# Increasing polymer molecular weight enables low-donor-content, efficient and scalable semi-transparent organic solar cells

## TL;DR

Increasing the molecular weight of a polymer donor in solar cells improves efficiency and transparency, enabling better performance in semitransparent organic solar cells.

## Contribution

High-molecular-weight polymers enable efficient and scalable semitransparent solar cells with low donor content.

## Key findings

- High molecular weight PTB7-Th improves charge mobility and allows low donor content while maintaining high efficiency.
- Improved percolation and acceptor crystallinity explain the enhanced performance in high-molecular-weight blends.
- Scalability is affected by cell area, and the concept applies to other polymers and acceptor molecules.

## Abstract

The advent of non-fullerene acceptors has enabled high efficiencies in organic photovoltaics (OPVs). The active layer of such devices typically consists of a narrow-bandgap molecular acceptor (A), with strong light absorption in the near-infrared region, combined with a polymer donor (D) that harvests visible photons. Reducing the donor content is a good strategy to increase transparency, but often leads to lower power conversion efficiencies (PCEs) due to loss in absorption efficiency and, importantly, a worsening of the electrical properties. Here, we tackle this compromise by investigating if the improved electrical properties granted by high polymer molecular weight (Mw) can simultaneously result in a high PCE and high visible transparency, thus leading to efficient semitransparent OPVs. We investigate the polymer : non-fullerene blend PTB7-Th : IEICO-4F as a function of blend ratio for two polymer Mw values. We show that increasing the PTB7-Th Mw from 57 kDa to 125 kDa promotes a film morphology that enhances charge carrier mobility. Moreover, we demonstrate that using high-Mw PTB7-Th enables high PCEs in blends with as low a polymer content as 28%. Interestingly, we find that this behaviour can be explained by improved percolation (granted by higher Mw) and higher acceptor crystallinity. In order to assess the scalability of the system, we compared devices fabricated in nitrogen or in air, and investigated the use of xylene as a greener solvent, the effect of increasing the cell area, the use of semitransparent electrodes and the fabrication of modules, identifying cell area as the most critical factor that negatively impacts PCE. Finally, we show the generality of the concept by extending it to two other polymers and two other acceptor molecules.

Increasing the donor’s molecular weight improves transport and enables high-efficiency solar cells with reduced donor content, which in turn improves transparency, leading to boosted semitransparent photovoltaics.