title: Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices.
abstract: 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 (VOC), we find that the effective energy gap (Eeff), the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO-D) and lowest unoccupied molecular orbital of the PCBM acceptor (ELUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated VOC 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 VOC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Qing-Dan
authors.initials: QD
authors.lastname: Yang
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Ho-Wa
authors.initials: HW
authors.lastname: Li
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Yuanhang
authors.initials: Y
authors.lastname: Cheng
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Guan
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Taili
authors.initials: T
authors.lastname: Liu
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Tsz-Wai
authors.initials: TW
authors.lastname: Ng
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.|Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Chun-Sing
authors.initials: CS
authors.lastname: Lee
authors.affiliation: Department of Physics and Materials Science, City University of Hong Kong , Hong Kong SAR, P. R. China.
authors.forename: Sai-Wing
authors.initials: SW
authors.lastname: Tsang
keywords: bulk heterojunction; energy diagram; photoemission spectroscopy; photovoltage loss; photovoltaics