Dual Triplet Sensitization Strategy for Efficient and Stable Triplet–Triplet Annihilation Upconversion Perovskite Solar Cells

Open AccessCCS ChemistryRESEARCH ARTICLE11 May 2022Dual Triplet Sensitization Strategy for Efficient and Stable Triplet–Triplet Annihilation Upconversion Perovskite Solar Cells Wangping Sheng†, Jia Yang†, Xiang Li, Jiaqi Zhang, Yang Su, Yang Zhong, Yanda Zhang, Lingyun Gong, Licheng Tan and Yiwang Chen Wangping Sheng† College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 †W. Sheng and J. Yang contributed equally to this work.Google Scholar More articles by this author , Jia Yang† College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 †W. Sheng and J. Yang contributed equally to this work.Google Scholar More articles by this author , Xiang Li College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Jiaqi Zhang College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Yang Su College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Yang Zhong College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Yanda Zhang College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Lingyun Gong College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author , Licheng Tan *Corresponding authors: E-mail Address: email protected E-mail Address: email protected College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Google Scholar More articles by this author and Yiwang Chen *Corresponding authors: E-mail Address: email protected E-mail Address: email protected College of Chemistry and Chemical Engineering, Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031 Institute of Advanced Scientific Research (iASR), Key Lab of Fluorine and Silicon for Energy Materials and Chemistry of Ministry of Education, Jiangxi Normal University, Nanchang 330022 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201798 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The novel triplet–triplet annihilation (TTA) upconversion (UC) field of rubrene (Rub) and dibenzotetraphenylperiflanthene (DBP) sensitized by bulk metal halide perovskite, integrated with copper-2,3,9,10,16,17,23,24-octafluorophthalocyanine (F8CuPc) as cosensitizer, have been investigated in perovskite solar cells (PVSCs) to minimize sub-bandgap photon transmission loss. The firm hydrogen bonding interaction (F···H–N between F8CuPc and MA+), cation-π interaction (MA+ with Rub), and the hydrophobic characteristic of additives enable F8CuPc:Rub:DBP dually-sensitized p-i-n PVSCs based on MAPbI3 and Cs0.05(FA0.83MA0.17)0.95Pb(Br0.17I0.83)3 absorbers to attain champion efficiencies of 20.83% and 21.51%, respectively. Furthermore, due to the excellent photochemical and thermal stability of F8CuPc, the corresponding PVSCs can maintain nearly 80% of the original efficiencies exposed to air with 50∼70% relative humidity over 1100 h and N2 at 85 °C for 300 h. Download figure Download PowerPoint Introduction Organic-inorganic halide perovskite materials have attracted monumental attention from both scientific and industrial communities owing to their impressive optical and electronic traits.1–3 The certified power conversion efficiency (PCE) of perovskite solar cells (PVSCs) has soared from 3.8%4 to the current 25.7%.5 The amelioration of the photovoltaic performance of PVSCs is based on device structure optimization and defects passivation.6 However, the PCE of PVSCs is still restrained by the Shockley-Queisser limit. As is well known, the common perovskite photoactive material CH3NH3PbI3 (MAPbI3) can only absorb the light with a wavelength region in 300–800 nm, which only accounts for 45∼50% of the entire solar spectrum.7 The low energy photons beyond the visible light range are insufficient to excite the semiconductor, implying the severe squandering of energy and potential space for boosting PCE.8 Intriguingly, the photon upconversion (UC) can convert two low-energy photons into a high-energy photon for the further utilization of near-infrared (NIR) light. By permitting the collection of sub-bandgap photons, UC exhibits the potential of spectral widening in the photovoltaic field, which is regarded as a promising strategy to overcome the Shockley-Queisser limit.9 The current methods for UC implementation are mainly via the ladder-like electronic structure of the lanthanide-based nanoparticles or second harmonic generation in nonlinear crystals.10–12 Song et al.13 have synthesized the core/shell structured NaYF4:Yb3+, Er3+@NaYF4:Yb3+, and Nd3+ UC nanoparticles assembled above the SnO2 layer in the triple cations-based PVSCs, achieving the highest PCE of 20.5% based on various UC devices. Compared with the aforementioned traditional UC processes which require high photon flux, the diffusion-mediated triplet–triplet annihilation-based UC (TTA-UC) is ideally appropriate for solar applications at subsolar photon fluxes by capitalizing on the long-lived triplet states.14–16 The TTA process is currently observed in conjugated organic molecules (for instance, polyacene), where the interaction between adjacent spin-triplet states contributes to the presence of the singlet excited state.17 Direct optical excitation into a triplet state is prohibited because of the selection rules. Hence, the excitation of the spin-triplet state demands sensitizer. In the TTA-UC system, the sensitizer is used as the absorber to convert the optically excited singlet to the annihilator triplet. Conventional triplet sensitizers can be mainly categorized as three forms: (1) metal-organic complexes, where the triplet state is populated via intersystem crossing (ISC),18 (2) quantum confined semiconductors (CdSe or PbS nanocrystals), where the organic annihilator can be populated on account of exchange interaction among semiconductor nanocrystals,19,20 and (3) direct singlet-to-triplet absorption metal osmium complexes.21 The novel findings of bulk perovskite materials as an alternative triplet sensitizer has been explored in TTA-UC, which can avoid the drawback of poor exciton diffusion in conventional nanocrystals.22 The combination of long carrier lifetimes and low exciton binding energy makes bulk perovskite films more suitable for TTA-UC in comparison with NCs.23 However, the absorption capacity of single bulk MAPbI3 perovskite sensitizer in the NIR region is inadequate and insufficient for the utilization of NIR light. An efficient triplet sensitizer should be able to absorb a broad spectral range. Metallo-phthalocyanines have the advantage of a large molar extinction coefficient (ε∼105 M−1 cm−1) in the Q band and can be tuned in the range of 650–800 nm to effectively collect NIR photons.24 Therefore, to convert more NIR light into visible light, we firstly employ a photochemically and thermally stable F8CuPc as cosensitizer into PVSCs, which exhibit intense absorption in the NIR region and the multiple fluorine atoms that can form hydrogen bonding with MA+.25,26 Moreover, the suitable energy level alignment between F8CuPc and rubrene (Rub) annihilator is favorable to energy transfer. The annihilator Rub and singlet energy collector dibenzotetraphenylperiflanthene (DBP) have formerly been demonstrated to accomplish effective NIR-to-visible TTA-UC when combined with nanocrystal and metal-organic complexes.27,28 That is, two triplet states are annihilated upon adjoining Rub molecules and then produce a higher energy singlet of DBP to finish the UC process. These visible photons emitted from DBP which were reabsorbed by the perovskite layer to generate more charge carriers, thus effectively enhancing the photocurrent of PVSCs. Furthermore, the novel dual triplet sensitization approach exhibits multiple functions including effective trap passivation, released residual stress, and suppressed ion migration. Herein, the F8CuPc:Rub:DBP-incorporated devices based on MAPbI3 display excellent photovoltaic performance (PCE of 20.83%) accompanied with prominent environmental stability in comparison to the reference device (17.55%). Commendably, the dually triplet sensitized PSVCs without encapsulation can both retain nearly 80% of the initial efficiencies exposed to air with 50∼70% relative humidity (RH) over 1100 h and in nitrogen atmosphere at 85 °C for 300 h, respectively. Meanwhile, the champion efficiency of CsFAMA-based PSVCs is further improved to 21.51%, which is the highest PCE for the PVSCs based on omnigenous UC materials to date. Experimental Methods Materials and sample preparation The solution of NiOx nanoparticles was prepared by dispersing them in deionized water (20 mg mL−1). The MAPbI3 and CsFAMA perovskite precursor was prepared with the reported ratio.6 PC61BM (20 mg mL−1) solution was prepared by dissolving it in chlorobenzene. The bathocuproine (BCP) solution was prepared by dissolving it in ethanol (0.5 mg mL−1). PVSCs fabrication First, the indium tin oxide (ITO) glass substrates were sequentially cleaned with acetone, distilled water, and ultrasonic baths. The NiOx solution was spin-coated on ITO at 2000 rpm for 30 s, followed by annealing at 120 °C for 30 min in air. Then the perovskite solution was spin-coated onto the NiOx layer at 4000 rpm for 30 s, followed with a dropping of conduct band (CB) antisolvent (200 μL, with/without Rub:DBP and F8CuPc:Rub:DBP) after 10 s. The substrate was immediately annealed at 100 °C for 10 min (MAPbI3) or 1 h (Cs0.05 (FA0.83MA0.17)0.95Pb(Br0.17I0.83)3) to form perovskite films. Then the PC61BM solution was spin-coated on perovskite films at 2000 rpm for 30 s. The BCP solution was spin-coated at 4500 rpm for 30 s. Finally, 100 nm Ag was deposited on top by evaporation in high vacuum. Characterization Current density–voltage (J–V) characteristics were measured using a source meter (Keithley 2400, United States), equipped with a light source (100 mW/cm2) under AM 1.5 G filter. The standard silicon solar cell was corrected from National Renewable Energy Laboratory (NREL), and the currents were detected under the solar simulator (Enlitech, Taiwan, China, 100 mW cm−2, AM 1.5 G irradiation). The ultraviolet–visible (UV–vis) spectra was conducted using a SHIMADZU UV-2600 spectrophotometer (Japan). The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were recorded by an Edinburgh Instruments FLS920 spectrometer (United Kingdom). Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu IRPrestige-21 spectrometer (Japan). Scanning electron microscopy (SEM) measurements were conducted on SU8020 scanning electron microscope (Japan). X-ray diffraction (XRD) measurements were recorded by a Rigaku D/Max-B X-ray diffractometer (Japan) with Bragg-Brentano parafocusing geometry. The samples were coated on the highly conductive ITO substrates. The ultraviolet photoelectron spectroscopy (UPS) measurements were performed in an ESCALAB 250Xi, Thermo Fisher (United States) (by using Al Kα X-ray source) under high vacuum (10−9 mbar), and the energy resolution was 100 meV. X-ray photoelectron spectroscopy (XPS) measurements were also carried out using a Thermo Scientific ESCALAB 250Xi photoelectron spectrometers (United States), and the energy resolution was 450 meV. Results and Discussion The addition of cosensitizer enables us to harvest photons from the broader wavelength range to improve the feasibility of UC for photovoltaics. Herein, we employ F8CuPc and bulk MAPbI3 perovskite to sensitize Rub and fabricate a dual-sensitizer TTA-UC system to upconvert NIR photons to visible light (Figure 1a). In Supporting Information Figure S1, the ultraviolet–visible (UV–vis) absorption spectrum of F8CuPc exhibits broad light absorption, extending to the NIR region. The distinguishing light absorption scopes of F8CuPc and MAPbI3 are ideal for the intention of this work by permitting the utilization of the larger proportion of the solar spectrum. In particular, it should be emphasized that all types of practical solar cells are definitely resistant to a certain amount of thermal stress. For this reason, highly chemically and thermally stable metallo-phthalocyanines sensitizer is a prerequisite for the further application of the dual triplet sensitization strategy in PVSCs. The addition of F8CuPc cosensitizer not only passivates the defects in grain boundaries (GBs)/on the surface of the perovskite, but it also forms a hydrophobic and thermotolerant structure for excellent device stability (Figure 1b).29,30 The thermogravimetric analysis (TGA) curves prove that the dually sensitized perovskite has superior thermal stability compared with the reference (Figure 1c). To deeply understand the passivation influence of the dual sensitization approach, we have conducted XPS analysis. Initially, the fluorine element from F8CuPc is identified in the corresponding sample, verifying the presence of cosensitizer (Figure 1d). In addition, the variation of Pb peaks toward lower binding energy for F8CuPc:Rub:DBP-treated perovskite demonstrates the coordination between the uncoordinated Pb in perovskite and the N atom in F8CuPc, which enables the passivation of defects in GBs (Figure 1e).31 To further verify the chemical interaction formed at the interface between F8CuPc and perovskite, the FT-IR spectra are carried out and reveal that the tough hydrogen bonding between fluorine atoms and CH3NH3+ (MA+) can immobilize MA+ and ensure a surprising thermal stability (Figure 1f). The strongly enhanced thermal stability of PVSCs should be attributed not only to the thermal stability of F8CuPc itself but also to the strong chemical interaction between F8CuPc and perovskite. The bulk perovskite and F8CuPc dually sensitized TTA UC scheme’s energetic diagram accompanied by its constituents is depicted (Figure 1g). Two triplet sensitization mechanisms exist simultaneously: (I) sequential electron and hole transfer resulting in a bound Rub triplet and (II) a Dexter-type triplet energy transfer (TET) process. Due to strong spin-orbit coupling, the cosensitizer F8CuPc undergoes ISC to a long-lived spin triplet, and the energy is transmitted to the Rub triplet state via a Dexter process.32 Recombination of two adjacent Rub triplet states can contribute to the generation of a higher energy singlet state. The singlet is subsequently collected by DBP through Förster resonance energy transfer (FRET).33 Upon relaxation of the excited singlet state in DBP, the upconverted photons are emitted which can then be reabsorbed by the perovskite absorber. Figure 1 | (a) The PVSC device architecture. (b) Schematic diagram of defect passivation and enhanced stability mechanism for F8CuPc:Rub:DBP-incorporated device. (c) TGA curves of the reference and F8CuPc:Rub:DBP-treated perovskite. (d and e) XPS spectra of F 1s and Pb 4f for the perovskite films with and without F8CuPc:Rub:DBP. (f) FT-IR spectra of reference and F8CuPc:Rub:DBP-treated perovskite. The hydrogen bonding of F⋯H forces the N–H stretching vibration band at ∼3190 cm−1 shift to lower wavenumber. (g) Scheme for the TTA-UC mechanism dually sensitized by MAPbI3 perovskite and F8CuPc. (1) Band diagram of MAPbI3 (sensitizer I) and Rub show the holes can be readily extracted from the valance band of MAPbI3 to the highest occupied molecular orbital (HOMO) of Rub. Due to the existence of holes in Rub, free electrons transfer to the triplet state (T1) by Coulomb interaction. (2) A ground-state F8CuPc (sensitizer II) molecule absorbs a low-energy photon, then undergoes ISC to the triplet state. (3) The energy from F8CuPc triplet is transferred through the Dexter-type TET process to Rub annihilator ground state, which is then transferred to its triplet state. (4) TTA occurs among adjoining Rub triplet states. (5) The derived singlet state can be collected by the DBP. (6) MAPbI3 can reabsorb the visible light emitted from DBP via FRET process. Download figure Download PowerPoint The charge transfer dynamics of perovskite are further investigated by employing steady-state PL and TRPL spectra measurements. In Supporting Information Figures S2a and S2b, the F8CuPc:Rub:DBP-derived perovskite deposited with 6,6-Phenyl-C61-butyric acid methyl ester (PCBM) displays a lower PL intensity and shorter carrier lifetime (the corresponding parameters are listed in Supporting Information Table S1), accompanied by a bluer PL emission peak, indicating a reduction of trap states in perovskite films.34 The stronger quenched PL demonstrates the more effective charge extraction and reduced recombination process in the F8CuPc:Rub:DBP-modified PVSCs, and the lessened weighted average carrier lifetime indicates effective charge-carrier transmission. Furthermore, the carrier extraction behavior is also influenced by the alignment of energy levels (Figure 2a). The UPS measurement is utilized to analyze the electronic structures of the reference, Rub:DBP, and F8CuPc:Rub:DBP-treated perovskite films (Figures 2b–2d), and the band structure values are exhibited in Supporting Information Table S2. The CB of the reference perovskite is −3.88 eV while the CB for Rub:DBP and F8CuPc:Rub:DBP-incorporated perovskites are −3.89 and −3.91 eV, respectively. Since the lowest unoccupied molecular orbital (LUMO) level of the carrier transport layer PCBM is −4.20 eV, the incorporation of F8CuPc:Rub:DBP can minimize the energy level mismatch between PCBM and perovskite and show better-matched energy band alignment, contributing to the open circuit voltage (Voc) enhancement. Moreover, the incorporation of F8CuPc:Rub:DBP has modified the characteristics of carrier transport, as verified by the PL and TRPL spectra. For further confirmation, the space charge-limited current (SCLC) measurements are carried out to evaluate the trap density and carrier mobility ( Supporting Information Figure S3). The trap state density is determined by the trap-filled limited voltage (VTFL), and the carrier mobilities can be derived from the Mott-Gurney law.35 The electron trap densities ntrap of the reference, Rub:DBP, and F8CuPc:Rub:DBP-incorporated devices are 9.89 × 1015, 6.73 × 1015, and 5.94 × 1015 cm−3, respectively. The lowest trap density for the F8CuPc:Rub:DBP-modified device shows the superior perovskite quality and passivation effect. The electron mobility of the F8CuPc:Rub:DBP-incorporated device is 7.67 × 10−1 cm2·V−1·s−1, which is higher than those of the reference and Rub:DBP-incorporated samples (4.86 × 10−1 cm2·V−1·s−1 and 4.95 × 10−1 cm2·V−1·s−1, respectively). The corresponding parameters are shown in Supporting Information Table S3. The aforementioned consequences imply that the introduction of F8CuPc:Rub:DBP can passivate defects and prompt the efficient transmission of charge carriers, which is anticipated to improve Voc. Figure 2 | (a) Schematic illustration of the carrier transport in PVSCs with the Rub:DBP and F8CuPc:Rub:DBP treatment. UPS spectra for the (b) reference perovskite, (c) perovskite with Rub:DBP, and (d) perovskite with F8CuPc:Rub:DBP. Cross-sectional SEM images of PVSCs based on (e) reference, (f) Rub:DBP, and (g) F8CuPc:Rub:DBP-treated perovskite layers, respectively. Download figure Download PowerPoint To examine the efficacy of F8CuPc:Rub:DBP on the perovskite film morphology, the top view SEM and cross-sectional SEM are used. In Supporting Information Figure S4a, the reference sample is composed of relatively small grains. At the low Rub concentration of 1 or 5 mg mL−1 ( Supporting Information Figures S4b and S4c), the MAPbI3 perovskite grains are still obvious, along with bigger grain size. These longer needles coating the perovskite film are likely ascribed to the Rub agglomerates. The underlying perovskite film becomes blurry at higher Rub concentration ( Supporting Information Figure S4d). Furthermore, compared with the disordered grains in the reference PVSCs, the cross-sectional SEM results of the Rub:DBP, and F8CuPc:Rub:DBP-modified devices demonstrate the vertically extended and larger dense grains, which facilitate the carrier transport (Figures 2e–2g). The XRD test is utilized to confirm the vertical orientation growth of perovskite grains. In Supporting Information Figure S5, all the films show the same diffraction peaks relevant to MAPbI3, demonstrating that the Rub:DBP and F8CuPc:Rub:DBP additives can only exist in GBs/on the surface of the perovskite. Integrated with cross-sectional SEM results, the strongest (110) peak suggests that the perovskite grains with superior crystallinity vertically traverse the entire F8CuPc:Rub:DBP-modified perovskite the enhanced of (110) and peaks further demonstrates the orientation of perovskite Moreover, the X-ray are to the perovskite structure and the The reference perovskite displays characteristic diffraction at and relevant to the and of MAPbI3 (Figure The for the (110) for the reference perovskite shows the diffraction with in and out of the a In the a diffraction with a in the perovskite with which demonstrates crystallinity compared with the Rub:DBP-incorporated sample (Figures and In addition, Supporting Information Figure displays the integrated from the along the (110) at 10 the F8CuPc:Rub:DBP-modified perovskite film demonstrates the growth orientation along the and the intensity of is lower than those of the reference and the existence of a crystallinity with a vertical growth To the of Rub:DBP and F8CuPc:Rub:DBP incorporation on the surface we have the X-ray diffraction to analyze the residual on the perovskite surface (the of to of 30 In the of the reference and the characteristic peaks shift to lower by from 10 to which are mainly attributed to the by (Figures and By a peak shift in F8CuPc:Rub:DBP-incorporated perovskite is which indicates released (Figure we can demonstrate that the effective passivation of the uncoordinated defects and the formed hydrogen owing to the interaction between F8CuPc:Rub:DBP and the perovskite are for the Furthermore, the F8CuPc:Rub:DBP-incorporated perovskite film can the residual based on the lower of ( Supporting Information Figure Figure | and with values of and reference, and e) Rub:DBP and and F8CuPc:Rub:DBP-modified perovskite films. Download figure Download PowerPoint To verify the aforementioned FRET the absorption of DBP and the PL spectrum of Rub are shown in Figure The emission band of Rub at nm and with the DBP absorption As the Rub and DBP spectrum are a FRET process Then the DBP energy to the perovskite via the FRET process which to the of the perovskite thus the Furthermore, the of F8CuPc:Rub:DBP on the photovoltaic performance of PVSCs is on the concentration of Rub is 10 mg and the Rub:DBP is at The curves of PVSCs with of F8CuPc the concentration of 2 mg mL−1 ( Supporting Information Figure and Table which is in the Figure shows the curves of reference, Rub:DBP and F8CuPc:Rub:DBP-modified devices. champion photovoltaic parameters are in Table the reference device a PCE of with a of cm−2, a of and a of Compared with the reference and the PCE of the F8CuPc:Rub:DBP-incorporated device has to with a of a of cm−2, and an of accompanied by a ( Supporting Information Figure The of the photovoltaic parameters in and after F8CuPc:Rub:DBP is mainly ascribed to the carrier transport, and lessened defect To understand the source of the of from the F8CuPc:Rub:DBP dually-sensitized PVSCs, the quantum efficiencies are the of PVSCs with F8CuPc:Rub:DBP is higher compared with the reference and PVSCs over the nm region (Figure Furthermore, the of the F8CuPc:Rub:DBP-modified device is enhanced in the NIR region. the peak in nm, which to the DBP emission wavelength ( Supporting Information Figure Moreover, the device F8CuPc:Rub:DBP higher photocurrent and PCE at a (Figure To the for the of the absorption and UC PL spectra are carried In Figure a absorption in the wavelength range is observed in the F8CuPc:Rub:DBP-modified sample owing to the larger and the FRET process. In Figure the steady-state UC PL spectra under nm excitation for the reference, and dually-sensitized samples are The visible emission from DBP is at nm, which the FRET process between DBP and the perovskite. To the of the of PVSCs photovoltaic performance via dually-sensitized the PCE can be in Supporting Information Figure The dually-sensitized PVSCs demonstrate excellent PCE with a demonstrating To understand the carrier recombination in current and have been carried In Supporting Information Figure the of for the reference and modified PVSCs are The F8CuPc:Rub:DBP-modified device shows the lowest current density compared with the reference and The current and charge carrier recombination can contribute to the on photovoltaic Furthermore, the and measurements on intensity are shown in Supporting Information Figure The values are from are and for the reference, Rub:DBP, and F8CuPc:Rub:DBP-incorporated PVSCs, demonstrating the suppressed recombination by the incorporation of F8CuPc:Rub:DBP. The light intensity a of