC₇₀ and β-DTC₇₀ (Scheme 1) fullerene derivatives were synthesized according to our previously reported methodology.^{36, 37} The α-DTC₇₀ isomer was prepared by a Bingel cyclopropanation reaction of malonate derivative 3 and fullerene C₇₀. The product was obtained in 55% yield after chromatographic purification. For the synthesis of the β-DTC₇₀ isomer, C₇₀ was first reacted with anthracene to yield the α−6-α’ C₇₀-BA isomer, which was used as a template to direct the subsequent cyclopropanation reaction to the C_s-symmetric tris-adduct isomer (4, Scheme 1). A thermal retro Diels-Alder reaction of 4 resulted in the selective synthesis of the β-DTC₇₀ isomer with a 63% overall yield (see materials and methods for details). The pure compounds were characterized by means of ¹H-NMR and ¹³C-NMR spectroscopy, UV-Vis and matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) (Figures S1–S10).

Scheme 1:

Synthesis of α-DTC₇₀ and β-DTC₇₀.

We recently reported that the functionalization of C₆₀ with thiophene groups substantially improved their solubility in organic solvents such as chlorobenzene (CB), and PCE values higher than those of PC₆₁BM based-devices were obtained for C₆₀-thiophene based devices. Additionally, devices fabricated with the compounds bearing the thiophene groups showed slower degradation, which was attributed to the specific interactions between the sulphur atoms present in the addend and the free lead cations from the surface of the perovskite layer.³⁷

The feasibility of the α-DTC₇₀ and β-DTC₇₀ compounds to be used in inverted PSCs as ETMs was anticipated by estimation of their HOMO-LUMO energy values from UV-vis and cyclic voltammetry (CV) (Figures S7 and S8). Both isomers exhibited electrochemical and chemical reversibility evidenced by three well-defined and independent reduction and oxidation waves. The reduction onset potentials ($E_{red}^{on}$) for both isomers were estimated from the CV measurements using the equation $\text{LUMO}=-\text{e}\left(E_{red}^{on}+4.8\right)\left(\text{eV}\right)$ and these are summarized in Table 1. The energy level diagram in Figure 1a shows that the HOMO and LUMO energy levels of the new derivatives match well with that of the perovskite material. The two isomers exhibit deep enough HOMO values to block the holes and low enough LUMO values to allow the fast transport of electrons, indicating that they can be used efficiently as ETMs in PSCs.

Table 1.

Optical bandgap, onset reduction and HOMO/LUMO energy levels of α-DTC₇₀ and β-DTC₇₀.

Compound	λabs (nm)	Eg (eV)	$E_{red}^{on}$ (V)	LUMO (eV)	HOMO (eV)
α-DTC70	695	1.78	−0.92	−3.88	−5.66
β-DTC70	710	1.75	−0.93	−3.87	−5.62

Figure 1.

a) Energy level diagram, PC₇₁BM values were taken from the literature,²¹ b) surface SEM image of the perovskite layer and c) cross section SEM of the perovskite device.

X-ray diffraction (XRD) was performed to characterize the formation of the perovskite layer. As observed in Figure S11 the perovskite is completely formed, as indicated by the characteristic diffraction planes 100, 112, 211, 202, 220, 310, 312, 322 that suggest the formation of a tetragonal phase perovskite.³⁸ To study the quality of the films, scanning electron microscopy (SEM) was recorded to investigate the morphology of the perovskite layer, which revealed good homogeneity with no visible pin holes, and an average grain size of 200 nm (Figure 1b). The cross-section SEM (Figure 1c) measurements showed the indium tin oxide (ITO) transparent electrode (50 nm), poly(3,4-ethylenedioxythiophene):poly-styrene sulfonate (PEDOT: PSS) as hole transporting material (25 nm), perovskite active layer (300 nm), and the fullerene ETL(80 nm).

To study the performance of α-DTC₇₀ and β-DTC₇₀ as ETMs, PSCs planar devices with an inverted configuration (ITO/PEDOT:PSS/perovskite/α-DTC₇₀ or β-DTC₇₀/Ag) were fabricated. (see supporting information for fabrication details).

Current-voltage (J-V) curves for both isomers and PC₇₁BM as control were obtained under AM 1.5G irradiation (100 mW cm⁻²) in air. The photovoltaic parameters such as FF, J_sc, and V_oc are listed in Table 1. As shown in Figure 2 and Table 3 a maximum PCE value of 15.9% was achieved for devices based on α-DTC₇₀ whereas PCE values of 15.1% and 8.80% were obtained for PC₇₁BM and β-DTC₇₀ based devices, respectively. The J_sc and FF values are higher for the α isomer, which is attributed to its good electron mobility (Figure S13) and high solubility in dichlorobenzene, whereas in the case of the β isomer the electron mobility is slightly lower. Although the differences are not very pronounced, the electron mobility follows the same trend as the PCE (Table S1). It has been reported for similar C₇₀ β isomers, that the packing in the solid state is not as compact and efficient as compared to the α isomer, yielding to a lower photovoltaic performance.³⁹

Figure 2.

a) J-V curves and b) EQE spectra of PSCs fabricated with α-DTC_70, β-DTC₇₀ and PC₇₁BM as ETMs.

Table 3.

Summary of device performance. The calculated J_sc values were obtained from the EQE curves. Values in parentheses represent the best PCEs measured.

Compound	Calculated Jsc (mA/cm2)	Jsc (mA/cm2)	Voc (V)	FF	PCE (%)
PC71BM	20.94	21.01	0.891	80.9	14.9±0.3 (15.1)
α-DTC70	21.95	22.00	0.874	82.6	15.3±0.5 (15.9)
β-DTC70	13.97	14.09	0.812	76.9	8.29±0.5 (8.80)

Figure S12 shows the PCEs based on statistical histograms obtained from 25 devices. The average efficiencies for α-DTC₇₀, β-DTC₇₀ and PC₇₁BM based devices are 15.3% ±0.5%, 8.29± 0.5% and 14.9%±0.3, respectively, showing narrow distributions for the three compounds, which is an indicative of the good performance reproducibility.

The J_sc values were also investigated by external quantum efficiency (EQE) as shown in Figure 2b. The values obtained from EQE were in agreement with the ones derived from the J-V curves, which suggest that the high J_sc and FF values for the α-DTC₇₀ isomer can be related to a better ability to extract electrons in the visible spectral window. Also, we can conclude from the spectra that α-DTC₇₀ can efficiently convert more than 80% of the incident photons into electrons in the visible range when compared to β-DTC₇₀ and PC₇₁BM.

Steady state photoluminescence (SSPL) measurements and time resolved photoluminescence (TRPL) were performed to demonstrate that the α and β derivates are good at extracting electrons, passivating the perovskite layer, and decreasing the electron-hole recombination. These measurements were collected from devices with the structure glass/perovskite and glass/perovskite/fullerene.

Figure 3a shows that the perovskite layer exhibited a high PL signal, as the result of electron-hole recombination events, while the perovskite/fullerene devices showed a very efficient quenching of the photoluminescence, a clear indication of the ability of these compounds to reduce recombination processes. These results were very similar to those obtained for perovskite/PC₇₁BM devices. To study the kinetics of the electron-hole recombination, TRPL was also collected by monitoring the emission peak (780 nm) as a function of time. Figure 3b shows that the decay time for α-DTC₇₀ is faster than that of PC₇₁BM and β-DTC₇₀, which indicates that electrons are more efficiently transferred from the perovskite layer to α-DTC₇₀ than to β-DTC₇₀ or PC₇₁BM. These results could explain the devices performance since a faster decay time indicates a faster injection of electrons into the ETL, thus decreasing the recombination events.

Figure 3.

a) SSPL and b) TRPL of the perovskite, perovskite/α-DTC_70, perovskite/β-DTC₇₀ and perovskite/PC₇₁BM layers.

To further investigate the electrical and charge transport properties at the perovskite/fullerene interface, electrochemical impedance spectroscopy (EIS) was performed for PSCs based on the new derivatives and PC₇₁BM (Figure S14 and Table S2). The equivalent circuit used to fit the Nyquist plots is illustrated in Figure S14, where the series resistance (R_s) corresponds to the resistance of the two electrodes and the charge-transport resistance (R_ct) and the recombination resistance (R_rec) correspond to the resistances at the fullerene/perovskite interfaces.^{40, 41} Our results show that PSCs based on α-DTC₇₀ display a lower R_ct and larger R_rec values than devices based on β-DTC₇₀ and PC₇₁BM. Since very similar electron mobilities were measured for all the fullerene-based devices, the higher PCE values obtained for α-DTC₇₀-based devices must be the result of improved interfacial contact and charge transfer between the perovskite and the α-isomer. These results are also consistent with the higher PL quenching observed for perovskite/α-DTC₇₀ films.

Stability studies were performed on encapsulated devices under 50% humidity at ambient conditions. The devices were stable for up to 7 days and the ones containing α-DTC₇₀ retained 80% of their initial PCE, compared to 42% and 40% for β-DTC₇₀ and PC₇₁BM, respectively. As shown in Figure 4, α-DTC₇₀ devices showed comparatively better photostability than β-DTC₇₀ and PC₇₁BM devices, which can be attributed to better packing in the solid state, thus allowing higher environmental resistance.

Figure 4.

Stability Studies of the PSCs fabricated with α-DTC_70, β-DTC₇₀ and PC₇₁BM as the ETMs.

To better understand the interaction between α-DTC₇₀ and β-DTC₇₀ fullerene adducts and the perovskite surface, we performed periodic density functional theory (DFT) calculations. Previous studies on PC₆₁BM-like derivatives showed that these compounds interact with the surface via the fullerene and also via the addend.³⁸ The latter occurs through a direct interaction between the Pb²⁺ ions and the carbonyl or ester substituents. In the C₇₀ adducts, the situation is similar but the lower symmetry of the C₇₀ cage increases somewhat the complexity because the possible different orientations of the addends with respect to the surface. Mono functionalization of C₇₀ affords preferentially C₁-C₂ adducts (α type regioisomers), whereas the second most reactive bond is C₅-C₆ which yield β regioisomers.⁴² As shown in Figure 5, α and β isomers exhibit different local symmetry, with the α isomer showing a symmetry plane through the C₁-C₂ (red) bond. It is worth mentioning that for β-DTC₇₀ regioisomers, two different adsorption modes on the perovskite surface are possible depending on the way the C=O groups of the malonate are attached to the Pb²⁺ sites of the surface. We define the β−1 mode when the C=O group on the side of the pentagon is the one bonded to the Pb²⁺ atom, whereas the β−2 mode corresponds to the situation when the attached C=O is the one on the side of the hexagon (Figure 5).

Figure 5.

α and β motifs for the [6,6] Bingel adducts of C₇₀. The red line represents the addend carbon-carbon bond position on the fullerene. For a representation of β₁ and β₂ isomers see Figure S15.

Table 4 collects adsorption energies and structural data for several orientations of fullerene adducts deposited on the perovskite surface. Initially, we have analyzed in detail the different orientations of a pristine C₇₀ in contact with the surface. We have two fullerene alignments, cap and equatorial and three surface sites, hollow, top and bridge. Figure S16 summarizes the adsorption energies computed for the different pristine C₇₀ structures, which range between −1.07 eV and −1.81 eV. The optimal alignment corresponds to the fullerene interacting with a hollow site through the poles. The second most stable orientation is the bridge-equatorial one with an adsorption energy of −1.59 eV, only 0.22 eV higher than the lowest energy minimum. Indeed, for the α-DTC₇₀ adduct, we have found that the most exothermic adsorption energy occurs when the fullerene is interacting at a bridge site, as shown in Figure 6b.

Table 4.

Adsorption energies and shortest distances for the different isomers of the fullerene adducts and adsorption sites.

Isomer	Eadsa)	REb)	dPb-Oc)	d Surf-C70d)
α-DTC70 -hollow	−1.90	0.67	2.869	3.138 < 3.226 < 3.411
α-DTC70 -top	−2.23	0.35	2.615	3.226 < 3.516 < 3.742
α-DTC70 -bridge	−2.36	0.22	2.638	3.100 < 3.304 < 3.679
β1-DTC70-hollow	−2.54	0.04	2.655	3.393 < 3.405 < 3.776
β1-DTC70-top	−2.43	0.15	2.709	3.175 < 3.394 < 3.640
β1-DTC70-bridge	−2.54	0.04	2.689	3.284 < 3.346 < 3.795
β2-DTC70-hollow	−2.01	0.57	2.857	3.315 < 3.338 < 3.528
β2-DTC70-top	−2.58	0.00	3.497	3.130 < 3.359 < 3.515
β2-DTC70-bridge	−2.48	0.10	3.009	3.201 < 3.516 < 3.526

^a)Adsorption energies in eV;

^b)relative energies with respect to β₂-DTC₇₀-top.

^c)shortest Pb-O distance in Å;

^d)shortest contacts between the carbon atoms of the fullerene and the perovskite surface in Å.

Figure 6.

a) Schematic representation of three types of adsorption sites of the fullerene on the perovskite surface and b) two views of the lowest energy structures (bridge isomer) for α-DTC₇₀.

The top and the hollow sites are less exothermic by 0.35 and 0.67 eV. The change in the fullerene orientation with respect to the pristine fullerene is due to the carbonyl-lead interaction that fixes the fullerene on the perovskite surface. The adsorption energies for the fullerene adducts are in general between 0.10 and 0.77 eV larger than in the pristine fullerene. This can be rationalized if we consider that the strong O−Pb²⁺ interactions easily overcome the loss of fullerene-surface interactions imposed by the addend constraints.

A similar analysis was done for the β regioisomers, for which we have found three orientations of the fullerene on the surface in an energy range of only 0.04 eV. The most exothermic adsorption energies are displayed by two β₁-DTC₇₀ adducts in hollow and bridge sites and one β₂ isomer on top. Indeed, β₂-DTC₇₀-top corresponds to the adduct with the shortest carbon-lead distance (3.130 Å). In β₁-DTC₇₀-bridge and β₁-DTC₇₀-hollow sites, the fullerene is found at longer separations from the surface, but the loss of interaction is compensated by the stronger Pb-addend interactions, revealed by shorter Pb-O distances (Table 4). Due to the small energy differences, it is reasonable to believe that in our device the β₁-DTC₇₀ isomers are occupying different adsorption sites on the perovskite surface.

The electronic structure of lead-halide perovskites has been recurrently analyzed by means of DFT calculations showing that the valence band is mainly formed by antibonding states derived from hybridizations of 5p orbitals of iodine and 6s orbitals of lead, while the conduction band is mainly constituted by the empty 6p orbitals of lead.^43–46 Given that the electron transfer from the excited perovskite to the fullerene empty orbitals has probably to involve the conduction band of the perovskite, we have analyzed the fullerene−lead contacts. In agreement with the experimental results, which exhibit a higher efficiency for the α-DTC₇₀ isomer, Figure 7 shows that the shortest contacts are observed for this isomer. Nevertheless, the contact distances between carbon atoms and lead are not very different and likely other factors, such as fullerene adduct compactness and packing could be relevant in the electron-transport and overall device performance properties.

Figure 7.

Schematic representation for the lowest energy α-DTC₇₀ and β-DTC₇₀ adducts adsorbed on the MAPbI₃ surface. The values in red and cyan correspond to the shortest lead-fullerene and lead addend distances (in Å), respectively. A 3D view of the metal adduct contacts can be found in Figure S17.

Conclusions

In conclusion, we have regioselectively synthesized new α- and β-C₇₀ fullerene derivatives by following our previously reported methodology. The new compounds were fully characterized and tested in inverted PSCs as ETMs. Our results show significant differences between the isomers. The α-DTC₇₀ exhibited an enhanced of the overall device performance with higher PCE, J_sc and FF values and a significantly improved ability to extract electrons as evidenced by PL. Additionally, the long term stability was also improved when devices were prepared with the α isomer, retaining 80% of the initial PCE for up to 7 days after fabrication. The different device performances of the two C₇₀ isomers was rationalized by DFT calculations. The electron mobilities together with EIS measurements, the packing differences reported in the literature between α and β isomers, and the DFT calculations in this article provide a reasonable rationalization for the drastically different PCEs observed experimentally.