The inorganic perovskite CsPbI3 that exists in the form of a quantum dot (QD) shows a stable cubic structure, attracting much attention for its application in solar cells. However, too many grain boundaries in the perovskite QD (PQD) layer block the transport of carriers, resulting in the potential loss of solar cells. Herein, we devise a gradient-band alignment (GBA) homojunction, which is constructed from three layers of PQDs with different band-gaps to form a gradient energy alignment. The GBA structure facilitated the charge extraction and increased the carrier diffusion length of the PQD layer because of the additional driving force for the electrons. In addition, the homojunction made from the same substance could minimize the lattice mismatch of the active layer. As a result, the champion solar cell based on the GBA homojunction layer achieved a high open voltage V OC of 1.25 V and a power conversion efficiency (PCE) of 13.2%.

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Potassium thiocyanate as a cheap additive effectively eliminates the hysteresis effect of perovskite solar cells.

The mismatched energy-level alignment and interface defects of the SnO2 nanoparticles’ electron transport layer (ETL) and perovskite layer worsen the efficiency of the perovskite solar cell. Herein, we devise a multiple-function surface engineering of SnO2 nanoparticles. TBA+ ions improve the dispersion and stability of colloidal T-SnO2 nanoparticles and act as a bridge between the ETL and perovskite layer through the electrostatic interaction with anions, thus suppressing the charge recombination and reducing the energy loss. I– ions passivate oxygen vacancies of SnO2 nanoparticles but also halide vacancies of the perovskite layer. Furthermore, the conduction band edge of T-SnO2 is enhanced to match the energy alignment with the perovskite, which reduces the energy offset for electron transfer. As a result, the champion solar cell based on T-SnO2 presented a power conversion efficiency of 21.71% with a V OC of 1.15 V and negligible hysteresis, which are much higher than those of the reference device.

Tin oxide (SnO2) nanocrystals‐based electron transport layer (ETL) has been widely used in perovskite solar cells due to its high charge mobility and suitable energy band alignment with perovskite, but the high surface trap density of SnO2 nanocrystals harms the electron transfer and collection within device. Here, an effective method to achieve a low trap density and high electron mobility ETL based on SnO2 nanocrystals by devising a difunctional additive of potassium trifluoroacetate (KTFA) is proposed. KTFA is added to the SnO2 nanocrystals solution, in which trifluoroacetate ions could effectively passivate the oxygen vacancies (OV) in SnO2 nanocrystals through binding of TFA− and Sn4+, thus reducing the traps of SnO2 nanocrystals to boost the electrons collection in the solar cell. Furthermore, the conduction band of SnO2 nanocrystals is shifted up by surface modification to close to that of perovskite, which facilitates electrons transfer because of the decreased energy barrier between ETL and perovskite layer. Benefiting from the decreased trap density and energy barrier, the perovskite solar cells exhibit a power conversion efficiency of 21.73% with negligible hysteresis.