Effect of molecular structure of benzo[1,2-b:4,5-b′]dithiophene-based push-pull type donor polymers on performance panchromatic organic photodiodes

Highlights

• The effect of PTB polymers on the performance of OPD is investigated. Panchromatic response from 300 to 900 nm was observed.

• The bulk heterojunction OPD based on rr-PTBS:IT-4F with 3 v/v% of 1,8-diiodooctane additive showed superior performance.

• With the wavelength of 600 nm (red light), highest specific detectivity of ~10¹² Jones and EQE of ~73% at −2 V was observed.

Abstract

Herein, the molecular structure variation effect in well-known PTB polymer series based on the performance of organic photodiode (OPD) is explained. PTB7, PTB7-Th, and rr-PTBS polymers, which comprise of the same molecular structure as thieno[3,4-b]thiophene (TT) and benzo[1,2-b:4,5-b′]dithiophene (BDT) but distinct structural alterations, are used as donor moieties with 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-4F) as a non-fullerene acceptor for the bulk heterojunction OPD active layer. Panchromatic spectra response at a broad spectrum from 300 to 900 nm was observed in the OPDs with the above-mentioned combination. The bulk heterojunction conventional OPD based on a relatively thin rr-PTBS:IT-4F active layer with 3 v/v% of 1,8-diiodooctane additive at a wavelength of 600 nm (red light region) showed superior performance with the highest specific detectivity in the range of ~10¹² Jones, an average responsivity of 312.94 mA/W and EQE of ~73% at −2 V. In this study, a stronger lamellar stacking and a much-pronounced face-on orientation with enhanced morphology of rr-PTB:IT-4F blend film is ascribed to its superior device performance compared to others.

Introduction

From its inception, the domain of organic electronics has shown great prospects in the acceptance of technologically relevant electronic devices complementary to their inorganic analogous [1]. Organic photodiodes (OPDs), one of the most promising families of this technology has proven to be vital in the development of a range of products including conventional photo sensor for CMOS image sensor of digital camera, portable fingerprint sensors, motion or object detectors, and e-skin which is also known as systems for medical imaging [[2], [3], [4]]. In its similitude to organic photovoltaics (OPVs), in regard to its general working mechanism and structural make-up, OPDs research also thrive on solution-processed bulk heterojunction (BHJ) theme, composed of electron-donating and electron-accepting organic semiconducting polymers and small molecules which are intimately blended as active matrix [[5], [6], [7]]. The properties of the active layers have been significant among the various determinants that were studied for device performance [5].

For the ideal photoactive film of the BHJ photodiode, carefully selected electron donor and acceptor materials having a large absorption cross-section, broader absorption profile, and high carrier mobility are required [8]. In addition, suitable energetic alignment between the donor and acceptor, providing adequate offset for charge separation from exciton is considered [9]. Lastly, the morphology of the BHJ active layer, which forms an interface within the exciton diffusion length and simultaneously facilitates both electron and hole transport without recombination, plays an important role in achieving high-performance OPDs. Thus, it is imperative for a large donor/acceptor interface to form a bicontinuous network having the domain width about twice that of the exciton diffusion length for the exciton dissociation and transport of separated charges to respective electrodes in the devices [5,[9], [10], [11]].

Especially for solar cell application, many high-performance π-conjugated molecules have been designed, synthesized, and tested. Among these, a class of low bandgap donor polymers called the PTB polymer series, which are composed of thieno[3,4-b]thiophene (TT) as an electron-accepting unit and benzo[1,2-b:4,5-b′]dithiophene (BDT) as an electron-donating unit has been studied and have been proven to be benchmark materials for photovoltaic applications [[12], [13], [14]]. These PTB polymers showed low band-gap properties because of the quinoid resonance structure stabilization that could improve hole mobility owing to their rigid structure. With structural optimization such as incorporating a fluorine atom into a TT (i.e., PTB7) and a heterocyclic side chain into a BDT to achieve a two-dimensional conjugated polymer (i.e., PTB7-Th), OPV performance has drastically improved because of the increase in the highest occupied molecular orbital (HOMO) level, extended absorption range, and improved hole mobility [5,15]. The comprehensive optimization of moieties and processes has led to the development of excellent photovoltaic properties with a fullerene-derivative acceptor (PCBM) [15,16]. Thus, PTB7 and PTB7-Th have been used as standard donor materials for OPVs. However, despite the advancement in OPV technology, studies on the correlation between PTB polymer structure and the OPD performance has rarely been reported.

Recently, non-fullerene acceptors (NFAs) are being studied as a promising electron acceptor alternative to fullerenes and their derivatives [17]. Particularly, fused-ring donor-cored NFAs with the acceptor–donor–acceptor (A-D-A) structural architecture have been extensively developed by many research groups. The NFAs have several distinct advantages such as easy structural change, tunability of frontier energy levels, high molecular planarity, and complementary absorption spectra. In particular, ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-n-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b’]dithiophene) and its derivatives are the most widely used NFAs, i.e., they exhibit high power conversion efficiency (PCE) of over 10% [18]. In addition, fluorination (IT-4F) or methylation (IT-M and IT-DM) of the end acceptor of ITIC showed more optimized performance because of the energy level or morphological optimization with donor polymers [19,20]. However, ITIC and its derivative acceptors containing 1-(dicyanomethylene)-3-indanone showed low PCE with PTB7-based standard donor polymers because of the existential high energy loss and absorption wavelengths overlapping between the ITIC-based acceptor and PTB7-based donor [11,18].

Herein, we elucidate the effect of the PTB polymer structure using three representative polymer semiconductors based on molecular design in this series—the one-dimensional PTB7, the two-dimensional PTB7-Th, and the two-dimensional regioregular PTB7-Th-based copolymer named rr-PTBS—as donors with 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-4F) as a non-fullerene small molecular acceptor (Fig. 1) on the basis of OPD performance. The rr-PTBS copolymer consisting of TT and BDT with alternate ethylhexyl-thiophene and ethyhexyl-thio-thiophene side chains show comparatively better hole transport ability, a better bi-continuous bulk heterojunction morphology with strong face-on orientation when blended with PCBM as the acceptor [16]. The rr-PTBS copolymer showed similar intrinsic properties when a bulk heterojunction (BHJ) was prepared with IT-4F. The structural variation of donor materials in the BHJ layer, had a synergistic effect on the OPD performance. Among the three PTB-based donor materials, OPDs with rr-PTBS:IT-4F active layer with minimal thickness (less than 200 nm) and in a simple conventional OPD stack with no elaborate interlayer engineering in device, showed optimal performance with an average detectivity of ~10¹² Jones, an average responsivity of 312.94 mA/W and an external quantum efficiency (EQE) of ~73% under red light (~600 nm) at −2 V bias voltage which is very promising compared with reported OPD performance utilizing either very thick active layers, intricate device architecture and interlayer engineering [3,[21], [22], [23], [24], [25], [26]].