Organic Photovoltaics

Project 1: Increasing spectral absorption and power conversion efficiency of organic solar cells with new nonfullerene acceptors

PI: Frederic Laquai, Iain McCulloch, Derya Baran, Aram Amassian, Pierre Beaujuge

State-of-the-art single junction organic bulk heterojunction solar cells that use fullerene derivatives as acceptors have now reached power conversion efficiencies beyond 12% (2017). However, a major limitation in terms of further increasing the external quantum efficiency (EQE) of organic PV devices based on such fullerene acceptors is the incomplete spectral absorption of the active layers linked to the modest absorption coefficients of fullerenes in the visible spectrum. In this respect, developing alternative (nonfullerene) acceptors with large optical absorption strength and high electron mobility is the way forward to enhance the power conversion efficiency of organic PV devices beyond the limits currently set by traditional fullerene-based acceptors. 

More specifically, in this project new sets of small molecular and polymer acceptors will be developed (McCulloch group) and their efficiency and stability tested in devices (McCulloch / Baran). The materials will provide a test bed to establish structure-property relations by:
  1. studying the interfacial and bulk energetics as well as donor acceptor interactions by computational approaches (CPF with Denis Andrienko, MPIP),
  2. mapping of energetic landscape of BHJs using STM techniques (Amassian), and iii.) efficiency-limiting photophysical processes by ultrafast transient spectroscopy including all-optical and electro-optical measurement techniques (Laquai).  

Project 2: Investigating the degradation mechanisms of solution processed semiconductors as a route to high efficiency-long term stable solar cell devices

PI: Derya Baran

To be compatible with manufacturing processes, a solar cell device should be comprised of scalable materials as well as being fabricated by easy/scalable processing. It is known in the literature that fullerene derivatives suffer from dimerization and crystallization upon light irradiation in bulk-heterojunction solar cells. In addition, most of the high efficiency fullerene solar cells have been achieved with solvent additives. Yet, mixing high boiling point co-solvents induce phase demixing during aging process and is detrimental to stability. Recently, non-fullerene acceptors emerged as alternatives to fullerenes and showed superior performances as well as stabilities compared to fullerene derivatives. However, understanding the origin of this better stability is still not well understood. In addition, there is still need for exploring the aging and degradation of non-fullerene acceptors as well as their solar cell devices for long-term high efficiency organic solar cells. This project aims to identify intrinsically stable donor and acceptor materials and devices for long-term stable organic solar cells. The project involves the characterization of materials and devices in terms of their stability and degradation under different stress conditions such as light, oxygen and thermal using various characterization techniques.  

Project 3: Fully-printed bulk heterojunction solar cells with best-in-class efficiency

PI: Aram Amassian, Frederic Laquai, Derya Baran

We seek to successfully translate champion lab-based solar cells to an entirely scalable and green solvent approach that ultimately leads to slot-die coated large area devices (Amassian & Baran). We seek to do so while maintaining performance parity on small device area and scaling up to large device area (1 cm2) with minimal losses. The scale-up and green processing targets the ETL and HTL layers (Amassian & Laquai). To do this successfully, we will map out process kinetics and aggregation behavior in lab-based and scalable processes used for coating the active layer, as well as the ETL and HTL (Amassian). We will also establish a feedback loop which includes device fabrication and characterization (Amassian, Baran) as well as more advanced device physics (Laquai) to identify sources of loss in order to achieve champion performance. The effort will include the development of a solution-processed top transparent electrode which should enable high-performance semi-transparent OPVs. This collaborative effort between the Amassian, Baran and Laquai groups should yield state of the art solar cells, as well as the methodology to scale-up devices yet maintain performance parity with World-record solar cells. 

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