Mirko Seri

Conjugated anthracene derivatives as donor materials for bulk heterojunction solar cells: olefinic versus acetylenic spacers

Substituting olefinic for acetylenic pi-spacers in anthracene-based conjugated semiconductor donors leads to appreciable increases in the power conversion efficiencies of the resulting bulk heterojunction solar cells.

Molecular-Shape-Controlled Photovoltaic Performance Probed via Soluble π-Conjugated Arylacetylenic Semiconductors

The synthesis and characterization of a new series of anthracene-based derivatives and their use as donors in bulk-heterojunction solar cells is reported. It is found that when using well-defined building blocks in constructing the chromophore, the donor molecular shape dramatically affects organic photovoltaic (OPV) performance in a previously unrecognized way.

Conjugated polymers based on benzodithiophene and fluorinated quinoxaline for bulk heterojunction solar cells: thiophene versus thieno[3,2-b]thiophene as π-conjugated spacers

Two conjugated donor–acceptor copolymers based on a benzodithiophene donor unit and a fluorinated quinoxaline acceptor unit, spaced with either thiophene or thieno[3,2-b]thiophene π-bridges, were designed and synthesized. The effect of different π-bridges and of the processing conditions on optical, electrical, morphological and photovoltaic properties of the polymer:fullerene blend films were investigated. The polymer containing the thieno[3,2-b]thiophene π-bridge (PBDTFQ-TT) showed a red-shifted absorption and an enhanced charge carrier mobility, as compared to its analogue with the thiophene π-bridge (PBDTFQ-T), due to its narrower optical gap (by ∼0.1 eV) and stronger inter-chain interactions, favored by the structural planarity and increased linearity of the polymer backbone, as also supported by DFT calculations. The blend of PBDTFQ-TT and PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), compared to the PBDTFQ-T:PC61BM one processed under the same conditions (by blade-coating technique), showed greatly enhanced photovoltaic performance, with more than doubled power conversion efficiency (PCE up to 5.60% for the best device) due to the increased short-circuit current density and fill factor. However, similar PCEs were also achieved for PBDTFQ-T:PC61BM-based devices by optimizing the processing conditions through the addition of 1,8-diiodooctane (DIO) as the solvent additive. Through morphological and electrical analysis of the films, produced with and without additive, it was observed that the addition of DIO greatly enhances the self-organization, and consequently the charge mobility, of the thiophene π-bridge-based polymer, while it was detrimental for the nanoscale morphology and photovoltaic performances of the thieno[3,2-b]thiophene π-bridge-based polymer in the corresponding blend.

Enhanced Ultraviolet Stability of Air-Processed Polymer Solar Cells by Al Doping of the ZnO Interlayer.

Photostability of organic photovoltaic devices represents a key requirement for the commercialization of this technology. In this field, ZnO is one of the most attractive materials employed as an electron transport layer, and the investigation of its photostability is of particular interest. Indeed, oxygen is known to chemisorb on ZnO and can be released upon UV illumination. Therefore, a deep analysis of the UV/oxygen effects on working devices is relevant for the industrial production where the coating processes take place in air and oxygen/ZnO contact cannot be avoided. Here we investigate the light-soaking stability of inverted organic solar cells in which four different solution-processed ZnO-based nanoparticles were used as electron transport layers: (i) pristine ZnO, (ii) 0.03 at %, (iii) 0.37 at %, and (iv) 0.8 at % aluminum-doped AZO nanoparticles. The degradation of solar cells under prolonged illumination (40 h under 1 sun), in which the ZnO/AZO layers were processed in air or inert atmosphere, is studied. We demonstrate that the presence of oxygen during the ZnO/AZO processing is crucial for the photostability of the resulting solar cell. While devices based on undoped ZnO were particularly affected by degradation, we found that using AZO nanoparticles the losses in performance, due to the presence of oxygen, were partially or totally prevented depending on the Al doping level.

2D π-conjugated benzo[1,2-b:4,5-b′]dithiophene- and quinoxaline-based copolymers for photovoltaic applications

Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for a high device open circuit voltage and LUMO energy levels higher than those of PC61BM and PC71BM for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us to obtain a high 2D π-conjugation which promotes red shifted absorption profiles, low HOMO energy levels (<−5.6 eV) and enhanced environmental and thermal stability. Moreover, the introduction of the fluorine atom in the polymer backbone allows us to obtain efficient OPV devices, based on as-cast P(2)-FQ-BDT-4TR:PC61BM blend, showing a JSC of −10.2 mA cm−2, VOC of 0.90 V, FF of 58% and PCE of 5.3%, without the need for any additional thermal treatment.

Eciency enhancement of P3HT:PCBM solar cells containing scattering Zn-Al hydrotalcite nanoparticles in the PEDOT:PSS layer

Abstract In this article we report on a new hybrid (organic-inorganic) composite material based on hydrophilic, electrically inert and semi-transparent hydrotalcite (HT) nanoparticles and a pHneutral formulation of PEDOT:PSS. The application of this composite material as electrically and optically active buffer layer in P3HT:PC61BM bulk heterojunction (BHJ) solar cells is reported. Two different synthetic routes are explored to obtain HTs having discoid shape, with a diameter of around 150- 200 nm and a thickness of ~20 nm, to be easily embedded in ~50 nm thick PEDOT:PSS films. The good affinity between HTs and the sulfonate groups of the PEDOT:PSS allows to obtain homogeneous HTs/PEDOT:PSS films, for HT concentrations of 0.25% and 0.50% by weight (vs. PEDOT:PSS). At these particle loads the electrical and morphological features of doped and undoped PEDOT:PSS films are nearly identical, while providing a significant effect on the visible light scattering properties of the composite films. We demonstrate ~12% improvement in power conversion efficiency (PCE) for P3HT:PC61BM solar cells incorporating HTs in the PEDOT: PSS layer, which mainly originates from increased shortcircuit current densities (JSC).

Integration of a silk fibroin based film as a luminescent down-shifting layer in ITO-free organic solar cells

We report here a study on the integration of the silk fibroin (SF) protein in organic solar cells. The intrinsic low toxicity, natural availability, biodegradability, water processing, good film forming properties and capability to be doped with functional molecules of SF biopolymer inspired us to integrate it as a transparent and inert or functional bottom layer in organic solar cells. Water stable, optically transparent, smooth and homogeneous SF thin films (thickness ∼400 nm) were successfully prepared on glass and characterized. Then ITO-free bulk heterojunction (BHJ) solar cells employing P3HT:PC61BM as a standard active layer and a highly conductive PEDOT:PSS formulation as a semi-transparent anode were deposited over the SF films. As a result, the power conversion efficiency (PCE) of all silk-integrated BHJ solar cells was comparable to the references on bare glass. The ability of SF to act as a host matrix for functional moieties was exploited to give to the SF layer the functionality of a Luminescent Down-shifting film (LDS), as confirmed by the spectral response measurements, by using a water soluble stilbene derivative (Stb). The photovoltaic performance of all SF-based devices was significantly stable over time, overcoming the problems of the ITO-based reference cells after 70 days. Finally, flexible SF-integrated ITO-free solar cells were successfully fabricated on PET substrates.

Efficient as-cast bulk-heterojunction solar cells based on a tert-butyl substituted methanofullerene acceptor

We report on the implementation of a modified methanofullerene PCBtB ([6,6]-phenyl-C61 butyric acid tert-butylester) as electron acceptor for solution-processable bulk-heterojunction (BHJ) solar cells based on P3HT (poly(3-hexylthiophene)). The results demonstrate how a small modification of the steric hindrance of the benchmark molecule PCBM, leaving the electronic structure unchanged, significantly influences the main properties of the BHJ active layer. In particular, we investigate the effect of the replacement of the PC61BM methyl end-group with a tert-butyl substituent on the optical properties, OPV parameters, nanoscale morphology, D:A phase segregation, and charge carrier mobility in bulk-heterojunction with P3HT. We find that the PCBtB-based blends present a finer self-organization, higher D:A phase segregation and good charge carrier mobility, affording interesting photovoltaic responses and highlighting a marked potential of PCBtB to tailor the active BHJ layer. The best BHJ solar cells based on as-cast P3HT:PCBtB blend films exhibit power conversion efficiencies (PCEs) of ∼3.1%, under a simulated AM1.5G illumination of 100 mW cm−2, which are greater than the analogous as-cast P3HT:PCBM based devices.

Synthesis and characterization of benzodithiophene and benzotriazole-based polymers for photovoltaic applications

Summary Two high bandgap benzodithiophene–benzotriazole-based polymers were synthesized via palladium-catalysed Stille coupling reaction. In order to compare the effect of the side chains on the opto-electronic and photovoltaic properties of the resulting polymers, the benzodithiophene monomers were substituted with either octylthienyl (PTzBDT-1) or dihexylthienyl (PTzBDT-2) as side groups, while the benzotriazole unit was maintained unaltered. The optical characterization, both in solution and thin-film, indicated that PTzBDT-1 has a red-shifted optical absorption compared to PTzBDT-2, likely due to a more planar conformation of the polymer backbone promoted by the lower content of alkyl side chains. The different aggregation in the solid state also affects the energetic properties of the polymers, resulting in a lower highest occupied molecular orbital (HOMO) for PTzBDT-1 with respect to PTzBDT-2. However, an unexpected behaviour is observed when the two polymers are used as a donor material, in combination with PC61BM as acceptor, in bulk heterojunction solar cells. Even though PTzBDT-1 showed favourable optical and electrochemical properties, the devices based on this polymer present a power conversion efficiency of 3.3%, considerably lower than the efficiency of 4.7% obtained for the analogous solar cells based on PTzBDT-2. The lower performance is presumably attributed to the limited solubility of the PTzBDT-1 in organic solvents resulting in enhanced aggregation and poor intermixing with the acceptor material in the active layer.

Open circuit voltage decay as a tool to assess the reliability of organic solar cells: P3HT:PCBM vs. HBG1:PCBM

We developed a model that explain the open circuit voltage decay in polymeric solar cells to analyze the effects of the degradation induced by accelerated life-tests. The model accounts for the polaron pairs separation and recombination, the Langevin recombination, and the trap assisted recombination. The model permits to calculate several parameters. Among them, for the first time, we were able to extrapolate the Energy Gap of the blend from electrical, non-destructive measurements. We performed electrical stresses on organic solar cells based on two different active layers (P3HT:PC61BM, HBG1:PC61BM) applying specific characterizations, including open circuit voltage decay. We showed the better reliability of HBG1 based devices with respect to the analogous systems containing P3HT as donor material.