A study published in Materials studied the use of 3D printing technology to fabricate electron transport layers for photovoltaic devices.

To study: Inkjet Printed Electron Transport Layers for Perovskite Solar Cells. Image Credit: Thongsuk7824 / Shutterstock.com

3D printing is one of the key emerging technologies of the 21^st century. Recently, 3D printing has been explored extensively for the fabrication of components and materials in solar cells, another key technology that helps address the climate change emergency.

Electron transport layers

Electron transport layers are crucial components in perovskite photovoltaic solar cell devices. They capture the photo-generated electrons and transport them to the conductive contact layer. The most widely used material for electron transport layers in perovskite solar cells is TiO₂, due to its superior thermal and chemical stability, low cost, abundance and chemical robustness.

In addition, its conduction band aligns well with perovskite materials.

(a) Top view ; (b) SEM images in cross section; and (vs) XRD model of an inkjet printed c-TiO₂ lying down. Image credit: Lu, D et al., Materials

The preferred structure for a TiO₂ the electron transport layer is a bilayer of a compact TiO₂ film and a mesoporous TiO₂ lying down. This morphology makes it possible to achieve a power conversion efficiency greater than 25%. However, challenges exist with this material which affects its performance under prolonged periods of UV lighting, as light can degrade it. Additionally, inefficient charge transfer results in charge recombination and hysteresis problems.

Many strategies have been proposed and studied to overcome these problems, with varying degrees of success. These include doping, graphene / TiO fabrication₂ composite materials, interface engineering and surface passivation.

Not all of these strategies effectively suppress the phenomenon of sweep direction hysteresis. For this reason, alternative transport materials are being explored as they can provide solutions to these challenges.

Alternative electron transport materials

Recent studies have demonstrated the improved charge transport properties of alternative materials such as SrTiO₃ and SnO₂. SrTiO₃ displays a bandgap similar to TiO₂ and good alignment of the bands with the perovskites. Recombination problems are reduced due to the high electronic mobility of the material.

Studies have shown that due to the larger size of SrTiO₃ nanoparticles, electron transport layer materials based on them exhibit improved open-circuit voltage and lower short-circuit density.

However, there are some challenges with SrTiO₃. Graphene / SrTiO₃ composite electron transport layers compensate for the low current density of the material. Designing perovskite solar cells with compact SrTiO₃The electron transport layers based on smaller particles improve their stability and improve electron transport.

Other strategies such as doping have been explored to improve the stability and photovoltaic performance of solar cells with these electron transport layers.

Cross-sectional SEM images of the device (a) S1, (b) S2, (vs) S3, (D) S4, (e) S5, and (F) spin coating, TiO₂based on PSC. Image credit: Lu, D et al., Materials

SnO₂ possesses favorable chemical and electrical properties which make it an attractive alternative material for electron transport layers. It has a deep conduction band and high electronic mobility. These two properties improve the extraction and transport of electrons from the perovskite layer.

In addition, it retains its stability under prolonged UV illumination and has a wide bandgap. Recent studies have explored techniques such as doping and the use of gradient interleaves to improve the stability and photovoltaic performance of SnO.₂based on perovskite solar cells.

Using 3D printing to fabricate electron transport layers of perovskite solar cells

3D printing is a technology that has gained prominence in recent years in a variety of industries due to its low cost, durability, ease of operation, part customization, and manufacturing processes. manufacturing without additives. 3D printing processes are easily scalable to industrial levels.

Using 3D printing for the fabrication of large-scale perovskite solar cells can reduce waste and consumption of raw materials. It is much more durable and cost effective than traditional spin coating methods.

3D printing has been widely explored in recent years for the fabrication of perovskite solar cell components and materials, including perovskite electrodes and absorbers. In the study published in Materials, the authors specifically focused on 3D inkjet printed electron transport layers.

Study identified a lack of studies on inkjet printed TiO₂ electron transport layers for photovoltaic solar cells, very few studies on 3D printed SnO₂ electron transport layers and no literature on the printed SrTiO₃ layers.

(a) PCE, (b) VOC, (vs) JSC, and (D) FF of mp-SrTiO based PSCs₃ ETL inkjet printed with SrTiO₃ nanoparticle inks of different concentrations. Data are obtained from 9 devices of each type. Image credit: Lu, D et al., Materials

The authors specify that to their knowledge, only one study was carried out on 3D printed SnO.₂ electron transport layers. The best reported device efficiency was 18.8%. Therefore, there is an eminent research possibility to increase the number of printed electron transport layers in the future.

The study presented various 3D printing processes for electron transport layers that will improve their properties. Areas explored include optimization of ink design, film uniformity and device performance. In addition, the importance of the solvents used to facilitate the dispersion of nanoparticles was investigated in the study. A key research finding was that co-solvent inks exhibit beneficial drying properties that help create uniform film formation.

The authors said that the electron transport layer printing processes developed in this research will be used in future work that will explore the use of fully 3D printed perovskite solar cells.

Further reading

Lu, D et al. (2021) Inkjet Printed Electron Transport Layers for Perovskite Solar Cells [online] Materials 14 (24) | mdpi.com. Available at: https://www.mdpi.com/1996-1944/14/24/7525

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