The evolution and emergence of organic solar cells and hybrid organic-silicon heterojunction solar cells have been deemed as promising sustainable future technologies, owing to the use of π-conjugated polymers. In this regard, the scope of this review article presents a comprehensive summary of the applications of π-conjugated polymers as hole transporting layers (HTLs) or emitters in both organic solar cells and organic-silicon hybrid heterojunction solar cells. The different techniques used to synthesize these polymers are discussed in detail, including their electronic band structure and doping mechanisms. The general architecture and principle of operating heterojunction solar cells is addressed. In both discussed solar cell types, incorporation of π-conjugated polymers as HTLs have seen a dramatic increase in efficiencies attained by these devices, owing to the high transmittance in the visible to near-infrared region, reduced carrier recombination, high conductivity, and high hole mobilities possessed by the p-type polymeric materials. However, these cells suffer from long-term stability due to photo-oxidation and parasitic absorptions at the anode interface that results in total degradation of the polymeric p-type materials. Although great progress has been seen in the incorporation of conjugated polymers in the various solar cell types, there is still a long way to go for cells incorporating polymeric materials to realize commercialization and large-scale industrial production due to the shortcomings in the stability of the polymers. This review therefore discusses the progress in using polymeric materials as HTLs in organic solar cells and hybrid organic-silicon heterojunction solar cells with the intention to provide insight on the quest of producing highly efficient but less expensive solar cells.

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α-cellulose isolated from African star apple seed shell (ASASS) was used as the raw material for the production of food-grade carboxymethyl cellulose (CMC). Preparation of CMC from ASASS cellulose was carried out by an etherification process, using sodium hydroxide and monochloroacetic acid (MCA) at various temperature. The cellulose and CMC were characterized by FTIR, XRD and SEM techniques. Degree of substitution (DS) was determined using potentiometric titration. Solubility increased with DS of CMC increased together with varied conditions but decreased at a certain level. The optimum results of varied conditions were 0.8015 DS for 30% NaOH concentrations; 0.6259 DS for 50℃ temperature and 0.9254 DS for the amount of MCA. The FTIR spectra revealed the functional group modification of native cellulose to CMC, with absorptions at-1-1 1409.49 cm and 1640.05 cm indicating ether and carboxyl functional group respectively. The results of XRD analysis showed that crystallinity of isolated cellulose decreased in the CMC and SEM showed the elongated long fibres in cellulose while the fibrils in CMC are short and rough. The obtained product is well suited for pharmaceutical and food additives.

The hexathienylbenzene-co-poly(3-hexylthiophene-2,5diyl) (HTB-co-P3HT) conducting polymer was synthesized by oxidative co-polymerization of hexathienylbenzene (HTB) and 3-hexylthiophene using iron chloride (FeCl3) as an oxidant. The effect of chlorobenzene, toluene and chloroform on the optoelectronic characteristics of the polymer was investigated. The study revealed that spectroscopic and electrochemical responses of HTB-co-P3HT are affected by the nature of the solvent. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of HTB-co-P3HT were determined from cyclic voltammetry (CV) and were compared to those of (6,6)-Phenyl C71 butyric acid methyl ester (PC71BM) and it was found that the LUMO energy levels of HTB-co-P3HT in toluene were lower than those for chlorobenzene and chloroform. The electrochemical impedance spectroscopy (EIS) analysis also revealed the thin film of HTB-co-P3HT prepared using toluene as the most conductive. However, the photovoltaic parameters of the HTB-co-P3HT organic photovoltaic cells (OPVs) departed from the favored toluene and noted chlorobenzene as being the advantageous solvent. We obtained a power conversion efficiency (PCE) of 0.48%, fill factor (FF) of 27.84%, current density (JSC) of 4.93 mA.cm−2 and open circuit voltage (VOC) of 0.35 V in chlorobenzene, a PCE of 0.30%, FF of 26.08%, JSC of 5.00 mA.cm−2 and VOC of 0.23 V in chloroform and finally, a PCE of 0.33%, FF of 25.45%, JSC of 5.70 mA.cm−2 and VOC of 0.23 V in toluene.