A phthalocyanine-based polycrystalline interlayer simultaneously realizing charge collection and ion defect passivation for perovskite solar cells

Abstract

In the quest for sustainable energy solutions, perovskite solar cells have emerged as a promising avenue due to their remarkable efficiency and cost-effectiveness. However, their widespread adoption is hampered by performance degradation issues primarily attributed to ion migration and vacancy formation within halide perovskite films. To mitigate the impact of ion defects, a passivation layer, which typically acts as a barrier for carriers, is employed. Nonetheless, the requirement for extreme thinness to avoid increasing series resistance complicates the manufacturing process. In this study, we introduced gallium phthalocyanine hydroxide (OHGaPc), a p-type organic semiconductor with Lewis base functionality, as a passivation layer to mitigate performance degradation in perovskite solar cells. We demonstrated that this material passivates halide vacancies by being a Lewis base and promotes efficient charge transport as a p-type semiconductor. This dual functionality of OHGaPc not only enhances the stability and performance of perovskite solar cells but also simplifies the manufacturing process by obviating the need for ultra-thin insulating films. Our findings underscore the significance of leveraging the properties of Lewis bases and p-type semiconductors in improving charge extraction and overall cell efficiency, setting a new direction in the development of durable and efficient perovskite solar cells.

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Introduction

Organic–inorganic hybrid perovskite solar cells have garnered significant interest as a next-generation photovoltaic technology. Since the first perovskite solar cell was reported in 2009,¹ the efficiency of perovskite solar cells has rapidly increased over the past decade, exceeding 26%.² The conversion efficiency of perovskite solar cells has reached a level comparable to that of silicon solar cells. However, their long-term stability against light exposure remains a commercial challenge.³ Particularly, halide perovskite crystals, which contain halide ions that are relatively easily oxidized, tend to readily form the iodide anion vacancies upon light irradiation.⁴ Moreover, because the A site often contains organic cations, it is prone to the formation of cation vacancies at the A site due to external factors such as light, heat, and water.⁵ The formation of these ion vacancies facilitates ion migration, which has been reported as a serious issue for the durability of the devices. Theoretical calculations have shown that halide ions are the predominant migrating species.⁶ Factors that promote the migration of iodide anions include the MA vacancy ⁷ It has been reported that an increase in the number of these leads to an increase in the ionic conductivity of I⁻. Furthermore, the detached I⁻ diffuses through the perovskite crystal and within the device,⁸ causing electrode corrosion⁹ and degradation of the perovskite crystal,¹⁰ significantly lowering the durability.

One of the solutions to compensate for these ion vacancies and suppress migration is the introduction of a passivation layer.¹¹ Studies have indicated that low-dimensional perovskites containing organic halide compounds such as phenethylammonium iodide (PEAI) and n-octylammonium iodide (OAI),¹² as well as passivation materials like polymethyl methacrylate (PMMA),¹³ are suggested to compensate for ^12a However, these materials are insulators and act as energy barriers,¹⁴ requiring the formation of ultra-thin films of only a few nanometers, which makes thickness control challenging.

In this research, gallium phthalocyanine hydroxide (OHGaPc), a p-type organic semiconductor, was introduced as a passivation layer. OHGaPc, with its –OH groups functioning as Lewis bases, not only compensates for but also suppresses the formation of , , and thereby inhibiting ionic conductivity. The fact that it is a Lewis base, rather than a reducing agent, is of essential importance in the performance of these functions. Moreover, being a p-type material, it does not require the formation of ultra-thin films and efficiently transports holes from the perovskite crystal. The findings of this study combine the strategy of using Lewis bases to suppress ion vacancies in halide perovskites with the concept of employing a p-type semiconductor layer instead of an insulating film for the passivation layer. The approach presented in this research contributes to the ongoing development of the hole transport layer for perovskite solar cells.

Results and discussion

We propose the suppression mechanism for ion vacancies, predicated on the assumption that OHGaPc acts as a Lewis base (Fig. 1). Given the partial negative charge (δ⁻) associated with the OH group, it exhibits an affinity towards cations. Consequently, it is plausible to propose that cations, including Pb²⁺, MA⁺, and FA⁺, are stabilized through this interaction, thereby reducing their propensity for reduction. At this time, the OH group should also be attracted to This is obvious, as shown in Fig. 1, because the entity of is the space itself surrounded by Pb²⁺, MA⁺, and FA⁺. Therefore, as shown in Fig. 1, is estimated to be compensated. In the following discussion, we will elucidate the sequence of experiments conducted to affirm the effectiveness of the proposed compensation model.

Fig. 1 Conceptual representation of the model wherein OHGaPc compensates for as demonstrated in this study.

Liquid state ¹H-NMR measurement in deuterated dimethyl sulfoxide-d₆ (DMSO-d₆) as the solvent was carried out to investigate the interaction between OHGaPc and the perovskite precursor materials via hydrogen bonding, as depicted in Fig. 2, S1 and S2.† For FAI, the resonance peak attributed to the NH group was detected at 8.82 ppm.¹⁵ In the case of OHGaPc, the peak corresponding to the OH group was recorded at 5.74 ppm. Upon mixing FAI and OHGaPc, the NH group's resonance peak at 8.82 ppm split into two distinct peaks at 8.65 ppm and 9.00 ppm, while the resonance peak at 5.74 ppm associated with the OH group of OHGaPc vanished. Comparable patterns were also observed with MAI, MABr, and FABr, as shown in Fig. 2, S1 and S2.† These observations suggest that the OH group in OHGaPc functions as a Lewis base towards the NH group in FA and the NH₃ group in MA, leading to the formation of hydrogen bonds.

Fig. 2 Schematics and liquid-state partial ¹H-NMR spectra of OHGaPc with perovskite precursor materials FAI and MAI. (a) and (b) Pertain to FAI with OHGaPc; (c) and (d) to MAI with OHGaPc.

Investigating the interactions within the composite film of OHGaPc and perovskite crystals, we stacked a layer of OHGaPc atop the perovskite crystal layer. This configuration was analyzed using X-ray photoelectron spectroscopy (XPS). As illustrated in Fig. 3 and S3,† the binding energies for Pb 4f_7/2 and Pb 4f_5/2, situated at 138.9 and 143.75 eV respectively, demonstrated a downward shift of 1.05 eV due to the stacking. This shift was observed simultaneously with the disappearance of the Pb⁰ signal.¹⁶ A similar pattern was noted for I 3d_5/2 and I 3d_3/2, which also experienced a decrease in energy values by 1.15 eV.¹⁷ These phenomena were consistently observed in the XPS analysis of the OHGaPc/PbI₂ composite film (Fig. S4 and S5†). In XPS measurements, a decrease in binding energy signifies that electrons are more readily released. This indicates that OHGaPc serves as a Lewis base, donating an electron pair to both and . Such donation implies the stabilization of aligning with the observed absence of the Pb⁰ peak.

Fig. 3 XPS spectra of halide perovskite thin films with and without OHGaPc (a) for Pb for Pb 4f_7/2 and Pb 4f_5/2 peaks and (b) for I 3d_5/2 and I 3d_3/2 peaks.

The NMR and XPS results indicate that OHGaPc acts as a Lewis base. These results support the model shown in Fig. 1, as described in the following:

(1) For positively charged Pb²⁺FA⁺ and MA⁺ OHGaPc acts as a Lewis base and stabilizes them, thereby inhibiting the formation of , and

(2) The OH group in OHGaPc as a Lewis base is attracted to which has an effective positive charge, resulting in the compensation of This is physically equivalent to the hydrogen bonding of the OH groups in OHGaPc to and

We performed the grazing incidence wide-angle X-ray scattering (GIWAXS) measurement to evaluate their crystallinity and orientation. As shown in Fig. 4a, diffraction peaks were observed at q = 0.58 Å⁻¹ in the q_z direction and at q = 0.53 Å⁻¹ with an azimuthal angle of 20°.

Fig. 4 (a) The GIWAXS pattern of the OHGaPc thin film, (b) detailed crystal structure of OHGaPc,¹⁸ and (c) visualization of the π–π stacking interactions within the OHGaPc structure.

The GIWAXS pattern of OHGaPc on the perovskite thin films also exhibited similar characteristics (Fig. S6†). As illustrated in Fig. 4b, the crystal structure is a triclinic p̄1, with dimensions a = 11.52 Å, b = 12.75 Å, c = 8.89 Å, α = 95.87°, β = 96.00°, γ = 69.45°.¹⁸ The position of the diffraction peak corresponding to the (010) plane, calculated from b, is in agreement with the peak observed in the q_z direction. Furthermore, the azimuthal angle corresponds to 90°–γ, and the position of the (100) diffraction peak is consistent with the peak that has an azimuthal angle of 20°. These results indicate that the OHGaPc polycrystalline thin films exhibit (010) orientation. As indicated in Fig. 4c, a π–π stack is present with a centroid–centroid distance of 3.690 Å and a shift distance of 1.173 Å. Similarly, another π–π stack exhibits a centroid–centroid distance of 3.895 Å and a shift distance of 1.253 Å. In both instances, the π–π stacking distance is less than 4 Å, indicating conductivity. However, the orientation of the π–π stacking, which has a large proportion aligned with the in-plane direction of the film, may restrict mobility within the stacking direction.

In order to evaluate the effect of OHGaPc on the filling trapping level and carrier extraction, time-resolved PL (TRPL) measurements were performed on the OHGaPc/perovskite stacked film and the perovskite monolayer film. As shown in Fig. 5a, OHGaPc stacking reduced the intensity of band-to-band emission intensity from the perovskite crystals by 77%, substantiating the carrier extraction by OHGaPc. Concurrently, a blue shift in the emission peak alongside a reduction in the FWHM was observed (refer to the inset in Fig. 5a), indicative of a decrease in non-radiative recombination due to diminished trapping.¹⁹ This observation aligns with the finding that OHGaPc effectively compensates for

Fig. 5 (a) PL spectra of perovskite films and (b) time dependence of PL intensities of perovskite films with and without OHGaPc.

Fig. 5b displays the luminescence intensity as a function of time. These dependencies were fit using a biexponential decay model, as detailed in eqn (1).²⁰

where A₁ and A₂ are pre-exponential factors and τ₁ and τ₂ are time constants. It has been reported that the early decay component τ₁ is attributed to interband transitions, whereas the late decay component τ₂ corresponds to trap-assisted recombination, with findings indicating that an enhancement in carrier extraction efficiency is correlated with a reduction in τ₁ and an increase in A₁ and that a decrease in both τ₂ and A₂ is indicative of the suppression of trap-assisted recombination.²⁰

The parameters obtained from the fitting results are summarized in Table 1. The results indicate that with the stacking of OHGaPc, there was an increase in A₁, and a decrease in A₂, τ₁, and τ₂. These results suggest that the stacking of OHGaPc contributes to the reduction of trap density and effective carrier extraction. The fact that the VBM is primarily composed of I 5p and the ideal energy level for hole extraction in ultraviolet photoelectron spectroscopy (Fig. 6b and S7†) may contribute to both the suppression of trap sites and the rapid extraction of holes.

Table 1 Parameters of the TRPL spectra from experimental results

	A 1 τ 1 + A2τ2 (ns)	τ 1 (ns)	τ 2 (ns)	A 1 (%)	A 2 (%)
Without OHGaPc	1108.6	2.83	1117.2	0.75	0.25
With OHGaPc	511.2	2.78	521.5	0.79	0.21

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Fig. 6 (a) Device structure and (b) energy diagram of the perovskite solar cells with OHGaPc. (c) J–V curves (reverse scan) of perovskite solar cells with and without OHGaPc. (d) Durability test device for solar cells during light irradiation. (e) Time-dependent normalized current of perovskite solar cells under 1 sun light exposure, which was obtained by connecting a 100 Ω resistor and measuring the voltage at both ends. Depth profile of ions in perovskite solar cells measured by TOF-SIMS (f) with OHGaPc and (g) without OHGaPc. The solid and the dashed lines represent profiles before and after 80 hours of light irradiation, respectively.

To demonstrate the impact of OHGaPc on the improvement of device durability, we fabricated PSCs with an OHGaPc layer. The device structure is [ITO/SnO₂/perovskite/OHGaPc/spiro-OMeTAD/Au] (perovskite: Cs_0.1(FA_0.87MA_0.13)_0.9Pb(I_0.9Br_0.1)₃), as shown in Fig. 6a. We show the energy diagram of the solar cells in Fig. 6b. The current density–voltage (J–V) characteristics of PSCs were measured under simulated solar light (100 mW cm⁻²), as shown in Fig. 6c. Forward and reverse scans of the devices with and without OHGaPc are summarized in Fig. S8 and S9.† The best power conversion efficiency (PCE) obtained for the PSC with OHGaPc was 21.0% with a short circuit current density (J_sc) of 22.28 mA cm⁻², an open-circuit voltage (V_oc) of 1.174 V, and fill factor (FF) of 0.803. Otherwise, the PCE without the interlayer showed the best PCE of 21.8% (J_sc = 22.90 mA cm⁻², V_oc = 1.176 V, and FF = 0.810). The only diminished parameter was the current value compared to the PSC without the interlayer, signifying a PCE reduction. IPCE spectra and integrated J_sc of the device are shown in Fig. S10.† We confirmed consistent results with the device performance, indicating that the current density of the OHGaPc device slightly decreased. It is posited that the tens of nanometers thickness of the interlayer contributed to elevated resistance, as evidenced by cross-sectional SEM analysis (Fig. S11†). Thinning the thickness of OHGaPc, even if not to the extent of ultra-thin films, may potentially improve the conversion efficiency. Generally, the thickness of the passivation layer is a few atomic layers thick.^14a We summarized the thickness dependence of OHGaPc on the device performance in Table S1.† It was found that for each doubling of the OHGaPc film thickness from 130 nm, the conversion efficiency is reduced by about half. This indicates that the decrease in conversion efficiency is almost negligible if the thickness of the OHGaPc film is reduced by at least a fraction, suggesting that ultra-thin films with only several atomic layers are not necessary. The observation of only a slight increase in resistance, despite the film's thickness being in the tens of nanometers, robustly demonstrates the efficacious charge transport capability of OHGaPc. We performed a durability test of perovskite solar cells incorporating OHGaPc. The durability testing of the device under light irradiation was conducted using the measurement system shown in Fig. 6d and S12.† Under light irradiation, the device temperature rose to 32 °C. Fig. 6e shows the time-dependent output current with the intensity of one sun. In samples without OHGaPc, the output current decreased by 72% after 24 hours, whereas in samples with OHGaPc, the reduction was limited to 28%. From these results, it was confirmed that the introduction of OHGaPc contributes to the enhancement of device durability.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses were conducted to evaluate the effect on the reduction of ionic conductivity. The extent of ion migration was assessed by examining the ion distribution along the stacking direction of the films through TOF-SIMS (Fig. 6f and g). In the sample devoid of OHGaPc, a pronounced increase in ion intensity for I⁻ and Br⁻ was detected within the hole transport layer (HTL). This phenomenon suggests that ion migration of I⁻ and Br⁻ occurs within the HTL as a consequence of the light durability tests. Conversely, in the sample containing OHGaPc, neither the escalation of I⁻ and Br⁻ ion intensities nor any increase in ion intensity within the HTL was observed when OHGaPc served as the intermediate layer. This indicates that I⁻ and Br⁻ ions are blocked in the intermediate layer, thereby preventing their migration into the HTL. These findings suggest that the incorporation of OHGaPc mitigates ion migration. In addition to the compensating effect of ion deficiency, the physical presence of the layer is thought to have a blocking effect on the entry and exit of materials. These effects of the OHGaPc layer significantly enhance the device's performance in terms of light durability.

Conclusion

In summary, this study has successfully demonstrated the introduction of the molecular interfacial layer, OHGaPc as an effective passivation layer for perovskite solar cells. OHGaPc characterized both as a p-type semiconductor and a Lewis base capable of the compensation of plays a pivotal role in enhancing the performance of perovskite solar cells. Typically, phthalocyanine compounds are recognized as p-type semiconductors.²¹ In this research, we introduced an OH group into the phthalocyanine structure, enabling it to function as a Lewis base, that is, a passivation layer. The employment of materials that simultaneously exhibit properties of Lewis bases and p-type semiconductors, exemplified by OHGaPc, for the interfacing of ionic halide perovskite crystals with p-type organic semiconductors, has been shown to facilitate seamless charge extraction. Until now, passivation layers have acted as a source of series resistance while they suppress ionic conduction. This study's results present a solution to this problem, offering guidance for the development of future devices.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

TO and NN conceived this idea. NS, TM, AO, MI and TM supervised the project. TO, NS and NN designed and performed the experiments. ST performed the XPS measurements. AH performed the NMR. YM performed the SIMS measurements and helped with the data analysis. NS, KM and MT performed the IV measurements. NN, AH, and MK performed the stability test. YN performed the GIWAXS measurements and helped with the data analysis. TO and NS wrote the manuscript. All authors discussed the results and commented on the manuscript.

Conflicts of interest

Canon Inc. has filed patents (JP application no. 2023-184750 and 2022-167637) related to the subject matter of this manuscript. The remaining authors declare no competing interests.

Acknowledgements

NS was supported by the Science Research Promotion Fund of the Promotion and Mutual Aid Corporation for Private Schools of Japan. The GIWAXS measurements were performed at SPring-8 with the approval of the JASRI (proposal no. 2020A2133, 2021A2085, 2022A2078, 2022B2110, 2023A2358, 2023B1655, 2023B1687, 2023B1887, 2023B1889, 2023B1977, 2023B2056, and 2023B2318). We thank the Kanagawa Industrial Technology Research Institute (KISTEC) for device measurements. NS, YN and TM gratefully acknowledge the support of JSPS Grants-in-Aid for Scientific Research (No. 24K08273, 24K17778 and 24H00488).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02491e

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