Efficient Organic Solar Cells Using a High-Quality Crystalline Thin Film as a Donor Layer (p NA)

Efficient Organic Solar Cells Using a High-Quality Crystalline Thin Film as a Donor...

(Parte 1 de 2)

Efficient Organic Solar Cells Using a High-Quality Crystalline Thin Film as a Donor Layer

By Bo Yu, Lizhen Huang, Haibo Wang, and Donghang Yan*

Organic solar cells based on small molecules and conjugated polymers have attracted much attention due to their potential for providing low-cost solar energy conversion.[1–4] Recently, a power-conversion efficiency (PCE) of organic solar cells over 6% has been achieved[5,6] but this is still much lower than that of inorganic products. One of the major factors limiting the PCE is that the exciton diffusion length is much shorter than the optical absorption length in most of the organic semiconductors.[7] To date, the limitation was in some degree solved by smart device structures such as bulk heterojunction[6,8–9] and tandem cells.[5,10] For conjugated polymer solar cells, thermal annealing[9] or solvent treatment[8] not only form a better bulk heterojunction but also improve the carrier mobility, which may increase the exciton diffusion length and enhance the carrier transport efficiency. For small-molecular solar cells, directly depositing organic materials onto a heated substrate has been used to improve the carrier mobility and to form a better bulk heterojunction.[1–13] However, a heated substrate roughens the small-molecular active films and leads to a high contact resistance. Meanwhile, heat treatment increases the density of pinholes in small-molecular active films, which induces short circuits and reduces the device performance.[1]

Recently, Wang et al. developed a technique called ‘‘weak epitaxy growth’’ (WEG). By applying this technique, highperformance organic thin-film transistors (OTFTs) with singlecrystal-like carrier mobility for metal phthalocyanine thin films can be produced.[14] In addition, compared with non-WEG metal phthalocyanine thin films, the deep bulk traps decrease dramatically and the space charge region broadens from 10nm to 40nm in WEG thin films.[15] This indicates that free carriers can move a longer distance with less losses. In the WEG method, the inducing layer between the active layer and the substrate not only determines the quality of the active layer but also directly influences the device performances. For example, parahexaphenyl (p-6P), a typical inducing layer for fabricating high-performance OTFTs,[14,16–18] is not suitable to fabricate organic solar cells because of the mismatch of its energy level and low carrier mobility.

In this Communication, we report an efficient organic solar cell fabricated by the WEG method with 2,5-bis(4-biphenylyl)- bithiophene (BP2T) as the inducing layer, where the PCE reaches 3.07%. BP2T is an excellent inducing layer for organic solar cells fabricated by the WEG method. First, the highest occupied molecular orbital (HOMO) of BP2T is 5.3eV, which supports the formation of Ohmic contact with both poly(3,4-ethylenedioxythophene):poly(styrene sulfonic acid) (PEDOT:PSS) and zinc phthalocyanine (ZnPc), as shown in Figure 1. Second, BP2T is a good hole-transport materials and its carrier mobility is about 10 2cm2 Vs 1.[19] The additional BP2T layer will not increase the series resistance of devices. Third, BP2T can play a role as an electron-blocking layer because of the lowest unoccupied molecular orbital (LUMO) of 2.8eV. Finally, the high-quality ZnPc thin film can grow on the BP2T inducing layer.

An 8-nm-thick BP2T thin film was first deposited onto an indium tin oxide (ITO) substrate, smoothed by PEDOT:PSS at 1558C, and its morphology obtained by tapping-mode atomic force microscopy (AFM) is shown in Figure 2a. The BP2T thin film shows a layer-mode growth and covers the whole substrate. A 20-nm-thick ZnPc thin film was then deposited onto the surface w.MaterialsViews.com w.advmat.de

[*] Prof. D. H. Yan, B. Yu, L. Z. Huang, Dr. H. B. Wang

State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022 (P. R. China) E-mail: yandh@ciac.jl.cn

B. Yu, L. Z. Huang Graduate School of Chinese Academy of Sciences Beijing 100039 (P. R. China)

DOI: 10.1002/adma.200903023

Figure 1. a) The molecular structures of the organic materials used in the solar cells. b) Schematic energy level at flat-band condition.

w.advmat.de w.MaterialsViews.com of BP2T thin film at 1558C, and its AFM images with different scales are shown in Figure 2b and 2d. Compared with a polycrystalline ZnPc thin film directly grown on and ITO substrate at ambient temperature (Fig. 2c), a larger crystal and more continuous ZnPc thin film was achieved with the WEG method. The length of ZnPc fibers is in the same scale as the BP2Tmonodomain and the typical width of ZnPc fibers is about 0.2mm. The root-mean-square (RMS) roughness of surface decreases from 3.38 to 2.42nm. Selected area electron diffraction (SAED) was performed to investigate the in-plane orientation relationship of ZnPc/BP2T. Figure 3 shows the SAED pattern containing one [001] zone of BP2T (lattice parameters: a¼5.708A , b¼7.604A , c¼52.869A and b¼97.158) and one [100] zone of ZnPc. The c*-axis of ZnPc is parallel to the b*-axis of BP2T. This obviously indicates that ZnPc grown on each BP2T domain exhibits only one orientation. In the other words, a ZnPc thin film can epitaxially grow on a BP2T inducing layer. The epitaxy relationship between ZnPc and BP2T is:

Two typical types of ZnPc/C60 organic solar cells, i.e., planar heterojunction (PHJ) and planar–mixed heterojunction (PM-HJ), are investigated. For comparison, reference devices with the same configurations were fabricated on ITO substrates at room temperature. Table 1 summarizes the device parameters of all solar cells. Under the illumination of 100mWcm 2 (mismatch to air mass (AM) 1.5 Gis not corrected), the reference cell of a planar heterojunction (cell A) shows a typical photovoltaic response with a device performance comparable to the earlier reports; the circuit current density (Jsc) is 4.16mA cm 2, and the fill factor (F) is 0.5, so the PCE

(PCE¼Voc Jsc F/Pinc, where Pinc is the intensity of incident light) is 1.19%. In comparison, the PHJ device prepared by the WEG method (cell B) shows a higher F of

0.65, Jsc increases to 5.76mA cm 2, and Voc increases slightly to 0.56V, which lead to a nearly 100% increase in the device performance and the PCE reaches 2.10%.

The high F is attributed to the reduction of the carrier recombination. Compared with cell A, the saturation factor (ratio of photocurrent density at 1V to Jsc)[20] decreasing from 1.27 to 1.07 demonstrates less recombination in cell

B. In ZnPc thin films prepared by WEG, a hole field-effect mobility increasing from 0.02 to 0.32cm2 V 1s 1 has been demonstrated in previous reports.[14] The similar carrier mobi-

balance between holes and electrons. Hence, it improves the transport efficiency of free carriers dissociated by exciton at the donor/ acceptor interface and reduces the recombination. A similar result has been observed in polymer solar cells. Furthermore, the singlecrystal-like ZnPc thin film with less bulk traps can further decreases the carrier recombination. Due to the increase of transport efficiency and the decrease of the carrier recombination, photocurrent is improved accordingly. The external quantum efficiency (EQE) data shown in Figure 4a confirms the improvement of Jsc

Figure 2. Surface morphology images of organic films observed by AFM. a) 8nm BP2T on ITO/ PEDOT:PSS, b) 20nm ZnPc on BP2T (10mm 10mm), c) 20nm ZnPc on ITO, and d) 20nm ZnPc on BP2T (1mm 1mm).

Figure 3. Morphology (inset) and corresponding SAED pattern of the BP2T/ZnPc film. The area selected by the white circle was used for SAED. The SADE pattern indicates the epitaxy relationship between ZnPc and BP2T. The b*-axis of ZnPc coincides with a*-axis of BP2T and the c*-axis of ZnPc coincides with the b* axis of BP2T.

2 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–4 Final page numbers not assigned w.MaterialsViews.com w.advmat.de

in cell B. Photoresponse is enhanced from 400 to 800nm, which is the key factor in the higher photocurrent. Figure 4b shows the absorbance spectra of BP2T, ZnPc, and BP2T/ZnPc, respectively. It is worthy to note that the WEG method broadens the absorption of ZnPc thin film in the range of 750 800nm and more photons are absorbed. Compared with the EQE curves depicted in Figure 4a, photoresponse increases significantly in the corresponding region, thus, these photons can make an important contribution to the photocurrent. As we known, the Voc is dependent on the reverse saturation current, Is.[2] As illustrated in Figure 5a, Is of cell B is one order of magnitude lower than that of cell A due to the introduction of the BP2T layer. Laminar

growth of the BP2T layer without pinhole avoids the microscopic leak circuit in the device and Is becomes smaller. For organic solar cells with PM-HJ structure, the current– volage (I–V) characteristic curves are depicted in Figure 5b. Cell

C, as a reference cell, exhibits a Jsc of 8.19mA cm 2,a Voc of 0.52V and a F of 0.41.The introduction of a ZnPc:C60 mixed layer increases the Jsc because of the formation of bulk heterojunctions. However, a non-ideal interpenetrating network

without a direct percolation path for photogenerated carriers toward the respective electrode increases the recombination and reduces the F. In cell D, the mixed layer was deposited onto the ZnPc thin film fabricated by the WEG method at 1008C. The

The increase in Jsc and F implies a better interpenetrating network with continuous channels in the mixed layer. The reduction of the saturation factor from 1.30 to 1.09 reveals an interpenetrating interface that is formed in cell D. The EQE data shown in Figure 4a confirm the improvement of Jsc in cell D, since the value of EQE for both ZnPc and C60 are enhanced significantly witha peak valueofover60%at620nm.IntegratingtheEQEcurve of cell D in Figure 4a from 400 to 1000nm with the standard AM

1.5 G solar spectrum we obtain Jsc¼11.5mA cm 2. Using the Voc and FFdata for cell D in Table 1, a PCE of 3.54% could be achieved.

In conclusion, organic solar cells using weak-epitaxy-growth thin films as donor layers have been fabricated and the PCE is over 3%. The high-quality thin film prepared by the WEG method is the determinant of high-performance devices. The improvement in the short-circuit current and the fill factor is attributed to the single-crystal-like thin film with less bulk traps and a high carrier mobility. In addition, for sophisticated PM-HJ structure solar cells, the direct deposition of a ZnPc:C60 mixed layer onto a ZnPc WEG thin film at high temperatures can form a better interpenetrating network and its performance is further enhanced.

Experimental

The ITO-coated glass substrates with a sheet resistance of 15V & 1 were used as anode. The substrates were cleaned with detergent, then ultrasonicated in acetone, alcohol, and deionized water in sequence and, subsequently, dried in pure N2. PEDOT: PSS (H.C.Starck P VP Al 4083) was spin-coated at 3000rpm for 30s and, then, the substrate was dried at

1558C for 15min in air. ZnPc, C60, and 8-hydroxyquinoline aluminium (Alq3) were purchased from Aldrich Corp. BP2T was synthesized by a

Table 1. Summary of the cell performance extracted from I–V curves.

Cell Structure Voc [V] Jsc [mA/cm2] F PCE [%] Saturation Factor

Absorption(Normalized) Wavelength (nm)

ZnPc BP2T/ZnPc BP2T

Wavelength(nm)

PM-HJ by WEG Ref-PM-HJ PHJ by WEG Ref-PHJ

Figure 4. a) EQE of cell A (PHJ by WEG), cell B (Ref PHJ), cell C (PM-HJ by WEG), and cell D (Ref PM-HJ). b) Normalized absorption spectra of BP2T, ZnPc, and BP2T/ZnPc films on quartz/PEDOT:PSS substrates.

0Current density(mA/cm)Voltage (V)

Current density(mA/cm)Voltage (V)

Figure 5. a) I–V characteristic curves of cells A and B in the dark. b) I–V characteristic curves of cell A (PHJ by WEG), cell B (Ref PHJ), cell C (PM-HJ by WEG), and cell D (Ref PM-HJ) under 100mW cm 2 intensity of simulated AM 1.5 G solar illumination.

w.advmat.de w.MaterialsViews.com method described previously [23]. All materials were purified twice by thermal-gradient train sublimation prior to use. Al was used as cathode.

Two typical types of device were fabricated with configurations as follows: i) Planar heterojunction (PHJ) devices: ITO/PEDOT:PSS/BP2T

(8nm)/ZnPc (25nm)/C60 (40nm)/Alq3 (5nm)/Al (100nm). i) Planar–mixed heterojunction (PM-HJ) devices: ITO/PEDOT:PSS/BP2T

The substrate was kept at 1558C during deposition of BP2T and ZnPc and, susequently, cooled to 1008C for the deposition of the ZnPc:C60 mixed layer. Finally, C60, Alq3, and Al were deposited at room temperature in sequence. For comparison, reference devices were fabricated on ITO

substrates at room temperature without a BP2T layer.

All materials were thermally evaporated at a base pressure of 10 4Pa at a rate of about 0.2–0.3A s 1 for BP2T, 1–2A s 1 for other organic materials, and 5–10A s 1 for Al. Their thicknesses were monitored by a quartz-crystal microbalance during the film deposition. A shadow mask was used to define the area of the cathode to 3.14 mm2 for each device. The I–V curves were measured with a Keithley 2400 source-measure unit under 100mW cm 2 illuminations with an AM 1.5 G filter (SS150W solar simulator, Sciencetech Inc.). The illumination intensity was calibrated with a standard silicon photovoltaic traced to the National Renewable Energy Laboratory (NREL). The EQEwas measuredwithQ TestStation 2000(CrowntechInc.USA). The measurements were carried out at room temperature in air.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (50773079 and 50803063) and the National Basic Research Program (2009CB939702)

Received: September 3, 2009 Published online:

(Parte 1 de 2)

Comentários