Recently, perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency (25.2%) and low fabrication cost. However, the short lifetime under operation is the major obstacle for their commercialization. With efforts from the entire PSC research community, significant advances have been witnessed to improve the device operational stability, and a timely summary on the progress is urgently needed. In this review, we first clarify the definition of operational stability and its significance in the context of practical use. By analyzing the mechanisms in established approaches for operational stability improvement, we summarize several effective strategies to extend device lifetime in a layer-by-layer sequence across the entire PSC. These mechanisms are discussed in the contexts of chemical reactions, photo-physical management, technological modification, etc., which may inspire future R&D for stable PSCs. Finally, emerging operational stability standards with respect to testing and reporting device operational stability are summarized and discussed, which may help reliable device stability data circulate in the research community. The main target of this review is gaining insight into the operational stability of PSCs, as well as providing useful guidance to further improve their operational lifetime by rational materials processing and device fabrication, which would finally promote the commercialization of perovskite solar cells.

Towards commercialization: the operational stability of perovskite solar cells

Since 2009, perovskite solar cells (PSCs) have witnessed dramatic developments with the record power conversion efficiency (PCE) exceeding 25% within a single decade. One bottleneck for the commercialization of PSCs is the lack of stability due to the commonly used hole transport materials (HTMs) with dopants. These hygroscopic dopants not only deteriorate the long-term stability due to moisture ingress and ion diffusion, but also increase the complexity and total cost. Therefore, the development of dopant-free HTMs is of great significance. In this work, the recent progress in dopant-free HTMs, including organic small molecules, polymers, organometallic compounds, and inorganic materials, has been systematically reviewed and summarized. To bring current dopant-free HTMs closer to the ideal ones, detailed solutions for making dopant-free HTMs more efficient, stable, and cost-effective are analyzed and discussed. In the end, we try to answer the key scientific questions such as what makes HTMs dopant-free, how to obtain the maximum PCE for a new dopant-free HTM, and what is the most efficient way to find ideal dopant-free HTMs based on the literature survey and our own understanding. With continuous efforts in developing dopant-free HTMs, PSC commercialization can be realized in the near future.

Toward ideal hole transport materials: a review on recent progress in dopant-free hole transport materials for fabricating efficient and stable perovskite solar cells

Organic semiconductors have become essential parts of thin-film electronic devices, particularly as hole transport layers (HTLs) in perovskite solar cells (PSCs) where they represent one of the major bottlenecks to further enhancements in both device stability and efficiency. Small molecule 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) and polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) are two of the first successful HTLs used in PSCs, and have remained at the forefront of developing high efficiency devices for almost a decade. Since their first application, many investigations into the properties of spiro-OMeTAD and PTAA have contributed to a growing understanding of the mechanisms that enable their success as HTLs. This review summarizes and discusses the key electronic and morphological properties, doping strategies and mechanisms, and degradation pathways of both spiro-OMeTAD and PTAA. A critical comparison between the two materials is provided, highlighting both the similarities which explain their enduring popularity as well as key differences in electrical and morphological properties. From this analysis emerges an improved understanding of the fundamental properties that enable the persistent success of HTL materials, which are found to include not only hole conductivity, band gap, and morphology, but also interactions with dopants, the perovskite, and environmental stressors. The knowledge about these properties, which are critically summarized in this review, is also applicable to the many other types of organic electronic devices now employing spiro-OMeTAD and PTAA. A detailed examination of the properties of materials reveals a clear set of guiding principles for the development of future generation HTLs. Applying these design strategies to produce more advanced HTLs will be essential to further improve the stability, efficiency, and commercialization of PSCs.

/pdf/lessons-learned-from-spiro-ometad-and-ptaa-in-perovskite-2io5qbmlgt.pdf

Lessons learned from spiro-OMeTAD and PTAA in perovskite solar cells

Excess lead iodide (PbI2 ), as a defect passivation material in perovskite films, contributes to the longer carrier lifetime and reduced halide vacancies for high-efficiency perovskite solar cells. However, the random distribution of excess PbI2 also leads to accelerated degradation of the perovskite layer. Inspired by nanocrystal synthesis, here, a universal ligand-modulation technology is developed to modulate the shape and distribution of excess PbI2 in perovskite films. By adding certain ligands, perovskite films with vertically distributed PbI2 nanosheets between the grain boundaries are successfully achieved, which reduces the nonradiative recombination and trap density of the perovskite layer. Thus, the power conversion efficiency of the modulated device increases from 20% to 22% compared to the control device. In addition, benefiting from the vertical distribution of excess PbI2 and the hydrophobic nature of the surface ligands, the modulated devices exhibit much longer stability, retaining 72% of their initial efficiency after 360 h constant illumination under maximum power point tracking measurement.

Ligand-Modulated Excess PbI2 Nanosheets for Highly Efficient and Stable Perovskite Solar Cells

Nickel oxide for inverted structure perovskite solar cells

Hole transport layers (HTLs) play a crucial role in the efficiency and stability of perovskite solar cells (PSCs). The most efficient PSCs based on spiro-OMeTAD (Spiro) generally have stability pro...

NiOx/Spiro Hole Transport Bilayers for Stable Perovskite Solar Cells with Efficiency Exceeding 21%

Low-Temperature Oxide/Metal/Oxide Multilayer Films as Highly Transparent Conductive Electrodes for Optoelectronic Devices

Currently, chemical vapor deposition (CVD) has attracted widespread attention for preparing high efficiency and large-scale perovskite solar cells (PSCs). The surface morphology and crystallinity of precursor lead iodide (PbI2) films play a critical role in the subsequent CVD process. Here, we use a self-formed PbI2-dimethyl sulfoxide adduct in precursor PbI2 for fabricating a high quality perovskite film. The results show that a larger grain size and excessive PbI2 in the perovskite film effectively suppress carrier recombination, which is helpful to improve the device stability and efficiency. It is noted that the device maintained an excellent stability with almost the same efficiency after 1000 h (in a dry N2 glovebox, 25 °C). Finally, the PSC by using the CVD method exhibits a high power conversion efficiency of 17.9% with a bandgap of 1.60 eV, an open voltage (VOC) of 1140 mV, a short-circuit current density (JSC) of 20.1 mA/cm2, and a fill factor of 78%. This method provides guidance for preparing large-area solar cells or applications in tandem solar cells.

Self-formed PbI2-DMSO adduct for highly efficient and stable perovskite solar cells

High-Performance and Stable Perovskite-Based Photoanode Encapsulated by Blanket-Cover Method

Interface engineering has played an increasingly essential role in the development of perovskite solar cells (PSCs). Herein, we adopted an effective and simple one-step interface passivation method on a FA-based perovskite to fabricate efficient and stable planar PSCs. The surface defects are reduced by the perovskite interface passivation layer incorporated between the hole transport and perovskite absorber layers, and then non-radiative recombination is suppressed while interfacial carrier extraction is enhanced. The passivated planar PSCs demonstrates 20.83% power conversion efficiency (PCE), which is caused by the simultaneous enhancement of the fill factor and open-circuit voltage. In addition, the device also shows great ambient and thermal stability. It retains 94% of its original PCE after 1000 h under ambient air without encapsulation as well as 90% of its initial efficiency after 400 h under continuous heating at 65 °C with encapsulation. This research provides a strategy for the development of efficient and stable PSCs.

Xinghua Cui

Papers

NiOx/Spiro Hole Transport Bilayers for Stable Perovskite Solar Cells with Efficiency Exceeding 21%

Low-Temperature Oxide/Metal/Oxide Multilayer Films as Highly Transparent Conductive Electrodes for Optoelectronic Devices

Self-formed PbI2-DMSO adduct for highly efficient and stable perovskite solar cells

High-Performance and Stable Perovskite-Based Photoanode Encapsulated by Blanket-Cover Method

I/P interface modification for stable and efficient perovskite solar cells