A perylene diimide-based non-fullerene acceptor as an electron transporting material for inverted perovskite solar cells

Abstract

Herein we introduce a new perylene diimide dimer (diPDI) as a non-fullerene electron transporting layer (ETL) material for inverted perovskite solar cells. The solar cell device with n-doped diPDI as ETL exhibits a notable power conversion efficiency of 10.0%.

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Solar energy is a clean and renewable energy resource that can solve the problems of the worldwide energy crisis and global warming. Although the power conversion efficiencies (PCEs) of commercially available solar cells made of silicon, gallium arsenide, copper indium gallium selenide and cadmium telluride exceed 20%,¹ the energy production costs of such solar cells are very high as compared to the energy cost produced by fossil fuels, which motivates us to develop low-cost solar cells. Organic photovoltaics (OPVs) including dye sensitized solar cells (DSSCs) and polymer-based solar cells have intensively been developed for the past two decades to meet the demand for low cost photovoltaics.² Organic–inorganic hybrid perovskite solar cells have recently attracted tremendous attention since its first report in 2009 because of its superior intrinsic properties such as extremely long diffusion length of charge carriers, excellent charge carrier transport, and high absorption coefficient.³ Furthermore, thin films of organometal halide perovskite can be easily fabricated from abundant and inexpensive precursor materials. In spite of their short history, the photovoltaic performances of perovskite solar cells have advanced dramatically with surpassing a PCE of 20%.⁴ Hence, organometal halide perovskite has been recognized the most promising candidate for low-cost photovoltaics.

Solar cells made of organometal halide perovskite can be classified into two types according to the device architecture: one is the normal cell with the configuration of FTO/compact TiO₂/mesoporous TiO₂/perovskite/spiro-OMeTAD/Au similar to the DSSC device structure, and another is the inverted cell with the configuration of ITO/PEDOT:PSS/perovskite/PCBM/Al similar to the OPV device structure. The most remarkable advantage of inverted structure over normal structure for perovskite solar cells is its simple and low-cost fabrication, because inverted perovskite solar cells can be fabricated by low-temperature solution process while normal perovskite solar cells require high temperature (>450 °C) sintering process to form compact TiO₂ layer.

Design and synthesis of new hole transporting materials (HTMs) for normal perovskite solar cells have widely been studied by several groups in order to replace spiro-OMeTAD, because of relatively high production cost, poor stability and low hole mobility of spiro-OMeTAD,⁵ while new electron transporting layer (ETL) materials of inverted cell as an alternative to PCBM have rarely been reported. In OPV, non-fullerene electron acceptors based on polymers and small molecules have recently been developed because of their unique advantages over fullerene derivatives such as high absorption in visible range, tunable energy level, and low production cost.⁶ However, since inverted perovskite solar cells require sufficiently thick ETL to prevent direct contact between perovskite layer and metal cathode,⁷ the non-fullerene acceptors as ETL material of perovskite solar cells must have sufficiently high electrical conductivity and high electron mobility.

In our previous study, we successfully introduced an n-type dopant (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole: DMBI) to PCBM layer in inverted perovskite solar cells.⁸ The electrical conductivity of PCBM was remarkably increased due to n-doping of DMBI when a small amount of DMBI is added to PCBM, and as a consequence the device with thick (105 nm) layer of PCBM exhibits a high PCE of 13.8%, indicating that n-doping is an effective strategy to enhance the electrical conductivity of n-type material as ETL material. This result provides high possibility to use DMBI as an effective n-dopant for n-doping of non-fullerene electron acceptor, which can be used as ETL material of inverted perovskite solar cells.

In this study, we use di-perylene diimide (diPDI) as non-fullerene ETL material of inverted perovskite solar cells. When we dope diPDI by a small amount of DMBI, we have found that the charge transfer at the perovskite/diPDI interface becomes more effective than the case without n-doping to afford enhanced photovoltaic performance. When the n-doping of diPDI by DMBI is examined by photoluminescence (PL), UV-Vis-NIR absorption and ultraviolet photoelectron spectroscopy (UPS) experiment, it reveals that the electron accepting ability of diPDI is comparable to that of PCBM and that diPDI is effectively n-doped by DMBI. With 1 wt% DMBI doping, the device with diPDI as ETL exhibits a PCE of 10.0% with a J_SC of 21.6 mA cm⁻² under illumination of AM 1.5 G 100 mW cm⁻².

Among non-fullerene electron acceptors developed for OPVs, naphthalene diimide (NDI) and perylene diimide (PDI) derivatives have mostly been used due to their easy synthesis, easy tunability of frontier energy levels, excellent thermal and photochemical stability, and strong electron accepting ability.^6a–h,9 In this study, we choose diPDI as ETL material of inverted perovskite solar cells because of ease of synthesis, excellent performance as electron acceptor, and similar energy level to PCBM.^6a,9a diPDI is synthesized through 3 steps with a moderate yield (Fig. S1†). First, 3,4,9,10-perylenetetracarboxylic dianhydride is reacted with 4-heptylamine to yield PDI with 1-propylbutyl side groups. Then, the PDI is brominated for dimerization. Finally, two mono-brominated PDIs are coupled by homo-coupling reaction to yield diPDI. When PL spectra are measured to evaluate the suitability of diPDI as ETL material for inverted perovskite solar cell, as shown in Fig. 1a, the emission peak of perovskite (CH₃NH₃PbI₃) at 780 nm is quenched by diPDI as effectively as by PCBM, indicating that the charge transfer from perovskite to diPDI takes place effectively at the CH₃NH₃PbI₃/diPDI interface. When the transient PL was measured to examine the average charge carrier lifetimes (Fig. 1b), it reveals that pristine perovskite film shows an average charge carrier lifetime of 38.7 ns, while perovskite/diPDI, perovskite/PCBM films exhibit notably reduced average lifetimes of 1.2 ns and 2.2 ns, respectively. This result leads us to expect that diPDI exhibits the photovoltaic performance comparable to PCBM as an ETL material of inverted perovskite solar cells.

Fig. 1 (a) PL spectra (excited at 520 nm) and (b) transient PL spectra for pristine perovskite, perovskite/diPDI, and perovskite/PCBM films.

Solar cell devices with diPDI as ETL material are fabricated with an inverted configuration of ITO/PEDOT:PSS/CH₃NH₃PbI₃/diPDI/TiO₂/Al. The frontier energy levels of each layer are shown in Fig. 2a. Stepwise alignment of energy levels from CH₃NH₃PbI₃ layer to Al cathode facilitates transport and collection of electrons. The device with diPDI as ETL exhibits a moderate PCE of 7.1% in current density–voltage (J–V) curve (see Fig. 3a and Table 1). The moderate PCE is attributed to low electrical conductivity of diPDI (4.7 × 10⁻¹² S cm⁻¹), which results in poor charge transport in diPDI layer although the charge transfer at the CH₃NH₃PbI₃/diPDI interface is effective. In our previous study, we have found that the electrical conductivity of PCBM is remarkably enhanced upon n-doping by DMBI (from 3.8 × 10⁻⁹ S cm⁻¹ to 6.1 × 10⁻⁵ S cm⁻¹) and thereby the PCE of solar cells is increased from 8.9% to 13.8%.⁸ Similarly, when we add a small amount of DMBI to diPDI, the electrical conductivity of diPDI is increased from 4.7 × 10⁻¹² S cm⁻¹ to 7.6 × 10⁻⁸ S cm⁻¹.

Fig. 2 (a) Energy levels of each layer and (b) chemical structures of diPDI and DMBI. Fermi levels and HOMO energy levels of ETL were determined by UPS.

Fig. 3 (a) J–V curves, and (b) EQE spectra of the devices with pure diPDI and DMBI-doped diPDI with various dopant concentrations as ETL.

Table 1 Photovoltaic performances of devices with diPDI with various dopant concentrations as ETL

Dopant conc. (wt%)	JSC (mA cm^−2)	VOC (V)	FF	PCE^a (%)	Rsh^b (Ω cm^2)	Rs^b (Ω cm^2)
a Average PCE values based on at least 10 devices are indicated in parentheses.b The series and shunt resistance were calculated by the slopes of J–V curves.
0.0	17.2	0.80	0.51	7.1 (6.6)	447	9.7
0.5	21.1	0.85	0.51	9.2 (8.7)	571	9.7
1.0	21.6	0.86	0.54	10.0 (9.3)	526	7.2
3.0	21.0	0.85	0.54	9.6 (9.1)	555	8.7
5.0	20.2	0.81	0.54	8.8 (8.4)	559	8.7

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As the amount of DMBI added to diPDI is increased, the PCE of solar cell device increases first and then decreases showing a maximum at 1 wt% addition (see Fig. 3a and Table 1). When diPDI is doped by 1 wt% of DMBI, the solar cell exhibits the maximum PCE of 10.0% with a J_SC of 21.6 mA cm⁻², a V_OC of 0.86 V and a FF of 0.54. Especially, J_SC is largely increased upon n-doping due to an increased electrical conductivity of diPDI. Further addition of DMBI above 1% decreases the photovoltaic performance probably due to aggregation of DMBI.^8,10 External quantum efficiency (EQE) spectra also demonstrate the effect of n-doping on the current density (Fig. 3b): the devices with n-doped diPDI as ETL show higher EQE values than the device with un-doped diPDI as ETL.

To characterize the effect of n-doping, UV-Vis-NIR absorption and ultraviolet photoelectron spectroscopy (UPS) were measured (Fig. 4 and 5). Absorption spectra of DMBI-doped diPDI films show additional peaks around 700 nm and 800 nm, as shown in Fig. 4, while the peak intensity increases with the doping concentration, indicating that DMBI effectively dopes diPDI by donating an electron to diPDI to form diPDI radical anions.¹¹ When the Fermi level and the highest occupied molecular orbital (HOMO) energy level of diPDI were measured by UPS, the Fermi level of diPDI is up-shifted from −4.95 eV to −4.71 eV upon 1 wt% addition of DMBI, and further addition of DMBI up-shifts further the Fermi level up to −4.61 eV at 5 wt% addition. The up-shift of the Fermi level of diPDI upon addition of DMBI is another definite evidence of n-doping, while the HOMO energy level of diPDI is not significantly changed, as clearly demonstrated in Fig. 2a. Here, the work function is determined by subtracting the binding energy cutoffs in high binding energy region (Fig. 5a) from the He I photon energy (21.22 eV), and the energy gap between the Fermi levels and the HOMO energy level is determined from the binding energy cutoffs in low binding energy region (Fig. 5b). In short, the addition of a small amount of DMBI effectively dopes diPDI to form diPDI radical anion and up-shifts the Fermi level of diPDI, increasing the electrical conductivity, which enhances the electron transporting property of diPDI.

Fig. 4 UV-Vis-NIR absorption spectra of pure diPDI film and DMBI-doped diPDI films with various dopant concentrations. All films are fabricated by spin-coating the chlorobenzene solution on glass.

Fig. 5 UPS spectra of pure diPDI and DMBI-doped diPDI with various dopant concentrations in (a) high binding energy region and (b) low binding energy region. Binding energy cutoffs of pure diPDI and diPDI doped with 1 wt% DMBI are indicated.

When the J–V hysteresis curves of the devices are examined (Fig. S3†), all devices exhibit moderately stable hysteresis while stronger hysteresis is observed at higher doping concentration. Since it has been reported that one of possible reasons for hysteresis is the charge accumulation at the interface,¹² we have assumed that the hysteresis of our device with high doping concentration is caused by an increase of trap sites and accumulated charges due to aggregation of DMBI.

In summary, we first introduced a solution processible non-fullerene electron transporting material, diPDI, for inverted perovskite solar cells. As an alternative to PCBM, diPDI shows electron accepting ability comparable to PCBM, which is evidenced by PL quenching. The solar cell device with diPDI as ETL exhibits a moderate PCE of 7.1% with a J_SC of 17.2 mA cm⁻². Furthermore, when diPDI is doped by 1 wt% DMBI, the efficiency of device with the DMBI-doped diPDI as ETL is remarkably increased to 10.0% with a J_SC of 21.6 mA cm⁻². This enhancement is attributed mainly to increased electrical conductivity of diPDI and up-shift of the Fermi level of diPDI by n-doping. Effective n-doping of diPDI by DMBI was proved by UV-Vis-NIR absorption and UPS measurement. This result leads us to conclude that diPDI is a promising non-fullerene ETL material for inverted perovskite solar cells when diPDI is n-doped by DMBI. In short, this study motivates us to further pursue development of effective non-fullerene electron acceptors for high performance perovskite solar cells.

Acknowledgements

We thank the Ministry of Education, Korea for financial support through the Global Research Laboratory (GRL).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed information about materials, synthesis, device fabrication and characterization methods. See DOI: 10.1039/c5ra27620a

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