Pushing out disorder improves polymer solar cell efficiency

Photovoltaic (PV) technology, used to harvest electricity from sunlight, is becoming an essential component of future global energy production. Organic photovoltaics (OPV), such as polymer solar cells, are a desirable complement to traditional solar cells because they can be flexible, semitransparent, and applicable to cheap manufacturing processes such as screen-printing, inkjet, and roll-to-roll techniques. One key challenge for OPV is their relatively low power conversion efficiency, which is strongly dependent on the nanoscale morphology and the molecular alignment of the polymer structure in the photoactive layer, where sunlight is captured to generate electricity.

Although the efficiency of polymer solar cells has increased greatly in the last decade, disorder in the photoactive layer can still hinder the separation of charge generated by sunlight, and the subsequent transport of that charge to the cell's electrodes. The discrete and randomly distributed polymer phases in the active layer cause significant charge recombination, and disorder in the polymer chains themselves results in low carrier mobility.

Our research has focused on applying nanoimprint lithography (NIL), a well-established lithographic method, to increase the performance of polymer solar cells.^1–4 NIL is a top-down process, where a nano-sized stamp deforms a photoresist layer into the desired shape. Using NIL, we worked to optimize the structure of the active layer made by popular materials such as poly(3-hexylthiophene) or P3HT. The ideal structure would be bicontinuous, where both materials comprising the structure are continuously connected. It would also be interdigital, where the two materials interlock like fingers of folded hands, with a controlled vertical polymer chain orientation (see Figure 1). This structure would allow for simultaneous control of charge separation and transport, leading to a potential efficiency breakthrough.

Figure 1. Schematic of an ideal bicontinuous, interdigital polymer solar cell that can be achieved by nanoimprint lithography. The inset shows how charge transport occurs in our imprinted solar cell. Carrier pairs are generated at the interface between the acceptor and donor material from light absorption, which then separate and flow to their respective electrodes.

To fully realize the potential of nanoimprinted polymer solar cells, it is vitally important to understand the correlation between the structure and properties of the polymer nanostructures and heterojunctions formed by nanoimprint process. Our recent studies investigated the effects of various geometries of imprinted P3HT nanostructures on the polymer molecular alignment using grazing-incidence x-ray diffraction measurements, and on the performance of the phenyl-C61-butyric acid methyl ester (PCBM) infiltrated P3HT-nanostructured OPV devices.^{2, 4} Our results show that a highly favorable vertical chain alignment in P3HT can be achieved in imprinted P3HT nanostructures, a significant improvement over the orientation found in non-imprinted films. Figure 2 shows the imprinted nanograting structures in the P3HT polymer layer and the illustrated vertical chain alignment inside the nanogratings induced by NIL. We found that a polymer active layer with vertically aligned chains provided improved hole mobility,³ which resulted in higher current, higher power conversion efficiency, and improved fill factor.⁴ We believe that this chain alignment is caused by vertical flow of the polymer in the nano-cavities, the interaction between the hydrophobic side chains of P3HT, and the hydrophobic surface of the imprinting mold.²

Figure 2. (left) Electron micrograph of the imprinted P3HT nanogratings and (rignt) a schematic of the vertical chain alignment induced by nanoconfinement during the nanoimprint lithography process.

Our systematic study of how differing the nanostructured geometry affects the chain orientation of P3HT nanogratings revealed that proper alignment of the polymer chains only occurs within a certain range of imprinted feature sizes. Vertical orientations that favor hole-carrier transport were formed in imprinted nanogratings with heights greater than 170nm and widths between 60 and 210nm. We also fabricated P3HT/PCBM solar cells with different sizes of P3HT nanogratings to study their influence on device performance. Devices with the tallest fully aligned nanostructures had the highest power conversion efficiency. These nanostructures enabled the most efficient charge separation, charge transport, and light absorption.

Our recent studies of nanoimprinted polymer solar cells demonstrate that the NIL process is a promising technique for controlling the active layer morphology. Simultaneous control of charge separation and transport, two important factors limiting the performance of organic solar cells, is possible using this technique. To integrate NIL into future OPV industrial manufacturing in the long term, extensive research must be performed to test the NIL process window. This includes the ease of applying the NIL technique across a range of different conjugated polymers and the allowable range of processing conditions such as the temperature, time, and pressure used to form the structure. Of particular interest to our work is a class of emerging polymer materials with low band gaps, the energy required for a charge carrier to conduct. Our future work includes attempting to apply the rollable nanoimprint process to make solar cells, and to make smaller nanostructures for improved power conversion efficiency.⁴