In a paper recently published in the open-access journal ACS Applied Energy Materials, researchers studied the usage of polymeric films of polyethylenimine (PEI) for interfacial enhancement of electron-selective contacts. The modification of PEI increased the performance of conventional highly efficient heterojunction solar cells.
Study: Expanding the Perspective of Polymeric Selective Contacts in Photovoltaic Devices Using Branched Polyethylenimine. Image Credit: ZinetroN/Shutterstock.com
A notable achievement in crystalline silicon (c-Si) solar cells has resulted from expanding the existing sources for carrier-selective contacts. Such hole-transport and electron-layers (HTLs and ETLs) based on nitrides, organic polymers, and metal oxides can passivate the silicon surface and eliminate undesirable effects like Fermi-level pinning and Schottky barriers at the metal-semiconductor interface. Conventional techniques, on the other hand, are subject to high temperatures and intricate deposition procedures that include the usage of toxic gases in the production process.
Furthermore, defects introduced during doping contribute to recombination and parasitic absorption in the film. Organic molecule-based carrier-selective contacts have piqued the interest of researchers because of their highly adaptable chemical and electrical characteristics and lower costs of production. Branched polyethylenimine (b-PEI) is an amine-rich cationic polyelectrolyte. The lone electron pair in such amines can be adequately reactive to change the electrical junction’s characteristics, causing field effect and chemical passivation at the surface of the semiconductor.
About the Study
In this study, the team investigated the benefits of b-PEI as an electron-selective contact and proposed a clearer understanding of its functionality incorporating dipole layer conduction mechanisms and their influence on insulator/semiconductor/metal junctions. Additionally, b-PEI was used in a c-Si solar cell trial configuration as the ETL with efficiencies over 14%.
50 wt% b-PEI solution in water was taken as the electron-selective contact. Using ethanol as the solvent, PEI solutions with weight percentages ranging from 0.0001 to 0.1 wt% were produced. The substrates used were polished n-type wafers of c-Si (100) with a 2 Ω·cm resistivity. PEI solutions were spin-coated on silicon wafers for one minute at 5000 rpm, followed by annealing of the films at 80°C for two minutes. The PEI films were grown on bare silicon as well as passivating hydrogenated amorphous silicon (a-Si:H) layers.
Following spin coating, thermal evaporation of aluminum (Al) electrodes with a shadow mask was carried out to produce a pattern resembling the transfer length method (TLM). UV photoelectron spectroscopy (UPS) was utilized to study the change in work function (WF) produced by the PEI layer. To investigate the PEI sheet's electron selectivity, silicon solar cells with five distinct architectures were created. For comparison, the same reference devices excluding the b-PEI layer were also synthesized and studied.
The results showed that the specific resistance at contact was minimum for both non-passivated and passivated samples. The ideal thickness values for minimizing contact resistance were around 1 and 0.4 nm, respectively. Conversely, the non-passivated samples’ rapidly growing specific contact resistance over 1 nm indicated electron tunneling as the major conduction process.
Following PEI deposition, the lifetime of minority carriers for both passivated and bare c-Si samples was nearly unchanged. However, after applying a thin, semi-transparent Al capping, a significant rise by one order of magnitude was found. The lifetime increased with the semi-transparent layer of Al, suggesting that the metallic electrode plays a vital role in improving surface passivation.
The UPS spectra of PEI-capped samples showed that the silicon WF related to the interface dipole had decreased. The solvent also affected the UPS spectra. When compared to ethanol, the PEI solution in methanol resulted in an extra 0.15 eV shift in the WF value. XPS measurements revealed double and single carbon-oxygen bonds, which were associated with the creation of aldehyde and ethanolate groups when ethanol lost its proton.
Due to the WF alteration, the valence band displacement seen by XPS also hinted toward electron accumulation. This enhanced the MDS junction's performance as a layer for electron transport. Furthermore, the PEI-modified cathodes were employed in a dopant-free, proof-of-concept solar cell.
The efficiency of solar cells with direct contact with aluminum/silicon was rather low, with a very low fill factor (FF), open circuit voltage (Voc), and a high short-circuit current density (Jsc). The introduction of a thin intrinsic amorphous silicon film as an interlayer between the polymer and silicon improved the FF by 2% while retaining the same Voc and Jsc values. In this situation, the excellent a-Si:H passivation could enhance interfacial electron accumulation, further improving the contact quality. Overall, dopant-free, selective contact silicon solar cells showed conversion efficiencies of up to 14.2%.
To summarize, the researchers used b-PEI as a selective contact in the c-Si semiconductor/metal/insulator junctions. As a result, it was inferred that the modification of PEI causes a significant charge transfer to the silicon substrate from the Al contact. Simultaneously, electron accumulation favored electrical current transmission and reduced specific contact resistance.
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Ros E. et al., Expanding the Perspective of Polymeric Selective Contacts in Photovoltaic Devices Using Branched Polyethylenimine, ACS Applied Energy Materials, 2022, doi: https://pubs.acs.org/doi/10.1021/acsaem.2c01422
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