Application of Metal oxide Nanofibers and Thin Films in P3HT based Solar Devices for Efficiency Improvement

Abstract

Organic photovoltaic (OPV) devices are the focus of research among the other photovoltaic devices due to various advantages like their low cost, versatility of the materials used in terms of tunability of their electrical and optical properties and ease of fabrication. However, the challenges of high efficiency and stability are barrier toward commercialization of OPV. The most widely used donor polymer in OPV devices is Poly(3-hexylthiophene) (P3HT) due to excellent optical properties required for efficient photovoltaic devices. The solution processed metal oxide nanostructure provides excellent carrier transport properties by their high mobility and reasonably good optical properties for P3HT based solar devices. This thesis presents diverse device design to improve the performance of the P3HT based photovoltaic devices using metal oxide nanostructures such as nanofibers and thin films. Various techniques such as electrospinning (ES) and sol-gel were used to synthesize metal oxide nanostructures. The optical and electrical characterizations were used to analyze the performance of all the fabricated devices in this study, while 1.5G solar simulator under illumination of 100 mWcm−2 was used to study the power conversion efficiency (PCE). Electrospun nanofibers of zinc oxide (ZnO) and titanium oxide (T iO2) were synthesized by ES technique. The structural and optical properties of these structures were studied to investigate the feasibility of their application in solar devices. The performance of P3HT: Phenyl-C61-butyric acid methylester (PCBM) bulk heterojunction devices are considered to be poor due to excessive volume occupation by PCBM that results in low mobility and reduced solar light absorption in such devices. To overcome such challenges, a new device design was introduced where PCBM was replaced by electrospun nanofibers of ZnO and T iO2 as electron accepting material/structure in P3HT based hybrid solar devices. The PCE up to 0.93±0.01% was achieved by P3HT/T iO2 hybrid devices. On the concept of double heterojunction OPV, ZnO and T iO2 electrospun nanofibers were as well applied to P3HT:PCBM bulk heterojunction devices resulted in improved PCE up to 2.29±0.03% and 4.25±0.03% respectively. Furthermore, other devices were fabricated by interface modification through solution processed inter layers of ZnO, T iO2 and CuOx. The optical properties of the thin metal oxides layer were also investigated, convincing the application of these layers as an inter layer for improved device performance. The electron transport layers of ZnO and T iO2 were introduced in the P3HT:PCBM inverted OPV devices. CuOx as hole transport layer was also investigated in these devices, showing that PCE can reach up to 4.24±0.01% in combination with ZnO as an electron transport layer, the PCE further increased to 4.26±0.02% when ZnO was replaced by T iO2 layer. ZnO and T iOx layers were investigated as an amorphous optical spacer in normal OPV structures. The amorphous ZnO and T iOx optical spacer based bulk heterojunction (BHJ) devices have attained PCE of 3.35±0.03% and 3.48±0.02% respectively, the detailed study of the fabricated devices is included in this thesis. Significant progress in PCE of organic solar cells was achieved using semiconducting metal oxides as charge extraction inter-layers or an optical spacer either in inverted or normal structure devices. Both n- and p-type transition metal oxides with good transparency in the visible as well as infrared region also made good ohmic contacts to both donors and acceptors in OPV devices to improve their performance.