Highly efficient small molecule solar cells fabricated with non-halogenated solvents

Abstract

Though much progress has been achieved for bulk-heterojunction (BHJ) organic solar cells (OSCs), the best performing devices are processed with halogenated solvents. However, the halogenated solvents should be replaced for the practical applications of OSCs due to the toxicity of halogenated solvents to human beings and the environment. In this study, OSCs based on a porphyrin small molecule are fabricated with toluene and o-xylene, which show high PCEs up to 5.46% and 5.85%, respectively. Both PCEs are the highest for small molecule-based OSCs processed with non-halogenated solvents, demonstrating that porphyrin small molecules are very promising for solar cell commercial applications.

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1. Introduction

Bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted tremendous attention because of their advantages and their potentials for making low-cost, light-weight, semi-transparent and flexible devices through roll-to-roll printing.^1–9 The performance of OSCs has improved rapidly in the past few years and the power conversion efficiency (PCE) has overcome the 10% barrier, which is nearing the critical point of industrialization.^10–15

However, all these high PCE devices are fabricated with halogenated solvents such as 1,2-dichlorobenzene (o-DCB) and chlorobenzene (CB), which have strong negative impacts to human health and the environment due to their high toxicity. Therefore, halogenated solvents should be avoided in order to apply the lab-based results to large-scale commercial productions.^16–18 Though much lower performance was obtained for non-halogenated solvent-processed OSCs in the early years of organic photovoltaic (OPV) study,¹⁹ efforts and also significant progress have been made recently to find non-halogenated or non-hazardous solvents for processing the active layer of OSCs.^1,17,20–38 Among the various non-halogenated solvents, benzene derivatives, such as o-xylene and 1,2,4-trimethylbenzene, are promising due to the low toxicity, easier removal from environmental accumulation, and comparable properties but with lower cost comparing to the halogenated analogs,³⁹ and especially a power conversion efficiency up to 7.15% and 7.50% were achieved for the polymer solar cells fabricated with 1,2,4-trimethylbenzene or o-xylene solvents.^24,31

Small molecules (SMs) show many advantages such as well-defined molecular structure, easy synthesis, higher purity and more definite molecular weight distribution without batch derived variations compared with polymers,^40,41 but the study on the fabrication of SM-based OSCs with non-halogenated solvents are very limited and the PCEs were usually lower than 5.0%.^27,33 Since the materials with lower molecular weights usually show higher solubility than those with higher molecular weights of similar chemical structures, small molecules are more likely to be soluble in a wider variety of solvents and it is worthy to study the SM-based OSCs fabricated with non-halogenated solvents more intensely.

Recently, small molecules based on porphyrin unit arose among the most efficient OPV materials,^42–47 and a PCE higher than 7% was achieved for the BHJ OSCs based on the porphyrin small molecule 5,15-bis(2,5-bis-(2-ethylhexyl)-3,6-di-thienyl-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione-50-yl-ethynyl)-10,20-bis(4-octyloxy-phenyl)-porphyrin zinc (DPPEZnP-O) (Fig. 1) using CB as solvent and 1,8-diiodooctane as additive.⁴⁶ In this work, we fabricate the BHJ OSCs based on DPPEZnP-O small molecule from simple processing steps using non-halogenated solvent of toluene and o-xylene with 1% non-halogenated additive of 1-methyl naphthalene (MN) for the casting of the active layers, and achieve PCEs up to 5.46% and 5.85%, respectively, which are the highest to date for non-halogenated solvent processed small molecule OSCs.

Fig. 1 The chemical structure of DPPEZnP-O and the structure of OSCs devices.

2. Experimental

2.1. Materials

The small molecule DPPEZnP-O was synthesized according to the procedures reported by our group.⁴⁶ Toluene, o-xylene, 1-methyl naphthalene and phenyl-C₆₀-butyric acid methyl ester (PC₆₁BM) was purchased from Sigma-Aldrich.

2.2. Device fabrication and characterization

Solution-processed BHJ solar cells were fabricated as follows: indium tin oxide (ITO) coated glass substrates were cleaned prior to device fabrication by sonication in acetone, detergent, distilled water, and isopropyl alcohol. After treated with an oxygen plasma for 4 min, 40 nm thick poly(styrene sulfonate)-doped poly(ethylene-dioxythiophene) (PEDOT:PSS) (Bayer Baytron 4083) layer was spin-coated on the ITO-coated glass substrates at 2500 rpm for 30 s, the substrates were subsequently dried at 150 °C for 10 min in air and then transferred to a N₂-glovebox. The active layers were prepared from DPPEZnP-O : PC₆₁BM solutions in toluene or o-xylene with donor to fullerene weight ratio of 1 : 1 with an overall concentration of 24 mg mL⁻¹ in the presence of 1% 1-methyl naphthalene (MN). The solutions were stirred overnight at 70 °C in a N₂ glove box and then spin-casted on top of the PEDOT:PSS. The thicknesses of active layers were measured by a profilometer. The ultra-thin PFN layer was then deposited by spin casting from a 0.02% (w/v) solution in methanol (from 2000 rpm for 30 s). Finally, Al (∼90 nm) was evaporated in a high-vacuum chamber with base pressure < 3 × 10⁻⁶ mbar as the top electrode. Masks made from laser beam cutting technology with a well-defined area of 0.16 cm² were attached to define the effective area for accurate measurement.

The atomic force microscopy (AFM) measurements of the surface morphology of blend films were conducted on a NanoScope NS3A system (Digital Instrument). The values of power conversion efficiency were determined from J–V characteristics measured by a Keithley 2400 source-measurement unit under AM 1.5G spectrum from a solar simulator (Oriel model 91192). Solar simulator illumination intensity was determined using a monocrystal silicon reference cell (Hamamatsu S1133, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). EQE values of the encapsulated devices were measured by using an integrated system (Enlitech, Taiwan, China) and a lock-in amplifier with a current preamplifier under short-circuit conditions. The devices were illuminated by monochromatic light from a 75 W xenon lamp. The light intensity was determined by using a calibrated silicon photodiode.

3. Results and discussion

As shown in Fig. 2a and Table 1, though the BHJ OSCs with DPPEZnP-O as the donor and PC₆₁BM as the acceptor only show a PCE of 2.02% with a short circuit current (J_SC) of 6.39 mA cm⁻², an open circuit voltage (V_OC) of 0.74 V, and a fill factor (FF) of 42.8%, those fabricated with toluene in the presence of 1% non-halogenated additive MN without and under thermal annealing (ANN) at 100 °C for 10 min exhibit significantly enhanced PCEs to 4.74% and 5.46%, respectively, and the significant enhancement PCEs are contributed by the remarkable increase of the J_SC values to more than 14 mA cm⁻². The PCE of 5.46% is the highest among those of small molecule-based solar cells fabricated with non-halogenated solvent.^27,33

Fig. 2 J–V characteristics of the OSCs, in which DPPEZnP-O : PC₆₁BM blend films are spin-coated in different conditions of (a) toluene and (b) o-xylene.

Table 1 Device performance of the OSCs processed under different conditions

Processing condition	JSC (mA cm^−2)	VOC (V)	FF (%)	PCE (%)
a The best PCE.b The average value of PCE standard deviation of eight devices.
Toluene	6.39	0.74	42.8	2.02^a (1.75 ± 0.22)^b
Toluene + 1% MN	14.94	0.78	40.7	4.74^a (4.57 ± 0.15)^b
Toluene + 1% MN + ANN	14.70	0.77	48.3	5.46^a (5.31 ± 0.13)^b
o-Xylene	12.55	0.79	43.3	4.30^a (4.16 ± 0.12)^b
o-Xylene + 1% MN	15.51	0.77	43.6	5.21^a (5.08 ± 0.14)^b
o-Xylene + 1% MN + ANN	15.73	0.75	50.0	5.85^a (5.73 ± 0.11)^b

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Compared to those processed with pure toluene, the OSCs fabricated with pure o-xylene show an enhanced PCE to 4.30% with J_SC = 12.55 mA cm⁻², V_OC = 0.79 V and FF = 43.3%. And the devices fabricated with o-xylene in the presence of 1% MN show a J_SC enhancement to 15.51 mA cm⁻² and a similar FF of 43.6%, but a slight decrease of V_OC to 0.77 V. Furthermore, for the OSCs fabricated with o-xylene in the presence of 1% MN and then thermally annealed at 100 °C for 10 min, the FF improves dramatically to 50.0% with a slight increase of J_SC to 15.73 mA cm⁻² and a further slight decrease of V_OC to 0.75 V, leading to an improved PCE of 5.85%, which is the highest for small molecule-based solar cells fabricated with non-halogenated solvent. The high performance of the OSCs fabricated with toluene and o-xylene suggests that it is very promising to fabricated small molecule-based OSCs environment-friendly.

The external quantum efficiency (EQE) curves of the OSCs processed with o-xylene are shown in the Fig. 3, and the J_SC values integrated from the EQE spectra are 12.25, 14.78 and 15.40 mA cm⁻², which agree well with those measured from the corresponding OSCs reported above. The EQE curves of these devices show a broad wavelength range from 400 to 850 nm, and EQE values are enhanced significantly in the whole region from 450 to 850 nm for the devices fabricated in the presence of 1% MN compared with those fabricated with pure o-xylene. And in the range of 700–850 nm, the devices fabricated in the presence of 1% MN additive display an EQE up to 55%, which shows a 37% increase, compared to the EQE (40%) of devices fabricated with pure o-xylene. The EQE enhancement is attributed to that a small amount of MN can promote the miscibility of DPPEZnP-O and PC₆₁BM and thus facilitate the exciton separation and charge transfer, which is consistent with the efficient photoluminescence (PL) quenching of DPPEZnP-O by PC₆₁BM for the blend films fabricated with 1% MN additive reported below. For the devices fabricated with 1% MN and then annealed under 100 °C for 10 min, the EQE values increase in the region of 400 to 650 nm but slightly decrease in the region from 650 to 800 nm. For these thermally annealed-devices, the enhanced EQE values in the region of 400 to 650 nm are contributed by the more interpenetrating networks revealed by AFM (Fig. 4). Though the more interpenetrating networks should also lead to EQE enhancement in the region of 650 to 800 nm, the slightly EQE decreases in this region are mainly ascribed to the significant absorption reduce shown in Fig. 5.

Fig. 3 EQE curves of the devices fabricated with o-xylene.

Fig. 4 AFM height (up) and phase (bottom) images of DPPEZnP-O : PC₆₁BM blend films spin-casted from toluene (a and b), o-xylene (c and d), o-xylene + 1% MN (e and f) and o-xylene + 1% MN with annealing (g and h), respectively.

Fig. 5 Absorption spectra of DPPEZnP-O : PC₆₁BM blend films fabricated in different conditions (film thickness: 110 ± 5 nm).

Atomic force microscopy in tapping mode was employed to investigate the topography of the blend films processed under different conditions. As shown in Fig. 4a and b, the blend films processed with pure toluene show isolated and large-sized domains of roughly 200 nm also with very high root-mean-square (RMS) roughness of 12.6 nm, which can lead to the low performances for the corresponding solar cells. On the contrary, the blend films spin-casted with o-xylene exhibit very smooth surface with RMS of only 1.00 nm. The dramatic morphological differences indicate that solvents play a very important role for the degree of phase separation, and the better morphology for the blend films processed with o-xylene is ascribed to the better solubility of PC₆₁BM in o-xylene than that in toluene.³²

Though very smooth film morphology is obtained for o-xylene processed films, some little fullerene-rich grains still can be seen possibly because the solubility of PC₆₁BM is still not good enough in o-xylene. Non-halogenated solvent 1-methylnaphthalene, an analogue of Cl-naphthalene, is known to be very good for fullerene dissolvation.⁴⁸ As we expect, AFM topography images of the blend films casted in the presence of 1% MN (Fig. 4e and f) show more uniform morphology and smaller domain sizes with RMS roughness of only 0.55 nm, suggesting a finer phrase separation and more homogenous distribution of DPPEZnP-O and PC₆₁BM throughout the films. Furthermore, the films processed in the presence of 1% MN and then annealed at 100 °C for 10 min show slightly increased RMS roughness to 0.92 nm with better donor/acceptor interpenetrating networks, which are beneficial for device performance.

The absorption spectra of DPPEZnP-O : PC₆₁BM films with almost the same thickness spin coated from o-xylene, o-xylene + 1% MN and o-xylene + 1% MN with annealing are investigated. As shown in Fig. 5, these films show strong absorption in the range from 700 nm to 850 nm. And the films processed in pure o-xylene show a dominant absorption peak at 808 nm with a weak shoulder at ca. 730 nm, and this peak blue-shifts to 785 nm with increased intensity and the almost disappearance of the shoulder of 730 nm for the films spin-coated from o-xylene + 1% MN solution, suggesting more ordered H-aggregations, which contributed to the enhanced EQEs. Under thermal annealing, the dominant peak at 785 nm red-shifts to 806 nm with the reappearance of a shoulder peak at 720 nm, and the intensities decrease significantly from 700 to 800 nm but slightly increase from 800 to 900 nm, indicating the reduction of H-aggregations but the enhancement of J-aggregations, which is consistent with the EQE reduction and enhancement in 650 to 800 nm and 800 to 900 nm regions, respectively. Meanwhile, though the absorption from 300 to 600 nm is only changed slightly, the EQE in this region is significantly enhanced, indicating better morphology regarding to PCBM may form under thermal annealing, which benefits to the higher J_SC of 15.73 mA cm⁻². However, we also note that this NIR absorption peak shifted to 815 nm for the blend films fabricated in CB with DIO additive,⁴⁶ suggesting higher J-aggregation in CB with DIO additive processed films, which can be a very important reason for the lower PCE of the solar cells here. Though it is difficult to clearly explain the absorption changes, the absorption spectra indicate that the aggregation behaviors are different for the films fabricated under different conditions.

Photoluminescence (PL) spectra are also employed to investigate the miscibility of DPPEZnP-O with PC₆₁BM in blend films processed under different conditions. As shown in Fig. 6, excited at 570 nm, the neat film of DPPEZnP-O shows a strong fluorescence peak at 838 nm, which is quenched efficiently by PC₆₁BM but still with obvious PL intensity for the blend films processed with pure o-xylene, indicating that the D/A interface area is still not enough and a fraction of excitons from DPPEZnP-O decays radiatively without reaching the D/A interface possibly because the sizes of some small clusters of DPPEZnP-O in the blend film are beyond the exciton diffusion length. However, the blend films processed in the presence of 1% MN additive exhibit almost completely quenched PL spectrum, suggesting the miscibility of DPPEZnP-O with PC₆₁BM is very good, and the D/A interface area is large enough and the most excitons can migrate to the interface and are quenched by PC₆₁BM. Under thermal annealing at 100 °C for 10 min, the PL spectrum of the blend films does not change significantly but with slightly increase, further demonstrating that the D/A interface area is still large enough and the DPPEZnP-O domains are small enough so that most of the excitons can migrate to the interface and are quenched by PC₆₁BM. It should be noted that the PL intensity slightly increases and one more weak peak can be seen compared to that without thermal annealing, indicating that more ordered assemblies of DPPEZnP-O form under the thermal annealing,⁴⁹ which contributes to the enhanced OSC performance.

Fig. 6 Photoluminescence (PL) spectra of pure DPPEZnP-O and DPPEZnP-O : PC₆₁BM blend films with almost the same thickness processed under different conditions (film thickness: 110 ± 5 nm, λ_Ex = 570 nm).

4. Conclusions

In summary, to replace toxic halogenated solvents, we used two non-halogenated solvents of toluene and o-xylene to fabricate the OSCs based on DPPEZnP-O as the donor and PC₆₁BM as the acceptor, and the devices fabricated in the presence of non-halogenated additive 1-methyl naphthalene with thermal annealing show PCEs of 5.46% and 5.85%, respectively, which are the highest for the small molecule-based OSCs processed with non-halogenated solvent, demonstrating that small molecules are promising for fabricating solar cell environment-friendly.

Acknowledgements

This work was financially supported by the grants from International Science & Technology Cooperation Program of China (2013DFG52740, 2010DFA52150), the Ministry of Science and Technology (2014CB643500), and the National Natural Science Foundation of China (51473053, 51073060).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19054a

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