Absence of Charge Transfer State Enables Very LowV_OCLosses in SWCNT:Fullerene Solar Cells

Abstract

cells, [1][2][3][4] but also other types based on small molecule:fullerene [5,6] blends as well as solar cells incorporating singlewalled carbon nanotubes (SWCNTs) [7][8][9][10][11][12][13][14] were investigated with increased efforts. All of these new concepts have been introduced to tackle several drawbacks and outstanding issues related to polymer:fullerene solar cells. [15] Concepts such as small molecule:fullerene active layers aim to use simple, well-defined molecules instead of polymers, which always exhibit a distribution in chain length altering their properties. The application of nonfullerene acceptors allows for more freedom to align the energy levels at the donor-acceptor interface. The incorporation of SWCNTs in OSCs or other organic devices [16][17][18][19] (near-infrared (nIR) detectors, nIR lightemitting diodes (LEDs), and field-effect transistors (FETs)) is based on several advantageous key factors that allow to boost device performances. First, SWCNTs have shown to exhibit a high photochemical stability making them superior to conventional polymers. Second, SWCNTs exhibit a very high charge carrier mobility outperforming conventional organic materials by orders of magnitude. [20,21] The high absorption coefficients in the nIR wavelength regime [22,23] make SWCNTs well suited to IR-sensitive OSCs via a ternary concept or in a binary blend with SWCNTs as the main absorber in the IR region.Previously it was shown that SWCNTs work in a type-II heterojunction scheme acting either as an electron acceptor in combination with a polymer or as the electron donor in combination with C 60 or [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). [24] Concerning the combination with C 60 , it was already demonstrated that charge carrier generation and harvesting is highly efficient with internal quantum efficiencies (IQEs) in the range of 85%. [25] The use of nearly monochiral SWCNTs in a bilayer architecture with C 60 facilitated high fill factors (FFs) greater than 60% and a high peak external quantum efficiency (EQE) of 43% at 1050 nm. Employing a bulk heterojunction (BHJ) architecture in combination with multichirality SWCNTs achieved a broad absorption in the nIR with power conversion efficiencies (PCEs) surpassing 3% for all-carbon allotrope absorbers. [24,26] In a recent study performed by Shea et al. [27] on nearly single-chirality (6,5) Current state-of-the-art organic solar cells (OSCs) still suffer from high losses of open-circuit voltage (V OC ). Conventional polymer:fullerene solar cells usually exhibit bandgap to V OC losses greater than 0.8 V. Here a detailed investigation of V OC is presented for solution-processed OSCs based on (6,5) single-walled carbon nanotube (SWCNT): [6,6]-phenyl-C 71 -butyric acid methyl ester active layers. Considering the very small optical bandgap of only 1.22 eV of (6,5) SWCNTs, a high V OC of 0.59 V leading to a low E gap /q − V OC = 0.63 V loss is observed. The low voltage losses are partly due to the lack of a measurable charge transfer state and partly due to th...

An Oligothiophene–Fullerene Molecule with a Balanced Donor–Acceptor Backbone for High‐Performance Single‐Component Organic Solar Cells

“…The effective band gap ( E gap ) is determined by the crossing point between absorption spectra of films and EL spectra of relevant devices (Figure S13) . As shown in Table , in contrast to the Rh‐OH:PC 71 BM blend with a E gap of 1.83 eV, the E gap of Rh‐PC 71 BM is larger with a value of 1.91 eV.…”

“…Thee ffective band gap (E gap )i sd etermined by the crossing point between absorption spectra of films and EL spectra of relevant devices ( Figure S13). [44] As shown in Table 1, in contrast to the Rh-OH:PC 71 BM blend with a E gap of 1.83 eV,t he E gap of Rh-PC 71 BM is larger with av alue of 1.91 eV.Notably,the E loss (0.93 eV) of the Rh-PC 71 BM device is lower than that of the Rh-OH:PC 71 BM device which has a E loss E CT of 0.95 eV.Here,byapplying the FTPS signals and Equation (2), we further fitted and calculated the E CT values of 1.55 eV for Rh-PC 71 BM film and 1.45 eV for Rh-OH:PC 71 BM blend, respectively.I ti sf ound that the energy loss for the formation of the CT state (DE 1 )i nt he Rh-PC 71 BM device is 0.36 eV.H owever,t he Rh-OH:PC 71 BM binary device shows a DE 1 of 0.38 eV.The results suggest that the Rh-PC 71 BM device possess efficient exciton dissociation to overcome the exciton binding energy (E b )o ft he D/A moieties of the pristine Rh-PC 71 BM film. In addition, the E loss due to recombination, which consists of radiative recombination (DE 2 )and nonradiative recombination (DE 3 ), exhibits the same value (0.57 eV) in Rh-PC 71 BM and Rh-OH:PC 71 BM devices.N evertheless,t he DE 2 value (0.22 eV) of the Rh-PC 71 BM device,w hich is dependent on the additional radiative recombination below the E gap ,i sh igher than that of the Rh-OH:PC 71 BM binary device (DE 2 = 0.10 eV), indicating that the Rh-OH:PC 71 BM BHJ blend is better for overcoming the Coulomb binding energy of the CT state.The final term of the nonradiative recombination loss (DE 3 )c an be geminate or non-geminate and includes recombination through structural defects or energetic traps,triplet states,and Auger recombination, etc.…”

Ultrafast carrier dynamics at organic donor–acceptor interfaces—a quantum-based assessment of the hopping model

“…Organic molecular blends based on fullerene acceptors (FAs), have been in the focus of intensive research efforts in the organic solar cell community, due to their reliable performance in the solar energy conversion [1,2]. Despite the recent advent of non-fullerene acceptor systems [3,4] with power conversion efficiencies reaching 18% [5], FA-based blends remain important model systems to study key electronic and optoelectronic processes and limiting factors of organic photovoltaic devices [6][7][8][9][10][11][12].…”

Linking optical spectra to free charges in donor/acceptor heterojunctions: cross-correlation of transient microwave and optical spectroscopy

“…6,[17][18][19][20] In OPV devices, s-SWCNT films have shown high values for both internal and external quantum efficiency (IQE and EQE), [21][22][23] and it has also been suggested that CT states are absent, leading to low nonradiative open-circuit voltage (V oc,nonrad ) loss. 24 The photodynamics of exciton dissociation and charge recombination processes at heterojunctions between s-SWCNTs and various electron acceptors such as fullerenes, 6,17,25,26 perylene diimides (PDIs), 19 and two-dimensional (2D) molybdenum disulfide (MoS 2 ) monolayers 20 have been explored, and the charge separation processes therein occur rapidly, in the range of 120 fs-1.6 ps, with charge recombination times often exceeding 1 ms. The ultrafast charge separation and long-lived charge separated states from s-SWCNT-based heterojunctions are often attributed to a high degree of charge carrier delocalization in the s-SWCNTs phase, resulting in the fast extraction of charge carriers from the donor-acceptor interface after exciton dissociation.…”