Research Paper Sample on Organic Photovoltaic Devices - a Renewable Energy Source

Energy is a critical component of every issue facing the man in the 21st century such as climate change, sustainability of the economy, social development and so on. With the prediction that energy consumption is likely to increase to about 28 TW globally by the year 2050, the demand for clean, sustainable and renewable energy sources has become a top priority (Borbulevych et al., 2002). Organic photovoltaic (OPV) devices are a viable choice that meets the demands of clean energy sources and are essential in the development of decentralized energy (off-grid) zones. OPV devices possess electronic and mechanical properties that are highly profitable (Elstner et al., 1998). However, it is still challenging to create efficient mixtures hence this research paper looks into several calculations that have been used to increase the comprehensiveness of how density functional tight-binging (DFTB) technique works in OPV components modeling (Yu, Gao, Hummelen, Wudl, & Heeger, 1995). Molecular dynamics are used to get relevant configurations and equilibration of the systems. Self-consistent charge density functional tight binding (SCC-DFTB) is present as a computational feasible and reliable technique for modeling the properties and structure evolution of such systems (Thompson & Frechet, 2008).

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Structure evolution

The results of the study are indicative of various aspects regarding OPV systems. First of all, there is structure evolution provided by MD algorithm to study the dynamics of the molecules especially in systems with many degrees of freedom making it costly to do systematic geometry evolution (Liao et al., 2007). Geometric relaxation enables slight adjustment of bond lengths and angles in the structure without displacement of constituents (Yarovsky, 2002).

Morphology and band gap

It is essential to understand the morphology and band gap of the system. It is also important to take note that one-dimensional simulation is handy in the characterization of solar cells behavior. Using statistical analysis to analyse snapshots generated from the system the validity of the study can be increased. Properties are also evaluated to establish a solid ground for interpretation of higher dimensionality (Wurfel, 2005). It becomes evident that there is variation in the final energies behavior which is quite subtle. This is a representation of the systems energetic stability being driven by the other factors in addition to the position of the constituents (Valavala, Clancy, Odegard, & Gates, 2007). Mono-dimensional systems are seen to possess a larger mobility that a tri-dimensional system. Using 3D packing limits the movements on the structures and therefore diminishing the band gap fluctuation. The partial mobility is essential later on when it comes to stabilization during charge separation and charge migration (Santhanam & Sharon, 1988). When understanding morphology, it is essential to point out that there is usually a collaborative effect of fullerene clustering and in a time-dependent situation the preferred position of the fullerene derivatives is bound to vary (Robertson, 2006).

The performance of OPV devices is affected by the material properties of the donor and acceptors as they have a direct impact on the mobility of free charges, light absorption and the value of the open circuit voltage (Piliego et al., 2009). The band gap of the system is usually determined by the selected snapshot and spatial arrangement of the molecules within the complex. The polymeric backbone is apparently planar and interconnected with systems band; the fewer twists, the larger the band (Rand, Genoe, Heremans, & Poortmans, 2007). The effects are felt when broadening the energy bands. The variations of the band gaps are also in the same magnitude order.

Charge redistribution

Charge distribution and polarization effects are induced by the systems. Electrons and nuclei are bound together by the electrostatic force to form atoms. Similarly, the atoms and molecules in bulk material are held together by electrostatic forces. However, behavior matches the typical interaction effects among molecules in the bulk although the interaction is quite small compared to that (Patnaik & Pachter, 2002).

Density of states

Analysis of the total density of state (DOS) and the partial density of state (PDOS) of the systems being dealt with indicates that there is a shift of DOS of the isolated molecules and same molecules (Lu, Baek, & Dai, 2012). However, PDOS within the complex rarely overlap. Variations analysis indicates that DOS of the fullerene derivatives are displaced to less negative values while the polymer shift to the opposite direction which can be perceived as the buckyball part being less prone to receive more electrons from the complex (Maitland, Rigby, Smith, Wakeham, & Henderson, 1983). The buckyball may have already received some electronic density from the copolymer (Hourston, 1981).

Molecular orbitals

Bulk heterojunction scheme is representative of the plot of the molecular orbitals. The systems analysed depict equal behavior to that of expected molecular orbitals (Gratzel, 2005). Calculation of the orbitals using DFT for a clustered geometry reveals qualitative resemblance to those produced by DFTB. The calculations are time-consuming requiring about a quarter of a day using DFT running in 16 processors. However, when DFTB technique is used the information is obtained in half an hour using 2 processors and using less computation cost (Gratzel, 2001).

2D Packing

As earlier stated 3D packing limits the movements on the structures and therefore diminishing the band gap fluctuation. 2D packing provides a better understanding of electronic behavior (Berrio & Zuluaga, 2014). It exhibits tighter packing that favors small displacement of fullerene derivatives toward the polymer chain. Benzothiadiazole is still the most preferred position (Chiang et al., 1978). Fullerene-fullerene interaction is enhanced in 2D packing by slightly shifting the buckyballs rather than placing them directly over the rings. There is a similarity between the average distance polymer-fullerene of the systems in 2D packing to that of 1D packing (Gerischer, 1989).

Conclusion

The addition of groups with high dipoles to the PCBM does not have any influence on the properties under investigation for the systems ground state. This necessitates a study of both denser packing and excited states in order to generate a large picture (Aradi, Hourahine, & Frauenheim, 2007). Fullerenes functional group orientation is quite different depending on the snapshot under observation and this is essential in reorganization energies that follow after excitation (Arunachalam & Fleischer, 2008). To sum up, it is possible to connect some of the electronic properties of the structure while considering the morphological arrangement according to the proposed conceptual and computational framework.

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