Hexagonal-Tiled Indium Tin Oxide Electrodes To Enhance Light

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Hexagonal-Tiled ITO Electrodes to Enhance Light Trapping in Perovskite Solar Cells Yuqian Huang, Ranran Jin, Zhenzhong Xiong, Shaohang Wu, Ke Cheng, and Wei Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01371 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Applied Nano Materials

Hexagonal-Tiled ITO Electrodes to Enhance Light Trapping in Perovskite Solar Cells Yuqian Huang1, Ranran Jin2, Zhenzhong Xiong1, Shaohang Wu1 , Ke Cheng2* and Wei Chen1* 1

Wuhan National Laboratory for Optoelectronics, Huazhong University of

Science and Technology, Luoyu Road 1037, Wuhan, 430074, China. 2

Key Laboratory for Special Functional Materials of Ministry of Education,

Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, Henan University, Kaifeng 475004, PR China *Corresponding authors. E-mail address: [email protected] (W. Chen); [email protected] (K. Cheng) Keywords:

Perovskite,

Solar

Cells,

Hexagonal-tiled

Management, Light Trapping

ACS Paragon Plus Environment

ITO,

Photon

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The photon management and light trapping could significantly improve the short-circuit photocurrent densities (JSC) and power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) by scattering of the light or reducing the optical losses. However, it remains challenging to fabricate the textured substrate or window layer in PSCs. In this study, hexagonal-tiled ITO substrates are prepared by a simple and low-cost method in terms of photon management and microstructure. The hexagonal-tiled ITO surfaces not only can increase the absorption of light, but also beneficial to extraction of interface charge. Photovoltaic device used hexagonal-tiled ITO substrate with the suitable thickness exhibited a maximum PCE of 18.2%, with the VOC of 1.06 V, JSC of 21.65 mA cm-2 and FF of 0.791, which showed an encouraging improvement of over 7.2% in the JSC and a 7.4% enhancement in the PCE compared with the traditional ITO substrate.


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Metal halide perovskite solar cells (PSCs) have attracted extensive research interest for next-generation solution-processed photovoltaic devices because of low fabrication cost and high power conversion efficiency (PCE).1-3 The top-performing PSCs, which have reached a certified PCE of 22.7%,4 have the potential to outperform state-of-the-art high efficiency silicon-based solar cells. The peculiar historic progresses than others solar cells can be attributed to high absorption coefficient, tunable band gap, long charge carriers diffusion lengths and easy preparation method of material itself.5-10 With the development of the PSCs preparation technology, many innovative strategies have been applied in improving the PCE of PSCs, such as interface engineering,11 solvent additive project,12 with or without mesoporous scaffold design,13 and so on. These effective strategies could significantly enhance the electrical properties of the devices, decrease the carrier loss and obtain encouraging PCEs. However, the photon management and light trapping, which could significantly improve the short-circuit current densities (JSC) and PCEs of PSCs by scattering of the light or reducing the optical losses, has still a great challenge in PSCs for the difficulty of fabricating the textured substrates. The microcrystalline silicon ACS Paragon Plus Environment


thin-film solar cells prepared on hexagonal-tiled surfaces exhibit record JSC and PCE,14 so the photon management and light trapping could play the important role to further enhance the JSC and PCE of PSCs. Some exploratory works have been well studied by the researchers to prepare the textured substrates which could enhance the photon management and light trapping in PSCs.15-19 Yan’s group fabricated the patterned perovskite films based on a microsphere lithography SiO2 honeycomb scaffold template, yielded a maximum PCE of 10.3% with relatively high active layer average visible transmission of 38%.15 Yang’s group designed a mesoporous TiO2 nanobowl array by the sol–gel process and the polystyrene (PS) template, and achieved a PCE up to 12.02% with good stability, which is 37% higher than that of the planar counterpart.16 Fan’s group fabricated PSCs on the plastic substrates with inverted nanocone structures, the device PCE could be improved by 37%, and reached up to 11.29% on nanocone substrates.17 Jang’s group

assembly of HBL-free PSCs based on a

transformed FTO featuring a hierarchically porous surface formed by electrochemically etching of commercially purchased FTO, obtained a remarkable 19.22% efficiency with a low level of hysteresis.18 Song’s group report a effective means to improve perovskite grain sizes using a porous-PEDOT:PSS fabricated by the polystyrene template, the efficiency


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was significantly enhanced from 15.33% for the planar device to 17.32%.19 These works demonstrated that the textured substrates could significantly enhance the light trapping in PSCs and improve the PCEs. Based on the classical device structures of PSCs, the ITO, FTO, ZnO, TiO2, PEDOT:PSS and NiOx could be used to prepare the textured substrates. Because the uneven thickness of semiconductor layers could significantly affect the device performance of PSCs,20 using the ITO and FTO to fabricate the textured substrates is the optimal choice. Up to now, fabrication of the high-order textured ITO substrate is still a big challenge in PSCs. Here, we fabricate the highly ordered hexagonal-tiled ITO substrate using the polystyrene (PS) template and sputtering technology. We first fabricate the PS-coated ITO substrate, and then sputtering ITO on it.21 By changing the sputtering time, the textured ITO substrate with different depths can be obtained. These textured ITO could scatter the light and reduce the optical losses, and improve the photon management and light trapping. So it could significantly improve the JSC and PCEs of PSCs. On the other hand, it could increase the contact areas of P-type NiOx and perovskite films, and then promote the extraction of the charge. Photovoltaic device with textured-ITO/NiOx/MAPbI3/PCBM/BCP/Ag structure exhibited a maximum PCE of 18.2%, with the VOC of 1.06 V, JSC of 21.65 mA cm-2 and



FF of 0.791, which showed an encouraging improvement of over 7.2% in the JSC and a 7.4% enhancement in the PCE compared with the traditional ITO substrate. Our work provide a wonderful paths to improve the photon management and light trapping in PSCs, and demonstrate a competitive strategy to enhance the PCE of PSCs. Scheme 1 is a brief preparation flow chart of hexagonal-tiled ITO. First, a monolayer PS spheres are overlaid on the ITO substrate by the self-assemble process. Afterwards, the ITO with PS spheres is treated with heat and reactive ion etching (RIE) to increase the adhesion and the distance of PS spheres. After sputtering ITO and heat treatment process, the hexagonal-tiled ITO was obtained.

Scheme 1. The preparation flow chart of Hexagonal-Tiled ITO. The typical FESEM images of hexagonal-tiled ITO substrate fabricated by PS template with diameter of ~500 nm is shown in Figure 1. In order to


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investigate the influence of groove depth on optical properties and electric performance of PSCs, the sputtering times were carried out for 6 and 12 min, respectively. Correspondingly, the morphologies are showed in Figure 1a and 1b show the top and cross view FESEM images of hexagonal-tiled ITO substrate with the sputtering time of 6 min (named PS6), and a highly ordered hexagonal-tiled structure was observed. The hexagon groove can be seen clearly (Figure 1a). Figure 1c shows the cross section view FESEM image of PS6, it shows the width and depth of the hexagon were about 500 nm and 65 nm. When the sputtering time increase to 12 min (named PS12), the hexagon is more obviously observed for the depth increasing. The width of the hexagon in PS12 is still about 500nm as shown in Figure 1d and 1e, but the depth increase to about 190 nm (Figure 1f) for the increasing sputtering time. ITO groove depth and sputtering time is out of proportion. The similar width in PS6 and PS12 is attributed to use the same PS template, the sputtering ITO were deposited on the plane ITO surface though the gaps between the PS particles.



Figure 1. The top-view and cross-view FESEM images of PS6 (a, b and c) and PS12 (d, e and f). To further study the microstructure of plane ITO and hexagonal-tiled ITO substrate, we used atomic force microscope (AFM) measurement to assess regularity of the structure. Figure 2a and 2d show the plane ITO AFM images, and the typical AFM images of hexagonal-tiled ITO surface are illustrated in Figure 2b, 2e (PS6), 2c and 2f (PS12). The apparent hexagonal-tiled shape observed in the AFM images are consistent with the FESEM images (Figure 1). The AFM result shows the hexagonal-tiled ITO was highly ordered, the hexagon with the width of ~500 nm and depth with 67.69 nm for PS6 and 191.98 nm for PS12 average surface roughness. Such textured surface is beneficial in scattering the light, reducing the optical losses and increasing the contact areas of P-type NiOx and perovskite films.


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After deposited the NiOx films on the ITO substrates, the average surface roughness of plane ITO, PS6 and PS12 become smooth than the ITO substrates with NiOx (Figure 2g and 2j), but the highly ordered hexagon still remained in the PS6 (Figure 2h and 2k) and PS12 (Figure 2i and 2l). The depth of hexagon became smaller after deposited the NiOx films in PS6 and PS12, because the pit of hexagon easily deposited the thicker NiOx films than the top of hexagon.



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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Figure 2. The AFM and AFM 3D images of plane ITO substrate without (a and d) and with (g and j) NiOx films. The AFM and AFM 3D images of PS6 without (b and e) and with (h and k) NiOx films. The AFM and AFM 3D images of PS12 without (c and f) and with (i and l) NiOx films.


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The UV-visible transmit spectra of the plane and hexagonal-tiled ITO substrates with different sputtering times are shown in Figure 3. Compared with the transmit spectrum of plane ITO, the transmit spectrum of PS6 shows the obvious enhancement from 350 to 450 nm, which could be attributed to the optical scattering caused by the hexagonal texture (Figure 3a).22 The transmit spectrum of PS12 has an overall downward trend, this is mainly due to the increased absorption of ITO for the increasing of hexagonal depth.23 The hexagonal titled ITO could increase the diffuse transmittance, which will change the light propagation path and may increase the light absorption of the devices. For the NiOx-coated ITO samples, the UV-visible transmittance and diffuse transmittance are similar to the NiOx-free samples (Figure 3c and 3d). Based on the FESEM, AFM, UV-visible transmittance and diffuse transmittance results, the NiOx films did not obviously changed the structures and optical properties of hexagonal-tiled ITO substrate. Figure S1 is the reflection spectra of those samples. The reflect spectrum of the PS6 and PS12 are weakened at short wavelength and enhanced at long wavelength. This demonstrate that the hexagonal-tiled ITO substrate could significantly change the optical performance of the ITO substrate. Furthermore, the reflectance and absorbance spectra of NiOx films with different ITO substrates are shown in Figure S2 and S3. There are no significant difference in reflection and ACS Paragon Plus Environment


absorption between the ITO-NiOx and the PS6-NiOx samples, but the absorption rate of PS12-NiOx sample is significantly higher than them for the thicker ITO layers. The high absorption of light is detrimental to the absorption of perovskite films.24

Figure 3. UV-visible transmittance (a) and diffuse transmittance (b) of plane ITO and hexagonal-tiled ITO substrates. UV-visible transmittance (c) and diffuse transmittance (d) of NiOx-coated plane ITO and hexagonal-tiled ITO substrates. The XRD patterns of plane ITO and hexagonal-tiled ITO substrates are shown in Figure 4a. There is no significant difference in the crystallinity and


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conductivity of three types of ITO substrates. Meanwhile, the resistivity of the hexagonal titled ITO electrode (14.8 Ω/□ for PS6, 14.2 Ω/□ for PS12 ) is lower than the ITO reference (15.2 Ω/□). After deposition perovskite layers, the perovskite on PS6 substrate has better crystallinity than the plane ITO samples, but the perovskite on PS12 substrate shows poor crystallinity (Figure 4b). This is due to s suitable height of hexagonal can enhance the crystallinity of the perovskite. When the height of hexagonal is too high, the thin perovskite film in local area lead to the poor crystallinity. According the top-view SEM images (Figure 4c 、 4d and 4e), the hexagonal titled ITO substrates do not significantly affect the surface morphology of the perovskite layer. The average particle size of perovskite film on the plane ITO is ~200 nm. Interestingly, the average particle sizes of perovskite films on the PS6 and PS12 are ~230 and ~180 nm, respectively.



Figure 4. The XRD patterns of bare (a) and NiOx/perovskite-coated (b) three types of ITO substrates. The top-view SEM images of ITO-NiOx-PVK (c), PS6-NiOx-PVK (d), and PS12-NiOx-PVK (e). After deposition perovskite layers on the four types of substrates (glass, ITO-NiOx, PS6-NiOx and PS12-NiOx), steady-state photoluminescence (PL) spectra and time-resolved PL spectra were applied to investigate the ability of hexagonal-tiled ITO extract carriers from the perovskite layers. As shown in Figure 5a, the hexagonal-tiled ITO films shows more efficient steady-state PL quenching than glass and plane ITO, indicating they are demonstrate the better hole extraction capability from absorber layers. In addition, the steady-state PL peak is related to the recombination channel concerning the bandgap and trap state.25 The red-shift phenomenon of PS12-NiOx may ACS Paragon Plus Environment

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imply that it has higher trap density around the band-edge than the other films.26 Furthermore, the blue-shift of PS6-NiOx film demonstrate it has the lower trap density.27 Moreover, the time-resolved PL spectra is shown in Figure 5b. The PL lifetime of four samples (glass-PVK, ITO-NiOx-PVK, PS6-NiOx-PVK, and PS12-NiOx-PVK) were 58.29, 9.69, 6.02, and 4.89 ns, respectively. The PL lifetimes of PS6-NiOx-PVK and PS12-NiOx-PVK obviously decrease than the glass-PVK and ITO-NiOx-PVK. The decrease in PL lifetime for the PS6-NiOx indicates more highly charge extract ability than the plane ITO-NiOx, and PS12-NiOx shows the strongest charge extraction ability.28 The hexagonal-tiled ITO electrode could increase the contact area between perovskite absorber layer and NiOx, this is beneficial to the charge extract ability. Meanwhile, the prominent ITO could shorten the distance between the hole and ITO electrode, so it could extract the hole more easily. Figure 5c shows the current density-voltage curves of the best performing inverted planar devices based on three types of ITO substrates. The extracted performance parameters of these solar cells are shown in Table 1. The perovskite solar cell based on plane ITO has the VOC of 1.059 V, JSC of 20.17 mA cm-2, and FF of 0.792, resulting in a PCE of 16.95%. As for the PS6-based device, the best PCE of 18.21% was achieved, with a VOC of



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1.063 V, JSC of 21.65 mA cm-2 and FF of 0.791. However, the device based on PS12 substrate shows the performance of VOC = 1.039 V, JSC = 19.78 mA cm-2, FF = 0.750, and PCE = 15.41%. The hexagonal-tiled ITO substrates could enhance the light absorbance, but the thicker sputtering ITO could absorb the more light than the thin hexagonal-tiled ITO sample, and the top of hexagon easily deposited the thinner perovskite films. So the PS6-based device show the highest JSC. In addition, the cross-sectional FESEM image of

the

inverted

device

with

the

glass/ITO-PS6/NiOx/perovskite/PCBM/BCP/Ag structure is shown in the Figure 5d. The thicknesses of NiOx, perovskite, PCBM, BCP and Ag layers are about 30, 400, 50, 10 and 80 nm, respectively. For this cross-sectional FESEM image, we can find that the adhesion between perovskite layer and the PS6 substrate is very strong and without any crevices.


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Figure 5. The steady-state PL (a) and time-resolved PL spectra (b) of perovskite/glass,

perovskite/NiOx/ITO,

perovskite/NiOx/ITO-PS6,

and

perovskite/NiOx/ITO-PS12. (c) J-V characteristics of three types of PSCs. (d) Cross-sectional

FESEM

image

of

the

inverted

device

with

the

glass/ITO-PS6/NiOx/perovskite/PCBM/BCP/Ag structure.

Table 1. Performance parameters of three types of PSCs. Device ID

Jsc (mA cm-2)

Voc (V)

FF

PCE (%)

Series resistance (Ω cm2)

Shunt resistance (Ω cm2)

ITO PS6

20.17 21.65

1.059 1.063

0.79 0.79

16.95 18.21

6.2 4.1

2624 2858



PS12

19.78

1.039

0.75

15.41

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6.9

2218

For further investigated the role of hexagonal-tiled ITO substrates, we used more characteristic methods to detect the device internal changes of optical and electrical properties. The approximation absorption spectrum of the three types of absorbers were shown in the Figure 6a. The active layer in the PS6 device has a higher absorption rate than the other devices (from 400 to 700 nm). It is due to the hexagonal-tiled ITO scatter light and enhance the length of light travel through the absorber. It could significantly increase the JSC compared with the plane-ITO-based PSC. But another device with PS12 absorption is decreased. This is because the thicker hexagonal-tiled ITO has the higher absorption, and thus reduce the transmittance. Furthermore, the transmittance spectrum and reflectance spectrum of devices were tested (see Figure S4 and S5), respectively. The incident-photo-to-current conversion efficiency (IPCE) of the three types of devices are shown in Figure 6b. The devices of IPCE measured results are consistent with the results of Figure 6a. The theoretic short circuit density by fitting the IPCE data are 21.99, 20.11 and 19.85 mA cm-2, respectively, which were consistent with the JSC value measured under the solar simulator. The dark J-V curves of three types of devices are shown in Figure 6c. Through fitting the data using the equivalent circuit of Shockley diode, the ACS Paragon Plus Environment

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J0 is calculated.29 The J0 of plane ITO, ITO-PS6 and ITO-PS12 based devices are 1.01*10-10, 1.12*10-10 and 4.07*10-10 A/cm2, respectively. The ITO-PS6 device has a lower charge carrier recombination, but the ITO-PS12 device has a higher charge carrier recombination. The lower leakage density of solar cell indicates the higher rectification ratio and better diode quality.30 In addition, the carrier dynamic along the entire path in the solar cells were studied by the transient photocurrent technique (see Figure 6d). The photocurrent decay lifetimes are 2.24, 1.72, and 1.70 μs for the plane ITO, ITO-PS6 and ITO-PS12 based devices, respectively. The shorter delay time of ITO-PS6-based devices compared with plane-ITO-based device indicates the carrier extraction ability of NiOx with hexagonal-tiled ITO is better than the NiOx with plane ITO.31 This is agree with the PL and time-resolved PL results. Though ITO-PS12 based device has the lowest photocurrent decay lifetime, the efficiency has not been promoted due to higher absorber of ITO layer and more traps in the devices.



Figure 6. (a) The approximation absorption of active layers in three types of devices. (b) The corresponding incident photo to current conversion efficiency (IPCE) curves. (c) Dark J-V curves and (d) normalized transient photocurrent decay for the three types of devices. Figure 7a presents the histogram for PCEs extracted from the stabilized PCEs at the maximum power point of the plane ITO and ITO-PS6 based devices. PSCs used the plane ITO substrates with average stabilized PCE of 15.35% (maximum and minimum values of 16.95 and 14.26%, respectively) could be achieved. When we used the ITO-PS6 as the substrate, the performance dramatically improve to the average stabilized PCE of 17.13% (maximum and minimum values of 18.21 and 15.62%, respectively). This ACS Paragon Plus Environment

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demonstrate the hexagonal-tiled ITO could significantly enhance the device performance. PCEs used plane ITO and ITO-PS6 substrate were monitored under 500 hours light soaking ( 100 mW cm-2, provided by white LED light equipped with a UV blocking filter) in a N2 filled glove box. The and ITO-PS6 based cell retained 93.1% of its initial performance after 500 hours, but the efficiency of plane ITO based device obviously depredated. The development of the ITO-PS6 cells shows the higher stability and device performance compared with the traditional plane ITO cells.

Figure 7. (a) Histograms of PCEs extracted from a photocurrent density stabilized at the maximum power point during 100 s for the plane ITO and ITO-PS6 based PSCs. (b) Long-term photostability test for the plane ITO and ITO-PS6 based PSCs. In summary ， we introduced a simple way to fabricate highly-order hexagonal-tiled ITO substrates, which can be used for the preparation of perovskite solar cells as the skeleton of NiOx. The hexagonal-tiled ITO



substrates, to some extent, could significantly change the path of light transmission and enhance the light absorption. The ITO-PS6 with the suitable thickness of hexagon has the best performance in optics and electricity. The active layer used the ITO-PS6 substrate has the higher light absorption performance, and could increase the contact areas of the p-i junction which make it have the better hole extraction capability compared with the plane ITO substrate. Photovoltaic device used the ITO-PS6 substrate exhibited a maximum PCE of 18.2%, with the VOC of 1.06 V, JSC of 21.65 mA cm-2 and FF of 0.791, which showed an encouraging improvement of over 7.2% in the JSC and a 7.4% enhancement in the PCE compared with the traditional ITO substrate. In addition, the thickness of hexagonal-tiled ITO substrate could seriously affect the device performance. For the ITO-PS12 samples, though the thicker hexagonal ITO has good hole extraction capability, it make the excessive optical loss and more defects in the devices. Finally, we use the simplified design to prepare the highly-order hexagonal-tiled ITO substrate, and it has good commercialization potential arising from its good photon management and light trapping performances.

Supporting Information.


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Experimental details, supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *E-mail: [email protected] (W. Chen); [email protected] (K. Cheng) Conflict of interest There are no conflicts of interest to declare. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation (51672094, 51661135023, 51572070), the National Key R&D Program of China (2016YFC0205002), the Self-determined and Innovative Research Funds of HUST (2016JCTD111), the China Postdoctoral Science Foundation Grant (2016M602286), Guangdong Natural Science Foundation (2017A030313342), the Basic Research Project of Shenzhen Science and Technology Plan (JCYJ201005280434A). The authors appreciate Analytical and Testing Center of Huazhong University of Science and Technology for the sample measurements.

References



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Hexagonal-Tiled Indium Tin Oxide Electrodes To Enhance Light

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