Cuncun Wu

The Dawn of Lead‐Free Perovskite Solar Cell: Highly Stable Double Perovskite Cs2AgBiBr6 Film

Abstract Recently, lead‐free double perovskites have emerged as a promising environmentally friendly photovoltaic material for their intrinsic thermodynamic stability, appropriate bandgaps, small carrier effective masses, and low exciton binding energies. However, currently no solar cell based on these double perovskites has been reported, due to the challenge in film processing. Herein, a first lead‐free double perovskite planar heterojunction solar cell with a high quality Cs2AgBiBr6 film, fabricated by low‐pressure assisted solution processing under ambient conditions, is reported. The device presents a best power conversion efficiency of 1.44%. The preliminary efficiency and the high stability under ambient condition without encapsulation, together with the high film quality with simple processing, demonstrate promise for lead‐free perovskite solar cells.

High Crystallization of Perovskite Film by a Fast Electric Current Annealing Process

High-efficiency organic-inorganic hybrid perovskite solar cells have experienced rapid development and attracted significant attention in recent years. Crystal growth as an important factor would significantly influence the quality of perovskite films and ultimately the device performance, which usually requires thermal annealing for 10 min or more. Herein, we demonstrate a new method to get high crystallization of perovskite film by electric current annealing for just 5 s. In contrast to conventional thermal annealing, a homogeneous perovskite film was formed with larger grains and fewer pinholes, leading to a better performance of the device with higher open-circuit voltage and fill factor. An average power conversion efficiency of 17.02% with electric current annealing was obtained, which is higher than that of devices with a conventional thermal annealing process (16.05%). This facile electric current annealing process with less energy loss and time consumption shows great potential in the industrial mass production of photovoltaic devices.

Improving device performance of perovskite solar cells by micro–nanoscale composite mesoporous TiO2

In perovskite solar cells, the morphology of the porous TiO2 electron transport layer (ETL) largely determines the quality of the perovskites. Here, we chose micro-scale TiO2 (0.2 µm) and compared it with the conventional nanoscale TiO2 (20 nm) in relation to the crystallinity of perovskites. The results show that the micro-scale TiO2 is favorable for increasing the grain size of the perovskites and enhancing the light scattering. However, the oversized TiO2 results in an uneven surface. The evenness of the perovskites can be improved by nanoscale TiO2, while the crystallinity and compactness are not as good as those of the films based on micro-scale TiO2. To combine the advantages of both micro-scale and nanoscale TiO2, by mixing 0.2 µm/20 nm TiO2 with a ratio of as the composite ETL, the device average power conversion efficiency was increased to 11.2% from 9.9% in the case of only 20 nm TiO2.

Transparent electrode for top emission organiclight-emitting diode

Organic light-emitting diode (OLED) has been successfully used in mobile display (small area) and television display (large area). For small area display screens, the top emission structure is mostly adopted because of the limitation of display area. Top emission OLED (TEOLED) can get higher power efficiency at the same current density than traditional bottom emission devices because of its higher aperture-opening ratio. However, the most important part of top emission device is the preparation of transparent electrode. In summary, according to the method and material, TEOLED transparent electrode is divided into the following categories: (1) Transparent conductive oxide electrode (TCO); (2) ultrathin composite metal electrode; (3) dielectric/metal/dielectric (DMD) composite electrode; (4) nanomaterial electrodes, etc. The general oxide has great optical transmittance in the visible light range, but the thermal evaporation temperature is very high. It needs to be fabricated by sputtering, which damages the organic layer. The composite metal electrode can be made by thermal evaporation, which is simple and easy. It is adopted by most commercial products by now, however its transparency is relatively low. Nano materials have high transparency and can be fabricated for flexible electrodes, but because of the solution processing, it is easier to destroy the organic layer. DMD composite electrode not only has high transmittance, but also can use thermal evaporation method to form film, it is expected to become better choice of top electrode for TEOLED. In order to achieve the objective of producing top emitting OLED with excellent properties, it is most important to find enough transparent, high conductive, ohmic contact, film forming process without destroying organic layer and stable electrode material. Up to now, the researchers have proposed several methods for preparing barrier layer-ITO, pulsed laser deposition transparent conducting oxide, thermal evaporated ultrathin composite metal, DMD composite, nano-materials, etc. In this paper, the applications of these materials in the TEOLED transparent electrode are reviewed. By comparing the advantages and disadvantages of thin film electrode, the research progress of TEOLED transparent electrode is reviewed.

Solution-Processed Cu9S5 as a Hole Transport Layer for Efficient and Stable Perovskite Solar Cells

Organic-inorganic perovskite solar cells have seen tremendous developments in recent years. As a hole transport material, 2,2,7,7-tetrakis( N, N-di- p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD) is widely used in n-i-p perovskite solar cells. However, it may lead to the perovskite film degradation due to the dopant lithium bis((trifluoromethyl)sulfonyl)amide (Li-TFSI), which has strong hydrophilicity. Cu9S5 is considered as a superior p-type transport material, which also has a favorable energy level matching with the highest occupied molecular orbital of Spiro-OMeTAD. Herein, a solution-processed organic-inorganic-integrated hole transport layer was reported, which is composed of the undoped Spiro-OMeTAD and Cu9S5 layer. Since there is no Li-TFSI doping, it is extremely conductive to the long-term stability of the solar cells. In the meantime, we proposed a method to adjust the lowest unoccupied molecular orbital (LUMO) of SnO2 via nitrogen implantation (N:SnO2). The LUMO of SnO2 can be tuned from -4.33 to -3.91 eV, which matches well with the LUMO of CH3NH3PbI3 (-3.90 eV), and thus helps to reduce hysteresis. The modified hole and electron transport layers were applied in n-i-p perovskite solar cells, which achieve a maximum power conversion efficiency (PCE) of 17.10 and 96% retention of PCE after 1200 h in air atmosphere without any encapsulation.

Ion Implantation-Modified Fluorine-Doped Tin Oxide by Zirconium with Continuously Tunable Work Function and Its Application in Perovskite Solar Cells

In recent years, perovskite solar cells have drawn a widespread attention. As an electrode material, fluorine-doped tin oxide (FTO) is widely used in various kinds of solar cells. However, the relatively low work function (WF) (∼4.6 eV) limits its application. The potential barrier between the transparent conductive oxide electrode and the hole transport layer (HTL) in inverted perovskite solar cells results in a decrease in device performance. In this paper, we propose a method to adjust WF of FTO by implanting zirconium ions into the FTO surface. The WF of FTO can be precisely and continuously tuned between 4.59 and 5.55 eV through different dopant concentration of zirconium. In the meantime, the modified FTO, which had a WF of 5.1 eV to match well the highest occupied molecular orbital energy level of poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonate, was used as the HTL in inverted planar perovskite solar cells. Compared with the pristine FTO electrode-based device, the open circuit voltage increased from 0.82 to 0.91 V, and the power conversion efficiency increased from 11.6 to 14.0%.