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8Статья поступила в редакцию 14.06.13. Ред. рег. № 1684 The article has entered in publishing office 14.06.13. Ed. reg. No. 1684
УДК (PACS) 621.311.6
FABRICATION AND STUDY OF ORGANIC PHOTOVOLTAIC CELLS BASED ON THE POLYMER/FULLERENE BULK HETEROJUNCTION
V.A. Gevorkyan, S.G. Grigoryan, A.M. Arzumanyan, E.A. Beghloyan, N.R. Mangasaryan
Russian-Armenian (Slavonic) State University (RAU) Department of Materials Technology and Structures of Electronic Technique 0051, Yerevan, Armenia, Hovsep Emin St., 123, Tel.: +374 91 29 95 20, e-mail: vgev@rau.am
Заключение совета рецензентов 17.06.13 Заключение совета экспертов 19.06.13 Принято к публикации 24.06.13
Results regarding synthesis of two different architectures of organic photovoltaic cells based on the polymer/fullerene bulk heterojunction are reported. The first device consists of front electrode of conducting ITO transparent layer, bulk heterojunction and back reflective electrode. In order to increase the efficiency of the solar cell we improved the architecture of the first device including an additional TiO2 layer between bulk heterojunction and back electrode. The TiO2 layer creates an additional potential barrier for holes, repulsing them back to the opposite electrode and thus improving the charge separation between electrodes. These devices were obtained by spin-coating of P3HT/PCBM blends dissolved in organic solvents on ITO substrates preliminary coated with hole-conducting layer of PEDOT:PSS. For TiO2 layer deposition sol-gel technique was used. The influence of solvent nature, P3HT:PCBM ratio, annealing temperature, and speed of spin coating on the current-voltage characteristics of the photovoltaic devices have been studied. The obtained results have shown that the solar cells efficiency strongly depends on technological conditions of the device structure formation. The conditions that provide device efficiency not less than 3% were found.
Key words: polymer solar cells, P3HT:PCBM, TiO2 layer, heterojunction, photovoltaic.
ПОЛУЧЕНИЕ И ИССЛЕДОВАНИЕ ФОТОВОЛЬТАИЧЕСКИХ ЯЧЕЕК, ОСНОВАННЫХ НА ПОЛИМЕР/ФУЛЛЕРЕН ОБЪЕМНЫХ ГЕТЕРОПЕРЕХОДАХ
В.А. Геворкян, С.Г. Григорян, А.М. Арзуманян, Э.А. Беглоян, Н.Р. Мангасарян
Российско-Армянский (Славянский) государственный университет кафедра "Технологии материалов и структур электронной техники" 0051, Ереван, ул.Овсеп Эмина 123, Тел.: +374 91 29 95 20, e-mail: vgev@rau.am
Referred 17.06.13 Expertise 19.06.13 Accepted 24.06.13
В данной работе сообщается о результатах синтеза двух различных архитектур органических фотовольтаических ячеек, изготовленных на основе полимер/фуллерен объемных гетеропереходов. Первая фотовольтаическая ячейка состоит из верхнего электрода на основе проводящего прозрачного слоя (ITO), объемного гетероперехода и отражающего нижнего металлического электрода. Для увеличения эффективности мы усовершенствовали архитектуру первой солнечной ячейки за счет введения дополнительного слоя TiO2 между объемным гетеропереходом и нижним электродом. Слой TiO2 создает дополнительный потенциальный барьер для дырок, отражая их обратно к противоположному электроду, улучшая тем самым разделение носителей заряда между электродами. Солнечные ячейки были получены путем центрифугирования P3HT/PCBM, растворенной в органическом растворителе, на подложки ITO, предварительно покрытые слоем PEDOT:PSS. Для осаждения слоя TiO2 использовался метод золь-геля. Исследовалось влияние на вольтамперные характеристики солнечных ячеек, соотношения P3HT:PCBM, типа растворителя, температуры термоотжига и скорости центрифугирования. Полученные результаты показали, что эффективность солнечных ячеек сильно зависит от технологических условий формирования приборной структуры. Найдены технологические условия, обеспечивающие эффективность солнечных ячеек не ниже 3%.
Ключевые слова: полимерные солнечные элементы, P3HT:PCBM, слой TiO2, гетеропереход, фотовольтаические ячейки.
Introduction
Photovoltaic cells having active layers based on organic polymers, in particular polymer-fullerene composites are of interest as potential sources of renewable electrical energy [1]. Such cells offer the advantages implied for polymer-based electronics, including low cost fabrication in large sizes and low weight on flexible substrates. This technology enables efficient "plastic" solar cells which would have major impact. Although encouraging progress has been made in recent years with highest 3-4% power conversion efficiencies reported under AM 1.5 however this efficiency is not sufficient to meet reasonable specifications for commercialization. The need to improve the light-to-electricity conversion efficiency requires the new approaches including new materials and new device architectures.
The design of active layer architecture determines by the physical phenomena in polymers. Incident light that is absorbed within the photoactive layer of a polymer solar cell leads first to the creation of an electron-hole pair - the exiton. These exitons diffuse during their lifetime with diffusion lengths generally limited to about 5-20 nm in organic materials [2-6]. If an exition does not eventually separate into electron and hole, it eventually recombines by emitting a photon or via thermalization. Hence, an effective exiton dissociation mechanism is required to separate exitons which have binding energies ranging between 0.1 and 1 eV [7-11]. Exiton dissociation in current polymer solar cells relies on the gradient of potential across a donor/acceptor interface. As a result of the photoinduced charge transfer, the positively charged hole remains on the donor material whereas the electron is located on the acceptor. This is schematically depicted in Fig. 1 for a soluble derivative of poly(paraphenylenevinylene) as donor and C60 as acceptor [12].
Fig. 1. Photoinduced charge transfer from a donor (here PPV) to an acceptor (here C6o) serves as a highly efficient charge separation mechanism in most polymer solar cells [12] Рис. 1. Перенос фотоносителей от донора (здесь PpV) к акцептору (здесь C6o), являющийся высокоэффективным механизмом разделения носителей заряда во многих полимерных солнечных ячейках [12].
Fig. 2. Structure of regioregular "head-to-tail" type
polythiophenes Рис. 2. Структура пространственно-регулярного политиофена типа «голова к хвосту»
The bulk heterojunction composed of blends of low band-gap polymer and soluble derivatives of fullerene have been demonstrated to achieve up to 4% efficiency of conversion solar energy to electricity [13]. The transition of photogenerated electrons from donor (polymer) to acceptor (fullerene) is very efficient in the bulk heterojunction due to its large interface for charge transfer.
Among the candidates of polymers for photovoltaic applications, polythiophenes are found to be very promising due to their low band gap (2,14 eV) and high hole mobility [1].
Polyconjugated polymers on the basis of thiophene and its derivatives of regular structure (Fig.2) show interesting electrical and optical properties, at the same time they have high thermal and good environmental stability.
Conductivity of doped polythophenes is connected with electrons delocalization along the polymer chain and bipolarons formation, where polythophene is a donor and doping agent is an acceptor of electrons (p-doping).
In the polymer donor-acceptor solar cells electron transitions take place at light absorption resulting in the charge separation and free electrons and holes formation and as a result of this electrical current flow. Charge separation efficiency in polythophene based donor-acceptor systems strongly depends on structure, molecular weight and acceptor nature. Developing so called bulk heterojunctions, in which fullerene molecule С6о is an electron acceptor, was a breakthrough in creation of polythophene based plastic solar cells. Due to the high tendency of highly organized nanostructural fullerene particles formation in the polymer matrix, effective charge separation takes place in these systems. Unlike unsubstituted polythiophene, which is soluble only in solvent mixtures of arsenic trifluoride and arsenic pentafluoride, poly(3-butyl-, 3-hexyl- and 3-octylthiophenes) are soluble in common organic solvents such as toluene, o- and p-xylenes, chlorobenzene and usually are used in these systems. A monosubstituted derivative of fullerene С60 with organic radical (Ся) is used, which unlike fullerene itself is readily soluble in the same solvents as poly(3-butyl-, 3-hexyl- and 3-
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octylthiophenes), which makes easy formation of bulk heterojunction as nanosize films from the same solution.
In this work we report our results regarding synthesis of two different architectures of organic photovoltaic cells based on polythiophene/fullerene bulk heterojunction. The first device consists of front electrode of conducting ITO transparent layer, bulk heterojunction and back reflective electrode. In order to increase the efficiency of the solar cell we improved the architecture of the first device including an additional TiO2 layer between bulk heterojunction and back electrode.
Experimental
Regioregular "head-to-tail" type poly(3-hexyl-2,5-diylthiophene) (P3HT) with 45000-65000 average
molecular weight and a fullerene derivative (1-(3-methoxycarbonyl)propyl- 1-phenyl [6,6]C61) (PCBM) were used in our studies for fabrication solar cells on polythiophene/fullerene basis (Fig. 3).
These compounds were purchased from Aldrich Chemical Co. and were used without preliminary purification. All solar cell samples were prepared on glass substrates (25x25mm) coated with indium tin oxide (ITO) conductive layer with ~ 8-12 ^/square surface conductivity (anode). The substrates were cleaned off in ultrasonic bath by deionized water, containing surfactants mixture of non-ionic Synperonic 91/8 and amphoteric Dehyton PK 45 in 1:1 weight ratio, and were washed thoroughly by deionized water after. The substrates were dried at 800C for 1.5-2h.
PEDOT
w [-f—)
P3HT
sor
PSS
SOjH
PCBM
Fig. 3. Structures of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)-(PEDOT-PSS), regioregular poly(3-hexyl-2,5-diylthiophene)-(P3HT) and fullerene derivative- (1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61)-(PCBM). Рис. 3. Структуры поли(3,4-этилендиокситиофен)-поли(стиролсульфоната)- (PEDOT-PSS), региорегулярного поли(3-гексил-2,5-диилтиофена)-(Р3НТ) и производного фуллерена-(1 -(3-метоксикарбонил)пропил-1 -фенил[6,6]-(PCBM).
The cleaned ITO samples were spin coated with PEDOT:PSS from 1.5% water dispersion (Aldrich), which was preliminary filtered off through Schott glass filter with №4 pore size.
The dependence of PEDOT:PSS layer thickness upon the rpm of spin-coating was determined using surface profile meter that was calibrated to a Si-SiO2 ellipsometry standard. The results of these measurements have shown that PEDOT:PSS layers are very uniform and have « 55.5 nm thickness at 5000 rpm (Fig.4).
The PEDOT:PSS coated samples were dried at 1400C for 10 min in air. The P3HT:PCBM active layer of 100180 nm thickness was spin-coated from solution of chlorobenzene or chlorobenzene/nitrobenzene mixture. In the former case chlorobenzene solution consists of 1% wt. P3HT and 0.8% wt. PCBM, in the latter case the
solution consists of 4% wt. nitrobenzene and 1.66% wt. P3HT/PCBM of 3:2 weight ratio. P3HT and PCBM were dissolved by stirring for 1h with magnetic stirrer at 600C. Every time before spin coating the P3HT/PCBM solution was treated in ultrasonic bath for complete homogenization. For obtaining the active layer of various thicknesses, 50 mg of the solution was spin coated with microdosing pipette at 1500-4000 rpm for 60 sec. At drying the colour of the film was gradually changed from brown to golden cherry. The film was dried from 140 to 1750 C for 10 min in N2 flow. The thickness profile of PEDOT-PSS/P3HT :PCBM layer is shown in Fig. 5.
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Fig. 4. Thickness profile of PEDOT:PSS layer Рис. 4. Профиль толщины слоя PED0T:PSS
Fig. 6. Architecture of bulk heterojunction solar cell Рис. 6. Архитектура солнечной ячейки на основе объемного гетероперехода
Fig. 7. Energy diagram of bulk heterojunction (P3HT:PCBM) solar cell
Рис. 7. Энергетическая диаграмма солнечной ячейки на основе объемного гетероперехода (P3HT:PCBM)
Fig. 5. Thickness profile of PED0T:PSS/Р3НТ:PCBM layer Рис. 5. Профиль толщины слоя PED0T:PSS/ Р3НТ:РСВМ
The schematic diagram of the device architecture and energy diagram are shown in Fig.6 and Fig.7 respectively.
In order to increase the efficiency of the solar cell we improved the architecture of the first device including an additional amorphous TiO2 layer between bulk heterojunction and back reflecting electrode (Fig.8).
Fig. 8. Architecture of bulk heterojunction solar cell with hole blocking Ti02 layer Рис. 8. Архитектура солнечной ячейки на основе объемного гетероперехода с запорным для дырок слоем Ti02
The TiO2 layer creates an additional potential barrier for holes, repulsing them back to the opposite electrode and thus improving the charge separation between electrodes (Fig. 9).
Furthermore, TiO2 layer between the active layer and electrode should improve the stability of polymer solar cell because TiO2 layer is expected to inhibit the reaction between the active layer and metallic electrode. The deposition of amorphous TiO2 layer was carried out by sol-gel technique. The active layer after drying in air for 10 min was spin coated at 5000 rpm using 0,2M TiO2 gel. The TiO2 gel was prepared by dissolving 0.568 g of titanium tetraisopropoxide in 9.5 ml of 2-methoxyethanol, containing 0.144 ml of concentrated nitric acid and 0.144 ml of deionized water. After deposition of TiO2 layer the multilayer structure was dried in air for 30 min and heat treated at 120°С-140°С for 10 min. Sample thickness was determined using surface profile meter.
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Fig. 9. Energy diagram of bulk heterojunction P3HT:PCBM/ TiO2 solar cell
Рис. 9. Энергетическая диаграмма солнечной ячейки на основе объемного гетероперехода (P3HT:PCBM)/TiO2
Fig. 10. Photo of 8 solar cells formed on ITO/PEDOT-PSS/Р3НТ:PCBM multilayer structure Рис. 10. Фотография 8 солнечных ячеек на основе многослойной структуры ITO/PEDOT-PSS/РЗНТ^СВМ
Fig. 11. The schematic view of solar cell cross section with electrical pathways Рис. 11. Схематическое изображение поперечного сечения солнечной ячейки с токопроводящими электродами
The metallic electrode on the surface of PEDOT-PSS/P3HT:PCBM (TiO2) layer was vapor- deposited through a mask, leaving 8 solar cells with an active area of 0.035 cm2 (Fig. 10). Different metals (In, Cu, Ag and Al) were tested as electrodes. The schematic view of solar cell cross section with electrical pathways is shown
in Fig. 11. Electrical pathways were connected to the regions of metallic electrode located beyond the polymer layer. This architecture prevents the polymer layer from damage which may occur at connection of electrical pathways.
Discussion and conclusion
Our studies proved other authors observations that efficiency of solar cells to a greater extent depends on structure, morphology, the active layer thickness, presence of protection layer between cathode and the active layer as well as the cathode material. For making the active layer with improved structure and morphology the following factors, such as the solvent nature, concentration and conditions of P3HT/PCBM solution preparation, the active layer spin coating speed, temperature and conditions of films drying are very important. A role of PEDOT-PSS layer between anode and the active layer is not limited only by its high hole conductivity but probably this layer is an active matrix for growing the bulk heterojunction in the form of P3HT/PCBM film of certain structure due to content of the polythiophene derivative (PEDOT). Another important factor is a presence of intermediate active layer between cathode and the active layer. In US patent [14] the TiO2 intermediate layer is formed from gel consists of titanium isopropoxide, 2-methoxyethanol and monoethanolamine of 1:5:0,5 volume ratio, however such gel results in formation of stains on active layer due to partial dissolving and washing off the active layer components. In [15] TiO2 layer is spin coated from ethanol based titanium tetraisopropoxide gel. However the best results are obtained in case of the above described 2-methoxyethanol-H2O-HNO3 -Ti(/-OC3H7)4 sol-gel system. The TiO2 layer creates an additional potential barrier for holes, repulsing them back to the opposite electrode and thus improving the charge separation between electrodes.
As a metallic electrode we have tested Cu, Al, In and Ag. The best results showed solar cells with Ag metallic electrode with thickness ~200 nm.
The solar cells with P3HT:PCBM active layer formed from chlorobenzene solution and chlorobenzene/nitrobenzene mixture have the same efficiency. The solar cells with TiO2 layer showed higher efficiency than without it. The plots of dark and illuminate current versus voltage for two PEDOT-PSS/P3HT:PCBM (TiO2) solar cells that are differed in heat-treatment temperature are shown in Fig.12. The solar illumination intensity was 50 mW/cm2.
Heat-treatment temperature rise from 1400C to 1750C results in both short current and open circuit voltage increasing (Fig. 10). These results have shown that the heat-treatment temperature dramatically influences on the solar cells efficiency. These conditions provide the device efficiency not less than 3%.
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Fig. 12. Dark and illuminate I-V curves of ITO / PEDOT:PSS / P3HT:PCBM /TiO2/ Ag devices. The solar illumination intensity ~50 mW/cm2. a - active layer: 1.7% chlorobenzene/nitrobenzene P3HT:PCBM solution, which was spin-coated at1500 rpm; device was heat-treated at 1750 for 20 min in N2 flow; b - the active layer: 1.7% chlorobenzene/nitrobenzene P3HT:PCBM solution, which
was spin-coated at1500 rpm; device was heat-treated at 1400 for 10 min in N2 flow. Рис. 12. Темновые и световые вольтамперные характеристики фотовольтаической ячейки на основе ITO / PEDOT:PSS / P3HT:PCBM /TiO2/ Ag. Интенсивность солнечного излучения ~50 мВт/см2. а - активный слой: активный слой создавался путем центрифугирования при 1500об/мин 1.7% раствора хлорбензола/нитробензола P3HT:PCBM, который затем подвергался термообработке в атмосфере N2 при температуре 1750С в течение 20 мин; б - активный слой: активный слой создавался путем центрифугирования при 1500об/мин 1.7% раствора хлорбензола/нитробензола P3HT:PCBM, который затем подвергался термообработке в атмосфере N2 при температуре 1400С в течение 10 мин.
References
1. Hoppe H., Sariciftci N.S. // Polymer Solar Cells. Adv. Polymer Sci. DOI 10.1007/12_2007_ 121, Springer-Verlag Berlin Heidelberg. Published online: 17 October 2007.
2. Halls J.M., Pichler K., Friend R.H., Moratti S.C., Holmes A.B. Exciton diffusion and dissociation in a poly(p-phenylenevinylene)/C60 heterojunction photovoltaic cell // Appl. Phys. Lett., 1996, 68:3120.
3. Savanije T.J., Warman J.M., Goossens A. Visible light sensitization of titanium dioxide using a phenylene vinylene polymer // Chem. Phys Lett ,1998, 287:148.
4. Pettersson L.A., Roman L.S., Inganäs O. Modeling photocurrent action spectra of photovoltaic devices based on organic thin films // J. Appl. Phys., 1999, 86:487.
5. Haugeneder A., Neges M., Kallinger C., Spirkl W., Lemmer U., Feldmann J., Scherf U., Harth E., Gügel A., Müllen K. Exciton diffusion and dissociation in conjugated polymer/fullerene blends and heterostructures // Phys. Rev., 1999, B 59:15346.
6. Stoessel M., Wittmann G., Staudigel J., Steuber F., Blässing J., Roth W., Klausmann H., Rogler W., Simmerer J., Winnacker A., Inbasekaran M., Woo E.P. Cathodeinduced luminescence quenching in polyfluorenes // J. Appl. Phys., 2000, 87:4467.
7. Pope M., Swenberg C.E. Electronic processes in organic crystals and polymers. 2nd Edn., Oxford University Press, New York. 1999.
8. Sariciftci N.S. Primary photoexcitations in conjugated polymers: molecular exciton versus semiconductor band model // World Scientific, Singapore. 1997.
9. Chandross M., Mazumdar S., Jeglinski S., Wei X., Vardeny Z.V., Kwock E.W., Miller T.M. Excitons in poly(para-phenylenevinylene) // Phys. Rev., 1994, B 50:14702.
10. Campbell I.H., Hagler T.W., Smith D.L., Ferraris J.P. Direct measurement of conjugated polymer electronic excitation energies using metal/polymer/metal structures // Phys. Rev. Lett., 1996,76:1900.
11. Knupfer M. Exciton binding energies in organic semiconductors // Appl. Phys. A, 2003, 77:623.
12. Sariciftci N.S., Smilowitz L., Heeger A.J., Wudl F. Photoinduced electron transfer from a conducting polymer to buckminsterfullerene // Science, 1992258:1474.
13. Moule A.J., Meerholz K. Controlling morphology in polymer-fullerene mixtures // Adv. Mater, 2008, v.20, P. 240-245.
14. Kwangee Lee, Alan J. Heeger, Architecture for high efficiency polymer photovoltaic cells using an optical spacer, US Pat. Application # 20060292736A1, 12/28/2006.
15. Van Hal P.A., Wienk M.M., Kroon J.M., et al. Photoinduced electron transfer and photovoltaic response of a MDMO-PPV:TiO2 bulk-heterojunction // Adv. Mater., 2003, v.15, p. 118-121.
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