Non-conjugated polymer electrolytes (nCPEs) are applied as electron transporting layers (ETLs) in organic photovoltaic (OPV) devices with conventional structures and used as competent interfacial layers. The new nCPEs are prepared from dihydroxyl pyrrolidinium bromide ionic liquid monomers and m-diphenylene diisocyanate to create linear polyurethane ionenes with main-chain ionic salt moieties. The ultraviolet photoelectron spectroscopy (UPS) results reveal that the formation of an interfacial dipole by the nCPEs facilitates electron transfer from the photoactive layer to the electrode. The OPV devices with the new nCPEs show a power conversion efficiency (PCE) comparable to that with the well-known conjugated polymer electrolyte poly(9,9-bis(3’-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide (PFN), which possesses charged sites on its pendants, as an ETL. The OPV device with the synthesised nCPE exhibits superior stability compared to the PFN-based OPV devices. Our results suggest that the nCPEs containing ionic salt structures on polymer main chains are more suitable ETLs for OPV devices than PFN-based layers with pendant salt moieties.
Bibliographical noteFunding Information:
Ka Yeon Ryu and Shafidah Shafian were equally contributed to this work. This research was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning ( NRF-2019M1A2A2072417 and NRF-2020R1A2C1101905 and NRF-2018M3D1A1058624 ).
Pyrrolidinium(Br)-co-PEG polyurethane (PPEG-PU) A mixture of 1,1-bis(11-hydroxyundecyl)pyrrolidinium salt (0.500 equiv.), PEG-1000 (0.500 equiv.), MDI (1.00 eq.) and diazabicyclo[2.2.2]octane (130 ppm) in dried DMF (15 mL) was stirred at 90°C for 60 h under N2. After cooling to room temperature, ethyl acetate was added to precipitate the polymer product. After removing the upper layer by decanting, the residue was dissolved in dichloromethane. The solution was added into ethyl acetate and the product was collected by filtration. This precipitation step was repeated at least two more times. Drying in a vacuum oven at 70°C gave the highly viscous gel pyrrolidinium(Br)-PEG polyurethane copolymer (Fig. S1 of Supporting Information).Meanwhile the reduction of FF mainly due to the change in resistance of the devices. To investigate the changes in internal resistance, electrical impedance spectroscopy (IES) measurements were conducted of as prepared and stored devices with various nCPEs layers at Voc under 1 sun illumination with measuring frequency range in between 1 Hz and 5 MHz represented using Nyquist plots and the equivalent circuit model is defined in Fig. S10 of Supporting Information. Since, the same photoactive layer was used, the difference in the diameter of the semicircle to be related to the resistance at the interface of photoactive layer/ETL [24,25] The diameter of fresh samples was 76.64, 80.65 and 91.27 Ω and aged device was 165.53, 170.18 and 237.58 Ω with PPEG-PU, P-PU and PFN layers respectively (Table S2 of Supporting Information). The diameter of the semicircle of the fresh device with PPEG-PU and P-PU are smaller than that of the devices with PFN, showing that charge transfer resistance of device with PFN layers was the highest among all. After being stored in ambient air for 48 h, the semicircle diameter difference in between aged and fresh devices was observed as 88.89 and 89.53 Ω for device with PPEG-PU and P-PU layers and 146.31 Ω for device with PFN layer. Higher different was found in device with PFN layers indicating the increase in resistance of the interfacial layer after being stored for certain amount of time. Based on the equivalent circuit model of the device, the Rs refers to the resistance of HTL and the electrodes. The shunt pair of R1 and C1 refers to the photoactive layer and R2 and C2 pair corresponds to the electrical contact in between photoactive layer and Al interfaces. The fitting parameter of Rs, R1, C1, R2 and C2 were tabulated in Table S3 of Supporting Information. The value of R2 in fresh device was 43.07, 38.03 and 67.38 Ω and for aged device was 130.50, 129.90 and 204.60 Ω with PPEG-PU, P-PU and PFN layers respectively. Here clearly shows the value of R2 was the highest with PFN as ETL layer for both fresh and aged devices. The result is in consistent with the PCE results. The arc size and transport time constant, τ value of device with PPEG-PU and P-PU interlayer was smaller compared to the device utilizing PFN interlayers proves that conjugated PPEG-PU and P-PU has better conductivity and smaller interfacial resistance. According to the stability, contact angle and IES results, nCPEs with hydrophobic main chains and pyrrolidinium salt units in the main chains should be used for ETLs for stable solar cells.Ka Yeon Ryu and Shafidah Shafian were equally contributed to this work. This research was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning (NRF-2019M1A2A2072417 and NRF-2020R1A2C1101905 and NRF-2018M3D1A1058624).
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