• Energy Research

Abstract We investigate a family of low band-gap polymers based on the common acceptor moiety 2,3-bis-(3-octyloxyphenyl)quinoxaline (Q) combined with thiophene (T) or the fused thiophene systems: benzo[2,1-b:3,4-b′]-dithiophene (BDT) or dithieno[3,2-b,2′,3′-d]-thiophene (DTT). The photochemical stability of the three polymers was examined and compared to P3HT. They were found to be substantially more robust than P3HT with a ranking of DTTQ>BDTQ>TQ1⪢P3HT, indicating that the fused ring systems of DTT and BDT impart a large degree of photochemical stability than thiophene. Furthermore devices with normal and inverted geometry were prepared and tested in air. The normal geometry devices showed the highest efficiencies compared to the inverted, in particular owing to a higher Voc, with TQ1 being the most efficient with a power conversion efficiency (PCE) of 1.5% (1000 W m−2, AM1.5 G). For the inverted devices TQ1 and DTTQ showed the best PCEs of 0.9%.

AbstractContinuous flow methods are utilized in conjunction with direct arylation polymerization (DArP) for the scaled synthesis of the roll‐to‐roll compatible polymer, poly[(2,5‐bis(2‐hexyldecyloxy)phenylene)‐alt‐(4,7‐di(thiophen‐2‐yl)‐benzo[c][1,2,5]thiadiazole)] (PPDTBT). PPDTBT is based on simple, inexpensive, and scalable monomers using thienyl‐flanked benzothiadiazole as the acceptor, which is the first β‐unprotected substrate to be used in continuous flow via DArP, enabling critical evaluation of the suitability of this emerging synthetic method for minimizing defects and for the scaled synthesis of high‐performance materials. To demonstrate the usefulness of the method, DArP‐prepared PPDTBT via continuous flow synthesis is employed for the preparation of indium tin oxide (ITO)‐free and flexible roll‐coated solar cells to achieve a power conversion efficiency of 3.5% for 1 cm2 devices, which is comparable to the performance of PPDTBT polymerized through Stille cross coupling. These efforts demonstrate the distinct advantages of the continuous flow protocol with DArP avoiding use of toxic tin chemicals, reducing the associated costs of polymer upscaling, and minimizing batch‐to‐batch variations for high‐quality material.

Abstract The oxygen permeability of the transparent organic anode poly(3,4,-ethylene dioxythiophene) with paratoluenesulphonate as the anion (PEDOT:pTS) was determined to be 2.5 ± 0.7 × 10 - 15 cm 3 (STP) cm cm - 2 s - 1 Pa - 1 , and is thus comparable in magnitude to the oxygen permeability of polyethyleneterephthalate (PET). The oxygen diffusion through bilayers of polyethylene (PE) and PEDOT:pTS and bilayers of PET and PEDOT:pTS was established. The bilayer structures were applied as the carrier substrate and the transparent anode in polymer-based photovoltaic devices employing a mixture of poly(1-methoxy-4-(2-ethylhexyloxy)- p -phenylenevinylene) (MEH-PPV) and [6,6]-phenyl- C 61 -butanoicacidmethylester (PCBM) as the active layer and aluminium as the cathode. The oxygen permeability of the layers and the aluminium cathode was correlated with the lifetime of the solar cell devices. It was found that the performance of the devices with PET as the carrier substrate degraded more slowly due to the lower oxygen and water permeability, whereas devices using PE as the carrier substrate gave devices with a very short lifetime. It was found that PEDOT:pTS on its own is a not a significant barrier for oxygen in the context of photovoltaic devices where long lifetimes are anticipated. It is concluded that the large oxygen permeability of the barrier layers contribute to the short device lifetimes while other permeates such as water also contribute to device degradation.

Abstract Four new copolymers of 1,4-dialkoxybenzene, thiophene, bithiophene and benzothiadiazole have been prepared and investigated for optical properties and in photovoltaic devices. The structures were chosen to show the effect of successively introducing an acceptor moiety, longer alkoxy side chains and finally, substituting thiophene for bithiophene. The absorption spectra and IPCE showed that these are low band gap polymers that can harvest light in the visible spectrum (400 to 700 nm) and that photoelectrons are generated in the whole range. The photovoltaic devices produced short circuit current densities ( J sc ) of 2.6 to 4.6 mA/cm 2 under AM1.5 G illumination. The open circuit voltage ( V oc ) was 0.56 to 0.64 V and the efficiencies ( η ) of 0.8% to 2.2% depending on the structure.

Abstract One of the prime objects of organic solar cell research has been to improve the power conversion efficiency. Unfortunately, the accurate determination of this property is not straight forward and has led to the recommendation that record devices be tested and certified at a few accredited laboratories following rigorous ASTM and IEC standards. This work tries to address some of the issues confronting the standard laboratory in this regard. Solar simulator lamps are investigated for their light field homogeneity and direct versus diffuse components, as well as the correct device area determination using various types of masks, are discussed. The main findings are that the light field inhomogeneity and a large diffuse component in combination with masking give rise to a substantial variation in the measured efficiency. As a result the device efficiency can easily be over or underestimated. We offer a set of recommendations for the device masking and efficiency characterization that enable accurate determination of the efficiency in a standard OPV characterization laboratory.

Abstract Printing of the silver back electrode under ambient conditions using simple laboratory equipment has been the missing link to fully replace evaporated metal electrodes. Here we demonstrate how a recently developed roll coater is further developed into a single machine that enables processing of all layers of the polymer solar cell without moving the substrate from one machine to another. The novel approach to polymer solar cells is readily scalable using one compact laboratory scale coating/printing machine that is directly compatible with industrial and pilot scale roll-to-roll processing. The use of the techniques was successfully demonstrated in one continuous roll process on flexible polyethyleneterphthalate (PET) substrates and polymer solar cells were prepared by solution processing of five layers using only slot-die coating and flexographic printing. The devices obtained did not employ indium–tin-oxide (ITO) or vacuum evaporation steps making it a significant step beyond the traditional laboratory polymer solar cell processing methods involving spin coating and metal evaporation.

Aqueous nanoparticle dispersions of a series of three low-band-gap polymers poly[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b')dithiophene-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)(2,1,3-benzothiadiazole)-5,5'-diyl] (P1), poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P2), and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (P3) were prepared using ultrasonic treatment of a chloroform solution of the polymer and [6,6]-phenyl-C(61)-butyric acid methyl ester ([60]PCBM) mixed with an aqueous solution of sodium dodecylsulphate (SDS). The size of the nanoparticles was established using small-angle X-ray scattering (SAXS) of the aqueous dispersions and by both atomic force microscopy (AFM) and using both grazing incidence SAXS (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS) in the solid state as coated films. The aqueous dispersions were dialyzed to remove excess detergent and concentrated to a solid content of approximately 60 mg mL(-1). The formation of films for solar cells using the aqueous dispersion required the addition of the nonionic detergent FSO-100 at a concentration of 5 mg mL(-1). This enabled slot-die coating of high quality films with a dry thickness of 126 ± 19, 500 ± 25, and 612 ± 22 nm P1, P2, and P3, respectively for polymer solar cells. Large area inverted polymer solar cells were thus prepared based on the aqueous inks. The power conversion efficiency (PCE) reached for each of the materials was 0.07, 0.55, and 0.15% for P1, P2, and P3, respectively. The devices were prepared using coating and printing of all layers including the metal back electrodes. All steps were carried out using roll-to-roll (R2R) slot-die and screen printing methods on flexible substrates. All five layers were processed using environmentally friendly methods and solvents. Two of the layers were processed entirely from water (the electron transport layer and the active layer).

AbstractRoll‐processed, indium tin oxide (ITO)‐free, flexible, organic tandem solar cells and modules have been realized and used in round‐robin studies as well as in parallel inter‐laboratory stability studies. The tandem cells/modules show no significant difference in comparison to their single‐junction counterparts and the use of round‐robin studies as a consensus tool for evaluation of organic solar cell parameters is judged just as viable for the tandem solar cells as for single‐junction devices. The inter‐laboratory stability studies were conducted according to testing protocols ISOS‐D‐2, ISOS‐D‐3, and ISOS‐L‐2, and in spite of a much more complicated architecture the organic tandem solar cells show no significant difference in stability in comparison to their single‐junction counterparts.

Abstract Solar cells that convert sunlight into electrical power have demonstrated a large and consistent growth through several decades. The growth has spawned research on new technologies that potentially enable much faster, less costly and environmentally friendly manufacture from earth abundant materials. Here we review carbon based solar cells through a complete analysis of all the data that has been reported so far and we highlight what can be expected from carbon based technologies and draw scenarios of how it can be made of immediate use.