Organic photovoltaics (OPV) present a nascent and exciting field, particularly in the green energy sector, where they possess the potential to become the cheapest source of electricity in the world. [1] Their lightweight, flexible and semi-transparent nature allows for building-integrated photovoltaics and agricultural applications such as roofs of greenhouses to generate electricity while still allowing adequate light for plant growth. Nevertheless, OPVs are not yet commercially available due to ineffectively generating free charges and poor device lifetime. [2] 

Herein, this study investigates a novel narrow bandgap non-fullerene acceptor COTIC-4Cl that strongly absorbs light in the near-infrared region (NIR), while remaining transparent in the visible region. [3] Molecular doping of COTIC-4Cl with p- (F6TCNNQ) and n- (NDMBI) dopants were explored by performing conductivity measurements, UV-Vis and FTIR spectroscopy. UV-Vis spectroscopy revealed the presence of polaronic signals in the NIR region, confirming successful doping. Additionally, the UV-Vis spectra exhibited an increase in light absorption for the N-doped COTIC-4Cl films, suggesting improved intermolecular packing. Furthermore, FTIR spectroscopy was conducted under both inert and ambient conditions, revealing a change in baseline absorbance intensity. This variation suggests humidity doping, where water creates trap states that allow electron injection, thereby altering the vibrational modes and electronic environment, which in turn enhances the absorbance intensity. 

To support the spectroscopy data, doped bulk heterojunction (BHJ) devices were fabricated with a conventional device architecture that is yet to be reported in literature. These doped devices resulted in improved device performance efficiency compared to the control device. Moreover, controlling the dopant distribution in BHJ films remains a challenge as efficient doping requires precise placement of the dopant within the BHJ structure. By confining the dopants to their respective layers through sequential deposition (SD), a favorable vertical distribution is achieved, enhancing charge transport. [4] Simultaneously doping both the donor and acceptor layers with p- and n-dopants, respectively, creates a p-i-n-like structure that enhances the internal electric field and improves charge carrier extraction. This approach possesses the potential to produce devices that do not require interfacial layers to facilitate charge extraction.