Polymer hole-transport material improving thermal stability of inorganic perovskite solar cells

Abstract

Cesium-based inorganic perovskite solar cells (PSCs) are paid more attention because of their potential thermal stability. However, prevalent salt-doped 2,2′,7,7′-tetrakis(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) as hole-transport materials (HTMs) for a high-efficiency inorganic device has an unfortunate defective thermal stability. In this study, we apply poly(3-hexylthiophene-2,5-diyl) (P3HT) as the HTM and design all-inorganic PSCs with an indium tin oxide (ITO)/SnO₂/LiF/CsPbI_3−xBr_x/P3HT/Au structure. As a result, the CsPbI_3−xBr_x PSCs achieve an excellent performance of 15.84%. The P3HT HTM-based device exhibits good photo-stability, maintaining ~80% of their initial power conversion efficiency over 280 h under one Sun irradiation. In addition, they also show better thermal stability compared with the traditional HTM Spiro-OMeTAD.

Graphical abstract

Keywords

inorganic perovskite solar cell (PSC) / hole-transport material (HTM) / stability

1 Introduction

Photoacoustic microscopy (PAM) has been recognized as a kind of rapid development of medical imaging technology [ 1– 3]. PAM has been applied in brain functional imaging [ 4], detection of early breast tumor [ 5] and dynamic change of oxygen saturation [ 6, 7], monitoring of cerebrovascular activity as an noninvasive manner in small animals, which can provide high sensitivity and high specificity [ 8], and detection of differentiate atherosclerotic plaques [ 9], monitoring of vascular damage during tumor photodynamic therapy [ 10]. In PAM imaging, laser pulses are delivered into biologic tissues. Some of the delivered energy has been absorbed and converted into heat, leading to photoacoustic (PA) effect, inducing transient thermoelastic expansion and thus releasing wideband ultrasonic. The generated ultrasonic waves, namely PA signals, are then detected by ultrasonic transducers to reconstruct images. Then, we can get the distribution of light absorption information in tissues [ 11– 14]. With the characteristic of point source excitation, PAM imaging can be obtained by scanning the biologic tissues point by point. To achieve the above procedure, we can move the imaging tissue with two-dimensional stepper motor [ 15] or move the scanning excitation beam with two-dimensional galvanometer [ 16, 17] and digital micromirror device [ 18, 19]. Considering from the scanning speed and stability, PAM imaging generally adopts the method of moving the scanning excitation beam.

Here, we developed the high-resolution PAM for imaging erythrocytes and blood vessels. In this system, the laser was through a two-dimensional scanning galvanometer, the scanning lens was focused on intermediate image plane. Then, the PAM and optical microscopy simultaneously image the intermediate image plane. Thus, the focus of photoacoustic imaging and the interested imaging area can be quickly found by observation of optical image. The developed PAM has the ability to provide a lateral resolution of 1.0 μm at 532 nm with a × 10 objective. The PA image of blood vessels and capillaries in the mouse ear was reconstructed by the maximum amplitude projection algorithm, and in vitro red blood cells was clearly distinguished, which means it has the potential of clinical application for imaging of erythrocytes and blood vessels. What’s more, our system was the reflection mode, so that optical excitation and ultrasonic detection were the same side of the sample. This configuration makes the proposed PAM much more flexible and convenient in imaging of some thick samples with different structure and function.

2 Methods

The schematic of the experimental setup is shown in Fig. 1. An Nd:YAG laser (LS-2134, LOTIS TII, Belarus), working at the wavelength of 532 nm and this exciting laser has a full width at half maximum (FWHM) of 10 ns with a 15 Hz repetition. The laser, through a two-dimensional scanning galvanometer (6231H; Cambridge Technology, Inc., Lexington, MA, USA), and the scanning lens was focused on intermediate image plane. Then, the light was collimated with a tube lens and focused with the long working distance objective. The focused light passed through a custom-made hollow focused ultrasound transducer and irradiated the biologic tissues to generate photoacoustic signals. The hollow focused ultrasound transducer, which has a center frequency of 50 MHz, was used to receive the PA signals. The charge coupled device (CCD) was used to get the optical microscopy image of the sample. According to the reversibility principle of beam path, the PAM and optical microscopy simultaneously image the intermediate image plane. The focus of photoacoustic imaging and the interested imaging area can be quickly found by observation of optical image. Through the signal amplifier and a dual-channel data acquisition card (NI5124, National Instruments Corp., Austin, TX, USA), the computer recorded the PA signals from each point. Finally, the photoacoustic image was reconstructed by the software of Labview 2012. The sampling rate of the data acquisition card was 200 M samples/s.

Fig.1 Schematic of high-resolution PAM. GM= Galvo scanner; SL= scanning lens; TL1= tube lens 1, TL2= tube lens 2, M= intermediate image plane; BS = beam splitter; OBJ = objective ( × 10, NA= 0.3; × 20, NA= 0.5; × 40, NA= 0.65); UTD= ultrasonic transducer; Amp= amplifier; DAQ= data acquisition

Full size|PPT slide

3 Experiments and results

3.1 Lateral resolution of the system

Lateral resolution of the high-resolution PAM is principally depended on numerical aperture (NA) of the objective, which determines optical focal diameter of the nearly diffraction limit [ 15, 16]. To evaluate the lateral resolution at the laser wavelength of 532 nm, a sharp edge submerged in water was imaged with the PAM system. After scanning across the edge of a surgical knife blade at a 0.2 μm steps, the PA signal values were fitted by a sigmoidal-shaped function, then we can get the fitted edge spread function (ESF) of the system. The optical image and the PAM image of the edge are shown in Figs. 2(a) and 2(b), respectively. Figure 2(c) shows the photoacoustic signal of the white solid line in Fig. 2(b), the black square is the experimental measurement and the red solid line is the fitted ESF. The line spread function (LSF) shown in Fig. 2(d) is the derivative with respect to the ESF. So the lateral resolution of the system, which was evaluated by the FWHM of the LSF, was 1.0 μm with a × 10 objective, approached to the theoretical value 0.51λ/NA ≈ 0.9 μm. The experimental results showed that the transverse spatial resolution of the PAM system can reach the nearly diffraction-limit of the objective.

Fig.2 Lateral resolution of the high-resolution PAM. (a) Optical image of a knife; (b) photoacoustic image of the knife; (c) photoacoustic signal of the white solid line in (b), black square= experimental measurement, ESF= edge spread function, red solid line= theoretical fit; (d) LSF derived from the fitted ESF. LSF= line spread function

Full size|PPT slide

3.2 Testifying the lateral resolution of the system

We first testified the resolution of our high-resolution PAM system by imaging of carbon fibers with different magnification objectives. The carbon fibers we used has a diameter of 7 μm. Figures 3(a), 3(b) and 3(c) present the photoacoustic image of the carbon fibers with the objective magnifications of × 10, × 20, × 40, using the scanning steps of 5, 3 and 1 μm, respectively. And the top right corner of each image is the guided optical image. From this experiment, the carbon fiber was clear imaged. So we can quickly find the focus of photoacoustic imaging and the interested imaging area through the observation of the optical image.

Fig.3 PAM imaging of the carbon fibers. (a), (b), (c) present the photoacoustic image of the carbon fibers with the objective magnification of × 10, × 20, × 40, respectively. The top right corner of each image is the guided optical image. The bar in each picture was 10 μm

Full size|PPT slide

3.3 In vitro imaging of a single erythrocyte

Furthermore, we demonstrated the high-resolution ability of PAM system by imaging a single erythrocyte. The in vitro red blood cells (RBCs) were drawn from the rabbit ear arteries containing anticoagulant. As shown in Fig. 4(a), the PA imaging of a single erythrocyte can be clearly distinguished with high contrast. The diameter of the erythrocyte is about 5-6 μm. An optical image of a rabbit blood sample at the same place is shown in Fig. 4(b) for comparison. Each erythrocyte in photoacoustic image can be perfectly corresponded to the optical image. It is worth mentioning that the optical image in Fig. 4(b) is quite ambiguous. This is mainly because we have covered 500 μm depth of silicone oil on the blood smear. The experimental result proves that the fabricated high-resolution PAM microscopy system is able to image single erythrocyte.

Fig.4 Red blood cells imaged in vitro using our high-resolution PAM. (a) Photoacoustic image of red blood cells; (b) optical image corresponding to (a)

Full size|PPT slide

3.4 In vivo imaging of RBCs and microvasculature in a mouse ear

The high-resolution PAM system was further applied in in vivo imaging blood vessels of a BALB/c mouse. The hair of the mouse ear was chemically depilated before imaging and the mouse were intraperitoneally anesthetized with a mixture of a-chloralose and urethane to keep the mouse with no movement during the experiment. At the edge of a mouse ear, an interesting area with a size of 2 mm × 2 mm was chosen to image. Then using our high-resolution PAM, this selected area was scanned with ~ 5 μm steps. Figure 5(a) is blood vessels in a mouse ear imaged in vivo using our high-resolution PAM. As shown in Fig. 5(a), there is a pair of accompanying venule and arteriole, whose detailed and further complicated branches can be clearly identified. And some smaller microvasculature also can be distinguished in the square. Figure 5(b) is the local amplification of the square area in Fig. 5(a). That one by one point labeled with arrows is likely red blood cell arrayed one by one in microvasculature. The experimental results show that the high resolution PAM system is able to image blood vessels and red blood cells in vivo, which illustrates this system has clinical potential for imaging of blood vessels and red blood cells.

Fig.5 Blood vessels in a mouse ear imaged in vivo using our high-resolution PAM. (a) PA image of the microvasculature in a mouse ear with large field; (b) local amplification of the square area in (a)

Full size|PPT slide

4 Discussion

The experimental results suggest that the PA images of cellular and vascular can be clearly reconstructed by using the high resolution PAM system. There is a pair of accompanying venule and arteriole, whose detailed and further complicated branches can be clearly identified. Some smaller microvasculature also can be distinguished. And likely red blood cells arrayed one by one in microvasculature was also shown. In our experiment, the repetition rate of the pulsed laser influenced the speed of imaging. For this reason, the system cannot accomplish real-time dynamic imaging and monitoring, which means that a laser with higher repetition frequency is required for future work. In addition, an improved transducer with low-resistance electronic devices as well as higher sensitivity would help to reduce the excitation laser power in the future.

5 Conclusions

This paper reported a high-resolution PAM system to image erythrocytes and blood vessels with high contrast. In the present study, the developed system has the ability to provide lateral resolution of 1.0 μm at 532 nm with a × 10 objective. We used a sharp edge to measure the lateral resolution of the PAM and testify the stability with carbon fibers. Also, using this system, blood vessels and capillaries in the mouse ear even a single erythrocyte can be clearly imaged. Therefore, the high-resolution PAM can accomplish imaging of microvasculature and a single erythrocyte in vivo, demonstrating its great potential of clinical applications for imaging of erythrocytes and blood vessels.

References

[1] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of Applied Chemical Science, 2009, 131(17): 6050–6051

[2] National Renewable Energy Laboratory (NREL). Best Cell Efficiencies, available at the website of nrel.gov/pv/cell-efficiency (accessed: January 2019)

[3] Im J H, Jang I H, Pellet N, Grätzel M, Park N G. Growth of CH₃NH₃PbI₃ cuboids with controlled size for high-efficiency perovskite solar cells. Nature Nanotechnology, 2014, 9(11): 927–932

[4] Stoumpos C C, Malliakas C D, Kanatzidis M G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorganic Chemistry, 2013, 52(15): 9019–9038

[5] Dong Q, Fang Y, Shao Y, Mulligan P, Qiu J, Cao L, Huang J. Electron-hole diffusion lengths>175 mm in solution-grown CH₃NH₃PbI₃ single crystals. Science, 2015, 347(6225): 967–970

[6] D’Innocenzo V, Grancini G, Alcocer M J, Kandada A R, Stranks S D, Lee M M, Lanzani G, Snaith H J, Petrozza A. Excitons versus free charges in organo-lead tri-halide perovskites. Nature Communications, 2014, 5(4): 3586

[7] Zhu H, Miyata K, Fu Y, Wang J, Joshi P P, Niesner D, Williams K W, Jin S, Zhu X Y. Screening in crystalline liquids protects energetic carriers in hybrid perovskites. Science, 2016, 353(6306): 1409–1413

[8] Svane K L, Forse A C, Grey C P, Kieslich G, Cheetham A K, Walsh A, Butler K T. How strong is the hydrogen bond in hybrid perovskites? Journal of Physical Chemistry Letters, 2017, 8(24): 6154–6159

[9] Chen H, Xiang S, Li W, Liu H, Zhu L, Yang S. Inorganic perovskite solar cells: a rapidly growing field. Solar RRL, 2018, 2(2): 1700188

[10] Sim K M, Swarnkar A, Nag A, Chung D S. Phase stabilized α-CsPbI₃ perovskite nanocrystals for photodiode applications. Laser & Photonics Reviews, 2018, 12(1): 1700209

[11] Mei A, Li X, Liu L, Ku Z, Liu T, Rong Y, Xu M, Hu M, Chen J, Yang Y, Grätzel M, Han H. A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability. Science, 2014, 345(6194): 295–298

[12] Wang P, Zhang X, Zhou Y, Jiang Q, Ye Q, Chu Z, Li X, Yang X, Yin Z, You J. Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells. Nature Communications, 2018, 9(1): 2225

[13] Wang Y, Dar M I, Ono L K, Zhang T, Kan M, Li Y, Zhang L, Wang X, Yang Y, Gao X, Qi Y, Grätzel M, Zhao Y. Thermodynamically stabilized β-CsPbI₃–based perovskite solar cells with efficiencies>18%. Science, 2019, 365(6453): 591–595

[14] Wang K, Jin Z, Liang L, Bian H, Bai D, Wang H, Zhang J, Wang Q, Liu S. All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15. Nature Communications, 2018, 9(1): 4544

[15] Beal R E, Slotcavage D J, Leijtens T, Bowring A R, Belisle R A, Nguyen W H, Burkhard G F, Hoke E T, McGehee M D. Cesium lead halide perovskites with improved stability for tandem solar cells. Journal of Physical Chemistry Letters, 2016, 7(5): 746–751

[16] Wang Y, Zhang T, Kan M, Zhao Y. Bifunctional stabilization of all-inorganic α-CsPbI₃ perovskite for 17% efficiency photovoltaics. Journal of the American Chemical Society, 2018, 140(39): 12345–12348

[17] Yang F, Hirotani D, Kapil G, Kamarudin M A, Ng C H, Zhang Y, Shen Q, Hayase S. All-inorganic CsPb_1−xGe_xI₂Br perovskite with enhanced phase stability and photovoltaic performance. Angewandte Chemie International Edition, 2018, 130(39): 12927–12931

[18] Sanchez S, Christoph N, Grobety B, Phung N, Steiner U, Saliba M, Abate A. Efficient and stable inorganic perovskite solar cells manufactured by pulsed flash infrared annealing. Advanced Energy Materials, 2018, 8(30): 1802060

[19] Zeng Q, Zhang X, Feng X, Lu S, Chen Z, Yong X, Redfern S A T, Wei H, Wang H, Shen H, Zhang W, Zheng W, Zhang H, Tse J S, Yang B. Polymer-passivated inorganic cesium lead mixed-halide perovskites for stable and efficient solar cells with high open-circuit voltage over 1.3 V. Advanced Materials, 2018, 30(9): 1705393

[20] Bian H, Bai D, Jin Z, Wang K, Liang L, Wang H, Zhang J, Wang Q, Liu S F. Graded bandgap CsPbI_2+xBr_1-x, perovskite solar cells with a stabilized efficiency of 14.4%. Joule, 2018, 2(8): 1500–1510

[21] Swarnkar A, Marshall A R, Sanehira E M, Chernomordik B D, Moore D T, Christians J A, Chakrabarti T, Luther J M. Quantum dot-induced phase stabilization of α-CsPbI₃ perovskite for high-efficiency photovoltaics. Science, 2016, 354(6308): 92–95

[22] Liu J, Wu Y, Qin C, Yang X, Yasuda T, Islam A, Zhang K, Peng W, Chen W, Han L. A dopant-free hole-transporting material for efficient and stable perovskite solar cells. Energy & Environmental Science, 2014, 7(9): 2963–2967

[23] Franckevičius M, Mishra A, Kreuzer F, Luo J, Zakeeruddin S M, Gratzel M. A dopant-free spirobi[cyclopenta[2,1-b:3,4-b′]dithiophene] based hole-transport material for efficient perovskite solar cells. Materials Horizons, 2015, 2(6): 613–618

[24] Liu Y, Chen Q, Duan H S, Zhou H, Yang Y, Chen H, Luo S, Song T B, Dou L, Hong Z, Yang Y. A dopant-free organic hole transport material for efficient planar heterojunction perovskite solar cells. Journal of Materials Chemistry A, Materials for Energy and Sustainability, 2015, 3(22): 11940–11947

[25] Jung E H, Jeon N J, Park E Y, Moon C S, Shin T J, Yang T Y, Noh J H, Seo J. Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene). Nature, 2019, 567(7749): 511–515

[26] Ye Q, Zhao Y, Mu S, Ma F, Gao F, Chu Z, Yin Z, Gao P, Zhang X, You J. Cesium lead inorganic solar cell with efficiency beyond 18% via reduced charge recombination. Advanced Materials, 2019, 31(49): e1905143

[27] Shao Y, Xiao Z, Bi C, Yuan Y, Huang J. Origin and elimination of photocurrent hysteresis by fullerene passivation in CH₃NH₃PbI₃ planar heterojunction solar cells. Nature Communications, 2014, 5(1): 5784

[28] Zeng Q, Zhang X, Feng X, Lu S, Chen Z, Yong X, Redfern S A T, Wei H, Wang H, Shen H, Zhang W, Zheng W, Zhang H, Tse J S, Yang B. Polymer-passivated inorganic cesium lead mixed-halide perovskites for stable and efficient solar cells with high open-circuit voltage over 1.3 V. Advanced Materials, 2018, 30(9): 1705393

[29] Jiang Q, Zhao Y, Zhang X, Yang X, Chen Y, Chu Z, Ye Q, Li X, Yin Z, You J. Surface passivation of perovskite film for efficient solar cells. Nature Photonics, 2019, 13(7): 460–466

[30] Leijtens T, Ding I K, Giovenzana T, Bloking J T, McGehee M D, Sellinger A. Hole transport materials with low glass transition temperatures and high solubility for application in solid-state dye-sensitized solar cells. ACS Nano, 2012, 6(2): 1455–1462

[31] Abate A, Leijtens T, Pathak S, Teuscher J, Avolio R, Errico M E, Kirkpatrik J, Ball J M, Docampo P, McPherson I, Snaith H J. Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells. Physical Chemistry Chemical Physics, 2013, 15(7): 2572–2579

[32] Tiep N H, Ku Z L, Fan H J. Recent advances in improving the stability of perovskite solar cells. Advanced Energy Materials, 2016, 6(3): 1501420

Acknowledgements

This work was supported by the Beijing Municipal Science and Technology Commission (Nos. Z181100004718005 and Z181100005118002).

RIGHTS & PERMISSIONS

2020 Higher Education Press