有机薄膜太阳能电池的研究进展

近年来,由于硅系太阳能电池的高纯硅原料价格昂贵,将低成本的有机半导体材料用于太阳能电池的研究越来越引起学术界的高度重视.而且有机半导体材料具有轻质量、柔韧易加工性、可低成本大面积制备等突出优点,使之具有很强的竞争力.介绍了有机薄膜太阳能电池及材料的工作原理,综述几种重要有机半导体的真空沉积镀膜法以及电池结构的研究进展.     

高效率有机太阳电池的理论建模

采用综合考虑温度、电场强度、载流子浓度的普遍迁移率模型,利用实际太阳能光谱和非富勒烯材料的吸收系数来计算载流子的产生,结合漂移扩散方程、电流连续性方程等对高效率有机太阳电池进行理论建模。利用该模型计算了器件的电流-电压曲线、开路电压-光照强度曲线和短路电流-光照强度曲线。结果发现,利用该模型计算的电流-电压曲线与实验数据符合很好,其他两种曲线也与实验数据符合较好。此外,利用该模型分析了能量无序度对器件性能的影响,结果表明减小材料的能量无序度可以提高有机太阳电池的性能。     

Imaging the Electric Potential within Organic Solar Cells

The charge transport in organic solar cells is investigated by surface potential measurements via scanning Kelvin probe microscopy. Access to the solar cell's cross‐section is gained by milling holes with a focused ion beam which enables the direct scan along the charge transport path. In a study of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, the open circuit voltage is built up at the top contact. A comparison of the potential distribution within normal and inverted solar cells under operation exhibits strongly different behaviors, which can be assigned to a difference in interface properties.     

Near IR Sensitization of Organic Bulk Heterojunction Solar Cells: Towards Optimization of the Spectral Response of Organic Solar Cells

The spectroscopic response of a poly(3‐hexylthiophene)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT/PCBM)‐based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b´]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] [PCPDTBT] to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two‐step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible.     

有机/无机复合体异质结太阳电池

利用简单的低温工艺制备了纳米晶纤锌矿结构的ZnO,用高分辨透射电镜(HRTEM)、X射线衍射(XRD)和光致发光(PL)技术进行了表征.利用纳米晶ZnO和共轭聚合物2-甲氧基-5-(3,7.二甲基辛氧基)对苯撑乙烯(MDMO-PPV)制备了结构为ITO/PEDOT:PSS/ZnO:MDMO-PPV/Al的有机/无机复合体异质结太阳电池,作为对比,同时制备了ITO/PEDOT:PSS/MDMO-PPV/Al结构的纯有机聚合物电池.实验结果表明,添加纳米晶ZnO使其能量转换效率提高了约550倍.PL谱测试结果表明这是由于有高电子亲合能的ZnO提高了电子空穴对分离的能力.另外,光伏性能的提高可能也与ZnO引起的电子传输能力的提高有关.此外,本文分析了ZnO:MDMOPPV体异质结电池性能低于传统电池的原因,并提出了进一步提高其性能的方法.     

Solution‐Processed Organic Tandem Solar Cells

A solution‐processed polymer tandem cell fabricated by stacking two single cells in series is demonstrated. The two bulk‐heterojunction subcells have complementary absorption maxima at λ_max ～ 850 nm and λ_max ～ 550 nm, respectively. A composite middle electrode is applied that serves both as a charge‐recombination center and as a protecting layer for the first cell during spin‐coating of the second cell. The subcells are electronically coupled in series, which leads to a high open‐circuit voltage of 1.4 V, equal to the sum of each subcell. The layer thickness of the first (bottom) cell is tuned to maximize the optical absorption of the second (top) cell. The performance of the tandem cell is presently limited by the relatively low photocurrent generation in the small‐bandgap polymer of the top cell. The combination of our tandem architecture with more efficient small‐bandgap materials will enable the realization of highly efficient organic solar cells in the near future.     

Accurate Measurement and Characterization of Organic Solar Cells

Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral‐mismatch factors for different test‐cell/reference‐cell combinations. The test devices include both polymer/fullerene‐based bulk‐heterojunction solar cells and small‐molecule‐based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light‐bias intensity is reported. The current–voltage curves are measured under 100 mW cm^–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094‐1) generated from a source set with a reference cell and corrected for spectral error.     

有机/无机复合体异质结太阳电池

利用简单的低温工艺制备了纳米晶纤锌矿结构的ZnO,用高分辨透射电镜(HRTEM)、X射线衍射(XRD)和光致发光(PL)技术进行了表征.利用纳米晶ZnO和共轭聚合物2-甲氧基-5-(3,7.二甲基辛氧基)对苯撑乙烯(MDMO-PPV)制备了结构为ITO/PEDOT:PSS/ZnO:MDMO-PPV/Al的有机/无机复合体异质结太阳电池,作为对比,同时制备了ITO/PEDOT:PSS/MDMO-PPV/Al结构的纯有机聚合物电池.实验结果表明,添加纳米晶ZnO使其能量转换效率提高了约550倍.PL谱测试结果表明这是由于有高电子亲合能的ZnO提高了电子空穴对分离的能力.另外,光伏性能的提高可能也与ZnO引起的电子传输能力的提高有关.此外,本文分析了ZnO:MDMOPPV体异质结电池性能低于传统电池的原因,并提出了进一步提高其性能的方法.     

Efficient Organic Tandem Solar Cells based on Small Molecules

In this paper, two vacuum processed single heterojunction organic solar cells with complementary absorption are described and the construction and optimization of tandem solar cells based on the combination of these heterojunctions demonstrated. The red‐absorbing heterojunction consists of C₆₀ and a fluorinated zinc phthalocyanine derivative (F4‐ZnPc) that leads to a 0.1–0.15 V higher open circuit voltage V_oc than the commonly used ZnPc. The second heterojunction incorporates C₆₀ and a dicyanovinyl‐capped sexithiophene derivative (DCV6T) that mainly absorbs in the green. The combination of both heterojunctions into one tandem solar cell leads to an absorption over the whole visible range of the sun spectrum. Thickness variations of the transparent p‐doped optical spacer between both subcells in the tandem solar cell is shown to lead to a significant change in short circuit current density j_sc due to optical interference effects, whereas V_oc and fill factor are hardly affected. The maximum efficiency η of about 5.6% is found for a spacer thickness of 150‐165 nm. Based on the optimized 165nm thick spacer, effects of intensity and angle of illumination, and temperature on a tandem device are investigated. Variations in illumination intensity lead to a linear change in j_sc over three orders of magnitude and a nearly constant η in the range of 30 to 310 mW cm^?2. Despite the stacked heterojunctions, the performance of the tandem device is robust against different illumination angles: j_sc and η closely follow a cosine behavior between 0° and 70°. Investigations of the temperature behavior of the tandem device show an increase in η of 0.016 percentage points per Kelvin between ?20 °C and 25 °C followed by a plateau up to 50 °C. Finally, further optimization of the tandem stack results in a certified η of (6.07 ± 0.24)% on (1.9893 ± 0.0060)cm² (Fraunhofer ISE), i.e., areas large enough to be of relevance for modules.     

Organic Tandem and Multi‐Junction Solar Cells

The emerging field of stacked layers (double‐ and even multi‐layers) in organic photovoltaic cells is reviewed. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single‐layer bulk heterojunction photovoltaic cell. Furthermore, the low charge‐carrier mobilities of most organic materials limit the thickness of the active layer. Consequently, only part of the intensity of the incident light at the absorption maximum is absorbed. A tandem or multi‐junction solar cell, consisting of multiple layers each with their specific absorption maximum and width, can overcome these limitations and can cover a larger part of the solar flux. In addition, tandem or multi‐junction solar cells offer the distinct advantage that photon energy is used more efficiently, because the voltage at which charges are collected in each sub‐cell is closer to the energy of the photons absorbed in that cell. Recent developments in both small‐molecule and polymeric photovoltaic cells are discussed, and examples of photovoltaic architectures, geometries, and materials combinations that result in tandem and multi‐junction solar cells are presented.     

有机聚合物太阳电池电极的表面性能研究

采用多种方法对有机聚合物电池的电极进行表面处理,在测量接触角的基础上,应用几何平均法计算了电极样品的表面能和极性度,研究了处理方法对电极表面润湿性能的影响。结果表明,电极表面性能与其处理方法密切相关,等离子处理具有最小的接触角、最大的表面能和极性度,有效增强了电极表面的润湿性能,这一结果对于优化电极/活性层的界面性质,改善有机聚合物太阳电池的光伏性能具有非常重要的作用。     

Recent Progress in Flexible and Stretchable Organic Solar Cells

Flexible and stretchable organic solar cells (OSCs) have attracted enormous attention due to their potential applications in wearable and portable devices. To achieve flexibility and stretchability, many efforts have been made with regard to mechanically robust electrodes, interface layers, and photoactive semiconductors. This has greatly improved the performance of the devices. State‐of‐the‐art flexible and stretchable OSCs have achieved a power conversion efficiency of 15.21% (16.55% for tandem flexible devices) and 13%, respectively. Here, the recent progress of flexible and stretchable OSCs in terms of their components and processing methods are summarized and discussed. The future challenges and perspectives for flexible and stretchable OSCs are also presented.     

Experiment-Oriented Machine Learning of Polymer:Non-Fullerene Organic Solar Cells

Despite the capacity of conjugated materials for enhanced power conversion efficiency (PCE) of organic photovoltaics (OPV), a comprehensive survey of unexplored materials is beyond the reach of most researchers’ resources. In such instances, a data-driven approach using machine learning (ML) is an efficient alternative; however, bridging the gap between experimental observations and data science requires a number of refinements. In this investigation, using a random forest model based on an experimental dataset, a high correlation coefficient of 0.85 is achieved for the ML of polymer and non-fullerene small molecule acceptor OPVs and performed virtual screening of 200,932 conjugated polymers generated by the combinatorial coupling of donor and acceptor units. Further, to evaluate the effectiveness of the ML model, a series of conjugated polymers (based on benzodithiophene and thiazolothiazole) were designed, synthesized, and characterized with different alkyl chains. Among these, PBDTTzEH:IT-4F showed a PCE of 10.10%, which is in good correspondence with ML predictions with respect to the choice of alkyl chains. Thus, the current study demonstrates how ML can be utilized for developing OPVs using a relatively small number of experimental data points (566) and screening numerous molecular structures.     

Conventional Electron Microscopy and Electron Holography of Organic Solar Cells

Organic solar cells are a promising route towards large‐area and low‐price photovoltaic systems. The devices are composed of at least two layers: the hole‐transport layer and the electron‐transport layer. The light absorption can occur in one or both layers. At the interface of the layers the excitons are separated into charge carriers, and every layer deals with one type of carrier. Higher efficiencies of the separation process can be obtained by using a mixed layer containing both materials to obtain a very high interface area. Although the structure of the mixed layers used plays a crucial role for the device performance, until now the morphologies have not been elucidated. In order to correlate physical and optical findings with structure and morphology for the materials in question, electron microscopy experiments were performed on the single components as well as on the layer systems. The conventional electron microscope is a poor phase microscope. As consequence, weak‐phase objects like organic molecules have to be stained or imaged under strong defocus to produce an observable contrast. Artifacts caused by chemical staining and the appearance of Fresnel diffraction using the defocus technique represent the main problems of conventional microscopy. These artifacts can be avoided using electron holography. Holograms of ultrathin sections of thin layers composed of organic dye molecules were recorded. Subsequently, the phase images were reconstructed. In this manner, we succeeded in obtaining high‐contrast electron micrographs without applying staining or defocus. In addition, holograms of crystalline C₆₀ and zinc phthalocyanine were successfully recorded. Holography has been shown to be a useful tool to image beam‐sensitive and weak‐phase objects without artifacts.     

Organic Solar Cells by Annealing Stacked Amorphous and Microcrystalline Layers

Organic solar cells were fabricated by stacking aromatic amine and C₆₀ layers. The energy conversion efficiency of these solar cells was low because of poor photoabsorption by these layers and short diffusion length of excitons. However, the photocurrent density was increased by about 3 times by the application of heat treatment to the stacked organic layers at 140 °C, and the maximum energy conversion efficiency reached 1.1 % under AM 1.5, 100 mW cm^–2 simulated solar light. The internal quantum efficiency of the photocurrent after the annealing reached about 45 %. When the aromatic amine layer was about 100 nm thick, the organic layers after the annealing showed a wrinkled structure under an optical microscope. The annealing temperature needed for the formation of this structure was in good agreement with the temperature needed for the increase in the photocurrent. The morphological change caused by the annealing was attributed to infiltration of the amorphous aromatic amine compound into grain boundaries of the microcrystalline C₆₀ layer, resulting in expansion of the C₆₀ layer and the wrinkled structure of the organic layers. From observation by electron microscopy, the mixed form of these two compounds near the interface was found to be suited to solar cells because the C₆₀ and aromatic amine phases wedge each other in a direction normal to two electrodes. However, the annealing slightly lowered photovoltage of the solar cell. This effect was attributed to a partial contact of the C₆₀ layer with a counter electrode through the aromatic amine layer.     

Charge Density Dependent Nongeminate Recombination in Organic Bulk Heterojunction Solar Cells

Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility and protection of trapped charge carriers from recombination by a donor–acceptor phase separation until re‐emission from these deep states. In order to clarify which mechanism is dominant temperature‐ and illumination‐dependent charge extraction measurements are performed under open circuit and short circuit conditions at poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C₆₁ butyric acid methyl ester (P3HT:PC₆₁BM) and PTB7:PC₇₁BM (poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]]) solar cells in combination with current–voltage characteristics. It is shown that the charge carrier density n dependence of the mobility μ and the recombination prefactor are different for P3HT:PC₆₁BM at temperatures below 300 K and PTB7:PC₇₁BM at room temperature. Therefore, in addition to μ(n), a detrapping limited recombination in systems with at least partial donor–acceptor phase separation is required to explain the high recombination orders.     

Asymmetric and Halogenated Fused-Ring Electron Acceptor for Efficient Organic Solar Cells

Fused-ring non-fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC-M, this work systematically designs and synthesizes an asymmetric counterpart ITCC-M-2F, halogenated counterpart ITCC-Cl, and asymmetric and halogenated counterpart IDTT-Cl-2F. Among these NFAs, IDTT-Cl-2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB-T-2Cl, IDTT-Cl-2F-based OSCs yield the highest PCE of 13.3% with an open-circuit voltage of 0.96 V, short-circuit current of 19.20 mA cm^–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2-(2-chloro-6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.