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Efficient organic photovoltaic cells with vertically ordered bulk heterojunctions
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 484006 (http://iopscience.iop.org/0957-4484/24/48/484006) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 35.8.11.2 This content was downloaded on 30/11/2013 at 12:00
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 484006 (5pp)
doi:10.1088/0957-4484/24/48/484006
Efficient organic photovoltaic cells with vertically ordered bulk heterojunctions Bo Yu, Haibo Wang and Donghang Yan State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China E-mail: [email protected]
Received 11 March 2013, in final form 11 July 2013 Published 6 November 2013 Online at stacks.iop.org/Nano/24/484006 Abstract Nanoscale morphology has been proved to be the key parameter deciding the exciton dissociation and charge transportation in bulk heterojunction (BHJ) solar cells. In this paper, we report a kind of small molecular organic photovoltaic cell (OPV) with a vertically ordered BHJ prepared by the weak epitaxial growth method. By this method, zinc phthalocyanine (ZnPc) can easily be formed into a highly ordered and continuous thin film and C60 is inclined to become dispersed crystalline grains in ZnPc film. Furthermore, we can control both the size and distribution density of C60 crystalline grains in ZnPc thin film without destroying the order of the ZnPc thin film. The OPVs with the vertically ordered BHJ show a high fill factor and a power conversion efficiency over 3% has been achieved. (Some figures may appear in colour only in the online journal)
1. Introduction
donor and acceptor molecules could form isolated islands more or less during the phase separation process. Isolated islands play a role as recombination centres, reducing both the photocurrent and the fill factor (FF) of devices. Up to now, many methods have been used to obtain a better BHJ structure. Thermal annealing [15, 16], using additives [17] and solvent treatments [18] are typical methods that have been widely used in polymer BHJs solar cells successfully. For small molecular solar cells, directly co-depositing the D–A mixture on a heated substrate [19–21] can also form a better BHJ structure. However, in small molecular BHJs, both donor and acceptor continuous phases consist of low quality crystals or amorphousness. Abundant grain boundaries reduce the carrier transport efficiency and increase the recombination; accordingly, this kind of device usually has a low FF. Recently, McGehee et al introduced the concept of ordered BHJs [22]. As shown in figure 1, in an ordered BHJ, phase separation is only generated in the direction parallel to the substrate. In the vertical direction, donor and acceptor molecules stack in order. Hence, excitons can be dissociated efficiently and carrier transport is also enhanced. This structure is always used in polymer and inorganic nano-crystal hybrid solar cells but there are a few reports in small molecular organic solar cells [23, 24]. One reason is that highly ordered organic semiconductor thin film are always
Organic photovoltaic cells (OPVs), which can directly convert solar energy into electrical energy, have attracted attention because of their potential to produce low-cost and large-area flexible devices [1–6]. Since Tang invented the first donor–acceptor OPV [7], the power conversion efficiency (PCE) of OPVs increased steadily due to various novel materials and smart device structures [8–12]. Recently, a PCE over 10% has been reported [8] and it is not far from the efficiencies required for commercial applications. One of the most important milestones in OPV development is the introduction of bulk heterojunctions (BHJs) [13, 14]. This solves the problem of exciton diffusion length being much shorter than light absorption length in most organic semiconductors. In the BHJs, donor (D) and acceptor (A) materials are blended together and form an interpenetrating network because of the phase separation. For the ideal BHJ, the scale of phase separation is equal to the exciton diffusion length in materials. Hence, all photo-generated excitons can reach the D–A interface and dissociate to free electrons and holes. On the other hand, donor and acceptor phases must remain continuous. The continuous phase provides a direct transmission path for electrons and holes towards the corresponding electrode. However, in practical BHJs, 0957-4484/13/484006+05$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 484006
B Yu et al
(10 nm)/ZnPc:C60 (30 nm)/C60 (25 nm)/Alq3 (5 nm)/Al (100 nm). All materials were thermally evaporated at a ˚ s−1 base pressure of 10−4 Pa at a rate of about 0.2–0.3 A −1 ˚ for BP2T, 1–2 A s for other organic materials and 1 nm s−1 for Al. Their thicknesses were monitored by a quartz crystal microbalance during film deposition. The device area was 3.14 mm2 defined by a shadow mask. The current–voltage (I–V) curves were measured with a Keithley 2400 source-measurement unit under 100 mW cm−2 illuminations with an AM 1.5G filter (SS150W solar simulator, Sciencetech Inc.) and the illumination intensity was calibrated with a standard silicon solar cell traced to the National Renewable Energy Laboratory (NREL). The measurement conditions were at room temperature in air.
Figure 1. Device architecture of solar cells with a vertically ordered bulk heterojunction.
3. Results and discussion
prepared on an inorganic single-crystal substrate by epitaxial growth [25]. However, an isolated inorganic single crystal could not be used in OPVs because it could not transport carriers. In addition, co-deposition could destroy the ordered structure of the thin film and reduce the carrier transport efficiency. Our prior works have reported fabricating highly ordered ZnPc thin film on the conducting substrate by the weak epitaxial growth (WEG) method and preparing high performance OPVs based on ZnPc and C60 [26]. In small molecule organic solar cells, ZnPc-C60 is a typical system and many significant works have been reported [27–29]. In this paper, we focus on the growth behaviour after depositing C60 and ZnPc in sequence and co-depositing two materials on WEG ZnPc thin film. The result shows that C60 tends to form dispersed grains and ZnPc could form large area and continuous thin films composed of needle-like crystals. More importantly, the growth behaviour of two materials has little change even when co-depositing them at the same time. Therefore, we could easily prepare vertically ordered BHJs and adjust the phase separation scale by varying the size and amount of C60 grains in ZnPc thin film. Devices with a high FF indicate a better phase separation structure existing in vertically ordered BHJs. The PCE of optimized OPVs attains 3.1%.
First, we prepared the vertically ordered BHJs by depositing C60 and ZnPc in sequence on the surface of 5-bis(4biphenylyl)-bithiophene (BP2T) thin film. Here, BP2T was chosen as an inducing layer for obtaining high quality ZnPc WEG thin film [26, 30]. Figures 2(a) and (b) show the atomic force microscope (AFM) images of the morphological evolution of C60 on BP2T thin film at different substrate temperatures (Tsub ) of 165 ◦ C and 90 ◦ C, respectively. When the Tsub was fixed at 165 ◦ C, C60 molecules only grew at the edge of BP2T crystal domains and no C60 micro-crystal was observed at the surface of BP2T crystal domains (figure 2(a)). With decreasing Tsub , C60 molecules began to grow randomly at the surface of BP2T crystal domains. In addition, the size of C60 micro-crystals became smaller and their number increased at a lower Tsub (figure 2(b)). Then 10 nm thickness ZnPc was deposited on BP2T/C60 ; their AFM images are shown in figures 2(c) and (d), respectively. Needle-like ZnPc crystals regularly stacked on bared BP2T because of WEG behaviour and C60 micro-crystals were partially embedded in WEG ZnPc thin film. Hence, a vertically ordered BHJ was obtained. More importantly, we can easily adjust the scale of phase separation in BHJs by controlling the size and distribution density of C60 micro-crystals. The solar cells with the vertically ordered BHJ mentioned above were prepared. For comparison, a planar double-layer device without C60 micro-crystals was fabricated under the same conditions. Figure 3 depicts current–voltage (I–V) curves of two cells under an illumination of 100 mW cm−2 (a spectrum mismatch to air mass (AM) 1.5 Globe is not corrected). The reference cell shows a typical photovoltaic behaviour with an open-circuit voltage (Voc ) of 0.56 V, a FF of 0.65 and a short-circuit current density (Jsc ) of 3.95 mA cm−2 . As a comparison, the solar cell with a vertically ordered BHJ has a similar Voc (0.54 V) and FF (0.62), and the Jsc increases to 5.0 mA cm−2 . So the PCE (PCE = Voc Jsc FF/Pinc , where Pinc is the intensity of incident light) increases from 1.4% to 1.7%. The result indicates that more photo-induced excitons dissociated to free carriers with little recombination loss in a vertically ordered BHJ solar cell. Nevertheless, there are two problems in these kinds of cells. One is that the scale of phase separation in the vertically
2. Experimental details All devices were prepared on indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 /. The substrates were ultrasonicated in acetone, alcohol and deionized water in sequence and then dried by pure N2 . Poly(3,4ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT: PSS, H C Starck P VP Al 4083) was spin-coated on the ITO glass at 3000 rpm for 30 s and baked at 155 ◦ C for 15 min in air. ZnPc, C60 and 8-hydroxyquinoline aluminum (Alq3) were purchased from Aldrich Corp. The 2,5-bis(4-biphenylyl)-bithiophene (BP2T) was synthesized by the method reported earlier. All materials were purified twice by thermal gradient train sublimation prior to use. Two types of vertically ordered BHJ solar cells were fabricated with configurations as follows. Devices A: ITO/PEDOT:PSS/BP2T (8 nm)/C60 -ZnPc (10 nm)/C60 (40 nm)/Alq3 (5 nm)/Al (100 nm). Device B:ITO/PEDOT:PSS/BP2T (8 nm)/ZnPc 2
Nanotechnology 24 (2013) 484006
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Figure 2. Tapping-mode AFM images of C60 micro-crystals grow on the surface of BP2T at Tsub = 165 ◦ C (a) and Tsub = 90 ◦ C (b), WEG ZnPc grown on a C60 /BP2T surface at Tsub = 165 ◦ C (c) and Tsub = 90 ◦ C (d).
Dispersed C60 micro-crystals grown on BP2T indicate (figure 2(b)) that a C60 molecule tends to combine with a C60 molecule instead of a BP2T molecule. In contrast, a continuous ZnPc thin film on BP2T reveals that the ZnPc molecule combines easily with both ZnPc and BP2T molecules. Hence, when we blend ZnPc: C60 together, like molecules, such as ZnPc–ZnPc or C60 –C60 , prefer to aggregate and form a phase separation. Based on this, we fabricated the vertically ordered BHJs by co-depositing ZnPc and C60 on the surface of WEG ZnPc film. The variation in the morphology of ZnPc:C60 (5:1) mixed films at 150 and 100 ◦ C is shown in figures 4(a) and (b), respectively. As we expected, a visible phase separation is observed because of quite different morphology between needle-like ZnPc crystals and protuberant C60 micro-crystals. This phenomenon also indicates that, in the mixed layer, C60 molecules have no effect on the WEG mode of ZnPc molecules. Moreover, a low Tsub decreases the size of C60 micro-crystals but has little impact on ZnPc WEG behaviour. By holding Tsub = 100 ◦ C and increasing C60 ratio to 50% in the mixed layer (figure 4(c)), the size of C60 micro-crystals changes a little and the scale of the ZnPc phase becomes smaller. For further investigating the growth of the ZnPc:C60 mixed layer, partial material was peeled off the surface of the mixed layer and the section morphology was examined by a scanning electron microscope (SEM, figure 4(d)). It distinctly displays that C60 micro-crystals are embedded in WEG ZnPc film and a vertical phase separation structure is obtained. For optimizing the performance of devices, a ZnPc:C60 mixed layer was deposited at varying Tsub from 150 to 45 ◦ C
Figure 3. The current–voltage curves of solar cells with (circles) and without (squares) C60 micro-crystals under AM 1.5G simulated solar illumination of 100 mW cm−2 .
ordered BHJs is several hundreds of nanometres even when depositing C60 on BP2T at room temperature, which is still much larger than the exciton diffusion length in materials. The other is that the thickness of vertically ordered BHJ must be less than the height of C60 micro-crystals, which limits the thickness of active film and reduces the utilization of incident light. The method of depositing C60 and ZnPc in sequence hardly improves the PCE of the devices further, but it provides an important phenomenon which could help us to better understand the phase separation between ZnPc and C60 . 3
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Figure 4. (a) and (b) AFM images of ZnPc:C60 (5:1) co-depositing on WEG ZnPc at 150 ◦ C and 100 ◦ C, respectively; (c) AFM image of ZnPc:C60 (1:1) co-depositing on WEG ZnPc at 100 ◦ C; (d) SEM section image of ZnPc:C60 co-depositing on WEG ZnPc at 100 ◦ C.
Figure 5. The photovoltaic response of a vertically ordered BHJ solar cell as a function of the ZnPc:C60 co-depositing temperature.
and all device parameters extracted from I–V characteristic curves under simulated sunlight are summarized in figures 5(a) and (b). The Voc remains constant at 0.56 V because it is relatively dependent of the difference between the lowest unoccupied molecular orbital (LUMO) of C60 and the highest occupied molecular orbital (HOMO) of ZnPc. It is worth noting that the FF still keeps a value around 0.55 even though Tsub decreases from 120 to 45 ◦ C. It indicates that continuous transmission paths for both holes and electrons exist in the vertically ordered BHJ and a few photo-generated carriers are lost in the transport process. Unlike the Voc and the FF, the Jsc is evidently influenced by Tsub . Starting from 7.3 mA cm−2 at Tsub = 150 ◦ C, the Jsc reaches a maximum at Tsub = 100 ◦ C with a value of 10 mA cm−2 , and drops to 8.5 mA cm−2 at a low Tsub = 45 ◦ C. Hence, the PCE = 3.1% is obtained at Tsub = 100 ◦ C. In BHJs, Jsc is directly decided by the balance between exciton dissociation and the carrier transport. For BHJs prepared at high Tsub , large crystals are beneficial to carrier transport but reduce the exciton separating
interfaces. Correspondingly, low Tsub creates smaller crystals which provide more interfaces for exciton dissociation but limit carrier extraction from BHJs. Thus, as an optimum result, Tsub = 100 ◦ C is found.
4. Conclusions In conclusion, ZnPc:C60 mixed films with vertically ordered phase separation in the nanoscale have been fabricated. Different growth behaviour between dispersed C60 microcrystals and continuous WEG ZnPc thin film induces phase separation in the vertical direction. By varying the substrate temperatures, we could easily adjust the size of C60 micro-crystals in high quality ZnPc WEG thin film and obtain ordered mixed films. The upright and continuous phase reduces carrier recombination. On the other hand, extracting carriers by ordered structures is more efficient. Hence, organic solar cells with vertically ordered BHJs have a high FF and power conversion efficiency. 4
Nanotechnology 24 (2013) 484006
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Acknowledgments
[16] Padinger F, Rittberger R S and Sariciftci N S 2003 Effects of postproduction treatment on plastic solar cells Adv. Funct. Mater. 13 85–8 [17] Peet J, Kim J Y, Coates N E, Ma W L, Moses D, Heeger A J and Bazan G C 2007 Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols Nature Mater. 6 497–500 [18] Li G, Shrotriya V, Huang J S, Yao Y, Moriarty T, Emery K and Yang Y 2005 High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends Nature Mater. 4 864–8 [19] Pfuetzner S et al 2011 The influence of substrate heating on morphology and layer growth in C-60:ZnPc bulk heterojunction solar cells Org. Electron. 12 435–41 [20] Wynands D, Levichkova M, Leo K, Uhrich C, Schwartz G, Hildebrandt D, Pfeiffer M and Riede M 2010 Increase in internal quantum efficiency in small molecular oligothiophene:C-60 mixed heterojunction solar cells by substrate heating Appl. Phys. Lett. 97 073503 [21] Pfuetzner S, Meiss J, Petrich A, Riede M and Leo K 2009 Thick C60:ZnPc bulk heterojunction solar cells with improved performance by film deposition on heated substrates Appl. Phys. Lett. 94 253303 [22] Coakley K M and McGehee M D 2004 Conjugated polymer photovoltaic cells Chem. Mater. 16 4533–42 [23] Bu L J, Guo X Y, Yu B, Qu Y, Xie Z Y, Yan D H, Geng Y H and Wang F S 2009 Monodisperse co-oligomer approach toward nanostructured films with alternating donor–acceptor lamellae J. Am. Chem. Soc. 131 13242–3 [24] Matsuo Y, Sato Y, Niinomi T, Soga I, Tanaka H and Nakamura E 2009 Columnar structure in bulk heterojunction in solution-processable three-layered p-i-n organic photovoltaic devices using tetrabenzoporphyrin precursor and silylmethyl 60 fullerene J. Am. Chem. Soc. 131 16048–50 [25] Forrest S R 1997 Ultrathin organic films grown by organic molecular beam deposition and related techniques Chem. Rev. 97 1793–6 [26] Yu B, Huang L Z, Wang H B and Yan D H 2010 Efficient organic solar cells using a high-quality crystalline thin film as a donor layer Adv. Mater. 22 1017–20 [27] Kim H J, Kim J W, Lee H H, Lee B and Kim J J 2012 Initial growth mode, nanostructure, and molecular stacking of a ZnPc:C60 bulk heterojunction Adv. Funct. Mater. 22 4244–8 [28] Kim T M, Kim J W, Shim H S and Kim J J 2012 High efficiency and high photo-stability zinc-phthalocyanine based planar heterojunction solar cells with a double interfacial layer Appl. Phys. Lett. 101 113301 [29] Zhou Y, Taima T, Miyadera T, Yamanari T, Kitamura M, Nakatsu K and Yoshida Y 2012 Phase separation of co-evaporated ZnPc:C-60 blend film for highly efficient organic photovoltaics Appl. Phys. Lett. 100 233302 [30] Huang L Z, Liu C F, Yu B, Zhang J D, Geng Y H and Yan D H 2010 Evolution of 2,5-bis(4-biphenylyl)bithiophene thin films and its effect on the weak epitaxy growth of ZnPc J. Phys. Chem. B 114 4821–7
This work was financially supported by the National Natural Science Foundation of China (51133007) and The National Basic Research Program (2009CB623603).
References [1] Li G, Zhu R and Yang Y 2012 Polymer solar cells Nature Photon. 6 153–61 [2] Walker B, Kim C and Nguyen T Q 2011 Small molecule solution-processed bulk heterojunction solar cells Chem. Mater. 23 470–82 [3] Helgesen M, Sondergaard R and Krebs F C 2010 Advanced materials and processes for polymer solar cell devices J. Mater. Chem. 20 36–60 [4] Hains A W, Liang Z Q, Woodhouse M A and Gregg B A 2010 Molecular semiconductors in organic photovoltaic cells Chem. Rev. 110 6689–35 [5] Brabec C J, Gowrisanker S, Halls J J M, Laird D, Jia S J and Williams S P 2010 Polymer–fullerene bulk-heterojunction solar cells Adv. Mater. 22 3839–56 [6] Riede M, Mueller T, Tress W, Schueppel R and Leo K 2008 Small-molecule solar cells—status and perspectives Nanotechnology 19 424001 [7] Tang C W 1986 Two-layer organic photovoltaic cell Appl. Phys. Lett. 48 183–5 [8] You J B et al 2013 A polymer tandem solar cell with 10.6% power conversion efficiency Nature Commun. 4 1–10 [9] Small C E, Chen S, Subbiah J, Amb C M, Tsang S W, Lai T H, Reynolds J R and So F 2012 High-efficiency inverted dithienogermole–thienopyrrolodione-based polymer solar cells Nature Photon. 6 115–20 [10] He Z C, Zhong C M, Su S J, Xu M, Wu H B and Cao Y 2012 Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure Nature Photon. 6 591–5 [11] Chu T Y et al 2011 Bulk heterojunction solar cells using thieno[3,4-c]thieno[3,2-b:20 ,30 -d]silole copolymer with a power copyrrole-4,6-dione and dinversion efficiency of 7.3% J. Am. Chem. Soc. 133 4250–3 [12] Price S C, Stuart A C, Yang L, Zhou H and You W 2011 Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer–fullerene solar cells J. Am. Chem. Soc. 133 4625–31 [13] Yu G, Gao J, Hummelen J C, Wudl F and Heeger A J 1995 Polymer photovoltaic cells—enhanced efficiencies via a network of internal donor–acceptor heterojunctions Science 270 1789–91 [14] Halls J J M, Walsh C A, Greenham N C, Marseglia E A, Friend R H, Moratti S C and Holmes A B 1995 Efficient photodiodes from interpenetrating polymer networks Nature 376 498–500 [15] Ma W L, Yang C Y, Gong X, Lee K and Heeger A J 2005 Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology Adv. Funct. Mater. 15 1617–22
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Efficient organic photovoltaic cells with vertically ordered bulk heterojunctions
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 484006 (http://iopscience.iop.org/0957-4484/24/48/484006) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 35.8.11.2 This content was downloaded on 30/11/2013 at 12:00
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 484006 (5pp)
doi:10.1088/0957-4484/24/48/484006
Efficient organic photovoltaic cells with vertically ordered bulk heterojunctions Bo Yu, Haibo Wang and Donghang Yan State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China E-mail: [email protected]
Received 11 March 2013, in final form 11 July 2013 Published 6 November 2013 Online at stacks.iop.org/Nano/24/484006 Abstract Nanoscale morphology has been proved to be the key parameter deciding the exciton dissociation and charge transportation in bulk heterojunction (BHJ) solar cells. In this paper, we report a kind of small molecular organic photovoltaic cell (OPV) with a vertically ordered BHJ prepared by the weak epitaxial growth method. By this method, zinc phthalocyanine (ZnPc) can easily be formed into a highly ordered and continuous thin film and C60 is inclined to become dispersed crystalline grains in ZnPc film. Furthermore, we can control both the size and distribution density of C60 crystalline grains in ZnPc thin film without destroying the order of the ZnPc thin film. The OPVs with the vertically ordered BHJ show a high fill factor and a power conversion efficiency over 3% has been achieved. (Some figures may appear in colour only in the online journal)
1. Introduction
donor and acceptor molecules could form isolated islands more or less during the phase separation process. Isolated islands play a role as recombination centres, reducing both the photocurrent and the fill factor (FF) of devices. Up to now, many methods have been used to obtain a better BHJ structure. Thermal annealing [15, 16], using additives [17] and solvent treatments [18] are typical methods that have been widely used in polymer BHJs solar cells successfully. For small molecular solar cells, directly co-depositing the D–A mixture on a heated substrate [19–21] can also form a better BHJ structure. However, in small molecular BHJs, both donor and acceptor continuous phases consist of low quality crystals or amorphousness. Abundant grain boundaries reduce the carrier transport efficiency and increase the recombination; accordingly, this kind of device usually has a low FF. Recently, McGehee et al introduced the concept of ordered BHJs [22]. As shown in figure 1, in an ordered BHJ, phase separation is only generated in the direction parallel to the substrate. In the vertical direction, donor and acceptor molecules stack in order. Hence, excitons can be dissociated efficiently and carrier transport is also enhanced. This structure is always used in polymer and inorganic nano-crystal hybrid solar cells but there are a few reports in small molecular organic solar cells [23, 24]. One reason is that highly ordered organic semiconductor thin film are always
Organic photovoltaic cells (OPVs), which can directly convert solar energy into electrical energy, have attracted attention because of their potential to produce low-cost and large-area flexible devices [1–6]. Since Tang invented the first donor–acceptor OPV [7], the power conversion efficiency (PCE) of OPVs increased steadily due to various novel materials and smart device structures [8–12]. Recently, a PCE over 10% has been reported [8] and it is not far from the efficiencies required for commercial applications. One of the most important milestones in OPV development is the introduction of bulk heterojunctions (BHJs) [13, 14]. This solves the problem of exciton diffusion length being much shorter than light absorption length in most organic semiconductors. In the BHJs, donor (D) and acceptor (A) materials are blended together and form an interpenetrating network because of the phase separation. For the ideal BHJ, the scale of phase separation is equal to the exciton diffusion length in materials. Hence, all photo-generated excitons can reach the D–A interface and dissociate to free electrons and holes. On the other hand, donor and acceptor phases must remain continuous. The continuous phase provides a direct transmission path for electrons and holes towards the corresponding electrode. However, in practical BHJs, 0957-4484/13/484006+05$33.00
1
c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 484006
B Yu et al
(10 nm)/ZnPc:C60 (30 nm)/C60 (25 nm)/Alq3 (5 nm)/Al (100 nm). All materials were thermally evaporated at a ˚ s−1 base pressure of 10−4 Pa at a rate of about 0.2–0.3 A −1 ˚ for BP2T, 1–2 A s for other organic materials and 1 nm s−1 for Al. Their thicknesses were monitored by a quartz crystal microbalance during film deposition. The device area was 3.14 mm2 defined by a shadow mask. The current–voltage (I–V) curves were measured with a Keithley 2400 source-measurement unit under 100 mW cm−2 illuminations with an AM 1.5G filter (SS150W solar simulator, Sciencetech Inc.) and the illumination intensity was calibrated with a standard silicon solar cell traced to the National Renewable Energy Laboratory (NREL). The measurement conditions were at room temperature in air.
Figure 1. Device architecture of solar cells with a vertically ordered bulk heterojunction.
3. Results and discussion
prepared on an inorganic single-crystal substrate by epitaxial growth [25]. However, an isolated inorganic single crystal could not be used in OPVs because it could not transport carriers. In addition, co-deposition could destroy the ordered structure of the thin film and reduce the carrier transport efficiency. Our prior works have reported fabricating highly ordered ZnPc thin film on the conducting substrate by the weak epitaxial growth (WEG) method and preparing high performance OPVs based on ZnPc and C60 [26]. In small molecule organic solar cells, ZnPc-C60 is a typical system and many significant works have been reported [27–29]. In this paper, we focus on the growth behaviour after depositing C60 and ZnPc in sequence and co-depositing two materials on WEG ZnPc thin film. The result shows that C60 tends to form dispersed grains and ZnPc could form large area and continuous thin films composed of needle-like crystals. More importantly, the growth behaviour of two materials has little change even when co-depositing them at the same time. Therefore, we could easily prepare vertically ordered BHJs and adjust the phase separation scale by varying the size and amount of C60 grains in ZnPc thin film. Devices with a high FF indicate a better phase separation structure existing in vertically ordered BHJs. The PCE of optimized OPVs attains 3.1%.
First, we prepared the vertically ordered BHJs by depositing C60 and ZnPc in sequence on the surface of 5-bis(4biphenylyl)-bithiophene (BP2T) thin film. Here, BP2T was chosen as an inducing layer for obtaining high quality ZnPc WEG thin film [26, 30]. Figures 2(a) and (b) show the atomic force microscope (AFM) images of the morphological evolution of C60 on BP2T thin film at different substrate temperatures (Tsub ) of 165 ◦ C and 90 ◦ C, respectively. When the Tsub was fixed at 165 ◦ C, C60 molecules only grew at the edge of BP2T crystal domains and no C60 micro-crystal was observed at the surface of BP2T crystal domains (figure 2(a)). With decreasing Tsub , C60 molecules began to grow randomly at the surface of BP2T crystal domains. In addition, the size of C60 micro-crystals became smaller and their number increased at a lower Tsub (figure 2(b)). Then 10 nm thickness ZnPc was deposited on BP2T/C60 ; their AFM images are shown in figures 2(c) and (d), respectively. Needle-like ZnPc crystals regularly stacked on bared BP2T because of WEG behaviour and C60 micro-crystals were partially embedded in WEG ZnPc thin film. Hence, a vertically ordered BHJ was obtained. More importantly, we can easily adjust the scale of phase separation in BHJs by controlling the size and distribution density of C60 micro-crystals. The solar cells with the vertically ordered BHJ mentioned above were prepared. For comparison, a planar double-layer device without C60 micro-crystals was fabricated under the same conditions. Figure 3 depicts current–voltage (I–V) curves of two cells under an illumination of 100 mW cm−2 (a spectrum mismatch to air mass (AM) 1.5 Globe is not corrected). The reference cell shows a typical photovoltaic behaviour with an open-circuit voltage (Voc ) of 0.56 V, a FF of 0.65 and a short-circuit current density (Jsc ) of 3.95 mA cm−2 . As a comparison, the solar cell with a vertically ordered BHJ has a similar Voc (0.54 V) and FF (0.62), and the Jsc increases to 5.0 mA cm−2 . So the PCE (PCE = Voc Jsc FF/Pinc , where Pinc is the intensity of incident light) increases from 1.4% to 1.7%. The result indicates that more photo-induced excitons dissociated to free carriers with little recombination loss in a vertically ordered BHJ solar cell. Nevertheless, there are two problems in these kinds of cells. One is that the scale of phase separation in the vertically
2. Experimental details All devices were prepared on indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 /. The substrates were ultrasonicated in acetone, alcohol and deionized water in sequence and then dried by pure N2 . Poly(3,4ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT: PSS, H C Starck P VP Al 4083) was spin-coated on the ITO glass at 3000 rpm for 30 s and baked at 155 ◦ C for 15 min in air. ZnPc, C60 and 8-hydroxyquinoline aluminum (Alq3) were purchased from Aldrich Corp. The 2,5-bis(4-biphenylyl)-bithiophene (BP2T) was synthesized by the method reported earlier. All materials were purified twice by thermal gradient train sublimation prior to use. Two types of vertically ordered BHJ solar cells were fabricated with configurations as follows. Devices A: ITO/PEDOT:PSS/BP2T (8 nm)/C60 -ZnPc (10 nm)/C60 (40 nm)/Alq3 (5 nm)/Al (100 nm). Device B:ITO/PEDOT:PSS/BP2T (8 nm)/ZnPc 2
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Figure 2. Tapping-mode AFM images of C60 micro-crystals grow on the surface of BP2T at Tsub = 165 ◦ C (a) and Tsub = 90 ◦ C (b), WEG ZnPc grown on a C60 /BP2T surface at Tsub = 165 ◦ C (c) and Tsub = 90 ◦ C (d).
Dispersed C60 micro-crystals grown on BP2T indicate (figure 2(b)) that a C60 molecule tends to combine with a C60 molecule instead of a BP2T molecule. In contrast, a continuous ZnPc thin film on BP2T reveals that the ZnPc molecule combines easily with both ZnPc and BP2T molecules. Hence, when we blend ZnPc: C60 together, like molecules, such as ZnPc–ZnPc or C60 –C60 , prefer to aggregate and form a phase separation. Based on this, we fabricated the vertically ordered BHJs by co-depositing ZnPc and C60 on the surface of WEG ZnPc film. The variation in the morphology of ZnPc:C60 (5:1) mixed films at 150 and 100 ◦ C is shown in figures 4(a) and (b), respectively. As we expected, a visible phase separation is observed because of quite different morphology between needle-like ZnPc crystals and protuberant C60 micro-crystals. This phenomenon also indicates that, in the mixed layer, C60 molecules have no effect on the WEG mode of ZnPc molecules. Moreover, a low Tsub decreases the size of C60 micro-crystals but has little impact on ZnPc WEG behaviour. By holding Tsub = 100 ◦ C and increasing C60 ratio to 50% in the mixed layer (figure 4(c)), the size of C60 micro-crystals changes a little and the scale of the ZnPc phase becomes smaller. For further investigating the growth of the ZnPc:C60 mixed layer, partial material was peeled off the surface of the mixed layer and the section morphology was examined by a scanning electron microscope (SEM, figure 4(d)). It distinctly displays that C60 micro-crystals are embedded in WEG ZnPc film and a vertical phase separation structure is obtained. For optimizing the performance of devices, a ZnPc:C60 mixed layer was deposited at varying Tsub from 150 to 45 ◦ C
Figure 3. The current–voltage curves of solar cells with (circles) and without (squares) C60 micro-crystals under AM 1.5G simulated solar illumination of 100 mW cm−2 .
ordered BHJs is several hundreds of nanometres even when depositing C60 on BP2T at room temperature, which is still much larger than the exciton diffusion length in materials. The other is that the thickness of vertically ordered BHJ must be less than the height of C60 micro-crystals, which limits the thickness of active film and reduces the utilization of incident light. The method of depositing C60 and ZnPc in sequence hardly improves the PCE of the devices further, but it provides an important phenomenon which could help us to better understand the phase separation between ZnPc and C60 . 3
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Figure 4. (a) and (b) AFM images of ZnPc:C60 (5:1) co-depositing on WEG ZnPc at 150 ◦ C and 100 ◦ C, respectively; (c) AFM image of ZnPc:C60 (1:1) co-depositing on WEG ZnPc at 100 ◦ C; (d) SEM section image of ZnPc:C60 co-depositing on WEG ZnPc at 100 ◦ C.
Figure 5. The photovoltaic response of a vertically ordered BHJ solar cell as a function of the ZnPc:C60 co-depositing temperature.
and all device parameters extracted from I–V characteristic curves under simulated sunlight are summarized in figures 5(a) and (b). The Voc remains constant at 0.56 V because it is relatively dependent of the difference between the lowest unoccupied molecular orbital (LUMO) of C60 and the highest occupied molecular orbital (HOMO) of ZnPc. It is worth noting that the FF still keeps a value around 0.55 even though Tsub decreases from 120 to 45 ◦ C. It indicates that continuous transmission paths for both holes and electrons exist in the vertically ordered BHJ and a few photo-generated carriers are lost in the transport process. Unlike the Voc and the FF, the Jsc is evidently influenced by Tsub . Starting from 7.3 mA cm−2 at Tsub = 150 ◦ C, the Jsc reaches a maximum at Tsub = 100 ◦ C with a value of 10 mA cm−2 , and drops to 8.5 mA cm−2 at a low Tsub = 45 ◦ C. Hence, the PCE = 3.1% is obtained at Tsub = 100 ◦ C. In BHJs, Jsc is directly decided by the balance between exciton dissociation and the carrier transport. For BHJs prepared at high Tsub , large crystals are beneficial to carrier transport but reduce the exciton separating
interfaces. Correspondingly, low Tsub creates smaller crystals which provide more interfaces for exciton dissociation but limit carrier extraction from BHJs. Thus, as an optimum result, Tsub = 100 ◦ C is found.
4. Conclusions In conclusion, ZnPc:C60 mixed films with vertically ordered phase separation in the nanoscale have been fabricated. Different growth behaviour between dispersed C60 microcrystals and continuous WEG ZnPc thin film induces phase separation in the vertical direction. By varying the substrate temperatures, we could easily adjust the size of C60 micro-crystals in high quality ZnPc WEG thin film and obtain ordered mixed films. The upright and continuous phase reduces carrier recombination. On the other hand, extracting carriers by ordered structures is more efficient. Hence, organic solar cells with vertically ordered BHJs have a high FF and power conversion efficiency. 4
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Acknowledgments
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This work was financially supported by the National Natural Science Foundation of China (51133007) and The National Basic Research Program (2009CB623603).
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