Differential absorption optical coherence tomography with strong absorption contrast agents of gold nanorods

Abstract

Plasmon-resonant gold nanorods (GNRs) are demonstrated as strong absorption contrast agents for optical coherence tomography (OCT). OCT imaging of tissue phantoms doped with GNRs of different resonant wavelengths and concentrations is studied. To utilize the high absorption property of GNRs, a differential absorption OCT imaging is introduced to retrieve the absorption information of GNRs from conventional backscattered signals. It is shown that the contrast of the OCT image can be enhanced significantly when the plasmon resonant wavelength of the GNRs matches the central wavelength of the OCT source.

Keywords

optical coherence tomography (OCT) / plasmon resonance / gold nanorod (GNR) / differential absorption

1 Introduction

Organic–inorganic hybrid halide perovskite-based solar cells (PSCs) received tremendous attention in recent years owing to its high absorption coefficient, small exciton-binding energy, high carrier mobility, and extensive carrier-diffusion lengths [1–9]. Single-junction PSCs have achieved a power conversion efficiency (PCE) of 22.1% [10–14]. Tandem solar cells present a promising strategy to further enhance device performance due to the Shockley-Queisser limit for single-junction devices [15,16]. With its proper small band gap (E_g) of 0.9 to 1.2 eV, mixed lead-tin (Pb-Sn) perovskite is an ideal light-harvesting material for rear cell in tandem solar cells. Recently, PSCs based on a mixed Pb-Sn perovskite-based absorber with an efficiency of 17% have been demonstrated [17].

Two device architectures are commonly used for PSCs: the conventional n-i-p structure and the inverted p-i-n structure. In both structures, a perovskite active layer is sandwiched between a hole-transporting layer (HTL) and an electron-transporting layer (ETL). For conventional n-i-p structures, the most frequently used ETLs are transition metal oxides (TiO₂ and ZnO), whereas HTLs are mainly organic materials, such as 2,2′,7,7′-tetrakis [N,N-di(4-methoxyphenyl) amino]-9,9′-spirobifluorene (spiro-OMeTAD) [18–21]. Inverted devices generally employ NiO_x, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) as HTL, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), ZnO, or SnO₂ as the top ETL [22–25]. For inverted device structures, both ETL and HTL can use inorganic materials, thus facilitating better device stability. Numerous studies have attempted to optimize the contact between the active layer and the carrier-transporting layers, as well as between the carrier-transporting layers and the electrodes, to achieve quick carrier transport and less interface recombination [26–30]. Given the tedious device-fabrication process for each layer (especially the requirement of high-temperature annealing process for the fabrication of some oxide films), ETL- or HTL-free devices were developed to simplify device fabrication [26,31–37]. HTL-free inverted-structure solar cells based on both Pb and Sn perovskites have been reported with good performance and stability [34–38].

In the current work, we constructed an HTL-free planar Pb-Sn binary PSC by preparing perovskite film on bare indium tin oxide (ITO) for the first time. Homogeneous and mirror-like FAPb_0.5Sn_0.5I₃ films were developed through anti-solvent assisted crystallization and solvent engineering. The effect of the Pb-Sn ratio as well as the concentration of SnF₂ on film morphology and device performance were investigated. Based on the inverted-structure (ITO/perovskite/PCBM/BCP/Al), solar cells with a highest PCE of 7.94% under the illumination of AM 1.5G solar simulator were presented.

2 Experimental section

2.1 Perovskite precursor preparation

The perovskite precursor solution was obtained by mixing the solutions of FASnI₃ and FAPbI₃ at a specific ratio. FASnI₃ and FAPbI₃ solutions of 1-M^†

1 M= 1 mol/L

⁾ concentration were prepared in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (volume ratio of 3:2). The mole ratio of FAI to SnI₂/PbI₂ was 1:1. Furthermore, 0.1 M SnF₂ was added to the tin precursor to prevent oxidation of the Sn²⁺ to Sn⁴⁺ unless otherwise stated. The final solution was stirred at 70°C for 1 h and subsequently filtered with a PTFE filter (0.22 µm pore size) before use.

2.2 Perovskite film fabrication

For FAPb_1−xSn_xI₃ film (x>0.25) fabrication, the mixed precursor prepared above was spin coated onto the substrate via a one-step anti-solvent assisted crystalline process at 1000 and 5000 rpm for 10 and 30 s, respectively. In the second process, 800 µL toluene was slowly dropped in the middle of the substrate. The perovskite film was subsequently annealed at 70°C for 30 min.

For FAPbI₃ and FAPb_0.75Sn_0.25I₃ films, the annealing temperature was increased to 170°C and 100°C, respectively. All of the operations were performed in a glovebox (O₂≤1 ppm, H₂O≤1 ppm).

2.3 Device fabrication

ITO-coated glasses were sequentially cleaned with Triton X-100 detergent, acetone, deionized water, and isopropanol. The cleaned ITO glasses were treated with UVO for 3 min before being transferred into the glovebox. Perovskites were fabricated on top of the ITO glasses using the procedure described above. The PCBM film was prepared via a spin-coating process (speed of 2000 rpm and time of 60 s) using chlorobenzene as the solvent (concentration of 10 mg/mL), followed by annealing at 80°C for 5 min. Subsequently, a saturated solution of BCP in anhydrous methanol was spin coated at 6000 rpm for 15 s. Aluminum (of 100 nm thickness) was thermally evaporated as the top electrode at a rate of 1 Å/s.

2.4 Device characterization

Current-voltage (J-V) curves were measured using a Keithley 2400 source unit under AM1.5G solar simulator (Enli Tech, Taiwan) at 100 mW/cm² (1 sun). The light intensity was calibrated by a certified KG-5 Si diode. The spectral mismatch correction factor is 0.49%. The devices were measured in reverse scan (from 0.7 to 0 V, at increments of 0.01 V) and the delay time was 30 ms. J-V curves for all devices were measured by masking the active area using a metal mask with an area of 0.04 cm². The external quantum efficiency (EQE) spectra were obtained by a commercial system (Solar Cell Scan 100, Beijing Zolix Instruments Co., Ltd). Additionally, the cells were subjected to monochromatic illumination (150 W Xe lamp passing through monochromator filters). The light intensity was calibrated by a standard photodetector (QE-B3/S1337-1010BQ, Zolix). The light beam was chopped at 180 Hz, and the response of the cell was acquired using a Stanford Research SR830 lock-in amplifier.

3 Results and discussion

We firstly investigated the optical properties of the mixed Pb-Sn perovskites. As the Sn is added, the absorption spectra of FAPb_1−xSn_xI₃ (x = 0.25, 0.50, 0.75, 1.00) are all red shifted. Based on the absorption onset point, the band gaps of FAPb_0.75Sn_0.25I₃ and FAPb_0.5Sn_0.5I₃ are estimated to be 1.31 and 1.27 eV, respectively (Fig. 1(a)). The absorption tail mainly results from sub-band gap states [39]. Photoluminescence (PL) spectra show similar trend as the absorption spectra (Fig. 1(b)). The E_g values estimated from both absorption and PL are shown in Table 1. As the Sn content increases from 0% to 75%, the estimated E_g decreases from 1.507 to 1.282 eV, and recovers to 1.40 eV for FASnI₃. The small band gaps of mixed Pb-Sn perovskites can be attributed to the synergistic effect of the relatively higher valence band maximum of Sn and lower conduction band minimum of Pb. The multi-PL peaks of FAPbI₃ and FAPb_0.75Sn_0.25I₃ films were ascribed to phase impurity.

Tab.1 Band gaps for Pb-Sn perovskite alloys calculated from absorption edge and PL peak, respectively

FAPb1−xSnxI3	0	0.25	0.50	0.75	1.00
band gap from absorption/eV	1.490	1.310	1.270	−	−
band gap from PL/eV	1.507	1.355	1.286	1.282	1.400

Fig.1 Optical and structural characterization of FAPb1−xSnxI3. (a) Absorption spectra; (b) photoluminescence spectra; and (c) X-ray diffraction (XRD) patterns of FAPb1−xSnxI3 films with different Sn ratios; “#” indicates the diffraction peaks of ITO

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XRD experiment was conducted to study the structure of FAPb_1−xSn_xI₃ films (Fig. 1(c)). All of the samples show diffraction peaks near 14°, 25°, and 28°, corresponding to the (100), (110), and (200) planes of the cubic structure of FAPb_1−xSn_xI₃. High-temperature post-annealing was employed to convert FAPbI₃ and FAPb_0.75Sn_0.25I₃ to cubic structure. FAPbI₃ and FAPb_0.75Sn_0.25I₃ films show extra diffraction peaks corresponding to a triclinic structure (Fig. S1), possibly deriving from the high formation energy of a cubic structure, despite the high-temperature post-annealing employed to convert FAPbI₃ and FAPb_0.75Sn_0.25I₃ to a cubic structure. As the Sn content increased, the extra peaks disappeared, while a pure cubic structure was observed. We conclude that the addition of Sn reduced the formation energy of the cubic phase [40].

Given its uniform crystal structure and extended absorption range, we explored FAPb_0.5Sn_0.5I₃ films as a light absorber for solar cells based on inverted planar architecture: ITO/perovskite/PCBM/BCP/Al (Fig. 2(a)). In this study, PCBM and BCP serve as the ETL and HTL. According to the cross-sectional scanning electron microscopy (SEM) image (Fig. 2(b)), the thickness of the FAPb_0.5Sn_0.5I₃, PCBM, and BCP films are approximately 270, 55, and 45 nm, respectively. Figure 2(c) shows the band alignment diagram of the HTL-free devices. The conduction band and valence band position are estimated to be 4.00 and 5.25 eV, respectively [40]. The small difference between the work function of ITO and the valence band of the perovskite allows for effective hole injection. As shown in Fig. 2(d), our champion cell with 10% (molar ratio) SnF₂ achieves a maximum PCE of 7.94%, with an open-circuit voltage (V_oc) of 0.59 V, a short-circuit current density (J_sc) of 23.13 mA/cm², and a fill factor (FF) of 58%.

From the EQE spectra, the solar cells based on mixed Pb-Sn perovskite exhibit high EQE in a broad absorption range of up to 1050 nm (Fig. 2(e)), and the highest EQE reaches 80.4% at 520 nm. The integrated J_sc from the EQE spectrum (22.57 mA/cm²) matches well with the J_sc obtained in J-V curves measured under solar simulator (with a difference within 3%). This high J_sc demonstrates that holes can be effectively collected by ITO without the HTL. We subsequently investigated the stability of the devices under continuous simulated AM 1.5G illumination (Fig. S2), in a N₂-filled glovebox with O₂<2 ppm, H₂O<1 ppm. Device efficiency slightly increased initially, and then remained steady. The illumination-initiated improvement of efficiency can be ascribed to the drift of ions in perovskites driven by photovoltage-induced electric field, forming anin situ p-i-n homojunction [41].

Fig.2 Photovoltaic structure and performance of HTL-free devices based on FAPb0.5Sn0.5I3 films. (a) Schematics of the device architecture; (b) SEM cross-sectional image; (c) energy band diagram; (d) J-V characteristics; (e) EQE spectrum and integrated Jsc of the highest performance of HTL-free PSC

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To investigate the impact of the energy-level alignment between ITO and perovskite, we prepared a control sample using NiO_x as an HTL (Fig. S3). The XRD patterns of perovskite films prepared on ITO and NiO_x substrates exhibited similar cubic structures and good crystallinity (see Fig. 3(a)). The J_sc and FF of the PSC with NiO_x as HTL, however, are significantly lower compared with the HTL-free one. This condition may result from the deep work function of NiO_x, which affected the hole transfer from perovskite. To confirm this speculation, we examined the PL spectra of FAPb_0.5Sn_0.5I₃ films on ITO and NiO_x. As illustrated in Fig. 3(b), the emission intensity at 964 nm of the film on ITO is significantly weaker, implying worse hole transport between NiO_x/FAPb_0.5Sn_0.5I₃. We further compared the perovskite film morphology through SEM, as shown in Fig. 3(c). The FAPb_0.5Sn_0.5I₃ film exhibits higher surface coverage and larger crystal domain size on bare ITO (left). More pinholes and smaller grains appear when NiO_x (right) was employed, which would lead to direct contact between NiO_x and ETL and severe interface charge recombination.

Fig.3 Characterizations of FAPb0.5Sn0.5I3 films on different substrates. (a) XRD patterns (“#” indicates the diffraction peaks of ITO); (b) PL spectra; and (c) SEM images of the perovskite film on ITO and NiOx

Full size|PPT slide

SnF₂ is generally employed to stabilize the FASnI₃ perovskites from oxidation owing to its reducing character [42,43]. The addition of excess amount of SnF₂, however, will induce phase separation of the film, thus generating poor device performance [42,43]. We systematically investigated the impact of SnF₂ molar concentration on film morphology and device performance. As shown in Figs. 4(a)–4(e), pure FAPb_0.5Sn_0.5I₃ exhibits a full coverage and small grain size. Larger grain size along with pinholes appeared when 5 mol% SnF₂ was added. Upon adding 10 mol% of SnF₂, rock-like grains with large domain size emerged with a relatively high coverage ratio. When more SnF₂ was incorporated, both grain size and coverage rate decreased. The maximum value of the average grain size was around 280 nm with SnF₂ ratio of 10 mol% (Fig. 4(f)). We further compared the Sn⁴⁺ content in the FAPb_0.5Sn_0.5I₃ films with or without SnF₂ additive using X-ray photoelectron spectroscopy. To exclude the effect of surface oxidation, samples were etched before the measurement (Fig. S4 and Table S1). As shown in Table S1, the Sn⁴⁺ amount in the 10 mol% SnF₂-doped FAPb_0.5Sn_0.5I₃ film is significantly lower than that in pure FAPb_0.5Sn_0.5I₃, implying that the addition of SnF₂ prevents Sn²⁺ from oxidation.

Fig.4 Morphology of perovskite films with varying molar concentration of SnF2. SEM images of FAPb0.5Sn0.5I3 films on ITO with (a) 0 mol%, (b) 5 mol% , (c) 10 mol%, (d) 15 mol%, and (e) 20 mol% of SnF2; and (f) calculation of the average grain size for perovskites

Full size|PPT slide

The PSCs based on FAPb_0.5Sn_0.5I₃ with various molar concentrations of SnF₂ were fabricated. As shown in Fig. S5 and Table S2, the devices exhibited low J_sc and efficiency without SnF₂ additives. This condition can be attributed to the high defect density resulting from the oxidation of Sn²⁺ to Sn⁴⁺, causing significantly reduced carrier-diffusion lengths [42–44]. With the addition of SnF₂, the device performance is effectively improved by suppressing Sn oxidation. However, film morphology deteriorated when the concentration of SnF₂ is increased to 20%, leading to poor performance. Therefore, both oxidation and film morphology are important to device performance.

4 Conclusion

In this study, we constructed an inverted planar mixed Pb-Sn perovskite solar cell on bare ITO glass without an independent HTL. The Pb-to-Sn ratio and the concentration of SnF₂ were systematically optimized. Highly uniform reflective FAPb_0.5Sn_0.5I₃ thin films on ITO glass with a band gap of 1.28 eV were achieved. Solar cells based on HTL-free ITO/FAPb_0.5Sn_0.5I₃/PCBM/BCP/Al structure exhibit a promising PCE of 7.94% with a short-circuit current density of 23.13 mA/cm², EQE of 80%, and extended light-harvesting range to 1050 nm. The removal of HTL allows effective carrier transfer between the perovskite and the ITO electrode. It therefore provides an alternative simple structure for low-temperature, small band gap single-junction or tandem PSCs.

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Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (Grant No. 60688401) and partially supported by a multidisciplinary project of Zhejiang University and the Swedish Foundation for Strategic Research (SSF).

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2014 Higher Education Press and Springer-Verlag Berlin Heidelberg