Novel Hybrid Ligands for Passivating PbS Colloidal Quantum Dots to Enhance the Performance of Solar Cells
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We developed novel hybrid ligands to passivate PbS colloidal quantum dots (CQDs), and two kinds of solar cells based on as-synthesized CQDs were fabricated to verify the passivation effects of the ligands. It was found that the ligands strongly affected the optical and electrical properties of CQDs, and the performances of solar cells were enhanced strongly. The optimized hybrid ligands, oleic amine/octyl-phosphine acid/CdCl2 improved power conversion efficiency (PCE) to much higher of 3.72 % for Schottky diode cell and 5.04 % for p–n junction cell. These results may be beneficial to design passivation strategy for low-cost and high-performance CQDs solar cells.
KeywordsPbS Colloidal quantum dot Solar cells Ligands
Colloidal quantum dots (CQDs) solar cells as potential next-generation solar energy-harvesting devices have received considerable attention in the past several years [1, 2, 3, 4] owing to their low manufacturing cost (coated on substrates using drop-casting, spin-coating or ink-jet printing). Among all kinds of CQDs (such as CdTe [5, 6, 7, 8, 9], CdSe , PbS [11, 12, 13, 14], PbSe [15, 16, 17], CuInS2 , etc.) solar cells, PbS CQDs solar cells have lots of distinctive merits. For example, PbS CQDs solar cells can be prepared in ambient condition under low temperatures below 200 °C, and the electronic bandgap of PbS CQDs can be easily tuned by changing size due to its large exciton Bohr radius (~18 nm for PbS ), which enables the fabrication of multi-junction solar cells from single material.
PbS CQDs solar cells with negligible power conversion efficiency (PCE) were first reported in 2005 . There are two key factors which affect the performance of PbS CQDs solar cells. The important one is protection technique of as-prepared PbS CQDs. As the size decreases, the surface state of PbS will go up rapidly due to oxidation if there is no suitable ligand to protect. Devices based on no-protection PbS CQDs exclusively show large internal series resistance and low carrier mobility, resulting in low device performance. Meanwhile, since the long-chain carboxyl acid is usually attached to the surface of PbS, it is difficult to gain satisfactory device performance unless the carboxyl ligands are well removed during device fabrication processes. With development of nanotechnology, many efforts had been made to improve the PCE of PbS CQDs solar cells, including packing and passivation of CQDs [21, 22], adoption of new exchanging ligands , and design of new device structure . In order to improve surface passivation and therefore eliminate valence-band-associated trap states in CQDs thin film, Sargent et al. [25, 26, 27] first introduced a mixture of CdCl2 and tetradecyl phosphonic acid (TDPA) during synthesis process of PbS CQDs. They obtained solar cell device with ~8.0 % efficiency based on these hybrid passivated CQDs. On the other hand, new exchanging ligands such as mercaptopropionic acid (MPA) or di-thiol were introduced during device fabrication process to remove the long-chain carboxyl acid ligands on the surface of PbS CQDs. The thin film prepared by this method was compact and showed good carrier mobility.
In this paper, we developed a novel simple process of passivating PbS CQDs to improve the film quality and therefore to enhance the solar cells’ performance. Different hybrid ligands were introduced during PbS nucleation and QDs growth processes. Two kinds of solar cells based on PbS CQDs were fabricated to verify the effects of ligands passivation.
Oleic acid (OA, 90 %), lead oxide (PbO, 99.9 %), 1-octadecene (ODE), CdCl2, and oleic amine (OLA) were purchased from Alfa Aesar. Mercaptopropionic acid Hexamethyldisilathiane (TMS), octyl-phosphine acid (OPA), tetradecyl phosphonic acid (TDPA), and octodecyl-phosphine acid (ODPA) were purchased from Aladdin. All chemicals were used directly without any further purification.
2.2 Synthesis of PbS CQDs
PbS CQDs with different passivating ligands were defined as (A) without ligand, (B) CdCl2 + OLA + OPA, (C) CdCl2 + OLA + TDPA, and (D) CdCl2 + OLA + ODPA.
PbS CQDs with different ligands were synthesized by a solvent thermal method reported previously . Typically, 0.45 g PbO, 1.5 mL OA, and 16.5 mL ODE were loaded into a three neck flask at 120 °C and degassed for 6 h to remove any moisture and low boiling point organic solvents. Then, 0.20 mL TMS mixed with 5 mL ODE was quickly injected into the reaction system. The hotplate was removed away immediately, and the reaction was cooled down to room temperature. When the temperature was dropped down to 90 °C, different hybrid ligands A, B, C, or D were injected into the reaction system, and the reaction was cool down to room temperature. Then, 50 mL acetone was injected into the final reaction solution to centrifuge at 10,000 rpm for 5 min. Black powder product was collected and re-dissolved into a mixture of 2 mL toluene and 10 mL of ethanol and acetone (volume ratio 1:1) to centrifuge again to remove impurity. This procedure was repeated more than three times to obtain pure PbS CQDs.
The above-mentioned hybrid ligands B, C, and D were prepared by mixing 0.72 mmol CdCl2, 2 mL OLA, and respective 0.048 mmol OPA, 0.048 mmol TDPA, and 0.048 mmol ODPA. The mixtures were degassed and refluxed at 90 °C for 5 h until transparent solutions were formed.
2.3 Device Fabrication
The fabrication process of solar cells with p–n junction of FTO/ZnO/TiO2/PbS/Au was almost similar except that FTO was used as the substrate, and ZnO/TiO2 films were inserted between FTO and PbS by thermal decomposition of spin-casting Zn/Ti precursor. The detailed process was described in the literatures [21, 28]. The Au electrode was deposited on the PbS active layer via thermal evaporation.
Space charge limited current (SCLC) measurement was carried out to investigate hole mobility of passivated PbS CQDs thin film. The measurement process was similar to that of PbS CQDs solar cells with Schottky diode structure except that 10 nm MoOx and 80 nm Al were deposited on the substrate via evaporation. In this case, the thickness of PbS CQDs thin film was about 200 nm.
The morphology, structure, and surface state of PbS CQDs were characterized by transmission electron microscope (TEM, JEOL 2010), powder X-ray diffraction (XRD, Bruker D8), and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250), respectively. The optical properties of the samples were recorded by ultraviolet (UV) spectrophotometer (Shimadzu UV-3600). The current–voltage (J–V) curves of solar cells were measured by a source-measurement unit under AM 1.5G spectrum (Keithley 2400) with a solar simulator (Oriel model 91192). The SCLC measurement was carried out on a semiconductor parameter analyzer (Agilent 4155C).
3 Results and Discussion
Photovoltaic parameters of PbS CQDs Schottky diode cell with different hybrid ligands
Jsc (mA cm−2)
Atom percentage of element content of PbS CQDs solar cells with different hybrid ligands from XPS results
C 1s C–C
C 1s C=O
Cl 2p metal chloride
Pb 4f (4d)
S 2p3 (S 2p1) metal sulfide
S 2p3 (S 2p1) thiol
O 1s carbonates/sulfates
O 1s metal oxide
S 2p3 (S 2p1) sulfate
Photovoltaic parameters of PbS CQDs p–n junction cell with different hybrid ligands
Jsc (mA cm−2)
In summary, novel hybrid ligands were developed to passivate PbS CQDs, and the performance of as-prepared PbS CQDs solar cells with not only Schottky diode structure but also p–n junction structure was improved. The reason is that the hybrid ligands passivate surface defects well and prevent oxidation of PbS CQDs during the device fabrication process. In addition, the shorter the chain length of phosphine in hybrid ligands, the higher hole mobility and PCE were demonstrated in cells. Especially, the PbS CQDs cell with ligand B in Schottky diode structure has the highest PCE value compared with reported cells with other ligands. Our results provide an effective way to improve the performance of PbS CQDs solar cells.
We thank the financial support of the National Natural Science Foundation of China (No. 91333206, 61274062, and 11204106), National Science Foundation for Distinguished Young Scholars of China (Grant No. 51225301), and Guangdong Province Natural Science Fund (No. 2014A030313257).
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