Effect of Nanocrystalline Domains in Photovoltaic Devices with Benzodithiophene-Based Donor-Acceptor Copolymers
Effect of Nanocrystalline Domains in Photovoltaic Devices with Benzodithiophene-Based Donor-Acceptor Copolymers
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DOI：
10.1021/jp504010x
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发表时间：
2014-08-07
影响因子：
3.7
通讯作者：
Herz, Laura M.
中科院分区：
文献类型：
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作者：
Menelaou, Christopher;Tierney, Steve;Herz, Laura M.
我们研究了薄膜形态对一系列基于苯并二噻吩给体和苯并噻二唑受体单元的给体-受体共聚物的光伏性能的影响。光伏器件，将聚合物：富勒烯共混物显示最高的效率（高达6%）的那些聚合物表现出最低程度的结晶度的X射线衍射图案和相应的最低表面粗糙度的薄膜。我们发现，在薄的聚合物膜中的这种结晶域的存在相关以及与聚合物链聚集体的光谱特征已经存在于溶液中之前，铸造的膜。因此，聚合物溶解度和浇铸条件似乎是提高基于这种给体受体共聚物的光伏器件效率的关键因素。为了研究为什么微晶域的存在下降低设备的效率，我们测量激子扩散长度通过建模的时间依赖性的光致发光从薄的聚合物薄膜上沉积的激子淬灭层的二氧化钛。我们发现这些材料中的激子扩散长度很大（4-7.5纳米），并且随着聚合物结晶度的变化而显示出一些变化。然而，从聚合物：PCBM共混物的聚合物发射的超快（1 ps）淬灭表明，绝大多数激子迅速到达电荷解离界面，因此激子扩散并不代表限制因素。因此，我们认为，随后的电荷提取和寿命必须受到不利影响的结晶域的存在。我们认为，形成的微晶太小，提供显着的增强，在远程电荷载流子迁移率，而是引入域边界，阻碍电荷提取。因此，对于这类材料，需要以高溶解度和链熵为目标的聚合物设计，从而导致无定形膜形成。
We have investigated the effects of thin-film morphology on the photovolatic performance for a series of donor acceptor copolymers based on benzodithiophene donor and benzothiadiazole acceptor units. Photovoltaic devices incorporating polymer:fullerene blends show highest efficiencies (up to 6%) for those polymers exhibiting the least degree of crystallinity in X-ray diffraction patterns and a corresponding lowest surface roughness in thin films. We find that the existence of such crystalline domains in thin polymer films correlates well with spectral signatures of polymer chain aggregates already present in solution prior to casting of the film. Polymer solubility and casting conditions therefore appear to be crucial factors for enhancing efficiencies of photovoltaic devices based on such donor acceptor copolymers. To examine why the presence of crystallite domains lowers device efficiencies, we measured exciton diffusion lengths by modeling the time-dependent photoluminescence from thin polymer films deposited on an exciton quencher layer of TiO2. We find that exciton diffusion lengths in these materials are substantial (4-7.5 nm) and show some variation with polymer crystallinity. However, ultrafast (1 ps) quenching of the polymer emission from polymer:PCBM blends indicates that the vast majority of excitons rapidly reach the charge-dissociating interface, and hence exciton diffusion does not represent a limiting factor. We therefore condude that the subsequent charge extraction and lifetimes must be adversely affected by the presence of crystalline domains. We suggest that the formed crystallites are too small to offer significant enhancements in long-range charge carrier mobility but instead introduce domain boundaries which impede charge extraction. For this class of materials, polymer designs are therefore required that target high solubility and chain entropy, leading to amorphous film formation.