Potential Substitutes for Replacement of Lead in Perovskite Solar Cells: A Review.

Lead halide perovskites have displayed the highest solar power conversion efficiencies of 23% but the toxicity issues of these materials need to be addressed. Lead-free perovskites have emerged as viable candidates for potential use as light harvesters to ensure clean and green photovoltaic technology. The substitution of lead by Sn, Ge, Bi, Sb, Cu and other potential candidates have reported efficiencies of up to 9%, but there is still a dire need to enhance their efficiencies and stability within the air. A comprehensive review is given on potential substitutes for lead-free perovskites and their characteristic features like energy bandgaps and optical absorption as well as photovoltaic parameters like open-circuit voltage (V OC), fill factor, short-circuit current density (J  SC), and the device architecture for their efficient use. Lead-free perovskites do possess a suitable bandgap but have low efficiency. The use of additives has a significant effect on their efficiency and stability. The incorporation of cations like diethylammonium, phenylethyl ammonium, phenylethyl ammonium iodide, etc., or mixed cations at different compositions at the A-site is reported with engineered bandgaps having significant efficiency and stability. Recent work on the advancement of lead-free perovskites is also reviewed.

Effectiveness of Solvent Vapor Annealing over Thermal Annealing on the Photovoltaic Performance of Non-Fullerene Acceptor Based BHJ Solar Cells.

We explore two small molecules containing arms of dicyano-n-hexylrhodanine and diathiafulvalene wings terminated with benzothiadiazole linker, denoted as BAF-4CN and BAF-2HDT, respectively, as small molecule non-fullerene acceptors (SMNFAs) in organic solar cells. The proposed materials are mixed with a low band gap polymer donor PTB7-Th having broad absorption in the range of 400-750 nm to form solution-processed bulk heterojunctions (BHJs). The photoluminescence (PL) measurements show that both donor and acceptor can quench each other's PL effectively, implying that not only electrons are transferred from PTB7-Th → SMNFAs but also holes are transferred from SMNFAs → PTB7-Th for efficient photocurrent generation. Furthermore, solvent vapor annealing (SVA) processing is shown to yield a more balanced hole and electron mobility and thus suppresses the trap-assisted recombination significantly. With this dual charge transfer enabled via fine-tuning of end-groups and SVA treatment, power conversion efficiency of approximately 10% is achieved, demonstrating the feasibility of the proposed approach.

D-π-A-π-D Structured Diketopyrrolopyrrole-Based Electron Donors for Solution-Processed Organic Solar Cells.

Solution-processable D-π-A-π-D structured two organic small molecules bearing thienyl diketopyrrolopyrrole (TDPP) and furanyl diketopyrrolopyrrole (FDPP) as central acceptor units and cyano on the π-bridge and phenothiazine as the terminal donor units, coded as TDPP-PTCN and FDPP-PTCN, are designed and synthesized. The C-H arylation and Suzuki coupling protocols have been adopted for synthesizing the molecules. Solution-processed organic solar cells (OSCs) were constructed with these molecules as the donors and phenyl-C71-butyric acid methyl ester as the acceptor yielding power conversion efficiencies (PCE) of 4.0% for FDPP-PTCN and 5.2% for TDPP-PTCN, which is the highest PCE reported so far from the small molecular DPP-phenothiazine-based architecture for solution-based OSCs. The effect of heteroatom substitution on thermal stability and optoelectronic and photovoltaic performances is also systematically investigated herein. This work demonstrates that replacement of oxygen with sulfur in these kinds of small molecules remarkably improves the photovoltaic performance of OSCs.

A simple fluorene core-based non-fullerene acceptor for high performance organic solar cells.

A small molecule non-fullerene acceptor based on a fluorene core having a furan π-spacer and end capped with rhodanine (FRd2) is developed for solution processable bulk heterojunction organic solar cells (OSCs). The simplistic synthetic protocol reduces several reaction steps and hence production cost. Extended π-conjugation via furan units and the presence of electronegative rhodanine groups result in a power conversion efficiency of 9.4% in OSCs, which is the highest so far among these categories of molecules.

Dithienogermole-based solution-processed molecular solar cells with efficiency over 9.

A molecular donor of intermediate dimensions based on dithienogermole (DTG) as the central electron rich unit, coded as DTG(FBT2Th2)2, was designed and synthesized for use in bulk heterojunction, solution-processed organic solar cells. Under optimized conditions, a maximum power conversion efficiency (PCE) of 9.1% can be achieved with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor semiconductor component.