Bulk and contact resistance in P3HT:PCBM heterojunction solar cells

In this paper, the series resistance of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been studied. The series resistance of thermal annealed and un-annealed devices with different active layer thicknesses was measured. The series resistance of the organic solar cells consists of the bulk resistance of the active layer itself and the specific contact resistance between the active layer and the electrode. The bulk resistance and contact resistance were extracted from the measured series resistance using the vertical transmission line model (TLM) method. By fabricating solar cell devices with different active layer thicknesses, a relationship of the series resistance with thickness was established from which bulk and contact resistances were derived. We have also found that thermal annealing helps reduce both contact resistance and bulk resistance significantly; the contact resistance dropped by a factor of 2, while the bulk resistance decreased by a factor of 8. Results have shown that for an annealed P3HT:PCBM device that has an active layer thickness of 85 nm (optimum thickness for high efficiency), 17% of the total series resistance was due to the contact resistance, and bulk resistance contributed the rest 83%. The bulk resistance value for thermal annealed organic solar cell device with an active area of 0.1 cm² was found to be 150 Ω, and the measured specific contact resistance was 3.1 Ω cm². The measured bulk and contact resistance values are much higher as compared to the high efficiency silicon solar cells. Bulk resistance and contact resistance need to be further decreased in order to achieve higher organic solar cell efficiency.     

High Efficiency Large Area Back Contact Concentrator Solar Cells with a Multilevel Interconnection

Very high efficiencies have been demonstrated under concentration with silicon solar cells having interdigitated contacts on the backside. However, only laboratory cells of small dimension have reached very high efficiencies. The need for developing a multilevel metallization technology for back contact concentrator solar cells of large area is demonstrated. The particular features required for such a multilevel interconnection are studied and a process using anodic oxidation of aluminum is presented. Back contact silicon solar cells of 0.64 cm² have been processed in this technology resulting in 26.2% efficiencies at 10W/cm² (100 suns AM1.5, 25.5 ^°C). the highest efficiency reported to date for a solar cell of this area. The one-sun efficiency of this cell is 21.7% (AMI.5, 25.2^°C). We propose also a new design for the metallization of back contact cells which allows an increase in the size of the cell without increasing the series resistance.     

Performance of double junction a-Si solar cells by using ZnO:Al films with different electrical and optical properties at the n/metal interface

High-quality ZnO:Al films have been prepared by using RF-magnetron-sputtering method with resistivity ranging from 10⁻¹ to 10⁻⁴ Ω cm and transmittance above 90% in visible region. We have fabricated small area (1 cm²) double junction (a-Si/a-Si) solar cells using ZnO/Al and ZnO/Ag as back contact. The conversion efficiency of double junction a-Si solar cell increases from 9.9% to 10.9% by using ZnO/Al back contact and to 11.4% by using ZnO/Ag as back contact. Effect of variation of thickness of i-layer on performance of the cell has also been studied.     

Fabrication of flexible CdTe solar modules with monolithic cell interconnection

CdTe solar cells and modules have been manufactured on polyimide (PI) substrates. Aluminum doped zinc oxide (ZnO:Al) was used as a transparent conductive oxide (TCO) front contact, while a thin high resistive transparent layer of intrinsic zinc oxide (i-ZnO) was used between the front contact and the CdS layer. The CdS and CdTe layers were evaporated onto the ZnO:Al/i-ZnO coated PI films in a high vacuum evaporation system followed by a CdCl₂ activation treatment and a Cu–Au electrical back contact deposition. In some cases prior to the cell deposition, the PI film was coated with MgF₂ on the light facing side and the effects on the optical and electrical properties of TCO and solar cells were investigated. The limitations on current density of solar cells due to optical losses in the PI substrate were estimated and compared to the experimentally achieved values. Flexible CdTe solar cells of highest efficiencies of 12.4% and 12.7% were achieved with and without anti-reflection MgF₂ coating, respectively.Laser scribing was used for patterning of layers and monolithically interconnected flexible solar modules exhibiting 8.0% total area efficiency on 31.9 cm² were developed by interconnection of 11 solar cells in series.     

Cost effective process for high-efficiency solar cells

A new method for patterning the rear passivation layers of high-efficiency solar cells with a mechanical scriber has been developed and successfully adapted to fabricate high-efficiency passivated emitter and rear cell (PERC). Three types of the rear contact patterns: dot patterns with a photolithography process, line and dashed line patterns with a mechanical scriber process have been processed in order to optimize the rear contact structure. An efficiency of 19.42% has been achieved on the mechanical-scribed (MS)-PERC solar cell on 0.5 Ω cm p-type FZ-Si wafer and is comparable to that of conventional PERC solar cells fabricated by using photolithography process. The mechanical scriber process shows great potential for commercial applications by achieving high efficiency above 20% and by significantly reducing the fabrication costs without an expensive photolithography process. Low-cost Ni/Cu metal contact has been formed by using a low-cost electroless and electroplating. Nickel silicide formation at the interface enhances stability and reduces the contact resistance resulting in an energy conversion efficiency of 20.2% on 0.5 Ω cm FZ wafer.     

High-efficiency OECO Czochralski-silicon solar cells for mass production

To improve the economy of photovoltaics, efficiencies of solar cells have to be drastically increased without using complex technologies. This work demonstrates that with the obliquely evaporated contact metal-insulator-semiconductor (MIS)-n⁺p solar cell structure recently developed at ISFH efficiencies exceeding 21% can be obtained using only four simple fabrication steps: (i) mechanical surface grooving, (ii) P-diffusion, (iii) oblique vacuum evaporation of Al, and (iv) plasma silicon nitride deposition. Cell design and processing sequences are outlined together with the importance of MIS contacts as both low-cost and high efficiency features. The custom-made pilot line equipment for mass production of 20% efficient 10×10 cm² Cz silicon solar cells including Ga doped wafers is described.     

Industrially feasible multi-crystalline metal wrap through (MWT) silicon solar cells exceeding 16% efficiency

On the way to higher efficiencies, back contact solar cells seem to be a promising alternative to the conventional screen-printed solar cells. Especially, the metal wrap through (MWT) solar cell concept with only two additional process steps is appropriate for a fast transfer to industry. Hence, an industrially feasible process based on a new contact design was developed and tested at the pilot-line of the Photovoltaic Technology Evaluations Center (PV-TEC). A maximum cell efficiency of 16% is achieved. Compared with conventionally processed cells made of the same mc Si-block, an efficiency gain of 0.5% absolute is observed. Due to a cell interconnection on the back the serial resistance losses in the tabs decrease. Therefore, a fill factor of almost 77% and an efficiency of 15% for a MWT module prototype (16 MWT cells) is reached.     

Recent progress on CdTe/CdS thin film solar cells

The CdTe/CdS thin film solar cell is the most suitable to be fabricated on the form of thin films. The processes used to make all the films, which compose the cell, are quite simple and fast. An efficiency of 16.5% has been reached on laboratory scale and modules of 0.6 × 1.2 m² with efficiency larger than 8% are now fabricated and commercialized. A strong contribution to the development of this type of solar cells has been given by the Parma University group with the discovery of a new ohmic back contact for CdTe which is very stable in respect to any other ohmic contact used for CdTe, and by the development of a new all dry process to make the cell. An efficiency of 15.8% has been recently obtained on a 10⁻⁴ m² soda-lime glass without using any copper or any other metal of the first group of the periodic table of the elements at the back contact.     

Optimization of ZnO for front and rear contacts in a-Si solar cells

The enhancement of the reflection from the rear contact of p-i-n a-Si solar cells using ZnO combined with metals (Ag/Al) as a back reflector was demonstrated theoretically and experimentally. Futhermore, the incorporation of unreacted H₂O as source gas in the ZnO films was clearly observed through the thermal evolution measurement, suggesting the need for employing the pre-annealing technique for ZnO films before using them as a front contact in p-i-n a-Si solar cells. By using these approaches, the a-Si solar cells with glass/annealed-ZnO/delta-doped p/buffer/i/n/ZnO/metals(Ag/Al) structure were successfully fabricated and a conversion efficiency of 12.1% (AM-1.5, area 3×3 mm²) was obtained. Moreover, the solar cells with a structure of AR coated glass/SnO₂/delta-doped/p/buffer/i/n/ZnO/metals(Ag/Al) were also fabricated and by optimizing the use of the ZnO layer at the rear contact, a conversion efficiency of 12.6% was obtained. To make the ZnO films more appropriate for solar cells application, the growth rate of the ZnO films was increased by increasing the flow rate of diethylzinc used as a source gas.     

Large-size multi-crystalline silicon solar cells with honeycomb textured surface and point-contacted rear toward industrial production

In this paper, we present a multi-crystalline solar cell with hexagonally aligned hemispherical concaves, which is known as honeycomb textured structure, for an anti-reflecting structure. The emitter and the rear surface were passivated by silicon nitride, which is known as passivated emitter and rear (PERC) structure. The texture was fabricated by laser-patterning of silicon nitride film on a wafer and wet chemical etching of the wafer beneath the silicon nitride film through the patterned holes. This process succeeded in substituting the lithographic process usually used for fabricating honeycomb textured structure in small area. After the texturing process, solar cells were fabricated by utilizing conventional fabrication techniques, i.e. phosphorus diffusion in tube furnace, deposition of anti-reflection film and rear passivation film by chemical vapor deposition, front and rear electrodes formation by screen printing, and contact formation by furnace. By adding relatively small complicating process to conventional production process, conversion efficiency of 19.1% was achieved with mc-Si solar cells of over 200 cm² in size. The efficiency was independently confirmed by National Institute of Advanced Industrial Science and Technology (AIST).     

Dye sensitized solar cells on paper substrates

This article reports for the first time in the literature, a dye sensitized solar cells with 1.21% efficiency (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) on paper substrates. The current dye sensitized solar cell technology is based on fluorine doped SnO₂ (FTO) coated glass substrates. The problem with the glass substrate is its rigidity and heavy weight. Making DSSCs on paper opens the door for both photovoltaic and paper industries. The potential of using mature paper making and coating technologies will greatly reduce the current PV cost. Paper substrate based DSSCs not only offer the advantages of flexibility, portability and lightweight but also provide the opportunities for easy implantation to textile. In this study, a low temperature process is developed to coat uniform nickel on paper substrate as the metal contact to replace the traditional expensive FTO. The Ni paper showed excellent conductivity of 8-10 Ω/□. It is found that the control of metal oxide electrode morphology is critical to solar cell performance. The TiO₂ film has the tendency to crack on Ni coated paper, which resulted in the shunt of the device and no solar cell efficiency was obtained. ZnO film on the other hand had good morphology tolerance on Ni coated paper and yielded solar cell efficiency of 1.21% (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) under AM 1.5 (activation area is 0.16 cm²). The control sample of ZnO solar cell on FTO glasses has the efficiency of 2.66% (V_oc=0.64 V, J_sc=9.97 mA/cm² and F.F.=0.42).     

Novel materials for high-efficiency III–V multi-junction solar cells

As a result of developing wide bandgap InGaP double hetero structure tunnel junction for sub-cell interconnection, InGaAs middle cell lattice-matched to Ge substrate, and InGaP-Ge heteroface structure bottom cell, we have demonstrated 38.9% efficiency at 489-suns AM1.5 with InGaP/InGaP/Ge 3-junction solar cells by in-house measurements. In addition, as a result of developing a non-imaging Fresnel lens as primary optics, a glass-rod kaleidoscope homogenizer as secondary optics and heat conductive concentrator solar cell modules, we have demonstrated 28.9% efficiency with 550-suns concentrator cell modules with an area of 5445 cm². In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. We have obtained the following results: (1) improvements of lattice-mismatched InGaP/InGaAs/Ge 3-junction solar cell property as a result of dislocation density reduction by using thermal cycle annealing, (2) high quality (In)GaAsN material for 4- and 5-junction applications by chemical beam epitaxy, (3) 11.27% efficiency InGaAsN single-junction cells, (4) 18.27% efficiency InGaAs/GaAs potentially modulated quantum well cells, and (5) 7.65% efficiency InAs quantum dot cells.     

Bifacial configurations for CdTe solar cells

We present a different back contact for CdTe solar cell by the application of only a transparent conducting oxide (TCO), typically ITO, as a back electrical contact on all-PVD CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. Also devices with thin CdTe layers (from 2 μm down to 1 μm) have been prepared to improve the conversion efficiency on the back side illumination, which is limited by the collection of carriers far away from the junction and to reduce the amount of material in the CdTe device. Reproducible solar cells exceeding 10% efficiency on the front side illumination and exceeding 3% on the back side illumination are reported.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Large area thin film Si module

An initial efficiency of 14.1% (J_sc=13.6 mA/cm², V_oc=1.392 V, FF=74.3%) has been achieved for a-Si/transparent interlayer/poly Si solar cell (total area of 1 cm²). Both a-Si and crystalline Si films were fabricated by plasma chemical deposition at low temperature. The short circuit current was enhanced by the introduction of a transparent intermediate layer. An initial aperture efficiency of 11.7% has been achieved for 910×455 mm² a-Si/poly Si integrated solar cell submodule, where the laser-scribing techniques were applied for series interconnections. The results of our first run of 266 submodules in our pilot plant showed the average efficiency of 11.2%, which is applicable for mass production.     

Wet-etch texturing of ZnO:Ga back layer on superstrate-type microcrystalline silicon solar cells

Surface wet etching is applied to the ZnO:Ga (GZO) back contact in μc-Si thin film solar cells. GZO transparency increases with increasing deposition substrate temperature. Texturing enhances reflective scattering, with etching around 5-6 s producing the best scattering, whereas etching around 5 s produces the best fabricated solar cells. Etching beyond these times produces suboptimal performance related to excessive erosion of the GZO. The best μc-Si solar cell achieves FF=68%, V_OC=471 mV and J_SC=21.48 mA/cm² (η=6.88%). Improvement is attributed to enhanced texture-induced scattering of light reflected back into the solar cell, increasing the efficiency of our lab-made single μc-Si solar cells from 6.54% to 6.88%. Improved external quantum efficiency is seen primarily in the longer wavelengths, i.e. 600-1100 nm. However, variation of the fabrication conditions offers opportunity for significant tuning of the optical absorption spectrum.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

a-Si/mc-Si hybrid solar cell using silicon sheet substrate

A hybrid junction solar cell with amorphous silicon (a-Si) and multicrystalline silicon (mc-Si) was fabricated using a mc-Si sheet substrate, which is produced directly from molten silicon using a novel rotational solidification method. The efficiency of 11.6% was obtained for the hybrid junction cell, while 10.2% for the single junction cell made of a mc-Si sheet substrate, which confirmed that the hybrid structure is effective to improve the solar cell property made of a mc-Si substrate. With introducing light trapping structure, the efficiency was improved to be 12.0%. Moreover, the possibility of Jsc improvement was investigated using the advanced light trapping structure. Jsc of 15.6 mA/cm² was obtained and it was confirmed that the hybrid junction is a promising structure.     

A study on the fabrication of polycrystalline Si wafer by direct casting for solar cell substrate

Si-wafers for solar cells were cast in a size of 50 × 46 × 0.5 mm³ by a direct casting method. A graphite mold coated by boron nitride (BN) powder was used in order to prevent the reaction between carbon and the molten silicon. Without any coating, the reaction of the Si melt to the graphite mold was very severe. In the case of BN coating, SiC was formed in the shape of tiny islands on the surface of the Si wafer by the reaction between the Si-melt and the carbon of the graphite mold at high temperature. The grain size was about 1 mm. The efficiency of the Si solar cell was about 0.5% under AM1.5 conditions. It was lower than that of a Si solar cell fabricated with a common single- (sc, 3.0%) and poly-crystalline (pc, 1.0%) Si wafer, which showed much lower efficiency than that of other commercial pc- or sc-Si solar cell (10–15%).     

Analysis of series resistance of crystalline silicon solar cell with two-layer front metallization based on light-induced plating

Improving the front metallization quality of silicon solar cells should be a key to enhance cell performance. In this work, we investigated a two-layer metallization scheme involving light-induced plating (LIP) and tried to quantify its impact on the series resistance of the front grid metals and FFs on finished cells. To estimate the effect of LIP processing on a printed and fired seed layer, individual components of series resistance were measured before and after LIP processing. Among them, grid resistance and contact resistance were closely observed because of their large contribution to series resistance. To optimize the plating on the seed metal grid, the grid resistance of the two-layer metal grid structure was calculated as a function of cross section area of the plated layer. Contact resistivity of the grid before and after LIP processing was analyzed to understand the contact resistance reduction, as well. As a result, the efficiency of solar cells with 80 μm seed metal grid width increased by 0.3% absolute compared with conventional solar cells of 120 μm metal grid width. The total area of electrodes in conventional cells was 1800 mm² and electrodes area of LIP processed solar cells was 1400 mm². The efficiency gain was due to reduction of shadowing loss from 7.7% to 6.0% without the increase of resistance due to two-layer front metallization.