The effect of organic salt, tetrabutylammonium hexafluorophosphate doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF6 exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm2. The short circuit current density and the open circuit voltage of the doped device increased from 0.54 mA/cm2 to 6.41 mA/cm2 and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.
Organic solar cells have attracted considerable interest due to their great potential for the production of flexible and large-area solar cells at relatively low costs and easy-processing fabrication properties (Brabec et al., 2001b; Yu et al., 1995). Most commonly organic solar cells are fabricated on transparent conductive oxide (TCO) covered glass substrates using a hole injection layer between TCO and the bulk heterojunction organic layer and a low work function metallic electron contact.
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For organic solar cell with ohmic contacts, the maximum open circuit voltage (Voc) is governed by the energy difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor material and the highest occupied molecular orbital (HOMO) of the donor materials (Brabec et al., 2001a; Scharber et al., 2006). As for the non-ohmic contacts, the Voc depends on the difference of work function of electrodes according to metal-insulator-metal (MIM) model (Mihailetchi et al., 2003). The increase in Voc is attributed to enhancement in built-in electric field generated by the difference of work function of electrodes (Kinoshita et al., 2008). Besides, the increase of built-in electric field also facilitates the transportation of free photogenerated carries towards their corresponding electrodes, which in turn enhances the short circuit current density (Jsc) (Song et al., 2007). One of the approaches to modify the effective work function of electrodes is by insertion of a buffer layer at the interface between electrode layer (cathode and anode) and active layer. A hole injection layer such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is commonly inserted between anode and active layer (Brabec, 2004; Zhang et al., 2004). Besides, devices with additional LiF layer at the Al cathode/active layer interface are preferred (Ahlswede et al. 2007; Brabec et al. 2002). However, these methods may increase the production cost due to additional step in manufacturing process. As a result, it is worth to investigate some other approaches which are able to alter the effective work function of electrodes without involving multilayer structure. The doping of organic salt has previously been used to enhance the performance of single layer organic light emitting diodes by lowering the charge injection barriers at both interfaces simultaneously caused by accumulation of ionic species at the interface (Itoh et al., 2001; Lee et al., 2009; Sakuratani et al. 2001; Yap et al., 2008). However, the effect of organic salt doping in organic solar cells has yet to be discovered.
Bulk heterojunction organic solar cells based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) and (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) blend film has been extensively investigated over the past few years (Alem et al., 2004; Chang et al., 2006; Qiao et al., 2008). The present work reports on the effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF6) doping on the performance of a single layer organic solar cell with structure of ITO/MEHPPV:PCBM/Al. In contrast to the undoped device, the TBAPF6-doped device that has been given an electric field treatment showed improved solar cell performance under illumination with a halogen projector lamp at 100 mW/cm2. The Jsc of the electric field-treated doped device increased by almost ten times (from 0.54 mA/cm2 to 6.41 mA/cm2), whereas the Voc also increased by almost 100% (from 0.24 V to 0.50 V), as compared to those of the undoped device.
2. Materials and Methods
The donor polymer, MEHPPV with an average molecular weight of 40,000-70,000 g/mole and the organic salt, TBAPF6 were purchased from Aldrich Chemical Company, whereas the acceptor, PCBM was purchased from Luminescence Technology Corporation. All materials were used as received without further purification.
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The ITO-coated glass substrates were etched and patterned to serve as anode. The substrates were cleaned with acetone and 2-propanol in an ultrasonic bath each for 15 min. MEHPPV:PCBM in weight ratio of 1:2 were dissolved in chloroform solution with total solid concentration of 15 mg/mL. For the TBAPF6-doped solution, 20 wt% of TBAPF6 (weight percentage with respect to MEHPPV) was added to the solution with the same concentration of MEHPPV:PCBM. The solutions were then spin-coated onto the ITO surface with a typical spinning speed and time at 1000 rpm and 40 s, respectively. Lastly, 150-nm aluminium (Al) was deposited as cathode by using electron gun evaporation technique. The active area was 0.07 cm2. For absorption and photoluminescence studies, the organic layers were spin coated onto quartz substrates with the same solution and procedure used for active layers of the devices.
The thicknesses of the blend films were determined by using Dektak 150 surface profiler. The absorption and photoluminescence (PL) properties of the relevant blend films were investigated by using Perkin Elmer LAMBDA 900 UV-VIS spectrophotometer and Perkin Elmer LS55 luminescence spectrometer, respectively. The organic solar cells were characterized by current-voltage measurements under illumination with a halogen projector lamp at 100 mW/cm2 using Keithley 237 source measurement unit.
3. Results and discussion
Fig. 1 shows the absorption spectra of MEHPPV, MEHPPV:PCBM (1:2) and MEHPPV:PCBM (1:2):20 wt% TBAPF6 films. The MEHPPV film has a strong absorption in the region 400-600 nm with a peak at 505 nm. Both MEHPPV:PCBM (1:2) and MEHPPV:PCBM (1:2):20 wt% TBAPF6 films exhibited a similar broad absorption spectrum ranging from 300 nm to 600 nm. This observation indicated that the organic salt did not change the absorption property of the blend film. However, the absorption of MEHPPV:PCBM (1:2):20 wt% TBAPF6 blend film was slightly higher than that of MEHPPV:PCBM (1:2), possibly due to small increase of the thickness when additional organic salt was added to the solution. The thickness of the MEHPPV:PCBM (1:2) and MEHPPV:PCBM (1:2):20 wt% TBAPF6 was 150 ± 1 nm and 157 ± 4 nm, respectively.
The PL quenching in blend of donor-acceptor is a useful indication for the efficient charge transfer between the materials (Suresh et al., 2008). Fig. 2 shows the PL spectra of the MEHPPV, MEHPPV:PCBM (1:2) and MEHPPV:PCBM (1:2):20 wt% TBAPF6 films under 500 nm monochromatic excitation which is corresponding to the absorption wavelength of MEHPPV. For the MEH-PPV, there was an emission peak at the wavelength of 595 nm. The PL emission was significantly quenched in the MEHPPV:PCBM (1:2) and MEHPPV:PCBM (1:2):20 wt% TBAPF6. The strong PL quenching observed for both films is an evidence of the efficient photo-induced charge transfer at the interface between MEHPPV and PCBM with and without the presence of organic salt.
Fig. 3 shows the current density-voltage (J-V) characteristics of the undoped device and the device with TBAPF6 under illumination of a halogen projector lamp at 100 mW/cm2. The doped and the undoped devices showed a short circuit current density (Jsc) of 0.70 mA/cm2 and 0.62 mA/cm2, an open circuit voltage (Voc) of 0.28 V and 0.26 V, respectively. The device doped with TBAPF6 exhibited a small improvement in the photovoltaic performance compared with the undoped device.
The doped and the undoped devices were treated with a constant voltage at 10 V for 30 s where the ITO was positively biased with respect to Al. Fig. 4 shows the current density-voltage (J-V) properties of the undoped device and the device with TBAPF6 under illumination after undergoing electric field treatment. Interestingly, the device doped with TBAPF6 demonstrated a significant increment in the Jsc and Voc as compared with the undoped device. Under illumination of a halogen projector lamp at 100 mW/cm2, the undoped device showed a Jsc of 0.54 mA/cm2, Voc of 0.24 V, and a fill factor (FF) of 16%. With the doping of TBAPF6, the Jsc increased almost ten times to 6.41 mA/cm2. Besides, the Voc also improved significantly from 0.24 V to 0.50 V. However, the power conversion efficiency (5.77 x 10-4 %) and the FF (18%) of the doped device seem to be very low. This might possibly arise from the absorbance mismatch with the halogen projector lamp spectrum (Yahaya et al., 2009) and insufficiency of optimization of device processing condition, such as active layer thickness, choice of solvent and MEHPPV:PCBM ratio (Alem et al., 2004; Chang et al., 2006). Further improvement of the device performance is required and will be reported in the future.
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To understand the origin of photovoltaic parameters improvement due to organic salt doping, the current flow of the undoped and the TBAPF6-doped devices during the electric field treatment was investigated. Fig. 5 shows the time response of current density flows (J-t) curves of the undoped and the doped devices during the electric field treatment. Under steady voltage, the current density of the undoped device increased slightly in the first several second and remained constant with time after that. On the other hand, the current flows of the doped device increased significantly and could reach much higher value as compared to undoped device. It is considered that the separated organic salt ions moved slowly under the influence of the electric field toward their corresponding interfaces of the electrodes and the accumulated ions near the interface reduced the interfacial barriers for carrier injection between the electrode and the active layer, resulting in improved electrical characteristics of the doped device. The number of ions accumulated at the interface kept on increasing with time as the constant voltage was applied continuously.
These experimental results proved the strong influence of TBAPF6 doping and electric field treatment on the performance of single layer bulk heterojunction organic solar cell. A modified metal-insulator-metal (MIM) model was proposed for the system as shown in Fig. 6. Without doping, the built-in electric field and the Voc depend on the difference between the ITO and Al work functions (as shown in Fig. 6a) (Mihailetchi et al., 2003). When the ITO electrode of the doped device was positive-biased with a constant voltage at 10 V for 30 s, the PF6- and TBA+ ions moved under the influence of the electric field to the anode and cathode interfaces, respectively. At the ITO anode contact, the accumulation of separated negative PF6- ions increased the localized electric field at the interface. At the same time, the accumulation of the positive TBA+ ions lowered the localized electric field at the Al/active layer interface (as shown in Fig. 6b) (Yap et al., 2008). The accumulation of ions at the interfaces resulted in the increase of the built-in electric field in the bulk of the organic solar cell, which is also responsible for the increase of the Voc and Jsc (Kinoshita et al., 2008; Song et al., 2007). Without electric field treatment, the number of separated ions at the electrode interfaces was insignificant, as indicated by the minor increase of Jsc and Voc of untreated doped device compared to that of undoped device.
The effect of TBAPF6 doping followed by an electric field treatment on the photovoltaic properties of ITO/MEHPPV:PCBM/Al single layer bulk heterojunction organic solar cell has been examined. The TBAPF6-doped device treated with electric field gave a short circuit current density of 6.41 mA/cm2 and an open circuit voltage of 0.50 V under illumination of a halogen projector lamp at 100 mW/cm2. The short circuit current density and the open circuit voltage of the electric field treated device doped with TBAPF6 increased by almost one order of magnitude and 100%, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the electrode/active layer interfaces. Therefore, TBAPF6 doping has been shown to be a simple and cost-effective approach to increase the performance of organic solar cell.