Physics and Growth of Planar and Bulk Heterojunction Solar Cells based on Organic Small Molecules (Fysica en groei van vlakke en bulkheterojunctie zonnecellen gebaseerd op organische kleine moleculen)

Abstract

This thesis tried to improve on the efficiency of organic solar cells based on small molecules. Therefore, new materials, growth conditions and the bulk heterojunction architecture have been investigated.VI.1 Summary chapter 2Chloroboron (III) subnaphthalocyanine (SubNc) was introduced as a donor material for organic solar cells. The absorption spectrum of SubNc extends to the near infrared portion of the solar spectrum, with an optical gap of 1.7 eV. The SubNc/C60-based solar cells maintain a high Voc of 790 mV, producing a power conversion efficiency of 2.5%. This combination of a strong and narrow red absorption with a high Voc make SubNc/C60 solar cells an excellent candidate for the use in tandem cells. VI.2 Summary chapter 3Fluorinated fused subphthalocyanine dimer (FSubPcDimer) proved to be an exceptional novel acceptor material with complementary absorption to the donor material subphthalocyanine (SubPc). Compared to an optimized solar cell with C60 as an acceptor, this leads to an enhanced photocurrent while preserving the high open-circuit voltage characteristic of SubPc-based devices. In order to integrate this new material in a solar cell, we compare multiple solar cell architectures and find that it is crucial to optimize the electron-extracting contact. The best FFs are achieved by introducing a C60 interlayer in between FSubPcDimer and BCP, leading to a 4% efficient cell, which is very efficient high for cells with another acceptor than fullerene. The spectral response and reflection data show that the SubPc/FSubPcDimer interface is the photoactive one, whereas C60 functions as a transport and optical spacer layer which enhances electron extraction, and positions the active layers near the peak of the incident light intensity.VI.3 Summary chapter 4Here, the physics of CuPc:C60 bulk heterojunction devices is studied. The hole mobility of CuPc:C60 is determined from space charge limited currents, and set at μ0,p= 1.2 10-4 cm2/Vs, with field activation parameter γp= 1.3 10-3(cm/V)1/2. Comparison of external quantum efficiency measurements of normal and inverted cells reveals that only holes generated near the anode are collected. The voltage and light dependence of the photocurrent are consistent with the behaviour of space-charge limited solar cells.Numerical simulations (ATLAS, Silvaco) involving optical simulation, Onsager-Braun dissociation, Langevin recombination and electrical transport equations could reproduce the Jsc, FF and ηp dependence of the devices versus cell thickness.VI.4 Summary chapter 5Chloro-aluminum phthalocyanine films are grown on either FDTS or MoO3. At RT, the ClAlPc films grow amorphous, independent of the surface. When heated to 105°C, ClAlPc grows face-on on MoO3, with a phase I-like absorption profile. On FDTS, the film is characterized as edge-on, phase II. The different growth modes had a clear impact on planar HJ cells fabricated on top of the ClAlPc films. On FDTS, the induced phase is accompanied by a lower Voc, but also by a higher current, leading to 3 % efficient planar HJ cells. Next, ClAlPc and C60 are co-evaporated on FDTS and MoO3 substrates. In order to achieve an enhanced crystallinity, a thin pure templating layer of ClAlPc proved advantageous. When the films were used in bulk HJ cells, the more crystalline films grown on top of a templating layer showed higher FF than devices without templating layers. The use of the co-evaporated bulk HJs ensured high photocurrents, independent of the growth surface. Here, the higher Voc of devices on MoO3 was beneficial, with devices showing efficiencies greater than 4% as a result.nrpages: 146status: publishe