Performance enhancement of P3HT:PC61BM organic solar cells via acetic acid treatment approach / Lim Lih Wei

Lim , Lih Wei (2022) Performance enhancement of P3HT:PC61BM organic solar cells via acetic acid treatment approach / Lim Lih Wei. PhD thesis, Universiti Malaya.

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Abstract

In this work, an acid acetic solution treatment was developed to improve the performance of the organic solar cells (OSCs) based on bulk heterojunctions regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT):[6, 6]-Phenyl-C61-butyric acid butyl ester (PC61BM). After the deposition of active layer using spin-coating technique, additional treatment step where the solution of the diluted acetic acid with different concentrations (5 %, 10 %, 25 %, 50 %, 75 % and 100 %) was spun on top of the active layer. Current density-voltage (J-V) characteristics under light illumination was used to characterise the photovoltaic effects of the devices. The untreated device showed typical photovoltaics effect with current density (Jsc) of 2.0 mA/cm2, open circuit voltage (Voc) of 0.58 V, 46 % fill factor (FF) and 0.53 % power conversion efficiency (PCE). All the devices that were treated with acetic acid treatment showed significant enhancement in the Jsc and PCE outputs. The highest values of Jsc and PCE of the devices occurred at 50 % acetic acid concentration treatment. At this acid concentration, Jsc of 11.0mA/cm2 with PCE up to 2.04 % can be achieved. On the other hand, Voc remains constant as 0.58 V whereas FF decreased to 32 % due to drastic increase in Jsc. It can be seen that the rise of Jsc with Voc remain constant in Region III will make the slope of the J–V curve decreases correspondingly, leading to less “squareness” of J–V curve, therefore a lower FF . Hence, this inexpensive and simple technique have demonstrated that acid treatment greatly increased the Jsc of the devices and consequently, the PCE of the devices are enhanced up to three folds. In addition, the effect of acid acetic solution treatment on the device physics of polymer solar cells has been investigated through Schottky diode method, Cheung & Cheung method and Norde’s function to provide fundamental insight into electrical parameters of the OSCs. SCLC approach was performed to explain the enhancement of the current produced by OSC devices treated with acetic acid. The results indicate that the concentration of traps in the treated devices become lower compared to the untreated device. However, optical spectroscopy characterisations of the untreated and treated thin films exhibited similar pattern without any shift in the spectra suggesting that the acid treatment did not alter the molecular packing of the P3HT and PC61BM in the bulk heterojunction system. FESEM measurements revealed that the impurity particles on the active layer have been removed and visible cavities on the surface can be seen after it went through the treatment. The etching effect of the acetic acid may not only create cavities on the surface of active layer but at the same time dissolved the particles/impurities to provide clean surface. Therefore, purification on the active layer surface might have taken place. The reduction of the impurities results in lower series resistance and barrier height which in turn caused the enhancement performance of the OSCs.

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