Efficient Low‐Bandgap Polymer Solar Cells with High Open‐Circuit Voltage and Good Stability

Abstract

occupied molecular orbital (HOMO) level of the polymer, which has a direct impact on the V OC of the cell. In addition, it is important to achieve low-bandgap PSCs with great stability for the purpose of practical applications. While thermal stability of PSCs is related to thermal properties of the polymer, air stability can be affected by the HOMO level of the polymer. Polymers with deeper HOMO levels are generally more stable against the oxidation by air. Lastly, high-performance low-bandgap polymers should also exhibit high charge transport ability in order to achieve high fi ll factors (FF). [ 24,25 ] Among the reported low bandgap polymers, diketopyrrolopyrrole (DPP) has been a commonly used building block due to its excellent properties such as strong absorption covering near-infrared and visible regions, its electron defi cient nature and high charge carrier mobility. A polymer (DT-PDPP2T-TT) has been reported as the best low-bandgap polymer, which could achieve an impressive short-circuit current density ( J sc) of 20.07 mA cm −2 with diphenolether as additive. The open circuit voltage ( V OC ) of cell is still relatively low (0.67 V) due to the relatively electron-rich nature of thienothiophene-based donor unit. [ 13 ] Another high-performance low-bandgap polymer is C3-DPPTT-T (with a high effi ciency up to 8.8%), which offers an excellent J sc up to 23.5 mA cm −2 but still exhibits a V OC of 0.57 V. [ 17 ] To increase the V OC of DPP-based low bandgap polymers, an effective strategy is to copolymerize the DPP unit with donor building blocks that are less electron-rich than thiophene or thienothiophene. When a benzodithiophene unit was copolymerized with DPP, a signifi cantly higher V OC of 0.73 V was achieved, but the cell effi ciency was signifi cantly lower (6.6% for single junction cell). [ 10 ] Recently, a DPP-based D 1 -A-D 2 -Atype low bandgap polymer (PDPP3T alt TPT) was reported with the introduction of a phenyl ring between two DPP2T moieties. This polymer further increases the V OC to 0.75 V and can achieve an excellent PCE of 8.0%. [ 16 ] In this paper, we report a novel DPP polymer (named poly(difluorobenzothiadiazole-bisthiophene-alt -diketopyrrolopyrrole-bisthiophene) or PffBTT2-DPPT2) that combines a D-A 1 -D-A 2 copolymer strategy and our temperature-dependent aggregation polymer design. The chemical structure of PffBTT2-DPPT2 is shown in Figure 1 a. While the fi rst acceptor unit (A 1 ) is DPP, 5,6-difl uoro-2,1,3-benzothiadiazole (ffBT) is selected as the second acceptor unit (A 2 ), as it was shown in our previous studies to introduce excellent polymer properties for high-effi ciency PSCs. [ 8 ] Although the bandgap of PffBTT2-DPPT2 is small (1.43 eV), PSCs based on PffBTT2-DPPT2 can still achieve an impressive V OC of 0.81 V, indicating a very small V OC loss of 0.62 V. The bandgap and V OC loss of PffBTT2-DPPT2-based PSCs are very close to the optimal Polymer solar cells (PSCs) have been developed as a promising low-cost and environmentally friendly alternative to conventional inorg...

New Processable Phenanthridinone‐Based Polymers for Organic Solar Cell Applications

“…In these conditions, the open-circuit voltage of 0.88 V is one of the highest values ever seen for a DPP-based organic solar cell. [ 11 ] The unoptimized effi ciency value is also one of the highest obtained from a non-chlorinated solvent under ambient conditions. The highest performances, 6.11% in air and 7.15% under a nitrogen atmosphere, were reported by Tsai et al and Hoppe et al, respectively.…”

“…[ 9,10 ] Among other effi cient semiconducting polymers, several contain lactam units, such as diketopyrrolopyrrole (DPP), isoIndigo (iI), bidithienopyridinedione (BDTP) and thienopyrroledione (TPD) derivatives. [10][11][12][13][14][15][16][17] In this context, the objective of this work was to design and synthesize new low bandgap polymeric materials inspired by carbazole and lactam units for OSC applications. To achieve this goal, we focused on derivatives of the simple but rather unexplored 3,8-dibromo-5H-phenanthridinone (PTD).…”

A fluorine-induced high-performance narrow bandgap polymer based on thiadiazolo[3,4-c]pyridine for photovoltaic applications

“…[1][2][3][4] During the last decade, remarkable progress has been made in enhancing the power conversion efficiency (PCE) of the PSCs, such as development of high-performance polymer donors, [4][5][6][7][8][9][10][11][12][13][14] incorporation of efficient interfacial materials, [15][16][17] and advancement of device architectures. 6,[20][21][22][23] Compared with large and medium bandgap polymers, NBG polymers have strong and broad absorption in near-infrared region, which makes them an important component for constructing multicomponent and tandem solar cells. 6,[20][21][22][23] Compared with large and medium bandgap polymers, NBG polymers have strong and broad absorption in near-infrared region, which makes them an important component for constructing multicomponent and tandem solar cells.…”

“…18,19 Despite plenty of high-performance polymer donors having been reported, only a limited number of them are narrow bandgap (NBG, < 1.50 eV) polymers. 21,[24][25][26] However, most reported high-performance NBG polymers usually need complex morphology engineering and interfacial modifications during device fabrications, which may limit their further applications. 21,[24][25][26] However, most reported high-performance NBG polymers usually need complex morphology engineering and interfacial modifications during device fabrications, which may limit their further applications.…”

“…6,[20][21][22][23] Compared with large and medium bandgap polymers, NBG polymers have strong and broad absorption in near-infrared region, which makes them an important component for constructing multicomponent and tandem solar cells. 6,[20][21][22]27 Therefore, it's necessary to develop novel high-performance NBG polymers with easy device processing methods and simple device structures. 6,[20][21][22]27 Therefore, it's necessary to develop novel high-performance NBG polymers with easy device processing methods and simple device structures.…”