(Invited) Nanomaterials and Device Architecture Engineering for Enhanced Efficiency in Bulk Heterojunction Solar Cells

Polymer solar cells are emerging as an alternative inexpensive renewable source of green energy due to their interesting properties, such as, low–temperature based manufacturing, mechanical flexibility and solution processablility. However, efficiency still to be improved in order to make a viable technology.  Here we report on various approaches to increase the efficiency by engineering the materials or the device architecture. First, a modified bulk heterojunction (BHJ) solar cell in which a nanohybrid composite material made of lead sulfide (PbS) colloidal quantum dots (QDs) and multiwall carbon nanotubes (MWCNTs) was incorporated onto a standard regioregular poly(3–hexylthiophene) (rr–P3HT):phenyl–C₆₁–butyric acid methyl ester (PCBM) blend. This hybrid device exhibits a higher power conversion efficiency (PCE) of ~3.40% as compared to that of ~2.57% for a control rr–P3HT:PCBM BHJ solar cell made under the same experimental conditions. The increase in efficiency by 33% is mainly attributed to the extended quantum-dot-sensitization in the near–infrared (NIR) due to the absorbance of PbS–QDs/MWCNTs in the spectral range from 700 nm to 1500 nm. In a second approach, localized surface plasmon resonance (LSPR) phenomenon in metallic nano-particles/structures was used for improving the optical absorption of a constant thickness of the photoactive layer. LSPR occurs when the frequency of the electric field of the incident light resonates with oscillations of the free conduction electrons in metallic nanoparticles. Consequently, the illuminated particles are excited and strongly absorb/scatter the incident light. This produces a large enhancement, up to a factor of 100, in the local electric field surrounding the particles. Since the resonance frequencies of noble metals are located mostly in visible or near infrared region of the spectra, incorporating such metallic nanoparticles in BHJ solar cell could lead to an enhancement in their total absorption of light and hence enhance their efficiency. Here, we focus on studying the effect of incorporating gold nanorods layer into different locations of the BHJ solar cell. As deduced form the characterization data, the transverse and longitudinal resonance peaks of Au NRs with length of 40 nm and diameter of 10 nm are situated towards the NIR regime. Moreover, Au NRs were found to possess enhanced forward-scattering properties due to their unique shape. Different ways for inserting nanorods into our solar cells have been considered: embedding them in the photoactive layer, depositing them on the anode, and using them to form a layer on the cathode. We found that for each location of rods in our devices there was an optimal concentration of the rods to produce enhancement in the devices’ performance. Based on theoretical considerations, devices enhancement was related to either the far field or near field effect induced by the presence of rods. It was found that increasing or decreasing the rods density from the optimal one reduced the overall efficiency of resulting devices. Using the rod shape of gold nanoparticles to increase the device performance is indeed a promising approach since a fairly low density of the rods in the layer succeeded in increasing remarkably the devices efficiency by up to 21.3 %. Finally, engineering of both electrode interfaces through the introduction of ultra thin layers of donor– and acceptor– type materials at the anode/BHJ and BHJ/cathode interfaces, was done. The introduction of the P3HT and modified C₆₀ at the anode and cathode interfaces improve the hole and electron extraction from the BHJ layer to either electrode. From the experimental observations, we presume that upon interposing such interfacial layers at the anode/BHJ and BHJ/cathode interfaces could repair poor contact at the anode/hole–donor and electron–acceptor/cathode interfaces and prevent undesired vertical phase segregation. The PV cell fabricated from poly(3–hexylthiophene) (P3HT) as donor and C₅₅H₃₆O as acceptor, exhibit an optimal power conversion efficiency (PCE) of 4.14%, where when  C₇₂H₁₆S is used a highest PCE of 4.35% was observed.