Jun Ji

Photo-induced degradation of lead halide perovskite solar cells caused by the hole transport layer/metal electrode interface

Lead halide perovskite solar cells (PSCs) suffer from poor long-term stability, especially due to photo-induced degradation, as PSCs function under continuous sunlight. However, the origins of this instability have not been clearly explored. Herein, the photo-induced degradation of PSCs with mesoporous and planar architectures are investigated, respectively, and the main origin is proved to be correlated with the hole transport material (HTM)/metal (Au) electrode interface. The solar irradiation of PSCs causes significant deterioration of device performance, with the efficiency decreasing from approximately 18% to 2.46% for planar PSCs in 180 min. Electrical analysis of the PSCs and XPS measurements show that the deteriorated performance is induced by retarded carrier extraction from the HTM to the Au electrode, due to a broken interface binding. Accordingly, in situ renewal of the Au electrode was found to cause notable recovery (approximately 80%) of the device performance of both mesoporous and planar PSCs. In comparison, the material degradation of perovskite and the TiO2/perovskite interface were also studied; however, these showed minor effects on the photo-induced degradation of PSCs. These results indicate that the photo-induced degradation of PSCs is mainly caused by the HTM/Au interface. This study provides an important insight into the photo-induced degradation of PSCs, and is crucial for the fabrication of highly photo-stable PSCs.

Dual function interfacial layer for highly efficient and stable lead halide perovskite solar cells

The trap states and the intrinsic nature of polycrystalline organometallic perovskites cause carrier losses in perovskite solar cells (PSCs) through carrier recombination at the surface and subsurface of the perovskites, leading to lowered conversion efficiency. Herein, to reduce the carrier losses, an intelligent approach concerning surface passivation and interfacial doping of the perovskite is proposed by introducing an F4TCNQ interfacial layer. The trap states at the perovskite surface are efficiently suppressed, leading to a homogenous surface potential of perovskite, which avoids the surface carrier recombination. The Fermi level of the perovskite is shifted to its valence band by 0.2 eV, inducing an energy barrier for electron diffusion and contributing directly to a minimized carrier recombination at the subsurface of the perovskite film. Consequently, the performance of the PSCs is remarkably improved, with the average efficiency increased from 14.3 ± 0.9% to 16.4 ± 1.0% (with a maximum efficiency of 18.1%). Moreover, the PSCs with the dual function interfacial layer show enhanced long-term stability in ambient air without device encapsulation.

A TiO2 embedded structure for perovskite solar cells with anomalous grain growth and effective electron extraction

The structure of the perovskite solar cells (PSCs) is either mesoporous or planar. Here, a novel structure for highly efficient and stable PSCs is proposed, i.e., an embedded structure, which combines the advantages of the mesoporous and planar structures. The embedded structure utilizes a TiO2 nanoparticle embedded perovskite (CH3NH3PbI3) film as the absorption layer. The presence of TiO2 nanoparticles in the perovskite film could improve the electron extraction, and promote the formation of a compact perovskite layer with large grains. Consequently, the performance of the PSCs is significantly improved with the efficiency increasing from 16.6% for the planar structure to 19.2% for the embedded structure, which is the best performance of the MAPbI3-based PSCs. Furthermore, the TiO2 embedded perovskite films present better long-term stability than the pristine perovskite films, and the corresponding PSCs, which have no other chemical modifications, also show excellent stability with efficiency approaching 80% (for average) or 90% (for the best) after being exposed to air for 28 days without encapsulation.

Degradation of organometallic perovskite solar cells induced by trap states

The degradation of organometallic perovskite solar cells (PSCs) is the key bottleneck hampering their development, which is typically ascribed to the decomposition of perovskite (CH3NH3PbI3). In this work, the degradation of PSCs is observed to be significant, with the decrease in efficiency from 18.2% to 11.5% in ambient air for 7 days. However, no obvious decomposition or structural evolution of the perovskite was observed, except the notable degradation phenomenon of the device. The degradation of PSCs derives from deteriorated photocurrent and fill factor, which are proven to be induced by increased trap states for enlarged carrier recombination in degraded PSCs. The increased trap states in PSCs over storage time are probably induced by the increased defects at the surface of perovskite. The trap states induced degradation provides a physical insight into the degradation mechanisms of PSCs. Moreover, as the investigations were performed on real PSCs instead of individual perovskite films, the finding...

The path of mass transfer during Au thin film-assisted chemical etching by designed surface barriers

The mass transfer in metal-assisted chemical etching between the interfaces has been revealed directly by an epoxy protection method. The results show that the dissolution of Si occurs in the Au film surface instead of the Au–Si interface. A mass transfer path inside the Au film is proposed, in which the Si atoms dissolve in the Au film, and then diffuse across the Au lattice, and are oxidized and etched away at the Au film/solution interface. This model is proved by the oxidation products of Si atoms (SiO2 and SiF62−) on the surface of the Au thin film. In addition, the abnormal emission of H2 at the Au–Si interface indicates the probability of the diffusion of H atoms inside the Au film during the etching. This work provides a further insight into the mechanism of metal-assisted chemical etching.

Computational Study of Ternary Devices: Stable, Low-Cost, and Efficient Planar Perovskite Solar Cells

Although perovskite solar cells with power conversion efficiencies (PCEs) more than 22% have been realized with expensive organic charge-transporting materials, their stability and high cost remain to be addressed. In this work, the perovskite configuration of MAPbX (MA = CH3NH3, X = I3, Br3, or I2Br) integrated with stable and low-cost Cu:NiOx hole-transporting material, ZnO electron-transporting material, and Al counter electrode was modeled as a planar PSC and studied theoretically. A solar cell simulation program (wxAMPS), which served as an update of the popular solar cell simulation tool (AMPS: Analysis of Microelectronic and Photonic Structures), was used. The study yielded a detailed understanding of the role of each component in the solar cell and its effect on the photovoltaic parameters as a whole. The bandgap of active materials and operating temperature of the modeled solar cell were shown to influence the solar cell performance in a significant way. Further, the simulation results reveal a strong dependence of photovoltaic parameters on the thickness and defect density of the light-absorbing layers. Under moderate simulation conditions, the MAPbBr3 and MAPbI2Br cells recorded the highest PCEs of 20.58 and 19.08%, respectively, while MAPbI3 cell gave a value of 16.14%.

Ion‐Migration Inhibition by the Cation–π Interaction in Perovskite Materials for Efficient and Stable Perovskite Solar Cells

Migration of ions can lead to photoinduced phase separation, degradation, and current-voltage hysteresis in perovskite solar cells (PSCs), and has become a serious drawback for the organic-inorganic hybrid perovskite materials (OIPs). Here, the inhibition of ion migration is realized by the supramolecular cation-π interaction between aromatic rubrene and organic cations in OIPs. The energy of the cation-π interaction between rubrene and perovskite is found to be as strong as 1.5 eV, which is enough to immobilize the organic cations in OIPs; this will thus will lead to the obvious reduction of defects in perovskite films and outstanding stability in devices. By employing the cation-immobilized OIPs to fabricate perovskite solar cells (PSCs), a champion efficiency of 20.86% and certified efficiency of 20.80% with negligible hysteresis are acquired. In addition, the long-term stability of cation-immobilized PSCs is improved definitely (98% of the initial efficiency after 720 h operation), which is assigned to the inhibition of ionic diffusions in cation-immobilized OIPs. This cation-π interaction between cations and the supramolecular π system enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.

Superior Stability and Efficiency Over 20% Perovskite Solar Cells Achieved by a Novel Molecularly Engineered Rutin-AgNPs/Thiophene Copolymer

Abstract Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized mostly with expensive spiro‐MeOTAD hole‐transporting material. PSCs are demonstrated that achieve stabilized efficiencies exceeding 20% with straightforward low‐cost molecularly engineered copolymer poly(1‐(4‐hexylphenyl)‐2,5‐di(thiophen‐2‐yl)‐1H‐pyrrole) (PHPT‐py) based on Rutin–silver nanoparticles (AgNPs) as the hole extraction layer. The Rutin–AgNPs additive enables the creation of compact, highly conformal PHPT‐py layers that facilitate rapid carrier extraction and collection. The spiro‐MeOTAD‐based PSCs show comparable efficiency, although their operational stability is poor. This instability originated from potential‐induced degradation of the spiro‐MeOTAD/Au contact. The addition of conductive Rutin–AgNPs into PHPT‐py layer allows PSCs to retain >97% of their initial efficiency up to 60 d without encapsulation under relative humidity. The PHPT‐py/ Rutin–AgNPs‐based devices surpass the stability of spiro‐MeOTAD‐based PSCs and potentially reduce the fabrication cost of PSCs.