基于双载流子产生通道的高性能有机聚合物光探测器

Organic/polymer photo-detectors (OPDs), which can be used detect and measure the radiation in the corresponding portion of the electromagnetic spectrum, has long been a major focus of basic science research and various applications in broad areas, such as digital imaging, bio-diagnostics, optical communication, environmental monitoring, remote sensing, and night vision. Unlike conventional PDs which consist of p-n junction, OPDs rely on bulk-heterojunction to facilitate charge separation and photo diodes structure for charge collection. It has been established that charge generation OPDs is either through photo-induced electron transfer (PET) from donor to acceptor or photo-induced hole transfer (PHT) from acceptor to donor, or perfectly the combination of both. However, in the past two decades, the state-of-art OPDs has been dominated by the blends of conjugated polymer donor with fullerene acceptor, in which photo-induced charges (mainly electrons) transfer processes occur between the donor/acceptor interface, resulting in a spontaneous dissociation of excitons into free charges. In contrast, seldom has attention been given to the contribution to photocurrent from PHT, mainly due to spectral overlap in the absorption of the donor and acceptor. In this proposal, the applicant aim at the realization of highly sensitive OPDs featuring photocurrent generation from dual pathways: photo-induced electron transfer from a polymer donor to a non-fullerene acceptor and photo-induced hole transfer from the latter to the former, with comparable effiencies. Since the photo-induced hole transfer is critically dependent on the energy offset between the LUMO levels of the non-fullerene acceptor and the polymer, while photo-induced electron transfer occurs with high yield in all of blend from the polymer donor and non-fullerene acceptor, we will use a series of polymer donor with relatively shallow lowest unoccupied molecular orbital (LUMO) as electron donor and a narrow bandgap non-fullerene acceptor with relatively deep lying highest occupied molecular orbital (HOMO), or very low bandgap polymer as donor for this study. To obtain high performance OPDs, the applicant's team will combine device modeling, interface and device structure optimization, as well as device fabrication study. Through this project, it is expected that the key device performance parameters of PPDs, such as sensitivity (detectivity), response speed and linear dynamic range (LDR) be significantly improved, while some of which are comparable to or even better than that of their inorganic counterparts.

有机聚合物光电探测器(OPDs)是一种捕获光信号并将之转化为电信号输出的器件。当前OPDs的发展存在诸多制约性的因素。比如，绝大多数报道的高性能OPDs都是基于电子给体－受体体异质结结构，以此来实现激子的高效解离和产生光电流。其中的电子受体，往往是使用富勒烯衍生物(PCBM)。由于PCBM的吸收区主要在紫外区，这类OPDs中的光电流的产生过程，往往以光诱导电子转移为主, 而光诱导空穴转移效率较低。因而此类器件的内量子效率往往强烈依赖于波长，且载流子的收集渠道往往以单一的PET渠道或者单一的PHT通道为主。本项目拟通过使用窄带隙的新型电子受体材料与宽带隙的电子给体材料,或者以窄带隙电子给体材料为主要吸光材料，结合器件结构设计与界面优化，在OPDs中以双通道收集光生载流子，实现OPDs性能上的新突破。在此基础上，深入研究影响OPDs性能的关键因素，为以后研制更高灵敏度的器件提供坚实的基础。

本项目通过使用窄带隙的新型电子受体材料与宽带隙的电子给体材料,或者以窄带隙电子给体材料为主要吸光材料，结合器件结构设计与界面优化，在OPDs中以双通道收集光生载流子，努力实现OPDs性能上的新突破。在基于新型电子受体材料与宽带隙的电子给体材料的光探测体系中, 我们发现随着给体/受体能级差的减小，器件的辐射效率显著提高，且上述电压损失与器件的辐射效率的对数呈显著负相关。这一发现，为以辐射效率作为评判给体/受体之间的能级差大小提供了实验依据。进一步地，利用这个测量方法，结合器件性能和内量子效率的测量结果，统计得到相对最优的给受体能级差---并以辐射效率测量值的方式直观呈现。科研成果以 “Assessing Energy Offset between Electron Donor/acceptor Interface in Organic Solar Cells through Radiative Efficiency Measurement” (Yuan XIE et al. Energy Environ. Sci., 2019, 12, 3556 )为题发表,为筛选高性能的双通道收集光生载流子的材料体系提供可靠技术保障。我们实验研究了基于有机给体材料PBDTTT-C-T:受体材料FOIC的有机可见-近红外宽光谱相应有机光电探测器的性能。通过不同给受体比例分析对比，表明在PBDTTT-C-T:FOIC质量比为1:1.5时器件理想因子最低，即缺陷越少，SRH缺陷复合低，暗电流最低，因而散粒噪声最弱。通过电致发光与光致发光分析，PBDTTT-C-T:FOIC器件无论是光诱导电子传输通道与光诱导空穴传输通道，其激子拆分形成电荷驱动力都足以使给体PBDTTT-C-T吸收光后产生的电子高效地传输到受体FOIC，受体FOIC吸收光能量产生的空穴高效地传输到给体PBDTTT-C-T。在350-900 nm范围内，内量子效率曲线平滑，800-900 nm处大约有2.0 × 10^13 Jones比探测率，这是目前文献报导中几乎最高的，这表明基于PBDTTT-C-T:FOIC体系的有机光电探测器可以探测低至0.24 pW cm^-2光强的微弱光信号。研究成果发表在ACS Applied Materials & Interfaces, 2020, 12(15): 17769−17775.