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Hydrogen produced by water electrolysis, and electrochemical batteries are widely considered as primary routes for the long- and short-term storage of photovoltaic (PV) energy. At the same time fast power ramps and idle periods in PV power generation may cause degradation of water splitting electrochemical (EC) cells. Implementation of batteries in PV-EC systems is a viable option for smoothening out intermittence of PV power. Notably, the spreading of PV energy over the diurnal cycle reduces power of the EC cell and thus its overpotential loss. We study these potential advantages theoretically and experimentally for a simple parallel connected combination of PV, EC, and battery cells (PV-EC-B) operated without power management electronics. We show feasibility of the unaided operation of PV-EC-B device in a relevant duty cycle and explore how PV-EC-B system can operate at higher solar-to-hydrogen efficiency than the equivalent reference PV-EC system despite the losses caused by the battery.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2554-2557, Hydrogenated microcrystalline silicon oxide (μc-SiOx:H) was developed and implemented as a contact layer in hydrogenated amorphous thin film silicon (a-Si:H) single junction solar cells. Higher transparency, sufficient electrical conductivity, low ohmic contact to sputtered ZnO:Al and tunable refractive index make p-type μc-SiOx:H a promising alternative to the commonly used p-type μc-Si:H contact layers. In this work p-type μc-SiOx:H layers were fabricated with a conductivity up to the order of 10-2 S/cm and over 60% Raman crystallinity. Furthermore, we present p-type μc-SiOx:H with a broad range over which the optical properties (band gap E04 and refractive index n) can be tuned (2.1 eV < E04 < 2.8 eV and 1.6 < n < 2.6) by adapting deposition parameters like CO2/SiH4 ratio in the gas mixture. a-Si:H solar cells were produced with an conversion efficiency improvement from 9.4 to 9.7% by using p-type μc-SiOx:H compared to standard p-type μc-Si:H contact layer.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2543-2547, The texture of the transparent front contact is of major importance with respect to the optical performance of a thin-film silicon solar cell. In this study we address the method of angular resolved scattering as a tool to characterize the transparent conductive oxide surface texture. Using variously etched sputter-deposited aluminum doped zinc oxide films correlations between the scattering characteristics of the front contacts and the current generation of different solar cells are investigated. In order to simplify the characterization only discrete angles are used as a fingerprint. Thus, correlations between the front contact scattering and current of a-Si:H and μc- Si:H single junction solar cells as well as a-Si:H/μc-Si:H tandem junction solar cells are established. It is well known that the texture does not only influence the optical performance of the device but also the growth processes of the successive layers. Especially the growth of the μc-Si:H absorber is very sensitive to the front contact texture. Therefore the correlation between the light scattering based texture fingerprint of the front contact and the crystalline volume fraction of the μc-Si-H absorber layer is investigated.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2978-2982, For the deposition of a-Si:H solar cells, a single-chamber process was developed. A singlechamber fabrication of a-Si:H solar cells in an inline 40 × 40 cm2 deposition system with an initial efficiency of = 10.3 % is achieved. To avoid boron cross-contamination, usually a critical issue for single-chamber processes, reactor treatments prior to the i-layer deposition are required. Three types of treatments are compared for their effectivity and required treatment time, namely a CO2-plasma, a reactor evacuation and a cell deposition without plasma stop. The deposition without plasma-stop did not lead to state-of-the-art solar cell performances. A strong boron cross-contamination leads to losses in conversion efficiency, especially in the short wavelength range. The two other types of treatments led to the deposition of high-quality solar cells. The required treatment time was 15 min and 27 min for chamber evacuation and CO2-plasma treatment, respectively. For the “process chamber evacuation” with the substrate remaining in the recipient (in-situ), subsequent to the deposition of the p-layer the chamber is evacuated to 9×10-7 mbar prior to the deposition of the i-layer. For the device designs applied in this work, the influence on the boron cross-contamination is not significantly different.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3068-3072, Series of a-Si:H thin-films and p-i-n solar cells with intentionally contaminated intrinsic (i-) layers via controllable oxygen, nitrogen and air leaks were prepared by plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz. Two deposition regimes with different total gas flows (~ 370 sccm and ~ 1000 sccm) were studied. An important quantity is the critical concentration which is the concentration of an impurity causing a noticeable decay of solar cell performance. For a low total gas flow the critical oxygen concentration was determined at ~ 2-5 × 1019 cm-3. A shift of the critical oxygen concentration to a higher value of ~ 1 × 1020 cm-3 is observed for a-Si:H deposited at a higher total gas flow of ~ 1000 sccm. Above these critical concentrations the solar cell efficiency decreases due to losses in fill factor under red illumination. We also investigated for an air leak, if oxygen or nitrogen impurities are responsible for solar cells efficiency deterioration. We found that an oxygen incorporation in the i-layer is the most critical factor determining the a-Si:H solar cell quality for a regime with low gas flow. Interestingly, for material deposited with high total gas flow and intentionally contaminated via an air leak the solar cell performance deteriorated also at higher oxygen concentration (7 × 1019 cm-3), similar as is the case of pure oxygen leak.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2660-2664, Series of a-Si:H p-i-n solar cells with various oxygen concentrations were fabricated by plasmaenhanced chemical vapour deposition (PECVD) at 13.56 MHz. Increasing the total gas flow of silane and hydrogen and the deposition rate (via high RF power) results in suppression of oxygen incorporation in the intrinsic layer (ilayer). The tolerated oxygen flow in the deposition system can be increased by two orders of magnitude, leading to the possibility of depositing high-quality solar cells under a high base pressure conditions (up to 10-5 Torr). We observed that at low total process gas flow (~ 100 sccm) and low deposition pressure regime, the increase of the discharge power and hence the deposition rate reduces the oxygen incorporation in the i-layer. However, at a high total process gas flow (~ 400 sccm) and high deposition pressure regime no significant change of the incorporation of oxygen with increasing deposition rate is found. Impurity incorporation leads to deterioration in solar cell performance if a critical contamination level is exceeded for both, the initial states and after light soaking. Similar degradation losses in efficiency were found for cells deposited at low and high total gas flows although a two time higher deposition rate was applied in the high flow regime. The stabilized efficiency of these cells was ~ 7 % at a deposition rate of ~ 0.27 nm/s in spite of a high base pressure of ~ 10-5 Torr.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 2121-2124, In this study the influence of different types of contamination sources and species on thin film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma enhanced chemical vapor deposition (PECVD) at 13.56 MHz is examined. Nitrogen contaminants were introduced either by a simulated chamber leak or together with the process gases to simulate impure SiH4 and H2, e.g. a gas pipe leak. It is found that the decrease in solar cell quality starts at about 4·1018 nitrogen-atoms/cm3 (critical nitrogen contamination level) for both nitrogen sources. Nevertheless impure process gases lead to an incorporation probability, which is enhanced by a factor of about 3. This is assumed to be due to the different probability for the nitrogen atoms to approach the deposition zone or to be pumped out of the chamber unused. Furthermore a (virtual) chamber oxygen leak was investigated and a critical oxygen contamination level of about 2·1019 atoms/cm3 was observed.

Thin film p-i-n microcrystalline silicon (mu c-Si:H) solar cells were studied by photoluminescence (PL) spectroscopy and open circuit voltage, V-oc. By this means, the cause of the commonly observed increase of V-oc when the silane concentration in the gas phase for the preparation of the intrinsic absorber layers is raised is addressed. The hot wire chemical vapour deposition (HW-CVD) technique was used for the preparation of films and intrinsic absorber layers of solar cells with efficiencies higher than 9%. By monitoring V-oc and the PL energy, E-PL, as a function of temperature, information on the photogenerated carrier distributions and the splitting of the quasi-Fermi energies was gained and the effect of states in the gap was studied by comparison with results from a crystalline p-i-n diode. An increase of E-PL and V-oc for increasing SC and decreasing temperature is attributed to the shift of the excess carrier distributions to higher energies. It is proposed that this shift is limited by band tail states. It is proposed that increasing SC leads to a reduction of the density of band tail states, due to structural relaxation of the mu c-Si:H network by the presence of hydrogen or hydrogenated amorphous silicon.

A simple model is proposed to calculate photoluminescence (PL) spectra and open circuit voltages (V-oc) in thin film p-i-n microcrystalline silicon (mu c-Si:H) solar cells with different structural compositions, as a function of temperature. By using a new technique, namely voltage-modulated PL on solar cells, experimental data are obtained that can be directly compared with the model. The model is based on the distributions of electrons and holes in quasi-equilibrium conditions. Recombination between the two distributions determines the PL band (energy and width of the spectrum). A symmetrical density of states distribution (DOS) described by a superposition of a DOS like that in c-Si and band tail states for the conduction and valence bands is assumed, The best agreement between the model calculations and experimental results for two solar cells with different structural properties is obtained by using an E-o approximate to 0.031 eV for the slope of both exponential band tail states.