Optimization of perovskite by 3D twisted diketopyrrolopyrrole for efficient perovskite solar cells

Abstract

A solution-processable, aggregation-induced emission-type three-dimensional molecule TPE-DPP₄ was synthesized in a facile way. TPE-DPP₄ can function as a light-capturer, grain-boundary filler as well as an electron-donor for perovskite + TPE-DPP₄ bulk heterojunction hybrid film. The perovskite solar cells obtained with TPE-DPP₄ resulted in enhanced power conversion efficiency of 14.1% with 40% enhancement to the device compared with pristine perovskite.

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CH₃NH₃PbX₃-based materials (X = Cl, Br, I) possess some unique properties:¹ intense light absorption across the visible spectrum; high charge carrier mobility; direct band gaps; long carrier diffusion length; and small exciton binding energy.^2,3 Hence, perovskite solar cells (PSCs) based on CH₃NH₃PbX₃ materials have garnered increasing attention. The power conversion efficiency (PCE) of PSCs has developed rapidly from the first reported value of 3.81% in 2009⁴ to >20% recently.^5,6

However, the performance of PSCs is strongly dependent on interface engineering,^7,8 compositional optimization,^6,9–14 solvent selection,^15–17 and device fabrication.^18–20 The quality of perovskite morphology is also a major factor for determining the performance of PSCs. Some pioneering approaches have been postulated to improve the quality of perovskite morphology: the solution-based deposition method;²¹ vapor-based deposition method;^19,22–25 and addition of additive.^26–30 Recently, Chun-Guey Wu and coworkers employed [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) to prepare PbI₂–PCBM hybrid films from N,N-dimethylformamide (DMF) solution by a two-step spin-coating method to obtain high-quality bulk heterojunction (BHJ) perovskite PCBM films with a high fill factor (FF) of 82%. They also demonstrated that BHJ perovskite PCBM films possessed higher conductivity, greater balance in electron and hole mobility, and longer charge diffusion length than pure perovskite film. However, perovskite PCBM material also has several disadvantages: (i) PCBM is too expensive for roll-to-roll mass production; (ii) very poor solubility of PCBM in DMF (precipitation is observed in only 1 wt% concentration of PCBM); (iii) poor light absorption of PCBM limits further improvement in device performance.

Inspired by the success of PCBM as an additive in perovskite film and to overcome its disadvantages, in this work we developed a light-harvesting three-dimensional (3D) small molecule TPE-DPP₄ based on tetraphenylethylene (TPE) and diketopyrrolopyrrole (DPP) as a dopant in perovskite film. TPE is a typical aggregation-induced emission (AIE) molecule with a 3D structure,³¹ whereas DPP possesses high charge mobility, strong light absorption as well as good solubility.³² Therefore, with TPE as a core and DPP as an arm, 3D TPE-DPP₄ has a similar role to 3D fullerene for improving the quality of the perovskite layer by filling pinholes and vacancies between perovskite grains. Simultaneously, solubility and light absorption were also obviously enhanced. More interestingly, the 3D molecule TPE-DPP₄ could function as a light-absorber as well as an electron-donor to transfer electrons to perovskite, which makes the light-absorbing layer a perovskite + TPE-DPP₄ BHJ hybrid film. As a consequence, the prepared PSCs blend with TPE-DPP₄ obtained resulted in a PCE of 14.1% with 40% enhancement to the device compared with pristine perovskite (10.6%), accompanied with an overall improvement in device parameters. This method provides a new, effective strategy to improve device performance by appending solution-processed 3D molecules as additives to the perovskite layer.

The synthetic routes to TPE-DPP₄ are depicted in Scheme 1 (experimental details and the adopted synthetic route are provided in ESI†). A 3D twisted molecule based on TPE-DPP₄ was obtained through Suzuki coupling between DDP-based DPP boric acid ester (compound 2) and tetra-brominated TPE (TPE-Br₄). The resulting TPE-DPP₄ was purified by column chromatography to obtain the product as a blue-black powder and the yield reached 80%. The structure of TPE-DPP₄ was confirmed by time-of-flight mass spectroscopy as well as ¹H and ¹³C NMR spectroscopy (related data of the intermediates and final product are shown in Fig. S1 and S2 of ESI†). Compared with the synthetic route of TPE-DPP₄ depicted by Bhosale and coworkers³³ (in which the TPE was first converted to TPE boric acid ester, followed by Suzuki coupling with brominated DPP), our method is simpler with a much higher yield. Compared with PCBM, which has poor solubility in common organic solvents, TPE-DPP₄ has excellent solubility in DMF, hexane, dichloromethane, chloroform and tetrahydrofuran. The distinctly different solubility of TPE-PP₄ and PCBM in DMF were compared (Fig. S3, ESI†). TPE-DPP₄ (1 mg ml⁻¹) showed a clear and stable solution, whereas a large amount of precipitate was formed in 0.5 mg ml⁻¹ of PCBM. In addition, TPE-DPP₄ possessed a similar 3D molecular structure to that of PCBM, and the 3D configuration of TPE-DPP₄ was simulated by density functional theory (Fig. S4, ESI†).

Scheme 1 Synthetic routes of TPE-DPP₄.

The manufacturing procedure and architecture of PSCs are presented in Fig. 1. To ensure reproducibility of the performance, a conventional structure of FTO/compact-TiO₂(c-TiO₂)/mesoporous-TiO₂(mp-TiO₂)/CH₃NH₃PbI₃ + TPE-DPP₄/spiro-MeOTAD/Ag was employed. The perovskite hybrid film was fabricated by a two-step method. TPE-DPP₄ served as an additive with concentrations of 0.01 and 0.02 wt% incorporated into PbI₂ DMF solution to favor the growth of PbI₂ film in the first step, followed by dipping into CH₃NH₃I (MAI) solution in the second step and annealing at 105 °C for 10 min. TPE-DPP₄ also possesses a similar 3D structure to PCBM, so it may have a positive effect on the quality of CH₃NH₃PbI₃.

Fig. 1 The spin-coating process for fabrication of perovskite solar cells with TPE-DPP₄ using the structure of glass/FTO/c-TiO₂/mp-TiO₂/CH₃NH₃PbI₃ + TPE-DPP₄/spiro-MeOTAD/Ag.

The morphology of the PbI₂ + TPE-DPP₄ hybrid films was first analyzed by scanning electron microscopy (SEM). Fig. 2(a–c) shows the representative morphological evolution of PbI₂ films using different concentrations of TPE-DPP₄. Both the size and number of holes decreased when a small amount of TPE-DPP₄ was added to the PbI₂ solution. A denser film could be attributed to TPE-DPP₄ prolonging the precipitation/crystallization of the PbI₂ film. Meanwhile, TPE-DPP₄ could fill the hole of the PbI₂ film in a similar to that of PCBM. The morphology of the PbI₂ film (with and without TPE-DPP₄) had a considerable impact on the morphology of the resulting perovskite film after addition of MAI, as displayed in Fig. 2(d–f). Incompact packing of CH₃NH₃PbI₃ was significantly reduced and resulted in a more uniform and denser film when a small amount of TPE-DPP₄ was added to the PbI₂ solution. The perovskite + TPE-DPP₄ hybrid films had a larger grain size than the pure perovskite film, which would be beneficial for the generation and transportation of charge carriers. In addition, the morphology of the perovskite films with or without TPE-DPP₄ using atomic force microscopy (AFM) is shown in Fig. S5(a–c) (ESI†). The AFM results indicated that the film prepared with the TPE-DPP₄ additive was more uniform with a larger domain size than the untreated perovskite film. This morphology characterization indicate that the introduction of TPE-DPP₄ additive in the PbI₂ solution can improve the film uniformity and perovskite crystallinity, two key requirements for high-performance PSCs.

Fig. 2 Scanning electron micrographs of different film types. (a) Pure PbI₂ film. (b) PbI₂ + 0.01 wt% TPE-DPP₄ hybrid film. (c) PbI₂ + 0.02 wt% TPE-DPP₄ hybrid film. (d) Pure perovskite film. (e) Perovskite + 0.01 wt% TPE-DPP₄ film. (f) Perovskite + 0.02 wt% TPE-DPP₄ film.

To investigate the change in crystallinity, X-ray diffraction (XRD) was performed (Fig. 3(a)). XRD patterns showed that the perovskite + TPE-DPP₄ hybrid films and pure perovskite film had a similar profile, but the peak intensity was distinctly different between perovskite + TPE-DPP₄ hybrid films and the pure perovskite film. The TPE-DPP₄-incorporated perovskite films showed a higher diffraction peak intensity than the pure perovskite film at 15.2°, 29.2° and 32.7° assigned to (110), (220) and (310) diffractions of the tetragonal CH₃NH₃PbI₃ phase, respectively. The enhanced intensity of the diffraction peak manifested as improved crystallinity of the CH₃NH₃PbI₃ film. Extraordinarily, 0.01 wt% TPE-DPP₄ showed the highest diffraction peak intensity. When the concentration of TPE-DPP₄ increased from 0.01 wt% to 0.02 wt%, the peak intensity weakened, implying that high loading of TPE-DPP₄ slightly affected the formation of the perovskite film. XRD results were in accordance with the SEM images.

Fig. 3 (a) X-ray diffraction spectra of pure perovskite and perovskite + TPE-DPP₄ hybrid films with various TPE-DPP₄ concentrations. (b) UV/Vis absorption spectra of pure perovskite and perovskite + TPE-DPP₄ hybrid films with various TPE-DPP₄ concentrations. (c) Steady-state photoluminescence spectra of pure perovskite and perovskite + TPE-DPP₄ hybrid films with various TPE-DPP₄ concentrations. (d) Time-resolved photoluminescence measurements of pure perovskite film and perovskite + TPE-DPP₄ hybrid films with various TPE-DPP₄ concentrations.

The ultraviolet-visible (UV-vis) spectra of the pure perovskite film and perovskite + TPE-DPP₄ films were further characterized. As shown in Fig. 3(b), all the perovskite films had strong absorption in the visible light range of 400–800 nm. Relative to pristine perovskite film, the perovskite film with TPE-DPP₄ had stronger absorption, especially in the case of perovskite film with 0.01 wt% TPE-DPP₄. The improved crystallinity of the TPE-DPP₄-treated perovskite film as well as the good light absorption of TPE-DPP₄ both made this contribution, which can ensure considerate current density in device performance.^29,30

Fig. 3(c) shows the steady-state photoluminescence (PL) spectra of the pure perovskite film and perovskite + TPE-DPP₄ hybrid films. When the films were excited at 400 nm (which had limited depth of penetration in the perovskite films), the PL emission peak appeared at 800 nm. It is obvious that the addition of the TPE-DPP₄ additive dramatically quenched the intensity of the steady-state PL of the perovskite film, indicating a more efficient photo-generated electron separation in the perovskite + TPE-DPP₄ hybrid films. To explore the true origin of quenched PL, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level were determined by cyclic voltammetry (CV). As displayed in Fig. S6(a) and Table S1 (ESI†), the HOMO and LUMO energy levels of TPE-DPP₄ were estimated to be −5.26 eV and −3.63 eV, respectively. The valence band maximum and conduction band minimum of the CH₃NH₃PbI₃ film were −5.43 eV and −3.92 eV, respectively. It was clearly observed that the energy level of TPE-DPP₄ was well aligned with CH₃NH₃PbI₃ (Fig. S6(b), ESI†). Therefore, TPE-DPP₄ can act as an electron-donor to transfer electrons to CH₃NH₃PbI₃ and provide an additional electron separation channel for PSCs. The active layer worked as BHJ CH₃NH₃PbI₃ + TPE-DPP₄ hybrid films rather than a pure light-active CH₃NH₃PbI₃. Thereby, a more efficient electron transfer was observed by the dramatic PL quenching. To further understand the charge separation processes, the time-resolved PL was measured, as illustrated in Fig. 3(d). The perovskite +0.01 wt% TPE-DPP₄ film give a shortest PL lifetime (4.87 ns) compared with the pristine perovskite film (5.01 ns) and perovskite +0.02 wt% TPE-DPP₄ film (5.02 ns). The shortest PL lifetime was ascribed to the reduced charge trap state realized by enlarging the grain size and filling grain boundary of perovskite with the help of TPE-DPP₄.

To explore the effect of TPE-DPP₄ on PSCs performance, a device fabrication process is given in Fig. 1. The current density–voltage (J–V) curves of the device are shown in Fig. 4(a) and the relevant photovoltaic parameters are shown in Table 1. The photovoltaic performance varied with different concentrations of TPE-DPP₄. For the pure CH₃NH₃PbI₃ PSCs without TPE-DPP₄, a PCE of 10.06% with an open circuit voltage (V_OC) of 0.94 V, a short-circuit photocurrent density (J_SC) of 20.06 mA cm⁻², and a FF of 56.1% was observed. Intriguingly, the PCEs of the perovskite films with TPE-DPP₄ additive devices were all enhanced compared with that of the control device, and the best PCE of 14.1% was achieved in the device with 0.01 wt% TPE-DPP₄. In particular, all the device parameters improved with regard to FF (56.1% to 65.9%), J_SC (20.06 to 21.04 mA cm⁻²) and V_OC (0.94 to 1.02 V). The enhanced V_OC, J_SC and notable FF could be related to better crystallization and the perovskite film. The electron mobility of the pure and perovskite + TPE-DPP₄ hybrid films was determined by the space-charge limited current method, as shown in Fig. 4(b). Clearly, with the addition of TPE-DPP₄, the electron mobility of the perovskite + TPE-DPP₄ hybrid films (1.1 × 10⁻⁵ cm² V⁻¹ s⁻¹ for 0.01 wt% TPE-DPP₄ and 1.0 × 10⁻⁵ cm² V⁻¹ s⁻¹ for 0.02 wt% TPE-DPP₄) was one order of magnitude higher than that of pure perovskite film (1.0 × 10⁻⁶ cm² V⁻¹ s⁻¹). The improved mobility contributed to the higher J_SC and FF. In Fig. 4(c), the device incorporated with perovskite +0.01 wt% TPE-DPP₄ shows a PCE of 14.1% from the reverse scan, whereas the forward scan yields 12.1%. The perovskite +0.02 wt% TPE-DPP4 hybrid film had a similar hysteresis (Fig. S7, ESI†). Fig. 4(d) shows the device stability of a 0.01 wt% TPE-DPP₄-based PSC under ambient conditions. It can be seen that the photocurrent and PCE stabilizes within seconds to approximately 17.33 mA cm⁻² and 11.67%, probably due to TPE-DPP₄ reacting with moving ions, just like PCBM.^34,35 The integrated current density values from the external quantum efficiency (EQE) (Fig. S8, ESI†) curves of the perovskite, perovskite +0.01 wt% TPE-DPP₄ and perovskite +0.02 wt% TPE-DPP₄ were 17.86, 19.07 and 18.03 mA cm⁻², respectively. Furthermore, the maximum values of the J_SC from the EQE spectra were as almost the same as the J–V measurements even though there was little difference between the EQE and J–V measurements. This was because our tests were done under air atmosphere, leading to the decay of the device. Finally, the improved efficiencies of the devices were mostly attributed to the FF. Based on the above data and analysis, we believe that the photocurrents and efficiencies are credible.

Fig. 4 (a) J–V curves of perovskite solar cells based on pure perovskite and perovskite + TPE-DPP₄ films with various TPE-DPP₄ concentrations with a scan rate of 150 mV s⁻¹. (b) J–V curves of electron-only devices based on pure perovskite and perovskite + TPE-DPP₄ films with different TPE-DPP₄ concentrations. (c) Forward and reverse scan J–V curves for a perovskite +0.01 wt% TPE-DPP₄ perovskite solar cell. (d) Device stability of a 0.01 wt% TPE-DPP₄-based perovskite solar cell under ambient conditions.

Table 1 Photovoltaic parameters of perovskite + TPE-DPP₄ solar cells with various amounts of TPE-DPP₄ in perovskite film

Active layer	J SC/mA cm^−2	V OC/V	FF/%	η/%
Perovskite +0 wt%	20.06	0.94	56.1	10.6
Perovskite +0.01 wt%	21.04	1.02	65.9	14.1
Perovskite +0.02 wt%	20.61	1.01	55.6	11.6

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In conclusion, we developed a novel 3D molecule, TPE-DPP₄, as an additive for CH₃NH₃PbI₃ to improve the performance of PSCs. It was found that the TPE-DPP₄ appended to a CH₃NH₃PbI₃ film could effectively improve the crystallinity of CH₃NH₃PbI₃ by filling pinholes and vacancies between perovskite grains. TPE-DPP₄ can also act as an additional electron-donor and light-capturer in perovskite. As a result, the PCE of PSCs with a TPE-DPP₄ additive reached 14.1% with 40% enhancement compared with the pure perovskite device. This performance was not as high as the values reported by state-of-art devices but, due to the conventional device structure, higher efficiency could be achieved upon combination with device optimization. More importantly, this method provides a new, effective strategy to improve device performance by appending solution-processed 3D molecules as additives to the perovskite layer.

Acknowledgements

This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), National Natural Science Foundation of China (51302130, 51663015, and 51673092), and Graduate Innovation Fund Projects of Jiangxi Province (YC2016-S005).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6qm00210b

‡ B. Huang and Q. Fu contributed equally to this work.

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