Nanostructured Hybrid Polymer−Inorganic Solar Cell Active Layers

pubs.acs.org/NanoLett

Nanostructured Hybrid Polymer-Inorganic Solar Cell Active Layers Formed by Controllable in Situ Growth of Semiconducting Sulfide Networks Henry C. Leventis,† Simon P. King,† Anna Sudlow,‡ Michael S. Hill,‡ Kieran C. Molloy,‡ and Saif A. Haque*,† †

Department of Chemistry, Imperial College London, South Kensington Campus, Exhibition Road, SW7 2AY, U.K. and ‡ Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. ABSTRACT Nanostructured composites of inorganic and organic materials are attracting extensive interest for electronic and optoelectronic device applications. In this paper, we introduce a general method for the fabrication of metal sulfide nanoparticle/ polymer films employing a low-cost and low temperature route compatible with large-scale device manufacturing. Our approach is based upon the controlled in situ thermal decomposition of a solution processable metal xanthate precursor complex in a semiconducting polymer film. To demonstrate the versatility of our method, we fabricate a CdS/P3HT nanocomposite film and show that the metal sulfide network inside the polymer film assists in the absorption of visible light and enables the achievement of high yields of charge photogeneration at the CdS/P3HT heterojunction. Photovoltaic devices based upon such nanocomposite films show solar light to electrical energy conversion efficiencies of 0.7% under full AM1.5 illumination and 1.2% under 10% incident power, demonstrating the potential of such nanocomposite films for low-cost photovoltaic devices. KEYWORDS Organic solar cells, hybrid optoelectronics, inorganic nanoparticles, polymer semiconductors, electron transfer, quantum dots, metal xanthate

T

hin films comprising nanostructured organic and inorganic semiconductor components are currently attracting considerable interest for the development of photovoltaic devices1-4 and light-emitting diodes (LEDs).5,6 To date, significant attention has been paid to the use of nanocrystals (NCs) made from metal sulfides,7,8 selenides,2,9 and tellurides10 as light absorbers and electron acceptors in solution-processed polymer solar cells. This results in large part from their high electron mobility as well as the possibility of tuning their optical band gap into the near-infrared region, thereby offering the prospect of improved spectral coverage. The performance of such hybrid devices depends critically on the ability to control materials and interface structure at the nanometer length scale. For example, one of the major challenges facing the design of photoactive layers for efficient inorganic nanoparticle/polymer solar cells is the compromise which must be made between efficient charge photogeneration and charge carrier transport, while ensuring high nanocrystal solubility within the film processing solution.11,12 Typically, to achieve high charge separation yields and efficient charge transport, NCs must have first undergone ligand exchange, whereby as-prepared NCs (capped with, for example, trioctylphosphine oxide or oleic acid species) are exposed to shorter, incoming ligands.

However, this process reduces the nanocrystal solubility and can be detrimental to the homogeneity of the photoactive layer, often resulting in poor device performance. Strategies aimed at addressing this limitation have emerged in the literature, which include the use of alternative surfactants for nanocrystals, such as thiols,13,14 dendronized poly- and oligothiophenes15,16 and amine-functionalized block copolymers,17 as well as the synthesis of NCs within solutions of the conducting polymer, such that the polymer itself serves to passivate the semiconductor surface.8,18 In addition, the use of as-synthesized quantum dots (QDs) possessing thermally cleavable passivating ligands has been reported,12 which enables an in situ reduction of the passivating ligand length following active layer formation. One alternative strategy is to deposit films from polymer solutions which also contain a soluble precursor to the inorganic semiconductor component. Such an approach has been recently studied with the use of soluble zinc complexes, which, during and after the deposition process, decompose by reaction with water from the surrounding atmosphere to yield bicontinuous, interpenetrating ZnO and polymer networks within the resulting film.11,19 Moreover, impressive power conversion efficiencies of over 2% have been reported for ZnO/polymer solar cells using this fabrication approach.11 While such methods have been developed and successfully employed to fabricate a range of metal oxide/ polymer films, the realization of precursor-based routes to

* Corresponding author, [email protected]. Received for review: 11/12/2009 Published on Web: 03/12/2010 © 2010 American Chemical Society

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to that of CdS (28.1%). Decomposition of metal xanthates is known to occur via a Chugaev rearrangement,24 in which, in this case, C2H4, COS, and H2S are eliminated, none of which should be retained as film contaminants. Furthermore, the low decomposition temperatures for metal xanthates make their in situ decomposition compatible with relatively fragile host materials. Hybrid CdS/P3HT films were fabricated by spin-casting from a chlorobenzene solution containing the cadmium xanthate precursor (1) and P3HT. It has previously been reported that decomposition of Cd(S2COEt)2 in the capping agent trioctylphosphine oxide (TOPO) affords hexagonal CdS nanoparticles of mean diameter 4.2 nm,25 while in refluxing surfactant Brij 52, cubic CdS nanoparticles 4-7 nm along with ca. 20 nm aggregates are generated, growing to 140-200 nm aggregates with time.26 In the case of the chelated adduct Cd(S2COEt)2(TMEDA) (TMEDA, tetramethylethylenediamine), decomposition in refluxing Brij 52 gives only 4-7 nm particles irrespective of time, in which the TMEDA appears to be acting as a capping agent like TOPO.26 In the present case, it is likely that the P3HT polymer acts as the capping agent and controls nanoparticle size. The as-spun films were subsequently annealed at 150 °C to decompose the xanthate species to generate the metal sulfide network inside the polymer film. Decomposition of 1 g of 1 (MW ) 513.0 g/mol) is expected to generate 0.28 g of CdS; for ease of comparison, composition ratios quoted herein reflect the resultant CdS:polymer weight ratios following complex decomposition. Figure 2a shows the steady-state absorption spectra as a function of increasing CdS concentration; these data have been normalized by the fraction of photons absorbed at 540 nm. The appearance of a large absorption feature below ca. 500 nm can be seen as the CdS:P3HT ratio is increased. This feature is attributed to the absorption of CdS nanocrystals and is also seen in the absorption spectrum of a thermally annealed 1/polystyrene (PS) film used here as a control sample (inset of Figure 2b); in both cases, a first excitonic absorption maximum can be seen at ca. 450 nm, consistent with the formation of CdS NCs of ca. 5.5 nm in diameter.27 Moreover, it can be seen in Figure 2b that this feature is not present in unannealed films, confirming that CdS formation occurs only during thermal annealing. To obtain further evidence for the growth of CdS nanoparticles in the P3HT film, we performed transmission electron microscopy (TEM). Bright-field TEM images are shown in Figure 3; in the case of the cross-sectional view, the extremely dark region is a layer of gold which had been evaporated onto the surface of the active layer. The darker region within the active layer is believed to result from the presence of the CdS nanocrystal network, which appears to be formed primarily within the lower half of the film, closer to the bottom transparent conducting glass substrate. This assignment was confirmed using energy dispersive X-ray spectroscopy (EDS), whereby X-ray emission bands charac-

FIGURE 1. The structure of 1. Only one component of the disordered ethyl group is shown for clarity. Selected geometric data: Cd-N(1) 2.367(2), Cd-N(2) 2.374(2), Cd-S(1) 2.6564(8), Cd-S(2) 2.7234(8), Cd-S(3) 2.6695(8), Cd-S(4) 2.7100(8) Å; N(1)-Cd-N(2) 86.01(8), S(1)-Cd-S(2) 67.11(2), S(3)-Cd-S(4) 67.01(2)°.

metal sulfide/polymer nanocomposites remains limited to date. The development of such fabrication methods is now needed to make use of the superior light-harvesting properties of metal sulfide nanoparticles (relative to metal oxides) in solution-processed polymer solar cells. Herein, we address this issue and report a new route to the fabrication of such metal sulfide nanoparticle/polymer films. Our approach is based upon the controlled in situ thermal decomposition of a metal xanthate precursor inside a semiconducting polymer film. The metal xanthate is engineered to be air-stable, to be highly soluble (solution processable), and to decompose controllably within the temperature range typically used for the annealing of polymer active layers. Herein we show that such precursors can be used in the design of thin films comprising electron-transporting CdS nanocrystal networks in the hole-transporting polymer poly 3-hexylthiophene (P3HT), which exhibit high yields of charge photogeneration and impressive device power conversion efficiencies. The structure of the Cd(S2COEt)2(C5H4N)2 (1) precursor used in this study is shown in Figure 1. 1 was synthesized following a modified version of a previously published procedure, using CdCl2, KS2COEt, and pyridine.20 The formation of the complex was fully characterized by microanalysis and 1H, 13C NMR spectroscopy (see Supporting Information). The molecule adopts a cis, cis, cis configuration of ligands about an octahedrally coordinated metal, which differs from similar structures previously reported which incorporate trans-donors when they are monodentate, e.g., trans-Cd(S2COBu)2(L)2 L ) C5H5N,21 3-ClC5H4N.22 A donor adduct of Cd(S2COEt)2 was chosen as these are more soluble in typical organic solvents than their uncomplexed analogue; Cd(S2COEt)2 forms a two-dimensional polymeric network as a result of bridging xanthate groups.23 TGA shows that 1 begins decomposing at 50 °C and is complete by 150 °C; the final residual mass (31.9%) is close © 2010 American Chemical Society

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FIGURE 3. TEM images of CdS/P3HT composite layers. (a) Transmission electron micrograph of a cross section of a 2:1 weight ratio CdS/ P3HT blend film. The active layer was spin-coated from a chlorobenzene solution (containing P3HT and 1) and subsequently annealed at 150 °C before evaporation of a 60 nm thick Au layer. EDS was used to confirm the presence of Cd within region A, while Cd was found to be absent from region B. (b) Top-down transmission electron micrograph of a 2:1 weight ratio CdS/P3HT blend film.

phase while the P3HT is expected to serve as the hole transporting component. It is anticipated that the ca. 1.4 eV LUMO-LUMO and/or ca. 1.2 eV HOMO-HOMO energy offset (as estimated using literature values for P3HT and bulk CdS)30-32 is sufficient to facilitate charge pair generation. To investigate this, microsecond-to-millisecond transient absorption spectroscopy was employed to determine the charge generation yield and recombination dynamics following photoexcitation of the CdS/P3HT film. Details of the transient absorption apparatus have been described previously.32 Figure 4a shows the transient absorption spectrum (black data) of a CdS/P3HT sample obtained 10 microseconds after 567 nm pulsed laser excitation. For comparison, data obtained using a 1:1 weight ratio solution containing P3HT and the electron acceptor PCBM (phenyl C61 butyric acid methyl ester) are also shown (grey data) and are normalized by the fraction of photons absorbed at the excitation wavelength. The transient absorption spectra shown in Figure 4a show the presence of similar transient features in both the 2:1 weight ratio CdS/P3HT blend and the PCBM/P3HT blend, which are seen to peak at ca. 960 and ca. 1000 nm, respectively. Such features have been attributed to the transient absorption of the P3HT+ polaron.33-35 Moreover, these data clearly indicate that CdS/ P3HT nanocomposite film exhibits efficient charge pair generation upon photoexcitation. The data shown in Figure 4b follows the charge recombination reaction between the photogenerated holes in the P3HT and the electrons in the CdS, as a function of increasing CdS concentration. The data shown in Figure 4b were obtained by monitoring the decay of the P3HT+ polaron

FIGURE 2. Absorption spectra of CdS/polymer layers. (a) Absorption spectra, normalized to the absorbance at 540 nm, of 1/P3HT blends following thermal annealing at 150 °C. Weight ratios assume that 1 g of the precursor complex decomposes to yield 0.28 g of CdS. (b) Absorption spectra of 1/P3HT blends (with a CdS:P3HT weight ratio of 4.7:1) before (black dashed line) and after (black solid line) thermal annealing, and decomposition of the precursor complex to generate CdS, at 150 °C. Upper Inset: Absorption spectra of 1 /polystyrene blends before (red dashed line) and after (red solid line) thermal annealing at 150 °C. Lower Inset: Photographic images of 1/polystyrene (top) and 1/P3HT nanocomposite films (bottom) before (left) and after (right) thermal annealing and subsequent CdS formation.

teristic of electron transitions to the Cd L shell were seen at 3.2 and 3.9 keV upon exposure of region A (shown in Figure 3a) to the electron beam; no such signal was seen upon examination of region B, which we believe to be primarily composed of P3HT polymer. The top-down TEM image in Figure 3b shows the presence of CdS nanocrystalites (dark regions) in a P3HT (lighter regions) matrix, further demonstrating that our method results in the growth of a metal sulfide network in the polymer film. For further evidence for the formation of CdS nanoparticles in the P3HT film, Raman spectroscopy was performed (see Supporting Information). These data revealed Raman modes at 597 and 300 cm-1, consistent with the formation of CdS.28,29 We consider next the charge photogeneration yield in the CdS/P3HT nanocomposite films. In the CdS/P3HT architecture, CdS is intended to function as the electron transporting © 2010 American Chemical Society

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precursor in to the CdS. It is also apparent that the lifetime of the photoinduced charge separated state is extended with the use of CdS NC electron acceptors, possibly as a consequence of the improved screening of charges in these systems resulting from the high dielectric constant of the inorganic component. Figure 4b (inset) shows a log-log plot of a selected transient absorption trace of a CdS/P3HT sample (with a CdS: polymer weight ratio of 2:1); a linear fit corresponding to power law (∆OD ∼ t-R) is indicative of a decay mechanism with competing recombination versus transport dynamics. We find that all CdS/P3HT traces shown in Figure 4b exhibit power-law decays with an exponent in the range R ) 0.19-0.22, suggesting the presence of thermal traps limiting the diffusion of charges, as has been reported in MDMO-PPV:PCBM (R ) 0.3-0.4)36 and P3HT: PCBM (R ) 0.3-0.7)37 blends. Further, more detailed studies addressing the mechanisms of charge separation and recombination in such CdS/P3HT layers are currently underway and will be reported in due course. Photovoltaic devices were fabricated using an “inverted” architecture based upon the following components: ITO/ TiOx/active layer/PEDOT:PSS/Au.38 In this design, holes in the P3HT are collected at the top Au contact, while the photogenerated electrons in the CdS are collected at the bottom ITO/TiOx electrode. Active layers were deposited by spin coating from chlorobenzene solutions containing 1 and P3HT (equivalent to a CdS: P3HT weight ratio of 4.7:1) at 1000 rpm for 30 s and were subsequently annealed in a nitrogen glovebox at 150 °C prior to the application of the PEDOT:PSS layer. Figure 5 and Table 1 illustrate the current-voltage characteristics resulting from the use of this device architecture. While devices were seen to exhibit PCEs in excess of 0.7% (and short-circuit currents of 3.5 mA cm-2) under AM1.5 illumination, the efficiency rose with decreasing illumination intensity to values of 1.2% and ∼1.5% under 0.1 and ∼0.05 suns, respectively. Typical external quantum efficiency data (corrected for the nonlinear response of current with light intensity) are shown in Figure 5b; EQEs are seen to peak at ca. 400 nm (where the EQE was found to be 36.5%), in the region of the spectrum dominated by the CdS NC absorption band. This provides an indication that inorganic materials grown in situ can be used to generate current from the visible region of the solar spectrum, which was not achieved previously with the in situ growth of metal oxide networks. In summary we have reported a new fabrication method for a hybrid CdS/P3HT polymer nanocomposite film. Our approach is based upon the controlled in situ low temperature thermal decomposition of a metal xanthate precursor in a polymer film. More specifically we have demonstrated that this approach can be used to grow an interpenetrating network of CdS nanoparticles in a P3HT film. Such films are shown to exhibit high yield and long-lived charge photogeneration which is a prerequisite for efficient photovoltaic device function. Photovoltaic devices based upon such layers

FIGURE 4. Transient absorption spectroscopy of CdS/P3HT nanocomposite layers. (a) Transient absorption spectra of a CdS/P3HT sample (black data, weight ratio 2:1) and a PCBM/P3HT blend (grey data, weight ratio 1:1), 10 µs following photoexcitation at 567 nm (pump intensity 29 µJ cm-2). Data have been scaled by the fraction of photons absorbed at the pump wavelength. (b) Transient kinetics (following P3HT photoexcitation at 567 nm), obtained using a probe wavelength of 980 nm so as to monitor the recombination kinetics of P3HT+ polarons in CdS/P3HT composite films containing the following estimated CdS:P3HT weight ratios: 1:4 (red trace), 1:2 (green trace), 1.2:1 (dark blue trace), 2:1 (light blue trace), 3.1:1 (pink trace), and 4.7:1 (dark yellow trace). The transient kinetic behavior of an unannealed (1/P3HT) blend is also shown (grey trace), as are data obtained using a pristine polymer film (black trace) and a 1:1 weight ratio PCBM/P3HT blend (dark red trace). Data have been scaled by the fraction of photons absorbed at the pump wavelength. Inset: Data for a 2:1 weight ratio film presented on a log-log plot and fitted to a power law function (red line, ∆OD ∼ t-R), using a value of R ) 0.19.

band at 980 nm. The amplitude of the signal (magnitude of change in optical density m∆OD) in this figure is directly related to the number of photogenerated charge pairs. It is clear that upon increasing the CdS weight fraction, yields of charge photogeneration are seen to improve. Furthermore, similar yields of charge separation per absorbed photon are observed (300 ns following photoexcitation) with the use of a 4.7:1 CdS:P3HT weight ratio as are seen when PCBM is used as the electron acceptor. Also shown in Figure 4b are data obtained using an unannealed 1/P3HT film, where it can be seen that charge separation yields are extremely small and are comparable with those seen using pristine P3HT polymer films; this further indicating that the temperature annealing step is essential for the conversion of the © 2010 American Chemical Society

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expected through optimization of the active layer morphology as well as the use of alternative charge collecting electrodes. Further studies addressing device optimization and the charge transport properties in the in situ grown CdS/ polymer films are planned and will be reported in due course. We further note that our method should be applicable to the design and fabrication of alternative narrow band gap metal sulfide/polymer films such a PbS/polymer nanocomposites. In conclusion, the present findings demonstrate the potential and versatility of using metal xanthate precursors to grow inorganic nanoparticles in semiconducting polymer films. Acknowledgment. We acknowledge financial support from the Engineering and Science Research Council (EPSRC) Grand Challenges in energy program; S.A.H acknowledges support from the European Science Foundation ESF SONS EUROCORES program and thanks the Royal Society for a RS-URF. We also thank Pedro Atienzar for helpful discussions. The Royal Society is thanked for the provision of a University Research Fellowship (M.S.H.). Supporting Information Available. NMR data and microanalysis, TGA of 1, and crystallographic data for 1. This material is available free of charge via the Internet at http:// pubs.acs.org. REFERENCES AND NOTES (1) (2)

FIGURE 5. Performance of CdS/P3HT solar cells. (a) J-V characteristics under different illumination intensities. Devices were prepared in the “inverted” ITO/TiOx/active layer/PEDOT:PSS/Au configuration. The active layer was spin-coated at 1000 rpm from a chlorobenzene solution (P3HT and 1) and subsequently annealed at 150 °C before application of the PEDOT:PSS layer. (b)The dependence of shortcircuit current density (blue data, left axis) and open-circuit voltage (red data, right axis) upon illumination intensity. Inset: EQE of the same device, corrected for the nonlinear response of the short-circuit current with illumination intensity. When convoluted with the AM1.5 solar spectrum, an estimated short-circuit current density of 3.5 mA cm-2 is obtained.

(3) (4) (5) (6) (7) (8)

TABLE 1. Dependence of Key Device J-V Characteristics upon Illumination Intensity illumination intensity/mWcm-2

JSC/mA cm-2

VOC/mV

FF

PCE/%

100 67.8 49.5 31.8 26.5 21.5 15.7 10.2 4.4

3.54 2.72 2.01 1.41 1.15 1.01 0.73 0.51 0.27

611 599 590 578 569 565 551 535 430

33.3 35.0 36.3 39.4 41.1 42.0 44.6 44.4 50.7

0.72 0.84 0.88 1.00 1.01 1.11 1.12 1.21 1.53

(9) (10) (11) (12) (13) (14) (15) (16)

are shown to exhibit impressive device efficiencies of 1.2% under 10% AM 1.5 solar illumination. It is pertinent to note that the prototype devices reported here are unoptimized and further improvements in device performance can be © 2010 American Chemical Society

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