EFFECT OF MIXED METAL OXIDES ANODE MODIFICATION ON THE PERFORMANCE OF INVERTED TYPE HYBRID ORGANIC SOLAR CELL
BASED ON ZnO/P3HT
Nasehah Syamin Sabri1, Chi Chin Yap1, Muhammad Yahaya1, Muhamad Mat Salleh2 and Mohammad Hafizuddin Haji Jumali1
1School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
1Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
Corresponding author: [email protected]
ABSTRACT
The effect of mixed MoO3:WO3 anode buffer layer on the efficiency of inverted type hybrid solar cell has been investigated. A simple solution-processed mixture of MoO3
and WO3 was dissolved in 1-butanol. The mixed MoO3:WO3 solution was spin coated between the active layer of ZnO nanorod arrays (NRAs)/P3HT (deposited by spin coating technique) and Ag electrode (deposited by using thermal evaporation technique). The FTO/ZnONRAs/P3HT/ MoO3:WO3/Ag device was fabricated and a device without MoO3:WO3 buffer layer was also fabricated for comparison. The performance of the devices was analyzed through the current density-voltage (J-V) curve under illumination with a solar simulator at 100 mW/cm2. The device with MoO3:WO3 anode buffer layer exhibited a FF 51%, a Voc of 0.36 V, Jsc of 1.42 mA/cm2 along with PCE of 0.26%. The PCE was two times greater than the device without the anode buffer layer. These results indicate that the effectiveness of hole collection at Ag electrode can be enhanced by mixed anode buffer layer and the efficiency of the inverted organic solar cell has been improved.
Keywords: Anode modification; Mixed metal oxides; Organic solar cell; P3HT INTRODUCTION
Organic solar cells (OSCs) based on polymer have attracted much attention due to their low cost, ease of fabrication, flexibility and light weight [1-3]. Recently, OSCs with power conversion efficiency (PCE) of 12% have been reported [4]. However the poor stability of conventional OSC under atmospheric pressure conditions results in low PCE
Solid State Science and Technology, Vol. 26, No 1 (2018) 194-199 ISSN 0128-7389 | http://journal.masshp.net
function top electrode metal such as Ag is used as anode [6,7]. For better hole collection, an anode buffer layer is introduced due to its capability of driving out the photogenerated holes with minimum recombination losses and transporting the holes to the outer circuit with minimal electrical resistance [8].
In recent years, the study on anode buffer layer in inverted type OSCs has attracted significant attention due to its great promise in improving the device performance and stability [9-12]. Metal oxide buffer layers are often deposited by physical processes such as thermal evaporation [13] and sputtering techniques [14], which involve the vacuum process that can detract the advantage of ease large-area and low cost fabrication. To solve this problem, solution-based metal oxide approach which can significantly simplify the fabrication process and lower the cost is preferred. Many researches have been devoted in using the solution-based metal oxide buffer layers such as vanadium II oxide (V2O2), and molybdenum oxide (MoO3) [15,16] to modify the anode. It has been reported that the anode buffer layer would serve as the electron blocking layer and also be helpful to collect holes towards the electrode and subsequently enhance the device performance [10].
Nevertheless, the effects of mixing different kinds of metal oxides as anodic modification on the OSCs have seldom been reported. In this paper, we investigated the mixed oxides (mixing of MoO3 and tungsten oxide (WO3) nanoparticles) as the anodic modification through simple solution process. By using the mixed oxides anode buffer layer approach in inverted hybrid OSC, the PCE of the device has been improved significantly compared to that pristine device.
EXPERIMENTAL
ZnO nanorod arrays (NRAs) were synthesized by a low temperature hydrothermal method on ZnO seed layer-coated fluorine-doped tin oxide (FTO) glass substrates using the same procedures as described in previous report [17]. Subsequently, a chlorobenzene solution containing 35mg/ml of poly(3-hexylthiophene) (P3HT) was spin coated on the ZnO NRAs, forming a photoactive layer. Next, 1-butanol solvent containing a mixture of 0.2 mg/ml MoO3 and 0.2 mg/ml of WO3 nanopowders was casted onto the photoactive layer at 2000 rpm. Finally, silver film was deposited on top in a vacuum of 5× 10−5 torr. The device area was defined as 0.07 cm2 by a shadow mask. For comparison, a device without MoO3:WO3 was also fabricated. Current density–voltage (J–V) curves were measured with a Keithley 2401 source measurement unit. The surface morphology of the photoactive layers was measured by Veeco CP-II Scanning Probe Microscope (AFM). Figure 1 shows the schematic diagram of the structure and band diagram of the inverted type hybrid OSC with FTO/ZnONRAs/P3HT/ MoO3:WO3 /Ag structure.
Figure 1: Schematic of the (a) structure and (b) band diagram of FTO/ZnONRAs/P3HT/MoO3:WO3 /Ag solar cell
RESULTS AND DISCUSSION
Figure 2 shows the surface morphology of the photoactive layer without and with MoO3:WO3 anode buffer layer in 3D images using AFM. The rougher surface was clearly observed on the photoactive layer with MoO3:WO3 anode buffer layer compared to that without MoO3:WO3. This is consistent with the RMS roughness which increased
Solid State Science and Technology, Vol. 26, No 1 (2018) 194-199 ISSN 0128-7389 | http://journal.masshp.net
Figure 2: AFM images of ZnONRAs/P3HT layer (a) without and (b) with MoO3:WO3
anode buffer layer
Figure 3 shows the J-V characteristics of ZnONRAs/P3HT inverted type hybrid OSCs with and without MoO3:WO3 anode buffer layer under illumination of a simulated AM 1.5 G sunlight at 100 mW/cm2. With the introduction of MoO3:WO3 anode buffer layer,the open circuit voltage (Voc) of the device enhanced from 0.30V to 0.36V, short circuit current density (Jsc) enhanced from 1.11 to 1.42 mA/cm2, fill factor (FF) enhanced from 42% to 51%, and PCE enhanced from 0.14% to 0.26%. The mixed oxides anode buffer layer was beneficial for the hole collection towards the anode. The valence band of MoO3 (5.3 eV) is closed to the highest occupied molecular orbital (HOMO) level of P3HT (5.2 eV) as shown in Figure 1 (b), revealing that MoO3 will help in collecting holes towards Ag. Moreover, the high work function of WO3 (4.8 eV) would double up the efficiencies of holes collection at Ag electrodes [18] as proved by the decrease in series resistance (Rs). In addition, the device with MoO3:WO3 anode buffer layer promoted better ohmic contact at the photoactive layer/anode interface and reduced voltage loss in band bending, thus increasing the Voc. The reduced Rs of the device with MoO3:WO3 mixed oxide as anode modification is favorable for charge transport and collection, which also explains the improved FF and Jsc. The photovoltaic parameter data have been obtained for the devices with and without MoO3:WO3 anode buffer layer and are summarized in Table 1.
Figure 3: J-V characteristics of ZnONRAs/P3HT inverted type hybrid OSCs with and without MoO3:WO3 buffer layer under illumination
CONCLUSION
In conclusion, we have found that mixed metal oxides anode modification of the interface between the photoactive layer ZnO NRA/P3HT and Ag electrode can improve the overall performance of inverted hybrid OSCs. The PCE was improved from 0.14%
to 0.26% by the introduction of the MoO3:WO3 anode buffer layer. This anode modification can effectively enhance the hole collection and increase the PCE.
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