Interface engineering for high-performance polymer solar cells

Among promising solar energy conversion devices, bulk-heterojunction polymer solar cells (PSCs) are low cost, lightweight, and flexible. Typically, a PSC consists of a transparent electrode, a p-type semiconducting conjugated polymer blend with an n-type fullerene derivative serving as active layer, and a reflective electrode to generate a so-called sandwiched configuration. Much effort has gone into elevating PSC power conversion efficiency (PCE) as well as simplifying device processing.

Good contact between the active layer and electrodes is an essential factor in PSC performance. Incorporating new interface materials with the desired charge selectivity that are compatible with solution processing is one way to raise the PCE. However, most of the commonly used organic semiconducting materials have similar solubility in common organic solvents, and this may erode previously pristine layers when they are subsequently integrated. As a result, it is very challenging to make high-performance solution-processed multilayer PSCs. Water/alcohol-soluble conjugated polymers (WSCPs) combine unique solubilities and excellent intrinsic optoelectronic properties. Consequently, they have attracted the interest of researchers seeking to solve the problem of interface erosion.

WSCPs consist of a conjugated main chain and polar side chains.^1,2 To understand the relationship between structure and properties, we have developed and studied a series of WSCPs with various main and side chains (see Figure 1). These include amino- and ammonium-functionalized conjugated polymers,^3–5 amino N-oxide functionalized conjugated polymers,⁶ zwitterionic conjugated polyelectrolytes,⁷ and amino-functionalized conjugated metallopolymers.⁸

Figure 1. Chemical structures of our water/alcohol-soluble conjugated polymers (WSCPs) for polymer solar cells (PSCs), including those known as PFN-OX, PFN, and PFEN-Hg.

We found that the optoelectronic properties of WSCPs are highly affected by their main and side chain structures. All the WSCPs investigated are good interlayer materials that can overcome the problem of interface erosion of multilayer solution-processed optoelectronic devices.^3–9Interestingly, we found that these WSCPs not only improve electron collection from the active layer to the electron-collecting electrode in PSCs,²but also enhance electron injection from the high-work-function metal cathode to the active layer in polymer LEDs (PLEDs), with benefits for the production of color displays.^{10, 11}

The unique electron-injection properties of WSCPs have been mainly attributed to the dipole formation between the WSCPs' interlayer and metal cathode.¹² However, the dipole theory alone appears insufficient to explain the function of WSCP materials in the PSCs, since PLEDs and PSCs have opposing charge-injection/transporting directions. We investigated further and found that WSCP interlayers in PSCs fulfill three functions: first, to tune the cathode work function to enhance the open-circuit voltage; second, to dope the acceptor of the active layer (fullerene derivatives) at the interface to facilitate electron extraction; and third, to extract electrons and block holes to enhance the fill factor.¹³ Moreover, in conventional PSCs with a buffer layer of the conductive polymer PEDOT:PSS, we found that the interface between PEDOT:PSS and the active layer is modified by the highly polar solvent used to process the WSCPs, improving hole collection.¹⁴

Using WSCPs as interlayers in inverted devices based on different active layer materials (known as PTB7/PC₇₁BM or PBDTTT-C-T/PC₇₁BM), we have realized state-of-the-art single-junction PSCs with PCEs exceeding 9%.^{8, 15,16} The chosen WSCP material (either PFN, PFN-OXl, or PFEN-Hg in Figure 1) was directly spin-coated onto a pre-washed indium tin oxide (ITO) substrate, which lowered the work function of ITO so that it served as the cathode. We then spin-coated the active layer on top of the corresponding WSCP layer, followed by thermally evaporated molybdenum oxide and aluminum as the anode to form the PSCs. In addition, we have realized high-performance inverted PSCs with an active layer over 1μm thick by using WSCPs as interlayers and the polymers PBDT-DTNT/PC₇₁BM as the active layer.¹⁷

In summary, we have taken advantage of the ease of designing WSCPs to develop a series of them, all of which are highly soluble in strongly polar solvents and can be used as interlayers for high-performance PSCs. Using WSCPs as interlayer materials not only enhances performance but also simplifies fabrication. In the near future, we plan to develop new WSCPs with high conductivity and charge selectivity, which may further improve the performance of PSCs and offer new applications in other organic electronic fields.

The work was financially supported by the Ministry of Science and Technology (grant 2014CB643501), the Natural Science Foundation of China (grants 21125419 and 51361165301), and Guangdong Natural Science Foundation (grant S2012030006232).