光電所所訊

High Efficient Planar Heterojunction Perovskite Solar Cells via Sandwich Evaporation Technique

Professor Ching-Fuh Lin’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所林清富教授

Due to the gradual depletion of fossil fuels, the energy issue has received wide attention in this century. The world is making many efforts to develop alternative energy. Solar energy is an inexhaustible and readily available resources on earth. On the top of that, it is eco-friendly to our environment. For these advantages, the development of solar cells has become an excellent strategy to solve the shortage of energy. Among many types of solar cells, perovskite solar cells suddenly appear on the horizon and the conversion efficiency has been made significant progress from 3% to above 20% in just six years.

Although high-efficiency perovskite solar cells have been achieved, perovskite formed by solution process, in general, is very susceptible to the atmosphere and moisture. The perovskite grains usually become small and messy by the rapid formation of perovskite. In this research, we developed a new method using a homemade chamber, called Sandwich Evaporation Technique (SET), to form the perovskite layer, as shown in Fig.1. The process can be mostly conducted in the atmosphere and not be affected by the environment easily. The process includes the following steps. First, we raised a sandwich-like structure, MAI/PbI₂/MAI, to fabricate the active layer of perovskite to enhance the current density, as shown in Fig. 2. Second, the perovskite solar cells are fabricated with the traditional structure of ITO/PEDOT-4083/Perovskite/PCBM/PEI/Ag, which can successfully achieve the power conversion efficiency of 14.03%.

Fig. 1. The picture of the homemade chamber of Sandwich Evaporation Technique (SET) process.

Fig. 2. The picture of Sandwich Evaporation Technique (SET) process.

Furthermore, we modify those techniques that can make the better film of the active layer. IPA, CB have been applied on the substrates to slow down the reaction. Here this mechanism is refined for the SET process. Vials with solvent are placed inside the chamber directly. With the refined process, we are able to achieve 14.62% of perovskite solar cells with a higher current density and large crystals of perovskite, as shown in Fig. 3. This brand new method to fabricate the perovskie under atmosphere environment could potentially enable the mass production in the future using the Sandwich Evaporation Technique (SET), as shown in Fig.4.

Fig. 3. The SEM image of the perovskite.

Fig. 4. The schematic diagram of the Sandwich Evaporation Technique (SET).

Improved Internal-Quantum Efficiency of GaN-Based LEDs

Professor Chieh-Hsiung Kuan

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所管傑雄教授

Instead of the limited exposure feature sizes of conventional photolithography, the ELIONIX ELS-7000 E-beam lithography system and the wet-etching technology were conducted to carry out the investigation. All patterned-sapphire substrates (PSSs) were all arranged in squares with the period of 2000nm and with various post-duty cycles (PDCs) of 1, 4 and 8%, which is defined in Fig. 1(a), respectively. Each square had a hexagonal post. SEM images of accomplished PSSs are shown in Fig. 1(b)-(d). The PSSs and a conventional sapphire substrate (CSS) with polished surface were grown using low pressure metal organic chemical vapor deposition (LP-MOCVD). The light-emitting diodes (LEDs) were encapsulated with TO-46 metal can package with a chip size of 17×34 mil².

Figure 2 shows the residual compressive strain and the associated Raman width of the GaN-based PSS LEDs. With the increase of PDC, the residual compressive strain of the GaN-based PSS LEDs reduces first and then slightly increases as compared to the sample with the CSS. Fig. 3 shows the relative IQE versus PDC of the GaN-based PSS LEDs biased at injection currents of 120mA and 360mA, respectively. With the increase of PDC in contrast to the CSS, the reduced residual compressive strain leads to the enhanced relative IQE under the same crystalline quality.

Fig. 1. The SEM images of different PSSs and the definition of post-duty cycle

Fig. 2. The residual compressive strain with the associated Raman width. The inserted figure is the Raman spectra.

Fig. 3. The relative IQE of the LEDs, calculated from the inserted figure regarding the EQE versus PDC