Silicon nanowires in polymer nanocomposites for photovoltaic hybrid thin films

Materials Chemistry and Physics 132 (2012) 284–291

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Silicon nanowires in polymer nanocomposites for photovoltaic hybrid thin films S. Ben Dkhil a,b,∗ , R. Bourguiga a , J. Davenas b , D. Cornu c a

Laboratoire Physique des Matériaux, Structures et Propriétés Groupe Physique des Composants et Dispositifs Nanométriques, 7021 Jarzouna, Bizerte, Tunisia Ingénierie des Matériaux Polymères, IMP, UMR CNRS 5223, Université Claude Bernard – Lyon 1, 15, boulevard Latarjet, 69622 Villeurbanne, France c Institut Européen des Membranes, UMR CNRS 5635, Ecole Nationale supérieure de Chimie, Université de Montpellier, 1919 route de Mende, F34000 Montpellier, France b

a r t i c l e

i n f o

Article history: Received 17 June 2011 Received in revised form 27 October 2011 Accepted 11 November 2011 Keywords: Hybrid solar cells Silicon nanowire Poly(N-vinylcarbazole) Photoluminescence quenching

a b s t r a c t Hybrid thin films combining the high optical absorption of a semiconducting polymer film and the electronic properties of silicon fillers have been investigated in the perspective of the development of low cost solar cells. Bulk heterojunction photovoltaic materials based on blends of a semiconductor polymer poly(N-vinylcarbazole) (PVK) as electron donor and silicon nanowires (SiNWs) as electron acceptor have been studied. Composite PVK/SiNWs films were cast from a common solvent mixture. UV–visible spectrometry and photoluminescence of the composites have been studied as a function of the SiNWs concentration. Photoluminescence spectroscopy (PL) shows the existence of a critical SiNWs concentration of about 10 wt % for PL quenching corresponding to the most efficient charge pair separation. The photovoltaic (PV) effect has been studied under illumination. The optimum open-circuit voltage Voc and short-circuit current density Jsc are obtained for 10 wt % SiNWs whereas a degradation of these parameters is observed at higher SiNWs concentrations. These results are correlated to the formation of aggregates in the composite leading to recombination of the photogenerated charge pairs competing with the dissociation mechanism. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, there has been a growing interest for organic solar cells because of their potential for low-cost manufacturing of a next generation of plastic solar modules exhibiting several advantages such as flexibility, light-weight semi-transparent characteristics and ability to large scale production [1–4]. To increase the power conversion efficiency of organic solar cells, the most common strategy is the production of the socalled bulk (or dispersed) heterojunction (BHJ), in which donors and acceptors are blended to create a heterogeneous composite with a high-interface surface area. Organic materials generally have limited carrier transporting properties [5]. Thus the power conversion efficiency remains limited by the low dissociation probability of excitons and inefficient hopping carrier transport. A potential solution is the use of inorganic nanoparticles or nanostructures, such as nanorods or nanowires, as electron acceptors, to provide not only a large interface area between organic and inorganic components for excitons dissociation but also the high electron mobility of the inorganic phase [6,7]. Several semiconducting polymer/nanostructures have been studied

∗ Corresponding author at: Ingénierie des Matériaux Polymères, IMP, UMR CNRS 5223, Université Claude Bernard – Lyon 1, 15, boulevard Latarjet, 69622 Villeurbanne, France. Tel.: +33472431983. E-mail address: [email protected] (S. Ben Dkhil). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.11.014

in photovoltaic devices [8–10] involving poly(phenylene-vinylene) (PPV) derivative [10–12] or poly(N-vinylcarbazole) (PVK) [9,13] systems. PVK has been chosen for its good hole (hole mobility h is much greater than electron mobility el ) transport properties [14]. PVK has been widely used as an electronic and optical semiconducting polymer. PVK films are transparent over the visible spectrum enabling their integration in windows and available at a moderate cost for a mass production. Poly(N-vinylcarbazole) is also stable in a range of temperature that is compatible with the extrusion process (melt processing) to overcome the increasing regulations for the use of organic solvents in industrial settings. For these reasons PVK (and its derivatives) is a good candidate for photovoltaic applications where the device cost is of prime consideration as in low energy consuming buildings for example. Silicon nanowires are a relatively new silicon nanostructure showing many potential applications in the fields of electronics, sensing, lighting and energy production. Different techniques been investigated to grow silicon nanowires such as chemical vapor deposition (CVD), dry etching, laser ablation, vapor–liquid–solid (VLS) and wet chemical etching [15–19] allowing various potential applications. Several advantages are expected for the use silicon nanowires in photovoltaics. These advantages include a high absorbance of light inducing a lower material use in comparison to bulk silicon for equivalent sunlight trapping [20]. Nanowires have a high surface/volume ratio providing a large interface for charge pair dissociation and are natural pathways for electron transport.

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Fig. 1. (a) Chemical structure of poly(N-vinylcarbazole) (PVK); (b) HRTEM image of a single nanowire showing the crystalline Si core surrounded by an amorphous SiO2 shell.

In this study, we have been interested in the production of hybrid photovoltaic films by the dispersion of silicon nanowires (SiNWs) in a semiconducting polymer to combine the efficient optoelectronic properties of crystalline silicon with the good film forming properties of polymers. Our goal is to develop a simple and robust route for a large scale production of photovoltaic films which could induce a main drop of the cost of the PV electricity production. In this paper we present the fabrication of hybrid thin films based on n-type silicon nanowires dispersed in poly(Nvinylcarbazole) (PVK) as electron donor in the perspective of the development of low cost solar cells. The dissociation of the photogenerated charge pairs in PVK upon incorporation of the SiNWs in the blend has been evaluated by the quenching of the PVK photoluminescence. The relation between the morphology of the composite thin films (dispersion of SiNWs) and the charge transfer between SiNWs and PVK has been investigated. Finally, the optical properties of the composites thin films have been correlated with the photovoltaic properties characterized by current–voltage measurements under white light illumination.

2. Experimental details 2.1. Instrumentation The UV–vis spectra were performed over spectral range 190–900 nm using a PERKIN ELMER Lambda 35 spectrophotometer. Photoluminescence spectra have been performed with a “JOBIN YVON-SPEX Spectrum One” CCD detector, cooled at liquid nitrogen temperature. A monochromator was used to select an excitation wavelength corresponding to the maximum of the absorption band of PVK. The current–voltage characteristics under illumination with a 150 W Xe Oriel solar simulator were obtained with a Keithley 236 source and a PC card for acquisition. Characterizations of the composite morphologies were performed by scanning electron microscopy (SEM Hitachi S800) at a voltage of 15 kV. Transmission electron micrographs are obtained with a Topcon (model EMB-002B) microscope at 200 kV. The thicknesses of thin films are measured using a mechanical profilometer Veeco Dektak 150.

All these measurements were performed under room atmosphere conditions. 2.2. Materials All polymers and solvents were purchased from Sigma–Aldrich and used without further purification. Tetrahydrofuran (THF), which has a relatively high volatility (Teb ≈ 65 ◦ C), was chosen for its good solvent properties for both PVK and SiNWs. The molecular weight of the PVK used in this study was 90, 000 g mol−1 . The molecular structure of PVK is presented in Fig. 1(a). The SiNWs used in this study have been synthesized using the vapor–solid technique without any metal catalyst. An oxide assisted growth (OAG) mechanism [21] based on a dismutation reaction of the silicon monoxide formed from the mixture of Si and SiO2 at 1200 ◦ C under inert gas flow leads to a high production yields of SiNWs. The nanowires, exhibiting a diameter less than 20 nm and some 10 ␮m lengths, are deposited as a foam on the (carbon) collector of the reactor (Fig. 2(a)). SiNWs can be dispersed in a wide range of organic solvents (or in water) to produce a silicon ink for deposition on glass or ITO substrates. At the difference of the silicon nanowires grown by the popular V–L–S technique that has been extensively studied for the production ordered arrays of vertical silicon nanowires for photovoltaic applications, no catalyst was used in this study to avoid pollution by metal species (in particular gold) known to be at the origin of deep electronic levels acting as recombination centers and which are highly undesirable for photovoltaic devices [22,23]. The nanowire morphology, and more specifically their diameter, length, crystallinity and growth orientation can affect the electronic properties such as the band-gap and the electrical characteristics [24]. High resolution transmission microscopy (HRTEM) (Fig. 1(b)) has shown that silicon nanowires consist of a crystalline silicon core surrounded by an amorphous silicon oxide sheath of small thickness acting as a passivation layer of the silicon surface. 2.3. Substrate preparation Glass (suprasil) substrates were used for optical characterizations and ITO-coated glass substrates (ITO-thickness 100 nm) as

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Fig. 2. (a) Elaboration of silicon nanowires by the VS (vapor–solid) technique at T > 1200 ◦ C under gas flow; (b) images of SiNWs in the form of a foam obtained after optimization of experimental procedure; (c) SiNW solution in THF.

Fig. 3. SEM image of SiNWs before (a) and after (b) ultrasonic treatment at high power (250 W/20 min).

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Fig. 4. (a) Schematic representation of the PVK/SiNWs hybrid solar cells: seven cells have been fabricated simultaneously on each ITO substrate (Jp is the generated photocurrent); (b) image of a transparent device containing 10 wt% SiNWs in PVK; (c) representation of energy band diagram and possible charge transportation in the fabricated device with n-SiNWs and PVK.

anodes for electrical characterizations. All substrates were beforehand cleaned in an acetone and ethanol bath at 100 ◦ C, rinsed with isopropyl alcohol and dried under nitrogen. 2.3.1. PVK:SiNWs hybrid film elaboration The as produced SiNWs are obtained as a foam (Fig. 2(b)). The first step is to wash this foam in acetone for a few minutes and then filter the solution. The filtrate is recovered and dried at 80 ◦ C for 15 min. Finally we used a mortar to make the SiNWs powder. The SiNWs powder was dispersed in THF and submitted to a high power ultrasonic probe (250 W/20 min). THF is a rapidly evaporating solvent, which has been used since producing homogeneous SiNWs dispersion (Fig. 2(c)). In fact, extended precipitation effects were observed for commonly used organic solvents like ethanol, acetone, dichloromethane, orthodichlorobenzene and

toluene.Ultrasonic treatment at high power causes cuts of the nanowires (Fig. 3) which reduces their average length. This effect is advantageous in our case because too long nanowires may cause short circuits through the thin layer. PVK was also dissolved in THF at concentrations of 50 mg mL−1 and mixed at appropriate ratio to obtain the needed SiNWs concentration. Then, the dispersed SiNWs were blended with the PVK solution and sonicated for 20 min. After sonication, the solution was left to settle for a few minutes to allow any remaining impurities to sediment out. Homogeneous PVK/SiNWs composite films were prepared from this solution by spin coating (1500 rpm for 40 s) a solution of these two components in tetrahydrofuran (THF) on glass (suprasil) substrates for ultraviolet–visible (UV–vis) absorption or photoluminescence (PL) measurements. Just after spin-coating, the substrates were annealed at 100 ◦ C/10 min under vacuum at a pressure of 10−3 Torr and slowly cooled down.

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2.3.2. Device preparation To study the effect of the SiNWs concentration on photovoltaic performance, hybrid SiNWs/PVK devices with different amounts of SiNWs were prepared according to the following procedure. First, a 50 nm thick film of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) [PEDOT/PSS] was spincasted at 4400 rpm/30 s from an aqueous solution onto ITO-coated glass in laboratory atmosphere. The PEDOT/PSS layer was then dried in vacuum at 120 ◦ C for 20 min to remove the solvent. PEDOT/PSS is commonly used as a hole injection/transport layer for organic optoelectronic devices [25]. The active layer, consisting of a blend of PVK (50 mg mL−1 ) and SiNWs, was then spin coated onto the ITO/PEDOT/PSS layer and was then dried at 100 ◦ C for 10 min. Blends of PVK and SiNWs at different weight ratios of SiNWs ranging from 0 to 15% were prepared. The thickness of the PVK/SiNWs layers was about 100 nm. Back-contacts or a series of Al contacts (typical thickness ≈130 nm) were thermally evaporated on top of the active layer, at a pressure below 10−6 Torr for the electrical characterizations. The active areas of the diodes confined within the overlap of the electrodes were 0.24 cm2 . Fig. 4(a) shows the process used in the fabrication of hybrid solar cells with SiNWs as an electron acceptor and PVK as an electron donor. Energy levels relative to the vacuum level are shown in (Fig. 4(c)) for the components of the photovoltaic cell. The HOMO of PVK is positioned to inject holes into PEDOT:PSS and hence into the ITO electrode. The LUMO of PVK is well above the Fermi level of the n-Si nanowires and electron collection should occur efficiently at the silicon interface. Electrons generated in the nanowires will be collected at the Al electrode.

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Fig. 6. Photoluminescence spectra of PVK/SiNWs thin films (annealed at 100 ◦ C for 10 min under vacuum) with various SiNWs concentrations. Wavelength excitation at 300 nm.

peaks of PVK are identified at 235, 265, 300 and 345 nm. The 1 Lb (345 nm) and 1 La (300 nm) transitions and assigned to the delocalized electrons responsible for the PVK conductivity. These spectra are just the sum of the absorption spectra of the components of the composite films, without additional absorption in the investigated spectral range (200–400 nm). This indicates that charge transfers at the interfaces are negligible at the ground-state. The same spin-coating parameters have been used for all thin films. We can note that the film thickness increases from 60 to 95 nm for 0–25 wt.% SiNW content increase. This increase of the layer thickness with the SiNW content can be attributed to the increase of the blend viscosity. The degree of PL quenching depends mainly of the available interface area between donor and acceptor moieties for charge pair dissociation. It provides an indication of how well the SiNWs are mixed with the polymer and also depends on the quality of the interface between polymer and SiNWs. Fig. 6 shows the PL spectra of pure PVK and PVK/SiNWs composites, with different SiNW 4500 PVK/SiNWs ( 410 nm) 4200 3900

PL intensity (a.u.)

Fig. 5. Absorption spectra of PVK/SiNWs (different wt.% SiNWs) BulkHeterojunction after thermal annealing at 100 ◦ C for 10 min under vacuum. The inset curve shows the absorption spectra of SiNWs thin film.

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Before the fabrication and characterization of solar cells, the optical properties of the organic semiconductors: PVK, and the composite PVK/SiNWs with different SiNW compositions have been studied. Fig. 5 shows the absorption increase of SiNWs:PVK bulkheterojunction for an increasing SiNWs content. Characteristic

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Fig. 8. SEM images showing the surface morphology of SiNWs:PVK thin films with different SiNWs concentrations. Large aggregates are observed for the highest SiNW concentration.

PVK + SiNWs (wt.%) : 0% 1.5% 10% 15%

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Table 1 Photovoltaic parameters of ITO/PEDOT:PSS/PVK:SiNWs/Al devices with different weight ratios of SiNWs ranging from 0 to 15%. Jmax and Vmax are, respectively, the current and the voltage at the maximum power point of the J–V curve in the 4th quadrant. SiNWs mass fraction (%) 0 1.5 10 15

Voc (V) 0.40 0.58 0.89 0.70

Jsc (A cm−2 ) −7

0.88 × 10 1.41 × 10−7 2.44 × 10−7 1.99 × 10−7

concentrations, in the 350–500 nm wavelength range under excitation at 300 nm (corresponding to the edge of the conduction band of the polymer). The characteristic PL peak is observed around 410 nm for PVK which corresponds to the carbazole moiety of PVK. This peak is significantly quenched in the PVK/SiNW composite films with increasing SiNW concentrations. The PL quenching of the polymer fluorescence begin for a relatively low SiNW concentration (1.5 wt.%). An optimum PL quenching is observed for 10 SiNW wt.%.PL quenching in hybrid donor–acceptor blends upon photoexcitation of the donor is usually indicative of either energy or charge transfer from the donor to the acceptor [26–29]. Because there is a negligible overlap between the emission spectrum of the PVK and the electronic absorption spectrum of the SiNWs, it is likely that the PL quenching is due to charge-transfer rather than resonance energy transfer. Recombination centers could also contribute to photogenerated charge pair dissociation. It is in particular the case of surface defects like silicon dangling bonds, but such an effect can be considered as negligible in this study due to the surface passivation of silicon nanowires by a thin oxide layer. Interestingly, the films corresponding to the high SiNW concentrations (17 and 25 wt.%) show a slight increase in PL intensity in comparison to the film with 10 SiNW wt.%. This increase in emission intensity may be explained by the formation of SiNW aggregates resulting in smaller interfacial area with the polymer. This confirms that an optimum amount of SiNWs can provide the proper mixing and networking in the hybrid layer which is desirable for optimum PV conversion. The variation of the photoluminescence intensity versus SiNWs concentration, shown in Fig. 7 at a wavelength of 410 nm, exhibits a minimum around 10 wt.% of SiNWs. Above this concentration, the photoluminescence intensity increases in correlation with the increase of radiative charge recombinations.

3.2. Morphological study In order to understand the condition mechanisms involved in the SiNWs/PVK composites, it is essential to know how the SiNWs are organized and dispersed in the polymer matrix. Fig. 8 shows scanning electron microscopy (SEM) images of the morphology of PVK/SiNW composites films for different SiNWs concentrations. A more homogenous dispersion of SiNWs in PVK is observed for 10 wt.%. If the SiNW amount is increased and becomes greater than 10 wt.%, the formation of large SiNWs aggregates is observed. That can be explained by the tendency of SiNWs particles to agglomerate in solution as well as during spin-coating to reduce their free energy. This result is in good agreement with the variation of the photoluminescence, which was presented in Fig. 6, where a new increase of the PL intensity appears for SiNW weight fraction larger than 10%. Fig. 8 shows also that the number and size of large aggregates increases strongly when the concentration of SiNWs exceeds 10 wt.%. This evolution corresponds to the beginning of phase separation leading to a reduction of the total interface area and as a consequence of charge dissociation efficiency.

Vmax (V)

Jmax (A cm−2 )

FF

0.19 0.33 0.53 0.46

0.56 × 10−7 0.79 × 10−7 1.38 × 10−7 0.97 × 10−7

0.30 0.32 0.33 0.32

3.3. Characteristics of ITO/PEDOT:PSS/PVK:SiNWs/Al diodes under illumination Table 1 shows the photoelectric parameters of the solar cells as function of the SiNWs concentration in the SiNWs:PVK composite. Current–voltage characteristics of the devices under white light illumination at 19 mW cm−2 with a xenon lamp corrected for AM 1.5 solar spectrum are shown in Fig. 9. The electrical performance of a solar cell can be characterized by its open-circuit voltage Voc , its short-circuit current density Jsc and its fill factor FF. As shown in Fig. 9, the introduction of SiNWs increases both the open-circuit voltage Voc and the current density Jsc compared to the polymer alone. In our study, we found the optimum PV parameters, as short-circuit current density (Jsc = 244 × nA cm−2 ), and open-circuit voltage (Voc = 0.89 V), for 10 SiNWs wt.%. When the content of SiNWs is increased a degradation of these parameters is observed. Then, the ability to form electron transporting paths through the formation of percolation threshold is limited by the phase separation leading to the formation of large domains of aggregated SiNWs. 4. Conclusions In this study, we have shown that the combination of PVK and SiNWs leads to a good donor–acceptor system, appearing as a promising candidate to form optically active materials for photovoltaic applications. The quenching of the poly(N-vinylcarbazole) photoluminescence upon the addition of SiNWs implies that there is a process competing with the PVK radiative emission in the PVK/SiNWs hybrids which was attributed to electron transfer from photoexcited PVK to the SiNWs. So silicon nanowires have the potential to improve charge transfer and charge collection efficiency in photovoltaic devices. The main role of the composite morphology in controlling the charge transport through the active layer has been demonstrated.We have shown that it was possible to obtain hybrid solar cells based on organic materials which do not absorb in the visible. The choice of materials absorbing in the ultraviolet has been oriented by the need to develop transparent solar cells exhibiting thermal stability which could be fitted to windows and building structures for increased energy savings. The optimization of the photovoltaic of PVK/silicon nanowire hybrid devices is a step toward the realization of low cost solar cells. The low absorption of the visible spectrum of the polymer can explain the low current densities, because few excitons are created in the active layer. Future work will focus on the optimization of the hybrid layer morphology through thermal treatments and investigation of the factors limiting the photocurrents. Acknowledgements We thank the Photovoltaic ANR for its support for this work performed in the framework of the Physipo Project. We thank Pr. Arnaud Brioude (Laboratoire des Multimatériaux et Interfaces, UCBL) for the transmission electron micrographs of SiNWs. We are very grateful to the microscopy center of Lyon (CLYM) for TEM observations.

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