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Abstract: Research and development towards high efficiency plastic solar cells have been accelerating in recent years. Polymer-based bulk heterojunction solar cells are offering an attractive and inexpensive concept for large scale production by solution processing as well as advantageous flexible and aesthetic form factors. The thin film nano-morphology of bulk-heterojunction solar cells has been shown to dramatically influence the photovoltaic performance of the devices. This article reviews the different methods used to control the film nano-morphology of bulk-heterojunction solar cells focussing on the chemical additives during solution processing. All power conversion efficiency limiting mechanisms of bulk-heterojunction solar cells are discussed in detail. It is shown, how the formation of optimal percolation pathways between donor and acceptor influences the photovoltaic device performance. It is explained how the film nano-morphology relates to light absorption, free charge carrier generation as well as charge transport to the electrodes. [Copyright &y& Elsevier]

The article offers insights into fabrication of organic solar cells using conjugated polymer. The authors offer a general introduction to the materials, production techniques, working principles, critical parameters and stability of the organic solar cells. They then focus on conjugated polymer/fullerene bulk heterojunction solar cells. Lastly, they discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells.

Conjugated Polymers with a HOMO-LUMO transition <2 eV, i.e. a low bandgap, respectively, have interesting and desired properties for some thin film optoelectronic devices like light emitting diodes and solar cells. In this contribution we present the implementation of the novel copolymer PTPTB, consisting of alternating electron-rich N -dodecyl-2,5-bis(2′thienyl)pyrrole (TPT and electron-deficient 2,1,3-benzothiadiazole (B) units, in light emitting diodes (LEDs ) and photovoltaic devices. The LEDs show emission in the near infrared region, peaking at 800 nm. The electroluminescence yield can be significantly enhanced by blending with the wide bandgap polymer poly (para-phenylene-vinylene) derivative MDMO-PPV. Bulk heterojunction devices of PTPTB blended with the C 60 derivative PCBM shows AM 1.5 efficiencies around 1%. The low bandgap of PTPTB allows collecting photons up to 750 nm. [ABSTRACT FROM AUTHOR]

Abstract: Postproduction treatment of poly (3-alkylthiophene) based bulk-heterojunction solar cells is an important and required procedure in order to fabricate organic solar cells with highest power conversion efficiencies. Postproduction treatment, by means of annealing solar cells at elevated temperatures during which an external voltage is simultaneously applied, is not very suitable for large scale production schemes due to the need for well controlled environment and involvement of flexible substrates. A faster and easier method for improving the efficiency, like a simple addition of chemicals, would be highly desirable. We have studied the effect of alkyl thiol addition into bulk-heterojunction solar cells, based on poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) mixtures as reported first by Santa Barbara group [J. Peet, C. Soci, R.C. Coffin, T.Q. Nguyen, A. Mikhailovsky, D. Moses, G. C. Bazan, Appl. Phys. Lett. 89 (2006) 252105, J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat. Mater. 6 (2007) 497], which gives the same final results in even higher performance of organic solar cells through increased power conversion efficiency and thus substitutes the postproduction treatment. Red-shift in optical absorption is seen in the films with alkyl thiol, resembling the absorption of thermally annealed films. Based on steady-state current–voltage characteristics and transient charge carrier extraction by linearly increasing voltage (CELIV) measurements, the conductivity of thermally annealed and films with alkyl thiol is found to be an order of magnitude higher than in films spun from pristine blends of P3HT and PCBM. Charge carrier mobility measurements indicate significant increase in carrier mobility, consistent with the improved structural order and formation of interpenetrating network between the donor and acceptor in the bulk-heterojunction solar cells. [Copyright &y& Elsevier]

Abstract: We report herein large area (>10cm2), interconnected organic solar cell modules both on glass substrates as well as on flexible ultra-high barrier foils, reaching 1.5% and 0.5% overall power conversion efficiency under AM1.5 conditions. Series connection is described, as these modules consist of up to three cells. Using our flexible barrier material, a shelf lifetime of polythiophene-based solar cells of 6000h could be realized. Furthermore, we compare the photovoltaic performance of efficient conjugated polymer:fullerene solar cell modules with established technologies. Under typical indoor-office lighting, our modules are competitive with these systems. [Copyright &y& Elsevier]

Abstract: Solution-processed bilayer heterojunction hybrid solar cells have been fabricated using size-quantized PbS nanoparticles and poly (3-hexylthiophene) (P3HT). PbS was used as an electron-transporting layer whereas P3HT was used for hole transport. A photovoltaic device consisting of PbS and P3HT exhibited 3% incident photon to current efficiencies (IPCE) under 550-nm monochromatic irradiation. [Copyright &y& Elsevier]

Abstract: Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C61-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4mA/cm2 under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65eV (750nm), which corresponds to the absorption spectrum of the polymer. [Copyright &y& Elsevier]

Abstract: We report the photophysical behavior and photovoltaic performance of a poly(3,4-propylenedioxythiophene) (PProDOT) derivative, namely poly-[3,3-dihexyl-3,4-dihydro-2H-thieno(3,4-b)(1,4)dioxepine] (PProDOT(Hx)2), as an electron donor in bulk heterojunction solar cells blended with the acceptor 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM). Devices composed of a 1:4 (w:w) ratio of PProDOT(Hx)2/PCBM and spin coated from chlorobenzene were characterized by measuring current–voltage characteristics under simulated Air Mass 1.5 (AM1.5) conditions as well as the spectrally resolved photocurrent (IPCE). The influence of different preparation parameters like various blend ratios, spin coating from different solvents, and changing the metal contacts was studied. It was found that the photoluminescence of the polymer is quenched by a factor greater than 100 using blends consisting of PProDOT(Hx)2 and PCBM (3:2, w:w). Additionally, the photoactive blends were characterized by photoinduced absorption spectroscopy and the results suggest that charge transfer is occurring from PProDOT(Hx)2 to PCBM. Results from atomic force microscopy reveal that a bicontinuous network, with domain sizes on the order of 100–200nm, results when a 1:4 blend of PProDOT(Hx)2/PCBM is spin coated from chlorobenzene. [Copyright &y& Elsevier]