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Abstract
We reported the basic optical properties of a triple cation CsFAMA perovskite film and its application in the inverted p-i-n solar cells. The exciton binding energy of 42 meV and the refractive index of 2.4 is obtained from the temperature-dependent photoluminescence spectra and spectroscopic ellipsometry measurement, respectively. The results indicate that CsFAMA lead halide perovskite is an excellent light-absorbing material. The inverted p-i-n CsFAMA perovskite solar cells with PMMA passivation layer are studied, and the optimized PCE can be increased to 16.90% with a negligible hysteresis effect. The long-term and thermal stabilities of CsFAMA perovskite solar cells can be improved after PMMA passivation, which maintains 81% (40% relative humidity, 25 °C, 720 h) and 91% (50% relative humidity, 100 °C, 6 h) of initial efficiencies, respectively. This work provides a promising method for stable and low-cost inverted perovskite solar cells.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The organic-inorganic hybrid metal halide perovskite light-harvesting materials are regarded as promising candidates for next-generation energy materials due to their excellent optoelectronic properties, such as large absorption coefficient, long electron-hole diffusion length, high carrier mobility, tunable bandgap, and so on [1–3]. The triple cation Cs0.05FA0.79MA0.16PbI2.52Br0.48 (CsFAMA) perovskite was reported and widely accepted as the light absorption material for typical n-i-p perovskite solar cells (PSCs), due to its narrow bandgap and good stability [4]. However, the basic optical constants of the triple cation CsFAMA lead halide perovskite film, such as refractive index, extinction coefficient, and exciton binding energy have not been studied, which are crucial parameters for evaluating its feasibility for the high-performance optoelectronic devices. Moreover, few reports could be found for the stable and efficient inverted p-i-n CsFAMA perovskite solar cell devices that can be easily fabricated by the one-step spin-coating method [5–7]. Recently, the surface passivation method is used to improve the performance of perovskite solar cells by depositing ligand materials to passivate the perovskite layer. For example, the trioctylphosphine oxide (TOPO) and triphenylphosphine oxide (TPPO) ligands were used to passivate the surface defects of perovskite films via the chemical interactions between O atom of P = O bond and defect sites in perovskite films [8,9]. In addition, the ultrathin poly (methyl methacrylate) (PMMA) passivation layer was incorporated into the n-i-p perovskite solar cells to increase the open-circuit voltage (Voc) and improve stability by reducing surface traps/defects and increasing carrier extraction [10]. Compared with TPPO and TOPO molecules, the hydrophobic PMMA polymer could protect effectively perovskite from oxygen and moisture, and exhibit better tensile strength and transparency [11]. We can speculate that PMMA can also passivate the CsFAMA perovskite active layer in the inverted p-i-n solar devices.
In this work, we investigate the basic optical constants of the triple cation CsFAMA perovskite film and improve the efficiency and stability of the CsFAMA perovskite solar cells by inserting a PMMA passivation layer. The spectroscopic ellipsometry (SE) and temperature-dependent PL spectra reveal the refractive index, extinction coefficiency, and exciton binding energy of the CsFAMA perovskite. In addition, the CsFAMA perovskite solar cells (PSCs) with the PMMA passivation layer are systematically studied. The best PCE can be increased to 16.90% when 4 mg/ml PMMA solution is used to passivate the perovskite films, which is attributed to the improved surface morphology and the increased short-circuit current (Jsc). Moreover, the long-term stability of the CsFAMA PSCs can be improved after PMMA passivation, which retains 81% of its initial efficiency after 720 h of storage at room temperature with 40% relative humidity (RH), and also maintains about 91% of initial efficiency even under a high temperature of 100 °C for 6 h with 50% RH. The results demonstrate that the PMMA passivated CsFAMA PSCs can present an improved efficiency and stability.
2. Experimental section
In this experiment, the etched ITO substrates were cleaned with detergent solution, deionized water, acetone, and anhydrous ethanol for 15 min, respectively, and then dried in an oven. The PEDOT: PSS solution was spin-coated on the ITO substrates at 4500 rpm for 50 s and then annealed at 140 °C for 30 min. The CsFAMA film was deposited on the ITO/PEDOT: PSS substrates via the one-step spin-coating method at 1000 rpm and 5000 rpm for 10 s and 30 s, respectively [12]. And the chlorobenzene (CB) antisolvent was used to improve the crystal growth, then the perovskite films were annealed at 120 °C for 30 min. The PMMA solution was used to passivate the surface of perovskite films. The PC61BM electron transport layer (ETL) was spin-coated on the PMMA at 3000 rpm for 20 s. Bphen solution was further spin-coated on the ETL at 4000 rpm for 60 s and annealed. Finally, the Ag electrode was fabricated by thermal evaporation. The basic optoelectronic properties of the CsFAMA perovskite films were characterized. The refractive index and extinction coefficiency of spectroscopic ellipsometry (SE-VM, Wuhan Eoptics Technology Co., Ltd 245-1000 nm), as well as the exciton binding energy of the temperature-dependent photoluminescence system (SP 2500i, Acton, Janis 150c) were characterized. The photovoltaic properties and stabilities were measured by the current density-voltage (J-V) measurements (Newport, Oriel Class AAA, Keithley 2400). The detailed device fabrication procedures can be found in the supporting information.
3. Results and discussion
3.1 Optical properties of CsFAMA film
Figure 1(a-b) shows the surface morphologies of the triple cation CsFAMA film. The scanning electron microscope (SEM) and atomic force microscopy (AFM) images indicate that the CsFAMA perovskite thin film has a conformal and homogeneous surface morphology.
Spectroscopic ellipsometry (SE) is employed to comprehensively study the basic optical parameters of the CsFAMA perovskite film over a broad spectral range. Please note that the CsFAMA perovskite thin film is deposited on a silicon substrate. The refractive index (n) and extinction coefficient (κ) are obtained by analyzing the ellipsometric parameters (Ψ and Δ), the experimental and the fitted results are shown in Fig. 2(a-b). In the SE analysis, a four-phase optical model consisting of ambient/surface roughness layer/perovskite film/Si substrate is used. The fitted thickness of the CsFAMA light-absorbing layer is 298 nm. The simulated spectra of Ψ and Δ are in good agreement with the experimental spectra, as shown in Fig. 2(a). In addition, the refractive index of the CsFAMA perovskite film is 2.44, which exhibits a lower refractive index than that of MAPbI3, FAPbI3, and FA0.85Cs0.15PbI3 perovskite films [13]. The relationship between the refractive index (n) and the reflection coefficient (R) of $n = \frac{{1 + R}}{{1 - R}} + \sqrt {\frac{{4R}}{{{{({1 - R} )}^2}}}{ - {\kappa}^2}} $ indicates that the lower refractive index can result in a lower reflection (i.e. high transmission) and higher light absorption at the perovskite active layer [14,15]. The results demonstrate that the CsFAMA perovskite film can be regarded as an excellent light-absorbing material. The absorption coefficient is proportional to the extinction coefficient according to α=4πκ/λ, where λ represents wavelength, and the absorption onset is about 1.6 eV, which corresponds to the bandgap of CsFAMA film [16]. The sharp absorption edge and n spectra are related to the enhanced absorption of discrete excitonic states, as shown in Fig. 2(b) [13].
Fig. 2. (a) Ellipsometric experimental spectra (points lines) and simulated spectra (solid lines) of the CsFAMA perovskite thin film deposited on a Si substrate with a fixed incident angle of 65°. (b) Wavelength-dependent refractive index and extinction coefficient of CsFAMA film.
The temperature-dependent photoluminescence (PL) spectra of the as-prepared CsFAMA film are characterized to study the exciton-related PL properties in the range of 5 K-300 K, as shown in Fig. 3. The decreased PL intensities and the blue-shifted PL peak positions can be observed with the increase in temperature. The decreased photoluminescence intensities are attributed to the temperature-activated exciton dissociation [17]. The relationship between integrated PL intensity and temperature can be fitted by the following Arrhenius equation [18]: $I(T )= \frac{{{I_0}}}{{1 + Aexp({ - {E_B}/{K_B}T} )}}\; $ where I (T) and I0 are the integrated PL intensities at temperatures T and 0 K, respectively. A is a constant, EB is the exciton binding energy, and kB is the Boltzmann constant. The fitted exciton binding energy EB of the CsFAMA perovskite film is 42 meV, which is comparable with the reported exciton binding energy of MAPbI3 (2-50 meV) [19,20–25], however, it is higher than the room-temperature thermal activation energy of 26 meV [26]. The relatively high exciton binding energy may be attributed to the doping of halide ions (Br- or Cl-) or the excessive PbI2 [27,28], the excessive PbI2 is commonly used to improve the performance of perovskite solar cells [29]. The result agrees well with the strong PbI2 diffraction peak in the XRD pattern shown in the following context. Despite the high exciton binding energy of the CsFAMA film, there can be still a large population of free carriers at room temperature in the active layer of the perovskite solar cells [30]. The results demonstrate that the CsFAMA perovskite can be regarded as an excellent light absorption material for PSCs.
Fig. 3. Temperature-dependent PL spectra of CsFAMA film and the relationship of integrated PL intensity and 1/Temperature (inset).
3.2 PMMA passivates device
The structural properties of planar p-i-n CsFAMA perovskite solar devices are systematically analyzed when the PMMA passivation layer is deposited on the light-absorbing layer. Various PMMA concentrations (0, 2, 4, 6, 8, and 10 mg/ml) are used to optimize the efficiency and stability of the CsFAMA solar cells, as shown in Fig. 4(a). The structure of the inverted p-i-n perovskite solar cell is ITO/PEDOT: PSS/CsFAMA/PMMA/PCBM/Bphen/Ag, where the CsFAMA perovskite film and the Bphen layer are the light absorption layer and the buffer layer, respectively. Figure 4(b) shows the cross-sectional SEM image of the inverted CsFAMA device with an ultrathin PMMA passivation layer, the thickness of each functional layer is 25 nm, 300 nm, 80 nm, and 100 nm for HTL, CsFAMA/PMMA, PCBM/Bphen layer and Ag electrode, respectively. The thickness of the triple cation CsFAMA perovskite film in the SEM image agrees well with that measured in the spectroscopic ellipsometry spectra. The thickness of PMMA passivation thin film cannot be measured because it is only several nanometers. The energy level diagram of the passivated CsFAMA solar cell is shown in Fig. 4(c). Inserting an ultrathin PMMA passivation layer between the CsFAMA perovskite film and the PCBM electron transport layer can form an effective tunneling junction, which helps to suppress the charge recombination, passivate perovskite surface as well as increase device efficiency. The insulating PMMA polymer can help to extract the photogenerated electrons and block holes, because the barrier height of tunneling for electrons is less than that of holes [31–33].
Fig. 4. (a) Schematic diagram of the device structure with PMMA passivation layer. (b) Cross-sectional SEM image of CsFAMA devices. (c) The corresponding energy level of CsFAMA PSC.
Figure 5 shows X-ray diffraction (XRD) patterns of the CsFAMA films with different PMMA passivation concentrations. The unshifted diffraction peak positions indicate that the perovskite phase is not affected after the PMMA modification. In addition, the diffraction peak at 12.9 ° represents the PbI2 phase, which is attributed to the partial decomposition of the perovskite layer. The perovskite degradation in the air will result in a stronger PbI2 diffraction peak [34–36]. Here the peak intensities of PbI2 decrease when the PMMA passivation layer is inserted between the perovskite layer and the ETL, which suggests the polymer PMMA can prevent the degradation of the perovskite films and improve the stability of CsFAMA perovskite solar cells.
To evaluate the effect of the PMMA passivation layer on the photovoltaic performance, the current density-voltage curves (J-V) are characterized under the forward scan direction with a standard AM 1.5 G illumination (100 mW/cm2), as shown in Fig. 6(a). The highest short-circuit current density (Jsc) and the largest open-circuit voltage (Voc) are obtained when 4 mg/ml PMMA solution is deposited on the CsFAMA perovskite film. In addition, the decreased efficiencies of CsFAMA devices can be observed with the increase in PMMA concentrations. When the concentration of PMMA to more than 4 mg/ml, the Jsc, Voc, and efficiency of the solar cells can decrease, which is attributed to that higher PMMA concentrations can cause aggregation of PMMA molecules, which decreases the tunneling effect of electrons, resulting in low carrier collection efficiencies. The results demonstrate that the efficiencies of devices are dependent on the concentrations of PMMA passivation solution and the optimal concentration of PMMA solution is 4 mg/ml. The photovoltaic parameters of CsFAMA devices with different PMMA deposition concentrations are summarized in Table 1
Fig. 6. (a) J-V curves of the CsFAMA PSCs with various PMMA passivation concentrations. (b) Hysteresis analysis of the devices at the reverse and forward scans. (c) The triple cation CsFAMA solar cell.
Table 1. The photovoltaic parameters of CsFAMA devices with different PMMA passivation concentrations.
The CsFAMA solar cell with the optimal PMMA modification concentration shows a power conversion efficiency (PCE) of 16.90%, with a Voc of 0.95 V, a Jsc of 25.82 mA/cm2, and a fill factor (FF) of 69.06%. In contrast, the pure CsFAMA device exhibits a PCE of 14.91%, with a Voc of 0.91 V, a Jsc of 24.12 mA/cm2, and an FF of 67.90%. The significantly increased photovoltaic parameters can be attributed to the improved electron extraction and transport after PMMA passivation.
The J-V curves of the CsFAMA perovskite solar cell and the CsFAMA PSCs with 4 mg/ml PMMA passivation layer are measured under the reverse and forward scan directions with AM 1.5 G illumination and 100 mW/cm2, as shown in Fig. 6(b). The J-V curves of the pure CsFAMA device suffer severe hysteresis behavior, however, the passivated CsFAMA perovskite solar cell shows a negligible hysteresis behavior. The reduced hysteresis is attributed to that the solution-processed insulating PMMA can fill the perovskite surface vacancies and passivate Pb2+ defects to prevent the connection between the ETL and the HTM by the carbonyl (C = O) group with the negative electrostatic potential [37]. The results demonstrate that the PMMA passivation layer can reduce the shunt-leak current paths and suppress the hysteresis behavior of CsFAMA devices. We provide the picture of the triple cation CsFAMA solar cell, as shown in Fig. 6(c). The ITO substrates are 20 × 20 mm with an active area of 0.0725 cm2. We fabricated sixteen cells for each PMMA passivation concentration to measure the J-V curves. The average performance parameters for the devices with different PMMA passivation concentrations are shown in Fig. 7(a-d). The performance of the devices is stable and repeatable.
Stability is an important parameter to evaluate the performance of perovskite solar cells. The passivated CsFAMA device maintains 91% of its initial efficiency after 6 h being stored at 100 °C with relative humidity (RH) of 50%, while the pure CsFAMA device shows poor stability, and only 78% of its initial efficiency is retained at the same condition, as shown in Fig. 8(a). The results are attributed to that the PMMA passivation layer can effectively hinder the loss of iodide in the CsFAMA perovskite films in high temperature and moisture by forming the intermediate adduct PMMA-PbI2 [38]. When the pure CsFAMA device is heated for 6 h, the performance has been decreased obviously. In addition, the improved long-term stability of the CsFAMA perovskite solar cells with PMMA passivation layer at room temperature is demonstrated, maintaining about 81% of its initial efficiency with 40% relative humidity for 720 h, and the relative humidity of 40% is the average humidity of the storage environment, however, the pure CsFAMA device shows a serious degradation process and the normalized efficiency reduces to 68% at the same condition, as shown in Fig. 8(b), which indicates that the PMMA passivation layer can protect CsFAMA perovskite film from oxygen and moisture. The result is attributed to the hydrophobicity of the polymer PMMA molecules. As shown in Fig. 8(c-d), the water contact angles of the CsFAMA film and the CsFAMA/PMMA film are 60.6 ° and 91.0 °, respectively. The significantly increased water contact angle of the CsFAMA film after the optimized PMMA passivation demonstrates that the hydrophobicity of the PMMA layer can improve effectively the long-term stability of CsFAMA perovskite solar cells.
Fig. 8. Stability of the unencapsulated CsFAMA perovskite solar cells: (a) The normalized PCE at 100 °C for 6 h with a relative humidity of 50%. (b) The normalized PCE under ambient conditions with 40% RH for 720 h; (c) The contact angles of the CsFAMA film and the CsFAMA/PMMA film (d).
The SEM and AFM images of the CsFAMA perovskite film with the optimized PMMA passivation layer are characterized, as shown in Fig. 9(a). The passivated CsFAMA perovskite film shows a smoother surface morphology and a lower roughness of 10 nm than the unpassivated CsFAMA perovskite film, which is attributed to the forming of an intermediate adduct PMMA-PbI2 after the interaction between the polymer PMMA and the perovskite film [37,39]. The insert of an ultrathin PMMA layer can help to reduce shunt current, accelerate charge transport as well as improve device performance, as verified by the improved Jsc and the increased stabilities [40,41]. Figure 9(b) shows the optical absorption of CsFAMA perovskite films with various PMMA concentrations. The unchanged optical absorption of the CsFAMA films indicates that the bandgap of the perovskite light-absorbing layer is not affected after the deposition of the transparent PMMA passivation layer.
Fig. 9. (a) SEM image of the CsFAMA/PMMA film and AFM image (inset). (b) UV-vis absorption spectra of the CsFAMA perovskite films with various PMMA concentrations.
The steady-state photoluminescence (PL) and time-resolved PL (TRPL) spectra of CsFAMA perovskite films are shown in Fig. 10(a-b). The significant PL quenching effect is observed when the PMMA passivation layer is used to passivate the CsFAMA perovskite thin film, as shown in Fig. 10(a). The results indicate that the PMMA passivation layer can decrease the photoluminescence recombination of electron-hole pairs and increase the electron extraction and transport at the interface of the CsFAMA/ETL [42,43], which agrees with the increased Jsc after PMMA passivation as shown in Table 1. The time-resolved PL decay curves are obtained from the CsFAMA perovskite films before and after 4 mg/mL PMMA passivation, as shown in Fig. 10(b). The measured TRPL decay curves can be well fitted based on the following triple-exponential decay equation [44]:$f(t )= \mathop \sum \limits_{i = 1}^{i = 3} {A_i}\exp\left( { - \frac{t}{{{\tau_i}}}} \right) + {y_0}$ where τi is the photoluminescence lifetime component of the CsFAMA perovskite films, the Ai and y0 are constants. The average carrier lifetimes (τave) can be calculated according to the equation:${\tau _{ave}} = \frac{{\sum {A_i}\tau _i^2}}{{\sum {A_i}{\tau _i}}}$, the detailed carrier lifetime parameters of the CsFAMA perovskite films before and after the optimized PMMA passivation are listed in Table 2. The decreased PL decay lifetime of the CsFAMA perovskite film with PMMA modification indicates that the less radiative recombination process is affected by the more effective carrier extraction in perovskite films, while the pure CsFAMA thin film suffers more carrier radiative recombination [45,46]. The TRPL lifetime of perovskite films is strongly affected by the carrier recombination dynamics and efficient carrier extraction (electron or hole) processes. The mixed PMMA: PCBM passivation layer is used between the SnO2 ETL and perovskite layer of the n-i-p solar cell in the Ref. [7], the perovskite is laterally deposited on the PMMA: PCBM, the PMMA passivation layer can suppress the non-radiative recombination, thus result in a longer TRPL lifetime. In our inverted p-i-n device structure, the PMMA layer is deposited on the perovskite layer to improve the perovskite thin film quality, and obtain the smoother perovskite morphology, increased Jsc and quenched PL intensities, here shortened the TRPL lifetime of the passivated perovskite film are determined by the carrier extraction processes [33,47,48]. The average carrier lifetimes of the CsFAMA film and the CsFAMA/PMMA film are 304.2 ns and 85.6 ns, respectively. The shorter average carrier lifetime of the CsFAMA film after PMMA modification demonstrates that the efficiency of electrons extraction and transport is effectively improved [49–52].
Fig. 10. (a) Steady-state PL spectra of CsFAMA films with different PMMA concentrations. (b) Time-resolved PL spectra of CsFAMA films with and without the optimal PMMA passivation.
Table 2. Summary of the TRPL parameters of the CsFAMA film and passivated CsFAMA film.
4. Conclusions
In summary, the spectroscopic ellipsometry and temperature-dependent PL spectra demonstrate that the triple cation CsFAMA perovskite is an excellent light absorption material. The inverted p-i-n CsFAMA perovskite solar cells are studied, and an ultrathin PMMA passivation layer is incorporated into the devices to significantly improve the efficiency and stability of the CsFAMA solar cells. At the optimized PMMA concentration, the passivated CsFAMA device achieves an improved PCE of 16.9% with negligible hysteresis behavior and increased stabilities. The improved performance can be attributed to the increased electron extraction and transport at the interface of the CsFAMA/ETL, which is confirmed by the shortened PL lifetime and decreased PL intensity. The stability of the CsFAMA perovskite solar cells is improved after PMMA passivation because of the hydrophobic property of the PMMA molecules. The results offer a feasible method to improve the efficiency and stability of the inverted perovskite solar cells.
Funding
National Natural Science Foundation of China (11874185); University Postgraduate Programme (KYCX21_3471).
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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