Chen Weijie

The transport properties of double and triple cation perovskite have been explored by SSPG and SSPC, under various conditions. The addition of cesium has been shown to significantly enhance the diffusion length of the perovskite layers, this improvement being reversibly annihilated by a vacuum stress. This behavior can be explained considering the interaction between the iodine vacancies and oxygen.

In this study, the transport properties of high quality double and triple cation perovskite are explored under different environmental stresses by Steady State Photocurrent (SSPC) and Steady State Photocarrier Grating (SSPG) measurements. SSPG allows a direct measurement of the ambipolar diffusion length that is significantly higher for the cesium containing perovskite (1.4 micron compared to 0.57 micron for a double cation compound). This positive effect is shown to be reversibly annihilated as the samples are put under vacuum, and to be accelerated with the increase of the temperature. The degradation of the material properties appears together with the activation of a defect trap level and a rise of the doping level in the material. This behavior is discussed considering the interaction between the iodine vacancies contained in the perovskite and oxygen which is known to lead to trap passivation.

We developed highly efficient ternary all‐polymer solar cells by incorporating a narrow‐bandgap electron‐donating polymer (PNTB) into blended films of the wide‐bandgap electron‐donating copolymer PBTA‐BO and the electron‐accepting copolymer N2200. The device based on the ternary blend film exhibites a substantially higher power conversion efficiency than its binary counterparts, which is attributable to a complementary absorption profile, efficient energy transfer, enhanced charge mobility, and improved morphology.

Ternary all‐polymer solar cells (all‐PSCs) attract considerable research attention owing to the simplicity of the single‐junction device architecture and the broad absorption range of the light‐harvesting layer. However, the difficulty in controlling the morphology of ternary blended films makes it challenging to develop high‐performance ternary all‐PSCs. Herein, we report on the development of efficient ternary blended all‐PSCs by incorporating the narrow‐bandgap electron‐donating polymer PNTB, which contains a naphtho[1,2‐c:5,6‐c′]bis([1,2,5]thiadiazole) moiety, into blend films comprising the wide‐bandgap electron‐donating copolymer PBTA‐BO and the electron‐accepting copolymer N2200. The resulting ternary blended devices reached an impressively high power conversion efficiency of 10.09%, which obviously outperforms those obtained from binary blended counterparts. The improved photovoltaic performance is attributable to the combined effects of the extended absorption profile, a favorable film morphology, and more efficient charge transfer. Of particular interest is that these ternary blend films are processed using a non‐halogenated solvent, 2‐methyltetrahydrofuran, which is promising for practical applications. These findings lend credence to the ternary approach as a facile and promising strategy for achieving high‐performance all‐PSCs.

A PEDOT:PSS semitransparent gate has enabled the control of the surface carrier density in metal‐insulator‐semiconductor structures. This allows to demonstrate the direct dependence of interface recombination with surface carrier population using large‐area photo‐conductance lifetime measurements. This easy and reliable technique can also enable investigations into insulators built‐in potential, work function differences, semiconductor surface charge dynamics, and dielectric conduction mechanisms.

This communication reports a technique to control the surface carrier population of silicon during photo‐conductance decay measurements, by using a semi‐transparent PEDOT:PSS gate. The potential of this technique has been demonstrated by characterizing carrier‐dependent surface recombination of 1  cm n‐type float zone silicon, passivated with dielectric stack layers of either SiO₂, SiO₂/SiN_x, a‐Si/SiO_x, a‐Si/SiO_x/SiN_x, AlO_x, or AlO_x/SiN_x. Carrier density at the Si‐dielectric interface has been controlled from heavy inversion to heavy accumulation regimes despite leakage currents. This has provided insightful information into the recombination activity at the silicon surface.

A facile passivation process is introduced to reduce the surface deep traps of Pb vacancy (V _Pb) and I interstitial (I _i) for CsPbI₂Br perovskite film. The dissociative Pb²⁺ in Pb(NO₃)₂ solution would combine with I _i (the excess I‐ions) and fill to V _pb of the perovskite surface, resulting in the less nonradiative recombination and defect states density of the perovskite film.

Cesium halide perovskite CsPbX₃ has emerged to be a promising candidate for photovoltaic materials due to their componential and thermal stability. During the fabrication of CsPbX₃ films, rich halide ions could cause deep trap states on the surface of the perovskite film, leading to much charge recombination. Herein, Pb²⁺ solution post‐processing strategy is introduced to passivate the deep trap states of CsPbI₂Br films. The dissociative Pb²⁺ in the solution effectively combines with the excess halide ions on the perovskite surface to reduce the deep trap states of Pb vacancy (V _Pb) and I interstitial (I _i). As a result, the average photoluminescence lifetimes τ _ave of the perovskite film prolonged nearly double after passivation. The trap density of perovskite is effectively decreased from 8 × 10¹⁶ to 6.64 × 10¹⁶ cm⁻³. The CsPb₂Br solar cell shows an open‐circuit‐voltage as high as 1.29 V and power conversion efficiency of 12.34% with small hysteresis. The postprocessing method would provide an avenue to improve further the efficiency of inorganic perovskite solar cells via reducing surface traps.

Carboxyl‐substituted perylene (PTCA) has been successfully applied as the electron‐transport layer in perovskite solar cells. By the carboxyl groups, PTCA can effectively connect the perovskite layer and FTO, thus reducing the interface barriers induced by weak contact, resulting in a high PCE of 16.09%. In addition, the PTCA‐based devices exhibit remarkable stability under illumination in ambient conditions without encapsulation.

Carboxyl‐substituted perylene (PTCA) has been successfully applied as the electron‐transport layer in perovskite solar cells. The large rigid π–π conjugated plane structure in PTCA endows it excellent electronic transmission performance. By the carboxyl groups, PTCA can effectively connect the perovskite layer and FTO, thus reducing the interface barriers induced by weak contact, resulting in a high PCE of 16.09%. In addition, the PTCA‐based devices exhibit remarkable stability under illumination in ambient conditions without encapsulation.

Atmospheric pressure spatial atomic layer technique (s‐ALD) has been adopted to introduce a ZnO buffer layer in the p‐i‐n planar perovskite solar cell architecture. The s‐ALD layer successfully prevents damages during ITO sputtering deposition, enabling the fabrication of efficient and stable semitransparent bifacial perovskite solar cells.

The replacement of the conventional top metal contact with a semi‐transparent conducting electrode such as sputtered indium‐tin oxide (ITO) is strictly required to adopt the perovskite solar cell (PSC) in hybrid tandem photovoltaic applications. In order to prevent sputtering damages on the perovskite absorber and the organic materials adopted in p‐i‐n planar architecture, an atmospheric pressure spatial atomic layer deposited (s‐ALD) ZnO buffer layer has been included. The use of a 45 nm thick s‐ALD layer enables the fabrication of a PSC with a power conversion efficiency (PCE) of 14.7%, with a similar PCE when illuminated from the ITO/s‐ALD ZnO side. When adopted in a four terminal configuration with a c‐Si solar cell (PCE of 18.6%), a 2.5% absolute PCE gain is observed with respect to the stand alone c‐Si. Finally, the semi‐transparent PSC shows an excellent shelf life, and only −4% degradation on the tracked maximum power point when encapsulated and aged at 65 °C in an inert atmosphere after 1500 h.

The hole extraction property of the hole transport layer based on TAPC small molecule via polymer assistance is largely improved. The average power conversion efficiency is enhanced from 17.66 ± 0.52% to 19.03 ± 0.53%, and the champion efficiency reaches 21.01%.

In this paper, inverted perovskite solar cells (PSCs) employing a novel polymer‐assisted small molecule layer as hole transport layer (HTL) are reported and the effect of mixed HTL on the device performance is investigated. It is the first time that the small molecule HTL is doped with a polymer HTL. The introduction of appropriate content of polymer into the small molecule layer will lead to a much smoother surface for the mixed HTL and largely reduced charge recombination, and most importantly, the energy level alignment is more matched with that of the perovskite via optimization of the doping content. Therefore, the hole transfer property is largely improved for the perovskite/mixed HTL composites. After the optimization of the polymer content in the mixed HTLs, an average power conversion efficiency (PCE) of 19.03 ± 0.53% is achieved, and the champion device exhibits a PCE of >21%. This work provides an effective strategy for the development of highly efficient inverted PSCs based on small molecule HTLs.

Publication date: November 2018

Source: Nano Energy, Volume 53

Author(s): Chao Ding, Yaohong Zhang, Feng Liu, Yukiko Kitabatake, Shuzi Hayase, Taro Toyoda, Kenji Yoshino, Takashi Minemoto, Kenji Katayama, Qing Shen

Abstract

Graphical abstract

In planar-structure perovskite solar cells (PSCs), a spike structure is formed between the perovskites and the electron selective layer (ESL), i.e., the energy level of the conduction band of the ESL is higher than that of the perovskite absorber. This spike structure can lead to less photovoltage loss for the PSCs while maintaining high electron injection, which results in a higher open-circuit voltage and a greater short-circuit current, as well as a higher efficiency for planar PSCs.

Transformation from crystalline precursor to perovskite in PbCl₂-derived MAPbI₃

Transformation from crystalline precursor to perovskite in PbCl₂-derived MAPbI₃, Published online: 27 August 2018; doi:10.1038/s41467-018-05937-4