Solution‐processed colloidal quantum dot (CQD) solar cells harvesting the infrared part of the solar spectrum are especially interesting for future use in semitransparent windows or multilayer solar cells. To improve the device power conversion efficiency (PCE) and stability of the solar cells, surface passivation of the quantum dots is vital in the research of CQD solar cells. Herein, inorganic CsPbI3 perovskite (CsPbI3‐P) coating on PbS CQDs with a low‐temperature, solution‐processed approach is reported. The PbS CQD solar cell with CsPbI3‐P coating gives a high PCE of 10.5% and exhibits remarkable stability both under long‐term constant illumination and storage under ambient conditions. Detailed characterization and analysis reveal improved passivation of the PbS CQDs with the CsPbI3‐P coating, and the results suggest that the lattice coherence between CsPbI3‐P and PbS results in epitaxial induced growth of the CsPbI3‐P coating. The improved passivation significantly diminishes the sub‐bandgap trap‐state assisted recombination, leading to improved charge collection and therefore higher photovoltaic performance. This work therefore provides important insight to improve the CQD passivation by coating with an inorganic perovskite ligand for photovoltaics or other optoelectronic applications.

Broader contextUltra-flexible and lightweight solar cells with high power output per weight have attracted much attention due to their high potential for utilization in applications such as spacecraft, aircraft, personal pack load and wearable electronic devices. PbS colloidal quantum dots (CQD) are promising candidates for the fabrication of flexible and lightweight solar cells due to their nanocrystal character, which enables functioning energy conversion even in the case when the solar cell is under extreme deformation. Moreover, the PbS CQD possesses the advantages of solution-processability, size-dependent optoelectronic properties and a broad light absorption spectrum covering the ultraviolet-visible-near infrared wavelength region. In this study, we report an ultra-flexible and extremely lightweight PbS CQD solar cell. The solar cell is fabricated on a 1.3 mm-thick flexible polyethylene naphthalate foil substrate and an Ag nanowire network with strong mechanical properties and a large aspect ratio and is used as a transparent and conductive front-electrode. The thickness of the full solar cell is less than 2 mm and the device gives B10% power conversion efficiency with an extremely low weight of 6.5 g m À2 , resulting in a high power-per-weight output ofThe demonstrated CQD solar cell shows good mechanical properties and works during large compression-stretching deformation. In particular, the solar cell also exhibits promising stability both under continuous illumination and after storage under ambient conditions. These results reveal that the CQDs are very promising materials for realizing flexible, efficient and extremely lightweight solar cells that makes it possible for utilization of solar energy in many new applications.

In this work, high-performance four-terminal solution-processed tandem solar cells were fabricated by using dyesensitized solar cells (DSSCs) as top-cells and lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs) as bottom-cells. For dye-sensitized top-cells, three different dye combinations were used while the titanium dioxide (TiO 2 ) scattering layer was removed to maximize the transmission. For the PbS bottom-cells, quantum dots with different sizes were compared. Over 12% power conversion efficiency has been achieved by using the XL dye mixture and 890 nm PbS QDs, which shows a significant efficiency enhancement when compared to single DSSC or CQDSC subcells.

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Recent progress in colloidal quantum dot (CQD)-based solar cells indicates that low-toxicity materials such as AgBiS 2 nanocrystals (NCs) show potential in replacing toxic PbS and CdS CQDs in solar cell applications. In this study, an investigation on the importance of the composition and sensitivity toward synthesis conditions was performed by adjusting concentrations and ratios of Ag and Bi precursorsfirst, by varying the ratio of Ag toward Bi precursors and, second, by varying the concentration of Ag with a constant ratio toward Bi precursors in the solution. Furthermore, elemental XPS studies and TEM imaging together with solar cell analysis indicated a strong correlation between the concentration of Ag precursor and the NC properties and, moreover, the solar cell properties based on these NCs. In short, a large amount of Ag precursor resulted in smaller Ag-rich NCs, which resulted in solar cells with high photovoltage but low photocurrent density, while a lower amount of Ag precursor resulted in larger NCs and solar cells with a lower photovoltage. The Ag:Bi:S ratio of 0.72:0.9:1 resulted in almost stoichiometric NCs but with a slight excess of Ag, which in turn resulted in solar cells with the highest performance. This work therefore gives insight into how the elemental composition and size of the NCs can be tuned by the precursor ratios and how this, in turn, affects the performance of the solar cell devices.

A hot-injection synthesis method was used to synthesize low-toxicity Ag 2 S colloidal quantum dots (CQDs) with strong and broad light absorption as an ultra-thin photo-absorber in CQD heterojunction solar cells. By using iodide and sulfur linkers it was possible to accomplish efficient charge carrier extraction, resulting in a high photocurrent due to the broad absorption spectrum. Transient photovoltage decay measurements were used to obtain information about trap states in the CQDs and the effect on the lifetime of the photoinduced carriers. The devices show very promising stability under constant long-term illumination and they are stable under ambient storage conditions with low losses to the performance over a period of over two months. These results show that Ag 2 S CQDs have high potential within solar cell applications, and point the direction for further improvements.[a] V.

A facile heat-up synthesis route is used to synthesize environmentally friendly Ag 2 S colloidal quantum dots (CQDs) that are applied as light absorbing material in solid state p-i-n junction solar cell devices. The as-synthesized Ag 2 S CQDs have an average size of around 3.5 nm and exhibit broad light absorption covering ultraviolet, visible, and near infrared wavelength regions. The solar cell devices are constructed with a device architecture of FTO/TiO 2 /Ag 2 S CQDs/hole transport material (HTM) /Au using a solution-processed approach. Different HTMs, N2,N2,N2 ,N2 ,N7,N7,N7 ,N7 -octakis(4-methoxyphenyl)-9,9 -spirobi(9H-fluorene)-2,2 ,7,7 tetramine (spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl) (P3HT), and poly((2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl)-2,5-thiophenediyl) TQ1 are studied for maximizing the device photovoltaic performance. The solar cell device with P3HT as a hole transport material gives the highest performance and the solar cell exhibit broad spectral absorption. These results indicate that Ag 2 S CQD have high potential for utilization as environmentally friendly light absorbing materials for solar cell application and that the hole transport material is critical to maximize the solar cell photovoltaic performance.