Improved All-Polymer Solar Cell Performance of n‑Type Naphthalene Diimide−Bithiophene P(NDI2OD-T2) Copolymer by Incorporation of Perylene Diimide as Coacceptor Sandeep Sharma, †,‡ Nagesh B. Kolhe, †,‡ Vinay Gupta,* ,§,∥ Vishal Bharti, ‡,∥ Abhishek Sharma, ‡,∥ Ram Datt, ‡,∥ Suresh Chand, §,∥ and S. K. Asha* ,†,‡,§ † Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune, India 411008 ‡ Academy of Scientific and Innovative Research, New Delhi, India 110025 § CSIR-Network Institutes of Solar Energy, New Delhi, India ∥ National Physical Laboratory, New Delhi, India 110012 * S Supporting Information ABSTRACT: Naphthalene diimide−bithiophene P(NDI2OD-T2) is a well-known donor−acceptor polymer, previously explored as n-type material in all-polymer solar cells (all-PSCs) and organic field effect transistor (OFETs) applications. The optical, bulk, electrochemical, and semiconducting properties of P(NDI2OD-T2) polymer were tuned via random incorporation of perylene diimide (PDI) as coacceptor with naphthalene diimide (NDI). Three random copolymers containing 2,2′- bithiophene as donor unit and varying compositions of naphthalene diimide (NDI) and perylene diimide (xPDI, x = 15, 30, and 50 mol % of PDI) as two mixed acceptors were synthesized by Stille coupling copolymerization. Proton NMR spectra recorded in CDCl 3 showed that the π−π stacking induced aggregation among the naphthalene units could be successfully disrupted by the random incorporation of bulky PDI units. The newly synthesized random copolymers were investigated as electron acceptors in BHJ all-PSCs, and their performance was compared with P(NDI2OD-T2) as reference polymer. An enhanced PCE of 5.03% was observed for BHJ all-PSCs (all-polymer solar cells) fabricated using NDI-Th-PDI30 as acceptor and PTB7-Th as donor, while the reference polymer blend with the same donor polymer exhibited PCE of 2.97% efficiency under similar conditions. SCLC bulk carrier mobility measured for blend devices showed improved charge mobility compared to reference polymer, with PTB7- Th:NDI-Th-PDI30 blend device exhibiting the high hole and electron mobility of 4.2 × 10 −4 and 1.5 × 10 −4 cm 2 /(V s), respectively. This work demonstrates the importance of molecular design via random copolymer strategy to control the bulk crystallinity, compatibility, blend morphology, and solar cell performance of n-type copolymers. ■ INTRODUCTION The research based on solution processable organic bulk- heterojunction (BHJ) solar cells is progressing very fast. Continuous efforts have been made to increase the power conversion efficiency (PCE) of BHJ solar cell via development in both the active layer material design (n-type and p-type) and the device engineering. 1−3 In all-polymer BHJ solar cells (PSCs), both components of the active layer, i.e., electron donor and acceptor, are polymeric semiconductors which have potential advantages over the extensively studied donor polymer/acceptor fullerene composite solar cells. 4,5 Although significant progress has been made in the development of polymer/fullerene composite solar cell in terms of their high device efficiency over >9%, the use of fullerene acceptor (PC 61 BM and PC 71 BM) has some disadvantages like relatively weak absorption ability in the visible region, high cost of synthesis, and morphological instability of polymer/fullerene blend over time and temperature that limits the performance of Received: July 20, 2016 Revised: October 7, 2016 Published: October 19, 2016 Article pubs.acs.org/Macromolecules © 2016 American Chemical Society 8113 DOI: 10.1021/acs.macromol.6b01566 Macromolecules 2016, 49, 8113−8125