Effects And Mechanism Of Treatments In The Active Layer On The Improved Performance Of Polymer Solar Cells

It is a pretty helpful and widely used technique to enhance the efficiency of polymer solar cell by adding liquid additive or solid additive into the active layer. In this thesis, the morphology, physical processes, energy band structure and optoelectronic properties are studied in order to further understand the working mechanism and device physics in polymer solar cells.Using solvent additive1,8-diiodooctane(DIO) and thermal annealing can increase the efficiency of P3HT:PC61BM bulk heterojunction solar cells. By using Grazing incidence X-ray diffraction (GIXRD), we evaluated and compared the structure evolution of P3HT:PC61BM blend films used in solar cells after thermal annealing and solvent additive adding. Although both treatments contribute to the formation of the nano-morphology of P3HT:PC61BM blends, the two methods have different driving force for the morphology evolution. The blend films with DIO adding show much narrower interplanar spacing than the blend films with thermal annealing; however, the annealed blend films do have longer coherence length than the DIO added blend films which may result in more efficient charge transport. And then the well-ordered morphology evolution of the P3HT:PC61BM active layers combined with the thermal annealing and processing additive was analyzed. The results show that thermal annealing controls the crystallization time of chain stacking of P3HT molecules accompanied by the diffusion and aggregation of PC61BM molecules. The device annealed at70℃showed the highest PCE among all devices compared with the as-spun device. It is found that Voc is mainly influenced by the interplanar spacing of P3HT lamellar, and Jsc and FF have close relationship with the P3HT crystalline domains with better packing order and PC61BM aggregation which are attributed to the charge-carrier transportation.Using solvent additives1-chloronaphthalene (CN) can increase the efficiency of P3HT:PC61BM bulk heterojunction solar cells. Both P3HT and PC61BM are soluble in CN. It is observed that the absorption of additive-added blends has a higher intensity and is red-shifted than that of the P3HT:PC61BM blend. The PL intensity increases which suggest that the conjugation length increases or the domain size of P3HT increases. Large domains with serious phase separation influence the interface area between P3HT and PC61BM. Excitons are generated in both the P3HT phase and the PC61BM phase. By using transient absorption spectroscopy (TAS), in all the film blends with or without additive, strongly bound interfacial CT states are formed by a large fraction of the excitons indicating geminate recombination may occur. It is indicated that this enhanced fraction of CT states with CN added comes from the more crystalline P3HT phases and the slower CT states and mobile charges decay of blend with CN adding indicates reduced recombination losses from early time recombination.Using gold nanoparticles added into the active layer can improve the efficiency of PTB7:PC71BM bulk heterojunction solar cells. Gold nanoparticles block the photon absorption of the active layer. Along with the addition of increasing concentrations, the absorption strength of the solar cell is declining. Since all devices are morphology optimized, the dissociation efficiency is almost the same. By analyzing the capacitor-voltage curves, Mott-Schottky curves and impedance spectroscopy, accompanied with the corresponding band structure change in the device under different voltage bias, it is found that the addition of low concentrations of gold nano-particles in the active layer can reduce bimolecular recombination rate, improve the carrier mobility and improve the carrier collection efficiency, thereby improving the short-circuit current and the efficiency of the solar device. And when a relatively high concentration of gold nanoparticles is added in the active layer, the carrier mobility of the device is the highest; bimolecular recombination decline; the carrier collection efficiency is also improved to some extent. But gold nanoparticles induced defect density of states to increase and substantial decline of the absorption spectrum is the main impact of the device efficiencies. So the key study of the concentration of such gold nanoparticles added in the active layer is to find a balance between the absorption and the improved carrier characteristics. Finally, it is found that a series of gold nanoparticles can enhance the performance of PTB7:PC71BM bulk heterojunction solar cell and they all have similar physical mechanism to the solar cell.