Lightweight and flexible solar cells are highly interesting materials for use in new applications, such as spacecraft, aircraft and personal pack load. PbS colloidal quantum dots (CQDs) exhibit a broad and strong light absorption spectrum covering the ultraviolet-visible-near infrared region, allowing for incorporation of very thin CQD films into solar cells with high power conversion efficiency (PCE) from solar light to electricity. Herein, we report an extremely lightweight and ultra-flexible CQD solar cell constructed on a polyethylene naphthalate substrate with a thickness of 1.3 mu m. A solution-processed Ag nanowire network with excellent mechanical, optical and electrical properties was prepared as the front-electrode in the solar cell. The thickness of the complete CQD solar cell is less than 2 mm, and similar to 10% PCE with a weight of 6.5 g m(-2) is achieved, resulting in a power-per-weight output of 15.2 W g(-1). The flexible solar cell possesses durable mechanical properties and maintains high-level photovoltaic performance under extreme deformation and after repeated compression-stretching deformation. Moreover, the flexible CQD solar cell shows impressive stability both under continuous illumination and after storage under ambient conditions. These results reveal that solution-processed CQDs are compatible with an ultra-flexible substrate for the construction of ultra-lightweight infrared light-converting CQD solar cells with possibilities for new exciting solar energy applications.

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.

A facile heat-up synthesis route is used to synthesize environmentally friendly Ag₂S colloidal quantum dots (CQDs) that are applied as light absorbing material in solid state p-i-n junction solar cell devices. The as-synthesized Ag₂S 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/TiO₂/Ag₂S 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 Ag₂S 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.

A hot-injection synthesis method was used to synthesize low-toxicity Ag2S 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 Ag2S CQDs have high potential within solar cell applications, and point the direction for further improvements.