Understanding the Trap Characteristics of Perovskite Solar Cells via Drive-Level Capacitance Profiling.

Perovskite solar cells (PSCs) are gaining significant interest as the future of photovoltaics owing to their superior performance and cost-effectiveness. Nevertheless, traps in PSCs have emerged as issues that adversely affect the efficiency and stability of the devices. In this study, the methylammonium chloride (MACl) additive and phenyltrimethylammonium iodide (PTMAI) posttreatment were applied to passivate bulk and surface defects. Furthermore, variations of the traps' quantitative spatial arrangement have been monitored by using the drive-level capacitance profiling (DLCP) analysis. A similar magnitude of trap reduction was observed for the bulk perovskite layer and two interfaces (electron transport layer (ETL)/perovskite and hole transport layer (HTL)/perovskite) with an optimal concentration of the MACl additive. However, the effect of perovskite posttreatment in reducing the trap density was much more noticeable at the HTL/perovskite interface compared to the bulk and ETL/perovskite regions. This observation was reinforced by the outcomes of the 500 h thermal stability tests at 60 °C from seven independent batches, which demonstrated a substantial suppression of trap accumulation, particularly at the HTL/perovskite interface, by an order of magnitude.

Incorporation of Lithium Fluoride Restraining Thermal Degradation and Photodegradation of Organometal Halide Perovskite Solar Cells.

Because of the facile formation of defects in organometal halide perovskites, the defect passivation has become an important prerequisite for the stable and efficient perovskite solar cell (PSC). Regarding that ionic defects of the perovskites play a significant role on the performance and stability of PSCs, we introduce lithium fluorides as effective passivators based on their strong ionic characteristics and small ionic radii. Both Li+ and F- are observed to successfully incorporate within the perovskite layer, improving the device performances with the best efficiency over 20%, while the hysteresis effects are significantly reduced, confirming the passivation of perovskite defects. Moreover, LiF restrains both thermal degradation and photodegradation of PSCs, where over 90% of the initial efficiencies have been retained by LiF-incorporated devices for more than 1000 h under either 1 sun illumination or 85 °C thermal condition. As the trap density of states is analyzed before and after the thermal stress, not only the mitigation of electronic traps as fabricated but also the dramatic relaxation of traps during the postannealing step is observed with the LiF incorporation. From this work, LiF has shown its potential as a promising ionic passivator, and the phenomenal achievement of device stability by LiF provides a clear insight to overcome the stability issues of PSCs, a key to the commercialization of next-generation photovoltaics.

CuCrO2 Nanoparticles Incorporated into PTAA as a Hole Transport Layer for 85 °C and Light Stabilities in Perovskite Solar Cells.

High-mobility inorganic CuCrO2 nanoparticles are co-utilized with conventional poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) (PTAA) as a hole transport layer (HTL) for perovskite solar cells to improve device performance and long-term stability. Even though CuCrO2 nanoparticles can be readily synthesized by hydrothermal reaction, it is difficult to form a uniform HTL with CuCrO2 alone due to the severe agglomeration of nanoparticles. Herein, both CuCrO2 nanoparticles and PTAA are sequentially deposited on perovskite by a simple spin-coating process, forming uniform HTL with excellent coverage. Due to the presence of high-mobility CuCrO2 nanoparticles, CuCrO2/PTAA HTL demonstrates better carrier extraction and transport. A reduction in trap density is also observed by trap-filled limited voltages and capacitance analyses. Incorporation of stable CuCrO2 also contributes to the improved device stability under heat and light. Encapsulated perovskite solar cells with CuCrO2/PTAA HTL retain their efficiency over 90% after ~900-h storage in 85 °C/85% relative humidity and under continuous 1-sun illumination at maximum-power point.

Interfacial Modification and Defect Passivation by the Cross-Linking Interlayer for Efficient and Stable CuSCN-Based Perovskite Solar Cells.

The study of the inorganic hole-transport layer (HTL) in perovskite solar cells (PSCs) is gathering attention because of the drawback of the conventional PSC design, where the organic HTL with salt dopants majorly participates in the degradation mechanisms. On the other hand, inorganic HTL secures better stability, while it offers difficulties in the deposition and interfacial control to realize high-performing devices. In this study, we demonstrate polydimethylsiloxane (PDMS) as an ideal polymeric interlayer which prevents interfacial degradation and improves both photovoltaic performance and stability of CuSCN-based PSC by its cross-linking behavior. Surprisingly, the PDMS polymers are identified to form chemical bonds with perovskite and CuSCN, as shown by Raman spectroscopy. This novel cross-linking interlayer of PDMS enhances the hole-transporting property at the interface and passivates the interfacial defects, realizing the PSC with high power-conversion efficiency over 19%. Furthermore, the utilization of the PDMS interlayer greatly improves the stability of solar cells against both humidity and heat by mitigating the interfacial defects and interdiffusion. The PDMS-interlayered PSCs retained over 90% of the initial efficiencies, both after 1000 h under ambient conditions (unencapsulated) and after 500 h under 85 °C/85% relative humidity (encapsulated).

From Nanostructural Evolution to Dynamic Interplay of Constituents: Perspectives for Perovskite Solar Cells.

Moving away from the high-performance achievements in organometal halide perovskite (OHP)-based optoelectronic and photovoltaic devices, intriguing features have been reported in that photocarriers and mobile ionic species within OHPs interact with light, electric fields, or a combination of both, which induces both spatial and temporal changes of optoelectronic properties in OHPs. Since it is revealed that the transport of photocarriers and the migration of ionic species are affected not only by each other but also by the inhomogeneous character, which is a consequence of the route selected to deposit OHPs, understanding the nanostructural evolution during OHP deposition, in terms of the resultant structural defects, electronic traps, and nanoscopic charge behaviors, will be valuable. Investigation of the film-growth mechanisms and strategies adopted to realize OHP films with less-defective large grains is of central importance, considering that single-crystalline OHPs have exhibited the most beneficial properties, including carrier lifetimes. Critical factors governing the behavior of photocarriers, mobile ionic species, and nanoscale optoelectronic properties resulting from either or all of them are further summarized, which may potentially limit or broaden the optoelectronic and photovoltaic applications of OHPs. Through inspection of the recent advances, a comprehensive picture and future perspective of OHPs are provided.

Route to Improving Photovoltaics Based on CdSe/CdSexTe1-x Type-II Heterojunction Nanorods: The Effect of Morphology and Cosensitization on Carrier Recombination and Transport.

One-dimensionally elongated nanoparticles with type-II staggered band offset are of potential use as light-harvesting materials for photovoltaics, but only a limited attention has been given to elucidate the factors governing the cell performance obtainable from such materials. Herein, we describe a combined strategy to enhance charge collection from CdSe/CdSexTe1-x type-II heterojunction nanorods (HNRs) utilized as light harvesters for sensitized solar cells. By integrating morphology- and composition-tuned type-II HNRs into solar cells, factors that yield interfaces favorable both for the electron injection into TiO2 and hole transfer to electrolyte are examined. Furthermore, it is shown that a more efficient photovoltaic system results from cosensitization with CdS quantum dots (QDs) predeposited on a TiO2 scaffold, which improves charge collection from HNRs. Electrochemical impedance spectroscopy (EIS) analysis suggests that such a synergistically enhanced system benefits from the decreased recombination within HNRs and facilitated charge transport through the cosensitized TiO2 electrode, even with the activation of a recombination path presumably related to the photogenerated holes in CdS QDs.