Organic photovoltaics will become 30 years old relatively soon. In spite of the impressive development achieved throughout these years, especially in terms of reported power conversion efficiencies, there are still important technological and fundamental obstacles to circumvent before they can be implemented into reliable and long-lasting applications. Regarding device processing, the synthesis of highly soluble polymeric semiconductors first, and fullerene derivatives then, was initially considered as an important breakthrough that would definitely change the fabrication of photovoltaics once for all. Nowadays, the promise of printing solar cells by low-cost and high throughput mass production techniques still stands. However, the potential and the expectation raised by this technology is such that it is considerably difficult to keep track of the most significant progresses being now published in different and even monographic journals. There is therefore the need to compile the most remarkable advances in well-documented reviews than can be used as a reference for future ideas and works. In this letter, we review the development of polymeric solar cells from its origin to the most efficient devices published to date. After analyzing their fundamental limits, we separate these achievements into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: Active materials, strategies -fabrication/processing procedures- that can mainly modify the active film morphology and result in improved efficiencies for the same starting materials, and all the different cell layout/architectures that have been used in order to extract as high photocurrent as possible from the Sun. The synthesis of new donors and acceptors, the use of additives and post-processing techniques, buffer interlayers, inverted and tandem designs are some of the most important aspects that are in detailed reviewed in this letter. All have equally contributed to develop this technology and leave it at doors of commercialization. (C) 2015 Elsevier B.V. All rights reserved.
In the present review, the main degradation mechanisms occurring in the different layer stacking (i.e. photoactive layer, electrode, encapsulation film, interconnection) of polymeric organic solar cells and modules are discussed. Bulk and interfacial, as well as chemical and physical degradation mechanisms are reviewed, as well as their implications and external or internal triggers. Decay in I-V curves in function of time is usually due to the combined action of sequential and interrelated mechanisms taking place at different locations of the device, at specific kinetics. This often makes the identification of specific root causes of degradation challenging in non-model systems. Additionally, constant development and refinement in terms of type and combination of materials and processes render the ranking of degradation mechanisms as a function of their probability of occurrence and their detection challenging. However, it clearly appears that for the overall stability of organic photovoltaic devices, the actual photoactive layer, as well as the properties of the barrier and substrate (e. g. cut of moisture and oxygen ingress, mechanical integrity), remain critical. Interfacial stability is also crucial, as a modest degradation at the level of an interface can quickly and significantly influence the overall device properties. (C) 2011 Elsevier B. V. All rights reserved.
We study external quantum efficiency (eta(EQE)) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet-triplet annihilation and triplet-triplet annihilation dominate eta(EQE) roll-off because of the relatively long 2CzPN triplet lifetime of 273 mu s. This work yields a method to relax the exciton quenching process in TADF based OLEDs. (C) 2013 Elsevier B. V. All rights reserved.
A roll-to-roll process enabling fabrication of polymer solar cells comprising five layers on flexible substrates is presented. The device geometry is inverted and allow for fabrication on both transparent and non-transparent flexible substrates. The process is illustrated ill this work by formation of a bottom electrode comprising silver nanoparticles on a 130 micron thick polyethyleneternaphthalate (PEN) substrate. Subsequently an electron transporting layer of zinc oxide nanoparticles was applied from solution followed by an active layer of P3HT-PCBM and a hole transporting layer of PEDOT:PSS. These first four layers were applied by slot-die coating. The final electrode was applied by screen printing a grid structure that allowed for transmission of 80% of the light. The materials were patterned into stripes allowing for formation of a single cell device and serially connected modules comprising 2, 3 and 8 stripes. All five layers in the device were processed from solution in air and no vacuum steps were employed. An additional advantage is that the use of indium-tin-oxide (ITO) is avoided in this process. The devices were tested under simulated sunlight (1000 W m(-2), AM1.5G) and gave a typical performance 0.3% in terms of power conversion efficiency (PCE) for the active layer. The low PCE was due to poor transmission of light through the back electrode. (C) 2009 Elsevier B.V. All rights reserved.
We investigate the chemical and structural properties of solution-processed thin films of P3HT blended with p-type dopant F4TCNQ. The maximum in-plane electrical conductivity of doped films is observed at a molar doping fraction of 0.17, in agreement with the binding mechanism of F4TCNQ: P3HT complexes. Through the use of X-ray diffraction, a previously unreported crystalline phase is observed for P3HT films doped above a critical threshold concentration. This crystalline phase involves the incorporation of F4TCNQ molecules into ordered polymer regions and ultimately improves charge dissociation, leading to higher carrier density in thin film. Finally, optical absorption and X-ray diffraction reveal that the chemical state of P3HT in solution has a dramatic impact on the electrical and structural properties of the blended films. (C) 2013 Published by Elsevier B.V.
The electrical properties of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of sorbitol have been studied in detail. Although it is well known that sorbitol enhances the conductivity of PEDOT:PSS thin films by three orders of magnitude, the origin and consequences of sorbitol treatment are only partly understood and subject of further study. By thermal annealing of spin coated PEDOT:PSS/sorbitol films and simultaneously monitoring the conductivity, we demonstrate that the strong increase in conductivity coincides with evaporation of sorbitol from the film. Hence, sorbitol is a processing additive rather than a (secondary) dopant. Scanning Kelvin probe microscopy reveals that sorbitol treatment causes a reduction of the work function from 5.1 eV to 4.8-4.9 eV. Sorbitol also influences the environmental stability of the films. While the conductivity of the pristine PEDOT:PSS films increases by about one order of magnitude at similar to 50% RH due to an ionic contribution to the overall conductivity, films prepared using sorbitol exhibit an increased environmental stability with an almost constant conductivity up to 45% RH and a slight decrease at 50% RH. The higher stability results from a reduced tendency to take up water from the air, which is attributed to a denser packing of the PEDOT:PSS after sorbitol treatment. (C) 2008 Elsevier B.V. All rights reserved.
A study on p-doping of organic wide band gap materials with Molybdenum trioxide using current transport measurements, ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy is presented. When MoO3 is co-evaporated with 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP), a significant increase in conductivity is observed, compared to intrinsic CBP thin films. This increase in conductivity is due to electron transfer from the highest occupied molecular orbital of the host molecules to very low lying unfilled states of embedded Mo3O9 clusters. The energy levels of these clusters are estimated by the energy levels of a neat MoO3 thin film with a work function of 6.86 eV, an electron affinity of 6.7 eV and an ionization energy of 9.68 eV. The Fermi level Of MoO3-doped CBP and N,N'-bis(1-naphtyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (alpha-NPD) thin films rapidly shifts with increasing doping concentration towards the occupied states. Pinning of the Fermi level several 100 meV above the HOMO edge is observed for doping concentrations higher than 2 mol% and is explained in terms of a Gaussian density of HOMO states. We determine a relatively low dopant activation of similar to 0.5%, which is due to Coulomb-trapping of hole carriers at the ionized dopant sites. (C) 2009 Elsevier B.V. All rights reserved.
In this paper, we elucidate the role of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the degradation of polymer:PCBM ((6,6)-phenyl C-61-butyric acid methyl ester) solar cells. The study is done on unencapsulated cells exposed to ambient conditions in dark. The cell degradation results from reduced carrier extraction, and an investigation of the various interfaces within the cell allows us to correlate this to oxidation of the low work function metal cathode. We further show that this oxidation is caused by water vapor diffusion from the edges through the hygroscopic PEDOT:PSS layer. We demonstrate that only the hygroscopic nature of PEDOT:PSS, and not its acidity, has a detrimental impact. The oxidation of the cathode progresses in synchrony with the water ingress into the PEDOT:PSS layer from the edges of the device towards the central part, and results in a progressive constriction of the active area. When the PEDOT:PSS layer is replaced by an evaporated layer of MoO3, the device lifetime is improved considerably even with highly reactive metal cathodes. Finally, we provide a quantitative relationship between device lifetime and the level of humidity in the ambient, thus establishing a suitable accelerated shelf-life test for organic solar cells and their encapsulation. (C) 2011 Elsevier B.V. All rights reserved.
Charge carrier diffusion and recombination in an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) with indium tin oxide (ITO) and aluminium contacts have been analyzed in the dark by means of impedance spectroscopy. Reverse bias capacitance exhibits Mott-Schottky-like behavior indicating the formation of a Schottky junction (band bending) at the P3H:PCBM-Al contact. Impedance measurements show that minority carrier (electrons) diffuse out of the P3HT:PCBM-Al depletion zone and their accumulation contributes to the capacitive response at forward bias. A diffusion-recombination impedance model accounting for the mobility and lifetime parameters is outlined. Electron mobility results to be 2 x 10(-3) cm(2) V-1 s(-1) and lifetime lies within the milliseconds timescale. (C) 2008 Elsevier B.V. All rights reserved.
Manufacturing of flexible ITO-free polymer solar cell modules by roll-to-roll methods (R2R) is described. Inverted devices with top illumination were built on a Kapton foil and an Aluminum/Chromium bi-layer system was used as electron contact. The layer structure was Kapton/Al/Cr/P3HT:PCBM/PEDOT:PSS/Ag (printed) and devices were encapsulated. Small area cells (3 cm(2) active area) were first carefully optimized investigating the influence of a number of discrete parameters on performance. A maximum power conversion efficiency of 1.4% was achieved under 1 sun illumination (AM 1.5G, 1000 W m(-2)). Optimized lab-scale single devices were then transferred to a full R2R process combining slot-die coating and screen printing. All the layers were processed from solution under ambient conditions. Two different concepts were explored: (i) serially connected stripe modules (to reduce the Ohmic losses) and (ii) monolithic modules (to achieve high geometric fill factor and increase the flexibility of the process). For this second concept, the only layer that needs to be patterned is the silver grid electrode and the grid pattern design can then be readily tuned. As an example, four different patterns were used and the resultant performances compared. Modules comprising 16 serially connected cells gave total area efficiencies up to 0.5% (235 cm(2) - 1% on the active area) while the best monolithic ones gave 0.35% (100 cm(2) - 0.4% on the active area). The freshly prepared devices consistently showed an inflection point in the IV curve indicative of a rather poor photovoltaic behavior. Upon light exposure and repeated IV scans the inflection point partially disappeared, and performance significantly increased. (C) 2011 Elsevier B.V. All rights reserved.
This review collects recent five-year publications on low bandgap semiconducting polymers, which are composed of electron donor (D) and electron acceptor (A) units, exhibiting the power conversion efficiency (PCE) higher than 6%. When the photovoltaic performances of different types of D-A semiconducting copolymers are compared after the copolymers are classified into several categories according to the type of A-units, it is realized that diketopyrrolopyrrole (DPP)-based copolymers exhibit high J(SC)s owing to low bandgaps and low V(OC)s due to high-lying HOMO levels, while thienopyrroledione (TPD)-based copolymers exhibit high V(OC)s due to their deep HOMO levels and low J(SC)s because of wide bandgaps. Benzothiadiazole-and thienothiophene-based copolymers show intermediate values of V-OC and J(SC) between DPP-and TPD-based ones. For further enhancement of photovoltaic performance, DPP-based copolymers may be designed to have deeper HOMO level with the minimum widening of bandgap while TPD-based polymers may be designed to have lower bandgap with the minimum rise of HOMO level. Hence, the energy level tuning must be considered so as to minimize the adverse effect. (C) 2016 Elsevier B.V. All rights reserved.
Organic solar cells require suitable anode surface modifiers in order to selectively collect positive charge carriers and improve device performance. We employ a nickel metal organic ink precursor to fabricate NiO hole transport layers on indium tin oxide anodes. This solution deposited NiO annealed at 250 degrees C and plasma treated, achieves similar OPV device results reported with NiO films from PLD as well as PEDOT:PSS. We demonstrate a tunable work function by post-processing the NiO with an O-2-plasma surface treatment of varied power and time. We find that plasma treatment is necessary for optimal device performance. Optimal devices utilizing a solution deposited NiO hole transport layer show lower series resistance and increased fill factor when compared to solar cells with PEDOT: PSS. (C) 2010 Elsevier B.V. All rights reserved.
We investigated the influence of oxygen on the performance of P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) solar cells by current-voltage, thermally stimulated current (TSC) and charge extraction by linearly increasing voltage (CELIV) measurement techniques. The exposure to oxygen leads to an enhanced charge carrier concentration and a decreased charge carrier mobility. Further, an enhanced formation of deeper traps was observed, although the overall density of traps was found to be unaffected upon oxygen exposure. With the aid of macroscopic simulations, based on solving the differential equation system of Poisson, continuity and drift-diffusion equations in one dimension, we demonstrate the influence of a reduced charge carrier mobility and an increased charge carrier density on the main solar cell parameters, consistent with experimental findings. (C) 2010 Elsevier B.V. All rights reserved.
The past decades have driven a great deal of interest for developing low-cost electroluminescent devices. In this aim, highly emissive phosphors based on Earth-abundant metals and presenting the advantage of environment-benignancy are actively researched. Based on these requirements, copper(I) complexes have been identified as favorable candidates that could advantageously replace the well-established iridium(III) complexes. (C) 2015 Elsevier B.V. All rights reserved.
This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH3) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV-visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of pi-pi interactions between graphene and PEDOT:PSS. The ink-jet printed graphene-PEDOT:PSS gas sensor exhibits high response and high selectivity to NH3 in a low concentration range of 25-1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH3 molecules via pi electrons network. The NH3-sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted. (C) 2014 Elsevier B.V. All rights reserved.
The mobility is an important parameter for organic solar cell materials as it influences the charge extraction and recombination dynamics. In this study, the time of flight technique is used to investigate the charge mobility of the important organic photovoltaic materials PC71BM, PTB7 and their blend. The electron mobility of PC71BM is in the region of 1 x 10(-3) cm(2)/Vs for the neat fullerene film, and has a positive electric field dependence. At room temperature the hole mobility of PTB7 is 1 x 10(-3) cm(2)/Vs for the neat film and 2 x 10(-4) cm(2)/Vs for their blend. The hole mobility of the blend reduces by a factor of a thousand when the sample is cooled from room temperature to 77 K. This finding is compared with the device performance of efficient PTB7:PC71BM solar cells for varying temperature. At 77 K the solar cell efficiency halved, due to losses in fill factor and short circuit current. Bimolecular and trap-assisted recombination increase at low mobility (low temperature) conditions, whereas at high mobility conditions the open circuit voltage reduces. The power conversion efficiency as a function of temperature has a maximum between 260 K and 295 K, revealing an optimized mobility at room temperature. (C) 2015 The Authors. Published by Elsevier B.V.
The lowest unoccupied molecular orbital (LUMO) energies of a variety of molecular organic semiconductors have been evaluated using inverse photoelectron spectroscopy (IPES) data and are compared with data determined from the optical energy gaps, electrochemical reduction potentials, and density functional theory (DFT) calculations. A linear fit to the electrochemical reduction potential (relative to an internal ferrocene reference) vs. the LUMO energy determined by IPES gives a slope and intercept of -1.19 +/- 0.08 eV/V and -4.78 +/- 0.17 eV, respectively, and 0.92 +/- 0.04 and -0.44 +/- 0.11 eV, respectively, based on the DFT calculated LUMO energies. From these fits, we estimate the LUMO and exciton binding energies of a wide range of organic semiconductors. (C) 2009 Elsevier B.V. All rights reserved.
This article focuses on the electronic structure of the poly(3-hexylthiophene):phenyl-[6,6]-C61 butyric acid methyl ester (P3HT:PCBM) blend, widely used in bulk heterojunction (BHJ) solar cells. Given the fact that the surface of the blend film is a nearly pure P3HT wetting layer, we use a lift-off method to access the originally buried surface, which is rich in both P3HT and PCBM and thus representative of the BHJ. The combination of direct and inverse photoemission spectroscopy on this surface leads to a determination of the energy gap between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. The gap is similar to 1.4 eV, which implies a 0.5-0.6 eV interface dipole barrier between the two materials. The energy gap is found to be stable versus in situ annealing up to 100 degrees C. (C) 2010 Elsevier B.V. All rights reserved.
Data rates of plastic transponder chips have been limited to a few kHz, limited by the inherent low mobility of organic semiconductors. However, a target application for plastic RFID tags is Electronic Product Coding (EPC), which will require, at a base carrier frequency f(c) = 13.56 MHz, a data rate of f(c)/512 = 52.969 kb/s. In this work, we show that the compatibility of organic semiconductors with high-k gate dielectrics allows boosting the current drive of transistors in functional circuits to EPC compatible clock rates. We demonstrate an 8 bit RFID transponder chip with critical dimension of 2 mu m having a data rate of 50 kb/s at V-DD = 18 V. (c) 2010 Elsevier B.V. All rights reserved.
Charge carrier mobility is a figure of merit commonly used to rate organic semiconducting materials for their suitability in applications such as solid-state lighting or photovoltaics. Although large variations are found in published mobility values on identical materials, there is little open discussion in the literature of the reproducibility of these results. We address this with an interlaboratory study of mobility measurements performed on a set of organic semiconductors using the space-charge limited current method. We found mobility measured on nominally identical devices could vary by more than one order of magnitude, with the largest sources of variation being poor electrodes and film thickness variation. Moreover, we found that mobility values extracted from identical data by different scientists would typically vary by a factor of 3. We propose a protocol for analysis and reporting that was found to reduce this analysis variation to as little as 20%. We also present general guidelines for improving the reproducibility of benchmark mobility measurements. Crown Copyright (C) 2014 Published by Elsevier B.V.