Surface properties of colloidal quantum dots (CQDs) are critical for the transportation and recombination of the photoinduced charge carrier in CQD solar cells, therefore dominating the photovoltaic performance. Herein, PbS CQD passivated using liquid-state ligand exchange (LSLX) and solid-state ligand exchange (SSLX) strategies are in detail investigated using photoelectron spectroscopy (PES), and solar cell devices are prepared to understand the link between the CQD surface properties and the solar cell function. PES using different energies in the soft and hard Xray regime is applied to study the surface and bulk properties of the CQDs, and the results show more effective surface passivation of the CQDs prepared with the LSLX strategy and less formation of lead-oxide. The CQD solar cells prepared with LSLX strategy show higher performance, and the photoelectric measurements suggest that the recombination of photoinduced charges is reduced for the solar cell prepared with the LSLX approach. Meanwhile, the fabricated solar cells exhibit good stability. This work provides important insights into how to fine-tune the CQD surface properties by improving the CQD passivation, and how this is linked to further improvements of the device photovoltaic performance.

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

Quantum dot solar cells have not yet achieved optimal device performances and to direct development there is thereforea need to understand the device function of present solar cell structures in more detail. Understanding where photovoltage isgenerated in a device and where energy losses occur is a key aspect of this. We have previously shown that time-resolved core levelphotoelectron spectroscopy can be used to follow the photovoltage rise and decay at a specific interface from pico- to microsecondtimescales. Here, we extend this study and investigate the photovoltage generation in different parts of a PbS quantum dot solar cellthrough sample design. We show that thick absorbing quantum dot layers are required for generating a high photovoltage at theinterface between n-type PbS quantum dots and p-type quantum dots. Furthermore, we show that the full photovoltage is only generatedwhen a gold contact is deposited on the quantum dots and that the presence of this contact also leads to significantly slowercharge recombination.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they arepromising for application in photovoltaics and other optoelectronic devices. The ultrafastcharge transport is intriguing, offering insight into the mechanism of electron tunnelingprocesses within the material. In this study we investigated the charge transfer times of PbSquantum dots of different sizes and non-quantized PbS reference materials by comparing thepropensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays.We show that charge transfer times in PbS quantum dots decrease with excitation energy andare similar at high excitation energy for quantum dots and non-quantized PbS. However, atlow excitation energies a distinct difference in charge transfer time is observed with thefastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots.Our observations can be explained by iodide ligands on the quantum dots creating a barrierfor charge transfer, which reduces the probability of interparticle transfer at low excitationenergies. The probability of intraparticle charge transfer is limited by the density of availablestates which we describe according to a wavefunction in a quantum well model. The strongerquantum confinement effect in smaller PbS quantum dots is manifested as longer chargetransfer times relative to the larger quantum dots at low excitation energies.