Polymer-Based Solar Cells

Polymer-based solar cells, also known as organic photovoltaic cells, have been around since the 1990s. But their performance, and their efficiency by end of 2010, to convert light to electricity, after much research, has reached only approximately 8%. This is not good enough to compete with inorganic solar cells, like those based on cadmium teluride, that convert 10–15% of light to electricity. This single digit value of organic solar cells pales even further when compared with some highly specialized, high-priced state-of-the-art inorganic devices with conversion efficiencies topping 40%. The promise of low-cost organic solar cells, however, has encouraged intense research in many laboratories in efforts to improve the efficiency. Such research usually focuses on solution processable organic polymers that can be converted to semiconductors [262]. The polymers used are regarded as intrinsic wide band gap semiconductors, where the band gaps are above 1.4 eV. This can be compared to insulators, where the band gaps are below 3 eV. Doping of the film forming materials is done to introduce extrinsic charge carriers and convert them into organic semiconductors. Such charge carriers, as explained in Sect. 10.2, can be positive, p-type, or negative, n-type. Originally, a donor-acceptor bilayer device of two films was used as an n–p junction in solar cells. Thus, they were fabricated as sandwich structures. An example would be one where a transparent substrate is first coated with a conductor, like indium-tin oxide. A conducting polymer like, poly (ethylene dioxythiphene), doped with polystyrene-sulfonic acid, would then be applied from and aqueous solution. The indium-tin oxide actsas an electrode for hole injection or extraction. The polymer is then covered with a conductor, an aluminum foil. The doped polymer can be illustrated as follows:

The construction of the above-described solar cell can be illustrated as follows:

The donor material, containing a chromophore, absorbs the light energy and generates excitons. Excitons are high-energy couples where the energetic electrons are bound to positively charged electron vacancies or holes. To produce electric current, the electron-hole pairs must migrate to the interface between the electron donor and electron acceptor materials. Upon reaching the interface, the electron-hole pairs splits into separate mobile charges. The charges then diffuse to their respective electrodes. The electrons are transported by the electron-accepting material to the cathode and the holes by the hole-accepting material (electron donor) to the anode. To put it in other words, the Coulomb-correlated electron-hole pair, the excitons, diffuse to the donor-acceptor interface where exciton dissociation occurs via an electron-transfer process to the n-type layer. With the aid of an internal electric field, the n-type layer then carriers the electrons in the opposite direction. The electric field in turn generates the photocurrent and the photo voltage. Such devices are known as planar heterojunction cells. Such an arrangement, however, is not very efficient, because the excitons can decay back to the ground level before they diffuse into to the n-type layer. To overcome the difficulty, the concept of a bulk heterojunction was introduced [263]. By blending donor and acceptor materials together an interpenetrating bicontinuous network of junctions, large donor-acceptor interfacial areas can be achieved. This results in an enhanced quantum efficiency of charge separation and in efficient charge collection. Gaudiana [264] likened the morphology of a bulk heterojunction active layer to a sponge. The solid part represents the nano-sized interconnected bits of acceptors. The polymer is represented by the holes that are intimately connected to other holes throughout the sponge and never far from a solid region. Blending the phases on that scale, in effect, distributes small regions of interface throughout the photoactive layer. As a result, excitons need only to diffuse only a short distance before quickly reaching a donor–acceptor interface where they can dissociate into separate charges. An advancement in efficiency of polymeric solar cells, from 3 to 5%, came in 2009 when it was observed that promising efficient charge transfer materials can be prepared from combinations of poly (alkyl-thiophenes) donors with 1-(3-methoxycarbonyl) propyl-1phenyl-[6,6]-methanofullere acceptors [265]. Mild heating disperses the acceptor molecules among the donor molecules:

This led to exploration of many other combinations of various other polymers with different derivatives of fullerine and with various chromophores. In an attempt to lower highest occupied molecular orbitals (HMO) of the polymer with stronger electron-withdrawing groups, new polymers were developed. The results were summarized in a review [265].

As a result of the research, several research groups reported attaining 5% efficiencies with the combinations of poly (3-hexyl thiophene) with 1-(3-methoxycarbonyl) propyl-1phenyl- [6,6] methanofullere. Numerous other conducting polymers including copolymers containing fluorene, carbazole, cyclopentadithiophene were investigated. The efficiency of organic solar cells is usually defined as:

where Pin is the input energy of solar radiation. The output short circuit current density is J_sc and Voc is the open circuit voltage. FF is the fill factor. Much of the research effort to date has been based on attempts to increase Voc. This is based on empirical correlation between the magnitude of the open circuit voltage and the difference in energies between HMO of the donor and LUMO of the acceptor. It is expected that by lowering the HMO of the donor, Voc can be increased [266]. To achieve this goal, two research teams headed by Yu and by Yang tested a series of copolymers prepared by reacting a benzodithiophene derivative with various thienothiophenes. The aim was to lower the polymers’ HMO by attaching successively stronger electron-withdrawing groups to the polymer backbone. The result was that by replacing an alkoxy group that was adjacent to a carbonyl group with an alkyl chain at the same position, the group lowered the HOMO level by roughly 0.1 eV. They lowered the level by another 0.1 eV by adding a fluorine atom. Solar cells prepared with this polymer were found to be 6.8% efficient [267]. Subsequently, Yu et al. reported slightly improved conversion efficiency of over 7% [268]. It is interesting that at the time of the publication of the review, it was reported in Chem. and Eng News [269] that Heeger and Gong developed a broad-spectrum donor acceptor combination that can detect photons throughout the whole light spectrum, from the ultra-violet to the infra-red. The combination of the two materials can be illustrated as follows:

According to a subsequent write up in Chem. and Eng. News [270], a private laboratory called “Solamer” claimed, without disclosing details, to have achieved efficiency of 8.13%. They also stated that they hope to achieve efficiency of 10% by end of 2011. Syntheses of various polymers for solar cells were also reviewed by Cheng et al. [271] where they point out that there is a need to develop better p-type materials that have good film properties and act as very efficient chromophores, with good hole mobility and suitable molecular orbitals levels. They also point out that magnitude of the band gap and the energy positions of the HOMO and LUMO energy levels are the most important characteristics for determining the optical and electrical properties of a given conjugated polymer. These, of course, will in turn greatly influence the ultimate photovoltaic performance and conversion of light energy to electrical energy. The wavelength of the maximum photon flux density of the solar spectrum is located at approximately 700 nm, which corresponds to a low energy of 1.77 eV. The absorption spectrum of a conjugated polymer should cover both the red and near-infra-red ranges to match the greater part of the terrestrial solar spectrum and absorb the maximum photon flux. Thus, it is highly desirable to develop conjugated polymers with broader absorptions through narrowing their optical band gap. At the same time, these materials must efficiently absorb light. The overall high extinction coefficients of the polymers are also of critical importance. Following are presented some of the published results from recent and current research, to illustrate the bulk of the effort in the field. The examples are chosen at random and there is no implication that these are the best ones published to date. Li and coworkers [272] reported synthesis and photovoltaic properties of three donor-acceptor copolymers containing bithiazole acceptor. One of them was illustrated as follows:

The other two copolymers were similar. The copolymer that yielded the best results had the carbazole replaced with the following molecule:

The results indicate that donor units of carbazole influence the band gaps, electronic energy levels, and photovoltaic levels. The hole mobility was measured at 3.07x10^-4 cm²/V s. Honda et al. [273] reported injecting a photosensitizer dye into a bulk heterojunction solar cell, based on regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl) propyl-1phenyl- [6,6] methanofullere. The dye photosensitizer was illustrated as follows:

They reported that injection of the dye resulted in an increase in the photocurrent. Hiorns et al. [274] reported preparation of a block copolymer that incorporated fullerene molecules into the backbone of the polymer. They observed a band gap of 2.3 and 2.2 eV for the block copolymer:

Li and coworkers [275] reported syntheses of four alternating copolymers of carbazole and triphenylamine with conjugated side chain acceptor groups:

The four copolymers that were synthesized contained different acceptor end groups, aldehyde, monocyano, dicyano, and 1,3-diethyl-2-thiobarbituric acid. Through changing the acceptor groups, the electronic properties and energy levels of the copolymers were effectively tuned. Their results indicate that it is an effective approach to tuning the bandgaps in conjugated polymers. The polymers were used as donors in polymer solar cells. They reported, however, conversion efficiency of only 2.76%. You and coworkers [276] reported syntheses of two low-band gap polymers based on benzo (1,2 b:4,5-b) dithiophene:

where R₁ = 3-butylnonyl and R₂ = nonyl and in the send polymer, R₁ = 3-hexylundecyl and R₂ = hydrogen. Both polymers were reported to have performed well in preliminary bulk heterojunction solar cells, reaching power conversion efficiency greater than 4%. Jenekhe, Watson, and coworkers [277] reported synthesizing three new donor-acceptor conjugated polymers incorporating thieno[3,4-c] pyrrole-4,6-dione acceptor and dialkoxybithiophene or cyclopentadithiophene donor units. The thieno[3,4-c] pyrrole-4,6-dione acceptor containing materials were studied in bulk heterojunction solar cells and organic field-effect transistors. The polymers had optical band gaps of 1.50–1.70 eV. The highly electron-rich character of dialkoxybithiophene in these polymers, however, destabilizes their HMO and significantly affects the photovoltaic efficiency with power conversion efficiencies below 1.5%. On the other hand, cyclopentadithiophene copolymers achieved a better power conversation efficiency greater than 3%. Sharma and coworkers [278] reported synthesis of two low-band gap copolymers. One consists of alternating dihexyloxyphenylene and a-[4-(diphenylamine) phenyl methylene]-4-nitrobenzene aceto nitrile. The other one consists of alternating dihexyloxyphenylene and a,a0-[(1,4-phenylene) dimethylidyne] bis (-4-nitrobenzene acetonitrile):

They reported that these copolymers showed broad absorption curves with long-wavelength absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for both polymers. Both polymers were studied for photovoltaic response in bulk heterojunction solar cells. They observed an overall power conversion efficiency of 3.15 and 2.60% for the cast polymers. Further improvement led up to 4.06 and 3.35% for the devices based on thermally annealed materials. Wei and coworkers [279] used Stille polycondensation to prepare a series of low-band gap copolymers, by conjugating the electron-accepting pyrido[3,4-b] pyrazine moieties with electron rich benzo[1,2-b:3,4-b] di thiophene or cyclopentadithiophene units. All resulting polymers exhibited excellent thermal stability and sufficient energy offsets for efficient charge transfer and dissociation. The band gaps of the polymers could be tuned in the range 1.46–1.60 eV by using the two different donors, which have different electron-donating abilities. The three-component copolymers, incorporating the thiophene and bithiophene segments, respectively, absorbed broadly, covering the solar spectrum from 350 to 800 nm. The best device performance resulted in power conversion efficiency of 3.15%. The polymeric materials were illustrated as follows:

Liu et al. [280] synthesis and evaluation of n-conjugated copolymers were based on a soluble electro active benzo[1,2-b:4,5-b] difuran chromophore. The comonomer units consisted of thiophene/ benzo [c][1,2,5] thiadiazole/9-phenylcarbazole. These copolymers cover broad absorption ranges from 250 to 700 nmwithnarrow optical band gaps of 1.71–2.01 eV. The band gaps and the molecular electronic energy levels can be tuned by copolymerizing the benzo[1,2-b:4,5-b] difuran core with different n-conjugated electron-donating or withdrawing units in different ratios.

Bulk heterojunction solar cell devices were fabricated by Liu and coworkers, using the copolymers as the electron donor and ([6,60]-phenyl-C₆₁-butyric acid methyl ester) as the electron acceptor. The preliminary research has revealed power conversion efficiencies of 0.17–0.59% under AM 1.5 illumination (100 mW/cm2). Thompson and coworkers [281] point out that despite the correlation between the absorption and JSC, most polymers used in currently high-performing solar cells have limited absorption breadths and rely largely on band gaps. As a result, there is a heavy reliance on fullerenes (especially on PC61BM,1-(3-methoxycarbonyl) propyl-1phenyl-[6,6]-methanofullere) to absorb photons in the short wavelength range and considerable absorption losses in the longer wavelengths. Although fullerenes absorb in the short wavelength region and are thus complementary to many polymers, they point to the evidence that 60% of excitons formed in the PC61BM phase decay before being harvested and do not contribute to JSC [282]. This research group mixed graphene oxide that acts as a surfactant with fullerenes C₆₀ and single-walled carbon nanotubes in water, coated a glass slide with the solution, and heated it to reduce the graphene oxide to graphene. They claimed that chips using this photovoltaic layer were much more efficient at converting light into electricity that the organic devices developed with covalent chemistry. Simultaneous to doing research on polymeric materials, research is also being carried out on improving the construction of solar cells. One strategy is to stack two light-absorbing materials in a tandem cell to harvest greater fraction of the solar spectrum. By inserting gold nanoparticles between the two layers, Yang and coworkers coupled the two tandem cells and demonstrated a considerable boost inefficiency of light conversion to electricity [283].

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يواصل أعمال الإدامة لأحد أبواب حرم مرقد أبي الفضل العباس (عليه السلام)

ضمن فعاليات ملتقى النورين.. المجمع العلمي يقيم ندوة فكرية في بابل

المجمع العلمي يحيي ليلة القدر الأولى في مقام الإمام المهدي (عجّل الله فرجه)

مركز السيد جعفر الحلي الطبي يقدم الاستشارات والخدمات الصحية في ليالي القدر المباركة

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Photo-Conducting Polymers That Are Not Based on Carbazole

Photoconductive Polymers Based on Carbazole

Photo-Conducting Polymers

Liquid Crystalline Alignment

Application to Optical Data Storage

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams