Hybrid photovoltaic devices based on chalcogenide nanostructures [PDF]

Ana F. Nogueira*a a Chemistry Institute, University of Campinas - UNICAMP, P.O. Box 6154, Campinas,. SP, Brazil, 1308397

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Invited Paper

Hybrid photovoltaic devices based on chalcogenide nanostructures Jilian N. de Freitasa, João Paulo C. Alvesa, Lasantha Koralab, Stephanie L. Brockb and Ana F. Nogueira*a a Chemistry Institute, University of Campinas - UNICAMP, P.O. Box 6154, Campinas, SP, Brazil, 13083970; b Department of Chemistry, Wayne State University, Detroit, MI, USA 48202. ABSTRACT Solar cells based on the combination of conjugated polymers and fullerenes are among the most promising devices for low-cost solar energy conversion. Significant improvements in the efficiency have been accomplished, but some bottlenecks still persist. The substitution of fullerenes by inorganic semiconductor nanoparticles, especially CdSe and CdS, has been investigated as a promising alternative. In this work, we highlight two aspects to be considered in the pursuit of more efficient devices. By comparing different polymer/CdSe systems, we show how the polymer structure can be used to tune the charge transfer from the polymer to CdSe. Even if this process is efficient, the charges will be trapped in the inorganic phase if the charge carrier transport of the nanoparticles is poor. An elegant way to improve the electron hopping is to form an electrically integrated network of nanoparticles. The use of chalcogenide aerogels is a new alternative which may be interesting for applications requiring maximal transport of charge and is also discussed here. Keywords: CdSe, aerogel, P3HT, PCBM, polyfluorene, hybrid solar cells

1. INTRODUCTION Organic solar cells based on conjugated polymers are among the most promising devices for low-cost solar energy conversion. The “classical” device consist of a bulk-heterojunction of a polymer-fullerene network, usually poly(3-hexylthiophene) (P3HT), and a soluble fullerene derivative, such as [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The introduction of small alkyl thiol molecules, novel low-band gap polymers, PC71BM and optimizations of solvent conditions and device architecture are responsible for significant improvements in the efficiency, reaching ~ 7 % [1,2]. However, bottlenecks still persist: morphology control, the mismatch with the solar spectrum and stability. Hybrid solar cells assembled with the substitution of fullerenes by a vast variety of inorganic semiconductor nanoparticles have been investigated as promising alternatives. Probably, the most used materials are CdE nanostructures (E= S, Se, Te). In 2002, Huynh et al. [3] combined CdSe nanorods (7 nm x 60 nm) with P3HT. The best device presented Jsc of 5.7 mA cm-2, Voc of 0.7 V and FF of 40 %, with power conversion efficiency of 1.7 %. Later, it was found that, by optimizing the solvent mixture during film deposition, even higher conversion efficiencies could be obtained [4,5]. Other CdSe structures, such as tetrapods [6] and hyperbranched nanocrystals [7] have also been used in combination with P3HT or a poly(pphenylenevinylene) (PPV) derivative, resulting in devices with efficiencies around 1-2 %. Sun et al. [8] compared the performance of solar cells assembled with CdSe nanorods or tetrapods and observed that the branched nanoparticles lead to more efficient devices. The elongated and branched particles provide more extended electric pathways, resulting in more efficient devices. In 2005, Sun et al. [9] used CdSe tetrapods in combination with P3HT and films spin-cast from 1,2,4-trichlorobenzene solutions resulted in devices

Organic Photovoltaics XIII, edited by Zakya H. Kafafi, Christoph J. Brabec, Paul A. Lane, Proc. of SPIE Vol. 8477, 847711 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.928845

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with efficiencies of 2.8 %, which is among the highest values reported for this kind of device. Recently, a record efficiency of 3.2 % was reported by Dayal et al. [10] for a bulk heterojunction solar cell of CdSe tetrapods and a low band gap polymer, (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PC PDTBT). Meanwhile, some attempts to make hybrid solar cells using spherical CdSe nanoparticles resulted in devices with less than 1 % of efficiency. In 2006, Choi et al. [11] reported hybrid solar cells based on a mixture of CdSe spherical nanoparticles with P3HT or a PPV derivative and obtained Jsc of 2.16 μA cm-2, Voc of 1.0 V, FF of 20 % and η of 0.05 % in the best case. In 2007, Tang et al. [12] used spherical nanoparticles covered with 2-mercaptoacetic acid in solar cells in combination with a PPV derivative. The devices delivered Jsc of only 2.6 μA cm-2, Voc of 0.58 V and FF of 28 %. Han et al. [13] used spherical CdSe nanoparticles crystallized in zinc blend structure, passivated with 1-octadecene and oleic acid, and combined with a PPV derivative in solar cells. After annealing, the devices delivered Jsc of 2.0 mA cm-2, Voc of 0.90 V, FF of 47 % and η of 0.85 %. Recently, a record efficiency of 2 % obtained by using a combination of spherical CdSe nanoparticles with P3HT was reported by Zhou et al. [14]. The authors achieved this efficiency by treating the nanoparticles with hexanoic acid, which removed the excess of surfactants accumulated around the nanoparticles surfaces. Comprehensive reviews on polymer/CdSe systems can be found in the literature [15-19]. An analysis of the results reported by different groups reveals that the characteristics of the nanoparticles (i.e., size, shape and type of capping ligands) are crucial to the performance of the device. This is also the case for devices assembled with metal nanoparticles, as recently shown by Wang et al. [20]. Morphology is another parameter that plays a major role in hybrid systems, as observed for “standard” polymer/fullerene systems. In this work, we discuss two different strategies that can also be used to tune the properties of hybrid devices: modification of the polymer structure and the nanoparticle network.

2. EXPERIMENTAL 2.1 Materials Synthesis of CdSe nanoparticles: CdSe nanoparticles were synthesized according to the procedure by Peng et al. [21]: CdO (0.11 g, 0.89 mmol), tetradecylphosphonic acid (TDPA) (0.43 g 0.15 mmol) and TOPO (7 g) were loaded into the reaction flask and heated to 120 °C under argon and degassed for 20 min. The mixture was heated to 340 °C, until the solution became colorless. A stock selenium solution (1 mmol of Se dissolved in 5 mL of trioctylphosphine) was quickly injected and the mixture was kept at 340 °C for 2 h. Brownish CdSe nanoparticles were isolated by precipitation with methanol, and purified from two cycles of dissolution with toluene and precipitation with methanol. Synthesis of aerogels: TOPO-capped CdSe nanoparticles were shaken vigorously in a solution of mercaptoundecanoic acid (MUA) in methanol (0.16 mol L-1; pH ~ 10 adjusted with tetramethylammonium hydroxide) to achieve ligand exchange. The MUA-capped nanoparticles were washed with ethyl acetate and dispersed in methanol. Gelation was initiated by adding a tetranitromethane solution (3 wt% in methanol). Wet gels were aged several days and exchanged with acetone to remove byproducts, followed by liquid CO2 exchange in a critical point drier and drying by supercritical extraction to afford CdSe aerogel powders. Preparation of hybrid films: Nanocomposite films (thickness ~90-120 nm) consisting of polymer/CdSe, polymer/PCBM or ternary system polymer/CdSe/PCBM were obtained by co-dissolving the materials in toluene or chlorobenzene and spin-casting the dispersions (800 rpm, 40 s). Three different polymers were used: commercially available regioregular P3HT (Aldrich), commercially available poly[2-methoxy-5(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV, Aldrich) and poly(9,9-n-dihexyl-2,7-

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fluorenilenevinylene-alt-2,5-thienylenevinylene) (PFT). A detailed characterization of PFT can be found elsewhere [22]. PCBM was purchased from Nano-C and used as received. 2.2 Characterization UV–vis absorption spectra were measured with a Cary 3G UV–Visible Spectrophotometer. High resolution transmission electron microcopy (HRTEM) analyses were conducted in the bright field mode using a JEOL FasTEM 2010 HR TEM analytical electron microscope operating at an accelerating voltage of 200 kV. Cyclic voltammetry was carried out in a conventional three electrode-cell (electrolyte: 0.1 mol L-1 of tetrabutylammonium tetrafluoroborate in anhydrous acetonitrile) with Ag/AgCl as reference electrode, a platinum wire as counter electrode and a polymer-coated platinum substrate (1 cm2) as working electrode. Chronoamperometry was carried out in a three-electrode cell using Ag/AgCl as reference electrode, a Pt wire as counter-electrode, and a 0.1 M aqueous solution of Na2SO4 saturated with O2 as electrolyte. The current characteristics were recorded as function of time, while repeated on-off cycles of light illumination (100 mW cm-2) were applied. All the electrochemistry measurements were recorded at 30 mV s-1, using an Eco Chemie Autolab PGSTAT 10 potentiostat. The working electrodes consisted of films of the materials with active area of 1 cm2, prepared by spin-casting. To make bulk-heterojunction solar cells, a poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer was spin-cast onto pre-etched indium tin oxide (ITO) coated glass substrates. The substrates covered with PEDOT:PSS were annealed on a hot plate for 10 min at 150 °C in air. The photoactive layer was then spin-cast on top, resulting in films with typical thicknesses of 90-120 nm. A metal electrode consisting of 70 nm Al was deposited by thermal evaporation in vacuum (~10-6 mbar). The active area of the solar cells was ~ 0.1 cm2. The devices were irradiated with a solar simulator adjusted to give 100 mW cm-2 (AM 1.5) and the current-voltage (J-V) curves were measured using an Eco Chemie Autolab PGSTAT 10 potentiostat.

3. RESULTS AND DISCUSSION Figure 1a shows the absorption characteristics of films of P3HT, PFT, MDMO-PPV and TOPO-capped CdSe nanoparticles. As can be seen, the absorption of CdSe is complementary to the absorption of polymers PFT and MDMO-PPV in the visible region, while the absorption of the inorganic nanoparticles and P3HT are coincident in wavelengths longer than 450 nm. All the materials are expected to act as lightharvesters when used in the active layer of solar cells. Figure 1b shows the J-V curves obtained for devices assembled with mixtures of TOPO-capped CdSe nanoparticles with P3HT, PFT or MDMO-PPV. In all cases, the devices delivered low values of photocurrent and fill factor (Jsc < 50 μA cm-2, FF ≤ 0.25), indicating poor diode behavior. In principle, this could be attributed to limitations inherent to TOPO-capped nanoparticles/polymer systems. TOPO-capped CdSe nanoparticles have been considered inefficient for photovoltaic applications. Such robust capping molecules are used to prevent aggregation during the synthesis. They increase the solubility of the nanoparticles in organic solvents and improve their physical interaction with polymer matrixes. It is generally accepted that TOPO hinders efficient charge transfer and may also prevent charge transport in the nanoparticle phase (inter-particle charge transport) [23,24], leading to devices with low performance. On the other hand, the quenching of polymer emission by interaction with TOPO-capped CdSe nanoparticles has been reported, and this may be considered as a first indication of the existence of charge transfer between these materials, either by the injection of electrons (from the polymer to CdSe) or by injection of holes (from CdSe to the polymer) [25,26].

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Nomalized Absorption / a. u.

400

500

600

700

Wavelegnth / nm

0.01

Current / mA cm

(b)

P3HT PFT MDMO-PPV CdSe

-2

(a)

0.00 -0.01 -0.02 P3HT PFT MDMO-PPV

-0.03 -0.04 -0.05 -0.2

0.0

0.2

0.4

0.6

0.8

Voltage / V Figure 1. Absorption spectra (a) of CdSe, P3HT, PFT and MDMO-PPV films and J-V characteristics (b) of devices assembled with the configuration ITO/PEDOT:PSS/polymer + CdSe/Al, where polymers P3HT, PFT and MDMO-PPV were used (active area ~0.1 cm2, irradiation ~100 mW cm-2).

Figure 2 shows the J-V characteristics of solar cells assembled with the combination of P3HT/CdSe/PCBM and PFT/CdSe/PCBM. For comparison, the curves obtained for devices assembled with P3HT/PCBM and PFT/PCBM are also shown. The photocurrent (Jsc), voltage (Voc) and fill factor (FF) are much higher for the P3HT/PCBM-based device compared to the PFT/PCBM-based device. This is related to enhanced light-harvesting of P3HT (Figure 1a), and also enhanced charge transport of this polymer. Previous investigations revealed that PFT has charge carrier mobility of ~1.7 x 10-6 cm2 V-1 s-1 [22]. Figure 2 clearly illustrates that, upon addition of CdSe, the systems based on P3HT and PFT present a different behavior. For PFT, there is a significant increase of both Jsc and Voc, while for P3HT a dramatic decrease of these parameters is observed. In 2010, we reported a systematic investigation of the PFT/PCBM/CdSe systems. It was shown that both the concentration and size of the inorganic nanoparticles affected the device performance [27,28]. At optimized conditions, there is a dramatic increase of Jsc, leading to devices with higher efficiency. In 2011, Peterson et al.[29] investigated a ternary system of P3HT/PCBM/CdSe. The addition of CdSe nanoparticles in the P3HT/PCBM system promoted a decrease in Voc and FF, which was attributed to space charge build up on the CdSe surface or at an interface of the nanoparticle with another material of the device. Devices

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nanoparticles [24,31,32]. Other ways are to shorten long chain surface groups by thermal cleavage[33] or remove excess surface groups by performing additional washing steps [14]. Another elegant approach is to synthesize the nanoparticles in situ. This method involves the deposition of hybrid films from polymer solutions which also contain a soluble precursor of the inorganic nanoparticles [34,35], eliminating the need of capping ligands. Here, we would like to introduce a new alternative: the use of CdSe aerogels. In the aerogel, the nanoparticles are really “connected” together (through Se-Se bonds), forming an electrically integrated network, which can improve electron hopping in this phase. Such architectures are of interest for applications requiring maximal transport of charge (through the gel network) and small molecules (through the interconnected pore network). Chalcogenide sol-gel methods have been used to assemble II-VI and IV-VI nanoparticles into mesoporous colloidal networks with inorganic particle interfaces that do not present the barriers to electrical transport, yet remain quantum-confined [36,37,38]. Figure 4 schematically represents the sol-gel transformation of inorganic nanoparticles, initiated by oxidation of surface thiolate ligands, for example. Removal of surface groups is followed by the solvation of Cd2+ ions, exposing Se2- on the surface. This material undergoes further oxidation, giving diselenide or polyselanide species, which link the nanoparticles together and form a gel network. Supercritical drying allows the synthesis of aerogels [39].

Oxidation of thiolate ligands

Sol

Supercritical drying

Wet gel

Aerogel

Figure 4. Schematic representation of the synthesis of aerogels from nanoparticles capped with thiolate ligands.

Figure 5a displays a microscopy image of a CdSe aerogel network. Figure 5b shows the chronoamperometry measurements of P3HT/CdSe aerogel hybrid films containing different concentrations of aerogel. Films based only on P3HT produce significantly lower currents than the photoelectrodes containing the hybrid films. There is a ca. 5-fold increase in the photocurrent upon addition of only 20 wt% of CdSe aerogel. Further increasing the concentration of inorganic material to 33 wt% and 50 wt% also leads to enhancements in the current. This is a result from effective charge carrier separation within the hybrid films. The charge transfer between P3HT and the aerogel network is expected to increase the formation of polarons in the polymer phase, and decrease the recombination, leading to enhanced current collection. These preliminary results suggest that CdSe aerogels are potential materials for photovoltaic applications.

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PCBM phase. Thus, CdSe may act only as a sensitizer. As seen in Figure 2, for PFT-based devices this approach is effective to enhance the efficiency of the device, even when using TOPO-capped nanoparticles. We found that these nanoparticles participate in charge transfer processes, as observed in other reports.

-3.0 eV

-3.1 eV

LUMO -3.6 eV

HOMO

-4.2 eV

-3.6 eV

-5.2 eV -5.6 eV P3HT

CdSe

-5.6 eV PCBM

CdSe

-5.4 eV PFT

Figure 3. Scheme of energy levels and electron transfer processes expected in P3HT/CdSe/PCBM and PFT/CdSe/PCBM devices. In PFT-based ternary system, a balance between electron injection from PFT into CdSe and PCBM, and from CdSe into PCBM, is expected. This is related to the presence of fluorene units in the polymer chain, which do not strongly complex the CdSe nanoparticles. In P3HTbased ternary system, a strong complex formation between thiophene units and CdSe nanoparticles is expected, leading to preferential electron injection from this polymer into CdSe. This fact deactivates the charge injection from P3HT (or from CdSe) into PCBM.

From Figure 2 it also evident that this approach is sensitive to the structure of the polymer. In fact, the polymer structure can be a crucial parameter for all devices containing CdSe, but it has not received much attention so far. To the best of our knowledge, the highest efficiency of a CdSe/polymer-based device was obtained using PCPDTBT (a polymer containing fluorene and thiodiazole units). The enhanced efficiency of this system was mainly attributed the enhanced light-harvesting of the low band gap polymer. Although the optical properties are certainly an important factor, the contribution of how strongly this polymer can interact with the nanoparticles should also be considered. This will determine not only the yield of charge transfer, but also morphology, as it should have impact on how strongly the polymer evolves the nanoparticles. For P3HT/CdSe systems, for example, we observed that the presence of CdSe nanoparticles induces a local organization of polymer chains [30]. Therefore, when designing efficient polymer/inorganic nanoprticles systems, the polymer structure must be considered. As mentioned before, when strongly coordinating polymers such as P3HT are used, the charges will be trapped in the CdSe nanoparticles if this material has too many defects, traps and low charge carrier mobility. To accomplish higher charge carrier transport, many groups work with ligand-exchanged

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nanoparticles [24,31,32]. Other ways are to shorten long chain surface groups by thermal cleavage[33] or remove excess surface groups by performing additional washing steps [14]. Another elegant approach is to synthesize the nanoparticles in situ. This method involves the deposition of hybrid films from polymer solutions which also contain a soluble precursor of the inorganic nanoparticles [34,35], eliminating the need of capping ligands. Here, we would like to introduce a new alternative: the use of CdSe aerogels. In the aerogel, the nanoparticles are really “connected” together (through Se-Se bonds), forming an electrically integrated network, which can improve electron hopping in this phase. Such architectures are of interest for applications requiring maximal transport of charge (through the gel network) and small molecules (through the interconnected pore network). Chalcogenide sol-gel methods have been used to assemble II-VI and IV-VI nanoparticles into mesoporous colloidal networks with inorganic particle interfaces that do not present the barriers to electrical transport, yet remain quantum-confined [36,37,38]. Figure 4 schematically represents the sol-gel transformation of inorganic nanoparticles, initiated by oxidation of surface thiolate ligands, for example. Removal of surface groups is followed by the solvation of Cd2+ ions, exposing Se2- on the surface. This material undergoes further oxidation, giving diselenide or polyselanide species, which link the nanoparticles together and form a gel network. Supercritical drying allows the synthesis of aerogels [39].

Oxidation of thiolate ligands

Sol

Supercritical drying

Wet gel

Aerogel

Figure 4. Schematic representation of the synthesis of aerogels from nanoparticles capped with thiolate ligands.

Figure 5a displays a microscopy image of a CdSe aerogel network. Figure 5b shows the chronoamperometry measurements of P3HT/CdSe aerogel hybrid films containing different concentrations of aerogel. Films based only on P3HT produce significantly lower currents than the photoelectrodes containing the hybrid films. There is a ca. 5-fold increase in the photocurrent upon addition of only 20 wt% of CdSe aerogel. Further increasing the concentration of inorganic material to 33 wt% and 50 wt% also leads to enhancements in the current. This is a result from effective charge carrier separation within the hybrid films. The charge transfer between P3HT and the aerogel network is expected to increase the formation of polarons in the polymer phase, and decrease the recombination, leading to enhanced current collection. These preliminary results suggest that CdSe aerogels are potential materials for photovoltaic applications.

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Current / μA

0.0 -1.0 -2.0 P3HT:CdSe 1:0 1:0.25 1:0.5 1:1

-3.0 -4.0

0

100

200

300

400

500

Time / s Figure 5. Photocurrent responses under white light (λ >300 nm, 100 mW cm-2) of films of P3HT and P3HT:CdSe aerogel, containing different loadings of inorganic material, deposited onto ITO-glass substrates, in the presence of a 0.1 M Na2SO4 electrolyte saturated with O2.

When regular CdSe nanoparticles are applied in solar cells, the best photovoltaic responses are obtained at high loadings of nanoparticles (~ 80-90 wt %) [7,23,35]. This is because a large amount of nanoparticles is required for the formation of a percolation network in the inorganic phase. In the case of aerogels, one can expect that this effect is minimized, and that lower loadings of aerogel will allow effective charge transport, since the percolation network in these materials are pre-formed. A systematic investigation of the effect of the concentration of aerogels in hybrid films using photoelectrochemistry measurements and transient absorption spectroscopy will be presented elsewhere [40].

4. CONCLUSIONS Polymer/CdSe systems are attractive for application in low-cost, hybrid organic-inorganic solar cells. Many parameters considered determinants for device efficiency, such as the nanoparticles size and shape, type of capping ligands and the polymer band gap, have been intensively investigated and optimized. On the other hand, other important parameters have been less explored. In this work, we highlighted the importance of the chemical structure of the polymer considering a different perspective. While the polymer structure has been the subject for designing and tuning the absorption and emission properties, less attention has been given to how this parameter directly affects the interaction with inorganic nanoparticles. The formation of a charge transfer complex is strongly dependent on the polymer structure, and this will determine if the system has efficient charge transfer or not, even when nanoparticles covered with robust capping molecules (such as TOPO) are used. It will also affect the organization of polymer phase around the nanoparticles and, therefore, affect morphology. Only a few studies have been dedicated to systematic investigations of the nanomorphology of such hybrid systems. Besides, the exchange of capping ligands has been the main focus for improvement of charge carrier transport through the inorganic phase. New concepts should also be explored. Here the use of CdSe aerogels is introduced as an alternative. The aerogels consist of tridimensional electrically integrated networks in which enhanced charge carrier transport is achieved not by using smaller or conjugated molecules, but through direct bonding the individual nanoparticles.

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ACKNOWLEDGEMENTS The authors thank Fapesp (fellowship 2009/15428-0), CNPq and INEO (National Institute of Organic Electronic/CNPq/FAPESP) for financial support.

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