**Jin Woo Choi 1, Jong Woo Jin 1, Denis Tondelier 1, Yvan Bonnassieux <sup>1</sup> and Bernard Ge**ff**roy 1,2,\***


Received: 7 December 2018; Accepted: 25 December 2018; Published: 7 January 2019

**Abstract:** We propose a novel method to pattern the charge recombination layer (CRL) with a low-temperature solution-processable ZnO layer (under 150 ◦C) for organic solar cell applications. Due to the optimal drying process and thermal annealing condition, ZnO sol-gel particles formed a three-Dimensional (3D) structure without using a high temperature or ramping method. The generated 3D nano-ripple pattern showed a height of around 120 nm, and a valley-to-valley distance of about 500 nm. Based on this newly developed ZnO nano-ripple patterning technique, it was possible to pattern the CRL without damaging the underneath layers in tandem structure. The use of nano-ripple patterned ZnO as the part of CRL, led to the concomitant improvement of the power conversion efficiency (PCE) of about 30%, compared with non-patterned CRL device.

**Keywords:** organic tandem solar cell; 3D nano-ripple pattern; ZnO sol-gel; charge recombination layer; low temperature solution process
