Solution-processed Thin Film Solar Cells Research Articles

Stabilized Perovskite Quantum Dot Solids via Nonpolar Solvent Dispersible Covalent Ligands.

The ligand exchange procedure of CsPbI3 perovskite quantum dots (PQDs) enables the fabrication of thick and conductive PQD solids that act as a photovoltaic absorber for solution-processed thin-film solar cells. However, the ligand-exchanged CsPbI3 PQD solids suffer from deterioration in photovoltaic performance and ambient stability due to the surface traps, such as uncoordinated Pb2+ sites on the PQD surface, which are generated after the conventional ligand exchange process using ionic short-chain ligands dissolved in polar solvents. Herein, a facile surface stabilization is demonstrated that can simultaneously improve the photovoltaic performance and ambient stability of CsPbI3 PQD photovoltaic absorber using covalent short-chain triphenylphosphine oxide (TPPO) ligands dissolved in a nonpolar solvent. It is found that the TPPO ligand can be covalently bound to uncoordinated Pb2+ sites and the nonpolar solvent octane can completely preserve the PQD surface components. Owing to their synergetic effects, the CsPbI3 PQD photovoltaic absorber stabilized using the TPPO ligand solution dissolved in octane exhibit higher optoelectrical properties and ambient stability than the control absorber. Consequently, CsPbI3 PQD solar cells composed of PQD photovoltaic absorbers fabricated via surface stabilization strategy provide an improved power conversion efficiency of 15.4% and an enhanced device stability.

Interface Engineering in Solution-Processed Thin-Film Solar Cells

ConspectusAlong with the rapid industrialization of human society over the past century, incessant energy consumption and endless damage to the environment have aroused growing attention for seeking clean and renewable energy sources. Photovoltaics (PV) that can directly harvest and transform sunlight into electricity have shown great potential in achieving this goal. Especially for solution-processed thin-film solar cells, their extremely cost-effective and facile processing methods compatible with different substrates at large scales exhibit unique advantages over conventional PVs based on crystalline silicon. Various types of solution-processed thin-film PVs have been achieving or already exceeded 15% power conversion efficiency (PCE) through the numerous efforts of researchers. Organic solar cells (OSCs) and organic–inorganic hybrid perovskite solar cells (PVSCs) are the most well-known emerging solution-processed thin-film solar cells that have attracted great interest recently (the PCE of PVSCs soared form 3.8% to over 25% in the past decade). Usually, photogenerated excitons will form as a response to illumination in the active layer, then dissociate into charge carriers, travel in between layers, and finally get collected by electrodes of the device. Besides the broad exploration of active layer materials, suitably matched charge-transporting layers and electrodes also play a vital role in achieving high PCE and stability in PV devices. Furthermore, interfaces between different functional layers created during solution processing need to be carefully addressed to ensure efficient charge transport and prevent degradation. The utilization of proper interfacial materials to modify the chemical and electrical properties at interfaces has become an effective strategy to enhance the performance of PV. Therefore, it is important to develop a comprehensive understanding into the correlation between interfacial properties and charge carrier dynamics and establish molecular design principles for interfacial materials to realize commercialization of emerging PVs.In this Account, we first introduce the fundamental roles of interfaces in PVs, including the modulation of film formation, together with management of charge transport and recombination. Detailed analysis of interfaces and related surface science are also discussed to provide better understanding. Then, we highlight our research on interfacial materials with different functionalities including self-assembled monolayers, dopants, functional molecules for post-treatment, and composite materials. Combining with the fundamental mechanisms and design criteria of interfacial materials, new materials and interface engineering strategies can be integrated into other PV systems to achieve high PCE and stability. In the end, the main challenges and future perspectives toward the commercialization of solution-processed thin-film PVs are discussed to inspire more novel material designs and interface engineering.

Microstructural and Nanostructural Evolution of Light Harvester Perovskite Thin Film under the Influence of Ultrasonic Vibrations.

A key step of inexpensive and scalable perovskite thin-film formation is defect-free fabrication through low-cost and facile post-treatment processes. Methods using high annealing temperatures are not favorable for the scale-up of solution-processed thin-film solar cells, particularly on plastic/flexible substrates. This contribution analyzes the effect of ultrasonic vibrations, a recently developed low-cost post-treatment process, on thin-film quality. Ultrasonic vibrations were applied to as-spun CH3NH3PbI3 perovskite thin films prepared with various solvents and antisolvents deposited on substrates with compact and mesoporous textures. Then, mechanisms of solvent evaporation, nucleation, and crystallization of perovskite grains were characterized during ultrasonic vibration. These studies demonstrate that ultrasonic vibration at low temperature facilitates heterogeneous crystallization of perovskite grains with a higher conversion of nuclei into crystal, compared with the conventional annealing process. Topographic scanning electron microscopy images confirm the dense and fully covered thin films after the evaporation of solvent. Furthermore, it is shown that crystal orientation does not change with the choice of solvent, eliminating the effect of solvent on the deposition of thin-film perovskites with this method. Therefore, this ultrasonic vibration post-treatment method is applicable to any solution-processed material and deposition technique, and it can be used to fabricate a range of thin-film devices and printed electronics.

Solution-Processed CdTe Thin-Film Solar Cells Using ZnSe Nanocrystal as a Buffer Layer

The CdTe nanocrystal (NC) is an outstanding, low-cost photovoltaic material for highly efficient solution-processed thin-film solar cells. Currently, most CdTe NC thin-film solar cells are based on CdSe, ZnO, or CdS buffer layers. In this study, a wide bandgap and Cd-free ZnSe NC is introduced for the first time as the buffer layer for all solution-processed CdTe/ZnSe NC hetero-junction thin-film solar cells with a configuration of ITO/ZnO/ZnSe/CdTe/MoOx/Au. The dependence of the thickness of the ZnSe NC film, the annealing temperature and the chemical treatment on the performance of NC solar cells are investigated and discussed in detail. We further develop a ligand-exchanging strategy that involves 1,2-ethanedithiol (EDT) during the fabrication of ZnSe NC film. An improved power conversion efficiency (PCE) of 3.58% is obtained, which is increased by 16.6% when compared to a device without the EDT treatment. We believe that using ZnSe NC as the buffer layer holds the potential for developing high-efficiency, low cost, and stable CdTe NC-based solar cells.

On the interface between kesterite absorber and Mo back contact and its impact on solution-processed thin-film solar cells

Thin-film solar cells with a kesterite-type Cu2ZnSnSxSe4−x absorber have been prepared via a two-step process based on a metal salt solution with subsequent annealing in Se-containing nitrogen-atmosphere. Particular attention is paid to the interface between absorber and molybdenum, where the formation of secondary phases could be proven by a model experiment. To inhibit this formation, different back contact configurations of Mo and TiN have been tested. It is shown that TiN is an effective chalcogen barrier and avoids the formation of MoSxSe2−x, but influences the crystallinity of the absorber layer in a negative way, thereby also decreasing the open-circuit voltage. An effect on short-circuit current density and series resistance was not observed. The best results were obtained with an additional, thin Mo-layer on top of the TiN.

Plasmonic Nanoparticle Enhancement of Solution-Processed Solar Cells: Practical Limits and Opportunities

Solution-processed solar cells are particularly suited to benefit from absorption enhancement by plasmonic nanoparticles, due to their transport-limited film thicknesses and the ease with which metal nanoparticles can be integrated into the materials. Despite practical demonstrations of performance enhancements, the overall benefits have so far been limited in scope to photocurrents well below the theoretical limits. In this Perspective, we critically evaluate the prospects for plasmonic enhancements in solution-processed thin-film solar cells. We give an overview of recent work, focusing on embedded plasmonic nanoparticles in organic, perovskite, and colloidal quantum dot solar cells. We then develop an intuitive effective medium model for embedded plasmonic nanostructures in photovoltaic thin films, evaluate the model in the context of previous results in the field, and use the model to provide a framework for identifying the most promising avenues for realizing plasmonic performance enhancements in sol...

Three dimensional web-like fibrous CuInS2 film

To ensure effective collection of photo-generated minority electrons in a CuInS2 thin film solar cell, a three-dimensional nanostructure was constructed by electrospinning a copper-indium solution precursor onto a molybdenum substrate. The electrospun nanofibers adhered onto the underlying layers before undergoing sulfurization at 500°C to produce a three-dimensional web-like fibrous CuInS2 film. This fibrous film was characterized by scanning electron microscopy, X-ray diffraction, and Auger electron spectroscopy, thereby characterizing the 3D fibrous nanostructure of the CuInS2 film. This 3D structure could lead to next-generation solar cells optimized for maximal charge separation to enhance photovoltaic efficiency of solution-processed thin film solar cells.