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This paper investigates the use of emitter windows with varying passivation layers in an intensity range between 1 and 10−3 suns. The results are compared with a cleaved sample without emitter windows. It is found that the passivation of the nondiffused region outside the emitter windows is very important to reduce recombination. The surface passivation schemes investigated are the three most commonly used for solar cells: aluminum oxide, silicon dioxide, and silicon nitride. The aluminum oxide and silicon dioxide resulted in a reduction in edge recombination of 8 and 4.56 times, respectively. The silicon nitride passivation resulted in worse performance than the unpassivated sample, as a result of increased recombination. The impact of the thickness of the region outside of the emitter was investigated by reducing the outside area from a 2-mm border to a 200-μm border. The aluminum oxide sample was hardly influenced, while the silicon dioxide passivated sample suffered as the carrier was now able to travel to the edge and recombine. The performance of the silicon nitride passivated sample was improved with a reduction of the outside region. However, the performance is still reduced compared with the control sample with unpassivated emitter edges.

This paper discusses the influence of different solar cell loss mechanisms at low light intensities and presents a simple method for the analysis of solar cell performance under various illumination intensities below 1 sun. Suns–PL and ${\rm Suns}-V_{{\rm oc}}$ are used to measure the intensity-dependent pseudo I–V curves of symmetric test structures and of finished silicon solar cells in an intensity range between 1 sun and 10−3 suns. The solar cell parameters from the pseudo I–V curves are compared with the parameters evaluated by intensity-dependent measurements of the whole I–V curve. The pseudo efficiency and pseudo fill factor are found to be in good agreement with the real values at low intensities as the influence of the series resistance vanishes. Based on this finding, we compare the passivation quality of silicon dioxide and silicon nitride in combination with emitter windows on test structures. Above 0.1 suns, both passivation layers show similar performance. Below 0.1 suns, the pseudo fill factors and pseudo efficiencies of the silicon nitride passivated sample are strongly reduced compared with the sample with silicon dioxide. The open-circuit voltage starts differing below 0.01 suns.

We present a comprehensive study on the rear contact formation of rear surface-passivated silicon solar cells by full-area screen printing and alloying of aluminum pastes on the locally opened passivation layer. We show that the point contact distance has a significant influence on the local alloying process for the contact formation resulting in different structural and electrical contact properties when applying conventional Al pastes. Increasing the distance leads to 1) high contact depths resulting in an enlargement of the contact area and 2) severely reduced thicknesses of the Al-doped p+ regions in the contact points, leading to a strong increase in recombination within the contact points. This inadequate contact formation can be directly linked to the deficiently low percentage of silicon that dissolves into the Al-Si melt during alloying. We demonstrate that by intentionally adding Si to the Al paste, the contact point geometry can be significantly improved and particularly becomes independent of the contact distance. Further investigations on the internal reflectance and the specific resistivity of the rear contact suggest an upper limit for the Si content added to the Al paste. In summary, we present a simple way to significantly improve the rear contact formation of rear surface-passivated silicon solar cells.

Surface-passivated and surface-unpassivated aluminum-alloyed p+-layers are characterized. By varying the firing conditions and the thickness of the screen-printed aluminum paste, different sheet resistances Rsh of the p+-layer were fabricated. The emitter saturation current density J0e plotted versus Rsh follows distinctly different trends for the passivated and unpassivated samples. An aluminum paste with a boron additive achieves a much higher doping concentration and a lower sheet resistance but nevertheless follows the same J0e curves as the pure Al paste. The aluminum p+-layer was quantitatively analyzed with microphotoluminescence and Fano-Raman measurements. The latter shows an increased defect recombination at the interface between the p+-layer and the moderately doped Si bulk. The lower Shockley-Read-Hall lifetime in this region can be attributed to a high defect concentration in the most highly doped layer, represents an impediment to the reduction of J0e for Al-doped emitter regions, and needs to be optimized in future investigations.

The power conversion efficiency of photovoltaic devices depends strongly on the light spectrum and light intensity. Both parameters vary significantly between different environments and locations - indoors as well as outdoors. Since placement of energy harvesting devices is governed more by the intended functionality of the system than by optimizing with respect to light conditions, the knowledge of light conditions in different environments is necessary for the design and dimensioning of photovoltaic energy converters. With this paper we want to present a simple approach to systematically measure and analyze light conditions and energy availability in different environments and to provide the basis for the development of a database of `typical' light conditions. This will help to understand the energy availability and the dimensioning and design of photovoltaic devices in energy harvesting systems.

Photovoltaic micro energy harvesting for self-sufficient sensor system applications is a promising new market for silicon photovoltaics. Therefore, silicon solar cells need to work under weak light conditions both outdoors, e.g. in the forest, or indoors. Hence, the spectral distribution of the given conditions differs as well with the application. We present a new investigation of real conditions and a first evaluation for a frequently used office. Even during winter the contribution of the artificial light sources does not exceed 35%. The spectrum is mainly determined by sun light. In order to proof the feasibility of solar cells for micro energy harvesting simulations of the efficiency limits for different spectra and intensities were made. The simulation of the theoretical efficiency limits show that silicon, which is best suited for AM1.5, can achieve even higher efficiencies for artificial light sources. Concerning the technological feasibility, PC1D simulation show that a PERC structure can be optimized resulting in efficiencies of 20% at an intensity of 1 sun and still 14% at an intensity of 1/1000 sun. The efficiencies even rise for artificial spectra of energy saving lamps, halogen bulbs or LEDs.

Commercial photovoltaic cells used in energy harvesting applications suffer from low power conversion efficiencies at low light intensities. Typical efficiencies are in the range of 2 to 5% at 100 Lux. This is because the cells are typically optimized and specified with respect to the AM 1.5 sun spectrum at 1000 W/m2 light intensity, which is far away from the required operating range. This paper infers design rules for photovoltaic cells with special attention to the low intensity range. It discusses the major parameters for the optimization of photovoltaic cell for different material classes like crystalline silicon, amorphous silicon and III-V materials. Measurement results for an optimized silicon cell with more than 15% efficiency at 100 Lux are presented.

The optimization of photovoltaic devices for versatile conditions is necessary to improve the energy capture for indoor applications, such as self sufficient sensors. However, the design rules of standard outdoor solar cells are not applicable for cells which are used indoors due to differing conditions from the standard testing conditions (STC). We will discuss the substantial influences on cell efficiencies and their impact on the design of photovoltaic cells with indoor applications. We show that in order to reduce losses due to recombination the influence of impurity recombination outweighs the Auger recombination. Therefore, good quality material and a reduction of the doping level are favorable. Thedo pant reduction implies an improvement of the saturation current density J_01and is realizable because the seriesresistance R_s has no influence on the efficiency at indoor intensities. The crucial parameter is the shunting resistance R_sh and it has to have a value of at least 50 k?cm2.

The common design rules for outdoor solar cells do not apply necessarily for indoor photovoltaic cells due to differences in spectra and intensities. Since no general standard for indoor conditions is currently available, the first aim of this paper is to present a statistical approach encompassing the diversity of available intensities and spectra which shall enable a systematic derivation of a standard for indoor conditions. In a second step, the influences of injection-and spectra-dependent parameters on cell efficiencies are studied with the focus on indoor conditions. It is shown that the parallel resistance has to be increased above 50 kΩcm2 in order to achieve an efficiency above 10% at intensities below 10−3 W/cm2 (∼0.01 suns). The influence of the series resistance is negligible in this intensity range, allowing values in the order of 10 Ωcm2 without appreciable efficiency reduction. This gives room for wider design variations with respect to base and emitter doping, optimization of bulk-, surface- and emitter recombination, contact optimization and back contact cell designs. The combined evaluation of possible photovoltaic cell parameters show that efficiencies of more than 10% at intensities as low as 10−4 W/cm2 are accessible.

Recent research focuses on developing photovoltaic cells that are optimized for indoor conditions and therefore suitable for photovoltaic energy harvesting in micro systems. The available conditions for indoor photovoltaic cells are investigated and discussed concerning intensity and spectra. The influence of the parallel and series resistances on the efficiency of crystalline and amorphous Silicon devices with decreasing intensity are discussed and compared to simulations with the two diode model. For low intensities as present indoors (< 1 mW cm−2) the parallel resistance restricts the achievable efficiency and needs therefore to be optimized.

The passivation mechanisms and qualities of Al2O3, SiNx, SiO2 and a-Si:H( i ) on p - and n -type silicon are investigated by quasi-steady-state photoluminescence measurements. This technique allows effective lifetime measurements in an extremely large injection range between 1010 cm-3 and 1017 cm-3. The measurements are discussed focusing on injections below 1012 cm-3 in order to determine the most effective passivation layer for solar cells arranged for indoor applications. Fixed negative charges in the passivation layer cause field-effect passivation due to band bending leading to either accumulation or inversion at the passivation layer/silicon interface. Accumulation causes a stable passivation quality at low level injection. Inversion leads to effective lifetime losses similar to the losses in the space charge region. On p -type silicon the most effective surface passivation at low injections is provided by Al2O3 or a-Si:H( i ). The n -type silicon samples passivated with a-Si:H( i ) show the best effective lifetimes. SiNx and SiO2 show lifetimes one order of magnitude below a-Si:H( i ). Al2O3 on n -type is the most effective passivation at high injections around 1015 cm-3. Due to inversion losses at low level injections the passivation quality decreases more than two orders of magnitude for injections around 1010 cm-3.

The surface passivation of rear aluminum-alloyed p+ emitters is highly beneficial to increase the efficiency of back-junction n-type silicon solar cells, thus however demanding the application of locally defined emitter contacts. The formation of the rear contact by full-area screen-printing and alloying of Al-pastes on the locally opened contact points in the passivation layers exhibits two main problems: (i) increase of the contact depth leading to an enlargement of the contact area and (ii) low Al-p+ emitter thicknesses underneath the point contacts, both implying the danger of emitter shunts. In this study we therefore focus on controlling the formation and structural properties of contact points by systematically modifying the composition of the rear Al paste. By examining the contact geometry over a broad range of pitches, we demonstrate that the contact point depth and the Al-p+ emitter thickness in the contact region are directly linked to the percentage of Si that is dissolved into the Al-Si melt during alloying. For conventional Al pastes, the Si percentage in the melt was calculated to be far too low, so that we provided additional Si by manually adding Si powder to the Al paste. Thus, we could significantly reduce the contact depth and significantly enlarge the Al-p+ thickness in the point contacts, respectively. A first quantitative evaluation of the electrical properties was carried out, showing that the saturation current density is decreased by increasing the Si content of the paste, very likely due to the decreased contact area and improved electron shielding. In summary, we demonstrate that the local rear contact formation by alloying of full-area screen-printed Al pastes can be considerably improved by intentionally adding Si to the paste. The results of this investigation are highly interesting for applications to both n-type and p-type Si solar cells with passivated rear side.

We present a detailed study on the rear contact formation of rear-surface-passivated silicon solar cells by full-area screen printing and alloying of aluminum pastes on the locally opened passivation layer. We demonstrate that applying conventional Al pastes exhibits two main problems: (1) high contact depths leading to an enlargement of the contact area and (2) low thicknesses of the Al-doped p+ Si regions in the contact points resulting in poor electron shielding. We show that this inadequate contact formation can be directly linked to the deficiently low percentage of silicon that dissolves into the Al-Si melt during alloying. Thus, by intentionally adding silicon to the Al paste, we could significantly improve the contact geometry by reducing the contact depth and enlarging the Al-p+ thickness in the contact points, enabling a simple industrially feasible way for the rear contact formation of silicon solar cells.

The use of a fiber-coupled terahertz TDS system offers new possibilities for angle-resolved reflection spectroscopy. We put the focus especially on angle-resolved scattering from roughened surfaces of both dielectric and metallic surfaces.