Fully solution processed p-i-n organic solar cells with an industrial pigment – Quinacridone

Organic Electronics 12 (2011) 1126–1131

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Fully solution processed p-i-n organic solar cells with an industrial pigment – Quinacridone Teresa L. Chen, John Jun-An Chen 1, Luis Catane, Biwu Ma ⇑ The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States

a r t i c l e

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Article history: Received 11 February 2011 Received in revised form 16 March 2011 Accepted 25 March 2011 Available online 9 April 2011 Keywords: Organic solar cells Industrial pigment Quinacridone p-i-n Solution processing Thermal treatment

a b s t r a c t We report solution processed organic solar cells with quinacridone (QA), an industrial pigment, as the electron donor. Applying simple spin casting and thermal annealing, trilayer devices with a pure donor (p) layer, a bulk heterojunction (i) layer, and a pure acceptor (n) layer have been fabricated. Tert-butoxycarbonyl quinacridone (t-BOC QA), a soluble yellow precursor of industrial red pigment of quinacridone, was synthesized by replacing the H atom of the NH group on QA with a t-BOC group. Uniform thin films were prepared by spin casting t-BOC QA solutions, which could be converted into insoluble thin films by thermal treatment to remove the solubilizing groups. This conversion allowed for the subsequent depositions of multiple layers without the use of orthogonal solvents. The p-i-n devices showed much higher device performance than their bilayer and simple bulk heterojunction counterparts, exhibiting power conversion efficiencies (PCEs) as high as 0.83%. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic solar cells (OSC) have great future prospects for their low cost in both materials use and device processing [1]. Solution processing of small molecule industrial pigments represents a highly attractive means of achieving low cost OSCs with abundant stable materials. Indeed, this approach could avoid a number of drawbacks associated with vapor deposition of small molecules and solution processing of polymer materials, e.g., the need for expensive high vacuum systems for the former, and the issues of purity and complexity of polymer synthesis for the latter [2,3]. Recently, a great amount of research efforts have been on the development of soluble small molecules for efficient bulk heterojunction solar cells, which operate in the same way as polymer/fullerene blends [4–10]. The PCEs of these kinds of cells have steadily improved from 1% to above 4% over the last few years. To obtain solution processable

⇑ Corresponding author. E-mail address: [email protected] (B. Ma). Present address: Department of Chemistry, University of Southern California, United States. 1

1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.03.039

small molecules with well defined structures and high purity, functionalizing well-known industrial pigments, such as diketopyrrolopyrrole (DPP) and quinacridone (QA), represents one of the most successful approaches [4,5]. Nevertheless, the preparation of these solution processable small molecules still involves a complicated multi-step synthesis, which can lower the cost effectiveness. Utilizing materials that can be easily prepared at low-cost is one of the key challenges for the research community. Another effective approach to solution processed small molecule solar cells involves the transformation of films containing soluble small molecule precursors into insoluble ones to realize layer-by-layer deposition [11], which well mimics the conventional vapor deposition processing [12–14]. To date, the most efficient solution processed small molecule solar cells have been achieved via this approach, wherein a thermally transformable phthalocyanine and silyl fullerene derivative were sequentially deposited from solution to form p-i-n trilayer devices, exhibiting overall PCEs as high as 5.2%. The success of tetrabenzoporphyrin precursor has inspired our investigations on other thermally transformable soluble precursors for application in organic solar cells, especially those based on high performance industrial pigments [15].

T.L. Chen et al. / Organic Electronics 12 (2011) 1126–1131

Herein, we present the use of a soluble QA precursor for the fabrication of organic solar cells via solution processing. As one of the most important low-cost industrial pigments with exceptional color and weather fastness, QA has been used extensively in a variety of applications, e.g., automobile coatings, artist’s paints, and ink for printers [16]. Recently, the application of QA in electronic devices, i.e., organic light emitting diodes, solar transistors and solar cells, has gained a great deal of attention [17–24]. Most of these QA based devices have been fabricated via high vacuum vapor deposition due to its low solubility. More recently, a few soluble QA derivatives have been developed, affording solution processed QA based devices with high performance [5,24]. For instance, the organic field effect transistors (OFETs) fabricated by t-BOC QA have been found to exhibit nearly equivalent device characteristics to vacuum deposited devices. The objective of our present investigation is to evaluate the performance of QA based organic solar cells prepared via solution processing and thermal treatment of t-BOC QA.

2. Experimental 2.1. Materials and equipments Pre-patterned ITO-coated glass substrates (R = 20 X/sq) were purchased from Thin Film Devices, Inc. Phenyl-C61butyric acid methyl ester (PC60BM) was purchased from Nano-C Inc. Poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) (Baytron PH) was purchased from H.C. Starck. Pristine QA was purchased from Aldrich. t-BOC QA was prepared according to the procedures described in literature, i.e., stirring the pigment in tetrahydrofuran (THF) at room temperature with two equivalents of di-(t-butyl)-dicarbonate and N,N0 -dimethylaminopyridine as a catalyst for 24 h, producing t-BOC QA at a yield of 65%. TGA was carried out on Thermogravimetric Analyzer (TGA) Q5000 IR (TA instruments). UV–Vis data were obtained using CARY 5000 UV–Vis-NIR spectrophotometer with film samples spin-coated on glass substrates. Film thickness was measured with a Veeco Dektak 150 surface profilometer. Tapping-mode atomic force microscopy (AFM) was carried out on a Veeco Nanoscope V scanning probe microscope. SEM was performed on Zeiss Gemini Ultra-55 Analytical Scanning Electron Microscope.

2.2. Device fabrication and testing Organic solar cells with three device architectures, bilayer, bulk heterojunction, and p-i-n structure, were fabricated. The pre-patterned ITO-coated glass substrates were cleaned using the following procedure: sonication in soap solution, rinsing in deionized water, sonication in acetone and isopropanol, respectively, for 15 min, and UV ozone cleaning for 10 min. A thin-layer (30 nm) of PEDOT:PSS (Baytron PH) was spin-coated onto the ITO glass substrate at 4000 RPM for 40 s and then baked at 140 °C for 15 min in air.

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For p-n devices, t-BOC QA solutions with concentrations varying from 2 to 14 mg/mL (thickness 15–70 nm) and PC60BM with concentrations of 8–14 mg/mL (thickness 30–70 nm) were used. The optimal condition was found to be 10 mg/mL of t-BOC QA (57 nm) and 12 mg/mL of PC60BM (60 nm) both in chloroform. The t-BOC QA film was baked at 170 °C for 15 min on the hot plate prior to spin casting of the PC60BM layer. For i devices, the photoactive layers were prepared by spin casting a blend t-BOC QA and PC60BM chloroform solution with varying weight ratios and concentrations of 6–20 mg/mL (40–110 nm thick). The optimal t-BOC QA:PC60BM weight ratio was found to be 1:1.5 with a concentration of 16 mg/mL (87 nm). The blend film was then baked at 170 °C for 15 min on the hot plate. For p-i-n devices, t-BOC QA (p), t-BOC QA:PC60BM (i), and PC60BM (n) were varied from 2 to 4 mg/mL (17– 32 nm), 10 to 20 mg/mL (55–117 nm), and 6 to 20 mg/mL (22–41 nm), respectively, to find the optimal conditions. The optimal condition was found to be 2 mg/mL (18 nm) in chloroform for t-BOC QA, 15 mg/mL (80 nm) in chloroform for t-BOC QA:PC60BM, and 16 mg/mL (33 nm) in chlorobenzene for PC60BM. The p and i layer were each baked at 170 °C for 15 min on the hot plate prior to deposition of the subsequent layer. All active layers were cast at 2000 rpm and passed through a 0.2 lm polytetrafluoroethylene filter prior to spin casting. After the depositions of organic layers, an Al cathode (100 nm) was thermally evaporated under high vacuum (107 Torr) through a shadow mask defining an active device area of 0.03 cm2. The current density–voltage (J–V) curves were measured using a Keithley 236 sourcemeasure unit under AM 1.5 G solar illumination at 100 mW cm2 (1 sun) using a Thermal-Oriel 300 W solar simulator. External Quantum Efficiencies (EQEs) were obtained with a monochromator and calibrated with a silicon photodiode.

3. Results and discussion Soluble t-BOC QA precursor is prepared by replacing the H atom of the NH group of QA with a t-butoxycarbonyl (t-BOC) group in high yield. The soluble precursor can be converted into insoluble parent pigment QA in high purity and quantitative yield by thermal treatment, as shown in Fig. 1a. This process has been well characterized by UV–Vis spectroscopy, TGA and SEM. (See Supplementary data) Spin casting of t-BOC QA solutions in chloroform allowed for the formation of uniform thin yellow colored films. Thermal treatment of those films at 170 °C for 15 min resulted in the generation of polycrystalline red colored films of QA pigment, as evidenced by absorption spectra (Fig. 1b) and SEM (Fig. 1c and d). The change of photophysical and morphological properties is attributed to the NH  O hydrogen-bonding effect of QA. The field effect mobility of the regenerated QA film was previously reported to be p-type with a value of 8  106 cm2 V1 s1 [24]. The optical and electronic properties of QA suggest that it can act as an electron donor for organic solar cells.

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Fig. 1. (a) Molecular structures of soluble t-BOC QA and insoluble QA, (b) UV–Vis absorption spectra of 32 nm thick neat films of t-BOC QA spin-coated and after baking at 15 min at 170 °C (QA), (c) SEM image of spin-coated neat film of t-BOC QA, (d) SEM image of regenerated film of QA.

Fig. 2. Solution processing of organic solar cells with three different device architectures: bilayer (p-n) device, bulk heterojunction (i) device, and trilayer (p-i-n) device. The fabrication processes involve spin coating, thermal treatment and vapor deposition of Al cathode.

The thermal transformation of solution processed thin films into insoluble ones allows for solution processing of multilayer structures. As illustrated in Fig. 2, bilayer organic solar cells were fabricated by depositing the acceptor

layer of PC60BM from solution on the top of the regenerated QA layer. Fig. 3a shows the current density–voltage (J–V) characteristics of the optimized p-n device with a structure of ITO/PEDOT:PSS(30 nm)/QA(57 nm)/PC60BM

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Fig. 3. (a) The J–V curve under 1 sun illumination for the optimized p-n device that shows a PCE of 0.61%; (b) The EQE curve of the p-n device.

(60 nm)/Al. The device showed a modest PCE of 0.61% with Voc = 0.53 V, Jsc = 2.50 mA/cm2, and FF = 0.46 (Table 1). Fig. 3b shows the external quantum efficiency of this device, with photocurrent generation in a broad range from 300 nm to 750 nm. An EQE of 10–15% is very typical for planar heterojunction devices. The photocurrent at short wavelengths (300–500 nm) and long wavelengths beyond 610 nm is mainly contributed by PC60BM. The contribution from QA is mainly in the region of 500–600 nm, where QA exhibits high absorption intensity as shown in Fig. 1b. To further improve the device performance, we have adapted a more advanced device architecture, i.e., trilayer p-i-n structure, which contains an additional mixing layer (i) sandwiched between the pure donor (p) and acceptor (n) layers. The fabrication process for p-i-n device is shown in Fig. 2. As control, a single layer (i) device was also fabricated. The i layer was prepared by spin casting of blends of t-BOC QA and PC60BM followed by thermal treatment to convert soluble t-BOC QA into insoluble QA. We have investigated various ratios of t-BOC QA to PC60BM and different thicknesses of i layer to obtain optimized device

performance. Spin casting of pure PC60BM solution on the top of the regenerated QA:PC60BM mixing layer might dissolve the existing PC60BM component, creating porous structures while filling the pores at the same time. Nevertheless, a neat film of PC60BM can be generated with careful control of the concentration of the PC60BM solution. The device characteristics of i and p-i-n devices are shown in Fig. 4. A PCE of 0.83% has been achieved for the p-i-n device with Voc = 0.79 V, Jsc = 2.34 mA/cm2, and FF = 0.44 (Table 1). In contract, the i device exhibited a much lower PCE of 0.57%. The improvements in open circuit voltage and fill factor account for the enhanced device performance of p-i-n over i device. As demonstrated in a number of systems, high open circuit voltage can be achieved by reducing the electron leakage current though the introduction of carrier blocking layers between the organic layers and the electrodes [25–29]. As shown in Fig. 4b, reduced dark current was observed for the p-i-n device, indicating the blocking effects induced by the neat film layers of QA and PC60BM. Moreover, the improved organic/electrode interface properties is likely to lower the series resistance affording a higher fill factor, as evidence by the higher current density under forward bias of >1 V. Interestingly, the EQEs for both i and p-i-n devices showed little-to-no change as compared to the p-n device, which are significantly lower than typical values of 40–60% for typical bulk heterojunction devices with nanophase separations [30]. This is likely due to the morphology of the blends of QA and PC60BM, i.e., micron sized phase separation is formed in the i layer, similar to what has been observed in the blends of diethylhexyl functionalized QA and PC70BM [5]. This suggests that further improvement of device performance can be realized with better control of the phase separation between QA and acceptor materials. In addition, the p-i-n device has a slightly lower EQE than the i device. Provided that both devices have very similar absorption spectra (See Supplementary data), this can be explained by that a portion of light was harvested by the neat films to generate excitons, which were not able to reach the donor/acceptor interfaces to produce the photocurrent. Another interesting finding is that both i and p-i-n devices exhibited much higher Voc than the p-n device (See Supplementary data for the dark current curves as well). This might be attributed to the different nature of the donor/acceptor interfaces for the devices. For the p-n device, the interfaces consist of QA that was regenerated from the neat film of t-BOC QA and solution processed PC60BM. While for the devices with an i layer, the interfaces consist of QA regenerated from the blends of t-BOC QA and PC60BM, and solution processed and/or leftover thermally annealed PC60BM. Thermal treatment of the blends likely produces QA domains with less crystallinity and encourages the phase separation between QA and

Table 1 Device parameters for OSCs in three device structures. Devices

Structures

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

Bilayer (p-n) BHJ (i) Trilayer (p-i-n)

QA (57 nm)/PC60BM(60 nm) QA:PC60BM (1:1.5, 87 nm) QA(18 nm)/QA:PC60BM (1:1.5, 80 nm)/PC60BM(33 nm)

0.53 0.70 0.79

2.50 2.54 2.34

0.46 0.32 0.44

0.61 0.57 0.83

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conversion efficiencies of these cells of up to 0.83% are still below those of state-of-the-art, our work is a proof-of-concept for the great potential of solution processing of highly stable industrial pigments for solar cells. On-going research is to develop effective approaches to realizing controlled nanophase separation within the blends of pigment donors and fullerene acceptors. Acknowledgements This work was performed at the Molecular Foundry, Lawrence Berkeley National Laboratory, and was supported by the Office of Science, Office of Basic Energy Sciences, Scientific User Facilities Division, of the US Department of Energy under Contract No. DE-AC02—05CH11231. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel.2011. 03.039. References

Fig. 4. (a) The J–V curves under 1 Sun illumination for the optimized p-i-n and i devices that exhibit PCE of 0.83% and 0.57% respectively; (b) The dark current characteristics of the optimized p-i-n and i devices; (c) The EQE curves of the i and p-i-n device.

PC60BM, which would lead to weaker donor/acceptor interactions and subsequently higher open circuit voltages [31].

4. Conclusions In summary, we have demonstrated a simple device fabrication approach for p-i-n junction organic solar cells based on an ‘‘old’’ industrial pigment, quinacridone. The pristine insoluble quincridone molecule was easily functionalized with solubilizing t-BOC groups, yielding a solution processable material with suitable properties for photovoltaic application. Although the overall power

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