Near-IR Absorbing D-A-D Zn-Porphyrin-based Small Molecule Donors

Subscriber access provided by Karolinska Institutet, University Library

Organic Electronic Devices

Near-IR Absorbing D-A-D Zn-Porphyrin-based Small Molecule Donors for Organic Solar Cells with Low Voltage Loss Virginia Cuesta, Rahul Singhal, Pilar de la Cruz, Ganesh D Sharma, and Fernando Langa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20917 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 49 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 Applied Materials & Interfaces

Near-IR Absorbing D-A-D Zn-Porphyrin-based Small Molecule Donors for Organic Solar Cells with Low Voltage Loss Virginia Cuesta,† Rahul Singhal,‡ Pilar de la Cruz, *,† Ganesh D. Sharma,*,‡,§ and Fernando Langa*,†.

†Universidad

de Castilla-La Mancha, Institute of Nanoscience, Nanotechnology and Molecular

Materials (INAMOL), Campus de la Fábrica de Armas, 45071 Toledo, Spain

‡Department

of Physics, Malviya National Institute of Technology (MNIT), Jaipur

§Department

of Physics, The LNM Institute of Information Technology (Deemed University),

Rupa ki Nangal, Jamdoli, Jaipur (Raj.) 302031, India.

KEYWORDS: Photovoltaics, bulk heterojunction solar cells, small molecule, porphyrin, D-A-D structure.

ABSTRACT ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Page 2 of 49

Two D-A-D small molecules with a DPP acceptor core and Zn-porphyrin donor with different electron-donating substituents, namely 2,6-bis(dodecyloxy)phenyl and 5-hexylthieno[3,2b]thiophen-2-yl at meso positions, VC4 and VC5, were synthesized and their optical and electrochemical properties were investigated. The results reveal that both molecules are suitable as donors for organic solar cells in which PC71BM is employed as the acceptor. Overall power conversion efficiencies of 8.05% (Jsc = 13.83 mA/cm2, Voc = 0.91 V and FF = 0.64) and 8.89% (Jsc = 16.98 mA/cm2, Voc = 0.79 V and FF = 0.663) were obtained, respectively. The high Voc value for the VC4-based OSC correlates with the deeper HOMO, whereas the high Jsc value for VC5 may be attributed to the extended absorption spectrum towards the longer wavelength region. Moreover, the relatively high FF value for VC5-based OSCs as compared to the VC4 counterparts may be related to the more balanced charge transport in the active layer, reduced charge recombination and efficient charge collection. The energy loss for VC5 is smaller (0.52 eV) that that for VC4 (0.56 eV).

Introduction

Organic solar cells (OSCs) consisting of a bulk heterojunction (BHJ) active layer are promising for the production of light weight and flexible photovoltaic devices by low-cost solution processing roll to roll fabrication technology.1–9 The BHJ active layer is composed of a blend of ACS Paragon Plus Environment

Page 3 of 49 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


donor (polymers or small molecules) and acceptor (fullerene derivatives or non-fullerene small molecules)10 that provides an appropriate nanoscale morphology and interpenetrating network for efficient exciton dissociation, charge transfer and charge transport. OSCs processed with a single BHJ active layer based on conjugated polymers as donors have provided overall power conversion efficiency (PCE) values of 11–12% and 13–14% on employing fullerene derivatives11 and small molecule non-fullerene acceptors,12–15 respectively. In recent years, OSCs based on small molecules (SMs) as the donor component in the BHJ active layer have attracted significant attention due to their advantages over their conjugated polymer counterparts, which include well-defined chemical structures, reproducible synthesis and negligible batch to batch variations.16–20 In addition to these advantages, the shorter length of SMs results in a lower entropic barrier and also reduces the chain entanglements, thereby enhancing the molecular packing and aggregation in the solid state, which in turn leads to improved inter-chain transportation. In the relatively short time in which such devices have been investigated, the PCE values of OSCs based on SMs donor have reached about 11%21–23 and this is comparable to values obtained for polymer counterparts. Indeed, the astonishing 17.3% record efficiency recently achieved in tandem OSCs has stimulated research in this field.24

ACS Paragon Plus Environment


Page 4 of 49

The PCE of the OSC is governed by three parameters, namely short circuit photocurrent (JSC), open circuit voltage (VOC) and fill factor (FF). In order to improve the JSC, the optical band gap (Eg) of the active material should be low and more light should be absorbed in a broader wavelength region extended up to the near infrared, since 50% of photon flux lies beyond 700 nm. However, a reduction of Eg generally leads to a decrease in VOC and an increase in the energy loss (Eloss). The Eloss is defined as Eloss = Eg – qVOC and this is a significant parameter for the assessment of the performance of OSCs.25–27 Most OSCs suffer from high Eloss (more than 0.8 eV) and if the Eloss is less than 0.6 eV, then the quantum efficiencies often drop considerably.28–31 Therefore, it is necessary to design new organic semiconducting materials with low Eloss while retaining high Jsc and VOC values.32

Inspired by photosynthesis in nature, porphyrins and their derivatives have been explored as SM donors because chlorophylls are strong chromophores and porphyrins are analogues of chlorophylls. Such molecules have been explored as donors for BHJ OSCs since they possess high absorption coefficients and their optical and electrochemical properties can be easily tuned by functionalization of the periphery. The overall PCEs of the OSCs based on porphyrin derivatives as donors in molecules with an A-D-A configuration have been significantly improved to around 9% through molecular design, and device optimization indicates that the porphyrin ACS Paragon Plus Environment

Page 5 of 49 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


macrocycle is an excellent building block for materials employed as donors for OSCs.33,34,43– 49,35–42

Recently, we designed a D-A-D small molecule based on Zn-porphyrin and achieved an

overall PCE of 8.03%, which is superior to the efficiency achieved on using an A-D-A molecule with the same A and D components.50 Encouraged by this finding, we report here two new DA-D small molecules, denoted as VC4 and VC5 (Chart 1), in which the electron richness of the substituent in the meso-position of the porphyrin macrocycle has been modulated. We investigated the optical and electrochemical properties of this system and subsequently used the compounds as donors in conjunction with PC71BM as the electron acceptor for the fabrication of solution-processed BHJ OSCs.

C6H13 S S C6H13

C6H13

S

N

N

S

Zn S

N

N

S

S

N O

O S

N

S

S

Zn N

S

N

N

N

C6H13 S VC4

S C6H13


S

C6H13


C12H25O C12H25O

Page 6 of 49

OC12H25 N

N Zn

C12H25O C12H25O

N

S

N

N O

OC12H25

C12H25O

O N

OC12H25 N

S

N

OC12H25

Zn

N

VC5

C12H25O

N

OC12H25 OC12H25

Chart 1. New D-A-D small molecules VC4 and VC5.

After optimization of the active layers, overall PCE values of 8.05% and 8.89% were achieved for VC4 and VC5, respectively. Although the VC4-based OSCs exhibit high VOC values due to the deeper HOMO energy level, the VC5-based OSCs showed higher JSC values when compared to the VC4 counterpart and this may be related to the low optical bandgap and the photoresponse being extended to the near-IR region (up to 950 nm). Moreover, the high FF values obtained for the VC5-based active layer may also be one of the reasons for the higher PCE when compared to the VC4 counterpart. The energy loss values for the OSCs based on VC4 and VC5 are 0.56 eV and 0.52 eV, respectively. The lower energy loss for VC5 may be related to the larger dipole moment of VC5 and the lower exciton binding energy when compared to VC4. On the basis of these results, it is necessary to design the small molecules with both high dielectric constant and dipole moment which can reduce the exciton binding energy and


Page 7 of 49 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


then need a low frontier energy level offset for exciton dissociation into free charge carriers, leading to low energy losses

Results and discussion The synthesis of the target molecules VC4 and VC5 was achieved by a double Sonogashira coupling reaction between the DPP dibromo derivative 4 and the corresponding porphyrins 3a and 3b before deprotection of the TMS group (Scheme 1), according to the procedure described before by our group for similar D-A-D systems based on porphyrins.50

R

R 1) BF3.Et2O /CHCl3 2) DDQ R CHO +

N H

+ TMS

CHO

N

NH R

3) Et3N

CH Cl3/ Me OH

HN

N

1a,b R

N

N

TMS

Zn

R

N

N R

3a (quantitative) 3b (85%)

2a (7%) 2b (10%)

OC12H25 R = C6H13

Zn(OAc)2

TMS

S S

OC12H25 a

b

Br 3a,b

+

N S

O S

O

N

Br

1) TBAF / CHCl3

VC4 (30%)

2) Pd(PPh3)2Cl2, CuI, Et3N /Toluene (80ºC)

VC5 (46%)

4

Scheme 1. Synthetic procedure to obtain VC4 and VC5.

The synthetic strategy for the preparation of the new molecules is depicted in Scheme 1. The first step was the synthesis of aldehydes 1a51,52 and 1b.53 The reaction between pyrrole, 3ACS Paragon Plus Environment


Page 8 of 49

(trimethylsilyl)-2-propynal and the corresponding aldehydes 1a or 1b, under standard conditions, led to the formation of free-base porphyrins 2a and 2b in 7% and 10% yield, respectively, after purification (see experimental section); Zn-porphyrins 3a and 3b were prepared by treatment of 1a–b with Zn(AcO)2.

Deprotection of 3a–b was performed with TBAF in chloroform/methanol at room temperature; after washing with water and extraction with chloroform the terminal alkyne was reacted, without further purification, with DPP-based derivative 454 under Sonogashira coupling conditions catalyzed by Pd(PPh3)Cl2 and CuI (Scheme 1) to yield VC4 and VC5 in 30% and 46%, respectively, after purification. For further details of the synthetic and purification procedures see the Supporting Information.

The chemical structures of all new compounds were confirmed by spectroscopic techniques, including FT-IR, 1H NMR,

13C

NMR, and mass spectrometry (see Supporting Information,

Figures S1–S25). Both of the target compounds show good solubility in common organic solvents such as THF, benzonitrile, acetonitrile and o-DCB at room temperature. This enabled good solution processability of the resulting OSCs. Thermogravimetric analysis (Figure S26 and S27) under a nitrogen atmosphere showed that both porphyrin derivatives exhibited excellent


Page 9 of 49 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


thermal stability with decomposition temperatures (5% weight loss) of 395°C and 372°C for VC4 and VC5, respectively.

The optical absorption spectra of VC4 and VC5 in THF solution are shown in Figure S28 (Table 1). Both compounds showed three absorption bands. The absorption bands located at 452 nm and 438 nm, respectively, correspond to the characteristic Soret band, whereas the absorption bands at 708 nm (VC4) and 750 nm (VC5) correspond to the Q band.

Between the two afore mentioned bands there is another absorption band, in the 550–600 nm wavelength region, and this corresponds to the DPP moiety. The Soret band can be attributed to the -* transition of the conjugated backbone and the Q band in the longer wavelength region is ascribed to the intramolecular charge transfer (ICT) band. The redshift in the Q band for VC5, with respect to that in VC4, may be attributed to more efficient ICT between the donor and acceptor moieties.



b) 1.2

a) 1.2

solution film

0.8 0.6 0.4

0.8 0.6 0.4 0.2

0.2 0.0

solution film

1.0

Normalized absorption

1.0 Normalized absorption

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

Page 10 of 49

0.0 400

500

600

700

800

900

400

500

600

700

800

900

Wavelength (nm)

Wavelength (nm)

Figure 1. Normalized optical absorption spectra of (a) VC4 and (b) VC5 in dilute THF solution and thin film.

The optical absorption spectra of VC4 and VC5 in dilute THF solution and thin film cast from THF are shown in Figures 1a and 1b, respectively. The absorption bands in the thin film were significantly redshifted when compared to those in solution, and this is due to the more ordered molecular stacking in the solid state. The optical bandgaps, estimated from the onset absorption edge in the absorption spectra in thin film, are around 1.47 eV and 1.31 eV for VC4 and VC5, respectively, and this decreases as the electron-donating ability of the Zn-porphyrin increases due to the rising ICT effect.

10 ACS Paragon Plus Environment

Page 11 of 49 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


The electrochemical properties of the target compounds VC4 and VC5 were studied by Cyclic Voltammetry (CV). The results, Eox and Ered, are summarized in Table 1 along with the HOMO/LUMO energy levels estimated from the onset oxidation/reduction, respectively (Figure 2). The cyclic voltammograms of VC4 and VC5 show reversible oxidation and reduction waves for the corresponding first potentials (Figures S30 and S31). Compound VC5 shows a lower oxidation potential (0.19 V) than VC4 (0.37 V) and this indicates that the electron-donating ability is higher for VC5. Table 1. Optical properties and redox properties of compounds VC4 and VC5.

VC4

λabs/nm (log

Eox1

Ered1

EHOMO

ELUMO

Eg

ε)a

(V)b,c

(V)b,c

(eV)d

(eV)e

(eV)f

452 (5.4)

0.37

–1.53

–5.47

–3.57

1.9

0.19

–1.54

–5.29

–3.56

1.73

584 (4.7) 708 (4.8) VC5

438 (5.6) 584 (4.8) 750 (4.9)

a)10-7M

in THF; b)10–3 M in ODCB-acetonitrile (4:1) versus Fc+/Fc (Eox = 0.05 V) glassy

carbon, Pt counter electrode, 20 °C, 0.1 M Bu4NClO4, scan rate=100 mV s–1; c) reversible 11 ACS Paragon Plus Environment


processes; f)E

g

d)

estimated from EHOMO = –5.1 – E1ox;

e)

Page 12 of 49

estimated from ELUMO = –5.1 – E1red;

= EHOMO – ELUMO.

As the HOMO energy level of the D-A-D conjugated molecular system is dependent upon the electron-donating ability of D, the significant up-shift in the HOMO energy level of VC5 is attributed to the stronger electron-donating ability of the meso substituent in the Zn-porphyrin in VC5 as compared to VC4; on the other hand, the LUMO energy levels of both compounds are almost the same. It should be noted that the deeper value of the HOMO energy level for VC4 is beneficial for a higher VOC in the corresponding OSCs.

Figure 2. Structure of the devices (left) and HOMO and LUMO energy levels of VC4, VC5 (calculated from the corresponding Eox and Ered values) and PC71BM (right). 12 ACS Paragon Plus Environment

Page 13 of 49 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


Theoretical calculations (Gaussian g09W, DFT, B3LYP/6-31G*) were carried out to gain a deeper understanding of the geometry (Figure S32) and electronic properties (Figure S33) of VC4 and VC5. Both D-A-D systems present a quasi-planar structure along the -conjugated system, with dihedral angles of less than 10o, thus favoring the hole mobility. Nevertheless, the dihedral angle between the porphyrin substituent R (Scheme 1) and the macrocycle is higher for VC5 (  90°), with R perpendicular to the porphyrin macrocycle, due to the steric impediment of the ortho alkoxy chains. Regarding the electronic densities, the HOMOs spread along the whole -conjugated system while the LUMOs are centred on the DPP fragment in both VC4 and VC5. Therefore, there is an overlap between the frontier orbitals and this favours electron delocalization. The photovoltaic properties of VC4 and VC5 as electron donor materials were investigated, along with PC71BM as electron acceptor, in solution-processed BHJ OSCs. The optimization of the device performance was achieved by variation of the donor/acceptor weight ratios (1:05, 1:1 1:1.5 1:2 and 1:2.5) and the thickness of active 13 ACS Paragon Plus Environment


Page 14 of 49

layers (controlling the spin coating speed from 1500 rpm to 2500 rpm) using THF as solvent. It was found that a donor to acceptor weight ratio of 1:2 gave the best photovoltaic performance for both VC4:PC71BM and VC5:PC71BM, with overall PCE values of 4.17% (JSC = 9.12 mA/cm2, VOC = 0.94 V and FF = 0.486) and 4.90% (Jsc = 11.87 mA/cm2, VOC = 0.81 V and FF = 0.51) for VC4 and VC5, respectively (Table 2).

Table 2. Photovoltaic parameters of BHJ OSCs constructed with VC4 and VC5 as donors along with PC71BM as the acceptor, processed under different conditions.

VOC (V)

FF

PCE (%)a

9.12

0.94

0.486

4.17 (4.09) (0.08)

(0.07)

(0.03)

(0.012)

11.87

0.81

0.51

(0.09)

(0.02)

(0.013)

VC4:PC71BM

13.83

0.91

0.64

(SVA)

(0.06)

(0.04)

(0.015)

VC5:PC71BM

16.98

0.79

0.663

(SVA)

(0.09)

(0.03)

(0.015)

Active layer

JSC (mA/cm2)

VC4:PC71BM (as cast) VC5:PC71BM (as cast)

aAverage

4.90 (4.83) (0.07)

8.05 (7.97) (0.08)

8.89 (8.83) (0.06)

of 8 devices 14 ACS Paragon Plus Environment

Page 15 of 49 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


The moderate PCE values, which are a consequence of the low Jsc and FF values, may be related to the poor nanoscale morphology of the active layer limiting the exciton dissociation and charge transport. In order to improve the morphology of the active layer, various post-deposition treatments, i.e., solvent additive (SA), thermal annealing (TA) and solvent vapor annealing (SVA), were employed. SA and TA treatments were both applied but the PCE was only improved slightly. We subsequently tried SVA treatment (in a CS2 environment for 40 s) for these active layers as reported in the literature55,56 due to the high vapor pressure of CS2 when compared to THF.57,58 After this treatment the PCE of the OSCs was significantly improved.



Page 16 of 49

Figure 3. (a) Current–voltage characteristics under illumination (1.5 AM, 100 mW/cm2) and (b) IPCE spectra of the OSCs based on optimized (SVA treated) VC4:PC71BM and VC5:PC71BM active layers.

The J-V characteristics under illumination for the optimized OSCs are shown in Figure 3a and the photovoltaic parameters are compiled in Table 2. The PCE values for VC4:PC71BM- and VC5:PC71BM-based OSCs were 8.05% (Jsc = 13.83 mA/cm2, Voc = 0.91 V and FF = 0.64) and 8.89% (Jsc = 16.98 mA/cm2, Voc = 0.79 V and FF = 0.663), respectively. The increase in the PCE is related to the enhancement in the Jsc and FF after the SVA treatment of the active layers. The slight reduction in the VOC after SVA


Page 17 of 49 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


treatment of the active layer may be related to the enhanced crystalline ordering of the donor component in the active layer59 or a lowering of the quasi-Fermi levels for electron and hole transport due to depleted steady carrier density.60 As indicated by the IPCE spectra of the OSCs (Figure 3b), the IPCE values are higher for the SVA-treated active layer OSCs when compared to those of the as-cast counterparts. Additionally, the absorption of the active layers became broader with increased absorption intensity after the SVA treatment and this contributes to the enhanced Jsc. Moreover, the VC5-based OSC showed a higher and broader IPCE response region than VC4, which is a consequence of the redshifted absorption spectrum of VC5 when compared to that of VC4. The Jsc values estimated from the integration of IPCE spectra are around 13.74 mA/cm2 and 16.87 mA/cm2 for the OSCs processed with optimized VC4 and VC5 active layers, respectively, and these values are in good agreement with those observed in the

J-V characteristics under illumination.



Page 18 of 49

Figure 4. Dark current–voltage curves of hole-only devices using optimized (SVA-treated) VC4:PC71BM and VC5:PC71BM active layers.

The performance of the OSCs, especially Jsc and FF, can be significantly affected by the charge transport. The hole (µh) and electron (µe) mobilities in the active layers (both as-cast and SVA-treated) were measured from the dark J-V characteristics of hole-only and electron-only devices, respectively, and these were fitted with the space charge limited current (SCLC) model (Figure 4 shown only for optimized active layers). As-cast VC4:PC71BM and VC5:PC71BM active layers showed h/e values of around 5.71 × 10–5/2.45 × 10-4 cm2/Vs and 7.39 × 10–5 / 2.52 × 10–4 cm2/Vs, respectively, and


Page 19 of 49 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


these increased to 1.29 × 10–4 /2.53 × 10-4 cm2/Vs and 1.67 × 10–4/2.62 × 10–4 cm2/Vs, respectively, upon SVA treatment. The increase in h after SVA treatment led to more balanced charge transport and this in turn led to enhanced Jsc and FF values.61 Moreover, the VC5:PC71BM active layer exhibited a higher h than the VC4:PC71BM blend irrespective of processing conditions. This finding is consistent with the Jsc and FF values for the corresponding OSCs. We proceeded to investigate the exciton dissociation, charge generation and collection properties by varying the photocurrent density (Jph) with effective voltage (Veff) in optimized OSCs. The results are shown in Figure 5 (Jph = JL – JD, where JL and JD are the current densities under illumination and in the dark, respectively, and Veff = Vo – Va, where

Va is the applied voltage and Vo for which Jph is zero).62 The variation of Jph with Veff for OSCs based on as-cast active layers is shown in Figure S34 (supplementary information). It was observed that the Jph value did not reach the saturation value, even at the highest

Veff value, for as-cast OSCs. This finding demonstrates that the internal electric field is not sufficient to extract the charge carrier towards the electrode and leads to significant electron–hole recombination prior to collection by the electrodes.63 19 ACS Paragon Plus Environment


Page 20 of 49

Figure 5. Variation of photocurrent density (Jph) with effective voltage (Veff) for the OSCs based on the optimized active layer.

It can be seen from Figure 5 that initially the Jph increases in a linear manner with Veff and then tends to saturate at high Veff values and Jphsat is only limited by the incident photons. The saturation value Jphsat depends upon the maximum exciton generation rate (Gmax) and can be estimated from Jphsat = qGmaxL (where q is elementary charge and L is the thickness of the active layer). The higher Gmax value for the VC5 (1.02 × 1028 and 1.31 × 1028 m–3 s–1 for as-cast and SVA-treated) based OSCs as compared to that of the VC4


Page 21 of 49 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


(0.78 × 1028 and 1.10

× 1028 m–3s–1 for as-cast and SVA-treated, respectively)

counterpart, irrespective of the active layer processing conditions, is consistent with the broader absorption range of VC5. The exciton dissociation efficiency (Pdiss) and charge collection efficiency (Pcoll) values were estimated from Jph/Jphsat at short circuit conditions and maximum power point, respectively. The Pdiss/Pcoll values are 0.85/0.64, 0.88/0.69, 0.92/0.73 and 0.95/0.78 for the OSCs based on VC4 (as cast), VC5 (as cast), VC4 (SVA) and VC5 (SVA), respectively. The higher values of both Pdiss and Pcoll for VC5-based OSCs when compared to VC4, irrespective of the processing conditions, indicate that the exciton dissociation and charge collection are more efficient in the OSCs based on VC5 and these are further improved by SVA treatment and are consistent with the charge carrier mobilities. In order to obtain information about the recombination mechanism in the OSCs based on VC4 and VC5, the JSC was measured at different illumination intensities (Pin) and the results are shown in Figure 6a for the OSCs based on optimized active layers to investigate the effect of SVA on the charge recombination mechanism. The relationship between JSC and Pin (Figure 6a) can be expressed as

J sc  ( Pin ) 

, where  is the exponent 21



Page 22 of 49

factor.64 The value of  is unity when most of the free charge carrier after exciton dissociation is swept out and collected by electrodes.

Figure 6. Variation of (a) Jsc and (b) Voc with illumination intensity (Pin) for optimized active layers. 22 ACS Paragon Plus Environment

Page 23 of 49 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


The  values for the OSCs based on as-cast/SVA-treated active layers are 0.88/0.93 and 0.90/0.96 for VC4:PC71BM and VC5:PC71BM active layers, respectively. The high  value for VC5 when compared to VC4, irrespective of the processing conditions, suggests that the bimolecular recombination is suppressed in the former case and this is further suppressed upon the SVA treatment. The suppression of bimolecular recombination leads to an improvement in the Jsc and FF values. The variation of Voc with Pin was also measured in order to gain information on the recombination mechanisms65 and the results are shown in Figure 6b. The relationship is described as (equation 1)

Voc  nkT ln( Pin ) q

(1)

where k is Boltzmann’s constant, T is the temperature and q is the electronic charge. When the n value is around unity the bimolecular recombination is dominant in the OSCs. However, if the value of n is around two, then trap-assisted or Shockley–Read–Hall (SRH) trap-assisted recombination is the dominant process. The calculated n values for ascast/SVA-treated OSCs are around 1.39/1.26 and 1.31/1.17 for VC4 and VC5, respectively. The devices processed with SVA-treated active layers showed higher n 23 ACS Paragon Plus Environment


Page 24 of 49

values when compared to as-cast counterparts and this demonstrates that the trapassisted recombination is significantly suppressed due to the improved nano-scale phase separation morphology and more balanced charge transport induced by SVA treatment. The crystallinity and the molecular ordering the donor in the active layer also have a significant influence on the FF and Jsc values of the OSCs. As a consequence, in order to obtain information about these characteristics we recorded the X-ray diffraction (XRD) patterns of the VC4:PC71BM and VC5:PC71BM active layers before and after SVA treatment. The diffractograms are shown in Figure 7.


Page 25 of 49 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


Figure 7. X-ray diffraction patterns of the optimized VC4:PC71BM and VC5:PC71BM thin films.

It can be seen that all active layers (as cast and SVA treated) showed a strong diffraction peak at 2 = 4.96, which is related to the lamellar distance of 2.03 nm; however, the (010) peak, located at 2 = 22.96 (VC4) and 23.26 (VC5), corresponds to the - stacking distance of 0.39 nm and 0.35 nm, respectively. The intensity of both the diffraction peaks at 2=4.96 and - stacking peaks is intense for SVA treated film as compared to that for as cast counter parts, indicating that the crystalline nature increased after the SVA treatment. This indicates that VC5 has a denser molecular packing than VC4. Moreover, the intensity of both XRD peaks, i.e., (100) and (010), for VC5 are stronger than those for VC4 and this indicates that the crystallinity of the former is better. In addition to these two peaks, a weak diffraction peak at around 2 = 18.20 is also evident in both active layers and this corresponds to PC71BM. The increase in the degree of crystallinity for VC5:PC71BM, along with the more compact - stacking distance, is



Page 26 of 49

beneficial for charge transport and this is one of the reasons for the high FF value for the OSC. The VOC value for the optimized VC4-based OSC (0.91 V) is higher than that for the VC5 counterpart (0.79 V) and this trend correlates with the lower-lying HOMO energy level of VC4. However, the JSC value of the VC5-based OSC is significantly higher than that for the VC4-based device although the LUMO offset values for both of these blends are almost the same (0.53 eV) but VC5 has a lower optical band gap and the absorption spectrum is more towards the near infrared region than that of VC4, thus signifying enhanced light harvesting ability for the VC5:PC71BM-based OSCs when compared to the VC4:PC71BM counterparts. In most OSCs one of the key issues is the trade-off between JSC and VOC and optimization of both Jsc and Voc is an important aspect. In an effort to overcome aforementioned issue, the photon energy loss (Eloss) was evaluated;

Eloss is defined as Eg – qVOC, where Eg is the optical bandgap of the donor or acceptor, whichever is small.66 Here we have used the optical bandgap values for VC4 and VC5 of 1.47 eV and 1.31 eV, respectively, for the estimation of Eloss. The Eloss values for the optimized OSCs employing VC4 and VC5 are 0.56 eV and 0.52 eV, respectively. 26 ACS Paragon Plus Environment

Page 27 of 49 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


Recently, Yang et al. reported a low energy loss of 0.54 eV with a high VOC value of 1.15 V for all-small-molecule OSCs employing a non-fullerene acceptor,67 whereas we achieved an even lower energy loss of 0.52 eV for OSCs based on a porphyrin small molecule as donor and PC71BM as acceptor.50 Although the difference between the HOMO-LUMO offset for VC5:PC71BM (1.19 eV) and VC4:PC71BM (1.37 eV) is 0.18 eV, the fact that the difference in the Voc for VC5 and VC4 is only 0.12 V may be related to a lower photon energy loss and the narrow optical bandgap for VC5. The major energy losses in the OSCs originate from the non-radiative recombination and the presence of charge transfer states (a driving force for charge separation of the exciton).68,69 We estimated theoretically the dipole moment of both molecules in the ground/excited state and the values are 0.47/0.35 and 0.54/0.47 D for VC4 and VC5, respectively. Both dipole moments for VC5 are higher than those for VC4 and this can be attributed to the stronger electron-donating ability of the 2,6-bis(dodecyloxy)phenyl substituents of the porphyrin relative (VC5) to 5-hexylthieno[3,2-b]thiophen-2-yl (VC4). We have measured the dielectric constant of the VC4 and VC5 thin films employing the impedance spectroscopy on ITO/PEDOT:PSS/VC4 or VC5/Al structure. The values estimated from 27 ACS Paragon Plus Environment


Page 28 of 49

this measurement are 3.6 and 4.2 at frequency of 10 kHz, respectively. Since the exciton binding energy is inversely proportional to the dielectric constant of the material, indicating that the exciton binding energy for VC5 is relatively low as compared to VC4. It has been reported in the literature that when the magnitude of the difference between the ground and excited state dipole moments (µge) for a D-A molecular system is large then the coulombic binding energy of the excitons is low and this facilitates the charge separation more efficiently.70 We estimated the value of µge by employing the following expression and considering changes in the dipole along each coordinate axis. 𝛥𝜇𝑔𝑒 = [(𝜇𝑔𝑥 ― 𝜇𝑒𝑥)2 + (𝜇𝑔𝑦 ― 𝜇𝑒𝑦)2 + (𝜇𝑔𝑧 ― 𝜇𝑒𝑧)2]1/2

The µge value for VC5 (0.68 D) is higher than that for VC4 (0.24 D) and this indicates that the exciton binding energy is relatively lower for VC5 than VC4. This confirms that a low value of either LUMO offset or HOMO offset is enough for exciton dissociation and charge transfer in the BHJ active layer. An increased value of the dipole moment also leads to an increase in the dielectric constant of VC5, which may decrease the exciton binding energy and decrease the Eloss


Page 29 of 49 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


in the corresponding OSC. Moreover, the higher dipole moment value may also lead to a high charge carrier mobility, as confirmed from the mobility results. The larger dipole moment resulted in better self-assembly of the molecules and this led to the formation of longer supramolecular chains and corroborated the improved percolation pathways for charge carriers.71 Moreover, the high dipole moment and dielectric constant for VC5 may also reduce the non-radiative recombination and lead to a reduction in Eloss.62

CONCLUSIONS Two new D-A-D small molecules (VC4 and VC5) were synthesized and these compounds have the same core DPP acceptor unit and Zn-porphyrin terminal units with different electron-donating substituents at meso positions. The effect that electrondonating strength had on the optical absorption and HOMO and LUMO energy levels was investigated. On the basis of these properties, we employed the compounds as electron donors in conjunction with PC71BM as the electron acceptor in solution-processed BHJOSCs. After optimization (i.e., donor to acceptor weight ratio in the active layer and SVA treatment in CS2 for 40 s), the resultant OSCs based on VC4:PC71BM and VC5:PC71BM 29 ACS Paragon Plus Environment


Page 30 of 49

active layers showed overall PCE values of 8.05% and 8.89% with low energy losses of 0.56 eV and 0.52 eV, respectively. The VC4-based OSC gave a higher Voc than the VC5 counterpart and this result is related to the deeper HOMO level of VC4 when compared to VC5. The Jsc value of the VC5 OSC is higher than that for the VC4 counterpart and this result is attributed to the broader absorption spectrum of VC5, which extended up to 950 nm, and the larger dipole moment, which led to a high exciton generation rate and efficient charge transfer. Moreover, the relatively high FF value for VC5 may be related to the high charge carrier mobility, more balanced charge transport and low non-radiative recombination within the VC5:PC71BM active layer and also to the high charge collection efficiency. Finally, small molecules based on porphyrins with a D-A-D structure provide an excellent approach to obtain highly efficient BHJ solar cells. Moreover, these porphyrin based molecules can be used as donor for non-fullerene acceptors and work is in progress will be communicated shortly.

ASSOCIATED CONTENT


Page 31 of 49 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


Supporting Information.

Experimental details, the synthesis of VC5 and VC6, 1H NMR,

13C

NMR, FT-IR and

MALDI-TOF mass spectra, TGA and DSC analysis, theoretical calculations and electrochemical studies. This material is available free of charge via the Internet at http://pubs.acs.org..

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (F. L.).

*E-mail: [email protected] (G. D. S.).

*E-mail: [email protected] (P. d. l. C.).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 31 ACS Paragon Plus Environment


Page 32 of 49

ACKNOWLEDGMENT F.L. and P.C. thank MINECO (Spain) (CTQ2016-79189-R) and the Junta de Comunidades de Castilla-la Mancha (SBPLY/17/180501/000254) for financial support and V.C. acknowledges an FPU grant to MECD (FPU15/02170). Prof. G.D.S. and R.S. thank

the

Department

of

Science

and

Technology

(Government

of

India)

(DST/TMD/SERI/D05), for financial support.

REFERENCES (1)

Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666–12731.

(2)

Lin, Y.; Zhan, X. Oligomer Molecules for Efficient Organic Photovoltaics. Acc.

Chem. Res. 2016, 49, 175–183.

(3)

Chen, W.; Zhang, Q.; Lin, Y. Z.; Wang, W. B.; Gao, M. L.; Li, L. L.; Zhang, J.; Zhan, X. W.; Lee, T. H.; Huang, M. J.; et al. Recent Progress in Non-Fullerene Small Molecule Acceptors in Organic Solar Cells (OSCs). J. Mater. Chem. C 2017, 5, 32 ACS Paragon Plus Environment

Page 33 of 49 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


1275–1302.

(4)

Hu, H.; Chow, P. C. Y.; Zhang, G.; Ma, T.; Liu, J.; Yang, G.; Yan, H. Design of Donor Polymers with Strong Temperature-Dependent Aggregation Property for Efficient Organic Photovoltaics. Acc. Chem. Res. 2017, 50, 2519–2528.

(5)

Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials. Chem. Rev. 2016, 116, 7397–7457.

(6)

Zhang, K.; Hu, Z.; Sun, C.; Wu, Z.; Huang, F.; Cao, Y. Toward Solution-Processed High-Performance Polymer Solar Cells: From Material Design to Device Engineering. Chem. Mater. 2017, 29, 141–148.

(7)

McDowell, C.; Bazan, G. C. Organic Solar Cells Processed from Green Solvents.

Curr. Opin. Green Sustain. Chem. 2017, 5, 49–54.

(8)

Zhang, S.; Ye, L.; Zhang, H.; Hou, J. Green-Solvent-Processable Organic Solar Cells. Mater. Today 2016, 19, 533–543.



(9)

Page 34 of 49

Hedley, G. J.; Ruseckas, A.; Samuel, I. D. W. Light Harvesting for Organic Photovoltaics. Chem. Rev. 2017, 117, 796–837.

(10) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science (80-. ). 1995, 270, 1789–1791.

(11) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027.

(12) Fei, Z.; Eisner, F. D.; Jiao, X.; Azzouzi, M.; Röhr, J. A.; Han, Y.; Shahid, M.; Chesman, A. S. R.; Easton, C. D.; McNeill, C. R.; et al. An Alkylated Indacenodithieno[3,2- b ]Thiophene-Based Nonfullerene Acceptor with High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies Greater than 13% with Low Voltage Losses. Adv. Mater. 2018, 30, 1705209.

(13) Xu, X.; Yu, T.; Bi, Z.; Ma, W.; Li, Y.; Peng, Q. Realizing Over 13% Efficiency in


Page 35 of 49 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


Green-Solvent-Processed Nonfullerene Organic Solar Cells Enabled by 1,3,4Thiadiazole-Based Wide-Bandgap Copolymers. Adv. Mater. 2018, 30, 1703973.

(14) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem.

Soc. 2017, 139, 7148–7151.

(15) Li, S.; Ye, L.; Zhao, W.; Yan, H.; Yang, B.; Liu, D.; Li, W.; Ade, H.; Hou, J. A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells. J. Am. Chem. Soc. 2018, 140, 7159–7167.

(16) Wang, Z.; Zhu, L.; Shuai, Z.; Wei, Z. A-π-D-π-A Electron-Donating Small Molecules for Solution-Processed Organic Solar Cells: A Review. Macromol. Rapid Commun. 2017, 38, 1700470.

(17) Du, J.; Biewer, M. C.; Stefan, M. C. Benzothiadiazole Building Units in SolutionProcessable Small Molecules for Organic Photovoltaics. J. Mater. Chem. A 2016,

4, 15771–15787.



Page 36 of 49

(18) Collins, S. D.; Ran, N. A.; Heiber, M. C.; Nguyen, T.-Q. Small Is Powerful: Recent Progress in Solution-Processed Small Molecule Solar Cells. Adv. Energy Mater. 2017, 7, 1602242.

(19) Ilmi, R.; Haque, A.; Khan, M. S. High Efficiency Small Molecule-Based Donor Materials for Organic Solar Cells. Org. Electron. 2018, 58, 53–62.

(20) Po, R.; Roncali, J. Beyond Efficiency: Scalability of Molecular Donor Materials for Organic Photovoltaics. J. Mater. Chem. C 2016, 4, 3677–3685.

(21) Deng, D.; Zhang, Y.; Zhang, J.; Wang, Z.; Zhu, L.; Fang, J.; Xia, B.; Wang, Z.; Lu, K.; Ma, W.; et al. Fluorination-Enabled Optimal Morphology Leads to over 11% Efficiency for Inverted Small-Molecule Organic Solar Cells. Nat. Commun. 2016, 7, 13740.

(22) Wan, J.; Xu, X.; Zhang, G.; Li, Y.; Feng, K.; Peng, Q. Highly Efficient Halogen-Free Solvent Processed Small-Molecule Organic Solar Cells Enabled by Material Design and Device Engineering. Energy Environ. Sci. 2017, 10, 1739–1745.


Page 37 of 49 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


(23) Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.; Yang, X.; Feng, H.; et al. A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am.

Chem. Soc. 2015, 137, 3886–3893.

(24) Meng, L.; Zhang, Y.; Wan, X.; Li, C.; Zhang, X.; Wang, Y.; Ke, X.; Xiao, Z.; Ding, L.; Xia, R.; et al. Organic and Solution-Processed Tandem Solar Cells with 17.3% Efficiency. Science (80-. ). 2018, 361, 1094–1098.

(25) Janssen, R. A. J.; Nelson, J. Factors Limiting Device Efficiency in Organic Photovoltaics. Adv. Mater. 2013, 25, 1847–1858.

(26) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. The Energy of Charge-Transfer States in Electron Donor-Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells. Adv. Funct. Mater. 2009, 19, 1939–1948.

(27) Gao, K.; Xiao, L.; Kan, Y.; Yang, B.; Peng, J.; Cao, Y.; Liu, F.; Russell, T. P.; Peng, X. Solution-Processed Bulk Heterojunction Solar Cells Based on Porphyrin Small



Page 38 of 49

Molecules with Very Low Energy Losses Comparable to Perovskite Solar Cells and High Quantum Efficiencies. J. Mater. Chem. C 2016, 4, 3843–3850.

(28) Li, W.; Hendriks, K. H.; Furlan, A.; Wienk, M. M.; Janssen, R. A. J. High Quantum Efficiencies in Polymer Solar Cells at Energy Losses below 0.6 EV. J. Am. Chem.

Soc. 2015, 137, 2231–2234.

(29) Wang, M.; Wang, H.; Yokoyama, T.; Liu, X.; Huang, Y.; Zhang, Y.; Nguyen, T.-Q.; Aramaki, S.; Bazan, G. C. High Open Circuit Voltage in Regioregular Narrow Band Gap Polymer Solar Cells. J. Am. Chem. Soc. 2014, 136, 12576–12579.

(30) Gao, K.; Li, L.; Lai, T.; Xiao, L.; Huang, Y.; Huang, F.; Peng, J.; Cao, Y.; Liu, F.; Russell, T. P.; et al. Deep Absorbing Porphyrin Small Molecule for HighPerformance Organic Solar Cells with Very Low Energy Losses. J. Am. Chem. Soc. 2015, 137, 7282–7285.

(31) Chen, S.; Xiao, L.; Zhu, X.; Peng, X.; Wong, W.-K.; Wong, W.-Y. SolutionProcessed New Porphyrin-Based Small Molecules as Electron Donors for Highly


Page 39 of 49 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


Efficient Organic Photovoltaics. Chem. Commun. 2015, 51, 14439–14442.

(32) Liu, X.; Du, X.; Wang, J.; Duan, C.; Tang, X.; Heumueller, T.; Liu, G.; Li, Y.; Wang, Z.; Wang, J.; et al. Efficient Organic Solar Cells with Extremely High Open-Circuit Voltages and Low Voltage Losses by Suppressing Nonradiative Recombination Losses. Adv. Energy Mater. 2018, 8, 1801699.

(33) Xiao, L.; Chen, S.; Gao, K.; Peng, X.; Liu, F.; Cao, Y.; Wong, W.-Y.; Wong, W.-K.; Zhu, X. New Terthiophene-Conjugated Porphyrin Donors for Highly Efficient Organic Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 30176–30183.

(34) Qin, H.; Li, L.; Guo, F.; Su, S.; Peng, J.; Cao, Y.; Peng, X. Solution-Processed Bulk Heterojunction Solar Cells Based on a Porphyrin Small Molecule with 7% Power Conversion Efficiency. Energy Environ. Sci. 2014, 7, 1397–1401.

(35) Gao, K.; Miao, J.; Xiao, L.; Deng, W.; Kan, Y.; Liang, T.; Wang, C.; Huang, F.; Peng, J.; Cao, Y.; et al. Multi-Length-Scale Morphologies Driven by Mixed Additives in Porphyrin-Based Organic Photovoltaics. Adv. Mater. 2016, 28, 4727–4733.



Page 40 of 49

(36) Liang, T.; Xiao, L.; Gao, K.; Xu, W.; Peng, X.; Cao, Y. Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells. ACS Appl. Mater.

Interfaces 2017, 9, 7131–7138.

(37) Morán, G.; Arrechea, S.; Cruz, P. de la; Cuesta, V.; Biswas, S.; Palomares, E.; Sharma, G. D.; Langa, F. CuSCN as Selective Contact in Solution-Processed Small-Molecule Organic Solar Cells Leads to over 7% Efficient Porphyrin-Based Device. J. Mater. Chem. A 2016, 4, 11009–11022.

(38) Bucher, L.; Desbois, N.; Koukaras, E. N.; Devillers, C. H.; Biswas, S.; Sharma, G. D.; Gros, C. P. BODIPY–diketopyrrolopyrrole–porphyrin Conjugate Small Molecules for Use in Bulk Heterojunction Solar Cells. J. Mater. Chem. A 2018, 6, 8449–8461.

(39) Kesters, J.; Verstappen, P.; Kelchtermans, M.; Lutsen, L.; Vanderzande, D.; Maes, W. Porphyrin-Based Bulk Heterojunction Organic Photovoltaics: The Rise of the Colors of Life. Adv. Energy Mater. 2015, 5, 1500218. 40 ACS Paragon Plus Environment

Page 41 of 49 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


(40) Liu, C.; Zhang, L.; Xiao, L.; Peng, X.; Cao, Y. Doping ZnO with Water/AlcoholSoluble Small Molecules as Electron Transport Layers for Inverted Polymer Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 28225–28230.

(41) Mishra, R.; Regar, R.; Singhal, R.; Panini, P.; Sharma, G. D.; Sankar, J. Porphyrin Based

Push–pull

Conjugates

as

Donors

for

Solution-Processed

Bulk

Heterojunction Solar Cells: A Case of Metal-Dependent Power Conversion Efficiency. J. Mater. Chem. A 2017, 5, 15529–15533.

(42) Zhang, A.; Li, C.; Yang, F.; Zhang, J.; Wang, Z.; Wei, Z.; Li, W. An Electron Acceptor with Porphyrin and Perylene Bisimides for Efficient Non-Fullerene Solar Cells.

Angew. Chemie Int. Ed. 2017, 56, 2694–2698.

(43) Hadmojo, W. T.; Yim, D.; Aqoma, H.; Ryu, D. Y.; Shin, T. J.; Kim, H. W.; Hwang, E.; Jang, W.-D.; Jung, I. H.; Jang, S.-Y. Artificial Light-Harvesting n-Type Porphyrin for Panchromatic Organic Photovoltaic Devices. Chem. Sci. 2017, 8, 5095–5100.

(44) Wang, H.; Xiao, L.; Yan, L.; Chen, S.; Zhu, X.; Peng, X.; Wang, X.; Wong, W.-K.;



Page 42 of 49

Wong, W.-Y. Structural Engineering of Porphyrin-Based Small Molecules as Donors for Efficient Organic Solar Cells. Chem. Sci. 2016, 7, 4301–4307.

(45) Xiao, L.; Liang, T.; Gao, K.; Lai, T.; Chen, X.; Liu, F.; Russell, T. P.; Huang, F.; Peng, X.; Cao, Y. Ternary Solar Cells Based on Two Small Molecule Donors with Same Conjugated Backbone: The Role of Good Miscibility and Hole Relay Process.

ACS Appl. Mater. Interfaces 2017, 9, 29917–29923.

(46) Xiao, L.; Gao, K.; Zhang, Y.; Chen, X.; Hou, L.; Cao, Y.; Peng, X. A Complementary Absorption Small Molecule for Efficient Ternary Organic Solar Cells. J. Mater.

Chem. A 2016, 4, 5288–5293.

(47) Xiao, L.; Liu, C.; Gao, K.; Yan, Y.; Peng, J.; Cao, Y.; Peng, X. Highly Efficient Small Molecule Solar Cells Fabricated with Non-Halogenated Solvents. RSC Adv. 2015,

5, 92312–92317.

(48) Xiao, L.; Lai, T.; Liu, X.; Liu, F.; Russell, T. P.; Liu, Y.; Huang, F.; Peng, X.; Cao, Y. A Low-Bandgap Dimeric Porphyrin Molecule for 10% Efficiency Solar Cells with


Page 43 of 49 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


Small Photon Energy Loss. J. Mater. Chem. A 2018, 6, 18469–18478.

(49) Mahmood, A.; Hu, J.-Y.; Xiao, B.; Tang, A.; Wang, X.; Zhou, E. Recent Progress in Porphyrin-Based Materials for Organic Solar Cells. J. Mater. Chem. A 2018, 6, 16769–16797.

(50) Cuesta, V.; Vartanian, M.; Cruz, P. de la; Singhal, R.; Sharma, G. D.; Langa, F. Comparative Study on the Photovoltaic Characteristics of A–D–A and D–A–D Molecules Based on Zn-Porphyrin; a D–A–D Molecule with over 8.0% Efficiency. J.

Mater. Chem. A 2017, 5, 1057–1065.

(51) Jang, Y. J.; Lim, B. T.; Yoon, S. B.; Choi, H. J.; Ha, J. U.; Chung, D. S.; Lee, S.-G. A Small Molecule Composed of Anthracene and Thienothiophene Devised for HighPerformance Optoelectronic Applications. Dye. Pigment. 2015, 120, 30–36.

(52) Gupta, A.; Ali, A.; Gao, M.; Singh, T. B.; Bilic, A.; Watkins, S. E.; Bach, U.; Evans, R. A. Small Molecules Containing Rigidified Thiophenes and a Cyanopyridone Acceptor Unit for Solution-Processable Bulk-Heterojunction Solar Cells. Dye.



Page 44 of 49

Pigment. 2015, 119, 122–132.

(53) Lee, C. Y.; Hupp, J. T. Dye Sensitized Solar Cells: TiO 2 Sensitization with a Bodipy-Porphyrin Antenna System. Langmuir 2010, 26, 3760–3765.

(54) Hendriks, K. H.; Heintges, G. H. L.; Gevaerts, V. S.; Wienk, M. M.; Janssen, R. A. J.

High-Molecular-Weight

Regular

Alternating

Diketopyrrolopyrrole-Based

Terpolymers for Efficient Organic Solar Cells. Angew. Chemie Int. Ed. 2013, 52, 8341–8344.

(55) Wang, J.-L.; Wu, Z.; Miao, J.-S.; Liu, K.-K.; Chang, Z.-F.; Zhang, R.-B.; Wu, H.-B.; Cao, Y. Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing. Chem. Mater. 2015, 27, 4338– 4348.

(56) Kumar, C. V.; Cabau, L.; Viterisi, A.; Biswas, S.; Sharma, G. D.; Palomares, E. Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6%


Page 45 of 49 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


Efficiency Based on a Benzodithiophene Donor Core and Dicyano Acceptor Units.

J. Phys. Chem. C 2015, 119, 20871–20879.

(57) Más-Montoya, M.; Janssen, R. A. J. The Effect of H- and J-Aggregation on the Photophysical and Photovoltaic Properties of Small Thiophene-Pyridine-DPP Molecules for Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2017, 27, 1605779.

(58) Li, Y.; Lee, D. H.; Lee, J.; Nguyen, T. L.; Hwang, S.; Park, M. J.; Choi, D. H.; Woo, H.

Y.

Two

Regioisomeric

π-Conjugated

Small

Molecules:

Synthesis,

Photophysical, Packing, and Optoelectronic Properties. Adv. Funct. Mater. 2017,

27, 1701942–n/a.

(59) Xie, Y.; Zhou, W.; Yin, J.; Hu, X.; Zhang, L.; Meng, X.; Ai, Q.; Chen, Y. PostAnnealing to Recover the Reduced Open-Circuit Voltage Caused by Solvent Annealing in Organic Solar Cells. J. Mater. Chem. A 2016, 4, 6158–6166.

(60) Wang, J.-L.; Liu, K.-K.; Liu, S.; Xiao, F.; Liu, F.; Wu, H.-B.; Cao, Y. Developing High-



Page 46 of 49

Performance Electron-Rich Unit End-Capped Wide Bandgap Oligomeric Donor by Weak Electron-Deficient Central Core Strategy. Sol. RRL 2018, 2, 1700212.

(61) Proctor, C. M.; Kher, A. S.; Love, J. A.; Huang, Y.; Sharenko, A.; Bazan, G. C.; Nguyen, T.-Q. Understanding Charge Transport in Molecular Blend Films in Terms of Structural Order and Connectivity of Conductive Pathways. Adv. Energy Mater. 2016, 6, 1502285.

(62) Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Charge Transport

and

Photocurrent

Generation

in

Poly(3-Hexylthiophene):

Methanofullerene Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2006, 16, 699–708.

(63) Li, Y.; Ko, S.-J.; Park, S. Y.; Choi, H.; Nguyen, T. L.; Uddin, M. A.; Kim, T.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Quinoxaline–thiophene Based Thick Photovoltaic Devices with an Efficiency of ∼8%. J. Mater. Chem. A 2016, 4, 9967–9976.

(64) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in Polymer-Fullerene Bulk


Page 47 of 49 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


Heterojunction Solar Cells. Phys. Rev. B 2010, 82, 245207.

(65) Leong, W. L.; Cowan, S. R.; Heeger, A. J. Differential Resistance Analysis of Charge Carrier Losses in Organic Bulk Heterojunction Solar Cells: Observing the Transition from Bimolecular to Trap-Assisted Recombination and Quantifying the Order of Recombination. Adv. Energy Mater. 2011, 1, 517–522.

(66) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. The Energy of Charge-Transfer States in Electron Donor-Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells. Adv. Funct. Mater. 2009, 19, 1939–1948.

(67) Yang, D.; Wang, Y.; Sano, T.; Gao, F.; Sasabe, H.; Kido, J. A Minimal NonRadiative Recombination Loss for Efficient Non-Fullerene All-Small-Molecule Organic Solar Cells with a Low Energy Loss of 0.54 EV and High Open-Circuit Voltage of 1.15 V. J. Mater. Chem. A 2018, 6, 13918–13924.

(68) Naveed, H. B.; Ma, W. Miscibility-Driven Optimization of Nanostructures in Ternary Organic Solar Cells Using Non-Fullerene Acceptors. Joule 2018, 2, 621–641.



Page 48 of 49

(69) Azzouzi, M.; Yan, J.; Kirchartz, T.; Liu, K.; Wang, J.; Wu, H.; Nelson, J. Nonradiative Energy Losses in Bulk-Heterojunction Organic Photovoltaics. Phys. Rev. X 2018,

8, 31055.

(70) Carsten, B.; Szarko, J. M.; Lu, L.; Son, H. J.; He, F.; Botros, Y. Y.; Chen, L. X.; Yu, L. Mediating Solar Cell Performance by Controlling the Internal Dipole Change in Organic Photovoltaic Polymers. Macromolecules 2012, 45, 6390–6395.

(71) Intemann, J. J.; Yao, K.; Ding, F.; Xu, Y.; Xin, X.; Li, X.; Jen, A. K.-Y. Enhanced Performance of Organic Solar Cells with Increased End Group Dipole Moment in Indacenodithieno[3,2-b]Thiophene-Based Molecules. Adv. Funct. Mater. 2015, 25, 4889–4897.

Table of contents (TOC)


Page 49 of 49 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



Near-IR Absorbing D-A-D Zn-Porphyrin-based Small Molecule Donors

Recommend Documents