【作者】 李妍；

【导师】 侯延冰；

【作者基本信息】 北京交通大学 ， 光学， 2009， 博士

【摘要】 近年来,由于有机聚合物太阳能电池的设计性、成本低、重量轻、光吸收率高以及可制备大面积柔性器件等优点,备受人们的关注。然而,目前有机聚合物太阳电池的光电转换效率低下,使之不能商业化、实用化。而制约其转换效率提高的主要因素是:在聚合物中形成的激子束缚能大、激子扩散长度短、载流子迁移率低和电荷在电极处的无效收集。考虑到上述几个方面的原因,在本论文中我们对有机聚合物太阳能电池进行了改进,利用聚合物/无机材料复合体系,对该体系的光伏特性进行了研究。主要的研究内容如下:1为了增大聚合物中激子的分离界面、提高载流子的传输几率,我们引入了无机材料-TiO₂纳米管,采用ITO/PEDOT:PSS/聚合物:TiO₂/Al结构,制备了聚合物/无机TiO₂纳米管本体异质结结构光伏器件。研究取得以下结果:ITO/PEDOT:PSS/MEH-PPV:TiO₂/Al光伏器件。实验表明,TiO₂纳米管的加入增强了器件的光伏特性,并且随着TiO₂纳米管混合比例的提高,器件的短路光电流和光电转换效率也相应的提高了。ITO/PEDOT:PSS/MEH-PPV:C₆₀（+TiO₂）/Al光伏器件。器件的光伏特性与TiO₂纳米管的浓度紧密相关。适量的TiO₂纳米管将有效地连接C₆₀分子,为载流子尤其是电子的传输提供更直接的导带路径,进而提高载流子的传输几率。ITO/PEDOT:PSS/P3HT:PCBM（+TiO₂）/Al光伏器件。SEM电镜显示,TiO₂纳米管在薄膜中的分布是无序的,纵向和斜向分布为电子的传输提供了不间断的路径,减少了电子在传输过程中的跳跃次数,提高器件的转换效率;而横向分布的TiO₂纳米管则为载流子复合提供了场所,阻碍了载流子的传输,降低器件的转换效率。2为了进一步分离载流子的传输路径,减小载流子复合几率,以及加强载流子在电极处的收集效率,我们采用聚合物-富勒烯和无机半导体TiO₂以及CdS设计出聚合物—无机半导体层间复合结构光伏器件,并研究了该复合体系器件的光伏特性。ITO/PEDOT:PSS/MEH-PPV:C₆₀/TiO₂/LiF/Al光伏器件。TiO₂层的插入使器件的短路电流和填充因子得到了明显的提高,在光强为16.7mW/cm²的500nm单色光照射下,器件的短路电流为2.35mA/cm²,填充因子为0.284,光电转换效率提高了20%。器件的光电转换效率与无机半导体TiO₂层的厚度和激活层中MEH-PPV:C₆₀质量比有关,当TiO₂层厚度为20nm同时MEH-PPV:C₆₀质量比为2:1时,光电转换效率达到最大值。为了进一步证明器件效率的提高是来自于TiO₂层的插入。我们还制备了ITO/PEDOT:PSS/MEH-PPV/TiO₂/LiF/Al器件,结果显示器件的短路电流和填充因子提高明显。ITO/PEDOT:PSS/MEH-PPV:C₆₀/CdS/Al光伏器件。该器件则显示出了显著增大的短路电流,在光强为16.7mW/cm²的500nm单色光激发下,器件的短路电流达到了4.67mA/cm²,光电转换效率更是达到了5.3%。显著增大的光电流不仅是由于CdS层的引入改变了器件激活层中载流子的产生和传输区域,CdS层自身对入射光的吸收也是使光电流增大的一个重要因素。同样,器件的效率随无机半导体CdS层的厚度变化产生明显的不同,随CdS层厚度（40-10nm）的减小,器件的光电转换效率提高。ITO/PEDOT:PSS/P3HT:PCBM/CdS/Al光伏器件。由于CdS的插入改变了器件的串、并联电阻,不仅使器件的光电流和光电转换效率显著增大,而且器件的填充因子也明显得到了加强。3为了进一步研究热退火处理对聚合物光伏器件的作用,进一步加强聚合物链在薄膜中的取向作用以及载流子在薄膜中的传输效率,我们一方面研究了热处理对P3HT:PCBM薄膜表面形貌的影响。另一方面利用反向电场对ITO/PEDOT:PSS/MEH-PPV:C₆₀/Al器件进行热退火处理,并对处理后器件的光伏特性进行了研究。SEM电镜结果表明,热退火处理提高了P3HT:PCBM薄膜的表面粗糙度,而且后处理器件比之前处理器件表面粗糙度的提高更加明显。表面粗糙度的增加将增强入射光在界面处的反射作用,增强入射光在激活层中的吸收,进而提高器件的光电流和转换效率。在120℃热退火的同时对ITO/PEDOT:PSS/MEH-PPV:C₆₀/Al器件施加一个反向电压的作用,短路电流和光电转换效率提高5倍以上。器件光伏特性的改善来自于薄膜中热动分子沿着外加电压方向的有序分布。同时器件光伏特性与反向电压幅值和退火时间密切相关,采用-6V 10min可以使器件的光电转换效率达到最大。4为了加强聚合物器件中载流子尤其是电子在阴极处的收集效率,我们采用Ag材料制备了Ag/Al复合电极。Ag/Al复合电极有效地阻止了聚合物与Al电极的接触,防止了Al₂O₃层的形成,平衡了器件中载流子的传输,提高了器件的填充因子。而且Ag/Al复合电极对激活层还可以起到一定的保护作用,减缓器件光伏特性的衰减过程。更多还原

【Abstract】 Recently,organic polymer solar cells have attacted much attention due to their unique optical,electronic,low cost and flexible properties.However,it is difficult for polymer photovoltaics（PV） to realize commercialization resulting from the low efficiency.The limited factors for increasing efficiency are:the bound-energy of exciton is high,the disffusion length of exciton is short,the mobility of carrier is low and charge collection efficiency near the interface of polymer/electrode is uneffective. In this dissertation,we use polymer/inorganic hybrid systems to instead of single polymer systems in this work,and make much reseach on hybrid systems.The main investigating contents included:1 In order to increase dissociation interface of excitons and enhance transport of carriers,we introduce TiO₂-nanotubes to PV device and use the structure of ITO/ poly（3,4- ethylenedioxythiophene） doped with poly（styrenesulfonate） （PEDOT:PSS）/polymer:TiO₂/Al for fabricating bulk heterojunction devices.The obtained results are:PV device with the structure of ITO/PEDOT:PSS/poly[2-methoxy-5-（2’-ethylhexoxy-1,4-phenylenevinylene）MEH-PPV:TiO₂-nanotubes/Al.The experimental results show that photovoltaic characteristics of the device are improved with increasing concentration of TiO₂-nanotubes.PV device with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀ （+TiO₂-nanotubes）/Al.The performance of device is dependent on the concentration of TiO₂-nanotubes,appropriate concentration of TiO₂-nanotubes can connect moles of C₆₀ which afford more direct band pathway for carrier transport especially electron transport.PV device with the structure of ITO/PEDOT:PSS/Poly（3-hexylthiophene）（P3HT）: [6,6]-phenyl C61-butyric acid methyl ester（PCBM）（+TiO₂-nanotubes）/Al.SEM images show that TiO₂-nanotubes arrangement in the film is disordered;vertical arrangement and sideling arrangement of TiO₂-nanotubes afford direct pathways for carrier transport and reduce the numbers of carrier hopping,thus increase the conversion efficiency（η） of the device.Howevr,transverse arrangement of TiO₂ nanotubes afford recombination center for carriers and block the paveways for carrier transport resulting to lowerdηof the device. 2 In order to optimize the route of photogenerated carriers,reduce carrier recombination and enhance collection of electrons at the cathode,we use polymer: fullerence and inorganic materials-TiO₂ or cadmium sulfide（CdS） to fabricate polymer-inorganic hybrid PV device,and investigate the photovoltaic properties of this kind of PV devices.We design the devices with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀/ TiO₂/LiF/Al.These devices show promising photovoltaic characteristics with higher I_sc and FF for introducing TiO₂ layer.I_sc reached to 2.35mA/cm²,FF reached to 0.284 andηincreased by 20%under the illumination of 500nm light with the intensity of 16.7mW/cm².Besides,both the thickness of TiO₂ layer and the weight ratios of MEH-PPV:C₆₀ can influence the photovoltaic properties of the device. When the thickness of TiO₂ film is 20nm and weight ratios of MEH-PPV:C₆₀ is 2:1, we can obtain the optimumη.For further clarifying that the increasedηcomes from TiO₂ layer,we also fabricate device with the structure of ITO/PEDOT:PSS/MEH-PPV/ TiO₂/LiF/Al.The results show that the introduction of TiO₂ increases I_sc and FF obviously.ITO/PEDOT:PSS/MEH-PPV:C₆₀/CdS/Al.The device shows a promising I_sc which reached to 4.67mA/cm² andηreached to 5.3%under the illumination of 500nm light with the intensity of 16.7mW/cm².So high I_sc not only come from the change of zone and amount of photogenerated carriers,but also the absorption of CdS in the visible light.The thickness of CdS layer also affectηof the device:the thiner the thickness is,the higher theηis in the range we are interested in.ITO/PEDOT:PSS/ P3HT:PCBM /CdS/Al.The device not only shows higher I_sc andη,but also improved FF because of the change of shunt resistance and series resistance.3 In order to investigating the effect of annealing on photovoltaic properties of solar cells and morphology of polymer.We investigate the influence of annealing on interfacial morphology.Besides,PV device with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀/Al is annealed under presence of reverse bias.SEM image of P3HT:PCBM film show that roughness of the film increased by annealing,and the post-annealed device indicated the rougher interface than pre-annealed device.The increased interfacial roughness can enhance photocurrent and efficiency of the device.Under the presence of a reverse bias,the device with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀/Al is treated at 120℃.After thermal treatment, I_sc andηof PV devices are improved more than 5 times.Attributing to the polarization in MEH-PPV layer,the resulting devices show improved photovoltaic properties.Additionally,the bias value and time also affect the photovoltaic properties of the device,the highestηcan be obtain when the bias is -6V and the time is 10min.4 We introduce Ag/Al hybrid electrode to the PV device for effective collection of electrons at the cathode.Ag/Al hybrid electrode prevents the interaction happening between Al and the polymer active layer,and play an active role in balancing carrier transport.Thus Ag/Al hybride electrode enhanced the FF of PV device.更多还原