Abstract
Commercially available organic photovoltaics (OPV) are commonly fabricated using printing and coating techniques that allow for low-cost, high throughput processing of large-area OPV devices. However, the power conversion efficiency (PCE) of scaled-up OPVs is often lower than that of small-area ones. This is because the deposition techniques typically being used in industry are different to those used in research laboratories (printing/coating vs. spin-coating). Thus, detailed studies of functional materials are required to tailor the characteristics of photoactive D/A blends of OPVs in order to preserve high PCE values for scaled-up device sizes. Therefore, the aims of this thesis were to enhance the PCEs of OPV cells made using a well-known donor material (P3HT), and to develop a structured approach to fabricating large-area OPVs thus easing the transfer of fabrication procedures from laboratory to industry. To achieve the first goal, indene-C70-bis-adduct (IC70BA) was chosen as an acceptor material for a photoactive blend with P3HT. A review of P3HT:ICBA-based solar cells indicated a significant variation of reported device PCE values (average of 4.66±1.45%). The majority of reported device efficiencies were measured for OPVs with photoactive areas rarely exceeding 0.1 cm2. Therefore, a detailed study of the intrinsic characteristics of the IC70BA molecule and the morphology of the P3HT:IC70BA blends was carried out in order to design the optimal fabrication conditions for achieving higher PCEs and up-scaled device areas. Record PCEs approaching 7% were accomplished in this thesis for OPVs with photoactive areas of 0.43 cm2. This was achieved by understanding the correlation between the isomeric properties of the IC70BA molecule and the resulting D/A blend morphology depending on the fabrication conditions used. The second goal of this thesis was accomplished by designing a slot-die coating equipment that allows for the deposition of functional materials over large-areas. Different solubilised materials were deposited in ambient conditions on glass and plastic substrates in order to fabricate OPV devices. Two different photoactive D/A systems were used: P3HT:IC70BA and PCDTBT:PC70BM. OPV cell and module PCEs approaching 4% were achieved for devices with photoactive areas of about 35 cm2. The quality of the slot-die coated layers was investigated using LBIC, PL, and Raman mapping. This will allow for future improvements in the coating process and, therefore, increased device PCEs and operational lifetimes. In conclusion, the results obtained in this thesis show a way of fabricating efficient large-area OPVs without the use of the spin-coating deposition technique. The study of materials and the development of deposition procedures allows for an accessible transfer of research outcomes from the laboratory to industry.