Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices

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• 作者：Qing-Dan Yang ; Ho-Wa Li ; Yuanhang Cheng ; Zhiqiang Guan ; Taili Liu ; Tsz-Wai Ng ; Chun-Sing Lee ; Sai-Wing Tsang
• 刊名：ACS Applied Materials & Interfaces
• 出版年：2016
• 出版时间：March 23, 2016
• 年：2016
• 卷：8
• 期：11
• 页码：7283-7290
• 全文大小：478K
• ISSN：1944-8252

文摘

Energy level alignment at the organic donor and acceptor interface is a key to determine the photovoltaic performance in organic solar cells, but direct probing of such energy alignment is still challenging especially for solution-processed bulk heterojunction (BHJ) thin films. Here we report a systematic investigation on probing the energy level alignment with different approaches in five commonly used polymer:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PCBM) BHJ systems. We find that by tuning the weight ratio of polymer to PCBM the electronic features from both polymer and PCBM can be obtained by photoemission spectroscopy. Using this approach, we find that some of the BHJ blends simply follow vacuum level alignment, but others show strong energy level shifting as a result of Fermi level pinning. Independently, by measuring the temperature-dependent open-circuit voltage (V_OC), we find that the effective energy gap (E_eff), the energy difference between the highest occupied molecular orbital of the polymer donor (E_HOMO–D) and lowest unoccupied molecular orbital of the PCBM acceptor (E_LUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated V_OC at 0 K. Consequently, the photovoltage loss of various organic BHJ photovoltaic devices at room temperature is in a range of 0.3–0.6 V. It is believed that the demonstrated direct measurement approach of the energy level alignment in solution-processed organic BHJ will bring deeper insight into the origin of the V_OC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.