On the Interplay between CT and Singlet Exciton Emission in Organic Solar Cells with Small Driving Force and Its Impact on Voltage Loss. Issue 31 (28th June 2022)

• Record Type: Journal Article
• Authors: Fritsch, Tobias; Kurpiers, Jona; Roland, Steffen; Tokmoldin, Nurlan; Shoaee, Safa; Ferron, Thomas; Collins, Brian A.; Janietz, Silvia; Vandewal, Koen; Neher, Dieter
• Is Part Of: Advanced energy materials. Volume 12:Issue 31(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-28
• Subjects: external quantum efficiency -- organic photovoltaics -- ternary blends -- voltage losses; Energy harvesting -- Materials -- Periodicals; Energy conversion -- Materials -- Periodicals; Energy storage -- Materials -- Periodicals; Photovoltaics -- Periodicals; Fuel cells -- Periodicals; Thermoelectric materials -- Periodicals; 621.31
• DOI: 10.1002/aenm.202200641
• Languages: English
• ISSNs: 1614-6832

Abstract

Abstract: The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S1 ) determines the recombination pathway and the resulting open circuit voltage ( V OC ) of organic solar cells. By combining a well‐aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state ( E CT ) is varied by 130 meV while leaving the S1 energy of the polymer ( E S 1 \[{E_{{{\rm{S}}_1}}}\] ) unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when E CT \[{E_{{\rm{CT}}}}\] approaches E S 1 \[{E_{{{\rm{S}}_1}}}\], while the V OC remains limited by the non‐radiative decay of the CT state. It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non‐radiative voltage loss because it lowers the radiative V OC limit ( V OC, rad ), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S1 will not improve the V OC, but rather reduce the V OC, rad . It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons. Abstract : The interplay between charge transfer and singlet exciton emission is studied in a ternary organic blend to understand the role of the two states in the non‐radiative voltage loss. The presence of the singlet exciton reduces the non‐radiative voltage loss, but not because it is more emissive but rather because its stronger absorption reduces the radiative open circuit limit.