Vol. 136 (2019) ACTA PHYSICA POLONICA A No. 4 Proceedings of the 48th International School and Conference on the Physics of Semiconductors “Jaszowiec 2019” Inﬂuence of Active Layer Processing on Electrical Properties and Eﬃciency of Polymer–Fullerene Organic Solar Cells W. Mech * , J. Borysiuk, A. Wincukiewicz, R. Bożek, P. Trautman, M. Tokarczyk, M. Kamińska and K.P. Korona Institute of Experimental Physics, Faculty of Physics, University of Warsaw, L. Pasteura 5, PL-02093 Warsaw, Poland New method of polymer-fullerene composite crystallization was examined as a possible way for eﬃciency en- hancement of polymer organic solar cells. Since the structure of bulk-heterojunction is crucial for its eﬃciency, there is a strong need for new methods that can control a crystallization process. In this work we studied the crys- tallization process taking place under thermal annealing in ambient conditions, as well as annealing in a solvent atmosphere. Two polymer compounds, P3HT and PTB7, performing as donor materials were used. The ac- ceptor material in the fabricated bulk heterojunction was [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The spin-coated layers were investigated by optical absorption and X-ray diﬀraction. We observed signiﬁcant changes in the crystal structure of some of the annealed layers. The constructed solar cells were examined with use of current–voltage characterization method under AM1.5G sun irradiation, and by photocurrent spectroscopy. In the case of P3HT:PCBM cells signiﬁcant increase of eﬃciency was observed, especially after annealing in solvent atmosphere. DOI: 10.12693/APhysPolA.136.579 PACS/topics: 72.40.+w, 72.80.Le, 78.70.Ck 1. Introduction Organic photovoltaic cells with a polymer:fullerene bulk heterojunction active layer are inexpensive sources of renewable energy, having maximum reported eﬃciency of about 11% [1]. The most popular acceptor materials are fullerene derivatives, while there is a broad range of proposed donor materials ranging from the most popu- lar polythiophenes [2–8] to less popular but promising polyazomethines [9, 10]. The main research on improve- ment of polymer: fullerene cell eﬃciency is focused on two approaches: (i) searching for better polymer that would have a smaller energy gap, more suitable highly occupied molecular orbital (HOMO) and lowly unoccu- pied molecular orbital (LUMO) energy levels regarding the acceptor respective ones, and increased carrier mo- bilities; (ii) improvement of crystal structure in order to obtain better electrical parameters (higher conductivity, lower leakage). Lower energy gap results in absorption in the wider range of solar radiation since the energy of the absorbed photon must be higher or equal to energy gap of donor material (P3HT — 1.9 eV, PTB7 — 1.6 eV). The HOMO and LUMO energy levels of donor and ac- ceptor materials should be carefully chosen since the well matched mutual energy levels improve exciton sep- aration. Good structure of active layer is equally of high importance. The most of light absorption is ex- * corresponding author; e-mail: wojciech.mech@fuw.edu.pl pected to occur inside donor molecules. The incoming photon creates an exciton there, that latter diﬀuses to the donor:acceptor interface, where it dissociates in a charge- transfer process, i.e., the hole stays at the donor material while the electron is transferred to the acceptor material. Finally, the separated charges are transported by molecu- lar networks to electrodes. Improvement of crystal struc- ture should lead to better carrier mobility, which is gen- erally low, like in hopping transport. On the other hand, because of low mobility, it is crucial to obtain layer struc- ture with the shortest possible carrier paths and crystal- lization should help in this aspect, too. In our research we examined the inﬂuence of thermal [11–13] and sol- vent annealing [2, 3, 12, 14–21] techniques on improving optical, crystal, and electrical parameters of P3HT:PTB7 organic cells. The main goal of the solvent annealing techniques is elongation of crystallization time of organic compounds by annealing in solvent vapor environment. The lower rate of crystallization is due to decrease of solvent evap- oration from the deposited layer. The conducting conju- gated polymers has high tendency to self-organize, but in standard spin-coating procedures crystallization time is not long enough (only a few seconds) for material or- dering. Thanks to the solvent annealing technique this time can be extended to several minutes. 2. Samples preparation and experiment details The solar cells based on conducting conjugated poly- mers use bulk heterojunction (BHJ) conﬁguration for electron and hole generation and their separation. (579)