It could successfully control the band-bending of the perovskite semiconductor, which led to improvement of the photovoltaic performance.

To abate the issue of moisture-assisted phase transition of CsPbI 2 Br, caused by hygroscopic dopants used in the hole-transporting material (HTM), developing dopant-free HTMs is necessary. In this work, a new polymer, PDTDT, is developed as a dopant-free HTM for CsPbI 2 Br solar cells, and the device performance and stability are systematically compared with cells employing dopant-free P3HT. CsPbI 2 Br solar cells using PDTDT show an efficiency of 17.36% with V OC of 1.42 V and FF of 81.29%, which is one of the highest values for CsPbI 2 Br cells. Moreover, a record-high efficiency of 34.20% with V OC of 1.14 V under 200 lux indoor light illumination and efficiency of 14.54% (certified efficiency of 13.86%) for a 1 cm 2 device under one sun are accomplished. Importantly, PDTDT shows superior/comparable device stability to P3HT, promising its potential to be an alternative to popular doped Spiro-OMeTAD and P3HT HTM.

The emerging CsPbI3 perovskites are highly efficient and thermally stable materials for wide‐band gap perovskite solar cells (PSCs), but the doped hole transport materials (HTMs) accelerate the undesirable phase transition of CsPbI3 in ambient. Herein, a dopant‐free D‐π‐A type HTM named CI‐TTIN‐2F has been developed which overcomes this problem. The suitable optoelectronic properties and energy‐level alignment endow CI‐TTIN‐2F with excellent charge collection properties. Moreover, CI‐TTIN‐2F provides multisite defect‐healing effects on the defective sites of CsPbI3 surface. Inorganic CsPbI3 PSCs with CI‐TTIN‐2F HTM feature high efficiencies up to 15.9 %, along with 86 % efficiency retention after 1000 h under ambient conditions. Inorganic perovskite solar modules were also fabricated that exhibiting an efficiency of 11.0 % with a record area of 27 cm2. This work confirms that using efficient dopant‐free HTMs is an attractive strategy to stabilize inorganic PSCs for their future scale‐up.

Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3 +, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.

Organometal halide perovskites have demonstrated remarkable achievements in solar cell applications and have attracted tremendous attention as next‐generation photovoltaic materials. Regardless of the unprecedented success, the degradation of the perovskite has caused the performance of the perovskite solar cells to be unreliable and prevented their commercialization. However, the detailed degradation mechanism of the perovskite has yet to be elucidated. In this study, the entire procedure of the thermally induced degradation of methylammonium lead iodide (MAPbI3) is reported using real‐time in situ transmission electron microscopy. The in situ investigation directly illustrates the detailed process of precipitating trigonal PbI2 grains during thermal degradation and confirms that trigonal PbI2 is precipitated from the amorphized MAPbI3 layer via intermediate states. The intermediate states and their stackings enable the generation of 3D linear‐empty spaces that can be utilized as passages by elements during the decomposition and intercalation of the perovskite. This report will provide critical clues for the commercialization of the perovskite‐based solar cells and for further investigation of the synthesis of the perovskite, which is not fully understood.

In the inverted structure perovskite solar cells, a buffer layer is generally used at the interface between the n-type semiconductor layer and the metal electrode, but its design guidelines have not yet been established. Here, a series of inverted perovskite solar cells have been fabricated with the controlled thickness of bathocuproine (BCP) buffer layers deposited by thermal evaporation and validated the BCP buffer layer evaluation tool. The ideal factor was calculated from the gradient in the plot of Voc against the log of Jsc, and the effect of the BCP buffer layer on charge recombination was verified. Since the ideal factor greatly decreased from 5 to 1.4 by introducing the BCP buffer layer, it was confirmed that the interface between the n-type semiconductor layer and the metal electrode gradually changed from a Schottky barrier diode to an ohmic contact. On the other hand, it was found that an excessive BCP film thickness causes the series resistance to increase and induced recombination. Finally, as a result of optimizing the perovskite layer and the BCP buffer layer, respectively, the performance exceeding 17% was obtained. This study provides insight into the improvements in the conversion efficiency of perovskite solar cells by optimizing the thickness of the buffer layer using the ideal factor.

Co-sensitized dye-sensitized solar cells using black dye and a pyridine-anchor dye (NI5 or YNI-2) showing site-selective adsorption behaviour at the TiO2 surface have been prepared for the first time to reduce the competitive adsorption between the two dyes.

Perovskite and textured silicon solar cells were integrated into a tandem solar cell through a stacking method. To consider the effective structure of silicon solar cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of two- and four-terminal structures by perovskite layers on crystal silicon wafers. The reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7 to 0.8%), which resulted in increases in average incident photon to current conversion efficiency values (from 83.0 to 88.0%) and current density (from 13.7 to 14.8 mA/cm). Using the texture surface of silicon heterojunction (SHJ) solar cells, the significant conversion efficiency of 21.4% was achieved by four-terminal device, which was an increase of 2.4% from that of SHJ solar cells alone.