Polymer solar cells are nowadays one of the most relevant approaches for future low cost solar cells as they combine low cost processing from solution and high efficiency. Among all the novel high efficient donor polymer synthesized for organic bulk heterojunction solar cells, polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-d iyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-di yl (PTB7-Th) are world record donor materials that allow efficiencies over 9 % and 10%, respectively, in single junction devices.1,2 These devices use photoactive layers of very thin thickness below 100 nm as charge transport properties of the PTB7 based blends are poor.3 Indeed, the low tendency of PTB7 to aggregate reduces the ability of the fullerene to form a connected network making to electron collection a Limiting factor in polymer:fullerene solar Cell efficiency.4 Processing of such thin layers demand highly reliable printing techniques. Therefore new strategies for developing high efficient thick polymer blend layers allowing more robust solar cell processing are highly desirable. In order to understand and optimize of the performance of the devices and charge transport properties of the photoactive layers, we studied ambipolar charge transport properties of PTB7:PC70BMC and PTB7-Th:PC70BMC blends by using at first the analysis of the transfer characteristic of organic field effect transistor (OFET) as well as space charge limited current (SCLC) analysis by fabrication of electron and hole only devices. Furthermore scanning transmission microscopy (STEM) coupled with an electron energy loss spectroscopy (EELS) was used correlate morphology with performance of the polymer blends. By producing solar cells using ZnO based electron extraction layers in normal device structures leading to 7.6% for PTB75 and 9.2 % for PTB7-Th, we studied in detail the relation between charge transport, morphology and power conversion efficiency. Our results indicate that optimization of layer morphology and fullerene concentration is essential for improved and balanced ambipolar charge transport. Thus active layers with thickness up to 200 nm could be processed with fill factor over 60% leading to clear improvement in photocurrent generation compared to thin layer devices and high solar cell efficiencies.
Keywords: bulk heterojunction, polymer solar cells, charge transport, PTB7, SCLS [1] LIAO, S.-H., JHUO, H.-I., YEH, P.-N., CHENG, Y. S., LI, Y.-L., LEE, Y.-H., SHARMA, S., CHEN. S.-A. 2014. Scientific Report, 2014.DOI: 10.1038/srep06813 [2] HE, Z., ZHONG, C., SU, S., XU, M., WU, H., CAO Y. Nat. Photonics, 2012, 6, 591 . [3] GUERRERO, A., MONTCADA, N. F., AJURIA, J., ETXEBARRIA, I., PACIOS, R., GARCIA_BELMONTE, G.. J. Mater. Chem. A 2013, 1, 12345–12354. [4] FORSTER, S. DELEDALLE, F., MITANI, A., KIMURA, T. KIM, K-B., OKACHI, T., KIRCHARTZ, T., OGUMA, J., MIYAKE, K., DURRANT, J. R., DOI, S., NELSON, J. Adv. Energy Mater. 2014., DOI: 10.1002/aenm.201400311 [5] BEN DKHIL, S. DUCHE, D. GACEUR M., THAKUR, A., ABOURA, F. B., ESCOUBAS, L., SIMON, J.-J., GUERRERO, A., BISQUERT J., GARCIA_BELMONTE, G., BAO, Q., FAHLMAN, M., VIDEOT-ACKERMANN, C., ACKERMANN, J. Ad. Energy Mater. 2014, DOI: 10.1002/aenm.201400805.