As solar cells approach the radiative limit, they can achieve significantly higher efficiencies and exhibit new effects owing to the significant number of radiatively emitted photons. One such effect is an increase in voltage by optically limiting the angles at which light is emitted from a cell to a narrow range around normal incidence. In materials such as GaAs where radiative recombination is dominant, a significant portion of the dark current results from radiatively emitted photons escaping the solar cell. Optically limiting the angles of emitted light recycles many of these emitted photons back to the cell, where they are re-absorbed. Thus, as fewer photons ultimately escape the cell, radiative dark current is reduced, and the cell voltage and efficiency are increased, with efficiencies >36% possible for Auger limited GaAs cells. Despite clear theoretical predictions, only recently, with the introduction of cells lifted off the growth substrate, have GaAs cells begun to closely approach the radiative limit so that enhanced photon recycling by limiting emission angle may be observed.
As proof of concept, we have demonstrated enhanced photon recycling and open-circuit voltage (Voc) experimentally using a nanophotonic element that limits the emission angle only over the narrow wavelength range of emitted light in GaAs, so that diffuse light may still enter the cell. This angle restrictor consists of an optimized dielectric multilayer with 19 nanoscale alternating high and low index layers and less than 3 um total thickness. Integrating sphere measurements on the fabricated structure showed excellent normal incidence transmission and near unity reflectivity at oblique angles (>20°) for radiatively emitted wavelengths. By simply placing this narrow band angle restrictor in optical contact with a high quality GaAs cell, we observed a clear Voc increase at constant current. Measuring the Voc increase with angle restriction across four cells with external radiative efficiencies of 3-16%, we found that cells closest to the radiative limit showed the largest voltage increase (3.7mV) with angle restriction, as more radiatively emitted photons are available to be recycled. Furthermore, we found excellent agreement between the measured Voc increases and a realistic detailed balance and optical model. Consistent with theory, current-voltage measurements in the dark showed a reduction in dark current with angle restriction. In particular, the high-voltage (J_01) dark current component associated with radiative recombination showed a marked (>10%) decrease, consistent with the observed Voc increase under illumination, while other fitted parameters showed no change with angle restriction. Finally, using spacers to vary the height of the angle restrictor, we found that more closely coupling the angle restrictor to the cell leads to higher Voc, emphasizing the optical nature of the enhancement.