Donor−Acceptor Polymers Incorporating Alkylated Dithienylbenzothiadiazole for Bulk Heterojunction Solar Cells: Pronounced Effect of Positioning Alkyl Chains

Abstract

4,7-Di(thiophen-2-yl)benzothiadiazole (DTBT) has been used to construct a number of donor-acceptor low band gap polymers for bulk heterojunction (BHJ) photovoltaics with high efficiency numbers. Its strong tendency to π-stack often leads to polymers with low molecular weight and poor solubility, which could potentially be alleviated by anchoring solubilizing chains onto the DTBT unit. A systematic study of the effect of positioning alkyl chains on DTBT on properties of polymers was implemented by investigating a small library of structurally related polymers with identical conjugated backbone. This series of donoracceptor polymers employed a common donor unit, benzo[2,1-b:3,4-b 0 ]dithiophene (BDT), and modified DTBT as the acceptor unit. Three variations of modified DTBT units were prepared with alkyl side chains at (a) the 5and 6-positions of 2,1,3-benzothiadiazole (DTsolBT), (b) 3-positions of the flanking thienyl groups (3DTBT), and (c) 4-positions (4DTBT), in addition to the unmodified DTBT. Contrary to results from previous studies, optical and electrochemical studies disclosed almost identical band gap and energy levels between PBDT-4DTBT and PBDT-DTBT. These results indicated that anchoring solubilizing alkyl chains on the 4-positions of DTBT only introduced a minimum steric hindrance within BDT-DTBT, maintaining the extended conjugation of the fundamental structural unit (BDT-DTBT). More importantly, the additional high molecular weight and excellent solubility of PBDT-4DTBT led to a more uniform mixture with PCBM, with better control on the film morphology. All these features of PBDT-4DTBT led to a significantly improved efficiency of related BHJ solar cells (up to 2.2% has been observed), triple the efficiency obtained from BHJ devices fabricated from the "conventional" PBDT-DTBT (0.72%). Our discovery reinforced the importance of high molecular weight and good solubility of donor polymers for BHJ solar cells, in addition to a low band gap and a low HOMO energy level, in order to further enhance the device efficiencies.

Structure‐Property Optimizations in Donor Polymers via Electronics, Substituents, and Side Chains Toward High Efficiency Solar Cells

“…The PBDT-DTBT series demonstrates that the optimum location for side chains should cause the least steric disturbance to the planarity of the polymer backbone. [ 84,85 ] In this series, PBDT-4DTBT, which is alkylated at the four-position of the thienyl groups, exhibited the highest effi ciency in its BHJ solar cells (Table 7 ). Similar to the control polymer Table 7.…”

“…Reproduced with permission. [84] Copyright 2010, American Chemical Society. (nonalkylated PBDT-DTBT), PBDT-4DTBT maintains the most planar backbone as evidenced by its small calculated dihedral angles and low band gap.…”

Intra- and Intermolecular Steric Hindrance Effects Induced Higher Open-Circuit Voltage and Power Conversion Efficiency

“…θ2 and θ3 usually determine whether effective conjugation and CT transition states exist in the DTBT system. 39 In order to elucidate how the substituents could affect θ2 and θ3 in the D−A polymers, we designed two π−π conjugating small molecules 2,5-bis(4′-hexyl-2′-thienyl)thiophene (hTTT) and 2,5-bis(4′-cyclohexyl-2′-thienyl)thiophene (chTTT); in addition, D−A-type oligmers hDTBT and chDTBT were also included (Figure 2, inset). In the oligomer thiophene systems, their absorption originated from the π−π conjugation was very sensitive to the twisted angle of the adjacent thiophenes.…”

Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development

“…By alternating DTBT or thiadiazolo [3,4-c]pyridine as strong acceptor,with naphtho[2,1-b:3,4-b']-dithiophene (NDT), dithieno[3,2-f:2',3'-h]quinoxaline (QDT) or BDT as weak donor, polymers P55-61 were developed H. Zhou, 2009;H.…”