Wide band gap polymers for organic tandem solar cells

Rene Janssen^a, Martijn Wienk^a, Ruurd Heuvel^a

^aEindhoven University of Technology (TU/e), PO Box 513, Eindhoven, 5600, Netherlands

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Ruurd Heuvel, 385
Publication date: 5th February 2015

Organic single junction solar cells suffer from both thermalisation and transmission losses due to a mismatch between the band gap and the solar spectrum. To decrease these losses, multiple junctions with different band gaps can be stacked on top of each other and thereby creating a tandem or multi-junction solar cell. However, not only the band gap but also the absolute energy levels of the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) are of importance as these determine the energy that is lost upon charge separation. Tuning these levels in such a way that the energy loss is as small as possible while still ensuring a good charge separation is therefore a formidable task. To this extend, a wide band gap polymer reported by Woo et al. has been reproduced and adapted to match the energetic needs as imposed by tandem and triple junction solar cells. When using the reported poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) in an optimal performing solar cell, an open circuit voltage (V_oc) of 0.79 V is obtained for both literature and in-house prepared references devices. The energy loss (e_g - qV_oc) with respect to a band gap of 1.76 eV is then calculated to be over 0.9 eV. With an ideal energy loss of 0.6 eV to ensure good charge separation, this leads to believe that another 100 or 200 mV of V_oc can at least be gained. Energy level engineering of PPDT2FBT has been done by changing donor and/or acceptor strength. In this way a lowering of the HOMO and LUMO level was aimed for, thereby lowering the energy loss. Decreasing the LUMO level was done by substituting the fluorine on benzothiadiazole for cyanide as was shown by Heeney et al., thereby increasing acceptor strength. Reducing the donating character of the alkoxy substituted phenyldithiophene was carried out by substituting the ester analogue, thus lowering the HOMO level. Using these strategies three new polymers have been synthesized and their performance tested in solar cells.

Nguyen, T. L.; Choi, H.; Ko, S.-J.; Uddin, M. A.; Walker, B.; Yum, S.; Jeong, J.-E.; Yun, M. H.; Shin, T. J.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Semi-Crystalline Photovoltaic Polymers with Efficiency Exceeding 9% in a ~300 nm Thick Conventional Single-Cell Device Energy Environ. Sci., 2014, 7, 3040–3051 Casey, A.; Han, Y.; Fei, Z.; White, A. J. P.; Anthopoulos, T. D.; Heeney, M. Cyano substituted benzothiadiazole: a novel acceptor inducing n-type behaviour in conjugated polymers J. Mater. Chem. C, 2015, 3, 365-275

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