Inkjet-printed Functional Materials for Perovskite Solar Cells

Abstract

Fabrication of lab-scale perovskite solar cells (PSCs) is dominated by the spin coating method, which wastes most of the precursor materials and is not compatible with large-scale manufacturing of PSCs. Inkjet printing provides a solution to upscaling of the fabrication of PSCs in a low-cost, waste-free, and sustainable way. In this thesis, we demonstrate the effectiveness of the inkjet technique in fabrication of functional materials for PSCs. The printing processes for depositing functional materials, i.e., electron transporting layers (ETLs), perovskite absorber layers as well as hole transporting layers (HTLs), are developed. We also strive to enhance the power conversion efficiency (PCE) of devices using the inkjet-printed ETLs and perovskite layers. Diverse measurements and analysis are conducted to provide insights into the enhancement mechanisms. The work undertaken in this thesis is presented as follows:

The printing processes for depositing TiO₂, SrTiO₃, and SnO₂ ETLs are developed. A cosolvent system is found to be beneficial for the formation of effective ETLs and the eventual device performance. A PCE of 17.37% is realized for the PSC device with an inkjet-printed SnO₂ ETL, outperforming both the SrTiO₃-based (15.73%) and TiO₂-based (12.42%) devices.

SnO_x ETLs are synthesized and deposited via an inkjet printing process. The effects of the annealing temperature for post-processing of the deposited precursor layer on the properties of the resulting SnO_x ETLs and their photovoltaic performance are discussed. The low-temperature amorphous SnO_x ETLs outperform the high-temperature crystalline SnO₂ ETLs, achieving a high PCE of 17.55%.

Elemental doping is conducted to modify SnO_x ETLs. Effects of doping on the properties of the SnO_x ETLs and the ETL/perovskite interfaces are investigated in detail. Cu doping exerts a negative influence on the photovoltaic performance of SnO_x ETLs. Surprisingly, a tunable hysteresis, transforming from normal hysteresis to inverted hysteresis, is observed with increasing Cu doping level. Ce doping leads to substantially improved properties. The incorporation of Ce into SnOx enables increased conductivity, improved energy level alignment at the ETL/perovskite interface, and suppressed recombination within the perovskite layer. The devices with Ce-doped SnO_x ETLs achieve enhanced efficiency compared to the undoped devices. Interface modification is also performed using a bilayer ETL structure to modify the SnO_x/perovskite interface. The effects of inserting a nanoparticle SnO₂ (NP-SnO₂) layer or a nanoparticle SrTiO₃ (NP-STO) layer at the SnO_x/perovskite interface are discussed.

Perovskite films are deposited via inkjet printing under ambient conditions, which is a significant challenge for this humidity-sensitive material. A large-grained perovskite film with full surface coverage is realized using strategies of in-situ heat treatment, self-vapor-annealing treatment, and solvent engineering. The effects of these strategies on the nucleation and crystallization of perovskite films are discussed. A PCE of 13.44% is achieved for the all-inkjet-printed PSC device with an inkjet-printed ETL, an inkjet-printed perovskite layer, and an inkjet-printed HTL. The additive engineering strategy is also applied to hinder premature crystallization of the perovskite materials. The uniformity of the inkjet-printed perovskite layer is significantly improved although it is not directly conducive to photovoltaic performance.

Overall, this thesis provides guidance in fabrication of effective functional materials via inkjet printing in a scalable and sustainable way.

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