Effect of ZnCdTe-Alloyed Nanocrystals on Polymer–Fullerene Bulk Heterojunction Solar Cells
The photovoltaic properties of solar cell based on the blends of poly[2-methoxy-5-(2-ethylhexoxy-1,4-phenylenevinylene) (MEH-PPV), fullerene (C60), and ZnCdTe-alloyed nanocrystals were investigated. Comparing the spectral response of photocurrent of the MEH-PPV:C60(+ZnCdTe) nanocomposite device with that of the devices based on MEH-PPV:C60and pristine MEH-PPV, one can find that the nanocomposite device exhibits an enhanced photocurrent. In comparing the composite devices with different ZnCdTe:[MEH-PPV + C60] weight ratios of 10 wt% (D1–1), 20 wt% (D1–2), 40 wt% (D1–3), and 70 wt% (D1–4), it was found that the device D1–3exhibits the best performance. The power conversion efficiency (η) is improved doubly compared with that of the MEH-PPV:C60device.
KeywordsZnCdTe Alloy Nanocrystals Heterojuction Solar cell
In the past decades, polymer solar cells have attracted more and more scientific attention due to their advantages, such as low cost, light weight, easy fabrication, and the possibility to fabricate flexible devices, compared with inorganic solar cells [1, 2, 3, 4, 5]. However, the limited dissociation of photoexcitation at weak electric field and poor charge transport in the polymer affects their applications. Along with the discovery of photo-induced fast charge transfer between conjugated polymer and C60, the possibility of all plastic photovoltaic cells with conjugated polymers and C60 derivatives as basic materials resulted in a large short-circuit current and a high power conversion efficiencies [6, 7, 8, 9]. Although more effective dissociation of photogenerated excitons can occur on the interfaces of polymer and C60 in polymer–fullerene bulk heterojunction solar cells, electron mobility is still poor and imbalance between electron and hole results in carrier recombination during transmission in solar cells. A good photoconductive device requires not only efficient charge separation, but also efficient transport of charge carriers to the electrodes. The blends of conjugated polymer and inorganic nanocrystals have exhibited excellent photoconductive properties because the inorganic semiconductor nanocrystals can efficiently dissociate the photogenerated excitons and have high electron mobility . Moreover, the dependence of the band gap of nanocrystals on particle radius can make it absorb light with tunable wavelengths and the compensated light harvest of nanocrystal and conjugated polymer enlarged the absorption range [1, 11, 12]. In recent years, binary nanocrystals such as CuS, ZnO, TiO2, CdS, and CdSe have been investigated extensively for their technological applications in optoelectronics [10, 13, 14, 15, 16, 17], but few study have been reported for the application of ternary-alloyed nanocrystals synthesized in aqueous solutions in the solar cells.
The band gap of ternary-alloyed nanocrystals can be adjusted by controlling not only particle radius but also the composition of ternary-alloyed nanocrystals [18, 19, 20]. Most of devices based on polymer–fullerene blend configurations suffer from reduced Voc because of the large band offset between the electron donor and acceptor materials. In order to increase the Voc and efficiency in such devices, the donor–acceptor band offset must be optimized. This has been previously demonstrated in polymer–fullerene blend devices, where the electron affinity of the fullerene derivative was modified leading to a change in the Voc. The application of ZnCdTe-alloyed nanocrystals can provide continuous control of the band offset over a large range by controlling not only particle radius but also the composition and leads to increased Voc values in hybrid devices.
We prepared ZnCdTe nanocrystals in aqueous solutions according to the method that have been reported . Then, the phase-transfer method was used to transfer ZnCdTe nanocrystals into a xylene solution, which is combined with the blend of MEH-PPV + C60 in xylene to fabricate composite devices. The comparison of the spectral response of photocurrent of the MEH-PPV:C60 (+ZnCdTe) nanocomposite device with that of the pristine MEH-PPV and MEH-PPV:C60 devices was made. The results from voltage–current characteristics showed that the conversion efficiency of devices with the 40% weight ratios of ZnCdTe nanocrystals is doubled compared with the devices without blending ZnCdTe nanocrystals.
Synthesis of ZnCdTe-Alloyed Nanocrystals
ZnCdTe nanocrystals were synthesized according to the method we have reported . In brief, thioglycolic acid was added into the mixed precursor solutions of Cd(CH3COO)2 2H2O and Zn(CH3COO)2 2H2O, which was adjusted to pH = 9 by using the solution of 2 mol/L sodium hydroxide, followed by adding a freshly prepared oxygen-free NaHTe solution with vigorous stirring under nitrogen atmosphere. Then, the solution was heated to 105 °C for 3 h, and ZnCdTe-alloyed nanocrystals were formed. Finally, ZnCdTe-alloyed nanocrystals were precipitated by addition of isopropyl alcohol and then centrifuged when the solution was cooled to room temperature. The ZnCdTe powder was obtained after drying in N2 atmosphere.
Fabrication of the Nanocomposite Device
The ZnCdTe-alloyed nanocrystals were redissolved in distilled water. Cetyltrimethyl ammonium bromide (CTAB) was used to precipitate the alloyed nanocrystals as the surfactant. And then the precipitation was re-dispersed in xylene after being dried and combined with the xylene solution of MEH-PPV:C60with different nanocryshals concentrations. ITO-coated glass substrate was first cleaned for sample preparation by detergent and distilled water in an ultrasonic bath, respectively. Poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spin-coated on top of ITO-coated glass substrates as the polymer anode to smoothen the substrate surface, and this was followed by thermal treatment for several minutes.
The spectral response of photocurrent and current–voltage curves were recorded by a Keithley 2410 source measure unit both in the dark and under illumination. The incident light from a Xe lamp was passed through a monochromator in SPEX Fluorolog-3 spectrophotometer to select a wavelength at 500 nm with an intensity of 16.7 mW/cm2. The spectral responses of photocurrent were also obtained by illumination with the light from a Xe lamp, which was passed through a monochromator to select wavelengths between 400 and 700 nm. Transmission electron microscopy (TEM) images were performed on a HITACHI H-700 transmission electron microscope. High-resolution transmission electron microscopy (HR-TEM) study was carried out on a PHILIPS TECNAI F30 microscope at an acceleration voltage of 300 kV. X-ray photoelectron spectra (XPS) were recorded by a VG MKII X-ray photoelectron spectrometer with monochromatized Al Ka radiation as the excitation source. X-ray diffraction (XRD) pattern was measured by Philips APD-10 X-ray diffractometer with graphite monochromatized Cu Ka radiation (λ = 0.154178 nm).
Results and Discussion
HR-TEM image of ZnCdTe nanocrystals (Fig. 2b) shows regular lattice fringes with a spacing of 0.35 nm, which corresponds to the (111) plane of ZnCdTe.
Figure 2c shows the XPS spectra of ZnCdTe nanocrystals. The features at 405 and 412 eV for the ZnCdTe nanocrystals are known to stem from Cd 3d. The peaks at 573 and 583 eV are from Te 3d, and the peak of 1021 and 1045 eV also appear with the presence of Zn 2p states, respectively. This result can confirm the existence of cadmium, zinc, and tellurium species in the ZnCdTe nanocrystals.
Figure 2d shows the XRD pattern of the obtained alloyed nanocrystals. XRD patterns show obvious broadening phenomena, which indicate the small size of the alloyed nanocrystals . The crystal structures of ZnCdTe can be assigned as cubic structures, with the presence of characteristic (111), (220), and (311) peaks in the XRD patterns.
Figure 2e shows the absorption spectrum of ZnCdTe alloy nanocrystals. The absorption peak of ZnCdTe nanocrystals locates at 450 nm.
Photocurrent–voltage characteristics of composite devices with MEH-PPV:C60 (+ZnCdTe) as the active layer, along with the undoped MEH-PPV:C60 (D1–0) device in comparison of the composite devices with different ZnCdTe:[MEH-PPV + C60] weight ratios of 10 wt% (D1–1), 20 wt% (D1–2), 40 wt% (D1–3), and 70 wt% (D1–4)
In summary, we have presented the photovoltaic properties of the MEH-PPV:C60(+ZnCdTe nanocrystals) composite device. The experimental results show that the devices with blending ZnCdTe nanocrystals exhibited an enhanced photocurrent. We investigated the effect of different ZnCdTe:[MEH-PPV + C60] weight ratios on the performance of the devices. When the weight ratio is not higher than 40%, the short current densityJscand the open-circuit voltageVocincreased with the increasing weight ratio of ZnCdTe nanocrystals. The characteristics of the devices degraded when the concentration of ZnCdTe nanocrystals is too high up to 70%. The power conversion efficiency is doubled and the short current is close to triple by blending ZnCdTe nanocrystals with the concentration of 40%.
This work was supported by Trans-Century Training Program Foundation for the Talents of Natural Science by the State Education Commission, Key Project of Chinese Ministry of Education (no. 105041), National Natural Science & Foundation Committee of China (NSFC) (Project nos. 90401006 and 10434030), State key project of basic research (2003CB314707) and the Key Laboratory of Specially Functional Materials and Advanced Manufacturing Technology, South China University of Technology, Ministry of Education, China.