In this paper, we study experimentally the enhancement of parameters of a typical organic thin film solar cell (OSC) achieved by an originally proposed light trapping structure (LTS). Our LTS is an array of silver nanoantennas supporting collective modes, what provide subwavelength light enhancement in the substrate. Low losses in the metal result from the advantageous optical field distribution that is achieved in the broad range of wavelengths. We study the enhancement of all the main photovoltaic (PV) parameters observed in OSC with our LTS, such as power conversion efficiency (PCE), which is effectively increased by 18%, fill factor (FF), that can also be effected positively and short-circuit current Isc improvement. Although the Ag nanoantenna occupies up to 40% of the photoactive OSC area, it provides the nearly twice enhanced optical absorption, resulting in overall higher Isc and FF. Improvements of the optical and electrical design of OSC architectures are discussed for further enhancement of performance.