Showing papers by "Baojie Yan published in 2019"

Phosphate-Passivated SnO2 Electron Transport Layer for High-Performance Perovskite Solar Cells.

Abstract: Tin oxide (SnO2) is widely used in perovskite solar cells (PSCs) as an electron transport layer (ETL) material. However, its high surface trap density has already become a strong factor limiting PSC development. In this work, phosphoric acid is adopted to eliminate the SnO2 surface dangling bonds to increase electron collection efficiency. The phosphorus mainly exists at the boundaries in the form of chained phosphate groups, bonding with which more than 47.9% of Sn dangling bonds are eliminated. The reduction of surface trap states depresses the electron transport barriers, thus the electron mobility increases about 3 times when the concentration of phosphoric acid is optimized with 7.4 atom % in the SnO2 precursor. Furthermore, the stability of the perovskite layer deposited on the phosphate-passivated SnO2 (P-SnO2) ETL is gradually improved with an increase of the concentration. Due to the higher electron collection efficiency, the P-SnO2 ETLs can dramatically promote the power conversion efficiency (PCE) of the PSCs. As a result, the champion PSC has a PCE of 21.02%. Therefore, it has been proved that this simple method is efficient to improve the quality of ETL for high-performance PSCs.

Engineering of hole-selective contact for high-performance perovskite solar cell featuring silver back-electrode

Abstract: Replacement of commonly gold (Au) with other non-precious metals is a crucial step for low-cost perovskite solar cells (PSCs). However, severe performance degradations always occurred when we get rid of the Au electrode, mainly because of the deteriorated hole-transporting properties. Here, we reported that the efficiency of the PSCs featuring silver (Ag) back-electrode can be increased from 17.02 to 18.62% after introducing a thin molybdenum trioxide (MoO3) film in between the Spiro-OMeTAD and the Ag electrode. Characterizations from impedance spectrum and dark current density showed that the addition of MoO3 can effectively reduce the series resistance and reverse saturation current density of the devices, thereby leading to improved fill factor and open-circuit voltage. The enhancements in hole transport and extraction at the interface were further confirmed by the external quantum efficiency and photoluminescence tests. Our results suggested that silver electrode with MoO3 is a promising design strategy for efficient and cost-effective PSCs.

An Expanded Cox and Strack Method for Precise Extraction of Specific Contact Resistance of Transition Metal Oxide/ n- Silicon Heterojunction

Abstract: Specific contact resistance $(\rho _{c})$ , commonly extracted by Cox and Strack method (CSM) and transfer length method (TLM), is one of the most important properties of carrier-selective contacts. However, in most cases, the hole-selective contacts (HSCs) deposited on n-type silicon (n-Si) substrate are Schottky heterojunction other than Ohmic contact, impeding the accurate extraction of $\rho _{c}$ by CSM and TLM. In this paper, an expanded CSM is proposed to precisely extract the $\rho _{c}$ of MoOx/n-Si heterojunction, achieving a generally lower coefficient of variation. The current transport characteristic and the $\rho _{c}$ value of MoOx/n-Si heterocontact are further verified by technology computer aided design (TCAD) simulation. The results demonstrate that the expanded CSM enables a more precise $\rho _{c}$ extraction, a better preparation technology compatibility, and a wider range of application, compared to TLM.

Enhanced perovskite crystallization by the polyvinylpyrrolidone additive for high efficiency solar cells

Abstract: Recently, perovskite solar cells (PSCs) have drawn a lot of attention due to their booming power conversion efficiency (PCE). However, the photovoltaic performance of PSCs is usually limited by the quality of the perovskite film. Here, trace polyvinylpyrrolidone (PVP) is introduced into the perovskite precursor solution as an additive for the crystalline defect passivation. The PVP with carbonyl delays the transition of the perovskite from the δ-phase to α-phase, which slows down the crystallization process. Thus, the crystallization rate of the α-phase perovskite can be controlled by forming coordination and a better film quality is achieved. As a result, we achieve a champion PCE of 19.63%, which is much higher than the 17.47% of the reference PSC. This study shows the mechanism of the material quality improvement by the additive during the perovskite film formation, where three processes are proposed to describe the film crystallization. We prove that the addition of PVP is an effective approach to improve the quality of the perovskite film, providing a valuable guidance for the development of high efficiency PSCs.

Metal-Enhanced Adsorption of High-Density Polyelectrolyte Nucleation-Inducing Seed Layer for Highly Conductive Transparent Ultrathin Metal Films

Abstract: In recent years, ultrathin Ag films (UTAFs), which are attractive owing to its extremely low resistance, relatively high transparency, excellent mechanical flexibility and mature mass production, have been reported as potential candidates to replace traditional indium tin oxide (ITO). To achieve a high-quality UTAF, a nucleation-inducing seed layer (NISL) is required to address the issue of irregular Ag islands growth. However, the structures of films so deposited are still far from being ideal and consist of rough surfaces with high densities of voids and grain boundaries when the film thickness is less than ~6 nm. Here, a hybrid structure composed of a gold (Au)/polyethyleneimine (PEI) bilayer is employed as a high-density NISL for the fabrication of an UTAF. Compared to the conventional single-layered PEI NISL that physisorbed on the substrate via the weak electrostatic attraction between the negatively charged substrate and the positively charged amine groups in PEI, our novel bilayered Au/PEI NISL exhibits a much higher density of nucleation sites due to the formation of strong coordinate covalent bonds between the Au atoms and amine groups. As a result, the percolation threshold thickness of the UTAF based on the Au/PEI bilayer can be reduced to as low as 3 nm. After capping with a high-refractive-index tantalum pentoxide (Ta2O5) anti-reflection layer, the resultant Au/PEI/8 nm Ag/30 nm Ta2O5 (APAT) electrode exhibits an excellent optoelectrical performance with a sheet resistance of 9.07 Ω/sq and transmittance of 92.9% in the spectral range of 400-800 nm as well as outstanding long-term environmental and mechanical stabilities. The findings demonstrate a novel strategy for the development of high-performance UTAF-based transparent electrodes.

Design and simulation of perovskite solar cells with Gaussian structured gradient-index optics.

Abstract: To unlock the full potential of the perovskite solar cell (PSC) photocurrent density and power conversion efficiency, the topic of optical management and design optimization is of absolute importance. Here, we propose a gradient-index optical design of the PSC based on a Gaussian-type front-side glass structure. Numerical simulations clarify a broadband light-harvesting response of the new design, showing that a maximal photocurrent density of 23.35 mA/cm2 may be expected, which is an increase by 1.21 mA/cm2 compared with that of the traditional flat-glass counterpart (22.14 mA/cm2). Comprehensive analysis of the electric field distributions elucidates the light-trapping mechanism. Furthermore, PSCs having the Gaussian index profile display superior optical properties and performance compared to those of the uniform index counterpart under varying conditions of perovskite layer thicknesses and incident angles. The simulation results in this study provide an effective design scheme to promote optical absorption in PSCs.