• Energy Research

Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells. Donor–acceptor systems with low energy-level offset enable high power efficiency in organic solar cells yet it is unclear what drives charge generation. Classen et al. show that long exciton lifetimes enable efficient exciton splitting and thus generation of free charges while also suppressing voltage losses.

This review highlights the importance of controlling the crystallization dynamics for the deposition of high-quality photovoltaic perovskite layers on larger-area coatings.

Energy losses of photovoltaic (PV) plants because of soiling are a problem in all regions, including Germany. Soft soiling, caused by a uniform dust film, is shading a PV module and is reducing yield depending on the thickness of the debris layer. Hard soiling, caused by agglomerations of dust or dirt, only covers parts of a PV module, and it causes local shading. A spot check of modules from different PV power plants in Germany revealed a power reduction because of soft- and hard soiling of up to 6%. Determination of soiling type and amount is a prerequisite for decisions about cleaning and for optimizing yield. In this article, we show that luminescence imaging can be used to detect and characterize soiling and quantify losses. For this purpose, we compare luminescence images before and after cleaning and we show that soiling becomes detectable and power losses quantifiable from difference images. We also show that the impact of hard soiling can be quantified from a single photoluminescence (PL) image. This technique may enable a fully automated quantification of power losses because of hard soiling in PV- modules and strings in the future.

Lead–tin (Pb/Sn) mixed perovskites are considered as promising photovoltaic materials owing to their adjustable bandgap and excellent optoelectronic properties. The low‐bandgap perovskite solar cells (PSCs) based on lead–tin mixed perovskites play a critical role in the overall performance of perovskite‐based tandem devices. Nevertheless, the current record efficiencies for Pb/Sn PSCs are mostly reported in devices with p–i–n configuration rather than n–i–p, which restricts the further development of conventional perovskite‐based tandem solar cells. Herein, this work systematically investigates the influence of the interlayers on the performance of low‐bandgap PSCs by analyzing the energy losses in both n–i–p and p–i–n devices. Quasi‐Fermi level splitting (QFLS) analysis of pristine films and films covering charge extraction layers reveals that the electron transport layer/perovskite interface is dominating the VOC losses. A joint experimental–simulative approach quantitatively determines the interface defect density to be more than one order in magnitude larger for the n–i–p architecture. Among the polymeric hole transport layers investigated for n–i–p devices, poly(3‐hexylthiophen‐2,5‐diyl) (P3HT) exhibits the most favorable energy‐level alignment to Pb/Sn perovskites. These results clarify the nature of VOC losses in Pb/Sn perovskites and highlight the necessity to develop electron extraction layers with a significantly reduced interface defect density.

Commercialization of printed photovoltaics requires knowledge of the optimal composition and microstructure of the single layers, and the ability to control these properties over large areas under industrial conditions.

Laser‐induced forward transfer (LIFT) is presented as a new, contactless, and roll‐to‐roll compatible method for the deposition of silver top electrodes for organic solar cells (OSCs). Employing a nanosecond laser, highly reflective silver electrodes are printed by LIFT onto OSCs with P3HT:o‐IDTBR bulk heterojunction layers. Upon optimization of ink composition, source film thickness, and laser parameters, the resulting organic solar cells reach efficiencies of more than 5%, which are similar to those of reference devices with vapor‐deposited silver electrodes.

The reversible device performance of organic solar cells is caused by light-induced long-persistent radicals, and can be released with activation energy provided by thermal annealing.