【作者】 徐苗；

【导师】 彭俊彪；

【作者基本信息】 华南理工大学 ， 材料物理与化学， 2010， 博士

【摘要】 适用于溶液加工的聚合物-富勒烯体异质结光伏器件由于其具有质量轻,成本低廉,易于大面积制备以及可用于制备柔性器件等方面的优势,在太阳电池和光电探测器等应用领域受到了人们广泛的关注。由于有机聚合物半导体材料的载流子迁移率、激子扩散长度以及载流子寿命等方面特性均远远低于无机半导体材料,因此基于有机半导体材料的光电器件一般表现出具有较低的光电转换效率以及较慢的工作速度。这严重的制约了有机半导体材料在光伏器件中的应用。而体异质结概念的提出,大大提高了有机半导体器件内部光生激子的拆分效率,这使得光电转换效率得到大大的增强。当前,进一步提高光伏器件的光电转换效率,以及挖掘有机体异质结器件的应用潜力是人们的努力方向。本论文尝试使用具有宽带隙的共轭聚合物聚(3,6-(N-(2-乙基己基))咔唑)(PCz)作为给体材料,与富勒烯的衍生物—6,6苯基-碳61-丁酸甲酯(PCBM)共混后,制备了体异质结型光伏器件以实现对紫外光(UVA)波段的探测。我们对器件的电流-电压(J-V)特性、光-电流响应特性、瞬态响应特性以及工作寿命方面进行了测量与分析。其中,零偏压时,在波长为346 nm处,紫外探测器的外量子效率达到18 %。这个数值可以与有机—无机杂化,以及一些无机器件的性能相比拟。通过时间响应曲线的测量,发现有效面积为4 mm2的紫外探测器响应时间小于500 ns,可以满足较快响应器件的应用的需求。另外,大多数有机聚合物在经紫外波段光线照射后会出现分子键断裂的现象,这会造成有源层失效。因此,我们采用的PCz材料制备的探测器,测量其在紫外光下连续照射下的工作寿命,结果发现在8 mW/cm2的紫外光下经过320小时连续照射后,紫外探测器的光电转换性能基本保持不变,表现出了很好的光稳定性。近年来,随着聚合物半导体材料已经被广泛应用于各种光电器件中,人们对它的认识及研究逐渐深入。但聚合物半导体的载流子迁移率作为影响器件性能的重要物理参数却一直很难测定。由于聚合物半导体材料中载流子迁移率非常低,因此适用于测量无机半导体材料载流子迁移率的霍尔效应法不能有效的测量聚合物材料。目前,能够直接探测载流子在聚合物半导体材料中扩散时间的方法为时间渡越(TOF)法。但是由于聚合物半导体薄膜的激子束缚能较高,并且内部具有大量的陷阱态和缺陷态。因此,使用TOF法测试聚合物半导体材料迁移率时很难得到良好的瞬态光电流波形,这使得迁移率的测定非常困难。我们在TOF测量中,在靠近ITO与聚合物中间引入了一层由待测聚合物与C60共同组成的共混层。在共混层中,聚合物与C60之间存在大量的D/A界面,使得该层内光生的激子能够有效的被拆分,因此我们称其为电荷分离层(CSL)。CSL的存在使得TOF测量中瞬态光电流波形得到了大大的增强,使渡越时间的确定更加准确。该方法适合于窄带隙聚合物太阳电池给体材料空穴迁移率的测量。本论文的另一个工作是从聚合物薄膜制膜方法入手,探讨了不同制膜方法对有机体异质结太阳电池性能的影响。我们分别使用滴涂法与旋涂法制备了基于MEH-PPV: PCBM共混薄膜的太阳电池。通过对性能的比较发现,采用滴涂法制备的MEH-PPV: PCBM太阳电池具有较高的能量转换效率。我们对不同制备方法制得的MEH-PPV薄膜的迁移率、表面形貌以及吸收光谱进行了研究,发现使用滴涂法制备的MEH-PPV薄膜在纳米尺度具有更高的有序度,并且能够使薄膜的吸收光谱发生红移。由于滴涂法在成膜原理上与丝网印刷法以及喷墨打印法等制膜方法相同。因此,对滴涂法的研究可以为利用丝网印刷法或喷墨打印法制备大面积、低成本、高性能的薄膜太阳电池提供重要的参考。另外,在第五章中,本论文还利用矩阵光学原理,使用数学软件Matlab,对聚合物体异质结太阳电池内部光电场的分布进行了模拟计算。对光电场分布的模拟计算可以为太阳电池器件结构的优化以及器件工作中物理过程的理解提供必要的帮助。在基于P3HT:PCBM的体异质结太阳电池中,对具有不同厚度有源层的器件进行了模拟计算发现,有源层中入射光能量分布的拟合结果与实验能量转换效率的结果相符。同时,我们还通过模拟计算得到,在太阳电池中加入光学修饰层可以使得器件内部光电场重新分布。因此通过选择合适的材料以及调节光学修饰层的厚度可以使得有源层中光能量的分布实现优化。更多还原

【Abstract】 Polymer-fullerene bulk-heterojunction (BHJ) photovoltaic devices have attracted great attention in recent years due to their low-cost, simple manufacting process to realize large area devices and technical compatibility with flexible devices. However, Organic semiconductor devices always exhibit the poor transient response characteristic and the low efficiency of photo/electric conversion because of their low charge-carrier mobility, short exciton diffusing length and short charge carrier lifttime. The poor performance in charge transportation restricts the widely application of organic semiconductors. Now, a simple, yet successful technique is the solution-processed bulk-heterojunction that leads the organic materials to a potential alternate to the inorganic counterpart in photovoltaic devices.In Chapter 2, A high-performance polymer ultraviolet (UV) detector based on a simple sandwich structure consisting of the wide-band-gap conjugated polymer of poly(3,6-(N-2-ethylhexyl)carbazole) (PCz) as the electron donor (D) blending with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor (A) was fabricated. The detector with the structure of ITO/PEDOT:PSS/PCz:PCBM(2:1 by w/w)/Al is sensitive to UV light, and the zero-bias photoresponsivity in UV range reaches as high as about 50 mA/W. The corresponding quantum efficiency is 18% (el/ph) under illumination with 346 nm UV light. The ratio of the photocurrent to dark current (Dynamic range) is 1.3×105 at zero bias under illumination of 355 nm UV light with power of 1 mW/cm2. The transient behavior and stability of the detector were also characterized and discussed.A charge carrier mobility of polymer films with the time-of-flight (TOF) technique by using a fullerene layer was measured and the TOF photocurrent waveform can be remarkably improved. The 80-nm-thick fullerene layer is functioned as a charge-separation layer (CSL) which was placed between ITO electrode and the polymer layer of MEH-PPV (poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene]). In the CSL, the photo-generated holes and electrons can be efficiently separated, resulting in an enhanced current signal and great improvement of TOF waveform. The sample structure with fullerene layer exhibits a great advantage to measure hole mobilities of polymers with low energy band gap.Hole mobilities of poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4- phenylenevinylene]( MEH-PPV )films fabricated by spin-casting and drop-casting have been measured by time-of-flight (TOF) technique. The non-dispersive hole transport current waveform is obtained. The results exhibit that the hole mobility of MEH-PPV prepared by drop-casting is higher than that from the film prepared by spin-casting. The solar cells based on MEH-PPV and fullerene derivative blend films prepared by spin-casting and drop-casting, respectively, were fabricated. The power-conversion efficiency (PCE) of the drop-casting device has a great improvement over 35%, compared with that of spin-casting one. The improvement attributes to the stronger absorption and more balance of electrons and holes transport in MEH-PPV polymer films.In Chapter 5, we have modeled experimental short-circuit photocurrent action spectra of thin film bulk-heterojunction photovoltaic cells. Modeled was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated exctions, which are dissociated by charge transfer action at the donor/acceptor interface. To calculate the distribution of optical electrical field inside cells, we have adopted the matrix modeling technique, which allows the accuracy simulation of all of the interference effects in the multilayered thin films. For P3HT: PCBM bulk-heterojunction photovoltaic cells, we have shown that the experimental power-conversion efficiency (PCE) is modulated by varying the thickness of active layer, which obeys calculation results of optical electric field distribution. The calculated internal optical electric field distribution can be used for studying the influence of the geometrical structure on the performance and optimizing the PCE, in the thin film photovoltaic cells. Additionally, we found that the internal optical electric field distribution would be reformed by introducing the optical spacer into bulk-heterojunction photovoltaic cells. Therefore, by introducing the suitable optical spacer would optimize the photocurrent action spectra to realize the higher PCE of photovoltaic cells.更多还原