Open-Circuit Voltage Losses in Selenium-Substituted Organic Photovoltaic Devices from Increased Density of Charge-Transfer States

详细信息    查看全文

• 作者：Dana B. Sulas ; Kai Yao ; Jeremy J. Intemann ; Spencer T. Williams ; Chang-Zhi Li ; Chu-Chen Chueh ; Jeffrey J. Richards ; Yuyin Xi ; Lilo D. Pozzo ; Cody W. Schlenker ; Alex K.-Y. Jen ; David S. Ginger
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：October 13, 2015
• 年：2015
• 卷：27
• 期：19
• 页码：6583-6591
• 全文大小：609K
• ISSN：1520-5002

文摘

Using an analysis based on Marcus theory, we characterize losses in open-circuit voltage (V_OC) due to changes in charge-transfer state energy, electronic coupling, and spatial density of charge-transfer states in a series of polymer/fullerene solar cells. We use a series of indacenodithiophene polymers and their selenium-substituted analogs as electron donor materials and fullerenes as the acceptors. By combining device measurements and spectroscopic studies (including subgap photocurrent, electroluminescence, and, importantly, time-resolved photoluminescence of the charge-transfer state) we are able to isolate the values for electronic coupling and the density of charge-transfer states (N_CT), rather than the more commonly measured product of these values. We find values for N_CT that are surprisingly large (鈭?.5 脳 10²¹鈥?.2 脳 10²² cm^鈥?), and we find that a significant increase in N_CT upon selenium substitution in donor polymers correlates with lower V_OC for bulk heterojunction photovoltaic devices. The increase in N_CT upon selenium substitution is also consistent with nanoscale morphological characterization. Using transmission electron microscopy, selected area electron diffraction, and grazing incidence wide-angle X-ray scattering, we find evidence of more intermixed polymer and fullerene domains in the selenophene blends, which have higher densities of polymer/fullerene interfacial charge-transfer states. Our results provide an important step toward understanding the spatial nature of charge-transfer states and their effect on the open-circuit voltage of polymer/fullerene solar cells.