Where does the energy go in an organic solar cell?
I'm Huotian Zhang, a device physicist and postdoctoral researcher at the University of Potsdam. I combine ultrafast and steady-state spectroscopy with device modelling to trace how light becomes current in organic semiconductors, and where it is lost on the way. Before Potsdam I did my PhD and postdoc in Linköping, with stays at Imperial College London and FAU Erlangen.
Recent highlights
Overcoming the fill-factor limit of organic solar cellsThe internal field caps fill factor as voltage loss shrinks. Longer-lived excitons lift the cap.Nature Photonics 2026 · explainer20.5% efficiency via co-crystallizable acceptorsA guest acceptor co-crystallises with L8-BO. Ordered packing gives 20.5% ternary cells.Advanced Materials 2026 · explainerConformationally locked macrocyclic acceptorsRing acceptors soften backbone vibrations. PLQY reaches 14.5%; cells reach 17.1%.Advanced Materials 2025 · explainer
Voltage losses
Why do organic solar cells deliver less voltage than their light absorption allows?
Barrierless free charge generation in PM6:Y6Generation ignores field and photon energy, with Ea only 6 meV. Y6 electrostatics offset the Coulomb pull.Advanced Materials 2020 · explainerReducing voltage losses in A-DA′D-A acceptor solar cellsNon-radiative loss costs 200–300 meV. Brighter blends are the route past 20%.Chem 2020 · explainerOrigins of the open-circuit voltage in ternary cellsThermal population sets the voltage loss. Best guest: bright, with matched CT energy.Nature Energy 2023 · explainerOptical gaps as a reference for voltage lossesFour gap definitions give four voltage losses. Best cells still lose about 0.6 V.Advanced Energy Materials 2018 · explainer
Charge dynamics
What happens in the first nanoseconds after a photon is absorbed?
The role of thermally activated charge separationSmall offsets make charge separation need heat. Mid-offset blends keep both current and voltage.Advanced Energy Materials 2024 · explainerCharge separation versus hole back transferSmall LUMO offsets slow electron transfer. Small HOMO offsets let holes slip back.Organic Electronics 2022 · explainerTriplet–triplet annihilation in non-fullerene cellsOrdered Y11 domains speed up triplet fusion. A voltage gain needs microsecond triplets.Advanced Energy Materials 2023 · explainerFrom generation to extractionCharges followed from femtoseconds to microseconds. About 100 meV HOMO offset is enough.J. Phys. Chem. C 2020 · explainer
Molecules, with chemists
Physics that feeds back into molecular design.
A multiple D–A conjugated macrocycleTwo BTP-C3 acceptors and two thiophenes close a ring. Brighter emission gives 14.2% cells.Angewandte Chemie 2023 · explainerLuminescent triphenylamine-functionalised acceptorA mixed acceptor phase lowers non-radiative loss. Ternary cells reach 20.32% at 0.927 V.Energy & Environmental Science 2025 · explainerHigh crystallinity and photoluminescence, >20%One side chain branches further out. Packing and PLQY both improve: 20.42% cells.Nature Materials 2025 · explainer
Beyond solar cells
The same spectroscopy, applied to other light-emitting and light-sensing materials.
White-light emission in ZIF glassesMelting, pressing and annealing shift ZIF-62 glow from blue to cold white. LEDs reach 4.2 lm/W.Advanced Materials 2026 · explainerSelf-driven single-component organic photodetectorOne neat DCV2-5T film detects light without bias. Dark current falls to 3.4·10⁻¹¹ A/cm².Advanced Materials 2024 · explainerLower thresholds for perovskite lasersPTAA pulls holes out, so hot carriers cool faster. The ASE threshold drops 22.9%.Advanced Materials 2023 · explainer
Tools & teaching
Things built so others can measure and learn.
JV ConsoleOpen-source software for analysing solar-cell current–voltage measurements.Software · GitHub
TAS GuideLab manual for transient-absorption spectroscopy.Lab manual · Linköping
Guide for SpectrographLab manual for optical measurements.Lab manual · Linköping
Organic transistor moduleLecturer and examiner, graduate polymer lab classes.Teaching · Potsdam 2026
Traditional papers
The complete list of journal articles is in the CV and on Google Scholar.