Porphyrin–Diindenothieno[2,3-b]thiophene Alternating Copolymer—
A Blue-Light Harvester in Ternary-Blend Polymer Solar Cells
Yu-Chun Wu,
1Yi-Hsiang Chao,
1Chien-Lung Wang,
1Chun-Ta Wu,
1Chain-Shu Hsu,
1Yu-Ling Zeng,
2Ching-Yao Lin
21
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan
2Department of Applied Chemistry, National Chi Nan University, 1 University Road, Puli, Nantou 54561, Taiwan
Correspondence to: C.-S. Hsu (E-mail: [email protected]) or C.-Y. Lin (E-mail: [email protected])
Received 16 July 2012; accepted 31 July 2012; published online 14 September 2012 DOI: 10.1002/pola.26340
ABSTRACT:Porphyrin, despite chosen by Nature as light har-vesting units, hasn’t revealed its full potentials as a structural unit in porphyrin-incorporated polymers (PPors). A novel PPor was synthesized to investigate the origins of the low perform-ances of PPor-based polymer solar cells (PSCs). The polymer features broad absorption in the blue-light region, because the diindenothieno[2,3-b]thiophene (DITT) unit extended the conju-gation in the polymer backbone. PPor-DITT/PC71BM based
PSCs have a high Voc(0.79 V). Their low Jscand fill factor (FF)
were attributed to the un-optimized morphology, as indicated by the photoluminescence quenching and atomic force micros-copy (AFM) experiments. Using PPor-DITT as a blue-light har-vesting dopant in an amorphous host leverage the strong 400– 550 nm absorption of PPor-DITT and circumvent the difficulties
in reaching optimized morphology in the PPor/PCBM thin films. An addition of 2 wt % of PPor-DITT in ternary-blend PSCs resulted in a 10 % increase of external quantum effi-ciency (EQE) in the blue-light region. However, in a crystalline host, the dopant decreased the crystallinity of the host and led to large drops in FF and power conversion efficiencies (PCEs). The study provides an alternative route and expands the appli-cation of PPors in PSCs as a blue-light harvester in ternary-blend PSCs using amorphous polymers as host.VC 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 5032– 5040, 2012
KEYWORDS:blends; bulk heterojunction; conjugated polymers; morphology; polymer blends; porphyrin; solar cells
INTRODUCTION Technology of effectively utilizing solar energy is regarded as one of the most important issues in this century, as it will offer a clean, inexhaustible, and envi-ronmentally friendly energy source. Polymer solar cell (PSC) is a promising candidate for the solar radiation-to-electricity conversion process. It has drawn great attentions due to their potential advantages in producing large-area, light-weight, and flexible photovoltaic devices using low-cost solu-tion processes.1–4 Bulk heterojunction (BHJ) PSCs fabricated by blending p-type conjugated polymer as a donor (D) rial and n-type fullerene-derivative as an acceptor (A) mate-rial in the active layer is one of the most useful device archi-tectures, which attain both maximum internal donor– acceptor (D–A) interfacial area for efficient charge separation and bicontinuous network for the transport of charge car-riers.5–7 To achieve high power conversion efficiencies
(PCEs), molecular structures of polymers are modified to modulate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy lev-els to optimal values. Conjugated polymers with deep-lying
HOMOs are advantageous in generating large open-circuit voltage (Voc),8–11whereas the LUMO energy level must be at
least 0.3–0.4 eV higher than the LUMO energy level of fuller-ene-derived acceptor for effective electron transfer.12,13 As an essential light-harvesting moiety in natural photosyn-thetic systems, porphyrin derivatives, which features rigid, two-dimensional planar p-conjugated structure, intense Soret band absorption, tunable optical and redox properties by appropriate metalation,14,15and ultrafast photoinduced elec-tron transfer to fullerene,16,17 are attracting structure units to be incorporated into conjugated polymers. So far, the ex-ploration of main-chain porphyrin-incorporated polymers (PPors) used in PSC applications remains at its early stage. The major obstacles come from the low Jscand FF of these
PPor-based PSCs, which are mainly attributed to the strong yet narrow absorption bands of the PPors, and the unopti-mized morphologies in the PPor/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) active layer. The averaged Jscand
FF of the PSCs fell in the rage of 0.12–5.03 mA cm2 and
Additional Supporting Information may be found in the online version of this article.
0.24–0.39, respectively, which limited the maximum PCE of the PSCs to 1%.18–23 Improving Jsc by broadening the
absorption band and light-harvesting abilities of PPors were attempted. Zhan and coworkers18 enhanced the coplanarity of the PPor conjugated backbone and extending the conjuga-tion length by easing the steric hindrance among the in-chain porphyrin units. Tan et al.19 reported random copolymers
containing porphyrins, thiophenes, and 2,1,3-benzothiadia-zole moieties with wide absorption band from 450–750 nm. Djurisˇic´ and coworkers20 also reported platinum polyyne polymers containing zinc porphyrinate chromophores, show-ing a Jscup to 3.42 mA cm2and PCE up to 1.04%. Although
broader absorption spectra were successfully demonstrated, significant enhancement in the Jsc of the resulted PSCs was
not reached. Thus, the mechanism of the unsatisfactory Jscof
PPor-based PSCs is not simply caused by the inefficient har-vesting of solar radiation and remains unclear. Recently, instead of using porphyrin as a main-chain D unit in D–A conjugated polymers, Xiaoyu Li, Yongfang Li, and Haiqiao Wang’s groups synthesized an edge-fused quinoxalino[2,3-b]porphyrin moiety as a novel A units. Through copolymer-ized the unit with carbazole-based D moiety, the resulting D– A conjugated polymers with laterally extended porphyrin units exhibits broad absorption spectrum from 400 to 750 nm and reached Jsc of 8.32 mA cm2, FF of 0.45, and PCE
of 2.53.24
In fact, among all the physical properties of these PPors, the most pronounced and common feature is their exceptional strong Soret band absorption. Thus, leveraging this strength of PPors is expected to enhance the solar radiation-to-elec-tricity conversion at blue-light region in the conventional bi-nary polymer-fullerene system and may circumvent the poor FF of the PPor-contained PSCs. In this study, we demonstrate a novel concept of using a newly synthesized PPor, PPor-diindenothieno[2,3-b]thiophene (PPor-DITT; Scheme 1), as a blue-light harvesting dopant in a ternary-blend PSC. To expand the absorption in the blue-light region, the DITT comonomer unit is incorporated into the PPor conjugated backbone. The thermal, optical, and electrochemical proper-ties of the PPor-DITT have been characterized, and the BHJ PSCs utilizing blends of PPor-DITT/[6,6]-phenyl-C71-butyric
acid methyl ester (PC71BM) were fabricated to investigate
the intrinsic photovoltaic properties of PPor-DITT. In addi-tion, the origin of the low Jsc and FF of the PPor-DITT/
PC71BM binary-blend PSCs are elucidated via
photolumines-cence (PL) quenching and morphological studies. To ensure efficient exciton dissociation and avoid charge trapping in a ternary-blend system, the prerequisite is to carefully design a cascade energy levels alignment among the components.25–28 A crystalline conjugated polymer, P3HT, and an amorphous low-band-gap polymer, PTPTPTDPP (Fig. 1) were chosen as the host donor polymers, because first, their HOMO and LUMO energy levels form an effective cascade energy level alignment between PPor-DITT and PC71BM; and second, the
crystalline and amorphous natures of the host polymers were observed to be a critical factor in the FF of the resulting PSCs. PSCs containing ternary blends in a device
configuration indium tin oxide (ITO)/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS)/PPor-DITT: P3HT:PC71BM/Ca/Al [ternary blend 1 (TB1)] and ITO/
PEDOT:PSS/PPor-DITT:PTPTPTDPP:PC71BM/Ca/Al (TB2) were
fabricated and characterized.
EXPERIMENTAL
Measurement and Characterization
1
H NMR spectrum was recorded on a Varian-300MHz spec-trometer. Gel permeation chromatography (GPC) was meas-ured using a Viscotek GPC system equipped with a Viscotek T50A differential viscometer and Viscotek LR125 laser re-fractometer. Three 10-cm American Polymer columns were connected in series in the order of decreasing pore size (105, 104, and 103 Å); polystyrene standards were used for
cali-bration, and THF was used as the eluent. Differential scan-ning calorimetry (DSC) was performed on a TA Q200 Series DSC and operated at a scan rate of 10C min1.
Thermogra-vimetric analysis (TGA) was carried out using a Perkin Elmer Pyris 7 instrument at a scan rate of 10 C min1. UV–vis spectra were measured using an HP 8453 spectrophotome-ter. PL spectra were obtained using an ARC SpectraPro-150 luminescence spectrometer. The cyclic voltammograms (CVs) was conducted on a Bioanalytical System analyzer. A carbon glass coated with a thin polymer film was used as the work-ing electrode and an Ag/AgCl as the reference electrode, whereas 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile was used as the electrolyte. The CV curves were calibrated using ferrocene as the standard, whose oxidation potential is set at4.8 eV with respect to zero vacuum level. Fabrication of Photovoltaic Devices
ITO/glass substrates were cleaned sequentially in the ultra-sonic bath of detergent, deionized water, acetone, and isopro-pyl alcohol for 15 min. The substrates were then covered by a 30-nm thick layer of poly(3,4-ethylenedioxythiophene):po-ly(styrenesulfonate) (PEDOT:PSS, Al4083 provided by H. C. Stark) by spin-coating. After annealing in air at 150 C for
30 min, the samples were cooled down to room temperature. All the polymers were dissolved in orthodichlorobenzene, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM,
pur-chased from Nano-C) was added to reach the desired ratio. The solution was then heated at 70 C for 1 h and stirred
overnight at room temperature. The solution was filtered through a 0.45-mm filter before spin-coating, and the sub-strate was transferred into a glove box. The active layer was then spin-cast at different spin rates to achieve the opti-mized device performance. The experimental conditions are listed in Table S1, Supporting Information. No thermal annealing was treated for PPor-DITT/PC71BM binary blend
and PPor-DITT/PTPTPTDPP/PC71BM ternary blend. The
ter-nary blend of PPor-DITT/P3HT/PC71BM was annealed at
150 C for 15 min. The spin rate is set at 1000 rpm. The cathode made of calcium (35 nm) and aluminum (100 nm) were evaporated through a shadow mask under a base pres-sure (<106 Torr). Finally, the devices were encapsulated, and J–V curves were measured in air. Each device is consti-tuted of 4 pixels defined by an active area of 0.04 cm2.
FIGURE 1 Chemical structures of P3HT and PTPTPTDPP. SCHEME 1 Synthetic route of PPor-DITT.
Characterization of Photovoltaic Devices
The devices were characterized under the irradiation of AM 1.5 G simulated light with intensity 100 mW cm2. Solar illu-mination conforming the JIS Class AAA was provided by a SANEI Electric 300 W solar simulator equipped with an AM 1.5 G filter. The light intensity was calibrated with a Hama-matsu S1336-5BK silicon photodiode. Current–voltage (J–V) characteristics of PSC devices were obtained by a Keithley 2400 sourcemeter. The performances presented in this arti-cle are the average of 4 pixels of each device. IPCE spectra were measured using a lock-in amplifier with a current pre-amplifier under short-circuit conditions with illumination by monochromatic light from a 250 W quartz-halogen lamp (Osram) passing through a monochromator (Spectral Prod-ucts CM110).
Synthesis of the Alternating Copolymer-PPor-DITT All reagents and chemicals were purchased from commercial sources (Aldrich, Lancaster, or TCI) and used without further purification. The synthetic route of PPor-DITT is shown in Scheme 1. The monomers Br-Por (5,15-dibromo-10,20-bis(3,4,5-tris(dodecyloxy)phenyl) Zinc(II) porphyrin) and B-DITT (diboronic ester diindenothieno[2,3-b]thiophene) in Scheme 1 were synthesized according to the previous literature.18,29,30
Synthesis of PPor-DITT
To a 50-mL round-bottomed flask, Br-Por (179.0 mg, 0.1 mmol), B-DITT (139.0 mg, 0.1 mmol), Pd(PPh3)4 (2.3 mg,
2 103mmol), K
2CO3(41.5 mg, 0.3 mmol), Aliquant 336
(10.0 mg, 0.025 mmol), degas toluene (17 mL), and degas H2O (3 mL) were introduced. The reaction mixture was
stirred at 90C under nitrogen for 72 h. The end-capping re-agent bromobenzene (1 equiv) was added to the solution and stirred for 1 day, and then another end-capping reagent 4,4,5,5-tetramethyl-2-phenyl-1,3,2-dioxaborolane (1 equiv) was added to the solution and stirred for another 1 day. Af-ter cooling to room temperature, the solution was added into methanol dropwise. The precipitate was collected by fil-tration and washed by Soxhlet extraction with hexane and methanol sequentially for 2 days. The residual was collected by dissolving in THF. The Pd-thiol gel (Silicycle) was added to above THF solution to remove the Pd catalyst. After filtra-tion and removal of the solvent, the polymer was redissolved in THF again and added into methanol to precipitate out. Af-ter drying under vacuum for 1 day, the polymer was obtained as a brown powder (108 mg, 38%).
GPC (THF, polystyrene standard) Mn¼ 6900 g mol1, Mw¼
12,700 g mol1, PDI ¼ 1.85.1H NMR (300 MHz, CDCl3, d):
9.17–8.90 (br, 8H, pyrrolic-H), 7.61–6.81 (br, 26H, Ar-H), 4.37–4.20 (br, 12H,AOCH2A), 4.08–3.69 (br, 8H, AOCH2A),
2.50–0.71 (br, 198H, alkyl-H).
RESULTS AND DISCUSSION
Synthesis and Thermal Properties of PPor-DITT
PPor-DITT was polymerized from the comonomers, Br-Por and B-DITT, by Suzuki coupling reaction,31–34 as shown in Scheme 1. The polymer is highly soluble in common organic
solvents, such as chloroform, toluene, and chlorobenzene. The number-average molecular weights (Mn), weight-average
molecular weights (Mw), and polydispersity index of
PPor-DITT are 6.9 kDa, 12.7 kDa, and 1.85, respectively. The 5% weight loss temperature of PPor-DITT (Td) is at 375 C
determined by TGA as shown in Figure S2, Supporting Infor-mation. The Td value of PPor-DITT is higher than 350 C,
indicating a sufficient thermal stability for the PSC applica-tions. The DSC thermograms of PPor-DITT in Figure S3, Sup-porting Information, indicates a glass transition temperature (Tg) at 53C.
Optical Properties
Figure 2 shows the UV–vis absorption spectra of PPor-DITT and the comonomers (Br-Por and B-DITT) in THF. As expected, Br-Por and PPor-DITT share a sharp Soret band at 432 nm and weak Q-bands at 564 and 602 nm. The B-DITT monomer revealed the absorption band with clear vibronic structure in the region of 330–430 nm. The absorption band of B-DITT is not observed in the absorption spectrum of PPor-DITT, and the Soret band of PPor-DITT (400–500 nm) covers much broader than Br-Por, indicating that the como-nomer unit, DITT, effectively extended the conjugation of the conjugated backbone. In the solid film of PPor-DITT, the Soret band further extended to 550 nm (Fig. 2). The broad-ened Soret band, as well as stronger Q-bands, may be attrib-uted to a closer packing of the polymers in the solid state. Thus, via incorporation of DITT units into the PPor back-bone, an alternating copolymer (PPor-DITT) covering widely in the light region and suitable to be used as the blue-light harvester is obtained.
Electrochemical Properties
Figure S4 Supporting Information, shows CVs of PPor-DITT in the oxidation process. The HOMO energy level of PPor--DITT was calculated from the onset oxidation potentials (Eonset
ox ), and the LUMO energy level was approximately
esti-mated by subtracting the optical band-gap values (Eopt g ) from
the HOMO level. The Eopt
g deduced from the absorption onset
were determined to be 2.34 eV for the Soret band and 2.12
FIGURE 2 UV–vis absorption spectra of PPor-DITT and the monomers.
and 1.95 eV for the Q-bands in the solid state. Therefore, the energy level of HOMO was estimated to be5.26 eV, whereas the LUMO energy levels were calculated to be 2.92 eV for the Soret band and 3.14 and 3.31 eV for the Q-bands. With this result, to form a cascade energy level alignment and attain effective charge transfer in the ternary-blend system, the well-known crystalline P3HT and a well-performed amor-phous low-band polymer PTPTPTDPP35 were chosen as the host polymers in the ternary-blend PSCs. The energy levels of all components are illustrated in Figure 3.
Photovoltaic Characteristics
BHJ PSCs of PPor-DITT in a binary-blend system were fabri-cated on the basis of ITO/PEDOT:PSS/PPor-DITT:PC71BM/
Ca/Al configuration, and the characteristics were measured under AM 1.5 G illumination at 100 mW cm2. The J–V curve of the PSC is shown in Figure 4(a). With PPor-DITT/PC71BM
at the optimized blend ratio of 1:3, a PCE of 0.78%, with a Voc of 0.79 V, a Jscof 2.98 mA cm2, and a FF of 0.33 were
obtained, as listed in the inset of Figure 4(a). Figure 4(b) presents the external quantum efficiency (EQE) spectrum of the device. A broad band around the 400–550 nm matched the absorption of PPor-DITT indicating that the PPor-DITT/ PC71BM blend has the ability to carry out solar
radiation-to-current conversion in the UV to blue visible light region. However, the maximum EQE values are around 20%, and the band does not extend to long-wavelength region, which lim-ited the Jscand PCE of the PPor-DITT based PSCs.
Identical to the results in the literature, the performance of the PPor-DITT based PSCs is also limited to the Jscand PCE,
though Vocas high as 0.79 V has been successfully reached.
To further investigate this common obstacle, the PL quench-ing experiments and the AFM topology of PPor-DITT/ PC71BM blend were studied. As shown in Figure 5(a)the PL
emission band of pristine PPor-DITT located between 600– 750 nm, and the emission is completely quenched in the thin-film of PPor-DITT/PC71BM blend. The results indicate
that the electron transfer from the photoexcited PPor-DITT to PC71BM can effectively take place, and the generation of
charge carriers in the blend should be efficient. Similar effi-cient processes of generating long-lived charge carriers (Porþfullerene) from photoexcited porphyrin-fullerene dyad have been reported and well-studied.16,17However, the AFM image of the PPor-DITT/PC71BM thin-film [Fig. 5(b)]
shows a featureless smooth topography with a low rough-ness of 0.32 nm. The morphology may cause inefficient charge transport in the PPor-DITT/PC71BM blend, because
the ideal interpenetrating bicontinuous network was not reached.36–38 So far, all the PPor based PSCs suffered from
low Jsc and FF, even in the copolymer systems, which cover
wide UV–vis absorption range.18,19Our results confirmed the efficient generation of charge carriers in the PPor-DITT/ PC71BM blend and identified one of the major factors for the
low Jscand FF as the inefficient charge transport in the
noni-deal morphology of the active layer. Therefore, further enhancement in the PPor-based PSCs may rely on the improvement of the PPor/PC71BM morphology, which is
cur-rently studied in our group.
To leverage the strong 400–550 nm absorption band of PPor-DITT and circumvent the difficulties in reaching opti-mized morphology in the PPor/PCBM thin film, PPor-DITT
FIGURE 4 (a) J–V characteristics and (b) EQE spectra of ITO/PEDOT:PSS/PPor-DITT:PC71BM/Ca/Al under illumination of AM1.5
solar simulator at 100 mW cm2.
was used as blue-light harvester and an additive in a ter-nary-blend system. PSCs containing ternary blends in a de-vice configuration ITO/PEDOT:PSS/PPor-DITT:P3HT:PC71BM/
Ca/Al (ternary blend 1(TB1)) and ITO/PEDOT:PSS/PPor-DITT:PTPTPTDPP:PC71BM/Ca/Al (TB2) were fabricated.
Ta-ble 1 lists the average values of Jsc,Voc,FF, and PCE of TB1
under simulated AM 1.5G illumination (100 mW cm2) with the overall Polymer:PC71BM weight ratio fixed at 1:1,
whereas the PPor-DITT:P3HT ratio was varied. The J–V curves of these devices are shown in Figure 6(a). As the ra-tio of PPor-DITT in the ternary blend increased, the Voc
val-ues of the three-component PSCs almost remained around the reasonable value of 0.6 V. However, the values of Jsc
decreased with the increase of PPor-DITT ratio. The results were further investigated using UV–vis absorption spectra and EQE spectra of the ternary blends. As shown in Figure 6(b)in the P3HT:PC71BM binary blend, an absorption
maxi-mum at 520 nm and the two absorption shoulders at 550 and 600 nm belong to the characteristic absorption band of crystalline P3HT can be clearly observed.39 Previous study indicates that the absorption maximum shifts from 520 to 450 nm and the absorption shoulder at 550 nm disappears when the crystallinity of P3HT decreases, and the coplanarity of the conjugated backbones is distorted. The same phenom-enon is observed as the PPor-DITT:P3HT ratio reached 0.2:0.8 [the triangle curve in Fig. 6(b)]. Thus, the blue-shift in the absorption spectra of the ternary blends clearly indi-cates that the amorphous component, PPor-DITT, although promoted the absorption at 400–500 nm, significantly decreased the crystallinity of P3HT. The decrease in JSC and
FF is understood as the deterioration in the photon-to-cur-rent conversion of the active layer due to the decrease of P3HT crystalline.36 Consequently, as can be seen in Figure 6(c)the EQE spectra revealed a descending trend as PPor-DITT weight ratio increased.
Because the deterioration of Jsc and FF caused by the
decrease in the crystalline is so severe that the possible
effects brought by the PPor-DITT component could be blanked out, BHJ materials based on an amorphous conju-gated polymer, PTPTPTDPP (Fig. 1) were investiconju-gated. PTPTPTDPP is a well-performed amorphous low-band-gap material for BHJ PSCs, which demonstrated a high PCE around 4.0%, when a weight ratio of PTPTPTDPP:PC71BM¼
1:4 was used.35 In addition, its HOMO and LUMO levels are in between the corresponding energy levels of the PPor-DITT and those of the PC71BM. Such a cascade energy level
alignment was designed, so that effective energy transfer from the photoexcited PPor-DITT dopants to the host poly-mer, PTPTPTDPP, may take place and facilitate the following charge separation and transport. Ternary-blend PSCs in the configuration of ITO/PEDOT:PSS/PPor-DITT:PTPTPTDPP: PC71BM(TB2)/Ca/Al were fabricated. Table 2 lists the
aver-age values of Jsc,Voc,FF, and PCE of TB2-based PCSs obtained
under simulated AM 1.5G illumination (100 mW cm2) with the overall Polymer:PC71BM weight ratio fixed at 1:4,
whereas the PPor-DITT: PTPTPTDPP ratio was varied. The J–V curves of these devices are shown in Figure 7(a). TB2-based PCSs possess higher Voc values than the TB1-based
PSCs due to the lower-lying HOMO level of PTPTPTDPP. Notably, on the contrary to the TB1-based PSCs, TB2-based PSCs have high FF values (>0.57) in all blending ratios (Ta-ble 2). Therefore, choosing an amorphous host polymer
FIGURE 5 (a) PL emission spectra of PPor-DITT and PPor-DITT/PC71BM at the weight ratio of 1:3 in thin film. (b) Topographic AFM
image (scale: 5 5 mm2) of devices incorporating PPor-DITT/PC
71BM blend at the optimized weight ratio of 1:3. Its
root-mean-square roughness is determined to be 0.32 nm.
TABLE 1 Photovoltaic Properties of PPor-DITT:P3HT:PC71BM
Ternary-Blend PSCs Polymer:PC71BM (w/w) Voc(V) Jsc (mA cm2) FF PCE (%) P3HT:PC71BM (1:1) 0.61 7.77 0.56 2.65 PPor-DITT:P3HT:PC71BM (0.1:0.9:1) 0.59 7.21 0.49 2.08 PPor-DITT:P3HT:PC71BM (0.2:0.8:1) 0.59 6.11 0.32 1.15
avoids the drastic morphological changes accompanied with the compositional changes of the ternary blend and prevents the significant drops of FF. Most importantly, from a blending ratio of PTPTPTDPP:PC71BM ¼ 1:4 to PPor-DITT:PTPTPTD
PP:PC71BM¼ 0.1:0.9:4, an addition of 2 wt % of PPor-DITT,
resulted in an averaged 10% increase of EQE in the region of 400–450 nm [Fig. 7(c)]. Thus, the PPor-DITT dopant at
FIGURE 6 (a) J–V characteristics, (b) UV–vis absorption spec-tra, and (c) EQE spectra of PPor-DITT:P3HT:PC71BM
ternary-blend solar cells.
TABLE 2 Photovoltaic Properties of PPor-DITT:PTPTPTDPP: PC71BM Ternary-Blend PSCs Polymer:PC71BM (w/w) Voc(V) Jsc (mA cm2) FF PCE (%) PTPTPTDPP:PC71BM (1:4) 0.70 9.35 0.60 3.93 PPor-DITT:PTPTPTDPP:PC71BM (0.1:0.9:4) 0.72 7.98 0.59 3.39 PPor-DITT:PTPTPTDPP:PC71BM (0.2:0.8:4) 0.72 6.52 0.57 2.68
FIGURE 7 (a) J–V characteristics, (b) UV–vis absorption spec-tra, and (c) EQE spectra of PPor-DITT:PTPTPTDPP:PC71BM
low concentration can effectively enhance the photo-to-cur-rent conversion in the blue-light region. Nevertheless, as shown in Figure 7(b)the intramolecular charge transfer absorption band of PTPTPTDPP decreased as the fraction of PPor-DITT raised, which led to a decrease in the overall Jsc
out-put and PCE of the PSC (Fig. 7). Further raise of the fraction of PPor-DITT to a blend ratio of PPor-DITT:PTPTPTDPP:PC71BM
¼ 0.2:0.8:4 resulted in a significant decrease in overall EQE, which limits PPor-DITT from high dopant concentrations.
CONCLUSIONS
In summary, a novel porphyrin-based polymer, PPor-DITT, featured broad absorption in the blue-light region (400–550 nm) was synthesized and used as a blue-light harvester in the ternary-blend PSCs utilizing either a crystalline conju-gated polymer (P3HT) or an amorphous one (PTPTPTDPP) as a host. The BHJ PSCs based on the binary blend of PPor-DITT/PC71BM at the blend ratio of 1:3 show a high Voc of
0.79 V, but low Jscof 2.98 mA cm2and FF of 0.33. The
lim-ited Jscand FF were elucidated by PL quenching experiments
and AFM topology of the PPor-DITT/PC71BM blend, which
indicate an efficient electron transfer from the photoexcited PPor-DITT to PC71BM but an inefficient charge transport due
to the nonideal morphology of the blend. These results iden-tify the breakthrough of the PCE of PPor-based PSCs may rely on the optimization of the active layer morphology. The ternary-blend experiments in this study demonstrated the first example of using PPor as blue-light harvester dopant in PSC applications. The amorphous nature of PPor-DITT makes it a more suitable dopant in an amorphous host. Within the crystalline host (P3HT), PPor-DITT resulted in a significant decrease of P3HT crystalline and led to great drops in Jsc,FF,
and PCE of the PSCs. On the contrary, in the amorphous host (PTPTPTDPP), the PPor-DITT dopant effectively enhanced the photo-to-current conversion in the blue-light region, with the FF well sustained at different dopant concentrations. Drop of the Jsc in the PPor-DITT:PTPTPTDPP:PC71BM
ter-nary-blend PSCs was attributed to the decrease of the host concentration as indicated by the EQE experiments. Further optimization of the ternary composition and morphology are under process.
ACKNOWLEDGMENTS
The authors thank the National Science Council and the ‘‘ATP Program’’ of the Ministry of Education, Taiwan, for financial support. They also thank Yen-Ju Cheng and Sheng-Wen Cheng for providing the B-DITT monomer.
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