Methods for enhancing the yield and performance of organic solar cells
Creators: name: Pang, Ai Lin Christina
Publisher: name: Imperial College London
Year: 2015

description: Significant interest has been directed towards achieving high performance, large area roll-to-roll printed organic solar cells (OSCs). This thesis focuses on optimisation of the transparent electrode layer and application of a new dielectric material, SU-8, on top of the electron-transporting interlayer in inverted OSCs. The aim of this work is to achieve higher yields and efficiencies in OSCs and to elucidate the physical origins of the improved performance. One of the main challenges for large area flexible devices has been the brittleness of indium tin oxide (ITO), the most popular transparent electrode and the scarcity of indium which leads to cost fluctuations. Of all the alternative electrodes so far developed, silver nanowires (AgNWs) are arguably the most promising, having been found to possess comparable sheet resistance and transmittance characteristics. When AgNWs are incorporated in devices, however, their high roughness often results in poor yield and performance. The first part of the thesis describes a series of optimisation steps performed on each layer in AgNW-based OSCs so as to develop a reliable fabrication protocol. When the protocol was employed, a significant improvement in performance and yield was achieved, such that the AgNW-based devices were then comparable to ITO-based devices. In the next section, SU-8 was first applied to sol-gel-derived zinc oxide (ZnO) layer of inverted solar cells based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM), and a ~14% enhancement in device efficiency was found. The film and surface properties of SU-8-modified ZnO were analysed. The planarising effect of the SU-8 layer and the hydrophobic nature of the SU-8 surface were found to assist with the wetting of the organic blend layer, leading to improved interlayer-active layer contact. Carrier recombination inside the device was investigated through transient photovoltage measurements. These showed a longer carrier lifetime in SU-8-containing devices than in SU-8-free devices. Double injection measurements showed the ambipolar mobilities to be unaffected by the presence of SU-8. These findings indicate the device enhancement is due to slower recombination dynamics in the presence of SU-8. When tested in polycrystalline ZnO, similar enhancements in performance were found, suggesting the SU-8 could be employed with other electron-transporting layers for performance enhancement. In the final section of the thesis, SU-8 was applied to bulk heterojunction (BHJ) devices based on another polymer polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) and a small molecule 7,7′-[4,4-Bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]bis[6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (p-DTS(FBTTh2)2). These devices showed similar enhancements in efficiency, although different reasons are suggested for the two cases. For PTB7 devices, there was likely better polymer orientation or phase separation, while in p-DTS(FBTTh2)2 devices, larger crystallite sizes, smaller d-spacing and reduced shunting was the likely cause of the enhanced device performance. These results suggest SU-8 may be beneficial for other BHJ OSCs.