Mathieu Turbiez

Towards 15% energy conversion efficiency: a systematic study of the solution-processed organic tandem solar cells based on commercially available materials

Owing to the lack of scalable high performance donor materials, studies on mass-produced organic photovoltaic (OPV) devices lag far behind that on lab-scale devices. In this work, we choose 6 already commercially available conjugated polymers and systematically investigate their potential in organic tandem solar cells. All the devices are processed under environmental conditions using doctor-blading, which is highly compatible with mass-production coating technologies. Power conversion efficiencies (PCE) of 6–7% are obtained for OPV devices based on different active layers. Optical simulations based on experimental data are performed for all realized tandem solar cells. An efficiency potential of ∼10% is estimated for these compounds in combination with phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor. In addition, we assume a hypothetical, optimized acceptor to understand the limitation of donors. It is suggested that a PCE of >14% is realistic for tandem solar cells based on these commercially available donor materials. Along with the demonstration of novel intermediate layers we believe that this systematic study provides valuable insight for those attempting to realize the high efficiency potential of tandem architectures.

Mixed π-conjugated oligomers of thiophene and 3,4-ethylenedioxythiophene (EDOT)

Abstract Conjugated oligomers based on various combinations of thiophene and 3,4-ethylenedioxythiophene (EDOT) moieties have been synthesised. Comparison of the optical and electrochemical properties shows that the introduction of a bis-EDOT core in the middle of the system produces a decrease of the HOMO–LUMO gap which is attributed to an enhancement of the planarity and rigidity of the π-conjugated system.

Linearly extended tetrathiafulvalene analogues with fused thiophene units as π-conjugated spacers

A new series of linearly extended tetrathiafulvalene analogues with thienothiophene and dithienothiophene π-conjugating spacers has been synthesized. Electronic absorption spectra present a vibronic fine structure typical for rigid conjugated systems. Investigation of the electrochemical behaviour of the new donors by cyclic voltammetry reveals the successive generation of stable radical cation and dication species. The crystallographic structure of a single crystal of a dication salt of TT-TTF(ClO4)2 has been analysed by X-ray diffraction. The dication presents a syn conformation stabilised by S⋯S intramolecular interactions. The quinoid structure expected for the spacer for the +2 oxidation state is clearly revealed by the bond lengths.

Fully printed organic tandem solar cells using solution-processed silver nanowires and opaque silver as charge collecting electrodes

We report in this work efficient, fully printed tandem organic solar cells (OSCs) using solution-processed silver as the reflective bottom electrode and silver nanowires as the transparent top electrode. Employing two different band-gap photoactive materials with complementary absorption, the tandem OSCs are fully printed under ambient conditions without the use of indium tin oxide and vacuum-based deposition. The fully printed tandem devices achieve power conversion efficiencies of 5.81% (on glass) and 4.85% (on flexible substrate) without open circuit voltage (Voc) losses. These results represent an important progress towards the realization of low-cost tandem OSCs by demonstrating the possibility of printing efficient organic tandem devices under ambient conditions onto production relevant carrier substrates.

A solution-processed barium hydroxide modified aluminum doped zinc oxide layer for highly efficient inverted organic solar cells

Inverted organic solar cells (iOSCs) with air stable interface materials and top electrodes and an efficiency of 6.01% are achieved by inserting a barium hydroxide (Ba(OH)2) layer between the aluminum doped zinc oxide (AZO) electron extraction layer and the active layer. A low bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (pDPP5T-2) and phenyl-C61-butyric acid methyl ester (PC61BM) were chosen as the active layer compounds. Compared to the control device without Ba(OH)2, insertion of a few nm thick Ba(OH)2 layer results in an enhanced VOC of 10%, JSC of 28%, FF of 28% and PCE of 80%. Modification of AZO with a solution processed low-cost Ba(OH)2 layer increased the efficiency of the inverted device by dominantly reducing the energy barrier for electron extraction from PC61BM, and consequently, reduced charge recombination is observed. The drastic improvement in device efficiency and the simplicity of fabrication by solution processing suggest Ba(OH)2 as a promising and practical route to reduce interface induced recombination losses at the cathode of organic solar cells.

Roll to roll compatible fabrication of inverted organic solar cells with a self-organized charge selective cathode interfacial layer

We successfully demonstrate a simple approach to printing efficient, inverted organic solar cells (OSCs) with a self-organized charge selective cathode interface layer based on the small-molecule Phen-NaDPO. Different from previous studies, Phen-NaDPO molecules were blended into a polymer/fullerene blend, comprising a low bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer pDPP5T-2 and phenyl-C61-butyric acid methyl ester (PC61BM), and processed by doctor blading in air. We observed a spontaneous, surface energy driven migration of Phen-NaDPO towards the ZnO interface and a subsequent formation of electron selective and barrier free extraction contacts. In the presence of 0.5 wt% Phen-NaDPO, a PCE of 5.4% was achieved for the inverted device based on an ITO/ZnO cathode. Notably, the photovoltaic performances remained at the same level with increasing the Phen-NaDPO concentration in the active layer from 0.25 to 1 wt%. Furthermore, this approach could be proven to effectively work with other cathodes such as bare ITO and ITO/AZO. The self-organization of Phen-NaDPO through spontaneous vertical phase separation is mainly attributed to its high surface energy and strong interaction with the cathode material. The present results highlight that a self-organized cathode interfacial material processed from a “ternary” active layer is fully compatible with the requirements for roll-to-roll fabrication of inverted organic solar cells.

Poly(3,6-dimethoxy-thieno[3,2-b]thiophene): a possible alternative to poly(3,4-ethylenedioxythiophene)(PEDOT)

Electropolymerization of the title compound leads to a conjugated polymer with redox potential, band gap, optical transparency in the doped state and stability similar to those of PEDOT.

A universal method to form the equivalent ohmic contact for efficient solution-processed organic tandem solar cells

The highly transparent, conductive and robust intermediate layer (IML) is the primary challenge for constructing efficient organic tandem solar cells. In this work, we demonstrate an easy but generic approach to realize the fully functional, solution-processed IMLs. In detail, solution-processed silver-nanowires are packed at low concentration between hole- and electron-transporting layers to convert an otherwise rectifying interface into an ohmic interface. The IMLs are proven to be of ohmic nature under applied bias, despite the unipolar charge selectivity of the single layers. Ohmic recombination within IMLs is further proven in organic tandem solar cells fabricated by doctor-blading under ambient conditions. The tandem solar cells based on PCDTBT:[70]PCBM as the bottom cell and pDPP5T-2:[60]PCBM as the top cell give a power conversion efficiency of 7.25%, which is among the highest values for solution-processed organic tandem solar cells fabricated by using a roll-to-roll compatible deposition method in air.

Bis-EDOT end capped by n-hexyl or n-hexylsulfanyl groups: the effect of the substituents on the stability of the oxidized states

Three bis-EDOT derivatives end capped with n-hexyl or n-hexylsulfanyl groups have been synthesized. The oxidation processes of these compounds have been studied using cyclic voltammetry and UV-Vis-NIR spectroscopy. The strong influence of the sulfur atoms of the hexylsulfanyl chains for stabilizing the oxidation states is evidenced. Thus, it is shown that the two compounds grafted with one or two hexylsulfanyl groups are reversibly oxidized into cation radical and dication states. By contrast, compounds end capped with hexyl groups present a fast σ-dimerization of the radical cation. The reduction of the σ-dimer to give the neutral bis-EDOT derivative is slow.