Modification of hybrid active bilayer for enhanced efficiency and stability in planar heterojunction colloidal quantum dot photovoltaics
© Heo et al.; licensee Springer. 2013
Received: 21 October 2013
Accepted: 14 November 2013
Published: 20 November 2013
Solution-processed planar heterojunction colloidal quantum dot photovoltaics with a hybrid active bilayer is demonstrated. A power conversion efficiency of 1.24% under simulated air mass 1.5 illumination conditions is reported. This was achieved through solid-state treatment with cetyltrimethylammonium bromide of PbS colloidal quantum dot solid films. That treatment was used to passivate Br atomic ligands as well as to engineer the interface within the hybrid active bilayer.
KeywordsHybrid active bilayer Colloidal PbS quantum dot photovoltaics Planar structure Ambient air stability
Much of the recent effort to develop photovoltaics (PV) has focused on third-generation PV. The third-generation PV is defined by cost and power conversion efficiency (PCE) greater than the Shockley-Queisser limit of 32% . It can be reached through device architecture innovations, multiple-carrier generation using impact ionization, and new materials. Colloidal quantum dots (CQDs) have been proposed as useful materials for third-generation PV because of their ability to generate multiple excitons. Also, by changing the physical dimensions of CQDs, band gaps can be tuned from the visible to the infrared region using low-cost solution-processed fabrication. CQD PV has been studied in various ways using the following: Schottky CQD solar cells , depleted heterojunction CQD solar cells , and CQD-sensitized solar cells . The highest PCE of CQD-based PV, 6%, has been achieved with depleted heterojunction CQD solar cells ; this PCE makes CQDs competitive with organic materials for the PV industry. CQD-based PV has lower cost per area and benefits from greater process flexibility compared with Si-based PV. However, some issues must still be overcome for PV applications. They are especially sensitive to humidity, light, and oxygen [6, 7]. This sensitivity is the main cause of inferior charge transport, demanding a new strategy to solve these issues. Concurrent use of CQDs and organic compounds in devices has been one approach; these materials have typically been blended together [8–10]. To date, though, the PCE of a bilayer-based PV device has been much lower than that of blend-based PV because of poor morphology at the bilayer interface. In one example of a bilayer approach, Spoerke et al. reported that bilayer-based PV made with CdS CQDs and poly(3-hexylthiophene) (P3HT) had a PCE of 0.11% under simulated air mass (AM) 1.5 conditions .
Here, we introduce a planar heterojunction (PHJ) device architecture that has a ‘hybrid active bilayer,’ i.e., PbS CQD solid films layered with a blend of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). This architecture offers broad absorption and efficient charge transport. Also, our study of the hybrid active bilayer clearly indicates its suitability as a new material for third-generation multijunction devices. Moreover, we have established an important dual role for solid-state treatment with cetyltrimethylammonium bromide (CTAB) used for atomic ligand passivation of PbS CQDs in a PHJ device. CTAB treatment serves to passivate the Br atomic ligands as well as engineer the interface within the hybrid active bilayer, leading to improved PCE and stability. We focused on the behavior of PbS CQDs to understand these phenomena.
Lead chloride (PbCl2, 98%), elemental sulfur, zinc acetate (Zn(Ac)2 · 2H2O), oleylamine (OLA, technical grade 70%), oleic acid (OA, technical grade 90%), 2-methoxyethanol, CTAB (99%), chlorobenzene (reagent, 99%), and toluene (anhydrous, 99.8%) were obtained from Sigma-Aldrich Corporation (St. Louis, MO, USA). Ethanol and methanol were purchased from Duksan Chemicals Co., Ltd. (Ansan-si, South Korea). P3HT and PCBM were purchased from Rieke Metals (Lincoln, NE, USA). All chemicals were used as received without further purification.
Nanocrystal synthesis and device fabrication
A slurry of excess PbCl2 in OLA (1:2 molar ratio) was prepared at 100°C under a flow of N2. The temperature was increased to 120°C for 30 min. At the same time, elemental sulfur was dissolved in OLA (0.1:0.2 molar ratio) at 80°C over 30 min. The sulfur-OLA solution was added to the PbCl2-OLA slurry, and the temperature was raised to the growth temperature of 100°C and held there for 30 min. The mixture was then removed and quenched by pouring into cold toluene. OA was added to the PbS CQD suspension (20:3 volume ratio) at room temperature to exchange the OLA ligands for OA. The suspension was ultrasonicated and then centrifuged to remove the excess PbCl2. Ethanol was added to the retained supernatant to precipitate the quantum dots. The suspension was centrifuged, the supernatant was discarded, the precipitate was redispersed in toluene, and ethanol was added. The PbS CQDs containing OA ligands were isolated by centrifugation. Treatment with a methanol solution of CTAB was used to exchange OA ligands for the Br- ones in the PbS CQD solid films using layer-by-layer spin coating. A three-step spin coating cycle was used: (1) 50 mg/mL of the PbS CQD solution was spin-coated, (2) 0.5 mL of the CTAB methanol solution was coated onto the PbS CQD solid films, and (3) the films were washed with methanol. Experiments were conducted at room temperature in air and without annealing during the ligand exchange process. This spin coating cycle was repeated seven times. OA-treated PbS CQD solid films, on the other hand, were made by simply spin coating PbS CQDs seven times, without using the other steps. Solution-processed ZnO thin films were spin-coated onto an indium tin oxide (ITO) substrate and annealed at 500°C for 4 h. The two types of PbS CQD solid films were then deposited. Chlorobenzene dispersions of P3HT and PCBM were spin-coated onto PbS CQD solid films in an argon-filled glove box and annealed at 120°C for 10 min. Layers of MoO3 (3 nm) and Au (100 nm) were deposited onto the active layer by thermal evaporation.
The PbS CQDs were characterized by high-resolution transmission electron microscopy (HRTEM; Titan, FEI Co., Hillsboro, OR, USA). Current density-voltage characteristics were measured using an electrochemical analyzer (IviumStat, Ivium Technologies, Eindhoven, The Netherlands). An AM 1.5 solar simulator (Sun 2000, ABET Technologies, Milford, CT, USA) at 100 mW/cm2 intensity was used for illumination measurements. Absorption spectra were measured with a spectrophotometer (Cary 5G, Varian Inc., Palo Alto, CA, USA). This instrument was equipped with two light sources, i.e., a deuterium arc lamp and a quartz tungsten halogen lamp. X-ray photoelectron spectroscopy (XPS) spectra were measured using a commercial spectrometer (K-alpha, Thermo VG, Thermo Fisher Scientific, Waltham, MA, USA).
Results and discussion
PHJ device performance
CTAB-treated cell (0 day)
CTAB-treated cell (3 days)
OA-treated cell (0 day)
OA-treated cell (3 days)
In conclusion, we have described an approach to improve VOC and stability in a PHJ device using a hybrid active bilayer. The interface of this bilayer was modified by solid-state treatment with CTAB. The optimal CTAB-treated cell had a PCE of 1.24% under AM 1.5 conditions and maintained almost the same value (1.06%) over 3 days. Optical absorption spectra and XPS confirmed that Br atomic ligand passivation helped to prevent oxidation, while OA-treated PbS CQD solid films rapidly oxidized in ambient air at room temperature. A dipole layer between the PbS CQD layers formed as a consequence of the solid-state treatment with CTAB. For these reasons, the CTAB-treated cell had almost double the VOC compared to the OA-treated cell. The possibility of using PbS CQDs as a multijunction with organic materials has been demonstrated in this study. We suggest that PbS CQDs be further explored as new materials for third-generation PV.
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