Efficiency enhancement of ZnO based inverted BHJ solar cells via interface engineering using C₇₀ modifier

The interface quality of ZnO and the photoactive polymer blend is of utmost importance in the performance of organic-inorganic hybrid photovoltaic devices. The chemically prepared ZnO electron transporting layer often produce surfaces unacceptable for efficient electron extraction and understate the photovoltaic performance. Herein, we propose a facile interfacial modification technique to enhance the charge collection efficiency of ZnO cathode electrode by efficiently bridging the superficial troughs and ridges of ZnO with the photoactive PCDTBT: PC₇₁BM polymer blend. The investigations show that vacuum sublimated C₇₀ interlayer efficiently fills the gaps between ZnO and the polymer blend reducing accumulation of the charges at the interface and thus minimizing the recombination probability. It also plays a very crucial role in passivating ZnO electrode against interfacial traps due to adsorbed chemical species. The inclusion of C₇₀ interlayer into the devices led to a substantial increase in device performance with PCE reaching close to 4%, an increment by a factor of 2 compared to the control devices. Our investigations aim towards showing the efficacy of C₇₀ small molecule in significantly enhancing the PCE of ZnO based BHJ solar cells fabricated and measured in ambient conditions rather than setting benchmark efficiency for the configured device. However, better performances for the devices are conceivable by performing the fabrication and measurement in controlled inert atmosphere.