In the rapidly developing field of solar cell technology, hybrid nanocrystal/polymer cells have become the focus of intensive research due to their low cost, light weight, flexibility, high electron mobility, and stability.1–3 Improving the performance of these devices depends crucially on finding effective electron acceptor nanocrystals and clarifying their working mechanisms.
Consider, for example, CdSe and CdTe nanocrystals. While both of these type II–VI semiconductors are considered good photovoltaic materials for inorganic solar cells, CdSe significantly outperforms CdTe in hybrid nanocrystal/polymer solar cells.3,4 How can the presence of Se or Te in the nanocrystals account for such a difference in photovoltaic properties? We addressed this question by investigating a series of ternary CdSexTe1-x nanocrystal5 electron acceptors, where x is the Se fraction. By trading off Se and Te content, we were able to show how cell performance depends on the concentration of these components. 6
The compositions of the synthesized ternary nanocrystals we studied were x = 0 (CdTe), 0.23 (Na1), 0.53 (Na2), 0.78 (Na3), and 1 (CdSe). Each was a 9:1 mixture by weight of nanocrystals and the polymer poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), also known as MEH-PPV.7 The active hybrid polymer/nanocrystal layer was sandwiched between an aluminum cathode, and an anode of indium tin oxide/poly(3,4-ethylene dioxythiophene/polystyrene sulfonate (ITO/PEDOT:PSS).
The photovoltaic properties of hybrid solar cell devices with ternary nanocrystal compositions. The illumination is 80mW/cm2 of AM1.5 (standard sunlight spectrum at Earth's surface). Voc: open-circuit voltage. Jsc: short-circuit current. FF: Fill factor. η: Conversion efficiency.
From the current density-voltage curves of the devices, we determined open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and power conversion efficiency (η). As indicated in Table 1, the photovoltaic quality, as measured by these properties, increased with increasing Se content. Comparing the endpoints, the conversion efficiency at x=1 (CdSe) is almost 400 times greater than at x=0 (CdTe).
Donor and acceptor band-level matching is a key factor responsible for efficient charge separation in high-performance solar cells. We used cyclic voltammetry to determine the nanocrystal and MEH-PPV band levels, given by the energies of each component's highest occupied molecular orbital (HOMO; lower part of Figure 1) and lowest unoccupied molecular orbital (LUMO; upper part of Figure 1).
Figure 1. Shown is a diagram of the band-energy levels of the polymer (MEH-PPV) and nanocrystals. The uppermost levels correspond to the lowest unoccupied molecular orbitals, while the lower levels correspond to the highest occupied molecular orbitals. Also shown are the levels of the aluminum cathode (Al) and the ITO/PEDOT:PSS anode (PEDOT).
Comparing energy levels in Figure 1, all nanocrystal LUMO levels are lower than the polymer's. However, only the CdTe HOMO level exceeds the polymer HOMO level, indicating that change separation is forbidden between the two components, and allowed for the other nanocrystals.
Efficient charge separation requires optimal energy differences between electron donor (polymer) and acceptor (nanocrystal). Figure 1 shows that the energy gap between the nanocrystals and the polymer increased with Se content for both HOMO and LUMO levels, explaining why the photovoltaic properties of the devices improved with increased Se content in CdSexTe1-x.
The dependence of hybrid solar cell performance on nanocrystal composition is due to the charge separation process, which depends on the degree of matching between donor and acceptor band levels. In providing insights on the key factors that influence hybrid solar cell performance, our work contributes to the design of improved nanocrystals for hybrid solar cell fabrication.