Solution-processable bismuth iodide nanosheets as hole transport
layers for organic solar cells
Karunakara Moorthy Boopathi
a,b,c, Sankar Raman
d, Rajeshkumar Mohanraman
a,b,e,
Fang-Cheng Chou
d, Yang-Yuang Chen
e, Chih-Hao Lee
b, Feng-Chih Chang
f,g,
Chih-Wei Chu
c,h,n,1aNanoscience and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan bDepartment of Engineering and Systems Science, National Tsing Hua University, Hsinchu 30013, Taiwan
c
Research Center for Applied Science, Academia Sinica, Taipei 115, Taiwan
d
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
e
Institute of Physics, Academia Sinica, Taipei 115, Taiwan
f
Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
g
Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan 32043, Taiwan
hDepartment of Photonics, National Chiao Tung University, Hsinchu, 300, Taiwan
a r t i c l e i n f o
Article history: Received 6 June 2013 Received in revised form 23 August 2013 Accepted 27 October 2013 Available online 15 November 2013 Keywords:
Bismuth iodide nanosheet Hole transport layer Low temperature Solution-processable Organic photovoltaics
a b s t r a c t
In this paper we demonstrate the use of low-temperature-solution-processable bismuth iodide (BiI3)
nanosheets as hole transport layers in organic photovoltaics with an active layer comprising poly(3-hexylthiophene) (P3HT) mixed with a fullerene derivative. The performance of the resulting devices was comparable with that of corresponding conventionally used systems incorporating polyethylenediox-ythiophene:polystyrenesulfonate (PEDOT:PSS). UV–vis spectroscopy revealed that the transparency of a BiI3layer in the visible (4620 nm) and near-infrared range is greater than that of a PEDOT:PSS layer.
X-ray photoemission spectroscopy of a BiI3film revealed signals at 158.8, 164, 618.6, and 630 eV—
characteristic of Bi 4f7/2, Bi 4f5/2, I 3d5/2, and I 3d3/2, respectively—that indicated a stoichiometric BiI3film.
Wet milling of BiI3crystals resulted in the formation of nanosheets, the presence of which we confirmed
using scanning electron microscopy. The resultant power conversion efficiency of the device was approximately 3.5%, with an open-circuit voltage of 0.56 V, a short-circuit current density of 10.4 mA cm–2, and afill factor of 60.1% under AM1.5G irradiation (100 mW cm2).
& 2013 Elsevier B.V. All rights reserved.
1. Introduction
Harnessing natural energy might be the best approach toward satisfying today's growing world energy demands, with solar energy the undisputed frontrunner among all such sources. Organic photovoltaics (OPVs) based on polymeric materials are promising candidates for harvesting solar energy for various reasons, including simple solution-processability, light weight
[1], mechanicalflexibility and transparency[2,3], and the ability to produce solar panels on large scale at low cost [4–6]. In particular, solution-processed bulk heterojunction (BHJ) solar cells are receiving much attention because of their superior mechanical robustness, easy blending, and high power conversion efficiencies (PCEs). Devices incorporating blends of regioregular poly(3-hex-ylthiophene) (P3HT) as the electron donor and phenyl C61-butyric
acid methyl ester (PCBM), a soluble fullerene derivative, as the acceptor have reached PCEs of 3–5%[7,8]. P3HT forms long, thin conducting nanowires and PCBM forms more-homogeneous nano-crystalline films when annealed. The importance of annealing during processing cannot be overestimated; it increases phase separation, crystallization, and the photophysical and transport properties of the active layer[7].
A buffer layer is an important constituent between the electrode and active layer; it plays the crucial role of extracting and transport-ing the photogenerated carriers (holes or electrons). While it allows movement of one kind of the carrier, it blocks the passage of the other through an energy mismatch, often resulting in a dramatic increase in PCE [9–11]. Polyethylenedioxythiophene:polystyrene-sulfonate (PEDOT:PSS) is used widely as a standard anode buffer layer as well as standalone indium tin oxide (ITO)-free anode in OPVs because of its excellent transporting properties, high trans-parency, and smooth textured surface [12–15]. Nevertheless, because of its acidic and hygroscopic nature, PEDOT:PSS can interact physically and chemically with adjoining layers, thereby degrading device performance [16,17]. In the quest for replacements for Contents lists available atScienceDirect
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0927-0248/$ - see front matter& 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.solmat.2013.10.031
nCorresponding author: Chih-Wei Chu at Research Center for Applied Science,
Academia Sinica, Taipei 115, Taiwan.
E-mail address:[email protected] (C.-W. Chu).
PEDOT:PSS, researchers have identified various metal oxide hole transport materials such as tungsten oxide (WO3)[18–20],
molyb-denum oxide (MoO3)[20–22], nickel oxide (NiOx)[23], and
vana-dium oxide (V2O5) [20,24] for OPVs with improved device
performance. However, these materials require stringent conditions of high vacuum, high temperature processing and high power consumption which increases the resulting cost of fabrication. Solution processable alternatives like graphene oxide (GO)[25,26]
are attracting interest due to their various advantages.
Herewith we show synthesis of solution processable BiI3
nanosheets and effectively demonstrated as hole transport layer (HTL) for OPVs. BiI3is a layered semiconducting material having a
wide band gap (ca. 2 eV) [27–30]; it has potential applications in room-temperature
γ
-ray detectors [31] and X-ray digital imaging sensors[32]. Recently, appreciable interest has been shown in the optical properties of BiI3 because of its strong intrinsic opticalanisotropy [27,33,34]. BiI3 adopts a layered structure (Fig. 1) with
Bi3þions establishing six-fold coordination with I–ions, which adopt non-linear two-fold coordination with I–Bi–I angles close to 901. The Bi–I bonds are highly ionic with the 6p electrons of the Bi atoms transferred to the I atoms. The I–Bi–I layers are held together through weak van der Waals forces, allowing BiI3 crystals to be cleaved
readily along the [00l] direction; such weak van der Waals bonding does, however, make this material soft and difficult to handle.
2. Experimental 2.1. Chemicals
Sodium tellurite (99%), bismuth(III) nitrate pentahydrate (99.99%), iodine (99.99%), polyvinylpyrrolidone (PVP; MW¼40,000), ethylene glycol (EG, 99%), hydrazine monohydrate (64–65%), acetic acid (99.7%), isopropanol (99.5%), ethanol (99.99%), and acetone (99.9%) were purchased from Sigma–Aldrich and used without further purification.
2.2. Synthesis of BiI3crystals
A solution of NaTeO3 (0.3 M) in EG (5 mL), a solution of
Bi(NO3)3
9H2O (0.3 M) in EG (3.5 mL), PVP (0.5 g), acetic acid(3 mL), and hydrazine monohydrate (0.5 mL) were added to EG (50 mL) and stirred for 20 min. The resulting homogeneous solution
was transferred to a 100-mL Teflon-lined stainless-steel autoclave. The sealed vessel was then heated at 1601C for 2 h before cooling to room temperature. Acetone (20 mL) was added and then the product was separated through centrifugation (12,000 rpm, 1 h) and washed sev-eral times with a mixture of acetone and ethanol. Thefinal product was dried in an oven at 801C overnight. A solution of Bi2Te3(0.1 M) in
water (25 mL) and a solution of I2 (0.6 M) in water (25 mL) were
mixed and then stirred for 30 min; the resulting homogeneous solution was transferred to a 100-mL Teflon-lined stainless-steel autoclave. The sealed vessel was then heated at 1801C for 10 h before cooling to room temperature. Deionized water (100 mL) was added and then the product separated through centrifugation (12,000 rpm, 1 h) and washed several times with a mixture of water and ethanol (75:25 mL). Thefinal product was dried in an oven at 80 1C overnight.
The synthesis was based on the disproportionation of I2:
Bi2Te3þ6I2þ6H2O-2BiI3þ3H2Teþ3I2þ3H2Oþ3/2O2 (1)
Elementary step
6I2þ6H2O-6HIþ6 HIO (2)
6 HIO-3 I2þ3H2Oþ3/2 O2 (3)
Bi2Te3þ6 HI-2BiI3þ3H2Te (4)
In the hydrothermal process, the pH of the reaction system decreased to less than 1. Thus, it is believed that single crystals of BiI3 were soluble in hot water under strong acidic conditions.
In addition, the instability of HIO [see Eq. (3)] meant that the reaction did not produce BiOI, consistent with the X-ray diffraction (XRD) data. The influence of the reaction time and temperature on the preparation of crystalline BiI3was also investigated; the optimal
conditions for the formation of highly crystalline BiI3 were a
temperature of 180–190 1C for 10–15 h. If the reaction temperature was below 1701C or the reaction time was less than 6 h, the yield of BiI3diminished and the as-synthesized BiI3was poorly crystalline.
2.3. BiI3nanosheets: preparation and characterization
The resultant BiI3 crystals were dispersed in isopropanol
(0.25 wt%) and ground to a fine powder at room temperature using a homemade grinder operated at 2000 rpm for 120 min[35]. The solvent and weight-percentage of BiI3crystals were optimized
based on device performance (seeFigs. S1 andS2andTables S1
andS2in Supplementary information). No surfactant or modifying agents were added during grinding. The resultant suspension containing BiI3 nanosheets was kept for a long period to check
its stability; no further precipitation was observed. Powder XRD patterns were recorded at room temperature—using a Bruker D8 X-ray diffractometer equipped with a diffracted beam monochro-mator set for Cu K
α
radiation (λ
¼1.54056 Å)—in the 2θ
range 10– 801 with a step size of 0.016551 and step time of 0.4 s. Transmis-sion spectra of the films were measured using a Jacobs V-670 UV–Vis spectrophotometer. Scanning electron microscopy (SEM) images were recorded using an FEI Noval 200 scanning electron microscope (15 kV). X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 Versa Probe equipped with an Al Kα
X-ray source (1486.6 eV). Atomic force microscopy (AFM) images of spin-coated BiI3 films were recorded using a Vecco di Innovainstrument operated in the tapping mode. 2.4. Device fabrication and characterization
ITO-Coated glass substrates (o10
Ω
sq–1, RiTdisplay) were cleaned through ultrasonication—once in detergent (20 min) and subsequently twice in deionized (DI) water (20 min each)—and then dried under N2gas and before placing in an oven overnight.Fig. 1. Crystal structure of BiI3; the Bi and I atoms are displayed as green and red
spheres, respectively. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)
Immediately prior to spin-coating of the BiI3layer, the substrates
were treated with ultraviolet (UV)/ozone for 15 min to clean the surfaces and also to improve the surface adhesion. BiI3nanosheets
were spin-coated onto the ITO surfaces at different spin speeds (1000–5000 rpm) for 1 min and then annealed at 100 1C for 30 min. The nanosheet-coated substrates were transferred to a glove box for coating with the active layer. A blend of P3HT:PCBM (1:1) in 1,2-dichlorobenzene (DCB) was spin-coated (600 rpm, 1 min) to form a thin film (ca. 200 nm) of the active layer. The active layer-coated substrates were kept under a Petri glass dish for 30 min for controlled solvent evaporation and then they were placed on hot plate for thermal annealing at 1301C for 30 min. To complete the structure of the device (Fig. 2), the top contact was formed through sequential thermal evaporation of Ca (30 nm) and Al (60 nm) through a shadow mask under vacuum (pressure: 1 10–6Torr). The active area of each device was 10 mm2
. The devices were illuminated with a solar simulator (Thermal Oriel 1000 W), which provided a simulated AM 1.5 spectrum (100 mW cm–2), inside a glove box using a Xe lamp. The light intensity was calibrated using a mono-silicon photodiode with a KG-5 color filter (Hamamatsu). Devices were encapsulated in transparent glass using UV gel and treated with UV light to prevent oxidation during the measurement of the external quan-tum efficiency (EQE). The EQE spectra were recorded under short-circuit conditions; the light source was a 450-W Xe lamp (Oriel Instruments, model 6123NS). The light output from the mono-chromator (Oriel Instruments, model 74100) was focused on the photovoltaic cell and the EQE curve was measured.
Fig. 2. (a) Schematic representation of the device structure of a fabricated solar cell. (b) Chemical structures of P3HT and PCBM.
Fig. 3. XRD patterns of the as-prepared BiI3single crystals, nanosheets, powder
and the BiI3standard.
Fig. 4. Transmission spectra of bare ITO and of ITO coated with layers of BiI3and
Fig. 6. SEM images of BiI3on glass substrates: (a,b) before grinding and (c, d) after grinding.
3. Results and discussion
Fig. 3(a) presents a single-crystal XRD pattern indicating that the BiI3 material crystallized in a single phase that could be
indexed to a space group of rhombohedral R-3 structural symme-try with cell parameters (a¼7.519 Å; c¼20.720 Å) in good agree-ment with published results[36].
Fig. 3(b) shows the room-temperature powder XRD pattern of BiI3
nanosheets prepared by homemade wet grinder; all the peaks are indexed with reference data (JCPDS, No. 00-048-1795), no impurity phase present in the exfoliated BiI3nanosheets.Fig. 3(c) displays the
room-temperature powder XRD pattern of a powder sample obtained using the hydrothermal method; all of the signals in this pattern can be indexed to a hexagonal structure, with no traces of any impurities. We refined the structural parameters using the Rietveld technique with quality refinement parameters. The refined lattice parameters (a¼7.525 Å; c¼20.710 Å) were consistent with the values reported in the literature (JCPDS, No. 00-048-1795). We detected no impurities (e.g., BiOI). Each unit cell consists of three I–Bi–I layers stacked along the [00l] direction; within each I–Bi–I layer, three close-packed atomic sheets are stacked in the sequence I–Bi–I[34]. Because stacking of the layers is rarely perfect, stacking faults are commonly found in BiI3crystals[37,38].
Fig. 4displays transmission spectra of bare ITO, PEDOT:PSS-coated ITO, and BiI3-coated ITO. Among these systems, the PEDOT:PSS-coated
ITO exhibited the highest optical transmittance in the range 380– 500 nm because the smoothness of the ITO surfaces increased after modification with the PEDOT:PSS (40 nm) layer. The solution-processed BiI3(8 nm) layer on the ITO was highly transparent in
the visible region; its transparency was better than that of the PEDOT: PSS-coated ITO in the range 620–900 nm. The highly transparent BiI3
layer might, therefore, have various applications as an active layer material that can absorb near-IR wavelengths and, thereby, improve device performance.
We recorded XPS spectra to measure the elemental composi-tion and surface characteristics of the BiI3 film. The XPS survey
spectrum of the BiI3film contained predominant peaks for Bi and I
atoms along with peaks representing Si, C, and O elements (see
Fig. S3, Supplementary information). We attribute the XPS peak centered at a binding energy of 284 eV to adventitious hydrocarbon (C 1s) arising from the XPS instrument. The strong signals for O and Si atoms in the XPS spectrum emanated from the glass substrate; no other impurities were evident in thefilm, in good agreement with the XRD data.Fig. 5reveals the Bi 4f7/2and 4f5/2peaks (at 158.8 and
164 eV, respectively) and I 3d5/2 and 3d3/2 peaks (at 618.6 and
630 eV, respectively) that are characteristic of Bi3þ and I–species,
respectively. Taken together, these results confirm that a stoichio-metric BiI3film was formed on the substrate.
We prepared samples for SEM characterization by drop-coating BiI3onto a glass substrate and then further annealing the system at
1001C for 15 min, followed by sputtering with gold. We recorded SEM images of the samples obtained before and after grinding the BiI3
crystals.Fig. 6(a) and (b) displays images of the samples incorporating the non-ground bulk BiI3 crystals; they reveal layered structures
stacked one over another. Similarly, Fig. 6(c) and (d) presents the structures of the samples prepared using the ground sample, revealing fine textured sheets that formed a smooth film on the substrate. The stacked BiI3layers were separated and formed nanosheets as a result
of shear stresses and mechanical forces during the wet milling process. We spin-coated (4000 rpm) the BiI3 nanosheets onto UV-O3
pretreated ITO-coated glass substrates and then annealed them at 1001C for 30 min.Fig. 7(a) and (b) presents AFM images of the bare ITO and the BiI3-modified ITO, respectively. Initially, the root mean
square (rms) roughness of the ITO surface was 3.69 nm; it decreased to 2.6 nm after modification with the BiI3film. Such an increase in
smoothness would tend to decrease the surface scattering of the
irradiated light flux from the device. The Tapping Mode (TM) deflection images of the BiI3-modified ITO inFig. 7(c) (low
magni-fication) and (d) (high magnimagni-fication) suggest quite uniform coverage of the BiI3nanosheets on ITO.
We fabricated devices having the structure glass/ITO/BiI3/P3HT:
PCBM/Ca/Al (see Fig. 2) for further investigation of the photo-voltaic performance and the EQE when using BiI3as the HTL. We
recorded the J–V characteristics of P3HT:PCBM OPVs prepared with BiI3 (0.25 wt% in isopropanol) deposited at various spin
speeds (1000–5000 rpm) to investigate the effect of the thickness of the BiI3layer on the device performance [Fig. 8(a)] under light
from a solar simulator operated at 100 mW cm–2 (AM 1.5G).
Table 1lists the corresponding device parameters.
Among all of the tested systems, the device incorporating the BiI3layer spin-coated at 4000 rpm exhibited the highest PCE (ca.
3.5%), with an open-circuit voltage (Voc) of 0.56 V, a short-circuit
current density (Jsc) of 10.4 mA cm–2, and afill factor (FF) of 60.10%.
A traditional OPV containing PEDOT:PSS as the HTL exhibited a PCE of approximately 3.83%, with a value of Vocof 0.60 V, a value of
Jsc of 10.09 mA cm–2, and a FF of 63.26%. Thus, the PCE of the
device featuring the BiI3 layer was comparable with that of
the device containing the PEDOT:PSS buffer layer. Increasing the thickness of the BiI3layer led to relative decreases in the value of
Jscand the PCE (Table 1). The limiting performance was reached at
a spin speed of 4000 rpm; thereafter, increasing the spin speed degraded the device performance, possibly because of erosion of
Fig. 8. (a) J–V characteristics of devices incorporating BiI3 as the HTL layer,
deposited at various spin rates. (b) EQE spectra of devices prepared with (BiI3or
the BiI3layer on the ITO surface. To confirm whether the BiI3layer
functioned as an efficient HTL, we prepared a device in which we spin-coated the active layer directly onto the ITO surface. The poor PCE (ca. 1.06%), with a value of Voc of 0.32 V, a value of Jsc of
9.15 mA cm–2, and a FF of 36.20%, confirmed that the BiI3
nanosheets did indeed play an effective role, forming an efficient HTL between the ITO and the active layer.
Fig. 8(b) presents EQE spectra of solar cells featuring a BiI3or
PEDOT:PSS layer, or no HTL. Among these systems, the maximum EQE (56%) was reached at a wavelength of 555 nm for the device incorporating the BiI3layer deposited at a spin rate of 4000 rpm.
The maximum EQE for the PEDOT:PSS-containing device was 55% at 500 nm, while that for the device lacking an HTL was 50.5% at 580 nm. The EQEs of the PEDOT:PSS-modified device were better than those of the BiI3-modified device within the wavelength
range 300–500 nm; above 500 nm, however, the BiI3-modified
device was more efficient. These results are consistent with the transmittance of each material inFig. 4.
Fig. 9displays the dependence of the device performance on the annealing temperature. All the samples were annealed for 30 min in air atmosphere. Although we observed no appreciable variations in PCE for the devices prepared with or without annealing, the PCE of the device obtained after annealing at 1001C was greater than those treated at other annealing temperatures. For annealing temperatures above 1001C, the PCEs of devices decreased because the thin BiI3
nanosheets were less stable on the substrate. Accordingly, BiI3
nanosheets appear to be useful materials for preparing flexible substrates at low temperatures.
4. Conclusion
We have demonstrated that solution-processable BiI3nanosheets
can be used as HTLs in conventional OPV devices. We synthesized BiI3
crystals through hydrothermal processing and then ground them using a simple and cost-effective wet milling method to form the BiI3nanosheets. We optimized the efficiency by varying the thickness
of the BiI3nanosheets HTL, the concentration of BiI3in the solution
used for spin-coating, as well as the annealing temperature of the HTL. The low-temperature-solution-processed BiI3nanosheets appear to be
compatible with organic materials, suggesting their applicability in various organic electronics.
Acknowledgments
We thank Mr. Hua-Yang Liao and Professor Jing-Jong Shyue (RCAS, Academia Sinica) for help with the SEM and XPS measure-ments and Mr. Ankur Anand (IAMS, Academia Sinica) for help with the preparation of this manuscript. C.-W.C. thanks the National Science Council (NSC), Taiwan (NSC101-2221-E-001–010, NSC101-2120-M-009-001), Ministry of Education of Taiwan (through the ATU program) and the Thematic Project of Academia Sinica, Taiwan (AS-100-TP-A05), forfinancial support.
Appendix A. Supplementary information
Supplementary data associated with this article can be found in the online version athttp://dx.doi.org/10.1016/j.solmat.2013.10.031. References
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Table 1
Detailed parameters of the performance of devices incorporating BiI3 HTLs
deposited at various spin rates, incorporating PEDOT:PSS as the HTL, and prepared without an HTL.
Spin rate (rpm) Voc(V) Jsc(mA cm–2) FF (%) PCE (%)
1000 0.59 5.83 63.95 2.20 2000 0.59 6.67 63.02 2.48 3000 0.58 8.38 62.75 3.05 4000 0.56 10.40 60.10 3.50 5000 0.55 10.01 57.40 3.16 PEDOT:PSS 0.60 10.09 63.26 3.83 W/O HTL 0.32 9.15 36.20 1.06
Fig. 9. J–V characteristics of photovoltaic devices incorporating BiI3as the HTL
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