Performance optimization studies of solution processed bulk-heterojunction solar cells

Organic Solar Cells (OSCs), which rely on the concept of bulk-heterojunction, stand out due primarily to their simple construction, mechanical flexibility and exceptional ease of processing. These characteristics make them potential candidates to substitute for the expensive photovoltaic counterparts. Among other OSCs, devices containing poly(3-hexylthiophene) (P3HT) and phenyl C61 butaric acid methyl ester (PCBM) as photo-active layer have shown promising results. However, the power conversion efficiency (PCE) is still lower than the required commercialization mark (∼10%).;Devices with structure glass/ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al, annealed and un-annealed with device area ∼0.4 cm2 (unless otherwise stated), have been studied. An investigation of the device processing variables has led to the conclusion that the optimum loading of PCBM in the blend for optimum performance is in the range of 1:1 to 1:2. Characterization of the active layer with UV-vis absorption, PL spectra and XRD reveal that the addition of PCBM to P3HT matrix is detrimental for the self-organization of P3HT chains (crystallinity) and it also increases the resistivity. Similarly, 1,2 dichlorobenzene (DCB) has been found to be the best solvent among other solvents such as chloroform (CF) and chlorobenzene (CB), for optimum PCE. The rho(T) data from the samples (pristine P3HT and P3HT/PCBM blends) exhibit anisotropy in conduction where it follows the variable range hoping (VRH) in the lateral (parallel to film) and polaronic behavior in vertical (perpendicular to film) transport. The activation energy obtained from the fit to polaronic model is 329 meV for P3HT/ PCBM blend (1:1).;Furthermore, the photovoltaic parameters extracted from a lumped circuit analysis of voltage and temperature dependence of photocurrent, JL(V), in P3HT/PCBM OSCs, completely describe the illuminated J-V data from far reverse bias to beyond the open circuit voltage (Voc). A simple model for carrier collection has been adopted to describe the voltage dependence of the photocurrent, with only one adjustable parameter, Lc/D, the ratio of the carrier collection length to the active-layer thickness. The resistive and collection losses have been quantified, which allowed the intrinsic junction behavior to be uncovered. It has been concluded that fill factor (FF) in such cases is limited more by JL(V) losses than resistive losses (Rs) and only Rs correction are not sufficient for quantifying complete losses. The linear dependence of Voc on temperature has been used to determine the effective bandgap of the blend. The observed lower values of the band gap than expected indicate the presence of mid-band-gap states. The blocking resistance Rb shows an Arrhenius behavior with temperature (200--300 K) giving the activation ∼ 207 meV.;XPS data from (P3HT/PCBM)/Al and (P3HT/PCBM)/LiF/Al surfaces suggest that the interaction of Al with sulfur S of the thiophene ring of P3HT, is more favorable as compared to carbon in the polymer. This is revealed by the appearance of new S 2p spin-orbit doublets at lower binding energy as in-situ Al deposition continues. The AFM micrographs of the LiF-deposited-polymer surfaces suggest that LiF (∼1 nm) does not provide complete coverage. This is also evident from the fact that the induced S peaks appear with Al evaporation on both (P3HT/PCBM) and (P3HT/PCBM)/LiF surfaces. The chemical shift of F1s core level peaks with Al evaporation indicates the band bending in LiF. In addition, the angle-resolved XPS suggests that the oxidation of Al is at the interface.;Solar cells fabricated from P3HT and various electron acceptors (PC 60BM, PC70BM, TiO2 and single wall carbon nanotubes (SWCNT)), the PC70BM in combination with PC60BM (PC 70BM:PC60BM=0.6:0.4) gives the best performance. This is due to the absorption of PC70BM in the longer wavelength regime as compared to PC60BM, which is supported by the EQE(%) as well as UV-vis absorption data. Moreover, in the case of single wall carbon nanotubes (SWCNT) and TiO2 nanoparticles as electron acceptor, the devices delivered smaller PCE (%) as compared to the P3HT/PC60BM devices alone. The reason for the low performance in this case is due to the to the agglomeration of SWCNT and TiO2 which are not soluble in chlorobenzene and result in low values of the PV parameters, in particular the short circuit current density, Jsc. We designed and constructed a spray coating system in the lab to deposit films/active layers for the solar cells via spray coating. With this technique P3HT/TiO2 hybrid solar cells as well as layered OSCs have been developed, where P3HT layer has been inserted between the blend layer and PEDOT layer which improved the PCE. (Abstract shortened by UMI.)...