Статья 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-
International Scientific Journal for Alternative Energy and Ecology № 06 (128) 2013
© Scientific Technical Centre «TATA», 2013
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 i
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