A–π–D–π–A Electron‐Donating Small Molecules for Solution‐Processed Organic Solar Cells: A Review

Organic solar cells based on semiconducting polymers and small molecules have attracted considerable attention in the last two decades. Moreover, the power conversion efficiencies for solution‐processed solar cells containing A–π–D–π–A‐type small molecules and fullerenes have reached 11%. However, the method for designing high‐performance, photovoltaic small molecules still remains unclear. In this review, recent studies on A–π–D–π–A electron‐donating small molecules for organic solar cells are introduced. Moreover, the relationships between molecular properties and device performances are summarized, from which inspiration for the future design of high performance organic solar cells may be obtained.     

Small molecule organic semiconductors on the move: promises for future solar energy technology

This article is written from an organic chemist's point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.     

Diketopyrrolopyrrole‐Based and Tetracyano‐Bridged Small Molecules for Bulk Heterojunction Organic Solar Cells

Research on bulk heterojunction organic solar cells has rapidly grown over the past two decades, and device performance has reached power conversion efficiencies over 13 %. In this focus review, we highlight design strategies used for the development of diketopyrrolopyrrole‐ and tetracyano‐based molecular donors. We also describe how tetracyano‐bridged non‐fullerene acceptors can be developed by a click‐type [2+2]‐cycloaddition–electrocyclic ring‐opening reaction of acetylene‐bridged small molecules with tetracyanoethylene by simple modification.     

Triplet Tellurophene‐Based Acceptors for Organic Solar Cells

Triplet materials have been employed to achieve high‐performing organic solar cells (OSCs) by extending the exciton lifetime and diffusion distances, while the triplet non‐fullerene acceptor materials have never been reported for bulk heterojunction OSCs. Herein, for the first time, three triplet molecular acceptors based on tellurophene with different degrees of ring fusing were designed and synthesized for OSCs. Significantly, these molecules have long exciton lifetime and diffusion lengths, leading to efficient power conversion efficiency (7.52 %), which is the highest value for tellurophene‐based OSCs. The influence of the extent of ring fusing on molecular geometry and OSCs performance was investigated to show the power conversion efficiencies (PCEs) continuously increased along with increasing the extent of ring fusing.     

Recent development of perylene diimide-based small molecular non-fullerene acceptors in organic solar cells

This review summarizes the recent progress of perylene diimide (PDI) derivatives used as the acceptor materials in non-fullerene organic solar cells. The resulting structure-property correlations and design strategies of this type of acceptors are discussed and commented, which will help to constructing high-performance PDI-based acceptor materials in the future. The problems at present and the effort direction are also pointed out in this review.     

可溶液加工给体-受体有机小分子太阳能电池材料研究进展

近年来,有机小分子体异质结太阳能电池因其制备工艺简单、廉价、轻便及柔性等优点而备受关注.理想的有机小分子给体材料是提高有机太阳能电池光电转换效率的基础.系统地综述了可溶液加工有机小分子太阳能电池给体材料的研究进展,并对其发展趋势和应用前景做了展望.     

Small molecular non-fullerene electron acceptors for P3HT-based bulk-heterojunction solar cells

Three small molecules with the same arms and different cores of perylene diimide(PDI)or indaceno[2,1-b:6,5-b']dithiophene(IDT)were designed and synthesized as the acceptor materials for P3HT-based bulk-heterojunction(BHJ)solar cells.The impacts of the different cores on the optical absorption,electrochemical properties,electron mobility,film morphology,photoluminescene characteristics,and solar cell performance were thoroughly studied.The three compounds possess a broad absorption covering the wavelength range of 400–700 nm and relatively low lowest unoccupied molecular orbital(LUMO)energy levels of?3.86,?3.81 and?3.99 eV.The highest power conversion efficiency of 0.82%was achieved for the BHJ solar cells based on SM3 as the acceptor material,the compound with a PDI core.     

Synthesis of tetracyanoethylene‐substituted ferrocene and its device properties

Small organic molecules are promising candidates for cheaper, flexible and good‐performance sources for organic solar cells (OSCs) due to their easy fabrication, low cost and slightly cheaper processing. However, the lower power conversion efficiency of OSCs is the main problem for their applications. Ferrocene structures could be the best candidates for the active layers of OSCs due to their unique properties such as thermal and chemical stability. The electrochemical, electro‐optical and solar cell performances of 2,5‐dicyano‐3‐ferrocenyl‐4‐ferrocenylethynylhexa‐2,4‐dienedinitrile (DiCN‐Fc) structures were investigated. First, the electrochemical and electro‐optical properties were examined for finding the highest occupied and lowest unoccupied molecular orbital values and bandgap of DiCN‐Fc. The best photovoltaic performance was obtained with 7 wt% of DiCN‐Fc loading, with a power conversion efficiency of about 4.27%. In the light of our investigations, ferrocenyl‐substituted small organic molecules could contribute to the development of organic photovoltaic devices.     

Polymer Donors for High‐Performance Non‐Fullerene Organic Solar Cells

Over the past few years, non‐fullerene organic solar cells have been a focus of research and their power conversion efficiencies have been improved dramatically from about 6 % to over 14 %. In addition to innovations in non‐fullerene acceptors, the ongoing development of polymer donors has contributed significantly to the rapid progress of non‐fullerene organic solar cell performance. This Minireview highlights the polymer donors that enable high‐performance non‐fullerene organic solar cells. We show the impressive photovoltaic devices results achieved by some of important classes of conjugated polymer systems in non‐fullerene organic solar cells. We discuss the molecular design strategies as far as developing matching polymer donors for non‐fullerene acceptors. We conclude with a brief summary and outlook for advances in donor polymers required for commercialization.     

可溶液加工的D-A-D型有机小分子的烷基链效应及光电性能

设计合成了3种可溶液加工的基于噻吩给体和2-吡喃-4-亚基丙二氰(PM)受体的新型Donor-Acceptor-Donor(D-A-D)型有机小分子TPT-N, TPT-S和TPT-D. 研究了噻吩给体单元上烷基链的数目对分子的溶解性、 光物理(吸收特性)、 热稳定和光电性能的影响. 结果表明, 随着烷基链的增加, 分子的溶解性增加, 成膜性能提高; 分子在溶液中的吸收光谱发生红移, 薄膜的吸收谱带变窄, 分子的最高占有分子轨道(HOMO)能级提高. 以D-A-D型有机小分子为给体, 富勒烯C₆₀衍生物-苯基-C₆₁-丁酸甲酯(PCBM)为受体制备了结构为ITO/PEDOT∶PSS/D-A-D∶PCBM/LiF/Al的体异质结太阳能电池. 研究结果表明, 基于单烷基链的TPT-S的太阳能电池具有相对较高的能量转换效率. 说明在D-A-D型有机小分子太阳能电池材料中, 烷基链的数目是决定材料性能及器件性能的重要因素之一.     

Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Small Molecules for Highly Efficient Solution‐Processed Organic Solar Cells

Two small molecules named BT‐TPD and TBDT‐TTPD with a thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) unit were designed and synthesized for solution‐processed bulk‐heterojunction solar cells. Their thermal, electrochemical, optical, charge‐transport, and photovoltaic characteristics were investigated. These compounds exhibit strong absorption at 460–560 nm and low highest occupied molecular orbital levels (?5.36 eV). Field‐effect hole mobilities of these compounds are 1.7–7.7×10^?3 cm² V^?1 s^?1. Small‐molecule organic solar cells based on blends of these donor molecules and a acceptor display power conversion efficiencies as high as 4.62 % under the illumination of AM 1.5G, 100 mW cm^?2.     

Small molecule semiconductors for high-efficiency organic photovoltaics

Organic photovoltaic cells (OPVs) are a promising cost-effective alternative to silicon-based solar cells, and possess light-weight, low-cost, and flexibility advantages. Significant progress has been achieved in the development of novel photovoltaic materials and device structures in the last decade. Nowadays small molecular semiconductors for OPVs have attracted considerable attention, due to their advantages over their polymer counterparts, including well-defined molecular structure, definite molecular weight, and high purity without batch to batch variations. The highest power conversion efficiencies of OPVs based on small molecular donor/fullerene acceptors or polymeric donor/fullerene acceptors are up to 6.7% and 8.3%, respectively, and meanwhile nonfullerene acceptors have also exhibited some promising results. In this review we summarize the developments in small molecular donors, acceptors (fullerene derivatives and nonfullerene molecules), and donor-acceptor dyad systems for high-performance multilayer, bulk heterojunction, and single-component OPVs. We focus on correlations of molecular chemical structures with properties, such as absorption, energy levels, charge mobilities, and photovoltaic performances. This structure-property relationship analysis may guide rational structural design and evaluation of photovoltaic materials (253 references).     

Tetrahydroxy‐Perylene Bisimide Embedded in a Zinc Oxide Thin Film as an Electron‐Transporting Layer for High‐Performance Non‐Fullerene Organic Solar Cells

By introduction of four hydroxy (HO) groups into the two perylene bisimide (PBI) bay areas, new HO‐PBI ligands were obtained which upon deprotonation can complex Zn^II ions and photosensitize semiconductive zinc oxide thin films. Such coordination is beneficial for dispersing PBI photosensitizer molecules evenly into metal oxide films to fabricate organic–inorganic hybrid interlayers for organic solar cells. Supported by the photoconductive effect of the ZnO: HO‐PBI hybrid interlayers, improved electron collection and transportation is achieved in fullerene and non‐fullerene polymer solar cell devices, leading to remarkable power conversion efficiencies of up to 15.95 % for a non‐fullerene based organic solar cell.     

D-A structural protean small molecule donor materials for solution-processed organic solar cells

This review summarizes the high performance small molecule donors of organic solar cells in various classes of typical donor-acceptor (D-A) structures and discusses their relationships briefly.     

Development of naphtho[1,2-b:5,6-b']dithiophene based novel small molecules for efficient bulk-heterojunction organic solar cells

Two new small molecules with a rigid planar naphtho[1,2-b:5,6-b']dithiophene (NDT) unit were designed and synthesized. Solution processed bulk-hetereojunction organic solar cells based on blends of the small molecules and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) exhibited promising photovoltaic device performance with a maximum power conversion efficiency up to 2.20% under the illumination of AM 1.5G, 100 mW cm(-2).     

How Does Bridging Core Modification Alter the Photovoltaic Characteristics of Triphenylamine-Based Hole Transport Materials? Theoretical Understanding and Prediction

Perovskite solar cells have gained immense interest from researchers owing to their good photophysical properties, low-cost production, and high power conversion efficiencies. Hole transport materials (HTMs) play a dominant role in enhancing the power conversion efficiencies (PCEs) and long diffusion length of holes and electrons in perovskite solar cells. In hole transport materials, modification of π-linkers has proved to be an efficient approach for enhancing the overall PCE of perovskite solar cells. In this work, π-linker modification of a recently synthesized H−Bi molecule ( R ) is achieved with novel π-linkers. After structural modifications, ten novel HTMs ( HB1–HB10 ) with a D−π−D backbone are obtained. The structure–property relationship, and optoelectronic and photovoltaic characteristics of these newly designed hole transport materials are examined comprehensively and compared with reference molecules. In addition, different geometric parameters are also examined with the assistance of density functional theory (DFT) and time-dependent DFT. All the designed molecules exhibit narrow HOMO–LUMO energy gaps (E_g=2.82–2.99 eV) compared with the R molecule (E_g=3.05 eV). The designed molecules express redshifting in their absorption spectra with low values of excitation energy, which in return offer high power conversion efficiencies. Further, density of states and molecular electrostatic potential analysis is performed to locate the different charge sites in the molecules. The reorganizational energies of holes and electrons are found to have good values, suggesting that these novel designed molecules are efficient hole transport materials for perovskite solar cells. In addition, the low binding energy values of the designed molecules (compared with R ) offer high current charge density. Finally, complex study of HB9:PC₆₁BM is also undertaken to understand the charge transfer between the molecules of the complex. The results of all analyses advocate that these novel designed HTMs are promising candidates for the construction of future high-performance perovskite solar cells.     

Recent studies on small molecular and polymeric hole-transporting materials for high-performance perovskite solar cells

A decade of significant research has led to the emergence of photovoltaic solar cells based on perovskites that have achieved an exceptionally high-power conversion efficiency of 26.08%. A key breakthrough in perovskite solar cells (PSCs) occurred when solid hole-transporting materials (HTMs) replaced liquid electrolytes in dye-sensitized solar cells (DSSCs), because HTMs play a crucial role in improving photovoltaic performance as well as cell stability. This review is mainly focused on the HTMs that are responsible for hole transport and extraction in PSCs, which is one of the crucial components for efficient devices. Here, we have reviewed small molecular as well as polymeric HTMs that have been reported in the last two years and discussed their performance based on the analysis of their molecular architectures. Finally, we include a perspective on the molecular engineering of new functional HTMs for highly efficient stable PSCs.     

All‐Polymer Solar Cells: Recent Progress,Challenges, and Prospects

For over two decades bulk‐heterojunction polymer solar cell (BHJ‐PSC) research was dominated by donor:acceptor BHJ blends based on polymer donors and fullerene molecular acceptors. This situation has changed recently, with non‐fullerene PSCs developing very rapidly. The power conversion efficiencies of non‐fullerene PSCs have now reached over 15 %, which is far above the most efficient fullerene‐based PSCs. Among the various non‐fullerene PSCs, all‐polymer solar cells (APSCs) based on polymer donor‐polymer acceptor BHJs have attracted growing attention, due to the following attractions: 1) large and tunable light absorption of the polymer donor/polymer acceptor pair; 2) robustness of the BHJ film morphology; 3) compatibility with large scale/large area manufacturing; 4) long‐term stability of the cell to external environmental and mechanical stresses. This Minireview highlights the opportunities offered by APSCs, selected polymer families suitable for these devices with optimization to enhance the performance further, and discusses the challenges facing APSC development for commercial applications.     

Mn-doped quantum dot sensitized solar cells: a strategy to boost efficiency over 5%

To make Quantum Dot Sensitized Solar Cells (QDSC) competitive, it is necessary to achieve power conversion efficiencies comparable to other emerging solar cell technologies. By employing Mn(2+) doping of CdS, we have now succeeded in significantly improving QDSC performance. QDSC constructed with Mn-doped-CdS/CdSe deposited on mesoscopic TiO(2) film as photoanode, Cu(2)S/Graphene Oxide composite electrode, and sulfide/polysulfide electrolyte deliver power conversion efficiency of 5.4%.     

Molecular Acceptors Based on a Triarylborane Core Unit for Organic Solar Cells

Triarylboranes that exhibit p–π* conjugation serve as versatile building blocks to design n-type organic/polymer semiconductors. A series of new molecular acceptors based on triarylborane is reported here. These molecules are designed with a boron atom that bears a bulky 2,4,6-tri-tert-butylphenyl (Mes*) substituent at the core and strong electron-withdrawing 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) units as the end-capping groups that are linked to the core by bithiophene bridges. Butyl or butoxy groups are introduced to the bithiophene units to tune the optoelectronic properties. These molecules show nearly planar backbones with highly localized steric hindrance at the core, low LUMO/HOMO energy levels, and broad absorption bands spanning the visible region, which are all very desirable characteristics for use as electron acceptors in organic solar cell (OSC) applications. The attachment of butyl groups to the bithiophene bridges brings about a slightly twisted backbone, which in turn promotes good solubility and homogeneous donor/acceptor blend morphology, whereas the introduction of butoxy groups leads to improved planarity, favorable stacking in the film state, and a greatly reduced band gap. OSC devices based on these molecules exhibit encouraging photovoltaic performances with power conversion efficiencies reaching up to 4.07 %. These results further substantiate the strong potential of triarylboranes as the core unit of small molecule acceptors for OSC applications.