Modular Fabrication of Hybrid Bulk Heterojunction Solar Cells Based

Article pubs.acs.org/JPCC

Modular Fabrication of Hybrid Bulk Heterojunction Solar Cells Based on Breakwater-like CdSe Tetrapod Nanocrystal Network Infused with P3HT Jaehoon Lim,†,‡ Donggu Lee,§,‡ Myeongjin Park,§ Jiyun Song,§ Seonghoon Lee,∥ Moon Sung Kang,*,⊥ Changhee Lee,*,§ and Kookheon Char*,† †

School of Chemical and Biological Engineering, The National Creative Research Initiative Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy & Environment, §Department of Electrical and Computer Engineering, Inter-University Semiconductor Research Center, and ∥School of Chemistry, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea ⊥ Department of Chemical Engineering, Soongsil University, Seoul, 156-743, Korea S Supporting Information *

ABSTRACT: We demonstrate the modular fabrication of nanocrystal/polymer hybrid bulk heterojunction solar cells based on breakwater-like CdSe tetrapod (TP) nanocrystal networks infused with poly(3-hexylthiophene) (P3HT). This fabrication method consists of sequential steps for forming the hybrid active layers: the assembly of a breakwater-like CdSe TP network followed by nanocrystal surface modification and the infusion of semiconducting polymers. Such a modular approach enables the independent control of the nanoscopic morphology and surface chemistry of the nanocrystals, which are generally known to exhibit complex correlations, in a reproducible manner. Using these devices, the influence of the passivation ligands on solar cell characteristics could be clarified from temperature-dependent solar cell experiments. We found that a 2-fold increase in the short-circuit current with 1hexylamine ligands, compared with the value based on pyridine ligands, originates from the reduced depth of trap states, minimizing the trap-assisted bimolecular recombination process. Overall, the work presented herein provides a versatile approach to fabricating nanocrystal/polymer hybrid solar cells and systematically analyzing the complex nature of these devices. ing work in this area was first performed by Alivisatos and coworkers,4 who demonstrated nanocrystal/polymer hybrid solar cells based on a blend of semiconductor CdSe nanorods and poly(3-hexylthiophene) (P3HT). Thereafter, multiple approaches to improve solar cell efficiency have been taken: (i) modifying nanocrystal structures (e.g., spherical dots,5−9 rods,10−12 tetrapods,13−17 and hyperbranched nanocrystals18), (ii) proposing new device architectures,19 and (iii) developing new nanocrystal surface passivation9,12,20 and hybridization21−27 processes. More recently, employing low-band-gap conducting polymers16,17,28 or organic nanowires in a hybrid structure8 was found to result in a significant enhancement in PCE up to 4.7%, reflecting that there is still much room for further improvement of device performance. Among various nanocrystals available, tetrapod (TP) semiconductor nanocrystals are a promising structure for improving the performance of hybrid solar cells. This is because the four branches of a tetrapod that are three-dimensionally extended increase the absorption cross section29 and also provide a

1. INTRODUCTION Colloidal semiconductor nanocrystals are promising candidates for photovoltaic cell applications because of their superior optical and electrical properties, which have been realized recently by low-cost solution processes. Control of the nanocrystal size/shape correlates with modulation of the optical band gap of these materials, from the ultraviolet to the near-infrared range, with a fairly large absorption coefficient (>105 cm−2 M−1).1 Excellent electrical conduction through arrays of nanocrystals (mobility > 20 cm2 V−1 s−1) can also be achieved by chemical modification of the nanocrystal surface.2,3 However, the power conversion efficiencies (PCEs) of solar cells employing such colloidal semiconductor nanocrystals as active materials still need to be improved. One possible reason for their poor device performance is that the size of typical nanocrystals is smaller than their exciton Bohr radius, which prevents facile dissociation of photogenerated excitons. Consequently, a strategy to introduce an additional layer having an energy offset with respect to the nanocrystal has been developed, which could trigger exciton dissociation and chargecarrier extraction. In particular, the hybridization of colloidal semiconductor nanocrystals with organic materials that form a type-II band alignment has drawn significant interest. Pioneer© 2014 American Chemical Society

Received: November 14, 2013 Revised: January 24, 2014 Published: January 31, 2014 3942

dx.doi.org/10.1021/jp411214n | J. Phys. Chem. C 2014, 118, 3942−3952

The Journal of Physical Chemistry C

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Scheme 1. Schematic Illustration of the Modular Fabrication of CdSe TP/P3HT Hybrid Solar Cells

conductive pathway to enhance overall charge transport through TP ensembles.30 Also, by forming hybridized structures with conducting polymers, the increased interfacial area between TPs and conducting polymers is expected to facilitate exciton dissociation. More importantly, the porous network formed by TPs, which resembles tetrapod-breakwaters, results in an ideal structure for forming a bulk heterojunction with the hybridizing organic counterpart. To fully utilize the benefits offered by TPs for solar cell applications, the proper elimination of bulky ligands attached to the TP surface, which are originally employed to prevent TP aggregation during synthesis, storage, and processing, is essential. This is because the bulky and insulating natures of typical surface ligands prevent efficient interfacial contact between TPs and conducting polymers within the hybrid structure. However, the steps for ligand removal or exchange have unfortunately hampered advances in TP/polymer hybrid solar cell research. Specifically, poor colloidal stability prevents reliable and reproducible formation of TP/polymer hybrid active layers in terms of nanoscopic morphology, particularly if the ligand removal/exchange processes are performed before the formation of binary dispersions of the two materials. On the other hand, if the ligand removal/exchange steps are introduced after the formation of TP/conducting polymer hybrid films, independent control on the nanoscopic morphology and surface chemistry of the TPs, which are the two critical factors determining the final solar cell performance, is not possible. This is because TP surface modification in such a manner would inevitably influence the interparticle distance or spatial distribution of TPs within the final hybrid structure. Consequently, systematic analysis of TP/polymer hybrid solar cell operations could not be carried out, and as a result, rational guidelines for improving TP/polymer hybrid solar cell performance could not be provided. In this report, we propose a so-called modular fabrication method for assembling TP nanocrystal/polymer hybrid

bulkheterojunction solar cells. Unlike prior approaches based on the binary dispersion of nanocrystals and polymers with preor postchemical treatments of the nanocrystals, our modular fabrication separates the formation of a breakwater-like nanocrystal network (step 1), the modification of the nanocrystal surface by chemical treatments (step 2), and the intrusion of polymers into the nanocrystal network (step 3) (Scheme 3). Accordingly, CdSe TP/P3HT hybrid bulkheterojuctions could be assembled with control of their nanoscopic morphology and surface chemistry. Consequently, the influence of nanocrystal surface modification on the as-assembled solar cell performance could be systematically examined through temperature-dependent current density (J) versus voltage (V) measurements at varying light intensities. Modifying nanocrystal surfaces with 1-hexylamine resulted in a solar cell PCE of 2.2%. These devices exhibited more than a 2-fold increment in short-circuit current compared to those with bare or pyridine-modified nanocrystal surfaces, which can be attributed to the reduced depth of trap states. We believe that the modular fabrication method presented herein provides a versatile framework for fabricating TP nanocrystal/conducting polymer hybrid bulkheterojunction solar cells reproducibly and analyzing the complicated nature of nanocrystal/polymer hybrid solar cells systematically.

2. EXPERIMENTAL SECTION 2.1. Materials. Cadmium oxide (CdO, 99.95%) was purchased from Alfa Aesar. Selenium (99.99%, powder), ntrioctylphosphine (TOP, 90%), oleic acid (OA, 90%), 1octadecene (ODE, 90%), zinc acetate dihydrate [Zn(ac)2· 2H2O, 98%], cetyltrimethylammonium bromide (CTAB, >99%), 1-hexylamine (99%), and pyridine (≥99.0%) were purchased from Sigma-Aldrich. Poly(3-hexylthiophene) (P3HT) was purchased from 1-Material Inc. All chemicals were used as received. 3943



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2.2. Synthesis of ZnO Nanoparticles. ZnO nanoparticles were synthesized according to the method reported by Pacholski et al.31 with modifications. First, 3 g of Zn(ac)2· 2H2O and 120 mL of methanol were placed in a three-neck round-bottom flask and heated to 60 °C. At 60 °C, 60 mL of a KOH solution containing 1.51 g of KOH was added dropwise to the Zn(ac)2·2H2O solution under strong agitation. The reaction mixture was kept at 60 °C for 2 h 15 min to yield a milky solution containing ZnO nanoparticles. The ZnO nanoparticles were then isolated by centrifugation at 4000 rpm, followed by repeated washing using methanol. Finally, the product was centrifuged again and redispersed in 5 mL of butanol. 2.3. Synthesis of CdSe TPs. A modification of the synthetic procedure of Lim et al.32 was employed for the synthesis of CdSe TPs. Cd and Se precursor solutions for arm growth were prepared as follows: cadmium oleate [Cd(OA)2] solution was prepared by dissolving 10 mmol of CdO in 7.8 mL of OA, 6.2 mL of ODE, and 1 mL of TOP in a round-bottom flask at 280 °C under a N2 flow. After the mixture became optically clear, the round-bottom flask was cooled to 50 °C, and 0.14 mmol of CTAB was added. Separately, 12 mmol of Se and 6 mL of TOP were mixed in a 50 mL round-bottom flask with a condenser and heated at 200 °C. Once the mixture became transparent, the TOPSe solution was cooled to room temperature. Then, 5 mL of TOPSe solution was added to the Cd(OA)2 solution, and the resulting mixture was stirred for 5 min. To prepare CdSe seeds with a zincblende crystal structure, a 0.5 mmol mL−1 Cd(OA)2 solution was prepared by reacting 5 mmol of CdO, 5 mL of OA, and 5 mL of ODE at 280 °C under a N2 flow for 15 min and then allowing the reaction mixture to cool to room temperature. Next, 1 mmol of Se and 10 mL of ODE were loaded into a 100 mL three-neck round-bottom flask and heated to 300 °C under a N2 flow. When the Se/ODE solution became optically clear, 4 mL of Cd(OA)2 solution (2 mmol) was rapidly injected into the solution and reacted at 270 °C for 15 min. Finally, 6 mL of ODE was added to the seed solution, and the reaction mixture was cooled to room temperature. Using the as-prepared injection solution and the CdSe seeds, CdSe TPs were synthesized by the continuous precursor injection approach. For a typical synthesis to attain 76-nm-long CdSe TPs, 2.5 mL of seed solution, 2.25 mL of OA, 1.5 mL of TOP, 23.75 mL of 1-ODE, and 0.21 mmol of CTAB were placed in a three-neck round-bottom flask and then heated to 260 °C. At this elevated temperature, 30 mL of injection solution was injected into the reactor in a stepwise manner: 0.5 mL min−1 for 6 min, 0.6 mL min−1 for 6 min 40 s, 0.7 mL min−1 for 8 min 35 s (the temperature was set to 280 °C at this step), and 0.8 mL min−1 for 8 min 45 s. After precursor injection had been completed, the reaction was quenched rapidly. To purify the product, 2 mL of chloroform and an excess amount of acetone were added to the cooled reactor until the solution became turbid. After centrifugation at 3000 rpm, the supernatant was decanted, and the precipitate was redispersed in organic solvents such as chloroform, toluene, or hexane. The precipitation and redispersion steps were repeated until the samples were sufficiently purified. 2.4. Surface Modification of CdSe TPs with HBF4 and Oleylamine. Five milliliters of oleate-capped CdSe TP solution (40 mg/mL) was collected by precipitation using excess ethanol and after centrifugation at 3000 rpm. The

transparent supernatant was discarded, and the precipitate was dissolved with 5 mL of hexane. Then, 5 mL of N,Ndimethylformamide (DMF) and 0.5 mL of HBF4 solution (48 wt % in H2O) were added to the CdSe TP solution dissolved in hexane. After the mixture had been vortexed for less than 1 min, CdSe TPs were collected in the polar DMF phase, and oleic acid remained in the nonpolar hexane phase. After removal of the upper phase containing oleic acid, 5 mL of hexane was added, and the mixture was vortexed again to wash out oleate ligands in the DMF/H2O phase. After three repeated washing steps, the mixture was precipitated with an excess amount of acetone. To remove oleate ligands completely, HBF4 treatment was conducted once again with the same amounts of DMF, HBF4 solution, and hexane. To modify bare CdSe TPs with oleylamine ligands, the bare CdSe TPs were precipitated with excess acetone and redispersed in a chloroform/oleylamine mixture [chloroform/ oleylamine = 8:2 (v/v)]. After 10 min, excess acetone was added to the CdSe TP solution, and then the solution was centrifuged at 3000 rpm to remove remaining HBF4 or water. Finally, the preciptate was redispersed in 3 mL of chloroform/ oleylamine mixture [chloroform/oleylamine = 30:1 (v/v)]. 2.5. Fabrication of CdSe TP:P3HT Hybrid Solar Cells. First, a dispersion of ZnO nanocrystals was spin-coated onto an indium tin oxide (ITO) substrate at 2000 rpm for 60 s and dried in a N2 atmosphere at 100 °C for 30 min. Then, a CdSe TP solution was deposited onto the ZnO nanoparticle layer by spin-coating at 2000 rpm for 30 s, followed by annealing at 150 °C for 20 min under an Ar atmosphere. After being allowed to cool to room temperature, the film was washed five times with ethanol by spin-coating (2000 rpm). For the complete fixation of the CdSe TP networks, the washed substrate was annealed again at 150 °C for 10 min under an Ar atmosphere (step 1). Surface modification of the CdSe TP network was conducted by spin-coating a ligand solution [acetone/ligand = 9:1 (v/v)] onto the CdSe TP networks. After spin-coating, the resulting substrates were dried at 60 °C for 10 min under an Ar atmosphere (step 2). Then, 2 wt % P3HT in monochlorobenzene was spin-coated onto the CdSe TP networks with modified surface at 1000 rpm for 30 s (step 3). Finally, a 10-nm MoO3 layer and 100-nm Al electrodes were deposited subsequently by thermal evaporation. 2.6. CdSe TP and Film Characterization. Room-temperature UV−visible absorption spectra were measured using an Agilent 8454 UV−vis diode-array spectrometer. Transmission electron microscopy (TEM) images of CdSe TPs were recorded using JEOL JEM-2100 and JEM-3010 microscopes. Top and cross-sectional SEM images of CdSe TP networks and CdSe TP/P3HT hybrid films were recorded by scanning electron microscopy (Carl Zeiss SUPRA 55VP). The atomic contents of the CdSe TP:P3HT hybrid films were characterized by Auger electron spectroscopy (Perkin-Elmer PHI 660), using an instrument equipped with a LaB6 electron gun, an Ar ion beam, and a cylindrical mirror analyzer. Fourier transform infrared (FT-IR) spectra of CdSe TP network films were acquired using a Perkin-Elmer Frontier FT-IR spectrometer in transmission mode. All samples were fabricated on dual-sidepolished Si wafers for characterization, which prevents the accumulation of charge during the radiation of a high-energy beam or light scattering during the transmission of an infrared beam. 2.7. Characterization of Hybrid Solar Cells. Typical current density−voltage (J−V) characteristics of the solar cells 3944



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Figure 1. (a) Schematic of the ligand-exchange procedure of CdSe TPs from oleates to oleylamines. (b) FT-IR spectra of as-synthesized CdSe TPs with oleate ligands (red), TPs after treatment with HBF4 (blue), and TPs after capping with oleylamine ligands (green). (c) Absorption spectra of assynthesized CdSe TPs (red dashed line) and oleylamine-capped CdSe TPs (green solid line). The inset is a photograph of oleylamine-capped CdSe TPs dispersed in chloroform.

significant increase in interfacial contact area with conducting polymers. The arm diameter of these CdSe TPs was ∼8 nm. Such an arm thickness was necessary to improve the mechanical robustness of the network structure and to maximize the light absorption window. Although stable nanocrystal dispersions, necessary for further solution processing, could be achieved with as-synthesized TPs with oleate ligands attached to the nanocrystal surfaces, the original ligands were replaced with oleylamines. Oleylamine ligands, which also yield stable nanocrystal dispersions, have moderate affinity to the surface Cd atoms of CdSe TPs, and therefore, the removal of oleylamine ligands can be done simply by rinsing the nanocrystals with alcohols later in the fabrication steps.33 To replace the oleate ligands with oleylamine ligands, a two-phase ligand-exchange method was employed using tetrafluoroboric acid (HBF4) (Figure 1a). Vortexing a mixture of CdSe TPs (in hexane) and HBF4 (in DMF/water) resulted in the phase transfer of CdSe TPs from the hexane phase to the mixed DMF/water phase. The phase transfer occurred because the surface Cd sites become positively charged and surrounded by BF4− counterions, as HBF4 donates protons to the oleate ligands that are detached from the Cd surface as oleic acids.34 The removal of oleate ligands upon repeated HBF4 treatments was confirmed from the disappearance of the vibrational peaks originating from alkyl chains (2800−3000 cm−1) in the FT-IR spectra (Figure 1b). Even though the repulsive interactions

were measured using a Keithley 237 source measurement unit under AM1.5G conditions and an illumination intensity of 100 mW cm−2 generated by a Newport 91160A solar simulator. Incident light power was varied with neutral density filters. For temperature-dependent J−V measurements between 200 and 300 K, the solar cells were loaded onto the coldfinger of a closed-cycle He cryostat with a LakeShore 331 temperature controller. Incident photon-to-current conversion efficiency (IPCE) was measured using an Oriel QE/IPCE measurement kit consisting of a 300-W xenon lamp, a monochromator (74125), an order-sorting filter wheel, a Merlin lock-in amplifier (70100), and a chopper.

3. RESULTS AND DISCUSSION 3.1. Preparation of CdSe TPs and Surface Modification. Well-defined CdSe TPs with high morphological uniformity were synthesized following the continuous precursor injection (CPI) method.32 Careful control of both the injection rate of precursors and the reaction temperature yielded long oleate-capped CdSe TPs with excellent shape selectivity (over 90%) (see Figure S1, Supporting Information). The arm length of these TPs was ∼76 ± 3 nm. Such well-defined CdSe TPs with uniform-length arms are particularly beneficial for achieving highly conductive TP networks. Moreover, the resulting breakwater-like TP networks ensure the formation of bulk heterojunctions with polymer counterparts exhibiting a 3945



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Figure 2. Changes in the FT-IR spectra of CdSe TP network films (a) after ethanol washing (step 1) and (b) after surface modification with 1hexylamine (step 2). (c) Depth profiles of sulfur and cadmium signals in a CdSe TP/P3HT hybrid film (step 3) taken by Auger electron spectroscopy. (Top) Plan and (bottom) cross-sectional SEM images of (d) a bare CdSe TP network film (step 1), (e) a CdSe TP network film treated with 1-hexylamine (step 2), and (f) a CdSe TP/P3HT hybrid film (step 3). All scale bars in the SEM images are 200 nm.

large volume contraction occurring during the ligand-exchange process.35 The voids created during the CdSe TP network formation are thought to mitigate the significant volume contraction upon ligand removal. In step 2, the surfaces of bare TPs were further modified by spin-coating short ligands such as 1-hexylamine and pyridine directly onto the TP network prepared from step 1 (Figure 2b). The choice of 1-hexylamine and pyridine is an interesting set of surface ligands, because these amines are known to cause opposite effects on the nanocrystal surface state, even though both ligands contain the same nitrogen atoms that interact with nanocrystal surfaces through σ-electron donation. Whereas 1hexylamine can effectively reduce the surface states,36 pyridine ligands are thought to form traps by delocalizing holes within the aromatic rings.37 After TP surface modification, a negligible change in the nanoscopic morphology was observed (Figure 2e). We also note that the surface modification did not cause a significant change in the absorption spectra (Figure S2a, Supporting Information). As the final step, voids left within the CdSe TP network were filled with conducting polymers by spin-coating a P3HT solution onto the surface-treated TP network. This final process guaranteed the creation of a bicontinuous TP/polymer hybrid structure, which is critical to serve as a solar cell active layer. The planar and cross-sectional scanning electron microscopy (SEM) images shown in Figure 2f clearly demonstrate that the TP network is fully covered with conducting polymers. Atomic depth profiles of the TP/P3HT hybrid film obtained from

among charged TPs might not yield completely stable dispersions of CdSe TPs, the electrostatic repulsion was sufficient to prevent irreversible nanocrystal aggregation. The charged CdSe TPs were then quickly collected by centrifugation and redispersed in chloroform containing oleylamines. As the oleylamines form coordination bonds with CdSe TP surfaces, TPs can be well-dispersed in organic solvents such as chloroform. Also, the vibrational peaks due to alkyl chains reappeared in the FT-IR spectra as evidence of TPs capped with oleylamines. We also note that the optical spectra of the CdSe TPs were unchanged upon ligand exchange from oleates to oleylamines (Figure 1c). 3.2. Modular Fabrication of CdSe TP/P3HT Hybrid Solar Cells. The modular approach to fabricating CdSe TP/ P3HT hybrid solar cells is illustrated in Scheme 1. In step 1, an interconnected CdSe TP network (∼200 nm thick) was first formed by spin-coating a dispersion of oleylamine-capped CdSe TPs in chloroform onto a transparent cathode. The oleylamine ligands were then removed by thermal annealing and repeated ethanol washing. Greater than 90% removal of alkyl chains after the repeated ethanol washings was confirmed from FT-IR spectroscopy (Figure 2a). In addition, the absorption spectra (Figure S2a, Supporting Information) and the highly porous morphology (Figure 2d) of the CdSe TP network were hardly changed after ligand removal, confirming the robustness of the TP network structure. We also note that the resulting CdSe TP network was devoid of microcracks, which have often been observed from closely packed nanocrystal films because of the 3946



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Figure 3. Current−voltage characteristics of CdSe TP/P3HT hybrid solar cells: TP surface treated with 1-hexylamine (red) and with pyridine (blue) and bare TP surface (black).

Auger electron spectroscopy, shown in Figure 2c, also confirm the complete infusion of P3HT within the TP network. From the top portion of the hybrid film during the measurement of Auger electrons, we noted a dominant signal from sulfur over that from cadmium. As the probing position penetrated into the hybrid film, yielding an intense Cd signal, the signal from sulfur was retained, implying the abundance of P3HT within the CdSe TP network. In addition, the mechanical robustness of the CdSe TP network was again confirmed by removing the P3HT domains from the CdSe TP/P3HT hybrid layer using chloroform: The recovered CdSe TP network showed an absorption spectrum and film morphology analogous to those of the original TP network (Figure S2b,c, Supporting Information). For the fabrication of actual solar cells, the hybrid TP/P3HT active layers were assembled in the inverted solar cell configuration utilizing ITO substrates as cathodes to extract electrons and Al electrodes to collect holes. The inverted device structure was employed because our modular approach first requires the formation of a CdSe TP network layer, which is ntype, on an ITO substrate. Also, for effective charge collection along with reduced exciton quenching, a hole-blocking layer (∼20-nm-thick ZnO nanoparticle layer) and an electronblocking layer (∼10-nm-thick MoO3 layer) were placed between the active layer and the two respective electrodes (Scheme 1). We noticed that the deposition of the ZnO nanoparticle layer facilitated the stable anchoring of the TP network onto the substrate, perhaps through enhanced atomic fusion between the ZnO nanoparticles and TPs. The entire solar cell fabrication was carried out under an inert argon atmosphere to prevent undesired oxidation or degradation. 3.3. CdSe TP/P3HT Hybrid Solar Cell Performance. Figure 3 displays the current density (J) versus voltage (V) characteristics obtained under a vacuum for the CdSe TP/ P3HT hybrid solar cells that were treated with pyridine and 1hexylamine. Also, the J−V curve of a hybrid solar cell fabricated with a bare TP network is included in the figure as a reference. From the device containing the TP network treated with 1hexylamine, more than a 2-fold increase in the short-circuit current (JSC) was obtained, along with small reductions in the open-circuit voltage (VOC) and fill factor (FF), when compared to the reference device. The average solar cell performances are summarized in Table 1, The best device, treated with 1-

Figure 4. (a) High-binding-energy cutoff region and (b) valence-band region in UPS spectra of CdSe TP network films: bare (black), modified with pyridine (blue), and modified with 1-hexylamine (red). All spectra were referenced to an Au substrate with a work function of 5.1 eV. (c) Energy level diagram of CdSe TP/P3HT hybrid solar cells with different passivation ligands.

hexylamine, yielded a power conversion efficiency (PCE) of 2.24%, a JSC of 7.56 mA cm−2, a VOC of 0.63 V, and an FF of 47.1%, which is comparable to the best CdSe NC/P3HT hybrid solar cells treated with short alkylamines reported so far.7,19 Meanwhile, the solar cell using a TP network treated with pyridine exhibited a slight increase in JSC, along with small reductions in both VOC and FF, when compared with the reference device. To understand the change in VOC upon surface modifications, UV-photoelectron spectroscopy (UPS) measurements were performed. Figure 4 shows the high-binding-energy cutoff region (Figure 4a) and the valence-band region (Figure 4b) in the UV-photoelectron spectra for CdSe TP networks treated with two different ligands. A larger shift in the high-bindingenergy cutoff region toward a lower energy value was observed for the CdSe TP network treated with 1-hexylamine compared with the TP network treated with pyridine. Meanwhile, the optical band gap obtained from absorption measurements remained constant (see the inset of Figure S2a, Supporting Information). Together, these results indicate that the relative positions of the valence-band maximum (VBM) and conduction-band minimum (CBM) for the CdSe TP networks change upon different chemical treatments. Compared with those of the bare TP network, the energy levels of TP networks treated with 1-hexylamine and pyridine ligands were lowered by 0.24 and 0.13 eV, respectively (Figure 4c). Because VOC in heterojunction solar cells is typically determined by the energy difference between the CBM of an acceptor layer and the VBM of a donor layer, the results from the UPS measurements could favorably explain the reduced VOC values for solar cell samples treated differently. The shift in the energy levels for the solar cell devices treated differently is attributed to the formation of 3947



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Table 1. Summary of Valence-Band Maximum (VBM), Conduction-Band Minimum (CBM), Short-Circuit Current (JSC), OpenCircuit Voltage (VOC), Fill Factor (FF), and Power Conversion Efficiency (PCE) Values of the Hybrid Solar Cells, Based on a Total of 12 Devices for Bare and Pyridine-Treated TPs and 38 Devices for 1-Hexylamine-Treated TPs bare pyridine 1-hexylamine

VBM (eV)

CBM (eV)

JSC (mA cm−2)

VOC (V)

FF (%)

PCE (%)

PCEbest (%)

5.5 5.6 5.7

3.7 3.8 3.9

2.84 ± 0.35 3.51 ± 0.29 6.42 ± 0.68

0.78 ± 0.04 0.67 ± 0.05 0.62 ± 0.06

49.1 ± 3.1 47.4 ± 2.8 45.4 ± 4.0

1.09 ± 0.12 1.11 ± 0.13 1.80 ± 0.18

1.31 1.39 2.24

different interfacial dipoles between the exchanged ligands and nanocrystal surfaces, which either raise or lower the local vacuum level.38 We note that the results given in the present study are inconsistent with the results from previous reports.39 Figure 5a displays the incident photon-to-current efficiency (IPCE) of the differently treated hybrid solar cells at varying wavelengths. Over the entire wavelength range, the IPCEs were higher for the active layers containing TP networks treated with either 1-hexylamine or pyridine, compared with those for devices containing pristine TP networks. Between these two,

the hybrid layers containing TP networks treated with 1hexylamine ligands exhibited a pronounced enhancement compared to those treated with pyridine. These results are consistent with the higher JSC values obtained from the devices treated with 1-hexylamine ligands compared to those treated with pyridine ligands. Interestingly, the shape of the IPCE curves exhibited a clear discrepancy with the absorption spectra of the active layers, as displayed in Figure 5b. As an IPCE curve reveals the photocarrier generation efficiency (which is a product of the photon absorption efficiency and the exciton dissociation efficiency) and the carrier extraction efficiency of solar cells as a function of wavelength, the distinct difference in the shapes of the two curves indicates that exciton dissociation and/or carrier extraction limits the performance of CdSe TP/P3HT hybrid solar cells. Because the carrier extraction at the electrodes is independent of the illumination wavelength, the discrepancy should originate from ineffective exciton dissociation at the CdSe TP/P3HT interfaces. The origin of poor exciton dissociation in these hybrid layers can be deduced from the detailed features of the IPCE curves. It is clear that the highest IPCE is achieved at wavelengths below 440 nm, which should be mainly attributed to the absorption of CdSe TPs rather than P3HT. Also, a small but notable photon-to-current conversion was observed at wavelengths of 640−700 nm. This finite current generation should also originate from the absorption of CdSe TPs, as the energy in this wavelength range is smaller than the optical band gap of P3HT. Meanwhile, the IPCE in the range between 440 and 640 nm (wavelengths that are absorbed dominantly by P3HT domains) was not as pronounced as one would expect from the location of the absorbance maximum. Overall, these results imply that excitons generated in the P3HT domains are less efficiently dissociated to free electrons and holes than those populated in the CdSe TP domains. We confirmed, however, that the formation of the hybrid films indeed makes a meaningful contribution to the enhancement of exciton dissociation, although not fully optimzed as of now. This was verifed through photoluminescence (PL) decay expermients. The exciton lifetime was reduced from ∼930 ps for neat P3HT films to ∼550 ps for the hybrid films (Figure S3, Supporting Information), a signature of exciton dissocation at the interfaces. 3.4. Temperature-Dependent CdSe TP/P3HT Hybrid Solar Cell Performance. To examine the influence of surface modification of CdSe TPs on hybrid solar cell performance more thoroughly and systematically, we carried out temperature (T)-dependent J−V measurements (200−300 K) under varying light power (Ilight, 3−100 mW cm−2) and analyzed device characteristics. Basically, the hybrid solar cells follow typical polymer/fullerene solar cell characteristics (Figure S5, Supporting Information); JSC and FF show positive temperature coefficients, whereas VOC displays a negative temperature coefficient.40,41 This implies that thermally assisted hopping

Figure 5. (a) Incident photon-to-electron efficiency (IPCE) curves: TPs without surface modification (black), treated with pyridine (blue), and treated with 1-hexylamine (red). (b) UV−visible absorption spectra of CdSe TP/P3HT active layers treated with 1-hexylamine (solid red), P3HT only (dash-dotted green), and CdSe TP network only (dashed black). 3948



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conduction42 in organic domain governs the overall photocurrent of the present hybrid solar cells. At the same time, the increase in VOC is in good agreement with an analytical equation for VOC based on the Shockley diode equation. Intercepts of the linear extrapolation of VOC values (T → 0 K) are very closed to ECT/q (where ECT is the energy of interfacial charge-transfer states and q is an elementary charge),43−45 estimated from UPS measurements (Figure S5a, Supporting Information). The light-power-dependent JSC reveals the nature of recombination processes within solar cells (JSC ∼ Ilightα, where α is a scaling exponent). A value of unity indicates that carrier recombination is determined by a monomolecular process, whereas an α value of 0.5 is associated with a bimolecular process.46,40 Deviation in the scaling exponent α from unity toward a smaller value, therefore, implies the presence of energetic traps that capture carriers, serving as recombination centers. At room temperature, we found that the scaling exponent α was ∼0.9 for a solar cell containing a TP network treated with 1-hexylamine and ∼0.8 for the rest of the device samples tested (Figure 6a). The variance in α for the devices with different surface ligands implies that trap states are altered by binding moieties in the surface ligands. As the temperature was lowered, the α values decreased monotonically, whereas the α value for TP devices containing 1-hexylamine ligands remained higher than those for other devices at given temperatures. These results imply that more frequent carrier trapping takes place at reduced temperatures but trapping from the 1-hexylaminetreated TP surface is less frequent than that from other devices at the given temperatures. As the active layer of the CdSe TP/ P3HT hybrid solar cells is a disordered system in which charge transport should be thermally assisted, more trapping and thereby more recombination of charge carriers are likely to occur at low temperatures. Overall, the enhanced JSC for the hybrid solar cells modified with 1-hexylamine compared to that of the other devices can then be attributed to the reduced bimolecular recombination due to efficient passivation of trap states in TP network by 1-hexylamine ligands. In fact, the effective surface passivation with 1-hexylamine ligands can be readily confirmed by comparing the shunt resistance of hybrid solar cells under dark (Figure 3). The shunt resistance for the device containing 1-hexylamine ligands (9884 kΩ cm2) turned out to be larger than those with pyridine ligands (3815 kΩ cm2) or bare the nanocrystal surface (611 kΩ cm2). We note that the FF values are rather smaller for the solar cells containing 1-hexylamine ligands when compared with other systems, although these ligands efficiently passivate the trap states. The origin of the slight decrease in FF needs to be investigated further. At this point, we consider contributions from additional factors beyond trap densities for determing the efficiency of carrier extraction or FF. For example, the analysis of FF as a function of Ilight (that is directly associated with photogenerated carrier concentration) shows that the attainment of a higher density of photogenerated carriers lowers the FF value of the devices (Figure S4a,b, Supporting Information). These results indicate that FF cannot be simply understood by trap densities for the hybrid solar cells but by considering other factors such as effective mobility and density of carriers, which can also influence the charge transport and extraction for these types of solar cells. To analyze the trapping nature of carriers and their influence on JSC, we employed the Arrhenius relation between JSC and T

Figure 6. (a) Temperature dependence of the scaling exponents and (b) light intensity dependence of activation energies of CdSe TP/ P3HT hybrid solar cells: without surface modification (black open triangles), modified with pyridine (blue open squares), and 1hexylamine (red open circles).

with different Ilight values, JSC(T,Ilight) = J0(Ilight) exp(−EA/kBT), as shown in Figure S6 (Supporting Information). Here, J0 is a pre-exponential factor determined by photogeneration, recombination, and transport of carriers; EA is the activation energy; and kB is the Boltzmann constant.40 According to the theories typically applied to organic solar cells, EA is associated with the depth of trap states relative to the transport levels of charge carriers. As summarized in Figure 6b, we found that the solar cells passivated with 1-hexylamine exhibited lower EA values than other devices. Meanwhile, the solar cells treated with pyridine showed EA values similar to (at low light intensities) or higher than (at high light intensities) those of the reference devices. The lowered EA and the reduced depth of trap states are perhaps due to the altered energy levels of surface Cd atoms in the presence of 1-hexylamine. That is, the strong σ-electron donation of 1-hexylamine modulates the nonbonding orbitals of surface Cd atoms that are located near the CBM of CdSe nanocrystals and reduces the depth of trap states.36 In contrast, the relatively weak σ-electron donation of pyridine [supported 3949



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by its lower basicity (pKb ∼8.7) compared with that of primary amines (pKb ∼3.3)] does not contribute to reducing the trap depth substantially. Instead, pyridine is thought to delocalize holes within the aromatic ring to generate hole traps37,47 and increase EA. Interestingly, the EA values consistently increased with increased light intensity for all of the solar cell devices tested in the present study. If one assumes that EA is determined solely by the depth of trap states, EA is expected to be independent of Ilight,40,48 as is often observed in organic or quantum-dot photovoltaic research. However, our results do not follow such behaviors. Therefore, an alternative explanation is necessary to describe the operation of TP/polymer hybrid solar cells. The origin of the such an intriguing observation is not yet clear. However, the unusual light-intensity-dependent EA values of our hybrid solar cells might arise from the physical nature of TP nanocrystals with long arms. In other words, the exciton dissociation in our solar cells containing tetrapod arms longer than the Bohr radius would be different from that in typical so-called excitonic solar cells including organic and quantum-dot solar cells. Unlike the Frenkel-type excitons in typical excitonic solar cells, which are dissociated only at the interfaces, the excitons in our system would more resemble Wannier-type excitons. Further detailed investigation is necessary to clarify the photophysical processes in hybrid systems involving TPs with various arm dimensions (i.e., length and diameter).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-2-828-7100. Fax: +822-812-5378. *E-mail: [email protected]. Phone: +82-2-880-9093. Fax: +822-877-6668. *E-mail: [email protected]. Phone: +82-2-873-7523. Fax: +82-2-873-7523. Author Contributions ‡

J.L. and D.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government Ministry of Science, ICT & Future Planning (MSIP): the National Creative Research Initiative Center for Intelligent Hybrids (No. 2010-0018290), the WCU Program of C2E2 (R31-10013), the Technology Development Program to Solve Climate Changes (No. NRF-2009-C1AAA001-20090093282), the Basic Science Research Program of the National Research Foundation (NRF) of Korea (2012011603), and the Brain Korea 21 Plus program. This work is also supported by the Human Resources Development Program (no. 20124010203170) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).

4. CONCLUSIONS We have demonstrated the modular fabrication for CdSe TP/ P3HT hybrid bulk heterojunction solar cells and investigated the influence of surface passivation of CdSe TPs on solar cell performance. The modular fabrication enabled the independent realization of inorganic CdSe TP networks devoid of insulating ligands, further modification of nanocrystal surfaces, and finally filling of semiconducting polymers into the TP networks in a sequential manner. As a result, the modular approach allowed a reliable and controllable framework for clarifying the relationship between the surface chemistry of nanocrystals and device characteristics to be established. We confirmed that the passivation of CdSe TPs with 1-hexylamine ligands showed the best short-circuit current and power conversion efficiency. The improvements in device characteristics associated with the hybrid solar cell containing TPs treated with 1-hexylamine ligands, when compared with that containing TPs treated with pyridine ligands, are attributed to the suppressed bimolecular recombination process within the active layer due to the reduced depth of trap states. We believe that the modular fabrication presented herein provides a versatile platform for fabricating solution-processable nanocrystal/conducting polymer hybrid solar cells that is easily applicable to different semiconductor nanocrystals and other novel conducting polymers.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of CdSe TPs, absorption spectra and SEM images of CdSe TP networks for each fabrication step, plots of temperature- and light-intensity-dependent JSC values of CdSe TP/P3HT hybrid solar cells. This material is available free of charge via the Internet at http://pubs.acs.org. 3950



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Modular Fabrication of Hybrid Bulk Heterojunction Solar Cells Based

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