CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells

https://doi.org/10.1016/j.solmat.2004.07.047Get rights and content

Abstract

The development of lightweight, flexible polymeric solar cells which utilize nanostructured materials has been investigated. Incorporation of quantum dots (QDs) and single wall carbon nanotubes (SWNTs) into a poly(3-octylthiophene)-(P3OT) composite, has been shown to facilitate exciton dissociation and carrier transport in a properly structured device. Optimization towards an ideal electron acceptor for polymeric solar cells that exhibits high electron affinity and high electrical conductivity has been proposed in the form of QD-SWNT complexes. Specifically, the synthesis of CdSe-aminoethanethiol-SWNT complexes has been performed, with confirmation by microscopy (SEM, TEM, and AFM) and spectroscopy (FT-IR and optical absorption). Polymer composites containing these complexes in P3OT have been used to fabricate solar cells which show limited efficiency due to recombination and surface effects, but an open-circuit voltage (VOC) of 0.75 V. However, evaluation of the optical absorption spectra for these nanomaterial-polymeric composites has shown a marked enhancement in the ability to capture the available irradiance of the air mass zero (AM0) spectrum.

Introduction

Considerable effort is currently underway to develop polymeric solar cells as an alternative to crystalline technology, due to the potential reduction in processing cost, improved scalability, and opportunity for lightweight, flexible devices [1], [2]. This effort is based on the fact that conducting polymers are capable of photo-induced charge transfer [3]. The polymers which have been used most commonly for this application are poly(3-hexylthiophene)-(P3HT), poly(3-octylthiophene)-(P3OT), and poly(para-phenylenevinylene)-(PPV) [4], [5], [6], [7]. Conducting polymers like these have the ability to generate excitons (bound electron–hole pairs) upon optical absorption. The exciton is typically coupled to a phonon, which provides the energy for mobility in the polymer chains [8]. Such intrachain diffusion of neutral excitons in the polymer is however limited by self-trapping and coulombic interactions [8]. The exciton size (average electron–hole separation) is ∼0.5 nm and given a typical dielectric constant for conducting polymers (ε=35), the corresponding exciton binding energy is ∼0.5-1 eV [9], [10]. Also, it has been shown that the diffusion lengths of excitons are quite low (less than 10 nm in most cases before recombination), typically on the order of several monomers [8], [11]. If a localized potential energy difference exists which is greater than the exciton binding energy (coulombic attraction and spatial location on polymer backbone [12]), dissociation into the free electron and hole carriers can occur. This dissociation process usually arises from sufficient localized electric fields (105–106 V/cm), resulting in carrier promotion to a delocalized state, transport by interchain hopping, or charge transfer to an electron-accepting impurity [8], [10]. Transient absorption spectroscopy has shown that such charge transfer effects will take place on the picosecond timescale in the absence of recombination [13]. The efficiency of the charge transfer process has been described on the basis of zero-point oscillations whereby an on-chain hole can act as a potential barrier for recombination. Such a mechanism is supported by sufficient molecular ordering and interfacial dipole due to differences in electronegativity of the electron acceptor and polymer [9]. The separated hole has carrier mobility through on-chain transport using the highly coupled, π-conjugation network [10]. The overall quantum yield of exciton dissociation is largely influenced by the localized electric field and temperature on the effective carrier mass (meff) of the material. This effect is most pronounced in ‘heavy’ carriers where the poor mobility leads to a higher probability of recombination [10]. In the case of polymeric solar cells, additives are incorporated with a sufficiently high electron affinity to dissociate the exciton into free carriers before recombination which can contribute to the photocurrent in a suitable device structure [14]. The use of nanomaterials as the electron accepting impurities in these polymer systems is currently an area of very active research and appropriate selection should maximize exciton dissociation and promote efficient carrier transport in the device.

The most widely investigated nanomaterials for polymeric solar cells have been semiconducting nanocrystals, fullerenes, and single wall carbon nanotubes (SWNTs) [1], [5], [6], [7], [11], [15], [16], [17]. In particular, CdSe quantum dots (QDs), nanorods, and tetrapods, have all shown viability as successful polymer solar cell additives [5], [16]. Under AM1.5 illumination, 90% w/w CdSe nanorods in P3HT and 86% w/w CdSe tetrapods in OC1C10-PPV, have been reported to produce a power conversion efficiency of 1.7% and 1.8%, respectively [5], [16]. Fullerenes have also been a popular additive for polymeric solar cells, with multiple variations in the chemical structure shown to promote high electron acceptor properties [15], [18], [19]. Recent results have demonstrated a power conversion efficiency of 2.5% under AM1.5 illumination for a device with an active material of methanofullerenes incorporated into a modified-PPV polymer with LiF/metal electrodes [7], [20]. The latest nanomaterial to show success as an additive in polymeric solar cells has been SWNTs [6], [21]. Incorporation of SWNTs into P3OT has demonstrated high open-circuit voltages (Voc), nearly 1.0 V, although the power conversion efficiencies are currently well below 1% [6], [21], [22].

The high electron affinity of QDs and fullerenes enables the dissociation of polymeric excitons, leading to an improvement in these types of devices. Also, since the electrical conductivity (σe) of the photoactive polymers routinely employed is low, the QD or fullerene additive is primarily responsible for electron transport to the negative electrode [14]. However, these materials have a low aspect ratio (high percolation threshold in the polymer), and have therefore necessitated very high doping levels (consistently >75% w/w) which can negatively impact the mechanical properties of the polymer composite [5], [16], [18]. In comparison, the low percolation threshold of SWNTs coupled with their extraordinary σe (104 S/cm for metallic (10,10) SWNTs [23]) allow for significant enhancement of electron transport at even very low doping levels (<1% w/w) [22]. Therefore, if a material which encompasses both high electron affinity and high σe can be utilized as an additive, namely in the form of a quantum dot-single wall carbon nanotube (QD-SWNT) complex, a further enhancement in the conversion efficiency of polymeric solar cells is expected.

The ideal cascade of energy transitions for an optimal nanomaterial-polymer solar cell would include photon absorption by the components over the entire air mass zero (AM0) spectrum, and dissociation of the excitons by the highest electron affinity material near the exciton [14]. The dissociation process will be based on a sufficient potential energy difference between polymer and nanomaterial energy levels as compared to the exciton binding energy [8]. Fig. 1a illustrates the energy levels in relation to the vacuum level for the valence, conduction, and exciton binding levels of the P3OT polymer, CdSe QDs, and semiconducting SWNTs (S-SWNT). Also, the workfunction of metallic SWNTs (M-SWNT) indicates an appropriate potential energy level for exciton dissociation when used in conjunction with the P3OT polymer, CdSe QDs, and S-SWNTs. Although the P3OT's exciton binding energy is relatively high, 0.5 eV [10], each of the nanomaterials discussed would have sufficient electron affinity to dissociate the electron–hole pair based on the difference in potential energy. The efficiency of dissociation and charge transfer from the polymeric exciton relies on the ability to delocalize the electron and eliminate the coulombic interactions between the localized hole on the polymer chain and the electron acceptor [9]. Since the QDs rely on a hopping conduction to transport the electrons to the negative electrode [16], an alternative would be a ballistic conductor like SWNTs that transports electrons with minimal coulombic interactions, preventing recombination. In comparison to the polymer, dissociation of the exciton from CdSe QDs shows a reduced energy barrier (0.1 eV [24]) and the electron transport through the SWNTs would presumably be most efficient with appropriate coupling of the QDs to the SWNTs. Engineering of a suitable polymeric photovoltaic device requires an understanding of the potential energy levels associated with the components such that the material junctions are tailored to promote efficient photo-conversion into free carriers.

The cascade of energy transitions, exciton dissociation, and carrier transport for a proposed photovoltaic device scheme, including a p-type polymer with QDs and SWNTs as additives (QD-SWNT-Polymer) is depicted in Fig. 1b. The energy levels have been adjusted in relation to the vacuum level and equilibrated at the Fermi energy. In addition, the electronic transition associated with each component's estimated band gap is clearly shown. Although the schematic represents a series of planar junctions, the reality in a nanomaterial-polymer composite is actually a complex three-dimensional network of junctions. The photo-induced excitons in the polymer are expected to be dissociated by the nearest high electron affinity material, either the QD or SWNT. Ultimately, the holes are transported by the polymer to the positive electrode and the dominant electron path is through the percolating SWNTs to the negative electrode. Additionally, the QDs can produce excitons upon optical absorption [25] and be dissociated by neighboring or chemically bonded SWNTs [26]. In addition to the absorption by the polymer and QDs, it has been shown that semiconducting SWNTs can also absorb light and create bound electron–hole pairs which could contribute to the photoconductivity in these types of devices [27], [28].

It is important to recognize that the relationship for the Voc in a nanomaterial-polymer composite device is proposed to derive from the energetic difference in the HOMO level of the polymer and the conduction band of the nanomaterial additive [21]. Therefore, control over the nanomaterial's electron affinity during synthesis lends itself as an important parameter that can significantly impact the device performance. The electron affinity of the QDs can be tailored based upon the selection of semiconductor material used [29]. Further control of the energy bandgap for the QDs and SWNTs can be achieved by tuning the diameter distribution during synthesis [30], [31]. For example, CdSe QDs have a range of electron affinities reported from 3.5–4.5 eV and diameters from 1–9 nm [32], [33], [34]. Due to the fact that the polymer, QDs, and SWNTs may each absorb in a different spectral region, the possibility exists that these nanomaterials could be combined in such a way as to produce a series of junctions in a polymeric solar cell which would be analogous to a conventional triple-junction solar cell [35]. Although a mixture of QDs and SWNTs in a polymer may prove worthwhile, coupling of the QDs to SWNTs [36], [37], [38], [39] would presumably provide the most efficient combination of exciton dissociation and carrier transport.

In the present work, investigation of an optimal energy cascade for QD-SWNT-polymer solar cells has been initiated. The covalent attachment of CdSe QDs to SWNTs through an organic coupling reaction has been confirmed by FT-IR spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The ability to capture a larger energy range of the AM0 solar spectrum through incorporation of nanomaterials in P3OT was also examined. Incorporation of the prepared CdSe-SWNT complexes into P3OT has been used to construct and test a novel polymeric solar cell.

Section snippets

Experimental

Synthesis of the single wall carbon nanotubes (SWNTs) was performed using an Alexandrite laser vaporization process, previously described in detail [22]. The raw soot was purified using conventional nitric acid and thermal oxidation steps, to achieve SWNT mass fractions of >95% w/w in the overall sample [22]. The necessary preparation of carboxylic acid-functionalized SWNTs prior to covalent attachment with the semiconductor quantum dots was done by ultrasonication in a 4:1 mixture of

Results and discussion

The ability to systematically control synthesis conditions can enable the proper tuning of nanomaterial properties. The laser vaporization process employed in this study has the advantage of modifying SWNT properties such as diameter by variation in parameters like temperature or laser pulse power of the reactor [31]. The ability to shift the absorption energy of the SWNTs based on diameter lends itself as a viable method for capturing a significant portion of the NIR and visible regions in the

Conclusions

Evaluation of QD-SWNT complexes for polymeric solar cells has been performed, including a proposed device structure with enhanced optical absorption and improved carrier transport. The synthesis of CdSe-AET-SWNT complexes using covalent bonding strategies has been supported by FT-IR spectroscopy, AFM, and TEM analysis. Incorporation of CdSe-AET-SWNT complexes into P3OT has been used to fabricate a nanomaterial-polymeric solar cell with an observed photoresponse under 1 Sun AM0 illumination.

Acknowledgements

The authors wish to thank William VanDerveer and Stephen Fahey for their assistance during this work. This project was financially supported by NASA Glenn Research Center (grant nos. NCC3-937 and NAG3-2595) and the National Science Foundation (grant no. ECS-0233776).

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