Christopher P. Thompson

Device characterization of (AgCu)(InGa)Se 2 solar cells

Ag-alloying of Cu(InGa)Se<inf>2</inf> thin films presents the possibility to increase the bandgap with improved structural properties as a result of a lower melting temperature. (AgCu)(InGa)Se<inf>2</inf> films were deposited by elemental co-evaporation and the resulting solar cell behavior was characterized. While the bandgap in the highest efficiency Cu(InGa)Se<inf>2</inf> cells is ∼1.15 eV, Ag alloying allows the bandgap to be increased to 1.3 eV with an increase in V<inf>OC</inf>, no loss in device efficiency, and fill factors up to 80%. With high Ga content to increase bandgap > 1.5 eV, Ag alloying improves solar cell efficiency. Analysis of the device behavior shows that the basic mechanisms controlling (AgCu)(InGa)Se<inf>2</inf> solar cells and limiting performance with wide bandgap are comparable to those with Cu(InGa)Se<inf>2</inf>. Finally the effect of Na in (AgCu)(InGa)Se<inf>2</inf> devices is shown to be comparable to that with Cu(InGa)Se<inf>2</inf> including a decrease in V<inf>OC</inf> attributed to interface recombination with insufficient Na.

Temperature dependence of VOC in CdTe and Cu(InGa)(SeS)2-based solar cells

The temperature and intensity dependence of VOC in CdTe and Cu(InGa)(SeS)2 polycrystalline thin film solar cells was examined. VOC was measured from 100–330K and from 0.1 to 1 sun illumination. Two distinct regimes of temperature dependence are commonly observed: a linear regime at higher temperatures with slope −0.5 to −3 mV/K and a logarithmic intensity dependence; and a saturation regime at lower temperatures, with little intensity or temperature dependence. The T=0 K intercept extrapolated from the linear regime around 300K is related to the activation energy of the dominant recombination mechanism and is equal to the absorber bandgap for Shockley-Read-Hall recombination, or in some cases, from heterojunction interface recombination, which is less than the absorber bandgap. In this work, the temperature dependence of VOC will be characterized for CdTe and Cu(InGa)Se2 devices with differences in composition and processing conditions. Analysis will focus on the activation energy of the recombination mechanism and saturation at lower temperatures which indicates a maximum separation of the quasi Fermi levels as thermally activated SRH recombination is frozen out. The saturation voltage is ∼1 V for a typical CdTe device (Eg=1.45 eV), ∼1V for low bandgap Cu(InGa)Se2 (Eg=1.15), and ∼1.1 V for wider bandgap Cu(InGa)Se2 (Eg=1.38 eV).

Bandgap gradients in (Ag,Cu)(In,Ga)Se2 thin film solar cells deposited by three-stage co-evaporation

(Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells are optimized at bandgaps greater than 1.2 eV by varying composition profile of the absorber layer using a three-stage evaporation process. Numerical modeling and cumulative process data provides insight into the process. Silver alloying CIGS changes the optimized bandgap profile by reducing carrier concentration, and reducing bandgap gradients. The minimum bandgap position is controlled by the point when the film reaches I/III stoichiometry during the second stage of the three-stage process. We achieved a 19.9% efficient solar cell with VOC = 732 mV at a bandgap of 1.2 eV based on quantum efficiency.

The Effects of Device Geometry and TCO/Buffer Layers on Damp Heat Accelerated Lifetime Testing of Cu(In,Ga)Se

In Cu(In,Ga)Se2 solar cells encapsulated with polyethylene terephthalate (PET) or glass top sheets, the effects of damp heat (D-H) accelerated lifetime testing (ALT) depend on water vapor transmission rate (WVTR) of both transparent conducting oxide (TCO) and the intrinsic zinc oxide (i-ZnO) buffer, as well as device geometry. PET top sheets have a WVTR of ~10 g/m2·day, and glass has a WVTR of 0. Previously, coupons encapsulated with PET degraded to 50% of initial efficiency after 1000 h D-H ALT. We show that PET encapsulated coupons degrade at the same rate as glass encapsulated coupons after 2000 h D-H ALT to 92% of initial efficiency. The only change from previous work is that, here, i-ZnO covers the entire coupon surface, not the just active area. The WVTR of the i-ZnO/TCO stack is 2 × 10-3 g·H2O/m2·day. A set of unencapsulated devices went through D-H ALT, one where scribing was used to define the active area of the device and another without scribing; both were protected only by 50-nm i-ZnO. The bare-unscribed device performed as well as the previous glass and PET encapsulated coupons after 1500 h D-H ALT; the bare-scribed device degraded to 78% of initial efficiency, indicating that TCO integrity is a critical ALT parameter.

_{\bf 2}

Modification of Ga depth profiles in Cu(InGa)Se₂ films on polyimide substrates has coincided with device efficiencies approaching 19% at temperatures at least 100°C lower than on glass substrates. In the present study, (AgCu)(InGa)Se₂ films with nominal bandgaps E_g ≈ 1.4 eV have been deposited on polyimide substrates using various three-stage deposition processes to modify Ga depth profiles. Devices were characterized by J-V and quantum efficiency measurements for comparison to their Ga profiles. The highest-efficiency device had η = 17.9% with device parameters V_OC = 744 mV, J_SC = 32.2 mA/cm², and FF = 74.7%.

Solar Cells

The effect of the back surface patterning process on solar cell performance is evaluated for an interdigitated back contact silicon heterojunction (IBC-SHJ) structure. Three different patterning processes, two step photolithography, two step photolithography with laser fired base contact, and one step photolithography are investigated, allowing independent evaluation of the interfaces between the intrinsic amorphous silicon (i.a-Si:H) layer that passivates the c-Si and p- and n-a-Si:H layers. The fill factor of the IBC-SHJ solar cell critically depends on the quality of i/p a-Si:H interface, while the open circuit voltage is governed by the surface passivation quality of the entire back surface.

Control of Ga profiles in (AgCu)(InGa)Se 2 absorber layers deposited on polyimide substrates

Cu(In, Ga)Se2 (CIGS)-based solar cells from six fabricators were characterized and compared. The devices had differing substrates, absorber deposition processes, buffer materials, and contact materials. The effective bandgaps of devices varied from 1.05 to 1.22 eV, with the lowest optical bandgaps occurring in those with metal-precursor absorber processes. Devices with Zn(O, S) or thin CdS buffers had quantum efficiencies above 90% down to 400 nm. Most voltages were 250–300 mV below the Shockley–Queisser limit for their bandgap. Electroluminescence intensity tracked well with the respective voltage deficits. Fill factor (FF) was as high as 95% of the maximum for each devices respective current and voltage, with higher FF corresponding to lower diode quality factors (∼1.3). An in-depth analysis of FF losses determined that diode quality reflected in the quality factor, voltage-dependent photocurrent, and, to a lesser extent, the parasitic resistances are the limiting factors. Different absorber processes and device structures led to a range of electrical and physical characteristics, yet this investigation showed that multiple fabrication pathways could lead to high-quality and high-efficiency solar cells.

Sensitivity of surface passivation and interface quality in IBC-SHJ solar cells to patterning process

Cu(In,Ga)(S,Se)2 solar cells, prepared by a selenization/sulfization reaction are characterized by GD-OES, JV, and capacitance spectroscopy. A numerical model of the solar cells is used to optimize the compositional profile: the S/(S+Se) ratio at the interface, and the thickness of the S-containing layer near the surface and the Ga/(In+Ga) minimum. Device behavior is modeled using the composition profiles, and an acceptor-like defect 160-230 meV above the valence band. Comparing devices from separate processes, we found that differences in composition and defects measured with capacitance account for the differences performance. Ga/(In+Ga) at the surface determines the optimal S/(S+Se) surface ratio.

Comparison of CIGS Solar Cells Made With Different Structures and Fabrication Techniques

Light or LASER beam induced current (LBIC) mapping is a useful technique for imaging of spatial non-uniformities within photovoltaic devices. Imaging integrated modules, measured photocurrents can vary widely from one cell to the next depending upon the voltage bias applied to the primary anode and cathode. Such variation with bias suggests the possibility of characterizing individual interconnected cells using LBIC. We have analyzed the voltage bias sensitivity of ILBIC to obtain specific quantitative values of shunt resistance (Rsh) and unique signatures of high and low diode saturation current (Jo). This allows LBIC to be applied to failure analysis, quality control, and accelerated life testing of completed modules where electrical access to individual cells is not possible. The application of this method of analysis can be used for all integrated module photovoltaic technologies, not just thin films.

Characterization and numerical modeling of Cu(In,Ga)(S,Se)2 solar cells

Thin film materials for photovoltaics such as cadmium telluride (CdTe), copper-indium diselenide-based chalcopyrites (CIGS), and lead iodide-based perovskites offer the potential of lower solar module capital costs and improved performance to microcrystalline silicon. However, for decades understanding and controlling hole and electron concentration in these polycrystalline films has been extremely challenging and limiting. Ionic bonding between constituent atoms often leads to tenacious intrinsic compensating defect chemistries that are difficult to control. Device modeling indicates that increasing CdTe hole density while retaining carrier lifetimes of several nanoseconds can increase solar cell efficiency to 25%. This paper describes in-situ Sb, As, and P doping and post-growth annealing that increases hole density from historic 1014 limits to 1016–1017 cm−3 levels without compromising lifetime in thin polycrystalline CdTe films, which opens paths to advance solar performance and achieve costs below conventional electricity sources.