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Solar & Alternative Energy

Highly efficient organic LED and solar cells using electrically doped layers

Dopant molecules can increase the conductivity of organic semiconductors by orders of magnitude, allowing them to outperform inorganic devices.
6 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200705.0761

Organic semiconductors are currently the object of intensive research because of their attractive application possibilities, such as flat-panel displays based on organic light emitting diodes (OLED), or organic solar cells. Their main advantages are low cost and a capacity for large-area deposition, even on flexible substrates. Additionally, the large variability of organic compounds allows tailoring the materials for specific applications. However, they have charge carrier mobilities significantly lower than for example silicon, which leads to rather low conductivities. This especially true for thin films. We have recently shown that conductivities can be raised by several orders of magnitude by adding a few percent of dopant molecules, thus avoiding ohmic losses and allowing efficient contacts with various types of electrode materials. Our studies were focused on evaporated layers doped by coevaporation with a molecular dopant, either for p-type1,2 or n-type3,4 charge transport. We used stable materials and performed careful analyses of their semiconducting properties.

Our results show that these electrical doping concepts can be successfully applied to the fabrication of green OLED devices with the highest efficiencies reported so far, even outperforming current inorganic GaN devices. We have also successfully introduced small organic molecules as dopants in OLED devices.5–8 Our work has also shown that using large organic molecules as dopants provides excellent stability and low doping concentrations when compared to doping with metals such as Li or Cs. Our first pin OLEDs6 displayed extremely low voltages of 2.55V for green devices, which is close to the thermodynamic limits for the voltage.9 Using the concept of the double emission layer shown in Figure 1(a) and incorporating phosphorescent emitters, as proposed in the pioneering work of Forrest et al.,10 we were able to design highly efficient OLEDs with small roll off at high luminance, as can be seen in Figure 1(b).8

Figure 1. (a) Energy diagram and layer sequence of a double emission layer organic light emitting diode: In between a doped hole transport layer (HTL) and an electron transport layer (ETL), an emission zone consisting of an electron blocking layer (EBL), two emission layers (EML) and a hole blocking layer (HBL) is embedded. Both emission layers contain the same emitter dye (dashed line). However, while the EML-1 host is preferably hole transporting, the EML-2 host mainly conducts electrons such that a broad exciton creation profile leads to a wide emission zone. (b) Quantum efficiency data of a green single emission layer (S-EML) and of D-EML devices.

This pin structure has now been further developed commercially. Recent results for monochromic pin OLEDs are described elsewhere.11 The Novaled group has also recently reported 132lm/W for a green OLED with a pin type structure and Ir(ppy)3 as the emitter. Their performance data is shown in Figure 2 and exceeds by about a factor of two that of the best currently available green InGaN devices.12 The concept of doped charge transport layers has also been successfully extended to other colors, including a highly efficient white OLED.13

Recently, we were able to demonstrate that highly stable doped OLEDs could be realized.14 In red devices, we showed that the lifetime limitation was mostly due to the choice of emitter host and charge blockers, instead of arising from the electrically doped transport layers. The most stable devices reached extrapolated lifetimes of several million hours when driven at 100 cd/m2 (see Figure 3).

Figure 2. Performance data of a green bottom-emitting pin organic light-emitting diode incorporating the phosphorescent emitter Ir(ppy)3 and Novaled-proprietary charge-carrier transport materials and dopants.12
Figure 3. (a) Lifetime decay curves of red pin organic light emitting diodes with different hole blockers driven at four different current densities (5, 10, 20, 30 mA/cm2), referring to different initial luminance. (b) Lifetime extrapolation for the devices shown in (a) using the stretched exponential decay model. For 100 cd/m2, we reach extrapolated lifetimes well above one million hours for different red devices.14 (Click to enlarge.)

Solar cells are another area where doped organic semiconductors are very useful. Due to the rather narrow absorption of cells, it is generally believed that stacked, ‘tandem’ solar cell concepts are needed to achieve higher power conversion efficiencies. A monolithic tandem cell requires a charge carrier recombination contact between individual cells. It has been shown that using n- and p-type doped regions in contact can produce efficient recombination contacts.15

Karsten Walzer
Institut für Angewandte Photophysik, Technische Universität Dresden
Dresden, Germany