When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.
Optimizing the post-production annealing conditions of polymer:fullerene bulk heterojunction solar cells is vitally
important, not only for fine-tuning the morphology - thus increasing the efficiency - but also for retaining the desired
morphology during long-term operation. However, optimal conditions for annealing temperatures and times can only be
chosen, once thermal transition temperatures and annealing kinetics of the blends are well-known. For instance, for
systems with glass transition temperatures (Tg) lower than the maximum device operation temperature of 80°C, the
mobility needed for morphology coarsening is present, leading to efficiencies decreasing in the course of time. Using
advanced fast-scanning thermal analysis techniques, the formation of nuclei and growth of crystals during heating or
cooling can be reduced or avoided, and thus, the fast crystallization processes occurring during annealing of the
polymer:fullerene blends can be followed. In this study, non-isothermal and isothermal crystallization kinetics of the
P3HT:PCBM (poly(3-hexyl thiophene: [6,6] -phenyl C61 - butyric acid methyl ester) and P3HT:bis-PCBM blends are
investigated and compared by using Rapid Heating Cooling Calorimetry (RHC).
Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by
using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated
in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of
standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is
observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from
12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal
annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar
cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to
25 nm wide and 0.5 to >4 μm long and mainly crystalline. By means of temperature control the fiber content in the
casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer
in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers,
a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T,
P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal
annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved
to be a potentially competitive material compared to P3HT.
This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.
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