Influence of Aggregation on the Performance of All‐Polymer Solar Cells Containing Low‐Bandgap Naphthalenediimide Copolymers

Abstract

The authors present efficient all‐polymer solar cells comprising two different low‐bandgap naphthalenediimide (NDI)‐based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near‐field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large‐scale phase‐separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donor–acceptor heterojunction.

Improved All-Polymer Solar Cell Performance of n-Type Naphthalene Diimide–Bithiophene P(NDI2OD-T2) Copolymer by Incorporation of Perylene Diimide as Coacceptor

“…29,30 Later studies showed that the aggregation in P(NDI2OD-T2) could be suppressed significantly in the early stage of film formation by using more polar aromatic solvents. 31 Indeed, the solar cell devices prepared from such nonaggregated solution showed improved PCE due to good intermixing of donor and acceptor components, thereby efficiently harvesting photogenerated excitons at donor−acceptor interface. Beyond the solvent induced morphological control, the structural variations also plays a vital role in improving the blend morphology, bulk crystallinity, molecular orientation, highest occupied and lowest unoccupied molecular orbital (HOMO−LUMO) energy level, and charge transport properties of polymer.…”

Development of Polymer Blend Solar Cells Composed of Conjugated Donor and Acceptor Polymers

“…The well-known composite of P3HT and a fullerene derivative, PCBM, has little absorptivity in the NIR region longer than 650 nm. Therefore, a variety of LBPs have been synthesized, consequently, being available for choosing polymers with a preferable bandgap and adequate HOMO/LUMO potentials as electron donor and acceptors [9][10][11]. The absorption spectrum of N2200 in Fig.…”

Charge Transport Orthogonality in All‐Polymer Blend Transistors, Diodes, and Solar Cells

“…S1) that CN suppresses the pre-aggregation of P(NDI2OD-T2) in solution, [18] thus improving the solar cell performance mainly by a more efficient photogeneration of free charge carriers in the film bulk. [19] Indeed, because light-induced excitons in BHJ-PSCs usually have short exciton diffusion lengths of only a few nanometers (typically less than 10 nm for several organic semiconductors [16] ), a fine donor/acceptor phase separation results in efficient exciton dissociation. The improved vertical charge transport for the film processed from the xylene:CN mixture is corroborated by a two/ten fold increase in space-charge limited current mobilities (µe = 2.2  10 -4 cm 2 V -1 s -1 and µh = 3.4  10 -4 cm 2 V -1 s -1 ).…”