Disordered semiconductors exhibit poor electronic transport properties due to their amorphous nature. Low carrier mobility and lifetime limits the diffusion length to 100 ~ 300 nm. Thus, conventional p-n junction photovoltaic design that is composed of a large quasi-neutral region and a small depletion region (Figure 1a) leads to poor charge extraction. To overcome this material limit, a p-i-n junction has been adopted in amorphous silicon solar cells. The internal electric field extends throughout the intrinsic absorber layer and assists carriers to be efficiently extracted to their respective electrodes (Figure 1b). Films composed of quantum dots (QD) show electronic transport properties similar to disordered semiconductors and may also benefit from the p-i-n “drift” device architecture, which has not been applied to QD solar cells to date.

This project aims to implement two major advantages of the p-i-n heterojunction using the QD as an intrinsic absorber layer. The first goal is to augment the width of the depletion region. This widening enhances the light absorbance of the device, allowing more photogenerated carriers and thereby increasing short-circuit current (J_sc). The second advantage to exploit is the ability to dope n- and p-type layers without having to affect the intrinsic absorber layer. Increasing the Fermi level difference by doping both window layers would increase the build-in electric field, enhancing open-circuit voltage (V_oc). Figure 1c and d show schematic illustrations of the device structure and energy diagram, respectively. Figure 2a and b show the device characteristic studied for each separate junction. Rectifying behavior is observed in both p-i, i-n and p-i-n junctions. Figure 2d and e show the JV curve under illumination with and without the intrinsic QD absorber layer. Future studies will focus on V_oc, J_sc, and fill factor (FF) as a function of p-type and n-type layer doping as well as intrinsic layer thickness.

1. M. Zeman, “Solar Cells,” TU Delft OpenCourseWare, Delft University of Technology, 2011. [Online]. Available: http://ocw.tudelft.nl/courses/microelectronics/solar-cells/readings/

Colloidal lead sulfide quantum dot (PbS QD) solar cells have recently shown attractive improvements in efficiency [1]. PbS QDs have been investigated in various device architectures—including Schottky junction, depleted planar heterojunction, and ordered bulk heterojunction photovoltaic (PV) architectures—with the aim of maximizing light absorption while maintaining efficient charge transport ^[1] . However, the thickness of the PbS QD layer in the aforementioned architectures remains limited by the relatively short exciton diffusion length characteristic of all QD films.  We developed a novel architecture in which PbS QDs and zinc oxide (ZnO) nanoparticles are co-deposited from solution to form a bulk heterojunction (BHJ). Much like a typical polymer BHJ solar cell, these PbS QD PV devices can decouple the light absorption, which is proportional to device thickness, from the exciton diffusion length ^[2] . We fabricated these BHJ PbS QD:ZnO solar cells in solution by layer-by-layer deposition in which each step is followed by ligand exchange as shown in Figure 1a. By conducting preliminary optimization of the processing condition, we obtained the current density as a function of the applied voltage graph shown in Figure 1b. Further studies aim at improving device performance while studying the morphological conformation of PbS and ZnO domains.

Figure 1: a) Schematic of the solution process for fabrication of bulk heterojunction lead sulfide quantum dots (PbS QD) and zinc oxide (ZnO) solar cells. The devices are fabricated by layer-by-layer deposition in which each step is followed by ligand exchange. b) JV characteristics under dark (squares) and AM 1.5 illumination (circles) for planar (control – red) and bulk heterojunction (blue) PbS QD:ZnO devices.

1. J. Tang and E. H. Sargent, “Infrared colloidal quantum dots for photovoltaics: Fundamentals and recent progress,” Advanced Materials, vol. 23, no. 1, pp. 12–29, Jan. 2011.
2. C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. J. Jia, and S. P. Williams, “Polymer-fullerene bulk-heterojunction solar cells,” Advanced Materials, vol. 22, pp. 3839–3856, Aug. 2010.