Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells

CHAPTER 14

Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells Golap Kalita*,†, Masayoshi Umeno**, and Masaki Tanemura† *Center for Fostering Young and Innovative Researchers, Nagoya Institute of Technology, Nagoya, Japan, **Department of Electronics and Information Engineering, Chubu University, Kasugai, Japan, †Department of Frontier Materials, Nagoya Institute of Technology, Nagoya, Japan

Chapter Outline 14.1 Introduction 495 14.2 Material and Methodology

496

14.2.1 Bottom-Up Technique: Chemical Vapor Deposition 497 14.2.2 Top-Down Technique: Electrochemical Etching 498

14.3 Applications in Hybrid Solar Cells 14.4 Recent Trends 502

500

14.4.1 Efficient Silicon Nanocone-Polymer Solar Cells 502 14.4.2 Silicon Nanowires/Polymer Solar Cells on Glass 504

14.5 Conclusion 505 References 505

14.1 Introduction In the last few years, inorganic nanostructures and conducting polymers have gained significant attention for novel and low-cost electronic device applications. Inorganic nanostructures exhibit unique size-dependent electrical, optical, mechanical, and thermal properties with great importance for future electronics and energy-related device applications [1 11]. On the other hand, conducting polymers with metallic or semiconducting properties have great potential for all solution-based roll-to-roll production of flexible organic electronics [12 20]. The organic-inorganic hybrid materials represent a new class of materials that may combine desirable physical properties and characteristics of both organic and inorganic components within a single composite [21 26]. The mixing of polymers and inorganic nanomaterials is opening pathways for engineering flexible Nanostructured Polymer Blends. DOI: http://dx.doi.org/10.1016/B978-1-4557-3159-6.00014-6 © 2014 Elsevier Inc. All rights reserved.

495

496 Chapter 14 composites and tailoring the macroscopic performance of the material. The organicinorganic blend has been extensively studied for photo- and electro-responsive devices, such as photodetectors, light-emitting diodes (LEDs), and solar cells [27 29]. Significant efforts have been devoted toward optimizing the bulk hybrid structure of a solar cell that incorporates inorganic nanocrystals inside the polymer semiconductors. The unique geometry of nanocrystals leads to a large surface area to volume ratio, which is exploited in an inorganic-organic hybrid photovoltaic device to achieve efficient photo exciton dissociation, carrier collection, and transportation. Various inorganic nanostructures acting as electron acceptors, such as Si, III VI, and II IV group materials, have been investigated along with conjugated polymers or small molecules as electron donor material [30 32]. Si nanostructures such as nanocrystals, nanocones, and nanowires were fabricated from Si wafers to fabricate efficient hybrid solar cells at very low processing cost [32 35]. The advantage of nanostructures synthesized from Si wafer is that the electrical properties are directly inherited from the host Si wafer. Nanostructures with selective carrier concentration, conductivity, crystallographic orientation, and type (such as n-type Si nanostructures for n-type Si wafers) can be fabricated with respect to the wafer. On the other hand, fabrication of other inorganic nanostructures require complicated intermediate steps such as template-assisted growth and nanowire growth on an intermediate layer on top of the metal oxide [36 38]. The growth of a nanostructure on an Si wafer is much simpler and effective for device application. Among the one-dimensional nanostructures, Si nanowires are widely considered as an important class of nanoscale building blocks for high-performance device applications. Presently, Si nanowires are investigated widely for solar cell fabrication to achieve high power conversion efficiency and simple, low-cost manufacturing, which can be competitive to conventional crystalline Si solar cell technology. The next section details fabrication processes of various Si nanostructures and hybridization with conducting polymers for device fabrication.

14.2 Material and Methodology Synthesis of Si nanostructures with controlled shape, diameter, length, and electronic properties are essential for device fabrication. There are two basic approaches to synthesis of Si nanostructures: bottom-up and top-down approaches. The bottom-up approach involves nucleation of Si atoms and consequently growth of various nanostructures. The bottom-up approach can be realized by vapor-liquid-solid (VLS) growth, oxide-assisted growth (OAG), and other related methods [39 46]. The Si nanowires’ growth process by the VLS technique is generally carried out with an Au catalytic layer. The Si precursor decomposes on the Au layer to form an Si-metal alloy, then the Si diffuses in the Si-metal

Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells 497 alloy droplet and subsequently precipitation of Si atoms form Si nanowires. The top-down approach is achieved by reduction of the bulk Si wafer by lithography and etching, such as electron beam lithography (EBL) and reactive ion etching (RIE). The other approach is metal-assisted electrochemical etching of an Si wafer to synthesize Si nanowires and other structures [47 55].

14.2.1 Bottom-Up Technique: Chemical Vapor Deposition The chemical vapor deposition process (CVD) via the VLS process is widely used to synthesize dense, high aspect ratio and vertically aligned Si nanowires. In the CVD process, Au or another suitable metal layer is used as a catalyst layer, and a gaseous source such as silane (SiH4) or Si tetrachloride (SiCl4) provides the gaseous reactant (Figure 14.1(a)) [56,57]. The CVD process enables control growth of Si nanowires with control diameter, density, and length with site-selective synthesis. Si nanowires with smaller diameters have been grown using H2 as a carrier gas of the Si precursor materials [43]. In the growth process, H2 can mitigate the radical growth through suppression of reactant adsorption on substrate surface and dissociation of precursor materials (SiH4) [43,57]. The H2 carrier gas can passivate the nanowire’s surface and thereby reduce the surface roughness. Au has been widely used as the catalyst to synthesize Si nanowires in a CVD process by the VLS mechanism (Figure 14.1(b) and (c)) [44]. However, the Au contamination in this process is unavoidable. Alternative catalysts such as Cu, Al, and Pt have been developed to grow Si nanostructures [41,42,57]. Good control over the size, position, and uniformity of vertical, epitaxially aligned, large-area Si nanowires’ arrays on an Si wafer has been achieved using Cu as a catalyst and SiCl4 as an Si precursor [41]. Using a patterned oxide buffer layer in the growth process, prevention of the catalyst migration has been demonstrated.

(a)

(b)

(c)

SiCl4

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Hot wall reactor

Figure 14.1 (a) Schematic diagram of high-temperature CVD for Si nanowire synthesis [56]; (b) and (c) show cross-sectional scanning electron microscopic (SEM) images of Si nanowires grown from 50 and 30 nm Au colloidal catalysts, respectively, by the VLS process (scale bars 1 µm) [44].

498 Chapter 14

14.2.2 Top-Down Technique: Electrochemical Etching The bottom-up technique outlined in the previous subsection enables the synthesis of Si nanowires with high aspect ratios and control lengths over a large area; however, high temperature growth and practical application remain a challenge. Peng et al. demonstrated synthesis of an Si nanowire array by electroless etching (EE) of an Si wafer at room temperature in an HF-AgNO3 solution [57 60]. Highly crystalline Si nanostructures such as nanowires, nanocone, nanoholes, and porous nanostructures can be fabricated using the EE process. One of the most important aspects of this process is that the fabricated nanostructures possess similar crystallographic orientation and carrier type as the base Si wafer. In the EE process, a microscopic mechanism is proposed based on metal-induced local oxidation and dissolution of Si in the aqueous oxidizing HF solution. For electrochemical etching a solution of HF and AgNO3 or Fe(NO3)3 is prepared, where Ag or Fe plays the role of etching catalyst. Electrochemical etching is based on the galvanic displacement of Si by the Ag1-Ag0 reduction on the Si wafer surface (Figure 14.2(a)). Briefly, Ag1 reduces onto the Si wafer surface by injecting holes into the Si valence band and oxidizing the surrounding lattice, which is subsequently etched by the HF solution. The nanowires are vertically aligned to the bottom surface of the host silicon wafer, as the etching occurs vertically to the wafer surface (Figure 14.2(b)). Depending on the etching condition and rate

Figure 14.2 (a) Schematic of tunneling motion of Ag (or other metal) particles in Si matrix leading to formation of Si nanostructures [57]. (b) Cross-section SEM image of the Si nanowire array fabricated by the electroless etching (EE) process [32]. (c) Top surface of the nanowire array presenting a highly porous surface [32].

Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells 499 of reaction, the length or size of the nanostructures can be controlled. The nanowires formed on the surface of the Si wafer are covered by a thick layer of metal dendrite film. The dendrite film can be removed by ringing in deionized water and sonication. The residual metal particles and native oxide layer can be removed by further processing in a nitric acid bath and diluted HF solution, respectively. The large area top surface of the nanowires has a cap-like structure at the free end and presents high porosity (Figure 14.2(c)). The EE process is a simple, low temperature, scalable, and fast approach to synthesis Si nanostructure. The EE process has great significance for practical application as a potential low-cost technique as compared to the conventional CVD-VLS based bottom-up technique. The Si nanowires’ structural morphology can be characterized by transmission electron microscopy (TEM). TEM studies give the diameter distribution and crystalline nature of the nanowires. The nanowires synthesized by the EE process generally have a diameter distribution of around 20 to 250 nm, which significantly depend on the etching reaction process [32]. The crystalline nature of the nanowires can be easily determined from the TEM study and present a highly crystalline structure with the same crystallographic orientation as that of the host Si wafer (Figure 14.3(a) and (b)). Similarly, the crystalline nature of the nanowire or other nanostructures can also be evaluated by the X-ray diffraction (XRD) analysis. The surface morphology of the Si nanowires can also be investigated by the TEM study. The nanowires derived by the EE process show a surface roughness during the rapid etching and oxidation process. In contrast to the smooth surface of typical VLS grown, gold catalyzed nanowires, the nanowires prepared by electroless etching are much rougher. The surface roughness of the nanowires could be attributed to lattice faceting of silicon during Ag-assisted step-by-step HF etching. The presence of surface roughness in Si nanowires derived by EE process contributes to enhancement in thermoelectric properties. The electrical conductivity of the electroless etched nanowires is significantly higher than

Figure 14.3 (a) TEM image of n-Si nanowires fabricated by the electroless etching process, (b) HRTEM image of the nanowires presenting a single crystal nature of the nanowires [32]. In the inset, a fast Fourier transmission (FFT) image of the nanowires shows the plane of orientation similar to the host n-Si wafer [111].

500 Chapter 14 that of the original bulk Si wafer due to surface defects. This is in sharp contrast to the nearly insulating porous Si prepared by electrochemical etching, which shows electrical conductivity several orders of magnitude lower than that of the original Si wafer.

14.3 Applications in Hybrid Solar Cells The process of Si nanostructures fabricated by EE has great potential for fabrication of hybrid solar cells. The nanomaterials can be embedded in a conducting polymer, thereby creating a heterojunction between the inorganic nanostructures and the organic molecules. Consequently, a hybrid system of the Si nanowires and organic materials provides a solar cell with significantly higher efficiency. This type of hybrid solar cell has been fabricated and demonstrated by incorporating a conducting polymer in Si nanowires (Figure 14.4(a)). The polymer solution can be cast on the top of the nanowires and infiltrated to create a heterojunction structure. The length of the nanowires can be controlled (as discussed in Section 14.2.2) and thereby the thickness of the hybrid solar cell can be determined. In the fabricated device, the Si nanowire array acts as an efficient electron collector and transporter in the active layer of the fabricated heterojunction device. In the demonstrated hybrid solar cell the nanowires are n-type (selecting n-Si wafer for fabrication) as is the original host n-Si Light

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Figure 14.4 (a) Schematic diagram of the fabricated hybrid solar cell with n-Si nanowires and poly (3-octylthiophene) (P3OT). (b) J-V characteristics of the device fabricated with the structure P3OT/n-Si nanowires without incorporation of carbon nanotubes in the dark and illuminated (AM1.5 100 mW/cm2). (c) J-V characteristics of the device fabricated with the structure P3OT 1 MWNTs/n-Si nanowires with incorporation of carbon nanotubes in the dark and illuminated (AM1.5 100 mW/cm2) [32].

Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells 501 wafer, which is favorable for photo-induced charge collection from the conducting polymer. Significant enhancement of device performance and conversion efficiency has been obtained with the n-Si nanowire arrays compared to that of a device fabricated on flat Si wafers (Figure 14.4(b) and (c)). This is due to the efficient charge collection and transportation through the interpenetrating nanowire array. On the other hand, in a device fabricated with the flat n-Si wafer, there can be photo exciton dissociation at the polymer and n-Si surface only, which leads to poor exciton dissociation and electron collection. Incorporating carbon nanotubes (CNTs) along with the conducting polymer, efficient hole transport can be achieved in the fabricated device which will enhance overall device performance [32]. Similarly, heterojunction solar cells with Si nanowires have been studied that are combined with other conducting polymers such as regioregular poly(3-hexyl thiophene) (P3HT) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) [61 69]. Poly (3-octylthiophene) (P3OT) and P3HT are semiconducting polymers that are extensively studied for organic solar cell fabrication with an absorption band in the visible light region. The semiconducting polymers contribute solely to the exciton generation and thereby obtain a photovoltaic action. In a heterojunction device fabricated with the semiconducting polymer and Si nanowires, the light absorption and photo-excitation can occur in both the polymers and nanowires (Figure 14.5(a)). However, the poor carrier mobility and Ag pads (front contact)

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Figure 14.5 (a) Schematic diagram of Si nanowires-PEDOT:PSS solar cell. (b) Cross-sectional view of the nanowires. (c) PEDOT:PSS-coated Si nanowires of the fabricated solar cell (scale B1 µm). (d) J-V characteristics of the Si nanowires-PEDOT:PSS heterojunction devices with different Si nanowires lengths. (e) Si nanowires’ length-dependent energy conversion efficiencies [66].

502 Chapter 14 conductivity (B10 S/cm) in the semiconducting polymers limit the conversion efficiency of the solar cell. In contrast, the metallic PEDOT:PSS-conducting polymer provides higher carrier mobility and conductivity (,1000 S/cm). Highly transparent and conducting thin films with low sheet resistance can be fabricated by spin coating the PEDOT:PSS solution. In a fabricated PEDOT:PSS/Si nanowire hybrid solar cell, maximum incident light transmits through the PEDOT:PSS layer and reaches the nanowire’s interface to generate the photo-exciton (Figure 14.5(b) and (c)). The PEDOT:PSS and Si nanostructures’ heterojunction can be explained by the Schottky junction principle as the PEDOT:PSS shows metallic characteristics. Most of the incoming light is absorbed in the Si; thus, the efficiency of the hybrid Si/PEDOT:PSS solar cell is much higher than that of the other hybrid solar cells. A significantly higher efficiency comparable to other organic and inorganic solar cells can be achieved by optimizing the Si nanowire length, and this yields the best conditions for photon absorption and charge carrier collection (Figure 14.5(d) and (e)).

14.4 Recent Trends 14.4.1 Efficient Silicon Nanocone-Polymer Solar Cells Recent development shows that efficient hybrid solar cells can be fabricated by modifying the Si nanostructure morphology. Periodic Si nanocones fabricated by nanosphere lithography were combined with PEDOT:PSS to achieve highly efficient solar cells (Figure 14.6(a)). In this approach, SiO2 nanoparticles synthesized by a modified Stober process were deposited as a monolayer on an Si wafer with a thickness of 500 µm. A monolayer of nanoparticles deposited on the Si substrate by using the Langmuir Blodgett assembly technique shows uniform coverage over a large area. Subsequently, oxygen (O2) and trifluoromethane (CHF3) plasmas were used to reduce the SiO2 nanoparticles’ diameter, followed by chlorine (Cl2) and hydrogen bromide (HBr) plasma etching for the pattern transfer into the Si substrate (Figure 14.6(b)) [35]. The fabricated nanocone structure with an aspect ratio (height/diameter of a nanocone) of less than 2 was an optimized shape for the light absorption enhancement as it provided both excellent antireflecting and light-scattering effects. Completing the fabrication process of Si nanostructures, the Si substrate was cleaned with a piranha and HF acid solution. In particular, the HF acid cleaning step was critical as it removed any native SiO2 layer on top of the Si and any residue of SiO2 nanoparticles that might have been left after the etching process. A hybrid photovoltaic device was fabricated by spin coating a PEDOT:PSS on the nanocone structures’ solution in air. The polymer deposited by the spin-coating method formed a conformal film over the Si nanocone structure (Figure 14.6(c)). As the conformal coating of PEDOT:PSS over the Si nanocone was achieved, other intermediate organic

Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells 503 (a)

Si nanocones by colloidal lithography

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Figure 14.6 (a) Schematic diagram of the fabrication of an Si nanocone/PEDOT:PSS photovoltaic device. (b) Cross-sectional SEM images of the Si nanocones fabricated with the nanosphere lithography and etching process. (c) Formation of a conformal heterostructure with the spin-coated PEDOT: PSS. (d) J-V characteristics of the Si nanocone/PEDOT:PSS solar cell in comparison to a planar device with or without the Au grid [35].

materials were not required for full coverage. The device structure was completed with a finger-grid thin film of gold (Au) as a top electrode as the PEDOT:PSS film is not conductive enough to be used as a top electrode. In the fabricated solar cell with improved optical properties with the novel Si nanostructures, there is significant enhancement in the short-circuit current density (Jsc) (Figure 14.6(d)). It has been observed that a Jsc of up to 39.1 mA/cm2 can be achieved by

504 Chapter 14 the optimal nanocone structure for a 10-µm-thick Si solar cell, which is close to the theoretical limit [35]. This technique of fabricating a hybrid Si nanocone/polymer solar cell is quite promising as it has a simple low-temperature manufacturing process and competes well with conventional solar cell technology.

14.4.2 Silicon Nanowires/Polymer Solar Cells on Glass Poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyleste (P3HT:PCBM) solar cells were fabricated on glass substrate by incorporating Si nanowires. The device was fabricated by transferring the nanowires synthesized by an electrochemical etching process. The Si nanowire arrays were pressed into the P3HT:PCBM film on a hot plate under N2 atmosphere. Subsequently, a homemade machine with lateral force was used to separate the silicon wafer from the P3HT:PCBM film (Figure 14.7(a) (c)); as a result the Si nanowires were retained and inserted into the P3HT:PCBM film. The absorption spectra of the Si nanowires blended P3HT:PCBM film showed absorption beyond 650 nm with improved light harvesting from 650 to 1100 nm. Compared with the ZnO nanorod blended P3HT: PCBM hybrid solar cell, the combination of Si nanowires and P3HT:PCBM blend has the advantage of light harvesting in the near-infrared and visible wavelength. In contrast, the (a)

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Figure 14.7 (a) A schematic of the hybrid solar cell fabricated with a blend of Si nanowires and P3HT:PCBM and the setup for transferring the nanowires onto the P3HT:PCBM blend. (b) SEM images of the nanowires on the Si wafer (in the inset is a cross-sectional image). (c) J-V characteristics for the solar cells with and without the Si nanowires under simulated AM1.5 illumination [67].

Blend of Silicon Nanostructures and Conducting Polymers for Solar Cells 505 band gap of ZnO is higher than 3 eV, so it cannot effectively absorb the solar spectrum in the near-infrared and visible regions. The J-V characteristics of the solar cells with and without the Si nanowires under simulated AM1.5 illumination showed significant difference in Jsc. Incorporating the Si nanowires, the fabricated device has improved performance with a Jsc of 11.61 mA/cm2, open circuit voltage (Voc) of 425 mV, FF of 0.39, and a conversion efficiency of 1.93%. The increase of Jsc can be attributed to faster and more direct pathways which improve the charge carrier collection and transport. This also leads to a reduction in the series resistance. The highdensity Si nanowires also increase the interface areas of Si-P3HT, improving the dissociation of excitons. Again, the Si nanowires can compensate the absorption of the P3HT:PCBM blend with improved light harvesting in the near-infrared region [67]. The studies clearly show significant potential of Si nanowires for hybrid organic-inorganic solar cells. However, the unavoidable polymer degradation in the presence of moisture, water, and its thermal stability is a further challenge in this type of device technology.

14.5 Conclusion In this chapter, the blending of conducting polymers with inorganic nanostructures and associated technological advantages were discussed. There are various inorganic nanostructures to combine with conducting polymers for fabrication of hybrid optoelectronics devices. Si nanostructures’ synthesis by the bottom-up and top-down technologies was explained. We looked at how hybrid solar cells can be fabricated by combining Si nanocrystals, nanowires, and nanocones with conducting polymers for high efficiency solar cells at very low processing cost. Conducting polymers such as poly (3-octylthiophene) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) were solution casted on the nanostructured Si surface to fabricate a heterojunction. The Si nanostructure polymer blend hybrid solar cells showed significant potential, with conversion efficiency achieving more than 10%. Inorganic nanostructures’ conductive polymer blends are a key technology, combining low temperature, easy manufacturing processes, and competing well with conventional solar cell technology.

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