Annette Petrich

Imbalanced mobilities causing S-shaped IV curves in planar heterojunction organic solar cells

We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carriermobilities(hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[ 1 , 2 , 3 -cd : 1 ′ , 2 ′ , 3 ′ -lm]perylene), with the acceptors C 60 and N , N ′ -dimethylperylene-3,4:9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interfaceeffects but also the photoactive material itself can cause S-kinks.

Improved bulk heterojunction organic solar cells employing C70 fullerenes

We show that the fullerene C70 is suitable to replace fullerene C60, which is commonly used as electron transporter and acceptor in small-molecule organic solar cells. It is shown that the higher absorption of C70 leads to high external quantum efficiencies of over 50% in the spectral range of 500–700 nm. By optimizing the energy level alignment to hole transport layers, the absorption, and the ratio of C70:zinc phthalocyanine (ZnPc) in a bulk heterojunction solar cell, an efficiency of η=2.87% is achieved. This is a substantial improvement over an identical solar cell employing C60 having η=2.27%. The efficiency increase is due to a higher photocurrent, while fill factor and open-circuit voltage for C70 and C60-containing organic solar cells remain comparable.

Thick C60:ZnPc bulk heterojunction solar cells with improved performance by film deposition on heated substrates

We study the influence of different substrate temperatures during the deposition of the ZnPc:C60 blend layer in bulk heterojunction organic solar cells. It is shown that substrate heating during evaporation leads to a significant improvement in the solar cell performance mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved morphology resulting in better charge carrier percolation pathways within the ZnPc:C60 blend, leading to reduced transport losses. Using this method, blend layer thicknesses of 150 nm are possible without loss in FF, which requires a three-dimensional interpenetrating network without isolated clusters. When heating the substrate up to 110 °C, an efficiency of 2.56% is achieved compared to 1.59% for an identical device prepared at room temperature.

Investigation of C60F36 as low-volatility p-dopant in organic optoelectronic devices

We demonstrate highly efficient small molecule organic light emitting diodes and organic solar cells based on the p-i-n-type structure using the fluorinated fullerene molecule C60F36 as p-dopant in the hole transport layer. We present synthesis, chemical analysis, and energy level investigation of the dopant as well as the conductivity of organic layers consisting of a matrix of N,N,N′,N′-tetrakis 4-methoxyphenyl-benzidine(MeO-TPD) or N,N′-[(Diphenyl-N,N′-bis)9, ?> 9,-dimethyl-fluoren-2-yl]-benzidine(BF-DPB) doped by the fullerene compound. State of the art organic p-i-n devices containing C60F36 show efficiencies comparable to devices with the commonly used p-dopant2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). The advantages of the fullerene based dopant are the low volatility and high thermal stability, which is beneficial for device operation under elevated temperature. These properties make C60F36 highly attractive for the usage as p-dopant in a broad spectrum of organic p-i-n device...

53.3: Influence of Material Purification by Vacuum Sublimation on Organic Optoelectronic Device Performance

In organic optoelectronic devices, in particular organic solar cells and organic electroluminescent devices, reduction of impurities is a critical issue. A frequently used process for oligomers (small-molecules) is sublimation. We demonstrate that the sublimation process is effective to purify organic molecular compounds. Examples are shown for simplified device structures that allow easy evaluation of the impurity effect. New compact vacuum sublimation systems have been developed to process and handle material volumes in excess of 200 cm3.

Characterisation of different hole transport materials as used in organic p-i-n solar cells

To reach higher performances in organic solar cells, each layer has to be optimised with respect to its purpose. In the case of a p-i-n structured solar cell, the layers are the absorber system, the doped electron and hole transport layers, and the bottom and top contacts. This work focuses on the investigation and characterisation of the transparent hole transport materials PV-TPD, PV-TPDoM, Di-NPB, and MeO-Spiro-TPD, as used in organic p-i-n solar cells. The motivation is to replace the hole transport material MeO-TPD, which has been used so far despite its morphological instability at elevated temperatures, with an energetically and morphologically more suitable material. The hole transport materials were investigated for dopability, hole mobility, absorption, reflection, cyclic voltammetry, and glass transition temperature. Further specific material properties were determined with simplified structures, e.g. m-i-p diodes, and the standard solar cells, consisting of the fullerene C60 as acceptor and ZnPc as the donor material. The Di-NPB has turned out to be the best choice with respect to its intrinsic properties and device parameters. The deep lying HOMO, the high hole mobility of μ = 1.9 • 10-4 cm2/V s, the morphological stability of Tg = 158°C, and the excellent results of the C60:ZnPc bulk heterojunction solar cell makes the Di-NPB highly suitable for replacement of the MeO-TPD in organic

Improved photon harvesting by employing C70 in bulk heterojunction solar cells

To achieve higher efficiencies in organic solar cells, ideally the open circuit voltage (VOC), fill factor (FF) as well as the short current density (JSC) have to be further improved. However, only a few suitable acceptor molecules, e.g. C60, are currently available for the photoactive layer. Despite a good electron mobility on the order of 1×10-3 cm2/Vs the absorption of C60 in the visible sun spectrum is low. From polymer based solar cells it is known that the fullerene derivative [70]PCBM used in the photoactive layer shows a significant enhancement in JSC compared to [60]PCBM. This work investigates the application of fullerene C70 as acceptor in comparison to the well known C60 in vacuum processed small molecule solar cells. C70 shows a broadened and red shifted absorption (abs. maximum around 500 nm) compared to C60. By fabricating p-i-i solar cells we show that the stronger absorption of C70 leads to enhanced photon harvesting and increased external quantum efficiency. The bulk heterojunction p-i-i solar cell containing C70 as acceptor and ZnPc as donor, co-evaporated with an optimized ratio of 2:1, and a layer thickness of 30 nm shows improved solar cell parameters: a 30% larger photocurrent of 10.1 mA/cm2 is obtained. The VOC of 0.56 V and FF of 55% remain comparable to C60-containing p-i-i solar cells. Therefore, the solar cell performance is mainly improved by JSC and leads to a mismatch corrected power conversion efficiency of 3.12%. Thus, we show that C70 is an alternative fullerene to C60 for solar cell applications.

Recent progress in organic solar cells based on small molecules

We report on a series of organic solar cells based on heterojunctions of oligothiophene derivatives with varying chain length and C60 fullerenes. Devices are based on either p-i-n or p-i-i structure. In the first the intrinsic photovoltaic active layer is sandwiched between a p-type and n-type doped organic wide-gap layer for hole and electron transport respectively. In the latter the electron transport layer is replaced by a thin layer of wide-gap material as exciton blocker. Through optimization of transport and absorber layers we are able to reach in devices with single heterojunctions an open circuit voltage Voc of about 1V, a short circuit current density Jsc of about 5.6mA/cm2 and a fill factor FF above 50% under an AM1.5 illumination with 1000W/m2. However, still only a small part of the available solar spectrum is used. Thus, based on these materials stacked solar cells have been made to further improve the light absorption. The thickness of each layer is optimized using optical simulations to match the currents delivered by each of the solar cells in the stack. Through the incorporation of a very efficient recombination zone between the stacked solar cells the resulting Voc nearly reaches the sum of the Voc of the two serially connected solar cells.