Introducing Cathode Interlayer To Improve Photovoltaic Performance Of Polymer Solar Cells

Polymer solar cells(PSCs) with the advanages of ligh weight, low cost, solution-processed and large-area manufacturing, have received great attention from both academic and industrial scientists. However, PSCs are still inferior to inorganic photovoltaics in efficiency and stability. In this dissertation, we mainly focus on improving the efficiency and stability of PSCs by cathode interface modification.First, we study the device performance of PSCs and field-effect transistors(FETs) based on the “donor-acceptor” conjugated polymer, FBT-Th4(1,4). The absorption spectras of FBT-Th4(1,4) in solution and solid thin film are almost identical, indicating the strong aggregation of FBT-Th4(1,4) chains. Based on FBT-Th4(1,4) as semiconducting layer, we fabricate FETs with bottom-gate and top-contact device structure. Through using interface modification of dielectrics and adjusting fabrication process, we achieve high hole mobility of 1.92 cm2V-1s-1. And for PSCs based on FBT-Th4(1,4) donor, we study the device performance with comparing conventional and inverted device structure, using cathode interface modification, and controlling morphology of active layer. Finally, based on FBT-Th4(1,4):PC71BM with the ratio of 1:2.0, using 1,2-dichlorobenzene with CN additive of 5% volume content, and with PFN cathode interlayer, we achieve high-efficiency inverted PSCs. For the active layer with thickness of 230 nm, the device efficiency is 7.64%, and for the active layer with thickness in the range of 100–440 nm, the device efficiency are all above 6.5%. The superior results further facilitate the development of high-efficiency and thick-film PSCs, and provide the research basis for printed PSCs.Second, we firstly introduce a water-soluble non-conjugated polyelectrolyte(C-PAM) with ultra-high molecular weight of 3 million as cathode interlayer for inverted PSCs. In comparison with conventional cathode interlayer polymer, C-PAM with ultra-high molecular weight and polar pendant can offer unique and excellent interface contact between ITO and active layer: numorous polar pendant can form strong interface adsorption with ITO(interface A), and C-PAM can also form compatible and good interface contact with active layer(interface B) comparing with inorganic ITO. C-PAM cathode interlayer can also effectively reduce ITO’s work function by UPS measurement.Based on PECz-DTQx:PC71BM active layer, we fabricate inverted PSCs with neat C-PAM and Cs F doped C-PAM as cathode interlayer, separately. For the devices with bare ITO cathode and Zn O cathode interlayer, the efficiency are 2.10% and 4.45%, respectively. However, the devices with neat C-PAM and 5% Cs F doped C-PAM cathode interlayer respectively show the efficiency of 4.15% and 5.25%. In addition, we focus on the efficiency of inverted PSCs with different cathode interlayer as a function of storage time in air under ambient condition. After storage in air of 9 months, the device with Zn O cathode interlayer shows the decreased efficiency of initial 56%. After storage in air of 12 months, the device with neat C-PAM cathode interlayer shows the decreased efficiency of initial 54%. However, for the device with 5% Cs F doped C-PAM cathode interlayer after storage in air of 15 months, the efficiency retains initial 64%. The device with neat C-PAM and 5% Cs F doped C-PAM cathode interlayer both show superior air stability. The outstanding results demonstrate water-soluble polymer with ultra-high molecular weight is a promising cathode interlayer for high-efficiency and excellent-stability inverted PSCs.In the final part, we firstly introduce water-soluble polymer with ultra-high molecular weight of 2.8 million(PDMC) and alcohol-soluble polymer(PFN) as double cathode interlayer(PDMC/PFN) for inverted PSCs. Water-soluble PDMC with ultra-high molecular weight and polar pendant, can offer strong interface adsorption with ITO. Water-soluble PDMC can provide the conditions to fabricate PFN cathode interlayer on PDMC and form PDMC/PFN double cathode interlayer. Compared with ITO based on separate PDMC or PFN cathode interlayer, ITO based on PDMC/PFN cathode interlayer shows the smoothest surface morphology, the most decreased work function, and strong surface hydrophobicity.Based on PTB7:PC71BM active layer, we fabricate inverted PSCs with separate PDMC, PFN cathode interlayer, and PDMC/PFN double cathode interlayer, respectively. The devices based on separate PDMC and PFN cathde interlayer show the efficiency of 7.72% and 8.61%, respectively. However the device based on PDMC/PFN cathode interlayer shows the best efficiency of 9.01%. We also focus on the efficiency of inverted PSCs with different organic cathode interlayer as a function of storage time in air under ambient condition. After storage in air of 60 days, the efficiency of device based on separate PDMC and PFN cathode interlayer are respectively initial 90% and 88%, but the efficiency of device based on PDMC/PFN double cathode interlayer still remains 95% of initial efficiency, showing the best air stability. The excellent results demonstrate the strategy of water-soluble polymer with ultra-high molecular weight and alcohol-soluble polymer as double cathode interlayer is promising for high-efficiency and excellent-stability inverted PSCs.