Rational Design of High Performance Conjugated Polymers for Organic Solar Cells

A new class of organic photovoltaic materials: poly(rod-coil) polymers having alternative conjugated and non-conjugated segments

A new class of organic photovoltaic donor materials, poly(rod-coil) polymers, has been proposed and studied.

Design of donor–acceptor star-shaped oligomers for efficient solution-processible organic photovoltaics

This contribution describes recent progress in the design, synthesis and properties of solution-processible star-shaped oligomers and their application in organic photovoltaics. Even though alternative chemistry has been used to design such oligomers, the most successful approach is based on a triphenylamine donor branching center, (oligo)thiophene conjugated spacers and dicyanovinyl acceptor groups. These are mainly amorphous low band-gap organic semiconductors, though crystalline or liquid crystalline ordering can sometimes be realized. It was shown that the solubility, thermal behavior and structure of such molecules in the bulk strongly depend on the presence and position of alkyl groups, as well as on their length. The photovoltaic properties of solution-processed molecules of this type are now approaching 5% which exceeds those of vacuum-sublimed devices. The design rules and future perspectives of this class of organic photovoltaic molecules are discussed.

A triple bond side-chained 2D-conjugated benzodithiophene based photovoltaic polymer

High molecular weight triple bond side-chained benzodithiophene polymer solar cells show a high open circuit voltage of nearly 1 V.

High-crystalline medium-band-gap polymers consisting of benzodithiophene and benzotriazole derivatives for organic photovoltaic cells

Two semiconducting conjugated polymers were synthesized via Stille polymerization. The structures combined unsubstituted or (triisopropylsilyl)ethynyl (TIPS)-substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5-b']dithiophene (BDT) as a donor unit and benzotriazole with a symmetrically branched alkyl side chain (DTBTz) as an acceptor unit. We investigated the effects of the different BDT moieties on the optical, electrochemical, and photovoltaic properties of the polymers and the film crystallinities and carrier mobilities. The optical-band-gap energies were measured to be 1.97 and 1.95 eV for PBDT-DTBTz and PTIPSBDT-DTBTz, respectively. Bulk heterojunction photovoltaic devices were fabricated and power conversion efficiencies of 5.5% and 2.9% were found for the PTIPSBDT-DTBTz- and PBDT-DTBTz-based devices, respectively. This difference was explained by the more optimal morphology and higher carrier mobility in the PTIPSBDT-DTBTz-based devices. This work demonstrates that, under the appropriate processing conditions, TIPS groups can change the molecular ordering and lower the highest occupied molecular orbital level, providing the potential for improved solar cell performance.

Influence of charge carrier mobility and morphology on solar cell parameters in devices of mono- and bis-fullerene adducts

Herein, we analyze charge carrier mobility and morphology of the active blend layer in thin film organic solar cells and correlate them with device parameters. A low band gap donor-acceptor copolymer in combination with phenyl-C61-butyric acid methyl ester (PCBM) or two bis-adduct fullerenes, bis-PCBM and bis-o-quino-dimethane C60 (bis-oQDMC), is investigated. We study the charge transport of polymer:fullerene blends in hole- and electron-only devices using the space-charge limited current method. Lower electron mobilities are observed in both bis-adduct fullerene blends. Hole mobility, however, is decreased only in the blend containing bis-oQDMC. Both bis-adduct fullerene blends show very high open circuit voltage in solar cell devices, but poor photocurrent compared to the standard PCBM blend for an active layer thickness of 200 nm. Therefore, a higher short circuit current is feasible for the polymer:bis-PCBM blend by reducing the active layer thickness in order to compensate for the low electron mobility, which results in a PCE of 4.3%. For the polymer:bis-oQDMC blend, no such improvement is achieved due to an unfavorable morphology in this particular blend system. The results are supported by external quantum efficiency measurements, atomic force microscopy, transmission electron microscopy and UV/vis spectroscopy. Based on these results, the investigations presented herein give a more scientific basis for the optimization of solar cells.

Efficiency of bulk-heterojunction organic solar cells

During the last years the performance of bulk heterojunction solar cells has been improved significantly. For a large-scale application of this technology further improvements are required. This article reviews the basic working principles and the state of the art device design of bulk heterojunction solar cells. The importance of high power conversion efficiencies for the commercial exploitation is outlined and different efficiency models for bulk heterojunction solar cells are discussed. Assuming state of the art materials and device architectures several models predict power conversion efficiencies in the range of 10-15%. A more general approach assuming device operation close to the Shockley-Queisser-limit leads to even higher efficiencies. Bulk heterojunction devices exhibiting only radiative recombination of charge carriers could be as efficient as ideal inorganic photovoltaic devices.

Precise Synthesis of Poly(fluorene vinylene)s Capped with Chromophores: Efficient Fluorescent Polymers Modified by Conjugation Length and End-Groups

Poly(9,9-di-n-octylfluorene-2,7-vinylene)s containing F-BODIPY [PFV-(F-BODIPY)₂] and Zn(II)-porphyrin have been prepared by olefin metathesis of the vinyl group in the PFV chain ends followed by subsequent Wittig-type coupling. An introduction of tris(2,5-dialkoxy-1,4-phenylene vinylene) into PFV-(F-BODIPY)₂ has also been achieved. The emission properties can be modified by the PFV chain length, chromophore, and the middle segment: PFV-(F-BODIPY)₂ exhibits white-light emission with high quantum efficiency.

N-acyldithieno[3,2-b:2',3'-d]pyrrole-based low-band-gap conjugated polymer solar cells with amine-modified [6,6]-phenyl-C61-butyric acid ester cathode interlayers

Efficient low-band-gap polymers are one key component for constructing tandem solar cells with other higher-band-gap materials to harvest wide absorption of the solar spectrum. The N-acyldithieno[3,2-b:2',3'-d]pyrrole (DTP) building block is used for making low-band-gap polymers. It is attractive because of its strong donating ability and relatively low highest-occupied-molecular-orbital level in comparison with the N-alkyl DTP building block. However, additional solubilizing groups on the accepting units are needed for soluble donor-acceptor polymers based on the N-alkanoyl DTP building block. Combining N-benzoyl DTP with a 4,7-dithieno-2,1,3-benzothiadiazole building block, a polymer with a low band gap of 1.44 eV, delivers a high short-circuit current of 17.1 mA/cm(2) and a power conversion efficiency of 3.95%, which are the highest for the devices with DTP-containing materials. Herein, an alcohol-soluble diamine-modified fullerene cathode interfacial layer improved the device efficiency significantly more than the mono-amine analogue.

Quantitative structure-property relationship modeling of Grätzel solar cell dyes

With fossil fuel reserves on the decline, there is increasing focus on the design and development of low-cost organic photovoltaic devices, in particular, dye-sensitized solar cells (DSSCs). The power conversion efficiency (PCE) of a DSSC is heavily influenced by the chemical structure of the dye. However, as far as we know, no predictive quantitative structure-property relationship models for DSSCs with PCE as one of the response variables have been reported. Thus, we report for the first time the successful application of comparative molecular field analysis (CoMFA) and vibrational frequency-based eigenvalue (EVA) descriptors to model molecular structure-photovoltaic performance relationships for a set of 40 coumarin derivatives. The results show that the models obtained provide statistically robust predictions of important photovoltaic parameters such as PCE, the open-circuit voltage (V(OC)), short-circuit current (J(SC)) and the peak absorption wavelength λ(max). Some of our findings based on the analysis of the models are in accordance with those reported in the literature. These structure-property relationships can be applied to the rational structural design and evaluation of new photovoltaic materials.

Acceptor-donor-acceptor-based small molecules with varied crystallinity: processing additive-induced nanofibril in blend film for photovoltaic applications

A series of acceptor-donor-acceptor-based small molecules (SMs) with varied crystallinity were successfully synthesized. The processing additive can induce the SMs to self-organize as nanofibrils with higher crystallinity and controlled scales of nanofibrils, which have significant influence on the photovoltaic performance.

Rational design of benzotrithiophene-diketopyrrolopyrrole-containing donor-acceptor polymers for improved charge carrier transport

Two new donor-acceptor polymers containing benzo[2,1-b:3,4-b':5,6-c'']trithiophene (BTT) as donor and diketopyrrolopyrrole (DPP) as acceptor are synthesized and applied in OFETs. By tuning the alkyl substituents of the polymers, a striking difference in packing order, thin-film arrangement, and charge carrier transport is observed. The polymer without substituents at the BTT exhibits a hole mobility two orders of magnitude higher than that with alkyl chains therein.

Real function of semiconducting polymer in GaAs/polymer planar heterojunction solar cells

We systematically investigated GaAs/polymer hybrid solar cells in a simple planar junction, aiming to fundamentally understand the function of semiconducting polymers in GaAs/polymer-based heterojunction solar cells. A library of semiconducting polymers with different band gaps and energy levels were evaluated in GaAs/polymer planar heterojunctions. The optimized thickness of the active polymer layer was discovered to be ultrathin (~10 nm). Further, the open-circuit voltage (Voc) of such GaAs/polymer planar heterojunctions was fixed around 0.6 V, regardless of the HOMO energy level of the polymer employed. On the basis of this evidence and others, we conclude that n-type GaAs/polymer planar heterojunctions are not type II heterojunctions as originally assumed. Instead, n-type GaAs forms a Schottky barrier with its corresponding anode, while the semiconducting polymer of appropriate energy levels can function as hole transport layer and/or electron blocking layer. Additionally, we discover that both GaAs surface passivation and thermal annealing can improve the performance of GaAs/polymer hybrid solar cells.

Organic ternary solar cells: a review

Recently, researchers have paid a great deal of attention to the research and development of organic solar cells, leading to a breakthrough of over 10% power conversion efficiency. Though impressive, further development is required to ensure a bright industrial future for organic photovoltaics. Relatively narrow spectral overlap of organic polymer absorption bands within the solar spectrum is one of the major limitations of organic solar cells. Among different strategies that are in progress to tackle this restriction, the novel concept of ternary organic solar cells is a promising candidate to extend the absorption spectra of large bandgap polymers to the near IR region and to enhance light harvesting in single bulk-heterojunction solar cells. In this contribution, we review the recent developments in organic ternary solar cell research based on various types of sensitizers. In addition, the aspects of miscibility, morphology complexity, charge transfer dynamics as well as carrier transport in ternary organic composites are addressed.