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.
In this Proceedings paper, we report on the synthesis of a family of polythiophene-based conjugated polyelectrolytes, both homopolymers and random copolymers varying in the building block ratio and counter ions, toward a better fundamental understanding of the structure-property relations of these ionic derivatives in organic photovoltaics. One of the ionic homopolymers was successfully implemented as a donor material in fully solution-processed efficient bi-layer solar cells (up to 1.6% PCE in combination with PC71BM) prepared by the low impact meniscus coating technique. On the other hand, these imidazolium-substituted polythiophenes were also applied as materials for electron transport layers (ETLs), boosting the I-V properties of PCDTBT:PC71BM solar cell devices up to average PCE values of 6.2% (~20% increase), which is notably higher than for previously reported ETL materials. Advanced scanning probe microscopy techniques were used to elucidate the efficiency enhancing mechanism.
Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration
planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to DTU and characterized
simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun
simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally
identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for
a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the
devices. Characterization includes current-voltage curves, light beam induced current (LBIC) imaging, dark lock-in thermography (DLIT),
photoluminescence (PL), electroluminescence (EL), in situ incident photon-to-electron conversion efficiency (IPCE), time of flight secondary
ion mass spectrometry (TOF-SIMS), cross sectional electron microscopy (SEM), UV visible spectroscopy, fluorescence microscopy, and
atomic force microscopy (AFM). Over 100 devices with more than 300 cells were used in the study. We present here design of the device
sets, results both on individual devices and uniformity of device sets from the wide range of characterization methods applied at different
stages of aging under the three illumination conditions. We will discuss how these data can help elucidate the degradation mechanisms as well
as the benefits and challenges associated with the unprecedented size of the collaboration.
Long alkyl chain ligands such as oleic acid (OLA) which cover the as-prepared PbS nanodots act as an insulating layer
that impedes efficient charge transfer in PbS nanodots:polymer hybrid solar cells. The replacement of OLA with tailored
ligands of an appropriate chain length is needed to achieve a noticeable enhancement of photovoltaic performance.
Several studies have centered on the ligand exchange prior to casting the PbS film1,2,3. However, this post synthesis
approach requires careful consideration for the choice of a ligand as clustering of the nanodots has to be avoided.
Recently, a new approach that allows direct chemical ligand replacement in a blended mixture of PbS:P3HT has been
demonstrated 4,5,6. In this contribution, the latter approach (post-fabrication) was compared with the post-synthesis ligand
exchange. We investigated the effect of the ligand exchange processes to the charge separation dynamics in the
P3HT:PbS blends by steady-state and time-resolved photoluminescence (PL). Hexanoic acid and acetic acid were used
as a short-length ligand for the post fabrication approach while decylamine, octylamine and butylamine were used for the
post-synthesis approach. As expected, decreasing the chain length of the ligand led to an increase of the P3HT
fluorescence quenching. The absence of enhancement of PbS luminescence due to energy transfer from P3HT and the
dependence of the quenching efficiency on the bulkiness of the ligands coating the QDs suggest that the quenching of the
P3HT fluorescence is dominated by electron transfer to PbS quantum dots (QDs). In addition, the fluorescence
quenching is also less prominent in the P3HT with higher regioregularity (RR) suggesting an enhanced phase separation
in the blend due to more densely packed nature of conjugated polymer with higher RR.