Light-trapping is necessary to achieve low-cost and high-efficiency thin-film Si photovoltaics devices. Many nano-architectures leading to light scattering have greatly improved efficiencies although their ideal characteristics remain uncertain. C. Battaglia and co-workers from the École Polytechnique Fédérale de Lausanne recently used UV-nanoimprint lithography to fabricate 12% efficiency micromorph (a-Si :H/μc-Si :H) tandem cells with identically nanostructured ZnO and In2O3:H front electrodes.
As described in the February 9th issue of Nano Letters (DOI: 10.1021/nl1037787; p. 661), the researchers used a high-resolution replication process to copy the nanostructured interface of a randomly oriented pyramidal ZnO electrode (master) onto an In2O3:H electrode (replica). The master is used to mold a UV-sensitive solgel stamp, which in turn is used to obtain a positive replica of the master (on glass). This is the transparent substrate onto which a In2O3:H electrode was sputtered. Tandem Si cells were then deposited by plasma-enhanced chemical vapor deposition.
External quantum efficiencies (EQE) and current–voltage characteristics show that the replica exhibits comparable overall performances as the master, as opposed to a flat reference structure without light-trapping. The nanostructuring allows light scattering in the device, as proved by the disappearance of interferences in the EQE spectra. Efficiencies of the nanostructured electrodes devices reach 12%, against 7% for the flat substrate device. This increase is due to the doubling of the short-circuit current (almost 26 mA/cm2 for the nanostructured replica).
On the material side, the larger bandgap of In2O3:H compared to ZnO implies a larger EQE at short wavelengths, whereas the EQE is improved at long wavelengths due to the low free carrier absorption of In2O3:H with respect to ZnO. These EQE improvements both lead to an increase in short-circuit current. Moreover, the nanotextured replica-In2O3:H interface acts as an anti-reflection layer, in contrast to the flat glass-ZnO interface.
