Alcohol-soluble Commercial Polymer Modified Transparent Cathode For Inverted Structure Polymer Solar Cells

As a promising approach to provide clean energy, Polymer based solar cells (PSCs)have been attracting growing interests due to their unique advantages, such as lowfabrication cost, lightweight, simple processing, solution-processable and highmechanical flexibility. In this thesis, the commercial alcohol-soluble polymers wereinvestigated as cathode modifier for inverted polymer solar cells. The experiment resultsshowed that the power conversion efficiency (PCE) of the PSCs can be improved byinserting such polymer layer between the active layer and the cathode.PSCs with the interfacial layer of PVA, PVB, PVP or PEG were made by themethod of spin-coating. The results showed that the performance of devices was worsewith PVB or PVA as modified layer, while the performance was improved with PVP orPEG as modifier layer. With the PVP concentration of0.5mg/ml in methanol solutionand annealed in glove box after spin-coating, we got the optimized device performance.Compared to the unmodified one, the PCE, EQE and the Voc of the device increasedfrom2.41%to4.07%,45%to70%and0.4V to0.53V respectively. We analyzed themechanism as followed: by inserting PVP as modifier, dipole was formed between thecathode and the active layer, better energy alignment was formed, lower inversesaturation leakage current due to the higher carrier selectivity at the cathode, and betterdiode characteristic due to the higher rectification ratio.Experiments showed that the performance of the device can be improved byintroducing the interfacial layer of PEG. With the PEG concentration of1mg/ml inmethanol solution and annealed in glove box after spin-coating, we got the optimizeddevice performance. Compared to the unmodified one, the PCE, EQE and the Voc of thedevice increased from2.39%to4.94%,42.38%to61.13%and0.4V to0.55Vrespectively. By inserting PEG as modifier, better contact was formed between thecathode and the active layer, better energy alignment was formed, lower inversesaturation leakage current due to the higher carrier selectivity at the cathode, and betterdiode characteristic due to the higher rectification ratio. From the SEM pictures wecould see the morphology of the active layer and we found less phase separation wasobtained after the PEG modification which was benefit to the device performance. Fromthe UPS technique we could see that the cathode with lower work function was obtained with the PEG modified. The optimized results with PEG as interfacial layer were: PCEof5.22%was obtained with the device based on P3HT:PCBM as active layer,7.83%was obtained based on PTB7:PCBM as active layer.