高分子混摻異質接面太陽能電池光載子動態行為分析

Abstract

本研究主要利用暫態分析技術對高分子太陽能電池中光載子的動態行為進行探討。近年來，高分子太陽能電池的光電轉換效率已有大幅度的進展，主要來自於其施體與受體混摻的奈米形貌優化，顯示控制施體與受體的混摻形貌對於光載子的動態行為有著重大的影響。 在本研究中首先針對高分子與無機二氧化鈦奈米粒子混摻太陽能電池進行分析，應用不同濃度的二氧化鈦奈米粒子的潻加量，在50wt%時可得到最好的元件效率，同時也對應到最平衡的電子與電洞遷移率及最佳的載子生命週期。接著藉由控制二氧化鈦奈米粒子的尺寸及形狀，並使用掃描式電子穿透顯微鏡做3-D結構的影像重建，可以得知棒狀的奈米粒子能夠提供電子更有效率的傳導路經，且較容易形成較施體與受體材料分別的連續相，使得載子有更長的生命週期，因此在電荷的傳導及收集上，棒狀奈米粒子比球狀更有效率，能夠減少更多不必要的損失。本研究也利用化學方式來修飾二氧化鈦奈米粒子表面，除了增進與有機高分子的相容性，並有效地增進了電荷轉移能力及減少了電子電洞在異質接面上的再結合損失，其光電轉換效率可被提升到2.2%。 利用石墨稀氧化物來取代傳統電洞傳導層的應用也在本研究中有詳盡的描述。中性的石墨稀氧化物可利用溶液製備方式將其塗布於氧化錫銦電極上，做為一電洞傳導層並抑制電子往陽極的傳輸，可有效地減少電子與電洞的再結合速率，使得元件光電轉換效率可與以傳統導電高分子做為電洞傳導層的元件相匹敵。

In this thesis, photocarrier dynamics in bulk heterojunction polymer solar cell was comprehensively investigated with various transient techniques. Power conversion efficiency of polymer solar cells have been remarkably improved by the optimization of donor/acceptor blending nanomorphologies, which are vital for efficient photocurrent generation. It also significantly affects the phtotocarrier dynamics such as exciton dissociation, carrier mobility and transport recombination. This thesis not only aims to correlate carrier dynamics and solar cell performance by morphology control but also providing a promising methodology for the future study of improvement in power conversion efficiency. In chapter 3, TiO2 concentration dependent carrier dynamics was investigated in P3HT/TiO2 nanorods hybrid BHJ PVs. The best power conversion efficiency was found at 50wt% of TiO2 nanorods where the amount of TiO2 are enough to create a bi-continuous transport phase and resulted in balanced electron and hole mobility and also a longest carrier lifetime. Further in chapter 4, we investigated the interplay of 3-D morphologies by STEM-HAADF and the photocarrier dynamics of P3HT/TiO2 NPs and NRs hybrids BHJ PVs. The anisotropic TiO2 nanorods can not only reduce the probability of the inter-particle hopping transport of electrons by providing better connectivity with respect to the TiO2 nanoparticles, but also tend to form a large-scale donor-acceptor phase-separated morphology. The presence of dimensionality of TiO2 nanocrystals ensures the formation of favorable morphology for polymer/inorganic hybrid solar cells and results in more effective mobile carrier generation and more efficient and balanced transport of carriers. Furthermore, in chapter 5, we present that through interface modification with effective molecules, the photovoltaic performance of P3HT/TiO2 NRs can be largely improved by enhancing charge separation and suppressing interface recombination rate in the polymer/inorganic hybrids. Similar process of surface modification can was also successfully demonstrated in P3HT/nanostructured ZnO hybrid solar cells. The utilization of graphene oxide (GO) thin films as the hole transport and electron blocking layer in organic photovoltaics (OPVs) is demonstrated in chapter 6. The incorporation of GO as a hole transport layer leads to the decrease in recombination and leakage currents. Solar cell performance is comparable to devices fabricated with PEDOT:PSS as the hole transport layer. Indicating that GO could be a simple solution-processable alternative to PEDOT:PSS as the effective hole transport and electron blocking layer in OPV and light-emitting diode devices.