Ingo Riedel

Quality control of polymer solar modules by lock-in thermography

We have characterized lateral imperfections of photovoltaic modules based on solution processed polymer-fullerene semiconductor blends by means of lock-in thermography (LIT). The active layer of the solar cell modules is based on the heterogeneous organic semiconductor system poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester and the power conversion efficiency of the modules reached nearly 2% under irradiation of an AM 1.5 solar simulator. Applying highly sensitive LIT allowed us to detect several kinds of laterally distributed defects originating from imperfections in the respective functional layers as well as in the quality of encapsulation. We show that LIT is a powerful method for the quality control of large area polymer solar cells and modules, enabling fast feedback for optimization of production parameters.

Influence of particle size in hybrid solar cells composed of CdSe nanocrystals and poly(3-hexylthiophene)

Inorganic semiconductor nanoparticles, such as CdSe quantum dots, are considered to be a promising alternative to fullerene derivates for application as electron acceptors in polymer-based bulk heterojunction solar cells. The main potential advantage is the strong light absorption of CdSe nanoparticles with a spectral bandwidth, which can even be tuned, due to the quantum size effect. However, the impact of the particle size on the performance of polymer/CdSe solar cells has remained largely unexplored so far. Therefore, the influence of particle size in hybrid solar cells using a blend of poly(3-hexylthiophene) (P3HT) and quasi-spherical CdSe nanoparticles on relevant cell parameters and the overall solar cell performance is systematically studied in the present work. As the most important result, an increase of the open-circuit voltage (VOC) can be found for smaller nanoparticles and can be explained by an “effective bandgap” model. In contrast, no significant changes of the short-circuit current densit...

Electronic Properties of Polymer-Fullerene Solar Cells

We studied the electronic transport properties of conjugated polymer/fullerene based solar cells by means of temperature and illumination intensity dependent current-voltage characteristics, admittance spectroscopy and light-induced electron spin resonance. The short-circuit current density increases with temperature at all light illumination intensities applied, i.e., from 100 mW/cm 2 to 0.1 mW/cm 2 (white light), whereas a temperature independent behavior was expected. An increase of the open-circuit voltage from 850 mV to 940 mV was observed, when cooling down the device from room temperature to 100 K. The fill factor depends strongly on temperature with a positive temperature coefficient in the whole temperature range. In contrast, the light intensity dependence of the fill factor shows a maximum of 52% at intermediate illumination intensities (3 mW/cm 2 ) and decreases subsequently, when increasing the intensity up to 100 mW/cm 2 . Further studies by admittance spectroscopy revealed two frequency dependent contributions to the device capacitance. One, as we believe, originates from trapping states located at the interface between composite and metal electrode with an activation energy of E A =180 meV, and the other one is from very shallow bulk states with E A =10 meV. The origin of the latter is possibly the thermally activated conductivity. The photo-generation of charge carriers and their fate in these blends have been studied by light-induced electron spin resonance. We can clearly distinguish between photo-generated electrons and holes in the composites due to different spectroscopic splitting factors (g-factors). Additional information on the environmental axial symmetry of the holes located on the polymer chains as well as on a lower, rhombic, symmetry of the electrons located on the methanofullerene molecules has been obtained. The origin of the signals and parameters of the g-tensor have been confirmed from studies on a hole doped polymer.

Detection of a MoSe2 secondary phase layer in CZTSe by spectroscopic ellipsometry

We demonstrate the application of Spectroscopic Ellipsometry (SE) for identification of secondary phase MoSe2 in polycrystalline Cu2ZnSnSe4 (CZTSe) samples. A MoSe2 reference sample was analyzed, and its optical constants (e1 and e2) were extracted by SE analysis. This dataset was implemented into an optical model for analyzing SE data from a glass/Mo/CZTSe sample containing MoSe2 at the back side of the absorber. We present results on the n and k values of CZTSe and show the extraction of the thickness of the secondary phase MoSe2 layer. Raman spectroscopy and scanning electron microscopy were applied to confirm the SE results.

Charge transfer and transport in polymer-fullerene solar cells

The development of polymer-fullerene plastic solar cells has made significant progress in recent years. These devices excel by an efficient charge generation process as a consequence of a photoinduced charge transfer between the photo-excited conjugated polymer donor and acceptor-type fullerene molecules. Due to the paramagnetic nature of the radical species, the photo-induced charge transfer can be analyzed by the help of light-induced electron spin resonance spectroscopy. Upon looking at an interpenetrating donor-acceptor composite consisting of the polymer MDMOPPV and the fullerene derivative PCBM, we disclose two well separated line groups having a strongly anisotropic structure. The line shape can be attributed to an environmental axial symmetry of the polymer cation and a lower rhombohedric symmetry of the fullerene anion. Since the signals were found to be independent of one another with different spin-lattice relaxation times, the radical species can be discriminated via separate characterization procedures. In order to study the bulk charge transport properties, we carried out admittance spectroscopy on the polymer-fullerene solar cell device including a transparent semiconductor oxide front contact (ITO/PEDOT:PSS) and a metal back contact (Al). The temperature- and frequency-dependent device capacitance clearly uncovers two different defect states, the first, having an activation energy of 9 meV, indicates a shallow trap due to a bulk impurity, the latter, having an activation energy of 177 meV, can be assigned to an interfacial defect state located between the polymer-fullerene composite and the metal back contac

Toward highly efficient photogeneration and loss-free charge transport in polymer-fullerene bulk heterojunction solar cells

Different material combinations of two conjugated polymers, each blended with the methanofullerene acceptor phenyl-C61 butyric acid methyl ester (PCBM) have been evaluated focusing on their potential for application as absorber material in polymer-fullerene bulk-heterojunction solar cells. Devices based on these solution processable composite materials have been studied by means of temperature dependent profiling of the photocurrent. In combination with measurements of the incident photon conversion efficiency, this technique probes the charge carrier recombination losses within the absorber material. Samples based on material composites with a low mobility-lifetime (μτ) product of the charge carriers (OC1C10-PPV: PCBM) exhibit a thermally activated photocurrent throughout the temperature range from 100 K to 350 K. The latter issue is attributed to the presence of shallow traps inside the bulk of the absorber limiting the photocurrent by recombination and scattering of the charge carriers with defects. Accordingly, the active layer thickness must be kept low at the expense of optical absorption. In contrast, the photocurrent in devices based on absorber materials with a high μτ product, P3HT: PCBM, saturates at a certain temperature and becomes constant, reflecting that all photogenerated charge carriers are efficiently extracted within their lifetime prior to recombination. Thus, solar cells with absorber materials demonstrating a high μτ product, have the potential to be designed with relatively thick absorber films above 100 nm. A large active layer thickness is a prerequisite for industrial deposition techniques, e.g., screen-printing, and improves the mechanical stability of large area flexible solar cells. As consequence of a high μt product the increase of the active layer thickness to L=350 nm in P3HT: PCBM photovoltaic devices results in a higher density of photogenerated charge carriers due to improved light absorption. Consequently, a strongly increased short-circuit current density of up to 15.2 mA/cm2 was obtained for devices with absorber thickness of 350 nm which rising the power conversion efficiency up to 3.1 %.

Electronic properties of polymer-fullerene solar cells studied with light-induced electron spin resonance and admittance spectroscopy

Within recent years, the development of polymer-fullerene plastic solar cells has made significant progress. In such devices, an efficient charge generation takes place via photoinduced charge transfer between the photoexcited conjugated polymer and acceptor-type fullerene molecules. Due to the paramagnetic nature of the radical species, the photoinduced charge transfer can be studied by means of light induced electron spin resonance (LESR) techniques. We carried out W-band (95 GHz) LESR at high magnetic field strengths. Two well separated line groups with a strong anisotropic structure were detected for the composite MDMOPPV: PCBM. From the line shape analysis, we obtained an environmental axial symmetry for the positive polaron P+ and a lower, rhomboedric symmetry for the fullerene anion. The signals were found to be independent of each other with different spin-lattice relaxation times; hence, the radical species can be investigated separately. In order to study the bulk transport properties, we carried out admittance spectroscopy on the ITO/PEDOT:PSS/MDMO-PPV:PCBM/Al device. Two frequency-dependent contributions to the device capacitance with the activation energies 9 meV and 177 meV were found. For the very shallow trap state, we assume a bulk impurity, whereas the latter one is assigned to an interfacial defect state, located at the composite- aluminium interface.

CuIn(Ga)Se 2 based thin film solar cells with electrodeposited absorber on flexible steel foils

In this work we studied the electrical properties of CuIn(Ga)Se2 solar cells electrodeposited on steel substrate with chromium as diffusion barrier and molybdenum back electrode. The open circuit voltage (VOC=435 mV) of the samples lies approx. 80 mV below the value commonly observed for devices based on neat CuInSe2 [1]. From current-voltage measurements (I-V) under variable temperature and light intensity we found that the activation energy Ea of the recombination current J0 equals the absorber band gap Eg. Hence, interface recombination via defects at the heterojunction seems to be less likely to explain the VOC loss. Another explanation for this performance limit could be the presence of deep recombination centers in the volume of the absorber layer caused, e.g., by iron diffusion from the substrate. Admittance (AS) and deep level transient spectroscopy (DLTS) revealed the presence of such deep states close to mid-gap position which may facilitate Shockley-Read-Hall (SRH) recombination. To verify or falsify that the observed recombination centers originate from iron diffusion we performed the same experiments on CuIn(Ga)Se2 solar cells prepared on Ti substrates. In contrast to our expectations we found the same mid-gap states in both sample configurations which point towards an origin being independent of the substrate.

Current-voltage characteristics of polymer-fullerene solar cells

We have studied the influence of temperature and light intensity on the current-voltage characteristics of polymer-fullerene bulk-heterojunction solar cells. The open-circuit voltage varies linearly with temperature in the range 200K-300K and approaches a value of 930mV at T=80K. Strictly positive temperature coefficients were found for both, the short-circuit current density and the fill factor. These cause the device efficiency to increase steadily with temperature up to T=330K. The short-circuit current density increases almost linearly with light intensity. The fill factor is not significantly influenced by the incident light intensity in the temperature range from 260K to 330K. At lower temperatures, a negative slope of the fill factor is observed. Since the maximum power point varies sublinearly with light intensity, a decrease of the power efficiency is obtained at light intensities higher than 3mW/cm/sup 2/.

Investigation of Cu(In,Ga)Se2 Solar Cell Performance Deviations in Nominally Equal Absorbers

Cu(In,Ga)Se2 (CIGSe) solar cells were fabricated independently by industrial scale co-evaporation in two separate production lines with the same nominal composition and thickness of the absorber film. Although the device properties were believed to be the same we observed substantial deviations of the respective values of the open circuit voltage (ΔVOC = 40 mV) and of the fill factor (ΔFF= 4%), whereas the short circuit current was essentially the same. We performed fundamental device analysis, space charge and defect spectroscopy, transient photoluminescence as well as in-depth profiling of the chemical gradients of the absorber films. Using the results from the experiments we set up a simulation baseline which allowed us to conclude that the apparent deviations can be related to the presence of deep recombination centers with different concentration within the CIGSe absorber as well as to variations of the band gap grading.