POLYMERIC MATERIALS FOR SOLAR CELLS
2.2. DESCRIPTION OF NOVEL ORGANIC MATERIALSMATERIALS
AM1.5G. This standard describes parameters like an integrated photon flux of 4.31 × 1021 1/s × m and an integrated power density of 1000 W/m2 (100 mW/cm2), distributed over an extensive range of wavelengths (280–4000 nm) needed for the characteristics of SCs. The P3HT layer: [60]PCBM could absorb, at best, 44.3% of the available power and 27% of the available photons. Regardless of this, the real effectiveness value for an organic solar cell founded on P3HT:[60]PCBM doesn’t exceed 5% (Dennler et al., 2009).
To further enhance the effectiveness of SCs, it is essential to develop donor polymers that engage light in an even lengthier wavelength region than P3HT, that is, the absorption limit should lie at wavelengths larger than 700 nm. Such polymers must have a bandgap (the difference in the energies of the HOMO and LUMO (lowest unoccupied molecular orbital) of less than 2 eV (Al-Ibrahim et al., 2005).
The number of identified donor polymers giving acceptable light transformation efficiency in PV cells is still little. In addition to the synthesis of novel polymers, work is also underway to attain new fullerene compounds for the drive of utilizing them rather than the [60]PCBM in PV cells.
In this regard, the goal of our chapter was the development of novel acceptor components founded on modified fullerene donors and С60, founded on soluble derivatives of polyaniline for usage in organic SCs.
2.2. DESCRIPTION OF NOVEL ORGANIC
et al., 2003; Haraguchi, 2011). For example, a new DBA or donor-bridge- acceptor kind conjugated block copolymer structure had been effectively characterized, synthesized, and SCs founded on the new materials had been preliminarily verified displacing better performance of the block copolymer structure vs the donor/acceptor simple blend structure. Additionally, dye- sensitized polymer and inorganic/organic hybrid nanostructured SCs were also examined as dyes absorb more sunlight photon and had extra available gaps and energy levels that could better match the solar spectrum than old SCs (Eo et al., 2012, 2014).
Extensive usage of inorganic-founded solar cell technology as an alternate energy source is still restricted due to the high energy consumption or high cost related to the elaborate fabrication procedures involving high vacuum, raised temperature as well as a recent shortage of feedstock materials (Biglova et al., 2015). Polymeric and organic SCs provide many competitive benefits, comprising appropriate fine turning of materials chemical structures, materials durability, energy gaps (Eg), frontier orbitals (HOMOs and LUMOs), and low cost and flexibility for solution-based huge- scale industrial fabrications and processing, comprising well-established polymer solution printing methods or an R2R (roll-to-roll) tinny film processing protocol (Miftakhov et al., 2014). Additionally, polymeric, and organic semiconductors display much greater optical absorption coefficients likened to their inorganic counterparts, therefore opening opportunities for the production of very tinny solar films or sheets that could protect a large number of materials (Biglova et al., 2017). The existing best-reported polymer-founded solar cell has a power transformation efficiency of around 8–10% under one Sun at AM 1.5, the cells normally contain a mixture of donor kind polymer with an acceptor (usually fullerene derivatives) (Mehrotra et al., 1997; Nayak et al., 1997). Though fullerenes are not cost- effective so far, the morphology (like donor/acceptor phase domain size and ordering) of a physical mixture is not easy to control (Kondratiev et al., 2011).
The critical achievement factors for polymeric and organic SCs comprise the improvement of photon capture through energy gap engineering, specifically in the most concentrated sunlight radiations of 1–2 eV, and the development of charge generation and transport through polymer morphology engineering, as it is now obvious that the photo-induced charge detachment is crucially affected by the donor/acceptor domain size, and charge mobility is crucially affected by the polymer morphology (Otero et al., 2012).
The complete photoelectric power transformation efficiency of a polymeric/organic solar cell is determined by at least the following five critical steps (Tarver et al., 2009):
• Exciton generation or photon capture;
• Exciton dispersion to donor/acceptor interface;
• Carrier generation or exciton dissociation at the donor/acceptor interface;
• Carrier diffusion to specific electrodes; and
• Carrier gathering by the respected electrodes.
Till yet, none of the five steps had been improved, which accounts for the comparatively low power transformation efficiencies of OPVs (the best stated/announced efficiency is around 10%). Though all these five steps could be and *must be improved through systematic enhancements in materials design, processing, synthesis, and fabrications (Borole et al., 2004, 2006).
In the first step of photon capture (also happening in the photosynthesis of natural plants), a basic need is that the materials optical excitation energy gap (Eg) must look at the incident photon energy. In most organic materials, the energy gap defaults to the difference between the LUMO and the HOMO frontier orbital levels (Zade and Bendikov, 2006).
For terrestrial solar cell applications, it is appropriate that the energy gaps of the extent of the material a broad range of 1.0 to 2.0 eV. Many extensively utilized conjugated semiconducting polymers utilized in the organic solar cell had energy gaps larger than 2.0 eV. This is because the photon capture for organic PV cells is yet required to be optimized at AM 1.5. This ‘photon loss’ issue is very usual in most of the presently reported OPV materials. One benefit of organic materials is the processability of their gaps and energy levels through molecular design and synthesis. Thus, sufficient opportunity exists for development (Abdrakhmanov et al., 1988;
Hadziioannou and Malliaras, 2006).
The existing main performance barriers for PSCs could be attributed to the three main losses comprising the photogenerated ‘exciton loss,’ the photogenerated ‘carrier loss,’ and the sunlight ‘photon loss,’ (Schmechel and Von Seggern, 2004). As mentioned previously, the most concentrated sunlight at the surface of the earth is in a photon energy extent of 1.02.0 eV, however, most usual stable conjugated polymers (like PPVs) had optical excitation energy gaps above 2 eV, so the ‘photon loss’ is the first
performance obstacle that needs to be resolved. Moreover, since sunlight is very comprehensive radiation with photon energy extending from UV to IR, a tandem style consecutively connected and parallel set cell structure with energy gaps steadily sliding from UV to IR along the light propagation direction is projected to yield very high-power transformation efficiency, as this would allow a broad capture of maximum solar photons and at the same time the summation of immersed photovoltage (Salikhov et al., 2007, 2008; Gadiev et al., 2011). Thus, the development of a diversity of sunlight energy matched (1.0–2.0eV), stable, chain-end functional, processable, and cost-effective conjugated polymer blocks and their joined block copolymers are crucial.
A high effectiveness photoelectric conversion not only needs efficient photon capture but also needs a donor/acceptor bicontinuous nanophase detached morphology for efficient charge carrier transport and exciton dissociation For example, a nanodomain sized ‘honeycomb” shaped column morphology seems to be able to decrease both the photon and exciton losses simultaneously (Sze and Ng, 2006). The major challenge is the synthetic chemistry that could covalently relate an acceptor conjugated block with a donor conjugated block through a non-conjugated bridge unit. Slight organic donor-bridge-acceptor structures had been comprehensively studied and revealed effective photo-induced charge separations. However, the small organic molecular DBA structures suffer from charge transport harms for solar cell device applications because of the absence of bicontinuous nanophase detached morphology. For this purpose, a (DBAB)n-type conjugated block copolymer structure has been effectively developed and revealed by us previously (Fowler, 1928). Precisely, a PV device comprised of a (DBAB)n-type block copolymer tinny film exhibits a substantial performance enhancement over its conforming D/A blend or donor/
acceptor under identical situations, where the donor is a ROPPV (oxyalkyl derivatized poly-p-phenylenevinylene) conjugated block, the acceptor is a sulfone-alkyl derivatized PPV conjugated block, and the bridge is a flexible and non-conjugated unit. XRD and AFM displayed nano phased well- organized packing in (DBAB)n block copolymer that was distracted in the D/A blend. We thus attribute the optoelectronic enhancement to the block copolymer inherent molecular self-assembly and nanophase morphology that consequences in the decrease of the carrier and exciton losses (Antoniadis et al., 1993; Biglova et al., 2017).
However, one restriction of the (DBAB)n structure is that the molecular size distribution (polydispersity) must be exactly slim to make good long-
range well-organized packing as displayed in the top scheme of Figure 2.2.
If the acceptor blocks or the donor blocks had extensive molecular size distribution, it would be tough to form ordered domain packing and size- matched in long ranges, merely small molecular mass fractions at a small domain might be probable. However, if a DBA-type block copolymer or donor-bridge-acceptor is developed, even if the acceptor blocks or the donor blocks might have extensive molecular size distributions, they would still be capable to pack nicely because of the free space or volume surrounding every block as displayed in the middle scheme of Figure 2.2 (Salikhov et al., 2013a). Moreover, additional acceptor or donor blocks added to the DBA system could further support the self-assembly of the DBA structure, this is because DBA could act as a backbone or surfactant or facilitate or guide the self-assembly of acceptor and donor blocks as shown in the lower scheme of Figure 2.2 (Torosyan et al., 2012; Patra et al., 2014).
Figure 2.2: Structure of potential solid-state stacking patterns of DBA and (DBAB) n-type block copolymers.
Source: https://www.sciencedirect.com/science/article/pii/
S1876610214013782.
2.3. CARBON NANOTUBE/POLYMER NANOCOMPOSITES
While numerous efforts are presently being followed in the field of donor polymer engineering, the acceptor material also is enhanced. In the photoactive layer, the conducting polymer donates to the photoconduction procedure and works as the exciton generation point, however, carbon nanotubes, a carbon allotrope same as the fullerenes, are also stated to contribute to photoconduction (Freitag et al., 2003).
The discovery of CNTs in 1991 by Ijima produced great interest in discovering the properties of CNTs and their applications. It was found that the integration of CNTs in the organic conducting polymers could increase the performance of the polymer PV cell. The research constantly enhances the performance (for example, short circuit current, open-circuit voltage, and efficiency) of the CNT conducting polymer merged PV cell.
CNTs could be applied to two significant features of OPV devices:
a transparent conductive layer to substitute ITO and the active layer as a combination with conductive polymers. When utilized as a composite material in the active layers (ALs), CNTs could improve device efficiency by numerous orders of magnitude. When light is excelled on an OPV, excitons produced in the conducting polymer diffuse by the polymer to reach the CNT
= conducting polymer junction. The final works as an exciton dissociation midpoint and the holes and electrons are transported to the electrodes by the conducting polymer and CNTs, respectively. Even though CNTs enhance the characteristics of OPVs, the power conversion effectiveness of the CNT and conducting polymer PV cell is still little related to that of the inorganic PV cell. The cause for this low effectiveness is that most of the excitons produced in the conducting polymer recombine earlier to reaching the CNT
= conducting polymer junction because of their small diffusion lengths (Figure 2.3) (Velasco-Santos et al., 2005; Min et al., 2010).
Figure 2.3: The improvement of the near-field near the nanoparticle surface.
Source: https://pubmed.ncbi.nlm.nih.gov/20168344/.
Decreasing the recombination rate of holes and electrons could enhance the efficiency of this PV cell, which is accomplished through depositing a very tinny (on the order of a rare 100 nanometers), consistently dispersed
CNT = conducting polymer photoactive layer. The CNTs utilized should be extremely crystalline and free from metal contaminations to avert the creation of charge traps in the photoactive layer. For the best performance of a PV cell, the CNTs should be consistently dispersed in the polymer, and it had been noticed that the polymer chains cover the walls of CNTs producing heterojunction alongside the wall of CNTs. This is essential for effective charge dissociation at the CNT = polymer junction due to the low exciton diffusion length. Another benefit of well-dispersed CNTs is the creation of percolation routes by CNTs through the composite, which increases electron transportation and decreases the recombination frequency of charges in the active layer. Saini et al. (2009) had presented that the integration of MWCNTs in the P3HT matrix could drastically enhance the morphological, thermal, optical, structural, and electrical properties of P3HT. Furthermore, Petra et al. (2014) confirmed that the existence of MWCNTs in P3HT could influence the dielectric conduct of P3HT polymer, in addition to the rise in melting temperatures and crystallization (Dalmas et al., 2005; Gulotty et al., 2013).