Hybrid morphology dependence of CdTe:CdSe bulk-heterojunction solar cells
© Tan et al.; licensee Springer. 2014
Received: 5 August 2014
Accepted: 16 October 2014
Published: 29 October 2014
A nanocrystal thin-film solar cell operating on an exciton splitting pattern requires a highly efficient separation of electron-hole pairs and transportation of separated charges. A hybrid bulk-heterojunction (HBH) nanostructure providing a large contact area and interpenetrated charge channels is favorable to an inorganic nanocrystal solar cell with high performance. For this freshly appeared structure, here in this work, we have firstly explored the influence of hybrid morphology on the photovoltaic performance of CdTe:CdSe bulk-heterojunction solar cells with variation in CdSe nanoparticle morphology. Quantum dot (QD) or nanotetrapod (NT)-shaped CdSe nanocrystals have been employed together with CdTe NTs to construct different hybrid structures. The solar cells with the two different hybrid active layers show obvious difference in photovoltaic performance. The hybrid structure with densely packed and continuously interpenetrated two phases generates superior morphological and electrical properties for more efficient inorganic bulk-heterojunction solar cells, which could be readily realized in the NTs:QDs hybrid. This proved strategy is applicable and promising in designing other highly efficient inorganic hybrid solar cells.
Solar cells based on nanoparticles have attracted intense attention in view of their compatibility with the solution synthesis of materials, low-cost fabrication of devices, and large area flexibility. Compared with their counterparts of organic solar cells which also possess these potentials, nanocrystal thin-film solar cells offer easy tuning of light response in a broad range by tuning the quantum size effect of colloidal nanoparticles. Up to now, tremendous attention has been paid to photovoltaic nanomaterials which could be adopted in thin-film solar cells, such as PbS [1–4], CuInS2 [5–7], and CdTe [8, 9].
With regard to the presently researched photovoltaic device with Schottky contact or bilayer heterojunction structure, it was suggested that the photocurrent was generated from charge separation driven by the built-in electric field in the depletion region which is located at the semiconductor-metal contact [10, 11] or p-n interface [12, 13]. Photogenerated excitons must diffuse a long way to the p-n depletion region before their splitting, which takes a high risk of recombination considering a relatively large quantum dot (QD) thickness as well as a small depletion width. To resolve this problem, a hybrid bulk-heterojunction (HBH) nanostructure was adopted , as what is commonly used in organic thin-film solar cells that possess highly efficient exciton splitting and charge transfer properties. Another critical problem adopting this structure in inorganic solar cell is that continuous charge transportation should also be required at the same time. Thus, it might not be a good idea if both spherical-shaped p- and n-type nanocrystals are blended together, which makes it difficult to form network pathways for electrons and holes. Nanocrystals with a hyperbranched shape should overcome the trade-off between efficient exciton separation and charge transportation and collection. Thus, in our previous work, the HBH concept was successfully introduced to fabricate all-inorganic QD solar cells based on CdTe nanotetrapods (NTs) and CdSe QDs, which can form a HBH nanostructure with a type-II energy band alignment .
It may be noticed that the morphology of inorganic nanoparticles has a profound impact on the performance of organic/inorganic hybrid solar cells [16, 17]. Similar to all-inorganic solar cells that also employ a hybrid bulk-heterojunction structure, how the hybrid morphology affects their properties should be explored, which, however, has not been reported on this new device. Here in this work, a focused investigation is carried out on the hybrid structure dependence of photovoltaic performance through variation in CdSe nanoparticle morphology while keeping the CdTe NTs unchanged. QD- or NT-shaped CdSe was introduced as an electron acceptor and transporter in the CdTe:CdSe hybrid. The hybrid structure in a nanoscale of the thin film is found to be closely related with the morphology of CdSe nanoparticles. Correspondingly, the charge dynamics behavior at the interface shows obvious difference in the two hybrids, which further results in variation in the photovoltaic performance of the two solar cells.
Synthesis of CdTe and CdSe nanoparticles
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 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 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 the CdSe QDs, a similar recipe and procedure were used just by replacing Te with 1.0 mmol Se powder. CdSe NTs were synthesized according to the procedure in the literature . CdO (1 mmol), oleic acid (OA, 6 mmol), and 20 ml 1-octadecene (ODE) were pumped at 140°C under N2 flow for 30 min. After that, the temperature was firstly raised to about 240°C where the solution turned clear and then decreased to 190°C at which a TOP-Se-hexadecyltrimethylammonium bromide (CTAB) solution (containing 1 ml TOP, 0.5 mmol Se, 0.05 mmol CTAB, and 3 ml toluene) was injected quickly. The injection caused the temperature to drop to about 165°C where the reaction was allowed and persisted for an hour to grow the CdSe NTs. Then, the heating mantle was removed and the solution was cooled to room temperature, after which 10 ml acetone was injected to collect the red precipitation by centrifugation at 4,500 rpm. All the three nanoparticles were purified with chlorobenzene/ethanol solvent/antisolvent for at least six times. The final products were dissolved separately in chlorobenzene to form solutions with desired concentration.
Fabrication of hybrid solar cells
The hybrid bulk-heterojunction solar cell was fabricated as follows: firstly, patterned indium tin oxide (ITO)-coated glass substrates were cleaned sequentially with soap water, deionized water, acetone, and isopropanol under ultrasonication for 20 min. Substrates were then dried under N2 flow, after which a compact TiO2 layer was deposited on top by spin coating a titanium-acetylacetone precursor and then sintering at 450°C for 90 min. The active layer was produced by spin coating several layers of CdTe NTs:CdSe QDs or CdTe NTs:CdSe NTs hybrid. The w/w ratio of CdTe to CdSe was varied in the hybrid. Following each spin coating, the substrates were treated with solvent containing 3-mercaptopropionic acid (MPA)/methanol solution (10% by volume). 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. The solar cell fabrication was finished by thermally depositing 5 nm MoO3 and thereafter 150 nm Au on top.
The morphology of CdTe and CdSe nanoparticles was confirmed by transmission electron microscopy (TEM) on a Hitachi H-800 (Hitachi, Ltd., Tokyo, Japan) at an acceleration voltage of 80 kV. The HBH thin-film surface and cross-sectional morphology were measured by field-emission scanning electron microscopy (FESEM, JEOL 7006 F, JEOL Ltd., Tokyo, Japan). Atomic force microscopy (AFM) test was carried out on a Solver P47 SPM (NT-MDT Co., Eindhoven, The Netherlands) under semi-contact mode. Absorption measurements were carried out on a Varian Cary ultraviolet-visible-infrared spectrophotometer (5000 model, Varian Inc., Palo Alto, CA, USA). Electrochemical impedance spectra were recorded on a CHI 660E electrochemical workstation (Chenhua Instruments, Inc., Shanghai, China). The current-voltage (I-V) measurements on CdTe:CdSe HBH solar cells were performed on a Keithley 2400 source in forward bias mode (Keithley Instruments Inc., Cleveland, OH, USA) under air mass (AM) 1.5 (100 mW/cm2) illumination.
Results and discussion
In conclusion, we have researched the influence of hybrid morphology on the photovoltaic performance of inorganic bulk-heterojunction solar cells. CdSe QDs or NTs were adopted together with CdTe NTs to form the hybrids that behave differently in film topology as well as charge transfer and transport. Compared to the CdTe NTs:CdSe NTs hybrid, interpercolation of CdTe NTs and CdSe QDs enables a flat and densely packed hybrid film, which ensures a better electrical contact between the hybrid active layer and the anode buffer layer. Besides, an interpenetrated heterojunction of NTs:QDs with large interface area facilitates electron transfer from CdTe NTs to the closely surrounding CdSe QDs, and electron transport also benefits from the dense and ordered assembly of QDs in the network. The structural and electrical advantage of the NTs:QDs hybrid makes it superior to the NTs:NTs hybrid in optic-electric conversion. Our research provides a designing strategy that should be considered for highly efficient inorganic hybrid bulk-heterojunction solar cells.
This work is supported by the National Natural Science Foundation of China (Grant no. 61306019), the National Basic Research Program of China (Grant no. 2014CB643503), and the Postdoctoral Science Foundation of China (Grant no. 2013 M541972). This work is also supported by the Natural Science Foundation of Henan Provincial Education Department (Grant no. 13B430912), the Scientific Research Found of Henan Provincial Department of Science and Technology (Grant no. 132300413210), and the Open Project of Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences.
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