Enhancing the Photovoltage of Polymer Solar Cells by Using a Modified Cathode

Abstract

Inexpensive solar cells with a low energy budget are interesting due to the need for alternative energy sources in the future.[1] Organic solar cells may become a low-cost alternative to Si-based solar cells because they are low weight, flexible, and could be produced by roll-to-roll printing techniques for large-area deposition. [2][3][4] Polymer solar cells are still hindered by their low efficiency, measured by the multiplication of short-circuit current (J sc ), open-circuit voltage (V oc ), and fill factor (FF) under solar illumination. J sc depends on the optical and electrical properties of the active layer, such as, absorption spectrum and charge mobility. V oc is proportional to the energy gap between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, but also influenced by the work function difference between electrodes as well as the thickness of the active layers. It is hard to increase V oc and J sc simultaneously in polymer solar cells, because larger photocurrents require a low bandgap absorber for maximum overlap with the solar spectrum, but to enhance V oc a high bandgap absorber is needed. It is common to use a thin layer of LiF between the active layer and the cathode to improve the V oc in polymer solar cells. [5][6][7][8] The role of the submonolayer of LiF has been suggested to be that of protecting the organic layer during metal deposition, modifying the work function of the cathode, and/ or introducing a dipole at the cathode interface, affecting charge injection. [9][10][11][12][13] The possibility to use roll-to-roll printing techniques is the biggest advantage of polymer electronic devices compared with their inorganic counterparts. However, LiF is vapor deposited in vacuum and is impossible to use in printing techniques.In order to increase V oc recently many efforts have been made to build tandem cells. The V oc is increased by stacking two cells on top of each other. [14][15][16][17][18][19] However, the significant loss of J sc makes the overall performance lower than that of a single cell. [19] So far few polymer tandem cells give higher efficiency than single cells. [18] In addition, the procedure to build tandem cells is very complicated involving multilayer deposition both via a solution process and a vapor process in vacuum, [18,19] which is not easy in printing methods.Here, we report a simple method to increase the V oc up to 200 mV and the power conversion efficiency (PCE) by 50 %, by using a thin layer of poly(ethylene oxide) (PEO) to modify the cathode in polymer solar cells. It was reported that a thin layer of PEO between the active layer and the Al or LiF/Al cathode improves the performance of polymer light-emitting diodes (PLED).[20] An enhanced photocurrent of solar cells based on poly[2-methoxy-5-(2'-ethyl-hexoxy)-1,4-phenylene vinylene] by blending a small amount of PEO and LiCF 3 SO 3 into the active layer was also demonstrated. [21] In this Communication, we demonstrate that a thin layer o...

Designs and Architectures for the Next Generation of Organic Solar Cells

“…Therefore, the choice of metals for the cathode will depend on the photoactive material used. For BHJ OSCs, a metal with workfunction higher than the LUMO of the acceptor will form an interface barrier for electrons which reduces extraction; a metal to be in the range 1-2 nm, showed significant increase in V oc (150-200 mV) [58]. All the OSCs with the PEO interlayer showed a higher PCE than those without.…”

Performance Study of Water/Alcohol Soluble Polymer Interface Materials in Polymer Optoelectronic Devices

“…与此同时, 非共轭的水/醇溶聚合物界面材料也在 有机光电器件上得到了成功的应用, 实现了高效的聚合 物发光器件 [24,25] , 聚合物太阳电池 [12,26,27] 等. 目前的研 究表明, 共轭及非共轭水/醇溶界面材料中含有的极性 基团, 在它们的独特界面修饰作用中起了关键作用, 可 以与金属或金属氧化物电极发生相互作用形成界面偶 极而降低电极功函数, 增强电极电子的注入与收集.…”

“…当使用共轭类界面材料 [17,26,32] . 但 是, 相对应其它界面材料修饰的器件, 基于 PEO 的器件 短路电流较低, 这可能是因为 PEO 的导电能力较差, 使 得器件的串联电阻升高, 阻碍了电子在阴极的收集和传 输, 从而使得电流降低.…”

High work-function hole transport layers by self-assembly using a fluorinated additive

“…Two successful strategies for improving V OC are adjusting the energy levels of the donor and acceptor materials [3][4][5][6][7] or by engineering the interfaces between layers. [8][9][10][11][12][13] In this paper we focus on the latter of these two approaches, specically engineering the hole transport layer between the anode and the active layer. Engineering of the active layer/ cathode interface has also proven successful in increasing V OC , with a variety of materials and methods available in the literature.…”