Low Voltage‐Loss Organic Solar Cells Light the Way for Efficient Semitransparent Photovoltaics

Abstract

Organic solar cells that are transparent to visible light are highly desirable for applications such as window treatments or solar greenhouse panels. A key challenge is to simultaneously transmit most photons between 400 and 700 nm while retaining a high short‐circuit current and power conversion efficiency (PCE). Here, organic bulk heterojunction (BHJ) solar cells consisting of a donor polymer (PM2) is reported and the non‐fullerene acceptor ITIC‐Th achieves a PCE of 9.3%, and the BHJ thin films exhibit an average visible transmittance over 40%. This value is achieved primarily due to a very high open‐circuit voltage (VOC) of 0.93 V, which represents a voltage loss of only 0.50 V relative to the material optical bandgap, Eopt. In PM2:PC61BM devices, this voltage loss increases to 0.62 V (VOC = 0.82 V). It is found that this difference in VOC is due to higher nonradiative recombination in the fullerene‐based solar cell, suggesting that non‐fullerene acceptors may lead to better performance in semi‐transparent devices. The optoelectronic properties associated with PM2:ITIC‐Th and PM2:PC61BM blends are further corroborated by different morphological features and local structures at the donor‐acceptor interfaces characterized by atomic force microscopy, X‐ray scattering, and solid‐state NMR spectroscopy techniques.

Subgap Absorption in Organic Semiconductors

“…Equation 6 is often applied to fit spectra obtained from subgap region EQE measurements to parametrize CT states. 20 , 52 − 58 As explained earlier, it is important to note that the spectral shape of the EQE does not always follow α( E ) due to the distortions caused by optical interference effects. While such distortions may appear minor when plotting the EQE on a logarithmic scale, these have been demonstrated to cause significant relative errors in the extracted E CT and λ CT values, being as large as 90%.…”

“…The corresponding α CT ( E ) spectral line shape is generally expected to take the Gaussian shape of the form α CT ( E ) = f α , CT E 4 π λ CT k T exp ( − ( E CT + λ CT − E ) 2 4 λ CT k T ) where the prefactor f α is related to the density of the CT states ( N CT ) and the associated oscillator strength ( f σ ) via f α,CT = N CT f σ,CT . Equation is often applied to fit spectra obtained from subgap region EQE measurements to parametrize CT states. ,− As explained earlier, it is important to note that the spectral shape of the EQE does not always follow α( E ) due to the distortions caused by optical interference effects. While such distortions may appear minor when plotting the EQE on a logarithmic scale, these have been demonstrated to cause significant relative errors in the extracted E CT and λ CT values, being as large as 90% …”

Additive-free molecular acceptor organic solar cells processed from a biorenewable solvent approaching 15% efficiency

“…Molecular-level origins of the different local structures and packing interactions in neat materials and blends can be characterized by ssNMR spectroscopy. 42,63,[79][80][81][82][83] To resolve the different packing interactions in acceptor morphology that contribute to the different PCE values in OPV devices, we have selected the PM7-D3:PTI04 and PM7-D3:Y12 blends, owing to the differences in their PCE values (Table 1). For the neat PM7-D3, PTI04, and Y12 compounds and the BHJ blends, the short-range order and intermolecular interactions are examined by analyzing the 1 H, 13 C, and 19 F chemical shifts (Fig.…”