Overcoming the Thermal Instability of Efficient Polymer Solar Cells by Employing Novel Fullerene‐Based Acceptors

Abstract

achieve the highest performance so far in combination with the commonly used methanofullerene derivatives like phenyl-C 61 -butyric acid methyl ester ([60]PCBM) or [6,6]-phenyl-C 71 -butyric acid methyl ester ([70]PCBM). [11] Functional fullerenes, on the one hand, can preserve the properties of pristine fullerenes, such as superior electron-transporting ability and high electron affinity. On the other hand, by properly attaching suitable addends, one can manipulate a wide variety of chemical, physical, or thermo dynamic properties, such as better solubility in organic solvents, higher lowest unoccupied molecular orbital (LUMO) level, and improved compatibility with organic donors. [12] These advantages endow functional fullerenes with extraordinary capability, acting as unique electron-accepting components in OPV devices. [2,13] Although PCBM is the most prominent acceptor in OPV devices, [5,14] the energy levels of PCBM should be further optimized in order to achieve high open circuit voltage (V OC ) in combination with the state-of-theart organic donors. [15] Fullerene derivatives with more suitable electronic levels are expected to reduce the bandgap to V OC loss and approach the theoretical efficiency limit. [13,16] PCBM bisadduct [17] and indene-C 60 bisadduct [18] made their glory debut for being well compatible with poly(3-hexylthiophene-2,5-diyl) (P3HT) and for achieving promising high V OC and PCE compared to the PCBM-based counterparts. However, resulting from constitutional isomer impurities and inefficient charge generation, fullerene bisadducts failed to produce similarly satisfying performance with the latest outstanding organic donors. [19] Another effective way of increasing the LUMO level of fullerene derivatives is to add an electron-donating group to the fullerene cage. [20] A series of newly developed functional fullerenes have shown higher photovoltaic performance than PCBM resulting from their high-lying LUMO levels. Itoh and co-workers carried out a systematic study on a series of fulleropyrrolidine derivatives, some of which possess higher V OC and efficiency than PCBM when blended with P3HT. [21] Troshin and co-workers reported that solar cells based on the novel pyrrolidinofullerene acceptors exhibited impressively high V OC and promising efficiency in combination with Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and P3HT compared to the PCBM-based references. [22] As the PCE of OPV devices was improved to over 11%, currently, more research efforts are required to address the long-term stability issue. Many factors may cause degradation Extensive research efforts around the world have been devoted to organic photovoltaics (OPV) over the last decade due to their unique advantages for commercial application, such as low cost, light weight, flexibility, and easy manufacture on large-scale. [1][2][3] By introducing the bulk heterojunction (BHJ) [4,5] structure into OPV devices, tremendous progress has been achieved to boost the powe...

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

“…This indicates an almost completely suppressed aggregation of PC 61 BM in PDTP-DFBT. This outstanding thermal stability may originate from a better miscibility of PDTP-DFBT and PC 61 BM versus PTB7-Th and PC 61 BM, and hence a more favorable conformation of the fullerene domains, 11,30 or from a higher glass transition temperature T g of PDTP-DFBT. 32 Optoelectronic properties of ternary blends PDTP-DFBT suppresses the aggregation of fullerenes in binary bulk-heterojunctions and hence obtains thermally stable organic solar cells.…”

“…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.…”

“…This indicates an almost completely suppressed aggregation of PC 61 BM in PDTP-DFBT. This outstanding thermal stability may originate from a better miscibility of PDTP-DFBT and PC 61 BM versus PTB7-Th and PC 61 BM, and hence a more favorable conformation of the fullerene domains, 11,30 or from a higher glass transition temperature T g of PDTP-DFBT. …”

Improved Glass Transition Temperature towards Thermal Stability via Thiols Solvent Additive versus DIO in Polymer Solar Cells

“…The corresponding morphology of active layer is thus believed to be stable. While Brabec et al proposed that chemical miscibility between donor and acceptor is a key factor in determining thermal stability of PTB7‐Th based systems despite of the use of DIO . In contrast, some researchers focused on restriction of crystallization and excessive aggregation of fullerenes to improve thermal stability of active layer .…”

Challenges to the Stability of Active Layer Materials in Organic Solar Cells

“…Later, Brabec and co‐workers reported two acceptors of pyrrolidinofullerene (PyF5) and methanofullerene (FAP1) possessing a significantly higher chemical compatibility with donor materials. The PyF5‐ and FAP1‐based devices are found to be extremely stable under annealing at 140 °C in inert atmosphere, which is attributed to high miscibility between donor and acceptor materials …”