[show abstract][hide abstract] ABSTRACT: Atomic layer deposition (ALD) was used to fabricate Al(2)O(3) recombination barriers in solid-state dye-sensitized solar cells (ss-DSSCs) employing an organic hole transport material (HTM) for the first time. Al(2)O(3) recombination barriers of varying thickness were incorporated into efficient ss-DSSCs utilizing the Z907 dye adsorbed onto a 2 μm-thick nanoporous TiO(2) active layer and the HTM spiro-OMeTAD. The impact of Al(2)O(3) barriers was also studied in devices employing different dyes, with increased active layer thicknesses, and with substrates that did not undergo the TiCl(4) surface treatment. In all instances, electron lifetimes (as determined by transient photovoltage measurements) increased and dark current was suppressed after Al(2)O(3) deposition. However, only when the TiCl(4) treatment was eliminated did device efficiency increase; in all other instances efficiency decreased due to a drop in short-circuit current. These results are attributed in the former case to the similar effects of Al(2)O(3) ALD and the TiCl(4) surface treatment whereas the insulating properties of Al(2)O(3) hinder charge injection and lead to current loss in TiCl(4)-treated devices. The impact of Al(2)O(3) barrier layers was unaffected by doubling the active layer thickness or using an alternative ruthenium dye, but a metal-free donor-π-acceptor dye exhibited a much smaller decrease in current due to its higher excited state energy. We develop a model employing prior research on Al(2)O(3) growth and dye kinetics that successfully predicts the reduction in device current as a function of ALD cycles and is extendable to different dye-barrier systems.
Physical Chemistry Chemical Physics 07/2012; 14(35):12130-40. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Conjugated polymers with nearly planar backbones have been the most commonly investigated materials for organic-based electronic devices. More twisted polymer backbones have been shown to achieve larger open-circuit voltages in solar cells, though with decreased short-circuit current densities. We systematically impose twists within a family of poly(hexylthiophene)s and examine their influence on the performance of polymer:fullerene bulk heterojunction (BHJ) solar cells. A simple chemical modification concerning the number and placement of alkyl side chains along the conjugated backbone is used to control the degree of backbone twisting. Density functional theory calculations were carried out on a series of oligothiophene structures to provide insights on how the sterically induced twisting influences the geometric, electronic, and optical properties. Grazing incidence X-ray scattering measurements were performed to investigate how the thin-film packing structure was affected. The open-circuit voltage and charge-transfer state energy of the polymer:fullerene BHJ solar cells increased substantially with the degree of twist induced within the conjugated backbone--due to an increase in the polymer ionization potential--while the short-circuit current decreased as a result of a larger optical gap and lower hole mobility. A controlled, moderate degree of twist along the poly(3,4-dihexyl-2,2':5',2''-terthiophene) (PDHTT) conjugated backbone led to a 19% enhancement in the open-circuit voltage (0.735 V) vs poly(3-hexylthiophene)-based devices, while similar short-circuit current densities, fill factors, and hole-carrier mobilities were maintained. These factors resulted in a power conversion efficiency of 4.2% for a PDHTT:[6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) blend solar cell without thermal annealing. This simple approach reveals a molecular design avenue to increase open-circuit voltage while retaining the short-circuit current.
Journal of the American Chemical Society 03/2012; 134(11):5222-32. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Regioregular pentacene-containing polymers were synthesized with alkylated bithiophene (BT) and cyclopentadithiophene (CPDT) as comonomers. Among them, 2,9-conjugated polymersPnBT-2,9 and PnCPDT-2,9 achieved the best performance in transistor and photovoltaic devices respectively. The former achieved the most highly ordered structures in thin films, yielding ambipolar transistor behavior with hole and electron mobilities up to 0.03 and 0.02 cm2 V−1 s−1 on octadecylsilane-treated substrates. The latter achieved photovoltaic power conversion efficiencies up to 0.33%. The impact of regioregularity and direction of conjugation-extension (2,9 vs. 2,10), on thin-film order and device performance has been demonstrated for the pentacene-containing polymers for the first time, providing insight towards future functional material design.
Journal of Materials Chemistry 02/2012; 22(10):4356-4363. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We demonstrate that poly(3,4-dialkylterthiophenes) (P34ATs) have comparable transistor mobilities (0.17 cm(2) V(-1) s(-1)) and greater environmental stability (less degradation of on/off ratio) than regioregular poly(3-alkylthiophenes) (P3ATs). Unlike poly(3-hexylthiophene) (P3HT), P34ATs do not show a strong and distinct π-π stacking in X-ray diffraction. This suggests that a strong π-π stacking is not always necessary for high charge-carrier mobility and that other potential polymer packing motifs in addition to the edge-on structure (π-π stacking direction parallel to the substrate) can lead to a high carrier mobility. The high charge-carrier mobilities of the hexyl and octyl-substituted P34AT produce power conversion efficiencies of 4.2% in polymer:fullerene bulk heterojunction photovoltaic devices. An enhanced open-circuit voltage (0.716-0.771 eV) in P34AT solar cells relative to P3HT due to increased ionization potentials was observed.
Journal of the American Chemical Society 10/2011; 133(42):16722-5. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: For organic semiconductors to find ubiquitous electronics applications, the development of new materials with high mobility and air stability is critical. Despite the versatility of carbon, exploratory chemical synthesis in the vast chemical space can be hindered by synthetic and characterization difficulties. Here we show that in silico screening of novel derivatives of the dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene semiconductor with high hole mobility and air stability can lead to the discovery of a new high-performance semiconductor. On the basis of estimates from the Marcus theory of charge transfer rates, we identified a novel compound expected to demonstrate a theoretic twofold improvement in mobility over the parent molecule. Synthetic and electrical characterization of the compound is reported with single-crystal field-effect transistors, showing a remarkable saturation and linear mobility of 12.3 and 16 cm(2) V(-1) s(-1), respectively. This is one of the very few organic semiconductors with mobility greater than 10 cm(2) V(-1) s(-1) reported to date.
[show abstract][hide abstract] ABSTRACT: Five new donor−acceptor copolymers containing the electron acceptor benzothiadiazole (BTZ) linked to the electron donors fluorene (FL) or cyclopentadithiophene (CPDT) via vinylene units were synthesized to study polymer structure−property relationships in organic photovoltaic devices. Both alternating (P) and random copolymers (P1−P4) were prepared via Suzuki and Stille polycondensations, respectively. The cyclopentadithiophene copolymers (P2 and P4) have smaller electrochemical band gaps (1.79 and 1.64 eV) compared to the fluorene-containing copolymers (2.08 and 1.95 eV for P1 and P3). However, the presence of CPDT raises the electrochemical HOMO energy levels (−4.83 and −4.91 eV for P2 and P4) compared to the FL copolymers (−5.06 and −5.15 eV for P1 and P3) leading to small open circuit voltages (Voc) in solar cells. The primary solution and thin-film UV−vis absorption peaks of P3 and P4, which do not contain alkylated thiophenes appended to the BTZ unit, are at lower energy and have larger absorption coefficients than their P1 and P2 counterparts. Detailed theoretical analyses of the geometric structure, electronic structure, and excited-state vertical transitions using density functional theory provide direct insight into the interplay between the structural modifications and resulting electronic and optical changes. A high molecular weight (Mn = 25 kg/mol) polymer with a large degree of polymerization (DPn = 21) was easily achieved for the random copolymer P1, leading to thin films with both a larger absorption coefficient and a larger hole mobility compared to the analogous alternating polymer P (Mn = 22 kg/mol, DPn = 18). An improved short circuit current and a power conversion efficiency up to 1.42% (Jsc = 5.82 mA/cm2, Voc = 0.765 V, and FF = 0.32) were achieved in bulk heterojunction solar cells based on P1.
[show abstract][hide abstract] ABSTRACT: Thiophene enriched fused-aromatic thieno[3,4-b]pyrazine systems were designed and employed to produce low band gap polymers (Eg = 1.0-1.4 eV) when copolymerized with fluorene and cyclopentadithiophene. The copolymers are mainly investigated for organic thin film transistor and organic photovoltaic applications. Molecular packing in the thin films of these polymers was investigated using Grazing incidence X-ray Scattering. Although both fluorene and cyclopentadithiophene polymers follow similar face to face [small pi]-[small pi] stacking, the latter polymers show much smaller lamellar d-spacings due to side-chain interdigitation between the lamellae. This lead to the higher charge carrier mobilities in cyclopentadithiophene polymers (up to 0.044 cm2/V.s) compared to fluorene polymers (up to 8.1 [times] 10-3 cm2/V.s). Power conversion efficiency of 1.4% was achieved using fluorene copolymer in solar cells with a fullerene derivative as an acceptor. Although the cyclopentadithiophene polymers show lower b
Journal of Materials Chemistry 01/2010; 20(28):5823-5834. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recent development of a fused aromatic thieno[3.4-b]pyrazine system and their application in optoelectronic devices are reviewed. Introduction of a fused aromatic unit followed by side chain engineering, dramatically enhanced the charge carrier mobility in thin film transistor devices and mobilities up to 0.2 cm2/Vs were achieved. The optoelectronic properties of these fused aromatic thienopyrazinepolymers (Eg = 1.3 to 1.6 eV, HOMO = −4.9 to −5.2 V) were tuned by introduction of various fused aromatic rings within thienopyrazine. By balancing the fundamental properties of these polymers, both high charge carrier mobilities and moderate PCEs in solar cells were achieved. Further, effects of copolymerizing units are discussed. Low band gap semiconducting polymer (Eg 1 eV) with high field effect mobility (0.044 cm2/Vs) was obtained using cyclopentadithiophene as copolymerizing unit. Finally, a molecular design approach to enhance the absorption coefficients is discussed, which resulted in improved power conversion efficiency in bulk heterojunction solar cells.
Journal of Materials Chemistry 01/2010; · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the performance of low-bandgap polymers with a new ACTP acceptor in organic transistors (max. field-effect mobility 0.2 cm 2 V À1 s À1), and solar cells (max. efficiency 1.4%). Semiconductive polymers may offer a route to photovoltaic and electronic function when characteristics such as light weight, flexi-bility, and low-temperature processing are required, as in bendable displays, solar panels, and smart textiles. 1,2 Key semiconductor material requirements for solar cells and transistors are: 3 (i) high optical absorption at wavelengths >500 nm, where solar irradiance peaks; 4,5 (ii) high field-effect mobility (m) >0.1 cm 2 V À1 s À1 , 6,7 and (iii) low-cost processing. 8 We report the performance of donor–acceptor copolymers designed to achieve these properties. A donor–acceptor structure reduces optical bandgap, enables absorption down to the near infrared, 9–12 and may raise power conversion efficiency (PCE) in solar cells through increased photocurrent. 4,13 Benzothiadiazoles 14–18 and thienopirazines 19–26 are commonly used as acceptors. Of these, thienopyrazine (TP) is advantageous because it allows modification of the pyrazine ring without introducing steric effects directly into the polymer backbone. Thus, alkyl and aryl groups can be added at positions 2 and 3 to tune structural and electronic properties of the acceptor. In this work we introduce a rigid naphthalene moiety fused to the 2 and 3 TP positions. This gives a planar, electron rich p-face of acenaphtho[1,2-b]thieno[3,4-e]pyrazine (ACTP), which may promote p–p stacking between polymer chains and lead to improved charge carrier mobility. ACTP monomer M1 was prepared from reaction of 3,4-dia-minothiophene dihydrochloride and acenaphthenequinone, followed by bromination. 27 ACTP-fluorene copolymers (ACTP-F) were made via Suzuki coupling (Scheme 1). 28 Five polymers were synthesized with different molecular weights (Mw) and side chains to study the effects of these variables on performance. The purified, dark-green powders were characterized by gel permeation chromatography (Table 1), differential scanning calorimetry, UV-vis absorption, and photoelectron spectroscopy, and have a low bandgap of ca. 1.6 eV, with an absorption onset of ca. 800 nm. We fabricated top-contact organic thin-film transistors (OTFT) from 1–5 to measure mobility. We drop-cast $30 nm polymer films on heavily-doped n-type Si wafers with SiO 2 (300 nm) treated with octadecyltrimethoxysilane (OTS). Films were annealed at different temperatures, and gold electrodes were deposited through shadow masks. Annealing at 200 C in N 2 yielded the best performance; lower temperatures gave lower mobilites, and higher temperatures caused film dewetting. Devices fabricated on phenyltrimethoxysilane treated surfaces did not dewet upon annealing, but their mobility was much lower. Polymers 1–5 show p-channel transport, with mobility as high as 0.2 cm 2 V À1 s À1 , of the same order of magnitude as the best polymer semiconductors reported. 29–34 The highest mobility was obtained with 1. Fig. 1 displays electrical characteristics of these OTFTs, under inert (a and c) and ambient conditions (b and d). There is no significant threshold voltage shift with changing environment, but mobility is higher in N 2 (Fig. 1 a and b). The output traces (Fig. 1 c and d) show somewhat late saturation and do not scale as expected in N 2 , but improve in air. This behavior was found in all polymers, and may be Scheme 1 Preparation of ACTP-F copolymers.
Journal of Materials Chemistry 01/2009; 19(5). · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Removing the adjacent thiophenegroups around the acceptor core in low band gap polymers significantly enhances solar cell efficiency through increasing the optical absorption and raising the ionization potential of the polymer.
Journal of Materials Chemistry 01/2009; · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Exciton harvesting is of fundamental importance for the efficient operation of organic photovoltaic devices. The quantum efficiencies of many organic and hybrid organic- inorganic devices are still limited by low exciton harvesting efficiencies. This problem is most apparent in planar heterostructures that suffer from a direct tradeoff between light absorption and exciton harvesting. One approach to overcome small diffusion lengths is the use of triplet excitons. We have an ongoing project investigating pentacene/C60 solar cells to determine if triplets are the dominant exciton species following photoexcitation. Simulations of exciton harvesting suggest that excitons in pentacene are triplets. These triplets are likely formed by an exciton fission route which further has implications for beating the Shockley-Queisser limit and experiments are ongoing to verify this. In addition to using triplets to harvest excitons over long distances, we have developed a new scheme using long range resonant energy transfer to harvest singlet excitons over 25 nm away from the donor-acceptor interface in organic solar cells using resonant energy transfer. These results represent dramatic improvements over our previous findings and show that this scheme holds promise for the future design of highly efficient organic photovoltaics. We present theory and experiment demonstrating a scheme to harvest singlet excitons over 25 nm away from a donor-acceptor interface using resonant energy transfer. Improvement in materials choice could yield effective diffusion lengths as large as 40 nm using long-range transfer, while minimizing the energy loss to less than 0.1 eV making this a promising approach for developing highly efficient organic photovoltaics. Finally, we have successfully synthesized several new low-band gap polymers. Several of these polymers have bandgap of 1.7 eV or lower. Evaluation of the performance of these new materials is underway.