Theoretical and Experimental Studies of Bulk-heterojunction Polymer Solar Cells

Abstract

The semi-empirical quantum chemical method was used to explore the electronic properties of conjugated polymers and fullerenes derivatives that commonly used in polymer photovoltaic cell. From our study we found that the energy gap of aromatic polythiophene obtained by using Austin Model 1 (AM1) and Modified Neglect of Differential Overlap (MNDO) was overestimated by more than 4 eV. The calculation results obtained by AM1 semi-empirical method show that substitution of polythiophene by alkyl groups in the 3-position does not change significantly the value of the energy gap. However, comparing the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of polythiophene with that of alkyl substituted polythiophene shows a noticeable energy shift; the HOMO energy of the polyalkylthiophenes increased by about 0.19, 0.21, 0.19 eV and the LUMO energy level increased by about 0.14, 0.17, 0.18 eV for poly (3-methylthiophene) (P3MT), poly (3-hexylthiophene) (P3HT) and poly (3-octylthiophene) (P3OT), respectively. The Austin model 1 (AM1) semi-empirical method was applied to obtain the optimized geometries, heat of formation, ionization potential, affinity potential and energy gaps of low band gap conjugated polymers. The theoretically obtained energy levels and energy gaps for the different polymers were compared. The geometrical and electronic structure (HOMO and LUMO levels) of some Fullerene derivatives was also studied by using AM1 and parameterization method 3 (PM3). The calculated LUMO levels were correlated with the reduction potential of the Fullerene derivatives found in the literature. The reduction potentials exhibit good linear relationships with the AM1 and PM3 calculated LUMO energy levels. We proposed a correction method to correct the calculated donor’s energy gaps, acceptors energy gaps, effective energy gaps and offset potentials in Polythiophene derivative/Fullerene bulk-heterojunction polymer solar cells. The corrected HOMO, LUMO energies and energy gaps of polythiophene derivatives match well with the experimental one. An effective medium model using AMPS-1D (Analysis of Microelectronic and Photonic Structure) simulation program was found to be applicable to simulate P3HT/PCBM bulk heterojunction solar cell. The results show that by increasing the interface band gap to 1.6 eV the efficiency increases to 8.29 %. By decreasing the optical band gap to 1.5 eV we obtained a iv maximum efficiency of 7.32 %. By combining both effects the maximum efficiency was found to be 9.34 % for an optical band gap 1.63 eV. We studied the optical and electrochemical properties of different conjugated polymers as well as the surface morphologies of thin films made from different conjugated polymer/Modified Fullerene composite. It was concluded from the UV-Vis absorption and PL spectra that the optical properties of the polymers films strongly affected by solvents used for spin coating. With Atomic Force Microscope (AFM) it has been demonstrated that the surface morphology of the thin films of polymer/Modified fullerene depends strongly on preparation condition (solvents). We fabricated and studied the performance of bulk-heterojunction polymer solar cells using P3HT, P3OT or MEH-PPV as electron donors blended with the acceptor [6, 6]-Phenyl C61- butyric Acid 3-ethylthiophene Ester (modified fullerene). We studied the effect of polymer/modified Fullerene ratios on the devices performance. Furtherwe investigated the effect of varying the spin coating speed on the device performance