PBDTBO, a conjugated polymer comprising benzo[1,2-b:4,5-b']dithiophene (BDT) and 5,6-bis(octyloxy)benzo[c][1,2,5]oxadiazole (BO) units, exhibits a deep HOMO energy level of -5.27 eV and excellent solubility. A device incorporating PBDTBO and [6,6]-phenyl-C(61)-butyric acid methyl ester (1:1, w/w) exhibited a power conversion efficiency of 5.7%.

This paper reports side-chain-engineered polymer donors and a small-molecule acceptor that are capable of simultaneous charge and energy transfer as the active layer for organic photovoltaics.

We used Stille coupling of electron-rich benzo[1,2b:4,5-b′]dithiophene (BDT) presenting conjugated alkylthiophene (T), alkylphenyl (P), or alkylfuran (F) side chains with electrondeficient alkoxy-modified 2,1,3-benzooxadiazole (BO) moieties to obtain a series of two-dimensional, conjugated, D−π−A polymers (PBDTTBO, PBDTPBO, and PBDTFBO). The side chains of the BDT units altered the solubility, conformations, and electronic properties of the synthesized conjugated polymers, allowing tuning of their photovoltaic properties when blended with fullerenes. Density functional theory calculations revealed that the presence of these side chain groups on the BDT donor units affected the torsion angles between the side chain groups and the conjugated main chains but resulted in only slightly different energy levels for the highest occupied molecular orbitals for these polymers, consistent with results obtained experimentally using cyclic voltammetry. These polymers displayed excellent thermal stabilities (5 wt % degradation temperatures: >330°C) and broad spectral absorptions (from 450 to 700 nm). Transmission electron microscopy images revealed that the morphologies of active layers comprising these two-dimensional conjugated polymers and the fullerene derivative PC 71 BM did, however, vary substantially depending on the structure of the side chains that affects the solubility of the polymers. As a result, the efficiencies of photovoltaic devices incorporating PBDTFBO, PBDTPBO, or PBDTTBO polymers and PC 71 BM varied greatly, from 3.6 to 5.9%. When using 1-chloronaphthalene (1 vol %) or 1,8-diiodooctane (1 vol %) as an additive for processing the active layer, the power conversion efficiencies (PCEs) of photovoltaic devices incorporating blends of PBDTFBO, PBDTPBO, or PBDTTBO and PC 71 BM (1:2) improved to 5.4, 6.4, and 7.4%, respectively, due to their optimized morphologies, with the PCE of 7.4% being among the highest values reported for conjugated polymers involving BO moieties. Thus, the photovoltaic properties of these conjugated polymers were highly tunable through slight modifications of their side chain structures.

In this study, we incorporated molybdenum disulfide (MoS) nanosheets into sol-gel processing of zinc oxide (ZnO) to form ZnO:MoS composites for use as electron transport layers (ETLs) in inverted polymer solar cells featuring a binary bulk heterojunction active layer. We could effectively tune the energy band of the ZnO:MoS composite film from 4.45 to 4.22 eV by varying the content of MoS up to 0.5 wt %, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene- alt-(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl] (PTB7-TH)/phenyl-C-butryric acid methyl ester (PCBM). In particular, the power conversion efficiency (PCE) of the PTB7-TH/PCBM (1:1.5, w/w) device incorporating the ZnO:MoS (0.5 wt %) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL, a relative increase of 15%. Incorporating a small amount of MoS nanosheets into the ETL altered the morphology of the ETL and resulted in enhanced current densities, fill factors, and PCEs for the devices. We used ultraviolet photoelectron spectroscopy, synchrotron grazing incidence wide-/small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy to characterize the energy band structures, internal structures, surface roughness, and morphologies, respectively, of the ZnO:MoS composite films.

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The effects of charge accumulation on fill factor (FF) are studied by using light intensity-dependent current-voltage and capacitance-voltage measurements based on poly-{bi(dodecyl)thiophenethieno[3,4-c]pyrrole-4,6-dione}:1(3-methyloxycarbony1)propyl-1-phenyl [6,6] (PBTTPD:PCBM) solar cells. We find that the FF of un-annealed device shows a non-monotonic dependence of light intensity: first an increase and then a decrease with increasing light intensity. This suggests that both bulk charge accumulation and recombination determine the FF before thermal annealing. Furthermore, the morphological enhancement of PBTTPD:PCBM film leads to a monotonic decrease of FF as increasing light intensity. This indicates that only surface charge accumulation plays an important role in the determination of the FF after thermal annealing. V

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