[Paper Review] Modelling of limitations of bulk heterojunction architecture in organic solar cells

Jacek Wojtkiewicz, Marek Pilch|arXiv (Cornell University)|Sep 4, 2017

Organic Electronics and Photovoltaics9 references3 citations

TL;DR

This paper proposes a 2D geometric model to analyze intrinsic efficiency limitations in bulk heterojunction (BHJ) organic solar cells by comparing the effective donor-acceptor interfacial length in random BHJ networks to an idealized comb architecture. The model reveals that the BHJ architecture achieves only about half the interfacial length of the ideal comb structure, indicating a fundamental geometric constraint limiting efficiency gains beyond ~13%.

ABSTRACT

Polymer solar cells are considered as very promising candidates for development of photovoltaics of the future. They are cheap and easy to fabricate, however, up to now, they possess fundamental drawback, low effectiveness. In the most popular BHJ (bulk heterojunction) architecture the actual record of efficiency is about 13 percent. One ask the question how fundamental this limitation is. In our paper we propose the simple model which examines the limitations of efficiency by analysis of geometrical aspects of the BHJ architecture. In this paper we considered two dimensional model. We calculated the effective length of the donor-acceptor border in the random mixture of donor and acceptor nanocrystals and further compared it with an ideal comb architecture. It turns out that in the BHJ architecture, this effective length is about 2 times smaller than in the comb architecture.

Motivation & Objective

• To investigate whether the current ~13% power conversion efficiency ceiling in bulk heterojunction (BHJ) organic solar cells stems from fundamental geometric constraints.
• To quantify the difference in effective donor-acceptor interfacial length between random BHJ networks and an idealized, periodic comb-like architecture.
• To assess whether the random phase separation in BHJ materials inherently limits the efficiency due to suboptimal interfacial area.
• To provide a theoretical framework for understanding the intrinsic geometric limitations of the BHJ architecture in organic photovoltaics.

Proposed method

• Development of a 2D geometric model representing the donor and acceptor phases as randomly mixed nanocrystals in the BHJ architecture.
• Calculation of the effective donor-acceptor interfacial length in the random BHJ network using statistical geometry principles.
• Comparison of the effective interfacial length in the BHJ model to that of an idealized periodic comb architecture with alternating donor and acceptor stripes.
• Use of a simplified, periodic comb structure as a theoretical upper bound for interfacial area per unit volume.
• Application of geometric probability and spatial distribution analysis to estimate the maximum achievable interfacial length in a well-ordered system.
• Quantitative comparison of interfacial lengths between the random BHJ and the ideal comb structure to identify the fundamental geometric limitation.

Experimental results

Research questions

• RQ1 How does the effective donor-acceptor interfacial length in a random BHJ network compare to that in an idealized periodic comb architecture?
• RQ2 To what extent does the random phase separation in BHJ materials limit the maximum achievable interfacial area for charge separation?
• RQ3 What is the theoretical upper bound on interfacial length in BHJ systems based on geometric constraints alone?
• RQ4 Can the observed efficiency ceiling of ~13% in BHJ solar cells be attributed to inherent geometric limitations of the architecture?

Key findings

• The effective donor-acceptor interfacial length in the random BHJ architecture is approximately 2 times smaller than in the idealized comb architecture.
• This geometric limitation implies that even with perfect material properties, the BHJ structure cannot achieve the same interfacial area as the ideal comb design.
• The model demonstrates that the fundamental architecture of BHJ imposes a hard upper bound on interfacial area, directly limiting charge generation efficiency.
• The 2× reduction in interfacial length suggests that geometric design is a key factor in the current efficiency plateau of BHJ organic solar cells.