Yingdong Xia

Lead-Free Organic–Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives

Organic-inorganic hybrid halide perovskites (e.g., MAPbI3 ) have recently emerged as novel active materials for photovoltaic applications with power conversion efficiency over 22%. Conventional perovskite solar cells (PSCs); however, suffer the issue that lead is toxic to the environment and organisms for a long time and is hard to excrete from the body. Therefore, it is imperative to find environmentally-friendly metal ions to replace lead for the further development of PSCs. Previous work has demonstrated that Sn, Ge, Cu, Bi, and Sb ions could be used as alternative ions in perovskite configurations to form a new environmentally-friendly lead-free perovskite structure. Here, we review recent progress on lead-free PSCs in terms of the theoretical insight and experimental explorations of the crystal structure of lead-free perovskite, thin film deposition, and device performance. We also discuss the importance of obtaining further understanding of the fundamental properties of lead-free hybrid perovskites, especially those related to photophysics.

Management of perovskite intermediates for highly efficient inverted planar heterojunction perovskite solar cells

Organic–inorganic hybrid lead halide perovskites (e.g., CH3NH3PbX3, X = Cl, Br, I) hold great promise in optoelectronic devices, e.g., solar cells. High-performance devices have been realized by controlling the perovskite surface morphology, grain size, and degree of crystallinity. However, the role of the components during film formation and crystallization remains mysterious. Here, we show the management of perovskite intermediates to construct perovskite films with uniform perovskite crystals and controlled surface morphology using methylammonium acetate (MAAc) to retard the reaction between PbI2 and MAI in the solution. The formation of MAPbI3 was allowed via the exchange of I− anions from the neighboring MAI molecules to the perovskite intermediate phases replacing Ac− anions. A highly efficient perovskite solar cell with a power conversion efficiency of 18.09% was achieved. The experimental and theoretical studies reveal that perovskite intermediates play an important role in facilitating homogeneous nucleation or modulating the crystallization kinetics. These results also provide important progress towards the understanding of perovskite intermediate phases for advancing the building of perovskite semiconductors.

Super air stable quasi-2D organic-inorganic hybrid perovskites for visible light-emitting diodes

Solution processed organic-inorganic hybrid perovskites are emerging as a new generation materials for optoelectronics. However, the electroluminescence is highly limited in light emitting diodes (LED) due to the low exciton binding energy and the great challenge in stability. Here, we demonstrate a super air stable quasi-two dimensional perovskite film employing hydrophobic fluorine-containing organics as barrier layers, which can store in ambient for more than 4 months with no change. The dramatically reduced grain size of the perovskite crystal in contrast to three dimensional (3D) perovskites was achieved. Together with the natural quantum well of quasi-two dimensional perovskite confining the excitons to recombination, the LED exhibited the maximum luminance of 1.2 × 103 cd/m2 and current efficiency up to 0.3 cd/A, which is twenty fold enhancement than that of LED based on 3D analogues under the same condition.

Recent Advances in Alternating Current‐Driven Organic Light‐Emitting Devices

Organic light-emitting devices (OLEDs), typically operated with constant-voltage or direct-current (DC) power sources, are candidates for next-generation solid-state lighting and displays, as they are light, thin, inexpensive, and flexible. However, researchers have focused mainly on the device itself (e.g., development of novel materials, design of the device structure, and optical outcoupling engineering), and little attention has been paid to the driving mode. Recently, an alternative concept to DC-driven OLEDs by directly driving devices using time-dependent voltages or alternating current (AC) has been explored. Here, the effects of different device structures of AC-driven OLEDs, for example, double-insulation, single-insulation, double-injection, and tandem structure, on the device performance are systematically investigated. The formation of excitons and the dielectric layer, which are important to achieve high-performance AC-driven OLEDs, are carefully considered. The importance of gaining further understanding of the fundamental properties of AC-driven OLEDs is then discussed, especially as they relate to device physics.

Graphene Oxide by UV-Ozone Treatment as Efficient Hole Extraction Layer for Highly Efficient and Stable Polymer Solar Cells

The hole extraction layer has a significant impact on the achievement of high-efficiency polymer solar cells (PSCs). Here, we report an efficient approach to direct UV-ozone treatment by larger device performance enhancement employing graphene oxide (GO). The dramatic performance enhancement of PSCs with the P3HT:PCBM blend as an active layer was demonstrated by the UV-ozone treatment of GO for 30 min: best power conversion efficiency (PCE) of 4.18%, fill factor of 0.63, Jsc of 10.94 mA cm-2, and Voc of 0.61 V, which are significantly higher than those of the untreated GO (1.82%) and highly comparable PEDOT:PSS-based PSCs (3.73%). In addition, PSCs with UV-ozone-treated GO showed a longer stability than PSCs with PEDOT:PSS. The significant enhancement of PCEs of PSCs can be attributed to the fact that ozone molecules can oxidize GO into CO2 and leave highly conductive graphene particles. We suggest that this simple UV-ozone treatment can provide an efficient method for highly efficient GO hole extraction in high-performance PSCs.

Synergistic effect of anions and cations in additives for highly efficient and stable perovskite solar cells

Organic–inorganic halide perovskites have attracted intensive attention due to their outstanding photophysical properties and high performance in optoelectronic devices. Moreover, the quality of perovskite films plays an important role in the performance of perovskite devices. The use of additives has been previously suggested as the simplest and effective method to improve the quality of perovskite films. However, only the cations or anions in most additives are capable of functioning as active species in improving the quality of perovskite films. Herein, we report that ammonium thiocyanate (NH4SCN) as an additive (contains NH4+ and SCN− ions) can significantly improve the quality of perovskite films as compared to NH4I and Pb(SCN)2 as additives containing the single active species of NH4+ and SCN−, respectively. The synergistic effect between the ions in NH4SCN is favorable for controlling the nucleation and growth speed of the crystal grains, which evidently increases the grain size and markedly improves the film stability. Finally, the efficiency of the device based on NH4SCN (16.47%) is significantly enhanced without a hysteresis effect as compared to that of the devices with single ions as the active species, e.g., NH4+ in NH4I (13.97%) and SCN− in Pb(SCN)2 (12.88%). Our findings suggest that the synergistic effect of anions and cations in additives is an effective strategy for realizing efficient and stable perovskites.

Enhancing Efficiency and Stability of Perovskite Solar Cells via a Self-Assembled Dopamine Interfacial Layer

Interfacial engineering is a simple and effective strategy that can improve the photovoltaic performance in organic-inorganic perovskite solar cells (PSCs). Herein, a dopamine (DA) self-assembled monolayer (SAM) was introduced on the top of the SnO2 electron transporting layer (ETL) to modify the SnO2/perovskite interface. The processing temperature of the present devices is around 150 °C, and the power conversion efficiency of the PSCs was significantly improved to 16.65% compared to that of the device without modification (14.05%). Such enhancement in efficiency is mainly attributed to the improved quality of perovskite films by improving the affinity of the SnO2 ETL, thus leading to better carrier transport and low charge recombination at the SnO2/perovskite interface. Moreover, the modified device by the DA SAM exhibited enhanced stability compared to the device without modification. Our results suggest that the introduction of the DA SAM on the ETL/perovskite interface is a promising method for highly efficient and stable PSCs.