Polymer solar cells have a wide range of advantages, and their bulk heterojunction formations, especially the solution-processed type, contribute to improved power conversion efficiency. Traditionally, PEDOT:PSS is used as a hole transport layer in an polymer solar cells (PSCs) owing to its potential advantages such as ease in processability, smooth surface topography, and matching work function with HOMO of many donor-type organic semiconductors. But the acidity and hygroscopic nature of the PEDOT:PSS makes it inefficient electron-blocking performance, and thus negatively contribute to the performance and stability of PSCs under ambient conditions. The transparent metal oxides (TMOs) are being used to replace PEDOT:PSS requires expensive and complicated vacuum based processes, the toxicity and scarcity of some materials such as NiOx and VOx makes it immaterial for PSCs application. So it is essential to focus on materials that are based on earth abundant elements, low-cost, nontoxic and environmentally stable HTMs for highly efficient OSCs.
Professor Sudip Batabyal and colleagues at Amrita University in India demonstrated the capability of applying solution-processed copper cadmium tin sulfide nanoparticles as low-cost inorganic hole transporting materials in polymer solar cells, and studied the effect of particles structure on the functioning of the cells. Their research is published in Solar Energy Materials and Solar Cells
The authors prepared 4 different samples by spin coating a cernyite solution on an indium tin oxide substrate and varied the number of layers in the samples from 1 to 4. A reference cell was fabricated using poly(3,4-ethylenedioxythiophene): polystyrenesulfonate as the hole transporting layer.
Structural features of the particles showed that they had a size of about 7-12nm, and a tetragonal structure. The band gap values for the as-deposited thin films were 1.65eV for 1 layer, 1.60eV for 2 layers, and 1.35eV for both 3 and 4 layers. This shows that the band gap values decrease with an increase in thickness for the as deposited thin films, which confirms that particles agglomerate and their sizes increase with an increase in layer thickness. Further, the absorption spectrum of the as-deposited thin films was observed to cover the entire visible spectrum and increases with an increase in thickness, which shows the nanoparticles have an additional contribution in photocurrent.
The authors calculated the surface roughness values for the samples which were 23.54 nm for 1 layer, 13.11 nm for 2 layers, 11.07 nm for 3 layers, and 21.00 nm for 4 layers. The as-deposited thin film having 3 layers had the minimum surface roughness which implies a more uniform coating over its surface.Although increasing the number of layers improves the compactness in the thin films, it reaches a point where particles aggregate which in effect deters transportation of charge carriers and thus reduces the device’s overall performance. Therefore, the 3 layers were observed to be the optimum number for use of the nanoparticles as a hole transporting layer.
The research team also noted that the solar cell power conversion efficiency improves with an increase in thickness of the as developed thin film and then it decreases significantly. This power conversion efficiency is comparable to that of the poly(3,4-ethylenedioxythiophene): polystyrenesulfonate hole transporting layer. Furthermore, the sample having 3 layers of the nanoparticles had the highest power conversion efficiency as compared with the other samples.This is as a result of the uniform thickness of the film and its compactness, which generates an optimum interface as well as improves the dynamics of the charge transfer.
Comments from Authors:
Metal sulfide nanomaterials have drawn drastic attention because of their exotic electronic properties and high specific surface areas that are potentially useful in photovoltaic applications. For the first time, the compound chalcogenides were used as a hole transport layer in a polymer solar cells. In addition to the transporting property of the p-type buffer layer it is also expected in generation of excitons resulting in increased photo generated currents. The inorganic materials are known for its stability than the organic materials and in future it is interested to work in this direction.
Suresh Kumar, KallolMohanta, Sudip K. Batabyal. Solution processed Cu2CdSnS4 as a low-cost inorganic hole transport material for polymer solar cells. Solar Energy Materials and Solar Cells, 161 (2017) 157-161.Go To Solar Energy Materials and Solar Cells