Transition from Photoconductivity to Photovoltaic Effect in P3HT

Mar 6, 2012 - photoconductive gain to the photovoltaic effect with an increase of the ... photoconductivity of colloidal CuInSe2 NCs in the prototypic...
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Transition from Photoconductivity to Photovoltaic Effect in P3HT/ CuInSe2 Composites Yang Yang,†,‡ Haizheng Zhong,*,† Zelong Bai,† Bingsuo Zou,† Yongfang Li,*,‡ and Gregory D. Scholes*,§ †

School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing, 100081, China ‡ CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Department of Chemistry, 80 St. George Street, Institute for Optical Sciences, and Center for Quantum Information and Quantum Control, University of Toronto, Toronto, Ontario, M5S 3H6 Canada S Supporting Information *

ABSTRACT: We report the investigation of composites of poly-(3-hexylthiophene) (P3HT) and CuInSe2 nanocrystals (NCs). CuInSe2 NCs were synthesized at gram scale through a colloidal route. Cyclic voltammetry was used to determine the HOMO−LUMO energy levels, as well as the energies of trap states intrinsic to CuInSe2 NCs. The results suggest that nanocrystal−polymer charge transfer is foreseeable in the P3HT and CuInSe2 system because of their type II band alignment. We studied optical properties of the hybrid composite films of P3HT and CuInSe2 NCs as a function of P3HT/CuInSe2 NCs weight ratios. The quenching of photoluminescence spectra with the increase of CuInSe2 ratio in the composite was interpreted to result from charge transfer between the polymer and CuInSe2 NCs. We further studied photodiode devices that exhibited a transition from photoconductive gain to the photovoltaic effect with an increase of the P3HT/CuInSe2 weight ratio. Performance of the photovoltaic devices was strongly limited by intrinsic traps associated with the CuInSe2 NCs.

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currently being studied. 41−45 Sariciftci and co-workers previously investigated blends of P3HT with CuInSe2 nanoparticles for photovoltaic applications,46 and Wan et al. studied blends of hexagonal CuInSe2 nanoplates and a P3HT matrix for photodetector applications.47,48 We recently developed a method to synthesize near-stoichiometric colloidal CuInSe2 NCs with a diameter of 3−5 nm and improved optical properties compared to previous materials.49 By controlling the synthetic conditions, CuInSe2 NCs forming necklace-like assemblies can be produced at the gram scale.49 Here, we investigate these NCs as electron acceptors for NCs/P3HT photodetectors and solar cells. We used cyclic voltammetry (CV) to determine the conduction and valence band levels, as well as the energies of trap states associated with CuInSe2 NCs. A type II band alignment between P3HT and CuInSe2 NCs suggests the possibility of photoinduced charge transfer in the hybrid composite, which was studied by steady photoluminescence (PL) spectroscopy. The quenching of PL with an increase of CuInSe2 ratio in the composite was interpreted as electron transfer between the polymer as donor and CuInSe2 NCs as

INTRODUCTION Colloidal semiconductor nanocrystals (NCs) have been investigated as potential components in photovoltaics and photodetectors, for example, as electron donors in the case of dye-sensitized solar cells1,2 or electron acceptors in the case of bulk heterojunction polymer solar cells.3−8 Various composite systems comprising conjugated polymers to serve as hole conductors, that include poly(phenylvinylene), polythiophene, and polyfluorene, in conjunction with NCs of CdSe,9−11 CdTe,12−14 CdS,15 CdSexTe1−x,16 CdTe-CdSe,17 ZnO,18−20 TiO2,21−23 PbS,24−27 PbSe,28−30 HgTe,31 and CuInS232−35 have been reported. In these systems, semiconductor NCs were usually incorporated into the polymer at a high weight ratio, forming interconnected networks envisioned to provide carrier pathways, and the effect of NC shape in these networks has been examined.10,36 It has been suggested that NCs composed of earth-abundant elements are needed,37 and to that end, a better understanding of how different materials perform in devices is needed.38 Here, we investigate hybrid solar cells and photoconductivity of colloidal CuInSe2 NCs in the prototypical polymer host region regular poly(3-hexylthiophene) (P3HT). We find that surface trap states that are characteristic of these NCs are a limiting factor in their performance. CuInSe2 is known to have a good performance in thin-film solar cells,39,40 and it appears to be less toxic than other NCs © 2012 American Chemical Society

Received: December 1, 2011 Revised: March 1, 2012 Published: March 6, 2012 7280

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using detergent, deionized water, acetone, and isopropanol. A layer of 30 nm PEDOT:PSS was spin-coated on the treated ITO substrate and was dried in a vacuum oven at 150 °C. P3HT and CuInSe2 NCs wih a particular ratio were dissolved in chlorobenzene and were spin-coated on top of the PEDOT layer and then annealed at a specific temperature for 30 min. Finally, the Al cathode (100 nm) was vacuum evaporated onto the annealed photoactive layer. The effective area of the device was about 4 mm2. The current−voltage (I−V) measurements of the photovoltaic devices were conducted using a computercontrolled Keithley 236 Source-Measure Unit. A xenon lamp with an AM 1.5G filter simulated a white-light source, and the optical power at the sample was 100 mW/cm2. The thicknesses of the photoactive layers were monitored by an Ambios Tech. XP-2 profilometer.

acceptor. We further studied the photodiode devices by sandwiching the composite film between indium tin oxide (ITO) and Al electrodes. It was found that the hybrid composite devices exhibited a transition from photoconductive gain to the photovoltaic effect with an increase of P3HT/ CuInSe2 ratio.

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EXPERIMENTAL SECTION Chemicals. Copper(I) iodide (Fluka, ≥98%), indium(III) acetate (Aldrich, 99.999%), selenium (Aldrich, 100 mesh, 99.5%), n-dodecanethiol (DDT, Alrich, ≥98%), n-octadecanethiol (ODT, Aldrich, 90%), oleic acid (OA, Aldrich, 90%), tri-noctylphosphine (TOP, Alrich, 90%), and 1-octadecene (ODE, Aldrich, 90%) were used as purchased and without further purification. Synthesis of CuInSe2 NCs. A typical synthesis of CuInSe2 NCs was performed as follows: CuI (0.96 g, 5 mmol) and In(OAc)3 (1.46 g, 5 mmol) were mixed with 5 mL of dodecanethiol and 40 mL of ODE in a 25 mL three-neck flask. The mixture was degassed at 120 °C for 30 min; then 5 mL of oleic acid was added into the solution and the solution was continuously degassed for another 30 min. After that, the solution was heated to 200 °C under an argon flow and 1 mL of TOPSe precursor (made by dissolving 10 mmol of Se powder in 2.5 mL of TOP and 2.5 mL of ODE) was swiftly injected into the flask and kept for 120 min at the same temperature. Afterward, the reaction solution was cooled to room temperature and precipitated by acetone, the flocculent precipitate formed was centrifuged, the upper liquid layer was decanted, and then the isolated solid was dispersed in chloroform and reprecipitated by adding methanol or acetone. The centrifugation−precipitation procedure was repeated several times for purification of the CuInSe2 NCs. Finally, the products were redispersed into toluene or chloroform or dried under vacuum. Typically, one batch produces ∼980 mg of CuInSe2 NC powder. Material Characterization. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were recorded using a JEOL2100F instrument equipped with a Gatan camera operated at 200 kV. Scanning transmission electron microscopy (STEM) images were acquired using a Hitachi HD-2000 at 200 kV. The ultraviolet−visible (UV−vis) absorption spectra were taken with a Hitachi U-3010 UV−vis spectrophotometer. Steady-state PL spectra were obtained using a Jasco V-570 spectrophotometer. Cyclic voltammograms (CVs) were recorded on a Zahner IM6e electrochemical workstation, using a glassy carbon disk as the working electrode (∼0.25 cm2), a Pt wire as the counter electrode, and Ag/Ag+ (Ag wires with 0.01 M AgNO3 in acetonitrile) as the reference electrode; 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF6) dissolved in acetonitrile was employed as the supporting electrolyte. All NC samples were purified and dispersed in chloroform. The working electrodes were polished, cleaned, and dried before depositing the NC samples. A drop of diluted NC solution was deposited onto the surface of the working electrode to form an NC film. The scan rate was set at 30 mV/s, and during all the experiments, the electrolyte solutions were thoroughly deoxygenated by bubbling high-purity nitrogen for 15 min and maintaining a nitrogen atmosphere over the solution. Device Fabrication. Devices were fabricated on indium tin oxide (ITO)-coated glass substrates that served as the anode. The ITO substrates were cleaned with an ultrasonic cleaner

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RESULTS AND DISCUSSION Figure 1a shows an overview image of CuInSe2 NCs with necklace structures. A representative high-resolution TEM

Figure 1. (a) Overview image and (b) representative high-resolution TEM image of CuInSe2 samples obtained from growth at 200 °C for 120 min.

image, shown in Figure 1b, indicates that the necklace structures are formed by smaller particles that are randomly oriented. The formation of necklace structures has been previous observed in assemblies of CdTe NCs with thiol ligands.50 In the synthesis, we use dodecanethiol as ligands to decrease the metal reactivity and control the particle size by capping on the preformed nanoparticles. As the reaction proceeds, the dodecanethiol ligands may partially decompose to liberate hydrogen sulfide and dodecene upon thermal heating.51 Loss of the ligands promotes aggregation of the nanoparticles into the necklace structure. The band gaps and energy levels are vital parameters for device design and material selection. Cyclic voltammetry has been used to determine the band gaps and energy levels of the 7281

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Figure 3. (a) UV−vis absorption spectra and (b) photoluminescence spectra of a hybrid composite of P3HT and CuInSe2 NCs with different ratios.

CuInSe2 NCs with an average diameter of ∼3.4 nm were purified as described in the Experimental Section. As a reference, bulk CuInSe2 (i.e., much larger NCs) were prepared by a similar method to that reported by Tang et al.42 These CuInSe2 NCs have an average size of 16 nm and are not expected to exhibit quantum confinement effects. The samples were deposited on a carbon electrode through drop-casting for CV measurements. Figure 2a compares CV curves of the small and large CuInSe2 NCs. The onset of oxidation and reduction peaks for the conduction band and valence band in the larger NCs were easily identified and are labeled by dotted lines. Relative to the Ag/Ag+ reference electrode, the oxidation and reduction potentials of the bulk like (16 nm) NCs were found to be 0.36 and −0.67 V, corresponding to HOMO and LUMO levels of −5.07 and −4.04 eV, which is consistent with the literature report by using photoelectron spectroscopic techniques.58,59 The electrochemical band gap of 1.03 eV is similar to the optical band gap.60 Finite-depth well effective mass approximation (EMA) calculations parametrized with values of energy levels measured for the bulk like NCs were used to predict the conduction and valence band-edge energies of CuInSe2 NCs as a function of diameter.60,61 Figure 2b shows the estimated valence and conduction band-edge energies of CuInSe2 NCs as well as their band gaps (eV) as a function of particle diameter (nanometers). From this theoretical estimation, the energy levels of ∼3.4 nm CuInSe2 NCs are predicted to be −3.59 eV for the conduction band-edge, −5.20 eV for the valence band-edge, and 1.61 eV for the band gap. In comparison to the large CuInSe2 NCs with a mean diameter of 16 nm, the CV curve (see Figure 2a) for the 3.4 nm CuInSe2 NCs shows several oxidation and reduction peaks, which correspond to the energies of −3.32, −3.63, −3.84, −4.06, −5.04, and −5.34 eV. By comparing with the theoretical estimated energy levels with the data from oxidation and

Figure 2. (a) CV curves of CuInSe2 NCs with an average diameter of 16 and 3.4 nm. (b) The valence and conduction band-edge energies as well as the band gaps (eV) of CuInSe2 NCs as a function of particle diameter (nm). (c) Analysis of the energy levels of CuInSe2 NCs with an average diameter of 16 and 3.4 nm from CV results.

conduction band and valence band in NCs as well as to locate deep trap states.52−56 The energy levels of the conduction and valence band edges can be calculated from the onset oxidation potential (Eox) and onset reduction potential (Ered), respectively, according to the following equations EHOMO = −Ip = −e(E ox + 4.71) eV

(1)

ELUMO = −Ea = −e(Ered + 4.71) eV

(2)

where the potential is measured relative to a Ag/Ag+ (0.01 M) reference electrode.57 7282

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Figure 4. I−V curves of the hybrid composite device with P3HT and CuInSe2 NCs weight ratios of (a) 19:1, (b) 1:2, (c) 1:6, and (d) 1:9 at room temerature under dark and illlumination of 100 mW/cm2 at room temperature.

is a hole trap state ∼300 meV above the valence band edge. It has been reported that I−III−VI semiconductors have inherent donor and acceptor levels due to the stoichiometry deviations, including vacancies (VCu, Vse) and site substitution (In on a Cu site or Cu on an In site).62 For the bulk like material, three donor levels at 8, 80, and 180 meV were observed. The acceptor levels were also observed at 12−30 and 65−98 meV above the valence band edge.63 Assuming that the intrinsic trap states of nanoscale materials are also due to the vacancies or site substitution with the bulk materials, the donor or acceptor levels should be located at similar positions above the valence band edge or below the conduction band edge. Here, the observed trap states in the ternary CuInSe2 NCs are located at a much higher or lower position than the expected levels of intrinsic defects. Therefore, the apparent trap states in CuInSe2 NCs are more likely related to the surface defects, which are well-known for II−VI semiconductor NCs, such as CdSe NCs.64 The properties of hybrid materials strongly depend on the relative alignment of HOMO and LUMO energy levels.65,66 The conjugated polymer P3HT has LUMO and HOMO levels of −2.74 and −4.76 eV, respectively.67 Therefore, P3HT and CuInSe2 NCs have a type II band alignment, so it is reasonable to expect charge transfer between P3HT and CuInSe2 NCs. PL quenching in the composite is consistent with that expectation. In Figure 3, we show the absorption spectra of hybrid P3HT and CuInSe2 composite films with different ratios of polymer to NCs. Pristine P3HT film absorbs light throughout the range of 400−650 nm with three distinguishable absorption features at 520, 555, and 605 nm. CuInSe2 NCs have a broader spectrum with an absorption edge at ∼800 nm. When the P3HT/ CuInSe2 ratio is high, the absorption is dominated by P3HT. Increasing the CuInSe2 NC concentration strongly enhances the absorption in the near-ultraviolet region. The pristine P3HT film shows clear PL emission in the range of 650−750 nm. We observed PL quenching in the composite with the

Table 1. List of the Performance of Hybrid Composite Devices with P3HT and CuInSe2 NCs Weight Ratio of 6:1 Fabricated with Different Annealing Temperatures annealing temp (°C) (20 min) 110 170 200 210 220 230 250 a

Jsc (mA/cm2)a 0.034 0.135 0.48 0.45 0.56 0.51 0.30

± ± ± ± ± ± ±

0.004 0.004 0.03 0.01 0.02 0.01 0.02

Voc (V)a

FF (%)a

PCE (‰)a

± ± ± ± ± ± ±

∼53 ∼40 ∼31 ∼24 ∼30 ∼28 ∼24

∼0.1 ∼0.2 ∼0.6 ∼0.5 ∼0.7 ∼0.6 0.3

0.43 0.42 0.43 0.44 0.44 0.42 0.35

0.02 0.02 0.02 0.02 0.02 0.02 0.02

Average value.

reduction peaks in the CV trace (see the blue solid line in Figure 2c), the energy levels of −3.63 and −5.34 eV can be attributed to the conduction and valence band-edge energies of ∼3.4 nm CuInSe2 NCs. The electrochemical band gap is thus 1.71 eV, which is consistent with the optical measurements (absorption band at 1.63 eV, emission peak at 1.55 eV; see the Supporting Information). The additional energy states/levels nearby the conduction and valence bands of 3.4 nm CuInSe2 NCs might be due to the NC size variations or the trap states. Trap states should be sensitive to certain temperature perturbations, but the NC sizes should not be sensitive to thermal perturbation.53 The CV curves measured at varied temperatures are provided in the Supporting Information. As the temperature is raised, the peak related to defect states exhibits a unique decrease behavior with the temperature increase. The temperature-induced quenching may be simply explained to that thermal energy induced the instability of holes trap states. The energy states of −3.84 and −4.06 eV are electron trap states that appeared ∼210 and ∼430 meV below the conduction band edge, whereas the energy state of −5.04 eV 7283

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necklace structures than found for hexagonal/spherical particles studied previously for photodetector applications.47,48 We observed that the hybrid composite devices exhibit a transition from photoconductive gain to a photovoltaic effect with decreasing P3HT/CuInSe2 ratio. When the P3HT/ CuInSe2 ratio reaches 1:2, an obvious photovoltaic effect is observed (see Figure 4b). The photovoltaic effect was significantly improved in the hybrid P3HT/CuInSe2 composite with weight ratios of 1:6 (see Figure 4c) and 1:9 (see Figure 4d). We further optimized the devices by tuning the annealing temperature. The device results are summarized in Table 1. The best performance, obtained with the annealing temperature at 220 °C, is VOC = 0.45 V, Jsc = 0.57 mA/cm2, FF = 29.7%, and 0.7‰ power conversation efficiency (PCE). Although the performance was limited by the ligands on the CuInSe2 NCs' surface, the result is still better than the previous reports for polymer and CuInSe2 NC hybrid solar cells.35,46 When the CuInSe2 concentration is low, the composite shows photoconductivity behavior, probably because the CuInSe2 NCs are dispersed in the polymer matrix and serve as electron traps (see Figure 5a). Under illumination, excitons dissociate at the polymer NC interface, electrons on CuInSe2 NCs, where they are likely to be trapped in the trap states with energies near the conduction band. The charge transport in the composite will, therefore, be hole-dominated.69 Under reverse bias, excess holes will be the charge carriers, and a photocurrent can be observed. When the CuInSe2 concentration is high, NCs form a percolated network and act as an acceptor for P3HT, providing an effective path for carrier extraction in the NC domain, and hence, the device exhibits photovoltaic behavior (see Figure 5b). After exciton dissociation at the polymer and NC interface, holes transport into the ITO electrode through polymer networks and the electrons transport into the Al electrode through NCs networks. CuInSe2 NCs were blended into P3HT without any ligand exchange; therefore, the power conversion efficiency was limited by inefficient electron transport.

Figure 5. Schematic diagram of the structure, energy levels, excitation, and carrier transport in the hybrid polymer and CuInSe2 NCs composite of (a) a P3HT/CuInSe2 weight ratio of 19:1 and (b) a P3HT/CuInSe2 weight ratio of 1:6.

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CONCLUSIONS CuInSe2 NCs that aggregate into a necklace structure were integrated into P3HT. CV measurements showed that P3HT and CuInSe2 NCs have a type II band alignment. The quenching of PL with an increase of CuInSe2 ratio in the composite was observed, implying charge transfer between P3HT and CuInSe2 NCs. Photodiode devices with the composite sandwiched between ITO and Al were studied, and we found that the hybrid composite devices exhibited a transition from photoconductive gain to photovoltaic effect with the increase of the P3HT/CuInSe2 weight ratio. In the composite with a lower CuInSe2 weight ratio, separated electrons were trapped by the CuInSe2 NCs and exhibited hole-dominated transport properties. We conclude that, while CuInSe2 NCs serve as a reasonable electron acceptor for P3HT, devices are limited by the intrinsic trap states characteristic of CuInSe2 NCs. New synthetic procedures might be able to address this issue. With increased CuInSe2 NCs in the composite, a photovoltaic effect was observed owing to the formation of electron transport pathways. However, the inefficient electron transport of CuInSe2 NCs largely limits the photovoltaic device performance. It can be greatly improved in the future by exchanging NC ligands before incorporating into devices.

CuInSe2 NCs mixed at 5 wt %. A further increase of the CuInSe2 percentage enhanced the PL quenching. For the composite with a P3HT/CuInSe2 weight ratio of 1:2, the polymer PL is almost completely quenched and no PL from the NCs is seen, which indicates that photoexcitation leads to charge transfer from P3HT to CuInSe2 NCs.68 To examine further the photoinduced charge separation, we fabricated devices of hybrid composite in a sandwich geometry with an ITO bottom electrode and an Al top electrode and studied the hybrid composite devices with different P3HT and CuInSe2 NCs weight ratios. Figure 4a shows current−voltage characteristics of devices with a weight ratio of 19:1 measured both in the dark and under illumination. Typical p−n junction behavior with rectifying current−voltage I−V characteristics was obtained in the dark, showing a rectification ratio (current ratio measured at +1 versus −1 V) of ∼500. Under light illumination, although the diode characteristic is still observed in the region of bias from −1.5 to 1.5 V, the device current increased sharply, by nearly 2 orders of magnitude, under reverse bias. That indicates photoconductivity under 100 mW/ cm2 simulated sunlight. The highest photocurrent-to-dark current ratio is about 700 at the negative bias of around 4 V. The on/off ratio is much higher for the CuInSe2 NCs with 7284

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical calculations, absorption and emission spectra, and CV curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.Z.), [email protected] (Y.L.), [email protected] (G.D.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under NSFC Research Grants (Nos. 51003005 and 91023039), Excellent Young Scholars Research Fund of Beijing Institute of Technology (No. 2010Y0913), and the 111 research base (BIT111-201101). The Natural Sciences and Engineering Research Council of Canada is also acknowledged for support of this research. The authors also thank Dr. Ankit Kumar for reading our manuscript.

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