CdTe-based Thin Film Solar Cell Research at the Institute of Energy Conversion

Why CdTe Solar Cells?

The promise of solar cells having high conversion efficiency and low production cost makes CdTe an attractive contender for terrestrial solar power generation.  The viability of CdTe for near-term low cost photovoltaics is demonstrated by the success of one thin film manufacturing company and the recent burst of CdTe-related R&D at over nine other commercial entities.

CdTe is a direct band gap compound semiconductor with a nearly optimal match to the solar spectrum for terrestrial photovoltaic energy conversion.  The direct band gap, E_g = 1.5 eV, and high absorption coefficient, α > 5 x 10⁵ cm^-1, account for the high quantum yield from the ultraviolet to the CdTe band gap, λ ~ 825 nm.  Short wavelength photons, with energy greater than E_g, are absorbed near the CdTe surface, making CdTe an attractive absorber layer material in thin-film solar cells, with 99% photon absorption occurring within 2 µm of film thickness.  Champion CdTe thin film solar cells have achieved conversion efficiencies exceeding 16%.

CdTe is a chemically stable compound which can be synthesized in thin film form by numerous techniques such as physical vapor deposition, vapor transport deposition, sputter deposition, electrodeposition, chemical bath deposition, sintering, and more.  The CdTe conductivity type and doping concentration is controlled by the influence of intrinsic defects related to Cd-Te stoichiometry and extrinsic dopants.  As a group II-VI compound with tetrahedral cation-anion bonding, CdTe can be alloyed in isostructural combinations with other group II and group VI elements, allowing variation of the band gap; for example, substituting Zn for Cd to form Cd_1-x Zn_xTe or Se for Te to form CdTe_1-ySe_y widens the band gap.  Substituting Hg for Cd to form the alloy Hg_1-xCd_x Te narrows the band gap, and has found wide application for infrared detectors.

CdTe Solar Cell R&D at the Institute of Energy Conversion

The Institute of Energy Conversion (IEC) has been involved in research and development of group II-VI semiconductor thin film photovoltaic devices since its founding in 1972.  R&D in the 1970’s and early 1980’s was focused on CdS/Cu₂S solar cells and methods for roll-to-roll coating onto flexible substrates.  A program for the development of polycrystalline CdTe thin film solar cells was initiated in 1985 and continues to the present.

The IEC CdTe program has been a significant contributor to the thin film PV community for over 20 years, having contributed original R&D, collaborated with other university and industrial groups, given support to industrial entities, trained researchers and maintaining active participation in the now defunct NREL CdTe R&D team, which provided extensive support to First Solar LLC and its predecessor Solar Cells, Inc as well as the now defunct BP Solar Apollo product.  Highlights of IEC CdTe achievements over the years which provided the experience needed to offer support include: development of low-temperature substrate configuration CdTe solar cells with evaporated CdTe which achieved >6% efficiency, development of transparent back contacts to CdTe based on ITO and ZnTe, fabrication of the first monolithic polycrystalline thin film solar cell using CuInSe₂ and CdTe, quantifying the effects of CdCl₂ treatment on the chemical and optoelectronic properties of the CdS-CdTe couple, quantifying 3-dimensional CdS-CdTe interdiffusion in polycrystalline thin films using x-ray diffraction, development of the chemical surface deposition for CdS and Cd_1-xZn_xS window layer deposition, development of a vapor transport deposition process for II-VI compounds, development of vapor treatments of polycrystalline films using metal halides, determining reversible and non-reversible components of stress-induced degradation of CdTe/CdS solar cells, determining electrical operation of CdTe/CdS devices by using bifacial analysis of cells with transparent contacts, and development of high throughput processes for cell fabrication.

Today IEC maintains a baseline cell fabrication process for fabricating CdTe/CdS solar cells onto commercially available tin-oxide coated glass.  The best cells made at IEC on commercial glass have efficiency >13% and use IEC-patented processes for CdS and CdTe film deposition and vapor treatment of the semiconductors.  Conformal and robust ultra-thin CdS films, with 80 nm thickness, are deposited by chemical surface deposition.  CdTe is deposited by vapor transport deposition at low vacuum (20-100 Torr) onto moving substrates at deposition rate from 1 to 80 microns per minute.

With the primary goal of defining a pathway towards low-cost CdTe modules with 15% efficiency, R&D emphasis at the IEC has been directed at device fabrication and analysis, high throughput processing methods, incorporation of sub-micron CdTe absorber layers, and development of flexible CdTe solar cells.

Chemical surface deposited CdS film on 10 cm x 10 cm plate of Pilkington TEC15 glass.

IEC vapor transport system for deposition of CdTe and alloys by using a carrier gas to entrain and transport Cd and Te₂ vapor to a substrate translating beneath.

Optical transmission of vapor transport deposited CdTe with different thickness.

Atomic force micrographs of surface topography of vapor transport deposited Cd_1-xZn_xTe films with different Zn content.

Rigaku D/Max 2200 diffractometer configured with parallel beam optics and hot stage to investigate formation of native oxides on CdTe.

Glancing incidence x-ray diffraction configuration used for in-situ measurement of CdTe-CdS thin film interface during thermal anneal with CdCl₂ vapor.

GIXRD scans and interpreted interface composition of VT CdTe during thermal anneal with CdCl₂ vapor.

Equilibrium phase diagram for the CdTe-CdS pseudobinary system determined by IEC using x-ray diffraction analysis, showing measured (solid markers) and modeled (open markers) phase boundaries.

Diode characteristics and quantum efficiency of IEC polycrystalline thin film solar cell with Cd0.9Zn0.1Te absorber layer deposited by vapor transport.  Cell structure is: SLG/SnO₂ (TEC15)/Ga₂ O₃ /CdS/CdTe/Cu/Ni.

IEC CdTe/CdS 10 cm x 10 cm mini-module fabricated using vapor transport deposited CdTe and chemical surface deposited CdS, with graphite back contact.