Bart Vermang

Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In,Ga)Se2 solar cells

Reducing absorber layer thickness below 500 nm in regular Cu(In,Ga)Se2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385 nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells.

Improved Rear Surface Passivation of Cu(In,Ga)Se

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (VOC) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al2O3 is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al2O3 rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in VOC compared with unpassivated reference cells.

_{\bf 2}

Recently, Cu(In,Ga)Se2 (CIGS) solar cells have achieved 21% world-record efficiency, partly due to the introduction of a postdeposition potassium treatment to improve the front interface of CIGS absorber layers. However, as high-efficiency CIGS solar cells essentially require long diffusion lengths, the highly recombinative rear of these devices also deserves attention. In this paper, an Al2O3 rear surface passivation layer with nanosized local point contacts is studied to reduce recombination at the standard Mo/CIGS rear interface. First, passivation layers with well-controlled grids of nanosized point openings are established by use of electron beam lithography. Next, rear-passivated CIGS solar cells with 240-nm-thick absorber layers are fabricated as study devices. These cells show an increase in open-circuit voltage (+57 mV), short-circuit current (+3.8 mA/cm2), and fill factor [9.5% (abs.)], compared with corresponding unpassivated reference cells, mainly due to improvements in rear surface passivation and rear internal reflection. Finally, solar cell capacitance simulator (SCAPS) modeling is used to calculate the effect of reduced back contact recombination on high-efficiency solar cells with standard absorber layer thickness. The modeling shows that up to 50-mV increase in open-circuit voltage is anticipated.

Solar Cells: A Combination of an Al

This work proves that blistering is the partial delamination of a thick enough Al<inf>2</inf>O<inf>3</inf> layer caused by gaseous desorption in the Al<inf>2</inf>O<inf>3</inf> layer upon thermal treatments above a critical temperature: the Al<inf>2</inf>O<inf>3</inf> layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al<inf>2</inf>O<inf>3</inf> samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H<inf>2</inf>O, illustrates that gaseous desorption from Al<inf>2</inf>O<inf>3</inf> and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al<inf>2</inf>O<inf>3</inf> layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (< 10 nm) Al<inf>2</inf>O<inf>3</inf> film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al<inf>2</inf>O<inf>3</inf> film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> layers can still be adequate passivation layers after being annealed in N<inf>2</inf> environment at temperatures up to 500–700 °C: (i) interface trap densities (D<inf>it</inf>) can remain below 1×10¹¹ cm⁻² and (ii) fixed charge densities (Q<inf>f</inf>) stay negative and in the order of −3×10¹² cm⁻² Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> passivation layers and performing an annealing step prior to capping and co-firing. Al<inf>2</inf>O<inf>3</inf>/SiN<inf>x</inf> passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.

_{\bf 2}

Atomic layer deposited (ALD) Al2O3 films on Cu(In, Ga)Se-2 (CIGS) surfaces have been demonstrated to exhibit excellent surface passivation properties, which is advantageous in reducing recombinatio ...

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A next generation material for Si surface passivation is atomic layer deposited (ALD) Al<inf>2</inf>O<inf>3</inf>. However, conventional time-resolved ALD is limited by its low deposition rate. Initially, a high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed, with Al<inf>2</inf>O<inf>3</inf> deposition rates up to 1.2 nm/s. Later, the spatial ALD technique has been transferred to an actual in-line process development tool (PDT) for commercial high-throughput ALD of Al<inf>2</inf>O<inf>3</inf>, resulting in a deposition rate of 30 nm/min. The passivation quality and uniformity of the spatially-separated ALD Al<inf>2</inf>O<inf>3</inf> films are evaluated on p- and n-type Si, applying quasi-steady-state photo-conductance, carrier density imaging and infrared lifetime mapping. In all cases, a spatial ALD Al<inf>2</inf>O<inf>3</inf> layer of only 10 nm reached an excellent passivation quality and uniformity, comparable to reference wafers passivated by equivalent temporal plasma-assisted or thermal ALD Al<inf>2</inf>O<inf>3</inf>. Effective surface recombination velocities as low as 1.1 or 2.9 cm/s were obtained after annealing at 350 °C or firing, respectively. Using spatial ALD Al<inf>2</inf>O<inf>3</inf> passivated local Al back surface field p-type Si solar cells, the sufficient passivation of this high-throughput Al<inf>2</inf>O<inf>3</inf> layer is evaluated: an average gain in open circuit voltage as compared to SiO<inf>x</inf> rear passivated i-PERC cells is obtained.

_{\bf 3}

This study deals with potential-induced degradation (PID) of Cu(In,Ga)Se2-based solar cells and different approaches to subsequent recovery of efficiency. Three different recovery methods were studied: 1) etch recovery, 2) accelerated recovery, and 3) unaccelerated recovery. After being completely degraded, the solar cells with CdS buffer layers recovered their efficiencies at different rates, depending on the method which was used. On the other hand, if Zn(O,S) was used as a buffer layer instead of CdS, the recovery rate was close to zero. The buffer layer type clearly influenced the sodium distribution during PID stressing and recovery, as well as the possibilities for recovery of the electrical performance.

Rear Surface Passivation Layer and Nanosized Local Rear Point Contacts

Thermal atomic layer deposition (ALD) of Al2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre-deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequently exhibits better surface passivation uniformity.

Introduction of Si PERC Rear Contacting Designto Boost Efficiency of Cu(In,Ga)Se2 Solar Cells

This work characterizes p-type Silicon surface passivation using a high-k material (Al2O3 or HfO2) combining capacitance voltage (CV) and lifetime measurements. For AI2O3 samples, the Silicon substrate bulk and surface quality is equivalent to CZ Silicon used in industrial solar cell processing. While AI2O3 has been proven to provide high quality surface passivation on p-type doped Silicon surfaces, the influence of the growth conditions and the post-deposition annealing is not yet completely understood. The dielectric thin film has been deposited by common techniques (ALD, PECVD) on H-/OHterminated Silicon surfaces (hydrophobic and hydrophilic, respectively). The impact of the roughness of the surface prior to the deposition has been also considered. Then, the passivation of each layer has been investigated as a function of different AI2O3 thicknesses (5 to 20 nm) and post-deposition annealing temperatures (300 to 800°C). CV measurements have been used to characterize chemical passivation (= interface trap density, Dit) and field effect passivation (= fixed charge density, Qf). Lifetime measurements have been used to assess the effective surface passivation. The results of both types of electrical characterization fit well together. (i) Prior post-deposition anneal, only either chemical passivation (ALO) or field effect passivation (PECVD) is adequate, resulting in lower effective lifetimes. (ii) At higher annealing temperatures, a negative net charge in the AI2O3 and a low Dit at the interface are measured, ideal for p-type CZ Silicon passivation and causing maximal effective lifetimes. (iii) At too high annealing temperatures, chemical passivation is destroyed resulting in decreasing effective lifetimes even though negative field effect remains in many cases. Another candidate as passivation layer on Silicon is HfO2. Being a new material in photovoItaics, it has been studied on FZ Silicon substrates and its electrical characterization has demonstrated interesting passivation properties at low anneal temperatures (also without thermal treatment).

On the blistering of atomic layer deposited Al 2 O 3 as Si surface passivation

This work investigates the formation of blisters during annealing an Al₂O₃ film grown by atomic layer deposition (ALD) on Si. This blistering phenomenon is shown to occur under an external load in the presence of a tensile residual stress: the total membrane stress is a super-imposition of the residual stress of the pre-stressed film and a concomitant stress due to change of blister profile. In the case of this Si/Al₂O₃ system, the film is indeed pre-stressed and the hydrostatic pressure is caused by (i) the effusion of H₂ and H₂O and (ii) Al₂O₃ being a diffusion barrier.