High‐Performance Semi‐Transparent Organic Photovoltaic Devices via Improving Absorbing Selectivity

Abstract

facades, roofs, etc. [1-4] Criteria to evaluate the performance of (semi-)transparent photovoltaic (ST-PV) include power conversion efficiency (PCE), averaged photopic transmittance (APT), and color rendering index (CRI), which indicates how good the original color is retained. [5] Since the light-absorbing and transmittance seem a paradox for PV devices, the light utilization efficiency (LUE), a product of PCE and APT (LUE = PCE × APT), is proposed to judge the performance of ST-PV. [4,5] Considering that the solar energy comprises photons of different energies or wavelengths (including both the visible and invisible photons), an ideal ST-PV device should make full use of the invisible photons to achieve high PCE, while allowing most of the visible photons to penetrate through for sake of high APT and CRI. [6-8] Thereafter, photovoltaic devices with good absorbing selectivity, that is, the ability to selectively absorb the near-infrared (NIR) photons and transmit the visible ones, are highly favored for high-performance ST-PV. Theoretically, the maximum LUE of single-junction ST-PV can reach over 20% with absolute selective absorbing property (Figure 1a). [9] While, the practical record LUE can only reach 5.7%, even with the tandem structure. [10] The huge gap can be attributed to the poor absorbing selectivity of ST-PV. [11-13] Particularly, energy band principle excludes most of the high-efficiency photovoltaic materials for ST-PV application. For example, although the inorganic photovoltaic materials, such as silicon, CIGS, CdTe, GaAs, etc, exhibit excellent performance approaching the S-Q limit and NIR absorbing properties, the energy band principle determines their strong absorption in the visible region (≈0% APT) and thus poor selective absorption. [14-16] As a result, these materials are seldom considered for ST-PV application. Fortunately, the semi-transparent organic photovoltaic (ST-OPV) device seems one exception to break the curse of energy band principle to fulfill good selective absorption. The organic semiconductors typically exhibit vibronic lightabsorbing features, which show peaks and valleys in different absorbing regions. Moreover, their absorption profiles can be easily tailored via modifying the molecular structures, rendering more space to realize good absorption selectivity. [17-20] Indeed, tremendous efforts have been devoted to explore high-performance ST-OPV. Particularly, developing the high-performance low band Semi-transparent organic photovoltaics (ST-OPVs) are promising solar windows for building integration. Improving the light-absorbing selectivity, that is, transmitting the visible photons while absorbing the invisible ones, is a key step toward high-performance ST-OPV. To achieve this goal, the optical properties of the active layer, transparent electrode, and capping layer are comprehensively tailored, and a highly efficient ST-OPV with good absorbing selectivity is demonstrated. First, a numerical method is established to quantify the absorbing selectivity of materials and d...

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