Abstract
We investigate the influence of the built-in voltage on the performance of organic bulk heterojuction solar cells that are based on a p-i-n structure. Electrical doping in the hole and the electron transport layer allows to tune their work function and hence to adjust the built-in voltage: Changing the doping concentration from 0.5 to 32 wt% induces a shift of the work function towards the transport levels and increases the built-in voltage. To determine the built-in voltage, we use electroabsorption spectroscopy which is based on an evaluation of the spectra caused by a change in absorption due to an electric field (Stark effect). For a model system with a bulk heterojunction of BF-DPB and C60, we show that higher doping concentrations in both the electron and the hole transport layer increase the built-in voltage, leading to an enhanced short circuit current and solar cell performance.
Highlights
Organic electronics have several advantages compared to inorganic technologies, allowing flexible, transparent, and large-area products
The electroabsorption spectroscopy (EA) spectra measured at the first and at the second harmonic frequency of a bulk heterojunction (BHJ) solar cell with 1 wt% doping concentration in the hole transport layer (HTL) and 32 wt% doping concentration in the electron transport layer (ETL) are shown in Figure 2 for several bias voltages
The results are shown for BHJ solar cells with 1.0 wt% and 17.5 wt% doping concentration in the HTL corresponding to work functions of 4.77 eV and 5.03 eV
Summary
Organic electronics have several advantages compared to inorganic technologies, allowing flexible, transparent, and large-area products. Systematic studies are possible by electroabsorption spectroscopy on organic solar cells based on the p-i-n concept.[14] Here, the intrinsic heterojunction is embedded between a p-doped hole transport layer (HTL) and an n-doped electron transport layer (ETL).[15] A variation of the doping concentration in the ETL as well as in the HTL allows to tune their work function.[16,17]. Using this concept, a systematic analysis of the built-in voltage is possible without the requirement of changing the electrode material. A clear increase of VBI is observed for larger doping concentrations in the charge carrier transport layers, leading to an increased short circuit current and improved device performance
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