Deep level transient spectroscopy (DLTS) study of P3HT:PCBM organic solar cells

The electronic structure of an organic photovoltaic bulk heterojunction cell strongly deviates from the typical textbook examples of a single sided junction used to explain electrical characterisation of defects in semiconductors. Therefore it is not so straightforward to assign the capacitance of this device or the charge in it to the presence of a depleted layer within this structure. However, conventional electronic spectroscopic techniques could give useful information to understand the electronic behaviour of the device. Therefore, in this work capacitance and charge DLTS have been performed on P3HT:PCBM solar cells. At 1MHz only negligible variation in the capacitance as a function of temperature and bias has been observed. As a result no spectrum could be recorded using a standard DLTS setup, registering the capacitance at this high frequency. To avoid this parasitic effect low frequency capacitance DLTS (40 kHz) has been performed, showing an anomalous signal with negative amplitude and an activation energy of 160meV, and a complementary positive signal could be observed altering the biases. Charge DLTS clearly revealed that both signals transients, conventional and with altered bias have the same time constants. A recent study has shown that such behaviour cannot be explained by the thermodynamic properties of capture and emission of carriers by a defect in bulk semiconductor. The validity of alternative explanations, including interface states, non-ideal ohmic contacts and effects of carrier hopping on charge mobility, will discussed

Structural Characterisation of Printable Noble Metal/Poly(Vinyl-­Alcohol) Nanocomposites for Optical Applications

This work was conducted under the aegis of the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (EP/I004173/1). Amin Abdolvand is an EPSRC Career Acceleration Fellow at the University of Dundee.In order to enable exploitation of noble metal/poly(vinyl-alcohol) nanocomposites for device fabrication, solutions of poly(vinyl-alcohol) suitable for piezo-driven inkjet printing techniques are identified and discussed in terms of their material properties. The printable poly(vinyl-alcohol) medium is then exploited as a host material through the formation of silver or gold nanoparticles in order to create nanocomposites that exhibit a surface plasmon resonance behaviour associated with the small metallic inclusions. To mitigate some of the material redistribution effects associated with the drying of printed droplets containing finely divided materials, the metallic nanoparticles are formed after the printing and drying process is completed, by way of an in-situ reduction of an appropriate metal salt by the poly(vinyl-alcohol)-host matrix itself, which takes place at modest temperatures compatible with most substrate materials. An obvious application for such nanocomposites is in optical elements whereby the surface plasmon resonance associated with the metal is the functional aspect of devices such as sensors or active optical elements. High Resolution Transmission Electron Microscopy was used to examine the dimensions, distribution, morphology and crystal structure of the silver and gold nanoparticles in detail allowing discussion of their suitability for these applications and what further optimisation may be necessary to adequately control their formation.Publisher PDFPeer reviewe

EU COST Action MP1307 - Unravelling the degradation mechanisms of emerging solar cell technologies

Organic and hybrid perovskite based solar cells have a huge potential to significantly contribute to a clean electricity supply of the future. However, so far they exhibit complex and hierarchical degradation paths and their understanding can only be acquired through the application of complementary chemical and physical characterization techniques. This limited device stability is the main hurdle for a successful and large scale market introduction of these emerging solar cell technologies. Our StableNextSol Action has created a highly interdisciplinary network of laboratories, as well as corresponding industry, overall more than 120 partners, with complementary analytical techniques for the study and understanding of the degradation mechanisms occurring in state-of-the-art devices. Our Action integrates and generates fundamental knowledge and expertise to foster disruptive innovations targeted to mitigate device failure and to propose and develop new concepts for more stable solar cells. Value is added to the entire value chain of photovoltaic research at European and international level, as well as variety decision makers in the public sector by supporting specialisation policy and standards still lacking in this research field. The outcome of the Action will contribute to resolve the global challenges facing the industry and this COST Action initiative has brought together all these expertises and resources to promote the cooperation between different sectors, academia, public authorities and industry

Organic co-evaporated films of a PPV-pentamer and C-60: model systems for donor/acceptor polymer blends:model systems for donor/acceptor polymer blends

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%

Organic Solar Cells Based on Evaporated Planar and Bulk Heterojunctions of a PPVpentamer and C₆₀

ABSTRACTThe technique of vacuum evaporation has been applied to deposit organic photovoltaic active layers. The five-ring PPV-type oligomer 2-methoxy-5-(2'-ethylhexyloxy)-1,4-bis((4',4”-bisstyryl)styrylbenzene) (MEH-OPV5) and C60 act as respectively donor and acceptor materials in planar heterojunction (MEH-OPV5/C60) and bulk heterojunction (MEH-OPV5:C60) devices. These devices were fabricated with ITO/PEDOT:PSS bottom electrodes and Al top contacts. The performance of both solar cell configurations has been compared. It was found that under AM1.5 illumination the MEH-OPV5/C60 cells exhibit a higher open-circuit voltage (∼ 1.00 V) than the MEH-OPV5:C60 devices (∼ 0.92 V). On the other hand, the limited exciton diffusion length in these materials was reflected in the lower short-circuit current density of the planar heterojunction cells as compared to the bulk heterojunction structures. Overall AM1.5 power conversion efficiencies reaching 2 % are reported. Also the influence of the organic layer thickness and the substrate temperature during deposition on the device performance has been addressed. Thick organic films generally induce a high series resistance that limits both the short-circuit current density and the fill factor. An elevated substrate temperature during deposition of the MEH-OPV5:C60 layers onto ITO/PEDOT:PSS led to the formation of nucleated islands of 100 - 150 nm diameter with holes in between. As a result, no reliable photovoltaic devices could be realized with such organic films. AFM analysis and spectral response measurements supported these findings.</jats:p