Chen Weijie

The effect of the zinc concentration on the performance of solution‐processed indium gallium zinc oxide (IGZO), as an electron transport layer for an inverted polymer solar cell based on PTB7:PC71BM, is investigated. The performances of the devices have been optimized by tuning the concentrations. The IGZO films significantly enhance the power conversion efficiency of the device from 6.22% to 8.72%.

Organic polymer semiconductor‐based polymer solar cells (PSCs) are drawing tremendous research interest for their superior electrical, structural, optical, mechanical, and chemical properties. During the last two decades, immense efforts have been made toward the development of PSCs. Generally, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as hole transport layer (HTL) of PSCs to improve hole extraction efficiency, but highly acidic PEDOT:PSS reduces device lifetime by destroying indium tin oxide (ITO) electrodes and active layers. To avoid this, some have attempted to develop inverted structured PSCs with different electron transport layers (ETLs); however, the power conversion efficiency (PCE) of these devices is limited owing to low electron mobility of their ETLs. Therefore, an attempt is made to improve the PCE of an inverted‐structured PSC by using indium gallium zinc oxide (IGZO) with optimized amount of indium (In), gallium (Ga), and zinc (Zn). Inverted PSCs with ZnO or IGZO (having various molar ratios of In, Ga, and Zn) as ETL with the structure ITO/ETL/PTB7:PC₇₁BM/MoO₃/Al are constructed. The PCE of the inverted PSC can be increased from 6.22% to 8.72% by using IGZO with an optimized weight ratio of In, Ga, and Zn as an ETL.

Chlorination of the β‐position of benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione can enhance the intermolecular interaction. Single‐crystal analysis demonstrates that TTO‐Cl‐β exhibits the smallest π‐π stacking distance of 3.23 Å, much smaller than that of TTO‐Cl‐α and TTO. Accordingly, PBBD‐Cl‐β based on TTO‐Cl‐β achieved an outstanding power conversion efficiency (PCE) of 16.20%, providing a new insight for the design of acceptor units.

Abstract

The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D‐A conjugated polymers PBBD‐Cl‐α and PBBD‐Cl‐β are synthesized based on two new building blocks (TTO‐Cl‐α and TTO‐Cl‐β) respectively by introducing the chlorine atom into α or β position of the upper thiophene of the highly electron‐deficient benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione moiety. Single‐crystal analysis demonstrates that the chlorine‐free TTO shows a π‐π stacking distance (d _π‐π) of 3.55 Å. When H atom at the α position of thiophene of TTO is replaced by Cl, both π‐π stacking distance (d _π‐π = 3.48 Å) and Cl···S distance (d _Cl‐S = 4.4 Å) are simultaneously reduced for TTO‐Cl‐α compared with TTO. TTO‐Cl‐β then showed the Cl···S non‐covalent interaction can further shorten the intermolecular π‐π stacking separation to 3.23 Å, much smaller than that of TTO‐Cl‐α and TTO. After blending with BTP‐eC9, PBBD‐Cl‐β:BTP‐eC9‐based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP‐eC9 (10.06%) and PBBD‐Cl‐α:BTP‐eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.

Self‐assembled P3HT‐COOH is an excellent hole extraction layer to fabricate robust, high‐performance, and extremely reproducible perovskite solar cells. The well‐aligned self‐assembled P3HT‐COOH generates a dipole layer between indium tin oxide and perovskite, substantially retarding interface charge recombination and producing highly sensitive devices to dim light. The enhanced crystallinity and preferred out‐of‐plane orientation play a key role to suppress the device degradation process.

Abstract

Crystallinity and crystal orientation have a predominant impact on a materials’ semiconducting properties, thus it is essential to manipulate the microstructure arrangements for desired semiconducting device performance. Here, ultra‐uniform hole‐transporting material (HTM) by self‐assembling COOH‐functionalized P3HT (P3HT‐COOH) is fabricated, on which near single crystal quality perovskite thin film can be grown. In particular, the self‐assembly approach facilitates the P3HT‐COOH molecules to form an ordered and homogeneous monolayer on top of the indium tin oxide (ITO) electrode facilitate the perovskite crystalline film growth with high quality and preferred orientations. After detailed spectroscopy and device characterizations, it is found that the carboxylic acid anchoring groups can down‐shift the work function and passivate the ITO surface, retarding the interface carrier recombination. As a result, the device made with the self‐assembled HTM show high open‐circuit voltage over 1.10 V and extend the lifetime over 4,300 h when storing at 30% relative humidity. Moreover, the cell works efficiently under much reduced light power, making it useful as power source under dim‐light conditions. The demonstration suggests a new facile way of fabricating monolayer HTM for high efficiency perovskite devices, as well as the interconnecting layer needed for tandem cell.

The mystery of the buried interface in perovskite photovoltaics is deciphered by combining advanced spectroscopy techniques with a lift‐off strategy. The findings open a new avenue to understanding performance losses and thus the design of unique passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

Abstract

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non‐exposed feature. Herein, the mystery of the buried interface in full device stacks is deciphered by combining advanced in situ spectroscopy techniques with a facile lift‐off strategy. By establishing the microstructure–property relations, the basic losses at the contact interfaces are systematically presented, and it is found that the buried interface losses induced by both the sub‐microscale extended imperfections and lead‐halide inhomogeneities are major roadblocks toward improvement of device performance. The losses can be considerably mitigated by the use of a passivation‐molecule‐assisted microstructural reconstruction, which unlocks the full potential for improving device performance. The findings open a new avenue to understanding performance losses and thus the design of new passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

Publication date: 17 February 2021

Source: Joule, Volume 5, Issue 2

Author(s): Jin Hyuck Heo, Fei Zhang, Chuanxiao Xiao, Su Jeong Heo, Jin Kyoung Park, Joseph J. Berry, Kai Zhu, Sang Hyuk Im

The mystery of the buried interface in perovskite photovoltaics is deciphered by combining advanced spectroscopy techniques with a lift‐off strategy. The findings open a new avenue to understanding performance losses and thus the design of unique passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

Abstract

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non‐exposed feature. Herein, the mystery of the buried interface in full device stacks is deciphered by combining advanced in situ spectroscopy techniques with a facile lift‐off strategy. By establishing the microstructure–property relations, the basic losses at the contact interfaces are systematically presented, and it is found that the buried interface losses induced by both the sub‐microscale extended imperfections and lead‐halide inhomogeneities are major roadblocks toward improvement of device performance. The losses can be considerably mitigated by the use of a passivation‐molecule‐assisted microstructural reconstruction, which unlocks the full potential for improving device performance. The findings open a new avenue to understanding performance losses and thus the design of new passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

Nature Materials, Published online: 04 January 2021; doi:10.1038/s41563-020-00865-5

Nature Materials, Published online: 04 January 2021; doi:10.1038/s41563-020-00859-3

Nature Energy, Published online: 04 January 2021; doi:10.1038/s41560-020-00747-9