"Exploration of graphene addition and boron doped nanocrystalline diamond transparent electrodes for charge transport enhancement in polymerbased solar cells and light emitting diodes."
Organic solar cells are an upcoming technology in the area of photovoltaic energy conversion. It distinguishes itself from classical silicon based solar cells according to the following characteristics: Mechanical flexibility (‘flexible’ solar cells), esthetic possibilities (freedom in design and color), improved response for indoor and diffuse light (interesting for indoor energy supply for mobile applications) and simple low cost processing possibilities (e.g. printable solar cells). These unique properties make organic solar cells interesting for certain niche applications (such as building integrated photovoltaics (BIPV)). Due to low efficiency and stability, currently only a few commercial applications are available and a major breakthrough is waiting. Research is performed all over the world for increasing the device efficiency and lifetime. Also environmentally green and robust production processes are being developed.
In organic solar cells, a combination of two components is normally used in the photo-active layer and the rare and expensive material Indium Tin Oxide (ITO) is used as transparent electrodes. The innovative routes explored in this work are: (I) the use of graphene as a 3rd component in the active layer and (II) the use of diamond as an electrode material in order to replace ITO.
The following research questions were investigated in this work:
- What effect will graphene have on thin polymer:fullerene solar cell devices when the graphene is used as a ternary component in the active layer at densities below percolation densities?
- Is it possible to replace Indium Tin oxide (ITO) by constructing transparent electrodes based on diamond and what are the physical characteristics of diamond electrodes? Is it possible to use these diamond based electrodes for Polymer Light Emitting Diodes (PLED) and Organic Photovoltaic (OPV) devices?
Graphene as a third component in donor:acceptor organic solar cells
In organic solar cells, a combination of two components is normally used in the photo-active layer: an electron donating material (e.g. conjugated polymer) and an electron accepting material (e.g. a fullerene derivate). In this work, the influence of the addition of a third component, graphene, is investigated. In particular, the effect of graphene for the regulation of the charge transport is explored.
In Chapter 2 we identified the threshold concentration for graphene in thin film solar cells and investigated the effects of graphene below this threshold voltage. When graphene is used above the threshold fraction, where graphene flakes start to cluster, this results in increased dark leakage currents, which finally degrade device performances. Light induced leakage currents are less prominent in this study. This is due to the fact that PEDOT:PSS layers remained passive with respect to the origin of the leakage currents. The addition of low concentrations of graphene in P3HT:PCBM solar cell devices induces a balance in hole- and electron transport mobilities. This induces increased efficient photocurrent escape from devices. The reduction of electron mobility is due to trapping of electrons from PCBM in graphene induced electron trapping centers. The enhancement of hole mobility results from enhanced P3HT crystallinity in the active layer after graphene addition. These two effects result in lower ambipolar charge mobility while electron and hole mobilities are balancing out. This results in turn to higher charge extraction efficiencies and ultimately to improved solar cell device performance.
BNCD:Cr/Au as transparent electrode for organic electrical applications
A key challenge for future opto-electronic applications is the replacement of expensive and scarce Indium Tin Oxide (ITO) with highly stable and transparent electrodes. In chapter 3, an alternative is developed for the integration of a gold metal grid within transparent BNCD layers. This is carried out with a procedure involving lithography, metal depositions and wet and dry etching. This procedure results in thin, highly transparent electrodes with sheet resistance values below 20 /sqr. These new electrodes are usable in applications involving charge injection, such as PLEDs, and charge extraction, such as organic solar cells. Here, they show the possibility to work with similar or even higher performance qualities compared to their ITO alternatives.
Solar cell properties are further investigated in Chapter 4. Here we describe a procedure using JV-characteristics for analyzing effects of dark and light induced leakage currents. We see that higher leakage currents are present in devices where BNCD/Cr:Au electrodes are used compared to their ITO counterparts. Kelvin probe measurements on BNCD:Cr/Au electrodes, covered with PEDOT:PSS show how PEDOT:PSS manipulates the work function of the underlying BNCD:Cr/Au layer. At positions where dark leakage currents are high, the PEDOT:PSS layers might be damaged. This causes increased local electron leakage from the Au electrode in the active layer. Under high internal electrical fields or at short circuit current, leaking electrons from the Cr/Au grid recombine with photo generated free holes in a bimolecular fashion. Therefore, drift lengths and related collection efficiency of photo-generated charge carriers are reduced, resulting in a light induced leakage current. Also the charge transfer energy, as measured with FTPS at short circuit current conditions, is affected. At open circuit voltage, leaking electrons reside in trapped states inside the band gap of the active layer inducing trap assisted recombination. Therefore, it must be emphasized that for the development of hybrid transparent electrodes, the effects of leakage currents should be reduced as much as possible. Also an electron blocking material, with high affinity to the metal grid of the electrode, should be used in order to reduce electron leakage from the Au-grid in the hole extraction region of the active layer. Good adhesion of the electron blocking layer to the BNCDelectrode can be useful in order to increase its work function, enhancing the open circuit voltage and device efficiency.