Abstract:Most of China's CO2 emissions are related to energy consumption in its cities. Thus, cities are critical for implementing China's carbon emissions mitigation policies. In this study, we employ an input-output model to calculate consumption-based CO2 emissions for thirteen Chinese cities and find substantial differences between production-and consumption-based accounting in terms of both overall and per capita carbon emissions. Urban consumption not only leads to carbon emissions within a city's own boundaries but also induces emissions in other regions via interregional trade. In megacities such as Shanghai, Beijing and Tianjin, approximately 70% of consumption-based emissions are imported from other regions. Annual per capita consumptionbased emissions in the three megacities are 14, 12 and 10 tonnes of CO2 per person, respectively. Some medium-sized cities, such as Shenyang, Dalian and Ningbo, exhibit per capita emissions that resemble those in Tianjin. From the perspective of final use, capital formation is the largest contributor to consumption-based emissions at 32-65%. All thirteen cities are categorized by their trading patterns: five are production-based cities in which production-based emissions exceed consumption-based emissions, whereas eight are consumption-based cities, with the opposite emissions pattern. Moreover, production-based cities tend to become consumption-based as they undergo socioeconomic development.

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A donor polymer based on 3-cyanothiophene, a structurally simple unit, is synthesized for organic solar cells, which exhibited prominent power conversion efficiency and excellent batch-to-batch reproducibility in a wide molecular weight range.

High-efficiency organic solar cells (OSCs) largely rely on polymer donors. Herein, we report a new building block BNT and a relevant polymer PBNT-BDD featuring B-N covalent bond for application in OSCs. The BNT unit is synthesized in only 3 steps, leading to the facile synthesis of PBNT-BDD. When blended with a nonfullerene acceptor Y6-BO, PBNT-BDD afforded a power conversion efficiency (PCE) of 16.1 % in an OSC, comparable to the benzo[1,2b:4,5-b']dithiophene (BDT)-based counterpart. The nonradiative recombination energy loss of 0.19 eV was afforded by PBNT-BDD. PBNT-BDD also exhibited weak crystallinity and appropriate miscibility with Y6-BO, benefitting of morphological stability. The singlet-triplet gap (DE ST) of PBNT-BDD is as low as 0.15 eV, which is much lower than those of common organic semiconductors (! 0.6 eV). As a result, the triplet state of PBNT-BDD is higher than the charge transfer (CT) state, which would suppress the recombination via triplet state effectively.

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The power conversion efficiencies (PCEs) of OSCs have reached 19% as the innovation of state-of-the-art nonfullerene acceptors. [2] On the contrary, the development of polymer donors lags much behind. Up to date, the polymer donors are limited to a handful of polymers and, more specifically, thiophene-substituted benzo [1,2-b:4,5-b']dithiophene (BDTT) polymers. [3] The side-chain conjugated substituents on BDTT could extend backbone conjugation and electron delocalization, enhance interchain π-π interaction, and improve charge transport of the polymers. Moreover, the absorption spectra, energy levels, and crystallinity of BDTTbased polymer donors can be finely tuned by both main-chain and side-chain engineering. [4] As a result, these kinds of polymers, such as PM6 and D18 (Figure 1a), [5] perform well in both fullerene and nonfullerene OSCs. [3a,4] However, BDTT-based polymers usually suffer from lengthy synthesis and tedious purification processes, which inevitably result in a high production cost and bring difficulties for the large scale synthesis. [6] Therefore, high-performance polymer donors with simple structure and facile synthesis are urgently needed to drive the further development of OSCs. [7] Among various polymer donors, poly(3-hexylthiophene) (P3HT) is one of the most extensively studied polymers due to its simple structure, low production cost, good self-assembly property, and high charge transport ability. [8] However, the PCEs of P3HT-based OSCs were restrained at about 10% due to the high-lying HOMO level of P3HT and excessively mixed active layer morphology. [9] An alternative strategy is incorporating electron-deficient units into the polymer main chain to form oligothiophene-based donor-acceptor polymers. This kind of polymer employs oligothiophene as a donor unit and thus the properties of the polymers can be adjusted by both the oligothiophene unit and acceptor unit. Moreover, these kinds of polymers can be synthesized more easily than BDTTbased donor-acceptor polymers. [10] A few state-of-the-art oligothiophene-based polymer donors such as PffBT4T-2OD and PNTT [11] (Figure 1a) have been developed, which exhibited good crystallinity and high hole mobility. Moreover, this kind of polymers usually exhibits temperature-dependent aggregation properties, which have provided valuable opportunities to regulate The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have increased rapidly owing to the development of non-fullerene acceptors (NFAs). However, the development of polymer donors lags behind significantly. Currently, the polymer donors are dominated by a handful of thiophenesubstituted benzo[1,2-b:4,5-b']dithiophene (BDTT) polymers, which suffer from lengthy synthesis and high production cost. Compared with BDTT-based polymers, oligothiophene-based donor-acceptor polymers feature much easier synthesis, which were the prevailing polymer donors in fullerene-based OSCs, but almost disappeared in non-fullerene OSCs. Herein, two oligothiophene-based donor-acceptor pol...

Polythiophenes (PTs) are promising electron donors in organic solar cells (OSCs) due to their simple structures and excellent synthetic scalability. However, the device performance of PT-based OSCs is rather poor due mainly to large photon energy losses and an unfavorable active layer morphology. Herein, the authors report a new PT, which is abbreviated as P5TCN-2F and features cyano-group substituents for high-efficiency OSCs. The cyano-group endows P5TCN-2F with a deep-lying highest occupied molecular orbital energy level, which thereby contributed to high opencircuit voltage in OSCs as a result of reduced non-radiative recombination energy loss. Moreover, the cyano-group leads to strong interchain interaction, improved polymer crystallinity, and appropriate miscibility with the prevailing non-fullerene acceptors. Consequently, P5TCN-2F offers over 15% power conversion efficiency when blended with various Y-series non-fullerene acceptors (Y6, Y6-BO, eC9, and L8-BO). Particularly, a champion efficiency of 16.1% is obtained by the P5TCN-2F:Y6 blend, which is largely higher than that of any previous PT-based OSCs. Moreover, the average figure of merit of the active layer based on P5TCN-2F is much superior to that of benzodithiophene-based polymers. These results suggest the renaissance of PT-based OSCs and have opened an avenue to access high-performance materials for the large-scale production of OSC modules.

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