Investigation of bulk hybrid heterojunction solar cells based on Cu(In,Ga)Se2 nanocrystals
© Yen et al.; licensee Springer. 2013
Received: 15 June 2013
Accepted: 10 July 2013
Published: 19 July 2013
This work presents the systematic studies of bulk hybrid heterojunction solar cells based on Cu(In, Ga)Se2 (CIGS) nanocrystals (NCs) embedded in poly(3-hexylthiophene) matrix. The CIGS NCs of approximately 17 nm in diameter were homogeneously blended with P3HT layer to form an active layer of a photovoltaic device. The blend ratios of CIGS NCs to P3HT, solvent effects on thin film morphologies, interface between P3HT/CIGS NCs and post-production annealing of devices were investigated, and the best performance of photovoltaic devices was measured under AM 1.5 simulated solar illumination (100 mW/cm2).
KeywordsHybrid solar cell Cu(In,Ga)Se2 Poly(3-hexylthiophene) Nanocrystal
Photovoltaic (PV) devices, converting photon into electricity as an elegant and clean renewable energy, have attracted tremendous attentions on research and developments. Among emerging PV technologies, organic photovoltaic devices (OPV) composed of polymer matrices can be considered as promising third-generation solar cell due to its exceptional mechanical flexibility for versatile applications [1, 2]. Moreover, the solution processes of OPV enables versatile and simple processes, including dip coating, ink jet printing, screen printing, and roll-to-roll method [3, 4]. Nonetheless, OPVs suffer from the low carrier mobility issues, which hinder the performance far behind to conventional inorganic solar cells. In order to promote carrier mobility in OPV systems, inorganic semiconductor materials was introduced into OPV as electron acceptor materials, so called hybrid solar cells . Hybrid solar cells utilize an advantage of intrinsically high carrier mobility from inorganic materials in organic matrices. By controlling of inorganic material into nanoscale, which can disclose the unique properties, such as enhanced absorption coefficient owing to quantum confinement , relatively high electron mobility, high surface area, and good thermal stability, providing alternative path for development of OPVs .
Typically, OPV composes of electron acceptors (e.g., [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)) and hole transport conjugated polymers (e.g., poly(3-hexylthiophene (P3HT))  as an active layer in the OPV. Owing to relative low carrier mobility and a similar band offset of most inorganic materials to PCBM. PCBM is usually replaced by inorganic nanomaterials as electron acceptor in most hybrid solar cells. Up to date, various inorganic semiconductors have been studied, including ZnO , TiO2, CdSe , CdS , PbSe , and PbS . Among them, metal sulfides or selenides (i.e., Cd and Pb) were extensively investigated. Examples have been reported by as Alivisatos et al., indicating P3HT/CdSe nanorod hybrid solar cells achieve a remarkable power-conversion efficiency (PCE) of 1.7% . Xu et al. have demonstrated a solar cell based on P3HT/PbSe NCs hybrids with a PCE of 0.13% . However, Cd and Pb are considered as hazard elements to environments, which limit the hybrid solar cell systems as the commercialized product.
In this study, we report a hybrid solar cell based on CIGS NCs with a conjugated polymer P3HT as matrix. Chalcopyrite series material CIGS is well known as a direct bandgap material with an intrinsic high optical absorbing coefficient. Such superior characteristic and tunable optical energy gap engineering that matches well with the solar spectrum makes CIGS a promising PV material in the near future . The blend ratios of CIGS NCs to P3HT, solvent effects on thin film morphologies, interface between P3HT/CIGS NCs and post-annealing of devices were investigated and the best performance of photovoltaic devices was measured. The approach combines non-toxic advantage of CIGS, benefitting a development in hybrid solar cells.
Synthesis of CIGS NCs
CIGS nanocrystals with stoichiometric of CuIn0.5Ga0.5Se2 was synthesized by chemical method. Oleylamine with 12 mL, 0.5 mmol of CuCl (0.0495 g), 0.25 mmol of InCl3 (0.0553 g), 0.25 mmol of GaCl3 (0.0440 g), and 1.0 mmol of elemental Se powder (0.0789 g) were mixed into a tri-neck beaker attached to the heating mantle. The beaker was purged by argon bubbling of oxygen and water at 130°C for 1 h. After purge, temperature was allowed to slowly increase to 265°C with slope of 2.3°C/min and held at 265°C for 1.5 h under vigorous stirring. The beaker was then cooled to room temperature by immersion into a cold water bath. The nanocrystals were extracted by a centrifugation process at 8,000 revolutions per minute (rpm) for 10 min by addition of 15 mL ethanol and 10 mL hexane. After two cycles of the centrifugation step, nanocrystals were precipitated and collected, while the supernatant was discarded. The extracted nanocrystals were re-dispersed in toluene or hexane for further device fabrication and characterization.
Fabrication of photovoltaic device
Photovoltaic devices with a typical sandwich structure were fabricated, where the active layers are constructed using the CIGS NCs in combination with P3HT. Briefly, a 40-nm thick layer of filtered poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS) was first spin-cast onto the indium tin oxide substrate with 400 rpm for 5 s and follow by 2,500 rpm for 40 s. Next, samples were dried at 120°C for 30 min under vacuum and transferred into a glove box filled by nitrogen gas. Then, an approximately 130-nm-thick P3HT/CIGS NC photoactive layer was deposited above the PEDOT/PSS layer by spin coating. The concentration of P3HT/CIGS NCs is 30 mg/mL using 1,2-dichlorobenzene as the solvent. The dried thin films were annealed at 120°C for 30 min. Finally, the Al electrodes (approximately 150 nm) were deposited by thermal evaporation, and through a shadow mask, resulted in a complete device with an active area of approximately 0.04 cm2.
Measurements and characterizations
Powder X-ray diffraction (XRD) pattern was recorded on a Shimadzu 6000 X-ray diffractometer (Kyoto, Japan) with monochromated Cu-Kα irradiation (λ is approximately 0.154 nm). Morphology, microstructures, and atomic compositions of CIGS NPs were performed by field-emission scanning electron microscopy (JSE-6500F, JEOL, Akishima-shi, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL-3000F 300 kV) equipped with electron dispersive spectrometer. UV–vis absorption spectra were acquired using an optical spectrometer (Hitachi, U-4100, Minato-ku, Japan). Fourier transform infrared (FTIR) spectra were obtained by a Perkin Elmer Spectrum RXI spectrometer (Waltham, MA, USA). Photoluminescence (PL) spectra were measured under ambient conditions on a F-7000 spectrofluorometer (Hitachi) with an excitation at 400 nm. Current–voltage behaviors (Keithley 2410 source meter, Cleveland, OH, USA) were studied by adopting a solar simulator (San-Ei Electric, Osaka, Japan) with the AM 1.5 filter under an irradiation intensity of 100 W/cm2.
Results and discussion
Characterization of as synthesized CIGS NCs
Optical and compositional studies of CIGS NCs
Device measurement of P3HT/CIGS NC hybrid solar cells under AM 1.5 at different mixing ratios
CIGS NCs (wt.%)
Solvent effects on CIGS NCs/P3HT hybrid solar cells
Effects of interface treatment between CIGS NCs and P3HT
Effects of thermal treatments on CIGS NCs/P3HT hybrid solar cell
This work investigated and discussed on the bulk heterojunction of solar cell based on the P3HT/CIGS NC hybrid active layer. Approaches such as blend ratios of CIGS NCs, solvent effects on the morphologies, interface between P3HT/CIGS NCs, and device thermal treatments have been investigated to enhance the power-conversion efficiency of the hybrid solar cells in detail. The best performance of devices was fabricated from a blend ratio of 1 to 3 by weight in P3HT to CIGS NCs, dichlorobenzene as solvent, pyridine as surfactant, yielding the highest PCE of approximately 0.017%.
Organic photovoltaic devices
[6,6]-Phenyl-C61 butyric acid methyl ester
Powder X-ray diffraction
Fourier transform infrared spectroscopy
High-resolution transmission electron microscopy
Full width at half maximum
Opened circuit voltage
This research was supported by the National Science Council through Grant no. 101-2622-E-007-011-CC2, 101-2622-E-492-001-CC2, NSC 101-2218-E-007- 009-MY3, NSC 100-2628-E-007-029-MY2, NSC 101-2623-E-007-013-IT, and the National Tsing Hua University through Grant no. 102N2022E1, 102N2051E1, and 102N2061E1. Y.L. Chueh greatly appreciates the use of facility at CNMM, National Tsing Hua University through Grant no. 102N2744E1.
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