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Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed. Hall of Fame ArticleAlthough the last decade has seen much progress in the field of OSC research, the development of semitransparent devices lags behind because of the lack of optimized photoactive materials. [22][23][24][25] The strong absorption of the active materials in the visible region causes difficulty when one tries to balance the power conversion efficiency (PCE) and transmittance. Encouragingly, the recent progress of fullerene-free OSCs has the potential to shed fundamental solutions to overcome these obstacles, hence becoming attractive research topics on developing narrow-bandgap nonfullerene acceptors to researchers. [26][27][28][29][30] We focus in this section on presenting recent progress in the design of narrow-bandgap organic semiconductors that have bandgaps less than 1.5 eV and have extended the boundaries of visible transparent/NIR absorbing OSC technology. Solution-Processable Organic SemiconductorsOrganic semiconductors are carbon-based materials with an electronically delocalized π-conjugated backbone. Such π-conjugated systems are created by a linear series of overlapping p z orbitals (π bonds). [31,32] As the parallel overlap of carbon p z orbitals increases with the molecular extension, the π bonds may further spread out into π bands, and this leads to a narrower energy bandgap. The energy of the highest occupied molecular orbital (HOMO) corresponds to the topmost π band, and the lowest π* band is referred to as the lowest unoccupied molecular orbital (LUMO). The energy gap between the HOMO and the LUMO dominates optical properties. Photoexcitations result in Coulombically bound excitons (electron-hole pairs) due to the low dielectric constant (ε ≈ 2−4) characteristic of organic semiconductors. [33,34] The exciton binding energy is in the range of 0.3−1 eV, thus requiring a high interfacial area between electron donor (D) and electron acceptor (A) components to promote exciton dissociation into free carriers. [35,36] This approach has led to the development of organic bulk heterojunctions (BHJs), which are cast from blend solutions of D and A components. [37][38][39] The excitonic nature of organic semiconductors offers an advantage in wavelength-specific light harvesting applications through a delicate manipulation of energy ...

Small-bandgap polymer solar cells (PSCs) with a thick bulk heterojunction film of 340 nm exhibit high power conversion efficiencies of 9.40% resulting from high short-circuit current density (JSC ) of 20.07 mA cm(-2) and fill factor of 0.70. This remarkable efficiency is attributed to maximized light absorption by the thick active layer and minimized recombination by the optimized lateral and vertical morphology through the processing additive.

Narrow bandgap n-type molecular semiconductors are relevant as key materials components for the fabrication near-infrared organic solar cells (OSCs) and organic photodetectors (OPDs). We thus designed nearly isostructural nonfullerene electron acceptors, except for the choice of solubilizing units, which absorb from 600 to 1100 nm. Specific molecules include CTIC-4F, CO1-4F, and COTIC-4F, whose optical bandgaps are 1.3, 1.2, and 1.1 eV, respectively. Modulation of intramolecular charge transfer characteristics was achieved by replacing alkoxy groups with alkyl groups on thiophene spacers that connect an electron-rich cyclopentadithiophene core to peripheral electron-poor fragments. OSCs incorporating CTIC-4F and CO1-4F with PTB7-Th achieve power conversion efficiencies of over 10% with short-circuit current densities as high as ∼25 mA·cm–2. The same blends achieve OPD responsivities of 0.52 A·W–1 at ∼920 nm. These findings highlight outstanding opportunities to tune further molecular design so that OPDs may ultimately compete with their silicon counterparts.

the continued research has led to power conversion efficiencies (PCEs) over 11% for single-junction devices. [7,8] Be that as it may, difficulties in tuning the molecular structure and electronic properties, as well as the comparatively high cost of production of fullerene-based acceptors are drawbacks that have triggered the search for nonfullerene acceptors (NFAs) as an alternative. [9][10][11][12][13][14][15][16] Noteworthy improvements have been made over the last few years, and state-of-the-art NFA-OSCs have been reported with PCEs over 16 %, thus outperforming their fullerene-based counterparts. [17,18] In tandem and ternary systems, PCEs reaching even up to 17.3% have been recently achieved. [19,20] Additionally, the reduction in the bandgap of NFAs opens up the possibility to fabricate semitransparent OSCs that could be applied in building-integrated photovoltaics or power generating greenhouses. [21][22][23][24] To further improve the performance of OSCs, loss-processes such as nongeminate recombination, the recombination of electrons and holes which do not originate from the same exciton, have to be curtailed. This includes bimolecular recombination (also known as Langevin recombination), where charge carriers recombine directly from band to band, as well as trap-assisted recombination (also known as Shockley-Read-Hall recombination), where states deep in the bandgap act as efficient recombination centers. A detailed understanding of these mechanisms responsible for the aforementioned recombination losses is required. [25] Whether or not there are considerable differences in recombination dynamics between NFA-OSCs and fullerene-based OSCs, has yet to be understood since most research in regard to recombination dynamics was performed only with OSCs employing fullerene acceptors. [26][27][28] Furthermore, the majority of studies describe recombination dynamics based on a numerical, drift-diffusion model under the assumption of an effective-medium for the BHJ active layer. [29][30][31][32][33][34][35][36][37][38] The concentration of charge carriers is the key differential parameter in the theory of recombination dynamics. However, only integral parameters (electrical conductivity, impedance, open-circuit voltage, V OC ) can be directly measured by experiments. Therefore, the primary challenge of the theoretical background of any method developed to quantify recombination processes in photovoltaic devices is based on finding the appropriate relationship between the measured In this study, a comprehensive analytical model to quantify the total nongeminate recombination losses, originating from bimolecular as well as bulk and surface trap-assisted recombination mechanisms in nonfullerene-based bulk heterojunction organic solar cells is developed. This proposed model is successfully employed to obtain the different contributions to the recombination current of the investigated solar cells under different illumination intensities. Additionally, the model quantitatively describes the experimentally measured ope...