Nanotetrapods: quantum dot hybrid for bulk heterojunction solar cells
© Tan et al.; licensee Springer. 2013
Received: 2 September 2013
Accepted: 19 September 2013
Published: 19 October 2013
Hybrid thin film solar cell based on all-inorganic nanoparticles is a new member in the family of photovoltaic devices. In this work, a novel and performance-efficient inorganic hybrid nanostructure with continuous charge transportation and collection channels is demonstrated by introducing CdTe nanotetropods (NTs) and CdSe quantum dots (QDs). Hybrid morphology is characterized, demonstrating an interpenetration and compacted contact of NTs and QDs. Electrical measurements show enhanced charge transfer at the hybrid bulk heterojunction interface of NTs and QDs after ligand exchange which accordingly improves the performance of solar cells. Photovoltaic and light response tests exhibit a combined optic-electric contribution from both CdTe NTs and CdSe QDs through a formation of interpercolation in morphology as well as a type II energy level distribution. The NT and QD hybrid bulk heterojunction is applicable and promising in other highly efficient photovoltaic materials such as PbS QDs.
KeywordsHBH nanostructure CdTe NTs CdSe QDs
Hybrid bulk heterojunction (HBH) nanostructure is commonly adopted in organic thin film solar cells where excitons are generated first after photon absorption[1, 2]. In these photovoltaic devices, the HBH structure enables a highly efficient exciton splitting or charge transferring through an interpenetrated nanoscale heterojunction distributed in the whole active layer. If optimization treatment to phase separation is carried out or efficient photovoltaic materials are adopted, not only the exciton splitting and charge transferring but also charge collection will benefit from the formation of interpenetrated and continuous transportation networks for holes and electrons[3–5]. Being profited from the HBH structure, the efficiency of organic hybrid solar cells has been remarkably improved[2, 6, 7].
During the research of thin film photovoltaic devices, it was found that HBH structure is not only a patent for organic or organic/inorganic hybrid photovoltaics. Inorganic thin film solar cells based on nanocrystals or quantum dots (QDs) also found their next step to better performance by introducing the HBH nanostructure mentioned above. Recently, it was found that the performance of PbS quantum dot solar cells was remarkably enhanced under a hybrid structure composed of PbS quantum dots and Bi2S3 nanoparticles. The key factor bringing such an exciting enhancement was attributed to a prolonged charge lifetime which allowed efficient charge separation and transport based on the formation of a nanoscale HBH. Another similar structure was fabricated by infiltrating PbS quantum dots into a porous TiO2 layer to form a depleted bulk heterojunction which was found beneficial to exciton splitting. In these devices, an electron donor-acceptor (D-A) model was introduced to discuss the work mechanism of solar cells with a HBH structure. Keeping this in mind, we think that it is reasonable to form interpenetrated and continuous two phases for the highly efficient exciton splitting and charge transportation. For this consideration, a novel HBH nanostructured solar cell was obtained by introducing CdTe nanotetrapod (NT)/CdSe QD hybrids as the photoactive layer and CdTe NTs as the anode buffer layer. Ligand treatment to the bulk heterojunction film composed of NT/QD hybrids ensures an efficient charge transferring and thereafter transporting in interpenetrated pathways. Remarkable photovoltaic performance is obtained with this hybrid composition. The novel HBH structure is commonly applicable and beneficial to other quantum dot-based solar cells with flexible, low-cost, and solution-processable manufacturing process.
Synthesis of CdTe NTs and CdSe QDs
CdTe NTs and CdSe QDs were synthesized according to the procedure in the literature with some modifications. A Cd precursor solution (containing 1 mmol of CdO dissolved in 3 mL of oleic acid (OA) and 3 g tri-n-octylphosphine oxide (TOPO)) was heated to 140°C and kept at this temperature for 1 h under nitrogen protection. In another flask, a Te source solution was formed by dissolving 0.5 mmol of Te powder in 3 mL tri-n-octylphosphine TOP. The Cd stock solution was heated to 260°C, and then the Te solution was quickly injected. The reaction proceeded for 3 to 4 min at 260°C to produce CdTe nanocrystals with a tetrapod shape. As to CdSe QDs, similar recipe and procedure were used just by replacing Te with 1.0 mmol of Se powder. Both CdTe NTs and CdSe QDs were purified with chlorobenzene/ethanol solvent/antisolvent for at least four times. The final products were dissolved separately in chlorobenzene to form a 40-mg/mL solution.
Fabrication of solar cells with CdTe/CdSe hybrid bulk heterojunction
The hybrid bulk heterojunction solar cells with a structure of ITO/CdTe/CdTe: CdSe/ZnO/Al was fabricated as follows: firstly, all patterned conductive indium tin oxide (ITO)/glass substrate were ultrasonically cleaned by soap and water, deionized water, acetone, and isopropanol for 15 min, respectively, and then dried at 110°C for 1 h in air. The active layer was produced by spin coating a 30-nm CdTe NTs layer firstly and then seven layers of CdTe/CdSe hybrid. The weight-to-weight ratio of CdTe NTs to CdSe QDs was controlled in the range of 6:1 to 1:2. Following each spin coating, the substrates were heat-treated at 150°C in air (sample A) or solvent treatment using 3-mercaptopropionic acid (MPA)/methanol solution (10% by volume) (sample B). For solvent treatment, two drops of MPA/methanol solution were dispensed onto the CdTe layer or CdTe/CdSe hybrid layer, and the substrate was spun at 2,500 rpm for 15 s after a 6-s wait. Three rinse steps with methanol were applied under the same operation. Afterward, the substrates were annealed at 150°C for 10 min. Finally, a ZnO buffer layer of about 20 nm is formed on the surface of the substrate by spin coating a ZnO quantum dot solution in isopropanol, as was usually done. The solar cell fabrication was finished by thermally depositing a 100-nm aluminum cathode on top.
The shape of CdTe NTs and CdSe QDs was characterized by transmission electron microscopy (TEM) on a Hitachi H-800 (Hitachi High-Tech, Tokyo, Japan) at an acceleration voltage of 80 kV. HBH thin film surface and cross-sectional morphology were measured by field emission scanning electron microscopy (JEOL 7006 F, JEOL Ltd., Tokyo, Japan). Atomic force microscopy (AFM) test was carried out on a Solver P47 SPM (NT-MDT, Moscow, Russia) under semi-contact mode. The crystal structure of hybrid was researched by Raman scattering on a Renishaw RW1000 (Renishaw, Wotton-under-Edge, UK) confocal microscope with a 514-nm line of Ar+ iron laser as exciting light. Absorption measurements were carried out on Varian Cary-5000 model (Agilent Technologies, Inc., Santa Clara, CA, USA) UV-visible infrared spectrophotometer. Electrochemical impedance spectra were recorded on a CHI 660E (CH Instruments, Austin, TX, USA) electrochemical workstation. The current–voltage (I-V) measurements on CdTe/CdSe HBH solar cells are performed on a Keithley 2400 source (Keithley Instruments Inc., Cleveland, OH, USA) in forward bias mode under AM 1.5 (100 mW/cm2) illumination. External quantum efficiency (EQE) measurements were carried out on Crowntech test station (Crowntech Inc., Macungie, PA, USA) with a Keithley 2000 multimeter and a standard silicon PV base cell.
Results and discussion
In this work, the CdSe QDs are supposed to fill in the gaps among the branched CdTe NTs. Also, it has suitable conduction and valence band distribution, enabling an effective transfer of holes as well as blocking of electrons. Meanwhile, the type 2 heterojunction at the CdTe/CdSe interface ensures the origin of photovoltaic effect when they are assembled together (Figure 1a(right)). Cross section of the device is shown in Figure 1b from which it is difficult to exactly identify the bottom CdTe NT layer because it has the same crystal phase with that of the above hybrids. The optimized hybrid film thickness is measured to be about 200 nm as seen from the scanning electron microscopy (SEM) image.
As can be seen, the OA-capped hybrid nanoparticle thin films exhibit a homogeneous topology, while clusters and agglomerates can be found on the hybrid film after MPA treatment. The surface height along the line part of the AFM image was shown at the corresponding bottom. Mainly, tiny and uniform nanoclusters are observed on the OA-capped hybrid surface, while larger sized nanostructures are demonstrated after MPA treatment, which means that aggregation of nanoparticles appears due to the removal of the long OA surfactant. Thus, ligand exchange correspondingly promotes a closer contact between the two phases from which charge transfer and transportation is benefited.
Raman spectrum is a useful tool as it provides short-ranged microstructure information that is further helpful to understand the electric behavior in the EIS result. As shown in Figure 5b, compared with the OA-capped HBH film, both the first and the second longitudinal optical phonon mode of CdTe can be observed around 165 cm-1 (1LO1) and 330 cm-1 (2LO1) after the NT/QD HBH film was treated with MPA (sample B). The same phenomenon happens with CdSe. The enhancement in Raman peak intensity was suggested to be correlated with molecule adsorption (with large polarity such as this) that induced the passivation of surface states[20–22]; herein, there was an adsorption of MPA on the surface of CdTe NTs and CdSe QDs through Cd-S bond which reduces the electron trapping state caused by the Cd dangling bond. This correspondingly results in a decreased charge trapping and recombination rate, as exhibited from the EIS analysis in Figure 5a. Interestingly, a slight blueshift of the 1LO1 mode from CdTe and 1LO2 mode from CdSe can be observed after MPA treatment, which, in accordance with TEM characterization in Figure 3, indicates a more densely packed microstructure in the hybrid film.
In conclusion, an efficient solar cell based on an all-inorganic HBH nanostructure composed of NTs and QDs is introduced. Both the CdTe NTs and CdSe QDs make a contribution to photovoltaic performance through their respective photoelectric response region. The interpercolated and continuous networks of CdTe NTs (as electron donor and hole acceptor) and CdSe QDs (as electron acceptor and hole donor) are a critical access in achieving a highly efficient charge transfer and transport. Ligand exchange process enables compacted contact between NTs and QDs which boosts the infiltration of CdSe QDs into the branched CdTe NTs and therefore enhances charge transfer at the heterojunction interfaces. This novel hybrid nanostructure will allow further improvement in photovoltaic performance of the efficient PbS QD solar cells, which is more interesting and exciting.
This work is supported by the Scientific Research Foundation of Henan Provincial Department of Science and Technology (grant no. 132300413210), China Postdoctoral Science Foundation (grant no. 2013 M541973), and the National Basic Research Program of China (grant no. 61306019). This work is also supported by the National Basic Research Program of China (grant no. 2012CB934200) and National Natural Science Foundation of China (grant nos. 50990064 and 61076009).
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