Self‐assembled monolayers for interface engineering in polymer solar cells

Abstract

Polymer solar cell (PSC) has been developed vastly in the past decade due to the advantages of low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, semitransparency, and solution processing. The performance and lifetime of PSCs are highly dependent on the properties of both active materials and their interfaces. The combination of the versatility of organic chemistry and the multitude of well-understood ligand-metal interactions allows self-assembled monolayers (SAMs) of organic molecules to direct control over the electronic and chemical properties at the inorganic-organic interfaces. Thus, SAMs are an attractive pathway to reconcile interfaces with tunable interface properties in PSCs. Hence, this review describes the application of SAMs in PSCs at different interfaces. First, SAMs as alternatives of traditional transporting materials to reduce the barrier at indium tin oxide (ITO)/ active layer interface due to the ability of tuning work function of ITO electrode are discussed. Second, the modifications of metal oxide by SAMs to control the electrical contacts at transporting layer/active layer interface are described. Third, tailoring the properties of the donor/acceptor interface by SAMs to improve the performance of PSCs are summarized. Finally, perspectives and challenges are pointed out for developing highly stable and highperformance PSCs by applying SAMs.

Tailoring Self‐Assembled Monolayers for High‐Performance Polymer Solar Cells with Improved Stability

“…The high WF with appropriate modification could provide Ohmic contact at the active layer/ITO interfaces to ensure the efficient charge extraction for high short‐circuit current density ( J SC ) and fill factor (FF) and realize maximum open‐circuit voltage ( V OC ). [ 20 ]…”

“…[ 3,19 ] As alternatives, self‐assembly monolayers (SAMs) of organic small molecules with different dipoles are another attractive alternative for indium tin oxide (ITO) modification to reduce the barrier at ITO/active layer interface with tunable interface properties to achieve high‐performance PSCs. [ 20–22 ] For example, Anthopoulos et al demonstrated a high PCE of 16.6% based on [2‐(9 H‐carbazol‐9‐yl)ethyl]phosphonic acid (2PACz)‐modified ITO (ITO‐2PACz) and poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PM6):N3 active layer due to the high work function (WF) of ITO‐2PACz (5.45 eV) and the nanofiber structure of active layer deposited on ITO‐2PACz. This is higher than PCEs of bare ITO (6.45%) and ITO/PEDOT:PSS–based (15.94%) devices.…”

“…[3,19] As alternatives, self-assembly monolayers (SAMs) of organic small molecules with different dipoles are another attractive alternative for indium tin oxide (ITO) modification to reduce the barrier at ITO/active layer interface with tunable interface properties to achieve highperformance PSCs. [20][21][22] (1 0 ,3 0 -di-2-thienyl-5 0 ,7 0 -bis(2-ethylhexyl) benzo[1 0 ,2 0 -c:4 0 ,5 0 -c 0 ]dithiophene-4,8-dione)] (PM6):N3 active layer due to the high work function (WF) of ITO-2PACz (5.45 eV) and the nanofiber structure of active layer deposited on ITO-2PACz. This is higher than PCEs of bare ITO (6.45%) and ITO/PEDOT:PSS-based (15.94%) devices.…”

“…[25] The SAMs show advantages of low cost, simple fabrication, and decreased parasitic absorption as compared to traditional transporting materials. [20] However, the effect on the stability is still controversial [23,26] and the possibility for large area devices are never studied, which are significantly important for the commercialization of PSCs.…”

Homojunction Doping for Efficient Hole Extraction in Polymer Solar Cells

“…However, the stability of such devices has not yet been fully investigated. [26] Finally, Xu et al have reported devices exhibiting better performances and stability using a new, less acidic, and solution-processable polymer called PCPDT-2Ph-H. [27] Here we demonstrate the fabrication of a solution-processed conventional (p-i-n) OPV structure embedding an HTL consisting of the same polymer used in the active layer: Poly[[6,7-difluoro[(2-hexyldecyl)oxy]-5,8-quinoxalinediyl]-2,5-thiophenediyl] (PTQ10, Figure 1a), a recent major polymer candidate for OPV with simplified chemical structure and lower cost synthesis. [28] Against the rules of solvent orthogonality, we show that coating the polymer:non-fullerene-acceptor (NFA) BHJ on top of the HTL consisting of doped PTQ10 is feasible.…”