Inorganic-organic hybrid solar cell: bridging quantum dots to conjugated polymer nanowires

Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.     

Controlled growth of a molecular bulk heterojunction photovoltaic cell.

The power conversion efficiency of organic photovoltaic cells has increased with the introduction of the donor-acceptor heterojunction that serves to dissociate strongly bound photogenerated excitons. Further efficiency increases have been achieved in both polymer and small-molecular-mass organic photovoltaic cells through the use of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the donor and acceptor materials. However, the random distribution of donor and acceptor materials in such structures can lead to charge trapping at bottlenecks and cul-de-sacs in the conducting pathways to the electrodes. Here, we present a method for growing crystalline organic films into a controlled bulk heterojunction; that is, the positions and orientations of donor and acceptor materials are determined during growth by organic vapour-phase deposition (OVPD), eliminating contorted and resistive conducting pathways while maximizing the interface area. This results in a substantial increase in power conversion efficiency compared with the best values obtained by 'random' small-molecular-weight BHJ solar cells formed by high-temperature annealing, or planar double heterojunction photovoltaic cells using the same archetypal materials systems.     

影响共混结构聚合物光电池性能的因素

共混结构聚合物光电池是一种极有前途的太阳能电池, 其性能主要由器件的开路电压Voc,短路电流Isc,填充因子FF,能量转换效率η等因素决定.笔者结合自己的实验经验,从溶剂、给体与受体材料的比例、器件后处理、电极等方面综述了这些因素对器件的性能影响,为提高聚合物光电池工艺提供了有意义的信息.     

High‐Performance Organic Bulk‐Heterojunction Solar Cells Based on Multiple‐Donor or Multiple‐Acceptor Components

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next‐generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple‐donor or multiple‐acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple‐donor or multiple‐acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple‐donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple‐acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.     

New diketopyrrolopyrrole based D–A–D π-conjugated molecules: Synthesis,optical, electrochemical,morphological and photovoltaic properties

Two solution processable donor–acceptor, π-conjugated molecules that consist of diketopyrrolopyrrole (DPP) central acceptor unit with dibenzofuran (DPP-DBF) or acenaphtene (DPP-ACN) donor substituents, were prepared by Suzuki coupling reaction. The optical, electrochemical and film forming properties of these D–A–D molecules were investigated and used as active materials in bulk heterojunction solar cells.     

Diketopyrrolopyrrole-based π-bridged donor-acceptor polymer for photovoltaic applications

We report the synthesis, properties, and photovoltaic applications of a new conjugated copolymer (C12DPP-π-BT) containing a donor group (bithiophene) and an acceptor group (2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), bridged by a phenyl group. Using cyclic voltammetry, we found the energy levels of C12DPP-π-BT are intermediate to common electron donor and acceptor photovoltaic materials, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. Whereas P3HT and PCBM are exclusively electron donating or accepting, we predict C12DPP-π-BT may uniquely serve as either an electron donor or an acceptor when paired with PCBM or P3HT forming junctions with large built-in potentials. We confirmed the ambipolar nature of C12DPP-π-BT in space charge limited current measurements and in C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT bulk heterojunction solar cells, achieving power conversion efficiencies of 1.67% and 0.84%, respectively, under illumination of AM 1.5G (100 mW/cm(2)). Adding diiodooctane to C12DPP-π-BT:PCBM improved donor-acceptor inter-mixing and film uniformity, and therefore enhanced charge separation and overall device efficiency. Using higher-molecular-weight polymer C12DPP-π-BT in both C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT devices improved charge transport and hence the performance of the solar cells. In addition, we compared the structural and electronic properties of C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT blends, representing the materials classes of polymer:fullerene and polymer:polymer blends. In C12DPP-π-BT:PCBM blends, higher short circuit currents were obtained, consistent with faster charge transfer and balanced electron and hole transport, but lower open circuit voltages may be reduced by trap-assisted recombination and interfacial recombination losses. In contrast, C12DPP-π-BT:P3HT blends exhibit higher open circuit voltage, but short circuit currents were limited by charge transfer between the polymers. In conclusion, C12DPP-π-BT is a promising material with intrinsic ambipolar characteristics for organic photovoltaics and may operate as either a donor or acceptor in the design of bulk heterojunction solar cells.     

Controlled phase separation for efficient energy conversion in dye/polymer blend bulk heterojunction photovoltaic cells

Low-cost photovoltaic energy conversion using conjugated polymers has achieved great improvement due to the invention of organic bulk heterojunction, in which the nanoscale phase separation of electron donor and acceptor favors realizing efficient charge separation and collection. We investigated the polymer photovoltaic cells using N, N′-bis(1-ethylpropyl)-3,4,9,10-perylene bis(tetracarboxyl diimide)/poly(3-hexyl thiophene) blend as an active layer. It is found that processing conditions for the blend films have major effects on its morphology and hence the energy conversion efficiency of the resulting devices. By optimizing the processing conditions, the sizes of donor/acceptor phase separation can be adjusted for realizing efficient charge separation and collection. The overall energy conversion efficiency of the photovoltaic cell processed with optimized conditions increases by nearly 40% compared to the normally spin-coated and annealed cell.     

Photovoltaic characterization of hybrid solar cells using surface modified TiO(2) nanoparticles and poly(3-hexyl)thiophene

We report on the photovoltaic performance of bulk heterojunction solar cells using novel nanoparticles of 6-palmitate ascorbic acid surface modified TiO(2) as an electron acceptor embedded into the donor poly(3-hexyl)thiophene (P3HT) matrix. Devices were fabricated by using P3HT with varying amounts of red TiO(2) nanoparticles (1:1, 1:2, 1:3?w-w ratio). The devices were characterized by measuring current-voltage characteristics under simulated AM 1.5 conditions. Incident photon to current efficiency (IPCE) was spectrally resolved. The nanoscale morphology of such organic/inorganic hybrid blends was also investigated using atomic force microscopy (AFM).     

Solution-processed bulk heterojunction solar cells based on interpenetrating CdS nanowires and carbon nanotubes

Incorporation of a bulk heterojunction is an effective strategy to enhance charge separation and carrier transport in solar cells, and has been adopted in polymeric and colloidal nanoparticle solar cells to improve energy conversion efficiency. Here, we report bulk heterojunction solar cells based on one-dimensional structures, fabricated by mixing CdS nanowires (CdS NWs) and single-walled carbon nanotubes (CNTs) to form a composite film with mutually interpenetrating networks through a simple solution-filtration process. Within the composite, the CNT network boosts charge separation by extracting holes generated from CdS NWs and also forms the transport path for carrier collection by the external electrode. At an optimized CNT loading of about 5 wt.%, the CdS NW/CNT bulk heterojunction solar cells showed three orders of magnitude increase in photocurrent and cell efficiency compared to a cell with the same materials arranged in a stacked layer configuration with a plain heterojunction. External quantum efficiency and photoluminescence studies revealed the efficient charge transfer process from photoexcited CdS NWs to CNTs in the mixed form. Our results indicate that the bulk heterojunction structure strategy can be extended to semiconductor NWs and CNTs and can greatly improve solar cell performance.      

Mechanism of recombination losses in bulk heterojunction P3HT:PCBM solar cells studied using intensity modulated photocurrent spectroscopy

Intensity modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies were used to study the mechanism of photoprocesses in P3HT:PCBM bulk heterojunction organic solar cells at various light intensities. The use of the frequency domain techniques allowed us to separate the bulk and interfacial processes and gain a valuable insight into the mechanism of losses in these devices. The results provide direct evidence that interfacial nongeminate recombination is one of the dominant loss and aging mechanisms in bulk heterojunction organic solar cells. The trapping of photoexcited holes in the P3HT phase was found to contribute to the increased recombination rate. The results suggest that promising ways of improving the efficiency of bulk heterojunction solar cells may be reducing the charge trapping both at and near the P3HT:PCBM interface, as well as improving the efficiency of charge extraction at contacts.     

Oligoselenophene derivatives functionalized with a diketopyrrolopyrrole core for molecular bulk heterojunction solar cells

Solution-processable oligoselenophenes functionalized with diketopyrrolopyrrole cores have been synthesized for use as the donor material in bulk heterojunction solar cells. The optical absorption of these materials extends to the edge of the visible spectrum. Power conversion efficiencies of 1.53 ± 0.04% for DPPS and 0.84 ± 0.04% for DPPDS were obtained under simulated 100 mW/cm(2) AM 1.5G irradiation for devices when PC(61)BM was used as an acceptor. DPPS showed hole mobilities of 4 × 10(-5) cm(2)/(V s) and a peak external quantum efficiency (EQE) of 25%, while DPPDS showed hole mobilities of 2 × 10(-5) cm(2)/(V s) and a peak EQE of 19%. To the best of our knowledge, these are the first oligoselenophenes that have been reported in molecular bulk heterojunction solar cells and this study could serve as a springboard for the design and optimization of high-performance selenophene-containing photovoltaics.     

Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols

High charge-separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make 'plastic' solar cells a compelling option for tomorrow's photovoltaics. Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power-conversion efficiency have, however, met with limited success. By incorporating a few volume per cent of alkanedithiols in the solution used to spin-cast films comprising a low-bandgap polymer and a fullerene derivative, the power-conversion efficiency of photovoltaic cells (air-mass 1.5 global conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology. This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.     

Solution-processed organic solar cells from dye molecules: an investigation of diketopyrrolopyrrole:vinazene heterojunctions

Although one of the most attractive aspects of organic solar cells is their low cost and ease of fabrication, the active materials incorporated into the vast majority of reported bulk heterojunction (BHJ) solar cells include a semiconducting polymer and a fullerene derivative, classes of materials which are both typically difficult and expensive to prepare. In this study, we demonstrate that effective BHJs can be fabricated from two easily synthesized dye molecules. Solar cells incorporating a diketopyrrolopyrrole (DPP)-based molecule as a donor and a dicyanoimidazole (Vinazene) acceptor function as an active layer in BHJ solar cells, producing relatively high open circuit voltages and power conversion efficiencies (PCEs) up to 1.1%. Atomic force microscope images of the films show that active layers are rough and apparently have large donor and acceptor domains on the surface, whereas photoluminescence of the blends is incompletely quenched, suggesting that higher PCEs might be obtained if the morphology could be improved to yield smaller domain sizes and a larger interfacial area between donor and acceptor phases.     

Solution‐Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

Recent research efforts on solution‐processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation–recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.     

Thermal annealing effect on the nanomechanical properties and structure of P3HT:PCBM thin films

Nanostructured polymer-fullerene thin films are among the most prominent materials for application in high efficient polymer solar cells. Specifically, poly(3-hexylthiophene) (P3HT) and fullerene derivatives (PCBM) blends are used as the donor/acceptor materials forming a bulk heterojunction. Although P3HT:PCBM properties have been extensively studied, less light has been set on its nanomechanical properties, which affect the device service life. In this work Atomic Force Acoustic Microscopy (AFAM), Atomic Force Spectroscopy and Nanoindentation were used to study the effect of the fullerene presence and the annealing on the P3HT:PCBM nanomechanical behavior. The P3HT:PCBM thin films were prepared by spin coating on glass substrates and then annealed at 100 °C and 145 °C for 30 min. Large phase separation was identified by optical and Atomic Force Microscopy (AFM) for the annealed samples. Needle-like PCBM crystals were formed and an increase of the polymer crystallinity degree with the increase of the annealing temperature was confirmed by X-ray diffraction. AFAM characterization revealed the presence of aggregates close to stiff PCBM crystals, possibly consisting of amorphous P3HT material. AFM force-distance curves showed a continuous change in stiffness in the vicinity of the PCBM crystals, due to the PCBM depletion near its crystals, and the AFM indentation provided qualitative results about the changes in P3HT nanomechanical response after annealing.     

新型三元聚合物给体材料的合成及在有机太阳能电池中的应用

以吡咯并[3,4-c]吡咯二酮(DPP)为A单元,苯并[1,2-b∶4,5-b′]二噻吩(BDT)和萘为D单元,合成了一种新型2D/A型三元共轭聚合物太阳能电池给体材料(PDPP-BDT-NT),通过核磁共振氢谱(1 H NMR)对其结构进行了表征,通过热重分析、紫外-可见吸收光谱(UV-Vis)、循环伏安法对其热学性质、光物理性能及能级结构进行了研究。PDPP-BDT-NT具有较好的热稳定性,热分解的温度为401℃,有较宽的吸收光谱,可覆盖300~900nm,最高占据轨道(HOMO)能级为-5.35eV。以聚合物PDPP-BDT-NT为给体材料,PC60BM为受体材料,制备了一系列有机聚合物太阳能电池,在大气质量(AM)为1.5G,功率为100mW·cm-2模拟的太阳光照射下,有机聚合物太阳能电池的光电转化效率(PCE)可达2.09%。甲醇处理后,有机聚合物太阳能电池的PCE可达2.34%。     

Polymerizable fullerene-based material for organic solar cells

A polymerizable fullerene derivative was designed and applied as the electron acceptor material in organic bulk heterojunction solar cells in combination with P3HT. This material was easily processed using environmentally friendly solvents such as toluene and xylene instead of conventional chlorobenzene. Moreover, toluene-processed solar cells yielded better performances than devices produced using chlorobenzene as a solvent. Solar cells based on the designed polymerizable fullerene derivative demonstrated highly stable operation even at elevated temperatures.     

Printing highly efficient organic solar cells

The technological attraction in organic solar cells is their compatibility to printing processes. However, up to today, nearly no literature on "printed" organic solar cells have been published and the major body of the research work was done by spin coating or blading techniques. Transferring the spin-coating or doctor blading process currently used for the fabrication of bulk heterojunction solar cell to a printing process holds morphological challenges that have not been observed or reported up to today. We highlight these challenges and we show that inkjet printing of organic bulk heterojunction solar cells requires completely novel approaches and skill sets compared to the current state of the art. By adjusting the chemical properties of the poly(3-hexylthiophene) polymer donor and by using our recently developed inkjet solvent mixture, we have gained control over the nanomorphology of poly(3-hexylthiophene):fullerene blends during the printing process and report a new record power conversion efficiency of 3.5% for inkjet printed poly(3-hexylthiophene):fullerene based solar cells.     

Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene-electron-acceptor-based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx-2 with quinoxaline-containing fused core and PBDB-TF as donor. The significant increase in photovoltaic performance of AQx-2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx-1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure–morphology–property relationship is established. The stronger π–π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx-2-based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next-generation high-performance OSCs.     

The role of solvent and morphology on miscibility of methanofullerene and poly(3-hexylthiophene)

A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases - a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) bilayer film - the evolution of the P3HT-PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry.