Highly efficient, green-solvent processable, and stable non-fullerene polymer solar cells enabled by a random polymer donor

Highlights

• Non-fullerene solar cells based on a random copolymer 2TRA were fabricated.

• A PCE of 12.10% was achieved for devices with 2TRA:IEICO-4F.

• The photovoltaic devices can be fabricated from various non-toxic/green solvents.

• The photovoltaic devices show thickness-insensitive properties in the range of 100–300 nm.

• The photovoltaic devices show appreciable long-term photo-stability.

Abstract

Rapid development of non-fullerene polymer solar cells (NF–PSCs) with high efficiency have gained wide research attention recently, however, high-performance large-area NF–PSCs are still lacking since most developed NF–PSCs use a thin active layer around 100 nm, which is not compatible with the fabrication of large-area devices. Herein, we fabricate high-performance and green-solvent processed NF–PSCs with thick active layer using a random copolymer (2TRA) and non-fullerene acceptors. Compared to its highly-crystallized analogue (2TRR), 2TRA with rearranged backbone enables better blend morphology, higher exciton generation rate, more efficient excitons dissociation, and less recombination in NF–PSCs. With O-IDTBR and IEICO-4F as non-fullerene electron acceptors, highly-efficient photovoltaic devices with PCE of 10.08% and 12.06% are achieved. Moreover, 2TRA-based NF–PSCs show good compatibility with multiple processing solvents, low sensitivity to the active layer thickness (up to 300 nm), and excellent photostability. The combined advantages of 2TRA-based solar cell devices indicate that 2TRA is a promising polymer donor for the application in large-area PSC modules through roll-to-roll technology, and the rearrangement design on polymer donors could be useful to design high-performance polymers for practical application.

Introduction

Polymer solar cells have received extensive attention as a promising solar energy-harvesting technology due to their advantages of low-cost, light-weight, and potential to fabricate flexible and large-area devices through roll-to-roll technique [[1], [2], [3], [4], [5], [6]]. Polymer solar cells consist of p-type electron donor and n-type molecular acceptor in the bulk-heterojunction active layer to absorb photos, generate excitons, and separate charges. Recent advances showed that low-band-gap non-fullerene molecules were the most promising electron acceptors to maximize the power conversion efficiency (PCE) of non-fullerene polymer solar cells (NF–PSCs) [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. Wide-bandgap polymer donors are generally paired with non-fullerene acceptors, both of which are critical to the photovoltaic performance of NF–PSCs, thus triggering the extensive development of polymer donors and molecular acceptors [[21], [22], [23], [24], [25], [26], [27], [28]]. Besides, the morphology between polymer donor and molecular acceptor should be optimized to improve the fill factor of NF–PSCs [[29], [30]]. Over the past several years, due to a great deal of achievements in the design of donor/acceptor materials and optimization of morphology, the PCE of NF–PSCs has been improved to over 16% [13,[31], [32], [33], [34], [35], [36]].

Regardless of the obtained breakthrough in PCE of NF–PSCs, developing high-performance large-area device towards practical application is still challenging. Large-area module devices are fabricated via scalable techniques, such as roll-to-roll printing [[37], [38], [39], [40]], requiring excellent applicability of NF–PSCs in a wide range of active layer thickness. However, most of the record efficiencies of NF–PSCs were achieved at an optimal active layer thickness around 100 nm, which makes it very challenging to fabricate large-area devices. Thus, the development of a thicker active layer with high tolerance on thickness variation is required to obtain better compatibility with scalable fabrication. In addition, it is of great importance to maintain an optimized morphology in the thick active layer to reduce the PCE losses [41,42]. Moreover, it is essential to employ non-toxic/green solvents to process polymer solar cells to reduce pollution during manufacture fabrication of NF-PSC [43,44]. Thus, it is strongly necessary to develop high-performance NF–PSCs that can work well in wide thickness and be processed from green solvents.

Herein, we reported novel donor/acceptor combinations that could enable high-performance thick NF–PSCs from green solvents by employing a low-band-gap random copolymer (2TRA, Fig. 1a). Compared to its regioregular counterpart 2TRR (Fig. 1a), 2TRA with rearranged backbones showed reduced crystallinity to enable optimal morphology with non-fullerene acceptors and to improve the PCE of NF–PSCs. It was found that the blends of 2TRA and non-fullerene acceptors had obvious photoluminescence quenching, high balanced electron/hole mobility, leading to significantly enhanced PCE of 10.08% for devices with O-IDTBR as acceptor, while its counterpart could only yield a low PCE of 0.71%. The detailed analysis of the photovoltaic devices showed that the 2TRA:O-IDTBR-based device displayed high exciton generation rate, efficient exciton dissociation as well as low recombination. Moreover, with a NIR electron acceptor IEICO-4F, a further improvement of PCE to be 12.10% was achieved for 2TRA based device. Further, the 2TRA based devices could be fabricated from various non-toxic/green solvents and showed extremely low sensitiveness to processing conditions. The 2TRA-based devices also showed remarkably low thickness–dependence in the range of 100–300 nm as well as appreciable long-term photo-stability, providing a promising approach in the application of large-area and stable polymer solar cells.