High-performance inorganic-organic hybrid perovskite solar cells

Abstract

Inorganic-organic hybrid perovskite solar cells (PSCs) have experienced a rapid development in the past few years and turned over a new leaf in solar cell research due to their high power conversion efficiency (PCEs), easy fabrication and low cost. Since the first report of the material used in solar cells in 2009, the power conversion efficiency of PSCs has now reached 22.7%, approaching the efficiencies of single crystalline silicon solar cells. The semiconducting inorganic-organic hybrid perovskites have shown great potential for photovoltaic applications because of their excellent optoelectronic and charge transport properties. In this thesis, the applications of CVD graphene as transparent electrodes in PSCs were firstly introduced. Semitransparent solar cells that can absorb light from both sides are promising for some special applications, such as building-integrated photovoltaics and tandem solar cells. However, the performance of semitransparent PSCs has been limited by the quality of transparent top electrodes. Semitransparent PSCs were fabricated by laminating stacked CVD graphene as top transparent electrodes on perovskite layers for the first time. The device performance was optimized by improving the conductivity of the graphene electrodes and the contact between the top graphene electrodes and the hole transport layer (HTL) on the perovskite films. The devices with double-layer graphene electrodes show the maximum PCEs of 12.02±0.32% and 11.65±0.35% from FTO and graphene side, respectively, which are relatively high compared with the reported semitransparent PSCs. This work indicates that CVD graphene is an ideal candidate for transparent electrodes of PSCs. Considering its excellent mechanical flexibility and convenient preparation, graphene electrodes are expected to be used in flexible PSCs by printing or roll to roll process, which may find applications to complement the rigid inorganic solar cells currently dominated in the market. Therefore, graphene is an ideal transparent-electrode material that can be used in various types of solar cells. Secondly, the utilization of ultrathin black phosphorus (BP) flakes as an effective interfacial layer between perovskite and HTL was studied. The interface between perovskite and HTL is crucial to the photovoltaic performance of PSCs. In this thesis, few-layer BP flakes were introduced as buffer layer at the perovskite/HTL interface. Most of the BP nanoflakes were found to be distributed at the perovskite grain boundary regions, demonstrating efficient charge transport and defect passivation effect. This structure offers better band alignment, improves the hole transport property and reduces the recombination loss at the interface, thus the PCEs were greatly enhanced by over 10%. This enables more reproducible device performances reaching a stabilized power output of ~19%. Besides, the device stability was also significantly improved due to the passivation effect of the BP flakes. This study opens up the avenue in using solution exfoliated thin BP flakes for interfacial engineering of photovoltaic devices. Besides, we have also introduced the utilization of laser annealing as an alternative approach to the traditional thermal annealing process for the crystallization of perovskite films. A continuous-wave laser diode with controllable output power was attached on an X-Y moving system, and the downward laser beam can scan through the perovskite surface line by line under the control of the computer program. Interestingly, it was found that the laser scanning parameters, including the laser power, laser wavelengths and scanning speeds, were closely related with the crystallization and morphology of the perovskite films. After systematical optimization, high-quality perovskite films with better crystallinity, larger grain size and lower density of defects were successfully fabricated. As a result, a high open circuit voltage of 1.15 V and a champion efficiency of 20.08% with small hysteresis was obtained with a 450-nm laser operating with an output power of 150 mW and scanning speed of 25 mm/min. The stability of devices fabricated with this laser-annealing approach was also greatly improved due to the improved crystallinity and morphology of the perovskite films. This preliminary work paves a way to prepare high-quality perovskite films with controllable morphology.