  Minimizing the material usage in thin-film solar cells can reduce
manufacturing costs and enable mechanically flexible implementations, but
concomitantly diminishes optical absorption. Coherent optical effects can help
alleviate this inevitable drawback at discrete frequencies. For example,
coherent perfect absorption guarantees that light is fully absorbed in a thin
layer regardless of material or thickness but only on resonance. Here we show
that omni resonance delivers such coherent enhancement over a broad bandwidth
by structuring the optical field to nullify the angular dispersion intrinsic to
resonant structures. After embedding an amorphous-silicon thin film
photovoltaic cell in a planar cavity, pre conditioning the incident light using
an alignment free optical arrangement severs the link between the resonant
bandwidth and the cavity photon lifetime, thereby rendering the cavity omni
resonant. Coherently enhanced near infrared absorption doubles the photocurrent
over the targeted spectral range 660 to 740 nm where every wavelength
resonates. These results may pave the way to transparent solar cells that
optimally harvest near infrared light.
