Tom Aernouts

Polymer based organic solar cells using ink-jet printed active layers

Ink-jet printing is used to deposit polymer:fullerene blends suitable as active layer for organic solar cells. We show that merging of separately deposited ink droplets into a continuous, pinhole-free organic thin film results from a balance between ink viscosity and surface wetting, whereas for certain of the studied solutions clear coffee drop effect occurs for single droplets; this can be minimized for larger printed areas, yielding smooth layers with minimal surface roughness. Resulting organic films are used as active layer for solar cells with power conversion efficiency of 1.4% under simulated AM1.5 solar illumination.

Extraction of bulk and contact components of the series resistance in organic bulk donor-acceptor-heterojunctions

Abstract Basic solar cell characteristics are examined in bulk donor-acceptor-heterojunction devices. Therefore, spin-coated organic blends of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) are used as active material sandwiched between a transparent IndiumTinOxide (ITO)-electrode and an Al backside contact. A comparison is made between cells with or without an extra interfacial layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode. Furthermore, the effect of the thickness of the active layer on the photovoltaic performance of the devices is studied. It is seen that applying this extra PEDOT/PSS layer results in an important increase of the contact component of the series resistance of the cells. At the same time, open circuit voltage improved for devices with an interfacial layer while the fill factor was higher for cells with no PEDOT/PSS film. For thinner active layers, in both cases the bulk component of the series resistance decreased. Nevertheless, a decrease of short circuit current is seen at reduced illumination. Shunt resistance shows a slight increase resulting in an improvement of open circuit voltage. Also, the fill factor increases for thinner active layers. For the thinnest standard devices a power conversion efficiency of over 3% could be measured under AM1.5 conditions.

Novel bis-C60 derivative compared to other fullerene bis-adducts in high efficiency polymer photovoltaic cells

We report the application of novel mono- and bis-o-quino-dimethane C60 (oQDMC60) adducts in bulk heterojunction photovoltaic devices. When blended with poly(3-hexylthiophene), the fullerene adducts presented here have an enhanced open-circuit voltage of 640 mV and 820 mV, while preserving high short-circuit current and fill factor, resulting in efficiencies of 4.1% and 5.2%, respectively. Detailed assessment of material properties relevant to photovoltaic devices such as energy levels, charge carrier mobility, absorption and solubility further complements the evaluation. Increased fullerene solubility hindering phase segregation in blends with bis-oQDMC60 has been circumvented by an in-depth morphology optimization assisted by absorption spectroscopy, X-ray reflectivity and atomic force microscopy. This optimized preparation could also serve as a guide for implementation of similar fullerene derivatives. Furthermore, we compare bis-oQDMC60 to previously reported fullerene bis-adducts to provide insight into this emerging class of materials.

Injection- and space charge limited-currents in doped conducting organic materials

Most conducting organic materials have a background p-type doping varying in the range 1015–1017 cm−3. We report results of a theoretical and experimental study of carrier transport in p-doped organic Schottky diodes. The theory given in this article shows that in a doped organic material with ohmic contacts the current is ohmic at low voltages. If the ohmic contact at the cathode is replaced by an Al Schottky contact the current varies exponentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltages. The voltage at which the change takes place depends on the doping concentrations. In the SCLC regime the current varies according to the well-known V2 law if there are no traps and the mobility is independent of the electric field. If either trapping or effect of field on mobility is important, the current varies as Vm, where m>2. We have investigated experimentally the I–V characteristics of Schottky diodes fabricated using the PPV-based oligomer 2,5-di-n...

Organic co-evaporated films of a PPV-pentamer and C60: model systems for donor/acceptor polymer blends

Abstract Organic solar cells based on an active layer of a spincast polymer donor/acceptor blend have proven to be very efficient. We present similar photovoltaic devices with organic layers that are formed using the technique of vacuum deposition. The donor and acceptor materials are, respectively, the five-ring PPV-type oligomer 2-methoxy-5-(2′-ethylhexyloxy)-1,4-bis((4′,4″-bisstyryl)styrylbenzene) (MEH-OPV5) and C60. An elevated substrate temperature during deposition of the single MEH-OPV5 layers on ITO-coated glass substrates yielded polycrystalline films with a rough surface, as was determined from AFM and XRD analysis. The co-evaporation of both materials, also at high substrate temperature, resulted in amorphous, but very smooth films exhibiting a good percolation of donor and acceptor. The dark I–V behaviour of single-layer and donor/acceptor-layer devices in an ITO/PEDOT/organic/Al configuration is compared. It was found that the PEDOT/C60 interface of an ITO/PEDOT/MEH-OPV5:C60/Al solar cell structure is responsible for the exponential rise of its dark I–V curve under forward bias. Sandblasting of the glass substrate was applied as a way to reduce the reflection of the incoming light and resulted in a significant increase of the short-circuit current. Standardised spectral response measurements confirmed this effect. The sandblasted bulk heterojunction photovoltaic devices were characterised under AM1.5 illumination and reached a power conversion efficiency of 2.2%.

The characterization of chloroboron (III) subnaphthalocyanine thin films and their application as a donor material for organic solar cells

Chloroboron (III) subnaphthalocyanine (SubNc) films have been characterized by ellipsometry, absorption, photoluminescence measurements, and atomic force microscopy. The films strongly absorb red light, as the extinction coefficient k peaks at 1.4 at a wavelength of 686 nm. Planar bilayer heterojunctions with fullerene (C60) on top of SubNc are measured under AM 1.5 simulated illumination at various light intensities, leading to an open-circuit voltage (Voc) of 790 mV and a power conversion efficiency of 2.5%. The external and internal quantum efficiencies peaked at 36% and 70%, respectively. The combination of a strong red absorption and high Voc make SubNc an interesting material for organic solar cells, in particular for tandem cells.

Scalable perovskite/CIGS thin-film solar module with power conversion efficiency of 17.8%

All-thin film perovskite/CIGS multijunction solar modules, combining a semi-transparent perovskite top solar module stacked on a CIGS bottom solar module, are a promising route to surpass the efficiency limits of single-junction thin-film solar modules. In this work, we present a scalable thin-film perovskite/CIGS photovoltaic module with an area of 3.76 cm2 and a power conversion efficiency of 17.8%. Our prototype outperforms both the record single-junction perovskite solar module of the same area as well as the reference CIGS solar module. The presented perovskite/CIGS thin-film multijunction solar module makes use of the “4-terminal architecture”, which stacks the perovskite solar module in superstrate configuration on top of the CIGS solar module in substrate configuration. Both submodules apply a scalable interconnection scheme that can accommodate scale-up towards square meter scale thin-film multijunction solar modules. In order to identify the future potential of the presented stacked perovskite/CIGS thin-film solar module, we quantify the various losses in the presented prototype and identify the key challenges of this technology towards very high power conversion efficiencies.

Role of Electron- and Hole-Collecting Buffer Layers on the Stability of Inverted Polymer: Fullerene Photovoltaic Devices

Systematic device performance and air stability comparison of inverted architecture polythiophene:fullerene photovoltaic cells with eight different electron-collecting layers (ECLs) and two hole-collecting layers are presented in this study. Regardless of the ECL, we achieved an efficiency of over 3.5% and lifetime of over 1000 h. These results indicate the relative interchangeability of various solution-processed ECLs. Long-term (>5000 h) air exposure revealed a secondary failure mechanism of inverted cells, which is assigned to hindered exciton harvesting. Notably, devices with a polymeric hole-collecting layer and Ag/Al electrode exhibited the longest lifetime (defined as 80% of the initial performance) of 4000 h, compared with 3000 h for MoO 3/Ag/Al.

Four-terminal organic solar cell modules with increased annual energy yield

The authors present experimental results on mechanically stacked organic solar modules and their advantage over standard tandem architectures. A four-terminal configuration of two single junction modules with complementary absorbing active layers uses the more efficient energy conversion of a tandem structure without the necessity of matching currents or voltages of electrically connected subcells. The presented combination of semitransparent and opaque solar cells consists of solution processed polymer-fullerene blends as active materials. A cost-effective mechanical scribing process is applied for the patterning of the deposited layers. The best devices have an efficiency of over 6.5% on an aperture area of 16 cm2 which equals a gain of 30% over the best single junction module fabricated by the same process. Optical simulations demonstrate a 32% increased annual energy output of a mechanically stacked device in comparison to a monolithic tandem structure using an equivalent geometry.

Efficient polymer solar cells via an all-spray-coated deposition

In this study, we report high performance organic solar cells with spray coated hole-injection and active layers. We were able to achieve high film quality, deduced from surface profilometry, atomic force microscopy and absorption measurements. With an automated ultrasonic spray coater we successfully deposit smooth and uniform PEDOT:PSS in optimum thicknesses of 40 nm and a mixture of P3HT and PCBM in thicknesses in the range 200–250 nm.