Enhanced thermal stability of organic solar cells comprising ternary D-D-A bulk-heterojunctions

Abstract

Ternary absorber blends have recently been identified as promising concepts to spectrally broaden the absorption of organic bulkheterojunction solar cells and hence to improve their power conversion efficiencies. In this work, we demonstrate that D-D-A ternary blends comprising two donor polymers and the acceptor PC 61 BM can also significantly enhance the thermal stability of the solar cell. Upon harsh thermal stress at 120°C for 2 h, the ternary solar cells show only a minor relative deterioration of 10%. Whereas the polymer/fullerene blend PTB7-Th:PC 61 BM is rather unstable under these conditions, its degradation was efficiently suppressed by incorporating the near infrared-absorbing polymer PDTP-DFBT. Spectroscopic ellipsometry investigations and an effective medium analysis of the ternary absorber blend revealed that the domain conformation in presence of PDTP-DFBT remains stable whereas the domain conformation changes in its absence. The ternary PTB7-Th:PDTP-DFBT:PC 61 BM solar cells yield thermally stable power conversion efficiencies of up to 6%.npj Flexible Electronics (2017) 1:11 ; doi:10.1038/s41528-017-0011-z INTRODUCTIONThe continuous improvement of materials and device architectures for organic bulk-heterojunction solar cells nowadays constantly enables power conversion efficiencies (PCEs) beyond 10%.1, 2 While the PCEs of state-of-the-art organic solar cells are sufficient to meet the demands of some first applications and hence to foster market entry, the device stability often drags behind. The stability of organic solar cells is often limited by a metastable bulk-heterojunction morphology, the diffusion of electrode or buffer layer components into adjacent layers, material decomposition through oxygen and water ingress, irradiation, mechanical stress or heat.3-5 Yet, excellent thermal stability of the organic solar cells is a prerequisite for most future applications. Besides thermal stress when the solar cell is exposed to the sun, the devices may have to endure harsh conditions during fabrication, e.g., when manufacturing car roofs, tiles or façade elements, typically requiring lamination temperatures of 120°C for several hours. 6,7 Upon thermal stress, one of the major degradation mechanisms is the aggregation of fullerene acceptors, commonly [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM) or [6,6]-phenyl C 61 -butyric acid methyl ester (PC 61 BM). 8,9 In the recent literature, several methods were discussed to hamper fullerene aggregation-for example sample irradiation during annealing, 10 structural modification of fullerenes 11 or replacing fullerenes with non-fullerene acceptors.12 Another promising concept to reduce degradation in fullerene-based solar cells is the use of third components which can improve the mechanical, photo, air and thermal stability of organic solar cells.13 Such third components can be crosslinkers, [14][15][16] In this work, we overcome the aggregation of the fullerene acceptor PC 61 BM during thermal annealing by employing ternary D-D-A blends comprisi...

Slot-Die-Coated Ternary Organic Photovoltaics for Indoor Light Recycling

“…After careful optimization of the additives based on the device performance of the binary systems (Figure S1 and Table S1), we decided to replace diphenyl ether, which was reported in our previous study, with AA, because this additive is not only considered green but has shown to improve the morphology of PC 61 BM-based active layers. 51 The control devices based on the binary blend (FBT:PC 61 BM) achieved a maximum PCE of 7.0% with the following photovoltaic parameters (J sc = 12.5 mA/cm 2 , V oc = 0.76 V, and FF = 73%), as shown in Table 1. PDI was added in different percentages (10−50%) to FBT:PC 61 BM maintaining a total donor/acceptor ratio of 1:1.5 (w/w) and a 20 mg/mL total concentration for all blend solutions.…”

Shining Light on Organic Solar Cells

“…[44] In particular, polymer:fullerene blends suffer from fullerene crystallization over time, enhanced by the temperature during operation. [45] Mitigation measures to suppress fullerene crystallization have been developed, using third compounds inside the bulkheterojunction [46,47] or cross-linking. [48,49] NFAs seemed to enable much better solar cell stabilities than fullerenescurrently setting the record with extrapolated device lifetimes of T 80 ¼ 10 years (80% of the initial PCE).…”

Thermal Stabilization of the Bulk‐Heterojunction Morphology in Polymer:Fullerene Solar Cells Using a Bisazide Cross‐Linker

“…Third components can also improve the thermal stability of organic solar cells . Such third components can be conventional fullerenes, modified fullerene derivatives, stable donor polymers, compatibilizers, or insulating polymers . Likewise, a lot of efforts went into the design of cross‐linkable materials to lock the as‐cast morphology in‐between donors, acceptors, or between both species.…”