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High quantum efficiencies in polymer solar cells at energy losses below 0.6 eV Weiwei Li, Koen H. Hendriks, Alice Furlan, Martijn M. Wienk, and René A. J. Janssen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5131897 • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 8, 2015
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High quantum efficiencies in polymer solar cells at energy losses below 0.6 eV Weiwei Li,†,‡,* Koen H. Hendriks,‡ Alice Furlan,‡ Martijn M. Wienk,‡ and René A. J. Janssen‡* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China Molecular Materials and Nanosystems, Institute of Complex Molecular Systems, Eindhoven University of Technolo‐ gy, P. O. Box 513, 5600 MB Eindhoven, The Netherlands and Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612 AJ Eindhoven, The Netherlands Supporting Information Placeholder ABSTRACT:
Diketopyrrolopyrrole‐based conjugated polymers bridged with thiazole units and different donors have been designed for polymer solar cells. Quantum efficiencies above 50% have been achieved with energy loss between optical band gap and open‐circuit voltage below 0.6 eV.
the synthesis and application of conjugated polymers, de‐ signed to have a low photon energy loss, and demonstrate efficient charge generation in PSCs where Eloss ≤ 0.6 eV. 0.9 0.8 0.7 0.6
Conjugated polymers have been intensively investigated as electron donor in combination with fullerene acceptor deriv‐ atives in polymer solar cells (PSCs). Power conversion effi‐ ciencies (PCEs) above 10% have recently been obtained in 1‐3 single junction solar cells. One of the main reasons that the performance of PSCs remains low compared to the best inor‐ ganic solution processed thin film solar cells, such as perovskite solar cells,4,5 is the significant loss in energy of the open‐circuit voltage (Voc) relative to the optical band gap (Eg), determined from the onset of the linear absorption.6 The minimum photon energy loss in PSCs defined as Eloss = 7 Eg − qVoc, has previously been suggested to be 0.6 eV. In practice, the most efficient PCSs have Eloss = 0.7‐0.8 eV. The trade‐off is that when reducing Eloss to 0.6 eV, the quantum efficiency for charge generation often drops dramatically. In perovskite solar cells, Eloss is less than 0.5 eV.8 In PSCs the photon energy loss originates from a loss (~0.5 eV) between Voc and the energy of the charge transfer (CT) state and from the energy difference between the singlet state (S1) of the polymer and the CT state (~0.1 eV). While the minimum value for the latter can be close to 0.1 eV, or even less, the energy loss from S1 to CT state is ~0.2‐0.3 eV in the best cells. It has often been suggested that the photon energy loss may assist charge separation and enhance the efficiency of photon conversion into free charges. In Figure 1 we show how the external quantum efficiency (EQE) relates to Eloss for a range of successful polymers. To further enhance the PCE of PSCs Eloss must be reduced, while keeping a high quantum efficiency for charge genera‐ tion. Hence, polymers that approach and possibly fall below 9 the 0.6 eV limit are required, but are rare. Here we present
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Figure 1. The maximum EQE within polymer absorption band vs. the energy loss (Eloss = Eg − qVoc) for published efficient conjugated polymer solar cells (see Supporting Information, SI, Figure S1 for details). The lines are guides to the eye. The most versatile design motif for tuning the optical band gap and redox energies of conjugated polymers is incorporat‐ ing electron rich and electron deficient building blocks into an alternating polymer backbone. Strong electron rich and electron deficient units lower the optical band gap, by reduc‐ ing the ionization potential (IP) and increasing the electron affinity (EA). The energy difference between the IP of the donor polymer and EA of the fullerene acceptor largely de‐ termine the Voc. In our design (Figure 2), we selected diketopyrrolopyrrole (DPP) units as electron deficient unit. DPPs have been suc‐ cessfully been applied in efficient near‐infrared absorbing PSCs when substituted with thiophene on the 3 and 6 posi‐ 10 tions. As we are interested in materials that provide a high Voc, it is important to have a high IP. This can be achieved by replacing the electron rich thiophene (T) rings with thiazoles (Tz) (Figure 2). The more electronegative nitrogen atom of
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Journal of the American Chemical Society thiazole increases the IP and EA of the polymer considerably. Recently, we reported on PDPP2TzT (Figure 2) where the EA was increased to such an extent that this polymer could be used as electron acceptor in all‐polymer solar cells.11 As elec‐ tron donor PDPP2TzT gives a very low PCE with [70]PCBM as acceptor. To better adjust the IP and EA we thought to replace the thiophene ring in PDPP2TzT, by more electron rich units such as benzodithiophene (BDT), bithiophene (2T), and dithienopyrrole (DTP) (Figure 2). With these new derivatives it is possible to make solar cells that still have Eloss ≤ 0.6 eV, while having a significant quantum efficiency for charge generation. The results show that it is possible to cre‐ ate conjugated polymers for efficient charge generation in PSCs even with energy loss
