Suppression of Recombination Energy Losses by Decreasing the

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Letter

Suppression of Recombination Energy Losses by Decreasing the Energetic Offsets in Perylene Diimide Based Non-fullerene Organic Solar Cells Huiting Fu, Yuming Wang, Dong Meng, Zetong Ma, Yan Li, Feng Gao, Zhaohui Wang, and Yanming Sun ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01665 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Energy Letters

Suppression of Recombination Energy Losses by Decreasing the Energetic Offsets in Perylene Diimide Based Non-fullerene Organic Solar Cells Huiting Fu,†,▽,‡ Yuming Wang,#,‡ Dong Meng,† Zetong Ma,†,▽ Yan Li,† Feng Gao,*,# Zhaohui Wang,*,† and Yanming Sun*,§ †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. §

Heeger Beijing Research and Development Center, School of Chemistry and Environment,

Beihang University, Beijing 100191, China #

Department of Physics Chemistry and Biology (IFM), Linköping University, Linköping SE-

58183, Sweden ▽

University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding Author *E-mail: [email protected] (F.G.). *E-mail: [email protected] (Z.W.). *E-mail: [email protected] (Y.S.).

ACS Paragon Plus Environment

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ACS Energy Letters

ABSTRACT: In this Letter, a range of non-fullerene OSCs comprising two perylene diimide (PDI)-based small molecule acceptors in combination with four representative polymer donors have been investigated and compared. In addition to significant differences in the power conversion efficiency, the energy losses of photovoltaic devices vary widely for these two PDIbased acceptors when paired with different donors. The sensitive Fourier-transform photocurrent spectroscopy (FTPS) and electroluminescence (EL) measurements have been performed to quantify their respective energetic offsets (ΔEoffset) and energy losses, aiming to understand the distinct energy losses in the studied organic blends. By comparing these results, we find that with decreasing ΔEoffset both non-radiative recombination loss and radiative recombination loss due to the charge-transfer state absorption are suppressed; as a result, the total energy loss is decreased. These observations offer a deep understanding of how the energetic offset affects the energy losses from the viewpoint of Shockey-Queisser (SQ) limit.

TOC GRAPHICS

S1

Al

ECT

ZrAcac

qΔVSQ

qVloss qΔVrad

Polymer Donor

+ Perylene Diimide Based Acceptor

Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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qΔVnon-rad Eg qVoc

PEDOT:PSS

Glass/ITO

S0


2

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ACS Energy Letters

Over the past three years, bulk-heterojunction (BHJ) organic solar cells (OSCs) based on nonfullerene small molecule acceptors have received increasing attention due to their preeminent advantages, such as enhanced photon-harvesting, superior morphological stability and easily tunable energetic levels compared with conventional fullerene acceptors.1-4 Thanks to the innovations in photovoltaic materials, the power conversion efficiencies (PCEs) of non-fullerene OSCs have been increased dramatically and now have reached over 14%.5-7 Despite the great progress made in this filed, the relatively large energy loss, defined as the difference between the optical gap (Eg) of the absorber material and the open-circuit voltage (Voc) of the solar cell,8 restricts the PCE for further breakthrough.9 Extensive efforts have been devoted to identifying the reasons responsible for the large energy losses in OSCs.10-12 Because of the inherent low dielectric constant of organic semiconductors, most photo-generated excitons are a tightly bound electron-hole pair that cannot simply be dissociated by the thermal energy as in the inorganic counterparts.13 For this reason, a reasonably large energetic offset, denoted as ΔEoffset, between donor and acceptor molecular states has been considered by the OSC community to provide a driving force for efficient charge separation at the BHJ interfaces.14 Although the existence of a large ΔEoffset decreases the Voc, the quantitative analyzation between the ΔEoffset and energy loss is often lacking. The energy loss analyzation based on Shockley-Queisser (SQ) limit15 gives the insights into the origins of large energy losses in OSCs. Among others, the strong non-radiative recombination has been identified as a key loss channel in OSC devices, which is attributed to the low electroluminescence quantum efficiency (EQEEL) yields of OSC blends.16 Different mechanisms were proposed to explain the low EQEEL. Benduhn et al.17 showed that the EQEEL was linked to the energy of charge transfer (CT) states by studying a large set of fullerene-based OSCs. It was


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also found that the light-induced traps in fullerenes could reduce the EQEEL and thus lead to the Voc degradation.18 Recently it was demonstrated that the EQEEL could be optimized by minimizing the ΔEoffset and improving the photoluminescence yield of the blend components, thus leading to the decreased energy losses.19 In this paper, a range of existing and unreported non-fullerene systems are fabricated and investigated, wherein two perylene diimide (PDI)-based acceptors (named TPH-Se20 and T-2,21 respectively) are combined with four representative donor polymers (P3HT,22 PDBT-T1,23 PTB7-Th,24 and PBDT-TS125) that have different lowest unoccupied molecular orbital (LUMO) energy levels and optical gaps. The results show that not only the PCEs but also the energy losses in these material blends vary significantly. To illustrate the distinct energy losses in these OSC devices, different energy loss origins are quantified by employing the sensitive Fourier transform

photocurrent

spectroscopy

(FTPS)

and

electroluminescence

(EL)

spectra

measurements. We provide a systematic analysis on these results, and find that the non-radiative recombination loss as well as the radiative recombination loss below gap is directly related to the energetic offsets. These observations offer a deep understanding of how the energetic offset makes the increase of energy losses from the viewpoint of SQ limit, further suggesting the validity of the approach to reduce energy loss and thus promote the Voc by minimizing the energy offset in non-fullerene OSCs. Optoelectronic Characterization of Neat Materials and Blends. The chemical structures of the four donor polymers and two PDI-based acceptor molecules used in this work are displayed in Figure 1a. The normalized ultraviolet-visible (UV-vis) thin-film absorption spectra of pure components and studied BHJ blends are shown in Figure 1b and Figure S1, respectively. It can be seen that each donor:acceptor combination exhibits complementary light absorption


4

(c)

0.6 0.4 0.2 0.0 400

500

600

800

-3.20

PDBT-T1

P3HT

-4

-5

-5.10 -5.36

TPH-Se

700

Wavelength (nm)

-3.43

PBDT-TS1

T-2 TPH-Se

0.8

-3

PTB7-Th

PTB7-Th PBDT-TS1

-6

-3.62 -3.67 -3.69 -3.79

TPH-Se

T-2

P3HT PDBT-T1

1.0

T-2

PDBT-T1

1.2

PBDT-TS1

P3HT

Normalized thin film absorbance

(b)

(a)

Energy levels (eV)

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ACS Energy Letters

PTB7-Th

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-5.22 -5.24 -5.98 -5.90

Figure 1. (a) Chemical structures of donor polymers and acceptor molecules used in this study. (b) Normalized thin film absorptions of the neat donor and acceptor materials. (c) Energy level diagram measured from thin films by cyclic voltammetry. profile in the visible region, thus in favor of high short-circuit current density (Jsc) for the corresponding OSCs. Interestingly, except the P3HT:T-2 blend, the absorption edges of all other blends present slightly blue-shifted with respect to those of neat polymer films. This may be related to the fact that the original nanostructure in the donor was perturbed upon the incorporation of an acceptor component. Because all the polymer donors have lower energy absorption compared with the non-fullerene acceptors, the energy of the polymer singlet states is adopted as the Eg to calculate the energy loss in the relevant OSC devices in this paper. Figure 1c depicts the energy level alignments of all the molecules that were measured by using cyclic voltammetry of thin films (Figure S2). The LUMO energy levels of TPH-Se and T-2 are –3.69 eV and –3.79 eV, respectively. For these four donors, from P3HT to PDBT-T1, PTB7-Th and PBDT-TS1, the LUMO positions are down-shifted in sequence, ranging from –3.20 to –3.67 eV.


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As such, the LUMO offset energies between donors and acceptors gradually decrease, providing the devices with different energetic offsets and energy losses. Photovoltaic Cell Performance. Photovoltaic devices comprising T-2 blended with these four donors and TPH-Se blended with PDBT-T1 or PBDT-TS1 were prepared with a conventional architecture of ITO/PEDOT:PSS/active layer/ZrAcac/Al. The active layers were spin-coated from chlorobenzene solutions with donor:acceptor weight ratio of 1:1 for all cases. The characteristic current density-voltage (J-V) curves and external quantum efficiency (EQE) spectra of these six material systems are shown in Figure S3, and the associated photovoltaic parameters are summarized in Table S1. For the P3HT:T-2 device, an inferior PCE of 0.92% with a Voc of 0.76 V and a low Jsc of 2.26 mA cm-2 was achieved. In contrast, after blending with other three donors, these non-fullerene OSCs based on T-2 yielded markedly enhanced PCEs: 4.18% for PDBT-T1, 6.18% for PTB7-Th, and 5.67% for PBDT-TS1, respectively. The poor performance of P3HT:T-2 device is largely due to the oversized domains formed in the active layer, evidenced by the atomic force microscopy (AFM, Figure S4) and transmission electron microscopy (TEM, Figure S5) results. While regarding the PDBT-1:T-2, PTB7-Th:T-2, and PBDT-TS1:T-2 blends, the AFM images indicate that they all exhibit uniform and smooth film morphology with small and similar root-mean-square (RMS) roughness values of 1.31, 1.15, and 1.25 nm, respectively. The TEM studies showed that small domains and favorable phase separation were formed for these three blend films, which are beneficial for exciton diffusion/dissociation, thus contributing to increased PCE values in these three blend devices. As for TPH-Se with a large conjugated configuration, the resulting devices exhibited decent photovoltaic performance in both combination with PDBT-T1 and PBDT-TS1. In particular, an impressive PCE of 9.17% with a Voc of 1.01 V was obtained for PDBT-T1:TPH-Se devices,


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ACS Energy Letters

comparable to the previously reported data.20 In addition to the dramatic differences in PCEs, the energy losses also changed significantly for both T-2- and TPH-Se-based systems when paired with different donor materials, as listed in Table S1. For example, the energy loss of P3HT:T-2 devices is 1.16 eV, which, however, drops to only 0.61 eV once P3HT is replaced with PBDTTS1; the PDBT-T1:TPH-Se blend has a larger energy loss (0.85 eV) than the PBDT-TS1:TPHSe blend (0.71 eV). Moreover, we notice that the energy loss is decreased if the LUMO offset is lessened. In order to explain the difference in energy loss (qΔVoc) for different material systems and the suppressed qΔVoc with the decrease of LUMO offsets, we determined the ΔEoffset and quantified the qΔVoc values of the six non-fullerene systems. Determination of the ΔEoffset. The ΔEoffset is often estimated from the difference in LUMOs or HOMOs of the donor and acceptor components.26,27 However, such estimation based on frontier orbital energies of the isolated donor and acceptor neglects the impacts of blend morphology and binding energy of the CT excitons.28,29 An alternative and preferable approach is to probe the interfacial energetics on complete photovoltaic devices through the intermolecular CT state, which is formed after photoinduced charge transfer leaving a hole on the donor phase and a nearby electron on the acceptor material.16,30-32 Then the ΔEoffset can be given by the energetic difference between the Eg and the energy of the CT state (ECT).33 We accordingly performed the highly sensitive FTPS and EL measurements to detect the absorption and emission from the CT states to determine the ECT and then the ΔEoffset. Figure 2 and 3 display the normalized FTPS-EQE and EL spectra of six blend devices together with those of pure polymer (low-gap component) devices for comparison, respectively. For the blends of P3HT:T-2, PDBT-T1:TPH-Se and PDBT-T1:T-2 with large LUMO offsets, a sub-gap absorption by CT states is clearly visible in the FTPS-EQE spectra (see Figure 2a-c). In


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ACS Energy Letters

101 10

EQE-FPTS (%)

0

101

(a)

10

0

10-1

10-1

-2

10-2

10

Pure P3HT Blend P3HT:T-2

10-3

10-4

10-5

10-5 1.5

2.0

2.5

3.0

3.5

4.0

10-1 Pure PDBT-T1 Blend PDBT-T1:TPH-Se

10-2

Pure PDBT-T1 Blend PDBT-T1:T-2

10-3 10-4

1.5 101

(d)

100

10-1

10-1

10

-2

10

-2

10

-3

10

-3

Pure PTB7-Th Blend PTB7-Th:T-2

2.0

2.5

3.0

3.5

4.0

1.5 101

(e)

100

Pure PBDT-TS1 Blend PBDT-TS1:TPH-Se

10-4

-5

10-5

2.0

2.5

3.0

3.5

4.0

3.0

3.5

4.0

1.5

2.0

2.5

3.0

3.5

Pure PBDT-TS1 Blend PBDT-TS1:T-2

10-3

10-4

1.5

2.5

(f)

10-2

-5

10

2.0

10-1

10-4 10

(c)

100

10-5

1.0 101

101

(b)

10-3

10-4

100

4.0

1.5

2.0

2.5

3.0

3.5

4.0

Energy (eV)

Figure 2. Normalized FTPS-EQE spectra of (a) pure P3HT and P3HT:T-2-based devices, (b) pure PDBT-T1 and PDBT-T1:TPH-Se-based devices, (c) pure PDBT-T1 and PDBT-T1:T-2based devices, (d) pure PTB7-Th and PTB7-Th:T-2-based devices,(e) pure PBDT-TS1 and PBDT-TS1:TPH-Se-based devices, and (f) pure PBDT-TS1 and PBDT-TS1:T-2-based devices.

P3HT:T-2 Pure P3HT

1.6 1.2

EL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.5

PDBT-T1:TPH-Se Pure PDBT-T1

2.8

1.2

PDBT-T1:T-2 Pure PDBT-T1

0.9

2.1 0.6

0.8

1.4

0.4

(a) 0.0

0.0 1.2

1.4

1.6

1.8

2.0

2.2

PTB7-Th:T-2 Pure PTB7-Th

0.3

0.7

(c)

(b) 0.0 1.2

1.4

1.6

1.8

2.0

2.2

1.2

1.2

PBDT-TS1:TPH-Se 1.2 Pure PBDT-TS1

0.9

0.9

0.9

0.6

0.6

0.6

1.2

0.3

1.4

1.6

1.8

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2.4

PBDT-TS1:T-2 Pure PBDT-TS1

(e)

(f) 0.0

0.0 1.2

1.6

0.3

0.3

(d) 0.0

1.4

1.2

1.4

1.6

1.8

2.0

2.2

1.2

1.4

1.6

1.8

2.0

2.2

Energy (eV)

Figure 3. Normalized EL curves of (a) pure P3HT and P3HT:T-2-based devices, (b) pure PDBTT1 and PDBT-T1:TPH-Se-based devices, (c) pure PDBT-T1 and PDBT-T1:T-2-based devices, (d) pure PTB7-Th and PTB7-Th:T-2-based devices,(e) pure PBDT-TS1 and PBDT-TS1:TPHSe-based devices, and (f) pure PBDT-TS1 and PBDT-TS1:T-2-based devices.


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ACS Energy Letters

the cases of PTB7-Th:T-2, PBDT-TS1:TPH-Se and PBDT-TS1:T-2 blends with small LUMO offsets, the FTPS-EQE onset is nearly identical to that of the polymer comprising the blend (Figure 2d-f), indicating that the ECT is very close to the energy of the polymer singlet exciton. In other words, the ΔEoffset is small for these three blends based OSCs. In terms of EL spectra, all blends except the PBDT-TS1:T-2 present an additional emission peak at the lower energy region (Figure 3a-e). In contrast, the EL spectra of the PBDT-TS1:T-2 blend and the PBDT-TS1 film are quite overlapping, which may be due to the hybridization of the CT states and singlet states in the systems with small ΔEoffset.19 The ECT can be determined by fitting the tail of EQE or EL spectra of the CT states with the following equations:33-35 ( E  ECT   ) EQE( E ) ~ exp( ) E 4 k

(1)

( E  ECT   ) EL( E ) ~ exp( ) E3 4 k

(2)

2

2

where E is the photon energy, k is the Boltzmann constant’s, T is the absolute temperature and λ is the reorganization energy. The EQE(E)/E and EL(E)/E3 represent the so-called reduced EQE and EL spectra, respectively, as plotted in Figure 4. Note that the EQE in Equation (1) is equivalent to the FTPS-EQE in this paper. Fits of FTPS-EQE and EL spectra of the CT states with respectively Equation (1) and (2) yield similar values for both ECT and λ in all cases except the PBDT-TS1:T-2 blend (see Figure 4). Besides, these ECT values coincide well with the intersection of the normalized fitting spectra, confirming the credibility of fitting results. As for the PBDT-TS1:T-2, since the CT peaks are not visible in the FTPS-EQE and EL spectra, the ECT is determined by taking the intersection of the normalized reduced FTPS-EQE and EL spectra of the blend film (Figure 4f). 17,36 Similarly, the energy of polymer singlet states (E g ) is


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Reduced EQE and EL (a.u.)

ACS Energy Letters

104 103 102 101 100 10-1 10-2 10-3

10

10-2 10-3

102

PDBT-T1:TPH-Se

101

10-1

=0.05 eV

EQE-fit ECT=1.27 eV =0.06 eV

(d)

1.2

1.4

1.6

10

-3

1.8

PTB7-Th:T-2 =0.14 eV

=0.14 eV

10

10-2

10-2

EQE-fit ECT=1.49 eV

=0.14 eV

1.2

1.4

1.6

1.8

2.0

2.2

(e) PBDT-TS1:TPH-Se =0.11 eV

EQE-fit ECT=1.70 eV

=0.35 eV

0.9

101 EQE-fit ECT=1.53eV

1.2

=0.33 eV

1.5

1.8

2.1

2.4

(f) PBDT-TS1:T-2

100 10-1

EL-fit ECT=1.53 eV

10-2

=0.11 eV

10-3 10

10-3

10-4

EL-fit ECT=1.70 eV

10-3

=0.15 eV

1.0

100 -1

PDBT-T1:T-2

10-1

EL-fit ECT=1.49 eV

0.8

101 EQE-fit ECT=1.59 eV

EL-fit ECT=1.59 eV

10-2

(c)

100

100

EL-fit ECT=1.27 eV

0

10-1

(b)

101

1.0

101

102

(a) P3HT:T-2

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ECT=1.62 eV

-4

10-5 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

1.0

1.2

1.4

1.6

1.8

2.0

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Energy (eV)

Figure 4. Determination of the ECT values for (a) P3HT:T-2, (b) PDBT-T1:TPH-Se, (c) PDBTT1:T-2, (d) PTB7-Th:T-2, and (e) PBDT-TS1:TPH-Se blends from the corresponding reduced FTPS-EQE (solid red lines) and EL (solid blue lines) spectra. The dashed red and blue lines are simultaneous fits using Equations (1) and (2), respectively. (f) The ECT for PBDT-TS1:T-2 is determined by taking the intersection of the reduced FTPS-EQE and EL spectra of the blend film (as the black arrow shown). identified as the energy of the crossing point between the normalized reduced FTPS-EQE and EL spectra of pristine polymers (Figure S6).33 The ECT, Eg and the resulting ΔEoffset values of these six material systems are listed in Table 1. As expected, the P3HT:T-2 and PDBT-T1:TPH-Se blends have a relatively large ΔEoffset with a value of 0.65 and 0.37 eV, respectively; while the ΔEoffset values of the blends of PTB7-Th:T-2, PBDT-TS1:TPH-Se and PBDT-TS1:T-2 are small or nearly negligible with a value of 0.05, 0.09 and 0 eV (less than 10 meV), respectively. It is worthy to note that these calculated ΔEoffset values are consistent with the variation trend of LUMO offset energies. Quantification of the Energy Losses. The SQ limit and variations thereof (if actual absorptance is considered) have proven to be useful to gain insights into the energy losses.15 According to the SQ limit, the energy losses (qΔVoc) in solar cells can be categorized into three


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ACS Energy Letters

Table 1. Energy loss analysis on different non-fullerene OSC systems in this study. Material Blends

Eg (eV)a

ECT (eV)

∆Eoffset (eV)

q∆Voc (eV)b

q∆V1 (eV)c

q∆V2 (eV)c

EQEEL

q∆V3 (eV)c

Voccal (V)d

Vocmea (V)e

P3HT:T-2

1.92

1.27

0.65

1.16

0.30

0.43

6.0×10-8

0.43

0.76

0.76

PDBT-T1:TPH-Se

1.86

1.49

0.37

0.85

0.29

0.17

2.0×10-7

0.40

1.00

1.01

PDBT-T1:T-2

1.86

1.70

0.16

0.78

0.29

0.12

9.0×10-7

0.36

1.08

1.08

-6

PTB7-Th:T-2

1.64

1.59

0.05

0.67

0.28

0.06

5.0×10

0.32

0.98

0.97

PBDT-TS1:TPH-Se

1.62

1.53

0.09

0.71

0.28

0.08

3.0×10-6

0.33

0.93

0.91

PBDT-TS1:T-2

1.62

1.62

0

0.61

0.28

0.06

2.5×10-5

0.27

1.01

1.01

a

The Eg values is determined from the crossing point between the normalized reduced FTPSEQE and EL spectra of the pure donors. bThe q∆Voc is defined as the difference between the Eg and the Voc measured in photovoltaic devices. cqΔV1, qΔV2, and qΔV3 stand for three contributions of the energy loss. dThe Voccal is calculated according to the Voccal = (Eg – q∆V1– q∆V2–q∆V3)/q. eThe Vocmea is obtained from the J-V curves directly. parts ( qΔVoc  qΔV1  qΔV2  qΔV3 ), and the calculation of each term is based on the FTPSEQE and EL measurements.8 The details of this calculation method are referred to the References 37 and 38. The maximum voltage ( qVocSQ ) for an idealized device in the SQ limit is reduced by qΔV1 with reference to Eg due to radiative recombination originating from the absorption above the optical gap ( qV1  Eg  qVocSQ ). This part of energy loss is unavoidable for any solar cells and depends only on the Eg of the absorber for the certain solar spectrum and temperature. As summarized in Table 1, there is a little difference in the qΔV1 for all six systems (between 0.28 and 0.30 eV). The second loss ( qV2  qVocrad,below gap ) is due to additional radiative recombination from the absorption below the gap. For OSCs, this term is mainly from the CT state absorption, namely, due to the existence of ΔEoffset. As we display in Figure 2, with decreasing ΔEoffset in these six systems, the redshifted EQE tail related to CT states vanishes gradually, increasing the


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sharpness of the EQE profile. As a result, the blends with significant ΔEoffset would suffer from much larger qΔV2 compared to those with negligible ΔEoffset (Table 1). The third loss term ( qV3  qVocnon-rad  kT ln(EQE EL ) ) is due to non-radiative recombination, where EQEEL is radiative quantum efficiency of the solar cell when charge carriers are injected in dark. Apparently, enhancing the EQEEL indicates reduced non-radiative recombination losses. Detailed EQEEL and the resulting qΔV3 values for these six investigated systems are also listed in Table 1. Similar to the variant qΔV2, the qΔV3 is quite different from each other. The EL spectra (Figure 3) reveal that it is easier to generate polymer singlet excitons during the carrier injection for the blends with smaller ΔEoffset, for example, PTB7-Th:T-2; PBDT-TS1:TPH-Se and PBDT-TS1:T-2. It is known that the EL emission from the pure polymer excitons is more efficient than that from CT states.37 Thus, the increased EQEEL in the three blends can be ascribed to the increased fraction of emission from the singlet excitons, resulting in a reduced q∆V3. While for the blends of P3HT:T-2, PDBT-T1:TPH-Se and PDBTT1:T-2 with larger ΔEoffset, a higher qΔV3 was observed. Notably, the PBDT-TS1:T-2 combination with the smallest ΔEoffset among these six blends delivered the lowest qΔV3 of 0.27 eV, which is comparable with the previously reported OSCs with small energy losses.37 Energy Loss Analysis on Different Material Systems. The calculated Voc (Voccal, Table 1) values according to the FTPS-EQE and EL data are in agreement with those (Vocmea) measured in our experiments, indicating the validity of this method for quantifying losses in Voc. The quantification results explain why the energy losses in T-2 and TPH-Se based non-fullerene OSCs vary substantially when combined with different donor materials. The larger energy losses in P3HT:T-2, PDBT-T1:TPH-Se and PDBT-T1:T-2 blends is mainly due to the higher qΔV2 and qΔV3 caused by the larger energetic offsets; whereas, these two terms drop for the blends of


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PTB7-Th:T-2, PBDT-TS1:T-2 and PBDT-TS1:TPH-Se with smaller ΔEoffset, leading to lower energy losses. We made a plot of ΔEoffset versus q∆V2 and q∆V3 as well as q∆Voc data of the study, as shown in Figure 5. It is clear that the q∆V2, q∆V3, and q∆Voc show a good correlation with the ΔEoffset. With the decreasing ΔEoffset, the q∆V2, q∆V3 and thus the total enegy losses are suppressed.

1.2 qVoc

0.6 0.4 0.2 0.0

1.0 0.8

q V3

q V2

0.6 0.4

Eg - qVoc (eV)

0.8

qV2 and qV3 (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Eoffset (eV)

Figure 5. The radiative recombination energy losses below gap (q∆V2, green sphere), nonradiative recombination energy losses (q∆V3, red sphere) and total energy losses (q∆Voc, blue sphere) versus ΔEoffset. It has recently been shown that the ΔEoffset could approach zero for some efficient OSCs based on non-fullerene acceptors, providing opportunities to improve the Voc.37,39-42 These recent developments imply that the ΔEoffset might not be the only origin of driving force in OSCs. Thus, minimizing the ΔEoffset in these particular material systems should be a valid approach to further improving the efficiencies of the non-fullerene OSCs. In these cases, a critical question in terms of the driving force for charge separation requires further investigations. In summary, we have investigated six types of non-fullerene OSCs with different ΔEoffset, involving two PDI-based small molecule acceptors (TPH-Se and T-2) and four polymer donors (P3HT, PDBT-T1, PTB7-Th and PBDT-TS1). Apart from significant differences in the efficiency, TPH-Se and T-2 based photovoltaic devices showed varied energy losses when


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combined with different donors. We have determined the associated ΔEoffset and quantified three terms of energy losses for each of these systems through the CT state. The results indicate that for both T-2 and TPH-Se based non-fullerene systems (P3HT:T-2, PDBT-T1:TPH-Se and PDBT-T1:T-2) the larger energy losses are mainly due to higher non-radiative recombination and radiative recombination originating from the CT state absorption; in contrast, these two loss origins decrease obviously in the blends (PTB7-Th:T-2, PBDT-TS1:T-2 and PBDT-TS1:TPH-Se) with smaller energy losses. Furthermore, we reveal the relationship of ΔEoffset and energy loss, that is, with the decreasing ΔEoffset the radiative recombination loss due to the CT absorption as wells as the non-radiative recombination loss is suppressed, as a result, the total energy loss is decreased. Therefore, an alternative approach to reduce energy loss and maximize the obtainable Voc is to minimize the ΔEoffset of the non-fullerene OSC devices by precisely adjusting the energy levels of photovoltaic materials. ASSOCIATED CONTENT Supporting Information. Experimental details, fabrication and testing of OSC devices, UV absorption spectra, cyclic voltammogram curves, J-V characteristics and photovoltaic parameters, morphology characterization details, AFM and TEM images, FTPS-EQE and EL measurements, PL spectra, and determination of Eg from the normalized reduced FTPS-EQE and EL spectra. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].


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ACS Energy Letters

Author Contributions ‡

H.F. and Y.W. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (NSFC)

(Nos.

21734009,

21734001,

21672221),

NSFC-DFG

joint

project

TRR61

(21661132006), the Chinese Academy of Sciences (XDB12010100), the Swedish Energy Agency Energimyndigheten (2016-010174), and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant No. SFO-Mat-LiU #2009-00971), Y.W. was supported by the China Scholarship Council. REFERENCES (1) Nielsen, C. B.; Holliday, S.; Chen, H. Y.; Cryer, S. J.; McCulloch, I. Non-fullerene Electron Acceptors for Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48, 28032812. (2) Zhang, G.; Zhao, J.; Chow, P. C. Y.; Jiang, K.; Zhang, J.; Zhu, Z.; Zhang, J.; Huang, F.; Yan, H. Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells. Chem. Rev. 2018, 118, 3447-3507. (3) Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K. Y.; Marder, S. R.; Zhan, X. Nonfullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 18003. (4) Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-generation Organic Photovoltaics Based on


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Performance

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Thieno[3,4-c]pyrrole-4,6-dione-based

Polymer:ITIC Solar Cells. Phys. Chem. Chem. Phys. 2017, 19, 23990-23998.


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