Efficient and Stable Low-Bandgap Perovskite Solar Cells Enabled by a CsPbBr3-Cluster Assisted Bottom-up Crystallization Approach.

Recently, low-bandgap formamidinium lead iodide FAPbI3-based perovskites are of particular interest for high-performance perovskite solar cells (PSCs) due to their broad spectral response and high photocurrent output. However, to inhibit the spontaneous α-to-δ phase transition, 15-17% (molar ratio) of bromide and cesium or methylammonium incorporated into the FAPbI3 are indispensable to achieve efficient PSCs. In return, the high bromide content will increase bandgap and narrow the spectral response region. If simply reducing the bromide content, the corresponding PSCs exhibit inferior operational stability due to α-to-δ phase transition, interface degradation, and halide migration. Herein, we report a CsPbBr3-cluster assisted vertically bottom-up crystallization approach to fabricate low-bromide (1% ∼ 6%), α-phase pure, and MA-free FAPbI3-based PSCs. The clusters, in the size of several nanometers, could act as nuclei to facilitate vertical growth of high quality α-FAPbI3 perovskite crystals. Moreover, these clusters can show further intake by perovskite after thermal annealing, which improves the phase homogeneity of the as-prepared perovskite films. As a result, the corresponding mesoporous PSCs deliver a champion efficiency of 21.78% with photoresponse extended to 830 nm. Moreover, these devices show remarkably improved operational stability, retaining ∼82% of the initial efficiency after 1,000 h of maximum power point tracking under 1 sun condition.

Dissolved-Cl2 triggered redox reaction enables high-performance perovskite solar cells.

Constructing 2D/3D perovskite heterojunctions is effective for the surface passivation of perovskite solar cells (PSCs). However, previous reports that studying perovskite post-treatment only physically deposits 2D perovskite on the 3D perovskite, and the bulk 3D perovskite remains defective. Herein, we propose Cl2-dissolved chloroform as a multifunctional solvent for concurrently constructing 2D/3D perovskite heterojunction and inducing the secondary growth of the bulk grains. The mechanism of how Cl2 affects the performance of PSCs is clarified. Specifically, the dissolved Cl2 reacts with the 3D perovskite, leading to Cl/I ionic exchange and Ostwald ripening of the bulk grains. The generated Cl- further diffuses to passivate the bulk crystal and buried interface of PSCs. Hexylammonium bromide dissolved in the solvent reacts with the residual PbI2 to form 2D/3D heterojunctions on the surface. As a result, we achieved high-performance PSCs with a champion efficiency of 24.21% and substantially improved thermal, ambient, and operational stability.

Ion-Diffusion Management Enables All-Interface Defect Passivation of Perovskite Solar Cells.

Perovskite solar cells (PSCs) have demonstrated over 25% power conversion efficiency (PCE) via efficient surface passivation. Unfortunately, state-of-the-art perovskite post-treatment strategies can solely heal the top interface defects. Herein, an ion-diffusion management strategy is proposed to concurrently modulate the top interfaces, buried interfaces, and bulk interfaces (i.e., grain boundaries) of perovskite film, enabling all-interface defect passivation. Specifically, this method is enabled by applying double interactive salts of octylammonium iodide (OAI) and guanidinium chloride (GACl) onto the 3D perovskite surface. It is revealed that the hydrogen-bonding interaction between OA+ and GA+ decelerates the OA+ diffusion and therefore forms a dimensionally broadened 2D capping layer. Additionally, the diffusion of GA+ and Cl- determines the composition of the bulk and buried interface of PSCs. As a result, n-inter-i-inter-p, i.e., five-layer structured PSCs can be obtained with a champion PCE of 25.43% (certified 24.4%). This approach also enables the substantially improved operational stability of perovskite solar cells.

Moisture-triggered fast crystallization enables efficient and stable perovskite solar cells.

Understanding the function of moisture on perovskite is challenging since the random environmental moisture strongly disturbs the perovskite structure. Here, we develop various N2-protected characterization techniques to comprehensively study the effect of moisture on the efficient cesium, methylammonium, and formamidinium triple-cation perovskite (Cs0.05FA0.75MA0.20)Pb(I0.96Br0.04)3. In contrast to the secondary measurements, the established air-exposure-free techniques allow us directly monitor the influence of moisture during perovskite crystallization. We find a controllable moisture treatment for the intermediate perovskite can promote the mass transportation of organic salts, and help them enter the buried bottom of the films. This process accelerates the quasi-solid-solid reaction between organic salts and PbI2, enables a spatially homogeneous intermediate phase, and translates to high-quality perovskites with much-suppressed defects. Consequently, we obtain a champion device efficiency of approaching 24% with negligible hysteresis. The devices exhibit an average T80-lifetime of 852 h (maximum 1210 h) working at the maximum power point.

Fullerene Derivative with Flexible Alkyl Chain for Efficient Tin-Based Perovskite Solar Cells.

Fullerene derivatives are considered excellent materials for the extraction and transportation of electrons in the production of efficient tin-based perovskite solar cells (TPSCs). However, it is not clear how the molecular structure of fullerene derivatives affects the efficiency and stability of TPSCs. In this study, the effects of fullerene derivatives, (6,6)-phenyl-C61-butyric acid hexyl ester (PCBH) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM), with different functional groups, on photovoltaic performance were investigated. The flexible alkyl chain of PCBH effectively improved the film morphology and stability, the electron extraction and transport capabilities, and the interface contact of fullerene and perovskite. As a result, the PCBH-based TPSC yielded a higher efficiency, of 9.21%, than the PCBM-based devices (7.54%). More importantly, the PCBH-based films exhibited higher stability and effectively suppressed the oxidation of Sn2+ by inhibiting oxygen permeation. Therefore, the PCBH-based devices exhibited significantly enhanced stability. This result indicates that optimizing the functional group of fullerene derivatives is crucial for improving the efficiency and stability of TPSCs.