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Charge carrier recombination and energy disorder in organic solar cells both have a profound impact on the open-circuit voltage of the device. In this paper, both traditional fullerene-(PC71BM) and nonfullerene-(O-IDTBR) based solar cells were fabricated using the same electron donor material (PTB7-Th). The room-temperature current density–voltage characteristics showed that despite the values of their power conversion efficiencies were very close, there was a huge open circuit voltage (Voc) difference between the PC71BM and O-IDTBR devices. To understand the sources of the Voc variation, characterization techniques such as impedance spectra, low temperature electrical characterization method, transient photovoltage, and electroluminescent spectra were carried out. Temperature-dependent Voc of the devices were measured in a large temperature range between 120 K and 300 K. The charge transfer state energy (ECT
) of the fullerene and the nonfullerene cells were determined to be 1.13 V and 1.34 V, respectively. Furthermore, the Mott-Schottky equation was applied to analyze the capacitance- voltage curves of the fabricated devices. Results showed that the built-in voltage (Vbi) of the O-IDTBR based cell (1.38 V) was much higher than that of the PC71BM cell (1.15 V). By analyzing the above data, it was easy to speculate that charge carrier recombination loss in the PC71BM device was more serious since the net electric field was relatively weak. Impedance spectra were used to measure the charge carrier recombination process in both devices. Fitting results through the equivalent circuit stated clearly that values of the recombination resistance in the O-IDTBR device were much higher in the test range, indicating that the charge carrier was less easy to recombine in the nonfullerene device. This speculation could be verified by the transient photovoltage (TPV) measurements since the carrier lifetime in the O-IDTBR device was much longer. The excited states in the devices were investigated by the electroluminescence spectra. Since the full width at half maximum (FWHM) of the O-IDTBR emission spectrum was narrower, the excited state energy distribution in the O-IDTBR device was more uniform. Based on the above analyses, the higher Voc in the O-IDTBR device was attributed to the mild charge carrier recombination and low energy disorder. -
Keywords:
- polymer solar cells /
- charge carrier recombination /
- energy disorder
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图 1 (a)电子受体材料PC71BM与O-IDTBR的化学结构式及光活性层材料的能级示意图; (b)基于PC71BM和O-IDTBR的电流密度-电压曲线
Figure 1. (a) Chemical structures of PC71BM、O-IDTBR and their energy level diagrams; (b) Current density-Voltage curves of the PC71BM and O-IDTBR based devices.
图 2 (a) 两种器件的开路电压随温度变化的曲线; (b)两种器件的Mott-Schottky曲线
Figure 2. (a) Voc-Temperature curves of the devices using different electron acceptors; (b) Mott- Schottky curves for the devices.
图 3 (a) 器件的复合电阻随光照强度的变化曲线, 插图: 阻抗谱的等效拟合电路; (b)器件的瞬态光电压曲线
Figure 3. (a) Recombination resistance as a function of the light intensities, inset: Equivalent circuit of the measured impedance spectrum; (b) transient photovoltage curves of the devices.
图 4 (a) PC71BM与 (b) O-IDTBR器件在不同注入电流下的电致发光光谱
Figure 4. Electroluminescence of the (a) PC71BM and (b) O-IDTBR based devices with various injection current.
图 5 (a) PC71BM器件与O-IDTBR器件的电致荧光发射过程示意图; (b)能量无序对开路电压的影响示意图
Figure 5. (a) Illustration of the fluorescence emission process in the polymer solar cells; (b) illustration of the impact of energy disorder on the open-circuit voltage.
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[1] Lee C, Lee S, Kim G U, Lee W, Kim B J 2019 Chem. Rev. 119 8028
Google Scholar
[2] Zhang S, Qin Y, Zhu J, Hou J 2018 Adv. Mater. 30 1800868
Google Scholar
[3] Yuan J, Zhou L Y, Zhang G C, Yip H L, Lau T K, Lu X H, Zhu C, Peng H J, Johnson P A, Leclerc M, Cao Y, Ulanski J, Li Y F, Zou Y P 2019 Joule 3 1140
Google Scholar
[4] Li S S, Ye L, Zhao W C, Yan H P, Yang B, Liu D, Li W N, Ade H, Hou J H 2018 J. Am. Chem. Soc. 140 7159
Google Scholar
[5] Sun C K, Pan F, Bin H J, Zhang J Q, Xue L W, Qiu B B, Wei Z X, Zhang Z G, Li Y F 2018 Nat. Commun. 9 743
Google Scholar
[6] Ma W, Tumbleston J R, Wang M, Gann E, Huang F, Ade H 2013 Adv. Energy Mater. 3 864
Google Scholar
[7] Zhao W C, Qian D P, Zhang S Q, Li S S, Inganäs O, Gao F, Hou J H 2016 Adv. Mater. 28 4734
Google Scholar
[8] Yang X N, Loos J, Veenstra S C, Verhees W J H, Wienk M M, Kroon J M, Michels M A J, Janssen R A J 2005 Nano. Lett. 5 4
[9] Cowan S R, Roy A, Heeger A J 2010 Phys. Rev. B 82 245207
Google Scholar
[10] Proctor C M, Kim C, Neher D, Nguyen T Q 2013 Adv. Funct. Mater. 23 3584
Google Scholar
[11] Sharma A, Chauhan M, Bharti V, Kumar M, Chand S, Tripathi B, Tiwari J P 2017 Phys. Chem. Chem. Phys. 19 26169
Google Scholar
[12] Noriega R, Rivnay J, Vandewal K, Koch F P V, Stingelin N, Smith P, Toney M F, Salleo A 2013 Nat. Mater. 12 1038
Google Scholar
[13] Jurchescu O D, Popinciuc M, van Wees B J, Palstra T T M 2007 Adv. Mater. 19 688
Google Scholar
[14] Credgington D, Hamilton R, Atienzar P, Nelson J, Durrant J R 2011 Adv. Funct. Mater. 21 2744
Google Scholar
[15] Blakesley J, Neher D 2011 Phys. Rev. B 84 075210
Google Scholar
[16] Gao F, Himmelberger S, Andersson M, Hanifi D, Xia Y X, Zhang S Q, Wang J P, Hou J H, Salleo A, Inganäs O 2015 Adv. Mater. 27 3868
Google Scholar
[17] Heumueller T, Burke T M, Mateker W R, Sachs-Quintana I T, Vandewal K, Brabec C J, McGehee M D 2015 Adv. Energy Mater. 5 1500111
Google Scholar
[18] Xie S K, Xia Y X, Zheng Z, Zhang X N, Yuan J Y, Zhou H Q, Zhang Y 2018 Adv. Funct. Mater. 28 1705659
Google Scholar
[19] He Z C, Xiao B, Liu F, Wu H B, Yang Y L, Xiao S, Wang C, Russel T P, Cao Y 2015 Nat. Photonics 9 174
Google Scholar
[20] Baran D, Ashraf R S, Hanifi D A, Abdelsamie M, Gasparini N, Röhr J A, Holliday S, Wadsworth A, Lockett S, Neophytou M, Emmott C J M, Nelson J, Brabec C J, Amassian A, Salleo A, Kirchartz T, Durrant J R, McCulloch I 2017 Nat. Mater. 16 363
Google Scholar
[21] Dennler G, Scharber M C, Brabec C J 2009 Adv. Mater. 21 1323
Google Scholar
[22] Gruber M, Wagner J, Klein K, Hörmann U, Opitz A, Stutzmann M, Brütting W 2012 Adv. Energy Mater. 2 1100
Google Scholar
[23] Fabregat-Santiago F, Garcia-Belmonte G, Mora-Seró I, Bisquert J 2011 Phys. Chem. Chem. Phys. 13 9083
Google Scholar
[24] Casalini R, Tsang S W, Deininger J J, Arroyave F A, Reynolds J R, So F 2013 J. Phys. Chem. C 117 13798
Google Scholar
[25] Zhou H Q, Zhang Y, Seifter J, Collins S D, Luo D, Bazan G C, Nguyen T Q, Heeger A J 2013 Adv. Mater. 25 1646
Google Scholar
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