Jong Baek Park

Effect of π-conjugated bridges of TPD-based medium bandgap conjugated copolymers for efficient tandem organic photovoltaic cells

Conjugated donor (D)–π–acceptor (A) copolymers, PBDT–TPD, PBDT–ttTPD, PBDTT–TPD, and PBDTT–ttTPD, based on a benzodithiophene (BDT) donor unit and thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) acceptor unit were designed and synthesized with different π bridges via Pd-catalyzed Stille-coupling. The π bridges between BDT and TPD were thiophene in PBDT–TPD and PBDTT–TPD, and 6-alkylthieno[3,2-b]thiophene in PBDT–ttTPD and PBDTT–ttTPD. The effects of the π bridges on the optical, electrochemical, and photovoltaic properties of the polymers were investigated, in addition to the film crystallinities and carrier mobilities. Copolymers with the 6-alkylthieno[3,2-b]thiophene π-bridge exhibited high crystallinity and hole mobility. Improved Jsc and FF were obtained to increase the overall power conversion efficiencies (PCE) in inverted single organic photovoltaic cells. A PCE of 6.81% was achieved from the inverted single device fabricated using the PBDTT–ttTPD:PC71BM blend film with 3 vol% 1,8-diiodooctane. A tandem photovoltaic device comprising the inverted PBDTT–ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% with a Voc of 1.58 V, a Jsc of 8.00 mA cm−2, and a FF of 74% under AM 1.5 G illumination at 100 mW cm−2. The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device. This work demonstrates that PBDTT–ttTPD may be very promising for applications in tandem solar cells. Furthermore, 6-alkylthieno[3,2-b]thiophene π-bridge systems in medium bandgap polymers can improve the performance of tandem organic photovoltaic cells.

Well-controlled thieno[3,4-c]pyrrole-4,6-(5H)-dione based conjugated polymers for high performance organic photovoltaic cells with the power conversion efficiency exceeding 9%

We have synthesized a series of conjugated D–π–A copolymers, PT-ttTPD and PBT-ttTPD, based on a (5-hexyltridecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (ttTPD) acceptor unit in order to develop better photovoltaic polymers based on the TPD moiety: an e-branched alkyl side chain on the TPD unit was coupled with 6-alkyl-thieno[3,2-b]thiophene (tt) π-bridge molecules. The Stille polymerization of the brominated ttTPD and stannylated simple thiophene (T) finally gave a promising PT-ttTPD polymer showing well-ordered inter-chain orientation in the BHJ active layer. PT-ttTPD-based OPVs exhibited a highest power conversion efficiency (PCE) of 9.21% (VOC = 0.86 V, JSC = 15.30 mA cm−2, FF = 70%).

Controlling the Morphology of BDTT-DPP-Based Small Molecules via End-Group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells

A series of narrow-band gap, π-conjugated small molecules based on diketopyrrolopyrrole (DPP) electron acceptor units coupled with alkylthienyl-substituted-benzodithiophene (BDTT) electron donors were designed and synthesized for use as donor materials in solution-processed organic photovoltaic cells. In particular, by end-group functionalization of the small molecules with fluorine derivatives, the nanoscale morphologies of the photoactive layers of the photovoltaic cells were successfully controlled. The influences of different fluorine-based end-groups on the optoelectronic and morphological properties, carrier mobilities, and the photovoltaic performances of these materials were investigated. A high power conversion efficiency (PCE) of 6.00% under simulated solar light (AM 1.5G) illumination has been achieved for organic photovoltaic cells based on a small-molecule bulk heterojunction system consisting of a trifluoromethylbenzene (CF3) end-group-containing oligomer (BDTT-(DPP)2-CF3) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. As a result, the introduction of CF3 end-groups has been found to enhance both the short circuit current density (JSC) and fill factor (FF). A tandem photovoltaic device comprising an inverted BDTT-(DPP)2-CF3:PC71BM cell and a poly(3-hexylthiophene) (P3HT):indene-C60-bisadduct (IC60BA)-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.30%. These results provide valuable guidelines for the rational design of conjugated small molecules for applications in high-performance organic photovoltaic cells. Furthermore, to the best of our knowledge, this is the first report on the design of fluorine-functionalized BDTT-DPP-based small molecules, which have been shown to be a viable candidate for use in inverted tandem cells.

New alkylthio-thieno[3,2-b]thiophene-substituted benzodithiophene-based highly efficient photovoltaic polymer

We introduced an alkylthio-substituent on the thieno[3,2-b]thiophene side chain of the polymer and synthesized a new low band gap 2D-conjugated polymer, PSTTBDT-FTT. It showed more red-shifted absorption spectra and higher hole mobility than PTTBDT-FTT without the thio-functionality. The power conversion efficiency (PCE) of the PSTTBDT-FTT device reached 8.61%. This work demonstrates that the newly synthesized PSTTBDT-FTT is a promising donor material for high efficiency OPVs.

Enhanced and controllable open-circuit voltage using 2D-conjugated benzodithiophene (BDT) homopolymers by alkylthio substitution

In this study, we explore the effects of alkylthiophene (T) and alkylthiothiophene (T-S) substituents on the benzo[1,2-b;4,5-b′]dithiophene (BDT) unit by comparing the BDTT homopolymer (PBDTT), the BDTT-alt-BDTT-S copolymer (PBDTT-BDTT-S), and the BDTT-S homopolymer (PBDTT-S) in terms of UV-visible absorption spectra, cyclic voltammetry (CV) results, computational calculations, and experimental results. The T-S substituent increased the hole mobility of the polymer and down-shifted the highest occupied molecular orbital (HOMO) energy level of the polymer, leading to slight red-shifting of the absorption spectrum. The organic photovoltaic (OPV) cells based on PBDTT-S as a donor and [6,6]-phenyl-C71-butylic acid methyl ester (PC71BM) as an acceptor demonstrated a high power conversion efficiency (PCE) of 7.05% under AM 1.5G illumination (100 mW cm−2). To the best of our knowledge, this PCE value is one of the highest values reported for homopolymer donor-based OPVs. Compared to the well-known P3HT homopolymer, which shows a similar absorption profile, PBDTT-S is a promising candidate for organic photodiodes.

Systematic control of heteroatoms in donor–acceptor copolymers and its effects on molecular conformation and photovoltaic performance

Heteroatom substitutions are an effective means of tuning the optoelectronic, conformational, and molecular packing properties of donor–acceptor conjugated copolymers, with a view to efficient photovoltaic performance. We investigate the effects of systematic sulfur/selenium substitutions into thiophene (donor) and benzothiadiazole (acceptor) units using complementary Raman spectroscopy, density functional theory, and X-ray diffraction to characterise the resulting copolymers. We find that, in each case, the heavy atom substitution is detrimental to photovoltaic performance and undertake to understand this in terms of fundamental optoelectronic properties of the copolymers. Specifically, we find that, due to mesomeric effects, the selenium atom donates electron density into the donor unit (selenophene) more strongly than sulfur, but also withdraws electron density more strongly from the benzene ring in the acceptor unit (benzoselenadiazole). In both cases, the selenium substitution reduces the optical energy gap but is unfavourable for intermolecular packing in thin films and so results in poor charge carrier mobility. We identify a complex interplay between the electronic properties of the substituted donor and acceptor units relating to the frontier molecular orbital energy levels, molecular conformation, and intermolecular packing of the copolymers. In particular, we find that the pairing of a strong acceptor unit with a weak donor unit results in relatively low electron density on the conjugated backbone, leading to high inter-unit torsion and weak optical absorption in the visible range. The methods and insights developed here have broad applicability to the design of other donor–acceptor copolymers for optoelectronic device applications.