Light trapping in photovoltaic devices with weak dielectric absorbers: Nanostructured dielectric and metal interfaces

Abstract Nanostructured designs provide new possibilities to improve light absorption in thin film photovoltaic devices. However, the underlying design rules are not well established. Most thin film photovoltaic absorbers are weak dielectrics in which light trapping is a challenge, owing to lower density of admissible trapped waveguide modes. We consider a nanostructured bulk heterojunction as a representative example of the weak dielectric systems. We show using a combination of theoretical simulations and experiments that such a weak dielectric photovoltaic system built on a nanostructured transparent substrate can enhance light trapping via nano-scale morphologies at both the dielectric light entry interface and at the rear absorber-metal electrode interfaces. Our simulations predict an enhancement exceeding 80% in the photocurrent density generated per unit absorber volume, for absorber thickness in the range 100–200 nm. This thickness range appears to be optimal for most thin-film solar cells. Our experiments based on monolithic nanostructured substrates showed good agreement with the predictions of the model. Broadband enhancement in photocurrent was obtained only for thinner absorbers which allowed the formation of nanoscale interfaces on the front and rear interfaces. Further, we have studied the role of the nano-scale curvature through simulations. It was seen that the convex nano-domes result in the best optoelectronic performance with ∼14% improvement in photocurrent and ∼15% improvement in the fill factor leading to about 30% improvement in the power conversion efficiency. This is due to an improved absorption in conjunction with about 38% drop in the series resistance. This study shows using that in the case of weak-dielectric absorbers, it is useful to have nanoscale interfaces of optimal curvature at both the front and rear absorber contacts. The associated design rules for simultaneous improvement of optical and electrical performances are established.