Solid state p-n heterojunction sensitized photovoltaic solar cell with electron and hole conductors

A solid state p-n heterojunction comprising an electron conductor and a hole conductor; it further comprises a sensitising semiconductor, said sensitizing semiconductor being located at an interface between said electron conductor and said hole conductor. In particular, the sensitizing semiconductor is in form of quantum-dots. A solid state sensitized photovoltaic cell comprises such a layered heterojunction between two electrodes.

Solid state heterojunction and solid state sensitized photovoltaic cell

Udo Bach, Michael Graetzel, Robert Plass

A solid state p-n heterojunction comprising an electron conductor and a hole conductor it further comprises a sensitising semiconductor, said sensitizing semiconductor being located at an interface between said electron conductor and said hole conductor. In particular, the sensitizing semiconductor is in form of quantum-dots. A solid state sensitized photovoltaic cell comprises such a layered heterojunction between two electrodes.

2005

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

Organic semiconductor materials have been widely applied in optoelectronic devices to replace their inorganic counterparts and explore new fields of applications. Due to their high extinction coefficients, chemical tunability and solubility, cyanine dyes are an interesting class of molecules to be employed in organic semiconductor devices. Their beneficial properties have been demonstrated in organic solar cells (OSC), photodetectors and recently in organic light-emitting electrochemical cells (OLEC). The focus of this thesis is to analyse different aspects regarding the dynamics of electronic and ionic charges in cyanine based devices. Models and conclusions are not restricted to cyanine based devices but can potentially be applied in various systems where combined ionic and electronic charges are present. In order to focus on electronic processes in OSCs, a combination of steady-state, transient and impedance analysis is used. It is resolved that trapped charges can be introduced at the dye/hole extracting layer interface by using specific solvents for the cyanine layer fabrication. These measurements also reveal the low hole mobility of cyanine films which in turn limits the possible upper layer thickness in OSCs. A frequently used way to enhance the conductivity of organic semiconductors is the addition of extra charges by chemical doping. This concept is employed for a near-infrared absorbing dye which is used as donor in a bilayer solar cell. The dependence of all figures of merit versus doping concentration and layer thickness is studied in detail. It is found that the performance of the optimised cell is not further improved by doping, but that it presents a good option if thicker layers are required, e.g. on rough substrates. Furthermore, the stability and degradation pathway of the p-doped species is analysed. This information is of general relevance as they also occur after charge transfer at the heterojunction interface of an OSC and during operation of OLECs. The dynamics of ionic charge carriers is separated into two contributions. In a first step, the steady-state diffusion profile of the counter ion within a cyanine/fullerene bilayer structure is scrutinised using different profiling techniques. An unexpected constant ion profile is found throughout the C60 layer. Simultaneously, the potential within the acceptor is measured by the Kelvin probe technique. A mechanism of combined electron and ion transfer from the C60 to the dye and vice versa is proposed which is in agreement with the experimental results. A prerequisite for OLEC operation is the mobility of ionic charge carriers. Their dynamics in cyanine salts under the effect of an external field is therefore studied in such devices. Several methods are developed and applied to characterise the dynamic p-i-n structure in detail, i.e. the position and width of the intrinsic region as well as the doping concentrations in p- and n-doped regions. The combination of all these methods helps to understand the transient behaviour of these cyanine devices but can also be adapted to other OLEC systems. A major drawback of cyanine dyes as emissive material is their low fluorescence quantum yield in solid films. To overcome this restriction and improve the OLEC efficiency, host-guest systems with various visibly emitting dyes are studied. The gain in fluorescence quantum yield in the film can directly be transferred to an increase in external quantum efficiency of the device.

EPFL2017

Nanoparticle sensitisation of solid-state nanocrystalline solar cell

Robert Plass

Over the past fifteen years dye-sensitised nanocrystalline solar cells have been the subject of intense research and development efforts. These systems provide a technically and economically credible alternative to classical p-n junction solar cells, reaching over 10 % certified efficiency under standard solar illumination conditions (AM 1.5, 1000 W/m2). Recently, the liquid electrolyte, commonly used in these dye-sensitised solar cells, could successfully be replaced by a novel solid hole-conducting material (spiro-OMeTAD). The absence of volatile solvents and corrosive components like iodine presents a clear advantage of these solid-state devices over their photoelectrochemical counterparts. Yet, their maximum overall efficiency of about 3 % clearly lacks behind the performance features of classical dye-sensitized solar cells. The objective of this work is the replacement of the usually used sensitiser (transition metal complexes and organic dyes) by quantum-dots. It was motivated by the possibility to achieve panchromatic light absorption due to the size-dependent properties of the quantum-dots. Some studies have been published using quantum-dots of inorganic low-bandgap semiconductors as sensitisers for mesoporous wide bandgap semiconductor electrodes in conjunction with liquid electrolytes. These systems often suffer from corrosion and photo-corrosion mainly due to the aggressive nature of the electrolyte employed. Solid-state organic-inorganic heterojunctions might provide the desired environment for quantum-dot sensitisation, as the environment is much less aggressive. A large variety of inorganic quantum-dot materials like PbS, CdS, PbSe and CdSe were scrutinized for their use as sensitisers. All particles were synthesised in situ on the TiO2 surface using two different techniques: dip-coating and chemical bath deposition. Lead sulphide was the most investigated due to its superior photovoltaic characteristics. High Resolution Transmission Electron Microscopy (HRTEM) of PbS sensitised TiO2 electrodes fabricated via dip-coating clearly showed the distinct particles on the surface of the semiconductor. The electron transfer dynamics following optical excitation of quantum-dot sensitized TiO2/spiro-OMeTAD heterojunctions were thoroughly studied. Fluorescence spectroscopy was used to prove the possible electron injection from the PbS into the TiO2. Nanosecond laser spectroscopy was applied to monitor the interfacial recombination of the injected electron and the oxidised hole-conductor. Femtosecond laser spectroscopy allowed measuring the ultra-fast kinetics in the system, including electron trapping, exciton recombination and PbS regeneration. An overall efficiency of 0.5 % at 0.1 sun (AM 1.5) has been reached with such systems. Chemical bath deposition (CBD) of PbS leads to the formation of a layer rather than distinct particles on the TiO2. Solar cells fabricated via CBD exhibited open-circuit voltages which were generally 100 mV higher then those of comparable devices made via dip-coating. The PbS layer acts as blocking layer, hindering the contact between TiO2 and the hole-conductor. This technique allowed achieving devices highly linear with respect to the illumination power. Different strategies were pursued to impede interfacial recombination, among which the introduction of self-assembled monolayers at the interface between the organic and inorganic phases was proven to be the most efficient. Nanosecond laser spectroscopy showed a strong decrease of the interfacial recombination kinetic rates in the presence of hexadecylmalonic acid or decylphosphonic acid monolayers. Short-circuit currents could be raised by a factor two to four using such type of molecules. The overall efficiency of the device could be strongly increased, reaching 1 % at 0.1 Sun (AM 1.5). CBD was also used to deposit PbSe and CdSe layers at the surface of TiO2. Fluorescence spectroscopy was used to probe electron injection from CdSe quantum dots to TiO2. Generally it was observed that metal sulfides were more efficient in sensitizing TiO2/OMeTAD heterojunctions then the respective selenides.