Role of Vertical Segregation in Semitransparent Organic Photovoltaics

Abstract

In this work the efficiency of semitransparent Organic photovoltaic (OPV) devices for low intensity applications is investigated as a function of the processing conditions.It is observed that a thermal treatment of the organic layer induces fullerene migration towards the active layer/air interface. This physical process gives rise to different vertical segregation profile of donor and acceptor molecules. Once the back contact is deposited the amount of fullerene covering the surface will determine the contact selectivity and leakage current of the device. Control of this leakage current may not be essential for devices fabricated for high illumination conditions applications. However, devices to be used under low illumination conditions may be highly influenced by the presence of this parasitic dark current which flows in the opposite direction to photogenerated current. At the proximity of the contacts the vertical segregation profile is inferred from optical and electrical measurements. In particular, External Quantum Efficiency (EQE) measurements carried out from a relatively opaque back contact provide local information on the materials spatially close to the light source.Alternatively, Capacitance-Voltage measurements enable calculation of the percentage of fullerene molecules covering the cathode contact. Overall, a versatile method that can be used in regular and inverted configuration is presented that explain the different behavior observed for devices to be used under low illumination conditions. Keywords: Organic photovoltaics, low illumination conditions, contact selectivity, Capacitance-Voltage, impedance spectroscopy. 2 I ntroductionOrganic Photovoltaics (OPV) have been studied for nearly 30 years and the technology is now on the verge of commercialization. With record power conversion efficiencies slightly exceeding 10 % OPV still lag behind other technologies such as Silicon-based photovoltaics that can provide 20 % in modules. 1 However, the benefits of OPV rely on the possibility to produce flexible and low weight products with a high degree of design freedom with very rapid payback times. 2 In addition, it has been claimed that OPV can overperform silicon technology under low light conditions. 3These advantages and the ease of handling in subsequent product-integration processes will enable its implementation into new consumer and portable electronics such as building-integrated photovoltaic (BIPV) products.valueadded applications may be envisaged such as the use of semitransparent windows BIPV that permit both natural or color designed room illumination with production of solar electricity. Such a semitransparent device could additionally function during the night as a room light scavenger working under low illumination conditions to recover part of the wasted illumination energy. A range of different techniques have been developed to study segregation of donor and acceptor molecules within the active layer. 15 Amongst the different types some provide 3D structural information of the...

Morphology versus Vertical Phase Segregation in Solvent Annealed Small Molecule Bulk Heterojunction Organic Solar Cells

“…sh in its turn can be influenced by the vertical phase segregation; high fullerene (electron transport material) cathode population reduces cell shunting as far as electrons can be easily transferred from the fullerene to cathode. Indeed, it is known that the thermal or solvent annealing can result in the vertical phase segregation in the particular components of BHJ [37,39]. To get deeper insight into the processes ongoing in the devices we performed capacitance-voltage measurements in order to reveal the changes of fullerene population on the top (cathode) contact.…”

“…To get deeper insight into the processes ongoing in the devices we performed capacitance-voltage measurements in order to reveal the changes of fullerene population on the top (cathode) contact. C-V measurement has proven to be a precise way to control a vertical segregation in working devices [37,39]. In its turn, correct contacts population is a significant parameter to reduce shunting in the solar cell [37][38][39][40][41], thus improving OSCs performance.…”

“…C-V measurement has proven to be a precise way to control a vertical segregation in working devices [37,39]. In its turn, correct contacts population is a significant parameter to reduce shunting in the solar cell [37][38][39][40][41], thus improving OSCs performance. DPP(TBFu) 2 material needs to be either thermally annealed [24,25] or solvent [27] annealed which results in reorganization of the solid state ordering of the material.…”

Reducing Recombination Processes in the Inverted-Solution-Processed Small-Molecule Solar Cells by the Inserted Fullerene Cathode

“…Introducing a 10-nm layer onto the cathode ensures its 100% electron transport material (fullerene) coverage. As seen from capacitance-voltage measurements [30], [37], [38], this reduces recombination processes in the solar cell, which can be observed in augmented FF and slight increase of J sc . Another assumption is that recombination processes can be reduced by the passivation of the interfacial trap states [40] by an inserted C 60 layer.…”

“…The flatband potential is obtained by the extrapolation of the slope until intercepting the X-axis. This approach, identifying cathode coverage, was proven by several cross-techniques in the previous reports (i.e., atomic force microscopy, transmission electron microscopy, and Kelvin Probe in [30], ellipsometry in [37], modified EQE measurements in [38], and electron energyloss spectroscopy in [39]). It is clearly seen that initial cathode coverage of the thermally annealed inverted devices is predominantly hole transport material (DPP(TBFu) 2 ), which is observed by the high value of flat-band potential that is comparable with the one of pure donor material [38].…”