Polymer design for solar cell – Current trend and future scenario

Materials for Photovoltaics: State of Art and Recent Developments

In recent years, photovoltaic cell technology has grown extraordinarily as a sustainable source of energy, as a consequence of the increasing concern over the impact of fossil fuel-based energy on global warming and climate change. The different photovoltaic cells developed up to date can be classified into four main categories called generations (GEN), and the current market is mainly covered by the first two GEN. The 1GEN (mono or polycrystalline silicon cells and gallium arsenide) comprises well-known medium/low cost technologies that lead to moderate yields. The 2GEN (thin-film technologies) includes devices that have lower efficiency albeit are cheaper to manufacture. The 3GEN presents the use of novel materials, as well as a great variability of designs, and comprises expensive but very efficient cells. The 4GEN, also known as "inorganics-in-organics", combines the low cost/flexibility of polymer thin films with the stability of novel inorganic nanostructures (i.e., metal nanoparticles and metal oxides) with organic-based nanomaterials (i.e., carbon nanotubes, graphene and its derivatives), and are currently under investigation. The main goal of this review is to show the current state of art on photovoltaic cell technology in terms of the materials used for the manufacture, efficiency and production costs. A comprehensive comparative analysis of the four generations is performed, including the device architectures, their advantages and limitations. Special emphasis is placed on the 4GEN, where the diverse roles of the organic and nano-components are discussed. Finally, conclusions and future perspectives are summarized.

Computer-Aided Screening of Conjugated Polymers for Organic Solar Cell: Classification by Random Forest

Owing to the diverse chemical structures, organic photovoltaic (OPV) applications with a bulk heterojunction framework have greatly evolved over the last two decades, which has produced numerous organic semiconductors exhibiting improved power conversion efficiencies (PCEs). Despite the recent fast progress in materials informatics and data science, data-driven molecular design of OPV materials remains challenging. We report a screening of conjugated molecules for polymer-fullerene OPV applications by supervised learning methods (artificial neural network (ANN) and random forest (RF)). Approximately 1000 experimental parameters including PCE, molecular weight, and electronic properties are manually collected from the literature and subjected to machine learning with digitized chemical structures. Contrary to the low correlation coefficient in ANN, RF yields an acceptable accuracy, which is twice that of random classification. We demonstrate the application of RF screening for the design, synthesis, and characterization of a conjugated polymer, which facilitates a rapid development of optoelectronic materials.

The Effect of Aromatic Diimide Side Groups on the π-Conjugated Polymer Properties

The presented study describes the method for the synthesis and characterization of a new class of conjugated copolymers containing a perylenediimide (PDI) and naphthalene diimide (NDI) side groups. The main conjugated backbone is a donor-acceptor polymer poly[3,6-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] containing thiophene and carbazole as donor units and benzothiadiazole as an acceptor unit. The presented compounds were synthesized in a multistep synthesis. The polymerization was carried out by Suzuki or Stille coupling reaction. Redox properties of the studied polymers were tested in different conditions. Electrochemical investigation revealed independent reduction of the main polymer chain and diimide side groups. UV-Vis spectroscopy revealed the overlap of two absorption spectra. The difference between the electron affinity of the polymer main chain and that of the diimides estimated electrochemically is approximately 0.3 eV.

Light-induced charge separation in a P3HT/PC70BM composite as studied by out-of-phase electron spin echo spectroscopy

A composite material of semiconducting polymer P3HT and fullerene derivative PC₇₀BM was studied by means of electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal was observed under laser irradiation of the composite at low temperature. We assume that during the charge separation process firstly the spin-correlated radical pairs in the singlet-polarized spin state are formed, and then the net polarization of radical pairs arises due to spin evolution. Both types of polarizations contribute to the out-of-phase ESE signal in the case of non-ideal microwave pulses. Analytical calculation of the echo shape for both types of initial polarization revealed that the contribution of the net polarization becomes zero after averaging over the whole EPR spectrum of the radical pair. This behavior was experimentally confirmed; thus the analysis of the out-of-phase ESE signal was simplified. Interspin distance distributions in the charge transfer state were obtained by modeling the out-of-phase ESE envelope modulation measured at different delays after laser flash T_DAF from 300 ns to 3.3 μs at a temperature of 65 K. Due to geminate recombination and diffusion of the radicals from the interface the distribution becomes significantly broader with larger distances prevailing at longer T_DAF values. The average distance between charges increases from 3.5 nm to 5.6 nm with an increase in T_DAF.

Two-acceptor one-donor random terpolymers comprising thiophene- and phenyl-capped diketopyrrolopyrrole for organic photovoltaics

A series of random terpolymers comprising two electron deficient phenyl (PDPP) and thiophene (ThDPP)-capped diketopyrrolopyrrole (DPP) in conjugation with the electron-donating thiophene moiety are synthesised using Stille coupling.

Synthesis of Dithienocyclohexanones (DTCHs) as a Family of Building Blocks for π-Conjugated Compounds in Organic Electronics

The development and widespread application of organic electronic devices require the availability of simple and cost-effective suitable materials. In this study, the preparation of a new class of conjugated compounds on the basis of a dithienocyclohexanone (DTCH) core is reported. Several synthetic strategies for the preparation of dialkyl DTCH derivatives are explored, with special emphasis on the establishment of a sustainable synthetic access. Two successful synthetic pathways, both consisting of five steps, are identified: the first one featuring readily available 3-thiophenecarboxaldeyde and the second one 3-ethynylthiophene as the starting materials. Both procedures are characterized by reasonably high overall yields (over 30%) and remarkably low E factors (<400). Preliminary evidences of the use of such building blocks in the micellar Suzuki-Miyaura cross-coupling reactions leading to promising molecular semiconductors are also given. Moreover, on a small molecule containing DTCH moiety, solar cell performance was investigated.

Recent Advances in Wide-Bandgap Photovoltaic Polymers

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

Spin dynamics of light-induced charge separation in composites of semiconducting polymers and PC60BM revealed using Q-band pulse EPR

Q-Band electron spin echo spectroscopy allows distinguishing light-induced polarons of different types in photovoltaic polymer/fullerene composites.