Ultra thin absorbers

(a) (b)

Figure 10.15: Experimental J −V curves of superstrate CIGSe solar cells with Au and MoO_3-x/Ag as the back contact,a) without any sodium presentb)with NaF PDT at 300‰ for both samples.

(a) (b)

(c)

Figure 10.16: a)Dependence of the power conversion efficiency on the CIGSe layer thickness.

The recombination at the back contact is set to zero. Incomplete absorption and back contact recombination leads to a reduced PCE for thin layers. b)Thickness dependence of the efficiency for devices with a MoO3−x/Ag back contact and differently strong Ga gradients at the back surface. c) Energy band diagram, which shows the Ga gradient at the back contact, which reduces the back contact recombination and increases the electron collection.

The maximum efficiency of 18.9 % for the MoO_3-x/Ag device, in the absence of back contact recombination, is reached for the total CIGSe thickness of 1µm. For thinner layers, the light absorption is incomplete and for thicker layers the bulk recombination is increased due to the small SCR at a charge carrier density of 1e+16 cm⁻³. For the MoSe_x/Mo stack in substrate devices the maximum PCE is lowered to 18.8 % and shifted to 1.4µm due to the reduced back contact reflectivity. At a total CIGSe thickness of 600 nm the PCE is reduced to only 18.6 % for the superstrate device and 18.0 % for the substrate device. For layers thinner than 500 nm, the PCE drops sharply. To reach an efficiency of 18.6 %, the thickness of the CIGSe layer has to be increases to 1.0µm.

It is questionable whether the back contact can be fully passivated in a real device.

But the back contact recombination can be also reduced by increasing the conduction band gradient ∆E_V,BC at the back contact from 250 meV to 500 meV. It even increases the PCE further due to the improved electron collection from the back. Such a steep Ga gradient is difficult to realize in substrate devices due to the inter-diffusion of In and Ga during the deposition. In superstrate devices though it was shown in Sec. 2.1 that it can be realized. This increases the maximum PCE to 19.0 % for a total thickness of 1µm and for 600 nm to 18.75 %.

The PCE could be increased even further due to the removal of the implemented Schottky contact (not shown here), which improves the V_OC value.

Experimentally, superstrate devices with thin and thick absorbers are difficult to com-pare due to the different interface formation. The chemical composition at the interface during the growth differs as well as the duration of the growth process. This leads to dif-ferent charge carrier collection efficiencies and difdif-ferentVOC values. Fig. 10.17a shows the J−V curve of a standard device with a 2.8µm thick CIGSe layer and the same device with a 0.75µm thick CIGSe layer. The photo current at negative voltages drops by 1.5 mA/cm² in the thinner device, which is exactly the expected value from the simulation. The VOC

and the FF are difficult to compare though, since the interface properties are not directly comparable. The FF of the thin device is slightly better, which compensates the loss in VOC. Still, the PCE for both devices are the same, as expected from the simulation.

Light scattering

Another option, to reduce the CIGSe thickness, is to increase the light pathway within the absorber by light scattering. This can happen at the front or at the back contact.

The back contact has a given roughness due to the CIGSe surface roughness. This is typically around 60 nm (RMS) for the co-evaporated layers prepared during this work.

The amount of diffuse scattering at the back contact can be calculated with the help of the simple scalar scattering theory [190] [191] as follows:

R_{dif f use} =R_tot·exp(−

2πσ_rms2n_CIGSe(λ)cos(θ) λ

2

), (10.1)

(a) (b)

Figure 10.17: J−V curve of superstrate devices with CIGSe layers fabricated at 520‰and witha)different thicknessb)plain and untreated ZnO as well as HCl etched ZnO.

with σ_rms describing the RMS surface roughness, n_CIGSe(λ) the wavelength dependent refractive index of CIGSe, θ the incoming angle to the surface normal. Equation 10.1 is only valid if diffraction effects dominate the light reflection, which is the case for surfaces whose facets are smaller or comparable to the wavelengths of the reflected light. For typical CIGSe layers this is given for light in the visible and infrared region. The percent-age of diffuse reflected light at the back contact is given in Fig. 10.19a. For the typical roughness of 60 nm the diffuse reflected light is 90 % in the infrared region. Assuming a cos² angle distribution of the reflected light, the average optical path of photons with λ = 1200 nm would increase by 30 % for one way through the absorber. The minimum thickness of 500 nm could be further reduced to 440 nm in order to achieve 19.1 % with the in the previous section described model. The MoSe_x/Mo reference would lead to a PCE of 18.3 %. To reach 19.1 % a thickness of 710 nm would be required.

In substrate devices the CIGSe surface roughness leads to light scattering at the front contact. The equation to describe the diffuse transmission is similar to Eq. 10.1, but the refractive index in the enumerator of the experiential is exchanged by half of the difference of the refractive index of ZnO and CIGSe:

T_{dif f use} =T_tot·exp(−

2πσ_rms(n_CIGSe(λ)−n_ZnO(λ))cos(θ) λ

2

) (10.2)

Fig. 10.19b shows the percentage of diffuse transmittance of the total transmittance for different values of the surface roughness. For the typical CIGSe roughness of 60 nm the diffuse transmittance is 6 % and 94 % of the infrared light is transmitted specular. Thus, the average optical pathway increases only 2 % for one way through the absorber, which does not reduce the minimum absorber thickness noticeably.

The fraction of diffuse transmittance through the ZnO can be controlled in superstrate devices by controlling the ZnO roughness prior to the CIGSe deposition. This is a standard

(a) (b)

Figure 10.18: a) Percentage of diffuse reflected light from the total reflected light at the CIGSe/Au interface in superstrate devices. b)Percentage of diffuse transmitted light from the total transmitted light at the ZnO/CIGSe interface in substrate devices.

procedure in amorphous silicon solar cells and usually achieved by HCl etching [192].

Achieving strong scattering for long wavelength photons with energies close to the band gap of CIGSe devices is more difficult, due to the lower band gap and the lower refractive index. To describe the scattering from a HCL etched ZnO it was found in [193] that the exponent within the exponential function of Eq. 10.2 has to be changed from 2 to 3, to account for the different roughness profile of the etched ZnO compared to the natural CIGSe surface profile. This leads to the following formula:

T_{dif f use} =T_tot·exp(−

4πσ_rms(n_CIGSe(λ)−n_ZnO(λ))cos(θ) λ

3

) (10.3)

With this equation the measured diffuse transmittance of a HCL etched ZnO substrate can be fitted to obtain the RMS roughness. Fig. 10.18a shows the measured and the cal-culated diffuse transmission spectra, which are in very good agreement. The HCl etching was performed for 60 s in a 1 wt.% HCl solution, as done in [194]. A RMS roughness of 46 nm is obtained from the fit. The calculated diffuse transmittance at the ZnO/CIGSe interface in the infrared only increases by 0.2 %. Fig. 10.18b shows the dependence of the diffuse transmittance at the CIGSe/ZnO interface. In order to get sufficient scattering to reduce the layer thickness, the ZnO RMS roughness has to be above 100 nm for CIGSe superstrate devices. This high roughness is likely to lead to shunting of the only 450 nm thick CIGSe layer. Further it will increase the series resistance. The sheet resistance of the 1µm thick test sample had a roughness of 46 nm and the sheet resistance increased from 6W/ to 13W/ due to the etching. This requires thicker ZnO layers, which in turn increase the absorption losses within the ZnO and with it the PCE of the device independent of the CIGSe thickness. The positive effect of the increased light scattering is therefore exceeded by the negative effects from the increased series resistance or light

(a) (b)

Figure 10.19: a)The total diffuse transmitted light at the HCl etched ZnO interface with air, fitted the function defined in Eq. 10.3. b)Percentage of the light which is diffuse transmitted at the HCl etched ZnO interface with CIGSe.

absorption.

Experimentally, the J−V curves of superstrate devices with etched ZnO layers, show very low fill factors and PCEs. Fig. 10.17b shows one example. Most likely this is caused by a combination of increased series resistance and increased interface recombination, due to the increased surface area. The parallel resistance, obtained from the darkJ−V curve, is however not reduced due to the 46 nm RMS roughness of the ZnO layer.