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UV- and NIR-protective semi-transparent smart windows based on metal halide solar cells Karunakara Moorthy Boopathi, Chintam Hanmandlu, Anupriya Singh, Yang-Fang Chen, Chao Sung Lai, and Chih Wei Chu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00152 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Energy Materials

UV- and NIR-Protective Semi-Transparent Smart Windows Based on Metal Halide Solar Cells Karunakara Moorthy Boopathi,† Chintam Hanmandlu,†, ‡ Anupriya Singh,†, § Yang-Fang Chen,§ Chao Sung Lai,‡ and Chih Wei Chu*, †, ∥, ξ †

Research Center for Applied Science, Academia Sinica, 128 Academia Road, Sec. 2, Nangang, Taipei

115, Taiwan (R.O.C). ‡

Department of Electronic Engineering, Chang Gung University, No. 259, Wenhua 1st Road, Guishan

District, Taoyuan City 33302, Taiwan (R.O.C.). §

Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan

(R.O.C.). ∥College

of Engineering, Chang Gung University, No. 259, Wenhua 1st Road, Guishan District, Taoyuan

City 33302, Taiwan (R.O.C.). ξ

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013,

Taiwan (R.O.C.).

Abstract In this study, a solution-processable lead iodide semiconductor having a wide band gap was investigated as a light absorbing material for various organic electron transport materials, in a search for low-cost semiconductor materials allowing the facile fabrication of efficient photovoltaic devices. A Tauc plot suggested a wide intrinsic optical bandgap of 2.4 eV for a thin film of PbI2, while X-ray diffraction revealed that the spin-coated PbI2 thin film had a hexagonal crystalline structure with preferable orientation along the (001) plane. The effect of the light intensity on the values of Voc and Jsc was studied to investigate the charge recombination mechanism of fabricated devices. An efficient bifacial solar cell was prepared featuring a thin Ag film sandwiched between BCP and MoO3 layers as a transparent rear electrode. The whole device featuring the BCP/Ag/MoO3 electrode exhibited a maximum transmittance of

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approximately 60% at visible region, less than 15% at UV region and less than 25% in NIR region. A power conversion efficiency of 2.19% was achieved for a device featuring an opaque electrode (Ca/Al), while the corresponding device featuring the transparent electrode (BCP/Ag/MoO3) provided values of 0.75 and 0.67% when illuminated from the front and rear, respectively. Thus, wide band gap metal halide materials potentially open up a new path for fabricating efficient and transparent photovoltaic devices having applications as buildingintegrated smart windows. It also effectively prevents the penetration of UV and NIR light, which is harmful for human health, into the building. Keywords: Wide bandgap semiconductor, transparent electrode, building integrated, smart window, bifacial solar cells. 1. Introduction Among the renewable energies, solar energy is considered the most promising because of its low cost and infinite resources. Photovoltaic (PV) devices that convert solar energy into electricity are among the most important technologies providing renewable energy. Traditionally, a PV module generates electricity when installed in an open area exposed to direct sunlight. In urban environments, however, the limited rooftop space of high-rise buildings inhibits the installation of large numbers of PV modules. To further lower the cost of PV installation, ground-mounted PV modules must change into building-integrated photovoltaics (BIPVs) to take advantage of the larger facade areas.1-4 Semi-transparent PVs play a crucial role in BIPVs because of they can admit light into building interiors, thereby replacing traditional windows.5 The preparation of high-performance semitransparent solar cells will require the transparent conductive film as electrode. H. W. Lin, et al. used microcavity embedded sandwich structure in organic solar cells


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to get highly transparent electrode.6, 7 Thin metals (few nanometers) with sandwich structure are considered to be a good transparent electrode due to low sheet resistivity and ease of fabrication.7 Finding suitable semiconducting active materials with wide band gaps is critical to fabricating transparent smart windows for BIPVs. Smart windows should not only generate electricity but also act as shields blocking harmful sunlight (UV or NIR) from entering buildings. Metal halide semiconductors have some unique electrical and optical properties that differ from those of conventional semiconductors (e.g., CdS, GaAs, CdTe, ZnO). Layered semiconducting metal halide materials (PbI2, HgI2, BiI3, SbI3) lead to a special confinement of charge carriers in multilayered or multi quantum well structures, allowing their potential use in detectors, sensors, photocatalysis, and PV applications.8-14 Lead iodide (PbI2) is a wide band gap semiconductor (Eg = ca. 2.4 eV)9 having an larger atomic weight, high electron (4600 cm2 V–1 s– 1

) and hole (3000 cm2 V–1 s–1) mobility15 and high light absorption coefficient (3.16×10–4 cm–1 at

495 nm)16 which makes it a good candidate for use in radiation detectors17 and lasers,18 and is a main precursor in the preparation of lead halide perovskite solar cells.19 PbI2 has a layered structure consisting of a repeating unit of a hexagonally closed packed layer of lead ions sandwiched between two layers of iodide ions.20, 21 Several groups have investigated the optical and excitonic properties of single-crystalline PbI2 films and thin layers.22-24 Recently, Lyubov et al. used a vapor deposition method to prepare Ag2PbI4/PbI2 blends as photoactive materials, and achieved power conversion efficiencies (PCEs) as high as 3.9%.25 Other than in perovskite-based solar cells, very few studies have been made into the use of PbI2 as a photoactive layer for solar cell applications. Further investigations on PbI2 as a photoactive layer will enhance our understanding of the photophysics of the organic metal halide perovskites and their applications.



In this study, we used solution-processing techniques to fabricate PV devices incorporating PbI2 as a photoactive layer with various organic electron transport layers (ETLs). First, the thicknesses of the PbI2 layer and ETLs were optimized to obtain reasonable PV performances with high visible transparency. A PCE of 2.2% was achieved for a device incorporating PbI2/N2200 as the active layer and an opaque electrode (Ca/Al). A bifacial solar cell featuring a transparent electrode (BCP/Ag/MoO3) yielded front and rear efficiencies of 0.75 and 0.67%, respectively. Furthermore, to mimic the operation of a bifacial solar window, both sides of the solar cell device were illuminated using two solar simulators; the illumination from front side was set at 1 sun, while variable light intensity was applied for the rear side illumination to imitate weaker indoor light. 2. Results and discussion Figure 1 provides a schematic representation of the device structure and an energy band diagram of the fabricated solar device featuring an opaque electrode; detailed fabrication procedures are provided in the Experimental section. Here, PbI2 was the main contributor to the light absorption, while N2200 and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) were the ETL and hole transport layer (HTL), respectively. Different types of ETL layers were investigated to ensure appropriate band alignment for efficient charge transfer with PbI2 [see Figure S1 in Supporting Information (SI)]. Figure 1c reveals the crystal structure of PbI2, with edge-sharing PbI6 octahedra leading to strong intralayer bonding in a hexagonal crystal lattice, but only weak interlayer bonding, thereby allowing the formation of different polytypes for the stacking of layers. Figure 1d presents the chemical structure of N2200.


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Figure 1 (a) Device architecture and (b) energy band diagram of a PbI2/N2200 hybrid solar cell featuring an opaque electrode (Ca/Al). (c) Crystal structure of PbI2; the Pb and I atoms are displayed as brown and yellow spheres, respectively. (d) Chemical structure of N2200. (e) XRD



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pattern of annealed PbI2 film on a glass/PEDOT:PSS substrate. (f) SEM image of spin-coated PbI2 film on a PEDOT:PSS-coated glass/ITO substrate, revealing a smooth and continuous film. 2.1. Characterization of crystalline phase and morphology Figure 1(e) presents the powder X-ray diffraction (XRD) pattern of the spin-coated PbI2 thin film on glass/PEDOT:PSS. The spin-coated and annealed PbI2 thin film exhibited diffraction peaks at values of 2θ of 12.75, 25.58, 38.71, and 52.38°, corresponding to the (001), (002), (003) and (004) planes, confirming the formation of a hexagonal crystal structure. The relatively high intensity of the peak at 12.75° confirms a predominant orientation along the (001) direction. All the diffraction peaks were readily indexed to a pure hexagonal phase of the PbI2 structure, as reported previously.26 The SEM image in Figure 1(f) reveals that the morphology of the PbI2 film was smooth and continuous on a PEDOT:PSS substrate. This continuous uniform PbI2 film was obtained when using dimethylsulfoxide (DMSO) as the solvent, due to its strong coordination to PbI2.27. When using dimethylformamide (DMF) as the solvent, a non-continuous film was obtained because of the ready crystallization of PbI2 (see Figure S1). 2.2. PV performance of PbI2 solar cell featuring an opaque Ca/Al electrode We

fabricated

devices

having

the

structure

glass/indium

tin

oxide

(ITO)/PEDOT:PSS/PbI2/N2200/Ca/Al (see Figure 1a) to investigate the PV performance and the external quantum efficiencies (EQEs). To obtain complete absorption in a semiconductor, the thickness of the sample should be on the order of a few micrometers, but this dimension is unrealistic for thin-film solar cells because of high defect-related carrier recombination.28,

29

Figure 2a displays the UV–Vis absorbance spectra of PbI2 thin films of various thicknesses. Increasing the film thickness enhanced the absorption of light in the range from 300 to 530 nm,


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but for wavelengths greater than 530 nm there was no considerable absorption because of the wide band gap of PbI2 (Eg = ca. 2.4 eV). Figure S1 presents the current density–voltage (J–V) curves of devices prepared using PbI2 dissolved in various solvents. The value of Jsc of the device prepared using DMSO as the solvent for PbI2 was higher than that of the device prepared using DMF, presumably because of the uniform and continuous film formed when using DMSO, thereby facilitating charge transfer through fewer deflections from grain boundaries (see Figure S2). A PCE of 1.29% was achieved for the PbI2 (DMSO) films, while for PbI2 (DMF) films gave a PCE of 0.54%. Figures S3–S5 and Tables S2 and S3 (SI) present the results of optimization of the device fabrication conditions. Figure 2b displays the J–V characteristics of the devices incorporating PbI2 films having thicknesses of 30, 40, 55, and 80 nm. Table 1 summarizes the PV parameters. The best device performance was achieved at a thickness of 55 nm, with a PCE of 2.19%, a value of J of 4.67 mA cm–2, a value of Voc of 1.01 V, and a fill factor (FF) of 46.9%. Thinner films led to comparably lower efficiency, mainly as a result of lower absorption, as supported by the data in Figure 2a. A thicker film (80 nm) also led to a decrease in PCE, presumably because of greater charge recombination within the film, due to shorter diffusion lengths and unequal charge mobility. The hysteresis behavior of the PbI2-based solar cells was investigated by measuring their J–V characteristics over various scan directions. A conventional PbI2 hybrid solar cell exhibited negligible hysteresis (Figure 2c). Figure 2d reveals a broad EQE from 300 to 530 nm, matching well the absorption spectra of the PbI2 film in Figure 2a. The value of Jsc integrated from the EQE spectrum (4.2 mA cm–2) was close to that measured using a solar simulator (4.76 mA cm–2).



Figure 2 (a) Absorption spectra of PbI2 films of various thicknesses; Inset showed absorption spectrum of N2200 neat film. (b) J–V characteristics of devices incorporating PbI2 films of various thicknesses. (c) Influence of scanning direction on the performance of the champion solar cell device by under AM 1.5 G irradiation. (d) EQE spectrum and integrated photocurrent density of the champion device. Table 1 PV performance of PbI2/N2200 hybrid solar cells incorporating PbI2 layers of various thicknesses.


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PbI2 thickness (nm)

Voc (V)

Jsc (mA cm–2)

FF (%)

PCE (%)

30

0.66

1.10

31.7

0.23

40

1.06

3.78

49.2

1.97

55

1.01

4.67

46.9

2.19

80

1.04

2.10

52.7

1.15

2.3. Effect of recombination in PbI2 solar cells To determine the effects of two competing recombination processes, namely bimolecular and trap-assisted recombination, the current–voltage characteristics of the devices were measured under light intensities ranging from 100 to 10 mW cm–2 [Figure 3(a)]. Figure 3(b) presents the variation in the value of Voc with respect to the natural logarithm of the light intensity. The slope of the line had a value of 2.04KBT/q. A simplified case of Shockley–Read–Hall (SRH) recombination predicts a value of Voc of 2KBT/q from the plot of Voc with respect to the natural logarithm of the light intensity; for bimolecular recombination, the predicted value of Voc would be KBT/q.30-32 Thus, in our system, SRH recombination through trap states or defect levels was predominant over bimolecular recombination.



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Figure 3 (a) J–V characteristics of PbI2/N2200 solar cells featuring an opaque electrode, measured under light intensities ranging from 100 to 10 mW cm–2. (b) Measured values of Voc of hybrid solar cells plotted with respect to light intensity on a logarithmic scale. Inset: Measured values of Jsc of hybrid solar cells plotted with respect to light intensity on a logarithmic scale. Fitting to a power law (eqn. 1) yielded the value of α. To gain a better understanding of the charge recombination kinetics, we studied the variation of Jsc with respect to the illumination intensity. Several researchers have observed a power law dependence of Jsc upon the light intensity, using equation (1) J α Iα

(1)

where a value of α of 1 indicates a system featuring weak bimolecular recombination and a value of α of less than 1 generally signifying a variation in mobility between carriers or energy barriers in the device.33 Linear fitting of the curve in Figure 3(b) inset gave a value of α of 0.86, suggesting further studies of the mobility of the carriers in the device. Our study reveals that recombination occurred mainly through the defect levels, and that variations in the mobility of charge carriers might be a significant reason for the low values of Jsc. Apart from this finding, a


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film having a higher density of grain boundaries might potentially limit the efficiency of the device because it would restrict carrier transport, leading to high carrier losses. 2.4. PV performance of bifacial PbI2 solar cell based on transparent BCP/Ag/MoO3 electrode Power-generating windows based on solar cells are very limited because they must be transparent in the visible regions of the solar spectrum. Figure 4a presents a bifacial solar device structure featuring BCP/Ag/MoO3 as the top transparent electrode. In our previous work, we have measured the optical (absorption and transmittance) and electrical (sheet resistance) properties of BCP/Ag/MoO3 electrodes of various thicknesses.34 For this study, we used the optimal thicknesses of BCP (8 nm), Ag (15 nm), and MoO3 (40 nm). The fabrication conditions were similar to those used to obtain the opaque electrode device, except for the top electrode; details are provided in the experimental section. Figure 4b displays a cross-sectional SEM image of the complete device featuring the 55-nm thin PbI2 layer, 30-nm N2200 layer, and top electrode (BCP/Ag/MoO3); this system exhibited the high transparency required for use in window applications. The photograph of a typical PbI2-based hybrid solar cell with a transparent top electrode reveals it had a greenish/yellow tinge and good transparency. To examine the optical properties, the transmittance of each layer was measured (Figure 4d). The transmittance of ITO/PEDOT:PSS/PbI2 revealed good transparency in the wavelength range from 800 to 530 nm, with less than 15% transparency in the range 300–450 nm; that is, most of the near-UV light was absorbed by the PbI2 active layer. After deposition of N2200 on the PbI2 film, a slight decrease in transmittance was observed in the range from 600 to 750 nm, due to the absorption of N2200. The whole device featuring the BCP/Ag/MoO3 electrode exhibited its maximum transmittance of 60% at a wavelength of 506 nm, with the transmittance decreasing at higher



wavelengths—for example, a transmittance of less than 25% for wavelengths greater than 700 nm, due to absorption by the top electrode in this region.34 Near-IR (NIR) light is absorbed effectively by BCP/Ag/MoO3 electrodes (Figure S6), making such devices suitable for smart window applications. By using this solar device as a building-integrated window, we could effectively prevent the penetration of UV and NIR light, which is harmful for human health, into the building. The J–V characteristics of the devices were measured under 1-sun light illumination (100 mW cm–2) from the sides of the ITO (front) and BCP/Ag/MoO3 (rear) electrode, respectively. A device performance of 0.75%—with a value of Jsc of 3.13 mA cm–2, a value of Voc of 0.66 V, and an FF of 36.3%—was achieved with illumination from the ITO side; a PCE of 0.67%—with a value of Jsc of 2.83 mA cm–2, a value of Voc of 0.65 V, and an FF of 36.4%—was achieved with illumination from the BCP/Ag/MoO3 side (Table 2). The rear efficiency is almost similar to the front efficiency, suggesting high transparency of the rear electrode in the UV–Vis region. The transparent top electrode absorbed light mainly in the NIR region, beneficial for a wide band gap semiconducting active layer.


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Figure 4 (a) Cartoon representation of the device architecture of a PbI2/N2200 hybrid solar cell featuring a transparent BCP/Ag/MoO3 electrode and (b) corresponding cross-sectional SEM image. (c) Photograph of a solar device featuring a transparent top electrode. (d) Transmittance



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spectra of PbI2, PbI2/N2200, and PbI2/N2200/BCP/Ag/MoO3 on ITO/PEDOT:PSS substrates. (e, f) J–V characteristics of a device featuring a transparent electrode under light illumination from (e) the front and rear and (f) both sides (using two solar simulators); the front-side illumination intensity was set at 1 sun, while variable sun intensity was used for the rear-side illumination. Table 2 PV performance of PbI2-based hybrid solar cells illuminated with light from different sides. Illumination side

Voc (V)

Jsc (mA cm–2)

FF (%)

PCE (%)

ITO side

0.66

3.13

36.3

0.75

BCP/Ag/MoO3 side

0.65

2.83

36.4

0.67

Furthermore, to mimic the bifacial operation of a solar cell device in the form of a solar window, both sides of the device were illuminated using two solar simulators; the front-side illumination intensity was set at 1 sun and variable light intensity was applied for the rear-side illumination (to imitate weaker indoor light), as displayed in Figure 4f. The power output increased linearly upon increasing the illumination intensity from the side of the BCP/Ag/MoO3 electrode; Table S5 lists the corresponding PV performance. The increase in the value of Voc was insignificant upon increasing the illumination intensity from the BCP/Ag/MoO3 side. The dependence of the value of Voc on the intensity would imply a minor role of trap-assisted Shockley–Read–Hall recombination.32 The PCE of device increased, from 066 to 0.81%, upon increasing the light intensity from the rear side. Using this architecture, indoor light could


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presumably be recycled effectively. Therefore, such transparent-electrode solar devices have potential utility in indoor light applications. 3. Conclusion Solution-processable, wide-band-gap, PbI2 semiconductor–based hybrid solar cells have been fabricated using opaque and transparent electrodes. A device having an opaque electrode exhibited a PCE of 2.19% when the thickness of the PbI2 layer was 55 nm. A study of the charge recombination kinetics, determined from the variations in the values of Voc and Jsc in response to the illumination intensity, suggested that SRH recombination through trap states or defect levels was predominant over bimolecular recombination. Semitransparent and bifacial PbI2-based hybrid solar cells were fabricated to feature a thermally evaporated BCP/Ag/MoO3 transparent electrode. Device performances of 0.75 and 0.67% were achieved upon illumination from the ITO and BCP/Ag/MoO3 sides, respectively. Although the efficiency remained below the average of regular solar cells, the flexibility of the application of such devices in smart windows, facades, and car glass roofs should generate a great deal of interest. By using such solar devices as building-integrated smart windows, we could effectively prevent the penetration of UV (by the PbI2 active layer) and NIR (by the BCP/Ag/MoO3 electrode) light, which are harmful to human health, into buildings.

Methods Materials Lead iodide (PbI2, 99.998%), DMSO, and chlorobenzene (CB) were purchased from Alfa Aesar and used without further purification. P2F-DO:NDI2OD-T2 (N2200), [6,6]-phenyl-C60-butyric



acid methyl ester (PC60BM), [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM), indene-C60 bisadduct (ICBA), and perylene diimide (PDI) were purchased from Lumitech (Taiwan). Solar cells ITO-coated glass substrates (

UV- and NIR-protective semi-transparent smart windows based on

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