Chen Weijie

Nature Energy, Published online: 13 April 2020; doi:10.1038/s41560-020-0598-5

Stacking multiple junctions with different bandgaps and operating under concentrated light allows solar cells to reach efficiencies beyond the limits of standard devices. Geisz et al. present a six-junction solar cell based on III–V materials with a 47.1% efficiency—the highest reported to date.

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Jian Kang, Shan Chen, Xiaole Zhao, Huajie Yin, Weiping Zhang, Mohammad Al-Mamun, Porun Liu, Yun Wang, Huijun Zhao

The bromination of non‐fullerene acceptors provides a promising alternative approach for the creation of high‐performance organic solar cells. BTIC‐2Br‐m ‐based devices exhibit an outstanding power conversion efficiency of 16.11% with an elevated open circuit voltage of 0.88 V, representing one of the highest efficiencies in brominated non‐fullerene acceptors.

Abstract

The concept of bromination for organic solar cells has received little attention. However, the electron withdrawing ability and noncovalent interactions of bromine are similar to those of fluorine and chlorine atoms. A tetra‐brominated non‐fullerene acceptor, designated as BTIC‐4Br, has been recently developed by introducing bromine atoms onto the end‐capping group of 2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) malononitrile and displayed a high power conversion efficiency (PCE) of 12%. To further improve its photovoltaic performance, the acceptor is optimized either by introducing a longer alkyl chain to the core or by modulating the numbers of bromine substituents. After changing each end‐group to a single bromine, the BTIC‐2Br‐m ‐based devices exhibit an outstanding PCE of 16.11% with an elevated open‐circuit voltage of V _oc = 0.88 V, one of the highest PCEs reported among brominated non‐fullerene acceptors. This significant improvement can be attributed to the higher light harvesting efficiency, optimized morphology, and higher exciton quenching efficiencies of the di‐brominated acceptor. These results demonstrate that the substitution of bromine onto the terminal group of non‐fullerene acceptors results in high‐efficiency organic semiconductors, and promotes the use of the halogen‐substituted strategy for polymer solar cell applications.

A nonhalogen organic salt sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite films for the first time, yielding an obvious enhancement of power conversion efficiency from 18.70% to 20.05% for perovskite solar cells, which originates from the surface passivation of the perovskite film with reduced trap state densities and suppressed interfacial charge recombination.

Ionic defects at the surfaces of organolead halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells (PSCs). Herein, sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite layer for the first time, leading to the efficient surface passivation of the perovskite film and consequently significant enhancements in both efficiency and stability of mixed‐cation PSC devices. Upon incorporating STS atop the perovskite layer, the power conversion efficiency of the Cs_0.05MA_0.12FA_0.83PbI_2.55Br_0.45 (abbreviated as CsMAFA) mesoporous‐structure mixed‐cation PSC devices improves from 18.70% to 20.05% with reduced hysteresis. The sulfonate (–SO₃ ⁻) anion of STS coordinates with the Pb²⁺ of CsMAFA perovskite, and the Na⁺ cation of STS electrostatically interacts with the anions (I⁻/Br⁻) of CsMAFA perovskite, resulting in the surface passivation of the CsMAFA perovskite film with reduced electron and hole trap state densities. In addition, STS modification induces an upshift of the valence band of perovskite, facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination. Moreover, such a trap state passivation of perovskite film leads to improvement of the ambient stability of PSC devices.

A CsBr buffer layer is inserted between NiO _x hole transport layer and perovskite layer to relieve the lattice mismatch induced interface stress and induce more ordered crystal growth. The results show that the addition of the CsBr buffer layer optimizes the interface between the perovskite and NiO _x , reduces interface defects and traps, and enhances the hole extraction/transfer.

Abstract

Recent research shows that the interface state in perovskite solar cells is the main factor which affects the stability and performance of the device, and interface engineering including strain engineering is an effective method to solve this issue. In this work, a CsBr buffer layer is inserted between NiO _x hole transport layer and perovskite layer to relieve the lattice mismatch induced interface stress and induce more ordered crystal growth. The experimental and theoretical results show that the addition of the CsBr buffer layer optimizes the interface between the perovskite absorber layer and the NiO _x hole transport layer, reduces interface defects and traps, and enhances the hole extraction/transfer. The experimental results show that the power conversion efficiency of optimal device reaches up to 19.7% which is significantly higher than the efficiency of the device without the CsBr buffer layer. Meanwhile, the device stability is also improved. This work provides a deep understanding of the NiO _x /perovskite interface and provides a new strategy for interface optimization.

Side group effect in ternary polymer solar cells is studied by adopting polymers with different side groups. With appropriate side group modification, high power conversion efficiency (PCE) over 13% is realized, which could mainly be attributed to the broadened photoresponse and optimized molecular stacking. The results demonstrate that side group plays a crucial role in determining the molecular stacking of ternary heterojunction.

Abstract

Ternary strategy is a promising approach to broaden the photoresponse of polymer solar cells (PSCs) by adopting combinatory photoactive blends. However, it could lead to a more complicated situation in manipulating the bulk morphology. Achieving an ideal morphology that enhances the charge transport and light absorption simultaneously is an essential avenue to promote the device performance. Herein, two polymers with different lengths of side groups (P1 is based on phenyl side group and P2 is based on biphenyl side group) are adopted in the dual‐acceptor ternary systems to evaluate the relationship between conjugated side group and crystalline behavior in the ternary system. The P1 ternary system delivers a greatly improved power conversion efficiency (PCE) of 13.06%, which could be attributed to the intense and broad photoresponse and improved charge transport originating from the improved crystallinity. Inversely, the P2 ternary device only exhibits a poor PCE of 8.97%, where the decreased device performance could mainly be ascribed to the disturbed molecular stacking of the components originating from the overlong conjugated side group. The results demonstrate a conjugated side group could greatly determine the device performance by tuning the crystallinity of components in ternary systems.

Addition of the n‐type dopant benzyl viologen (BV) into several best‐in‐class organic bulk‐heterojunctions (BHJ) is shown to consistently improve the power conversion efficiency (PCE) of the resulting solar cells. The presence of BV inside the BHJs increases the absorption coefficient, balances charge transport, and enhances the charge‐carrier density. These synergistic effects result in organic photovoltaics with a maximum PCE of 17.1%.

Abstract

Molecular doping is often used in organic semiconductors to tune their (opto)electronic properties. Despite its versatility, however, its application in organic photovoltaics (OPVs) remains limited and restricted to p‐type dopants. In an effort to control the charge transport within the bulk‐heterojunction (BHJ) of OPVs, the n‐type dopant benzyl viologen (BV) is incorporated in a BHJ composed of the donor polymer PM6 and the small‐molecule acceptor IT‐4F. The power conversion efficiency (PCE) of the cells is found to increase from 13.2% to 14.4% upon addition of 0.004 wt% BV. Analysis of the photoactive materials and devices reveals that BV acts simultaneously as n‐type dopant and microstructure modifier for the BHJ. Under optimal BV concentrations, these synergistic effects result in balanced hole and electron mobilities, higher absorption coefficients and increased charge‐carrier density within the BHJ, while significantly extending the cells' shelf‐lifetime. The n‐type doping strategy is applied to five additional BHJ systems, for which similarly remarkable performance improvements are obtained. OPVs of particular interest are based on the ternary PM6:Y6:PC₇₁BM:BV(0.004 wt%) blend for which a maximum PCE of 17.1%, is obtained. The effectiveness of the n‐doping strategy highlights electron transport in NFA‐based OPVs as being a key issue.

Hysteresis in perovskite solar cells is suppressed with the insertion of a phenyl‐C61‐butyric acid methyl ester (PCBM) layer. In situ PL imaging is employed to observe the ionic migration in the perovskite layer, perovskite/PCBM bilayer and PPCBM bilayer. The mobilizable PCBM molecules are able to diffuse into the perovskite and therein passivate iodine ions/vacancies, thus reducing the hysteresis.

Abstract

The power conversion efficiency of inorganic–organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical‐field‐induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in‐situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl‐C61‐butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter‐diffusion. By employing wide‐field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature‐dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X‐ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built‐in field modulation and interfacial barriers.

The active layer morphology of all‐small‐molecule organic solar cells (SM‐OSCs) is tuned by side chain engineering of the donor molecules and thermal annealing (TA) of the devices. An SM‐OSC based on A–D–A‐structured SM1‐F with fluorine and alkyl substituents as the donor and Y6 as the acceptor, and with TA, demonstrates a high power conversion efficiency of 14.07%.

Abstract

It is very important to fine‐tune the nanoscale morphology of donor:acceptor blend active layers for improving the photovoltaic performance of all‐small‐molecule organic solar cells (SM‐OSCs). In this work, two new small molecule donor materials are synthesized with different substituents on their thiophene conjugated side chains, including SM1‐S with alkylthio and SM1‐F with fluorine and alkyl substituents, and the previously reported donor molecule SM1 with an alkyl substituent, for investigating the effect of different conjugated side chains on the molecular aggregation and the photophysical, and photovoltaic properties of the donor molecules. As a result, an SM1‐F‐based SM‐OSC with Y6 as the acceptor, and with thermal annealing (TA) at 120 °C for 10 min, demonstrates the highest power conversion efficiency value of 14.07%, which is one of the best values for SM‐OSCs reported so far. Besides, these results also reveal that different side chains of the small molecules can distinctly influence the crystallinity characteristics and aggregation features, and TA treatment can effectively fine‐tune the phase separation to form suitable donor–acceptor interpenetrating networks, which is beneficial for exciton dissociation and charge transportation, leading to highly efficient photovoltaic performance.

The inverse temperature‐dependencies of the photovoltaic parameters in MAPbI₃ perovskite solar cells lead to obtaining a peak efficiency of 21.4% at 220 K. These T ‐varied behaviors are related to combined properties of improved interfacial charge extraction, reduced charge trap density, and suppressed nonradiative recombination at lower temperatures.

Abstract

Understanding the factors that limit the performance of perovskite solar cells (PSCs) can be enriched by detailed temperature (T )‐dependent studies. Based on p‐i‐n type PSCs with prototype methylammonium lead triiodide (MAPbI₃) perovskite absorbers, T ‐dependent photovoltaic properties are explored and negative T ‐coefficients for the three device parameters (V _OC, J _SC, and FF) are observed within a wide low T ‐range, leading to a maximum power conversion efficiency (PCE) of 21.4% with an impressive fill factor (FF) approaching 82% at 220 K. These T ‐behaviors are explained by the enhanced interfacial charge transfer, reduced charge trapping with suppressed nonradiative recombination and narrowed optical bandgap at lower T . By comparing the T ‐dependent device behaviors based on MAPbI₃ devices containing a PASP passivation layer, enhanced PCE at room temperature is observed but different tendencies showing attenuating T ‐dependencies of J _SC and FF, which eventually leads to nearly T ‐invariable PCEs. These results indicate that charge extraction with the utilized all‐organic charge transporting layers is not a limiting factor for low‐T device operation, meanwhile the trap passivation layer of choice can play a role in the T ‐dependent photovoltaic properties and thus needs to be considered for PSCs operating in a temperature‐variable environment.

SnO₂ and perovskite have been bridged with multifunctional graphdiyne. Such delicate interface modification boosted the performance of solar cells in energy band alignment, electron mobility improvement, controllable perovskite growth inducement, and interface defect passivation.

Abstract

The matching of charge transport layer and photoactive layer is critical in solar energy conversion devices, especially for planar perovskite solar cells based on the SnO₂ electron‐transfer layer (ETL) owing to its unmatched photogenerated electron and hole extraction rates. Graphdiyne (GDY) with multi‐roles has been incorporated to maximize the matching between SnO₂ and perovskite regarding electron extraction rate optimization and interface engineering towards both perovskite crystallization process and subsequent photovoltaic service duration. The GDY doped SnO₂ layer has fourfold improved electron mobility due to freshly formed C−O σ bond and more facilitated band alignment. The enhanced hydrophobicity inhibits heterogeneous perovskite nucleation, contributing to a high‐quality film with diminished grain boundaries and lower defect density. Also, the interfacial passivation of Pb−I anti‐site defects has been demonstrated via GDY introduction.

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Kevin J. Rietwyk, Boer Tan, Adam Surmiak, Jianfeng Lu, David P. McMeekin, Sonia R. Raga, Noel Duffy, Udo Bach

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Guozhen Liu, Haiying Zheng, Huifen Xu, Liying Zhang, Xiaoxiao Xu, Shendong Xu, Xu Pan

Publication date: July 2020

Source: Nano Energy, Volume 73

Author(s): Faguang Zhou, Zhizai Li, Huanyu Chen, Qian Wang, Liming Ding, Zhiwen Jin

Publication date: 20 May 2020

Source: Joule, Volume 4, Issue 5

Author(s): Felix Lang, Marko Jošt, Kyle Frohna, Eike Köhnen, Amran Al-Ashouri, Alan R. Bowman, Tobias Bertram, Anna Belen Morales-Vilches, Dibyashree Koushik, Elizabeth M. Tennyson, Krzysztof Galkowski, Giovanni Landi, Mariadriana Creatore, Bernd Stannowski, Christian A. Kaufmann, Jürgen Bundesmann, Jörg Rappich, Bernd Rech, Andrea Denker, Steve Albrecht

Publication date: 20 May 2020

Source: Joule, Volume 4, Issue 5

Author(s): Donghwan Koo, Sungwoo Jung, Jihyung Seo, Gyujeong Jeong, Yunseong Choi, Junghyun Lee, Sang Myeon Lee, Yongjoon Cho, Mingyu Jeong, Jungho Lee, Jiyeon Oh, Changduk Yang, Hyesung Park