Organic photovoltaics (OPVs) based on conjugated polymers are promising for low cost, scalable solar energy conversion. Much progress has been made in the last 10 years in increase the performance of bulk heterojuction (BHJ) OPVs based on donor polymer and fullerene acceptors with power conversion efficiencies (PCEs) now around 10 %. In contrast, the performance of all-polymer solar cells comprised of both donor and acceptor polymers and free of fullerenes, the PCE is still around 3-4 %. Most importantly, lack of suitable acceptor (n-type) polymers has limited the photocurrent and efficiency of polymer/polymer BHJ solar cells while many high-performance donor (p-type) polymers in polymer/fullerene system have been reported so far. Here we report highly efficient all-polymer solar cells enabled by a novel polymer/polymer blend systems comprised of new naphthalene diimide (NDI)-selenophene copolymer acceptors (PNDIS) and a thiazolothiazole-dithienylsilole copolymer donor (PSEHTT). We demonstrate all-polymer solar cells based on new PNDIS acceptor show high photovoltaic performance reported to date with efficiencies near 5% and photocurrent over 10 mA/cm2. The observed the highest photovoltaic properties of PSEHTT:PNDIS solar cells suggest that unipolar electron transport with high bulk mobility, good crystallinity, and molecular weight are all important factors in the design of suitable acceptor polymers in all-polymer BHJ solar cells. We show that the control of blend morphology led to much improved photovoltaic performance of polymer/polymer solar cells even exceeding similarly fabricated polymer/fullerene solar cells. We also report that tuning of the alkyl side chain composition and distribution on an n-type conjugated polymer backbone provides a means of maintaining solubility while increasing crystallinity and electron mobility, leading to enhanced photocurrent and performance of all-polymer solar cells. We find that the increased crystallinity and electron mobility of the NDI-selenophene alternating copolymer are achieved in conjunction with the optimum polymer/polymer blend morphology.

Highly efficient all-polymer photovoltaic devices

248th National Meeting (2014)

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