• Energy Research

Up-conversion (UC) is a promising approach to utilize sub-band-gap photons for solar cells (SCs). Due to the non-linear nature of UC, the optimal excitation power regimes between the solar cell semiconductor and the UC material correspond to a difference in solar concentration of more than an order of magnitude. This difference can be bridged with integrated optics by concentrating the photons transmitted through the solar cell to increase the power density and maximize the intensity of UC luminescence. To realize this, dielectric-filled compound parabolic concentrators (CPCs) were used as integrated optics on the rear side of a planar bifacial silicon solar cell together with a 25% Er3+ doped hexagonal sodium yttrium fluoride (β-NaYF4:Er) UC phosphor. An efficiency increase of 32% from 0.123% to 0.163% under sub-band-gap illumination is quantified by means of the first ever reported I-V characteristics for an up-conversion solar cell (UC-SC) based on c-Si. An enhancement in external quantum efficiency (EQE) is obtained from 1.33% for the non-concentrating reference UC-SC to 1.80% for a solar cell with integrated optics for an excitation at 1523 nm with an irradiance of 0.024 W/cm2, corresponding to a normalized EQE of 0.75 W/cm2. This demonstrates that CPCs are suitable for UC-SC as they increase the concentration in the forwards direction, while maintaining high collection efficiency of the UC emission in the reverse direction. In addition, such an approach enables the optimization of the solar concentration on the UC phosphor independently from the concentration required for the solar cell.

Despite being currently under-represented in IPCC reports, PV generation represents a growing share of power generation. This Perspective argues that underestimating PV potential led to suboptimal integration measures and that specific deployment strategies for emerging economies should be developed.

Terawatt-scale photovoltaics necessary for cost-efficient climate change mitigation, is associated with massive resource consumptions. Continued technological learning enables large-scale deployment without hitting hard resource constraints.

Abstract Upconversion of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly, but requires modification of the solar cells. In this paper, we present a calculation framework to assess the efficiency of a combined bifacial silicon solar cell upconverter device, which is then used to optimize the solar cell׳s front and rear side anti-reflection coatings. Our calculations show that an upconverter can increase the efficiency of an optimized solar cell by 3.0% relative. Subsequently, planar bifacial n -type silicon solar cells were fabricated with optimized anti-reflection coatings. An upconversion layer – containing the upconverter phosphor β -NaY 0.8 Er 0.2 F 4 embedded in the polymer perfluorocyclobutyl – was attached to the rear side of the solar cells and an external quantum efficiency arising from the upconversion of sub-band-gap photons of 1.69% was measured under 1508 nm monochromatic excitation with an irradiance of 1091 W/m 2 . This corresponds to a value of 0.15 (W/cm 2 ) −1 when normalized to the irradiance, constituting a five-fold increase compared to the previously best published normalized values that were achieved without optimized solar cells.

Abstract Rear side hexagonal sphere gratings are demonstrated as diffractive structures that enhance the light path length in the near infrared, where crystalline silicon solar cells suffer from weak absorption. Moreover, the rear side sphere grating can be added behind a solar cell with flat rear surface, giving an “electrically flat but optically rough” device with high efficiency potential. Here, a thin passivating tunnel-contact layer electrically separates the sphere grating from the cell's base. Solar cells with the rear side sphere grating have obtained a VOC of up to 710 mV and a FF of up to 81.9%. External quantum efficiency measurements show a current density gain of 1.4 mA/cm2 due to the sphere grating. This leads to an overall efficiency of up to 22.1% for the solar cells with planar front side and rear side sphere grating. Estimates for perovskite–silicon and III/V-silicon tandem devices show that the efficiency of tandems can be enhanced by up to 2.4% absolute with a sphere grating on the rear side. Thus, sphere gratings could improve Si-based tandem devices that are limited due to a low current in the Si bottom cell.

Upconversion (UC) of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly. We realized an upconverter solar cell device, by attaching an upconverter layer of β-NaYF4 doped with 25% Er3+ embedded in the polymer perfluorocyclobutyl to the rear side of a bifacial silicon solar cell. We determined the external quantum efficiency of such upconverter solar cell devices under broad-band sub-band-gap excitation. Under consideration of spectral mismatch, we calculated the expected increase of the short-circuit current density due to UC under the air mass 1.5 global illumination. We determined an enhancement of 2.2 mA/cm2 for a spectral excitation band ranging from 1450 to 1600 nm and a comparatively low solar concentration of 78 suns. Subsequently, a system of concentrator lens and upconverter solar cell device was characterized with a solar simulator. We determined an increase of the short-circuit current density due to UC of sub-band-gap photons of 13.1 mA/cm2 under a concentration of 210 suns. This corresponds to a potential relative increase of the solar cell efficiency of 0.19%.

A photonic light trap, i.e. a combination of an angularly selective filter and a light scattering process in a solar cell, results in potentially very efficient light trapping. Angularly selective filters are investigated theoretically and experimentally. One of the filters is used to realize a photonic light trap for a thin-film solar cell with amorphous-silicon absorber layer on a roughened superstrate. We experimentally demonstrate that the light absorption in this solar cell is enhanced by 25% in a wavelength range of 700–750 nm. Accordingly, the quantum efficiency of the solar cell demonstrates an increase of 25% in the same wavelength range.

Abstract We demonstrate diffractive rear side gratings to enhance the near infrared light trapping and thus the quantum efficiency of wafer based crystalline silicon solar cells. Binary crossed gratings with a period of 1 µm, produced via nanoimprint lithography and plasma etching, are electrically decoupled from the solar cell by a thin dielectric passivation layer, creating an electrically flat, but optically rough rear side. We fabricated solar cells with thicknesses of 250, 150 and 100 µm and demonstrate a short circuit current density gain due to the grating of 1.2, 1.6 and 1.8 mA/cm 2 for solar cells with planar front surface. For solar cells with pyramidally textured front surface the grating also leads to a small current density gain in the near infrared of approximately 0.3 mA/cm 2 according to EQE measurements, leading to the best cell's efficiency of 21.1%. By optical simulations we show the potential of the grating structure and identify losses in the fabricated solar cells.